The expression of NMDA receptors in the intermediolateral (IML) region of t
he upper thoracic spinal cord, was studied in 3 week old rats. The effect o
f section of the cervical sympathetic nerve on neuronal cell number and rec
eptor expression was examined up to two weeks after the operation. Age-matc
hed sham-operated and unoperated animals were used as controls. It was show
n using quantitative autoradiography with the NMDA receptor antagonist [H-3
]MK-801 (dizocilpine maleate), that there was a marked downregulation of re
ceptors in all groups of animals, beginning at approximately 4 weeks of age
. However after sympathectomy, which resulted in the death of 44% of neuron
es in the IML by 7 days, there was a significant increase in receptor densi
ty per neurone compared to sham-operated controls. In the control animals t
here was a significant increase in the Kd value of the binding between 21 a
nd 24 days after birth indicating an increased expression of a low affinity
receptor, but no such increase was seen after axotomy. The results are con
sistent with two populations of NMDA receptors being transiently expressed
in the IML in developing animals, and the higher affinity receptor being do
wn-regulated between 4 and 5 weeks of age. The presence of the high affinit
y receptor subtype may predispose neurones to die after axotomy. (C) 1999 E
lsevier Science B.V. All rights reserved.