The macaque inferior parietal lobule: cytoarchitecture and distribution pattern of serotonin 5-HT_1A binding sites

The inferior parietal lobule (IPL) of the macaque monkey integrates sensory information as a prerequisite for reaching and grasping movements. Electrophysiological data suggest that the convexity of the IPL is heterogeneous along its rostro-caudal axis. Cytoarchitecture reflects this functional diversity. However, various parcellations have been proposed so far, ranging from one homogeneous region to four areas. In order to obtain a more valid anatomical map of the IPL, we studied the distribution pattern of serotonin 5-HT_1A binding sites (i.e., the domain of synaptic transmission and cortical information processing) and cytoarchitecture with an observer-independent technique (thus avoiding the subjectivity inherent in visually guided microstructural parcellations). We cut with a cryostat microtome four unfixed macaque hemispheres and processed sections for 5-HT_1A binding sites with [³H]8-OH-DPAT receptor autoradiography or for cell bodies with a modified silver stain. We digitized the autoradiographs and the histological sections and extracted from the cortex of the IPL equidistant density profiles oriented vertically to the cortical layers. We then compared groups of neighboring profiles with multivariate statistics expecting to see a significant difference in profile shape at the interface between two cortical areas. These positions were compared between 5-HT_1A autoradiographs (neurochemical borders) and adjacent histological sections (cytoarchitectonic borders). Neurochemical and cytoarchitectonic borders showed a good topographical correspondence and revealed three areas arranged in a rostro-caudal sequence along the convexity of the IPL. Dorsally, the areas extend approximately 1 mm into the depth of the lateral bank of the intraparietal sulcus. Their ventral border lies on the convexity of the IPL close to the shoulder of the lateral sulcus. The three areas are in close agreement with areas PF, PFG, and PG as defined by Pandya and Seltzer (J Comp Neurol 204:196–210, 1982) and Gregoriou et al. (Program No. 919.5, Abstract Viewer/Itinerary Planner, Washington, Society for Neuroscience, 2003). The neurochemical and cytoarchitectonic data show that a complex structural framework underlies the functional heterogeneity along the rostro-caudal axis of the IPL, e.g., the representation of different types of arm and hand actions in different sectors of the monkey’s workspace.