Receptor architecture of the macaque monkey superior parietal lobule

The macaque monkey superior parietal lobule (SPL) is part of a neuronal network involved in the integration of information from visual and somatosensory cortices for execution of reaching and grasping movements. The cytoarchitecture of areas V6, V6Ad, V6Av, PE, PEc, PEci and PGm of the SPL has been described, but little is known about their receptor architectonic organization, although receptor analyses not only provide information concerning brain structure, but also crucial insights into its functional organization. Quantitative in vitro receptor autoradiography was applied to analyze the distribution patterns of 15 different receptors for glutamate, GABA, acetylcholine, serotonin, dopamine and adenosine in the SPL of three adult male Macaca fascicularis monkeys. For each area, mean (averaged over all cortical layers) receptor densities were visualized as a receptor fingerprint of each area. Multivariate analyses were conducted to detect clusters of areas according to the degree of (dis)similarity of their receptor organization. Differences in regional and laminar receptor distribution patterns confirm the location and extent of areas V6, V6Ad, V6Av, PE, PEc, PEci and PGm as found in cytoarchitectonical and functional studies. Receptor densities are higher in supra- than in infragranular layers of SPL areas, with the exception of kainate, M2, and adenosine receptors, which reach highest values in layers V-VI. The hierarchical cluster analysis shows a principal segregation of SPL areas from the primary sensory cortices. Areas PEc, PEci and PGm cluster with posterior cingulate area 31. Area V6Av clusters with visual V6, and V6Ad with MIP, while area PE with somatosensory area 2. These results are in accordance with the fact that V6Av contains more cells responsive to visual stimuli than does V6Ad, whereas the opposite holds true for cells responsive to somatosensory stimuli. They further emphasize the special receptor architecture of posterior parietal areas involved in reaching and grasping.