DIFFERENTIAL AGONIST INHIBITION IDENTIFIES MULTIPLE EPIBATIDINE BINDING-SITES IN MOUSE-BRAIN

The binding of [H-3]epibatidine, an alkaloid isolated from the skin of an Ecuadorean tree frog, was measured both in brain regions dissected from mouse brain and in tissue sections. Binding to each of 12 brain areas was saturable, but apparently monophasic; no indication of multi ple binding sites was obtained. However, inhibition of epibatidine bin ding by nicotine, acetylcholine, methylcarbachol and cytisine in olfac tory bulbs revealed a biphasic pattern consistent with the presence of two sites differentially sensitive to inhibition by these nicotinic a gonists. Cytisine displayed the greatest difference in inhibitory pote ncy between the two apparent sites. Subsequent analysis of the inhibit ion of epibatidine binding by cytisine in membranes prepared from 12 b rain areas also suggested the presence of two sites in each brain regi on. The estimated potency of cytisine at each site was similar in each brain region. However, the proportion of [H-3]epibatidine binding sit es that were more sensitive to inhibtion by cytisine and those sites l ess sensitive to inhibition by this agonist varied markedly among the brain regions. Quantitative autoradiographic analyses of mouse brain r evealed pattern of [H-3]epibatidine binding sites less sensitive to in hibition by cytisine that differed markedly from the pattern obtained with [H-3]nicotine. Among brain regions demonstrating substantial site s less sensitive to cytisine inhibition were the accessory olfactory n ucleus, medial habenula, interpeduncular nucleus, fasciculus retroflex us, superior colliculus, inferior colliculus and the pineal gland. The results indicate that epibatidine binds to at least two distinct nico tinic sites in mouse brain that may represent different nicotinic rece ptor subtypes, one of which appears to be identical to that measured b y the binding of other agonists such as nicotine or cytisine.