Effects of morphine and D-Ala, D-Leu enkephalin on the electrical activity of supraoptic neurosecretory cells in vitro

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Allopregnanolone in the brain: protecting pregnancy and birth outcomes

A successful pregnancy requires multiple adaptations in the mother's brain that serve to optimise foetal growth and development, protect the foetus from adverse prenatal programming and prevent premature delivery of the young. Pregnancy hormones induce, organise and maintain many of these adaptations. Steroid hormones play a critical role and of particular importance is the progesterone metabolite and neurosteroid, allopregnanolone. Allopregnanolone is produced in increasing amounts during pregnancy both in the periphery and in the maternal and foetal brain. This review critically examines a role for allopregnanolone in both the maternal and foetal brain during pregnancy and development in protecting pregnancy and birth outcomes, with particular emphasis on its role in relation to stress exposure at this time. Late pregnancy is associated with suppressed stress responses. Thus, we begin by considering what is known about the central mechanisms in the maternal brain, induced by allopregnanolone, that protect the foetus(es) from exposure to harmful levels of maternal glucocorticoids as a result of stress during pregnancy. Next we discuss the central mechanisms that prevent premature secretion of oxytocin and consider a role for allopregnanolone in minimising the risk of preterm birth. Allopregnanolone also plays a key role in the foetal brain, where it promotes development and is neuroprotective. Hence we review the evidence about disruption to neurosteroid production in pregnancy, through prenatal stress or other insults, and the immediate and long-term adverse consequences for the offspring. Finally we address whether progesterone or allopregnanolone treatment can rescue some of these deficits in the offspring.

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.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Effect of centrally administered opioid receptor agonists on CSF and plasma oxytocin concentrations in dogs

OBJECTIVE

To measure oxytocin concentrations in blood and CSF following central administration of opioid agonists in dogs.

ANIMALS

5 male dogs.

PROCEDURE

In a crossover design, CSF and blood were collected immediately before and 15 and 30 minutes after cisternal administration of D-Ala2, MePhe4, Gly-ol-enkephalin (DAMGO, a mu-receptor agonist); D-Pen, pCl-Phe4, D-Pen5-enkephalin (a delta-receptor agonist); U50488H (a kappa-receptor agonist); morphine; and saline (0.9% NaCl) solution.

RESULTS

Plasma oxytocin concentration was significantly increased 15 minutes after administration of DAMGO and 30 minutes after administration of U50488H, compared with concentrations obtained after administration of saline solution. Concentration of oxytocin in CSF was significantly decreased 30 minutes after administration of U50488H, compared with concentration after administration of saline solution.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that in male dogs, activation of centrally located mu and kappa receptors elicits an overall excitatory effect on neurons that regulate peripheral release of oxytocin, whereas activation of centrally located kappa receptors elicits an overall inhibitory effect on neurons that regulate central release. These results are in contrast to those reported for other species, in which opioids have a pronounced inhibitory effect on release of oxytocin from the neurohypophysis.

Chronic morphine treatment inhibits oxytocin release from the supraoptic nucleus slices of rats

Effect of chronic morphine treatment on oxytocin (OT) release from the long term-cultured organotypic slice of the supraoptic nucleus (SON) was investigated using radioimmunoassay. The co-localization of oxytocin and mu-opioid receptor in neurons within the SON was observed with the double-labeled methods of in situ hybridization combined with immunohistochemistry. After exposure to morphine for 6days, the OT levels in culture media were significantly decreased. Naloxone caused much greater release of OT in chronic morphine treatment group than in controls. Naloxone has no effect after acute morphine treatment. 90% of OT-ir (immunoreactive) neurons expressed mu-opioid receptor mRNA in the SON and 45% of the neurons that expressed mu-opioid receptor mRNAs were OT-ir neurons. These results indicated that the neurons within SON could develop dependence on morphine in vitro, and these effects might be exerted via mu-opioid receptor in oxytocin neurons of the SON.

Effects of the endogenous opioid peptide, endomorphin 1, on supraoptic nucleus oxytocin and vasopressin neurones in vivo and in vitro

We investigated the actions of the endogenous opioid tetra-peptide endomorphin 1, a selective mu-opioid receptor agonist, on oxytocin and vasopressin cell activity in vivo and in vitro. The activity of antidromically-identified supraoptic nucleus cells were recorded from urethane-anaesthetized female rats. The firing rates of both oxytocin and vasopressin cells were reduced by intracerebroventricular endomorphin 1 (5 - 100 pmol); this inhibition was prevented by intravenous naloxone (5 mg kg(-1)). A second group of rats was infused intracerebroventricularly with endomorphin 1 (27 pmol min(-1)) over 5 days. The firing rates of oxytocin and vasopressin cells in endomorphin 1 pre-treated rats were similar to those of endomorphin 1 naïve rats, indicating tolerance to the inhibitory effects of endomorphin 1. Intravenous naloxone induced similar modest and transient increases in the firing rate of oxytocin cells in endomorphin 1 pre-treated rats and endomorphin 1 naïve rats, indicating that endomorphin 1, unlike the mu-opioid alkaloid agonist, morphine, does not induce mu-opioid dependence in these cells. In vitro, whole-cell current clamp recordings were made from supraoptic nucleus cells in superfused coronal hypothalamic slices from young female rats. Endomorphin 1 (100 nM) inhibited the firing rate of oxytocin cells but had no significant effect on vasopressin cells at up to 10 microM. Inhibition of oxytocin cells was reversed by naloxone, and remained when synaptic transmission was blocked by superfusion with low Ca(2+)/Co(2+)-containing medium. Thus, endomorphin 1 directly inhibits oxytocin cells but inhibits vasopressin cells by indirect actions. Chronic endomorphin 1 administration induces mu-opioid tolerance in oxytocin and vasopressin cells but not mu-opioid dependence in oxytocin cells.

Chronic morphine treatment inhibits oxytocin synthesis in rats

The changes of oxytocin content and mRNA expression in some nuclei were investigated in morphine-dependent rats using radioimmunoassay (RIA) and in situ hybridization (ISH). After chronic administration of morphine, the oxytocin content in supraoptic nucleus (SON) and nucleus accumbens (NAc) decreased, and increased in the ventral tegment area (VTA) and locus coeruleus (LC), but did not change in other nuclei including the paraventricular nucleus (PVN), lateral septum (SEPTUM), raphe magnus nucleus (NRM) and periaquaductal gray (PAG). In morphine-L dependent rats, naloxone increased the levels of oxytocin in SON and PVN, but decreased that in LC. ISH first showed that chronic morphine treatment inhibited the oxytocin synthesis in SON but not in PVN. The present study demonstrates that chronic morphine treatment alters the brain oxytocin system, suggesting that oxytocin might contribute to the behavioral and neuroendocrine responses to morphine.

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 injection of pertussis toxin attenuates morphine withdrawal excitation of rat supraoptic nucleus neurones

Morphine inhibits oxytocin neurones via G(i/o)-protein-linked mu-opioid receptors. Following chronic morphine administration oxytocin cells develop dependence, shown by withdrawal excitation after administration of the opioid antagonist, naloxone. Here, inactivation of G(i/o)-proteins by pre-treatment of morphine-dependent rats with pertussis toxin injected into the left supraoptic nucleus reduced withdrawal-induced Fos protein expression within the injected nucleus by 41+/-10% compared to the contralateral nucleus, indicating that functional G(i/o)-proteins are essential for the development and/or expression of morphine dependence by oxytocin cells in the supraoptic nucleus. In another group of rats, pertussis toxin did not alter the responses to either systemic cholecystokinin administration or systemic hypertonic saline administration, indicating that pertussis toxin does not prevent oxytocin cells from responding to stimuli that are not mediated by G(i/o)-proteins. Finally, pertussis toxin reduced acute morphine inhibition of systemic hypertonic saline-induced Fos protein expression in the supraoptic nucleus, confirming that pertussis toxin effectively inactivates G(i/o)-proteins in the supraoptic nucleus. Thus, the expression of morphine withdrawal excitation by supraoptic nucleus oxytocin cells requires the functional integrity of G(i/o)-proteins within the nucleus.

Selective modulation of excitatory transmission by mu-opioid receptor activation in rat supraoptic neurons

Opioid peptides have profound inhibitory effects on the production of oxytocin and vasopressin, but their direct effects on magnocellular neuroendocrine neurons appear to be relatively weak. We tested whether a presynaptic mechanism is involved in this inhibition. The effects of mu-opioid receptor agonist D-Ala(2), N-CH(3)-Phe(4), Gly(5)-ol-enkephalin (DAGO) on excitatory and inhibitory transmission were studied in supraoptic nucleus (SON) neurons from rat hypothalamic slices using whole cell recording. DAGO reduced the amplitude of evoked glutamatergic excitatory postsynaptic currents (EPSCs) in a dose-dependent manner. In the presence of tetrodotoxin (TTX) to block spike activity, DAGO also reduced the frequency of spontaneous miniature EPSCs without altering their amplitude distribution, rising time, or decaying time constant. The above effects of DAGO were reversed by wash out, or by addition of opioid receptor antagonist naloxone or selective mu-antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-NH(2) (CTOP). In contrast, DAGO had no significant effect on the evoked and spontaneous miniature GABAergic inhibitory postsynaptic currents (IPSCs) in most SON neurons. A direct membrane hyperpolarization of SON neurons was not detected in the presence of DAGO. These results indicate that mu-opioid receptor activation selectively inhibits excitatory activity in SON neurons via a presynaptic mechanism.

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.

Phenotypic and state-dependent expression of the electrical and morphological properties of oxytocin and vasopressin neurones

Oxytocin and vasopressin secreting neurones of the hypothalamic supraoptic nucleus share many membrane characteristics and a roughly similar morphology. However, these two neurone types differ in the relative expression of some intrinsic and synaptic currents, and in the extent of their respective dendritic arbors. Spike depolarizing afterpotentials are present in both types, but more frequently give rise to prolonged burst discharges in vasopressin neurones. Oxytocin, but not vasopressin neurones, are characterized by a depolarization-activated, sustained outward rectifier which turns on near spike threshold, and which can produce prolonged spike frequency adaptation. When this sustained current is deactivated by small hyperpolarizing pulses, a rebound depolarization sufficient to evoke short spike trains follows the offset of these pulses. Both oxytocin and vasopressin neurones exhibit a transient outward rectification underlain by an Ia-type current. This transient rectifier delays spiking to depolarizing stimuli from a relatively hyperpolarized baseline, and is more prominent in vasopressin neurones. As a result, oxytocin neurones may be more reactive to depolarizing inputs. Both cell types receive glutamatergic, excitatory synaptic inputs and both possess R,S- alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subtypes. The AMPA receptor channel on both cell types is characterized by a relatively high calcium permeability and voltage-dependent rectification, characteristic of a diminished presence of the GluR2 AMPA subunit. However, AMPA-mediated synaptic transients are larger, and decay faster, in oxytocin compared with vasopressin neurones, suggesting a potential difference for synaptic integration. The characteristics of NMDA-mediated synaptic transients are similar in oxytocin and vasopressin neurones, but some data suggest NMDA receptors may be less involved in the glutamatergic activation of oxytocin neurones. In both cell types, synaptic release of glutamate often coactivates AMPA and NMDA receptors. The dendritic morphology of oxytocin and vasopressin neurones in female rats differs from one another and exhibits considerable plasticity as a function of endocrine state. In virgin rats, oxytocin neurones have more dendritic branches and a greater total dendritic length compared with lactation, when the arbor is much less extensive. A complementary change occurs in vasopressin dendrites, which are more extensive during lactation. This reorganization suggests that oxytocin neurones may be more electronically compact during lactation. In addition, such dramatic shifts in overall dendritic length imply that significant gains and losses in either the total number of synapses, or in synaptic density, are incurred by both cell types as a function of reproductive state.

mu-opioid receptor activation inhibits N- and P-type Ca2+ channel currents in magnocellular neurones of the rat supraoptic nucleus

1. The whole-cell voltage-clamp technique was used to examine opioid regulation of Ba2+ currents (IBa) through voltage-sensitive Ca2+ channels in isolated magnocellular supraoptic neurones (MNCs). The effects of local application of mu-, delta- or kappa-opioid receptor selective agonists were examined on specific components of high voltage-activated (HVA) IBa, pharmacologically isolated by use of Ca2+ channel-subtype selective antagonists. 2. The mu-opioid receptor selective agonist, DAMGO, suppressed HVA IBa (in 64/71 neurones) in a naloxone-reversible and concentration-dependent manner (EC50 = 170 nM, Emax = 19.5 %). The DAMGO-induced inhibition was rapid in onset, associated with kinetic slowing and voltage dependent, being reversed by strong depolarizing prepulses. Low-voltage activated (LVA) IBa was not modulated by DAMGO. 3. Administration of kappa- (U69 593) or delta-selective (DPDPE) opioid receptor agonists did not affect IBa. However, immunostaining of permeabilized MNCs with an antibody specific for kappa1-opioid receptors revealed the presence of this opioid receptor subtype in a large number of isolated somata. 4. mu-opioid-induced inhibition in IBa was largely abolished after blockade of N-type and P-type channel currents by omega-conotoxin GVIA (1 microM) and omega-agatoxin IVA (100 nM), respectively. Quantitation of antagonist effects on DAMGO-induced reductions in IBa revealed that N- and P-type channels contributed roughly equally to the mu-opioid sensitive portion of total IBa. 5. These results indicate that mu-opioid receptors are negatively coupled to N- and P-type Ca2+ channels in the somatodendritic regions of MNCs, possibly via a membrane-delimited G-protein-dependent pathway. They also support a scheme in which opioids may act in part to modulate cellular activity and regulate neurosecretory function by their direct action on the neuroendocrine neurones of the hypothalamic supraoptic neucleus.

Supraoptic oxytocin and vasopressin neurones show differential sensitivity to the neurosteroid pregnenolone sulphate

The neurosteroid pregnenolone sulphate (PS) interacts allosterically with ionotropic glutamate receptors and thereby could be an important modulator of activity within the hypothalamic magnocellular nuclei. The present in-vitro study therefore examined the effect of perifusion of PS (100 microM) on activity of supraoptic oxytocin (OT) and vasopressin (VP) neurones, in which firing was stimulated by local application of glutamate, NMDA or AMPA. In the presence of locally applied glutamate, PS significantly potentiated firing in putative VP neurones, but had little effect on putative OT neurones. In both cell types, PS increased firing in the presence of NMDA and depressed firing in the presence of AMPA. The action of PS on glutamate- and NMDA-stimulated firing was unaffected by addition of the GABA(A) receptor antagonist, picrotoxin (50 microM). The suppressive action of PS on AMPA-stimulated firing was, however, reversed by picrotoxin and therefore probably requires intact GABAergic transmission for its expression. When putative VP neurones were stimulated by local application of K+, in the presence of picrotoxin, PS evoked a small increase in the ongoing activity, although this did not reach significance. When the glutamate receptor antagonists, NBQX (20 microM) and AP5 (40 microM), were included in the medium, no change in K+ -stimulated firing was observed. Hence PS has no effect on activity of putative VP neurones in the absence of exogenous and endogenous glutamate excitation. In conclusion, PS selectively potentiates glutamate-stimulated activity in putative VP neurones, probably via NMDA receptors, thus providing a mechanism whereby this neurosteroid might exert rapid non-genomic effects on VP secretion. The lack of effect of PS in putative OT neurones probably relates to the relatively small involvement of NMDA receptors in mediating glutamate excitation in this cell type.

Inhibition of rat oxytocin and vasopressin supraoptic nucleus neurons by nociceptin in vitro

The effects of nociceptin (orphanin FQ) on the excitability of electrophysiologically-identified oxytocin and vasopressin neurons were investigated in rat hypothalamic supraoptic nucleus slices in vitro, using whole-cell patch-clamp recording techniques. Nociceptin inhibited the spontaneous discharge of 9/20 (45%) of supraoptic nucleus neurons tested, while in the remaining 11/20 neurons it inhibited firing rate and induced repetitive burst-firing. There were no differences between the effects of nociceptin on oxytocin and vasopressin neurons. When recordings were made using EGTA-containing patch pipettes, nociceptin caused inhibition in all 30 supraoptic nucleus neurons tested, and burst-firing was not seen. The inhibitory effects of nociceptin persisted in low Ca, Co medium, and were not antagonized by naloxone at concentrations sufficient to antagonize the inhibitory actions of morphine and U50488. The actions of nociceptin on supraoptic nucleus neurons are therefore likely to be mediated by postsynaptic opioid receptor-like (ORL1) receptors that are distinct from known opioid receptors. The inhibitory responses to nociceptin were also insensitive to naloxone benzoylhydrazone, which itself had no effect on the spontaneous discharge of the supraoptic nucleus neurons. Our findings demonstrate that endogenous nociceptin may have a functional role in regulating oxytocin and vasopressin secretion through its actions on hypothalamic supraoptic nucleus neurons.

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.

Glutamate excitation of oxytocin neurones in vitro involves predominantly non-NMDA receptors

Experiments were undertaken to compare effects of the NMDA and non-NMDA receptor antagonists, AP5 (40 microM) and NBQX (10 microM), on glutamate-induced firing in supraoptic oxytocin (OT) and vasopressin (VP) neurones in vitro. In putative OT neurones NBQX caused a significantly greater reduction in firing than AP5, whilst in putative VP neurones both antagonists reduced activity powerfully and to a similar extent. The relatively small effect of AP5 in putative OT neurones was unaffected by the removal of extracellular magnesium. These results suggest that glutamate-induced firing in putative OT neurones is predominantly controlled by non-NMDA receptors.

Developmental regulation of preproenkephalin mRNA in the ovine paraventricular nucleus: effects of stress and glucocorticoids

The opioid peptides have profound effects at several levels within the hypothalamo-pituitary-adrenal (HPA) axis. Activation of fetal HPA function occurs during late gestation, and as part of the fetal adaptive response to stress. Changes in the relative levels, localization and distribution of hypothalamic preproenkephalin (PENK) mRNA in the ovine hypothalamic paraventricular nucleus (PVN) during development were examined by in situ hybridization histochemistry. The effects of fetal hypoxemia applied in the presence or absence of concomitant cortisol, to establish negative feedback potential in late gestation were also investigated. PENK mRNA was present at low levels within the PVN, by d60 (term d147). During mid to late gestation, there was an increase in PENK mRNA levels from d60-80 to d100-120, then reaching a peak at d130-140. Levels then decreased dramatically during the last 5-7 days prior to parturition, but increased again in the newborn lamb. Throughout gestation, PENK mRNA was confined exclusively to the parvocellular region of the PVN. Cortisol infusion induced significant decreases (P < 0.05) in PENK mRNA, in normoxemic fetuses at d135 of gestation. The hypoxemic insult, which is known to stimulate plasma ACTH and cortisol, in these fetuses, did not significantly affect PENK mRNA. There was no significant difference in hypoxemia significantly decreased PENK mRNA compared to the saline-infused normoxemic fetuses. Together, these results indicate that the elevation of endogenous fetal cortisol, that occurs at the end of gestation, may act to inhibit expression of the PENK gene in the hypothalamic PVN of the developing ovine fetus.

Opioids influence neurohypophysial but not central oxytocin release following direct hyperosmotic stimulation of the supraoptic nucleus in urethane-anaesthetised rats

Microdialysis was used to apply an osmotic stimulus (0.5 M NaCl-aCSF) into both supraoptic nuclei (SON) to investigate the role of endogenous opioid peptides in the control of both central and peripheral oxytocin release in response to this stimulus. There were no differences in central peptide release during direct hyperosmotic stimulation between groups of rats given either vehicle, morphine (5 mg/kg) or naloxone (5 mg/kg) intravenously. Naloxone potentiated oxytocin release into blood; this suggests that endogenous opioid peptides at the level of the neurohypophysis, but not in the SON are important modulators of oxytocin release to this stimulus. However morphine blocked oxytocin release into blood indicative of a central inhibitory action on the firing rate of oxytocin neurones, contrasted with insensitivity to morphine of oxytocin secretion from the dendrites stimulated directly by hyperosmolarity.

Kappa-selective agonists decrease postsynaptic potentials and calcium components of action potentials in the supraoptic nucleus of rat hypothalamus in vitro

To investigate the effects of the endogenous kappa-receptor agonists dynorphin and leumorphin on neurons of the supraoptic nucleus in the rat hypothalamus, intracellular recordings were made from 62 supraoptic neurons in slice preparations. Bath application of dynorphin and leumorphin at 10(-7) M to 3 x 10(-6) M decreased the spontaneous firing rate with slight hyperpolarization of the membrane potential (-3.8 +/- 0.5 mV, mean +/- S.E.M.) but did not detectably change input resistance. The inhibitory effects were blocked by the relatively selective kappa-antagonist MR-2266. The synthetic kappa-receptor agonist U-50,488H had similar inhibitory effects on supraoptic neurons. Postsynaptic potentials evoked by electrical stimulation dorsal or dorsolateral to the supraoptic nucleus were suppressed by dynorphin and leumorphin. Morphine and [D-Ala, D-Leu]enkephalin, which are relatively selective to mu- and delta-receptors, respectively, influenced the postsynaptic potentials less. Dynorphin and leumorphin also decreased the duration of action potentials that were prolonged by either bath application of tetraethylammonium chloride at 5-10 mM or intracellular injection of Cs ions from the recording electrodes which were filled with 3 M cesium citrate. The prolongation was blocked by 1 mM MnCl2 and 2 mM CoCl2, which suggested that the components were due to voltage-dependent Ca2+ influx. The results suggest that endogenous kappa-receptor agonists inhibit neurosecretory cells of the supraoptic nucleus to suppress synaptic events and Ca2+ components of action potentials.

Microinjection of dynorphin into the supraoptic and paraventricular nuclei produces antidiuretic effects through vasopressin release

The mechanisms for the antidiuretic effects of dynorphin (DYN), an endogenous kappa-agonist, microinjected into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei were investigated. DYN decreased the urine outflow rate dose-dependently from 5 to 20 nmol in the SON and PVN, and it increased vasopressin release. Microinjection of des-Tyr-DYN (a non-opioid peptide) into the SON produced antidiuretic effects with similar potency to that of the DYN-induced effects. However, in the PVN, the effects of des-Tyr-DYN were very markedly weaker than those of DYN. The DYN-induced antidiureses in the SON were partially inhibited by phenoxybenzamine, timolol and atropine, but not by naloxone. Those in the PVN were partially inhibited by naloxone, timolol and atropine, but not by phenoxybenzamine. Synthetic specific kappa-agonists, U50, 488H and Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro- Arg-Leu-Arg-Gly 5-aminopentylamide (DAKLI), microinjected into the PVN also produced antidiuretic effects in a dose-dependent manner. The order of antidiuretic potency was DAKLI > DYN > U50,488H, which was the same as that of kappa-receptor binding affinity. The DAKLI-induced antidiureses in the PVN were not inhibited by naloxone. These results suggested that DYN caused antidiureses by vasopressin release, through adrenergic and cholinergic mechanisms in the SON and PVN. Only the DYN-induced effects in the PVN were mediated, at least partially, through opioid receptors, perhaps the kappa-subtype.

Effects of D-Ala2-D-Leu5-enkephalin, microinjected into the supraoptic and paraventricular nuclei, on urine outflow rate

The effects of D-Ala2-D-Leu5-enkephalin (DADL, a delta-opioid agonist), microinjected directly into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei, on urine outflow rate, urinary osmotic pressure, blood pressure, heart rate, respiratory rate and rectal temperature were investigated in water-loaded and ethanol-anesthetized rats. The microinjection of DADL into both the nuclei decreased urine outflow rate in a dose-dependent manner with an increase in urinary osmotic pressure, but did not change the other recorded parameters. The DADL-induced antidiuretic effect in the SON was inhibited by naloxone, but not by atropine, phenoxybenzamine, timolol nor a vasopressin antagonist, d(CH2)5-D-Tyr(Et)VAVP. The effect in the PVN was inhibited by naloxone, atropine, timolol and d(CH2)5-D-Tyr(Et)VAVP, but not by phenoxybenzamine. These results suggest that DADL causes antidiuretic effects mediated through opioid receptors in both the SON and PVN, and the underlying mechanisms are different between them. Involvement of delta-opioid receptors in the DADL-induced antidiureses was discussed.

Morphine tolerance and inhibition of oxytocin secretion by kappa-opioids acting on the rat neurohypophysis

1. The present study investigated the mechanisms by which endogenous opioids regulate oxytocin secretion at the level of the posterior pituitary gland. Effects of the selective kappa-agonist U50,488 on oxytocin secretion were studied in urethane-anaesthetized lactating rats. Oxytocin secretion in response to electrical stimulation (0.5 mA, matched biphasic 1 ms pulses, 50 Hz, 60-180 pulses) of the neurohypophysial stalk was bioassayed on-line by measuring increases in intramammary pressure, calibrated with exogenous oxytocin. Intravenous (I.V.) U50,488 inhibited electrically stimulated oxytocin secretion, without affecting mammary gland sensitivity to oxytocin. The inhibition was dose related, with an ID50 of 441 (+194, -136) micrograms/kg and was naloxone reversible. Antagonism of endogenous beta-adrenoceptor activation by propranolol (1 mg/kg) reduced the potency of U50,488. The selective mu-agonist morphine (up to 5 mg/kg), had no effect on electrically stimulated oxytocin secretion, but depressed the mammary response to oxytocin. 2. In lactating rats given intracerebroventricular (I.C.V.) morphine infusion for 5 days to induce tolerance and dependence, I.V. U50,488 still inhibited electrically stimulated oxytocin secretion, but the ID50 was reduced to 170 (+78, -54) micrograms/kg; thus at the posterior pituitary the sensitivity of kappa-receptors is enhanced rather than reduced in morphine-tolerant rats, indicating the absence of cross-tolerance. In these rats, naloxone produced a large, sustained, fluctuating increase in intramammary pressure indicating morphine-withdrawal excitation of oxytocin secretion; I.V. U50,488 diminished this response, confirmed by radioimmunoassay, demonstrating the independence of mu- and kappa-receptors regulating oxytocin secretion. 3. In pregnant rats, I.C.V. infusion of morphine from day 17-18 of pregnancy delayed the start of parturition by 4 h, but did not significantly affect the progress of parturition once established, indicating tolerance to the inhibitory actions of morphine on oxytocin secretion in parturition, and lack of cross-tolerance to endogenous opioids restraining oxytocin in parturition. 4. Neurointermediate lobes from control and I.C.V. morphine-infused virgin rats were impaled on electrodes and perifused in vitro. Vasopressin and oxytocin release from the glands was measured by radioimmunoassay. Each gland was exposed to two periods of electrical stimulation (13 Hz, for 3 min). Naloxone (5 x 10(-6) M) was added before the second stimulation; half the lobes from each I.C.V. treatment were exposed to 5 x 10(-5) M morphine throughout.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane properties of identified guinea-pig paraventricular neurons and their response to an opioid mu-receptor agonist: evidence for an increase in K+ conductance

Intracellular recordings were obtained from neurons in the paraventricular nucleus (PVN) of guinea-pig hypothalamic slices. Passive and active properties of the neurons were determined, and when possible, recorded neurons were injected with biocytin. The effects of a mu-receptor opioid agonist [D-Ala2, Nme-Phe4, Gly5-ol]enkephalin (DAGO) were studied in order to determine which types of cells have mu receptors and to test the hypothesis that an increase in K+ conductance causes mu-receptor-mediated inhibition in the PVN. The recorded cells inside the PVN were divided into two groups, primarily on the basis of the presence or absence of a low threshold Ca2+ spike (LTS). In one group of neurons, LTS potentials could not be evoked (non-LTS cells, n = 42). In another group of neurons (n = 35), LTS potentials with one or two Na+ spikes could be initiated with depolarizing pulses superimposed on steady hyperpolarizing currents; however, these neurons did not fire robust bursts (i.e. non-bursting LTS cells). The mean time constant of non-LTS cells (19.9 +/- 1.6 ms; mean +/- SEM) was significantly shorter than that of non-bursting LTS cells (26.7 +/- 2.1 ms). There were no differences in the mean resting membrane potential, spike amplitude, spike duration, input resistance, spike threshold and pattern of synaptic inputs between the two groups. Intracellular labeling with biocytin combined with cresyl violet counter-staining demonstrated that the two types of cells were located in the PVN.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the kappa-opioid agonist U50,488 on parturition in rats

1. The effects of the kappa-opioid agonist U50,488 on parturition were studied in the rat. 2. Given directly after the birth of the second pup U50,488 (5 mg or 10 mg kg-1, i.p.) delayed the birth of the subsequent 4 pups by ca. 100 min, acting like morphine (10 mg kg-1, i.p.). In controls given the vehicle i.p., the birth of the 4 pups after treatment took 45.4 +/- 4.6 min. The effects of U50,488 could be prevented by simultaneous naloxone injection (10 mg kg-1). Injection of either U50,488 or morphine at 1 mg kg-1, i.v. also significantly delayed parturition. The effects of U50,488 but not of morphine were fully prevented by preinjection with nor-binaltorphimine (0.5 mg kg-1, i.v.) showing selective kappa-opioid receptor-mediated inhibition by U50,488 of established parturition. 3. In rats with an indwelling jugular venous cannula, i.v. injection of U50,488 (5 mg kg-1) after the birth of the second pup slowed parturition in a similar way to i.p. injection and significantly reduced blood plasma oxytocin concentration measured by radioimmunoassay compared with vehicle-injected controls. 4. Bolus i.v. injections of oxytocin (4 mu once per 5 min) significantly reduced the delay in parturition caused by i.v. U50,488, but continuous i.v. infusion of oxytocin (4 mu 5 min-1) was less effective. 5. Since i.v. oxytocin did not immediately reverse the effects of U50,488 on parturition, direct effects of U50,488 on isometric uterine contractions in vitro were sought. U50,488 inhibited spontaneous or oxytocin-stimulated contractions of uteri from rats within 24 h after parturition in a dose-related manner; the inhibitory effect was not naloxone-reversible.6. Thus U50,488 inhibited established parturition in the rat in a Kappa-opioid selective manner by reducing oxytocin secretion. The inhibitory effect may well have been potentiated by a direct non-opioid depressant action on contractile activity of the uterus.

The maintenance of normal parturition in the rat requires neurohypophysial oxytocin

The neuropeptide oxytocin has long been known as a potent contractor of the uterus. However, it has remained difficult to attribute a definite role for neurohypophysial oxytocin in either the initiation or continuation of labour. Most recently, Lefebvre and colleagues have suggested that oxytocin produced in the uterus, rather than in the hypothalamus, may be more important in parturition since at term the uterus of the rat contains 70-fold more mRNA for oxytocin than the hypothalamus, and this disappears at about the time of parturition. Despite the high levels of mRNA the uterus contains only nanogram quantities of immunoreactive oxytocin per gram wet weight at term, compared to microgram quantities present in the pituitary. Here we show that activation of the neurohypophysial oxytocin system occurs, as reflected by expression of immunoreactivity for Fos in the hypothalamic supraoptic nucleus, and that this activation is indeed critical for normal parturition, since its inhibition results in a significant prolongation of parturition. In addition, we present evidence that pulsatile delivery of oxytocin into the circulation is important for the efficient progress of parturition, indicating that a major role of the neuronal circuits regulating oxytocin secretion for parturition, as is already known for suckling, is to produce an appropriately patterned hormonal output for efficient biological action.

Electrical stimulation of tooth pulp increases the expression of c-fos in the cat supraoptic nucleus but not in the paraventricular nucleus

Immunoreactivity to Fos protein was detected in the supraoptic (SON) and para-ventricular (PVN) nuclei of the cat using immunohistochemical methods. In the intact animal group, only a few Fos-positive neurons were observed in the PVN, but the SON did not contain any positive neurons. Intraperitoneal injection of pentobarbital sodium (Nembutal: 35 mg/kg) induced c-fos expression in the SON, but not in the PVN. Electrical stimulation of tooth pulp with an intensity that was 3 times the threshold of the jaw-opening reflex (200-600 microA) increased the number of Fos-positive neurons in the SON by up to 388% as compared with those of the Nembutal group, whereas the stimulation did not alter the number in the PVN. The increase was observed throughout the extent of the SON. In addition, morphine treatment (2 mg/kg, i. p.), 5 minutes before tooth pulp stimulation, considerably inhibited the increase in the SON. There were no significant differences among the 3 groups (intact, Nembutal, morphine) in the number of positive neurons in the PVN. These findings suggest that these hypothalamic nuclei have different functional roles and that the SON is involved in nociception and/or the consequent emotional and visceral reactions.

Chronic stress elevates enkephalin expression in the rat paraventricular and supraoptic nuclei

Numerous studies have implicated opioids in the regulation of hypothalamic functions. Dynorphin, which is co-expressed with vasopressin in the magnocellular neurons of the paraventricular and supraoptic nuclei, is co-regulated with vasopressin in response to hyperosmolality and appears to inhibit vasopressin and oxytocin release from the posterior pituitary. Enkephalin is present in paraventricular parvocellular neurons and its expression is elevated in response to various stresses. However, enkephalin's presence and roles in paraventricular and supraoptic magnocellular neurons are uncertain. By giving rats daily intraperitoneal injections of hypertonic saline for up to 12 days, we induced a marked increase in enkephalin expression in magnocellular neurons of the paraventricular and supraoptic nuclei, beyond what develops from drinking hypertonic saline. Our results suggest that enkephalin expression in both vasopressin and oxytocin neurons may increase in response to chronic stresses and provide another source of enkephalin in addition to the parvocellular neurons.

The opioid receptor subtypes mu and kappa, but not delta, are involved in the control of the vasopressin and oxytocin release in the rat

The effects of highly selective agonists and antagonists to the mu-, delta- and kappa-opioid receptor subtypes were studied on the vasopressin and oxytocin release in 24 h water-deprived male rats. The delta-agonist [D-Pen2,D-Pen5]enkephalin (dose range 0.01-5 mg/kg) did not affect plasma levels of either hormone 30 min after s.c. administration, whereas the mu-agonist DALDA (H-Tyr-D-Arg-Phe-Lys-NH2) over the same dose range strongly inhibited the release of both vasopressin and oxytocin, an effect that was maximal 30-60 min after s.c. injection. The same effect was found for s.c. administration of the kappa-agonist U-69,593. Intracerebroventricular (i.c.v.) administration of DALDA (0.5 and 5 micrograms/kg) but not U-69,593 suppressed both plasma hormone levels 30 min after injection. Also the effects of selective antagonists were tested over the s.c. dose range of 0.01-1 mg/kg. Whereas both the kappa-selective antagonist nor-binaltorphimine and the relatively mu-selective antagonist naloxone elevated oxytocin plasma levels (peak at 15 and 30 min after injection, respectively), the delta-selective antagonist naltrindole was without any effect. Nor-binaltorphimine, naloxone, and naltrindole did not affect vasopressin release. When the antagonists were administered i.c.v. (dose range 2.5-25 micrograms/kg), only the kappa-antagonist nor-binaltorphimine enhanced oxytocin and vasopressin release 30 min after injection. In conclusion, both mu- and kappa-opioid receptors are involved in the regulation of the secretion of vasopressin and oxytocin from the rat neural lobe; in contrast, delta-opioid receptors do not play a role.(ABSTRACT TRUNCATED AT 250 WORDS)

Seasonal changes in methionine-enkephalin immunoreactivity in the brain of a hibernator, Spermophilus columbianus

To identify the actual location of central endogenous opioid systems which may be involved in regulating the hibernation cycle, differences in the pattern of central methionine-enkephalin (Met-EK) immunoreactivity were compared between hibernating (body temperature, Tb = 7 degrees C) and non-hibernating (Tb = 37 degrees C) Columbian ground squirrels using the peroxidase-antiperoxidase technique. In non-hibernating animals, Met-EK-immunoreactive perikarya were observed in telencephalic (putamen, caudate nucleus, medial septum-diagonal band complex, amygdala) and diencephalic (periventricular hypothalamic nucleus, lateral hypothalamic area) regions, whereas immunoreactive fibers were found in the lateral septum, stria terminalis nucleus, various hypothalamic areas, arcuate nucleus, median eminence, thalamic intralaminar, periventricular nucleus and lateral habenular nucleus. Compared to the non-hibernating animal, a marked increase in the number of Met-EK-immunoreactive fibers was found in the lateral septal nucleus, the periventricular nucleus, the intralaminar thalamus and the paraventricular hypothalamus of hibernating ground squirrels. Since these changes in immunoreactivity were not observed in the artificially induced hypothermic ground squirrels (Tb = 7 degrees C), it is unlikely that the dissimilarity in immunoreactivity between animals from different hibernating phases is due to differences in their Tb. In combination with our previous studies, these results tend to suggest that hibernation may be brought about by an increase in endogenous opioid activity, especially in the lateral septal region.

Characterization of opioid-sensitive neurons in the anteroventral third ventricle region of polydipsic inbred mice in vitro

In previous studies we found that in the extremely polydipsic special strain of mice, STR/N, spontaneous drinking was greatly attenuated by injection of the opioid antagonists given intracerebroventricularly as well as subcutaneously. Therefore, we investigated, using hypothalamic slice preparations, responses of neurons in the anteroventral third ventricle region (AV3V) of the STR/N and its control, Swiss/Webster (S/W) mice to morphine and opiate peptides. An application of morphine at 10(-6) M to the circulating medium inhibited activities of 44% of AV3V neurons (45 of 102) in the STR/N, and 59% (76/129) in the S/W, demonstrating that morphine affected a smaller proportion of neurons of the polydipsic mice than that of controls. Opioid agonists for 3 receptor types, mu, delta and kappa, at 10(-6) to 10(-5) M inhibited AV3V neurons in both the STR/N and S/W mice, but to a different degree. No cell of either strain was excited by morphine or any of the opioids. The mu-receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO), was the most potent inhibitor of AV3V neurons; in the STR/N 53% (25/47), and in the S/W 77% (34/44) were inhibited. The kappa-agonist, dynorphin A-(1-13) (DYN), inhibited fewer cells in the STR/N (9%, 4/47), compared with the S/W (36%, 16/44). Only a few cells responded to the delta-agonist, [D-Pen2,5]enkephalin (DPDPE), in both strains. The inhibitory actions of the opiates were reversibly blocked by naloxone, and persisted under synaptic blockade. The threshold concentration of morphine or DAGO for inhibition of AV3V neurons was higher in the STR/N (approximately 10(-8) M for both morphine and DAGO) than in S/W mice (approximately 10(-9) M for morphine and less than 10(-9) M for DAGO). Although the AV3V also contains angiotensin II-sensitive neurons, they were not affected by morphine (10(-6) M). Similarly neurons inhibited by morphine were not excited by angiotensin II (10(-7) M); some neurons were unresponsive to both chemicals. We conclude that morphine and opiate peptides directly inhibit the AV3V neurons of both the STR/N and S/W strains of mice and the sensitivity of these neurons to the opiates is lower in the polydipsic inbred mice compared to their controls. The results, together with our behavioral studies, suggest involvement of the central opioid system in the polydipsia of the STR/N mice.

Morphine actions on supraoptic oxytocin neurones in anaesthetized rats: tolerance after i.c.v. morphine infusion

1. The effects of acute i.v. administration of morphine on putative oxytocin neurones of the supraoptic nucleus were studied in urethane-anaesthetized female rats which had been exposed to i.c.v. infusion of morphine (up to 50 micrograms h-1) or vehicle for 5 days. 2. In vehicle-infused rats, i.v. morphine inhibited the spontaneous activity of six out of seven putative oxytocin neurones. Increasing doses of morphine were given, from 1 microgram kg-1 to 5 mg kg-1. The median cumulative threshold dose to produce significant inhibition was 20 micrograms kg-1 (seven cells in six rats); six out of seven cells were inhibited at 161 micrograms kg-1. The highest doses tested inhibited by approximately 90% (excluding one unaffected cell). Inhibition was fully reversed by i.v. naloxone without overshoot, indicating a lack of acute dependence. 3. Injection of morphine i.c.v. inhibited firing at doses that were ineffective by i.v. injection and the effects of i.c.v. morphine were reversed by i.v. naloxone. 4. Acute morphine (500 micrograms kg-1 i.v.) reduced the plasma concentration of oxytocin, measured after 15 min by specific radioimmunoassay, by 34% (n = 14). 5. In lactating rats i.c.v. injection of morphine (1-2 micrograms) inhibited the activity of supraoptic neurones identified as oxytocinergic by their responses to suckling. 6. In seventeen rats infused with i.c.v. morphine the initial firing rate of twenty-eight spontaneously active, non-phasic neurones was significantly less, by 24%, than thirty-four similar cells in control rats, indicating incomplete tolerance to i.c.v. morphine. Morphine (up to 161 micrograms kg-1 given i.v.) inhibited none of nine active non-phasic neurones (P less than 0.01 compared to control rats), but at higher doses inhibited four of nine cells; the overall median threshold cumulative dose (1660 micrograms kg-1) was significantly greater than in vehicle-infused controls, indicating tolerance to i.v. morphine. In contrast with control rats, some cells (5/9) were modestly excited by low doses of morphine. Naloxone (5 mg kg-1 i.v.) produced withdrawal excitation: the firing rate of putative oxytocin neurones increased to approximately 260% of the pre-i.v. morphine value, indicating dependence in mechanisms regulating the firing rate of these neurones. 7. In morphine-infused rats, the basal firing rate of nineteen phasically active, putative vasopressin supraoptic neurones was not different in nineteen phasic cells in controls (6.4 +/- 0.7 vs. 4.2 +/- 0.6 Hz). 8. Thus morphine potently inhibits the firing of magnocellular oxytocin neurones in the female rat, inhibiting oxytocin secretion. Morphine tolerance and dependence develop during i.c.v. infusion of morphine for 5 days. Similar tolerance to and dependence upon endogenous opioids during pregnancy may be important in the preparation of oxytocin neurones for parturition.

Corticotropin-releasing hormone and opioid peptides in reproduction and stress

Increased knowledge on the mechanisms whereby corticotropin releasing hormone (CRH) and opioid peptides mediate the effects of stress has helped us to understand the relationship between stress and disturbed reproductive function. Increases of CRH and beta-endorphin in the hypothalamus in stressful situations inhibits the secretion of gonadotropins, oxytocin and vasopressin. This may lead to amenorrhea, which often is a consequence of intensive training or psychological stress, or it may disrupt parturition and lactation. There is a relationship between ovarian function and opioid peptides in the hypothalamus. Opioid peptides increase during puberty and fall at the menopause. Oestradiol and progesterone increase beta-endorphin concentrations in the luteal phase of the menstrual cycle, and this is followed by a rapid fall at menstruation. These changes may mediate symptoms typical of the premenstrual syndrome. Rather intensive exercise is required to increase plasma concentrations of beta-endorphin and corticotropin. During labour the amounts of beta-endorphin and corticotropin reach the values found in athletes during maximal exercise. The placenta produces increasing amounts of CRH towards the end of pregnancy which may help the mother and fetus to withstand the increased demands of labour. The placenta may thus be involved in the adaptation of the stress mechanism during pregnancy. CRH has also a paracrine function in different biological processes of the placenta and fetal membranes. It is possible to counteract the deleterious effects of stress on reproductive function by the administration of opiate antagonists. Induction of ovulation with naltrexone has been shown in patients with hypothalamic amenorrhea but the effect on fertility is not known.

Kappa-selective opioid receptor agonists leumorphin and dynorphin inhibit supraoptic neurons in rat hypothalamic slice preparations

Abstract To clarify the effects of opioid peptides, and in particular the effects of kappa-receptor agonists on the activity of supraoptic neurons, extracellular recordings were made from 71 spontaneously firing neurons in the rat hypothalamic slice preparation. Of 71 neurons, 28 showed a phasic firing pattern (phasic neurons: putative vasopressin neurons). The mean firing rate of phasic neurons was 2.6 spikes/s (intraburst firing rate 5.4 +/- 2.2 spikes/s). The mean firing rate of neurons classified as non-phasic neurons (putative oxytocin neurons) was 4.5 spikes/s. Following bath application of leumorphin (LM) at 10(-7) M, which has potent opioid activity at kappa-receptors, 17 (61%) of 28 phasic neurons were inhibited and 22 (51%) of 43 non-phasic neurons were inhibited. Excitation was observed in only one non-phasic neuron. The dose-dependence of the response to LM was tested in five supraoptic neurons. There was an inverse relationship between LM concentration and percent change in firing rate. The threshold concentration of LM was approximately 10(-8) M. The relatively selective kappa-receptor antagonist, MR-2266, completely blocked the LM-induced responses. Its effects were long-lasting and only partial recovery was observed 2 h after the application of MR-2266. Dynorphin had similar inhibitory effects on supraoptic neurons to those obtained with LM when tested on the same neurons. In another series of experiments the mu-receptor agonist morphine and the delta-receptor agonist [D-Ala, D-leu]-enkephalin (DADLE) were applied to 28 supraoptic neurons (12 phasic and 16 non-phasic neurons) at 10(-7) M and their actions compared directly with that of LM. Only two of 12 phasic neurons tested were inhibited by DADLE and none of five phasic neurons tested was inhibited by morphine, while eight of the 12 neurons tested were inhibited by LM. By contrast the non-phasic neurons tested were inhibited by application of each of the peptides; seven of 16 neurons tested were not only inhibited by LM, but also five of 11 neurons by DADLE and seven of 15 by morphine. The magnitude of the responses varied from cell to cell. These results suggest that LM acts at the same receptors as dynorphin, and that opioids acting preferentially at kappa-receptors inhibit both vasopressin and oxytocin neurons while delta- and mu-receptor agonists inhibit primarily oxytocin neurons.

Coexistence of corticotropin-releasing factor and enkephalin in the paraventricular nucleus of the rat

Corticotropin-releasing factor and enkephalin-containing neurons are found in the paraventricular nucleus of the rat hypothalamus. Their immunocytochemical distribution suggests that a subpopulation of neurons of the paraventricular nucleus might contain both peptides. The present study describes the coexistence of corticotropin-releasing factor and enkephalin in parvicellular neurons of the paraventricular nucleus. Immunocytochemical labeling of peptides was combined with in situ hybridization of mRNA to visualize peptide and mRNA labeling in the same tissue section. This resulted in several observations: (1) neurons labeling for the peptide corticotropin-releasing factor also contain the mRNA to synthesize corticotropin-releasing factor, (2) neurons labeling for the peptide enkephalin also contain the mRNA to synthesize the peptide enkephalin, (3) a subpopulation of corticotropin-releasing factor neurons contains the mRNA to synthesize enkephalin, and (4) a subpopulation of enkephalin neurons contains the mRNA to synthesize corticotropin-releasing factor. A large percentage of enkephalin immunoreactive neurons contains corticotropin-releasing factor mRNA, whereas a smaller percentage of corticotropin-releasing factor immunoreactive neurons contains enkephalin mRNA. These double-labeled neurons are present throughout the rostral-caudal extent of the paraventricular nucleus; the majority of these neurons is present in the medial parvicellular paraventricular nucleus.

Opioid receptor subtypes in the supraoptic nucleus and posterior pituitary gland of morphine-tolerant rats

Morphine, given acutely, inhibits oxytocin secretion in adult female rats, but chronic intracerebroventricular infusion for five to six days induces tolerance and dependence in the mechanisms regulating oxytocin secretion. One explanation for tolerance could be that there is a loss of opioid receptors. To test this hypothesis cryostat sections of selected brain regions and the pituitary, from six control and six intracerebroventricular morphine-infused rats, were processed for quantitative in vitro receptor autoradiography. [3H]Etorphine or [3H](-)-bremazocine were used as ligands, and DAGO, DPDPE and U50,488H as selective displacers from mu-, delta-, and kappa-receptors, respectively. Control incubations had naloxone determined specificity. The supraoptic nucleus (site of oxytocin-secreting magnocellular perikarya) contained both mu- and kappa-receptors in control rats (mean +/- S.E.M. binding of mu-selective [3H]etorphine was 91.8 +/- 25.4 fmol/mg of tissue, and of kappa-selective [3H](-)-bremazocine was 130.4 +/- 25.6 fmol/mg). Chronic morphine treatment caused a 83.9% decrease in binding in mu-selective conditions (P less than 0.05), but no significant change in kappa-selective binding. In the median preoptic nucleus (which projects to the supraoptic nucleus) mean +/- S.E.M. binding of [3H]etorphine decreased by 77.0% (P less than 0.01) in chronic morphine-treated rats, from the control value of 76.2 +/- 9.8 fmol/mg of tissue. In the posterior pituitary gland (site of the terminals of the oxytocin-secreting magnocellular perikarya) binding with [3H](-)-bremazocine in controls was over 90% lower than in the supraoptic nucleus. No changes followed chronic morphine treatment. Thus chronic morphine exposure reduces the numbers of available mu-receptors in the supraoptic nucleus, and of opioid receptors in the median preoptic nucleus, perhaps accounting for morphine-tolerance in relation to oxytocin secretion.

Direct effects of an opioid peptide selective for mu-receptors: intracellular recordings in the paraventricular and supraoptic nuclei of the guinea-pig

Responses to [D-Ala2, MePhe4, Gly-ol5]enkephalin, a selective agonist for mu-receptors, were recorded intracellularly from 26 neurons in slices of guinea-pig hypothalamus. Of eight cells tested in the supraoptic nucleus, all of which had electrical properties characteristic of magnocellular neuroendocrine cells, four were sensitive to the agonist applied in the perfusion bath or with microdrops. The main effect was a decrease or suppression of spontaneous firing. In the paraventricular nucleus, seven of 18 cells tested also had electrophysiological characteristics similar to magnocellular neurons: two of them were sensitive to the mu-agonist and the effect was similar to that observed in the supraoptic nucleus. The remaining paraventricular neurons displayed low-threshold Ca2+ spikes, and thus had electrophysiological characteristics different from putative magnocellular neurons. Ten of 11 cells with low-threshold Ca2+ spikes were hyperpolarized by more than 10 mV by the mu-agonist, and showed a 33 +/- 1.9% (S.E.M.) decrease in input resistance. In both types of cells, when synaptic transmission was blocked with tetrodotoxin, the mu-agonist could still induce a hyperpolarization, suggesting that the effect was in part direct. Hyperpolarization was also obtained when the Cl- reversal potential was shifted to more positive values by using KCl electrodes, thus excluding a Cl- conductance mechanism. These results provide evidence that opioid peptides can directly inhibit hypothalamic neurons, that the mechanism is an increase in K+ conductance, and that two types of hypothalamic neurons appear to have different sensitivities to a mu-agonist.

Intrinsic and synaptic mechanisms of hypothalamic neurons studied with slice and explant preparations

The use of slice and explant preparations has allowed major advances in our understanding of the membrane physiology of mammalian hypothalamic neurons. This article will review intracellular electrophysiological studies of neurons in or immediately surrounding the supraoptic and paraventricular nuclei. Considerable information is now available on the intrinsic membrane mechanisms that control action potential generation and burst firing in magnocellular neuroendocrine cells (MNCs) within these nuclei. Neurons surrounding the paraventricular nucleus have different electrical properties than the MNCs, including low-threshold Ca2+ spikes and pronounced anomalous rectification. Bicuculline and kynurenic acid strongly depress fast IPSPs and EPSPs in MNCs, thus suggesting that inhibitory and excitatory amino acids mediate fast synaptic transmission in the hypothalamus. The effects of neuromodulators, such as noradrenaline and opioid peptides, have also been examined. Noradrenaline excites supraoptic neurons and leads to phasic firing through an alpha-1 mechanism and decreased K+-conductance. Opioid peptides act directly on mu-receptors to hyperpolarize about half of the neurons through an increased K+-conductance. In conclusion, using the magnocellular neuroendocrine system as a model, in vitro slice and explant preparations have allowed the characterization of electrophysiological properties, the identification of neurotransmitters for synaptic events, and studies on the mechanism of action of neuromodulators.

Sensitivity of magnocellular oxytocin neurones to opioid antagonists in rats treated chronically with intracerebroventricular (i.c.v.) morphine

The identity of the subtype of opioid receptor mediating morphine dependence in relation to oxytocin neurones was investigated. Virgin female rats were implanted with a subcutaneous osmotic minipump to infuse morphine continuously (up to 50 micrograms/h) into a lateral cerebral ventricle. After 5 days of morphine infusion, rats were anesthetized with urethane, and the electrical activity of electrophysiologically identified supraoptic neurones was recorded extracellularly while opioid antagonists were injected i.v. Putative oxytocin cells were excited following low doses of naloxone HCl: 4/7 cells were excited by 1 microgram/kg, 6/7 cells by 2.5 micrograms/kg, and 11/13 cells by doses of 5-50 micrograms/kg. MR2266 ((-)-5,9 alpha-diethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphan: an antagonist with much greater affinity for kappa-subtype opioid receptors than naloxone) excited oxytocin cells less potently: none of 9 cells was excited by 10 micrograms/kg MR2266, 2/4 cells were by 25-50 micrograms/kg, 3/9 cells by 100 micrograms/kg and only 4/8 by 200-500 micrograms/kg. At low concentrations naloxone is selective for mu-subtype opioid receptors, hence the morphine dependence of oxytocin neurones is probably via mu-receptors. Naloxone methylbromide (MRZ), a quaternary ammonium derivative of naloxone, excited oxytocin cells in morphine-treated rats, but was at least 10 times less potent than naloxone. Thus part of the morphine-withdrawal excitation of oxytocin neurones may be mediated by mu-receptors outside the blood-brain barrier.