gamma-Aminobutyric acid antagonists stimulate vasopressin release from organ-cultured hypothalamo-neurohypophyseal explants

gamma-Aminobutyric acid (GABA) has been identified in axon terminals innervating neurons of the supraoptic nucleus and has been shown to inhibit the electrical activity of supraoptic neurons when applied iontophoretically. This study examines the effects of GABA and GABA antagonists on vasopressin (VP) release from organ-cultured explants of the hypothalamo-neurohypophyseal system (HNS). The GABA antagonists bicuculline and picrotoxin stimulated VP release in a concentration-dependent manner. These observations suggest that VP release by HNS explants is tonically inhibited by GABA. Exposure of HNS explants to GABA (10(-8)-10(-3) M) did not consistently alter basal VP release. This was true even when penicillin, which can block GABA-activated chloride channels, was omitted from the medium. Similarly, nipecotic acid, an agent that potentiates GABA activity by inhibiting GABA uptake, did not alter basal VP release; stimulation of VP release by acetylcholine and increases in osmolality was not diminished by the addition of 10(-5) M GABA. The failure of exogenous GABA to modify basal and stimulated VP release suggests that GABAergic inhibition of VP release is maximally activated by endogenously released GABA in cultured HNS explants. This is consistent with evidence for a local source of GABA in the supraoptic nucleus and suggests that one role of GABA in the regulation of VP release is that of a potent local inhibitory neurotransmitter.     

Angiotensin stimulation of vasopressin release from the rat hypothalamo-neurohypophyseal system in organ culture.

Angiotensin II (AII) stimulated vasopressin (VP) release from the rat hypothalamo-neurohypophyseal system (HNS) in organ culture in a concentration-dependent manner. Exposure to AII at 10(-8) M for 1 hr yielded a 1.8-fold increase in VP release over control release (P less than 0.01), while a 1-h exposure to 10(-5) M AII resulted in a 4-fold increment over control VP release by HNS explants maintained in organ culture for 3 days (P less than 0.01). Saralasin, an AII antagonist, blocked AII stimulation of VP release without significantly altering basal VP release by the HNS explants. Saralasin did not interfere with stimulation of VP release by acetylcholine or nicotine. Tetrodotoxin (10(-7) g/ml) also blocked AII stimulation of VP release. These findings suggest that action potentials are generated in response to AII stimulation of specific receptors in the HNS and are requisite for VP release in response to this stimulus.     

Effect of anteroventral third ventricle lesions on vasopressin release by organ-cultured hypothalamo-neurohypophyseal explants

Rats with electrolytic lesions of the tissue surrounding the third ventricle (AV3V) of the hypothalamus exhibit hypernatremia and chronic drinking deficits in response to hypertonic NaCl. These findings are suggestive of impaired osmoreception. The organ-cultured rat hypothalamo-neurohypophyseal system (HNS) previously has been shown to release vasopressin (VP) in response to osmotic stimuli. The ventral portion of the region damaged by AV3V lesions is included in the HNS explant. Thus, these studies were initiated to evaluate the ability of HNS explants which were obtained from rats previously prepared with AV3V lesions to respond to an increase in osmolality, acetylcholine, or angiotensin II with an increase in VP release. Following electrolytic ablation of the AV3V region or sham lesions and a 2-week recovery period, HNS explants were removed from rats with sham or AV3V lesions. The explants were maintained in organ culture for 4 days. On the third day in culture, increasing the osmolality of the culture medium from 295 to 315 mosm/kg H2O by the addition of NaCl resulted in a 2.5-fold increase in VP release from the explants with sham lesions, but did not significantly alter VP release from the explants with AV3V lesions. On the subsequent day in culture, acetylcholine (10(-5) M) stimulated VP release from the explants with AV3V lesions as well as the explants with sham lesions. Angiotensin II (10-5 M) also stimulated VP release from explants obtained from rats with both AV3V and sham lesions. These data suggest that the osmoreceptors which are involved in controlling VP release from the organ cultured HNS may be located in the region of the AV3V.     

Effect of extended exposure to hypertonicity on vasopressin messenger ribonucleic acid content in hypothalamo-neurohypophyseal explants

The feasibility of using organ-cultured explants of the rat hypothalamo-neurohypophyseal system (HNS) to study the mechanisms regulating the vasopressin (VP) mRNA content of the HNS was examined by evaluating the effect of exposure to hypertonicity on the VP mRNA content of these explants. Different effects were observed after a step increase in osmolality and a gradual increase in the same amount over 24 h. The VP mRNA content of control HNS explants determined from a RNA protection assay was 22 +/- 6 pg. It gradually decreased to 23% and 9% of the control value during 24 and 48 h in culture, respectively. Northern blot analysis revealed a single band of VP mRNA approximately 700 bases long in explants cultured for 36 h. Explants exposed to the step increase in osmolality were maintained in static culture. The control explants were placed directly into isotonic medium (299 mosmol/kg H2O). The explants exposed to the step increase were placed directly into hypertonic medium (greater than 304 mosmol/kg H2O). After 24 h in culture, basal VP release was measured, and all explants were then exposed to a further acute 15 mosm/kg H2O increase in osmolality. The highest basal release of VP was observed in the explants maintained under isotonic conditions (299 mosm/kg H2O). These explants significantly increased VP release in response to the acute increase in osmolality. Basal VP release was lower in explants maintained in hypertonic medium (greater than 304 mosmol/kg H2O), and these explants did not respond to the acute hypertonic pulse. VP mRNA content was significantly decreased in explants maintained for 24 or 48 h in hypertonic medium compared to that in explants maintained in isotonic medium (47 +/- 10% and 57 +/- 6%, respectively; P less than 0.01). No significant difference existed in the VP content of the posterior pituitary between the groups. To achieve a slow increase in osmolality, explants were perifused in individual chambers with medium at 2.1 ml/h. A gradual increase in osmolality (16 mosmol/kg H2O medium) was achieved by increasing the NaCl concentration in the perifusion medium. In response to this stimulus there was a significant increase in VP release, which was sustained for 9 h. VP mRNA content in the hypertonic group was 165 +/- 19% of that in control explants (P less than 0.001), but no difference existed in VP content in the posterior pituitary compared to that in time control explants.(ABSTRACT TRUNCATED AT 400 WORDS)     

17Beta-estradiol abrogates apoptosis in murine skeletal muscle cells through estrogen receptors: role of the phosphatidylinositol 3-kinase/Akt pathway

Estrogens can regulate apoptosis in various cellular systems. The present study shows that 17beta-estradiol (E2), at physiological concentrations, abrogates DNA damage, poly (ADP-ribose) polymerase cleavage, and mitochondrial cytochrome c release induced by H2O2 or etoposide in mouse skeletal muscle C2C12 cells. This protective action, which involved PI3K/Akt activation and Bcl-2 associated death agonist (BAD) phosphorylation, was inhibited by antibodies against the estrogen receptor (ER) alpha or beta isoforms, or transfecting siRNA specific for each isoform. The inhibition of the antiapoptotic action of E2 at the mitochondrial level was more pronounced when ER-beta was immunoneutralized or suppressed by mRNA silencing, whereas transfection of C2C12 cells with either ER-alpha siRNA or ER-beta siRNA blocked the activation of Akt by E2, suggesting differential involvement of ER isoforms depending on the step of the apoptotic/survival pathway evaluated. These results indicate that E2 exerts antiapoptotic effects in skeletal muscle cells which are mediated by ER-beta and ER-alpha and involve the PI3K/Akt pathway.     

Characterization of cholinergic control of vasopressin release by the organ-cultured rat hypothalamo-neurohypophyseal system.

Acetylcholine and nicotine stimulated vasopressin (VP) release from the organ-cultured rat hypothalamo-neurohypophyseal system (HNS). Nicotinic antagonists, hexamethonium, tetraethylammonium chloride, and trimethaphan blocked VP release in response to acetylcholine and nicotine. A muscarinic agonist, methacholine, was ineffective in eliciting VP release from HNS explants at a molar concentration equal to the maximally effective concentration of acetylcholine (10(-5) M). Atropine, a muscarinic antagonist, was an ineffective blocking agent for acetylcholine. These data indicate that the cholinergic receptor in the HNS explant is nicotinic rather than muscarinic in character.     

Cultured hypothalamic explants from spontaneously hypertensive rats have decreased vasopressin and oxytocin content and release

To investigate the central nervous system (CNS) changes in the spontaneously hypertensive rat (SHR), a tissue culture model was used to examine the content and release (24 hour) of the peptide hormones, vasopressin (VP) and oxytocin (OT), from brain explants. Nuclear regions consisting of the paraventricular (PVN) or supraoptic (SON) nuclei were microdissected from prehypertensive SHR and Wistar-Kyoto (WKY) rats. Media levels of VP and OT were measured at 1, 3, 4, and 7 days of culture. After three days of culture, the PVN explants from SHR secreted significantly less VP and OT (both reduced 80%) than did those from WKY. Release of both VP and OT in the SON explants was significantly lower (approximately 50% lower) in the SHR only at seven days of culture. Additionally, tissue content of the peptides was measured after 0, 1, 4, and 7 days of culture. Tissue content of VP and OT was decreased (40% or more) in the SHR in both nuclear regions after four and seven days of culture. In addition, nicotine was found to stimulate the release of VP from SON, but not PVN, cultures in both SHR and WKY explants. Immunohistochemical data showed that there was not a preferential loss of VP or OT neurons in explants from the SHR. Therefore, this in vitro model would indicate that there is a difference in the ability of cultured explants of PVN and SON from SHR and WKY (four-week-old) to synthesize and/or release the peptide hormones VP and OT.     

Dynorphin A inhibits and naloxone increases the electrically stimulated release of oxytocin but not vasopressin from the terminals of the neural lobe

Oxytocin release from the rat neurohypophysis is under endogenous opioid inhibition. It has recently been established that dynorphin precursor-derived peptides are colocalized with vasopressin (VP) in the secretory granules in nerve terminals of the neural lobe, and that the opiate receptors in the neural lobe are restricted to the kappa-subtype. Therefore, we hypothesized that dynorphin, which is copackaged and thus coreleased with VP, is the endogenous opioid that inhibits release from neighboring oxytocin (OT) terminals. To test this hypothesis we examined the effects of dynorphin-(1-8), dynorphin-(1-17), and naloxone on the electrically stimulated release of OT and VP from isolated rat neurointermediate lobes throughout a range of stimulus frequencies. Both dynorphin-(1-8) and -(1-17) (2 microM) produced a substantial reduction in OT release during a 4-Hz stimulus, and this effect was abolished by naloxone (10 microM). Neither form of dynorphin, however, affected OT secretion at a stimulus frequency of 12 or 30 Hz at concentrations up to 10 microM. Naloxone (10 microM) by itself did not affect OT release during the 4-Hz stimulus, but it produced a substantial increase in OT release at a stimulus frequency of 12 Hz. In contrast, neither form of dynorphin produced inhibition, nor did naloxone augment VP secretion at any frequency tested. Frequency-dependent secretion curves (4, 8, 12, 20, and 30 Hz) for OT and VP in the presence and absence of naloxone indicated that the degree of naloxone augmentation of OT release at a given stimulus frequency was positively correlated with the amount of VP release at that frequency. These data support the hypothesis that dynorphin released in parallel with VP during in vitro stimulations of the rat neurohypophysis simultaneously inhibits stimulated OT release.     

Ionotropic glutamate receptors in hypothalamic paraventricular and supraoptic nuclei mediate vasopressin and oxytocin release in unanesthetized rats

We report changes in plasma arginine vasopressin (AVP) and oxytocin (OT) concentrations evoked by the microinjection of l-glutamate (l-glu) into the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) of unanesthetized rats, as well as which local mechanisms are involved in their mediation. l-Glu microinjection (10 nmol/100 nl) into the SON increased the circulating levels of both AVP and OT. The AVP increases were blocked by local pretreatment with the selective non-N-methyl-d-aspartate (NMDA) receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) (2 nmol/100 nl), but it was not affected by pretreatment with the NMDA-receptor antagonist LY235959 (2 nmol/100 nl). The OT response to l-glu microinjection into the SON was blocked by local pretreatment with either NBQX or LY235959. Furthermore, the administration of either the non-NMDA receptor agonist (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) (5 nmol/100 nl) or NMDA receptor agonist NMDA (5 nmol/100 nl) into the SON had no effect on OT baseline plasma levels, but when both agonists were microinjected together these levels were increased. l-Glu microinjection into the PVN did not change circulating levels of either AVP or OT. However, after local pretreatment with LY235959, the l-glu microinjection increased plasma levels of the hormones. The l-glu microinjection into the PVN after the local treatment with NBQX did not affect the circulating AVP and OT levels. Therefore, results suggest the AVP release from the SON is mediated by activation of non-NMDA glutamate receptors, whereas the OT release from this nucleus is mediated by an interaction of NMDA and non-NMDA receptors. The present study also suggests an inhibitory role for NMDA receptors in the PVN on the release of AVP and OT.     

Central stimulatory influence of oxytocin on preovulatory gonadotropin-releasing hormone requires more than the median eminence

The study was designed to determine whether the ability of central oxytocin (OT) to stimulate gonadotropin-releasing hormone (GnRH) on the afternoon of proestrus (PE) in the cycling female rat is mediated at the level of GnRH terminals within the median eminence (ME), or at higher hypothalamic levels where GnRH cell bodies and axons are located. Determining the location of this OT effect in vivo has proven difficult. Therefore, an in vitro system utilizing ME or basal hypothalamic (BH) explants containing GnRH terminals, or GnRH neurons including the cell bodies, axons and terminals, respectively, were harvested from regular cycling female rats at 15:00 h on PE or diestrus (DI). The explants were allowed to preincubate in Krebs Ringer Bicarbonate Buffer containing glucose, ascorbic acid, calcium, and a metalloprotease inhibitor (KRBG) and enriched with 95% O(2)/5% CO(2) at 37 degrees C until a stable baseline release of GnRH was achieved (30 min). The 0.05 level of probability was used as the minimum criterion of significance in all experiments. The ability of OT (10(-15)--10(-9) M) to stimulate the release of GnRH was determined in both ME and BH explants on PE and DI. The results demonstrated a sensitive, dose-dependent ability of OT to stimulate GnRH release from PE BH explants which was observed only in PE. Furthermore, OT failed to significantly stimulate GnRH release from ME explants on either PE or DI. The data indicate that the PE BH explant paradigm can be used to examine the manner and mechanisms by which OT influences GnRH release on the afternoon of PE. Furthermore, the results indicate for the first time that the stimulatory action of OT by itself on preovulatory GnRH release in cycling female rats is not mediated at the level of the GnRH terminals within the ME, but requires neuronal interactions and mechanisms within the BH explants.     

Excitatory and inhibitory dopaminergic regulation of oxytocin secretion in the lactating rat: evidence for respective mediation by D-1 and D-2 dopamine receptor subtypes.

The present experiments were designed to test whether the previously reported excitatory and inhibitory effects of dopamine (DA) on the secretion of oxytocin (OT) in lactating rats are exerted at different DA receptor subtypes, and to examine whether one or both of these effects might occur at the level of the posterior pituitary. The basal release of OT in nonsuckled, lactating rats was increased after intravenous administration of the D-1 DA agonist SKF 38393, and this effect, as well as the suckling-induced release of OT, was prevented by treatment with the D-1 DA antagonist SCH 23390, suggesting that DA may exert an important stimulatory influence over OT secretion through an action at the D-1 DA receptor subtype. A small stimulation of basal PRL release was also produced by SKF 38393, but blockade of the D-1 DA receptor did not prevent the suckling-induced release of this hormone. Stimulation of the D-2 DA receptor with PPHT had no effect on basal OT release in nonsuckled rats, but this agent, as well as another D-2 DA agonist, bromocriptine, prevented the suckling-induced release of both OT and PRL. The inhibitory effect of D-2 DA receptor stimulation was blocked by the D-2 DA antagonist domperidone, which increased the basal release of both hormones when given alone. These observations confirm previous findings that inhibitory effects of DA on suckling-induced OT release are mediated through activation of the D-2 DA receptor. To test whether either dopaminergic effect occurs at the level of neurosecretory endings in the neurointermediate lobe (NIL), the stalk-NIL was isolated from lactating rats and perifused in vitro. The stalk-NIL junction was electrically stimulated for 4 s, and the effects of selective D-1 DA and D-2 DA agonists and antagonists on the basal and electrically evoked release of OT and vasopressin (VP) was assessed using the two stimulation (S2/S1) paradigm. Electrical stimulation produced marked increases in release of both neural lobe peptides in a Ca(2+)-dependent manner, and the electrically evoked release of OT, but not VP, was enhanced by the opiate antagonist naltrexone (10 microM). Consistent with the in vivo results, SKF-38393 (20 microM) produced a small, but statistically significant, increase in electrically induced OT release, while SCH 23390 (20 microM) was without significant effect. Neither drug affected the basal release of OT or the basal or electrically stimulated release of VP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Osmotic control of vasopressin release by rat hypothalamo-neurohypophyseal explants in organ culture

The rat hypothalamo-neurohypophyseal system (HNS) in organ culture has been used as an in vitro system for studying the osmotic control of vasopressin (VP) release. The HNS retains osmotically sensitive components as demonstrated by changes in the rate of VP release following alterations in the osmolality of the culture medium. Increasing the osmolality from 295 to 305 mosmol/kg H2O by the addition of NaCl resulted in a 2.5-fold increase in VP release. VP release was significantly decreased subsequent to reducing the osmolality from 295 to 280 mosmol/kg H2O by the addition of distilled water. Also, VP release was stimulated when the osmolality was increased to 300 mosmol/kg H2O by the addition of mannitol, but not by additions of urea or glucose which resulted in comparable increases in the tonicity of the culture medium. These studies demonstrate that the HNS in organ culture responds appropriately to osmotic challenges within the physiological range, and support Verney's concept of an osmoreceptor inasmuch as both NaCl and mannitol were effective osmotic agents.     

Effects of estrogens in vitro and in vivo on cartilage growth in the tilapia (Oreochromis mossambicus)

To study the effects of estrogens on cartilage growth in the tilapia Oreochromis mossambicus, an epiceratobranchial cartilage radioisotope incorporation assay was employed to measure proteoglycan synthesis and prechondrocyte proliferation by incorporation of radiolabeled sulfate and thymidine, respectively. Cartilage explants were cultured with estrogens with or without recombinant bovine insulin-like growth factor-I (IGF-I). In vitro experiments using the natural teleost estrogen, 17beta-estradiol (E2), showed a trend toward inhibition of sulfate incorporation and an inhibition of thymidine incorporation at higher doses (10 micrograms/ml), but not at physiological levels. E2 also showed a trend toward inhibition of sulfate and thymidine incorporation in the presence of IGF-I. Similar results were found with other estrogenic compounds in vitro: ethinylestradiol, diethylstilbestrol (DES), genistein, and nonylphenol. Ethinylestradiol inhibited sulfate and thymidine incorporation at 1000 ng/ml in the presence of IGF-I. DES inhibited thymidine incorporation at 1000 ng/ml in untreated or IGF-I-exposed cartilage. Genistein inhibited sulfate incorporation at 100 micrograms/ml in IGF-I-exposed cartilage and inhibited thymidine uptake at 1, 10, and 100 micrograms/ml in untreated and IGF-I-exposed cartilage. Nonylphenol inhibited sulfate uptake at 100 microM in untreated and IGF-I-exposed cartilage. Nonylphenol alone at 10 and 100 microM inhibited thymidine uptake. In IGF-I-exposed cartilage nonylphenol inhibited thymidine uptake at 100 microM. Fish receiving estrogen injections (E2 or DES) in vivo at a concentration of 2 micrograms/g body weight showed increased sulfate incorporation by cartilage in vitro. Stimulation in vivo by estrogens, in contrast to the inhibition by high doses in vitro, may be a result of the influence of estrogen on pituitary growth hormone release.     

In vitro effect of delta 9-tetrahydrocannabinol to stimulate somatostatin release and block that of luteinizing hormone-releasing hormone by suppression of the release of prostaglandin E2.

Previous in vivo studies have shown that delta 9-tetrahydrocannabinol (THC), the principal active ingredient in marijuana, can suppress both luteinizing hormone (LH) and growth hormone (GH) secretion after its injection into the third ventricle of conscious male rats. The present studies were designed to determine the mechanism of these effects. Various doses of THC were incubated with either stalk median eminence fragments (MEs) or mediobasal hypothalamic (MBH) fragments in vitro. Although THC (10 nM) did not alter basal release of LH-releasing hormone (LHRH) from MEs in vitro, it completely blocked the stimulatory action of dopamine or norepinephrine on LHRH release. The effective doses to block LHRH release were associated with a blockade of synthesis and release of prostaglandin E2 (PGE2) from MBH in vitro. In contrast to the suppressive effect of THC on LHRH release, somatostatin release from MEs was enhanced in a dose-related manner with a minimal effective dose of 1 nM. Since PGE2 suppresses somatostatin release, this enhancement may also be related to the suppressive effect of THC on PGE2 synthesis and release. We speculate that these actions are mediated by the recently discovered THC receptors in the tissue. The results indicate that the suppressive effect of THC on LH release is mediated by a blockade of LHRH release, whereas the suppressive effect of the compound on growth hormone release is mediated, at least in part, by a stimulation of somatostatin release.     

Estrogen signaling at the cell surface coupled to nitric oxide release in Mytilus edulis nervous system

In previous studies we have demonstrated release of nitric oxide (NO) in human tissues following exposure to estrogen. We now designed experiments to determine whether estrogen is present in the neural tissue of Mytilus edulis, a marine mollusk, and whether, as in vertebrates, it stimulates constitutive NO synthase activity. After HPLC purification of 17beta-estradiol (17beta-E(2)) from M. edulis ganglionic tissue, we confirmed the presence of 17beta-E(2) by RIA and ES-Q-TOF-MS analysis. We further found that when either exogenous or endogenous (purified HPLC fraction) 17beta-E(2) was added to pedal ganglia, there was immediate concentration-dependent NO release. Furthermore, 17beta-E(2) conjugated to BSA also stimulated NO release, suggesting mediation by a membrane surface receptor. Tamoxifen, an estrogen receptor antagonist, inhibited the action of both 17beta-E(2) and 17beta-E(2) conjugated to BSA, further supporting the presence of an estrogen receptor. In addition, by Western blot analysis with anti-ER-beta antibodies, we observed a 55-kDa protein in both the membrane and cytosolic fractions in pedal ganglia as well as in human leukocytes (that have been previously shown to express ER-beta). In summary, our results suggest that a physiological dose of estrogen acutely stimulates NO release within pedal ganglia via an estrogen cell surface receptor.     

N-Methyl-d-Aspartate-Stimulated Increases in Intracellular Calcium Exhibit Brain Regional Differences in Sensitivity to Inhibition by Ethanol

This study compared N -methyl- d -aspartate (NMDA)-stimulated increases in intracellular calcium in fura-2–loaded neurons dissociated from newborn rat brainstem (EC₅₀ in μM; 6.4), cerebellum (9.5), forebrain (6.3), and hippocampus (10.6). Ethanol inhibition of the response to 25 μM NMDA differed among the regions. The NMDA response in hippocampus was inhibited by 20 mM ethanol; cortex and cerebellum responses were inhibited by 80 mM ethanol, and no inhibition was seen in the brainstem. Addition of glycine (15 μM) failed to attenuate ethanol inhibition of the NMDA response. These results demonstrate that ethanol inhibition of NMDA-stimulated responses varies according to brain region. In contrast to previous findings from this laboratory using dissociated neurons from whole brain, the addition of glycine did not reverse the inhibitory effects of ethanol on NMDA-stimulated responses.     

Influence of dehydration on the expression of neuropeptide Y Y1 receptors in hypothalamic magnocellular neurons

Regulation of vasopressin (VP) and oxytocin (OT) secretion involves integration of neural signals from hypothalamic osmoreceptors, ascending catecholaminergic and peptidergic cell groups in the brain stem, and local and autoregulatory afferents. Neuropeptide Y (NPY) is one factor that stimulates the release of VP and OT from the supraoptic (SON) and paraventricular nuclei of the hypothalamus via activation of Y1 receptors (Y1R). The current studies were designed to assess the regulation and distribution of NPY Y1R expression in the SON of male rats that were either given 2% NaCl drinking water (24-72 h) or water deprived (48 h). Subjecting male rats to these conditions resulted in significant increases in both the number of cells expressing Y1R immunoreactivity (ir) and the amount of Y1R protein per cell within the SON. Y1R immunoreactivity was increased in the magnocellular but not medial parvocellular paraventricular nuclei, and Y1R mRNA levels were increased in the SON of salt-loaded rats. Subpopulations of both VP and OT cells in the hypothalamus express Y1R immunoreactivity and a greater percentage of VP-ir cells express Y1R after salt loading. To control for potential effects of dehydration-induced anorexia, a group of euhydrate animals was pair fed with animals consuming 2% NaCl. No detectable change in Y1R expression was observed in the SON of pair-fed animals, even though body weights were significantly lower than controls. These data demonstrate that NPY Y1R gene and protein expression are increased in the SON of salt-loaded and water-deprived animals and provide a mechanism whereby NPY can support VP/OT release during prolonged challenges to fluid homeostasis.     

Vasopressin and oxytocin receptors in vagina, myometrium, and oviduct of rabbits

In view of the presence of distinct oxytocin (OT) and vasopressin (VP) receptors in the male genital tract (porcine) we have reexamined the receptors for OT and AVP in the classical OT target tissue, female genital tract (rabbit). Neurohypophysial hormone receptors have been investigated in vagina, myometrium, and oviduct using quantitative ligand binding, adenylate cyclase, and contractility studies. Our results clearly indicate the presence of distinct OT and V1 VP receptors in the myometrium, while only the latter was detected in vagina and oviduct. In myometrium, estrogen treatment increases the density of OT and AVP receptors, while progesterone administration inhibits the estrogen effect. At the time of spontaneous delivery a dramatic (17-fold) increase was observed for the OT sites, while the AVP sites were unchanged. AVP receptors in vagina were sensitive to sex steroid administration and were reduced during pregnancy and delivery. Isometric contractility studies suggest that not just OT, but AVP can stimulate uterine strips, an effect that is partially reversible by the V1 antagonist d(CH2)5TyrMeAVP. In vagina only AVP is effective in inducing contractions at nanomolar concentrations. These results suggest a role for AVP as well as OT in regulation of the motility of female genital tract.