POST-ACTIVATION POTENTIATION IN THE NEOCORTEX .1. ACUTE PREPARATIONS

Long-term potentiation is widely studied as a memory model, and has be en demonstrated in a number of subcortical sites in both acute and chr onic preparations. In the neocortex, however, most of the demonstratio ns of LTP have been in neocortical slice or acute preparations, and ev en these have often required a drug-induced attenuation of inhibition before the LTP could be reliably expressed. In this paper we show that LTP can be reliably expressed in adult rats in a number of neocortica l sites, both ipsilateral and contralateral to the site of callosal st imulation. We also show that, when recording field potentials, LTP is expressed roughly equally at all cortical depths. In a third experimen t, we monitored input/output (I/O), paired-pulse inhibition and short- term potentiation effects over the course of LTP induction. The ipsila teral responses were, as expected, of shorter latency and larger ampli tude than contralateral responses. They also showed small spike-like c omponents that correlated with cell discharge. Nevertheless, the contr alateral responses tended to show the largest LTP effects. The paired pulse effect was mainly depression, lasting for up to 3000 ms, at both ipsilateral and control sites. The short-term potentiation components were best fit by two summed exponentials with time constants of about 70 s and 12 min. The LTP effect lasted at least two h which was the l ongest period monitored in these experiments.