Regional distribution of cholinergic muscarinic receptors in spinal cord

Quinuclidinylbenzilate ([3H]QNB) binding sites are present in the rat spinal cord. The binding site are muscarinic in character based on displacement of [3H]QNB by cholinoceptive drugs. They are distributed rather uniformly along the cord, although the receptor density is greater in gray matters than in white matter. Binding to white matter may be associated with glial cells. Within the gray matter, the receptor density is higher in the ventral horn than in the dorsal horn. In the thoracic region receptor density is about equal in the intermediate zone and ventral horn. Mid-thoracic transection of the cord does not change the receptor density or the dissociation constant of [3H]QNB in the lumbar cord. In contrast, treatment with the neurotoxin, 6-aminonicotinamide, which produces lesions of the cord, loss of motor control and paralysis, reduces the receptor density and affinity of [3H]QNB for lumbar gray matter but not white matter. The presence of [3H]QNB binding sites thrughout the spinal cord as well as the documented presence of acetylcholine-containing neurons, suggest that muscarinic receptors play a role in all phases of spinal cord physiology.