Electrophysiological and microiontophoretic analysis of the habenulo-hippocampal circuit.

In the cat, the effects of lateral habenula stimulation, at different ranges of frequency, on hippocampal units were studied. Habenular stimulation at low frequency excited, while at high frequency inhibited the greater part of hippocampal units. Moreover, in order to clarify the possible pathway involved in the habenulo-hippocampal circuit, the effects of iontophoretic acetylcholine and serotonin on hippocampal units were compared with those of habenular stimulation. Iontophoretic acetylcholine induced both excitatory and inhibitory responses while serotonin induced only inhibitory responses. Iontophoretic atropine blocked the effects of acetylcholine ejection but did not antagonize stimulation effects; ion-tophoretic methysergide induced an increase of basal firing of hippocampal units and antagonized both serotonin and habenular stimulation inhibition. The results suggest an influence of lateral habenula to the hippocampus which does not appear to be cholinergically-mediated. A possible involvement of the raphe as a relay station in the habenulo-hippocampal pathway is discussed.