POST-ACTIVATION POTENTIATION IN THE NEOCORTEX .4. MULTIPLE SESSIONS REQUIRED FOR INDUCTION OF LONG-TERM POTENTIATION IN THE CHRONIC PREPARATION

The neocortex in chronically prepared rats is very resistant to the in duction of long-term potentiation (LTP). In the first of two experimen ts described in this paper, we tried unsuccessfully to induce neocorti cal LTP within one session by coactivating basal forebrain cholinergic and cortical inputs to our neocortical recording site. In the second experiment, we tested a new procedure which involved the application o f repeated conditioning sessions over several days. This procedure was suggested by our finding that kindling-induced potentiation (KTP) of cortical field potentials could be reliably triggered but was slow to develop. We administered 30 high frequency trains per day to the corpu s callosum for 25 days. LTP in callosal-neocortical field potentials b ecame clear after about 5 days of stimulation and reached asymptotic l evels by about 15 days. After the termination of treatment, LTP persis ted for at least 4 weeks, the duration of our post-stimulation test pe riod. As in previous experiments on kindling-induced potentiation, the potentiation effects were clear in both early population spike compon ents and in a late (probably disynaptic) component. The monosynaptic E PSP component was often depressed, but this may have been due to compe ting field currents generated by the enhanced population spike activit y. We discuss these results in the context of theories emphasizing slo wer but more permanent memory storage in neocortex compared to the hip pocampus.