CHRONIC EMBRYONIC MK-801 EXPOSURE DISRUPTS THE SOMATOTOPIC ORGANIZATION OF CUTANEOUS NERVE PROJECTIONS IN THE CHICK SPINAL-CORD

The effect of altering neural activity on the development of the centr al projections of cutaneous and muscle sensory neurons was studied in the embryonic chick spinal cord. Animals were treated chronically with MK-801, a non-competitive N-methyl-D-aspartate receptor antagonist, d uring the period when both cutaneous and muscle sensory afferents form connections in the spinal cord. Daily applications of MK-801 began on embryonic day 5, 1 day before sensory collaterals penetrate the spina l cord gray matter, and continued until the animals were analyzed (at embryonic day 14). The patterns of cutaneous and muscle sensory nerve projections were determined by applying fluorescent tracers to individ ual, identified peripheral nerves. MK-801 treatment did not overtly al ter the pattern of muscle afferent projections. However, in the MK-801 -treated embryos, the somatotopic organization of cutaneous afferent p rojections was dramatically altered. Normally, the projections formed by the lateral femoral cutaneous and the medial femoral cutaneous nerv es are located immediately adjacent to one another in the lumbar dorsa l horn, with little overlap. In the MK-801-treated embryos, the projec tions from these two cutaneous nerves both expanded significantly with in dorsal horn laminae to become almost completely superimposed. These data suggest that MK-801: disrupts the development of the somatotopic organization of cutaneous afferent projections in the spinal cord.