INJECTIONS OF MUSCIMOL INTO THE MEDIAN RAPHE NUCLEUS PRODUCE HIPPOCAMPAL THETA-RHYTHM IN THE URETHANE-ANESTHETIZED RAT

It has previously been shown that serotonergic [5-hydroxytryptamine (5 -HT)] neurons of the median raphe nucleus (MR) are critically involved in the control of the hippocampal electroencephalogram (EEG). Activat ion of MR 5-HT neurons desynchronizes the hippocampal EEG, whereas inh ibition of MR 5-HT activity produces hippocampal theta rhythm. The MR contains an intrinsic population of gamma-aminobutyric acid (GABA) con taining neurons that synapse on 5-HT cells of the MR. The present stud y examined the effects on the hippocampal EEG of injections of the GAB AA agonist muscimol hydrobromide into the MR. Low doses of muscimol (0 .5 mu g) produced hippocampal theta rhythm at a mean latency of 6.81 m in and for a mean duration of 23.6 min. Higher doses (1.0 mu g and 3.0 mu g, respectively) produced theta at mean latencies of 2.24 min and 3.2 min and for mean durations of 31.84 min and 24.88 min. Injections of muscimol into regions adjacent to the MR generated theta at signifi cantly longer latencies or were without effect. The present results in dicate that MR injections of muscimol produce theta by inhibiting the activity of MR 5-HT neurons. It is concluded that MR GABAergic systems , via their influence on MR 5-HT cells, serve an important role in the control of the hippocampal EEG.