Estrogen-sensitive oxytocin binding sites are differently regulated by progesterone in the telencephalon and the hypothalamus of the rat

Hypothalamic Perineuronal Nets Are Regulated by Sex and Dietary Interventions

Perineuronal nets (PNNs) are widely present in the hypothalamus, and are thought to provide physical protection and ion buffering for neurons and regulate their synaptic plasticity and intracellular signaling. Recent evidence indicates that PNNs in the mediobasal hypothalamus play an important role in the regulation of glucose homeostasis. However, whether and how hypothalamic PNNs are regulated are not fully understood. In the present study, we examined whether PNNs in various hypothalamic regions in mice can be regulated by sex, gonadal hormones, dietary interventions, or their interactions. We demonstrated that gonadal hormones are required to maintain normal PNNs in the arcuate nucleus of hypothalamus in both male and female mice. In addition, PNNs in the terete hypothalamic nucleus display a sexual dimorphism with females higher than males, and high-fat diet feeding increases terete PNNs only in female mice but not in male mice. On the other hand, PNNs in other hypothalamic regions are not influenced by sex, gonadal hormones or dietary interventions. In summary, we demonstrated that hypothalamic PNNs are regulated in a region-specific manner and these results provide a framework to further investigate the potential functions of PNNs in regulating energy/glucose homeostasis at the interplay of sex, gonadal hormones and diets.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

The role of oxytocin in mating and pregnancy

The hormone oxytocin (OT) is released both centrally and peripherally during and after mating. Although research in humans suggests a central role in sexuality, the most reliable findings to date involve peripheral activation. This review will discuss these results and will particularly focus on understanding the most recent findings from fMRI data and the effects of exogenous peripheral OT administration. We will then consider hypotheses of the roles played by central and systemic OT release as well as their control and modulation in the female, summarizing recent findings from animal research. Finally, we will discuss the contribution of OT to the initiation of pregnancy in rodents. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Corticosterone release in oxytocin gene deletion mice following exposure to psychogenic versus non-psychogenic stress

Both anxiety-related behavior [J.A. Amico, R.C. Mantella, R.R. Vollmer, X. Li, Anxiety and stress responses in female oxytocin deficient mice, J. Neuroendocrinol. 16 (2004) 1-6; R.C. Mantella, R.R. Vollmer, X. Li, J.A. Amico, Female oxytocin-deficient mice display enhanced anxiety-related behavior, Endocrinology 144 (2003) 2291-2296] and the release of corticosterone following a psychogenic stress such as exposure to platform shaker was greater in female [J.A. Amico, R.C. Mantella, R.R. Vollmer, X. Li, Anxiety and stress responses in female oxytocin deficient mice, J. Neuroendocrinol. 16 (2004) 1-6; R.C. Mantella, R.R. Vollmer, L. Rinaman, X. Li, J.A. Amico, Enhanced corticosterone concentrations and attenuated Fos expression in the medial amygdala of female oxytocin knockout mice exposed to psychogenic stress, Am. J. Physiol. Regul. Integr. Comp. Physiol. 287 (2004) R1494-R1504], but not male [R.C. Mantella, R.R. Vollmer, J.A. Amico, Corticosterone release is heightened in food or water deprived oxytocin deficient male mice, Brain Res. 1058 (2005) 56-61], oxytocin gene deletion (OTKO) mice compared to wild type (WT) cohorts. In the present study we exposed OTKO and WT female mice to another psychogenic stress, inserting a rectal probe to record body temperature followed by brief confinement in a metabolic cage, and measured plasma corticosterone following the stress. OTKO mice released more corticosterone than WT mice (P<0.03) following exposure to this stress. In contrast, if OTKO and WT female and male mice were administered insulin-induced hypoglycemia, an acute physical stress, corticosterone release was not different between genotypes. The absence of central OT signaling pathways in female mice heightens the neuroendocrine (e.g., corticosterone) response to psychogenic stress, but not to the physical stress of insulin-induced hypoglycemia.

Oxytocin-induced excitation of neurones in the rat central and medial amygdaloid nuclei

Central oxytocin plays an important role in regulating emotionality. The amygdala expresses gonadal steroid-sensitive oxytocin binding sites in both the central and medial sub-nuclei, although the densities markedly differ between these nuclei. These studies examined the in vitro electrophysiological effects of oxytocin in the two amygdaloid nuclei and compared responses in female rats in different reproductive states (virgin, pregnant and lactating). Oxytocin (10(-9)-10(-6)M) caused a concentration-dependent increase in the firing rate of 20-36% of the neurones in both nuclei. Although autoradiographic studies using the oxytocin receptor antagonist [(125)I]d(CH(2))(5)[Tyr(Me)(2),Thr(4),Orn(8),Tyr-NH(2)(9)]-vasotocin showed a higher density of binding in the central nucleus of the amygdala than medial nucleus of the amygdala, neurones in the central nucleus of the amygdala had a much lower sensitivity to oxytocin: equivalent responses obtained with 10(-6)M in the central nucleus of the amygdala and 10(-8)M in the medial nucleus of the amygdala, and neurones in the central nucleus of the amygdala were insensitive to concentrations below 10(-6)M. Furthermore, repeated applications of oxytocin induced homologous desensitization in the central nucleus of the amygdala, but not medial nucleus of the amygdala-a single application of oxytocin producing long duration suppression of responses. This indicates that oxytocin has contrasting modes of action in the amygdala. Studies made across the reproductive cycle showed that lactating animals exhibited a larger proportion of oxytocin-responsive neurones in the medial nucleus of the amygdala and a smaller proportion in the central nucleus of the amygdala, compared with virgin or pregnant animals, indicating a peripartum shift in relative activation within the amygdala. However, changes in responses were not accompanied by changes in the density of oxytocin binding sites. These data show that oxytocin has a markedly different efficacy on neuronal activation in the central and medial sub-nuclei of the amygdala. The relative shift in excitatory responses between these two nuclei may underlie some of the neuroendocrine, behavioral and anxiolytic effects which have been ascribed to oxytocin in the periparturient rat.

Sexually dimorphic expression of oxytocin binding sites in forebrain and spinal cord of the rat

The central actions of oxytocin on reproduction-related functions and behaviors are strongly steroid-dependent and gender specific. This study characterizes sexual differences in the oxytocin binding site expression in forebrain and spinal cord of the rat. Using film autoradiography, we quantified the density of oxytocin binding sites in the ventromedial hypothalamic nucleus, the medial and central nuclei of the amygdala, the medial bed nucleus of the stria terminalis and the spinal cord dorsal horns both in adult male and female rats, and during development. In addition, neonatal castrated males and intact neonatal females treated with a single injection of testosterone (1 mg) were examined. Data showed a sexual dimorphism in the expression of oxytocin binding sites in the spinal cord dorsal horns and in restricted areas of the forebrain that are sensitive to gonadal steroids such as the ventromedial hypothalamic nucleus, but not in gonadal steroid insensitive sites such as the central nucleus of the amygdala. Adult males had higher oxytocin binding site densities in the ventromedial hypothalamic nucleus and dorsal horns than females. In the forebrain, but not in the dorsal horn, this sexual difference required a perinatal exposure to testosterone. Neonatal castration only abolished the sexual difference in the ventromedial hypothalamic nucleus of adults, but not in the dorsal horn. Furthermore, females that received a single injection of testosterone 1 day after birth showed significant increases in the density of oxytocin binding sites in the ventromedial hypothalamic nucleus, medial nucleus of the amygdala and medial bed nucleus of the stria terminalis. In addition, the findings suggest that the sexual difference in the ventromedial hypothalamic nucleus also requires gonadal hormones in adulthood. Our data support the hypothesis that sexually dimorphic oxytocin binding sites may contribute to the regulatory central actions of oxytocin in gender specific functions and behaviors such as nociception and reproduction.

Oxytocin attenuates stress-induced c-fos mRNA expression in specific forebrain regions associated with modulation of hypothalamo-pituitary-adrenal activity

We reported previously that the neuropeptide oxytocin attenuates stress-induced hypothalamo-pituitary-adrenal (HPA) activity and anxiety behavior. This study sought to identify forebrain target sites through which oxytocin may mediate its anti-stress effects. Ovariectomized, estradiol-treated rats received intracerebroventricular infusions of oxytocin (1 or 10 ng/hr) or vasopressin (10 ng/hr), and the patterns of neuronal activation after restraint stress were determined by semiquantitative mapping of c-fos mRNA expression. Oxytocin administration significantly attenuated the release of ACTH and corticosterone and the increase in corticotropin-releasing factor mRNA expression in the hypothalamic paraventricular nucleus (PVN) in response to 30 min restraint. Restraint also induced the expression of c-fos mRNA in selective regions of the forebrain, including the PVN, paraventricular thalamic nucleus, habenula, medial amygdala, ventrolateral septum (LSV), most subfields of the dorsal and ventral hippocampus, and piriform and endopiriform cortices. In most cases, this level of gene expression was unaffected by concomitant administration of oxytocin. However, in the PVN, LSV, and throughout all subfields of the dorsal hippocampus, restraint evoked no detectable increase in c-fos mRNA in animals treated with either dose of oxytocin. Vasopressin had no effects on either HPA axis responses or neuronal activation in response to restraint, indicating that the effects were highly peptide selective. These data show that central oxytocin attenuates both the stress-induced neuroendocrine and molecular responses of the HPA axis and that the dorsal hippocampus, LSV, and PVN constitute an oxytocin-sensitive forebrain stress circuit.

Neurons modulate oxytocin receptor expression in rat cultured astrocytes: involvement of TGF-beta and membrane components

We examined the effect of neurons on oxytocin (OT) receptors (OTR) and OTR gene expression in cultured astrocytes. The addition of neuron-conditioned medium induced an increase of both OTR binding and OTR mRNA level. This effect was enhanced after the medium was boiled or acidified. As it is known that transforming growth factor-beta (TGF-beta) can be released from carrier proteins by acid or heat, TGF-beta1 and 2 were tested and found to induce an increase of OTR binding. Furthermore, TGF-beta antibody abolished the stimulatory effect of normal or acidified neuron-conditioned medium. Neurons added to cultured astrocytes without contact mimicked the stimulatory effect of the conditioned medium. In contrast, neurons added with contact, induced a decrease in OTR binding and an increase of mRNA level, whereas neuronal membranes induced a decrease of both OTR binding and mRNA levels. In conclusion, the present data demonstrate that in vitro, neurons are able to modulate astrocytic OTR expression at the level of both protein and mRNA. They stimulate this expression through their release of TGF-beta and inhibit it by the action of unknown membrane components.

Like mother, like daughter: evidence for non-genomic transmission of parental behavior and stress responsivity

Considerable evidence demonstrates that the quality of the early environment influences patterns of development that, in turn, determine the health and productivity of the individual throughout their life span. However, the processes through which early life influences health are not clearly understood. Through the activation of the hypothalamo-pituitary-adrenal (HPA) axis and corticotropin-releasing hormone (CRH) pathways, prolonged or exaggerated responses to stress have profound effects on physiological and cognitive functions. Early maternal separation or handling of neonatal rats can program widespread and lifelong changes in various transmitter systems that regulate the HPA and CRH systems. Our studies show that a high level of maternal licking/grooming, and arched-back nursing correlates with reduced CRH mRNA expression and enhanced glucocorticoid negative feedback, and lower stress responses in the adult. This behavior is stably transmitted between generations and cross-fostering studies show that the offspring inherit the behavior from the nursing mother and not the biological mother. Such intergenerational transmission of maternal behavior is seen in rodents, primates and humans, and may underlie adaptive changes in the HPA axis. The neural basis of this inheritance pattern appears to reside in the central oxytocin system which determines features of maternal behavior. Through these various adaptive neural mechanisms the environmental demand on the mother is reflected in the quality of maternal care to her offspring. This, in turn, programs stress reactivity and maternal behavior patterns of the offspring. This not only determines certain health outcomes but also establishes the relationships between mother and offspring in the next generation. These findings suggest that for neurobiologists, the function of the family is an important level of analysis and the critical question is that of how environmental events regulate neural systems that mediate the expression of parental care.

Effect of gonadal steroids on the oxytocin-induced excitation of neurons in the bed nuclei of the stria terminalis at parturition in the rat

Experiments were undertaken to examine the role of ovarian steroids in peripartum programming of oxytocin sensitivity of limbic neurons implicated in oxytocin-induced facilitation of the milk-ejection reflex. In vivo recordings of neurons in the bed nuclei of the stria terminalis and ventrolateral septum of pre-parturient rats which had undergone prior ovariectomy and hysterectomy showed that oestradiol significantly increased the excitatory responses of bed nuclei/ventrolateral septum neurons to intracerebroventricular oxytocin, compared to oil-treated controls. Oestradiol also increased the excitation of bed nuclei neurons to the selective oxytocin agonist, [Thr4,Gly7]oxytocin in brain slices from steroid pre-treated ovariectomized hysterectomized rats, so that both the proportion of responsive neurons, and the magnitude of their responses were significantly increased. Parallel autoradiographic studies showed that oxytocin binding in the medial bed nuclei and ventrolateral septum was selectively increased following oestradiol treatment. Progesterone pre-treatment had no effect on either oxytocin sensitivity of bed nuclei/ventrolateral septum neurons recorded in vivo, or on oxytocin binding in the medial bed nuclei and ventrolateral septum, compared to oil-treated controls. Mean responses to [Thr4,Gly7]oxytocin in bed nuclei neurons recorded in slices from progesterone-treated rats were larger than controls, but this effect was highly variable. These results demonstrate that oestradiol greatly enhances oxytocin receptor expression and sensitivity of bed nuclei/ventrolateral septum neurons to oxytocin over the peripartum period, consistent with involvement of this steroid in enhancing oxytocin regulation of neuroendocrine and behavioural adaptations required for lactation.

Electrophysiological effects of oxytocin within the bed nuclei of the stria terminalis: influence of reproductive stage and ovarian steroids

The bed nuclei of the stria terminalis (BNST) is a target site for the central actions of oxytocin (OT) in promoting behavioural and neuroendocrine responses involved in female reproduction, and binding studies suggest that OT sensitivity may be modulated over the peripartum period. Electrophysiological recordings from brain slices in vitro showed that OT sensitivity of BNST neurones is relatively low in late pregnancy, but is high during lactation. In vivo studies over the immediate peri-partum period revealed that although BNST neurones can be excited by i.c.v. OT at day 22 of pregnancy, there is a 5-10 min delay in their response which is not present in lactation. This delay can be reversed by naltrexone, or lesioning the stria terminalis, and may involve an inhibitory opioid input to the BNST from the amygdala. Examination of the role of steroids in regulating OT responses of BNST neurones showed that oestradiol pre-treatment in late pregnant ovariectomized rats increased OT excitation of BNST neurones in vitro, and a similar result was observed with in vivo recordings. Progesterone also augmented OT excitation of BNST neurones in vitro, but no such effect was observed in vivo. This difference could indicate that an additional effect of progesterone is to potentiate extraneous inhibitory inputs to the BNST, or may reflect the ability of this steroid to suppress OT sensitivity by a direct membrane action. Changes in the response of BNST neurones to OT may have functional implications for the action of central OT in facilitating the neuroendocrine milk-ejection reflex (i.e. increasing milk-ejection frequency), an effect which first appears at around day 3 of lactation. Studies involving steroid treatment of late pregnant ovariectomized rats showed that this facilitatory mechanism can be induced to appear early (i.e. on day 22 of pregnancy) by oestradiol, but not progesterone treatment. Collectively, these results support this view, that the action of OT in the BNST is regulated by the changing levels of steroids towards the end of pregnancy, thereby ensuring appropriate neuroendocrine responses necessary for motherhood.

Changing pattern of oxytocin-induced excitation of neurons in the bed nuclei of the stria terminalis and ventrolateral septum in the peripartum period

Oxytocin acts within the limbic system (bed nuclei of the stria terminalis and ventrolateral septum) to induce maternal behaviour and to facilitate neuroendocrine activity at specific times during the peripartum period. Studies were undertaken to determine whether the timing of these effects arises from modulation of the oxytocin-induced excitation of limbic neurons. Extracellular activity of single units was recorded on urethane-anaesthetized rats and neurons were tested for responses to intracerebroventricular injection of 1.1 ng oxytocin. In the first part, animals were recorded on days 19 and 22 of pregnancy and on days 3 and 5 of lactation. No significant differences in the basal firing rates or in the proportion of oxytocin-responsive neurons were detected, but responses by neurons on day 22 of pregnancy occurred after a significant delay (10.7 +/- 2.0 min), resulting in a smaller overall response compared to the other groups. These differences in the pattern of response were not due to changes in density of oxytocin binding in the limbic areas studied, since autoradiographic detection of oxytocin binding sites using the iodinated antagonist [125I]d(CH2)5[Tyr(Me)2, Thr4, Orn3, Tyr-NH2(9)]-vasotocin showed no differences between the pregnant and postpartum animals. In the second part, parturient animals (day 22 of pregnancy) received intravenous injection of the long-acting opioid antagonist naltrexone, or unilateral knife-cut lesions to the stria terminalis, a source of inhibitory inputs (including enkephalinergic) to the bed nuclei of the stria terminalis and ventrolateral septum. Both treatments abolished the characteristic delay of oxytocin-induced excitation in non-treated animals on day 22 of pregnancy, and increased the overall excitatory response. Thus, during the peripartum period, a population of limbic neurons sensitive to oxytocin display a dynamically changing pattern of excitatory responses, apparently modulated by an endogenous opioid cone and independent of changes in oxytocin receptor expression. The attenuated neuronal response to central oxytocin seen on the day of parturition could account for the absence of a facilitatory effect of oxytocin on neuroendocrine activity at this time.

Vasopressin receptors in the mouse (Mus musculus) brain: sex-related expression in the medial preoptic area and hypothalamus

We have investigated the distribution of vasopressin binding sites in the brain of male and female adult mice using a radio-iodinated ligand and film autoradiography. Vasopressin receptors were uncovered in various regions of the brain including the basal nucleus of Meynert, the substantia innominata, the hypothalamic paraventricular nucleus, the substantia nigra pars compacta and the hypoglossal nucleus. A sex-related difference in the expression of vasopressin receptors was seen in the medial preoptic area/anterior hypothalamus corresponding to the rat sexually dimorphic nucleus in the rat and in the hypothalamic mammillary nuclei. In both structures the autoradiographic labeling is more intense in females than in males. These observations confirm that vasopressin binding sites are present in the hypothalamic preoptic area of most species examined so far and that sex-related expression of neuropeptide receptors could trigger sex-related behavioral differences.

Estradiol, progesterone, and prolactin regulate maternal nest-building in rabbits

Maternal nest-building in rabbits, expressed across the last third of pregnancy, consists of: digging a burrow, collecting straw and shaping it into a nest inside the burrow, plucking body hair and lining the straw nest with it. The sequential expression of these activities is correlated with specific changes in the plasma concentration of estradiol, progesterone (P), and prolactin (PRL). To further substantiate the participation of these hormones in the control of maternal nest-building we explored in ovariectomized (ovx) New Zealand white rabbits the capacity of several combinations of such hormones to stimulate digging, straw-carrying, and hair-pulling. Does given estradiol benzoate (EB; 5 micrograms/day from days 3 to 21) plus P (2 or 10 mg/day from days 4 to 16) dug into a substrate from the fourth day of the P treatment until withdrawal of this hormone. The intensity of this effect was greater in the group treated with the high dose of P. Straw-carrying and hair-pulling occurred after P withdrawal in a dose-response way. Food intake, which declines in pregnant females shortly before parturition, decreased to the same extent in both groups of ovx EB-treated does after P withdrawal. A significant increase in PRL plasma levels was observed on day 9 in does given EB plus 2 mg P/day and at two days following P withdrawal in does given EB plus 10 mg P/day. When such ovx EB/P-treated does were given bromocriptine to block PRL release (1 or 3 mg/Kg/day, from days 11 to 21) the expression of digging was unmodified. By contrast, bromocriptine abolished the display of straw-carrying and hair-pulling, and also prevented the decline in food intake normally following P withdrawal. The addition of ovine PRL to ovx EB/P-treated does given bromocriptine reduced the expression of digging, did not restore straw-carrying or hair-pulling, and provoked a sharp decline in food intake. The possible mechanisms of interaction between PRL and steroid hormones for the regulation of specific aspects of the pregnant doe's physiology and behavior are discussed.