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Abstract

The roles of the noradrenergic and serotonergic innervations of the hippocampus were investigated with respect to their involvement in the intracranial self-stimulation of this structure. In the first study, 6-hydroxydopamine-induced lesions of the dorsal tegmental noradrenergic bundle ascending to the forebrain had no effect on hippocampal self-stimulation in rats. In the second study, intragastric administration of para-chlorophenylalanine decreased hippocampal self-stimulation, suggesting the importance of a serotonin input in maintaining this behavior. Identical para-chlorophenylalanine treatments resulted in temporary depletions of brain serotonin which paralleled the changes in hippocampal self-stimulation. The maximal decreases in both the biochemical and behavioral measures occurred at 4 days' post-drug. Interpretations of this deficit in hippocampal self-stimulation in terms of gross sensory and/or motor changes were ruled out, as animals with lateral hypothalamic electrodes showed increases in self-stimulation paralleling the post-drug serotonin changes. An intrasessional analysis of the para-chlorophenylalanine-induced behavioral changes revealed that lateral hypothalamic self-stimulation was facilitated mainly during the first hr of the 2 hr test sessions, whereas the depression in hippocampal self-stimulation occurred primarily in the last hour of the sessions. The differential effects of para-chlorophenylalanine on lateral hypothalamic and hippocampal self-stimulation provide evidence against simple monoamine theories of reinforcement.