EVIDENCE THAT THE MEDIAN RAPHE NUCLEUS - DORSAL HIPPOCAMPAL PATHWAY MEDIATES DIAZEPAM WITHDRAWAL INDUCED ANXIETY

On the basis of our previous series of experiments we had postulated t hat the increased anxiety that occurred during diazepam withdrawal was mediated by increased 5-HT release in the hippocampus. The present se ries of experiments provide evidence for a major role of the median ra phe nucleus (MRN) dorsal hippocampal pathway. Rats were treated once d aily for 21 days with diazepam (2 mg/kg IP) and then tested after 24 h withdrawal in the social interaction test of anxiety. Relative to chr onically vehicle treated animals, those withdrawn from diazepam were s ignificantly more anxious and had significantly greater K+-evoked rele ase of [H-3]-5-hydroxytryptamine (5-HT) from slices of dorsal and of v entral regions of the hippocampus. Estimation of extracellular concent rations of 5-HT within the dorsal hippocampus, using in-vivo microdial ysis, showed doubling in the levels of 5-HT in the rats withdrawn from chronic diazepam treatment. This just failed to reach significance, b ut 33% of the rats showed dramatic increases (650%). It was not possib le to test these animals in the social interaction test, but it is pro posed that only the diazepam-withdrawn rats with raised extracellular levels of 5-HT would have displayed increased anxiety. 5-HT1A receptor agonists injected into the MRN decrease the MRN firing rate, and henc e the release of 5-HT in the dorsal hippocampus. As a further test of our hypothesis, we examined the effects of MRN injection of the 5-HT1A receptor agonist, 8-OH DPAT, on animals withdrawn from diazepam and t ested in the low light familiar condition of the social interaction te st. 8-OH DPAT (50-200 ng) dose-dependently reversed the anxiogenic eff ect of diazepam withdrawal, while having no effects in chronic vehicle -treated animals. These results provide clear evidence that the MRN-do rsal hippocampal 5-HT pathway is at least one of the pathways playing an important role in mediating diazepam withdrawal-induced anxiety.