REDUCTION OF THE EXTRACELLULAR LEVEL OF GLUTAMATE IN THE MEDIAN RAPHENUCLEUS ASSOCIATED WITH HIPPOCAMPAL THETA ACTIVITY IN THE ANESTHETIZED RAT

The relationship between hippocampal activity and the extracellular le vel of excitatory amino acids in the median raphe nucleus has been stu died in urethane anaesthetized rats, using the in vivo microdialysis t echnique. Dialysates were collected from the median raphe nucleus duri ng two to eight sampling periods of equal length (20 min) and hippocam pal electroencephalogram was continuously monitored. For each observat ion period, the average glutamate level in the median raphe nucleus wa s determined and the percentage of theta and non-theta segments in the hippocampal recordings was calculated. Theta synchronization, in thes e experiments, either developed spontaneously or it was elicited by in jection of anticholinesterase (Physostigmine or Sintostigmine, i.p.) o r by a series of short tail pinches. The relationship between hippocam pal activity and glutamate release in the median raphe nucleus was cha racterized by comparison of the direction of changes in these two para meters in consecutive sampling periods. We found that as long as theta /non-theta ratio changed spontaneously or under the effect of antichol inesterase (n=7), the extracellular level of glutamate in the median r aphe nucleus was elevated during periods dominated by desynchronized h ippocampal activity as compared with those mostly containing long and/ or frequently occurring theta segments. Such relationship was not obse rved in the adjacent reticular formation (n=4) and in the median raphe nucleus during sensory stimulation (n=2). The present data complete t hose found earlier indicating that the desynchronizing serotonergic in fluence originating from the brainstem is maintained by a tonic excita tory input to the median raphe nucleus. Since the majority of glutamat ergic afferents to the median raphe nucleus originates from the latera l habenula and the interpeduncular nucleus, known to connect limbic fo rebrain to the brainstem, theta associated changes in median raphe nuc leus glutamate levels might reflect descending forebrain influences, s uggesting therefore a feedback regulation of the hippocampal activity involving brainstem structures. (C) 1998 IBRO. Published by Elsevier S cience Ltd.