Electrophysiological evidence against negative neuronal feedback from the forebrain controlling midbrain raphe unit activity.

The hypothesis that the activity of serotonin (5-HT)-containing neurons of the midbrain raphe is subject to negative neuronal feedback regulation was examined. This hypothesis is based primarily on the observation that the administration of drugs which increase the synaptic availability of 5-HT depress midbrain raphe neuron discharge. Since the preponderance of midbrain raphe efferents are ascending, transections which interrupt both the main efferent outflow, as well as all inputs from anterior levels, ought to disrupt the functional integrity of a neuronal feedback loop. The effect of complete transections of the neuraxis placed just rostral to the midbrain raphe nuclei on the efficacy of two drugs which elevate synaptic serotonin, chlorimipramine and p-chloroamphetamine, was investigated in the chloral hydrate anesthetized rat. Such transections neither blocked nor attenuated the depressive effect of intravenously administered chlorimipramine (0.33 or 0.15 mg/kg) or p-chloroamphetamine (1.25 mg/kg) on midbrain raphe unit discharge. These results suggest that neuronal feedback involving the forebrain dose not mediate the depressive effect of drugs which elevate synaptic serotonin on midbrain raphe neuronal activity. An action at serotonergic synapses intrinsic to the midbrain raphe is suggested as an explanation for the persistence of drug effects in transected animals.