Sundararaman N, Vertes RP, Perry GW (2004) Neurotoxic lesions of serotonin containing cells of the median raphe nucleus produce constant hippocampal theta rhythm in behaving rats. Neuroscience 2004 Abstracts 196.15. Society for Neuroscience, San Diego, CA.
Summary: The median raphe nucleus (MR) is a major serotonin containing cell group with pronounced projections to the forebrain. The MR exerts strong desynchronizing actions on the EEG activity of the hippocampus. MR stimulation desynchronizes the hippocampal EEG (or blocks theta), and electrolytic MR lesions produce continuous theta. Evidence suggests that desynchronizing actions of MR on the hippocampal EEG are mediated by serotonergic (5-HT) cells of MR. Injections of pharmacological agents into MR that suppress 5-HT MR activity generate theta at short latencies and for long durations. We examined the effects of the selective destruction of 5-HT cells of MR using the 5-HT neurotoxin, anti-SERT-SAP (Advanced Targeting Systems) on the EEG activity of the hippocampus in behaving rats. Under deep sodium pentobarbital anesthesia, rats were chronically prepared with bipolar electrodes, bilaterally in the dorsal hippocampus, a cortical screw for recording the cortical EEG and an indwelling cannula placed 3-4 mm dorsal to MR for the injection of anti-SERT-SAP into MR. Following a 5-7 day period of recovery, hippocampal EEG activity was recorded daily for 7 days as rats freely moved about in a shielded enclose, and then re-assessed under the same conditions following neurotoxic lesions. We found that neurotoxic lesions of MR that resulted in a substantial destruction of 5-HT MR cells (80-90%) produced a continuous theta rhythm in rats; that is, during locomotor behavior as well as during states when theta is normally absent, such as immobility and grooming. For some rats, theta was equivalent during complete immobility and active movement. These results support earlier findings that 5-HT cells of MR are directly involved in the desynchronization of the hippocampal EEG, and indicate that the MR exerts a powerful modulatory influence on the hippocampus.
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