Laminar distributions of muscarinic acetylcholine, serotonin, GABA and opioid receptors in human posterior cingulate cortex

Experimental animal studies have demonstrated a number of receptor localizations on specific cortical afferents and neurons. The present study of human posterior cingulate cortex evaluates the laminar distributions of particular receptors and their likely association with components of the neuropil. Coverslip autoradiographic and single grain counting techniques were used followed by heterogeneity analysis in which the layer of peak binding and an index of heterogeneity were determined for each ligand. The index was calculated by determining specific binding by layer as a percentage of binding in all layers. The differences from an absolutely homogeneous distribution, i.e. 11.1% for each of nine layers, were subtracted and the absolute laminar differences summed to form the index. High indices of over 15 reflected heterogeneous binding patterns in neocortex. The binding of ligands for muscarinic acetylcholine, serotonin, opioid, GABA and beta adrenoceptors was evaluated. Pirenzepine binding peaked in layer II of area 23a but was extremely homogeneous with an index of heterogeneity of 8.9. In contrast, oxotremorine-M binding had a peak in layer IIIc and an index of 16.4, while AF-DX 116 binding peaked in layer IIIa-b and had an index of 30.6. Of the ligands for serotonin uptake and receptor binding paroxetine binding was evenly distributed in layers I–III and had a low index of heterogeneity of 9.8. Ketanserin binding was also homogeneous and, since it had an index of 8.9, this pattern was virtually the same as that for paroxetine. In contrast, serotonin and 8-hydroxy-2-(di- n-propylamino)tetralin binding peaked in layer II and had very high indices of 20.8 and 50.3, respectively, suggesting only a limited association with that of the paroxetine distribution. Finally, there were three layers which contained peaks in binding for ligands for opioid, GABA and beta adrenoceptors. Firstly, layer Ia had peak dynorphin-A binding, the latter of which had an index of 22.6. Secondly, Tyr- d-Ala-Gly-MePhe-Gly-ol and 2- d-penicillamine-5- d-penicillamine-enkephalin binding peaked in layer II and had indices of 8.6 and 17.4, respectively. Thirdly, muscimol and (−)-cyanopindolol binding peaked in layer IIIa-b and had indices of 29.6 and 11.1, respectively. When viewed in the context of experimental animal studies, it is likely that heterogeneities in oxotremorine-M and paroxetine binding are associated with the termination of the thalamic and raphe nuclei, respectively.