Subdivisions of human parietal area 5 revealed by quantitative receptor autoradiography: a parietal region between motor, somatosensory, and cingulate cortical areas

Brodmann's area (BA) 5 of the human superior parietal cortex occupies a central anatomical position between the primary motor (BA 4), somatosensory (area 3b and BA 2), cingulate (area 23c), and superior parietal association cortex (BA 7). We studied the regional and laminar distributions of the binding sites of 12 different neurotransmitter receptors (glutamatergic: AMPA, kainate, NMDA; GABAergic: GABAA, GABAB; cholinergic: muscarinic M2, nicotinic; adrenergic: alpha1, alpha2; serotoninergic: 5-HT1A, 5-HT2; dopaminergic: D1) in human postmortem brains by means of quantitative receptor autoradiography, since the structural and functional aspects of human BA 5 are widely unknown, and previous observations have demonstrated characteristic differences in receptor distribution between motor and somatosensory areas. Binding site densities were measured in the cytoarchitectonically defined BA 5 and surrounding regions. Similarities and differences of receptor distribution between cortical areas were studied by cluster analysis of mean binding site densities averaged over all cortical layers, univariate and multivariate statistics, and by density profiles representing laminar receptor distribution patterns. Based on regional heterogeneities of binding site densities and of the cytoarchitecture within BA 5, we suggest a subdivision into three subareas: medial area 5M, lateral area 5L, and area 5Ci in the region around the cingulate sulcus. BA 5 is therefore a heterogeneous cortical region, comprising three subareas showing receptor expression patterns similar to the adjoining higher order somatosensory, multimodal parietal, or cingulate regions. These findings suggest that human BA 5 constitutes a higher order cortical area, clearly distinct from the primary somatosensory and motor cortex.