• Adrenergic Neurotransmission: Introduction to the Neurotransmitters

  • Norepinephrine: transmitter released at most postganglionic sympathetic terminals

  • Dopamine: major CNS neurotransmitter of mammalian extrapyramidal system and some mesocortical and mesolimbic neurononal pathways.

  • Epinephrine: most important hormone of the adrenal medulla

• Catecholamine Synthesis, Storage, and Release

  • Aromatic L-amino acid decarboxylase (DOPA decarboxylase)

    1. Dopa leads to dopamine

    2. Methyldopa leads to a-methyldopamine (converted by dopamine β hydroxylase to the "false transmitter" α-norepinephrine)

    3. 5-hydroxy-L-tryptophanleads to5-hydroxytryptamine (5-HT)

  • Tyrosine Hydroxylase

    1. Tyrosine leads to DOPA

    2. Rate limiting step in pathway

    3. Ttyrosine hydroxylase is a substrate for cAMP-dependent and Ca²⁺ - calmodulin-sensitive protein kinase and protein kinase C

    4. Increased hydroxylase activity is associated with the phosphorylated enzyme

  • Catecholamine Storage

    • In adrenergically innervated tissue: norepinephrine is localized in post-ganglionic nerve terminals

      • Large dense core vesicles (corresponding to chromaffin granules)

      • Small dense core vesicles (containing norepinephrine, ATP, and membrane-bound dopamine β-hydroxylase

    • In the adrenal medulla, catecholamines are localized in chromaffin granules.

  • The most abundant catecholamine in the adrenal medulla is epinephrine.

  • The adrenal medulla has two cells types containing catecholamines:

    •  One type contains mainly norepinephrine

    •  The second type contains mainly epinephrine.

  • Epinephrine-containing cells express cytoplasmic phenylethanolamine-N-methyl transferase, allowing conversion of norepinephrine to epinephrine.

  • Norepinephrine:

    1. Synthesized in granules

    2. Diffuses out, is methylated in the cytoplasm to epinephrine

    3. Then reenters the chromaffin granules.

  •  About half of dopamine is formed in sympathetic neuronal cytoplasm is actively translocated into dopamine β-hydroxylase-containing vesicles, where the final step, conversion to norepinephrine occurs.

    • The remaining dopamine is converted to homovanillic acid.

  • Regulation of Adrenal Medullary Catecholamine Levels

    • An important factor controlling the rate of epinephrine synthesis and adrenal medullary epinephrine concentration is glucocorticoid concentration.

    • Glucocorticoids:

      1. Are secreted by the adrenal cortex

      2.  Are carried by an intra-adrenal portal vascular system to the adrenal medullary chromaffin cells

      3.  Induce synthesis of phenylethanolamine-N-methytransferase.

      4.  Also increase levels of tyrosine hydroxylase and dopamine β-hydroxylase.

    • Stress leads to increased corticotropin leads to increased cortisol, increased epinephrine

• Reuptake

  • Following release from adrenergic nerve endings, termination of norepinephrine effect is mainly due to reuptake into presynaptic terminals.

  • In tissues with wide synaptic gaps and in blood vessels, the effect of released norepinephrine is ended by:

    1. Enzymatic breakdown

    2. Diffusion away from receptors

    3. Extraneuronal uptake.

  • Neuronal norepinephrine reuptake requires two systems:

    •  A transport system that translocates norepinephrine from extraneuronal spaces into cytoplasm.

    •  A transport system that translocates norepinephrine from the cytoplasm into vesicles.

  • Translocation of norepinephrine from extraneuronal spaces (uptake I) into the cytoplasm is blocked by:

    1. Cocaine

    2. Tricyclic antidepressants (e.g. imipramine (Tofranil))

    • Imipramine (Tofranil) : Tricyclic Antidepressant

      • Inhibits norepinephrine and serotonin reuptake

      • Anticholinergic properties

      • Antihistaminic properties

      • Orthostatic hypotension due to α-receptor blockade

      • Sedation

      • Mild analgesic

    • Labeled Uses

      • Endogenous depression (an serotonin-specific reuptake inhibitor (SSRI) or other second generation agent is likely to be used first)

      • Occasionally, reactive depression

      • Treatment of enuresis in children older than six.

Shannon, M.T., Wilson, B.A., Stang, C. L. In, Govoni and Hayes 8th Edition: Drugs and Nursing Implications Appleton & Lange, 1995, pp. 616-619

• Mechamisms of Indirect Acting Sympathomimetics

  • An indirect acting sympathomimetic acts mainly by promoting norepinephrine release from nerve terminals.

  • Mechanism: These amines, all substates for uptake I, act by:

    • competing with noradrenergic vesicular transport systems, thus making norepinephine more available for release.

  • Indirect-acting agents, such as tyramine, produce tachyphylaxis in which repetitive doses of tyramine results in a progressively diminishing response.

    • Tachyphylaxis may result from depletion of a small pool of vesicular norepinphrine residing near the presynaptic membrane.

  • Uptake II is an extraneuronal (glia, heart, liver, etc )amine translocator that exhibits low affinity for norepinephrine and higher affinities for epinephrine and isoproterenol. This system is of limited physiological significance, unless Uptake I is blocked.

• Metabolic Transformation

  • Besides reuptake and diffusion away from receptor sites, catecholamine action can end due to metabolic transformation.

  • Two primary degradative enzymes:

    • Monoamine Oxidase (MAO)

    • Catechol-O-Methyl Transferase (COMT)

  • Inhibitors of MAO, such as pargyline, phenelzine (Nardil), and tranylcypromine (Parnate) increase norepinephrine, dopamine, and serotonin (5-HT) brain concentrations. 

    • These concentration increases may be responsible for antidepressant action of MAO inhibitors.

  • Monoamine Oxidase Inhibitor: Phenelzine [Nardil]

    • Hydrazine MAO inhibitor with amphetamine-like activity

    • Termination of drug action requires new MAO enzyme synthesis

    • May cause Hypertensive crisis

    • Labeled Uses

      1. Rreatment of endogenous depression

      2. Management of depressive phase of bipolar disorder

      3. Treatment of severe reactive depression not responsive to other drugs.

Shannon, M.T., Wilson, B.A., Stang, C. L. In, Govoni and Hayes 8th Edition: Drugs and Nursing Implications Appleton & Lange, 1995, pp. 904-905

• Catecholamine Release (Adrenal medulla)

  • Release steps: Chromaffin Granule Adrenal medulla

    • Preganglion fiber releases Ach →nicotinic receptor activation→depolarization→Ca²⁺ entry →exocytosis of granular content

    • Ca²⁺ influx is important in excitation (depolarization)--release coupling

Lefkowitz, R.J, Hoffman, B.B and Taylor, P. Neurotransmission: The Autonomic and Somatic Motor Nervous Systems, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp.112-137

• β-adrenergic receptors

  • Order of agonist potency

    • Isoproterenol > epinephrine > norepinephrine

  • β receptors are divided into two major categories: β₁ and β₂.

    • β₁ receptors → myocardium.

    • β₂ receptors → smooth muscle and most other sites.

  • The subdivision of beta receptors followed from the observation that in the heart norepinephrine and epinephrine were equipotent, whereas epinephrine was many fold (10 - 50) more potent at smooth muscle.

  • A β₃ receptor has been found that is strongly activated by norepinephrine compared to epinephrine and may explain "atypical" pharmacological properties of adipose tissue.

    • The β₃ -receptor is not blocked by propranolol, classified as a non selective beta-receptor blocker.

    • The β₃ -receptor is not blocked by propranolol, classified as a non selective beta-receptor blocker.

  • Activation of β₁, β₂ and β₃ receptors increases adenylyl cyclase activity (G_s mediated) resulting in a rise of intracellular cAMP.

    • Cardiac inotropic effects result from increases in Ca²⁺ concentration, due to:

      • phosphorylation of L-type Ca²⁺ channels

      • phosphorylation of sarcolemmal Ca²⁺ pumps

      • direct action G_s action on the L-type channel

    • Effects on the liver lead to activation of glycogen phosphorylation

  • β₂ receptor activation mediates relaxation of vascular smooth muscle

    •  β₂ receptor activation mediates relaxation of G.I. smooth muscle. α₂ adrenergic receptor activation acts presynaptically to reduce Ach release and promote G.I. smooth muscle relaxation.

    • The α₂ receptor effect is the more important.

• α-Adrenergic Receptors

  • Order of agonist potency

    • epinephrine > norepinephrine >> isoproterenol

    • Multiple α receptor subtypes have been identified.

    • Multiple forms were suggested when, after administration of an α-receptor antagonist, repetitive nerve stimulation resulted in increasing amount of norepinephrine release. This findings suggested a presynaptic α-receptor binding site.

    • Post-synaptic receptors→ α₁ .

    • Pre-synaptic receptors→ α₂ .

    • α₂ receptors are also present post-synaptically. This site is involved in the action of some centrally-acting antihypertensive agents, e.g. clonidine.

    • Some drugs, such as clonidine are more active at α₂ receptors.

• Clonidine (Catapres)

  • Clonidine acts in the brain at post-synaptic α₂ receptors, inhibiting adrenergic outflow from the brainstem. Inhibition of sympathetic outflow results in a decrease in blood pressure.

  • Clonidine reduces cardiac output (by reducing both stroke volume and heart rate) and peripheral resistance. Reduction in stoke volume occurs due to increased venous pooling (decreased preload).

  • Clonidine does not interfere with cardiovascular responses to exercise.

  • Renal blood flow and function is maintained during clonidine treatment.

  • Clonidine has minimal or no effect on plasma lipids. 

  • Adverse Effects

    • Dry Mouth (xerostomia)

    • Withdrawal syndrome upon abrupt discontinuation (increased blood pressure, headache, tachycardia, apprehension, tremors)

    • Bradycardia (in patients with SA nodal abnormality)

  • Some drugs such as methoxamine (Vasoxyl) or phenylephrine (Neo-Synephrine) are more active at α₁ receptors.

  • Multiple forms of both α₁ and α₂ receptors have been identified.

• Dopamine

  • D₁ receptor activation results in stimulation of adenylyl cyclase activity.

  • Smooth muscle relaxation (e.g. renal vasodilation) would result from increases in cAMP caused by activation of D₁ receptors 

    • Increased cAMP levels may facilitate inactivation of myosin light chain kinase, MLK (activated by calcium-calmodulin complexes which means that increased Ca²⁺ promotes MLK activation).

    • Note that only phosphylated myosin can bind to actin and that phosphorylation state is controlled by the enzyme myosin light chain kinase.

  • D₂ receptor activation inhibits cAMP production (inhibits adenylyl cyclase activity), increases K⁺ conductance and decreases calcium influx.

• Catecholamine Refractoriness

  • Following exposure to catecholamines, there is a progressive loss of the ability of the target site to respond to catecholamines. This phenomenon is termed tachyphylaxis, desensitization or refractoriness.

  • Regulation of catecholamine responsiveness occurs at several levels:

    • Receptors

    • G proteins

    • Adenylyl cyclase

    • Cyclic nucleotide phosphodiesterase

  • Stimulation of β-adrenergic receptors rapidly causes receptor phosphorylation and decreased responsiveness. The phosphorylated receptor exhibits:

    • Decreased coupling to G_s and

    • Decreased stimulation of adenylyl cyclase.

     

• Other Autonomic Neurotransmitters/Cotransmitters

  • ATP

    • ATP and catecholamines are found together in neuronal and adrenal medullary storage granules. ATP is released along with transmitters and, in certain cases, has an important role in synaptic transmission.

  • Vasoactive Intestinal Peptide (VIP)

    • Vasoactive intestinal peptide (VIP) is found in association with ACh in autonomic parasympathetic fibers innervating blood vessels and exocrine glands and cholinergic sympathetic fibers innervating sweat glands.

    • VIP may be involved in salivation, tracheal and the GI tract responsiveness to parasympathetic input.

  • Neuropeptide Y Family

    • The neuropeptide Y family includes NPY, pancreatic polypeptide, and peptide YY.

    • NPY in the periphery is associated with sympathetic fibers and assists in maintaining vascular tone.

    • NPY is a potent, long-lasting vasoconstrictor, especially of small vessels

  • Purines

    • Purines such as ATP and adenosine may be responsible for apparent non-cholinergic, non-adrenergic autonomic neurotransmission.

  • Nitric Oxide

    • Blood vessel endothelium is required for ACh-mediated smooth muscle relaxation.

    • The endothelial cell layer modulates vessel responsiveness to autonomic and hormonal influences.

    • Endothelial cell elaborate endothelium-derived relaxing factor (EDRF, nitric oxide) and a contracting factor.

    • Pharmacological actions of serotonin, histamine, bradykinin, purines, thrombin are mediated to some degree by stimulation of EDRF release.

    • Endothelial-released nitric oxide diffuses into vascular smooth muscle and activates guanylyl cyclase which increases cGMP.

    • Clinically, hypotension associated with endotoxemia may be mediated partially by increased release of nitric oxide. A similar mechanism is proposed for hypotension induced by cytokines.

Lefkowitz, R.J, Hoffman, B.B and Taylor, P. Neurotransmission: The Autonomic and Somatic Motor Nervous Systems, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp.112-137.Hoffman, B. B. Adrenoceptor-Activating & Other Sympathomimetic Drugs: Introduction to Antimicrobial Agents in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, p.118-122