Autoradiographic Localization of Putative Nicotinic Rat Brain Using 1251-Neuronal Bungarotoxin Receptors

Neuronal bungarotoxin (NBT), a snake venom neurotoxin, selectively blocks nicotinic receptors in many peripheral and central neuronal preparations. a-Bungarotoxin (aBT), on the other hand, a second toxin isolated from the venom of the same snake, is an ineffective nicotinic antagonist in most vertebrate neuronal preparations studied thus far. To examine central nicotinic receptors recognized by NBT, we have characterized the binding of 1a51-labeled NBT (Y-NBT) to rat brain membranes and have mapped the distribution of ‘*%NBT binding in brain sections using quantitative light microscopic autoradiography. The binding of Y-NBT was found to be saturable, of high affinity, and heterogeneously distributed in the brain. Pharmacological studies suggested that more than one population of sites is labeled by 1Z51-NBT. For example, one component of ‘%NBT binding was also recognized by aBT, while a second component, not recognized by arBT, was recognized by the nicotinic agonist nicotine. The highest densities of these aBT-insensitive, nicotine-sensitive sites were found in the fasciculus retroflexus, the lateral geniculate nucleus, the medial terminal nucleus of the accessory optic tract, and the olivary pretectal nucleus. aBT-sensitive NBT binding sites were found in highest density in the lateral geniculate nucleus, the subthalamic nucleus, the dorsal tegmental nucleus, and the medial mammillary nucleus (lateral part). The number of brain regions with a high density of YNBT binding sites, blocked either by arBT or by nicotine, is low when compared with results obtained using other approaches to studying the central distribution of nicotinic receptors, such as labeling with 3H-nicotine or labeling with cDNA probes to mRNAs coding for putative receptor subunits. It is proposed that Y-NBT labels a subpopulation of nicotinic receptors in the rat brain, and that, given its ability