Abstract
The effects of various neurochemicals were examined in intact, unanesthetized rainbow trout ( Oncorhynchus mykiss) to assess the role of branchial O 2-sensitive chemoreceptors in the cardio-ventilatory responses to exogenous neurochemicals. Cyanide stimulated ventilation and elicited bradycardia when given externally but only stimulated ventilation when injected internally. Norepinephrine increased heart rate, blood pressure and ventilatory rate but opercular pressure was not affected. Dopamine had no effect on either heart or ventilatory rate but increased blood pressure and decreased opercular pressure. Serotonin stimulated heart rate and ventilation but decreased blood pressure. Acetylcholine and nicotine stimulated all cardio-ventilatory variables. Muscarine decreased heart rate and blood pressure and had a biphasic effect on ventilation. These results, combined with the results from the preceding study, suggest that the cardio-ventilatory effects of exogenously administered (1) cyanide are entirely mediated by gill O 2 receptors, (2) serotonin, and cholinergic drugs could be partly mediated by O 2 receptors and (3) catecholaminergic drugs are not mediated by O 2 receptors.
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