ROLE OF HIPPOCAMPAL NO IN THE ACQUISITION AND CONSOLIDATION OF INHIBITORY AVOIDANCE-LEARNING

NITRIC oxide (NO), an unconventional neurotransmitter in the brain, ha s been postulated as a retrograde intercellular messenger necessary fo r the induction, but not the maintenance phase, of activity-dependent forms of synaptic plasticity in the hippocampus. Here we report on the effects of an inhibitory avoidance learning task on hippocampal NO sy nthase (NOS) activity and on the effects of intrahippocampal infusion of a NOS inhibitor in the acquisition and consolidation of this task i n rats. NOS activity increases by 45% in the hippocampus immediately a fter training (0 min) but not at 60 min after training. No changes wer e observed in cerebellar NOS activity. The bilateral intrahippocampal microinjection of nitro-arginine (NO-arg), an NOS inhibitor, provoked retrograde amnesia for the inhibitory avoidance when given 10 min befo re or immediately after training, but not 60 min after training. These results suggest that NO-regulated processes in the hippocampus play a n important role at the time of training or very shortly thereafter of an inhibitory avoidance learning.