Effects of LTP induction on hippocampal cellular excitability in the freely behaving developing rat brain

Our aim is to assess and quantify the exact effects of the induction of long term potentiation (LTP) on tonic inhibition and facilitation in a neural circuit within the hippocampal formation of neonatal rats. The neural circuit of interest in this paper is the perforant pathway-dentate gyrus synapse which serves as the first leg of the hippocampal trisynaptic circuit. A quantitative measure of the modulation of the perforant pathway/dentate gyrus circuit is the paired-pulse index which measures changes in the response of this circuit to a pair of pulses separated by varying interpulse intervals (IPI). It is known that such modulation influences the gating of neuronal transmission into and through the hippocampal formation; and, thereby, may play an important role in the development of learning and memory both in early life and throughout adulthood. Also important to this discussion is the finding that the dentate gyrus is one of the few areas of the rat brain which continues to generate new nerve cells well after birth. In an effort to quantify both age-related and LTP-dependent effects, LTP was induced using high frequency stimulation (HFS) of the perforant pathway-dentate gyrus synapse in freely behaving 10-12 day old, male Sprague-Dawley rats. Population spike amplitude measures which correspond to cellular discharge to a synaptic event were extracted from evoked field potentials recorded at the level of the molecular layer of the dentate granule cell population following induction of LTP in the same synapse. Preliminary results indicate the paired-pulse index was altered following induction of LTP.