EXTRACELLULAR RECORDINGS IN THE COLCHICINE-LESIONED RAT DENTATE GYRUSFOLLOWING 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 mu l 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 hippoca mpal slice preparations. Following stimulation of the host mid-molecul ar layer, evoked field potential responses, showing considerable varia tion, were recorded in both types of graft. Evoked responses in the le sioned 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 conn ectivity; stimulation of DG and CA1 graft areas occasionally evoked re sponses in the host CA1. Responses in the area of both DG and CA1 graf ts supported short-term potentiation following stimulation of the host mid-molecular layer but only DG graft areas supported long-term poten tiation of the population spike amplitude. In the area of both types o f transplant a tonic bicuculline-sensitive inhibition was present and paired-pulse stimulation paradigms provided some evidence for inhibito n. It is possible that responses recorded within the area of grafted t issue to stimulation of the host are attributable to host-graft connec tivity 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 sustai ning 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.