Distribution of nicotinic cholinergic receptors in the rat tel‐ and diencephalon: a quantitative receptor autoradiographical study using [³H]‐acetylcholine, [α‐¹²⁵l]bungarotoxin and [³H]nicotine

The topographical distribution of [α:‐¹²T]bungarotoxin [¹²⁵I]BTX, [³H]nicotine ([³H]Nic), [³H]acetylcholine ([³H]ACh) (in the presence of atropine) binding in rat teland diencephalon was investigated using a quantitative receptor autoradiographical technique. With the [³H|ACh and [³H]Nic radioligands, a strong labelling was observed in various thalamic nuclei, including the medial habenula, a moderate labelling in different areas of the cortex cerebri, the nucleus caudatus putamen, the nucleus accumbens and tuberculum olfactorium and a uniform weak labelling in the hypothalamus. When the binding data for [³H]Nic were plotted against binding data for [³H]ACh in various brain nuclei, a significant correlation was obtained. Considering [¹²⁵I]BTX, the strongest labelling was observed in the lateral mammillary nucleus and the hilus gyrus dentatus of the hippocampal formation. A weak labelling occurred in areas such as the nucleus causatus putamen, the thalamus and the cerebral cortex. No significant correlation was therefore obtained between the degree of [¹²⁵I]BTX binding in various brain nuclei and the degree of binding observed with [³H]Nic or [³H]ACh. The present results underline the view that the high‐affinity |³H]Nic and [³H]ACh binding sites label the same cholinergic nicotinic receptor binding site, while [¹²⁵I]BTX labels another subpopulation of nicotinic cholinergic receptors, predominantly found in discrete areas of the hypothalamus and the limbic cortex.