Activation of oxytocin neurones by systemic cholecystokinin is unchanged by morphine dependence or withdrawal excitation in the rat

Heralding a new era of oxytocinergic research: New tools, new problems?

According to classic neuroendocrinology, hypothalamic oxytocin cells can be categorized into parvo- and magnocellular neurons. However, research in the last decade provided ample evidence that this black-and-white model of oxytocin neurons is most likely oversimplified. Novel genetic, functional and morphological studies indicate that oxytocin neurons might be organized in functional modules and suggest the existence of five or more distinct oxytocinergic subpopulations. However, many of these novel, automated high-throughput techniques might be inherently biased and interpretation of acquired data needs to be approached with caution to enable drawing sound and reliable conclusions. In addition, the recent finding that astrocytes in various brain regions express functional oxytocin receptors represents a paradigm shift and challenges the view that oxytocin primarily acts as a direct peptidergic neurotransmitter. This review highlights the latest technical advances in oxytocinergic research, puts recent studies on the oxytocin system into context and formulates various provocative ideas based on novel findings that challenges various prevailing hypotheses and dogmas about oxytocinergic modulation.

α-Melanocyte-stimulating hormone inhibition of oxytocin neurons switches to excitation in late pregnancy and lactation

Oxytocin is secreted into the periphery by magnocellular neurons of the hypothalamic supraoptic and paraventricular nuclei (SON and PVN) to trigger uterine contraction during birth and milk ejection during suckling. Peripheral oxytocin secretion is triggered by action potential firing, which is regulated by afferent input activity and by feedback from oxytocin secreted into the extracellular space from magnocellular neuron somata and dendrites. A prominent input to oxytocin neurons arises from proopiomelanocortin neurons of the hypothalamic arcuate nucleus that secrete an alpha-melanocyte-stimulating hormone (α-MSH), which inhibits oxytocin neuron firing in non-pregnant rats by increasing somato-dendritic oxytocin secretion. However, α-MSH inhibition of oxytocin neuron firing is attenuated in mid-pregnancy and somato-dendritic oxytocin becomes auto-excitatory in late-pregnancy and lactation. Therefore, we hypothesized that attenuated α-MSH inhibition of oxytocin neuron firing marks the beginning of a transition from inhibition to excitation to facilitate peripheral oxytocin secretion for parturition and lactation. Intra-SON microdialysis administration of α-MSH inhibited oxytocin neuron firing rate by 33 ± 9% in non-pregnant rats but increased oxytocin neuron firing rate by 37 ± 12% in late-pregnant rats and by 28 ± 10% in lactating rats. α-MSH-induced somato-dendritic oxytocin secretion measured ex vivo with oxytocin receptor-expressing "sniffer" cells, was of similar amplitude in PVN slices from non-pregnant and lactating rats but longer-lasting in slices from lactating rats. Hence, α-MSH inhibition of oxytocin neuron activity switches to excitation over pregnancy while somato-dendritic oxytocin secretion is maintained, which might enhance oxytocin neuron excitability to facilitate the increased peripheral secretion that is required for normal parturition and milk ejection.

Local kisspeptin excitation of rat oxytocin neurones in late pregnancy

Abstract

The hormone, oxytocin, is synthesised by magnocellular neurones of the supraoptic and paraventricular nuclei and is released from the posterior pituitary gland into the circulation to trigger uterine contractions during parturition. Kisspeptin fibre density increases around the supraoptic nucleus over pregnancy and intracerebroventricular kisspeptin excites oxytocin neurones only in late pregnancy. However, the mechanism of this excitation is unknown. Here, we found that microdialysis administration of kisspeptin into the supraoptic nucleus consistently increased the action potential (spike) firing rate of oxytocin neurones in urethane‐anaesthetised late‐pregnant rats (gestation day 18–21) but not in non‐pregnant rats. Hazard analysis of action potential firing showed that kisspeptin specifically increased the probability of another action potential firing immediately after each action potential (post‐spike excitability) in late‐pregnant rats. Patch‐clamp electrophysiology in hypothalamic slices showed that bath application of kisspeptin did not affect action potential frequency or baseline membrane potential in supraoptic nucleus neurones. Moreover, kisspeptin superfusion did not affect the frequency or amplitude of excitatory postsynaptic currents or inhibitory postsynaptic currents in supraoptic nucleus neurones. Taken together, these studies suggest that kisspeptin directly activates oxytocin neurones in late pregnancy, at least in part, via increased post‐spike excitability.

Key points

Oxytocin secretion is triggered by action potential firing in magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei to induce uterine contractions during birth.

In late pregnancy, kisspeptin expression increases in rat periventricular nucleus neurones that project to the oxytocin system.

Here, we show that intra‐supraoptic nucleus administration of kisspeptin increases the action potential firing rate of oxytocin neurones in anaesthetised late‐pregnant rats, and that the increased firing rate is associated with increased oxytocin neurone excitability immediately after each action potential.

By contrast, kisspeptin superfusion of hypothalamic slices did not affect the activity of supraoptic nucleus neurones or the strength of local synaptic inputs to supraoptic nucleus neurones.

Hence, kisspeptin might activate oxytocin neurons in late pregnancy by transiently increasing oxytocin neuron excitability after each action potential.

Opioids and the hormone oxytocin

The neuropeptide Oxytocin (ΟΤ) is involved as a neurohormone, a neurotransmitter, or a neuromodulator in an extensive range of central and peripheral effects, complex emotional and social human behaviors, memory and learning processes. It is implicated in homeostatic, neuroadaptive processes associated with stress responses and substance use via interactions with the hypothalamic-pituitary-adrenal (HPA) axis and the dopamine mesolimbic reward stress system. This chapter reviews the preclinical and clinical literature on the complicated relationships between endogenous and exogenous opioids and ΟΤ systems and attempts to highlight key findings to date on the effectiveness of intranasal OT administration to treat opioid use disorders. OΤ seems to attenuate, even inhibit, the development of opioid use disorders in preclinical models but is still under clinical research as a promising pharmacological agent in the treatment of opioid use related behaviors. Evidence suggests a role for OT as an adjunctive or stand-alone treatment of behavioral, cognitive and emotional deficits associated with substance use, which may be responsible for seeking behavior and relapse. The mechanisms by which oxytocin acts to reverse the neural substrates of these deficits, partially due to substance induced alterations of the endogenous OT system, and thus modify the behavioral response to substance use are discussed. Other clinically relevant issues are also discussed.

A Predictive, Quantitative Model of Spiking Activity and Stimulus-Secretion Coupling in Oxytocin Neurons

Oxytocin neurons of the rat hypothalamus project to the posterior pituitary, where they secrete their products into the bloodstream. The pattern and quantity of that release depends on the afferent inputs to the neurons, on their intrinsic membrane properties, and on nonlinear interactions between spiking activity and exocytosis: A given number of spikes will trigger more secretion when they arrive close together. Here we present a quantitative computational model of oxytocin neurons that can replicate the results of a wide variety of published experiments. The spiking model mimics electrophysiological data of oxytocin cells responding to cholecystokinin (CCK), a peptide produced in the gut after food intake. The secretion model matches results from in vitro experiments on stimulus-secretion coupling in the posterior pituitary. We mimic the plasma clearance of oxytocin with a two-compartment model, replicating the dynamics observed experimentally after infusion and injection of oxytocin. Combining these models allows us to infer, from measurements of oxytocin in plasma, the spiking activity of the oxytocin neurons that produced that secretion. We have tested these inferences with experimental data on oxytocin secretion and spiking activity in response to intravenous injections of CCK. We show how intrinsic mechanisms of the oxytocin neurons determine this relationship: In particular, we show that the presence of an afterhyperpolarization (AHP) in oxytocin neurons dramatically reduces the variability of their spiking activity and even more markedly reduces the variability of oxytocin secretion. The AHP thus acts as a filter, protecting the final product of oxytocin cells from noisy fluctuations.

A switch from GABA inhibition to excitation of vasopressin neurons exacerbates the development angiotensin II-dependent hypertension

Hypothalamic magnocellular neurons secrete vasopressin into the systemic circulation to maintain blood pressure by increasing renal water reabsorption and by vasoconstriction. When blood pressure rises, baroreflex activation normally inhibits vasopressin neurons via activation of GABAergic inputs. However, plasma vasopressin levels are paradoxically elevated in several models of hypertension and in some patients with essential hypertension, despite increased blood pressure. We have previously shown that vasopressin neuron activity is increased early in the development of moderate angiotensin II-dependent hypertension via blunted baroreflex inhibition of vasopressin neurons. Here, we show that antagonism of vasopressin-induced vasoconstriction slows the development of hypertension and that local administration of a GABAA receptor antagonist inhibits vasopressin neurons during, but not before, the onset of hypertension. Taken together, our data suggest that vasopressin exacerbates the increase in blood pressure evident early in the development hypertension and that blunted baroreflex inhibition of vasopressin neurons is underpinned by an excitatory shift in their response to endogenous GABA signalling. This article is protected by copyright. All rights reserved.

Prolactin regulation of oxytocin neurone activity in pregnancy and lactation

KEY POINTS

During lactation, prolactin promotes milk synthesis and oxytocin stimulates milk ejection. In virgin rats, prolactin inhibits the activity of oxytocin-secreting neurones. We found that prolactin inhibition of oxytocin neurone activity is lost in lactation, and that some oxytocin neurones were excited by prolactin in lactating rats. The change in prolactin regulation of oxytocin neurone activity was not associated with a change in activation of intracellular signalling pathways known to couple to prolactin receptors. The change in prolactin regulation of oxytocin neurone activity in lactation might allow coordinated activation of both populations of neurones when required for successful lactation.

ABSTRACT

Secretion of prolactin for milk synthesis and oxytocin for milk secretion is required for successful lactation. In virgin rats, prolactin inhibits oxytocin neurones but this effect would be counterproductive during lactation when secretion of both hormones is required for synthesis and delivery of milk to the newborn. Hence, we determined the effects of intracerebroventricular (i.c.v.) prolactin on oxytocin neurones in urethane-anaesthetised virgin, pregnant and lactating rats. Prolactin (2 μg) consistently inhibited oxytocin neurones in virgin and pregnant rats (by 1.9 ± 0.4 and 1.8 ± 0.5 spikes s^-1 , respectively), but not in lactating rats; indeed, prolactin excited six of 27 oxytocin neurones by >1 spike s^-1 in lactating rats but excited none in virgin or pregnant rats (χ²₂  = 7.2, P = 0.03). Vasopressin neurones were unaffected by prolactin (2 μg) in virgin rats but were inhibited by 1.1 ± 0.2 spikes s^-1 in lactating rats. Immunohistochemistry showed that i.c.v. prolactin increased oxytocin expression in virgin and lactating rats and increased signal transducer and activator of transcription 5 phosphorylation to a similar extent in oxytocin neurones of virgin and lactating rats. Western blotting showed that i.c.v. prolactin did not affect phosphorylation of extracellular regulated kinase 1 or 2, or of Akt in the supraoptic or paraventricular nuclei of virgin or lactating rats. Hence, prolactin inhibition of oxytocin neurones is lost in lactation, which might allow concurrent elevation of prolactin secretion from the pituitary gland and activation of oxytocin neurones for synthesis and delivery of milk to the newborn.

Attenuated hypothalamic responses to α-melanocyte stimulating hormone during pregnancy in the rat

KEY POINTS

Increased appetite and weight gain occurs during pregnancy, associated with development of leptin resistance, and satiety responses to the anorectic peptide α-melanocyte stimulating hormone (α-MSH) are suppressed. This study investigated hypothalamic responses to α-MSH during pregnancy, using c-fos expression in specific hypothalamic nuclei as a marker of neuronal signalling, and in vivo electrophysiology in supraoptic nucleus (SON) oxytocin neurons, as a representative α-MSH-responsive neuronal population that shows a well-characterised α-MSH-induced inhibition of firing. While icv injection of α-MSH significantly increased the number of c-fos-positive cells in the paraventricular, supraoptic, arcuate and ventromedial hypothalamic nuclei in non-pregnant rats, this response was suppressed in pregnant rats. Similarly, SON oxytocin neurons in pregnant rats did not demonstrate characteristic α-MSH-induced inhibition of firing that was observed in non-pregnant animals. Given the known functions of α-MSH in the hypothalamus, the attenuated responses are likely to facilitate adaptive changes in appetite regulation and oxytocin secretion during pregnancy.

ABSTRACT

During pregnancy, a state of positive energy balance develops to support the growing fetus and to deposit fat in preparation for the subsequent metabolic demands of lactation. As part of this maternal adaptation, the satiety response to the anorectic peptide α-melanocyte stimulating hormone (α-MSH) is suppressed. To investigate whether pregnancy is associated with changes in the response of hypothalamic α-MSH target neurons, non-pregnant and pregnant rats were treated with α-MSH or vehicle and c-fos expression in hypothalamic nuclei was then examined. Furthermore, the firing rate of supraoptic nucleus (SON) oxytocin neurons, a known α-MSH responsive neuronal population, was examined in non-pregnant and pregnant rats following α-MSH treatment. Intracerebroventricular injection of α-MSH significantly increased the number of c-fos-positive cells in the paraventricular, arcuate and ventromedial hypothalamic nuclei in non-pregnant rats, but no significant increase was observed in any of these regions in pregnant rats. In the SON, α-MSH did induce expression of c-fos during pregnancy, but this was significantly reduced compared to that observed in the non-pregnant group. Furthermore, during pregnancy, SON oxytocin neurons did not demonstrate the characteristic α-MSH-induced inhibition of firing rate that was observed in non-pregnant animals. Melanocortin receptor mRNA levels during pregnancy were similar to non-pregnant animals, suggesting that receptor down-regulation is unlikely to be a mechanism underlying the attenuated responses to α-MSH during pregnancy. Given the known functions of α-MSH in the hypothalamus, the attenuated responses will facilitate adaptive changes in appetite regulation and oxytocin secretion during pregnancy.

Development of an excitatory kisspeptin projection to the oxytocin system in late pregnancy

KEY POINTS

Oxytocin release from the posterior pituitary gland stimulates uterine contraction during birth but the central mechanisms that activate oxytocin neurones for birth are not well characterized. We found that that kisspeptin fibre density around oxytocin neurones increases in late-pregnant rats. These kisspeptin fibres originated from hypothalamic periventricular nucleus neurones that upregulated kisspeptin expression in late pregnancy. Oxytocin neurones were excited by central kisspeptin administration in late-pregnant rats but not in non-pregnant rats or early- to mid-pregnant rats. Our results reveal the emergence of a new excitatory kisspeptin projection to the oxytocin system in late pregnancy that might contribute to oxytocin neurone activation for birth.

ABSTRACT

The hormone oxytocin promotes uterine contraction during parturition. Oxytocin is synthesized by magnocellular neurones in the hypothalamic supraoptic and paraventricular nuclei and is released into the circulation from the posterior pituitary gland in response to action potential firing. Systemic kisspeptin administration increases oxytocin neurone activity to elevate plasma oxytocin levels. Here, immunohistochemistry revealed that rats on the expected day of parturition (day 21 of gestation) had a higher density of kisspeptin-positive fibres in the perinuclear zone surrounding the supraoptic nucleus (which provides dense glutamatergic and GABAergic innervation to the supraoptic nucleus) than was evident in non-pregnant rats. Retrograde tracing showed the kisspeptin projections to the perinuclear zone originated from the hypothalamic periventricular nucleus. Quantitative RT-PCR showed that kisspeptin receptor mRNA, Kiss1R mRNA, was expressed in the perinuclear zone-supraoptic nucleus and that the relative Kiss1R mRNA expression does not change over the course of pregnancy. Finally, intracerebroventricular administration of kisspeptin increased the firing rate of oxytocin neurones in anaesthetized late-pregnant rats (days 18-21 of gestation) but not in non-pregnant rats, or in early- or mid-pregnant rats. Taken together, these results suggest that kisspeptin expression is upregulated in the periventricular nucleus projection to the perinuclear zone of the supraoptic nucleus towards the end of pregnancy. Hence, this input might activate oxytocin neurones during parturition.

Anti-opioid Effects of RFRP-3 on Magnocellular Neuron Activity in Morphine-naïve and Morphine-treated Female Rats

Neuropeptide FF receptors (NPFFR1 and NPFFR2) have been proposed to possess anti-opioid properties, and be involved in the development of opiate tolerance and dependence. However, there is no evidence to date supporting such opioid effects at the cellular level in vivo. Using in vivo electrophysiological recordings from vasopressin and oxytocin neurons in the supraoptic nucleus, we aimed to determine the effects of NPFFRs on opiate inhibition, tolerance, and dependence at a cellular level. Both vasopressin and oxytocin neurons are acutely inhibited by opioids and develop opiate tolerance. Oxytocin neurons also develop cellular opiate dependence and undergo withdrawal hyperexcitation upon cessation of opiate administration. Here, the classical μ-opioid receptor agonist, morphine robustly inhibited the spontaneous firing rate of vasopressin and oxytocin neurons, and this inhibition was attenuated by pretreatment with the NPFFR1 agonist, RFamide-related peptide-3. In rats infused with morphine for 6 d, vasopressin neurons were unresponsive to morphine, indicating the development of cellular tolerance, but pretreatment with the NPFFR antagonist, GJ14, restored acute morphine inhibition. In morphine-infused rats, RFamide related peptide-3 did not induce withdrawal excitation in oxytocin neurons and GJ14 did not reverse naloxone-precipitated withdrawal excitation. This is the first evidence of anti-opioid effects of the NPFFR system at a cellular level in vivo. Our results suggest that the anti-opioid properties of the NPFFR system reduce morphine sensitivity during tolerance but that it is not involved in dependence.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Induction of hypertension blunts baroreflex inhibition of vasopressin neurons in the rat

Vasopressin secretion from the posterior pituitary gland is determined by action potential discharge of hypothalamic magnocellular neurosecretory cells. Vasopressin is a potent vasoconstrictor, but vasopressin levels are paradoxically elevated in some patients with established hypertension. To determine whether vasopressin neurons are excited in hypertension, extracellular single-unit recordings of vasopressin neurons from urethane-anaesthetized Cyp1a1-Ren2 rats with inducible angiotensin-dependent hypertension were made. The basal firing rate of vasopressin neurons was higher in hypertensive Cyp1a1-Ren2 rats than in non-hypertensive Cyp1a1-Ren2 rats. The increase in firing rate was specific to vasopressin neurons because oxytocin neuron firing rate was unaffected by the induction of hypertension. Intravenous injection of the α1-adrenoreceptor agonist, phenylephrine (2.5 μg/kg), transiently increased mean arterial blood pressure to cause a baroreflex-induced inhibition of heart rate and vasopressin neuron firing rate (by 52 ± 9%) in non-hypertensive rats. By contrast, intravenous phenylephrine did not inhibit vasopressin neurons in hypertensive rats, despite a similar increase in mean arterial blood pressure and inhibition of heart rate. Circulating angiotensin II can excite vasopressin neurons via activation of afferent inputs from the subfornical organ. However, the increase in vasopressin neuron firing rate and the loss of inhibition by intravenous phenylephrine were not blocked by intra-subfornical organ infusion of the angiotensin AT1 receptor antagonist, losartan. It can be concluded that increased vasopressin neuron activity at the onset of hypertension is driven, at least in part, by reduced baroreflex inhibition of vasopressin neurons and that this might exacerbate the increase in blood pressure at the onset of hypertension.

Glial regulation of extrasynaptic NMDA receptor-mediated excitation of supraoptic nucleus neurones during dehydration

Magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) project to the posterior pituitary gland where they release the hormones, vasopressin and oxytocin into the circulation to maintain plasma osmolality. Hormone release is proportionate to SON MNC action potential (spike) firing rate. When activated by ambient extracellular glutamate, extrasynaptic NMDA receptors (eNMDARs) mediate a tonic (persistent) depolarisation to increase the probability of action potential firing. In the present study, in vivo single-unit electrophysiological recordings were made from urethane-anaesthetised female Sprague-Dawley rats to investigate the impact of tonic eNMDAR activation on MNC activity. Water deprivation (for up to 48 h) caused an increase in the firing rate of SON MNCs that was associated with a general increase in post-spike excitability. To determine whether eNMDAR activation contributes to the increased MNC excitability during water deprivation, memantine, which preferentially blocks eNMDARs, was administered locally into the SON by microdialysis. Memantine significantly decreased the firing rate of MNCs recorded from 48-h water-deprived rats but had no effect on MNCs recorded from euhydrated rats. In the presence of the glial glutamate transporter-1 (GLT-1) blocker, dihydrokainate, memantine also reduced the MNC firing rate in euhydrated rats. Taken together, these observations suggest that GLT-1 clears extracellular glutamate to prevent the activation of eNDMARs under basal conditions and that, during dehydration, eNMDAR activation contributes to the increased firing rate of MNCs.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Kisspeptin activation of supraoptic nucleus neurons in vivo

Oxytocin and vasopressin are synthesized by magnocellular neurosecretory cells in the hypothalamic supraoptic and paraventricular nuclei and are released from the posterior pituitary gland into the circulation. Intravenous administration of the ligand for the G protein-coupled receptor 54 receptor, kisspeptin-10, increases plasma oxytocin levels and intracerebroventricular kisspeptin-10 increases vasopressin levels, indicating that kisspeptin might play a role in various physiological functions via stimulation of oxytocin and vasopressin secretion. Because posterior pituitary hormone secretion is dependent on action potential (spike) discharge, we used in vivo extracellular single unit recording to determine the effects of kisspeptin-10 on supraoptic nucleus neurons in urethane-anaesthetized female rats. Intravenous kisspeptin-10 (100 μg) increased the firing rate of oxytocin neurons from 3.7 ± 0.8 to 4.7 ± 0.8 spikes/sec (P = 0.0004), but only a quarter of vasopressin neurons responded to iv kisspeptin-10, showing a short (<3 sec) high-frequency (>15 spikes/sec) burst of firing. By contrast, intracerebroventricular kisspeptin-10 (2 and 40 μg) did not alter oxytocin or vasopressin neuron firing rate. To investigate the pathway involved in the peripheral action of kisspeptin-10, we used i.p. capsaicin to desensitize vagal afferents, which prevented the i.v. kisspeptin-10-induced increase of oxytocin neuron firing rate. This is the first report to show that peripheral, but not central, kisspeptin-10 increases the activity of oxytocin neurons and a proportion of vasopressin neurons and that endogenous kisspeptin regulation of supraoptic nucleus neurons is likely via vagal afferent input, with kisspeptin acting as a hormone rather than as a neuropeptide in this system.

State-dependent changes in astrocyte regulation of extrasynaptic NMDA receptor signalling in neurosecretory neurons

Despite the long-established presence of glutamate NMDA receptors at extrasynaptic sites (eNMDARs), their functional roles remain poorly understood. Factors influencing the concentration and time course of glutamate in the extrasynaptic space, such as the topography of the neuronal–glial microenvironment, as well as glial glutamate transporters, are expected to affect eNMDAR-mediated signalling strength. In this study, we used in vitro and in vivo electrophysiological recordings to assess the properties, functional relevance and modulation of a persistent excitatory current mediated by activation of eNMDARs in hypothalamic supraoptic nucleus (SON) neurons. We found that ambient glutamate of a non-synaptic origin activates eNMDARs to mediate a persistent excitatory current (termed tonic I(NMDA)), which tonically stimulates neuronal activity. Pharmacological blockade of GLT1 astrocyte glutamate transporters, as well as the gliotoxin α-aminodadipic acid, enhanced tonic I(NMDA) and neuronal activity, supporting an astrocyte regulation of tonic I(NMDA) strength. Dehydration, a physiological challenge known to increase SON firing activity and to induce neuroglial remodelling, including reduced neuronal ensheathment by astrocyte processes, resulted in blunted GLT1 efficacy, enhanced tonic I(NMDA) strength, and increased neuronal activity. Taken together, our studies support the view that glial modulation of tonic I(NMDA) activation contributes to regulation of SON neuronal activity, contributing in turn to neuronal homeostatic responses during a physiological challenge.

Acute morphine administration and withdrawal from chronic morphine increase afterdepolarization amplitude in rat supraoptic nucleus neurons in hypothalamic explants

Supraoptic nucleus (SON) neurons secrete either oxytocin or vasopressin into the bloodstream from their axon terminals in the posterior pituitary gland. SON neurons are powerfully inhibited by the classical μ-opioid receptor agonist, morphine. Oxytocin neurons develop morphine dependence when chronically exposed to this opiate, and undergo robust withdrawal excitation when morphine is subsequently acutely antagonized by naloxone. Morphine withdrawal excitation is evident as an increased firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we used sharp electrode recording from SON neurons in hypothalamic explants from morphine naïve and morphine treated rats to determine the effects of morphine on the ADP, and to test the hypothesis that morphine withdrawal increases ADP amplitude in SON neurons. Acute morphine administration (0.05-5.0 μM) caused a dose-dependent hyperpolarization of SON neurons that was reversed by concomitant administration of 10 μM naloxone, or by washout of morphine; counter-intuitively, acute exposure to 5 μM morphine increased ADP amplitude by 78 ± 11% (mean ± SEM). Naloxone-precipitated morphine withdrawal did not alter baseline membrane potential in SON neurons from morphine treated rats, but increased ADP amplitude by 48 ± 11%; this represents a hyper-activation of ADPs because the basal amplitude of the ADP was similar in SON neurons recorded from explants prepared from morphine naïve and morphine treated rats. Hence, an enhanced ADP might contribute to morphine withdrawal excitation of oxytocin neurons.

Dehydration-induced modulation of kappa-opioid inhibition of vasopressin neurone activity

Dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland to reduce water loss in the urine. Vasopressin secretion is determined by action potential firing in vasopressin neurones, which can exhibit continuous, phasic (alternating periods of activity and silence), or irregular activity. Autocrine kappa-opioid inhibition contributes to the generation of activity patterning of vasopressin neurones under basal conditions and so we used in vivo extracellular single unit recording to test the hypothesis that changes in autocrine kappa-opioid inhibition drive changes in activity patterning of vasopressin neurones during dehydration. Dehydration increased the firing rate of rat vasopressin neurones displaying continuous activity (from 7.1 +/- 0.5 to 9.0 +/- 0.6 spikes s(1)) and phasic activity (from 4.2 +/- 0.7 to 7.8 +/- 0.9 spikes s(1)), but not those displaying irregular activity. The dehydration-induced increase in phasic activity was via an increase in intraburst firing rate. The selective -opioid receptor antagonist nor-binaltorphimine increased the firing rate of phasic neurones in non-dehydrated rats (from 3.4 +/- 0.8 to 5.3 +/- 0.6 spikes s(1)) and dehydrated rats (from 6.4 +/- 0.5 to 9.1 +/- 1.2 spikes s(1)), indicating that kappa-opioid feedback inhibition of phasic bursts is maintained during dehydration. In a separate series of experiments, prodynorphin mRNA expression was increased in vasopressin neurones of hyperosmotic rats, compared to hypo-osmotic rats. Hence, it appears that dynorphin expression in vasopressin neurones undergoes dynamic changes in proportion to the required secretion of vasopressin so that, even under stimulated conditions, autocrine feedback inhibition of vasopressin neurones prevents over-excitation.

Participation of opioid peptides in sucking-induced oxytocin and prolactin secretions in lactating goats

The role of opioid peptides in the secretion of oxytocin (OT) and prolactin (PRL) induced by sucking was studied in goats. Seven goats were isolated with their kids (four singletons and three twins) in individual corrals 3-4 weeks after parturition. On day 1 of the experiment, the kids were separated from the does for 7 h and were weighed before and 15 min after being reunited with their mothers to assess the amount of milk obtained by sucking. The does were blood-sampled 10 min before and at the end of the sucking period. On day 2, a similar protocol was followed, but naloxone was given immediately after the first blood sample. On day 3, the protocol was repeated but saline vehicle was injected instead of naloxone. On day 5, the naloxone experiment was repeated as on day 2. Milk ejection was evaluated as the difference in the weight of the kids before and after sucking for 15 min, and the maternal serum levels of OT and PRL were measured by radioimmunoassay. A significant decrease in the weight gain of the kids was obtained when the mothers were treated with naloxone on day 2. Consistently, serum levels of OT and PRL induced by sucking were significantly reduced; indicating that sucking-induced OT secretion for milk ejection in lactating goats is facilitated by opioid peptides. In a second experiment performed in the same animals 10 days later, the administration of OT, immediately after naloxone administration, prevented the decrease in the weight gain induced by naloxone, suggesting that the effect of the opioid antagonist on milk ejection in goats is a result of a reduced OT secretion. The results of this study confirm the importance of sucking-induced OT secretion for milk ejection in lactating goats, and indicate that OT and PRL secretion are regulated by opioid peptides in this species.

Rapid estradiol-17beta modulation of opioid actions on the electrical and secretory activity of rat oxytocin neurons in vivo

During pregnancy, emergence of endogenous opioid inhibition of oxytocin neurons is revealed by increased oxytocin secretion after administration of the opioid receptor antagonist, naloxone. Here we show that prolonged estradiol-17beta and progesterone treatment (mimicking pregnancy levels) potentiates naloxone-induced oxytocin secretion in urethane-anesthetized virgin female rats. We further show that estradiol-17beta alone rapidly modifies opioid interactions with oxytocin neurons, by recording their firing rate in anesthetized rats sensitized to naloxone by morphine dependence. Naloxone-induced morphine withdrawal strongly increased the firing rate of oxytocin neurons in morphine dependent rats. Estradiol-17beta did not alter basal oxytocin neuron firing rate over 30 min, but amplified naloxone-induced increases in firing rate. Firing pattern analysis indicated that acute estradiol-17beta increased oxytocin secretion in dependent rats by increasing action potential clustering without an overall increase in firing rate. Hence, rapid estradiol-17beta actions might underpin enhanced oxytocin neuron responses to naloxone in pregnancy.

Neuroactive steroids attenuate oxytocin stress responses in late pregnancy

In late pregnant rats neuroendocrine stress responses, expressed as increased oxytocin secretion and activation of the hypothalamo-pituitary-adrenal axis, are attenuated. These adaptations preserve the oxytocin store for parturition and prevent pre-term birth, and protect the fetuses from adverse programming by exposure to excess glucocorticoid. Mechanisms of adaptations for oxytocin neurones are reviewed, using challenge with systemic interleukin-1beta, simulating activation of immune signaling by infection, as a stressor of special relevance in pregnancy. In virgin rats, systemic interleukin-1beta stimulates the firing of oxytocin neurones, and hence oxytocin secretion, but interleukin-1beta has no effects in late pregnant rats. This lack of response is reversed by naloxone treatment just before interleukin-1beta administration, indicating endogenous opioid suppression of oxytocin responses in late pregnancy. This opioid presynaptically inhibits noradrenergic terminals impinging on oxytocin neurones. Finasteride pretreatment, inhibiting progesterone conversion to allopregnanolone, a positive GABA(A) receptor allosteric modifier, also restores an oxytocin response to interleukin-1beta. This finasteride effect is reversed by allopregnanolone treatment. In virgin rats allopregnanolone attenuates the oxytocin response to interleukin-1beta, which is exaggerated by naloxone. The effects of naloxone and finasteride in late pregnant rats in restoring an oxytocin response to interleukin-1beta are not additive. Accordingly, allopregnanolone may both enhance GABA inhibition of oxytocin neurone responses to interleukin-1beta, and induce opioid suppression of noradrenaline release onto oxytocin neurones.

Morphine withdrawal increases intrinsic excitability of oxytocin neurons in morphine-dependent rats

To determine whether intrinsic mechanisms drive supraoptic nucleus oxytocin neuron excitation during morphine withdrawal, we calculated the probability of action potential (spike) firing with time after each spike for oxytocin neurons in morphine-naive and morphine-dependent rats in vivo and measured changes in intrinsic membrane properties in vitro. The opioid receptor antagonist, naloxone, increased oxytocin neuron post-spike excitability in morphine-dependent rats; this increase was greater for short interspike intervals (<0.1 s). Naloxone had similar, but smaller (P=0.04), effects in oxytocin neurons in morphine-naive rats. The increased post-spike excitability for short interspike intervals was specific to naloxone, because osmotic stimulation increased excitability without potentiating excitability at short interspike intervals. By contrast to oxytocin neurons, neither morphine dependence nor morphine withdrawal increased post-spike excitability in neighbouring vasopressin neurons. To determine whether increased post-spike excitability in oxytocin neurons during morphine withdrawal reflected altered intrinsic membrane properties, we measured the in vitro effects of naloxone on transient outward rectification (TOR) and after-hyperpolarization (AHP), properties mediated by K+ channels and that affect supraoptic nucleus neuron post-spike excitability. Naloxone reduced the TOR and AHP (by 20% and 60%, respectively) in supraoptic nucleus neurons from morphine-dependent, but not morphine-naive, rats. In vivo, spike frequency adaptation (caused by activity-dependent AHP activation) was reduced by naloxone (from 27% to 3%) in vasopressin neurons in morphine-dependent, but not morphine-naive, rats. Thus, multiple K+ channel inhibition increases post-spike excitability for short interspike intervals, contributing to the increased firing of oxytocin neurons during morphine withdrawal.

Cellular mechanisms underlying neuronal excitability during morphine withdrawal in physical dependence: lessons from the magnocellular oxytocin system

Opiates are used clinically as analgesics, but their euphoric actions can lead to continued use and to dependence and addiction. While there are many factors involved in drug abuse, avoidance of stressful withdrawal symptoms is a key feature of addiction and its treatment. Fundamental to this is the need to understand the cellular processes that induce dependence and lead to the withdrawal syndrome. Many neurones in the brain express opioid receptors but only a few types of neurone develop dependence during chronic morphine exposure. The physiology of opiate-dependent cells is altered such that they require the continued presence of the drug to function normally and this is revealed, in cells that are inhibited by initial acute exposure to opiate, by a rebound hyperexcitation upon opiate withdrawal. Hypothalamic oxytocin neurones robustly develop morphine dependence and provide an exceptional opportunity to probe the cellular mechanisms underlying morphine dependence and withdrawal excitation. Although expression of morphine withdrawal excitation by oxytocin cells requires afferent inputs, the underlying mechanisms appear to reside within the oxytocin neurones themselves and probably involve changes in the intrinsic membrane properties of these neurones.

The role of nitric oxide in morphine dependence and withdrawal excitation of rat oxytocin neurons

Magnocellular oxytocin neurons develop morphine dependence after intracerebroventricular infusion for 5 days as revealed by their profound excitation following naloxone-induced withdrawal. Oxytocin neurons strongly express nitric oxide synthase (NOS) and nitric oxide (NO) inhibits their activity. This study investigated whether excitation of oxytocin neurons during morphine withdrawal involves reduced activity of NOS and NO. Neuron activity was measured in urethane-anaesthetized rats with blood sampling for oxytocin radioimmunoassay and extracellular single unit firing rate recording of supraoptic nucleus oxytocin neurons. To compare morphine-dependent and -naive rats oxytocin secretion was measured during stimulation by intravenous hypertonic saline infusion. Prior treatment with Nomega-nitro-l-arginine methyl ester, a NOS inhibitor, facilitated osmotically stimulated oxytocin secretion in both morphine-dependent and -naive rats. The facilitation was not different between these groups when corrected for the slower responses observed in morphine-dependent rats. Treatment of morphine-dependent rats with Nomega-nitro-l-arginine methyl ester also enhanced oxytocin secretion during naloxone-precipitated withdrawal. Oxytocin neurons excited by withdrawal were recorded during microdialysis application to the supraoptic nucleus of the NO donor sodium nitroprusside alone and in combination with the GABAA antagonist bicuculline. Sodium nitroprusside inhibited oxytocin neurons during naloxone-precipitated morphine withdrawal and, while bicuculline alone increased firing rate, it did not reduce the inhibition by sodium nitroprusside, in contrast with previous findings in naive rats. Together, these findings indicate that NO restraint of oxytocin secretion is not curtailed during morphine dependence and remains a potent inhibitor of withdrawal excitation despite reduced effectiveness on GABA innervation of the supraoptic nucleus. Hence there is no evidence that changes in NO regulation underlie excitation of oxytocin neurons during opiate withdrawal in morphine dependence.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Verapamil prevents withdrawal excitation of oxytocin neurones in morphine-dependent rats

We investigated whether the full expression of morphine withdrawal excitation by supraoptic nucleus (SON) oxytocin neurones is a property of the neurones themselves or a partial function of their afferent inputs, by interrupting synaptic input activity via central administration of the L-type Ca(2+) channel blocker verapamil. In morphine-dependent rats, withdrawal-induced release of oxytocin from the posterior pituitary was suppressed by prior administration of intracerebroventricular (i.c.v.) verapamil (160 microg), as was release of oxytocin within the SON measured by microdialysis. During morphine withdrawal the increased electrical activity of SON neurones was also reduced both by i.c.v. verapamil and microdialysis application of verapamil or nifedipine into the SON. Oxytocin secretion evoked by electrical stimulation of the pituitary stalk was unaffected by i.c.v. verapamil suggesting a central site of action. To determine whether the inhibitory actions of verapamil were specific to morphine withdrawal, we also investigated the effects of verapamil on other oxytocin-secreting stimuli. I.C.V. verapamil given to morphine-naïve rats abolished pituitary oxytocin release in response to activation of brainstem or rostral excitatory inputs by cholecystokinin (20 microg kg(-1), i.v.) and 1.5 M saline (4 ml kg(-1), i.p.) respectively, whilst in lactating rats, i.c.v. verapamil reduced suckling-induced release of oxytocin within the SON. These results suggest that verapamil has a central site of action on stimulated oxytocin release (including an action within the SON) and that both pre and post-synaptic L-type Ca(2+) channels are required for the full expression of morphine withdrawal in SON oxytocin neurones.

Local morphine withdrawal increases c-fos gene, Fos protein, and oxytocin gene expression in hypothalamic magnocellular neurosecretory cells

We measured stimulation of c-fos and oxytocin gene expression during excitation of oxytocin cells induced by systemic or local morphine withdrawal. Female rats were made morphine-dependent by intracerebroventricular morphine infusion over 5 d. Morphine withdrawal, induced by systemic injection of the opioid antagonist naloxone (5 mg/kg) in conscious or anesthetized rats, increased the density of c-fos messenger RNA and of oxytocin heterogeneous nuclear RNA in supraoptic nucleus cells compared with those of nonwithdrawn rats; c-fos messenger RNA was also increased in the magnocellular and parvocellular paraventricular nuclei of withdrawn rats. Morphine withdrawal increased the number of Fos-immunoreactive cells in the supraoptic and magnocellular paraventricular nuclei of conscious or pentobarbitone-anesthetized rats. Morphine withdrawal also increased Fos-immunoreactive cell numbers in the parvocellular paraventricular nucleus of conscious but not anesthetized rats. Central administration of the alpha(1)-adrenoreceptor antagonist benoxathian (5 microg/min) did not prevent morphine withdrawal-induced increases in the numbers of Fos-immunoreactive neurons in the supraoptic or magnocellular paraventricular nucleus. Unilateral microdialysis administration of naloxone (10(-5) M) into the supraoptic nucleus of anesthetized morphine-dependent rats increased Fos-immunoreactive cell numbers compared with the contralateral nucleus. Finally, we investigated whether dependence could be induced by chronic unilateral infusion of morphine into a supraoptic nucleus; systemic naloxone (5 mg/kg) increased Fos-immunoreactive cell numbers in the morphine-infused nucleus compared with the contralateral nucleus. Thus, morphine withdrawal excitation increases c-fos and oxytocin gene expression in supraoptic nucleus neurons. This occurs independently from excitation of their ascending noradrenergic inputs, and both dependence and withdrawal can be induced within the supraoptic nucleus.

Local morphine withdrawal increases c-fos gene, Fos protein, and oxytocin gene expression in hypothalamic magnocellular neurosecretory cells

We measured stimulation of c-fos and oxytocin gene expression during excitation of oxytocin cells induced by systemic or local morphine withdrawal. Female rats were made morphine-dependent by intracerebroventricular morphine infusion over 5 d. Morphine withdrawal, induced by systemic injection of the opioid antagonist naloxone (5 mg/kg) in conscious or anesthetized rats, increased the density of c-fos messenger RNA and of oxytocin heterogeneous nuclear RNA in supraoptic nucleus cells compared with those of nonwithdrawn rats; c-fos messenger RNA was also increased in the magnocellular and parvocellular paraventricular nuclei of withdrawn rats. Morphine withdrawal increased the number of Fos-immunoreactive cells in the supraoptic and magnocellular paraventricular nuclei of conscious or pentobarbitone-anesthetized rats. Morphine withdrawal also increased Fos-immunoreactive cell numbers in the parvocellular paraventricular nucleus of conscious but not anesthetized rats. Central administration of the alpha(1)-adrenoreceptor antagonist benoxathian (5 microg/min) did not prevent morphine withdrawal-induced increases in the numbers of Fos-immunoreactive neurons in the supraoptic or magnocellular paraventricular nucleus. Unilateral microdialysis administration of naloxone (10(-5) M) into the supraoptic nucleus of anesthetized morphine-dependent rats increased Fos-immunoreactive cell numbers compared with the contralateral nucleus. Finally, we investigated whether dependence could be induced by chronic unilateral infusion of morphine into a supraoptic nucleus; systemic naloxone (5 mg/kg) increased Fos-immunoreactive cell numbers in the morphine-infused nucleus compared with the contralateral nucleus. Thus, morphine withdrawal excitation increases c-fos and oxytocin gene expression in supraoptic nucleus neurons. This occurs independently from excitation of their ascending noradrenergic inputs, and both dependence and withdrawal can be induced within the supraoptic nucleus.

Opioid modulation of magnocellular neurosecretory cell activity

Magnocellular neurosecretory cells of the hypothalamic supraoptic and paraventricular nuclei secrete the hormones, oxytocin and vasopressin, into the systemic circulation from the posterior pituitary gland. Oxytocin is important for parturition and is essential for lactation. Vasopressin regulates body fluid homeostasis. The secretion of these hormones is altered in response to peripheral stimuli that are conveyed via projections from other parts of the brain. Endogenous opioid peptide systems interact with the magnocellular neurosecretory system at several levels to restrain the basal secretion of these hormones as well as their secretory responses to various physiological stimuli. The inhibition of basal secretion can occur at the level of the neurosecretory terminals where endogenous opioids inhibit the release of oxytocin, and at the cell bodies of magnocellular cells to modulate the activity pattern of vasopressin cells. The responses of the magnocellular neurosecretory system to physiological stimuli are also regulated by these mechanisms but in addition probably also by pre-synaptic inhibition of afferent inputs to magnocellular cells as well as direct effects on the cell bodies of afferent input cells to modulate their activity. Here, we review the mechanisms and functional consequences of opioid interactions with oxytocin and vasopressin cells.

Physiological pathways regulating the activity of magnocellular neurosecretory cells

Magnocellular oxytocin and vasopressin cells are among the most extensively studied neurons in the brain; their large size and high synthetic capacity, their discrete, homogeneous distribution and the anatomical separation of their terminals from their cell bodies, and the ability to determine their neuronal output readily by measurements of hormone concentration in the plasma, combine to make these systems amenable to a wide range of fundamental investigations. While vasopressin cells have intrinsic burst-generating properties, oxytocin cells are organized within local pattern-generating networks. In this review we consider the rôle played by particular afferent pathways in the regulation of the activity of oxytocin and vasopressin cells. For both cell types, the effects of changes in the activity of synaptic input can be complex.

Interruption of central noradrenergic pathways and morphine withdrawal excitation of oxytocin neurones in the rat

1. We have tested the hypothesis that morphine withdrawal excitation of oxytocin neurones that follows from administration of naloxone to morphine-dependent rats is a consequence of excitation of noradrenergic neurones. 2. Female rats were made morphine dependent by intracerebroventricular (i.c.v.) infusion of the opioid at increasing doses over 5 days. On the sixth day, the rats were anaesthetized with urethane or pentobarbitone and prepared for blood sampling to determine plasma oxytocin by radioimmunoassay or for in vivo extracellular recording of the firing rate of identified oxytocin neurones from the supraoptic nucleus. Morphine withdrawal was induced by intravenous (i.v.) injection of the opioid antagonist naloxone (5 mg kg-1). 3. In one group of rats the noradrenergic projections to the hypothalamus were lesioned by i.c.v. injection of 6-hydroxydopamine immediately prior to the induction of morphine dependence. In these rats the oxytocin secretion induced by i.v. cholecystokinin was reduced to 9 % of that seen in sham-lesioned rats but in contrast, no attenuation of morphine withdrawal-induced oxytocin secretion was observed. 4. i.c.v. infusion of the alpha1-adrenoreceptor antagonist benoxathian, at up to 5.3 microg min-1, dose- dependently inhibited the withdrawal excitation of oxytocin neurones in morphine-dependent rats under urethane anaesthesia, and benoxathian reduced withdrawal-induced oxytocin secretion to 37 % of that of vehicle-infused rats. i.c.v. benoxathian also inhibited the activity of oxytocin neurones in morphine-naïve rats. Similarly, microdialysis administration of 2 mM benoxathian directly onto the surface of the supraoptic nucleus reduced the activity of oxytocin neurones by 53 %. 5. Thus noradrenergic systems are not essential for the expression of morphine withdrawal excitation, since chronic neurotoxic destruction of the noradrenergic inputs to the hypothalamus did not affect the magnitude of withdrawal-induced oxytocin secretion. However, tonically active noradrenergic inputs influence the excitability of oxytocin neurones, and acute antagonism of this noradrenergic tone can powerfully impair the ability of oxytocin neurones to exhibit morphine withdrawal excitation.

Altered cholecystokinin binding site density in the supraoptic nucleus of morphine-tolerant and -dependent rats

The processes underlying the development of neuronal tolerance to and dependence upon opiates are not yet fully understood. To evaluate a possible role for cholecystokinin (CCK) in these processes, quantitative receptor autoradiography and in situ hybridisation histochemistry were used to study both the density and distribution of sulphated CCK octapeptide (CCK8S) binding sites and preproCCK peptide mRNA levels within the dorsal (oxytocin neurone-rich) supraoptic nuclei of rats given an intracerebroventricular (i.c.v.) infusion of morphine over 5 days, which is known to induce tolerance and dependence in mechanisms regulating oxytocin neurones. Specific CCK8S binding was significantly increased in the supraoptic nuclei of both morphine-dependent and salt-loaded (2% sodium chloride to drink for 48 h) rats compared to their respective controls (P < 0.05). In situ hybridisation histochemistry revealed no difference in preproCCK mRNA levels within supraoptic neurones of (i.c.v.) morphine-treated compared with either i.c.v. vehicle-treated or untreated control animals. These results suggest that CCK receptor mechanisms involved in the control of magnocellular oxytocin neurone activation are upregulated during chronic morphine treatment, and this may favour increased sensitivity to CCK, thereby offsetting the inhibitory actions of morphine, contributing to tolerance and perhaps to the withdrawal excitation characteristic of dependence.

Local opioid inhibition and morphine dependence of supraoptic nucleus oxytocin neurones in the rat in vivo

1. Single neurones of the rat supraoptic nucleus were recorded during microdialysis of naloxone onto the ventral surface of the nucleus in anaesthetized rats. We used this combination of techniques to test whether the acute or chronic effects of systemically or centrally applied opioids upon oxytocin cell activity were due to actions of the opioids within the nucleus itself. 2. Supraoptic nucleus oxytocin neurones were identified antidromically and by an excitatory response to intravenously injected cholecystokinin. Acute intravenous injection of the kappa-agonist U50488H or the mu-agonist morphine (1-5 mg kg-1) reduced the firing rate of identified oxytocin neurones by 97.7 +/- 4.8% (n = 6) and 94.1 +/- 4.1% (n = 7), respectively. The inhibition by each of these opioids was completely reversed after administration by microdialysis (retrodialysis) of the opioid antagonist naloxone (0.1-1.0 microgram microliter-1 at 2 microliters min-1) onto the exposed ventral surface of the supraoptic nucleus. 3. Retrodialysis of naloxone (0.1-10.0 micrograms microliter-1) onto the supraoptic nucleus of rats made dependent by intracerebroventricular morphine infusion for 5 days increased the firing rate of oxytocin neurones from 0.9 +/- 0.4 to 3.1 +/- 0.7 spikes s-1 (P < 0.05, n = 6). This increase in firing rate from basal was 58.5 +/- 15.1% of that following subsequent intravenously injected naloxone (5 mg kg-1). 4. Thus, the acute inhibition of supraoptic nucleus oxytocin neurones which results from systemic administration of opioid agonists primarily occurs within the supraoptic nucleus itself, since the antagonist naloxone was effective when given into the supraoptic nucleus. Furthermore, oxytocin neurones develop morphine dependence by a mechanism which is distinct from an action on their distant afferent inputs. Nevertheless, withdrawal excitation of these afferent inputs may enhance the magnitude of oxytocin neurone withdrawal excitation.

The role of afferent inputs to supraoptic nucleus oxytocin neurons during naloxone-precipitated morphine withdrawal in the rat

During prolonged exposure to morphine, oxytocin neurons of the rat supraoptic nucleus develop dependence, shown by hyperexcitation following morphine withdrawal. The present study investigated the role of afferent projections to the supraoptic nucleus in this withdrawal excitation. Rats were made morphine-dependent by continuous intracerebroventricular infusion of morphine at increasing doses (up to 50 microg/h). On the sixth day, rats were anaesthetized with pentobarbitone and morphine withdrawal was precipitated by intraperitoneal injection of naloxone (5 mg/kg). Fos-immunoreactivity in the supraoptic nucleus, and also in the median preoptic nucleus, organum vasculosum of the lamina terminalis and subfornical organ, which project to the supraoptic nucleus, increased following morphine withdrawal. However, retrograde tracing from the supraoptic nucleus showed that, of the neurons in these regions which project to the supraoptic nucleus, only 0.4-7.1% expressed Fos in response to morphine withdrawal. Following morphine withdrawal, Fos-immunoreactivity was present in 39.2% and 19.8% of the tyrosine hydroxylase-immunoreactive neurons of the A1/C1 and A2/C2 cell groups. Of the cells in these regions identified as projecting to the supraoptic nucleus, 11.3% in the region of the A2 cell group and 12.7% in the region of the A1 cell group expressed Fos after morphine withdrawal. In a second study, monoamine release was measured in the supraoptic nucleus of urethane-anaesthetized morphine-dependent and -naive rats. Retrodialysis of naloxone (10[-5] M) into the supraoptic nucleus induced a small increase in plasma oxytocin concentration in morphine-dependent rats (13.5+/-4.8 pg/ml increase) but not in naive rats (1.2+/-5.9 pg/ml decrease), with no significant change in monoamine release in either morphine-dependent or -naive rats. Intravenous injection of naloxone (5 mg/kg) 1 h later produced a further significant increase in plasma oxytocin concentration in morphine-dependent rats concomitant with a significant increase in noradrenaline release from the supraoptic nucleus. Thus, morphine-withdrawal excitation of supraoptic oxytocin neurons occurs concurrently with a modestly increased activity of their input from the brainstem, and very little activation in other known inputs.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.