Lesion of the median raphe nucleus: a combined behavioral and microdialysis study in rats.

The purpose of the present study was to investigate the behavioral consequences and the neurochemical correlates of a 5,7-dihydroxytryptamine (5,7-DHT) lesion of the median raphe nucleus (MRN) in rats. Anxiety-related behavior was assessed in the elevated plus maze test on days 5, 14, and 21 after lesioning. In general, behavior of MRN-lesioned rats was unchanged when compared with sham-lesioned or untreated controls. Neurochemically, microinjection of 5,7-DHT into the MRN resulted in 87.5% depletion of hippocampal 5-HT content. Using the in vivo microdialysis technique, the exposure of 5,7-DHT-lesioned rats to the elevated plus-maze failed to increase extracellular 5-HT release (94%) in the hippocampus, as shown in sham-lesioned (150%) or untreated controls (194%). Moreover, application of fenfluramine (10 mg/kg, i.p.) evoked a 10-fold increase in hippocampal extracellular 5-HT levels in sham-lesioned animals, whereas in 5,7-DHT lesioned rats 5-HT was only slightly increased. The results demonstrate, that a marked reduction of 5-HT release from the MRN is not necessarily accompanied by anxiolytic-like behavior.