LONG-TERM DEPRESSION AT THE MEDIAL PERFORANT PATH-GRANULE CELL SYNAPSE IN DEVELOPING RAT DENTATE GYRUS

Long-term depression (LTD) is a decrease in synaptic efficacy that may model the elimination of inappropriate synapses during brain developm ent. LTD might therefore be expected to be prominent in the juvenile h ippocampal dentate gyrus (DG), where the majority of neuronogenesis an d excitatory synapse production and pruning occur in the first postnat al month. Thus far, however, LTD in immature DG remains unexplored. Lo w-frequency stimulus induced homosynaptic LTD was studied at the media l perforant path-granule cell synapse in rats 8-30 days of age. LTD wa s most consistent and was of greatest magnitude in the youngest animal s, and was more robust in response to stimulation at 1 Hz than at 3 or 5 Hz. LTD was saturable by repetitive delivery of low-frequency stimu lation, and reversible by tetanic stimulation that induced long-term p otentiation (LTP). LTD of the field EPSP was not prevented by bath app lication of the NMDA receptor antagonist AP5, the mGluR antagonist MCP G, or the L-type voltage sensitive calcium channel antagonist nitrendi pine. In whole cell recordings LTD induction was blocked by hyperpolar ization of the postsynaptic neuron but not by calcium chelation with B APTA. Calcium chelation blocked LTP and simultaneously unmasked tetanu s induced LTD. These data demonstrate that LTD is prominent in immatur e DG, that LTP and LTD are complementary processes, and that LTD is li kely to be induced postsynaptically because it is voltage dependent, a lthough the mechanism of voltage dependence remains to be elucidated.