Early bi-parental separation or neonatal paternal deprivation in mandarin voles reduces adult offspring paternal behavior and alters serum corticosterone levels and neurochemistry

Although the effect of early social environments on maternal care in adulthood has been examined in detail, few studies have addressed the long-term effect on paternal care and its underlying neuroendocrine mechanisms. Here, using monogamous mandarin voles (Microtus mandarinus) that show high levels of paternal care, the effects of early bi-parental separation (EBPS) or neonatal paternal deprivation (NPD) on adult paternal behavior, serum corticosterone levels, and receptor mRNA expression in the nucleus accumbens (NAcc) and medial preoptic area (MPOA) were investigated. Compared to the parental care group (PC), we found that EBPS reduced crouching behavior and increased inactivity, self-grooming, and serum corticosterone levels in adult offspring; and NPD significantly reduced retrieval behavior and increased self-grooming behavior of offspring at adulthood. EBPS displayed more dopamine type I receptor (D1R) mRNA expression in the NAcc, but less oxytocin receptor (OTR) mRNA expression than PC in the MPOA. Both EBPS and NPD exhibited more mRNA expression of estrogen receptor alpha (ERα) than PC in the MPOA. In the EBPS group, increased serum corticosterone concentration was closely associated with reduced crouching behavior, and reduced expression of OTR was closely associated with altered crouching behavior and increased D1R expression. Our results provide substantial evidence that EBPS or NPD has long-term consequences and reduces paternal behavior in adult animals. Importantly the oxytocin system in the MPOA might interact with NAcc dopamine systems to regulate paternal behavior and EBPS may affect interactions between the MPOA and NAcc.

Neonatal paternal deprivation impairs social recognition and alters levels of oxytocin and estrogen receptor α mRNA expression in the MeA and NAcc, and serum oxytocin in mandarin voles

Paternal care is necessary for the healthy development of social behavior in monogamous rodents and social recognition underpins social behavior in these animals. The effects of paternal care on the development of social recognition and underlying neuroendocrine mechanisms, especially the involvement of oxytocin and estrogen pathways, remain poorly understood. We investigated the effects of paternal deprivation (PD: father was removed from neonatal pups and mother alone raised the offspring) on social recognition in mandarin voles (Microtus mandarinus), a socially monogamous rodent. Paternal deprivation was found to inhibit the development of social recognition in female and male offspring according to a habituation-dishabituation paradigm. Paternal deprivation resulted in increased inactivity and reduced investigation during new encounters with other animals. Paternal deprivation reduced oxytocin receptor (OTR) and estrogen receptor α (ERα) mRNA expression in the medial amygdala and nucleus accumbens. Paternal deprivation reduced serum oxytocin (OT) concentration in females, but had no effect on males. Our results provide substantial evidence that paternal deprivation inhibits the development of social recognition in female and male mandarin voles and alters social behavior later in life. This is possibly the result of altered expression of central OTR and ERα and serum OT levels caused by paternal deprivation.

Early social deprivation impairs pair bonding and alters serum corticosterone and the NAcc dopamine system in mandarin voles

Early life stress has a long-term negative impact on emotion, learning, memory and adult sexual behavior, and these deficits most likely impair pair bonding. Here, we investigated whether early social deprivation (ED) affects the formation of pair bonds in socially monogamous mandarin voles (Microtus mandarinus). In a partner preference test (PPT), ED-reared adult females and males did not show a preference for their partner, spent more time exploring the cage of an unfamiliar animal and directed high levels of aggression toward unfamiliar animals. In social interaction test, ED increased exploring behavior only in females, but increased movement around the partner and reduced inactivity in both males and females. Three days of cohabitation did not alter serum corticosterone levels in ED-reared males, but increased corticosterone levels in males that received bi-parental care (PC). Interestingly, serum corticosterone levels in ED- and PC-reared females declined after cohabitation. ED significantly increased basal serum corticosterone levels in males, but had no effect on females. ED significantly up-regulated the levels of dopamine and the mRNA expression of dopamine 1-type receptor (D1R) in the nucleus accumbens (NAcc) in females and males. ED suppressed dopamine 2-type receptor mRNA (D2R) expression in females, but increased this in males. After three days of cohabitation, levels of D1R mRNA and D2R mRNA expression changed in opposite directions in PC-reared voles, but in the same direction in ED-reared males, and only the expression of D2R mRNA increased in ED-reared females. Our results indicate that early social deprivation inhibits pair bonding at adulthood. This inhibition is possibly associated with sex-specific alterations in serum corticosterone, levels of dopamine and mRNA expression of two types of dopamine receptors in the NAcc.