INFLUENCE OF DIFFERENT LIGHTING REGIMEN ON THE MORPHOMETRIC CHARACTERISTICS OF NEURONS IN THE PARAVENTRICULAR NUCLEI OF THE RAT HYPOTHALAMUS
Authors: Bulik R.E. , Smetanyuk O.V. , Vlasova K.V. , Krivchanska M.I. 
Published: December 2021

Paraventricular nuclei (PVN) of the hypothalamus are the autonomic center forcoordination of functions and consists of a number of neuronal populations - sub-nuclei,which diﬀer in structural and functional features and the nature of nerve connectionswith various parts of the nervous and neuroendocrine systems. When studying stressreactions and actions of the stress-limiting factors, it is important to study subpopulationsof PVN neurons of the hypothalamus, synthesizing stress-releasing hormones, whichinitiate stress reactions of the body. One of the main factors that show a pronouncedeﬀect in the regulation of ACTH secretion is corticotropin-releasing factor (CRF). TheCRF-immunoreactive tracer was found mainly in the medial small-cell subnucleus of theparaventricular nuclei of the rat hypothalamus. Despite deep and comprehensive studiesof the hypothalamus, until now there are no unifed ideas about its individual reactivityand the degree of involvement of these structures into the stress reaction caused by theprolonged change in the lighting regime.Purpose - to fnd out the eﬀect of diﬀerent illumination regimes on the morphometriccharacteristics of the medial small-cell subnuclei of the paraventricular nuclei of thehypothalamus of mature and old rats.Material and methods. The experiments were conducted on 72 nonlinear white malerats, divided into 6 series of studies, in each of which the biomaterial sampling wascarried out at 14.00 and 02.00 h using morphometric and statistical research methods.Results. Morphometric assessment of the medial small-cell sub-nuclei of theparaventricular nuclei (msPVN) of the hypothalamus of rats revealed the diurnaldynamics of indices. It has been established that in both old and mature rats, theiraverage volume (p <0.05) signifcantly decreases at 2.00 compared to 14.00 undernormal lighting. The nuclear-cytoplasmatic ratio at 02.00 increases in comparison with14.00 respectively.When the animals were kept under constant illumination, more pronounced changesin the structures under study were established. In particular, the volume of neurons inthe msPVN of the hypothalamus decreases both mature and old rats compared withrats under standard illumination and even more in comparison with rats under lightdeprivation. In accordance with this, the NCR also changes. The average number ofneurons on the standard plane of the histological section also decreases in comparisonwith other illumination regimes. It is also necessary to point out that under conditionsof light stimulation in mature rats in msPVN, the average volume of neurons (p <0.05)noticeably decreases at 02:00 in comparison with 14:00, when the volume of theirnuclei does not change on average during the indicated study periods. This is reﬂected,respectively, in the NCR change, which increases at 02.00 in comparison with 14.00.As to the old rats, the average neuron volume does not decrease at 2.00 in comparisonwith 14.00 in msPVN neurons at light stimulation, as well as the volume of their neucleichanges, on average, is barely noticeable ,and according to it, NCR at 02.00 remainsstable in comparison with 14.00, and the average quantity of neurocytes in msPVN doesnot change on a standard plane of the histological section during various periods ofthe twenty four hours. At the same time, with various modifcations of the light regime,cytometric parameters of neurons under study in old rats, on average, are signifcantlylower (p <0.001) than those in mature rats.Conclusions. 1. The duration of the photoperiod signifcantly aﬀects the 24 hoursactivity of msPVN in the hypothalamus of mature and old rats. In particular, in bothinvestigated groups of rats, the average volume of neurons (p <0.05) reliably decreasesat 2.00 under normal illumination in comparison with 14.00, and, accordingly, the NCR at 02.00 increases compared to 14.00. All cytometric parameters of their neurons in oldrats are, on average, signifcantly lower (p <0.001) than in mature rats.2. Constant illumination causes more pronounced changes in the morphometricparameters of the msPVN of the hypothalamus than light deprivation. Thus, in maturerats in the msPVN, the average volume of neurons (p <0.05) decreases noticeably at02:00 in comparison with 14:00, which is reﬂected, respectively, in the change in NCR,which increases at 02:00 compared to 14:00. For old rats, under light stimulation, allthe studied cytometric parameters of their neurons, on average, are signifcantly lower(p <0.001) than those in mature rats.