Serotonergic Mediation of Reward Within the Medial Raphe Nucleus: Some Persistent Problems in Interpretation

Abstract

Adult male Sprague Dawley rats were implanted with unipolar stimulating electrodes aimed at the medial fore-brain bundle (MFB) or the medial raphe nucleus (MR). All MFB implanted subjects self-stimulated at high stable rates for at least three weeks. Only a minority (⅓) of MR rats self-stimulated at all. Rates for the MR group were considerably more variable, and could not be maintained for more than two weeks. Treatment with methysergide increased MFB self-stimulation but decreased MR self-stimulation. While this result suggests serotonergic mediation of self-stimulation this conclusion must be interpreted cautiously since parachlorophenylalanine (PCPA) reinstated self-stimulation in raphe animals which had spontaneously ceased responding.