Dynorphin (1-8) inhibits stimulated release of oxytocin but not vasopressin from isolated neurosecretory endings of the rat neurohypophysis

Unsupervised Network Analysis of the Plastic Supraoptic Nucleus Transcriptome Predicts Caprin2 Regulatory Interactions

The supraoptic nucleus (SON) is a group of neurons in the hypothalamus responsible for the synthesis and secretion of the peptide hormones vasopressin and oxytocin. Following physiological cues, such as dehydration, salt-loading and lactation, the SON undergoes a function related plasticity that we have previously described in the rat at the transcriptome level. Using the unsupervised graphical lasso (Glasso) algorithm, we reconstructed a putative network from 500 plastic SON genes in which genes are the nodes and the edges are the inferred interactions. The most active nodal gene identified within the network was Caprin2. Caprin2 encodes an RNA-binding protein that we have previously shown to be vital for the functioning of osmoregulatory neuroendocrine neurons in the SON of the rat hypothalamus. To test the validity of the Glasso network, we either overexpressed or knocked down Caprin2 transcripts in differentiated rat pheochromocytoma PC12 cells and showed that these manipulations had significant opposite effects on the levels of putative target mRNAs. These studies suggest that the predicative power of the Glasso algorithm within an in vivo system is accurate, and identifies biological targets that may be important to the functional plasticity of the SON.

Hypothalamic transcriptome plasticity in two rodent species reveals divergent differential gene expression but conserved pathways

We have addressed the question of how different rodent species cope with the life-threatening homeostatic challenge of dehydration at the level of transcriptome modulation in the supraoptic nucleus (SON), a specialised hypothalamic neurosecretory apparatus responsible for the production of the antidiuretic peptide hormone arginine vasopressin (AVP). AVP maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of AVP from SON axon terminals located in the posterior pituitary, and this is accompanied by a plethora of changes in the morphology, electrophysiological properties, biosynthetic and secretory activity of this structure. Microarray analysis was used to generate a definitive catalogue of the genes expressed in the mouse SON, and to describe how the gene expression profile changes in response to dehydration. Comparison of the genes differentially expressed in the mouse SON as a consequence of dehydration with those of the rat has revealed many similarities, pointing to common processes underlying the function-related plasticity in this nucleus. In addition, we have identified many genes that are differentially expressed in a species-specific manner. However, in many cases, we have found that the hyperosmotic cue can induce species-specific alterations in the expression of different genes in the same pathway. The same functional end can be served by different means, via differential modulation, in different species, of different molecules in the same pathway. We suggest that pathways, rather than specific genes, should be the focus of integrative physiological studies based on transcriptome data.

Voltage-dependent kappa-opioid modulation of action potential waveform-elicited calcium currents in neurohypophysial terminals

Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca(2+) channels results in less calcium influx into the terminals and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca(2+) channel kinetics, and the associated Ca(2+) influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCCs). In this series of experiments we test whether the kappa- and micro-opioid inhibition of Ca(2+) currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the kappa- and micro-receptors in neurohypophysial terminals. The kappa-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, micro-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of kappa-opioid inhibition during a physiologic action potential (AP) burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca(2+) channels due to specific G-protein modulation during a burst of APs may contribute to the fine-tuning of Ca(2+)-dependent neuropeptide release in other CNS terminals, as well.

Targeting of green fluorescent protein to secretory granules in oxytocin magnocellular neurons and its secretion from neurohypophysial nerve terminals in transgenic mice

Oxytocin (OT) is a hypothalamic nonapeptide that is synthesized as part of a larger precursor protein that also contains an approximately 10-kDa protein called neurophysin at its C-terminus. This precursor protein is trafficked through the regulated secretory pathway into secretory granules and then axonally transported to and secreted from nerve terminals in the neural lobe of the pituitary. In this paper, we show that the AI-03 transgene that contains enhanced green fluorescent protein (EGFP) fused to the end of the neurophysin at the C-terminus of the OT pre-prohormone, is expressed selectively in OT-magnocellular neurons and is trafficked to secretory granules in transgenic mice. The EGFP-containing secretory granules are then transported to OT-neurosecretory terminals in the neurohypophysis, where the EGFP fluorescence undergoes depolarization-induced calcium-dependent secretion. The endogenous fluorescence in the neural lobes is sufficiently intense to image secretory events in individual OT nerve terminals (neurosecretosomes) isolated from the posterior pituitaries in these transgenic mice.

The kappa opioid receptor and dynorphin co-localize in vasopressin magnocellular neurosecretory neurons in guinea-pig hypothalamus

The relationship between the cloned kappa opioid receptor, dynorphin, and the neurohypophysial hormones vasopressin and oxytocin was analysed in the guinea-pig hypothalamic magnocellular neurosecretory neurons. This analysis was performed in order to understand better which population of neuroendocrine neurons in the guinea-pig is modulated by kappa opioid receptors and its endogenous ligand dynorphin. Extensive co-localization was observed between kappa opioid receptor immunoreactivity and preprodynorphin immunoreactivity in neuronal cell bodies in the paraventricular and supraoptic nuclei. Cells positive for either the kappa opioid receptor or both the kappa opioid receptor and preprodynorphin were restricted to the vasopressin expressing neuronal population and not found in the oxytocin expressing neuronal population. The kappa opioid receptor and dynorphin were examined in the posterior pituitary and both were found to be extensively distributed. Staining for the kappa opioid receptor and dynorphin B co-localized in posterior pituitary. In addition, immunogold electron microscopy confirmed that kappa opioid receptor and dynorphin B immunoreactivity were found in the same nerve terminals. Ultrastructural analysis also revealed that kappa opioid receptor immunoreactivity was associated with both nerve terminals and pituicytes. Within nerve terminals, kappa opioid receptor immunoreactivity was often associated with large secretory vesicles and rarely associated with the plasma membrane. Our data suggest that the cloned kappa opioid receptor may directly modulate the release of vasopressin but not oxytocin in guinea-pig hypothalamic magnocellular neurosecretory neurons and posterior pituitary. Furthermore, we propose that this receptor is an autoreceptor in this system because our results demonstrate a high degree of co-localization between kappa opioid receptor and dynorphin peptide immunoreactivity in magnocellular nerve terminals.

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.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.

Mu, delta, and kappa opioid receptor mRNA expression in the rat CNS: an in situ hybridization study

The mu, delta, and kappa opioid receptors are the three main types of opioid receptors found in the central nervous system (CNS) and periphery. These receptors and the peptides with which they interact are important in a number of physiological functions, including analgesia, respiration, and hormonal regulation. This study examines the expression of mu, delta, and kappa receptor mRNAs in the rat brain and spinal cord using in situ hybridization techniques. Tissue sections were hybridized with 35S-labeled cRNA probes to the rat mu (744-1,064 b), delta (304-1,287 b), and kappa (1,351-2,124 b) receptors. Each mRNA demonstrates a distinct anatomical distribution that corresponds well to known receptor binding distributions. Cells expressing mu receptor mRNA are localized in such regions as the olfactory bulb, caudate-putamen, nucleus accumbens, lateral and medial septum, diagonal band of Broca, bed nucleus of the stria terminalis, most thalamic nuclei, hippocampus, amygdala, medial preoptic area, superior and inferior colliculi, central gray, dorsal and median raphe, raphe magnus, locus coeruleus, parabrachial nucleus, pontine and medullary reticular nuclei, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis and cuneatus, dorsal motor nucleus of vagus, spinal cord, and dorsal root ganglia. Cellular localization of delta receptor mRNA varied from mu or kappa, with expression in such regions as the olfactory bulb, allo- and neocortex, caudate-putamen, nucleus accumbens, olfactory tubercle, ventromedial hypothalamus, hippocampus, amygdala, red nucleus, pontine nuclei, reticulotegmental nucleus, motor and spinal trigeminal, linear nucleus of the medulla, lateral reticular nucleus, spinal cord, and dorsal root ganglia. Cells expressing kappa receptor mRNA demonstrate a third pattern of expression, with cells localized in regions such as the claustrum, endopiriform nucleus, nucleus accumbens, olfactory tubercle, medial preoptic area, bed nucleus of the stria terminalis, amygdala, most hypothalamic nuclei, median eminence, infundibulum, substantia nigra, ventral tegmental area, raphe nuclei, paratrigeminal and spinal trigeminal, nucleus of the solitary tract, spinal cord, and dorsal root ganglia. These findings are discussed in relation to the physiological functions associated with the opioid receptors.

Neuron-glia interactions in the release of oxytocin and vasopressin from the rat neural lobe: the role of opioids, other neuropeptides and their receptors

The release of the neurohormones oxytocin and vasopressin from the neural lobe into the circulation is regulated in a complex manner, which has only been partly elucidated. At the level of the neural lobe, regulation of release can occur by various endogenous compounds that act on specific receptors present on the nerve terminals themselves. In addition, release may be modulated by an alternative pathway in which the local glia cells, the pituicytes, are involved. It is especially the latter pathway that is discussed in detail in this commentary.

Technical aspects of opioid receptor localization: detection of opioid receptor proteins by immunocytochemistry or with a biotinylated dynorphin analog

Opioid receptors were localized at the cellular level, using either anti-opioid receptor antibodies or a biotinylated opioid ligand. In addition, a simple method was developed for selection of second antisera on their potencies to detect particular monoclonal antibodies (mAbs). Most anti-opioid receptor antibodies tested were not able to recognize the opioid receptor in frozen or fixed tissue sections, which was in contrast with their ability to recognize opioid receptors in isolated membrane fractions. However, one batch of anti-idiotypic mAbs gave a good immunocytochemical staining. Distribution of immunoreactivity suggested that these antibodies recognized more than one opioid receptor subtype. After very short fixation times, staining with a biotinylated kappa-selective ligand (DAKLIB) could be observed in the neural and intermediate lobe of pituitary. This binding could be displaced by non-biotinylated DAKLI. The pattern of DAKLIB staining in the neural lobe had the appearance of binding to both nerve fibres and astrocytes. The present results show successful staining of tissue sections with anti-idiotypic antibodies and with a biotinylated ligand. The specificity is discussed in the light of control experiments, pharmacological data and previous studies.

Characterization of opioid binding sites in the neural and intermediate lobe of the rat pituitary gland by quantitative receptor autoradiography

Previous studies have suggested an involvement of enkephalins in regulation of oxytocin (OXT) and vasopressin (AVP) release, which seems to disagree with the very low affinities of Met- and Leu-enkephalin for the kappa opioid receptor. As opioid receptors in the neural lobe exclusively exist of kappa receptors, we studied the binding characteristics of larger pro-enkephalin derived peptides for opioid binding sites in the neural lobe by means of light microscopic receptor autoradiography. In addition, the pharmacological characteristics of opioid binding sites in the neural lobe were compared with those in other parts of the pituitary. In the neural as well as the intermediate lobe both high and low affinity 3H-bremazocine binding sites were present. Binding to these sites was completely displaceable by both naloxone and nor-binaltorphimine suggesting that these sites represent kappa opioid receptors. Also with regard to selectivity and affinity characteristics to other ligands, opioid binding sites in the neural and intermediate lobe were quite similar. In the anterior lobe a very low level of bremazocine binding was present, which could not be displaced by nor-binaltorphimine. Displacement studies with pro-enkephalin and pro-dynorphin derived peptides showed that both groups of peptides could bind to opioid binding sites in the neural and intermediate lobe. Especially the relatively large pro-dynorphin and pro-enkephalin derived peptides, such as dynorphin 1-17 and BAM22, appeared to be very potent ligands for these opioid binding sites and were much more potent than smaller fragments, such as dynorphin 1-8, and Met- and Leu-enkephalin. These results contradict the existence of a mismatch in the neural (and intermediate) lobe with regard to the local type of opioid peptides and receptors present.

Immunocytochemical localization of neuropeptide FF (FMRF amide-like peptide) in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats by light and electron microscopy

Neuropeptide FF (F8Famide, FMRFamide-like, or morphine modulating peptide) immunoreactivity was localized by light and electron microscopy in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats. In Wistar rats neuropeptide FF was present in part of the magnocellular neurones of the paraventricular and supraoptic nuclei in which it was coexpressed with vasopressin. Neuropeptide FF containing fibres were present in the paraventricular and the supraoptic nuclei, and in the central part of the neural lobe. At the electron microscopic level, neuropeptide FF containing nerve terminals in the neural lobe formed synaptoid contacts exclusively with pituicytes. No neuropeptide FF containing neurovascular contacts or contacts with other neuronal structures were observed. In contrast with Wistar rats, neuropeptide FF was almost completely absent in cell bodies of the paraventricular and supraoptic nuclei, and in fibres of the neural lobe in Brattleboro rats. Only a few solitary cells could be observed in these structures. The present results demonstrate that neuropeptide FF coexists with vasopressin within the hypothalamo-neurohypophyseal system. As we did not observe neuropeptide FF containing neurovascular contacts, neuropeptide FF containing nerve terminals probably have a local function within the neural lobe. Neuropeptide FF may be involved in the modulation of oxytocin and vasopressin release, with the pituicyte as an intermediate cell.

Endogenous opioid regulation of oxytocin secretion through pregnancy in the rat

We have investigated the influence of endogenous opioids on oxytocin secretion during pregnancy. In blood-sampled conscious rats on days 18 and 21 of pregnancy plasma oxytocin concentration, measured by radioimmunoassay, was significantly increased compared to non-pregnant or post-partum rats. On days 15, 18 and 21 of pregnancy but not in non-pregnant, early pregnant or post-partum rats, the opioid antagonist naloxone caused a significant increase in plasma oxytocin compared to vehicle injection, indicating activation of an endogenous opioid restraint over oxytocin secretion. Electrically stimulated neural lobes isolated from 16- and 21-day pregnant rats released more oxytocin than those from non-pregnant rats. However, naloxone (10(-5) M) was less effective at potentiating, and the kappa-opioid agonist U50,488 (10(-5)M) was less effective at inhibiting, stimulated release at the end of pregnancy than in non-pregnant rats suggesting desensitization of oxytocin nerve terminals to actions of endogenous opioids. Neural lobes from male rats drinking 2% saline for 4 days also showed desensitization of oxytocin nerve endings to naloxone. Neither neural lobe content of dynorphin A(1-8), an endogenous kappa-opioid, nor prodynorphin mRNA expression, measured by in situ hybridization histochemistry in the supraoptic nucleus altered during pregnancy. However, neural lobe content of Met5-enkephalin significantly decreased by day 21 of gestation suggesting enhanced release.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Methionine5-enkephalin and opiate binding sites in the neurohypophysis of the bird, Gallus domesticus

Uncertainties with respect to the cellular localization, binding characteristics and function of Met-enkephalin in the neurohypophysis of mammalian species prompted us to examine the neurohypophysis of a non-mammalian species for opioid material and opioid binding sites. In extracts of the neurohypophysis of the domestic fowl we found immunoassayable Met-enkephalin, but could not detect dynorphin(1-8)-like material. Met-enkephalin immunoreactivity was co-localized with mesotocin in the same nerve endings. Stereospecific opiate binding was specifically located in neurosecretosomes (isolated neurosecretory terminals) of the mesotocin type, as shown by autoradiography. Enkephalins therefore may modulate mesotocin release in an autocrine manner. The neurohypophysis of this common bird appears to be a favorable model for studies of enkephalin function in the absence of dynorphin.

Activation of κ-opioid receptors inhibits depolarisation-evoked exocytosis but not the rise in intracellular Ca2+ in secretory nerve terminals of the neurohypophysis

Nerve endings of the magnocellular neurohypophysial neurones possess kappa-opioid receptors. Using a preparation of isolated terminals from the neurohypophysis we studied kappa-opioid effects on secretion of oxytocin and vasopressin and on intracellular Ca2+ concentration ([Ca2+]i) measured fluorimetrically or using digital video imaging with Fura-2. The dihydropyridine Ca(2+)-channel antagonist nicardipine reduced [Ca2+]i responses to K(+)-depolarisation (30-40 mM K+) by 55-75% and inhibited evoked secretion of oxytocin and vasopressin to a similar extent. The selective kappa-receptor agonist D-Pro10 Dynorphin A 1-11 (DPDYN) substantially inhibited K+ evoked secretion of oxytocin by 40-90% and secretion of arginine vasopressin (AVP) by 20-50%. DPDYN caused only a 10% reduction in the average total population [Ca2+]i response to K+ depolarisation. No sub-population of inhibitory responses was observed when samples of individual terminal [Ca2+]i responses were examined with imaging. Although kappa-receptors are coupled to Ca(2+)-channels at neuronal somata our data suggest that alternative effector mechanisms operate in these secretory nerve endings.

Effects of acute and chronic ketocyclazocine and its modulation by oxytocin or vasopressin on food intake in rats

The effects of acute and chronic ketocyclazocine (KCZ, a kappa receptor agonist) and its interactions with oxytocin (OXY) or vasopressin (AVP) were investigated on food intake in free-fed rats. Acute treatment with KCZ (1 mg/kg) produced a generalized hyperphagia during the light phase (0-6 h) without influencing dark phase (6-24 h) food intake. On chronic administration, tolerance developed to hyperphagic effect during light phase, whereas an enhancement in the food intake was seen during dark phase. OXY or AVP (both at 10 micrograms/kg) per se, did not affect the food intake response during either the light or the dark phase, after acute as well as chronic treatment. In the interaction studies, acute AVP or OXY attenuated the hyperphagia of KCZ during the light phase. On chronic treatment, both AVP and OXY blocked (a) the tolerance, and (b) the "reverse tolerance" to the food intake response to KCZ during light and dark phases, respectively. These results are discussed in light of complex opioid-OXY/AVP interactions during food intake in rats.

Modulation of oxytocin and vasopressin release from rat neurosecretosomes: the roles of VIP oxytocin and GABA

The effects of vasoactive intestinal polypeptide (VIP), of a selective oxytocin antagonist and of GABA on basal and stimulated oxytocin and vasopressin release from isolated neurosecretory endings were investigated. Superfusion of the secretosomes with VIP (10(-7) M) induced an increased basal and stimulated release of both oxytocin and vasopressin. Addition of the oxytocin antagonist induced a decrease of the stimulated oxytocin release as compared to the control which indicated a positive feedback mechanism of oxytocin on oxytocin release. In presence of GABA (1 or 50 microM) no change in basal or stimulated oxytocin and vasopressin release was observed.

Galanin lacks binding sites in the porcine pituitary and has no detectable effect on oxytocin and vasopressin release from rat neurosecretory endings

High concentrations of immunoreactive galanin-like material in rat hypothalamus, median eminence and neurohypophysis have been reported in the literature suggesting a regulatory role of galanin on hormone release from the anterior and posterior lobe of the pituitary. We studied binding of iodinated galanin to crude membrane preparations from porcine anterior hypothalamus, anterior and neurointermediate lobe of the hypophysis. In contrast to the hypothalamus where specific binding of 125I-galanin was found, there was no displaceable galanin binding in membranes of the anterior or neurointermediate lobe of porcine pituitaries. Effects of galanin on oxytocin and vasopressin release were investigated using isolated neurosecretory endings from rat neurohypophyses. Galanin had no detectable effect on the release of oxytocin or vasopressin.

Dendrosomes: a new preparation of isolated neurosecretory dendrites

To investigate secretion from dendrites of magnocellular neurosecretory neurons we have developed a preparation of isolated portions of the dendrites analogous to synaptosomes and neurosecretosomes. The dendrites of magnocellular neurons extend into the ventral glial lamina ventral and lateral to the nucleus. The ventral glial lamina is dissected out and gently homogenized to shear off portions of the dendrites. These re-seal to form granule-filled, membrane-bound structures which we refer to as 'dendrosomes'. Dendrosomes were recovered by differential centrifugation and identified by light and electron microscopy either in unlabelled preparations or by immunocytochemistry. Stimulation of dendrosomes with 56 mM potassium in the presence of tannic acid, which preserves exocytosed granule cores so that they can be visualized electron microscopically, demonstrates that, in response to a depolarizing stimulus, dendrosomes are capable of secreting by exocytosis the neuropeptides oxytocin (OT) and vasopressin (VP) and their co-packaged neurophysins.

Kappa-opioid receptor agonists differentially affect the release of neurohypophysial hormones

The influence of kappa-opioid receptor agonists and antagonists on release of oxytocin and vasopressin was examined in isolated rat neurointermediate lobes. Electrically evoked release of oxytocin and vasopressin was concentration-dependently inhibited by the specific kappa-receptor agonist U69593, whereas bremazocine only inhibited the secretion of oxytocin markedly. Treatment with naloxone enhanced the evoked release of oxytocin significantly without effect on vasopressin secretion. The U69593-mediated inhibition of oxytocin release was abolished by naloxone, whereas that of vasopression was unaffected. Naloxone did not reverse the bremazocine-induced inhibition of hormone release. The data support the theory of an inhibiting endogenous control over oxytocin secretion and show that the release of oxytocin and vasopresin is differentially affected by the two K-receptor agonists.

The influences of hyperosmolality and synaptic inputs on galanin and vasopressin expression in the hypothalamus

Galanin is a neuropeptide that is widely distributed throughout the rat central nervous system. It is co-localized with vasopressin in magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. Vasopressin biosynthesis is increased there by various hyperosmolar stimuli, including drinking 2% saline. We previously demonstrated that the chronically hyperosmolar Brattleboro rat has increased biosynthesis of galanin in the paraventricular and supraoptic nuclei. In this report we show using hybridization histochemistry that drinking 2% saline also increased galanin transcripts in these nuclei. We also demonstrate using hybridization histochemistry and immunohistochemistry that knife cuts that sever hypothalamo-hypophysial fibers transiently elevated galanin expression in the supraoptic nucleus ipsilateral to the lesion and depressed vasopressin expression ipsilaterally. Pituitary stalk transections also elevated galanin and decreased vasopressin transcripts. In addition, various knife cuts in the caudal hypothalamus were able to dissociate the expression of vasopressin and galanin, although co-localized and similarly affected by hyperosmolality in the supraoptic nucleus. Unilateral sagittal knife cuts that divided the posterior hypothalamus but avoided the hypothalamo-hypophysial pathway, as well as hemisections at the level of the premammillary area, resulted in ipsilateral elevations of galanin transcripts without significantly affecting vasopressin expression. These results indicate that independent intracellular signal transduction pathways exist for regulating expression of the two genes.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Coexisting peptides in hypothalamic neuroendocrine systems: some functional implications

1. Coexisting with oxytocin or vasopressin in the cell bodies and nerve terminals of the hypothalamic-neurohypophysial system are smaller amounts of other peptides. For a number of these "copeptides" there is strong evidence of corelease with the major magnocellular hormones. Guided by the location of their specific receptors we have studied the effects of three copeptides, dynorphin, cholecystokinin (CCK), and corticotropin releasing hormone (CRH), on the secretion of oxytocin and vasopressin from isolated rat neural lobe or neurointermediate lobe preparations in vitro. 2. Dynorphin is coreleased with vasopressin from neural lobe nerve terminals and acts on neural lobe kappa-opiate receptors to inhibit the electrically stimulated secretion of oxytocin. Naloxone augments oxytocin release from the neural lobe in a manner directly proportional to the amount of vasopressin (and presumably dynorphin) released. 3. Cholecystokinin, coreleased with oxytocin by neural lobe terminals, has been shown to have high-affinity receptors located in the NL and to stimulate secretion of both oxytocin and vasopressin. CCK's secretagogue effect was independent of electrical stimulation and extracellular Ca2+ and was blocked by an inhibitor of protein kinase C. 4. CRH, coreleased with OT from the neural lobe, has receptors in the intermediate lobe of the pituitary, but not in the neural lobe itself. CRH stimulates the secretion of oxytocin and vasopressin from combined neurointermediate lobes but not from isolated neural lobes. Intermediate lobe peptides, alpha and gamma melanocyte stimulating hormone, induced secretion of oxytocin and vasopressin from isolated neural lobes. Their effect was, like that of CCK, independent of electrical stimulation and extracellular Ca2+ and blocked by an inhibitor of protein kinase C. 5. Among the CRH-producing parvocellular neurons of the paraventricular nucleus, in the normal rat, approximately half also produce and store vasopressin. After removal of glucocorticoid influence by adrenalectomy, virtually all of the CRH neurons contain vasopressin. 6. The two subtypes of CRH neurosecretory cells found in the normal rat possess different topographical distributions in the paraventricular nucleus, suggesting the possibility of differential innervation. Stress selectively activates the vasopressin containing subpopulation of CRH neurons, indicating that there are separate channels of regulatory input controlling the two components of the parvocellular CRH neurosecretory system.

Oxytocin stimulates oxytocin release from isolated nerve terminals of rat neural lobes

Nerve endings from rat neural lobes isolated by homogenization were placed on a filter and constantly superfused. The effects of exogenous oxytocin and vasopressin (both added at 1 nM concentration) on basal and stimulated release of oxytocin and vasopressin were investigated. Stimulated release was evoked by 30 mM potassium and a simultaneous increase in osmolarity. A stimulatory effect of oxytocin on basal and evoked release of oxytocin was found while there was no effect on vasopressin release. The addition of vasopressin did not induce any change in the release of either hormone. The positive feedback mechanism observed with oxytocin might be active during the discharge of oxytocin which is known to occur in bursts.

Biphasic effect of a kappa-opioid receptor agonist on plasma oxytocin levels in rats

The effect of the kappa-opioid receptor agonist, bremazocine, on plasma oxytocin levels in rats was measured by a sensitive radioimmunoassay. Initially, a decrease in plasma oxytocin levels was seen 30 min after injection. This was in accordance with the bremazocine inhibition of oxytocin release after submaximal electrical stimulation seen in isolated neurointermediate lobes. The initial decrease in plasma oxytocin reversed, and 4 h after injection of bremazocine a 20-fold increase in the oxytocin level was seen. The rise in plasma oxytocin was paralleled by a rise in plasma sodium. The biphasic time course of the plasma oxytocin response can be explained by a combination of an inhibition of oxytocin release from the neurohypophysis and an increased water excretion leading to an elevation in plasma sodium, which may be responsible for the late rise in plasma oxytocin. Down-regulation of the opioid receptors may also contribute to the delayed rise in plasma oxytocin.