NMDA receptors and plasticity in adult primate somatosensory cortex

Topographic maps in adult primate somatosensory cortex are capable of dramatic reorganizations after peripheral nerve injuries. In the present experiments, we have deprived a circumscribed portion of the hand map in somatosensory cortex of our adult squirrel monkeys by transecting the median nerve to one hand, and evaluated the hypothesis that N-methyl-d-aspartate (NMDA) glutamatergic receptors are necessary for the reorganization that follows within four weeks. In one monkey, we confirm previous results demonstrating that the deprived cortex has regained responsiveness in its expanse four weeks after median nerve transection. However, in three monkeys in which NMDA receptors were concurrently blocked, most of the deprived cortex remained unresponsive. Thus, much of the cortical "recovery" that typically follows peripheral nerve injury in adult monkeys is apparently dependent on NMDA receptors and may well be due to Hebbian-like changes in synaptic strength. Perhaps the elimination of the normally dominant inputs to "median nerve cortex" permits the gradual strengthening of correlations between the activity of the formally impotent presynaptic and deprived postsynaptic elements. These enhanced correlations may also have been made possible by reductions in intracortical inhibition as a necessary but not sufficient condition.