Receptor autoradiography in thoracic spinal cord: Correlation of neurotransmitter binding sites with sympathoadrenal neurons

Receptor autoradiography was combined with the retrograde labeling of sympathoadrenal neurons by fast blue to determine whether opiate, serotonin, catecholamine, or cholinergic binding sites could be spatially correlated with preganglionic neurons in the rat intermediolateral cell column (IML) that projected to the adrenal gland. [³H]Dihydromorphine (DHM) was used for the visualization of μ opiate receptors, [³H]lysergic acid diethylamide (LSD) for serotonin receptors, [³H]para-aminoclonidine (pAC) for α₂-adrenergic receptors, and [³H]quinuclidinyl benzilate (QNB) for muscarinic cholinergic receptors. While qualitative assessment of autoradiograms indicated that α₂-adrenergic and muscarinic receptors were concentrated in the intermediate zone of the spinal cord, quantitation of grains in specific regions of the intermediate gray revealed that α₂-adrenergic and serotonergic receptors were more highly concentrated over sympathoadrenal preganglionic neurons than over other regions in IML or the adjacent intermediate gray matter. Information concerning the distribution of neurotransmitter-binding sites in other regions of thoracic spinal cord was also obtained. All ligands showed relatively dense binding sites in the superficial laminae of the dorsal horn, and all but [³H]DHM revealed similar densities of binding sites in the region adjacent to the central canal. Only [³H]QNB revealed a high density of binding sites in the ventral horn of the spinal cord.