[3H]epibatidine labels nicotinic receptors in rat brain: an autoradiographic study

Epibatidine, a frog skin alkaloid, is a nicotinic receptor agonist with potent analgesic activity. We used in vitro receptor autoradiographic techniques to map binding sites for [3H]epibatidine in rat brain, and those for another nicotinic agonist, [3H]cytisine. Both [3H]epibatidine and [3H]cytisine are excellent radioligands for autoradiography. [3H]Epibatidine in particular binds with very high affinity and extremely low nonspecific binding. In the present study, the distribution of binding was almost identical for both radioligands and closely resembled that of previously published autoradiographic studies with two other nicotinic ligands, [3H]acetylcholine and [3H]nicotine. Brain regions demonstrating the highest levels of [3H]epibatidine binding included the interpeduncular nucleus, medial habenular nucleus, fasciculus retroflexus, superficial gray layer of the superior colliculus and numerous thalamic nuclei, including the anteroventral, dorsal lateral geniculate and gelatinosus nuclei. Quantitative analysis revealed that [3H]epibatidine binding was greater than [3H]cytisine binding in several regions. In particular, [3H]epibatidine clearly labeled the optic nerve, optic chiasm and optic tract, whereas [3H]cytisine binding to these structures did not exceed background levels. Other regions in which [3H]epibatidine binding was greater than that of [3H]cytisine included the medial habenula, fasciculus retroflexus, olivary pretectal nucleus and superficial gray layer of the superior colliculus. The differences in labeling between these two nicotinic ligands may arise from receptor heterogeneity in brain nicotinic receptors and the ability of [3H]epibatidine to label more than one subtype of nicotine receptor.