Neurotransmitter receptors and cortical architecture.

Quantitative receptor autoradiography is a widely used technique for the demonstration of transmitter receptor distributions in anatomically defined brain regions. The present paper reviews some aspects of the relation between receptor distributions and cortical architecture in adult and early postnatal mammalian brains. 1. Changes in the distribution (mean density, laminar pattern) of transmitter receptors occur often at the borders of cyto- and myeloarchitectonically defined cortical areas. This corroborates the functional meaning of the classical cyto- and myeloarchitectonical parcellations. 2. Some, but not all receptor subtypes of one transmitter system and receptors of different transmitter systems show similar laminar patterns (codistribution) and colocalizations at the single cell level. Both aspects are important for a functional interpretation of cortical units (areas, layers, etc.) and neuronal cell types, because the interaction between numerous transmitters and their receptors is the neurochemical substrate of neurotransmission in vivo. Codistributions and colocalizations are prerequisites for such interactions. 3. Overshooting alpha 1-adrenoceptor production, heterochronous development of this receptor in the different layers of a cortical area, and different time tables of cortical histogenesis, increase in transmitter concentrations and local receptor maturation demonstrate that the processes regulating the development of transmitter receptors during postnatal ontogeny cannot be directly deduced from histogenetic events or transmitter maturation in the cortex. 4. Receptors of one transmitter (e.g., alpha 1-adrenoceptors) can be specifically associated with a neuronal projection system (e.g., hippocampal mossy fibers), which is using a different transmitter (e.g., L-glutamate). This points to complex interactions between different transmitter systems in an anatomically defined structure. Functional and architectonical cortical units (e.g., "barrels") show specific receptor distributions, and hitherto unknown modular structures (e.g., periodical concentrations of alpha 1-adrenoceptors in layer V of the rat posteromedial barrel subfield) can be visualized by quantitative receptor autoradiography combined with image analysis.