Role of NMDA receptors in adult primate cortical somatosensory plasticity

Abstract

We have previously shown that most of the reorganization that typically follows median nerve transection in adult squirrel monkeys is dependent on normally functioning N-methyl-D-aspartate (NMDA) receptors. Here, we have evaluated two additional hypotheses: (1) is the immediate "unmasking" found after median nerve transection NMDA receptor-dependent? and (2) are NMDA receptors necessary for both the initiation and maintenance of the second phase of reorganizational changes, or only the former? To address these issues, we implanted osmotic minipumps subcutaneously to deliver an NMDA receptor antagonist (3-((+/-)-2- carboxypiperazin-4-yl)propyl-1-phosphonic acid, CPP) systemically either before examining the immediate effects of median nerve transection, or after reorganization had presumably occurred. For the first set of experiments, NMDA receptor blockade was initiated either 1 or 4 weeks prior to multi-unit mapping in area 3b followed by transection of the median nerve and remapping of the cortex. In the second set of experiments, median nerve transection was followed 4 weeks later by either 1 or 4 weeks of NMDA receptor blockade prior to terminal mapping. We report that the immediate unmasking of new receptive fields after acute nerve injury is not prevented by NMDA receptor blockade; nor are completely reorganized cortical maps dependent upon NMDA receptors for their maintenance. We conclude that the immediate changes in cortical topography are not due to an NMDA receptor-dependent mechanism, but more likely due to release from tonic inhibition. Furthermore, the later phase of reorganization, as for some forms of hippocampal long-term potentiation (LTP), is dependent on normally functioning NMDA receptors for its initiation, but not for its maintenance.