Cardio-ventilatory control in rainbow trout: I. Pharmacology of branchial, oxygen-sensitive chemoreceptors.

The effects of various neurochemicals on O2-sensitive chemoreceptor afferent discharge in the glossopharyngeal nerve (cranial nerve IX) were examined in an isolated, perfused first gill arch preparation from rainbow trout. Afferent neural activity from O2 receptors in the first gill arch increased in response to hypoxic perfusate and NaCN. Adrenergic agonists (epinephrine, norepinephrine and isoproterenol) had little effect on neural activity. Dopamine and 5-hydroxytryptamine (serotonin) caused a brief, small burst in chemoreceptor activity followed by a mild inhibition of receptor discharge. Acetylcholine and nicotine were potent neurochemical stimulants; muscarine had only a slight effect. While atropine completely blocked the effects of acetylcholine on receptor discharge, it only slightly inhibited responses to hypoxia and NaCN. Thus, although cholinergic mechanisms appear more likely than either adrenergic or serotonergic mechanisms to alter cardiovascular and ventilatory reflexes in fishes through their effects on O2-sensitive chemoreceptor activity, the transduction process involved in O2-chemoreception appears to be complex and not dependent on any single one of the neurochemicals tested.