Oxytocinergic neuronal systems during mating, pregnancy, parturition, and lactation

Oxytocin and Steroid Actions

Biosynthesis and secretion of the hypothalamic nonapeptide oxytocin largely depends on steroid hormones. Estradiol, corticosterone, and vitamin D seem to be the most prominent actors. Due to their lipophilic nature, systemic steroids are thought to be capable of crossing the blood-brain barrier, thus mediating central functions including neuroendocrine and behavioral control. The actual mode of action of steroids in hypothalamic circuitry is still unknown: Most of the oxytocinergic perikarya lack nuclear steroid receptors but express proteins suspected to be membrane receptors for steroids. Oxytocin expressing neurons contain enzymes important for intrinsic steroid metabolism. Furthermore, they produce and probably liberate specific steroid-binding globulins. Rapid responses to steroid hormones may involve these binding proteins and membrane-associated receptors, rather than classic nuclear receptors and genomic pathways. Neuroendocrine regulation, reproductive behaviors, and stress response seem to depend on these mechanisms.

Regulation of Oxytocin Secretion

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.

The prairie vole (Microtus ochrogaster): an animal model for behavioral neuroendocrine research on pair bonding

Pair bond formation has been investigated much less than many other social behaviors, perhaps in part because traditional laboratory mice and rats do not exhibit this behavior. However, pair bonding is common among monogamous animals such as the prairie vole (Microtus ochrogaster). In this review, we discuss how the prairie vole has been used as a model system to investigate the neurobiology of pair bonding. Descriptions include neuroanatomical differences between monogamous and non-monogamous voles, as well as how manipulations of vasopressin, oxytocin, dopamine, and corticosterone systems affect pair bond formation. Also summarized are potential interactions among these systems that regulate pair bonding, and the extent of sexual dimorphism in underlying mechanisms. Pair bonding in prairie voles is an excellent model system for studying central processing of social information. Understanding the mechanisms underlying this behavior may provide important insights into human disorders associated with impaired social functioning.

Nucleus accumbens oxytocin and dopamine interact to regulate pair bond formation in female prairie voles

Although oxytocin (OT) and dopamine (DA) have been implicated in pair bond formation in monogamous prairie voles (Microtus ochrogaster), the nature of potential interactions between these two neurochemical systems and the brain circuits important for such interactions in the regulation of pair bonding have not been explored. Here, we demonstrated that access to both OT and DA D2-type receptors is necessary for pair bond formation, as blockade of either type of receptor prevented partner preferences induced by OT or a D2-type agonist. We also demonstrated that the nucleus accumbens (NAcc) is a brain area important for such OT-DA interactions. In NAcc, blockade of OT receptors prevented partner preferences induced by a D2-type agonist whereas blockade of D2-type, but not D1-type, DA receptors blocked OT-induced partner preferences. Together, our data suggest that concurrent activation of OT and DA D2-type receptors in NAcc is essential for pair bond formation in female prairie voles.

Role of oxytocin and vasopressin in the transitions of weaning in the rat

Sucklings (18-day-old) and weanlings (35-day-old) were injected icv with oxytocin or its antagonist (both 0.5 microg/1 microl), or vasopressin (1.0 ng/1 microl) or its antagonist (100 ng/1 microl), prior to 4-min observation in a behavioral maze with a sibling in one box and their anesthetized dam in the other. Oxytocin abolished nipple attachment in sucklings, decreased time spent with the dam, and increased self-grooming. The oxytocin antagonist had little influence on behavior. Vasopressin increased self-grooming while its antagonist reduced passive contact with the dam, increased active contact with her, and increased exploration and activity. We conclude that these neuropeptides have diverse roles during weaning, maintaining sucklings' behavior or promoting weaning, and subserving the transition from attachment to the dam to independence from her. We propose that these neurochemicals, and others, mediate the neural, affiliative, and affective changes of weaning, and that the term "weaning" should be understood to encompass these behavioral transitions.

Sexual motivation promotes oxytocin secretion in male rats

The present study examines plasma oxytocin levels in relation to performance of copulatory behavior in male rats. The animals were divided into three groups: A) home-cage controls, B) sexually naive and C) sexually experienced. Following 15 min of sexual interactions with a sexually proceptive female, brought into estrus by sequential injections of estradiol benzoate (12.5 micrograms animal-1, -48 h) and progesterone (0.5 mg animal-1, -6 h), the male rats were decapitated. Trunk blood was collected for preparation of plasma samples, and subsequent radioimmunoassay for oxytocin. Home-cage controls, not exposed to a sexually proceptive female, were decapitated at the same time as experimental animals. It was found that plasma oxytocin levels were significantly elevated in sexually naive rats following exposure to a sexually proceptive female, and that plasma oxytocin levels were highly correlated with intensity of copulatory performance in these animals. In addition, it was also found that plasma prolactin and glucose levels were increased, regardless of sexual experience, in comparison with home-cage controls. It is concluded that the emotional challenge, and the situation-specific demands for action, created by an encounter with a sexually proceptive female, are accompanied by an increased plasma concentration of oxytocin in sexually naive, but not sexually experienced, male rats.

Plasticity in GABAA receptor subunit mRNA expression by hypothalamic magnocellular neurons in the adult rat

The magnocellular hypothalamic neurons exhibit a substantial degree of structural and functional plasticity over the time of pregnancy, parturition, and lactation. This study has used in situ hybridization techniques to examine whether the content of alpha 1, alpha 2, beta 2, gamma 2 GABAA receptor subunit mRNAs expressed by these cells fluctuates over this period. A process of regional, followed by cellular and then topographical, analyses within the supraoptic (SON) and posterior paraventricular (PVN) nuclei revealed that an increase in magnocellular alpha 1 subunit mRNA content occurred during the course of pregnancy up to day 19, after which a decline in expression was detected on the day of parturition. Significant fluctuations of this nature were observed only in the oxytocin neuron-enriched regions of the SON and PVN. The expression of alpha 2, beta 2, and gamma 2 subunit mRNAs in the SON and PVN and of all subunit mRNAs in the cingulate cortex did not change over this period. During lactation, gamma 2 subunit mRNA content within the PVN increased significantly on day 14 of lactation as compared with day 7, and topographical analysis suggested that it involved principally magnocellular vasopressin neurons. These results demonstrate the cell-and subunit-specific regulation of GABAA receptor mRNA expression within the hypothalamic magnocellular system. In particular, they suggest that fluctuations in alpha 1 subunit expression may contribute to the marked variations in electrical activity exhibited by magnocellular oxytocin neurons at the time of parturition. More generally, they provide evidence in support of GABAA receptor plasticity within a physiological context in the adult rat brain.

Estimation of oxytocin mRNA in the human paraventricular nucleus in AIDS by means of quantitative in situ hybridization

Recently, a 40% reduction in the total of oxytocin immunoreactive neurons of the paraventricular nucleus of the hypothalamus in AIDS patients was reported. In the present study, we determined whether this decrease is associated with a diminished amount of oxytocin mRNA. We used in situ hybridization combined with densitometric image analysis for the quantitative assessment of oxytocin gene expression in the paraventricular nucleus of a group of AIDS patients (n=10) and a carefully matched control group (n=8). We found no significant difference (P=0.08) in the amount of oxytocin mRNA per total paraventricular nucleus between the two groups. In addition, no significant differences were found in the part of the volume of the paraventricular nucleus that was occupied by hybridized cells (P=0.12) or in the mean signal density (P=0.08). The findings do not support the hypothesis that the extensive decrease in oxytocin immunoreactive neurons of the paraventricular nucleus in AIDS is associated with a decrease in total oxytocin mRNA content in this nucleus. The data are compatible with the suggestion that in AIDS the biosynthesis of oxytocin is changed in an unknown way at the (post)transcriptional level.