Frequency-dependent examination of homosynaptic long-term depression in the freely moving rat

The strength and direction (LTP or LTD) of synaptic responses (i.e., synaptic plasticity) within the hippocampus resulting from activation of afferent inputs vary depending upon the frequency and pattern of such activation, as well, as, the order in which convergent afferents are activated. Specifically, such stimulation may result in LTP of activated synapses, or, conversely, in LTD of synaptic activity. The present study was designed to: (1) determine whether stimulation frequencies effective at inducing LTD in vitro and in the anesthetized preparations are also as effective in the freely moving adult rat, and (2) at what frequency does the shift from homosynaptic LTD to LTP occur. Population spike amplitude and EPSP slope measures were recorded in the dentate gyrus following low-frequency stimulation of the lateral perforant pathway at various frequencies during experiments conducted on adult freely moving rats. Preliminary results indicate that LTD can reliably be induced in freely moving animals and that there is a frequency-dependent shift in induction of LTP/LTD.