Long-term estrogen and progesterone and mating stimuli as regulators of female sexual receptivity in the Mongolian gerbil

Species differences in the immunoreactive expression of oxytocin, vasopressin, tyrosine hydroxylase and estrogen receptor alpha in the brain of Mongolian gerbils (Meriones unguiculatus) and Chinese striped hamsters (Cricetulus barabensis)

Species differences in neurochemical expression and activity in the brain may play an important role in species-specific patterns of social behavior. In the present study, we used immunoreactive (ir) labeling to compare the regional density of cells containing oxytocin (OT), vasopressin (AVP), tyrosine hydroxylase (TH), or estrogen receptor alpha (ERα) staining in the brains of social Mongolian gerbils (Meriones unguiculatus) and solitary Chinese striped hamsters (Cricetulus barabensis). Multiple region- and neurochemical-specific species differences were found. In the anterior hypothalamus (AH), Mongolian gerbils had higher densities of AVP-ir and ERα-ir cells than Chinese striped hamsters. In the lateral hypothalamus (LH), Mongolian gerbils also had higher densities of AVP-ir and TH-ir cells, but a lower density of OT-ir cells, than Chinese striped hamsters. Furthermore, in the anterior nucleus of the medial preoptic area (MPOAa), Mongolian gerbils had higher densities of OT-ir and AVP-ir cells than Chinese striped hamsters, and an opposite pattern was found in the posterior nucleus of the MPOA (MPOAp). Some sex differences were also observed. Females of both species had higher densities of TH-ir cells in the MPOAa and of OT-ir cells in the intermediate nucleus of the MPOA (MPOAi) than males. Given the role of these neurochemicals in social behaviors, our data provide additional evidence to support the notion that species-specific patterns of neurochemical expression in the brain may be involved in species differences in social behaviors associated with different life strategies.

Hormonal regulation of agonistic and affiliative behavior in female mongolian gerbils (Meriones unguiculatus)

Ovarian steroids and oxytocin (OT) have been implicated in the regulation of social behaviors. The purpose of the present study was to examine hormonal substrates of aggression and affiliation in the female Mongolian gerbil (Meriones unguiculatus), a highly social, monogamous rodent. Sexually naive adult females were paired with sexually experienced males for 48 h and their interactions videotaped. Females were gonadally intact and tested during vaginal estrus (INT) or ovariectomized and observed after the following treatments, administered by means of sc injections: EBEB (7 days of estradiol-benzoate); EBP (2 days of EB followed by progesterone), SALEB (saline, days 1-5 then 2 days of EB), OTEB (OT for days 1-5 then 2 days of EB); OTOIL (OT for days 1-5 then 2 days of OIL); or SALOIL (saline days 1-5 then 2 days of OIL). During the first hour of pairing INT females displayed higher levels of affiliation and lower levels of sniffing and agonistic behavior than SALOIL females. All hormonal treatments reduced agonistic behaviors when compared to SALOIL, although none of the hormonal treatments restored affiliation to INT levels. During the 48-h test overt aggression varied by treatment with INT, EBEB, EBP, and OTEB females displaying lower levels than SALOIL, while all groups displayed similar levels of affiliation. The results indicate that OT and E play a significant role in regulating male-directed aggressive behavior in females and that the presence of ovarian hormones as well as OT can increase affiliation during initial contact. Over a sustained period of cohabitation social cues appear to be more important in regulating affiliation than gonadal hormones.

Harderian letdown in male mongolian gerbils (Meriones unguiculatus) contributes to proceptive behavior

Adult female gerbils in estrus, like other female rodents, tend to engage in proceptive displays toward conspecific males. The displays, which may be interspersed with the more usual female agonistic activity, if the males are strangers, are preceded by female olfactory investigation of those head areas in the male gerbil where Harderian letdown accumulates most densely. This study explored the possibility that the male Harderian glands are a source of olfactory signals which promote proceptive behavior but suppress female agonistic behavior. Female gerbils in estrus were found to display significantly less than normal rates of proceptive behavior toward Harderianectomized males. The proceptive activity which was observed appeared to be slowed, but the typical pattern was retained. Female aggression, however, was not affected by their estrous condition or by the Harderian state of the males. Possibly Harderian letdown in male gerbils may inform females as to the reproductive competence of the males.