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.

Differential effects of psychological and immunological challenge on the hypothalamo-pituitary-adrenal axis function in adjuvant-induced arthritis

Inability to mount a suitable glucocorticoid response to a stressor can be life-threatening. Rats with hind-paw inflammation, associated with the development of adjuvant-induced arthritis (AA), are unable to mount a hypothalamo-pituitary-adrenal (HPA) axis response to acute stress. In the present study we have compared the effects of acute psychological stress (noise) and acute immunological challenge (lipopolysaccharide [LPS] injection), on the activation of the HPA axis in rats with the chronic inflammatory stress of AA. We conclude that the increase in HPA axis activity in AA is principally due to an increase in corticosterone pulse frequency and not to any alteration in pulse magnitude. The lack of response to acute stress can be accounted for by the increase in pulse frequency and the associated refractory period following each pulse, producing dramatic but specific changes in basal HPA function. These changes may account for the loss of responsiveness to acute stress, but not to acute immunological challenge, because the HPA axis is able to respond to LPS in male rats with AA. However, there appears to be an impaired adrenal responsiveness in female rats with AA that is not inherent, but occurs as a consequence of the development of inflammation.

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.

Vasopressin neurotransmission and the control of circadian rhythms in the suprachiasmatic nucleus

Vasopressin (VP) is one of the principal transmitters in the suprachiasmatic nucleus (SCN). Approximately 20% of neurones in the dorsomedial division of the SCN synthesize the peptide and a high proportion of SCN neurones (> 40%) are excited by VP acting through the V1 receptor. This suggests that VP may act as a feedback regulator of electrical activity within the nucleus. Such an intrinsic excitatory signal can be demonstrated by perifusion with a V1 antagonist which reduces spontaneous neural activity. As the synthesis and release of VP occurs in a circadian manner, this leads to a variable feedback excitation which may contribute to the circadian pattern of activity of the neural clock. This role in amplifying rhythmicity is supported by observations that animals deficient in VP show a reduced circadian amplitude of behavioural rhythms (e.g. locomotor and cortical electroencephalographic rhythms). VP expression declines during ageing and although aged animals show no change in the proportion of SCN neurones excited by VP, the rhythm of spontaneous electrical activity shows a progressive decline, consistent with the reduced endogenous excitatory feedback. However, the homozygous Brattleboro rat which lacks any VP expression still maintains rhythms of electrical activity, indicating that VP is not the sole factor generating circadian activity. The generation of this rhythmicity may depend upon the interaction of VP with other transmitter systems, such as the inhibitory transmitters somatostatin and GABA which show a circadian variation in efficacy. In addition to its role in feedback amplification of the endogenous rhythm of electrical activity, VP also functions as part of the efferent signal to the rest of the CNS where it potentially regulates a number of behavioural and physiological rhythms, including the circadian activity of the hypothalamo-pituitary-adrenal axis. Thus, the combined amplification and signalling functions makes VP an important component of the neuronal clock function in mammals.

Chronic treatment with oestradiol does not alter in vitro LTP in subfield CA1 of the female rat hippocampus

Population excitatory post-synaptic potentials (pEPSPs) were recorded in vitro from subfield CA1 of the hippocampus of female rats which had been ovariectomized and treated for 14 days with either oil or 17beta-oestradiol (10 microg/day). The currents applied to the Schaffer collateral-commissural input necessary to induce threshold, maximum and 50% maximum pEPSP responses did not differ between groups. Application of trains of pulses (0.1-1 s; 100 Hz) evoked post-tetanic and long-term (> 60 min) potentiation of pEPSP responses, the magnitude of which was related to stimulus duration in both groups. However, the degree of potentiation induced by near-threshold (0.1, 0.15 and 0.2 s) and saturating (1 s) stimuli did not differ between groups. Thus, despite reports that oestradiol can modulate synaptic spine density and glutamatergic and GABAergic components of the inputs to CA1, these data suggest that chronic oestradiol treatment has no effect on either the excitability or induction of LTP in the Schaffer collateral-commissural-CA1 pathway.

The pulsatile characteristics of hypothalamo-pituitary-adrenal activity in female Lewis and Fischer 344 rats and its relationship to differential stress responses

The dynamic patterns of basal and stimulated hypothalamo-pituitary-adrenal (HPA) activity of freely moving female Lewis and Fischer 344 rats were compared using an automated blood-sampling system. Both strains showed pulsatile corticosterone release throughout the 24 h cycle. Lewis rats showed clear circadian variation in both pulse frequency (8.4 +/- 0.4 pulses between 1700-2300 h vs. 5.3 +/- 0.8 pulses between 0500-1100 h; P < 0.05) and height (198 +/- 27 ng/ml between 1700-2300 h vs. 107 +/- 14 ng/ml between 0500-1100 h; P < 0.05). Fischer rats exhibited pulses of similar frequency and height to those in Lewis rats during the evening, but showed no circadian variation, resulting in higher mean daily corticosterone concentrations. Although both strains showed behavioral and HPA responses to white noise stress (10 min; 114 dB), Fischer rats showed much greater increases in total activity, grooming, and rearings, and two important differences in the corticosterone responses were observed. First, in Lewis rats a clear relationship existed between basal and stimulated HPA activities, in that a significant response was seen only when the stress coincided with the rising (secretory active) phase of a basal pulse. Noise stress coinciding with a falling (nonsecretory) phase elicited no significant response. In contrast, Fischer rats showed similar responses regardless of the underlying pulse phase. Second, after the peak response at 20 min (Lewis, 237 +/- 67 ng/ml; Fischer, 390 +/- 57 ng/ml), corticosterone levels fell rapidly in Lewis rats, but remained maximally elevated for 20 min in Fischer rats, resulting in a significantly greater integrated response. The corticosterone response to i.v. CRF was unaffected by pulse phase in both strains, suggesting that a suprapituitary mechanism mediates the phase-dependent response to stress in the Lewis strain. CRF-induced corticosterone levels rose more rapidly in Fischer rats, peaking at 10 min (473 +/- 95 ng/ml) compared with 30 min (390 +/- 75 ng/ml) in Lewis rats, suggesting greater pituitary sensitivity in this strain. Thus, differences in both central and pituitary control of the HPA axis contribute to the strain difference in stress responsiveness between female Lewis and Fischer rats.

Ultradian rhythm of basal corticosterone release in the female rat: dynamic interaction with the response to acute stress

The present study investigated the dynamic regulation of the hypothalamo-pituitary-adrenal axis and its significance to acute stress responsiveness in the female rat. An automated, frequent blood-sampling technique allowed the circadian rhythm of corticosterone to be resolved into a series of pulses. These were equally distributed (mean interval, 50.9 +/- 3.7 min) throughout the 24-h cycle, but their magnitude varied significantly, being higher between 1800-2200 h (137 +/- 9 ng/ml) than between 0600-1000 h (75 +/- 17 ng/ml). This pattern of release indicates continuous, but variable, activity of the axis throughout the day. The pulsatile ultradian rhythm suggested alternate periods of secretion and inhibition, which were found to have a profound effect on the corticosterone responses to acute stress. Noise stress (10 min, 114 decibels) evoked a transient increase in corticosterone, which reached a maximum (377 +/- 87 ng/ml) 20 min after onset. However, within this group (n = 26) the response varied depending on the underlying basal activity. When stress coincided with a rising (secretory) phase of a pulse, corticosterone concentrations rose to 602 +/- 150% of mean basal concentrations (P < 0.001). In contrast, when stress coincided with a falling (nonsecretory) phase of a pulse, a significantly smaller response, no greater than a basal pulse, was evoked. Thus, the alternate periods of secretion and inhibition generating basal hypothalamo-pituitary-adrenal activity are an important determinant of responses to acute stress.

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.

Region-specific immediate-early gene expression following the administration of corticotropin-releasing hormone in virgin and lactating rats

Central administration of corticotropin-releasing hormone (CRH) induces immediate-early gene (IEG) expression (c-fos and NGFI-B) in forebrain structures in a pattern similar to that observed following restraint stress. Lactating rats display modified neuroendocrine and behavioural responses to stress which have been hypothesized to be at least partially mediated through changes within the circuitry converging on the PVN, including CRH activated pathways. Quantitative measures of regional expression of c-fos and NGFI-B mRNA representative of two classical intracellular pathways, were used to define modification of the circuitry involved in the altered response to central CRH in the lactating female. Compared to saline controls, virgin female rats injected with 5 micrograms CRH i.c.v. displayed significantly increased immediate-early gene expression in the hypothalamic paraventricular nucleus (PVN), arcuate nucleus, lateral septum, bed nucleus of the stria terminalis, central, medial and cortical nuclei of the amygdala, and all subfields of the hippocampal formation. In lactating rats treated with CRH there was a significant increase in c-fos gene expression in the CeA and in the hippocampal subfields CA1, CA4 and dentate gyrus but not in the other areas examined. The i.c.v. administration of CRH significantly increased NGFI-B expression in the PVN, arcuate nucleus, medial amygdala and all hippocampal subfields of virgin rats. Lactating rats treated with CRH failed to show a significant increase in NGFI-B expression in the PVN, median eminence, arcuate nucleus, medial amygdala, CA2 and CA3 subfields of the hippocampus. These results further suggest that changes in specific neural circuits might at least partially underlie the modified responses to CRH and perhaps to stress in the lactating female.

Reduced response of the hypothalamo-pituitary-adrenal axis to alpha1-agonist stimulation during lactation

To determine whether altered noradrenergic activation of the hypothalamo-pituitary-adrenal (HPA) axis contributes to the attenuated neuroendocrine response to stress observed during lactation, the effect of intracerebroventricular injection of the alpha1-agonist methoxamine (100 microg) was compared between virgin and lactating rats. Virgin rats showed significant increases in plasma corticosterone after methoxamine, reaching 317 +/- 44 ng/ml at 10 min and remaining significantly elevated for more than 120 min, but lactating rats showed no significant increase in corticosterone levels. Furthermore, methoxamine induced an increase in paraventricular nucleus (PVN) CRF messenger RNA expression in virgin, but not lactating, animals. Both groups of rats exhibited comparable elevations in plasma PRL after methoxamine treatment. Arginine vasopressin messenger RNA expression within the parvocellular PVN was greater in the lactating animals than in the virgin controls, but methoxamine injection was without further effect. Studies performed on ovariectomized virgin rats and ovariectomized rats receiving estradiol or progesterone replacement failed to reproduce the attenuated HPA responses seen after methoxamine treatment, although methoxamine-induced PRL levels were greatly increased by estradiol, probably arising from an effect on hormone synthesis. In vitro electrophysiological recordings of PVN neurons in hypothalamic slices from proestrous virgin and lactating rats showed that 45-52% of neurons in both groups exhibited excitatory responses to 10(-4) M methoxamine, but there was a differential response to 10(-5) M methoxamine, with PVN neurons from lactating animals failing to show a response. These data show a selective down-regulation of alpha1-mediated activation of the HPA axis in lactating animals. This may contribute to the attenuated stress-induced activation of the HPA axis during lactation.

Central oxytocin administration reduces stress-induced corticosterone release and anxiety behavior in rats

Endocrine responses to noise stress and anxiety-related behaviors were measured in groups of ovariectomized, estradiol-treated female rats given central infusions of oxytocin. Control animals receiving isotonic saline showed a large increase in plasma corticosterone concentrations in response to 10 min of white noise. This response to noise stress was significantly and dose dependently decreased by oxytocin administered intracerebroventricularly at 10 or 100 ng/h for 5 days. Oxytocin also significantly decreased rearing behavior during this stress. When a second noise stress was given 3 days after cessation of oxytocin infusion, corticosterone responses did not differ between the control and previously oxytocin-infused animals. Administration of vasopressin had no significant effect on either the corticosterone or behavioral responses to noise stress. Anxiety-related behaviors were measured on the elevated plus-maze. No significant differences were seen in maze exploration between saline- and oxytocin-treated animals when housed and tested in the same environment. However, when animals were mildly stressed by testing in an unfamiliar environment, oxytocin-treated animals showed a higher proportion of open arm entries and spent significantly more time in the open arms of the maze. Thus, oxytocin exerts a central anxiolytic-like effect on both endocrine and behavioral systems and could play a role in moderating behavioral and physiological responses to stress.

Endocrine and behavioural responses to noise stress: comparison of virgin and lactating female rats during non-disrupted maternal activity

The behavioural and endocrine responses to a 10 min white noise stress have been characterized in female virgin and undisturbed lactating Sprague-Dawley rats. Animals were continuously video-taped and frequent blood samples were collected using an automated sampling system. Noise stress caused hypothalamo-pituitary-adrenal (HPA) activation, as indicated by a rapid increase in plasma corticosterone and ACTH in the virgins: corticosterone concentrations peaked 20 min after initiation of the stress before declining rapidly back to basal concentrations. In contrast, noise stress had no significant effect on either plasma corticosterone or ACTH concentrations in the lactating animals. However, 72 h after weaning the corticosterone response of the ex-lactating rats was of comparable magnitude, but longer duration to that seen in the virgins. Plasma prolactin concentrations were significantly higher in the lactating animals and declined in response to the noise whereas, a transient but reproducible increase was seen in the virgin group. In situ hybridization revealed a significantly lower basal expression of CRF mRNA in the paraventricular nucleus of lactating rats as compared to the virgins, but noise stress had no further effect. Virgin animals showed behavioural responses to the stress, including an increase in the total activity, exploratory behaviours (rearing) and displacement behaviours (grooming). Lactating animals also showed behavioural responses to the noise, but their activities were principally directed towards the pups. These data show that although lactating rats showed normal behavioural reactivity to a psychological stress they showed no statistically significant activation of the HPA axis, suggesting a dissociation of behavioural and neuroendocrine responses to this mild stress.

Pharmacological characterisation of oxytocin binding sites in the ovine pineal gland

Both oxytocin (OT) and [Arg8]vasopressin (AVP) are found within the ovine pineal gland and may function to modulate melatonin secretion. However, the receptors which mediate the actions of these peptides have yet to be characterised. Preliminary studies of ovine pineal microsomal cell membranes showed binding of [3H]OT (79+/-9 fmol/mg) 10 times greater than binding of [3H]AVP (8+/-3 fmol/mg). Saturation studies using either [3H]OT or the selective OT receptor ligand [125I]d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin (OTA) revealed high affinity, single site kinetics (Kd = 1.72+/-0.32 nM; Bmax = 68+/-18 fmol/mg). Binding of [3H]AVP was more effectively displaced by OT than AVP, suggesting that binding may be due to cross-reaction with the OT binding site. Displacement of [3H]OT using a range of selective agonists and antagonist analogues revealed pharmacological characteristics similar to [3H]OT binding sites in the ovine and rat uterus. These data show that the ovine pineal expresses a high density of OT binding sites which may participate in the regulation of melatonin secretion.

Evidence for independent hypertensive effects of oxytocin and vasopressin in the rat dorsal vagal complex

Infusion of OT or the V1 agonist [Phe2,Orn8]-vasotocin (POVT) (50 ng) into the dorsal vagal complex of urethane-anaesthetized male rats induced a sustained hypertension. Simultaneous infusion of the V1 antagonist d(CH2)5[Tyr(Me)2]-VP blocked the effect of POVT, but not of OT. The OT antagonist d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH(9)2]-vasotocin blocked the effect of OT, but not of POVT. The OT agonist [Thr4,Gly7]-OT had no effect. This suggests that OT-induced hypertension involves mechanisms independent of V1 and uterine-type OT receptors.

Characterisation of vasopressin V1a binding sites in the ovine olfactory bulb

In the ewe, [Arg8]vasopressin (AVP) release into the olfactory bulb (OB) modulates transmitter release necessary for the induction of olfactory memory. [3H]AVP binding to a microsomal preparation of ovine OB revealed saturable binding to a single class of high affinity sites (Kd = 2.03 +/- 0.18 nM). The density of binding sites was significantly greater in the lamb than ewe, but did not vary across the adult oestrous cycle. Displacement using AVP analogues showed that their relative affinities for the ovine V1a receptor were identical to the rat hepatic V1a receptor. These data demonstrate a single class of AVP binding sites in the ovine OB and the first pharmacological characterisation of the ovine V1a receptor.

Region-specific reduction in stress-induced c-fos mRNA expression during pregnancy and lactation

Hypothalamo-pituitary-adrenal (HPA) responses to stress are dramatically attenuated during lactation. To examine whether this is due to diminished stress-induced activation of specific areas of the brain involved in HPA responses, c-fos mRNA expression was employed as a marker of stress-induced neuronal activation. Regional levels of expression were quantified in female rats exposed to 30 min immobilisation stress during late pregnancy (days 19-21), early lactation (days 3-4) and mid-lactation (days 10-14), and compared with the levels in virgin females. Stress-induced levels of corticosterone were significantly lower in late pregnant and early lactating rats compared with the levels in virgin females, and this correlated with a marked attenuation of stress-induced c-fos mRNA expression in the parvocellular division of the PVN. This reduced activation suggests that neuroendocrine hyporesponsiveness during lactation may arise from an effect on afferent pathways to the PVN. Extrahypothalamic areas known to be important for HPA activation displayed three patterns of c-fos mRNA expression: (i) in the ventral tegmental area, dorsal vagal complex, pyriform cortex and all areas of the hippocampus (CA1, CA2, CA3, dentate gyrus), expression levels did not vary significantly with reproductive status; (ii) in the locus coeruleus (A6 catecholaminergic group), a peak of expression was detected in late pregnant animals; and (iii) in the medial amygdala, ventral part of the lateral septum and cingulate cortex expression was significantly reduced in pregnant and lactating animals, with a nadir in early lactation. The decreased expression of c-fos mRNA in these latter areas correlated with that in the parvocellular PVN, and suggests that their interaction may contribute to the reduced neuroendocrine responses of lactating rats.

Circadian rhythm of neuronal activity in suprachiasmatic nucleus slices from the vasopressin-deficient Brattleboro rat

In vitro extracellular recordings were made from tissue slices of suprachiasmatic nucleus from homozygous Brattleboro rats which are deficient in vasopressin. A high proportion (56%) of neurons were excited by application of exogenous vasopressin, indicating that the V1 receptors expressed by these neurons were functional. Basal activity of these vasopressin-sensitive neurons showed a marked circadian variation (higher during the subjective light phase) while vasopressin-insensitive neurons showed no significant variation, suggesting the presence of the V1 receptor identifies a population of highly circadian neurons. Suprachiasmatic neurons from both homozygous rats and their heterozygous (vasopressin-containing) litter mates displayed a circadian rhythm of spontaneous (basal) activity, with firing rates declining during the subjective dark phase, indicating that the endogenous pacemaker driving the circadian rhythm was not dependent upon the presence of vasopressin. However, the peak of spontaneous activity displayed during the subjective light phase was significantly lower in the vasopressin-deficient animals. These data show that the presence of endogenous vasopressin within the suprachiasmatic nucleus is not necessary for the generation of the circadian pattern of activity. However, vasopressin does function to amplify the rhythm by its excitatory effect during the light phase.

Endogenous opioid control of somatodendritic oxytocin release from the hypothalamic supraoptic and paraventricular nuclei in vitro

Oxytocin release was measured in a perifusion system from microdissected supraoptic (SO) and paraventricular (PV) nuclei of ovariectomised female rats. An initial period of electrical stimulation (S1) applied through a pair of platinum electrodes evoked an increase in peptide release, however, subsequent periods of stimulation (S2, S3, S4) were increasingly less effective, suggesting depletion of releasable stores. However, addition of the opioid antagonist, naloxone (5 x 10(-5) M), during periods S2 and S3 potentiated this stimulated oxytocin release, indicating the presence of an endogenous opioid inhibition. Tissue from ovariectomised animals pre-treated with progesterone for 3 days showed increased basal secretion but no naloxone-induced potentiation of electrically-stimulated release. However, increasing the naloxone concentration (5 x 10(-5) M) again revealed a potentiation, indicating that progesterone had caused a shift in the effective dose of the antagonist. These data demonstrate that, like their axon terminals in the neurohypophysis, the dendrites of magnocellular oxytocin neurones are under control of endogenous opioids, and that progesterone causes an increase in this opioid tone. This may function to regulate intranuclear oxytocin secretion in the pregnant and periparturient animal.

A combined immunocytochemical and retrograde tracing study of noradrenergic connections between the caudal medulla and bed nuclei of the stria terminalis

Region-specific noradrenergic inputs to the bed nuclei of the stria terminalis (BST) from the caudal medulla were studied using combined Fast Blue injections and tyrosine hydroxylase immunoreactivity (TH-ir). Injections into the rostral, dorsal, ventral and lateral BST resulted in predominantly ipsilateral retrograde labelling restricted to the mediodorsal and ventrolateral caudal medulla. Mediodorsal projecting neurones comprised the A2 TH-ir and a second non-aminergic group medial to A2. All ventrolateral retrogradely labelled neurons showed TH-ir and corresponded to A1. Injections into the caudal BST did not label the A2 and very few A1 neurones, indicating a paucity of noradrenergic inputs from this area of the medulla.

Electrical recordings of magnocellular supraoptic and paraventricular neurons displaying both oxytocin- and vasopressin-related activity

In suckled rats, some magnocellular neurons displayed both vasopressin-related phasic activity and oxytocin-related milk ejection bursts. Characteristics of basal activity and interspike intervals resembled those of vasopressin neurons. Bursts were coincident with those of oxytocin neurons and were facilitated by centrally injected oxytocin, but had lower maximum instantaneous frequency and often no after-inhibition. These data provide evidence of magnocellular neurones of mixed electrophysiological phenotype and complement reports of peptide coexistence.

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.

The effects of [Arg8]vasopressin and [Arg8]vasotocin on the firing rate of suprachiasmatic neurons in vitro

The excitatory effect of [Arg8]-vasopressin and its potential contribution to the circadian cycle of electrical activity in the suprachiasmatic nucleus of the rat was investigated using extracellular recordings from hypothalamic slices from virgin female rats. The majority of neurons tested for their responses to vasopressin and [Arg8]-vasotocin displayed coincident, dose-dependent excitation by both peptides, although the relative efficacy varied between neurons, with some showing a highly preferential excitation by vasotocin. Perifusion with the vasopressin receptor antagonist d(CH2)5[Tyr(OEt)2,Val4,Cit8]-vasopressin was able to block the majority of responses to vasopressin or vasotocin (20/25), and similar excitation could be induced by the selective agonist [Phe2,Orn8]-vasotocin, indicating a mainly V1 receptor-mediated effect. Few neurons (3/27; 11%) responded to the oxytocin-specific agonist, [Thr4,Gly7]-oxytocin, suggesting a low occurrence of oxytocin receptors. In addition to blocking the action of exogenous vasopressin, the V1 antagonist caused a reversible suppression of spontaneous basal activity in 7/25 cases, consistent with the presence of an endogenous excitatory vasopressin tone. In agreement with previous reports, the activity of suprachiasmatic nucleus neurons showed a significant correlation between spontaneous activity and the light-dark cycle, with activity decreasing during the subjective dark phase. When neurons were divided on the basis of their response to vasopressin and/or vasotocin, the peptide-sensitive neurons continued to show a strong correlation (r = 0.513, P < 0.01) while the insensitive neurons showed no correlation (r = 0.136, P > 0.05). These data confirm the presence of V1 type receptors in the suprachiasmatic nucleus and also indicate a small number of neurons possessing additional classes of receptor selective for either oxytocin or vasotocin. Contrary to previous reports, they also demonstrate that endogenous vasopressin tonically excites suprachiasmatic nucleus neurons. The fact that vasopressin-sensitive (but not vasopressin-insensitive) neurons show a level of basal activity correlated with time, suggests that this tone may contribute to the circadian cycle of electrical activity in the suprachiasmatic nucleus.

Suppression of suprachiasmatic nucleus neurone activity with a vasopressin receptor antagonist: possible role for endogenous vasopressin in circadian activity cycles in vitro

Neurones of the rat suprachiasmatic nucleus (SCN) were tested with [Arg8]vasopressin (AVP) and the AVP receptor antagonist, [d(CH2)5,d-Tyr(OEt)2,Val4,Cit8]-vasopressin in vitro. 52% of AVP-responsive neurones showed an antagonist-induced decrease in activity, indicative of the presence of an endogenous excitatory tone. The magnitude of this effect declined significantly between subjective light and dark phases, consistent with the possibility that circadian fluctuations in endogenous AVP excitation contribute to the cycle of electrical activity within the SCN. However, similar fluctuations in basal activity between the light and dark phases was observed for both antagonist-sensitive and -insensitive neurones, indicating that endogenous AVP was not the only factor determining the circadian cycle.

[Arg8]vasotocin excites neurones in the dorsal vagal complex in vitro: evidence for an action through novel class(es) of CNS receptors

Using extracellular recordings from brainstem slices in vitro, it was demonstrated that a high proportion (38/56) of neurones in the dorsal vagal complex of dioestrus, virgin female rats exhibit an excitatory response to [Arg8]-vasotocin (AVT). Pharmacological characterization suggests that these responses cannot be entirely explained by interaction with either of the currently known classes of central receptors for oxytocin (OT) and vasopressin (V1a). Comparison of the responses with those to the OT receptor-specific agonist [Thr4,Gly7]-OT (TGOT), showed that not all neurones that responded to TGOT also responded to AVT (3/27). Furthermore, while the effects of 10(-7) M TGOT could be blocked either by the broad-spectrum antagonist d(CH2)5[d-Tyr(OEt)2,Val4,Cit8]-vasopressin or by the selective OT receptor antagonist d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin, these peptides did not completely block the responses to AVT, indicating that AVT is unlikely to act through the central OT receptor. The responses to AVT and [Arg8]-vasopressin (AVP) indicated the presence of at least 2 classes of receptor with which these agonists could act. Of 42 neurones tested with both AVP and AVT, none responded to AVP in the absence of a response to AVT, while 7/42 responded to AVT without a response to AVP. This might be explained by AVP acting through only the V1 receptor, while AVT acts through both the V1 and its own novel class of receptor. This was substantiated by the fact that two OT/V1 receptor antagonists, d(CH2)5[d-Tyr(OEt)2,Val4,Cit8]-VP and d(CH2)5[Tyr(Me)2,Tyr-NH2(9)]-AVP, were unable to block completely all the responses to AVT at a dose which suppressed responses to both AVP and TGOT.(ABSTRACT TRUNCATED AT 250 WORDS)