Injection of MK-801 affects ocular dominance shifts more than visual activity

Purpose. Antagonists to the NMDA receptor reduce ocular dominance plasticity. Do they do this simply because they reduce activity in the visual cortex? We attempted to answer this question by comparing the effect of blocking NMDA receptors on ocular dominance plasticity with its effect on neural activity. Methods. Kittens were monocularty deprived for 8 days beginning at P27-P38. Seven experimental kittens were injected twice daily with MK-801 (∼0.15 mg/kg i.m.) at 4 hr intervals. Four hr after the second injection they were put in the dark overnight. Six control kittens were treated identically except that they were injected with saline. We recorded single units from the visual cortex on the ninth day after suture. Results. Ocular dominance histograms of the experimental kittens showed less shift toward the non-deprived eye (average weighted ocular dominance = 0.66) than did histograms from control animals (average weighted ocular dominance = 0.91). At the end of the recording session, we examined the effect of a similar dose of MK-801 on visual responsiveness. In 7 cells activity did not change substantially. Three cells showed long term reductions not related to the injection. Three cells snowed reduced activity that was related to the injection, but each of these showed substantial recovery after one hour. This suggests that activity was close to normal for 3 out of the 4 hours after injection of MK-801. Condition. A regime that does not reduce visually evoked activity significantly can produce a significant reduction in ocular dominance shifts. We suggest that our regime affected ocular dominance plasticity through a reduction of calcium entry to the cell without a significant reduction in activity.