A Study On The Neuroendocrine Mechanism Of Different Levels Of Sociality And Dominance Based On The Natural Occurrence Of Brown Voles

Sociability is important for social living animals, which is clearly beneficial for many species, often resulting in increased survival, enhanced fitness of the group, and progression of brain development and cognitive abilities. Previous studies about sociability mainly focus on comparative reseach of two different species and little attention has been paid to the intraspecific variation of sociability. Furthermore, the studies about intraspecific variation of sociability mainly focus on two separate populations, or comparative reseach of various treat, such as pharmacology, gene knockout and so on, but the intraspecific natural occuring variation of sociability and it underlying neuroendocrine mechanism are also poorly understood. Thus, the present study was divided into two parts. On one hand, through agonistic encounter tests, voles were divided into high attacking group and low attacking group, we aim to compare levels of sociability between male and female voles with different levels of aggression during agonistic encounter tests, and determine whether central nervous oxytocin(OT) and vasopressin (AVP) expression are affected by dominance and subordination in this species. On the other hand, through social preference tests, voles were divided into pro-social group and social avoidant group, we aimed to compare levels of central OT and hippocampus glucocorticoid receptor (GR) between male and female mandarin vole with different levels of social preference and social avoidance, and determine whether social preference and social avoidance effect emotional response in the open field tests and then explore whether Hypothalamic-pituitary-adrenal (HPA) axis is involved in the variation of social preference and aviodance.1. Comparison of sociability, central OT and AVP between voles with different levels of aggressionAgonistic encounter test:We found that both dominant male and female voles engaged in more approaching behavior, investigating behavior, self-grooming behavior and exploring behavior than subordinate ones, which means that male as well as female dominant voles display higher levels of sociality than subordinates. We also found that dominant males engaged in more investigating and approaching behavior in the first trial than in the third trial and showed a pattern of decline across trials. However, the investigating behavior and approaches of dominant females did not show a pattern of decline across trials, which means that males exhibit better social recognition abilities than females.Numbers of oxytocin-immunoreactivity (OT-ir) neurons:We found that the sex significantly affected the number of OT-ir neurons in the paraventricular nuclei of the hypothalamus (PVN), supraoptic nuclei of the hypothalamus (SON) and lateral hypothalamus (LH). However, we found that dominant males displayed significantly less OT-ir neurons than subordinate males in the PVN. Besides, dominant males displayed significantly less OT-ir neurons than dominant females in the PVN, SON and LH. Subordinate males expressed significantly fewer OT-ir neurons only in the PVN and LH compared to subordinate females.Numbers of vasopressin-immunoreactivity (AVP-ir) neurons:We found that dominance had a main effect on the number of AVP-ir neurons in the PVN, SON, LH and anterior hypothalamus (AH). Sex significantly affected the number of AVP-ir neurons in the SON, LH and AH. In addition, significant interactions between dominance and sex were found for OT-ir neurons in the PVN and SON. We found that dominant males displayed significantly more AVP-ir neurons than subordinate males in the PVN, SON, LH and AH. Dominant females exhibited significantly more AVP-ir neurons in the LH and AH compared to subordinate females. Besides, dominant males displayed significantly more AVP-ir neurons than dominant females in the PVN, SON, LH and AH. Subordinate males expressed significantly more AVP-ir neurons in the LH and AH compared to subordinate females.2. Comparison of central oxytocin and glucocorticoid receptor and serum adrenocorticotropic hormone and corticosterone between pro-social and social avoided mandarin voles.Social preference test:a significantly higher time of investigation of the social vs object stimulus was found in both males and females with high levels of sociability. In voles with low levels of sociability, no difference was found in time of investigation of the social vs object stimulus.Open field tests:We found that the sociability had a main effect on the percentage of time in central area, total distance and transitions, and the sex significantly affected the transitions. We also found that the percentage of time in central area and total distance were significantly higher in males with high levels of sociability compared with data from males with lower levels of sociability. The percentage of time in the central area was significantly higher in females with higher levels of sociability than females with lower ones.Elisa:We found that the sociability significantly affected the serum concentration of adrenocorticotropic hormone (ACTH) and corticosterone (CORT). We also found that both males and females with higher levels of sociability displayed significantly less serum ACTH and CORT. Sex differences were not found in serum concentration of ACTH and CORT.Numbers of OT-ir neurons:We found that the both sociability and sex significantly affected the number of OT-ir neurons in the PVN and SON. We also found that both males and females with higher levels of sociability displayed significantly more OT-ir neurons than voles with lower levels of sociability in the PVN and SON. Besides, males with both higher and lower levels of sociability displayed significantly less OT-ir neurons in the PVN and SON than females.Numbers of glucocorticoid receptor-immunoreactivity (GR-ir) neurons:We found that the sociability had a main effect on the number of GR-ir neurons in the dentate gyrus (DG), CA1and CA2-3. We also found that both males and females with higher levels of sociability displayed significantly more GR-ir neurons than voles with lower levels of sociability in the DG, CA1and CA2-3. Sex differences were not found in the number of GR-ir neurons in any brain regions.In summary, these results indicate that distinct parameters related to central nervous oxytocin and vasopressin are associated with behaviors during agonistic encounters in a sex-specific manner in mandarin voles. And natural occuring social preference and social avoidance are associated with levels of central oxytocin and hippocampus glucocorticoid receptor and the levels of serum adrenocorticotropic hormone and corticosterone.