Neuroendocrine Responses To Social Isolation And Paternal Deprivation During Different Postnatal Period In Mandarin Voles(Microtus Mandarinus)

The main characteristic of the postnatal development of the stress system in the rat is the so-called stress hypo-responsive period (SHRP). Lasting from postnatal day (pnd)4to pnd14, this period is characterized by very low basal corticosterone levels and an inability of mild stressors to induce an enhanced pituitary adrenocorticotropin (ACTH) and corticosterone release. During the recent years, the voles has become a generally used animal in social stress research, also due to the various social organizations in this genus. However, very few data are available on the ontogeny of the stress system in the monogamous voles. This study therefore describes the postnatal ontogeny of the hypothalamic-pituitary-adrenal (HPA) axis in mandarin voles (Microtus mandarinus).We measured pituitary adrenocorticotropin ACTH and corticosterone (CORT) in blood and also assessed the effects of stress on the neuropeptides arginine vasopressin (AVP) and oxytocin (OT) in the brain at postnatal days4,8,10,12,14and16. Based on the previous experiment, we choose the pups at pnd8,10,12and14to detect the HPA axis system response to stress after paternal deprivation. In this part we also examined pituitary adrenocorticotropin (ACTH) and corticosterone (CORT) in blood and assessed the effects of stress on the neuropeptides arginine vasopressin (AVP) and oxytocin (OT) in the brain. Our previous studies on mandarin voles have showed that neonatal isolation and paternal deprivation affect behaviors and brain functions of adult offspring via alteration in HPA axis and neuropeptides system of offspring. But, whether paternal deprivation affects offspring behavior and brain functions via alteration of early stress hormone or neuropeptides system also remains unclear. So, this study is aimed to detect the effects of social isolation and paternal deprivation on stress hormone and neuropeptides system during postnatal period in mandarin voles.1. Test the SHRP in mandarin volesCORT:At pnd4,8,10and12, basal corticosterone concentrations generally exhibited at a high level, but the levels were unstable. At pnd4,30min isolation could induce an increasing of serum CORT. But from pnd8to pnd12, isolation stress did not produce significant effect on serum CORT. At pnd14,16and18, the basal level of CORT was stable and30min of isolation stress did elicit a significant CORT response.ACTH:Adult-like basal ACTH levels were present at pnd4and30min isolation induced increasing of serum ACTH levels. But30min isolation did not alter ACTH levels significantly at any of other age. From pnd8to pnd12,30min isolation did not result in a change in ACTH levels. However, at pnds14,16and18,30min isolation did significantly increase serum concentrations of ACTH.AVP:the AVP-ir expression in PVN was relatively high and30min isolation did significantly increase AVP-ir density at pnd4. At pnd8, pnd10and pnd12, the basal levels of AVP-ir expression was lower than pnd4, but30min isolation also could increase the levels of AVP-ir expression. From pnd14to pnd18,30min isolation did not result in a significant change in AVP-ir expression.AVP-ir in the SON exhibited that the basal level from pnd4to pnd12was relatively low but after30min isolation, AVP-ir was significantly increased. At pnd14, the basal level expression of AVP increased and AVP-ir in the SON did not change significantly after30min isolation, as well as at pnd16and18.OT:the basal levels of OT-ir density in PVN gradually increased almost until pnd14, and the increases of OT-ir expression induced by30min isolation was only presented at pnd4and pnd8. At pnd16and pnd18, pups in the isolate condition had a lower OT-ir density in the PVN, but did not show significant changes.At pnd4isolation increased OT-ir neurons in the SON. The basal level increased by the age from pnd4to pnd12and almost until pnd14, the basal level of OT-ir density no longer increased. From pnd8to pnd18, isolation did not produce any changes in OT-ir neurons in the SON.2. Effects of paternal deprivation on stress hormones and peptides systemCORT:At pnd8, paternal deprivation did not produce significant effect on CORT level and saline injection did not further increase the circulating CORT level. But at pnds10,12and14after father deprivation we observed an elevation in basal CORT concentration. Further more, at these ages CORT level in pups showed statistically significant increase in response to saline injection.ACTH:At all ages, paternal deprivation significantly increased the basal level of ACTH and at pnd8, ACTH level in paternal deprived pups showed a slight, statistically significant increase and no ACTH response to saline injection occurred. After paternal deprivation, pups received saline injection showed a significant increase at pnd10,12 and14.AVP:paternal deprivation had a great effect on AVP-ir density in the PVN. At all ages, paternal deprived pups showed significantly higher AVP-ir density than those control groups. At pnd8, saline injection did not further increase the AVP-ir density in PVN, but at pnd10,12and14, AVP-ir densities in PVN were up-regulated by saline injection, however, the increases ranges decreased gradually.Paternal deprivation did not affect AVP-ir density in SON. Moreover, the pups received paternal deprivation and then followed with saline injection had a great increase in AVP-ir density in SON at pnd8,10,12and14.OT:paternal deprivation significantly decreased the OT-ir density in PVN at pnd10,12and14. There was lower density OT-ir in paternal deprivation pups than control group at pnd8, but the difference was not significant. At all ages, the pups received paternal deprivation and then followed with saline injection did not produce significant difference of changes in OT-ir density in PVN.Paternal deprivation significantly decreased OT-ir density compared with the control groups at pnd12and14, but saline injection did not further change the OT-ir density in SON. At pnd8, there was no significant difference between the control and paternal deprivation groups. However, saline injection could induce a significant increase in OT-ir density in paternal deprivation groups only at pnd8.In summary, this experiment clearly demonstrates that the postnatal development of HPA axis in the mandarin vole can be subdivided into two development phases:the SHRP and outside the SHRP (from pnd14afterwards.) and the early development of HPA axis can be regulated by paternal factors.