N-Methyl- d-aspartate receptors in motoneurones after unilateral axotomy in the neonatal rat

Abstract

Quantitative autoradiography was used to characterise the binding of the N-methyl- d-aspartate receptor antagonist [ 3H]dizocilpine maleate in the ventral horns of the lumbar spinal cord of normal, sham-operated, and axotomized neonatal rats. Specific binding sites were revealed on the cell membranes of the motoneurones. In the normal neonate both the B max and the K d values for the binding declined over the first 14 days of life. At 14 days the K d value was similar to that in adult rats. Unilateral sciatic nerve section was performed in neonates on the day of birth. Axotomy caused the death of approximately 53% of motoneurones. The B max and K d values for [ 3H]dizocilpine maleate binding declined in the first 2 weeks on operated and contralateral sides in both axotomized and sham-operated animals. However, the specific binding per motoneurone was significantly higher on the sectioned side in axotomized animals at all times examined after the day of birth. The results are consistent with two populations of NMDA receptors with different binding affinities being present in motoneurones at birth, and the lower affinity receptor gradually disappearing over the first few days. The lower affinity receptor may be responsible for the plasticity of motoneurones during embryonic and neonatal life, and for determining which motoneurones die after axotomy.