Intracellular recording from hippocampal CA1 interneurons before and after development of long-term potentiation.

Synaptic efficacy is modified following a brief train of high frequency stimulation (HFS) to a cell's afferent fibers (long-term potentiation, LTP). Modulation of synaptic efficacy could be achieved through changes in excitability of interneurons. To test this hypothesis, we used the in vitro hippocampal slice to record intracellularly from CA1 basket cell interneurons before and following a period of HFS to fibers in stratum radiatum. Stimulation of stratum radiatum consistently produced action potential discharge in the basket cell interneuron prior to the discharge of a population spike in stratum pyramidale. This result is consistent with findings that basket cells may receive direct excitatory afferent input from neurons extrinsic to the CA1 region, and mediate a powerful feed-forward inhibition. Interneuron responses were recorded under 3 experimental conditions, in which stimulus intensity was varied. The level of basket cell excitability, as assessed by responses to orthodromic input, was not decreased following a train of HFS. Rather, the evoked EPSP in basket cells was often enhanced following high frequency stimulation to the cell's afferent fibers. These findings support the notion that decrement in basket cell activity is not responsible for LTP in CA1 hippocampus.