ELECTROPHYSIOLOGICAL AND IONTOPHORETIC ASPECTS OF THE HABENULAR INFLUENCE ON HIPPOCAMPAL-NEURONS

In previous experimental studies, carried out on cats, we demonstrated that electrical stimulation of lateral habenula (LH) at 0.5-3.0Hz or 5-20Hz had a double effect (low frequency-excitation; high frequency-i nhibition) on the spontaneous firing rate of single hippocampal neuron es. Our results, in agreement with similar case studies, allowed us to hypothesise that in the habenular modulation of the hippocampus the r aphe nucleus is probably involved. In fact, all the effects of LH stim ulation were antagonised by the iontophoretic intrahippocampal applica tion of methysergide. In the present series of experiments, performed on rats, it was possible to demonstrate that LH stimulate 1-10Hz cause s an excitation of a progressively major number of hippocampal neurons depending upon the increase of frequency stimulation. The absence of habenulo-induced effects after a iontophoretic application of methyser gide on single hippocampal units suggests the involvement of the raphe nucleus. Furthermore, in consideration of recent anatomical evidences demonstrating an excitatory projection between LH and raphe nucleus, intraraphal N-methyl-D-aspartate (NMDA) application, performed through a Hamilton microsyringe, induces an inhibitory effect. All the result s suggest that in the raphe context it is possible to hypothesise the presence of an intrinsic interneurone, directly activated by the excit atory projection arising from the LH; this interneurone is likely inhi bitory on the serotonergic raphe-hippocampus efferent neurone. This fu nctional organization is responsible for the effect of LH stimulation at different frequencies as well as for the effects of intraraphal NMD A application. effects of intraraphal NMDA application.