PHYSIOLOGICAL EVIDENCE FOR HIPPOCAMPAL DISINHIBITION RESULTING FROM ACTIVATION OF THE MEDIAN RAPHE

The mechanism underlying median raphe (MR)-induced facilitation of hip pocampal synaptic transmission was investigated by recording stimulus- evoked field potentials and unitary responses in urethane-anesthetized rats. Stimulation of the MR 40 ms prior to perforant path (PP) activa tion significantly increased the magnitude of PP-evoked granule cell p opulation spikes (median increase = 78%) without affecting population EPSP slope. Injection of homocysteic acid into the vicinity of the MR also facilitated PP-evoked granule cell population spikes, in a dose-d ependent manner. Nineteen dentate hilar units were characterized as pu tative interneurons on the basis of their waveform characteristics and their response to PP stimulation. Electrical activation of the MR inh ibited spontaneous or PP-evoked activity in the majority (75%) of thes e cells; the remaining cells were unaffected. MR stimulation also inhi bited spontaneous activity in a large proportion (60%) of putative int erneurons in CA1. The present results provide evidence that neurons wi thin the raphe modulate hippocampal throughput by altering discharge o f non-principal cells. These data, thus, support the idea that disinhi bition is a common mechanism by which extrahippocampal structures modu late information now through the hippocampus. (C) 1997 Elsevier Scienc e B.V.