POST-ACTIVATION POTENTIATION AND DEPRESSION IN THE NEOCORTEX OF THE RAT .2. CHRONIC PREPARATIONS

Although long-term potentiation (LTP) has been demonstrated in a numbe r of subcortical sites in chronic preparations, there have been no dem onstrations of LTP in the neocortex of chronic preparations. Even neoc ortical slice and acute preparations often require a drug-induced supp ression of inhibition before LTP effects can be reliably induced. We h ave attempted to induce LTP in neocortical sites in 7 different experi ments using chronically prepared adult rats. We were unable to obtain any evidence, even a trend, for the induction of LTP. The following ma nipulations were tested: (1) standard stimulation train parameters tha t have been shown to be highly effective in subcortical and hippocampa l sites; (2) a 10-fold increase in the intra-train pulse durations; (3 ) variations in train pulse frequency (1 Hz to 300 Hz) and train durat ion (100 ms to 15 min); (4) co-activation of multiple inputs by stimul ation of combinations of cortical sites or cortical and thalamic sites ; (5) reduction of inhibition by administration of picrotoxin; 5) Hous ing of animals in an enriched environment; (6) utilization of the neoc ortical stimulation trains as a cue in a learning task; (7) applicatio n of pilocarpine to co-activate cholinergic systems. Although none of these manipulations produced LTP, the application of pilocarpine did f acilitate the induction of a long-lasting depression effect. These fin dings contrast with the results obtained from anesthetized rats and fr om studies using brain slices, where LTP can be reliably induced. Thes e results are discussed in light of other recent findings with respect to LTP and LTD effects.