Abstract

Long-term potentiation (LTP) of hippocampal synapses has been suggested to be a cellular mechanism for the learning and memory. However, few studies have reported a relationship between LTP and the behavioral index and the results have often been inconsistent with each other. The present study was conducted to see: 1) if LTP develops in the hippocampal dentate gyrus during classical conditioning of rabbit's nictitating membrane response (NMR) and 2) if stimulation of the medial septum, which is known to be an afferent structure, affects the induction of the LTP and the classical conditioning. Animals were pseudorandomlyassigned to one of four following experimental conditions: 1) CS(conditioned stimulus)-US(unconditioned stimulus), 2) CS-US with the electrical stimulation of medial septum, 3) unpaired presentation of CS and US 4) CS-US without test pulse to the perforant path. The results revealed that the three paired groups showed the robust acquisition compared with the unpaired group. The amplitude of evoked potential at hippocampal dentate gyrus to the test pulse of the perforant path during the acquisition was significantly increased in two paired groups with the test pulse, but not in the unpaired group. The electrical stimulation of the medial septum facilitated the behavioral acquisition without a change in evoked potential of dentate gyrus. The results of this study suggest that learning-related information from the enthorhinal cortex carried via perforant path induces LTP at hippocampus, and information to hippocampus carried via the medial septum modulates acquisition in conditioning of NMR, which can be independent of the induction mechanism of LTP.