Age specific features of nNOS immunoreactive neurons in rat neocortex

The aim of the study was to evaluate the morphological features of nNOS-positive (nNOS-IR) neurons in the dorsolateral cortex of the frontal lobe of the cerebral hemispheres in albino rats during 180 days of postnatal development.Material and methods. The study was performed on 40 outbred white Wistar rats of different ages, from 1 to 180 days. The object of the study was an area of the right cerebral hemisphere on the dorsolateral surface near the frontal pole (neocortex). On paraffin serial sections of the frontal lobe, an immunohistochemical reaction was performed with antibodies to nNOS and a detection system with horseradish peroxidase. Neuronal morphometry was performed by microphotographs using the ImageJ-Fiji (NIH) 1.51h program, measuring the sectional area of the  neuron   body,  the  area  of the  nucleus,  the  nuclear-cytoplasmic  ratio,  and  the  intensity  of the  reaction.The significance of differences was assessed by paired Student's t-test.Results. It was found that in mature rats in the frontal lobe cortex nNOS-IR was detected in large multi-polar cells with high activity of the enzyme located in the supragranular layers, spindle-shaped cells with long positive processes at the border with the white matter (type 1), and two varieties of low-positive neurons – accumulations in the VI layer and single ones in other layers (type 2). Polymorphism of nNOS-IR neurons manifests from the birth, but it was possible to distinguish all subpopulations only from the 21st day. Each subpopulation is distinguished by its own age dynamics of the studied parameters and the nature of the distribution of positivity. In addition, in 3–7 day old rat pups, numerous small neurons at the border of the cortex and white matter have transient immunoreactivity.Conclusion. Thus, the division of nNOS-IR neurons into two morphological types proposed in the works of predecessors does not correspond to the number of subpopulations that could be described in the dorsolateral region of the prefrontal cortex in rats. This diversity of nNOS-IR neurons is consistent with the numerous functions described for nitric oxide. For an objective characterization of various classes of nNOS-IR cortical interneurons, it is necessary to use additional data obtained from transcriptomic, histological, electrophysiological, and functional experiments, taking into account species, topographic, and age features. Only an extended approach will make it possible to selectively influence different types of cells and reasonably interpret the results of experimental studies.