Abstract
BACKGROUND: Dihydroergotamine (DHE) is an antimigraine drug that produces cranial vasoconstriction and inhibits trigeminal CGRP release; furthermore, it inhibits the vasodepressor sensory CGRPergic outflow, but the receptors involved remain unknown. Prejunctional activation of α(2A/2C)-adrenergic, serotonin 5-HT(1B/1F), or dopamine D(2)-like receptors results in inhibition of this CGRPergic outflow. Since DHE displays affinity for these receptors, this study investigated the pharmacological profile of DHE-induced inhibition of the vasodepressor sensory CGRPergic outflow. METHODS: Pithed rats were pretreated i.v. with hexamethonium (2 mg/kg·min) followed by continuous infusions of methoxamine (20 μg/kg·min) and DHE (3.1 μg/kg·min). Then, stimulus-response curves (spinal electrical stimulation; T(9)-T(12)) or dose-response curves (i.v. injections of α-CGRP) resulted in frequency-dependent or dose-dependent decreases in diastolic blood pressure. RESULTS: DHE inhibited the vasodepressor responses to electrical stimulation (0.56-5.6 Hz), without affecting those to i.v. α-CGRP (0.1-1 μg/kg). This inhibition by DHE (not produced by the methoxamine infusions): (i) was abolished by pretreatment with the combination of the antagonists rauwolscine (α(2)-adrenoceptor; 310 μg/kg) plus GR127935 (5-HT(1B/1D); 31 μg/kg); and (ii) remained unaffected after rauwolscine (310 μg/kg), GR127935 (31 μg/kg) or haloperidol (D(2)-like; 310 μg/kg) given alone, or after the combination of rauwolscine plus haloperidol or GR127935 plus haloperidol at the aforementioned doses. CONCLUSION: DHE-induced inhibition of the vasodepressor sensory CGRPergic outflow is mainly mediated by prejunctional rauwolscine-sensitive α(2)-adrenoceptors and GR127935-sensitive 5-HT(1B/1D) receptors, which correlate with α(2A/2C)-adrenoceptors and 5-HT(1B) receptors, respectively. These findings suggest that DHE-induced inhibition of the perivascular sensory CGRPergic outflow may facilitate DHE's vasoconstrictor properties resulting in an increased vascular resistance.