Goodman and Gilman Manual of Pharmacology and Therapeutics
Neurotransmission: The Autonomic and Somatic Motor Nervous Systems
ANATOMY AND GENERAL FUNCTIONS
The autonomic nervous system (ANS; a.k.a. the visceral, vegetative, or involuntary nervous system) regulates autonomic functions that occur without conscious control. In the periphery, it consists of nerves, ganglia, and plexuses that innervate the heart, blood vessels, glands, other visceral organs, and smooth muscle in various tissues.
DIFFERENCES BETWEEN AUTONOMIC AND SOMATIC NERVES
• The efferent nerves of the ANS supply all innervated structures of the body except skeletal muscle, which is served by somatic nerves.
• The most distal synaptic junctions in the autonomic reflex arc occur in ganglia that are entirely outside the cerebrospinal axis. Somatic nerves contain no peripheral ganglia, and the synapses are located entirely within the cerebrospinal axis.
• Many autonomic nerves form extensive peripheral plexuses; such networks are absent from the somatic system.
• Postganglionic autonomic nerves generally are nonmyelinated; motor nerves to skeletal muscles are myelinated.
• When the spinal efferent nerves are interrupted, smooth muscles and glands generally retain some level of spontaneous activity, whereas the denervated skeletal muscles are paralyzed.
VISCERAL AFFERENT FIBERS. The afferent fibers from visceral structures are the first link in the reflex arcs of the autonomic system. With certain exceptions, such as local axon reflexes, most visceral reflexes are mediated through the central nervous system (CNS).
Information on the status of the visceral organs is transmitted to the CNS through 2 main sensory systems: the cranial nerve (parasympathetic) visceral sensory system and the spinal (sympathetic) visceral afferent system. The cranial visceral sensory system carries mainly mechanoreceptor and chemosensory information, whereas the afferents of the spinal visceral system principally convey sensations related to temperature and tissue injury of mechanical, chemical, or thermal origin.
Cranial visceral sensory information enters the CNS by 4 cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. These 4 cranial nerves transmit visceral sensory information from the internal face and head (V); tongue (taste, VII); hard palate and upper part of the oropharynx (IX); and carotid body, lower part of the oropharynx, larynx, trachea, esophagus, and thoracic and abdominal organs (X), with the exception of the pelvic viscera. The pelvic viscera are innervated by nerves from the second through fourth sacral spinal segments. The visceral afferents from these 4 cranial nerves terminate topographically in the solitary tract nucleus.
Sensory afferents from visceral organs also enter the CNS from the spinal nerves and convey information concerned with temperature as well as nociceptive visceral inputs related to mechanical, chemical, and thermal stimulation. Those concerned with muscle chemosensation may arise at all spinal levels, whereas sympathetic visceral sensory afferents generally arise at the thoracic levels where sympathetic preganglionic neurons are found. The neurotransmitters that mediate transmission from sensory fibers have not been characterized unequivocally. Substance P and calcitonin gene-related peptide (CGRP), are leading candidates for neurotransmitters that communicate nociceptive stimuli from the periphery. Somatostatin (SST), vasoactive intestinal polypeptide (VIP), and cholecystokinin (CCK), also occur in sensory neurons. ATP appears to be a neurotransmitter in certain sensory neurons. Enkephalins, present in interneurons in the dorsal spinal cord, have antinociceptive effects both pre- and postsynaptically to inhibit the release of substance P. The excitatory amino acids glutamate and aspartate also play major roles in transmission of sensory responses to the spinal cord. These transmitters and their signaling pathways are reviewed in Chapter 14.
DIVISIONS OF THE PERIPHERAL AUTONOMIC SYSTEM. The ANS consists of 2 large divisions: the sympathetic and the parasympathetic (Figure 8–1).
Figure 8–1 The autonomic nervous system. Yellow, cholinergic; red, adrenergic; dotted blue, visceral afferent; solid lines, preganglionic; broken lines, postganglionic. The rectangle at right shows the finer details of the ramifications of adrenergic fibers at any 1 segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerves. The asterisk (*) indicates that it is not known whether these vasodilator fibers are motor or sensory or where their cell bodies are situated.
The neurotransmitter of all preganglionic autonomic fibers, most postganglionic parasympathetic fibers, and a few postganglionic sympathetic fibers is acetylcholine (ACh). Some postganglionic parasympathetic nerves use nitric oxide (NO) and are referred to as nitrergic. The majority of the postganglionic sympathetic fibers are adrenergic, in which the transmitter is norepinephrine (NE, noradrenaline). The terms cholinergic and adrenergic describe neurons that liberate ACh or NE, respectively. Substance P and glutamate may also mediate many afferent impulses.
SYMPATHETIC NERVOUS SYSTEM. The cells that give rise to the preganglionic fibers of this division lie mainly in the intermediolateral columns of the spinal cord and extend from the first thoracic to the second or third lumbar segment. The axons from these cells are carried in the anterior (ventral) nerve roots and synapse, with neurons lying in sympathetic ganglia outside the cerebrospinal axis. Sympathetic ganglia are found in 3 locations: paravertebral, prevertebral, and terminal.
The 22 pairs of paravertebral sympathetic ganglia form the lateral chains on either side of the vertebral column. The ganglia are connected to each other by nerve trunks and to the spinal nerves by rami communicantes. The white rami carry the preganglionic myelinated fibers that exit the spinal cord by the anterior spinal roots. The gray rami carry postganglionic fibers back to the spinal nerves for distribution to sweat glands and pilomotor muscles and to blood vessels of skeletal muscle and skin. The prevertebral ganglia lie in the abdomen and the pelvis near the ventral surface of the bony vertebral column and consist mainly of the celiac (solar), superior mesenteric, aorticorenal, and inferior mesenteric ganglia. The terminal ganglia are few in number, lie near the organs they innervate, and include ganglia connected with the urinary bladder and rectum and the cervical ganglia in the region of the neck. In addition, small intermediate ganglia lie outside the conventional vertebral chain, especially in the thoracolumbar region. They are variable in number and location but usually are in close proximity to the communicating rami and the anterior spinal nerve roots.
Preganglionic fibers from the spinal cord may synapse with the neurons of more than 1 sympathetic ganglion. Their principal ganglia of termination need not correspond to the original level from which the preganglionic fiber exits the spinal cord. Many of the preganglionic fibers from the fifth to the last thoracic segment pass through the paravertebral ganglia to form the splanchnic nerves. Most of the splanchnic nerve fibers do not synapse until they reach the celiac ganglion; others directly innervate the adrenal medulla (see below).
Postganglionic fibers arising from sympathetic ganglia innervate visceral structures of the thorax, abdomen, head, and neck. The trunk and the limbs are supplied by the sympathetic fibers in spinal nerves. The prevertebral ganglia contain cell bodies whose axons innervate the glands and smooth muscles of the abdominal and the pelvic viscera. Many of the upper thoracic sympathetic fibers from the vertebral ganglia form terminal plexuses, such as the cardiac, esophageal, and pulmonary plexuses. The sympathetic distribution to the head and the neck (vasomotor, pupillodilator, secretory, and pilomotor) is by means of the cervical sympathetic chain and its 3 ganglia. All postganglionic fibers in this chain arise from cell bodies located in these 3 ganglia; all preganglionic fibers arise from the upper thoracic segments of the spinal cord, there being no sympathetic fibers that leave the CNS above the first thoracic level.
Pharmacologically, the chromaffin cells of the adrenal medulla resemble a collection of postganglionic sympathetic nerve cells. Typical preganglionic fibers that release ACh innervate these chromaffin cells, stimulating the release of epinephrine (EPI, adrenaline), in distinction to the NE released by postganglionic sympathetic fibers.
PARASYMPATHETIC NERVOUS SYSTEM. The parasympathetic nervous system consists of preganglionic fibers that originate in the CNS and their postganglionic connections. The regions of central origin are the midbrain, the medulla oblongata, and the sacral part of the spinal cord. The midbrain, or tectal, outflow consists of fibers arising from the Edinger-Westphal nucleus of the third cranial nerve and going to the ciliary ganglion in the orbit. The medullary outflow consists of the parasympathetic components of the VII, IX, and X cranial nerves.
The fibers in the VII (facial) cranial nerve form the chorda tympani, which innervates the ganglia lying on the submaxillary and sublingual glands. They also form the greater superficial petrosal nerve, which innervates the sphenopalatine ganglion. The autonomic components of the IX (glossopharyngeal) cranial nerve innervate the otic ganglia. Postganglionic parasympathetic fibers from these ganglia supply the sphincter of the iris (pupillary constrictor muscle), the ciliary muscle, the salivary and lacrimal glands, and the mucous glands of the nose, mouth, and pharynx. These fibers also include vasodilator nerves to these same organs. The X (vagus) cranial nerve arises in the medulla and contains preganglionic fibers, most of which do not synapse until they reach the many small ganglia lying directly on or in the viscera of the thorax and abdomen. In the intestinal wall, the vagal fibers terminate around ganglion cells in the myenteric and submucosal plexuses. Thus, in the parasympathetic branch of the autonomic nervous system, preganglionic fibers are very long, whereas postganglionic fibers are very short. The vagus nerve also carries a far greater number of afferent fibers (but apparently no pain fibers) from the viscera into the medulla. The parasympathetic sacral outflow consists of axons that arise from cells in the second, third, and fourth segments of the sacral cord and proceed as preganglionic fibers to form the pelvic nerves (nervi erigentes). They synapse in terminal ganglia lying near or within the bladder, rectum, and sexual organs. The vagal and sacral outflows provide motor and secretory fibers to thoracic, abdominal, and pelvic organs (see Figure 8–1).
ENTERIC NERVOUS SYSTEM. The processes of mixing, propulsion, and absorption of nutrients in the GI tract are controlled through the enteric nervous system (ENS). The ENS consists of both afferent sensory neurons and a number of motor nerves and interneurons that are organized principally into 2 nerve plexuses: the myenteric (Auerbach) plexus and the submucosal (Meissner) plexus.
The myenteric plexus, located between the longitudinal and circular muscle layers, plays an important role in the contraction and relaxation of GI smooth muscle. The submucosal plexus is involved with secretory and absorptive functions of the GI epithelium, local blood flow, and neuroimmune activities. The ENS incorporates components of the sympathetic and parasympathetic nervous systems and has sensory nerve connections through the spinal and nodose ganglia (see Figure 46–1). Parasympathetic preganglionic inputs are provided to the GI tract via the vagus and pelvic nerves. ACh released frompreganglionic neurons activates nicotinic ACh receptors (nAChRs) on postganglionic neurons within the enteric ganglia. Excitatory preganglionic input activates both excitatory and inhibitory motor neurons that control processes such as muscle contraction and secretion/absorption. Postganglionic sympathetic nerves also synapse with intrinsic neurons and generally induce relaxation. Sympathetic input is excitatory (contractile) at some sphincters. Information from afferent and preganglionic neural inputs to the enteric ganglia is integrated and distributed by a network of interneurons. ACh is the primary neurotransmitter providing excitatory inputs between interneurons, but other substances such as ATP (via postjunctional P2X receptors), substance P (by NK3 receptors), and serotonin (via 5HT3 receptors) are also important in mediating integrative processing via interneurons.
The muscle layers of the GI tract are dually innervated by excitatory and inhibitory motor neurons with cell bodies primarily in the myenteric ganglia. ACh is a primary excitatory motor neurotransmitter released from postganglionic neurons. ACh activates M2 and M3 receptors in postjunctional cells to elicit motor responses. Pharmacological blockade of muscarinic cholinergic (mAChRs) receptors does not block all excitatory neurotransmission, however, because neurokinins (neurokinin A and Substance P) are also coreleased by excitatory motor neurons and contribute to postjunctional excitation. Inhibitory motor neurons in the GI tract regulate motility events such as accommodation, sphincter relaxation, and descending receptive relaxation. Inhibitory responses are elicited by a purine derivative (either ATP or β-nicotinamide adenine dinucleotide (β-NAD) acting at postjunctional P2Y1 receptors) and NO. Inhibitory neuropeptides, such as VIP and pituitary adenylyl cyclase-activating peptide (PACAP), may also be released from inhibitory motor neurons under conditions of strong stimulation.
COMPARISON OF SYMPATHETIC, PARASYMPATHETIC, AND MOTOR NERVES (FIGURE 8–2)
• The sympathetic system is distributed to effectors throughout the body, whereas parasympathetic distribution is much more limited.
• A preganglionic sympathetic fiber may traverse a considerable distance of the sympathetic chain and pass through several ganglia before it finally synapses with a postganglionic neuron; also, its terminals make contact with a large number of postganglionic neurons. The parasympathetic system has terminal ganglia very near or within the organs innervated and is generally more circumscribed in its influences.
• The cell bodies of somatic motor neurons reside in the ventral horn of the spinal cord; the axon divides into many branches, each of which innervates a single muscle fiber, so more than 100 muscle fibers may be supplied by 1 motor neuron to form a motor unit. At each neuromuscular junction, the axonal terminal loses its myelin sheath and forms a terminal arborization that lies in apposition to a specialized surface of the muscle membrane, termed the motor end plate (see Figure 11–3).
Figure 8–2 Wiring diagram for somatic motor nerves and the efferent nerves of the autonomic nervous system. The principal neurotransmitters, acetylcholine (ACh) and norepinephrine (NE), are shown inred. The receptors for these transmitters, nicotinic (N) and muscarinic (M) cholinergic receptors, t and adrenergic receptors, are shown in green.
• Somatic nerves innervate skeletal muscle directly at a specialized synaptic junction, the motor end plate, where ACh activates Nm receptors.
• Autonomic nerves innervate smooth muscles, cardiac tissue and glands. Both parasympathetic and sympathetic systems have ganglia, where ACh is released by the preganglionic fibers; ACh acts on Nnreceptors on the postganglionic nerves. ACh is also the neurotransmitter at cells of the adrenal medulla, where it acts on Nn receptors to cause release of EPI and NE into the circulation.
• ACh is the dominant neurotransmitter released by postganglionic parasympathetic nerves and acts on muscarinic receptors. The ganglia in the parasympathetic system are near or within the organ being innervated with generally a one-to-one relationship between pre- and post-ganglionic fibers.
• NE is the principal neurotransmitter of postganglionic sympathetic nerves, acting on α- or β-adrenergic receptors. Autonomic nerves form a diffuse pattern with multiple synaptic sites. In the sympathetic system the ganglia are generally far from the effector cells (e.g., within the sympathetic chain ganglia). Preganglionic sympathetic fibers may make contact with a large number of postganglionic fibers.
RESPONSES OF EFFECTOR ORGANS TO AUTONOMIC NERVE IMPULSES. In most instances, the sympathetic and parasympathetic neurotransmitters can be viewed as physiological or functional antagonists (Table 8–1).
Responses of Effector Organs to Autonomic Nerve Impulses
Most viscera are innervated by both divisions of the autonomic nervous system, and their activities on specific structures may be either discrete and independent or integrated and interdependent. For example, the effects of sympathetic and parasympathetic stimulation of the heart and the iris show a pattern of functional antagonism in controlling heart rate and pupillary aperture, respectively, whereas their actions on male sexual organs are complementary and are integrated to promote sexual function.
GENERAL FUNCTIONS OF THE AUTONOMIC NERVOUS SYSTEM. The ANS is the primary regulator of the constancy of the internal environment of the organism.
The sympathetic system and its associated adrenal medulla are not essential to life in a controlled environment, but the lack of sympathoadrenal functions becomes evident under circumstances of stress. In the absence of the sympathetic system: body temperature cannot be regulated when environmental temperature varies; the concentration of glucose in blood does not rise in response to urgent need; compensatory vascular responses to hemorrhage, oxygen deprivation, excitement, and exercise are lacking; and resistance to fatigue is lessened. Sympathetic components of instinctive reactions to the external environment are lost; and other serious deficiencies in the protective forces of the body are discernible. The sympathetic system normally is continuously active, the degree of activity varying from moment to moment and from organ to organ, adjusting to a constantly changing environment. The sympathoadrenal system can discharge as a unit. Heart rate is accelerated; blood pressure rises; blood flow is shifted from the skin and splanchnic region to the skeletal muscles; blood glucose rises; the bronchioles and pupils dilate; and the organism is better prepared for “fight or flight.” Many of these effects result primarily from or are reinforced by the actions of epinephrine secreted by the adrenal medulla.
The parasympathetic system is organized mainly for discrete and localized discharge. Although it is concerned primarily with conservation of energy and maintenance of organ function during periods of minimal activity, its elimination is not compatible with life. The parasympathetic system slows the heart rate, lowers the blood pressure, stimulates GI movements and secretions, aids absorption of nutrients, protects the retina from excessive light, and empties the urinary bladder and rectum.
Nerve impulses elicit responses in smooth, cardiac, and skeletal muscles, exocrine glands, and postsynaptic neurons by liberating specific chemical neurotransmitters. Neurohumoral transmission relates to the transmission of impulses from postganglionic autonomic fibers to effector cells. Evidence supporting this concept includes:
• Demonstration of the presence of a physiologically active transmitter and its biosynthetic enzymes at appropriate sites
• Recovery of the transmitter from the perfusate of an innervated structure during periods of nerve stimulation but not (or in greatly reduced amounts) in the absence of stimulation
• Demonstration that the putative transmitter is capable of producing responses identical to responses to nerve stimulation
• Demonstration that the responses to nerve stimulation and to the administered transmitter candidate are modified in the same manner by various drugs, usually competitive antagonists
While these criteria are applicable for most neurotransmitters, including NE and ACh, there are now exceptions to these general rules. For example, NO has been found to be a neurotransmitter; however, NO is not stored in neurons and released by exocytosis. Rather, it is synthesized when needed and readily diffuses across membranes. Synaptic transmission in many instances may be mediated by the release of more than 1 neurotransmitter.
STEPS INVOLVED IN NEUROTRANSMISSION
The sequence of events involved in neurotransmission is of particular importance because pharmacologically active agents modulate the individual steps.
AXONAL CONDUCTION. At rest, the interior of the typical mammalian axon is ~70 mV negative to the exterior. In response to depolarization to a threshold level, an action potential is initiated at a local region of the membrane. The action potential consists of 2 phases. Following depolarization that induces an open conformation of the channel, the initial phase is caused by a rapid increase in the permeability and inward movement of Na+ through voltage-sensitive Na+ channels, and a rapid depolarization from the resting potential continues to a positive overshoot. The second phase results from the rapid inactivation of the Na+ channel and the delayed opening of a K+ channel, which permits outward movement of K+ to terminate the depolarization.
The transmembrane ionic currents produce local circuit currents such that adjacent resting channels in the axon are activated, and excitation of an adjacent portion of the axonal membrane occurs, leading to propagation of the action potential without decrement along the axon. The region that has undergone depolarization remains momentarily in a refractory state.
The puffer fish poison, tetrodotoxin, and a close congener found in some shellfish, saxitoxin, selectively block axonal conduction by blocking the voltage-sensitive Na+ channel and preventing the increase in Na+ permeability associated with the rising phase of the action potential. In contrast, batrachotoxin, an extremely potent steroidal alkaloid secreted by a South American frog, produces paralysis through a selective increase in permeability of the Na+ channel, which induces a persistent depolarization. Scorpion toxins are peptides that also cause persistent depolarization by inhibiting the inactivation process. Na+ and Ca2+ channels are discussed in more detail in Chapters 11, 14, and 20.
JUNCTIONAL TRANSMISSION. The arrival of the action potential at the axonal terminals initiates events that trigger transmission of an excitatory or inhibitory impulse across the synapse or neuroeffector junction. These events, summarized by Figure 8–3, are:
Figure 8–3 Excitatory and inhibitory neurotransmission.
1. The nerve action potential (AP) consists of a transient self-propagated reversal of charge on the axonal membrane. The internal potential Ei goes from a negative value, through zero potential, to a slightly positive value primarily through increases in Na+ permeability and then returns to resting values by an increase in K+ permeability. When the AP arrives at the presynaptic terminal, it initiates release of the excitatory or inhibitory transmitter. Depolarization at the nerve ending and entry of Ca2+ initiate docking and then fusion of the synaptic vesicle with the membrane of the nerve ending.
2. Combination of the excitatory transmitter with postsynaptic receptors produces a localized depolarization, the excitatory postsynaptic potential (EPSP), through an increase in permeability to cations, most notably Na+. An inhibitory transmitter causes a selective increase in permeability to K+ or Cl–, resulting in a localized hyperpolarization, the inhibitory postsynaptic potential (IPSP).
3. The EPSP initiates a conducted AP in the postsynaptic neuron; this can be prevented, however, by the hyperpolarization induced by a concurrent IPSP. The transmitter is dissipated by enzymatic destruction, by reuptake into the presynaptic terminal or adjacent glial cells, or by diffusion. Depolarization of the postsynaptic membrane can permit Ca2+ entry if voltage-gated Ca2+ channels are present.
1. Release of the transmitter. The nonpeptide (small molecule) neurotransmitters are largely synthesized in the region of the axonal terminals and stored there in synaptic vesicles. An action potential causes the synchronous release of several hundred quanta of neurotransmitter. The influx of Ca2+, is a critical step; Ca2+ enters the axonal cytoplasm and promotes fusion between the axoplasmic membrane and those vesicles in proximity to it. The contents of the vesicles, are discharged to the exterior by a process termed exocytosis.
Receptors on soma, dendrites, and axons of neurons, respond to neurotransmitters or modulators released. Soma–dendritic receptors; when activated, they primarily modify functions of the soma–dendritic region such as protein synthesis and generation of action potentials. Presynaptic receptors; when activated, they modify functions of the terminal region such as synthesis and release of transmitters. Two main classes of presynaptic receptors have been identified on most neurons: Heteroreceptors are presynaptic receptors that respond to neurotransmitters, neuromodulators, or neuro-hormones released from adjacent neurons or cells. Autoreceptors are receptors located on or close to axon terminals of a neuron through which the neuron’s own transmitter can modify transmitter synthesis and release (see Figures 8–3, 8–4, and 8–6).
2. Combination of the transmitter with postjunctional receptors and production of the postjunctional potential. The transmitter diffuses across the synaptic or junctional cleft and combines with specialized receptors on the postjunctional membrane; this often results in a localized increase in the ionic permeability, or conductance, of the membrane. With certain exceptions, 1 of 3 types of permeability change can occur:
• A generalized increase in the permeability to cations (notably Na+ but occasionally Ca2+), resulting in a localized depolarization of the membrane, that is, an excitatory postsynaptic potential (EPSP)
• A selective increase in permeability to anions, usually Cl–, resulting in stabilization or actual hyperpolarization of the membrane, which constitutes an inhibitory postsynaptic potential (IPSP)
• An increased permeability to K+. Because the K+ gradient is directed out of the cell, hyperpolarization and stabilization of the membrane potential occur (an IPSP)
Electric potential changes associated with the EPSP and IPSP at most sites are the results of passive fluxes of ions down their concentration gradients. The changes in channel permeability that cause these potential changes are specifically regulated by the specialized postjunctional receptors for the neurotransmitter that initiates the response (see Figures 8–4, 8–6, and 11–4; and Chapter 14). High-conductance ligand-gated ion channels usually permit passage of Na+ or Cl–; K+ and Ca2+ are involved less frequently. The ligand-gated channels belong to a superfamily of ionotropic receptor proteins that includes the nicotinic, glutamate, and certain serotonin (5HT3) and purine receptors, which conduct primarily Na+, cause depolarization, and are excitatory; and GABA acid and glycine receptors, which conduct Cl–, cause hyperpolarization, and are inhibitory. Neurotransmitters also can modulate the permeability of K+ and Ca2+ channels indirectly. In these cases, the receptor and channel are separate proteins, and information is conveyed between them by G proteins (see Chapter 3).
3. Initiation of postjunctional activity. If an EPSP exceeds a certain threshold value, it initiates a propagated action potential in a postsynaptic neuron or a muscle action potential in skeletal or cardiac muscle by activating voltage-sensitive channels in the immediate vicinity. In certain smooth muscle types in which propagated impulses are minimal, an EPSP may increase the rate of spontaneous depolarization, cause Ca2+ release, and enhance muscle tone; in gland cells, the EPSP initiates secretion through Ca2+ mobilization. An IPSP, which is found in neurons and smooth muscle will tend to oppose excitatory potentials simultaneously initiated by other neuronal sources. Whether a propagated impulse or other response ensues depends on the summation of all the potentials.
4. Destruction or dissipation of the transmitter. At cholinergic synapses, which are involved in rapid neurotransmission, high and localized concentrations of acetylcholinesterase (AChE) rapidly hydrolyze the ACh. When AChE activity is inhibited, removal of the transmitter is accomplished principally by diffusion. Under these circumstances, the effects of released ACh are potentiated and prolonged (seeChapter 10).
Rapid termination of NE occurs by a combination of simple diffusion and reuptake by the axonal terminals of the released NE. Termination of the action of amino acid transmitters is by active transport into neurons and surrounding glia. Peptide neurotransmitters are hydrolyzed by various peptidases and dissipated by diffusion.
5. Nonelectrogenic functions. The activity and turnover of enzymes involved in the synthesis and inactivation of neurotransmitters, the density of presynaptic and postsynaptic receptors, and other characteristics of synapses are controlled by trophic actions of neurotransmitters or other trophic factors released by the neuron or target cells.
The neurochemical events that underlie cholinergic neurotransmission are summarized in Figure 8–4.
Figure 8–4 A cholinergic neuroeffector junction. The synthesis of ACh in the depends on the uptake of choline via a sodium-dependent carrier that can be blocked by hemicholinium. Choline and the acetyl moiety of acetyl coenzyme A, derived from mitochondria, form ACh, a process catalyzed by the enzyme choline acetyl transferase (ChAT). ACh is transported into the storage vesicle by another carrier that can be inhibited by vesamicol. ACh is stored in vesicles along with other potential cotransmitters (Co-T) such as ATP and VIP at certain neuroeffector junctions. Release of ACh and the Co-T occurs on depolarization of the varicosity, which allows the entry of Ca2+ through voltage-dependent Ca2+ channels. Elevated [Ca2+]in promotes fusion of the vesicular membrane with the cell membrane, and exocytosis of the transmitters occurs. This fusion process involves the interaction of specialized proteins associated with the vesicular membrane (vesicle-associated membrane proteins [VAMPs]) and the membrane of the varicosity (synaptosome-associated proteins [SNAPs]). The exocytotic release of ACh can be blocked by botulinum toxin. Once released, ACh can interact with the muscarinic receptors (M), which are GPCRs, or nicotinic receptors (N), which are ligand-gated ion channels, to produce the characteristic response of the effector. ACh also can act on presynaptic mAChRs or nAChRs to modify its own release. The action of ACh is terminated by metabolism to choline and acetate by acetylcholinesterase (AChE), which is associated with synaptic membranes.
SYNTHESIS AND STORAGE OF ACH. Two enzymes, choline acetyltransferase and AChE, are involved in ACh synthesis and degradation, respectively.
Choline Acetyltransferase. Choline acetyltransferase catalyzes the final step in the synthesis of ACh—the acetylation of choline with acetyl coenzyme A (CoA). Choline acetyltransferase is synthesized within the perikaryon and then is transported along the length of the axon to its terminal. Axonal terminals contain a large number of mitochondria, where acetyl CoA is synthesized. Choline is taken up from the extracellular fluid into the axoplasm by active transport. The final step in the synthesis occurs within the cytoplasm, following which most of the ACh is sequestered within synaptic vesicles.
Choline and Choline Transport. Choline availability is rate limiting to the synthesis of ACh and is provided from the diet as there is little de novo synthesis. Choline is taken up from the extracellular space by 2 transport systems: a ubiquitous low affinity, Na+-independent transport system that is inhibited by hemicholinium-3 with a Ki of 50 µM, and high affinity Na+ - and Cl–-dependent choline transportsystem that is also sensitive to inhibition by hemicholinium-3 (Ki 10-100 nM). This second transport system is found predominantly in cholinergic neurons. ACh is released from cholinergic neurons is hydrolyzed by acetylcholine esterase (AChE) to acetate and choline. Choline is recycled after reuptake into the nerve terminal and reused for ACh synthesis.
Storage of ACh. ACh is transported into synaptic vesicles by vesicular ACh transporter (VAChT) using the potential energy of a proton electrochemical gradient. The process is inhibited by the noncompetitive and reversible inhibitor vesamicol, which does not affect the vesicular ATPase.
RELEASE OF ACh. Exocytotic release of ACh and cotransmitters (e.g., ATP, VIP, NO) occurs upon depolarization of the nerve terminals. Depolarization of the terminals allows the entry of Ca2+ through voltage-gated Ca2+ channels and promotes fusion of the vesicular membrane with the plasma membrane, allowing exocytosis to occur.
ACh is stored in vesicles located close to presynaptic membranes. A multiprotein complex appears to attach the vesicle to the plasma membrane close to other signaling elements. Various synaptic proteins, including the plasma membrane protein syntaxin and synaptosomal protein 25 kDa (SNAP-25), and the vesicular membrane protein, synaptobrevin, form a complex designated as SNAP regulators (SNARES). SNARE proteins are involved in transmitter release which is blocked by botulinum neurotoxins.
ACETYLCHOLINESTERASE. At the neuromuscular junction, immediate removal is required to prevent lateral diffusion and activation of adjacent receptors. The time required for hydrolysis of ACh at the neuromuscular junction is less than a millisecond.
AChE is found in cholinergic neurons and is highly concentrated at the postsynaptic end plate of the neuromuscular junction. Butyrylcholinesterase (BuChE) is virtually absent in neuronal elements of the central and peripheral nervous systems. BuChE is synthesized primarily in the liver and is found in liver and plasma; its likely physiological function is the hydrolysis of ingested esters from plant sources. AChE and BuChE typically are distinguished by the relative rates of ACh and butyrylcholine hydrolysis and by effects of selective inhibitors (see Chapter 10).
CHARACTERISTICS OF CHOLINERGIC TRANSMISSION AT VARIOUS SITES
Skeletal Muscle. At the neuromuscular junction (see Figure 8–2), ACh stimulates the nicotinic receptor’s intrinsic channel, which opens for ~1 ms, admitting ~50,000 Na+ ions. The channel-opening process is the basis for the localized depolarizing end-plate potential (EPP) within the end plate, which triggers the muscle AP and leads to contraction.
Autonomic Effector Cells. Stimulation or inhibition of autonomic effector cells occurs on activation of mAChRs. In contrast to skeletal muscle and neurons, smooth muscle and the cardiac conduction system exhibit intrinsic activity (both electrical and mechanical) that is modulated but not initiated by nerve impulses.
In the heart, spontaneous depolarizations normally arise from the SA node. In the cardiac conduction system, particularly in the SA and AV nodes, stimulation of the cholinergic innervation or the direct application of ACh causes inhibition, associated with hyperpolarization of the membrane and a marked decrease in the rate of depolarization. These effects are due, at least in part, to a selective increase in permeability to K+.
Autonomic Ganglia. The primary pathway of cholinergic transmission in autonomic ganglia is similar to that at the neuromuscular junction of skeletal muscle. The initial depolarization is the result of activation of nAChRs, which are ligand-gated cation channels with properties similar to those found at the neuromuscular junction. Several secondary transmitters or modulators either enhance or diminish the sensitivity of the postganglionic cell to ACh (see Chapter 11).
Prejunctional Sites. ACh release is subject to complex regulation by mediators, including ACh itself acting on M2 and M4 autoreceptors, and activation of heteroreceptors (e.g., NE acting on α2A and α2Cadrenergic receptors) or substances produced locally in tissues (e.g., NO). ACh-mediated inhibition of ACh release following activation of M2 and M4 autoreceptors is a physiological negative-feedback control mechanism. At some neuroeffector junctions (e.g., the myenteric plexus in the GI tract or the cardiac SA node), sympathetic and parasympathetic nerve terminals often lie juxtaposed to each other. There, opposing effects of NE and ACh result not only from the opposite effects of the 2 transmitters on the smooth muscle or cardiac cells but also from the inhibition of ACh release by NE or inhibition of NE release by ACh acting on heteroreceptors on parasympathetic or sympathetic terminals.
Extraneuronal Sites. All elements of the cholinergic system are functionally expressed independently of cholinergic innervation in numerous non-neuronal cells. These non-neuronal cholinergic systems can both modify and control phenotypic cell functions such as proliferation, differentiation, formation of physical barriers, migration, and ion and water movements.
CHOLINERGIC RECEPTORS AND SIGNAL TRANSDUCTION
Acetylcholine elicits responses similar to those of either nicotine or muscarine depending on the pharmacological preparation. The physiological receptors for ACh are classified as having a “nicotine action” (nicotinic) or a “muscarine action” (muscarinic). Tubocurarine and atropine block nicotinic and muscarinic effects of ACh, respectively.
Nicotinic receptors are ligand-gated ion channels whose activation always causes a rapid (millisecond) increase in cellular permeability to Na+ and Ca2+, depolarization, and excitation. Muscarinic receptorsare G protein-coupled receptors (GPCRs). Responses to muscarinic agonists are slower; they may be either excitatory or inhibitory, and they are not necessarily linked to changes in ion permeability.
SUBTYPES OF NICOTINIC ACETYLCHOLINE RECEPTORS (TABLE 8–2). The nicotinic ACh receptors (nAChRs) exist at the skeletal neuromuscular junction, autonomic ganglia, adrenal medulla, the CNS and in nonneuronal tissues. The nAChRs are composed of 5 homologous subunits organized around a central pore (see Chapter 11). In general the nAChRs are further divided into 2 groups:
1. Muscle type (Nm), found in vertebrate skeletal muscle, where they mediate transmission at the neuromuscular junction (NMJ)
2. Neuronal type (Nn), found mainly throughout the peripheral nervous system, central nervous system, and also nonneuronal tissues
Characteristics of Subtypes of Nicotinic Acetylcholine Receptors (nAChRs)
Neuronal nAChRs are widely distributed in the CNS and are found at presynaptic, perisynaptic, and postsynaptic sites. At pre- and perisynaptic sites, nAChRs appear to act as autoreceptors or heteoreceptors to regulate the release of several neurotransmitters (ACh, DA, NE, glutamate, and 5HT) at several diverse sites throughout the brain.
SUBTYPES OF MUSCARINIC RECEPTORS (TABLE 8–3). Five distinct subtypes of muscarinic ACh receptors (mAChRs) have been identified, each produced by a different gene. Like the different forms of nicotinic receptors, these variants have distinct anatomic locations in the periphery and CNS and differing chemical specificities (see Table 8–3 andChapter 9).
Characteristics of Muscarinic Acetylcholine Receptor Subtypes (mAChRs)
The functions of mAChRs are mediated by interactions with G proteins. The M1, M3, and M5 subtypes couple through Gq/11 to stimulate the PLC-IP3/DAG-Ca2+ pathway, leading to activation of PKC and Ca2+-sensitive enzymes. Activation of M1, M3, and M5 receptors can also cause the activation of phospholipase A2, leading to the release of arachidonic acid and consequent eicosanoid synthesis; these effects of M1, M3, and M5 mAChRs are generally secondary to elevation of intracellular Ca2+. Stimulated M2 and M4 cholinergic receptors couple to Gi and Go, with a resulting inhibition of adenylyl cyclase, leading to a decrease in cellular cyclic AMP, activation of inwardly rectifying K+ channels, and inhibition of voltage-gated Ca2+ channels. The functional consequences of these effects are hyperpolarization and inhibition of excitable membranes. In the myocardium, inhibition of adenylyl cyclase and activation of K+ conductances account for the negative inotropic and chronotropic effects of ACh.
Norepinephrine (NE) is the principal transmitter of most sympathetic postganglionic fibers and of certain tracts in the CNS; dopamine (DA) is the predominant transmitter of the mammalian extrapyramidal system and of several mesocortical and mesolimbic neuronal pathways; and epinephrine (EPI) is the major hormone of the adrenal medulla. Collectively, these 3 amines are called catecholamines.
SYNTHESIS OF CATECHOLAMINES. The steps in the synthesis of catecholamines and the characteristics of the enzymes involved are shown in Figure 8–5 and Table 8–4.
Figure 8–5 Biosynthesis of catecholamines. The enzymes involved are shown in red; essential cofactors in italics. The final step occurs only in the adrenal medulla and in a few epinephrine-containing neuronal pathways in the brainstem.
Enzymes for Synthesis of Catecholamines
Table 8–4 summarizes the characteristics of these synthetic enzymes. These enzymes are not completely specific; consequently, other endogenous substances, as well as certain drugs, are also substrates. For example, 5-hydroxytryptamine (5HT, serotonin) can be produced from 5-hydroxy-L-tryptophan by aromatic L-amino acid decarboxylase (or dopa decarboxylase). Dopa decarboxylase also converts dopa into DA (see Chapter 13) and methyldopa to α-methyldopamine, which in turn is converted by dopamine β-hydroxylase (DβH) to methylnorepinephrine.
The hydroxylation of tyrosine by tyrosine hydroxylase (TH) is the rate-limiting step in the biosynthesis of catecholamines.
This enzyme is activated following stimulation of sympathetic nerves or the adrenal medulla. The enzyme is a substrate for PKA, PKC, and CaM kinase; phosphorylation is associated with increased hydroxylase activity. In addition, there is a delayed increase in TH gene expression after nerve stimulation. These mechanisms serve to maintain the content of catecholamines in response to increased transmitter release. TH also is subject to feedback inhibition by catechol compounds. TH deficiency has been reported in humans and is characterized by generalized rigidity, hypokinesia, and low cerebrospinal fluid (CSF) levels of NE and DA metabolites homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylethylene glycol.
The main features of the mechanisms of synthesis, storage, and release of catecholamines and their modifications by drugs are summarized in Figure 8–6.
Figure 8–6 An adrenergic neuroeffector junction. Tyrosine is transported into the varicosity and converted to dopa by tyrosine hydroxylase (TH); aromatic L-amino acid decarboxylase (AAADC) converts dopa to dopamine (DA). Dopamine is taken up into the vesicles of the varicosity by a transporter, VMAT2, that can be blocked by reserpine. Cytoplasmic NE also can be taken up by this transporter. DA is converted to NE within the vesicle by dopamine-p-hydroxylase (DβH). NE is stored in vesicles along with other cotransmitters, NPY and ATP, depending on the particular neuroeffector junction. Release of the transmitters occurs upon depolarization of the varicosity, which allows entry of Ca2+ through voltage-dependent Ca2+ channels. Elevated levels of Ca2+ promote the fusion of the vesicular membrane with the membrane of the varicosity, with subsequent exocytosis of transmitters, as described in the legend to Figure 8–4. NE, NPY, and ATP may be stored in the same or different vesicles and co-released. Once in the synapse, NE can interact with α and β adrenergic receptors to produce the characteristic response of the effector. The adrenergic receptors are GPCRs. α and β Receptors also can be located presynaptically where NE can either diminish (α2), or facilitate (β2) its own release and that of the cotransmitters. The principal mechanism by which NE is cleared from the synapse is via a cocaine-sensitive neuronal uptake transporter, NET. Once transported into the cytosol, NE can be re-stored in the vesicle or metabolized by monoamine oxidase (MAO). NPY produces its effects by activating NPY receptors, of which there are at least 5 types (YI through Y5), all GPCRs. NPY can modify its own release and that of the other transmitters via presynaptic receptors of the Y2 type; NPY is removed from the synapse by metabolic breakdown by peptidases. ATP produces its effects by activating P2X receptors or P2Y receptors (see Table 14–8). ATP can act prejunctionally to modify its own release viareceptors for ATP or via its metabolic breakdown to adenosine that acts on P1 (adenosine) receptors. ATP is cleared from the synapse primarily by releasable nucleotidases (rNTPase) and by cell-fixed ectonucleotidases.
In the case of adrenergic neurons, the enzymes that participate in the formation of NE are synthesized in the cell bodies of the neurons and then are transported along the axons to their terminals. In the course of synthesis, the hydroxylation of tyrosine to dopa and the decarboxylation of dopa to DA take place in the cytoplasm. DA then is actively transported into the DβH-containing storage vesicles, where it is converted to NE by DβH in sympathetic nerves (about 90%); the remainder is metabolized. DA is converted by MAO to an aldehyde intermediate DOPAL, and then mainly converted to 3,4-dihydroxyphenyl acetic acid (DOPAC) by aldehyde dehydrogenase and to a minor extent 3,4-dihydroxyphenylethanol (DOPET) by aldehyde reductase. DOPAC is further converted to HVA by O-methylation in nonneuronal sites.
The adrenal medulla has 2 distinct catecholamine-containing cell types: those with NE and those with primarily EPI. The latter cell population contains the enzyme phenylethanolamine-N-methyltransferase (PNMT). In these cells, the NE formed in the granules leaves these structures and is methylated in the cytoplasm to EPI. Epinephrine then reenters the chromaffin granules, where it is stored until released. EPI accounts for ~80% of the catecholamines of the adrenal medulla and NE ~20%. The level of glucocorticoids, secreted by the adrenal cortex, is a major factor that controls the rate of synthesis of EPI and the amount available for release from the adrenal medulla. The intra-adrenal portal vascular system carries the corticosteroids directly to the adrenal medullary chromaffin cells, where they induce the synthesis of PNMT. The activities of both TH and D H also are increased in the adrenal medulla when the secretion of glucocorticoids is stimulated.
In addition to de novo synthesis, NE stores in the terminal portions of the adrenergic fibers are also replenished by reuptake and re-storage of NE following its release. At least 2 distinct carrier-mediated transport systems are involved:
1. One across the axonal membrane from the extracellular fluid to the cytosol (the NE transporter [NET], previously called uptake 1)
2. The other from the cytosol into the storage vesicles (the vesicular monoamine transporter [VMAT2]) Sympathetic nerves as a whole remove ~87% of released NE by NET. More than 70% of recaptured NE is sequestered by VMAT2 into storage vesicles. In contrast, clearance of circulating catecholamines is primarily by nonneuronal mechanisms, with liver and kidney accounting for more than 60% of the clearance.
STORAGE OF CATECHOLAMINES. Catecholamines are stored in vesicles, ensuring their regulated release. VMAT2 is driven by pH and potential gradients that are established by an ATP-dependent proton translocase. For every molecule of amine taken up, 2 H+ ions are extruded. Monoamine transporters transport DA, NE, EPI, and 5HT. Reserpine inhibits monoamine transport into storage vesicles and ultimately leads to depletion of catecholamine from sympathetic nerve endings and in the brain.
There are 2 neuronal membrane transporters for catecholamines, the NE transporter (NET) and the DA transporter (DAT) (Table 8–5). NET is Na+-dependent and is blocked selectively by cocaine and tricyclic antidepressants (e.g. imipramine). This transporter has a higher affinity for NE than for EPI.
Characteristics of Plasma Membrane Transporters for Endogenous Catecholamines
Indirectly acting sympathomimetic drugs (e.g., ephedrine and tyramine) produce some of their effects by displacing and releasing NE from the nerve terminals to the effector cells. These agents are substrates for NET. As a result of their transport across the neuronal membrane and release into the axoplasm, they make NET available at the inner surface of the membrane for the outward transport of NE (“facilitated exchange diffusion”). These amines also mobilize NE stored in the vesicles by competing for the vesicular uptake process (VMAT2).
Three extraneuronal transporters (ENTs) handle a range of endogenous and exogenous substrates (see Table 8–5). The ENT (or uptake-2 and OCT3), is an organic cation transporter that exhibits lower affinity for catecholamines, compared to NET, and favors EPI over NE and DA. Other members of this family are the organic cation transporters OCT1 and OCT2 (see Chapter 5).
RELEASE OF CATECHOLAMINES. Details of excitation-secretion coupling in sympathetic neurons and adrenal medulla are not completely known, The triggering event is the entry of Ca2+, which results in the exocytosis of the granular contents, including EPI, ATP, some neuroactive peptides or their precursors, chromogranins, and D, H.
PREJUNCTIONAL REGULATION OF NOREPINEPHRINE RELEASE. Following their release from sympathetic terminals, the 3 sympathetic cotransmitters—NE, neuropeptide Y (NPY), and ATP—can feed back on prejunctional receptors to inhibit the release of each other. The α2A and α2C adrenergic receptors are the principal prejunctional receptors that inhibit sympathetic neurotransmitter release, whereas the α2B adrenergic receptors also may inhibit transmitter release at selected sites. NPY, acting on Y2 receptors, and ATP-derived adenosine, acting on P1 receptors, also can inhibit sympathetic neurotransmitter release. Numerous heteroreceptors on sympathetic nerve varicosities also inhibit the release of sympathetic neurotransmitters; these include: M2 and M4 muscarinic, 5HT, PG E2, histamine, enkephalin, and DA receptors. Enhancement of sympathetic neurotransmitter release can be produced by activation of β2 adrenergic receptors, angiotensin AT2 receptors, and nAChRs.
TERMINATION OF THE ACTIONS OF CATECHOLAMINES. The actions of NE and EPI are terminated by:
• Reuptake into nerve terminals by NET
• Dilution by diffusion out of junctional cleft and extraneuronal uptake by ENT, OCT1, and OCT2
METABOLISM OF CATECHOLAMINES. Following uptake, catecholamines are either metabolized by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). MAO metabolizes transmitter that is released within the nerve terminal. COMT, particularly in the liver, plays a major role in the metabolism of endogenous circulating and administered catecholamines. MAO and COMT can act sequentially in conjunction with aldehyde reductase, aldehyde dehydrogenase, and alcohol dehydrogenase to produce a variety of intermediates en route to vanillylmandelic acid, which is secreted into the urine (Figure 8–7).
Figure 8–7 Metabolism of catecholamines. NE and EPI are first oxidatively deaminated to a short-lived intermediate (DOPGAL) by monoamine oxidase (MAO). DOPGAL then undergoes further metabolism to more stable alcohol or acid deaminated metabolites. Aldehyde dehydrogenase (AD) metabolizes DOPGAL to 3,4-dihydroxymandelic acid (DOMA) while aldehyde reductase (AR) metabolizes DOPGAL to 3,4-dihydroxyphenyl glycol (DOPEG). Under normal circumstances DOMA is a minor metabolite with DOPEG being the major metabolite produced from NE and EPI. Once DOPEG leaves the major sites of its formation (sympathetic nerves; adrenal medulla), it is converted to 3-methoxy, 4-hydroxyphenylglycol (MOPEG) by catechol-0-methyl transferase (COMT). MOPEG is then converted to the unstable aldehyde (MOPGAL) by alcohol dehydrogenase (ADH) and finally to vanillylmandelic acid (VMA) by aldehyde dehydrogenase. VMA is the major end product. Another route for the formation of VMA is conversion of NE or EPI into normetanephrine or metanephrine by COMT either in the adrenal medulla or extraneuronal sites, with subsequent metabolism to MOPGAL and thence to VMA. Catecholamines are also metabolized by sulfotransferases.
The 2 enzymes have different subcellular localizations: MAO is associated chiefly with the outer surface of mitochondria; COMT is largely cytoplasmic. Both MAO and COMT are distributed widely throughout the body, however, little or no COMT is found in sympathetic neurons. The physiological substrates for COMT include L-dopa, all 3 endogenous catecholamines (DA, NE, and EPI), their hydroxylated metabolites, catecholestrogens, ascorbic acid, and dihydroxyindolic intermediates of melanin. Two different isozymes of MAO (MAO-A and MAO-B) occur in widely varying proportions in different cells in the CNS and in peripheral tissues. In the brain, MAO-A is located in all regions containing catecholamines. MAO-B, on the other hand, is found primarily in regions that are known to synthesize and store 5HT. MAO inhibitors are useful in the treatment of Parkinson disease and mental depression (see Chapters 15 and 22). Inhibitors of MAO (e.g., pargyline and nialamide) can cause an increase in the concentration of NE, DA, and 5HT in the brain and other tissues.
Adrenal medullary chromaffin cells contain both MAO and COMT; the COMT is mainly present as the membrane-bound form of the enzyme in contrast to the form found in the cytoplasm of extra-neuronal tissue.
CLASSIFICATION OF ADRENERGIC RECEPTORS. Adrenergic receptors are broadly classified as either α or β, with subtypes within each group (Table 8–6). The original subclassification was based on the rank order of potency of agonists:
• Epinephrine ≥ norepinephrine >> isoproterenol for α adrenergic receptors
• Isoproterenol > epinephrine ≥ norepinephrine for β adrenergic receptors
Characteristics for Adrenergic Receptor Subtypesa
MOLECULAR BASIS OF ADRENERGIC RECEPTOR FUNCTION. All adrenergic receptors are GPCRs that link to heterotrimeric G proteins. Each major type shows preference for a particular class of G proteins, that is, α1 to Gq, α2 to Gi, and β to Gs (see Table 8–6). The responses that follow activation result from G protein–mediated effects on the generation of second messengers and on the activity of ion channels, as discussed in Chapter 3. The pathways overlap broadly with those discussed for muscarinic ACh receptors.
α ADRENERGIC RECEPTORS. The α1 receptors (α1A, α1B, and α1D) and the α2 receptors (α2A, α2B, and α2C) are heptahelical proteins that couple differentially to a variety of G proteins to regulate smooth muscle contraction, secretory pathways, and cell growth (see Table 8–6).
α1 ADRENERGIC RECEPTORS. Stimulation of α1 receptors activates the Gq-PLCβ-IP3/DAG-Ca2+ pathway and results in the activation of PKC and other Ca2+ and calmodulin sensitive pathways such as CaM kinases, with sequelae depending on cell differentiation (e.g., contraction of vascular smooth muscle, activation of eNOS in vascular endothelium) (see Chapter 3). Activation of C1 receptor subtypes also stimulates several phospholipases (PLA2, PLD).
PKC phosphorylates many substrates, including membrane proteins such as channels, pumps, and ionexchange proteins (e.g., Ca2+-transport ATPase). α1 Receptor stimulation of PLA2 leads to the release of free arachidonate, which is then metabolized by cyclooxygenase (yielding prostaglandins) and lipoxygenase (yielding leukotrienes) (see Chapter 33); PLD hydrolyzes phosphatidylcholine to yield phosphatidic acid. In most smooth muscles, the increased concentration of intracellular Ca2+ causes contraction (see Chapter 3 and Figure 3–14). In contrast, the increased concentration of intracellular Ca2+that result from stimulation of α1 receptors in GI smooth muscle causes hyperpolarization and relaxation by activation of Ca2+-dependent K+ channels. α1 Receptors activate MAPKs, PI3K, and others to affect cell growth and proliferation.
α2 ADRENERGIC RECEPTORS. α Adrenergic receptors (α2A, α2B, and α2C) couple to a variety of effectors, generally inhibiting adenylyl cyclase (reducing signaling via the cyclic AMP-PKA pathway) and activating G protein–gated K+ channels (resulting in membrane hyperpolarization).
Activation of α2A receptors inhibits NE release from sympathetic nerve endings and suppresses sympathetic outflow from the brain, leading to hypotension. In the CNS, α2A receptors, probably produce the antinociceptive effects, sedation, hypothermia, hypotension, and behavioral actions of α2 agonists. The α2B receptor is the main receptor mediating α2-induced vasoconstriction, whereas the α2C receptor is the predominant receptor inhibiting the release of catecholamines from the adrenal medulla and modulating dopamine neurotransmission in the brain.
β ADRENERGIC RECEPTORS. β Receptors regulate numerous functions, including heart rate and contractility, smooth muscle relaxation, and multiple metabolic events in numerous tissues including adipose and hepatic cells and skeletal muscle (see Table 8–1). All 3 of the β receptor subtypes (β1, β2, and β3) couple to Gs and activate adenylyl cyclase (see Table 8–6). However, recent data suggest possible differences in downstream signals and events activated by the 3 β receptors.
β1, β2, and β3 receptors can differ in their intracellular signaling pathways and subcellular location. Stimulation of β2 receptors cause a transient increase in heart rate that is followed by a prolonged decrease. Following pretreatment with pertussis toxin, which prevents activation of Gi, the negative chronotropic effect of β2 activation is abolished. It is thought that these specific signaling properties of β receptor subtypes are linked to subtype-selective association with intracellular scaffolding and signaling proteins. β2 receptors normally are confined to caveolae in cardiac myocyte membranes; the importance of compartmentation of components of the cyclic AMP pathway are discussed in Chapter 3.
REFRACTORINESS TO CATECHOLAMINES. Exposure of catecholamine-sensitive cells and tissues to adrenergic agonists causes a progressive diminution in their capacity to respond to such agents. This phenomenon, variously termed refractoriness, desensitization, or tachyphylaxis (see Chapter 3). Multiple mechanisms are involved in desensitization, including receptor phosphorylation by both G-protein receptor kinases (GRKs) and by signaling kinases such as PKA and PKC, receptor sequestration and endocytosis, interaction with scaffold proteins, and activation of specific cyclic nucleotide phosphodiesterases. The β2 receptor system is the best studied in this regard.
Each step involved in neurotransmission (see Figures 8–3, 8–4, and 8–6) represents a potential point of therapeutic intervention. This is depicted in the diagrams of the cholinergic and adrenergic terminals and their postjunctional sites (see Figures 8–4 and 8–6). Drugs that affect processes involved in each step of transmission at cholinergic and adrenergic junctions are summarized in Table 8–7.
Representative Agents Acting at Peripheral Cholinergic and Adrenergic Neuroeffector Junctions
OTHER AUTONOMIC NEUROTRANSMITTERS
Most neurons in both the central and peripheral nervous systems contain more than 1 putative neurotransmitter (see Chapter 14). Although the parasympathetic and sympathetic components of the autonomic nervous system and the actions of ACh and NE still provides the essential framework for studying autonomic function, a host of other chemical messengers such as purines, eicosanoids, NO, and peptides modulate or mediate responses that follow stimulation of the ANS.
ATP. ATP is a cotransmitter with NE. The sympathetic nerves store ATP and NE in the same synaptic vesicles, and the 2 cotransmitters are released together. ATP and NE may also be released from separate subsets of vesicles and be subject to differential regulation. There is also evidence that ATP may be a cotransmitter with ACh in certain post-ganglionic parasympathetic nerves.
NPY. NPY peptides are distributed widely in the central and peripheral nervous systems. NPY is colocalized and coreleased with NE and ATP in most sympathetic nerves in the peripheral nervous system, especially those innervating blood vessels. Thus, NPY, together with NE and ATP, is likely a third sympathetic cotransmitter. The functions of NPY include (1) direct postjunctional contractile effects; (2) potentiation of the contractile effects of the other sympathetic cotransmitters; and (3) inhibitory modulation of the nerve stimulation–induced release of all 3 sympathetic cotransmitters.
VIP AND ACH. VIP and ACh coexist in peripheral autonomic neurons, possibly in separate populations of storage vesicles. There is evidence for their cotransmission in the regulation of salivation.
NONADRENERGIC, NONCHOLINERGIC (NANC) TRANSMISSION BY PURINES. There is evidence for the existence of NANC transmission in the ANS and of purinergic neurotransmission in the GI tract, genitourinary tract, and certain blood vessels. Indeed, ATP has fulfilled all the criteria for a neurotransmitter. Adenosine, generated from the released ATP by ectoenzymes and releasable nucleotidases acts as a modulator by causing feedback inhibition of release of the transmitter. Purinergic receptors are classified as adenosine (P1) receptors and ATP (P2X and P2Y) receptors. Adenosine receptors and P2Y receptors mediate their responses via G proteins, whereas the P2X receptors are a subfamily of ligand-gated ion channels (see Figure 14–8). Methylxanthines (e.g., caffeine and theophylline) preferentially block adenosine receptors (see Chapter 36).
MODULATION OF VASCULAR RESPONSES BY ENDOTHELIUM-DERIVED FACTORS; NO. An intact endothelium is necessary to achieve vascular relaxation in response to physiological ligands with Gq-linked receptors on endothelial cells and to exogenous ACh. In response to a variety of vasoactive agents and physical stimuli, endothelial cells produce and release a vasodilator called endothelium-derived relaxing factor (EDRF), now known to be NO. Less commonly, an endothelium-derived hyperpolarizing factor (EDHF) and endothelium-derived contracting factor (EDCF) are released. NO production contributes to vascular tone and thus can modulate the influence of α- and β-agonists and antagonists thereon. See Figure 3–14 for more information on the multiple signaling systems that can influence vascular tone.