Regionally specific distribution patterns of receptor densities differ between sensory, motor and multimodal association areas as well as between the various areas within each of these major functional groups. The receptor-based regional segregation of the cerebral cortex is further differentiated by the layer specific distribution patterns of receptor densities. Thus, the mapping of transmitter receptors provides data for understanding the molecular basis of cortical organization.
The thalamic input to the primary sensory cortical areas terminates in layer IV and supragranular layers. We hypothesized, that this common supramodal organizational principle is reflected by similar receptor expression and laminar distribution patterns in all primary sensory areas. As in V1, very high densities of GABAA, muscarinic M2 and a2 receptors are found in the supragranular layers and layer IV of the primary auditory area and primary somatosensory cortex. The cholinergic nicotinic receptor is nearly exclusively expressed in layer IV of the primary visual, auditory, and somatosensory areas. Thus, laminar distribution patterns shows that the thalamo-cortical input is controlled by a specific balance (“receptor fingerprint”) between a group of receptors. In sharp contrast to primary sensory areas, adjoining secondary sensory areas display significantly lower densities of GABAA, muscarinic M2 and 2 receptors, but markedly higher densities of other receptors.
Borders of cortical areas and layers defined by localized changes in receptor densities and laminar patterns are found (in most cases) precisely at precisely the same positions, where borders are detectable in cytoarchitectonic studies, but receptorarchitectonics also can reveal additional “subareas” (e.g., in the Broca region). Consequently, a multimodal approach including cyto- and receptorarchitecture as well as fiber tracking and functional neuroimaging is necessary to achieve a comprehensive insight into the organizational principles of the human cerebral cortex.