LONG-LASTING EFFECTS OF PARTIAL HIPPOCAMPAL KINDLING ON HIPPOCAMPAL PHYSIOLOGY AND FUNCTION

The objective of this project was to study the behavioral and physiological effects at 6-9 weeks after evoking 15 afterdischarges (ADs) in hippocampal CA1 (partial hippocampal kindling). Rats were trained on the open radial arm maze (RAM) with all eight arms baited, kindled, and then tested again on the RAM, followed by in vitro recordings at 8-9 weeks after kindling. Partial kindling was manifested by an increase in hippocampal AD duration. Enhancement of the commissural basal dendritic excitatory postsynaptic potential (EPSP) was observed for at least 1 day after the ADs. Kindled rats performed worse than control rats during the 1st but not during the 7th or 8th week after kindling. Rats that were slow in acquiring the RAM showed more RAM errors after kindling than those that showed fast acquisition. At 8-9 weeks after kindling, as shown by field potential recording in the hippocampal slice in vitro, kindled rats showed an increase in paired-pulse facilitation (PPF) of the EPSP in CA1 but a decreased PPF of the perforant path to dentate gyrus EPSP; no change in the PPF of the population spike was found in CA1 or DG. In a second group of rats that were not run on the RAM, at 6 weeks after kindling, PPF of the population EPSP and population spike were enhanced in the kindled rats compared to the control rats in CA1, but not in DG or CA3 in vitro (at 1.5, 2, or 4 times threshold intensity). In conclusion, partial hippocampal kindling induced persistent physiological effects for up to 8-9 weeks, and it is suggested that the normalization of the paired-pulse population spike response in CA1 and DG at more than 6 weeks after kindling may be accompanied by a recovery of RAM performance. (C) 1994 Wiley-Liss, Inc.