Evidence for opiate receptors on pituicytes

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.

Intracellular Ca2+ responses to nucleotides, peptides, amines, amino acids and prostaglandins in cultured pituicytes from adult rat neurohypophysis

The present study aimed to investigate the reactivity of cultured pituicytes from adult neurohypophysis to various bioactive substances using Ca2+ indicator dye Fura-2. A transient increase of intracellular Ca2+ [Ca2+]i was observed when pituicytes were treated with nucleotides (ATP, ADP, UTP, and UDP) and amines (5-HT2 and alpha2-agonist). Treatment with peptides such as endothelin-1 (ET-1), endothelin-3 (ET-3), bradykinin (BK), vasopressin (AVP), and angiotensin II (Ang II) also induced [Ca2+]i increase in pituicytes. Prostaglandin E2 (PGE2) and F2alpha (PGF2alpha) increased [Ca2+]i, but amino acids of GABA, glutamate (Glu), and taurine had no effect. Serum-free culture condition augmented [Ca2+]i responses to ATP, Ang II and 5-HT within 24 h. These results indicate that pituicytes express many of receptors for neurotransmitters or neuromodulators.

Astroglial modulation of neurotransmitter/peptide release from the neurohypophysis: present status

Reviewed in this article are those studies that have contributed heavily to our current conceptualizations of glial participation in the functioning of the magnocellular hypothalamo-neurohypophysial system. This system undergoes remarkable morphological and functional reorganization induced by increased demand for peptide synthesis and release, and this reorganization involves the astrocytic elements in primary roles. Under basal conditions, these glia appear to be vested with the responsibility of controlling the neuronal microenvironment in ways that reduce neuronal excitability, restrict access to neuronal membranes by neuroactive substances and deter neuron neuron interactions within the system. With physiological activation, the glial elements, via receptor-mediated mechanisms, take up new positions. This permissively facilitates neuron neuron interactions such as the exposure of neuronal membranes to released peptides and the formation of gap junctions and new synapses, enhances and prolongs the actions of those excitatory neurotransmitters for which there are glial uptake mechanisms, and facilitates the entry of peptides into the blood. In addition, subpopulations of these glia either newly synthesize or increase synthesis of neuroactive peptides for which their neuronal neighbors have receptors. Release of these peptides by the glia or their functional roles in the system have not yet been demonstrated.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

Is opioidergic activity responsible for the circadian variation observed in the gonadotrophin responsiveness of early follicular phase women?

OBJECTIVES

In women, the gonadotrophin response to gonadotrophin-releasing hormone (GnRH) displays a circadian rhythm during the early follicular phase (EFP), with GnRH-stimulated luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release found to be markedly decreased at night. Since the opioidergic inhibition of gonadotrophin secretion is selectively enhanced at night, we reasoned that the circadian changes in the gonadotrophin responsiveness to GnRH might be related to a nocturnal increase of opioidergic activity.

STUDY DESIGN

Eleven women with normal menstrual cycles were studied in the EFP on four different occasions in random order. Studies were conducted either during the day (0900-1300 h) or at night (2100-0100 h). During these times, GnRH (25 micrograms i.v.) was administered in conjunction with either saline (as control) or naloxone (4 mg i.v.).

MEASUREMENTS

Frequent blood samples were obtained before and after GnRH stimulation for determination of basal sex steroid and gonadotrophin concentrations by immunoradiometric assays.

RESULTS

While oestradiol levels were comparable (P > 0.3) at all times, progesterone concentrations were significantly (P < 0.01) higher during day than during night hours, with no difference between control and naloxone conditions. Gonadotropin responses to GnRH stimulation were not significantly different between day and night times, nor did they vary between control and naloxone conditions.

CONCLUSIONS

Opioidergic blockade imposed by naloxone did not noticeably change GnRH-stimulated gonadotrophin release at any of the study times. We therefore infer that mechanisms other than a nocturnal increase of opioidergic inhibition may account for eventual circadian changes in the gonadotrophin responsiveness of early follicular phase women.

Delta-opioid receptors on astroglial cells in primary culture: mobilization of intracellular free calcium via a pertussis sensitive G protein

Astrocytes in primary culture from rat cerebral cortex were probed concerning the expression of delta-opioid receptors and their coupling to changes in intracellular free calcium concentrations ([Ca2+]i). Fluo-3 or fura-2 based microspectrofluorometry was used for [Ca2+]i measurements on single astrocytes in a mixed astroglial-neuronal culture. Application of the selective delta-opioid receptor agonist, [D-Pen2, D-Pen5]-enkephalin (DPDPE), at concentrations ranging from 10 nM to 100 microM, induced concentration-dependent increases in [Ca2+]i (EC50 = 114 nM). The responses could be divided into two phases, with an initial spike in [Ca2+]i followed by either oscillations or a sustained elevation of [Ca2+]i. These effects were blocked by the selective delta-opioid receptor antagonist ICI 174864 (10 microM). The expression of delta-opioid receptors on astroglial cells was further verified immunohistochemically, using specific antibodies, and by Western blot analyses. Pre-treatment of the cells with pertussis toxin (100 ng/ml, 24 h) blocked the effects of delta-opioid receptor activation, consistent with a Gi- or Go-mediated response. The sustained elevation of [Ca2+]i was not observed in low extracellular Ca2+ and was partly blocked by nifedipine (1 microM), indicating the involvement of L-type Ca2+ channels. Stimulating neurons with DPDPE resulted in a decrease in [Ca2+]i, which may be consistent with the closure of the plasma membrane Ca2+ channels on these cells. The current results suggest a role for astrocytes in the response of the brain to delta-opioid peptides and that these opioid effects in part involve altered astrocytic intracellular Ca2+ homeostasis.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Ultrastructural immunocytochemical localization of mu-opioid receptors in rat nucleus accumbens: extrasynaptic plasmalemmal distribution and association with Leu5-enkephalin

mu-Opioid receptors and their endogenous ligands, including Leu5-enkephalin (LE), are distributed abundantly in the nucleus accumbens (NAC), a region implicated in mechanisms of opiate reinforcement. We used immunoperoxidase and/or immunogold-silver methods to define ultrastructural sites for functions ascribed to mu-opioid receptors and potential sites for activation by LE in the NAC. An antipeptide antibody raised against an 18 amino acid sequence of the cloned mu-opioid receptor (MOR) C terminus showed that MOR-like immunoreactivity (MOR-LI) was localized predominantly to extrasynaptic sites along neuronal plasma membranes. The majority of neuronal profiles containing MOR-LI were dendrites and dendritic spines. The dendritic plasma membranes immunolabeled for MOR were near sites of synaptic input from LE-labeled terminals and other unlabeled terminals forming either inhibitory or excitatory type synapses. Unmyelinated axons and axon terminals were also intensely but less frequently immunoreactive for MOR. Observed sites for potential axonal associations with LE included coexistence of MOR and LE within the same terminal, as well as close appositions between differentially labeled axons. Astrocytic processes rarely contained detectable MOR-LI, but also were sometimes observed in apposition to LE-labeled terminals. We conclude that in the rat NAC, MOR is localized prominently to extrasynaptic neuronal and more rarely to glial plasma membranes that are readily accessible to released LE and possibly other opioid peptides and opiate drugs. The close affiliation of MOR with spines receiving excitatory synapses and dendrites receiving inhibitory synapses provides the first direct morphological evidence that MOR selectively modulates postsynaptic responses to cortical and other afferents.

Primary astroglial cultures derived from several rat brain regions differentially express mu, delta and kappa opioid receptor mRNA

The existence of opioid receptors within glial cell membranes has been proposed by several laboratories based on biochemical and radioligand binding data. The recent cloning of the mu, delta and kappa receptors has enabled us to directly examine the issue of opioid receptor expression in rat brain astroglia by using solution hybridization/ribonuclease protection assays to analyze the total RNA obtained from primary cultures of cortical, striatal, cerebellar, hippocampal and hypothalamic astrocytes. The results indicate that all five glial cultures expressed mu, delta and kappa receptor mRNA. The rank order of receptor mRNA abundance, expressed collectively across all five cultures, was determined to be delta > or = kappa >> mu. An analysis of the glial distribution profile for each receptor type revealed that mu receptor mRNA levels were the most abundantly expressed in cortical cultures, while the greatest levels of delta receptor mRNA were found in the cortical and hypothalamic cultures, and significant kappa receptor mRNA levels were produced by the cortical, hypothalamic and cerebellar cultures. Furthermore, the five glial cultures each expressed different levels of total opioid receptor (mu + delta + kappa) mRNA. The rank order of total opioid receptor mRNA expression across different astroglial cultures was found to be cortex > hypothalamus > cerebellum = hippocampus > striatum. An analysis of the relative expression profiles for mu, delta and kappa receptor mRNA within each culture revealed that all cultures manifested relatively high levels of delta and kappa receptor mRNA, but relatively low levels of mu receptor mRNA. Generally, cortical, hippocampal and hypothalamic cultures were characterized by comparable levels of delta and kappa receptor mRNA, and little, if any, mu receptor mRNA. However, striatal cultures were characterized by a high level of delta receptor mRNA which was approximately twice and four times that of the kappa and mu receptor mRNA, respectively. In contrast, cerebellar cultures expressed predominantly kappa receptor mRNA at a level which was almost twice that of the delta receptor mRNA, and expressed very little mu receptor mRNA. These data show that primary astroglial cultures not only express mu, delta and kappa receptor mRNAs, but they do so in a manner dependent upon receptor type and brain region. This suggests a regional heterogeneity of astrocytes with respect to opioid receptor expression, a characteristic previously described only for neurons. Furthermore, it suggests the existence of an additional anatomical component in CNS opioid systems.

Localization of vasopressin mRNA and immunoreactivity in pituicytes of pituitary stalk-transected rats after osmotic stimulation

The presence of [arginine] vasopressin (AVP) mRNA and AVP immunoreactivity in pituicytes of the neural lobe (NL) of intact and pituitary stalk-transected rats, with and without osmotic stimulation, was examined. AVP mRNA was analyzed by Northern blotting, as well as by in situ hybridization in combination with immunocytochemistry using anti-glial fibrillary acidic protein (GFAP) as a marker for pituicytes. In intact rats, a poly(A) tail-truncated 0.62-kb AVP mRNA was detected in the NL and was found to increase 10-fold with 7 days of continuous salt loading. Morphological analysis of the NL of 7-day salt-loaded rats revealed the presence of AVP mRNA in a significant number of GFAP-positive pituicytes in the NL and in areas most probably containing nerve fibers. Eight days after pituitary stalk transection the NL AVP mRNA diminished in animals given water to drink, whereas in those given 2% saline for 18 h followed by 6 h of water, a treatment repeated on 6 successive days beginning 2 days after surgery, the 0.62-kb AVP mRNA was present. The AVP mRNA in the pituitary stalk-transected, salt-loaded rats showed an exclusive cellular distribution in the NL, indicative of localization in pituicytes. Immunoelectron microscopy showed the presence of AVP immunoreactivity in a subpopulation of pituicytes 7 and 10 days after pituitary stalk transection in salt-loaded animals, when almost all AVP fibers had disappeared from the NL. These data show that a subset of pituicytes in the NL is activated to synthesize AVP mRNA and AVP in response to osmotic stimulation.

Naloxone disinhibits magnocellular responses to osmotic and volemic stimuli in chronically hypoosmolar rats

Normonatremic and chronically hyponatremic rats were pretreated with naloxone (5 mg/kg) or isotonic (150 mM) NaCl, then were given i.v. injections of 2 M NaCl (2 ml) or were hemorrhaged (20 ml/kg). Baseline and post-stimulus blood samples were withdrawn through indwelling jugular venous catheters. Baseline levels of plasma vasopressin (AVP) and oxytocin (OT) were similar in both normonatremic and hyponatremic rats and did not change after naloxone pretreatment. Increases in plasma AVP and OT levels in response to both hypertonic saline and hemorrhage were markedly blunted in the hyponatremic rats compared to the normonatremic rats. Naloxone pretreatment caused augmented AVP and OT secretion in response to hypertonic saline stimulation and hemorrhage in both the normonatremic and hyponatremic rats; the magnitude of the naloxone augmentations in the hyponatremic rats were sufficient to normalize the OT response to hypertonic saline and both the OT and AVP responses to hemorrhage. Our results therefore suggest that endogenous opioids are likely involved in the inhibition of stimulus-induced AVP and OT release that accompanies chronic hypoosmolality.

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.

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.

A monoclonal anti-idiotypic antibody to opioid receptors labels desipramine-induced opioid binding sites on rat C6 glioma cells and attenuates thymidine incorporation into DNA

Treatment of rat C6 glioma cells with the tricyclic antidepressant desipramine induces opioid binding. Here the distribution of these opioid-binding sites on C6 cell membranes and a functional property were investigated. Immunohistochemical examination of C6 cells was performed using a monoclonal anti-idiotypic antibody to opioid receptors (Ab2AOR). Ab2AOR uniformly labeled > 97% of the cells exposed to desipramine over their entire surface. The opioid-receptor antagonist naltrexone completely blocked Ab2AOR binding. Ab2AOR, which has opioid agonist properties, also inhibited DNA synthesis in desipramine-treated but not in naive C6 cells. Similarly, morphine blocked C6 cell proliferation only after desipramine treatment. The antineurotrophic action of Ab2AOR was reversed by naltrexone and was insensitive to pertussis toxin. These findings demonstrate that Ab2AOR suppresses the proliferation of C6 glioma cells by binding to desipramine-induced opioid receptors.

Dynorphin 1-17 delays the vasopressin induced mobilization of intracellular calcium in cultured astrocytes from the rat neural lobe

Opioid peptides are present in nerve terminals in the rat neural lobe where they partially coexist with vasopressin. Morphological findings suggest that these neuropeptides are released onto pituicytes, which is in agreement with a possible role for the pituicyte in oxytocin and vasopressin release from the neural lobe. Pituicytes in culture respond to vasopressin with a mobilization of calcium from intracellular stores. In the present study this vasopressin induced increase in intracellular free calcium levels was both delayed and decreased by pre-exposure to dynorphin 1-17, while dynorphin 1-17 by itself did not affect basal calcium levels. All effects of dynorphin 1-17 could be blocked with naloxone. The present results suggest that opioid receptors are present on pituicytes and are coupled to a second messenger pathway by which opioid peptides may inhibit inositol phosphate dependent calcium mobilization by other neuropeptides, such as vasopressin.

POMC-derived peptide immunoreactivity in neural lobe axons of the human pituitary

The efferent projections of proopiomelanocortin (POMC) neurons in the arcuate nucleus and nucleus of the solitary tract have been extensively characterized in the rat, but are less well understood in the human brain. We report here that ACTH, alpha-MSH, beta-endorphin, and N-acetyl-beta-endorphin immunoreactive axons are localized in the neural lobe of the human pituitary gland, in congruence with prior evidence that beta-endorphin and other POMC-derived peptides modulate vasopressin and oxytocin secretion.

kappa-Opioid agonist modulation of [3H]thymidine incorporation into DNA: evidence for the involvement of pertussis toxin-sensitive G protein-coupled phosphoinositide turnover

A body of evidence has indicated that mu-opioid agonists can inhibit DNA synthesis in developing brain. We now report that kappa-selective opioid agonists (U69593 and U50488) modulate [3H]thymidine incorporation into DNA in fetal rat brain cell aggregates in a dose- and developmental stage-dependent manner, kappa agonists decreased thymidine incorporation by 35% in cultures grown for 7 days, and this process was reversed by the kappa-selective antagonist, norbinaltorphimine, whereas in 21-day brain cell aggregates a 3.5-fold increase was evident. Cell labeling by [3H]thymidine was also inhibited by the kappa-opioid agonist as shown by autoradiography. In addition, U69593 reduced basal rates of phosphoinositide formation in 7-day cultures and elevated it in 21-day cultures. Control levels were restored by norbinaltorphimine. Pertussis toxin blocked U69593-mediated inhibition of DNA synthesis. The action of kappa agonists on thymidine incorporation in the presence of chelerythrine, a protein kinase C (PKC) inhibitor, or in combination with LiCl, a noncompetitive inhibitor of inositol phosphatase, was attenuated in both 7- and 21-day cultures. These results suggest that kappa agonists may inhibit DNA synthesis via the phosphoinositide system with a pertussis toxin-sensitive G protein as transducer. In mixed glial cell aggregates, U50488 increased thymidine incorporation into DNA 3.1-fold, and this stimulation was reversed by the opioid antagonist naltrexone.

Intracellular calcium and hormone release from nerve endings of the neurohypophysis in the presence of opioid agonists and antagonists

Rat neural lobes and isolated nerve terminals from the neurohypophysis were stimulated in the presence of different opioid agonists and antagonists. The secretion of arginine vasopressin and oxytocin and rise in cytoplasmic calcium induced by depolarization were analyzed by radioimmunoassay and the fluorescent probe fura-2, respectively. The kappa-agonists dynorphin A(1-13) and dynorphin A(1-8) did not affect electrically evoked release of vasopressin, although oxytocin release was slightly reduced. U-50 488, a relatively specific kappa-receptor agonist, had no effect on the amount of vasopressin or oxytocin secreted, although it significantly reduced K(+)-evoked changes in [Ca2+]i in isolated nerve endings. Two kappa-receptor antagonists, MR 2266 and diprenorphin, alone had no effect on vasopressin and oxytocin secretion from isolated nerve endings depolarized with potassium. Opioid agonists less selective for the kappa receptors, etorphin and ethylketocyclazocin, were found to inhibit the release of both vasopressin and oxytocin significantly. Naloxone, a nonselective opiate receptor antagonist, alone had no effect on vasopressin release but potentiated the electrically evoked release of oxytocin. Naloxone also could overcome the inhibitory effect of etorphin on oxytocin and vasopressin release observed after electrical stimulation of the neural lobe. A number of inconsistencies therefore exist between the effects of opioid agonists and antagonists on neuropeptide release and on the evoked changes in [Ca2+]i. In view of these inconsistencies and the high concentrations of opioid agonists and antagonists necessary to modify release, we conclude that it is doubtful that opioid molecules have a physiological role in controlling neurohypophysial secretion.

Arginine vasopressin mobilises intracellular calcium via V1-receptor activation in astrocytes (pituicytes) cultured from adult rat neural lobes

An extremely close association exists between the membranes of the neurosecretory endings and the resident astrocytes (pituicytes) of the neurohypophysis. Indeed, synaptoid contacts involving neurosecretory vesicle-containing axons contacting pituicytes have been observed, suggesting pituicytes as targets of the products released from neurosecretory axons. We have investigated the effects of various neural lobe peptides on pituicytes in primary culture from adult neurohypophyses. Using Fura-2 loaded cells and dynamic ratio imaging, we have determined that arginine vasopressin (AVP) or V1- but not V2-receptor agonists, mobilise pituicyte intracellular Ca2+ ([Ca2+]i) in the absence of extracellular Ca2+. AVP was consistently effective at concentrations of 10 nM or higher in elevating [Ca2+]i by 200-1000 nM. These responses could be blocked by V1-antagonists and were shown to be associated with accumulation of phosphoinositides. Oxytocin was also found to mobilise [Ca2+]i but was effective only at higher concentrations than for AVP. Oxytocin-evoked [Ca2+]i elevations were also blocked by V1-antagonists. Raising [K+]0 was ineffective in changing [Ca2+]i suggesting that these cells lack voltage-gated Ca2+ channels. We conclude that pituicytes possess V1-receptors, activation of which mobilises [Ca2+]i, possibly functioning to initiate a Ca(2+)-activated K+ conductance which could contribute to further depolarisation of secretory terminals and facilitate exocytosis.

Localization of mu opioid receptors on the membranes of nerve endings and tanycytes in the guinea-pig median eminence by electron microscopic radioautography

The high density of opioid-containing nerve endings in the median eminence together with the absence of direct effects of opioids upon pituitary suggest a local action of opioids in the median eminence. The aim of this work was to address the occurrence of mu-opioid binding sites in the median eminence at the electron microscopic level, using the highly selective radioligand [125I]FK 33-824. mu-Opioid receptors were labeled in vitro on slightly prefixed slices of mediobasal hypothalamus. The labeling was essentially detected in the external part of the median eminence. Most of the silver grains overlaid membrane appositions. Two overall types of appositions were concerned: nerve terminal-nerve terminal or nerve terminal-tanycyte. Detailed analysis of the silver grain distribution indicated that mu receptors were observed on membranes of different types of nerve endings but also of tanycytes. All the binding sites were localized out of synaptic junctions since the median eminence is totally devoid of these structures. Our results suggest that in the median eminence, opioid peptides have a paracrine and/or autocrine action occurring at least via mu receptors located on nerve terminals but also on tanycytes.

Delta and kappa opiate receptors in primary astroglial cultures. Part II: Receptor sets in cultures from various brain regions and interactions with beta-receptor activated cyclic AMP

In a previous paper, delta and kappa opiate receptors were shown to be co-localized on the same cell in enriched primary cultures of astroglia from neonatal rat cerebral cortex. Activation of the receptors inhibited adenylate cyclase. In this work, the presence of opiate receptors was investigated in astroglial primary cultures from neonatal rat striatum and brain stem. Cyclic adenosine 3',5'-monophosphate accumulation was quantified in the presence of different opioid receptor ligands after stimulation of the cyclic adenosine 3',5'-monophosphate system with forskolin. Morphine was used as a mu receptor agonist. [D-Ala2, D-Leu5]-enkephalin or [D-Pen2, D-Pen5]-enkephalin were used as delta receptor agonists and dynorphin 1-13 or U-50,488H were used as kappa receptor agonists. Specific antagonists for the respective receptors were used. After striatum or brain stem cultures had been incubated in 10(-9)-10(-5) M of each [D-Ala2, D-Leu5]-enkephalin, [D-Pen2, D-Pen5]-enkephalin and Dynorphin 1-13 or U-50,488H, dose related inhibitions of the 10(-5) M forskolin stimulated cyclic adenosine 3',5'-monophosphate accumulation were observed. The changes were reversed to the forskolin-induced control level in the presence of the respective antagonists. 10(-9)-10(-5) M morphine did not significantly change the forskolin-induced accumulation of cyclic adenosine 3',5'-monophosphate in the cultures studied. Furthermore, cultures from cerebral cortex, striatum or brain stem were incubated with isoproterenol alone or together with morphine or [D-Ala2, D-Leu5]-enkephalin.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of pituitary stalk transection, hypophysectomy and thyroid hormone status on insulin-like growth factor 2-, growth hormone releasing hormone-, and somatostatin mRNA prevalence in rat brain

We have used in situ hybridization histochemistry to determine the effects of pituitary stalk transection, hypophysectomy and drug-induced changes in thyroid status on mRNA levels encoding insulin-like growth factor 2, somatostatin, and growth hormone-releasing factor in the choroid plexus, hypothalamic periventricular nucleus, and arcuate nucleus, respectively. Pituitary stalk transection and hypophysectomy in Sprague-Dawley rats decreased insulin-like growth factor 2 and somatostatin mRNA and increased growth hormone-releasing factor mRNA. In each case, the effect of hypophysectomy exceeded that of pituitary stalk transection. Treatment with propylthiouracil for 10 days decreased somatostatin mRNA, markedly increased growth hormone-releasing factor mRNA but had no significant effect on insulin-like growth factor 2 mRNA. Treatment with triiodothyronine had no effect on the mRNAs measured. These findings corroborate the clinical observation of abnormal somatic growth in disturbances of thyroid and growth hormone status and provide further evidence of the effects of these metabolic disturbances and of pituitary disconnection and hypophysectomy on insulin-like growth factor 2 mRNA prevalence.

Osmotic regulation of neuropeptide Y and its binding sites in the magnocellular hypothalamo-neurohypophysial pathway

The magnocellular hypothalamo-neurohypophysial system is, via a release of vasopressin from nerve terminals in the neurohypophysis to the peripheral blood, centrally involved in the regulation of body salt and water homeostasis. Furthermore, it has been shown that expression of neuropeptides co-existing with vasopressin or oxytocin in magnocellular neurons is influenced by salt loading. We here report, that neuropeptide Y (NPY)-immunoreactivity, which is normally not observed in the magnocellular neurons of the hypothalamic supraoptic and paraventricular nuclei of rats becomes immunohistochemically detectable after salt loading. Using a double-immunohistochemical procedure on the same brain sections, it is shown that NPY is co-existing with either vasopressin or oxytocin in these neurons. Within the neurohypophysis of normal rats, a moderate number of predominantly fine calibered NPY-immunoreactive nerve fibers most often coursing along vessels is observed in addition to a low number of large peptidergic terminals. In salt-loaded rats, however, the number of NPY-immunoreactive neurohypophysial large nerve terminals in apposition to vascular lumina is drastically increased. By using quantitative receptor autoradiography, it is demonstrated that in salt-loaded animals, the number of neurohypophysial NPY binding sites is decreased to nearly undetectable levels (0.054 +/- 0.02 fmol/mg) compared to a very high density of binding sites in normal animals (1.151 +/- 0.15 fmol/mg). This raises evidence that NPY containing hypothalamo-neurohypophysial neurons as well as peripherally released NPY may be involved in the regulation of water homeostasis via NPY receptors in the neurohypophysis.

Distribution of substance P-immunoreactive elements in the preoptic area and the hypothalamus of the rat

The localization and morphology of neurons, processes, and neuronal groups in the rat preoptic area and hypothalamus containing substance P-like immunoreactivity were studied with a highly selective antiserum raised against synthetic substance P. The antiserum was thoroughly characterized by immunoblotting; only substance P was recognized by the antiserum. Absorption of the antiserum with synthetic substance P abolished immunostaining while addition of other hypothalamic neuropeptides had no effect on the immunostaining. The specificity of the observed immunohistochemical staining pattern was further confirmed with a monoclonal substance P antiserum. The distribution of substance P immunoreactive perikarya was investigated in colchicine-treated animals, whereas the distribution of immunoreactive nerve fibers and terminals was described in brains from untreated animals. In colchicine-treated rats, immunoreactive cells were reliably detected throughout the preoptic area and the hypothalamus. In the preoptic region, labeled cells were found in the anteroventral periventricular and the anteroventral preoptic nuclei and the medial and lateral preoptic areas. Within the hypothalamus, immunoreactive cells were found in the suprachiasmatic, paraventricular, supraoptic, ventromedial, dorsomedial, supramammillary, and premammillary nuclei, the retrochiasmatic, medial hypothalamic, and lateral hypothalamic areas, and the tuber cinereum. The immunoreactive cell groups were usually continuous with adjacent cell groups. Because of the highly variable effect of the colchicine treatment, it was not possible to determine the actual number of immunoreactive cells. Mean soma size varied considerably from one cell group to another. Cells in the magnocellular subnuclei of the paraventricular and supraoptic nuclei were among the largest, with a diameter of about 25 microns, while cells in the supramammillary and suprachiasmatic nuclei were among the smallest, with a diameter of about 12 microns. Immunoreactive nerve fibers were found in all areas of the preoptic area and the hypothalamus. The morphology, size, density, and number of terminals varied considerably from region to region. Thus, some areas contained single immunoreactive fibers, while others were innervated with such a density that individual nerve fibers were hardly discernible. During the last decade, knowledge about neural organization of rodent hypothalamic areas and mammalian tachykinin biochemistry has increased substantially. In the light of these new insights, the present study gives comprehensive morphological evidence that substance P may be centrally involved in a wide variety of hypothalamic functions. Among these could be sexual behavior, pituitary hormone release, and water homeostasis.

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)

Effects of alcohol on beta-endorphin and reproductive hormones in the male rat

In an attempt to examine the relationship between alcohol-induced alterations in immunoreactive beta-endorphin (i-beta E) levels in the hypothalamic-pituitary-gonadal axis and the synthesis and release of reproductive hormones, male rats were treated with either an acute intraperitoneal injection of alcohol or were chronically exposed to an alcohol-containing liquid diet. Hypothalamic, pituitary, serum, and testicular levels of immunoreactive beta-endorphin (i-beta E) and serum levels of luteinizing hormone (LH) and testosterone were measured at various times after initiation of these treatments. Testicular interstitial fluid (TIF) volumes and levels of TIF i-beta E and testosterone were also measured 4 hr after acute treatment as an index of testicular release of these substances. Acute alcohol decreased pituitary levels of i-beta E and increased serum levels of the peptide for up to 1 hr after its injection, but did not alter hypothalamic or testicular levels. Acute alcohol markedly increased TIF i-beta E and decreased TIF testosterone and TIF volume. Sharp decreases in serum LH and testosterone were observed in association with these acute changes in i-beta E levels in the pituitary, blood, and testes. During chronic alcohol exposure serum testosterone levels were substantially depressed, but tolerance appeared to develop quickly to the chronic effects of alcohol on serum LH. Similarly, tolerance to alcohol's effects on i-beta E levels in the pituitary and serum also appeared to develop during chronic alcohol administration. However, hypothalamic and testicular i-beta E levels were markedly suppressed by chronic alcohol administration in contrast to the lack of effect observed after acute alcohol administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitter-mediated changes in the electrophysiological properties of pituicytes

Abstract Intracellular recordings were obtained from pituicytes in the neural lobe of the isolated rat pituitary. Like other glia, pituicytes lacked action potentials in response to depolarizing current injection, but they tended to have more positive resting membrane potentials and higher input resistances than astrocytes in other preparations. Dye-coupling typical of astrocytes was also demonstrated amongst pituicytes, and their morphologies were similar to those of pituicytes stained for glial fibrillary acidic protein. Action potentials, anode-break spikes or barium spikes were not observed in pituicytes, even under conditions that maximized the elicitation of Ca(2+)-dependent responses. This suggests that pituicytes either have no or a very low density of Ca(2+) channels or Ca(2+) currents that are too small to generate action potentials. Dynorphin A (1-13), a kappa-opioid agonist, produced long-lasting increases in pituicyte input resistance with no significant changes in resting membrane potential. Dynorphin's action was concentration-dependent and was blocked by the opioid antagonist naloxone. This is consistent with previous reports demonstrating kappa-opioid receptors on pituicytes in the neurohypophysis. The beta-adrenergic agonist isoproterenol (100 muM) reversed the increases in pituicyte input resistance produced by opioid application, with no significant changes in resting membrane potential. The fact that pituicytes responded to neurotransmitters suggests a functional link between pituicytes and neurosecretory nerve fibres.

Desipramine modulation of sigma and opioid peptide receptor expression in glial cells

Exposure of C6 glial cell cultures to desipramine induced the appearance of opioid receptors and up-regulated sigma receptors. Opioid binding was demonstrated with 3H-etorphine and 3H-dihydromorphine (DHM), but was not observed with the mu, delta and kappa ligands 3H-DAMGE, 3H-DADLE or 3H-(-)ethylketocyclazocine in the presence of specific blockers, respectively. Competition experiments with 3H-DHM and either (-)naloxone or (+)naloxone indicated the presence of authentic opioid receptors. In similar studies with beta-endorphin, its truncated form (1-27) or their N-acetyl derivatives, beta-endorphin proved to have the highest affinity. Opioid receptors in glial cell aggregates were primarily kappa, with few mu and delta sites. Desipramine increased Bmax values for kappa but not mu and delta.

Pharmacological assessment of the site of action of opioids on the release of vasopressin and oxytocin in the rat

Naloxone and its congener, methyl naloxone, were given subcutaneously (s.c.) or centrally (i.c.v.) to 24-h water-deprived male rats 30 min prior to decapitation and the effect on plasma levels of vasopressin (VP) and oxytocin (OT) was studied. The potency of s.c. applied methyl naloxone to increase plasma OT levels did not differ from that of naloxone. Injected i.c.v., neither methyl naloxone nor naloxone had a clear effect and they antagonized i.c.v. co-administered dynorphin A-(1-13) equipotently. Methyl naloxone or naloxone, s.c., antagonized the inhibitory action of simultaneous dynorphin A-(1-13) and beta-endorphin-(1-31) given i.c.v., although higher doses of methyl naloxone were required. The data indicate that the sites of inhibition of neurohypophysial hormone release due to beta-endorphin-(1-31) are more likely to be located mostly within the blood-brain barrier, to which methyl naloxone has less ready access, than are the sites of inhibition due to dynorphin A-(1-13).

Distribution, Isolation and Sequence Analysis of the C-Terminal Heptapeptide of Pro-Enkephalin A (YGGFMRF) from the Ovine Median Eminence

Abstract Using a polyclonal antiserum raised against the C-terminal heptapeptide of pro-enkephalin A, we have isolated the opioid heptapeptide Tyr-Gly-Gly-Phe-Met-Arg-Phe (MERF) from ovine median eminence and mapped its distribution in that structure. MERF-immunoreactivity was confined to the pars externa (neurosecretory zone) where it colocalized with corticotrophin-releasing factor in the majority of terminals. No larger, N-terminally extended forms of MERF were detected in median eminence extracts suggesting that pro-enkephalin is fully processed to its constituent enkephalin congeners, and that the bioactive products, including MERF, act at the level of the hypothalamus in regulating anterior pituitary function.

Binding sites for atrial natriuretic peptide (ANP) on cultured pituicytes: lack of effect of ANP on release of neurohypophysial hormones in vitro

Receptors for atrial natriuretic peptide (ANP) are known to be present in the posterior pituitary gland and this is a possible site of action of ANP to modulate neurohypophysial hormone release. Pituicytes cultured from adult rat neurohypophyses are shown to possess high affinity binding sites for ANP, suggesting that in vivo a population of neurohypophysial ANP receptors are present on these astrocytic glial cells. alpha-rANP (1-100 nM) did not modulate the basal or electrically stimulated release of oxytocin or vasopressin from the isolated neurohypophysis in vitro. The physiological significance of the glial ANP binding sites thus remains unknown.

Enhanced Neurohypophyseal Vasopressin Release is Associated with Increased Opioid Inhibition of Oxytocin Release

Abstract We tested the hypothesis of a cross-inhibition of oxytocin (OT) release by endogenous opioid peptides co-released with vasopressin (VP). This opioid cross-inhibition resulted in a selective block of OT release and hence in preferential release of VP. The effects of the opiate receptor antagonist naloxone were tested on neurohypophyseal VP release during dehydration, ethanol administration and sulphated cholecystokinin octapeptide (CCK-8S) application, assuming that the inhibition of pituitary OT release by endogenous opioids increases as neurohypophyseal VP output increases. A high VP output was found to coincide with increased inhibition of OT release: Subcutaneous injection of graded doses of naloxone (30 min prior to decapitation), augmented OT plasma levels significantly more in 24 h water-deprived male rats than in normally hydrated rats. Naloxone had no effect on VP release. Ethanol (10% in saline) administered intragastrically 50 min prior to decapitation and 20 min before subcutaneous naloxone (5 mg/kg) resulted in the inhibition of VP output. The ethanol treatment resulted in a rise in plasma OT levels that was additional to the effect of naloxone. These features were present in normally hydrated as well as in 24 h water-deprived animals, but were more pronounced in the latter group. Peripheral CCK-8S administration induces an abrupt and selective secretion of OT. Blocking the opioid inhibition of OT release with naloxone resulted in a significant rise of OT compared to that with CCK-8S alone. The magnitude of the opioid inhibition coincided with the activity of the VP system, and a higher dose of naloxone was needed to potentiate the CCK-8S effect on OT release in the water-deprived group than in euhydrated rats. No effect of CCK-8S and/or naloxone was found on VP plasma levels. The data indicate that opioid peptides co-released with VP (like dynorphin) may be responsible for cross-inhibition of OT release during dehydration. This suggests that dynorphin acts in a paracrine way, making it a strong candidate for this role.

Interaction of opiate peptides with dopamine effects on prolactin secretion and membrane electrical properties in anterior pituitary cells from lactating rats

Abstract Met-enkephalin and beta-endorphin induced a partial reversion of the dopamine inhibition of prolactin release from pituitary cells of lactating rats in primary culture. This effect of opiate peptides was dose-dependent with an EC50 of 40 +/- 8 nM and 45 +/- 7 nM and maximal blockade of dopamine inhibition of 60% and 68% for Met-enkephalin and beta-endorphin, respectively. Naloxone antagonized the effect of Met-enkephalin with an EC50 of 22 +/- 12 nM. Furthermore, this Met-enkephalin effect on dopamine inhibition of prolactin secretion appeared non-competitive since it reduced maximal inhibition without affecting the apparent affinity of dopamine. Finally, it should be noted that the two opiate peptides had no effect on spontaneous prolactin release. In electrophysiological experiments, local ejection of dopamine on tested cells induced an hyperpolarization concomitant with an increase of the membrane conductance. Ejection of Met-enkephalin or beta-endorphin alone did not modify the electrical properties of the cells (resting potential, membrane conductance and excitability). In contrast, both peptides blocked in a reversible manner the dopamine-induced electrical responses. These effects were antagonized by naloxone. However, this interaction of opiatepeptides with dopamine electrical response was not observed on all cells tested. We conclude that the blocking effect of opiates on dopamine-induced hyperpolarization may account, at least in part, for the ability of these peptides to interact with dopamine inhibition of prolactin release.

Delta and kappa opiate receptors in primary astroglial cultures from rat cerebral cortex

The effects of mu, delta, and kappa receptor-agonists on forskolin stimulated cyclic adenosine-3',5'-monophosphate (cAMP) formation were examined in astroglial enriched primary cultures from the cerebral cortex of newborn rats. Intracellular cAMP accumulation was quantified by radioimmunoassay. Morphine was used as a mu-receptor agonist, D-Ala-D-Leu-Enkephalin (DADLE) as a delta-receptor agonist and dynorphin 1-13 (Dyn) as a kappa-receptor agonist. Basal cAMP levels were unaffected by either the opiate agonists or the antagonists used. In the presence of the cAMP stimulator forskolin, morphine had no significant effect on the cytoplasmic cAMP levels. DADLE caused a dose related inhibition of the forskolin stimulated cAMP accumulation. The effects of this delta receptor stimulation was blocked with the selective antagonist ICI 174.864. In the presence of Dyn, the forskolin stimulated cAMP accumulation was inhibited in a dose related manner. This kappa receptor stimulation was blocked with the selective antagonist MR 2266. Co-administration of DADLE and Dyn resulted in a non additive inhibition of the forskolin stimulated accumulation of cAMP. These findings indicate that astroglial enriched cultures from the cerebral cortex of rats express delta and kappa-receptors co-localized on the same population of cells, and that these receptors are inhibitory coupled to adenylate cyclase.

The origin and regulation of posterior pituitary vasopressin ribonucleic Acid in osmotically stimulated rats

Abstract Vasopressin ribonucleic acid (VP RNA) accumulates in the posterior lobe of the rat pituitary in response to an osmotic stimulus. The accumulation of posterior pituitary VP RNA can be prevented by stalk transection or the intracerebroventricular injection of colchicine. The hypothalamic and pituitary VP RNAs however, have different structures and are independently regulated. Depletion of serotonin with parachlorophenylalanine blocks the osmotically induced accumulation of both VP and oxytocin RNA in the hypothalamus but does not affect the accumulation of VP RNA in the posterior pituitary gland. In Brattleboro rats, posterior lobe oxytocin RNA but not VP RNA, increases after osmotic stimulation. Although axonal transport of RNA may occur, our results are also consistent with VP RNA synthesis in pituicytes controlled by hypothalamic neuroendocrine factors released by neurohypophyseal terminals in response to osmotic stimuli.

Osmotic regulation of methionine enkephalin in the posterior pituitary of the rat

Methionine (Met) enkephalin is detectable in rat hypothalamic and neurointermediate lobe (NIL) tissue extracts using a specific radioimmunoassay. Reversed-phase HPLC revealed that only the pentapeptide form was present in the extracts. Total amounts of Met-enkephalin in extracts containing the median eminence (ME), supraoptic nucleus (SON), paraventricular nucleus (PVN), or NIL from control animals were 9.7 +/- 1.6, 0.9 +/- 0.2, 25.2 +/- 6.8 and 14.8 +/- 1.2 pmol respectively (means +/- S.E.M., n = 6). In animals given 340 mmol NaCl/l to drink for 5 days, no significant changes occurred in Met-enkephalin content in the SON or ME, but significant decreases were observed in the NIL and PVN (9.8 +/- 0.8 and 13.6 +/- 1.7 pmol, respectively). Amounts of Met-enkephalin in these tissues were further decreased after 12 days of 340 mmol NaCl/l (3.4 +/- 0.4 and 6.0 +/- 0.6 pmol). These data demonstrate that enkephalin immunoreactivity in the NIL is principally in the form of the Met-enkephalin pentapeptide, and that this peptide is released in response to increased plasma osmolality. The concomitant changes in the PVN and NIL tissue content suggest that the PVN is the source of NIL Met-enkephalin.

Pituitary astrocytes from the neural lobe of rats. A tissue culture and immunohistochemical study

Tissue culture preparations of adult and neonatal rat pituitary neural lobes were examined by use of cell-type specific immunohistochemical markers. Cultures obtained from explanted or dissociated adult tissue or explanted neonatal tissue produced cells immunoreactive for endothelial and fibroblast markers. In contrast, dissociated neonatal tissue produced, in addition, two distinct forms of astrocytic glial cells immunoreactive for glial fibrillary acidic protein, one of which was also immunoreactive for the ganglioside GD3.

μ-Opiate Receptors in Neuronal Primary Cultures from Cerebral Cortex

The presence of μ-opioid receptors was demonstrated as effects of receptor stimulation on PGE1-induced cAMP accumulation in neuronal-enriched primary cultures from rat cerebral cortex. Morphine was used as a μ-receptor agonist. There was a dose-dependent inhibition of the PGE1-stimulated cAMP accumulation by morphine, blocked by the μ-receptor antagonist naloxone. These findings suggest that these neuronal cultures express μ-receptors, possibly connected to adenylate cyclase via an inhibitory Gi-protein. The probable use of functional μ-receptors in neurotoxicological tests is discussed.

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.

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)

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.