The effects of serotonin (5-hydroxytryptamine; 5-HT) on continuously recorded dorsal and ventral aortic blood pressures, (Pda, Pva), arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), extracellular pH (pHa), buccal pressure (Pbuccal) and plasma catecholamine levels were investigated in rainbow trout, Oncorhynchus mykiss. Intra-arterial injections of serotonin (50–250 nmol kg-1) caused a rapid decrease in Pda and an increase in Pva, suggesting vasoconstriction of the branchial vasculature. The blood pressure changes were accompanied by a reduction in PaO2 (approximately 3–8 kPa depending on the dose injected), an increase in PaCO2 (approximately 0.03–0.07 kPa) and a decrease in pHa (approximately 0.02–0.12 pH units). These changes, indicative of impaired gas transfer, occurred despite obvious hyperventilation based on measurements of buccal pressure. After pre- treatment with the serotonergic receptor antagonist methysergide, injections of 100 nmol kg-1 serotonin caused an increase in PaO2, a reduction in PaCO2 and an increase in pHa. Methysergide treatment did not affect the usual serotonin-induced hyperventilation or the reduction in Pda but did abolish the rise in Pva; indeed, Pva was lowered significantly by serotonin after methysergide treatment. This reduction in Pva was eliminated by pre-treatment of fish with the combination of methysergide and sotalol (beta-adrenoceptor antagonist), suggesting an adrenergic component to the overall blood pressure response. Analysis of plasma catecholamines after injection of serotonin revealed that high doses (50, 100, 250 nmol kg-1) caused significant increases in concentrations of both noradrenaline and adrenaline. We conclude that the cardio-respiratory effects of exogenous serotonin injections are complex and arise from several integrated responses, including the direct action of serotonin on receptors within the branchial and systemic vasculatures, indirect action on ganglionic receptors, and the stimulation of catecholamine release from adrenergic nerves and/or chromaffin cells.

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