Investigation into the contractile response of melatonin in the guinea‐pig isolated proximal colon: the role of 5‐HT 4 and melatonin receptors

1 The interaction of melatonin (N‐acetyl‐5‐methoxytryptamine) with 5‐hydroxytryptamine 4 (5‐HT 4 ) receptors and/or with melatonin receptors (ML 1 , ML 2 sites) has been assessed in isolated strips of the guinea‐pig proximal colon. In the same preparation, the pharmacological profile of a series of melatonin agonists (2‐iodomelatonin, 6‐chloromelatonin, N‐acetyl‐5‐hydroxytryptamine (N‐acetyl‐5‐HT), 5‐methoxycarbonylamino‐N‐acetyltryptamine (5‐MCA‐NAT)) was investigated. 2 In the presence of 5‐HT 1/2/3 receptor blockade with methysergide (1 μ M ) and ondansetron (10 μ M ), melatonin (0.1 n M –10 μ M ), 5‐HT (1 n M –1 μ M ) and the 5‐HT 4 receptor agonist, 5‐methoxytryptamine (5‐MeOT: 1 n M –1 μ M ) caused concentration‐dependent contractile responses. 5‐HT and 5‐MeOT acted as full agonists with a potency (−log EC 50 ) of 7.8 and 8.0, respectively. The potency value for melatonin was 8.7, but its maximum effect was only 58% of that elicited by 5‐HT. 3 Melatonin responses were resistant to atropine (0.1 μ M ), tetrodotoxin (0.3 μ M ), and to blockade of 5‐HT 4 receptors by SDZ 205,557 (0.3 μ M ) and GR 125487 (3, 30 and 300 n M ). The latter antagonist (3 n M ) inhibited 5‐HT‐induced contractions with an apparent pA 2 value of 9.6. GR 125487 antagonism was associated with 30% reduction of the 5‐HT response maximum. Contractions elicited by 5‐HT were not modified when melatonin (1 and 10 n M ) was used as an antagonist. 4 Like melatonin, the four melatonin analogues concentration‐dependently contracted colonic strips. The rank order of agonist potency was: 2‐iodomelatonin (10.8) >6‐chloromelatonin (9.9) N‐acetyl‐5‐HT (9.8) 5‐MCA‐NAT (9.6) >melatonin (8.7), an order typical for ML 2 sites. In comparison with the other agonists, 5‐MCA‐NAT had the highest intrinsic activity. 5 The melatonin ML 1B receptor antagonist luzindole (0.3, 1 and 3 μ M ) had no effect on the concentration‐response curve to melatonin. Prazosin, an α‐adrenoceptor antagonist possessing moderate/high affinity for melatonin ML 2 sites did not affect melatonin‐induced contractions at 0.1 μ M . Higher prazosin concentrations (0.3 and 1 μ M ) caused a non‐concentration‐dependent depression of the maximal response to melatonin without changing its potency. Prazosin (0.1 and 1 μ M ) showed a similar depressant behaviour towards the contractile responses to 5‐MCA‐NAT. 6 In the guinea‐pig proximal colon, melatonin despite some structural similarity with the 5‐HT 4 receptor agonist 5‐MeOT, does not interact with 5‐HT 4 receptors (or with 5‐HT 1/2/3 receptors). As indicated by the rank order of agonist potencies and by the inefficacy of luzindole, the most likely sites of action of melatonin are postjunctional ML 2 receptors. However, this assumption could not be corroborated with the use of prazosin as this ‘ML 2 receptor antagonist’ showed only a non‐concentration‐dependent depression of the maximal contractile response to both melatonin and 5‐MCA‐NAT. Further investigation with the use of truly selective antagonists at melatonin ML 2 receptors is required to clarify this issue.British Journal of Pharmacology (1997) 121 , 1775–1781; doi: 10.1038/sj.bjp.0701287