REGENERATION OF THE GABAERGIC SEPTOHIPPOCAMPAL PROJECTION IN-VITRO

The formation of the GABAergic septohippocampal projection was studied in vitro. Slice cultures of the septal complex from young postnatal r ats were prepared and co-cultivated with hippocampal slices for up to four weeks. Then, the anterogradely transported tracer Phaseolus vulga ris leucoagglutinin was injected into the septal culture and the label ed fibers were traced into the hippocampal culture. Some fibers were i dentified as originating from GABAergic septal cells by double-labelin g with an antiserum against GABA using the postembedding immunogold pr ocedure. Our results showed that double-labeled terminals of GABAergic septohippocampal neurons established symmetric synapses exclusively w ith GABA-positive dendrites in one out of five co-cultures, but also c ontacted numerous GABA-negative structures in the remaining four co-cu ltures. These findings, together with light microscopic data from sect ions double-stained for Phaseolus and parvalbumin, indicate that the h igh target selectivity of the GABAergic septohippocampal pathway for G ABAergic interneurons in vivo is lost in most cases, at least under th e present in vitro conditions. It is hypothesized that this may be due to an immaturity of the connection, the lack of axon-guiding factors or an expansion of the septohippocampal GABAergic fibers in the absenc e of many extrinsic afferents, including GABAergic fibers. The simulta neous occurrence of anterogradely labeled, but GABA-negative, septohip pocampal terminals in the hippocampal target culture also suggests tha t the septohippocampal cholinergic projection developed in vitro, as w as shown before in other studies. Since most septohippocampal neurons have to be axotomized for culture preparation, the present results ind icate that GABAergic septohippocampal neurons from young postnatal rat s survive axotomy and are capable of regenerating a septohippocampal p rojection, including the formation of characteristic GABAergic synapse s on co-cultured hippocampal neurons. However, the characteristic targ et selectivity is rarely preserved.