Vasopressin (V1a) receptor binding, mRNA expression and transcriptional regulation by androgen in the Syrian hamster brain.

Arginine vasopressin plays an important role in the regulation of social behaviours in rodents. In the Syrian hamster, vasopressin injected directly into the brain stimulates scent marking and aggressive behaviour in a steroid dependent manner and is therefore a useful model for investigating steroid-peptide-behaviour interactions. In this study, we used in situ hybridization and radioligand binding assays on adjacent sections of hamster brains to compare the relative distribution of vasopressin (V1a) receptor mRNA and V1a receptor binding. V1a receptor mRNA and binding are abundant in the lateral septum, bed nucleus of the stria terminalis, medial preoptic nucleus, anterodorsal thalamus and suprachiasmatic nucleus. Moderate receptor binding and low levels of receptor mRNA are present in the central nucleus of the amygdala and a lateral zone from the medial preoptic area through the anterior hypothalamus. V1a receptor mRNA is anatomically more restricted in several areas compared to the ligand binding pattern, which is consistent with significant spread of receptor protein along neuronal processes. Comparison of V1a receptor ligand binding and mRNA in intact, castrated, and castrated-testosterone treated animals reveals that V1a receptors in the medial preoptic nucleus are regulated by androgen, most likely by an upregulation of V1a receptor gene expression in a cluster of neurones concentrated in the ventromedial part of this nucleus. This study confirms the presence of the V1a subtype of vasopressin receptors in behaviourally important regions of the hamster brain and suggests that transcriptional regulation by gonadal steroids may play a role in modulating behavioural sensitivity to vasopressin.

Conversion of testosterone to estradiol may not be necessary for the expression of mating behavior in male Syrian hamsters (Mesocricetus auratus).

Male sexual behavior is mediated in part by androgens, but in several species, mating is also influenced by estradiol formed locally in the brain by the aromatization of testosterone. The role of testosterone aromatization in the copulatory behavior of male Syrian hamsters is unclear because prior studies are equivocal. Therefore, the present study tested whether blocking the conversion of testosterone to estradiol would inhibit male hamster sexual behavior. Chronic systemic administration of the nonsteroidal aromatase inhibitor Fadrozole (2.0 mg/kg/day) for 5 or 8 weeks did not significantly increase mount latency or reduce mount frequency, intromission frequency, ejaculation frequency, or anogenital investigation relative to levels shown by surgical controls. However, Fadrozole effectively inhibited aromatase activity, as evidenced by the suppression of estrogen-dependent progesterone receptor immunoreactivity in the male hamster brain. The JZB39 anti-progesterone receptor antibody labeled significantly more neurons in brains of sham-treated hamsters than in brains of Fadrozole-treated hamsters. These data suggest that aromatization of testosterone to estradiol is not necessary for normal mating behavior in Syrian hamsters.

Injection of oxytocin into the medial preoptic-anterior hypothalamus increases ultrasound production by female hamsters.

Recent studies of hamsters have documented the facilitation of lordosis and other sociosexual responses by injections of oxytocin (OXT) into the medial preoptic area-anterior hypothalamus (MPOA-AH). These data suggest the regulation of social interaction and bonds by OXT. In turn, this suggests that OXT could act in the MPOA-AH to control other behaviors involved in the initiation or maintenance of social contact, including the ultrasonic vocalizations that female hamsters use to alert and attract potential mates. To test this possibility, we compared the ultrasound rates of 11 naturally estrous hamsters before and after injections of OXT (200 ng/200 nl of saline) or saline (200 nl) into the MPOA-AH. The data revealed a clear facilitation of ultrasound rate 30 min after OXT treatment. This result suggests the modulation of ultrasound rate by endogenous OXT acting within the MPOA-AH. It extends the range of social behaviors sensitive to control by OXT and supports the possibility that OXT acts within the MPOA-AH to facilitate a variety of behaviors involved in the establishment or maintenance of the social interactions required for successful reproduction. At the same time, these data extend earlier observations linking ultrasound production to the MPOA-AH, and begin to describe the peptidergic mechanisms controlling this form of reproductive behavior.

Glutamate and vasopressin interact to control scent marking in Syrian hamsters (Mesocricetus auratus).

In Syrian hamsters, vasopressin (AVP) in the medial preoptic-anterior hypothalamus (MPOA-AH) controls a form of scent marking called flank marking. Another neurochemical signal that may interact with AVP to control flank marking is glutamate. We tested the hypothesis that glutamate interacts with AVP in the MPOA-AH to regulate flank marking. On day 1, AVP was microinjected into the MPOA-AH. On day 2, AVP was microinjected as a cocktail combining either AP-5, a NMDA antagonist, or GAMS, a non-NMDA antagonist or propranolol, a beta norepinephrine antagonist. On day 3, AVP alone was microinjected. Hamsters engaged in high levels of marking in response to AVP alone or to a combination of AVP and propranolol. In contrast, the frequency of marking was significantly reduced in response to a combination of either AVP and AP-5 or AVP and GAMS. These data support the hypothesis that stimulation of flank marking by AVP within the MPOA-AH requires the activity of glutamate.

Effects of testosterone on the behavioral response to arginine vasopressin microinjected into the central gray and septum.

Arginine vasopressin (AVP) plays an important role in the control of a gonadal hormone-dependent communicative behavior in the Syrian hamster (Mesocricetus auratus) called flank marking. Previous studies have shown that gonadal hormones alter the amount of flank marking stimulated by the microinjection of AVP into the medial preoptic area-anterior hypothalamus (MPOA-AH). The purpose of the present study was to determine if testicular hormones alter the amount of flank marking stimulated by the microinjection of AVP into two other sites involved in the control of flank marking, the lateral septum-bed nucleus of the stria terminalis (LS-BNST) and the central gray. The data of the present study indicate that testicular hormones may influence the amount of AVP-stimulated marking in the central gray and LS-BNST; however, these effects are subtle and appear to occur primarily at high concentrations of AVP. When taken together with previous studies, these data indicate that gonadal hormones have greater effects on AVP-stimulated marking in the MPOA-AH than in the LS-BNST or central gray.