Immunocytochemical distribution of ionotropic glutamate receptor subunits in the spinal cord of the rabbit

Several histochemical and physiological studies in the literature suggest that ionotropic glutamate receptors are involved in various sensory and motor control mechanisms at the spinal level. The present immunocytochemical study used three specific antibodies to GluR2,4, GluR5,6,7 and to NMDAR1 to differentiate between the regional distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors throughout the rabbit spinal cord. All of these immunoreactivities were prominent in the superficial dorsal horn and motor column. Each antibody gave rise to regionally specific immunostaining patterns but which were similar at all spinal levels. Numerous small neurons in superficial laminae were immunostained with GluR2,4 antibody while only neuropilar elements were immunostained with the two other antibodies. Cell bodies of the intermediate zone and fibres in the motor column were particularly densely immunostained with GluR5-7. Such an immunostaining pattern, which was particularly abundant with the GluR5-7 antibody, suggests the presence, at the spinal level, of an extensive population of neurons exhibiting a high density of kainate receptors. Immunostaining with NMDAR1 antibody was less dense in comparison with the two others and especially in the motoneuron area. The present results provide the first immunohistochemical comparison between the respective regional distributions of the three types of ionotropic glutamate receptors in the spinal cord. Their parallel distributions throughout the spinal cord support the concept of a tight functional cooperation between NMDA and non-NMDA receptors which has been extensively described for spinal events.