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Prior studies have reported associations between plasma oxytocin and vasopressin and markers of social functioning. However, because most human studies have used cross-sectional designs, it is unclear whether plasma oxytocin and vasopressin influences social functioning or whether social functioning modulates the production and peripheral release of these peptides. In order to address this question, we followed individuals who experienced major changes in social functioning subsequent to the migration to a new country. In this study, 59 new international students were recruited shortly after arrival in the host country and reassessed 2 and 5 months later. At each assessment participants provided information on their current social functioning and blood samples for oxytocin and vasopressin analysis. Results indicated that changes in social functioning were not related to changes in plasma oxytocin. Instead, baseline oxytocin predicted changes in social relationship satisfaction, social support, and loneliness over time. In contrast, plasma vasopressin changed as a function of social integration. Baseline vasopressin was not related to changes in social functioning over time. These results emphasize the different roles of plasma oxytocin and vasopressin in responses to changes in social functioning in humans.

It is well established that social environment, particularly isolation, has a significant impact on social behaviors and neuroendocrine responses. Estrogen receptor alpha (ERalpha) expression in limbic structures and associated nuclei is related to the display of social behaviors. We hypothesized that the stress of isolation would cause changes in the pattern of ERalpha expression in the brain. Using a highly social (typically monogamous and biparental) rodent species, the prairie vole (Microtus ochrogaster), we housed juvenile voles with a sibling, stranger or in isolation for either 4 days or 21 days. Housing manipulations began following weaning from parents and group housed siblings. Rodents may be especially sensitive to manipulations of their social environment during this juvenile period. In particular, female prairie voles are induced ovulators, reliant upon exposure to an unrelated male (male urine) to become reproductively active. ERalpha immunoreactivity was quantified in the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BST), ventromedial nucleus of the hypothalamus (VMH) and medial amygdala (MeA). Significantly fewer ERalpha immunoreactive (ERalpha-ir) cells were labeled in the MPOA and BST of females isolated for 21 days compared with stranger housed females. Non-significant differences were shown in the VMH and MeA of females. No differences were found in voles isolated for 4 days. These results suggest that female prairie voles may be more sensitive than males to manipulations of their social environment during the juvenile period.

Social experiences, both positive and negative, may influence cardiovascular regulation. Prairie voles (Microtus ochrogaster) are socially monogamous rodents that form social bonds similar to those seen in primates, and this species may provide a useful model for investigating neural and social regulation of cardiac function. Cardiac regulation has not been studied previously in the prairie vole. Radiotelemetry transmitters were implanted into adult female prairie voles under anesthesia, and electrocardiographic parameters were recorded. Autonomic blockade was performed using atenolol (8 mg/kg ip) and atropine methyl nitrate (4 mg/kg ip). Several variables were evaluated, including heart rate (HR), HR variability and the amplitude of respiratory sinus arrhythmia. Sympathetic blockade significantly reduced HR. Parasympathetic blockade significantly increased HR, and reduced HR variability and the amplitude of respiratory sinus arrhythmia. Combined autonomic blockade significantly increased HR, and reduced HR variability and respiratory sinus arrhythmia amplitude. The data indicate that autonomic function in prairie voles shares similarities with primates, with a predominant vagal influence on cardiac regulation. The current results provide a foundation for studying neural and social regulation of cardiac function during different behavioral states in this socially monogamous rodent model.

This paper examines the developmental effects of the mammalian neuropeptide, oxytocin (OT). In adults, OT is the most abundant neuropeptide in the hypothalamus and serves integrative functions, coordinating behavioral and physiological processes. For example, OT has been implicated in parturition, lactation, maternal behavior and pair bond formation. In addition, OT is capable of moderating behavioral responses to various stressors as well as the reactivity of the hypothalamic-pituitary-adrenal (HPA) axis. Neonates may be exposed to hormones of maternal origin, possibly including peptides administered to the mother in the perinatal period to hasten or delay birth and in milk; however, whether peptide hormones from the mother influence the developing infant remains to be determined. In rodents, endogenous OT is first synthesized during the early postnatal period, although its functions at this time are not well known. Experiments in neonatal prairie voles have documented the capacity of OT and OT receptor antagonists to have immediate and lifelong consequences for social behaviors, including adult pair bonding and parental behaviors, as well as the reactivity of the HPA axis; most of these effects are sexually dimorphic. Possible mechanisms for such effects, including long-lasting changes in OT and vasopressin, are summarized.

ROBERTS, R. L., A. S. ZULLO AND C. S. CARTER. Sexual differentiation in prairie voles: The effects of corticosterone and testosterone. PHYSIOL BEHAV 62(6) 1379–1383, 1997.—Prairie voles (Microtus ochrogaster) exhibit low levels of physical sexual dimorphism and have endogenous basal corticosterone levels that are 5–10 times higher than those measured in rats; prairie voles also do not show a postnatal period of adrenal hyporesponsivity. On the basis of studies in rats suggesting that adrenal hyperactivity during the perinatal period could reduce masculinization or feminize sexual behavior, we hypothesized that adrenal hormones might influence sexual differentiation in prairie voles. We also examined the hypothesis that the effects of testosterone in prairie voles might differ from those reported in other rodents. Treatments with either corticosterone or testosterone propionate (TP) were given prenatally (gestational Days 12-20), via maternal injection, or postnatally (Days 1-6), by directly injecting the pups. Additional groups of males were castrated or sham-operated on postnatal Day 1, and a subgroup of castrated males received postnatal TP. Male and female sexual behavior was observed in adulthood following gonadectomy and hormone treatments. Corticosterone treatment was associated with high levels of mounting in both sexes and did not inhibit lordosis behavior in females. Postnatal TP treatment inhibited lordosis in females but did not facilitate mounting in either sex. Males that were castrated at birth showed unexpectedly high levels of mounting in response to adult androgens. The results of this study suggest that in prairie voles corticosterone is capable of masculinizing without defeminizing sexual behavior, whereas postnatal testicular secretions are not essential for, and may actually inhibit, masculinization in this species.

The environmental factors that arrest breeding in prairie voles during the middle of the breeding season are unknown. The role of water availability on reproductive function was examined by limiting water intake to 50% of ad lib water consumption for 10 weeks. At autopsy, testicular, epididymal and seminal vesicle masses were reduced in water restricted males as compared to animals with ad lib access to water. Body mass was also reduced in water restricted males. Plasma testosterone levels and the number of testicular and epididymal sperm were significantly reduced in water restricted voles as compared to animals drinking water ad lib, but plasma levels of luteinizing hormone were unaffected. Taken together, these data suggest that reduced water availability can inhibit male prairie vole breeding.

Neonatal lesions of the cerebral cortex in female rats did not eliminate female sexual behavior as measured by lordosis. However, lordosis in response to prolonged low levels of estradiol benzoate (1.0 μg/day for 6 days) was attenuated in lesioned females. Following estradiol benzoate plus progesterone (0.5 mg) treatment the probability of lordosis increased markedly in the decorticate females, but still remained below control levels. Decorticate females were mounted by the male at least as often as control females. Hopping and darting and rejection behaviors on the part of the female were virtually eliminated in the decorticate group. However, these females continued to direct sniffing behavior toward the male at levels above those of the controls.

Sexual development of female prairie voles does not occur unless females are exposed to stimuli from an unfamiliar male. Three experiments were conducted to determine whether a saturable high-affinity estradiol binding site (ERN) is present in cell nuclei of brain tissue from females exposed to male stimuli and whether serum estradiol and brain ERN are correlated with the duration of male exposure. Brain ERN were detected and found to be correlated with serum estradiol levels. Significant increases in brain ERN were observed within 18 hours after females were exposed to a male for either a continuous (uninterrupted) or transient (1 1/4 hour) period. A significant increase in serum estradiol was observed for females given continuous male exposure. The rate of increase of ERN in brain was significantly higher for females given continuous as opposed to transient male exposure. The results suggest that the continued presence of a male can accelerate the rate of increase of ERN in the brain, presumably through increases in circulating estradiol.

In the induced ovulator, Microtus ochrogaster (the prairie vole), daily injections of estradiol benzoate (EB 0.5 μg or 5.0 μg) facilitated high levels of sexual receptivity in ovariectomized females withing 48 hr of the onset of treatment. A lower level of EB (0.05 μg) with or without progesterone (0.5 mg) for five days did not induce lordosis. Four hr after sexually receptive EB-primed females received progesterone (0.5 mg) they either continued to show lordosis (EB 5.0 μg group) or showed an inhibition of lordosis (EB 0.5 μg group). Within 24 hr lordosis was markedly reduced in both groups. Progesterone-associated inhibitions were no longer apparent 48 hr after progesterone was discontinued. This general pattern of response to ovarian hormones is consistent with data described for other induced ovulators.