Nicotinic binding in rat brain: Autoradiographic comparison of [³H]acetylcholine, [³H]nicotine, and [¹²⁵I]-α-bungarotoxin

Three radioligands have been commonly used to label putative nicotinic cholinoceptors in the mammalian central nervous system: the agonists [³H]nicotine and [³H]acetylcholine ([³H]ACh - in the presence of atropine to block muscarinic receptors), and the snake venom extract, [¹²⁵I]-α-bungarotoxin ([¹²⁵I]BTX), which acts as a nicotinic antagonist at the neuromuscular junction. Binding studies employing brain homogenates indicate that the regional distributions of both [³H]nicotine and [³H]ACh differ from that of [¹²⁵I]BTX. The possible relationship between brain sites bound by [³H]nicotine and [³H]ACh has not been examined directly. We have used the technique of autoradiography to produce detailed maps of [³H]nicotine, [³H]ACh, and [¹²⁵I]BTX labeling; near-adjacent tissue sections were compared at many levels of the rat brain. The maps of high affinity agonist labeling are strikingly concordant, with highest densities in the interpeduncular nucleus, most thalamic nuclei, superior colliculus, medial habenula, presubiculum, cerebral cortex (layers I and III/IV), and the substantia nigra pars compacta/ventral tegmental area. The pattern of [¹²⁵I]BTX binding is strikingly different, the only notable overlap with agonist binding being the cerebral cortex (layer I) and superior colliculus. [¹²⁵I]BTX binding is also dense in the inferior colliculus, cerebral cortex (layer VI), hypothalamus, and hippocampus, but is virtually absent in thalamus. Various lines of evidence suggest that the high affinity agonist-binding sites in brain correspond to nicotinic cholinergic receptors similar to those found at autonomic ganglia; BTX-binding sites may also serve as receptors for nicotine and are possibly related to neuromuscular nicotinic cholinoceptors.