Involvement of glycine site associated with the NMDA receptor in hippocampal long-term potentiation and acquisition of spatial memory in rats.

The effects of 7-chlorokynurenic acid (7-Cl-Kyn), a selective antagonist at the glycine site associated with the N-methyl-D-aspartate (NMDA) receptor, on hippocampal long-term potentiation (LTP) and behavioral performances in a spatial learning task were investigated. Extracellular recordings of evoked potential (population spike) were made in rat hippocampal slices. Perfusion of 7-Cl-Kyn (10(-5) M) inhibited the induction of LTP following a tetanic stimulation (51 or 101 pulses at 100 Hz) both in the Schaffer/commissural-CA1 pyramidal cell synapses and in the perforant path-dentate granule cell synapses. Acquisition of a spatial memory in the Morris water maze was examined using rats chronically cannulated for application of drugs. The intact and vehicle-injected rats learned easily to escape onto a hidden platform with short latencies, while the rats given an injection of 7-Cl-Kyn (10(-8) mol/brain, i.c.v.) prior to every session took a longer time and a longer path to escape even after all 5 sessions of trials. Injection of 7-Cl-Kyn did not affect the swimming speed, an index of swimming ability. This is the first report providing direct evidence that endogenous glycine supports the processes of learning and memory.