Laminar distribution of nicotinic receptor subtypes in human cerebral cortex as determined by [3H](-)nicotine, [3H]cytisine and [3H]epibatidine in vitro autoradiography

The subregional localization of different nicotinic acetylcholine receptor subtypes in human cerebral cortex was estimated by quantitative in vitro autoradiography using the nicotinic ligands [H-3](-)nicotine, [H-3]cytisine and [H-3]epibatidine in large whole human forebrain hemispheres. Saturation experiments in frontal cortex revealed for [H-3](-)nicotine two binding sites with affinity constants (K-d) of 0.45 and 6.3 nM and binding site densities (B-max) of 3.0 and 14.2 pmol/g, for [H-3]cytisine one binding site with K-d of 0.19 nM and B-max of 21.8 pmol/g, and for [H-3]epibatidine one binding site with K-d of 0.011 nM and B-max of 20.0 pmol/g. The laminar binding distributions of the three ligands were compared in different cortical areas by creating binding profiles perpendicular to the entire cortical depth. The regional autoradiographic binding patterns of the three ligands were essentially similar, with higher receptor binding in cortical layers I, III and V. In the primary sensory cortex and inferior frontal sulcus, marked binding of all ligands was observed in layer III. [3H]Cytisine showed the lowest difference between maximal and minimal binding within the gray tissue in all other areas. In the primary motor cortex, [H-3]epibatidine and [H-3](-)nicotine showed high binding in layers III and V. The [H-3](-)nicotine binding was higher than that of the other ligands in layers I and VI of the primary motor cortex, the deeper layer V of the primary sensory cortex, layer III of the superior temporal sulcus and layer VI of the parietal cortex. A distinct band of binding of [H-3](-)nicotine and [H-3]epibatidine but not of [H-3]cytisine was found in layer IIIb of the occipital cortex and layer V of the superior temporal sulcus. [H-3]Epibatidine showed higher binding than the other ligands in ail layers of the medial frontal, superior frontal and superior temporal sulcus. The findings with the three nicotinic ligands suggest three binding sites in the cortex with different laminar distributions. All three ligands bound to an identical receptor site, most likely the alpha 4 nicotinic receptor subunit. The morphological distribution of [H-3]epibatidine and [H-3](-)nicotine binding indicate that they bind to an additional site, especially in the primary motor cortex, in layer IIIb of the occipital cortex and layer V of the superior temporal sulcus. High binding of [H-3](-)nicotine in layers I and VI of the primary motor cortex, the deeper layer V of the primary sensory cortex, layer III of the superior temporal sulcus and layer VI of the parietal cortex may indicate a third binding site. (C) 1998 IBRO. Published by Elsevier Science Ltd.