Septohippocampal adaptive GABAergic responses by AF64A treatment

A cholinergically disrupted laboratory animal has been produced by administration of the cholinotoxin ethylcholine aziridinium mustard (AF64A), which produced a dysfunction in the cholinergic forebrain system. After AF64A treatment, a reduction of choline acetyl transferase (ChAT) activity was measured in the hippocampal regions. ChAT activity was preferentially reduced in tissue samples of the dorsal with respect to the ventral hippocampus, and concomitantly with this reduction, a compensatory increase in ChAT activity in the medial septum was found. Tissue gamma-aminobutyric acid (GABA) content in the hippocampal and septal brain areas was not affected by AF64A, indicating a specific effect on the cholinergic septohippocampal projection. The rate of GABA accumulation induced by aminooxyacetic acid administration was higher in the dorsal hippocampus and medial septum of AF64A-treated animals, but not in their ventral hippocampus and lateral septum, where significant changes occurred in ChAT activity. Concomitantly with the changes in GABA metabolism, a significant Bmax increase and Kd reduction of 3H-flunitrazepam binding in the hippocampus of AF64A-treated animals were associated with changes in the ChAT activity. This finding suggests an increase of GABA input on the cholinergic somas of the medial septum and an uncompensated GABAergic interneuron activity in the hippocampus. In this study, we present an adaptive mechanism of homotypic compensatory metabolism by cholinergic somas, and a heterotypic response of the GABAergic septohippocampal projection system, which was elicited by AF64A administration.