Autoradiographic detection of oxytocin- and vasopressin-binding sites in various subnuclei of the bed nucleus of the stria terminalis in the rat. Effects of functional and experimental sexual steroid variations

Oxytocin Attenuates Methamphetamine-Induced Apoptosis via Oxytocin Receptor in Rat Hippocampal Neurons

Methamphetamine (METH) is a highly neurotoxic psychoactive substance that can directly damage the central nervous system through prolonged use. Oxytocin (OT) has attracted much attention because of its neuroprotective effect. The purpose of this study was to investigate whether OT is neuroprotective against METH-induced damage in rat hippocampal neurons. Our results revealed that pre-incubation with OT significantly prevented the damage of METH to hippocampal neurons, including the decrease of mitochondrial membrane potential and the increase of ROS (reactive oxygen species). OT pre-incubation attenuated the up-regulation of Cleaved-Caspase-3 expression and the down-regulation of Bcl-2/Bax expression induced by METH. Pre-incubation with OT prevented the decrease in oxytocin receptor density and P-CREB (phosphorylation of cAMP-response element binding) expression induced by METH in rat hippocampal neurons. Moreover, Pre-incubation of atosiban (ATO) significantly prevented these changes. In conclusion, our study proved that pre-administration of OT could significantly attenuate hippocampal neuron apoptosis induced by METH. Oxytocin receptor activation is involved in the preventive effect of OT on METH-induced apoptosis in rat hippocampal neurons.

Review of eating disorders and oxytocin receptor polymorphisms

BACKGROUND AND AIMS

Oxytocin, a nine amino acid peptide synthesised in the hypothalamus, has been widely recognised for its role in anxiolysis, bonding, sociality, and appetite. It binds to the oxytocin receptor (OXTR)-a G-protein coupled receptor-that is stimulated by the actions of oestrogen both peripherally and centrally. Studies have implicated OXTR genotypes in conferring either a risk or protective effect in autism, schizophrenia, and eating disorders (ED). There are numerous DNA variations of this receptor, with the most common DNA variation being in the form of the single nucleotide polymorphisms (SNPs). Two OXTR SNPs have been most studied in relation to ED: rs53576 and rs2254298. Each SNP has the same allelic variant that produces genotypes AA, AG, and GG. In this critical review we will evaluate the putative role of rs53576 and rs2254298 SNPs in ED. Additionally, this narrative review will consider the role of gene-environment interactions in the development of ED pathology.

FINDINGS

The OXTR SNPs rs53576 and rs2254298 show independent associations between the A allele and restrictive eating behaviours. Conversely, the G allele of the OXTR rs53576 SNP is associated with binging behaviours, findings that were also evident in neuroanatomy. One study found the A allele of both OXTR SNPs to confer risk for more severe ED symptomatology while the G allele conferred some protective effect. An interaction between poor maternal care and rs2254298 AG/AA genotype conferred increased risk for binge eating and purging in women.

CONCLUSIONS

Individual OXTR SNP are unlikely in themselves to explain complex eating disorders but may affect the expression of and/or effectiveness of the OXTR. A growing body of G x E work is indicating that rs53576G homozygosity becomes disadvantageous for later mental health under early adverse conditions but further research to extend these findings to eating pathology is needed. The GWAS approach would benefit this area of knowledge.

Oxytocin excites BNST interneurons and inhibits BNST output neurons to the central amygdala

The dorsolateral bed nucleus of the stria terminalis (BNST_DL) has high expression of oxytocin (OT) receptors (OTR), which were shown to facilitate cued fear. However, the role of OTR in the modulation of BNST_DL activity remains elusive. BNST_DL contains GABA-ergic neurons classified based on intrinsic membrane properties into three types. Using in vitro patch-clamp recordings in male rats, we demonstrate that OT selectively excites and increases spontaneous firing rate of Type I BNST_DL neurons. As a consequence, OT increases the frequency, but not amplitude, of spontaneous inhibitory post-synaptic currents (sIPSCs) selectively in Type II neurons, an effect abolished by OTR antagonist or tetrodotoxin, and reduces spontaneous firing rate in these neurons. These results suggest an indirect effect of OT in Type II neurons, which is mediated via OT-induced increase in firing of Type I interneurons. As Type II BNST_DL neurons were shown projecting to the central amygdala (CeA), we also recorded from retrogradely labeled BNST→CeA neurons and we show that OT increases the frequency of sIPSC in these Type II BNST→CeA output neurons. In contrast, in Type III neurons, OT reduces the amplitude, but not frequency, of both sIPSCs and evoked IPSCs via a postsynaptic mechanism without changing their intrinsic excitability. We present a model of fine-tuned modulation of BNST_DL activity by OT, which selectively excites BNST_DL interneurons and inhibits Type II BNST→CeA output neurons. These results suggest that OTR in the BNST might facilitate cued fear by inhibiting the BNST→CeA neurons.

Oxytocin inhibits the rat medullary dorsal horn Sp5c/C1 nociceptive transmission through OT but not V1A receptors

The medullary dorsal horn (MDH or Sp5c/C1 region) plays a key role modulating the nociceptive input arriving from craniofacial structures. Some reports suggest that oxytocin could play a role modulating the nociceptive input at the MDH level, but no study has properly tested this hypothesis. Using an electrophysiological and pharmacological approach, the present study aimed to determine the effect of oxytocin on the nociceptive signaling in the MDH and the receptor involved. In sevoflurane, anesthetized rats, we performed electrophysiological unitary recordings of second order neurons at the MDH region responding to peripheral nociceptive-evoked responses of the first branch (V1; ophthalmic) of the trigeminal nerve. Under this condition, we constructed dose-response curves analyzing the effect of local spinal oxytocin (0.2-20 nmol) on MDH nociceptive neuronal firing. Furthermore, we tested the role of oxytocin receptors (OTR) or vasopressin V_1A receptors (V_1AR) involved in the oxytocin effects. Oxytocin dose-dependently inhibits the peripheral-evoked activity in nociceptive MDH neurotransmission. This inhibition is associated with a blockade of neuronal activity of Aδ- and C-fibers. Since this antinociception was abolished by pretreatment (in the MDH) with the potent and selective OTR antagonist (L-368,899; 20 nmol) and remained unaffected after the V_1AR antagonist (SR49059; 20 nmol or 200 nmol), the role of OTR is implied. This electrophysiological study demonstrates that oxytocin inhibits the peripheral-evoked neuronal activity at MDH, through OTR activation. Thus, OTR may represent a new potential drug target to treat craniofacial nociceptive dysfunction in the MDH.

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.

Differential excitatory responses to oxytocin in sub-divisions of the bed nuclei of the stria terminalis

The lateral dorsal nucleus of the bed nuclei of the stria terminalis (BST-LD) expresses dense oxytocin binding while lower binding is detected in the medial anterior BST (BST-MA) and adjacent ventrolateral septum (VLS). However, in vitro examination of neuronal responses to oxytocin showed that the BST-LD exhibited small, transient responses which desensitized upon repeated challenge. In contrast, the BST-MA and VLS exhibited significantly larger responses with no significant desensitization. This inverse relationship between oxytocin binding density and electrophysiological responsiveness is also seen in the central and medial amygdaloid nuclei, which have respective associations with the lateral and medial divisions of the BST. Thus, excitatory responses to oxytocin vary markedly between BST sub-divisions and may reflect associations within the extended amygdala.

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.

The temporal course of expression of c-Fos and Fos B within the medial preoptic area and other brain regions of postpartum female rats during prolonged mother--young interactions

Maternal behavior is associated with an increase in the expression of c-Fos and Fos B within neurons of the medial preoptic area (MPOA) and ventral bed nucleus of the stria terminalis (vBST). Whether this increase wanes as the duration of mother-young interaction increases is unknown. By varying the length of mother-young interactions in postpartum rats, the authors found that within the MPOA/vBST, the levels of both c-Fos and Fos B, once elevated, remained significantly above control levels through 47 hr of pup exposure. The persistence of c-Fos and Fos B within the MPOA/vBST of females that remained with pups was almost unique in that only one other neural area, the anterior magnocellular part of the paraventricular hypothalamic nucleus, showed such a response. Because MPOA/vBST neurons are essential for maternal behavior, the results suggest that c-Fos and Fos B expression within these regions may be necessary to maintain their normal functional activity.

Previous maternal experience potentiates the effect of parturition on oxytocin receptor mRNA expression in the paraventricular nucleus

In sheep, central oxytocin release at parturition induces maternal behaviour which is thought to be mediated by changes in the expression of central oxytocin receptors. The distribution, effects of parturition, previous maternal experience and hormonal status on the distribution of an oxytocin receptor was investigated using immunocytochemistry and in situ hybridization. In ewes with no previous maternal experience, parturition induced significant increases in oxytocin receptor mRNA expression in the anterior olfactory nucleus, medial preoptic area, ventromedial hypothalamus, lateral septum, medial amygdala, bed nucleus of the stria terminalis and diagonal band of Broca. In maternally experienced ewes, parturition induced additional increases in two areas, the paraventricular nucleus and the Islands of Calleja. The changes in progesterone and oestrogen that occur during late pregnancy and parturition appear to contribute to increases in expression in the anterior olfactory nucleus, Islands of Calleja, medial preoptic area, ventromedial hypothalamus, bed nucleus of the stria terminalis and diagonal band of Broca, but not in the paraventricular nucleus, lateral septum and medial amygdala. These results demonstrate that progesterone and oestrogen priming enhance oxytocin receptor mRNA expression in a number of regions in the olfactory system, hypothalamus and limbic brain. These effects appear to be independent of maternal experience. Parturition increases oxytocin receptor mRNA expression in all the areas influenced by hormonal priming and the lateral septum, medial amygdala and paraventricular nucleus. Maternal experience also enhances expression of oxytocin receptor mRNA in the paraventricular nucleus and the Islands of Calleja. Because the paraventricular nucleus is the main source of oxytocin release in the brain, this upgrading of autoreceptors as a result of maternal experience may serve to enhance release of this peptide in projection sites regulating maternal behaviour.

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.

Effects of stress and glucocorticoids on CNS oxytocin receptor binding

Oxytocin receptors in several regions of the limbic system are regulated by gonadal steroids and play an important role in the mediation of maternal, sexual and affiliative behaviors. We have previously reported oxytocin receptor regulation by glucocorticoids in hippocampus and subiculum-neuroanatomical regions implicated in memory and stress regulation. In the current study we examined oxytocin receptor regulation by stress and high glucocorticoid concentration in adrenally intact male rats. Single prolonged stress and chronic non-habituating stress were used as experimental conditions in the first study, and chronic non-habituating and high dose corticosterone implants in the second. Oxytocin receptor concentration was assessed using in vitro receptor autoradiography with [125I]OVTA at the approximate KD concentration. Both stress paradigms increased oxytocin receptor binding (F = 3.7, df = 2, p = .03) across brain regions in the first study. Chronic non-habituating stress and corticosterone implants increased oxytocin receptor binding in the ventral hippocampus only (one-way ANOVA, F = 3.88, df = 2, p < .05). The current studies demonstrate that stress increases oxytocin receptor binding in areas of the CNS that are rich in glucocorticoid receptors, such as hippocampus. This suggests differential regulation of oxytocin receptors in CNS, depending upon their functional role in different regions. Oxytocin receptor modulation could mediate some of the long-term effects of stress on memory, and possibly play a role in the regulation of hypothalamo-pituitary-adrenal stress response. The ability of circulating glucocorticoids to up-regulate these receptors suggests a plausible mechanism for this stress-sensitive regulation.

Differential expression of oxytocin receptor mRNA in the developing rat brain

The embryonic and postnatal localizations of oxytocin receptor mRNA in the developing rat brain were studied by in situ hybridization histochemistry. The hybridization signal was first detected at embryonic-day 13 in the primordium of the dorsal motor nucleus of vagus. Other positive regions progressively appeared after this time. The developmental profile of oxytocin receptor gene expression could be classified into two types; transient expression and constant abundant expression. The caudate putamen, cingulate cortex, the anterior thalamic nuclei, and the ventral tegmental area belonged to the first type. In these regions, oxytocin receptor mRNA was expressed intensely only during the early postnatal period. The regions such as the anterior olfactory nucleus, tenia tecta, some amygdaloid nuclei, piriform cortex, the ventromedial hypothalamic nucleus, subiculum, the prepositus hypoglossal nucleus and the dorsal motor nucleus of vagus showed constant expression of oxytocin receptor mRNA at high levels throughout development and in the adult. These findings concurred well with those of the ontogenic studies using receptor binding autoradiography with a ligand specific to oxytocin. Thus, the transient expression of oxytocin receptor during development was regulated at the transcriptional level in several brain regions, and oxytocin may play a role in brain development as well as in neural transmission in the mature brain.

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

The localization at the cellular level and the regulation by progesterone of the estrogen-sensitive oxytocin binding sites was studied in the rat telencephalon and the hypothalamus by using quantitative film-autoradiography and histoautoradiography. Male rats (castrated or not) and ovariectomized females (estradiol supplemented or not) were used to characterize these sites and to precise their localization. They were detected in the striatal cell bridges, the olfactory tubercle, the principal nucleus of the bed nucleus of the stria terminalis and the medial nucleus of the amygdala of the telencephalon and in the medial preoptic, the ventromedial and the ventral premammillary nuclei of the hypothalamus. Estrogen administration in addition induced expression of oxytocin binding sites in the major island of Calleja, the anterior hypothalamic area and the terete nucleus. The density of the estrogen-sensitive oxytocin binding sites varied during the estrous cycle, but differently in the telencephalon and the hypothalamus. In the telencephalon it peaked at proestrus 9 h and was already decreased at proestrus 21 h, whereas in the hypothalamus it was similarly high at proestrus 9 h and proestrus 21 h, suggesting the intervention of progesterone in the regulation of the hypothalamic estrogen-sensitive oxytocin binding sites.

Involvement of the PVN and BST in 1K1C hypertension in the rat

The paraventricular nucleus of the hypothalamus (PVN) and the bed nucleus of the stria terminalis (BST) are two sources of central nervous system (CNS)-derived arginine vasopressin (AVP), a nonapeptide that has been implicated in central autonomic regulation and in particular in cardiovascular regulation, through its actions within the CNS. These experiments were designed to determine if either the PVN or the BST were involved in the development of Goldblatt one-kidney one-clip (1K1C) hypertension in the rat. In order to test this hypothesis, ibotenic acid lesions of the PVN or electrolytic lesions of the BST were undertaken in both normotensive (sham-operated) rats and in 1K1C rats. In both cases the development of 1K1C hypertension was inhibited over the 18-21 days following surgery. Lesions of the PVN did not alter normal blood pressure regulation in the sham-operated animals, whereas lesions to the BST did affect normal blood pressure regulation, resulting in a dramatic increase in blood pressure during the initial days following surgery. These studies suggest that the PVN and BST are involved in the development of 1K1C hypertension in the rat, moreover the BST may also play a role in central cardiovascular control.

Mortyn Jones Memorial Lecture. Limbic regions mediating central actions of oxytocin on the milk-ejection reflex in the rat

Central oxytocin administration has a profound facilitatory effect on the patterning of the milk-ejection reflex in the lactating rat. Lesion and microinjection studies indicate that this action is, in part, mediated via a population of limbic neurones in the bed nuclei of the stria terminalis and ventrolateral septum, which have been shown to possess oxytocin receptors and to be activated by selective oxytocin-receptor agonists in vitro. In vivo electrophysiological recordings reveal that some of these neurones display cyclical activity which is highly correlated to each milk ejection, and are rapidly activated following i.c.v. administration of oxytocin, coincident with the facilitation of milk ejection activity. A hypothetical model is proposed in which this population of limbic neurones serves to gate the activity of a pacemaker which, in turn, coordinates the bursting of hypothalamic magnocellular neurones. The oxytocin innervation of these neurones and their expression of oxytocin receptors increases in the postpartum period, and the resultant enhanced sensitivity leads to a greater facilitatory response during lactation. Inhibitory opioid and noradrenergic inputs which converge on these oxytocin-sensitive neurones may function to switch off the facilitatory circuit during periods of stress. Thus, this population of limbic neurones participates in the regulation of neuroendocrine activity during lactation by providing an appropriate degree of feedback to alter the patterning of the milk-ejection reflex.

Correlation between oxytocin neuronal sensitivity and oxytocin-binding sites in the amygdala of the rat: electrophysiological and histoautoradiographic study

Central nucleus (Ce), basomedial and medial nuclei of the amygdala (AMG), and some parts of the striato-pallidal system, present high densities of oxytocin (OT)-binding sites. In order to examine whether these OT-binding sites are functional receptors, the OT neuronal sensitivity and the presence of OT-binding sites were investigated using electrophysiological and autoradiographical techniques. To identify the AMG cells, electrical stimulation of the oval subnucleus of the bed nucleus of the stria terminalis (Ov) and of the parabrachial nucleus (Pb) were performed. Somatic and auditory sensory stimulations were also tested. OT was applied by iontophoresis during extracellular single unit recordings of cells which were localized in frontal brain sections subsequently used for histoautoradiographic detection of OT-binding sites. Cells responding to Ov nucleus stimulation were located in the AMG, mainly in the Ce nucleus, whereas those responding to Pb nucleus stimulation were distributed in the Ce nucleus and in the postero lateral part of the caudate putamen. Iontophoretic OT application excited 45% of the recorded cells (43/96) among which OT alone activated spontaneous firing rate of 30 and potentiated the L-Glutamate (GLU)-induced activation on 13. These OT-sensitive neurons were located mainly in the AMG and caudate putamen areas containing OT-binding sites. These results strongly suggest that OT-binding sites found in the AMG are functional receptors upon which OT could act as a neurotransmitter and as a neuromodulator to regulate autonomic functions.

Evidence for reciprocal connections between the dorsochiasmatic area and the hypothalamo neurohypophyseal system and some related extrahypothalamic structures

In the preceding article, a dorsochiasmatic area (DCh) was described that projects to both paraventricular (PVN) and supraoptic (SON) nuclei. The main afferents of the DCh, revealed by local injections of retrograde tracers, are the hypothalamic PVN and SON, lateral septal nuclei (LSV and SHy), bed nuclei of the stria terminalis (BST), anteroventral third ventricle region, particularly the median preoptic nucleus (MnPO), the subfornical organ, medial preoptic areas, arcuate hypothalamic nucleus, ventromedial hypothalamic nuclei, paraventricular thalamic nucleus, and, more caudally, several structures of the posterior hypothalamus and mesencephalon. The relations between DCh and BST, LSV, SHy, or MnPO appeared reciprocal. In view of their reciprocal relationships with the hypothalamo-neurohypophyseal system and some of their related extrahypothalamic structures, the DCh might be involved in the regulation of the vasopressin (AVP) and/or oxytocin (OT) systems, or in reproductive behavior.

Gonadal steroids have paradoxical effects on brain oxytocin receptors

Specific brain receptors for oxytocin have been described in several mammalian species. The distribution of these receptors differs greatly across species and in the rat, receptor binding in specific brain regions appears to depend upon gonadal steroids. This study used in vitro receptor autoradiography to examine the effects of testosterone on oxytocin receptor binding in the mouse forebrain. Three groups of male mice were compared: castrates treated with blank capsules, castrates treated with testosterone filled capsules, and intact males. Irrespective of steroid treatment, the distribution of oxytocin receptors in mouse forebrain differed markedly from patterns previously described in the rat. In addition to these species differences in receptor distribution, testosterone had effects in the mouse which differed from the induction of receptors previously reported in the rat. In the mouse ventromedial nucleus of the hypothalamus, binding in the untreated castrate males was approximately double that observed in either the intact or the testosterone-treated castrates. In other regions of the mouse brain, such as the intermediate zone of the lateral septum, binding to oxytocin receptors was increased with testosterone treatment. These results suggest that the brain oxytocin receptor varies across species not only in its distribution but also in its regional regulation by gonadal steroids. These apparently paradoxical changes in oxytocin receptor binding may result from either direct or indirect effects of gonadal steroids in mouse brain.

Oxytocin binding sites in rat limbic and hypothalamic structures: site-specific modulation by adrenal and gonadal steroids

Basal density and estrogen induction of oxytocin binding sites in limbic and hypothalamic structures of the rat brain were investigated by semi-quantitative autoradiography following chronic administration of dexamethasone or progesterone. The selective oxytocin receptor antagonist d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)] ornithine-vasotocin was used as a ligand for oxytocin binding sites. Estrogen administration increased ligand binding in all sites investigated. Dexamethasone treatment significantly increased ligand binding in the bed nucleus of the stria terminalis, lateral ventral septum and amygdala to an extent which was comparable to that of estradiol alone. In the hypothalamic ventromedial nucleus, dexamethasone significantly decreased basal levels of oxytocin binding. Estrogen administration subsequent to dexamethasone failed to cause a further increase in oxytocin binding in all structures investigated. Chronic progesterone treatment significantly increased basal oxytocin receptor density in the limbic structures, decreased it in the ventromedial nucleus, and prevented estrogen-induced increases in ligand binding in all areas studied with the exception of the medial preoptic area. These findings demonstrate that, in addition to gonadal steroids, glucocorticoids differentially and site-specifically modulate cerebral oxytocin binding sites. The evidence for glucocorticoid and gestagen influences on oxytocin receptors and their inducibility by estrogen may be relevant to the understanding of mechanisms leading to impairment of oxytocin-related behaviours.

Histoautoradiographic detection of oxytocin- and vasopressin-binding sites in the telencephalon of the rat

Localization of oxytocin- and vasopressin-binding sites has so far been studied in the rat brain by means of film autoradiographs. The disposal of iodinated ligands with high specificity has allowed us to develop histoautoradiography on emulsion-coated sections and to reinvestigate on a microscopic scale the distribution of these sites in the telencephalon (septum, striatopallidal system, amygdala and hippocampus). This technique showed that oxytocin and vasopressin labelling presented distinct distributions and coincided with delimited zones, corresponding to anatomical subdivisions defined on cytoarchitectural and immunocytochemical bases. Vasopressin sites were seen in the dorsal and intermediate parts of the lateral septum and the juxtacapsular nucleus of the bed nucleus of the stria terminalis. Oxytocin sites were located in the ventral and intermediate parts of the lateral septum, the oval and the principal nuclei of the bed nucleus of the stria terminalis and the septofimbrial nucleus. In the striatopallidal system, vasopressin sites were found in the accumbens nucleus and the fundus striati, whereas oxytocin sites were in the accumbens nucleus, the head, and the posterolateral parts of the caudate-putamen, the striatal cell bridges, and the olfactory tubercle. In the amygdala, vasopressin sites were not found, but oxytocin sites were located in the central, medial, and basomedial nuclei. In the hippocampus, vasopressin sites were located in the dentate gyrus (polymorph and molecular layers), and oxytocin sites, in the subiculum (molecular and pyramidal layers) and in the field CA1 of Ammon's horn (lacunosum moleculare and pyramidal layers). The localization of the binding sites at the microscopic level permitted us to reinvestigate whether or not correlation existed in a same area between innervation, electrophysiological effects, and presence of binding sites.

Electrical activity of neurons in the ventrolateral septum and bed nuclei of the stria terminalis in suckled rats: statistical analysis gives evidence for sensitivity to oxytocin and for relation to the milk-ejection reflex

Our previous results obtained by lesioning or stimulating the ventrolateral part of the lateral septum and the bed nuclei of the stria terminalis suggested that this area is involved in the control of milk ejection pattern in rats. The present study was undertaken with the aim of testing ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons as a putative link of the neuronal network controlling the bursting activity of oxytocin neurons in suckled lactating rats (anaesthetized with urethane). Ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons were recorded simultaneously with hypothalamic oxytocin neurons in either the paraventricular or supraoptic nucleus in rats with (n = 26) or without (n = 29) periodic milk ejections. Analysis of their firing pattern enabled differentiation of two subgroups: type I, characterized by numerous high frequency spikes, often grouped in clusters; and type II with very few or no high frequency clusters of spikes. The probability density function of the interspike intervals of both patterns could be modelled using a mixture of two log-normal distributions, the parameters of which differed significantly. The presence of absence of milk ejections did not influence the overall mean level of activity (2.0 +/- 0.5 and 1.9 +/- 0.4 spikes/s, respectively). However, the characteristics of the type I firing pattern were affected by the presence of the milk-ejection reflex. The average level of activity was not always constant and 16/55 ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons displayed cyclical activity (from 0.6 +/- 0.2 to 4.0 +/- 0.5 spikes/s) both in the presence (n = 8) and absence (n = 8) of the milk-ejection reflex. In five of eight neurons recorded during milk-ejection reflex, the cycles in firing were clearly correlated with the bursting of oxytocin neurons. These five neurons exhibited the type I firing pattern. The three remaining neurons and the eight neurons recorded in the absence of milk-ejection reflex displayed the type II firing pattern. Oxytocin (1-2 ng = 0.45-0.9 mU) was injected into the third ventricle (i.c.v.) in order to examine the possible involvement of ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons in the facilitatory effect of oxytocin on the reflex.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological actions of oxytocin in the dorsal vagal complex of the female rat in vitro: changing responsiveness during the oestrous cycle and after steroid treatment

The effect of the oxytocin-specific agonist Thr4,Gly7-oxytocin (TGOT) was tested on neurones in tissue slices of the dorsal vagal complex, obtained from virgin female rats at different stages of the oestrous cycle. The proportion of neurones excited by TGOT (0.1 microM) was independent of the day of the cycle, but both the basal activity and magnitude of response induced by TGOT were significantly reduced on the day of oestrus by comparison with dioestrus. This was due to a small but significant shift in the dose-response relationship. The magnitude of the excitation of neurones obtained from animals at proestrus did not differ significantly from either oestrus or dioestrus, but lay between the two. Ovariectomy 6 days prior to recording reduced the proportion of responsive neurones (35% vs. 69% at dioestrus), but had only a small effect on the amplitude of the averaged responses. Daily injection of 10 micrograms oestradiol benzoate had no additional effect on the proportion of responsive neurones (40%), but caused a marked suppression of the amplitude of the response at all doses (change in firing rate caused by 0.1 microM: 1.68 +/- 0.27 Hz vs. 2.69 +/- 0.39 Hz). In contrast, injections of 5 mg progesterone caused a small increase in the amplitude of the response. The data show that ovarian steroids have a marked effect on oxytocin-sensitive neurones of the dorsal vagal complex, causing dynamic changes in responsiveness over the oestrous cycle. This is discussed with respect to the effects of ovarian steroids on central oxytocin receptors and the possible involvement in regulating autonomic functions.

Post-partum increase in oxytocin-induced excitation of neurones in the bed nuclei of the stria terminalis in vitro

Extracellular recordings were made from neurones in the bed nuclei of the stria terminalis (BST) in brain slices from female rats on days 13-15 of pregnancy, during lactation or 5-6 days following weaning. The proportion of neurones excited by the oxytocin-specific agonist, Thr4, Gly7-oxytocin, did not change over the peri-partum period, but the magnitude of their response increased significantly during lactation. These data concur with reported changes in receptor density. It is suggested that increased oxytocin responsivity of BST neurones may subserve a regulatory function to maternal behaviour or neuroendocrine reflexes during lactation.

Oxytocin pretreatment enhances arginine vasopressin-induced motor disturbances and arginine vasopressin-induced phosphoinositol hydrolysis in rat septum: a cross-sensitization phenomenon

The recent observation that the central oxytocin (OT) receptor has high affinity for both OT and arginine vasopressin (AVP) raises the possibility that it may be involved in some of the central actions of AVP. Repeated intracerebroventricular (icv) injections of AVP in rats evoke an unusual sensitization phenomenon in that a first exposure to the peptide enhances the sensitivity (sensitization) of the brain to a second exposure. This report investigates the possibility that the OT receptor may be involved in the mediation of the phenomenon of sensitization, using OT, a specific OT receptor agonist, [Thr4,Gly7]OT, and a specific OT receptor antagonist, d(CH2)5,[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT (compound 6; cpd 6), as well as a V1 AVP receptor antagonist, d(CH2)5Tyr(Me)AVP. Peptides were injected icv in conscious, adult male Sprague-Dawley rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization and characterization of binding sites for vasopressin and oxytocin in the brain of the guinea pig

Using autoradiography on film, specific binding sites for arginine-vasopressin (AVP) and for oxytocin (OT) were localized in various areas of the brain of adult male guinea pigs. Vasopressin binding sites were detected with [3H]AVP or with [125I]VPA, a recently synthetized linear vasopressin antagonist radiolabeled with 125I. [125I]VPA and [3H]AVP yielded similar results, thus suggesting that AVP binding sites present in the guinea pig brain are V1 type receptors. Supporting evidence on this was obtained in competing studies using structural analogues allowing to discriminate V1 receptors from V2 and from OT receptors. Oxytocin binding sites were labeled with [3H]OT or with the iodinated OT antagonist [125I]OTA; both ligands yielded similar results. The localization in the guinea pig brain of AVP binding sites differed from that of OT binding sites. AVP binding sites were mainly detected in the olfactory bulb and throughout the cerebral cortex. Oxytocin binding sites were most noticeable in the hypothalamic ventromedial nucleus, in the amygdaloid complex and in restricted areas of the cerebral cortex. A comparison of the present data with those previously described in the rat, the mouse, the human and the hamster brain suggests that similar binding sites are present in these species, but that their anatomical distribution differs markedly. These data are discussed in relation to immunocytochemical and electrophysiological data which suggest that binding sites detected by autoradiography may represent, at least in part, functional neuronal receptors.