Background. Stressful influences, depending on their severity and duration, can cause the development of pathological conditions. Repeated episodes of stress cause functional and structural changes in the central nervous system and can cause the development of depressive conditions. Depression is one of the leading mental illnesses. One of the most stress-sensitive brain structures is the hippocampus. Objective. To study is to evaluate structural changes in the hippocampus, which is considered as a heterogeneous structure with separate dorsal and ventral regions, to evaluate the expression of inducible nitric oxide synthase, endothelial nitric oxide synthase, serine racemase, synaptophysin in a mild stress model.Methods. A study of the effects of mild stress was carried out on 16 adult male Wistar rats (age 12 months, body weight 350–400 g). After acclimatization, the rats were divided into two equal groups (n = 8): intact (control) and stressed. When keeping animals, modeling and removing them from the experiment, we were guided by the Regulations for Carrying Out Work Using Laboratory Animals and the Declaration of Helsinki. Experimental modeling of depression in animals was induced by mild stress exposure for 7 days (30 minutes daily). Euthanasia was performed in a CO2 incubator. The brain was fixed in neutral buffered 10% formalin. Paraffin sections were made in the frontal plane, stained with hematoxylin and eosin, thionin using the Nissl method and examined at a level from –2.40 to –3.96 mm relative to bregma using an Axio Lab A1 microscope (Carl Zeiss Microscopy GmbH, Germany). Photo documentation was carried out with an AxioCam 105 color camera (Carl Zeiss Microscopy GmbH, Germany). Using the Image Analysis module of the ZEN 1.1.2.0 program (Carl Zeiss Microscopy GmbH, Germany) in the pyramidal layer of the hippocampus. Statistical analysis was performed with Microsoft Office Excel 2016 (Microsoft, USA) and Prism 6 (GraphPad Software Inc., USA). Comparisons of two conditions were made by nonparametric Mann-Whitney-U test to avoid a statistical bias of unequal data distribution. The level of significance was set at p < 0.05. The summarized data were presented as a as mean ± standard error of mean.Results. Functional research methods and assessment of pathological changes in hippocampal neurons are presented. An increase in the relative number of wrinkled hyperchromatic pyramidal neurons in the dorsal cornu ammonis field 3 in stressed rats was noted by 23.6% (p < 0.05) compared to the control. There was an increase in the relative number of inducible nitric oxide synthase-immunopositive neurons in the dorsal cornu ammonis field 3 by 40% ( p < 0.05) and the relative area of inducible nitric oxide synthase-immunoreactive material by 35% (p < 0.05) in the pyramidal layer of cornu ammonis field 3 in stressed rats. A decrease in the relative area of synaptophysin-immunopositive material in stressed rats was found in the ventral cornu ammonis field 3 compared to the control group by 16.8% (p < 0.05); decrease in the relative area of serine racemase-immunopositive material in dorsal cornu ammonis field 3 by 4.3% (p < 0.05) and ventral cornu ammonis field 3 by 7.8% (p < 0.05).Conclusion. The results of the study demonstrate that mild stress is an adequate model of depression in rats. In animals exposed to mild stress, pronounced morphological signs of damage to hippocampal neurons were revealed; motor and indicative exploratory activity decreases. Differences were found in morphofunctional changes in the dorsal and ventral parts of the hippocampus under the influence of mild stress. In cornu ammonis field 3 of the dorsal hippocampus, in contrast to the ventral section, more pronounced signs of damage to pyramidal layer neurons were observed. The increase in the relative number of inducible nitric oxide synthase-immunopositive neurons and the relative area of inducible nitric oxide synthase-immunoreactive material in the cornu ammonis field 3 pyramidal layer in stressed rats indicates an increase in nitric oxide production and the participation of nitrooxide-dependent free radical mechanisms of damage to hippocampal neurons. The decrease in the relative area of synaptophysin-immunoreactive material in stressed rats may contribute to changes in synaptic plasticity. A decrease in the relative area of serine racemase-immunoreactive material in the dorsal and ventral parts of cornu ammonis field 3 is considered to be a sign of a possible decrease in N-methyl-D-aspartate-dependent neurotransmission in the hippocampus under stress.
