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 (1 50 mM) NaCI, 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 Baseline and post-stimulus blood samples were withdrawn through indwelling jugular venous catheters.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.     

Increased Fos Expression in Oxytocin Neurons Following Masculine Sexual Behavior

Induction of the c-fos protein product (Fos) was used to immunocytochemically identify oxytocin (OT) neurons that may be activated during copulatory interactions. Fos induction was quantified in sexually-experienced male rats after either (a) exposure to a testing arena recently vacated by an estrous female, (b) copulatory interactions such as mounting and intromission without ejaculation, or (c) mounting and intromissions culminating in ejaculation. In the parvocellular regions of the paraventricular nucleus of the hypothalamus (PVN), the number of neurons expressing Fos increased following either intromission (53%) or ejaculation (124%). Significant, but less striking, increases in the number of cells expressing Fos were noted in magnocellular regions of the PVN where intromission resulted in a 13% increase and ejaculation in a 49% increase in Fos. The number of perikarya immunoreactive for OT and AVP did not differ as a function of increasing sexual contacts. In control (novel arena) males, 33–73% of the Fos labeling occurred in OT cells. Sexual interactions did not enhance the number of double-labeled cells in most parvocellular regions. However, in lateral parvocellular regions located in the most caudal aspects of the PVN, 31% of the Fos-positive cells occurred in OT neurons in ejaculated males, while in control males none of the OT cells were double-labeled. This PVN subdivision is known to consist of neurons that project to the brain stem and spinal cord at lumbar levels which contain motor neurons that regulate penile reflexes. The present data suggest a possible neurochemical circuit which incorporates oxytocinergic neurons in the mediation of masculine sexual responses.     

Oxytocin in the Central Amygdaloid Nucleus Modulates the Neuroendocrine Responses Induced by Hypertonic Volume Expansion in the Rat

The present study investigated the involvement of the oxytocinergic neurones that project into the central amygdala (CeA) in the control of electrolyte excretion and hormone secretion in unanaesthetised rats subjected to acute hypertonic blood volume expansion (BVE; 0.3 M NaCl, 2 ml/100 g of body weight over 1 min). Oxytocin and vasopressin mRNA expression in the paraventricular (Pa) and supraoptic nucleus (SON) of the hypothalamus were also determined using the real time‐polymerase chain reaction and in situ hybridisation. Male Wistar rats with unilaterally implanted stainless steel cannulas in the CeA were used. Oxytocin (1 μg/0.2 μl), vasotocin, an oxytocin antagonist (1 μg/0.2 μl) or vehicle was injected into the CeA 20 min before the BVE. In rats treated with vehicle in the CeA, hypertonic BVE increased urinary volume, sodium excretion, plasma oxytocin (OT), vasopressin (AVP) and atrial natriuretic peptide (ANP) levels and also increased the expression of OT and AVP mRNA in the Pa and SON. In rats pre‐treated with OT in the CeA, previously to the hypertonic BVE, there were further significant increases in plasma AVP, OT and ANP levels, urinary sodium and urine output, as well as in gene expression (AVP and OT mRNA) in the Pa and SON compared to BVE alone. Vasotocin reduced sodium, urine output and ANP levels, although no changes were observed in plasma AVP and OT levels or in the expression of the AVP and OT genes in both hypothalamic nuclei. The results of the present study suggest that oxytocin in the CeA exerts a facilitatory role in the maintenance of hydroelectrolyte balance in response to changes in extracellular volume and osmolality.     

Cerebrospinal fluid neurohypophysial peptides in benign intracranial hypertension.

The cerebrospinal fluid (CSF) concentrations of arginine vasopressin (AVP) and oxytocin (OT) were investigated both in patients with benign intracranial hypertension and in age and sex matched controls. Twenty eight lumbar punctures were performed on 15 patients with benign intracranial hypertension as part of their routine investigation and therapy. All patients had raised intracranial pressure (27.4, SE 1.7 cm.CSF). CSF AVP levels were significantly elevated in benign intracranial hypertension (2.1, SE 0.3 pmol/l) compared with controls (0.7, SE 0.1 pmol/l, p less than 0.001) but CSF OT concentrations were similar in both groups. CSF osmolality and plasma AVP and osmolality were identical in patients and controls. There was no correlation between CSF AVP concentration and intracranial pressure. The selective elevation of AVP in CSF may be of importance in the pathogenesis of raised intracranial pressure in benign intracranial hypertension.     

Oxytocin Pretreatment Enhances Arginine Vasopressin-lnduced Motor Disturbances and Arginine Vasopressin-lnduced Phosphoinositol Hydrolysis in Rat Septum: A Cross-Sensitization Phenomenon

The recent observation that the central oxytocin (OT) receptor has high affinity for both OT and arginine vasopressin (AVP) raises the possibility that it may be involved in some of the central actions of AVP. Repeated intracerebroventricular (icv) injections of AVP in rats evoke an unusual sensitization phenomenon in that a first exposure to the peptide enhances the sensitivity (sensitization) of the brain to a second exposure. This report investigates the possibility that the OT receptor may be involved in the mediation of the phenomenon of sensitization, using OT, a specific OT receptor agonist, [Thr⁴, Gly⁷]OT, and a specific OT receptor antagonist, d(CH₂)₅, [Tyr(Me)², Thr⁴, Tyr-NH₂⁹]OVT (compound 6; cpd 6), as well as a V1 AVP receptor antagonist, d(CH₂)₅Tyr(Me)AVP. Peptides were injected icv in conscious, adult male Sprague-Dawley rats. The data showed that: 1) a first icv AVP injection (10 pmol/5μl) enhanced the sensitivity of the rat brain to the motor response of a second AVP injection (10 pmol/5 μl) given 24 h later; 2) injection of d(CH₂)₅Tyr(Me)AVP (100 pmol/5 μl icv) but not cpd 6, (100 pmol/5 μl icv) 2 min prior to the first AVP injection, blocked AVP-induced sensitization; 3) a first injection of OT or [Thr⁴, Gly⁷]OT (10 pmol/5 μl) enhanced the sensitivity of the brain to the motor actions of a subsequent AVP injection given 24 h later; 4) the magnitude of this cross-sensitization induced by OT pretreatment varied with dose and appeared to be ten times more potent than the sensitization induced by a first AVP injection; 5) injection of cpd 6 (100 pmol/5 μl) but not d(CH₂)₅Tyr(Me)AVP (100 pmol/5 μl icv) 2 min prior to the first OT injection (1 pmol/5 μl) blocked the cross-sensitization induced by OT; 6) an injection of OT (100 to 1,000 pmol/5 μl) or [Thr⁴, Gly⁷]OT (10 pmol/5 μl) in rats that had been cross-sensitized with OT or [Thr⁴, Gly⁷]OT pretreatment did not evoke enhanced motor responses; 7) OT injected 2 min prior to the second AVP injection in AVP-sensitized rats did not block the enhanced AVP-induced motor responses; 8) AVP-induced [³H]inositol monophosphate accumulation in septal slices was also enhanced in rats cross-sensitized by OT pretreatment. These results suggest that while pre-exposure of the rat brain to both AVP and OT alters the responsiveness of the rat brain to subsequent AVP exposures, AVP sensitization appears to be mediated via the V1 AVP receptor, whereas cross-sensitization by OT may be mediated via the OT receptor. The ability of OT to alter the responsiveness of the rat brain to subsequent AVP injection suggests a role for this peptide in modulating central AVP actions.     

Penetration of neurohypophyseal hormones from plasma into cerebrospinal fluid (CSF): Half-times of disappearance of these neuropeptides from CSF

The penetration of neurohypophyseal peptides after peripheral administration into the cerebrospinal fluid (CSF) was studied in freely moving rats. In addition, the clearance of these peptides from CSF was investigated. Increased concentrations of vasopressin (AVP) in CSF were detectable 2 min after s.c. injection of 5.0 micrograms of this peptide. Peak concentration was reached at 5 min after administration and this level declined slowly over the next hour. Administration of 5.0 micrograms oxytocin (OXT) s.c. or i.v. resulted in increased OXT levels in CSF within 10 min after application. After 60 min a significant elevation of OXT in CSF was no longer present. These data reveal that approximately 0.002% of the peripherally applied amount of AVP or OXT reached the central nervous system at 10 min after injection. AVP (2.5 ng) and OXT (5.0 ng) applied into one of the lateral brain ventricles reached the cisternal cavity within 2 min after administration. Both neuropeptides were cleared from the CSF with terminal half-times of 26 and 19 min for AVP and OXT, respectively. The present data demonstrate that neurohypophyseal hormones do cross the blood-brain barrier in amounts obviously sufficient to induce central actions.     

Sequential Exposure to Estrogen and Testosterone (T) and Subsequent Withdrawal of T Increases the Level of Arginine Vasopressin Messenger Ribonucleic Acid in the Hypothalamic Paraventricular Nucleus of the Female Rat

The hypothalamic peptides arginine vasopressin (AVP) and oxytocin (OT) have been implicated as mediators of socio-sexual behaviors in addition to their roles in osmolar homeostasis (AVP), milk ejection and uterine contractility (OT). Within 24  h of parturition, OT and AVP messenger ribonucleic acid (mRNA) levels increase in the hypothalamic paraventricular, and to a lesser degree, the supraoptic nucleus (PVN and SON) of the rat. We previously reported that the prepartum increase in OT mRNA is related to the spontaneous decline in progesterone levels prior to parturition. We also reported that increases in PVN and SON OT mRNA can be induced by exposing the ovariectomized rat to a steroid regimen that mimics the steroid milieu of pregnancy, namely sequential estrogen and progesterone and subsequent progesterone withdrawal. Levels of PVN and SON AVP mRNAs were not affected by progesterone withdrawal in late pregnant rats or the steroid regimen that increased OT mRNA in ovariectomized rats. These observations suggest that other factors, perhaps hormonal, may influence AVP mRNA levels. A decline in testosterone coincident with waning progesterone levels also occurs prepartum. Since peak levels of AVP mRNA prepartum coincide with the prepartum decline in testosterone, we questioned whether declining testosterone levels are important for the increase in AVP mRNA levels.     

Differential regulation of oxytocin and vasopressin messenger ribonucleic acid levels by gonadal steroids in postpartum rats

We previously reported that sequential estradiol and progesterone exposure followed by progesterone withdrawal increases oxytocin (OT), but not arginine vasopressin (AVP), messenger ribonucleic acid (mRNA) in the hypothalamic paraventricular nucleus (PVN) of the rat. Substitution of testosterone for progesterone and subsequent testosterone withdrawal in the estrogen-primed rat increases PVN AVP mRNA levels. At the end of pregnancy (day 21), rats are exposed to high estrogen and declining progesterone and testosterone concentrations. Coincident with these changes in circulating gonadal steroid hormones are increases in OT and AVP mRNAs. If progesterone levels are sustained at term, OT levels are attenuated and if testosterone is sustained, AVP mRNA levels are attenuated. Immediately postpartum, however, OT and AVP mRNA levels decline compared to term levels. To further determine the role of estrogen in the regulation of OT and AVP mRNAs, we performed two experiments. In the first experiment, we administered estrogen during the peripartum period to determine if estrogen supplementation prevents the relative attenuation of OT and AVP mRNAs that is seen after parturition. Day 18 pregnant rats were given estradiol-filled or empty capsules and sacrificed on day 2 of lactation. By Northern analysis, significant differences in PVN AVP, but not OT, mRNA were found between the estrogen- and sham-treated lactational animals, P < 0.02. In the second experiment, we determined if sustaining estrogen after progesterone is removed in steroid-treated ovariectomized rats is essential for the increase in OT mRNA. Ovariectomized rats were given either empty capsules or sequential estradiol- and progesterone-filled capsules and both were sustained for 12 days. When progesterone-filled capsules were removed, estradiol-filled capsules were either removed or left in place, and the animals were sacrificed 48 h later. PVN OT mRNA was analyzed by Northern blot hybridization. OT mRNA was increased in both of the steroid-treated groups to the same degree, compared to sham-treated animals, P = 0.04. In summary, estrogen supplementation during early lactation prevents the attenuation of PVN AVP, but not OT, mRNA after parturition. In the estrogen-primed ovariectomized rat, it is not necessary to sustain estrogen to see the effects of progesterone withdrawal upon PVN OT mRNA.     

Salivary Oxytocin and Vasopressin Levels in Police Officers With and Without Post‐Traumatic Stress Disorder

Post‐traumatic stress disorder (PTSD) is characterised by symptoms associated with maladaptive fear and stress responses, as well as with social detachment. The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) have been associated with both regulating fear and neuroendocrine stress responsiveness and social behaviour. However, there is only limited evidence for dysregulated peripheral OT and AVP levels in PTSD patients. The present study aimed to investigate basal salivary OT and AVP levels in trauma‐exposed male and female police officers with and without PTSD. Saliva samples were collected during rest and OT and AVP levels were determined using a radioimmunoassay. Men and women were analysed separately, having adjusted for differences in trauma history, and for hormonal contraception use in women. The results showed that male PTSD patients had lower basal salivary OT levels, and did not differ in AVP levels compared to male trauma‐exposed healthy controls after adjusting for childhood emotional abuse. There were no significant differences in basal salivary OT and AVP levels in women. Our findings indicate potential dysfunctioning of the OT system in male PTSD patients. Future studies are needed to replicate these findings and to further unravel the relationship between the OT and AVP systems, sex, trauma history and PTSD.     

Osmotic Inhibition of Prolactin Secretion in Rats

Chronic hyponatremia is known to cause inhibition of pituitary vasopressin (AVP) and oxytocin (OT) secretion in response to most physiological stimuli, as well as a marked inhibition of synthesis of these peptides. Because many studies have implicated neurohypophyseal peptides in the regulation of pituitary prolactin (PRL) secretion, we investigated the effects of chronic hyponatremia on basal and stimulus-induced PRL secretion in rats. Hyponatremia was induced by subcutaneous infusion of 1-deamino-[8-D-arginine]-vasopressin (dDAVP) (5ng/h) to rats fed a nutritionally balanced liquid diet, and plasma [Na+] was maintained ≤115 mmol/l for 10–12 days. After this period, hyponatremic rats and normonatremic controls fed the same diet without dDAVP were subjected to one of the following stimuli known to stimulate PRL release in rats: 3 min exposure to ether, hemorrhage (20 ml/kg), intravenous injection of 5-hydroxytryptophane (5-HTP, 10 mg/kg), or intravenous injection of estradiol (5 μg/kg). A baseline blood sample was collected before each stimulus, and 3–6 additional blood samples were collected at selected intervals after the stimulus. Baseline levels of plasma PRL were not different between normonatremic and hyponatremic rats. However, PRL responses induced by ether or estradiol, but not those induced by hemorrhage or 5-HTP, were very significantly blunted in the chronically hyponatremic rats. Plasma AVP and OT responses were measured as an index of magnocellular secretion, but did not correlate with the PRL responses for any of the stimuli tested. Our results therefore demonstrate that ether- and estradiol-induced PRL release can be osmotically inhibited, but the mechanisms underlying this inhibition appear to be relatively independent of effects on magnocellular AVP and OT secretion. This allows the possibility that either some parvocellular systems regulating PRL secretion are osmosensitive, or alternatively that other substances released from the neural lobe may selectively modulate pituitary PRL release in response to some, but not all, stimuli.     

Modulation of oxytocin secretion by ascending noradrenergic pathways: sexual dimorphism in rats

We have investigated the role of ascending noradrenergic pathways in the control of oxytocin (OT) and arginine-vasopressin (AVP) secretion during acute immobilization stress in male and female rats. 6-Hydroxydopamine-induced lesions of the ventral noradrenergic bundle (VNAB) resulted in a selective depletion of hypothalamic noradrenaline content. In sham-lesioned rats plasma levels of OT were raised following stress, the response being significantly greater in female compared with male animals. VNAB lesions were not associated with altered responses in female rats, whereas lesioned males exhibited markedly elevated OT stress responses. AVP secretion was not modulated in VNAB-lesioned rats of either sex. The results provide functional evidence of a sexually dimorphic inhibitory role of the VNAB in the control of OT secretion.     

Chronic hypoosmolality induces a selective decrease in magnocellular neurone soma and nuclear size in the rat hypothalamic supraoptic nucleus

The magnocellular neurones of the hypothalamo-neurohypophysial system (HNS) play a vital role in the maintenance of body homeostasis by regulating oxytocin (OT) and vasopressin (VP) secretion from the posterior pituitary. During hyperosmolality, OT and VP mRNA levels are known to increase by approximately two-fold, whereas during chronic hypoosmolality, OT and VP mRNA levels decrease to approximately 10-20% of basal levels. In these studies, we evaluated changes in cell size associated with these physiological conditions. Cell and nuclear sizes of neurones in the supraoptic nucleus (SON), the nucleus of the lateral olfactory tract (LOT) and the medial habenular nucleus (MHB) were measured from neurones identified by in situ hybridization histochemistry for beta(III)-tubulin mRNA, and measurements were made from OT and AVP magnocellular neurones in the SON after phenotypic identification by immunohistochemistry. Under hypoosmolar conditions, the cell and nuclear sizes of OT and VP magnocellular neurones decreased to approximately 60% of basal values, whereas cell and nuclear sizes of OT and VP neurones in hyperosmolar rats increased to approximately 170% of basal values. In contrast, neither hyperosmolality, nor hypoosmolality significantly affected cell and nuclear sizes in the LOT and MHB. These results confirm previous studies that showed that magnocellular neurones increase cell size in response to hyperosmolar conditions and, for the first time, demonstrate a marked decrease in cell size in the SON in response to chronic hypoosmolar conditions. These dramatic changes in cell and nuclear size directly parallel changes in OT and VP gene expression in the magnocellular neurones of the SON and, consequently, are consistent with the pronounced bidirectional changes in gene expression and cellular activity found during these osmotic perturbations. Our results therefore support the concept of global alterations in the synthetic activity of magnocellular OT and AVP neurones in response to extracellular osmolality.     

Cerebrospinal fluid pressure during cerebroventricular infusion of angiotensin and vasopressin

Rationale exists for suspecting that angiotensin (Ang) and arginine vasopressin (AVP) given by the intracerebroventricular (IVT) route can affect cerebrospinal fluid (CSF) pressure. This hypothesis was tested in conscious, unrestrained adult male Sprague-Dawley rats with IVT and left carotid arterial catheters. The rats were infused (IVT) for 30 min with artificial CSF followed by 30 additional minutes with CSF, Ang, (0.6 micrograms/h) AVP (5 or 50 ng/h), or AVP (5 or 50 ng/h) + Ang, (0.6 micrograms/h). Angiotensin evoked a central hypertensive effect (+ 16 mm Hg) and increased CSF pressure from 10 to 16 cm H2O (P less than 0.05). Neither dose of AVP affected blood or CSF pressures. The AVP (5 ng/h) prevented Ang-induced changes in blood and CSF pressures and AVP (50 ng/h) blocked only the Ang-induced rise in CSF pressure. These results raise the possibility that angiotensin and vasopressin participate in the regulation of CSF pressure.     

The effects of idazoxan and 8-OH-DPAT on sexual behaviour and associated ultrasonic vocalizations in the rat

Two experiments were performed studying the effects of 8-OH-DPAT and idazoxan on sexual behaviour and ultrasonic communication of male rats. In addition, the reactions of the females towards drug-treated males were studied. 8-OH-DPAT (a very specific 5-HT1A agonist) and idazoxan (an alpha 2-adrenergic antagonist) differentially affected sexual behaviour: 8-OH-DPAT (0.1 and 0.4 mg/kg IP) markedly facilitated ejaculations, a feature indicated by decreased numbers of mounts and intromissions preceding ejaculation and a reduction in ejaculation latency. This drug concomitantly reduced the postejaculatory refractory period. Idazoxan reduced the number of intromissions before ejaculation only at the highest dose (10 mg/kg IP), but did not markedly facilitate other parameters. Both drugs markedly and dose-dependently suppressed the postejaculatory 22 kHz ultrasounds normally recorded during the postejaculatory refractory period. Ultrasound frequencies above 30 kHz first appear at the end of the absolute refractory period, even when the refractory period is shortened by 8-OH-DPAT. Idazoxan increased the number of these 30 kHz ultrasounds, whereas 8-OH-DPAT had no effect on them. No effects were observed on ultrasound production (either 22 kHz or above 30 kHz) before an ejaculation. The behaviour of the females towards 8-OH-DPAT-treated males was also affected, with the females showing more darting and lordosis before and after ejaculation, but less sitting after ejaculation. Idazoxan treatment of the males resulted in more hopping and earwiggling of the females before ejaculation. Following ejaculation, females treated with the antagonist showed more darting, hopping, earwiggling and lordosis, but sitting was decreased. It has been suggested in the rat that the emergence of ultrasounds higher than 30 kHz indicates the end of the absolute refractory period and signals to the female that the male is capable of resuming sexual activity. The significance of 22 kHz ultrasound in sexual behaviour remains puzzling because these vocalizations could be easily uncoupled from the refractory period by drugs acting via different receptor mechanisms without disturbing sexual behaviour per se. A failure to produce postejaculatory sounds appears to disinhibit (proceptive) behaviour by the females.     

Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parurient rats

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were giben naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximatelyy doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increase and pituitary stores of this peptide are decreased by day 20 gestation, when labor has not yet begun. In contrast, OT secretion becomes elevated only during delivery. Inhibition of OT release by opioid peptides may: (1) allow preferential release of AVP during pregnancy; and (2) prevent depletion of pituitary stores of OT and neuronal fatigue during the 1–2 h period of parturition in the rat.     

Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parturient rats

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were given naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximately doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected by delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before and during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increases and pituitary stores of this peptide are decreased by day 20 of gestation, when labor has not yet begun.(ABSTRACT TRUNCATED AT 250 WORDS)     

Met5-enkephalin-Arg6-Gly7-Leu8 immunoreactivity in rat and human cerebrospinal fluid: influence of neuroleptic drugs and electroconvulsive shock

Met5-Enkephalin-Arg6-Gly7-Leu8 immunoreactivity was quantitated in both rat and human cerebrospinal fluid (CSF) by radioimmunoassay with a carboxy-terminal directed antiserum. The immunoreactivity in CSF was chromatographically characterized in both species and was found to consist almost exclusively of high molecular weight forms. In human CSF there was approximately 300 fmol/ml and in the rat 1,500 fmol/ml of immunoreactivity. The possibility of a rostro-caudal gradient was examined in the human by analyzing the first and the twenty-fifth ml of CSF drawn during a lumbar puncture: none was found. The immunoreactivity was fairly stable; no loss of immunoreactivity was observed after 24 h of incubation of rat CSF at 37 degrees C. Electroconvulsive shock (ECS) produced a significant elevation in CSF content but only after a course of chronic administration; a single acute ECS produced no increase. Human subjects with schizophrenia who were being treated with antipsychotic drugs had elevated levels of immunoreactivity in comparison to non-mediated patients and normals. The high levels of this immunoreactivity in CSF, its stability and the evidence that the content can change with physiological and pharmacological manipulation indicate that Met5-Enkephalin-Arg6-Gly7-Leu8 immunoreactivity can serve as a new and useful CSF marker for investigating the CNS enkephalin system in neurological or psychiatric disorders.     

Meal-induced changes in extracellular 5-HT in medial hypothalamus of lean (Fa/Fa) and obese (fa/fa) Zucker rats

Hypothalamic serotonin (5-HT) is involved in appetite regulation and sympathetic stimulation of thermogenesis. This study tested the hypothesis that the enhanced energetic efficiency of obese Zucker rats involves blunted serotonergic release within the medial hypothalamus (MH). We used microdialysis and HPLC-EC to measure dynamic changes in extracellular 5-HT levels in the MH of 10-13-week-old male lean (Fa/Fa) and obese (fa/fa) Zucker rats before and after a meal. No differences were noted in basal levels of 5-HT between lean and obese rats. Consistent with the suggestion that hypothalamic 5-HT plays a physiological role in feeding, extracellular 5-HT levels increased significantly in both lean and obese rats given a meal. This increase was observed in the 20 min interval in which they ate the 8.1 kcal meal and remained for an additional 60 min. The net release of 5-HT during the meal interval was comparable in the lean (1.46+/-0.38 fmol/microl) and obese (1.21+/-0.82 fmol/microl) rats. However, the 5-HT levels of the leans (1.80+/-0.29 fmol/microl) plateaued in the next 20 min interval, whereas they continued rising (2.74+/-0.53 fmol/microl) in obese rats and were significantly higher than those in the leans during the 40 and 60 min intervals after the meal was presented. This resulted in a total net release during the meal plus the next three 20 min intervals that was significantly higher in obese (9.83+/-1.16 fmol/microl) than in lean (5.59+/-0.85 fmol/microl) rats. Thus, the enhanced energetic efficiency of the obese Zucker rats may not be associated with attenuated serotonin release in response to a meal. Rather their enhanced release of 5-HT in the MH may reflect compensatory mechanisms for the elevated orexigen NPY, the reduction in meal-induced CCK release, and/or a functional resistance to 5-HT.     

Central and peripheral release of vasopressin and oxytocin in the conscious rat after osmotic stimulation

Vasopressin (AVP) and oxytocin (OXT) were measured by radioimmunoassay in push-pull perfusates and tissue samples of various brain areas, plasma and cerebrospinal fluid (CSF) of male rats in response to osmotic stimulation. Hypertonic saline caused a significant rise in plasma AVP and OXT and different changes in peptide contents, in the septum and hippocampus at 30 and 60 min after intraperitoneal injection. Push-pull perfusion (20 microliters artificial CSF/min, 30-min periods) of the septum and dorsal hippocampus of conscious, unrestrained animals revealed a significant, stimulus-evoked release of both AVP and OXT. This release was: (1) not always reflected by corresponding changes in the regional peptide content; (2) simultaneous with the peripheral release from the posterior pituitary; and (3) probably the result of synaptic/parasynaptic events as suggested by use of agents in the artificial CSF which either inhibit or facilitate the release from intact fibre terminals.