Differential effects of naloxone on the release of neurohypophysial hormones in normotensive and spontaneously hypertensive rats

The hypothalamo-neurohypophysial system is altered in the spontaneously hypertensive rat (SHR). We hypothesized that an aberrant regulation of vasopressin (VP) and oxytocin (OT) release by endogenous opioid peptides alters this neuroendocrine system in the SHR. Concentrations of the neurohypophysial hormones in plasma and the pituitary were measured in 17-week-old SHRs and two strains of normotensive controls. Wistar Kyoto (WKY) and Sprague-Dawley rats. Animals were decapitated 20 min after s.c. injection of saline (1 ml/kg) or naloxone hydrochloride (1 or 10 mg/kg). In addition, neurohypophysial hormones excreted during the day (08.00-17.30 h) and night (17.30-08.00 h) were determined in urine from 16-week-old animals kept in metabolic cages for 5 days. VP at extrahypothalamic sites was also measured as [VP] in acid extracts of the subfornical organ area, hippocampal commissure-fornix and choroid plexus. Hormones were quantified by radioimmunoassay. The pituitary content, plasma concentration, and urinary excretion of OT were reduced (P less than 0.05) in SHRs, whereas VP content was increased (P less than 0.05) in the pituitary and plasma, but unchanged in urine, of hypertensive animals. In extrahypothalamic tissues, [VP] in the hippocampal commissure-fornix was increased in the SHR. Naloxone elevated (P less than 0.05) the plasma concentration of OT in WKY animals and VP in SHRs. Neither [VP] nor [OT] in plasma was changed by naloxone in Sprague-Dawley rats. Pituitary stores of the neurohypophysial hormones were not altered by naloxone in either hypertensive or normotensive rats. In conclusion, endogenous opioid peptides tonically inhibit OT release in WKY rats, whereas VP release is decreased by opioid peptides in SHRs, 16-17 weeks of age. The neuromodulatory role of opioid peptides in the release of neurohypophysial hormones appears to be altered in the SHR such that VP release is suppressed and OT release is augmented.     

Differential effects on naloxone on the release of neurohypophysial hormones in normotensive and spontaneously hypertensive rats

The hypothalamo-neurohypophysial system is altered in the spontaneously hypertensive rat (SHR). We hypothesized that an aberrant regulation of vasopressin (VP) and oxytocin (OT) release by endogenous opiod peptides alters this neuroendocrine system in the SHR. Concentrations of the neurohypophysial hormones in plasma and the pituitary were measured in 17-week-old SHRs and two strains of normotensive controls, Wistar Kyoto (WKY) and Sprague-Dawley rats. Animals were decapitated 20 min after s.c. injection of saline (1 ml/kg) or naloxone hydrochloride (1 or 10 mg/kg). In addition, neurohypophysial hormones excreted during the day (08.00–17.30 h) and night (17.30–08.00 h) were determined in urine from 16-week-old animals kept in metabolic cages for 5 days. VP at extrahypothalamic sites was also measured as [VP] in acid extracts of the subfornical organ area, hippocampal commissure-fornix and choriod plexus. Hormones were quantified by radioimmunoassay. The pituitary content, plasma concentration, and urinary excretion of OT were reduced (P < 0.05) in SHRs, whereas VP content was increased (P < 0.05) in the pituitary and plasma, but unchanged in urine, of hypertensive animals. In extrahypothalamic tissues, [VP] in the hippocampal commissure-fornix was increased in the SHR. Naloxone elevated (P < 0.05) the plasma concentration of OT in WKY animals and VP in SHRs. Neither [VP] nor [OT] in plasma was changed by naloxone in Sprague-Dawley rats. Pituitary stores of the neurohypophysial hormones were not altered by naloxone in either hypertensive or normotensive rats. In conclusion, endogenous opioid peptides tonically inhibit OT release in WKY rats, whereas VP releas is decreased by opioid peptides in SHRs, 16–17 weeks of age. The neuromodulatory role of opioid peptides in the release of neurohypophysial hormones appears to be altered in the SHR such that VP release is suppressed and OT release is augmented.     

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

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.     

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)     

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.     

NO inhibition of the magnocellular neuroendocrine system in rats is independent of cGMP signaling pathway

Our objective was to test the hypothesis that the cGMP signal-transduction mechanism mediates nitric oxide's (NO) modulation of oxytocin (OT) and vasopressin (VP) secretion from the hypothalamo-neurohypophysial system. Three studies were conducted in adult male Sprague-Dawley rats: (1a) Euhydrated rats received an intracerebroventricular (icv) infusion (1 microl/min for 30 min) of artificial cerebrospinal fluid (aCSF), vehicle (2.6% dimethyl sulfoxide [DMSO]) or 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) (0.05 microg/microl), an inhibitor of soluble guanylyl cyclase (sGC). ODQ did not affect basal levels of plasma VP or OT; (1b) Rats dehydrated for 24 h received aCSF or 8-Br-cGMP (icv), a membrane-permeable analog of cGMP, and plasma hormones were measured 2 min later. 8-Br-cGMP did not significantly change VP or OT levels; (2) Rats ingested water or 2% NaCl for 4 days, and NO synthase (NOS) and sGC activities were measured in posterior pituitaries, the anatomical site of hormone secretion. Salt loading enhanced (P < 0.001) production of [(14)C]citrulline, the coproduct of NO synthesis, without altering cGMP; (3) One SON was microdialyzed with [(14)C]arginine and NOS and sGC activities were quantified in microdialysates during intravenous (iv) infusion of isotonic or hypertonic saline in awake and anesthetized rats. In awake rats, [(14)C]citrulline recovery, but not cGMP, increased (P < 0.05) during intravenous infusion of both isotonic and hypertonic solutions, and after insertion of microdialysis probe itself. In anesthetized rats, however, where basal NOS activity is low, intravenous infusion of hypertonic, but not isotonic solution, increased [(14)C]citrulline recovery without affecting cGMP. Thus, in the forebrain, neither NO produced basally nor during osmotic stimulation depends on cGMP to modulate plasma vasopressin and oxytocin secretion.     

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.     

The influence of stress on convulsive parameters in the mouse

Mice exposed to the stress of conspecific aggression for 10 min showed shorter latencies to convulsions induced by pentylenetetrazol but not by pilocarpine. This effect was short lived and was not influenced by pretreatment with naltrexone (5 mg/kg, SC). The onset of pilocarpine-induced convulsions in stressed mice was reduced by the opioid antagonist. Aggression stress did not change the incidence, duration or severity of convulsions triggered by the chemoconvulsants or electroshock. The results differ widely from those obtained using other stressogenic models such as cold-restraint or swim stress. This suggests that alterations of convulsive parameters and the involvement of opioid mechanisms in their mediation are critically dependent on the characteristics of the stressogenic procedure employed.     

Oestradiol potentiates hormone secretion and neuronal activation in response to hypertonic extracellular volume expansion in ovariectomised rats

Secretion of vasopressin (VP), oxytocin (OT) and atrial natriuretic peptide (ANP) is an essential mechanism for the maintenance of hydromineral homeostasis. Secretion of these hormones is modulated by several circulating factors, including oestradiol. However, it remains unclear how oestradiol exerts this modulation. In the present study we investigated the participation of oestradiol in the secretion of VP, OT and ANP and in activation of vasopressinergic and oxytocinergic neurones of the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus in response to extracellular volume expansion (EVE). For this purpose, ovariectomised (OVX) rats treated for 7 days with vehicle (corn oil, 0.1 ml/rat, OVX+O group) or oestradiol (oestradiol cypionate, 10 μg/kg, OVX+E group) were subjected to either isotonic (0.15 m NaCl, 2 ml/100 g b.w., i.v.) or hypertonic (0.30 m NaCl, 2 ml/100 g b.w., i.v.) EVE. Blood samples were collected for plasma VP, OT and ANP determination. Another group of rats was subjected to cerebral perfusion, and brain sections were processed for c‐Fos‐VP and c‐Fos‐OT double‐labelling immunohistochemistry. In OVX+O rats, we observed that both isotonic and hypertonic EVE increased plasma OT and ANP concentrations, although no changes were observed in VP secretion. Oestradiol replacement did not alter hormonal secretion in response to isotonic EVE, but it increased VP secretion and potentiated plasma OT and ANP concentrations in response to hypertonic EVE. Immunohistochemical data showed that, in the OVX+O group, hypertonic EVE increased the number of c‐Fos‐OT and c‐Fos‐VP double‐labelled neurones in the PVN and SON. Oestradiol replacement did not alter neuronal activation in response to isotonic EVE, but it potentiated vasopressinergic and oxytocinergic neuronal activation in the medial magnocellular PVN (PaMM) and SON. Taken together, these results suggest that oestradiol increases the responsiveness of vasopressinergic and oxytocinergic magnocellular neurones in the PVN and SON in response to osmotic stimulation.     

Central discriminative effects of morphine in rats: training via intracerebroventricular administration

There have been studies of the discriminative effects of intracerebroventricularly (ICV)-administered morphine (MOR) in rats trained to discriminate MOR systemically, but the converse has not been done. In this study, rats were trained to discriminate between ICV (1-10 microg/3 microl, 1 h) or subcutaneous (SC) (3.0 mg/kg, 30 min) injections of MOR vs. saline/vehicle in a discrete-trial avoidance/escape procedure. On generalization testing, subjects in both the ICV- and SC-trained groups responded on the MOR-appropriate lever at ICV MOR doses < or =1-3 microg, and at SC MOR doses 2 to 3 orders of magnitude higher (vs. ICV). Naltrexone (SC) blocked the stimulus effects of MOR (ICV) equipotently in both training groups. In ICV-trained subjects, levorphanol (SC), the mu-opioid selective peptide [D-Ala2, NMePhe4, Gly-ol]-enkephalin (DAMGO) (ICV), and the enkephalinase inhibitor N-[L-(1-carboxy-2-phenyl)ethyl]-L-phenylalanyl-beta-alanine (SCH 32615) (ICV) produced complete MOR-appropriate responding, whereas the dextrorotary enantiomer of levorphanol dextrorphan (SC; < or = 3.0 mg/kg) and the delta-opioid selective peptide [D-Pen2, D-Pen5]-enkephalin (DPDPE) (ICV, < or = 0.03 mg) did not. SC-trained subjects did not generalize to SCH 32615, which suggests qualitative differences in the discriminative stimulus effects of novel drugs as a function of the route of administration of the training drug. These data demonstrate that it is feasible to train rats to discriminate an opioid administered by the ICV route, and to perform extended tests of generalization to novel drugs (SC or ICV) in rats so trained.     

Ventral pallidal microinjections of receptor-selective opioid agonists produce differential effects on circling and locomotor activity in rats

Locomotor activity was investigated following microinjections of receptor-selective opioid agonists into the ventral pallidum (VP) of rats. In Expt. 1, male Long-Evans rats were treated with unilateral microinjections of the μ agonist [d-Ala²-MePhe⁴, Gly-ol⁵]-enkephalin (DAGO), the σ agonist [d-Pen²,d-Pen⁵]-enkephalin (DPDPE) or the κ agonist U50,488H, and the rate and duration of circling behavior were measured. DAGO (0.01, 0.1, 1.0 nmol) procedure a dose-dependent increse in contralateral circling; pretreatment with 1.0 mg/kg naltrexone blocked the circling induced by the highest dose. The behavioral effect was largest when injections were targeted at the VP rather than structures dorsal to the VP. In contrast to DAGO, intrapallidal DPDPE (0.01, 0.1, 1.0, 10.0 nmol) produced a slight increase in contralateral circling only at the highest dose and U50, 488H (0.01, 0.1, 1.0, 10.0 nmol) produced no effect. In Expt. 2, the effects of bilateral injections of DAGO, DPDPE and U50,488H were tested in photocell activity ☐es. DAGO produced a dose-dependent increase in locomotor activity and this increase was decreased by 1.0 mg/kg naltrexone. A slight increase in activity was observed with the highest dose of DPDPE, and a slight decrease was observed with the highest dose of U50,488H. These findings confirm that opiate actions in the VP contribute to opiate-induced locomotion and suggest that μ and to some extent σ receptors are involved in this behavior.     

Stress increases oxytocin release within the hypothalamic paraventricular nucleus

Evidence indicates that the hypothalamic paraventricular nucleus (PVN) and oxytocin (OT) neurons in particular play a role in the physiological response to stress. Microdialysis (MD) experiments were performed to determine whether OT is released into the PVN during shaker stress. Male rats were prepared with venous catheters and PVN guide cannulae. OT and vasopressin (VP) release into PVN and peripheral blood were measured under basal conditions and during and after shaker stress (10 min at 110 cycles/min). Stress produced a specific increase in PVN and plasma OT. Dialysate OT levels were 0.3±0.1, 2.8±1.2 and 1.3±0.6 pg/sample (control, stress and recovery, respectively). Plasma OT was significantly increased during stress (3.7±1.2 vs. 11.7±2.3 pg/ml, basal vs. stress, respectively). When MD probes were located outside the PVN, there was no increase in OT release, demonstrating site specificity. Stress produced no change in VP levels, either in dialysate or plasma. These results show that OT, but not VP, is released into the PVN and peripheral blood in response to shaker stress. The data raise the possibility that local release of OT into the PVN plays a role in the neuroendocrine stress cascade.     

Effects of chronic nicotine on cerebral glucose utilization in the rat

Effects of chronic nicotine on glucose utilization in 45 CNS regions were examined using the 2-deoxy-d-[1-¹⁴C]glucose technique in rats. Eitherl-nicotine (1.0 mg/kg) or saline was injected subcutaneously twice daily for 10 days and once the morning of day 11. On the following afternoon, rats received either nicotine challenge (0.3 mg/kg) or saline subcutaneously 2 min before an intravenous pulse of 2-deoxy-d-[1-¹⁴C]glucose. Drug-naive rats given nicotine challenge showed increases of glucose utilization in thalamic nuclei and components of the visual system. Tolerance to nicotine challenge in rats given nicotine chronically was seen in the ventral tegmental area, some components of visual pathways, the cerebellum, and vestibular nuclei; no regions showed sensitization. The percent of rats manifesting most nicotine-induced behaviors either increased (Straub tail, locomotor stimulation, tremors) or did not change (ataxia) over the 10 days of chronic treatment. Plasma from rats given nicotine chronically showed low concentrations of nicotine (12 ± 1ng/ml) and higher concentrations of cotinine (124 ± 6ng/ml) at the time of assay, with no apparent effect of chronic treatment on nicotine levels after the challenge dose. Changes in regional brain activity, as measured by the 2-deoxy-d-[1-¹⁴C]glucose technique, generally do not explain the observed sensitization to nicotine in behavioral assays.     

Endogenous opioid systems regulate cell proliferation in the developing rat brain

The role of endogenous opioid systems in modulating the proliferation of developing cerebellar cells was examined autoradiographically in 6-day-old rats. The blockade of endogenous opioid-opioid receptor interaction by naltrexone, a potent opioid antagonist, was accompanied within 1–2 h by an increased proportion of cells incorporating [³H]thymidine. When high doses of naltrexone (50 mg/kg) were administered this index was still elevated 12 h later; however, when low doses of naltrexone (1 mg/kg) were administered the index of labeled cells was decreased markedly. Injection of methionine-enkephalin, an endogenous opioid peptide, also resulted in a decrease in the proportion of cells incorporating [³H]thymidine. Concomitant injection of 1 mg/kg naloxone, however, blocked the inhibitory effects of methionine-enkephalin on cell division but did not itself affect cell generation. These studies demonstrate that endogenous opioid systems can regulate the proliferation of cell populations in the developing nervous system and do so through an inhibitory mechanism.     

Effects of adrenomedullin on adrenocorticotropic hormone (ACTH) release in pituitary cell cultures and on ACTH and oxytocin responses to shaker stress in conscious rat

Adrenomedullin (AM) is a potent vasodilator peptide, which is initially isolated from tissue of human pheochromocytoma. In addition to the effect on cardiovascular system, previous studies suggest that AM plays some roles as a neuropeptide in the brain. In the present study, we examined the effect of AM on in vitro adrenocorticotropic hormone (ACTH) secretion stimulated by corticotropin-releasing hormone (CRH), vasopressin (VP) or oxytocin (OT) in cultured rat corticotrophs and on the response of plasma ACTH, corticosterone (B) and OT to shaker stress in vivo. In contrast to the previous report, basal or CRH (10(-9) M)-stimulated ACTH secretion was not affected by coincubation with AM. Either of VP (10(-8) M) or OT (10(-8) M) significantly increased ACTH secretion in cultured rat anterior pituitary cells (156.7+/-24.9 in basal incubation vs. 267.8+/-15.0 in VP-stimulation, P<0.05, and 308.6+/-41.3 pg/ml in OT-stimulation, P<0.05). AM (10(-10) M) significantly inhibited OT-stimulated ACTH secretion. AM tended to inhibit VP-stimulated ACTH secretion, although the inhibitory effect was not statistically significant. Thus, it is likely that AM attenuates OT-stimulated ACTH secretion in corticotrophs. In vivo study, male Wistar rats were prepared with a guide cannula in the lateral ventricle and a catheter in femoral artery for blood sampling. AM (0.5, 1.0 microg in 5 microl) or normal saline (5 microl, control) was intracerebroventricularly (i.c.v.) injected in conscious rats. Shaker stress (110 cycles/min for 5 min) produced a significant increase of plasma ACTH (baseline: 106.4+/-48.6; vs. just after stress: 388.9+/-56.1 pg/ml, P<0.05) and B (baseline: 198.6+/-46.8 vs. 15 min after stress: 378.5+/-13.6 ng/ml, P<0.05) in the control group. Plasma OT tended to increase after stress, although the change was not significantly different (baseline: 29.8+/-6.5; just after stress: 65.6+/-18.2 pg/ml). I.c.v. injection of AM at 3 min before the stress did not significantly affect stress-induced changes of plasma ACTH, B and OT. These results suggest that AM has an inhibitory effect on OT-induced ACTH release in vitro and the inhibitory effect may be overwhelmed in ACTH and B response to shaker stress.     

beta-Funaltrexamine (beta-FNA) and the regulation of body and brain development in rats

The effects of beta-FNA, a highly selective and irreversible mu opioid receptor antagonist, in altering body and brain development in preweaning rats were determined. Animals given beta-FNA did not differ from controls in body weights, brain and cerebellar weights, macroscopic dimensions of the brain, the area of the cerebellum, or in organ weight. The dosage of beta-FNA utilized (5 mg/kg) blocked morphine-induced analgesia (2 mg/kg morphine sulfate, SC) for each injection period (i.e., 48 hr). In contrast to beta-FNA treatment, rats given naltrexone (50 mg/kg SC) in a regimen which completely blocked the opioid receptor throughout ontogeny exhibited marked increases in somatic and neurobiological growth. These results suggest that, in and by themselves, mu receptors selectively antagonized by beta-FNA do not play an important role in regulating development.     

Analysis of central opioid receptor subtype antagonism of hypotonic and hypertonic saline intake in water-deprived rats

Intake of either hypotonic or hypertonic saline solutions is modulated in part by the endogenous opioid system. Morphine and selective mu and delta opioid agonists increase saline intake, while general opioid antagonists reduce saline intake in rats. The present study evaluated whether intracerebroventricular administration of general (naltrexone) and selective mu (beta-funaltrexamins, 5–20 μg), mu₁ (naloxonazine, 50 μg), kappa (nor-binaltorphamine, 5–20 μg), delta (naltrindole, 20 μg), or delta, (DALCE, 40 μg) opioid receptor subtype antagonists altered water intake and either hypotonic (0.6%) or hypertonic (1.7%) saline intake in water-deprived (24 h) rats over a 3-h time course in a two-bottle choice test. Whereas peripheral naltrexone (0.5–2.5 mg/kg) significantly reduced water intake and hypertonic saline intake, central naltrexone (1–50 μg) significantly reduced water intake and hypotonic saline intake. Water intake was significantly reduced following mu and kappa receptor antagonism, but not following mu₁, delta, or delta₁ receptor antagonism. In contrast, neither hypotonic nor hypertonic saline intake was significantly altered by any selective antagonist. These data are discussed in terms of opioid receptor subtype control over saline intake relative to the animal's hydrational state and the roles of palatability and/or salt appetite.     

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.     

Inhibition of morphine withdrawal by the NMDA receptor antagonist MK-801 in rat is age-dependent

This study investigated the effects of the NMDA receptor antagonist MK-801 on the development of morphine dependence in 7-, 14-, and 21-day-old rat pups. For 6.5 days, starting at 1, 8, or 15 days of age, rats were pretreated with MK-801 (0.03 or 0.1 mg/kg, bid) or saline; 15 min later, morphine sulfate (10 mg/kg) or saline was injected to induce opiate dependence. On the afternoon of the seventh day, pups were injected with MK-801 (0.1 mg/kg) or saline and 15 min later with naltrexone (1 mg/kg) to precipitate withdrawal. Pups were then placed in a warm chamber with the litter and their behavior scan-sampled every 15 sec for a total of 15 min. MK-801 failed to inhibit morphine withdrawal in the 7-day-old rat, but did attenuate the development of morphine dependence in both the 14- and 21-day-old rats. These results suggest that the NMDA receptor is not functionally active in opiate withdrawal until around the second to third week of postnatal life in the rat and that there exists a transition period for the NMDA receptor to play a role in the development of opiate dependence and withdrawal.     

Beneficial effect of the opioid receptor antagonist naltrexone on hyper-sensitivity induced by spinal cord ischemia in rats: disassociation with MK-801

In this study we examined the effect of the long-acting opioid antagonist naltrexone on the allodynia-like effect of spinal ischemia in rats. The spinal cord ischemia was induced at midthoracic level by a recently developed photochemical technique using laser irradiation and photoactivatable intravascular dyes. An allodynia-like sensory disturbance, where the animals reacted by vocalization to non-noxious mechanical stimuli in the flank area, was consistently seen during several days after ischemia. Pretreatment with 10 and 20 mg/kg, but not 5 mg/kg naltrexone i.v. 10 min before irradiation decreased the incidence of allodynia. However, even the effect of the highest dose of naltrexone (20 mg/kg) was incomplete, which is in contrast to the effect of the NMDA receptor antagonist MK-801, which has been tested in the same model and found to completely prevent the incidence of allodynia at a dose of 0.5 mg/kg. Pretreatment with sub- or suprathreshold doses of naltrexone (5 and 20 mg/kg respectively) combined with a subthreshold dose of MK-801 (0.1 mg/kg) did not produce a synergistic effect. When naltrexone (20 mg/kg) was administered 10 min after induction of ischemia, it was totally ineffective in decreasing the occurrence and severity of allodynia. In contrast, MK-801 (0.5 mg/kg) still had a good protective effect when injected as this time. Histological examination showed slight morphological damage in the spinal cord in 38% of control rats after 1 min laser irradiation without pretreatment with naltrexone. No morphological abnormalities were observed in rats after pretreatment with naltrexone (20 mg/kg). The results suggest that opioid receptor antagonists and NMDA receptor antagonists prevent a consequence of transient spinal cord ischemia through different mechanisms. High doses of opioid antagonists may have anti-ischemic effects by improving local spinal cord microcirculation and therefore may have a role in preventing ischemia after traumatic spinal cord injury. On the other hand, the NMDA receptor may have a role in the secondary neuronal death resulting from ischemia. Thus, NMDA receptor antagonists may contribute to the prevention of tissue damage by antagonizing the excitotoxic action of glutamate and/or aspartate released by ischemia into the spinal cord. Finally, since only high doses of naltrexone had an effect in the present study, we cannot rule out the possiblity that this drug may act through non-opioid mechanisms.