Norepinephrine (NE), epinephrine (EPI), dopamine (DA), and serotonin (5-hydroxytryptamine; 5-HT) have historically been referred to as biogenic amines, and their neurotransmitter systems are similar in many respects. Neurotransmitter synthesis, vesicle transport and storage, uptake, and degradation share many enzymes and structurally similar transport proteins. Cocaine, reserpine, amphetamines, and monoamine oxidase inhibitors (MAOIs) affect all four types of neurons. In addition, these xenobiotics produce several different effects in the same system. For example, in the noradrenergic neuron, amphetamines work mainly by causing the release of cytoplasmic NE, but they also inhibit NE uptake, and their metabolites inhibit monoamine oxidase. Actions of xenobiotics that affect all biogenic amine neurotransmitters are described in the most detail for noradrenergic neurons. For the sake of brevity, similar mechanisms of action are simply noted in discussions of dopaminergic and serotonergic neurotransmission.
The locus ceruleus is the main noradrenergic nucleus and resides in the floor of the fourth ventricle on each side of the pons. Axons radiate from this nucleus out to all layers of the cerebral cortex, to the cerebellum, and to other structures. Norepinephrine demonstrates both excitatory and inhibitory actions in the CNS. Norepinephrine released from locus ceruleus projections in the hippocampus increases cortical neuron activity through β-adrenoceptor activation and G protein–mediated inhibition of K+ efflux. Norepinephrine released in outer cortical areas produces inhibitory effects mediated by α2-adrenoceptor agonism. Consistent with this, NE demonstrates anticonvulsant actions in animals. Carbamazepine's anticonvulsant action may be partly a result of inhibition of NE uptake.44 Despite antagonistic actions on different cortical neurons, electrical stimulation of the locus ceruleus produces widespread cortical activation and excitation. This overall effect probably explains a great deal of the hyperattentiveness and lack of fatigue that accompanies use of xenobiotics that mimic or increase noradrenergic activity in the brain. Locus ceruleus neuronal firing increases during waking and dramatically falls during sleep.
Synthesis, Release, and Uptake
Figure 13–5 is a representation of a noradrenergic neuron. Tyrosine hydroxylase is the rate-limiting enzyme in NE synthesis and is sensitive to negative feedback by NE. This enzyme requires Fe2+ as a cofactor and exists as a homotetramer and is upregulated by chronic exposure to caffeine and nicotine. Under normal dietary conditions tyrosine hydroxylase is completely saturated by tyrosine, and increasing dietary tyrosine does not appreciably increase dopa synthesis. Dopa undergoes decarboxylation by L-amino acid decarboxylase to DA. L-Amino acid decarboxylase (dopa decarboxylase) is not specific for dopa. For example, it also catalyzes the formation of serotonin from 5-HT.
Noradrenergic nerve ending. The postsynaptic membrane may represent an end organ or another neuron in the CNS. Brief examples of effects resulting from postsynaptic receptor activation are shown. Agents in Tables 13-4 and 13-5 produce effects by inhibiting transport of dopamine (DA) or norepinephrine (NE) into vesicles through VMA2 ; causing movement of NE an DA from vesicles into the cytoplasm ; activating or antagonizing postsynaptic α- and β-adrenoceptors [3 -5]; modulating NE release by activating or antagonizing presynaptic α2-autoreceptors , dopamine2 (D2) heteroreceptors , or α2-autoreceptors ; blocking uptake of NE (NET inhibition) ; causing reverse transport of NE from the cytoplasm into the synapse via NET by raising cytoplasmic NE concentrations ; inhibiting monoamine oxidase (MAO) to prevent NE degradation ; or inhibiting COMT to prevent NE degradation . AADC = aromatic L-amino acid decarboxylase; β-hydroxylase = dopamine-β-hydroxylase; COMT = catechol-O-methyltransferase; CNS = central nervous system; DOPGAL = 3,4-dihydroxyphenylglycoaldehyde; G = G protein; NET = membrane NE uptake transporter; NME = normetanephrine; VMA2 = vesicle uptake transporter for NE.
About one-half of cytoplasmic DA is actively pumped into vesicles by VMAT2. The remaining DA is quickly deaminated.
In the vesicle, DA is converted to NE by dopamine-β-hydroxylase. Vesicles isolated from peripheral nerve endings contain DA, NE, dopamine-β-hydroxylase, and ATP, and all of these substances are released into the synapse during Ca2+-dependent exocytosis triggered by neuronal firing. In neurons containing EPI as a neurotransmitter, NE is released from vesicles into the cytoplasm, where it is converted to EPI by phenylethanolamine-N-methyl-transferase. Epinephrine is then transported back into vesicles before synaptic release.84
Norepinephrine is removed from the synapse, mainly by uptake into the presynaptic neuron by the NE transporter (NET). Although this transporter has great affinity for NE, it also transports other amines, including DA, tyramine, MAOIs, and amphetamines. Once pumped back into the cytoplasm, NE can either be transported back into vesicles for further storage and release, or can be quickly degraded by monoamine oxidase (MAO), an enzyme expressed on the outer mitochondrial membrane.
MAO is present in all human tissues except red blood cells. It exists as 2 isozymes, MAO-A and MAO-B,101 each with relatively separate affinities for various substrates (Table 13–3). Neuronal MAO degrades cytoplasmic amines, including neurotransmitters, to prevent elevated cytoplasmic concentrations of biogenic amines. Hepatic and intestinal MAO prevent large quantities of dietary bioactive amines from entering the circulation and producing systemic effects.
Table 13–3. Characteristics of Monoamine Oxidase (MAO) Isozymes |Favorite Table|Download (.pdf)
Table 13–3. Characteristics of Monoamine Oxidase (MAO) Isozymes
Catechol-O-methyltransferase (COMT) is an intracellular enzyme widely distributed throughout the body, including the central nervous system, that is responsible for metabolism of DA, L-Dopa, NE, and EPI. In extraneuronal tissue, COMT metabolizes catecholamines, including those that have entered the systemic circulation.
The 2 main types of adrenoceptors are α-adrenoceptors and β-adrenoceptors. All adrenoceptors are linked to G proteins.
β-Adrenoceptors are divided into 3 major subtypes (β1, β2, and β3), depending on their affinity for various agonists and antagonists.25,65,68,86 β1-adrenoceptors and β2-adrenoceptors are linked to Gs, and their stimulation raises cAMP concentration and/or activates protein kinase A which, in turn, produces several effects, including regulation of ion channels. At least some β3-adrenoceptors may be coupled not only to Gs, but also to receptors for Gi/o proteins. Prolonged activation of β2 receptors in the heart causes them to become coupled to Gi.175
The β-adrenoceptors are polymorphic, with genetic variation in humans.23 Polymorphism influences response to medications, regulation of receptors, and clinical course of disease.23,68,86 In general, peripheral β1-adrenoceptors are found mainly in the heart (along with β2 receptors), whereas peripheral β2-adrenoceptors also mediate additional adrenergic effects.68 Presynaptic β2-adrenoceptor activation causes release of NE from nerve endings (positive feedback). β3-Adrenoceptors reside mainly in fat, but they also reside in skeletal muscle, gallbladder, and colon where they regulate metabolic processes. β3-Adrenoceptors' polymorphism may contribute to clinical expressions of non–insulin-dependent diabetes and obesity.23,154,171
α-Adrenoceptors are linked to G proteins that inhibit adenylate cyclase to lower cAMP levels, affect ion channels, increase intracellular Ca2+ through inositol triphosphate and diacylglycerol production, or produce other actions. These receptors are divided into two main types, α1 and α2, and at least six subtypes—α1A, α1B, α1D, α2A, α2B, and α2C—are described.39,65 Most α1 adrenoceptors are coupled to Gq, whereas most α2 adrenoceptors are coupled to Gi.
In peripheral tissue, α1-adrenoceptors reside on the postsynaptic membrane in continuity with the synaptic cleft. Stimulation of these receptors on blood vessels commonly results in vasoconstriction.
α2-Adrenoceptors reside on both sides of the synapse. Presynaptic α2-adrenoceptor activation mediates negative feedback, limiting further release of NE (Fig. 13–5). Postganglionic parasympathetic neurons (cholinergic) also contain presynaptic α2-adrenoceptors that, when stimulated, prevent release of ACh (Fig. 13–4).
Postsynaptic α2-adrenoceptors on vasculature also can mediate vasoconstriction. Initially, it was suggested that postsynaptic α2-adrenoceptors resided mainly outside of the synapse and mediated vasoconstrictive responses to circulating a agonists such as NE, whereas postsynaptic α1-adrenoceptors responded to NE released from nerve endings. However, it has been demonstrated that in at least some tissues (eg, saphenous vein), NE released following nerve stimulation produces vasoconstriction through action at α2-adrenoceptors, making the previous differentiation not as distinct.39,76 Because both α1- adrenoceptors and α2-adrenoceptors on noncerebral vasculature mediate vasoconstriction, a patient with hypertension from high concentrations of circulating catecholamines (eg, pheochromocytoma or clonidine withdrawal) or from extravasation of NE from an intravenous line commonly needs both α1-adrenoceptor and α2-adrenoceptor blockade to vasodilate adequately (eg, phentolamine). Stimulation of postsynaptic α2-adrenoceptors in the brainstem inhibits sympathetic output and produces sedation (Fig. 13–6). In fact, dexmedetomidine, an imidazole and potent α2A-adrenoceptor agonist, is used for sedation in intensive care patients, although hypotension and bradycardia occur as expected side effects.13
Central action of agents that activate α2-adrenoceptors or that bind to type 1 imidazoline binding sites. There are poorly understood interactions between imidazoline binding sites and α2-adrenoceptors that make delineation of specific effects difficult to attribute to specific receptor activation.
Xenobiotics producing pharmacologic effects that result in or mimic increased activity of the adrenergic nervous system are sympathomimetics (Table 13–4). Those with the opposite effect are sympatholytics (Table 13–5).
Table 13–4. Examples of Sympathomimetics |Favorite Table|Download (.pdf)
Table 13–4. Examples of Sympathomimetics
Selective α2-adrenoceptor antagonists
Imidazoline binding-site antagonists
Inhibit norepinephrine Uptake
Table 13–5. Examples of Sympatholytics |Favorite Table|Download (.pdf)
Table 13–5. Examples of Sympatholytics
Imidazoline binding-site agonistsb
Inhibitors of vesicle uptake
Xenobiotics whose sympathomimetic actions result from direct binding to α-adrenoceptors or β-adrenoceptors are called direct-acting sympathomimetics. Most do not cross the blood–brain barrier in significant quantities.
Xenobiotics that produce sympathomimetic effects by causing the release of cytoplasmic NE from the nerve ending in the absence of vesicle exocytosis are called indirect-acting sympathomimetics. Amphetamine is the prototype of indirect-acting sympathomimetics and is used for the discussion of what is known about their mechanism of action. In general, mechanisms of indirect release of NE by amphetamines, cocaine, phencyclidine, MAOIs, and mixed-acting xenobiotics noted in Table 13–4 are similar in that their actions depend on their ability to produce elevated cytoplasmic NE concentrations.
Amphetamine and structurally similar indirect-acting sympathomimetics move into the neuron mainly by the membrane transporter that pumps NE into the neuron. (Lipophilic indirect-acting sympathomimetics move into the neuron by diffusion.) From the cytoplasm, amphetamines are transported into neurotransmitter vesicles, where they buffer protons to raise intravesicular pH. As noted earlier, much of the vesicle's ability to concentrate NE (and other neurotransmitters) is a result of ion trapping of NE at the lower pH. The rise in intravesicle pH produced by amphetamines causes NE to leave the vesicle and move into the cytoplasm.155,156 Such movement may be caused by diffusion or reverse transport of NE by VMAT2. In the cytoplasm, amphetamines also compete with NE and DA for transport into vesicles, which further contributes to elevated cytoplasmic NE concentrations. In the case of amphetamine, the rise in cytoplasmic concentrations of NE may be enhanced by the ability of amphetamine metabolites to inhibit MAO, which impairs NE degradation.
Every time the Na+-dependent uptake transporter, NET, moves a bioactive amine (eg, tyramine) into the neuron where it is released, a binding site for NE on NET transiently faces inward and becomes available for reverse transport of NE out of the neuron. The normally low concentration of cytoplasmic NE prevents significant reverse transport. In the face of elevated cytoplasmic NE concentrations produced by indirect-acting sympathomimetics, NET moves NE out of the neuron and back into the synapse, where the neurotransmitter stimulates adrenoceptors (indirect action). This process is sometimes referred to as facilitated exchange diffusion, or displacement, of NE from the nerve ending. Evidence supporting reverse transport produced by amphetamines is that inhibitors of the transporter (eg, tricyclic antidepressants) prevent amphetamine-induced NE release.
While all indirect-acting smypathomimetics cause reverse NE transport by increasing cytoplasmic NE concentrations, those that move into the neuron by the membrane transporter (eg, amphetamines, MAOIs, DA, tyramine) further enhance reverse transport because their uptake may cause more NE binding sites on NET to face inward per unit time.
Although cocaine does inhibit NET, it also causes some NE release. In fact, cocaine similarly lessens pH gradients across vesicle membranes156 to raise cytoplasmic concentrations of NE. That cocaine produces less NE release than amphetamines is partly explained by cocaine-induced inhibition of the membrane transporter and by the fact that cocaine does not move into the neuron by active uptake (ie, does not increase the number of NE binding sites facing inward), but diffuses into the neuron. (Most of cocaine's severe sympathomimetic effects probably result from its action on the brain rather than peripheral nerve endings.162)
Phencyclidine (PCP) is a hallucinogen that possesses multiple pharmacologic actions. Like toxicity from many hallucinogens, PCP toxicity is accompanied by increased adrenergic activity, which results, in part, from PCP-induced decreases in pH gradients across the vesicle membrane156 and indirect release of NE. Like cocaine, PCP moves into the neuron by diffusion rather than uptake through the membrane transporter, at least partly explaining less PCP-induced NE release than typically occurs in amphetamine poisoning.
In addition to causing ACh release, black widow spider venom causes vesicle exocytosis of NE, producing hypertension and diaphoresis over the palms, soles, upper lip, and nose. All of the aforementioned indirectly acting sympathomimetics, except black widow spider venom, enter the CNS.
Mixed-acting sympathomimetics act directly and indirectly. For example, large doses of phenylpropanolamine indirectly cause NE release and act directly as α-adrenoceptor agonists. Intravenously administered DA indirectly causes NE release, explaining most of its vasoconstricting activity, but also directly stimulates dopaminergic and β-adrenoceptors. Direct α-agonism occurs at high doses. Except for DA, these xenobiotics cross the blood–brain barrier to produce central effects.
Inhibitors of NE uptake raise concentrations of NE in the synapse to produce excessive stimulation of adrenoceptors.
There are 2 main mechanisms of action for inhibitors of biogenic amine uptake: competitive and noncompetitive. Noncompetitive inhibitors, such as cyclic antidepressants, carbamazepine, venlafaxine, methylphenidate, and cocaine, bind at or near the carrier site on NET to prevent NET from moving NE and similar xenobiotics into or out of the neuron. These inhibitors are not transported into the neuron by this mechanism. Various xenobiotics used for their antimuscarinic effects also block NET noncompetitively. These include benztropine, diphenhydramine, trihexyphenidyl and orphenadrine.108 Atomexetine also inhibits NET.
The second mechanism, competitive inhibition of NET, characterizes most indirect-acting sympathomimetics, including amphetamines and structurally similar xenobiotics (eg, mixed-acting agents, MAOIs). These xenobiotics prevent NE uptake by competing with synaptic NE for binding to the carrier site on NET, the mechanism by which they move into the neuron. In fact, an additional adrenergic action of amphetamines, mixed-acting agents, MAOIs, and tyramine is to raise synaptic NE concentrations by competing for uptake, thereby compounding their indirect or direct actions.
MAOIs are transported by NET into the neuron, where they act through several mechanisms.101 Inhibition of MAO, their main pharmacologic effect, results in increased cytoplasmic concentrations of NE and some indirect release of neurotransmitter into the synapse. As a minor effect they also may displace NE from vesicles by raising pH in a manner similar to amphetamines. These actions explain the initial hyperadrenergic findings following MAOI overdose and probably also account for occasional and unpredictable adrenergic crises that occur despite dietary compliance.
Nonspecific MAOIs inhibit both isozymes of MAO, preventing intestinal and hepatic degradation of bioactive amines. A person taking such an MAOI who then is exposed to indirect-acting sympathomimetics (eg, tyramine in cheese, phenylpropanolamine, DA, amphetamines) has a much larger cytoplasmic concentration of NE to transport into the synapse and may, therefore, develop central and peripheral hyperadrenergic findings. MAOIs specific for the MAO-B isozyme are less likely to predispose to food or drug interactions by maintaining significant hepatic and intestinal MAO activity. Furthermore, reversible MAO-A specific inhibitors are also less likely to provoke this reaction because their reversibility allows competition of exogenous amines with the inhibitor, resulting in its displacement from the enzyme and normal metabolism of the bioactive amines.177 Isozyme specificity is lost as the dose of the MAOI is increased. In fact, selegiline, currently marketed as a selective MAO-B inhibitor, partially inhibits MAO-A activity at therapeutic doses. Specificity might lack importance when indirect-acting agents are administered parenterally (eg, intravenous DA or amphetamines). Linezolid is an antibiotic that produces weak MAO inhibition.
Occasionally, patients suffering from refractory depression respond to a combination of MAOIs and tricyclic antidepressants. This combination therapy is usually unaccompanied by excessive adrenergic activity because the inhibition of the membrane uptake transporter by the tricyclic antidepressant attenuates excessive reverse transport of elevated cytoplasmic NE concentrations produced by MAOIs.
Inhibitors of COMT are administered in the treatment of Parkinson disease to prevent the catabolism of concomitantly administered l-dopa. Entacapone only acts peripherally, whereas tolcapone also crosses the blood–brain barrier.
Yohimbine blocks α2-adrenoceptors to produce a mixed clinical picture. Peripheral postsynaptic α2 blockade produces vasodilation. Blockade of presynaptic α2-adrenoceptors on cholinergic nerve endings (Fig. 13–4) enhances ACh release, occasionally producing bronchospasm82 and contributing to diaphoresis. Similar presynaptic actions on peripheral noradrenergic nerves enhance catecholamine release (Fig. 13–5). Antagonism of central α2-adrenoceptors in the locus ceruleus results in CNS stimulation, whereas blockade of postsynaptic α2-adrenoceptors in the nucleus tractus solitarius may enhance sympathetic output (Fig. 13–6). The final result includes hypertension, tachycardia, anxiety, fear, agitation, mania, mydriasis, diaphoresis, and bronchospasm.87
Direct α-adrenoceptor and β-adrenoceptor antagonists are noted in Table 13–5. After overdose, β-adrenoceptor selectivity may be less significant. Some β-adrenoceptor antagonists also are partial agonists.
Xenobiotics That Prevent Norepinephrine Release
Xenobiotics that block the vesicle uptake transporter prevent the movement of NE into vesicles and deplete the nerve ending of this neurotransmitter, also preventing NE release after depolarization. Reserpine and ketanserin inhibit both VMAT1 and VMAT2, whereas tetrabenazine only inhibits VMAT2. Like guanethidine, reserpine causes transient NE release with the initial dose or early in overdose. β-Adrenoceptor antagonists block presynaptic β2-adrenoceptors to limit catecholamine release from nerve endings, although this does not appear to be their main mechanism of action.
Imidazoline and α2-Adrenoceptor Agonists
Numerous imidazoline derivatives (eg, clonidine) and structurally similar xenobiotics are used as centrally acting antihypertensives or long-acting topical vasoconstrictors. They are currently divided into first-generation agents (eg, clonidine) that are thought to act at both α2A-adrenoceptor and imidazoline binding sites, and second-generation agents (eg, rilmenidine) that express much greater affinity for imidazoline binding sites than for α2A-adrenergic receptors.
The ventromedial (depressor) and the rostral-ventrolateral (pressor) areas of the ventrolateral medulla (VLM) are responsible for the central regulation of cardiovascular tone and blood pressure. They receive afferent fibers from the carotid and aortic baroreceptors, which form the tractus solitarius via the nucleus tractus solitarius (NTS).75
The hypotensive actions of α2-adrenoceptor agonists were previously attributed entirely to brainstem α2-adrenoceptor activation, because stimulation of postsynaptic α2-adrenoceptors in the NTS decreased sympathetic output (Fig. 13–6).19 The discovery of imidazoline binding sites, however, led to a more complicated analysis. It was discovered that imidazolines and related xenobiotcis produced hypotension when applied to the VLM, whereas catecholamines capable of activating α2-adrenoceptors were claimed to be incapable of producing effects at this site. This led to the hypothesis that receptors specific for imidazolines, different from α2A-adrenoceptors, must exist. Decreased sympathetic output could result from activation of imidazoline binding sites in the VLM and from α2-adrenoceptor activation in the NTS; sedation and respiratory depression were attributed to α2-adrenoceptor activation in the locus ceruleus.46
Imidazoline binding sites have been characterized and subdivided into I1, I2 (with subtypes), and I3.41 I1 binding sites reside on neuronal plasma membranes and are involved in controlling systemic blood pressure. I2 sites are allosteric sites found on the external membrane of mitochondria and modulate MAO-A and MAO-B.19,41 The putative I3 sites are thought to modulate insulin secretion via ATP-sensitive potassium channels in β-islet cells.
The molecular structure of the imidazoline binding sites has not been identified. Endogenous ligands for these binding sites have been discovered: agmatine, imidazole-acetic acid ribotide, harmane, and other β-carbolines.10,60,132
Functional evidence suggests that there is significant interaction between the imidazoline sites and α2A-adrenoceptors, and that this interaction is necessary to trigger hypotensive effects.18,46 As examples, there appears to be a close relationship between "presynaptic" imidazoline sites and "downstream" α2A-adrenoceptors in the VLM mediating hypotension;59 α2A-adrenoceptors in the VLM appear to be activated as a consequence of imidazoline site activation. Although second-generation agents (rilmenidine and moxonidine) preferentially act via imidazoline binding sites, and although α2A-adrenoceptors are important for the hypotension produced by first-generation agents (clonidine and α-methyldopa), hypotension produced by all of these xenobiotics is dependent on central noradrenergic pathways.18,59 Some studies report that yohimbine, an α2-adrenoceptor antagonist, reverses the hypotensive effect of both clonidine and rilmenidine-like drugs when given at high doses. Thus, it appears that there is significant interaction between imidazoline sites and α2A-adrenoceptors, and that centrally acting antihypertensive agents with relatively high affinity for imidazoline binding sites may require both imidazoline specific sites and functional α2A-adrenoceptors to produce their hypotensive actions.
Ingestions of xenobiotics that activate α2A-adrenoceptors and imidazoline binding sites (Table 13–5) produce a mixed clinical picture. Peripheral postsynaptic α2-adrenoceptor stimulation produces vasoconstriction, pallor, and hypertension, often with reflex bradycardia (Fig. 13–5). Peripheral presynaptic α2-adrenoceptor stimulation prevents NE release (Fig. 13–5), whereas central α2-adrenoceptor stimulation in the locus ceruleus accounts for CNS and respiratory depression (Fig. 13–6). Stimulation of postsynaptic α2-adrenoceptors in the NTS and of central I1 receptors in the VLM are thought to inhibit sympathetic output and enhance parasympathetic tone, explaining hypotension with bradycardia (Fig. 13–6).75 Both first-generation and second-generation agents produce dry mouth.19,41
Inhibition of dopamine-β-hydroxylase, a copper-containing enzyme (Fig. 13–5), prevents the conversion of DA to NE, resulting in less NE release and less α-adrenoceptor and β-adrenoceptor stimulation with neuronal firing. Disulfiram and diethyldithiocarbamate, copper chelators, produce such inhibition.43 Because NE release mediates most of DA's ability to cause vasoconstriction, NE is the vasopressor of choice in a hypotensive patient taking disulfiram. MAOIs and α-methyldopa also inhibit dopamine-β-hydroxylase, although this is not their main mechanism of action.101
Dopamine is relatively contraindicated in hypotensive patients who have overdosed on MAOIs. First, DA acts indirectly and its administration might produce excessive adrenergic activity and exaggerated rises in blood pressure. Second, even if an adrenergic storm does not occur, most of dopamine's α-mediated vasoconstriction is secondary to NE release. In the presence of MAOIs, NE synthesis may be impaired from concomitant dopamine-β-hydroxylase inhibition, and DA may not reliably raise blood pressure if cytoplasmic and vesicular stores have been depleted. In the presence of impaired NE release or α-adrenoceptor blockade by any cause, unopposed dopamine-induced vasodilatation from action on peripheral DA and β-adrenoceptors may paradoxically lower blood pressure further. Norepinephrine and EPI can be used to support blood pressure relatively safely in patients taking MAOIs, because they have little or no indirect action and are metabolized by COMT when given intravenously.