3H]Acetylcholine Binding to Nicotinic Cholinergic Receptors in Brain: Localization and Regulation Visualized by Autoradiography

Abstract

[3H]Acetylcholine ([3H]ACh) labels nicotinic cholinergic receptors of high affinity in brain homogenates (1). The distribution of these sites can be studied in greater detail using autoradiographic techniques. In a recent autoradiographic study, a qualitative comparison was made of [3H]ACh and [3H]nicotine binding to nicotinic cholinergic receptors in rat brain sections (2). In the present study, autoradiography was used to determine the relative distribution of [3H]ACh binding sites throughout several levels of rat brain. Brain sections (24 ym) were thaw-mounted onto subbed slides, assayed for [3H]ACh binding, and exposed to tritiunv-sensitive film as previously described (2,3). Optical density measurements were made of specific brain regions relative to areas of white matter in the same slice. There was a seven-fold difference between areas with the highest and lowest levels of [3H]ACh binding. The greatest binding occurred in areas such as the interpeduncular nucleus, several thalamic nuclei, and medial habenula. Areas with moderate levels of binding included the superior colliculus, retrosplenial cortex, substantia nigra pars compacta, and caudate nucleus, while [3H]ACh binding was virtually absent in areas such as the inferior colliculus, hippocampus (stratum oriens), and entorhinal cortex. In previous studies using brain homogenates, we have observed that repeated exposure to the cholinesterase inhibitor OFP leads to a ‘down-regulation’ of [3H]ACh binding sites while repeated exposure to nicotine leads to an ‘up-regulation’ of these binding sites in brain areas such as the thalamus, cortex, caudate, and hypothalamus (4,5).