Norepinephrine & Epinephrine
The chemical transmitter present at most sympathetic post ganglionic endings is norepinephrine. It is stored in the synaptic knobs of the neurons that secrete it in characteristic small vesicles that have a dense core (granulated vesicles). Nor epinephrine and its methyl derivative, epinephrine, are also secreted by the adrenal medulla, but epinephrine is not a mediator at postganglionic sympathetic endings. As discussed in Chapter 6, each sympathetic postganglionic neuron has multiple varicosities along its course, and each of these varicosities appears to be a site at which norepinephrine is secreted.
There are also norepinephrine-secreting and epinephrine secreting neurons in the brain. Norepinephrine-secreting neurons are properly called noradrenergic neurons, although the term adrenergic neurons is also applied. However, the latter term should be reserved for epinephrine-secreting neurons. The cell bodies of the norepinephrine-containing neurons are located in the locus coeruleus and other medullary and pontine nuclei (Figure 1). From the locus coeruleus, the axons of the noradrenergic neurons descend into the spinal cord, enter the cerebellum, and ascend to innervate the paraventricular, supraoptic, and periventricular nuclei of the hypothalamus, the thalamus, the basal telencephalon, and the entire neocortex. The action of norepinephrine in these regions is primarily as a neuromodulator.
Fig1. Four diffusely connected systems of central neuromodulators. A) Noradrenergic neurons in the locus coeruleus innervate the spinal cord, cerebellum, several nuclei of the hypothalamus, thalamus, basal telencephalon, and neocortex. B) Serotonergic neurons in the raphe nuclei project to the hypothalamus, limbic system, neocortex, cerebellum, and spinal cord. C) Dopaminergic neurons in the substantia nigra project to the striatum and those in the ventral tegmental area of the midbrain project to the prefrontal cortex of the limbic system. D) Cholinergic neurons in the basal forebrain complex project to the hippocampus and the neocortex and those in the pontomesencephalotegmental cholinergic complex project to the dorsal thalamus and the forebrain. (Reproduced with permission from Boron WF, Boulpaep EL: Medical Physiology. St. Louis, MO: Elsevier; 2005.)
Biosynthesis & Release of Catecholamines
The principal catecholamines found in the body (norepinephrine, epinephrine, and dopamine) are formed by hydroxylation and decarboxylation of the amino acid tyrosine (Figure 2 and Figure 3). Some of the tyrosine is formed from phenylalanine, but most is of dietary origin. Phenylalanine hydroxylase is found primarily in the liver. Tyrosine is transported into catecholamine-secreting neurons via a Na+-dependent carrier. It is converted to dihydroxy-phenylalanine (dopa) and then to dopamine in the cytoplasm of the cells by tyrosine hydroxylase and dopa decarboxylase, respectively. The decarboxylase is also called amino acid decarboxylase. The rate-limiting step in synthesis of catecholamines is the conversion of tyrosine to dopa. Tyrosine hydroxylase is subject to feedback inhibition by dopamine and norepinephrine, thus providing internal con trol of the synthetic process.
Fig2. Biosynthesis of some common small-molecule neurotransmitters. A) Glutamate is synthesized in the Krebs cycle by the conversion of α-ketoglutarate to the amino acid via the enzyme γ-aminobutyric acid transferase (GABA-T) or in nerve terminals by the hydrolysis of glutamine by the enzyme glutaminase. GABA is synthesized by the conversion of glutamate by the enzyme glutamic acid decarboxylase (GAD). B) Acetylcholine is synthesized in the cytoplasm of a nerve terminal from acetyl-Co-A and choline by the enzyme choline acetyltransferase. C) Serotonin is synthesized from the amino acid tryptophan in a two-step process: the enzymatic hydroxylation of tryptophan to 5-hydroxytryptophan and the enzymatic decarboxylation of this intermediate to form 5-hydroxytryptamine (also called serotonin). D) Catecholamines are synthesized from the amino acid tyrosine by a multi-step process. Tyrosine is oxidized to dihydroxyphenylalanine (DOPA) by the enzyme tyrosine hydroxylase in the cytoplasm of the neuron; DOPA is then decarboxylated to dopamine. In dopaminergic neurons, the process stops there. In noradrenergic neurons, the dopamine is transported into synaptic vesicles where it is converted to norepinephrine by dopamine-β hydroxylase. In neurons that also contain the enzyme phenylethanolamine-N-methyltransferase, norepinephrine is converted to epinephrine.
Fig3. Biochemical events at a noradrenergic synapse. Tyrosine (Tyr) is transported into the noradrenergic nerve terminal by a Na+-dependent carrier (A). The steps involved in the conversion of Tyr to dopamine and dopamine to norepinephrine (NE) are described in Figure 2. Metyrosine blocks the action of tyrosine hydroxylase (TH), the rate-limiting step in the production of catecholamines. Dopamine is transported from the cytoplasm into the vesicle by the vesicular monoamine transporter (VMAT), which can be blocked by the drug reserpine. NE and other amines can also be carried by VMAT. Dopamine is converted to NE in the vesicle. An action potential opens voltage-sensitive Ca2+ channels to allow an influx of Ca2+, and the vesicles then fuse with the surface membrane to trigger expulsion of NE along with peptides (P) and adenosine triphosphate (ATP). This process involves synaptosome-associated proteins (SNAPs) and vesicle-associated membrane proteins (VAMPs); it can be blocked by drugs such as guanethidine and bretylium. NE released into the nerve terminal can act on G-protein—coupled receptors on the postsynaptic neuron or neuroeffector organ (eg, blood vessels). NE can also diffuse out of the cleft or be transported back into the nerve terminal by the norepinephrine transporter (NET). NET can be blocked by cocaine and tricyclic antidepressants. Autoreceptors and heteroreceptors on the presynaptic nerve ending modulate neurotransmitter release.
Once dopamine is synthesized, it is transported into the vesicle by the VMAT. Here the dopamine is converted to nor epinephrine by dopamine β-hydroxylase. Norepinephrine is the only small-molecule transmitter that is synthesized in synaptic vesicles instead of being transported into the vesicle after its synthesis.
Some neurons in the CNS and adrenal medullary cells also contain the cytoplasmic enzyme phenylethanolamine N-methyltransferase, which catalyzes the conversion of nor epinephrine to epinephrine. In these cells, norepinephrine leaves the vesicles, is converted to epinephrine in the cytoplasm, and then enters other vesicles for storage until it is released by exocytosis.
Catabolism of Catecholamines
Norepinephrine, like other amine and amino acid transmitters, is removed from the synaptic cleft by binding to post synaptic receptors, binding to presynaptic receptors, reuptake into the presynaptic neurons, or catabolism. Reuptake via a NET is a major mechanism to terminate the actions of nor epinephrine (Figure 4), and the hypersensitivity of sympathetically denervated structures is explained in part on this basis. After the noradrenergic neurons are cut, their endings degenerate with loss of NET to remove norepinephrine from the synaptic cleft. Consequently, more norepinephrine from other sources is available to stimulate the receptors on the autonomic effectors.
Fig4. Fate of monoamines secreted at synaptic junctions. In each monoamine-secreting neuron, the monoamine is synthesized in the cytoplasm and the secretory granules and its concentration in secretory granules is maintained by the two vesicular monoamine transporters (VMAT). The monoamine is secreted by exocytosis of the granules, and it acts on G-protein coupled receptors. In this example, the monoamine is norepinephrine acting on adrenoceptors. Many of these receptors are postsynaptic, but some are presynaptic and some are located on glia. In addition, there is extensive reuptake of the monoamine into the cytoplasm of the presynaptic terminal via a monoamine transporter, in this case the norepinephrine transporter (NET). (Modified with permission from Katzung BG, Masters SB, Trevor AJ: Basic and Clinical Pharmacology, 11th ed. New York, NY: McGraw-Hill; 2009.)
Epinephrine and norepinephrine are metabolized to bio logically inactive products by oxidation and methylation. The former reaction is catalyzed by monoamine oxidase (MAO) and the latter by catechol-O-methyltransferase (COMT). MAO is located on the outer surface of the mitochondria. MAO is widely distributed, being particularly plentiful in the nerve endings at which catecholamines are secreted. COMT is also widely distributed, particularly in the liver, kidneys, and smooth muscles. In the brain, it is present in glial cells, and small amounts are found in postsynaptic neurons, but none is found in presynaptic noradrenergic neurons. Consequently, catecholamine metabolism has two different patterns.
Extracellular epinephrine and norepinephrine are for the most part O-methylated, and measurement of the concentrations of the O-methylated derivatives normetanephrine and metanephrine in the urine is a good index of the rate of secretion of norepinephrine and epinephrine. The O-methylated derivatives that are not excreted are largely oxidized, and vanillylmandelic acid is the most plentiful catecholamine metabolite in the urine.
In the noradrenergic nerve terminals, some of the norepinephrine is constantly being converted by intra cellular MAO to the physiologically inactive deaminated derivatives, 3,4-dihydroxymandelic acid and its corresponding glycol. These are subsequently converted to their corresponding O-methyl derivatives, vanillylmandelic acid, and 3-methoxy-4-hydroxyphenylglycol.
α- & β-Adrenoceptors
Epinephrine and norepinephrine both act on α- and β-adrenergic receptors (adrenoceptors), with norepinephrine having a greater affinity for α-adrenoceptors and epinephrine for β-adrenoceptors. These receptors are metabotropic GPCR, and each has multiple subtypes (α1A , α1B , α1D , α2A , α2B , α2C, and β1 β3). Most α1-adrenoceptors are coupled via Gq proteins to phospholipase C, leading to the formation of IP3 and DAG, which mobilizes intracellular Ca2+ stores and activates protein kinase C, respectively. Thus, at many synapses, activation of α1-adrenoceptors is excitatory to the postsynaptic target. In contrast, α2-adrenoceptors activate Gi inhibitory proteins to inhibit adenylyl cyclase and decrease cAMP. Other actions of α2 adrenoceptors are to activate G-protein–coupled inward rectifier K+ channels to cause membrane hyperpolarization and to inhibit neuronal Ca2+ channels. Thus, at many synapses, activation of α2-adrenoceptors inhibits the postsynaptic target. Presynaptic α2-adrenoceptors are autoreceptors which, when activated, inhibit further release of norepinephrine from postganglionic sympathetic nerve terminals. β-Adrenoceptors activate a stimulatory GS protein to activate adenylyl cyclase to increase cAMP.
α1-Adrenoceptors are located on smooth muscle and the heart, and α2-adrenoceptors are located in the CNS and on pancreatic islets cells and nerve terminals. β1-Adrenoceptors are located in the heart and renal juxtaglomerular cells. β2-Adrenoceptors are located in bronchial smooth muscle and skeletal muscle. β3-Adrenoceptors are located in adipose tissue.
Pharmacology of Noradrenergic Synapses
Table 1 shows some of the common agonists that bind to adrenoceptors as well as some of the common adrenoceptor antagonists. Figure 7–9 also shows the site of action of various drugs that alter noradrenergic transmission. For example, metyrosine blocks the action of tyrosine hydroxylase, the rate-limiting step in the synthetic pathway for catecholamine production in the nerve terminal. Reserpine blocks the VMAT that moves dopamine into the synaptic vesicle. Also, bretylium and guanethidine prevent the release of norepinephrine from the nerve terminal. Cocaine and tricyclic antidepressants block the NET. In addition to the agonists listed in Table 1, some drugs mimic the actions of norepinephrine by releasing stored transmitter from the noradrenergic endings. These are called sympathomimetics and include amphetamines and ephedrine.
Table1. Pharmacology of a selection of receptors for some small-molecule neurotransmitters.
Dopamine
In some parts of the brain, catecholamine synthesis stops at dopamine (Figure 2), which can then be secreted into the synaptic cleft. Active reuptake of dopamine occurs via a Na+- and Cl–-dependent dopamine transporter. Dopamine is metabolized to inactive compounds by MAO and COMT in a manner analogous to the inactivation of norepinephrine. 3,4-Dihydroxyphenylacetic acid and homovanillic acid are conjugated, primarily to sulfate.
Dopaminergic neurons are located in several brain regions (Figure 1). One region is the nigrostriatal system, which projects from the midbrain substantia nigra to the striatum in the basal ganglia and is involved in motor control. Another dopaminergic system is the mesocortical system; it arises primarily in the ventral tegmental area, which projects to the nucleus accumbens and limbic subcortical areas. The mesocortical system is involved in reward behavior and addiction and in psychiatric disorders such as schizophrenia. Studies using positive emission tomography (PET) scanning in healthy humans show that a steady loss of dopamine receptors occurs in the basal ganglia with age. The loss is greater in men than in women.
Dopamine Receptors
Five dopamine receptors have been cloned, but they fall into two major categories: D1-like (D1 and D5 ) and D2-like (D2 , D3 , and D4 ). All dopamine receptors are metabotropic GPCR. Activation of D1-type receptors leads to an increase in cAMP, whereas activation of D2-like receptors reduces cAMP levels. Overstimulation of D2 receptors may contribute to the pathophysiology of schizophrenia. D3 receptors are highly localized, especially to the nucleus accumbens (Figure1). D4 receptors have a greater affinity than the other dopamine receptors for the “atypical” antipsychotic drug clozapine, which is used primarily to treat schizophrenia in individuals who do not respond to other therapies.
Serotonin
Serotonin (5-hydroxytryptamine; 5-HT) is present in highest concentration in blood platelets and in the gastrointestinal tract, where it is found in the enterochromaffin cells and the myenteric plexus. It is also found within the brainstem in the midline raphe nuclei, which project to a wide portion of the CNS including the hypothalamus, limbic system, neocortex, cerebellum, and spinal cord (Figure 1).
Serotonin is synthesized from the essential amino acid tryptophan (Figure2 and Figure 5). The rate-limiting step is the conversion of the amino acid to 5-hydroxytryptophan by tryptophan hydroxylase. This is then converted to serotonin by the aromatic L-amino acid decarboxylase. Serotonin is transported into the vesicles by the VMAT. After release from serotonergic neurons, much of the released serotonin is recaptured by the relatively selective serotonin transporter (SERT). Once serotonin is returned to the nerve terminal, it is either taken back into the vesicles or is inactivated by MAO to form 5-hydroxyindoleacetic acid (5-HIAA). This substance is the principal urinary metabolite of serotonin, and urinary output of 5-HIAA is used as an index of the rate of serotonin metabolism in the body.
Fig5. Biochemical events at a serotonergic synapse. Tryptophan is transported into the serotonergic nerve terminal by a Na+-dependent aromatic L-amino acid transporter. The steps involved in the conversion of tryptophan to serotonin (5-hydroxytryptamine, 5-HT) are described in Figure 7–1. 5-HT is transported from the cytoplasm into vesicles by the vesicular monoamine transporter (VMAT). 5-HT release occurs when an action potential opens voltage-sensitive Ca2+ channels to allow an influx of Ca2+ and fusion of vesicles with the surface membrane. 5-HT released into the nerve terminal can act on G-protein–coupled receptors on the postsynaptic neuron (not shown). 5-HT can also diffuse out of the cleft or be transported back into the nerve terminal by the 5-HT transporter. 5-HT can act on presynaptic autoreceptors to inhibit further neurotransmitter release. Cytoplasmic 5-HT is either sequestered in vesicles as described or metabolized to 5-hydroxyindole acetaldehyde by mitochondrial monoamine oxidase (MAO).
Tryptophan hydroxylase in the CNS is slightly different from the tryptophan hydroxylase in peripheral tissues and is coded by a different gene. This is presumably why knockout of the TPH1 gene, which codes for tryptophan hydroxylase in peripheral tissues, has much less effect on brain serotonin pro duction than on peripheral serotonin production.
Serotonergic Receptors
There are seven classes of 5-HT receptors (from 5-HT1 through 5-HT7 receptors), and all except one (5-HT3 ) are GPCR and affect adenylyl cyclase or phospholipase C (Table 1). Within the 5-HT1 group are the 5-HT1A , 5-HT1B , 5-HT1D , 5-HT1E , and 5-HT1F subtypes. Within the 5-HT2 group there are 5-HT2A , 5-HT2B , and 5-HT2C subtypes. There are two 5-HT5 subtypes: 5-HT5A and 5-HT5B . Some of the serotonin receptors are presynaptic and others are postsynaptic.
5-HT2A receptors mediate platelet aggregation and smooth muscle contraction. Mice in which the gene for 5-HT2C receptors has been knocked out are obese as a result of increased food intake despite normal responses to leptin, and they are prone to fatal seizures. 5-HT3 receptors are present in the gastrointestinal tract and the area postrema and are related to vomiting. 5-HT4 receptors are also present in the gastrointestinal tract, where they facilitate secretion and peristalsis, and in the brain. 5-HT6 and 5-HT7 receptors in the brain are distributed throughout the limbic system, and the 5-HT6 receptors have a high affinity for antidepressant drugs.
Pharmacology of Serotonergic Synapses
Table 1 shows some of the common agonists that bind to 5-HT receptors as well as some of the common 5-HT receptor antagonists. In addition, tricyclic antidepressants inhibit the uptake of serotonin by block of SERT, similar to what was described for their actions at noradrenergic synapses. Selective serotonin uptake inhibitors (SSRIs) such as fluoxetine are widely used in the treatment of depression.
Histamine
Histaminergic neurons have their cell bodies in the tuber omammillary nucleus of the posterior hypothalamus, and their axons project to all parts of the brain, including the cerebral cortex and the spinal cord. Histamine is also found in cells in the gastric mucosa and in heparin-containing cells called mast cells that are plentiful in the anterior and posterior lobes of the pituitary gland as well as at body surfaces. Histamine is formed by decarboxylation of the amino acid histidine. The three well-characterized types of histamine receptors (H1 , H2 , and H3 ) are all found in both peripheral tissues and the brain. Most, if not all, of the H3 receptors are presynaptic, and they mediate inhibition of the release of histamine and other transmitters via a G-protein. H1 receptors activate phospholipase C, and H2 receptors increase intracellular cAMP. The function of this diffuse histaminergic system is unknown, but evidence links brain histamine to arousal, sexual behavior, blood pressure, drinking, pain thresholds, and regulation of the secretion of several anterior pituitary hormones. In addition, a recently described histamine H4 receptor appears to play a role in regulating cells of the immune system.
ATP
ATP is an example of a small-molecule that is often co-localized and co-released from synaptic vesicles such as those in nor adrenergic postganglionic sympathetic neurons, and it has recently been identified as a neurotransmitter. ATP has been shown to mediate rapid synaptic responses in the autonomic nervous system and a fast response in the habenula. ATP binds to P2X receptors, which are ligand-gated ion channel receptors. P2X receptors have widespread distributions throughout the body, including the dorsal horn, which implicates a role for ATP in sensory transmission. Antagonists of P2X receptors are under development for the treatment of chronic pain. ATP also binds to P2Y and P2U receptors, which are GPCR.
