LIGAND SPECIFICITY OF NICOTINIC ACETYLCHOLINE-RECEPTORS IN RAT SPINAL-CORD - STUDIES WITH NICOTINE AND CYTISINE

Administration of nicotinic agonists to the spinal cord leads to a var iety of cardiovascular and behavioral responses. The discrete localiza tion of afferent and efferent fibers presents a system in which occupa tion of nicotinic receptor subtypes might be correlated with specific pharmacologic responses. To this end, we examined [H-3]cytisine and [H -3]nicotine binding to membranes isolated from regions of the rat spin al cord. [H-3]Cytisine showed saturable, noncooperative (n(H) similar or equal to 1) binding to a single-class of sites with a K-d of 0.44 /- 0.01 nM and total saturable sites of 19.9 +/- 0.9 fmol/mg of protei n. [H-3]Cytisine binding to membranes from intermediolateral cell colu mn and dorsal and ventral sections of the lumbosacral regions each rev ealed a single class of binding sites with virtually identical K-d val ues. However, the dorsal sections of the lumbar spinal cord contained a higher number of total binding sites than ventral lumbar or intermed iolateral sections. The rank order potencies of the nicotinic agonists competing for [H-3]cytisine binding was cytisine > l-nicotine > N-met hylcarbamylcholine > dimethylphenylpiperazinium > acetylcholine > d-ni cotine > carbamylcholine. Competitive antagonists also competed with h igh affinities (K-i as low as nanomolar) with the order of potencies b eing alpha-lobeline greater than or equal to dihydro-beta-erythroidine >> methyllycaconitine, whereas the channel blockers, mecamylamine and hexamethonium, only competed at concentrations greater than or equal to 100 mu M. Competitive ganglionic blockers such as d-tubocurarine or trimethaphan and neurotoxins such as alpha-bungarotoxin, alpha-cobrat oxin or neuronal bungarotoxin had weak affinities for cytisine sites. Similar to [H-3]cytisine, [H-3]nicotine also revealed a saturable sing le class of binding sites, but of lower affinity. The rank order of K- i values of the agonists, antagonists and neurotoxins for competing wi th [H-3]nicotine binding was similar to the order for [H-3]cytisine. N icotinic receptors in the spinal cord membranes show a specificity for both agonists and antagonists that differ from neuronal receptors in the ganglia or the regions of the brain characterized to date. These f indings, when correlated with the pharmacological responses documented in the companion article suggest that the spinal nicotinic receptors may define a new member of the neuronal nicotinic receptor family.