OTHER PERIPHERAL MEDIATORS: 5-HYDROXYTRYPTAMINE (5-HT) & PURINE PEPTIDES

1. OTHER PERIPHERAL MEDIATORS: 5-HYDROXYTRYPTAMINE (5-HT) & PURINE PEPTIDES

2. OVERVIEW • In this session we will discuss two types of mediators, both of which play a role as neurotransmitter in the brain and periphery and also probably function as local hormones. • These mediators are: • 5-HydroxyTryptamine (5-HT) • Purines (nucleosides and nucleotides)

3. Overview… • 5-HT has a longer pharmacological history than purines, and numerous drugs in current use act wholly or partly on 5-HT receptors, of which no fewer than 14 subtypes have been identified. • Purine pharmacology is much sparser

4. Overview… • In both cases, the physiological significance- and hence therapeutic relevance- of the various receptor subtypes is still being unravelled. (NB to unravel =to explain something that is difficult to understand) • Focus will be on the more secure hypotheses, recognizing that the picture is far from complete.

5. 5-HYDROXYTRYPTAMINE (5-HT)

6. Learning Objectives At the end of the session students must be able to understand and describe:- • Distribution, biosynthesis, and degradation • Pharmacological effects • Classification of 5-HT receptors • Drugs acting on 5-HT receptors • Clinical conditions in which 5-HT plays a role

7. 5-HYDROXYTRYPTAMINE • Serotonin was the name given in the last century to an unknown vasoconstrictor substance found in the serum after blood has clotted. • It was identified chemically as 5-hydroxytryptamine (5-HT) in 1948, and shown to originate from platelets. • It was subsequently found in the GIT and the CNS, and shown to function both as a neurotransmitter and as a local hormone in the peripheral vascular system.

8. DISTRIBUTION, BIOSYNTHESIS AND DEGRADATION OF 5-HT

9. Distribution of 5-HT • 5-HT occurs in the highest concentrations in three organs in the body-: • Wall of the intestine • Blood • CNS

10. Distribution of 5-HT… • In the wall of the intestine: Over 90% of the total amount in the body is present in enterochromaffin cells, which are cells derived from the neural crest, similar to those of the adrenal medulla, that are interspersed with mucosal cells, mainly in the stomach and small intestine.

11. Distribution of 5-HT… Some 5-HT also occurs in the nerve cells of myenteric plexus, where it functions as an excitatory neurotransmitter. • In blood: 5-HT is present in high concentration in platelets, which accumulate it from the plasma by an active transport system, and release it when they aggregate at the site of tissue damage.

12. Distribution of 5-HT (cont.) • In the CNS: 5-HT is a transmitter in the CNS and is found in high concentrations in localized regions of the midbrain.

13. THE BIOSYNTHESIS OF 5-HT • Though 5-HT is present in the diet, most of this is metabolized before entering the blood stream. • Endogenous 5-HT arises by biosynthesis, which follows a pathway similar to that of NA, except that the precursor amino acid is tryptophan instead of tyrosine.

14. Biosynthesis of 5-HT… • Is formed from amino-acid tryptophan which is present in the diet, but most is metabolized before entering the blood stream. • Tryptophan is converted to 5-hydroxytryptophan ( in chromaffin cells and neurons, but not in platelets) by the action of tryptophan hydroxylase ( an enzyme confined to 5-HT producing cells).

15. Biosynthesis of 5-HT (cont.) • The 5-hydroxytryptophan is then decarboxylated to 5-HT, by the non-specific amino acid decarboxylase that also participates in the synthesis of catecholamines and histamine. • 5-HT is transported into 5-HT-containing cells by a specific transport system.

16. Biosynthesis of 5-HT (cont) • Platelets (and neurones) possess a high affinity 5-HT uptake mechanism, and platelets become loaded with 5-HT as they pass through the intestinal circulation, where the local concentration is relatively high. • The mechanisms of synthesis, storage, release and re-uptake of 5-HT are very similar to those of NA, and many drugs affect both processes indiscriminately

17. 5-HT is often stored in neurons and chromaffin cells as a co-transmitter together with various peptide hormones, such as somatostatin, substance P or vasoactive intestinal polypeptide (VIP)

18. 5-HT Degradation • Degradation of 5-HT occurs mainly through oxidative deamination, catalysed by monoamine oxidase (MAO), followed by oxidation to 5-hydroxyindoleacetic acid (5-HIAA), which is excreted in the urine, and serves as an indicator of 5-HT production in the body. • This is used, for example in the diagnosis of carcinoid syndrome

19. Pharmacological Effects • The actions of 5-HT are numerous and complex, and there is considerable species variation. • This complexity reflects a profusion of 5-HT receptor subtypes, which has been revealed in recent years.

20. Pharmacological Effects-Sites of Action The main sites of action are: • GIT:- • 5-HT stimulates gastrointestinal motility, this being partly through a direct effect on the smooth muscles (5-HT2-receptors) and partly as a result of an indirect excitatory effect on enteric neurons (5-HT3 and 5-HT4-receptors).

21. 1. Site of action-GIT (cont.) • 5-HT also stimulates fluid secretion and elicits nausea and vomiting by stimulating smooth muscle and sensory nerves in the stomach (5-HT3- and 5-HT4-receptors)

22. Site of action-GIT (cont.) • The peristaltic reflex, evoked by increasing the pressure within a segment of intestine, is mediated, partly at least, by the release of 5-HT from chromaffin cells in response to the mechanical stimulus. • Chromaffin cells also respond to vagal stimulation by releasing 5-HT

23. 2. Site of action- Smooth Muscle • Elsewhere in the body (e.g. uterus and bronchial tree) smooth muscle is also contracted by 5-HT in many species, but only to a minor extent in humans

24. 3. Site of Action-Blood vessels • The effect of 5-HT on blood vessels depends on various factors, including the size of the vessel, the species and the prevailing sympathetic activity. • Large vessels, both arteries and veins are usually constricted by 5-HT, though the sensitivity varies greatly.

25. Site of Action-Blood vessels… • This is a direct action on vascular smooth muscle cells, mediated through 5-HT2A-receptors • Activation of 5-HT1 receptors causes constriction of large intracranial vessels, dilatation of which contributes to headache.

26. Site of Action-Blood vessels… • 5-HT also causes vasodilatation by acting on 5-HT1-receptors, partly by releasing nitric oxide from endothelia cells, and partly by inhibiting noradrenaline release from sympathetic nerve terminals. • Thus 5-HT2A receptors predominantly give rise to vasoconstriction, whereas 5-HT1 receptors produce dilatation

27. Site of Action-Blood vessels… • When 5-HT2A receptors are blocked by ketanserin the vasodilator effect is revealed. • If 5-HT is injected intravenously, the blood pressure usually rises, because of constriction of large vessels, then falls, because of arteriolar dilatation.

28. 4. Site of Action-Platelets • 5-HT causes platelet aggregation, via 5-HT2A-receptors, and the platelets that collect in the vessel release more 5-HT. • If the endothelium is intact, 5-HT release from adherent platelets causes vasodilatation, which helps to sustain blood flow;

29. Site of Action-Platelets… if it is damaged (e.g. by atherosclerosis), 5 -HT causes constriction and impairs blood flow further • These effects of platelet-derived 5-HT are thought to be important in vascular disease

30. 5. Site of Action-Nerve endings • 5-HT stimulates nociceptive ( pain-mediating) sensory nerve endings, an effect mediated mainly by 5-HT3-receptors. • If injected into the skin, 5-HT causes pain: given systemically, it elicits a variety of autonomic reflexes through stimulation of afferent fibres in the heart and lungs, which further complicate the cardiovascular response.

31. 6. Site of Action-CNS • 5-HT excites some neurons and inhibits others. • It also acts presynaptically to inhibit transmitter release from nerve terminals.

32. SUMMARY:ACTIONS AND FUNCTIONS OF 5-HT Important actions are: • Increased GIT motility (direct excitation of smooth muscle and indirect action via enteric neurons). • Contraction of smooth muscle (bronchi, uterus).

33. SUMMARY:ACTIONS AND FUNCTIONS OF 5-HT… • Mixture of vascular constriction (direct and via sympathetic innervation) and dilatation (endothelium dependent) • Platelet aggregation • Stimulation of peripheral nociceptive nerve endings • Excitation/inhibition of CNS neurons

34. POSTULATED PHYSIOLOGICAL AND PATHOPHYSIOLOGICAL ROLES • In periphery: peristalsis, vomiting, platelet aggregation and haemostasis, inflammatory mediator, sensitization of nociceptors and microvascular control. • In CNS: many postulated functions; include control of appetite; sleep; mood; hallucinations, stereotyped behaviour; pain perception and vomiting.

35. Clinical condition associated with disturbed 5-HT function includes migraine, carcinoid syndrome, mood disorders and anxiety

36. …TO CONTINUE…….

37. Classification of 5-HT Receptors • Currently, there are seven main receptor types:- • 5-HT1-7,with further subtypes (A-D) of 5HT1 and 5-HT2. • All are G- protein coupled receptors except 5-HT3 which is ligand-gated cation channel.

38. Classification of 5-HT Receptors… • 5HT1-receptor occur mainly in the CNS ( all subtypes i.e 5-HT1A-D) and some blood vessels (5-HT1D subtype). Effects are neural inhibition and vasoconstriction. Act by inhibiting adenylate cyclase. Specific agonist includes sumatriptan (used in migraine therapy) and buspirone (used in anxiety). Ergotamine is a partial agonist. Specific antagonists include spiperone and methiothepin.

39. Classification of 5-HT Receptors • 5-HT2-receptors occur in CNS and many peripheral sites (especially blood vessels; platelets; autonomic neurons). Neuronal and smooth muscle effects are excitatory. Some blood vessels dilated as a result of nitric oxide release from endothelial cells.

40. Classification Of 5-HT Receptors • 5-HT2-receptors act through phospholipase C/ inositol pathway. Specific ligands include LSD (lysergic acid diethylamide: agonist in CNS, antagonist in periphery). Specific antagonists: kentanserin, methysergide and cyproheptadine.

41. Classification of 5-HT Receptors • 5-HT3 receptors occur in peripheral nervous system; especially nociceptive afferent neuron and enteric neurons, and in CNS. Effects are excitatory, mediated via direct receptors-coupled ion channels. Specific agonist: 2-methyl-5-HT. Specific antagonist: ondansentron; tropisetron. Antagonists are used mainly as antiemetic drugs, but may also be anxiolytic.

42. Classification of 5-HT Receptors • 5-HT4 receptors occur mainly in enteric nervous system (also in CNS). Effects are excitatory, causing increased gastrointestinal motility. Act by stimulating adenylate cyclase. Specific agonist include metoclopramide (used to stimulate gastric emptying).

43. Classification of 5-HT Receptors • Little is known so far about the function and pharmacology of 5-HT5-7 receptors. • Many new receptor-selective agonists and antagonists are being developed

44. DRUGS ACTING ON 5-HT RECEPTORS • Important drugs that act on 5-HT receptors in periphery include-: • (1) 5-HT1D receptor agonist (e.g. sumatriptan) used for treating migraine. Selective 5-HT1A agonists, such as 8-OH-DPAT( 8-hydroxy-2-(di-n-propylamino) tetralin; are potent hypotensive agents, acting by central mechanism, but are not used clinically.

45. DRUGS ACTING ON 5-HT RECEPTORS • (2) 5-HT3-receptor antagonists (e.g. ondansetron, granisetron; tropisetron) used as anti-emetic drugs particularly for controlling the severe nausea and vomiting that occurs with many forms of cancer chemotherapy.

46. DRUGS ACTING ON 5-HT RECEPTORS • (3) 5-HT2-receptor antagonists (e.g. dihydroergotamine, methysergide, cyproheptadine, kentanserin, ketotifen, pizotifen). These ‘classical’ 5-HT antagonists act mainly on the 5-HT2-receptors. They are, however non-selective, and act also on targets, such as alpha-adrenoceptors andhistamine receptors.

47. DRUGS ACTING ON 5-HT RECEPTORS • Dihydrergotamine and methysergide belong to the ergot family and are used mainly for migraine prophylaxis. Ketotifen is sometimes used to treat asthma but the role of 5-HT receptors in this condition is unclear. Other 5-HT2 antagonists are used to control the symptoms of carcinoid tumours.

48. DRUGS ACTING ON 5-HT RECEPTORS • (4) 5-HT4-receptors agonists (e.g. metoclopramide; cisapride), which stimulate coordinated peristaltic activity (known as a ‘prokinetic action’) are used for treating gastrointestinal disorders

49. ERGOT ALKALOIDS • Many of them act on 5-HT-receptors, but not selectively, and their actions are complex and diverse. • Active substance produced by fungus (Claviceps purpurea) infecting cereal crops, responsible for occasional poisoning incidents. • The most important compounds are-:

50. ERGOT ALKALOIDS • Ergotamine, dihydroergotamine used in migraine. • Ergometrine, used in obstetrics to prevent postpartum haemorrhage. • Methysergide, used to treat carcinoid syndrome, and occasionally for migraine prophylaxis. • Bromocriptine, used in parkinsonism and endocrine disorders