Dopaminergic excitation of the goat carotid body is mediated by the serotonin type 3 receptor subtype

The purpose of the present study was to use chemoafferent recordings from the goat carotid body (CB) to pharmacologically identify the putative low affinity excitatory receptor for dopamine (DA). Close arterial injections of DA (1-50 μg kg^-1) induced a dose-dependent excitatory burst followed by inhibition of the CB chemoafferent activity. The inhibition is likely DA D₂ receptor-mediated as it was blocked by domperidone (0.5-1.0 mg kg^-1 iv). The initial high frequency burst of CB chemoafferent activity could not be attenuated by selective antagonists for the DA D_1-4 receptors but could be blocked by D-tubocurarine or the selective serotonin₃ (5-HT₃) receptor antagonists, tropisetron and MDL72222. The selective nicotinic antagonists, hexamethonium and vecuronium, were without effect. Selective blockade of the 5-HT₃-receptor subtype using tropisetron significantly reduced both normoxic and hypoxic unitary CB discharge. These results suggest that DA-mediated excitation of the goat CB chemosensitive afferents occurs via the 5-HT₃-receptor subtype and that the 5-HT₃-receptor may exert an excitatory modulation of CB output during normal physiological responses in the goat.