Ventral hippocampal glutamate receptors in the rat: possible involvement in learning mechanisms of an active...

The role of ventral hippocampus glutamate receptors on learning mechanisms and memory was studied in the rat. Adult male rats were unilaterally implanted in the ventral hippocampus with microinjection cannulas. The general experimental procedure used was the chemical stimulation of hippocampal neurons with glutamic acid alone or in combination with glutamate receptor antagonists during learning of an active avoidance response. The one-way active response consisted in avoiding an electric shock applied to the feet while an ultrasonic tone of 40 KHz was on. Two series of experiments were performed. In Experiment 1, the possible effect of glutamate on the evocation of the learned avoidance response was studied. In Experiment 2, the possible effect of glutamate on the acquisition of the avoidance response was analyzed. Experiment 1 showed that glutamate in the range 1-10 nmol did not interfere with the recall of the avoidance response, suggesting that glutamate has no effect on the hippocampal evocation processes. Experiment 2 showed that glutamic acid inhibits the acquisition process, increasing the latency time of escape and deteriorating the learning efficiency. This effect was antagonized by AP7, the NMDA-glutamate receptor antagonist, and increased by AP3, the metabotropic glutamate receptor antagonist. Present data suggest that metabotropic glutamate receptors facilitate and NMDA-glutamate receptors inhibit the learning hippocampal mechanisms in the rat.