Noradrenaline axon terminals in adult rat neocortex: an immunocytochemical analysis in serial thin sections

Peroxidase-antiperoxidase electron microscope immunocytochemistry with an antiserum against noradrenaline-glutaraldehyde-protein conjugate was used to identify cortical noradrenaline terminals (axonal varicosities) from the upper layers of the frontal, parietal and occipital cortex in adult rat. A large number of immunostained varicosities were examined in serial thin sections, and compared with a control population of randomly chosen unlabeled terminals from the same sections. Both groups of varicosities were measured and scrutinized for the presence of a junctional complex indicative of synaptic specialization. Cellular elements juxtaposed to the membrane of both types of varicosities were also identified and counted. Noradrenaline varicosities in all three cortical regions averaged 0.65 microns in diameter. In contrast to their unlabeled counterparts, these profiles rarely showed a membrane differentiation characteristic of a synaptic contact (junctional complex). The rare junctional complexes formed by cortical noradrenaline varicosities were invariably symmetrical and almost always found on dendritic shafts. The microenvironment of noradrenaline varicosities also differed, exhibiting a greater number of apposed axonal varicosities and a smaller number of dendritic spines than that of the random population. The proportion of noradrenaline varicosities making a synaptic contact (synaptic incidence) was determined by plotting the incidence of visible junctions as a function of the number of thin sections available for examination. As extrapolated for whole varicosities after linear transformation (double reciprocal plot), this proportion was 17% or 26% depending on the stringency of the criteria used in identifying the junctional complex. The same analysis provided a figure of 98% for the control population. The present study largely confirmed our initial radioautographic characterization of the cortical noradrenaline innervation as a mostly non-junctional system, and also indicated that these varicosities are set in a particular microenvironment. These new data further support the eventuality of a diffuse release of cortical noradrenaline in the extracellular space, compatible with both its neuromodulatory role and multiplicity of actions on diverse cellular targets in the cerebral cortex. The functions assigned to the coeruleocortical noradrenaline system must therefore be viewed as the product of a widespread and ubiquitously distributed neuronal organization characterized by loose intercellular relationships. This system might be capable of selectivity and specificity of action, however, owing to the distribution of its receptors, and in view of intrinsically or extrinsically driven control mechanisms triggered by the release of its own or other transmitters and which may also involve target-initiated feedback mechanisms.