A ROLE OF 5-HT2 RECEPTORS IN THE GILL VASCULATURE OF THE ANTARCTIC FISH PAGOTHENIA-BORCHGREVINKI

This study was conducted to describe the cardiovascular responses to i ntra-arterial injections of serotonin in the Antarctic fish Pagothenia borchgrevinki and to elucidate the underlying mechanisms, Immunohisto chemistry was used to localise serotonin-containing cells within the g ills, Simultaneous and continuous recordings of ventral and dorsal aor tic blood pressure, heart rate and ventral aortic blood flow (cardiac output) were made using standard cannulation procedures in combination with Doppler flow measurement, An extracorporeal loop with an in-line oxygen electrode allowed continuous measurements of arterial oxygen p ressure Pa-O2. Pre-branchial injection of serotonin (5-hydroxytryptami ne, 5-HT) or the 5-HT2 receptor agonist alpha-methylserotonin increase d the branchial vascular resistance and ventral aortic pressure, while the 5-HT1 receptor agonist piperazine was without effect, The branchi al vasoconstriction produced by serotonin injection was completely blo cked by the 5-HT1/5-HT2 receptor antagonist methysergide and the branc hial vasoconstriction produced by alpha-methylserotonin injection was completely blocked by the specific 5-HT2 receptor antagonist LY53857, The results suggest that the 5-HT2 receptor alone mediates the branchi al vasoconstriction. Serotonin also mediated a methysergide-sensitive reduction in Pa-O2, the reduction being greatest when the pre-injectio n Pa-O2 value was high, 5-HT-immunoreactive cells and nerve fibres wer e present within the gill tissues, All the 5-HT-immunoreactive cells w ere located on the efferent side of the filaments, but 5-HT-immunoreac tive nerve fibres were found lining both of the branchial arteries, Ou r findings demonstrate a potential serotonergic control system for the gills in Pagothenia borchgrevinki. In contrast to its effects on the branchial vasculature, serotonin produced a methysergide-insensitive d ecrease in the systemic vascular resistance, However, neither the spec ific 5-HT1 nor 5-HT2 receptor agonists produced a decrease in the resi stance of the systemic vasculature, The nature of the serotonergic rec eptor(s) inducing vasodilation in teleost fish is uncertain.