Pharmacological profile of the receptors that mediate external carotid vasoconstriction by 5-HT in vagosympathectomized dogs

Abstract

1. 5-Hydroxytryptamine (5-HT) can produce vasodilatation or vasoconstriction of the canine external carotid bed depending upon the degree of carotid sympathetic tone. Hence, external carotid vasodilatation to 5-HT in dogs with intact sympathetic tone is primarily mediated by prejunctional 5-HT1-like receptors similar to the 5-HT1D subtype, which inhibit the carotid sympathetic outflow. The present investigation is devoted to the pharmacological analysis of the receptors mediating external carotid vasoconstriction by 5-HT in vagosympathectomized dogs. 2. Intracarotid (i.c.) infusions for 1 min of 5-HT (0.3, 1, 3, 10, 30 and 100 micrograms) resulted in dose-dependent decreases in both external carotid blood flow and the corresponding conductance; both mean arterial blood pressure and heart rate remained unchanged during the infusions of 5-HT. These responses to 5-HT were resistant to blockade by antagonists at 5-HT2 (ritanserin) and 5-HT3/5-HT4 (tropisetron) receptors, but were partly blocked by the 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (0.3 mg kg-1); higher doses of methiothepin (1 and 3 mg kg-1) caused little, if any, further blockade. These methiothepin (3 mg kg-1)-resistant responses to 5-HT were not significantly antagonized by MDL 72222 (0.3 mg kg-1) or tropisetron (3 mg kg-1). 3. The external carotid vasoconstrictor effects of 5-HT were mimicked by the selective 5-HT1-like receptor agonist, sumatriptan (3, 10, 30 and 100 micrograms during 1 min, i.c.), which produced dose-dependent decreases in external carotid blood flow and the corresponding conductance; these effects of sumatriptan were dose-dependently antagonized by methiothepin (0.3, 1 and 3 mg kg-1), but not by 5-HT1D-like receptor blocking doses of metergoline (0.1 mg kg-1). 4. The above vasoconstrictor effects of 5-HT remained unaltered after administration of phentolamine, propranolol, atropine, hexamethonium, brompheniramine, cimetidine and haloperidol, thus excluding the involvement of alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors. Likewise, inhibition of either 5-HT-uptake (with fluoxetine) or cyclo-oxygenase (with indomethacin), depletion of biogenic amines (with reserpine) or blockade of calcium channels (with verapamil) did not modify the effects of 5-HT. 5. Taken together, the above results support our contention that the external carotid vasoconstrictor responses to 5-HT in vagosympathectomized dogs are mainly mediated by activation of sumatriptan-sensitive 5-HT1-like receptors. It must be emphasized, notwithstanding, that other mechanisms of 5-HT, including an interaction with a novel 5-HT receptor (sub)type and/or an indirect action that may lead to the release of a known (or even unknown) neurotransmitter substance cannot be categorically excluded.