POSTSYNAPTIC TARGETS OF GABAERGIC HIPPOCAMPAL-NEURONS IN THE MEDIAL SEPTUM DIAGONAL BAND OF BROCA COMPLEX

The termination pattern of hippocamposeptal nonpyramidal cells was inv estigated by injecting Phaseolus vulgaris leucoagglutinin (PHAL) into stratum oriens of the CA1 region. Electron microscopic analysis showed that the majority of the anterogradely labeled boutons formed symmetr ic synapses with dendrites and occasionally with cell bodies located i n the medial septum-diagonal band of Broca complex. We have demonstrat ed with postembedding GABA immunocytochemistry that the majority of PH AL-labeled axon terminals were GABAergic. The neurochemical character of the postsynaptic target cells was also investigated using immunocyt ochemical double staining. Our data showed that the majority of the la beled hippocamposeptal axons innervated parvalbumin-immunoreactive cel ls representing GABAergic projection neurons, and a smaller number of contacts were found on ChAT-positive neurons. Septohippocampal neurons identified by retrograde HRP transport were also shown to receive dir ect hippocamposeptal input. According to recent results, the lateral s eptum is unlikely to relay the hippocampal feedback to the medial sept um; therefore, the direct hippocampal projection to the medial septum, arising from GABAergic nonpyramidal cells, seems to be the only feedb ack pathway to the area containing septohippocampal neurons. A novel c ircuit diagram, based on our recent morphological-immunocytochemical f indings, is shown for the synaptic organization of the septo-hippocamp oseptal loop. We suggest that the GABAergic hippocamposeptal feedback controls the activity of septal (mostly GABAergic) projection neurons as a function of hippocampal synchrony. The newly discovered reciproca l interactions may give a better insight into septohippocampal physiol ogy.