EXTRACELLULAR RECORDINGS IN THE COLCHICINE-LESIONED RAT DENTATE GYRUS FOLLOWING TRANSPLANTS OF FETAL DENTATE GYRUS AND CA1 HIPPOCAMPAL SUBFIELD TISSUE

Grafts of fetal dentate gyrus (DG) and CA1 hippocampal subfield tissue were extruded into the dentate gyri of adult male Sprague-Dawley rats, 7-10 days after lesioning the granule cells with colchicine (0.06 mul of 7 mg/ml solution at each of 5 sites/hippocampus). Graft area-host and host-graft area connectivities were investigated 4-6 months post-transplantation by recording extracellular evoked responses in hippocampal slice preparations. Following stimulation of the host mid-molecular layer, evoked field potential responses, showing considerable variation, were recorded in both types of graft. Evoked responses in the lesioned DG without grafts were recorded in very few slices. Stimulation of the area of DG tissue grafts occasionally evoked responses in the host CA3/CA4 and there was no evidence for CA1 graft area-CA3/CA4 connectivity; stimulation of DG and CA1 graft areas occasionally evoked responses in the host CA1. Responses in the area of both DG and CA1 grafts supported short-term potentiation following stimulation of the host mid-molecular layer but only DG graft areas supported long-term potentiation of the population spike amplitude. In the area of both types of transplant a tonic bicuculline-sensitive inhibition was present and paired-pulse stimulation paradigms provided some evidence for inhibiton. It is possible that responses recorded within the area of grafted tissue to stimulation of the host are attributable to host-graft connectivity and similarly, responses recorded in the host to stimulation of the area of the graft may be attributable to graft-host connectivity. Only DG graft areas received host inputs which were capable of sustaining a long-term potentiation and establishing efferent contacts with the host CA3/CA4 subfield, suggesting that these would be more likely than CA1 grafts to reinstate normal functional circuitry.