Role of α₁-adrenoceptors in the reduction of external carotid blood flow induced by buspirone and ipsapirone in the dog

The effects of the 5-HT_1A receptor agonists with anxiolytic properties, buspirone and ipsapirone, in the external carotid bed of anaesthetized dogs were analyzed. Since these agonists produce several vascular effects via activation of both 5-HT receptors and α₁adrenoceptors, their effects were compared with those elicited by the 5-HT agonist, quipazine, and the α₁-adrenoceptor agonist, methoxamine. 1-Min intracarotid (i.e.) infusions of buspirone (300 μg/min), ipsapirone (40 μg/min), quipazine (300 μg/min) and methoxamine (15 μg/min) produced consistent decreases in external carotid blood flow (ECBF); since these changes in blood flow were not accompanied by modifications in systemic blood pressure, the agonists produced parallel increases in external carotid resistance. After interruption of the sympathetic tone by bilateral cervical vagosympathectomy, the vasoconstrictor responses to all the agonists remained unaffected. The intravenous (i.v.) administration of the nonselective 5-HT₁-like receptor antagonist, methiothepin (1-100 μg/kg), potently and dose-dependently antagonized buspirone-, ipsapirone- and quipazine-induced vasoconstriction; methiothepin similarly antagonized the vasoconstrictor responses to methoxamine. Interestingly, the aradrenoceptor antagonist, prazosin (1-100 μg/kg, i.v.), also antagonized the vasoconstrictor responses to buspirone, ipsapirone and methoxamine in a dose-dependent manner. Finally, buspirone (300 μg/min, i.c.) and ipsapirone (40 μg/min, i.e.) did not modify the responses to noradrenaline (10 μg/min, i.e.) or tyramine (100 μg/min, i.e.). It is concluded that canine external carotid vasoconstriction induced by buspirone and ipsapirone is mainly mediated by activation of α₁-adrenoceptors located in vascular smooth muscle. These data further highlight the ability of the above anxiolytics to produce significant vascular effects under in vivo conditions.