PAIRED PULSE LOW FREQUENCY STIMULATION PROMOTES RAPID ONSET POTENTIATION OF LATERAL PERFORANT PATH-DENTATE GYRUS SYNAPSES in vivo

Despite the prevalent occurrence of long term depression (LTD) in hippocampal neurons following low frequency stimulation (LFS; Bear & Abraham, 1996), homosynaptic LTD of perforant path-dentate gyrus (PP-DG) synapses has proven difficult to induce in adult intact animals (Errington et al, 1995; Abraham, 1996; Abraham et al, 1996). Motivated by previous studies demonstrating the effectiveness of paired pulse LFS in inducing LTD in hippocampal area CA1 of adult rats both in vivo and in vitro (Doyere et al, 1996; Kemp et al, 2000), we examined the effects of paired pulse LFS (900 paired pulses, 200 ms paired pulse interval) on PP-DG synaptic plasticity in adult (300-350 g, 10-12 week old) Sprague Dawley rats under sodium pentobarbital anesthesia. Following collection of a stable baseline of at least 20 minutes, LPP or MPP fibers were stimulated at a current intensity eliciting 50% of the maximum DG field response. LPP paired pulse LFS results in a sustained and rapid-onset increase in dentate field excitatory post synaptic potentials (fEPSPs, 119 + 7 percent change from baseline). In contrast, previous work has shown a slow-onset increase in DG fEPSPs following LPP 1 Hz LFS (900 pulses, Gonzalez et al, SfN Abstracts, 2009). MPP paired pulse LFS promotes a slow-onset potentiation of DG fEPSPs (132 + 11 percent change from baseline). Taken together, our results suggest that multiple forms of synaptic plasticity co-exist at perforant path-dentate gyrus synapses, the expression of which varies depending on the afferent stimulation pattern utilized.