Cardiovascular pharmacologyPharmacological evidence that histamine H3 receptors inhibit the vasodepressor responses by selective stimulation of the rat perivascular sensory CGRPergic outflow

This study has investigated whether pharmacological activation of Gi/o coupled histamine H3/H4 receptors inhibits the rat vasodepressor sensory outflow. For this purpose, 100 male Wistar rats were pithed, artificially ventilated and pretreated (i.v.) with: 25 mg/kg gallamine, 2 mg/kg/min hexamethonium and 20 μg/kg/min methoxamine, followed by i.v. continuous infusions of physiological saline (0.02 ml/min) or immepip (3.1, 10 or 31 μg/kg/min; a histamine H3/H4 receptor agonist). Under these conditions, electrical stimulation (0.56-5.6 Hz; 50 V and 2 ms) of the spinal cord (T9-T12) resulted in frequency-dependent vasodepressor responses, which were: (i) unchanged during the infusions of saline or immepip (3.1 μg/kg/min); and (ii) significantly but, surprisingly, not dose-dependently inhibited by 10 and 31 μg/kg/min immepip. Moreover, the sensory-inhibition by 10 μg/kg/min immepip (which failed to inhibit the vasodepressor responses by i.v. bolus injections of α-CGRP; 0.1-1 µg/kg) was: (i) essentially unaltered after i.v. administration of saline (1 ml/kg) or blocking doses of the antagonists ketotifen (100 μg/kg; H1), ranitidine (1000 μg/kg; H2) or JNJ7777120 (310 μg/kg; H4); and (ii) abolished after i.v. thioperamide (310 µg/kg; H3). In conclusion, our results suggest that immepip-induced inhibition of the vasodepressor sensory outflow is mainly mediated by prejunctional activation of histamine H3 receptors.

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