Absence of vasopressin expression by galanin neurons in the golden hamster: implications for species differences in extrahypothalamic vasopressin pathways

Vasopressin and oxytocin receptor systems in the brain: Sex differences and sex-specific regulation of social behavior

The neuropeptides vasopressin (VP) and oxytocin (OT) and their receptors in the brain are involved in the regulation of various social behaviors and have emerged as drug targets for the treatment of social dysfunction in several sex-biased neuropsychiatric disorders. Sex differences in the VP and OT systems may therefore be implicated in sex-specific regulation of healthy as well as impaired social behaviors. We begin this review by highlighting the sex differences, or lack of sex differences, in VP and OT synthesis in the brain. We then discuss the evidence showing the presence or absence of sex differences in VP and OT receptors in rodents and humans, as well as showing new data of sexually dimorphic V1a receptor binding in the rat brain. Importantly, we find that there is lack of comprehensive analysis of sex differences in these systems in common laboratory species, and we find that, when sex differences are present, they are highly brain region- and species-specific. Interestingly, VP system parameters (VP and V1aR) are typically higher in males, while sex differences in the OT system are not always in the same direction, often showing higher OT expression in females, but higher OT receptor expression in males. Furthermore, VP and OT receptor systems show distinct and largely non-overlapping expression in the rodent brain, which may cause these receptors to have either complementary or opposing functional roles in the sex-specific regulation of social behavior. Though still in need of further research, we close by discussing how manipulations of the VP and OT systems have given important insights into the involvement of these neuropeptide systems in the sex-specific regulation of social behavior in rodents and humans.

Species, sex and individual differences in the vasotocin/vasopressin system: relationship to neurochemical signaling in the social behavior neural network

Arginine-vasotocin (AVT)/arginine vasopressin (AVP) are members of the AVP/oxytocin (OT) superfamily of peptides that are involved in the regulation of social behavior, social cognition and emotion. Comparative studies have revealed that AVT/AVP and their receptors are found throughout the "social behavior neural network (SBNN)" and display the properties expected from a signaling system that controls social behavior (i.e., species, sex and individual differences and modulation by gonadal hormones and social factors). Neurochemical signaling within the SBNN likely involves a complex combination of synaptic mechanisms that co-release multiple chemical signals (e.g., classical neurotransmitters and AVT/AVP as well as other peptides) and non-synaptic mechanisms (i.e., volume transmission). Crosstalk between AVP/OT peptides and receptors within the SBNN is likely. A better understanding of the functional properties of neurochemical signaling in the SBNN will allow for a more refined examination of the relationships between this peptide system and species, sex and individual differences in sociality.

Vasopressin innervation of the mouse (Mus musculus) brain and spinal cord

The neuropeptide vasopressin (AVP) has been implicated in the regulation of numerous physiological and behavioral processes. Although mice have become an important model for studying this regulation, there is no comprehensive description of AVP distribution in the mouse brain and spinal cord. With C57BL/6 mice, we used immunohistochemistry to corroborate the location of AVP-containing cells and to define the location of AVP-containing fibers throughout the mouse central nervous system. We describe AVP-immunoreactive (-ir) fibers in midbrain, hindbrain, and spinal cord areas, which have not previously been reported in mice, including innervation of the ventral tegmental area, dorsal and median raphe, lateral and medial parabrachial, solitary, ventrolateral periaqueductal gray, and interfascicular nuclei. We also provide a detailed description of AVP-ir innervation in heterogenous regions such as the amygdala, bed nucleus of the stria terminalis, and ventral forebrain. In general, our results suggest that, compared with other species, the mouse has a particularly robust and widespread distribution of AVP-ir fibers, which, as in other species, originates from a number of different cell groups in the telencephalon and diencephalon. Our data also highlight the robust nature of AVP innervation in specific regulatory nuclei, such as the ventral tegmental area and dorsal raphe nucleus among others, that are implicated in the regulation of many behaviors.

The bed nucleus of the stria terminalis in the Syrian hamster (Mesocricetus auratus): absence of vasopressin expression in standard and wild-derived hamsters and galanin regulation by seasonal changes in circulating sex steroids

The bed nucleus of the stria terminalis (BNST) is a nucleus of the forebrain highly sensitive to sex steroids and containing vasopressin neurons implicated in several social- and reproduction-related behaviours such as scent-marking, aggression, pair bonding and parental behaviour. Sexually dimorphic vasopressin expression in BNST neurons has been reported in almost all rodents, with the notable exception of the Syrian hamster. In this species, vasopressin expression is completely absent in the BNST. Because almost all Syrian hamsters used in research are derived from a very small breeding stock captured in 1930, we compared commercially available Syrian hamsters with a recently captured, wild-derived breeding stock. We checked for vasopressin expression using in situ hybridization and immunohistochemistry. Vasopressin expression in BNST neurons was completely absent in both breeding stocks, confirming the absence of BNST vasopressin expression in Mesocricetus auratus and ruling out a breeding artefact. Because vasopressin expression in BNST neurons appears to be strictly dependent on circulating sex steroids, the absence of vasopressin expression in Syrian hamster BNST neurons might be due to an insensitivity of these neurons to sex steroids. BNST vasopressin neurons also express galanin. Although galanin expression in the BNST is not sexually dimorphic in the Syrian hamster, it appears to be regulated by sex steroids. In the Djungarian hamster, photoperiodically driven seasonal variations of circulating sex steroids result in a seasonal rhythm of galanin expression in BNST neurons. We analysed the sex steroid dependence of galanin expression in the Syrian hamster. Castration and short photoperiod-induced sexual quiescence both resulted in downregulation of galanin mRNA in cell bodies (BNST) and immunoreactivity in the fibres (lateral septum). Testosterone supplementation of short photoperiod-adapted animals was able to restore galanin expression. Thus Syrian hamster BNST neurons respond to circulating sex steroids and their seasonal variations as observed in other rodent species.

Distribution of vasopressin in the brain of the eusocial naked mole-rat

Naked mole-rats are eusocial rodents that live in large subterranean colonies in which one queen breeds with one to three males. All other animals are nonbreeding subordinates. The external features of male and female subordinates, including their genitalia, are remarkably monomorphic, as is their behavior. Because vasopressin (VP) is associated with social behaviors and sex differences in other species, its distribution in naked mole-rats was of interest. We used immunohistochemistry to examine VP in the brains of subordinate and breeding naked mole-rats of both sexes. As in other mammals, VP-immunoreactive (-ir) somata were found in the paraventricular (PVN) and supraoptic nuclei (SON) and VP-ir projections from these nuclei ran through the internal and external zone of the median eminence. However, naked mole-rats had very few VP-ir cells in the bed nucleus of the stria terminalis (BST) and none in the suprachiasmatic nucleus (SCN); the extensive network of fine-caliber VP-ir fibers usually seen in projection sites of the BST and SCN were also absent. Equally unexpected was the abundance of large-caliber VP-ir fibers in the dorsomedial septum. VP immunoreactivity was generally similar in all groups, with the exception of VP-ir cell number in the dorsomedial hypothalamus (DMH). Breeders had a population of labeled cells in the DMH that was absent, or nearly absent, in subordinates. Future studies on the function of VP in these areas are needed to determine how the atypical distribution of VP immunoreactivity relates to eusociality and the unusual physiology of naked mole-rats.

Distribution of vasopressin in the forebrain of spotted hyenas

The extreme virilization of the female spotted hyena raises interesting questions with respect to sexual differentiation of the brain and behavior. Females are larger and more aggressive than adult, non-natal males and dominate them in social encounters; their external genitalia also are highly masculinized. In many vertebrates, the arginine vasopressin (VP) innervation of the forebrain, particularly that of the lateral septum, is associated with social behaviors such as aggression and dominance. Here, we used immunohistochemistry to examine the distribution of VP cells and fibers in the forebrains of adult spotted hyenas. We find the expected densely staining VP immunoreactive (VP-ir) neurons in the paraventricular and supraoptic nuclei, as well as an unusually extensive distribution of magnocelluar VP-ir neurons in accessory regions. A small number of VP-ir cell bodies are present in the suprachiasmatic nucleus and bed nucleus of the stria terminalis; however, there are extensive VP-ir fiber networks in presumed projection areas of these nuclei, for example, the subparaventricular zone and lateral septum, respectively. No significant sex differences were detected in the density of VP-ir fibers in any area examined. In the lateral septum, however, marked variability was observed. Intact females exhibited a dense fiber network, as did two of the four males examined; the two other males had almost no VP-ir septal fibers. This contrasts with findings in many other vertebrate species, in which VP innervation of the lateral septum is consistently greater in males than in females.

Sexual differentiation of central vasopressin and vasotocin systems in vertebrates: different mechanisms, similar endpoints

Vasopressin neurons in the bed nucleus of the stria terminalis and amygdala and vasotocin neurons in homologous areas in non-mammalian vertebrates show some of the most consistently found neural sex differences, with males having more cells and denser projections than females. These projections have been implicated in social and reproductive behaviors but also in autonomic functions. The sex differences in these projections may cause as well as prevent sex differences in these functions. This paper discusses the anatomy, steroid dependency, and sexual differentiation of these neurons. Although the final steps in sexual differentiation of vasopressin/vasotocin expression may be similar across vertebrate species, what triggers differentiation may vary dramatically. For example, during development, estrogen masculinizes vasopressin expression in rats but feminizes its counterpart in Japanese quail. Apparently, nature consistently finds a way of maintaining sex differences in vasopressin and vasotocin pathways, suggesting that the function of these differences is important enough that it was conserved during evolution.