Effect of prolactin on the attractiveness of male odors to females in meadow voles: independent and additive effects with testosterone

Prosocial effects of prolactin in male rats: Social recognition, social approach and social learning

Prolactin (PRL) and oxytocin (OT) are pituitary hormones essential for lactation, but also promote sexual behavior. OT stimulates social behaviors, such as recognition, approach, and learning, but less is known about PRL in these behaviors. Since PRL and OT have complementary functions in reproduction, we hypothesized that PRL increases social recognition, approach, and learning. Male Long-Evans rats received ovine PRL (oPRL; 0.5, 2.0 or 5.0mg/kg), the PRL antagonist bromocriptine (0.1, 3.0 or 5.0mg/kg) or saline 20 mins before testing for recognition of familiar vs. unfamiliar stimulus males. Saline controls preferred the unfamiliar male (p<0.05), while bromocriptine blocked this preference. oPRL did not increase preference. To measure social approach, we determined if PRL restores approach 2h after defeat by an aggressive male. Defeated rats avoided the aggressive male. 2mg/kg oPRL, before or after defeat, restored approach towards the aggressive male (p<0.05). In non-defeated rats, oPRL or 3mg/kg bromocriptine had no effect. To determine if PRL increases social learning, we tested social transmission of food preference. Rats choose between two unfamiliar flavors, one of which they have previously been exposed to through interaction with a demonstrator rat. Vehicle controls preferred chow with the demonstrated flavor over the novel flavor. oPRL-treated rats were similar. Bromocriptine-treated rats failed to show a preference. When tested one week later, only oPRL-treated rats preferred the demonstrated flavor. The results suggest that PRL is required for social recognition and learning, and that increasing PRL enhances social memory and approach, similar to OT.

Photoperiodic and temporal influences on chemosensory induction of brain fos expression in female prairie voles

Prairie voles (Microtus ochrogaster) typically stop breeding during winter. Male prairie voles respond to winter day lengths with gonadal regression, whereas female voles are relatively unresponsive to photoperiod. Unlike commonly studied laboratory rodents, female prairie voles do not exhibit spontaneous oestrous cycles. Instead, females are induced into oestrus by chemosensory cues from conspecific male urine. The present study investigated the interaction among day length, chemosensory cues and the initial brain responses during oestrus induction in female voles. A single drop of male conspecific urine, saline or skimmed milk was applied to the nares of female prairie voles housed for 9 weeks in either long (LD 16 : 8 h) or short (LD 8 : 16 h) days. Animals were killed 0.5, 1, 2 or 24 h after chemosensory treatment and their brains were processed for Fos immunocytochemistry. Body mass and ovarian fat pad mass were higher, but uterine and ovarian mass were lower, in short-day compared to long-day females. Regardless of photoperiod, Fos- immunoreactivity increased in the granule layer of the accessory olfactory bulb (AOB), the supraoptic nucleus and bed nucleus of the stria terminalis (BNST) (anterior medial) in females treated with male urine compared to the two control groups. Fos staining intensified in the AOB, medial and posterocortical medial amygdala and BNST (posterior ventral), 1 h and 2 h after urine treatment. In the medial preoptic area, anterior and lateral hypothalamus, and ventromedial nucleus of the hypothalamus, Fos-immunoreactivity was elevated in females 2 h after receiving urine. Overall, long-day females displayed higher Fos expression in response to urine than females maintained in short days. These results identify a putative neural circuitry of oestrus induction in this species, and provide an approximate time line of activation in the brain circuit responsible for oestrus induction in prairie voles.