Role of alpha-adrenergic mechanism in effects of morphine on the hypothalamo-pituitary-adrenocortical and cardiovascular systems in the rat

The role of alpha-adrenergic mechanism in the acute effects of morphine in the hypothalamo-pituitary-adrenocortical (HPA) and cardiovascular (CV) systems, and the interrelationship between the HPA and CV responses to alpha-adrenoceptor antagonists and/or morphine were studied by peripheral administration of prazosin, a selective alpha 1-adrenoceptor antagonist, and yohimbine, a selective alpha 2-adrenoceptor antagonist, in conscious, unstressed or ether-stressed rats. The test substances were administered intravenously or intraperitoneally in chronically cannulated or noncannulated rats. In the i.v. experiment, morphine (1 mg/100 g BW) rapidly induced a pronounced bradycardia and a short-lasting fall in blood pressure (BP), followed by a rise in BP, and increased plasma corticosterone concentration. Prazosin (0.5 mg/kg BW) induced a rapid fall in BP and tachycardia, and increased plasma corticosterone concentration. Pretreatment with prazosin did not block the effect of morphine on the CV system, but abolished the morphine-induced increment in plasma corticosterone concentration. Yohimbine (0.5 mg/kg BW) induced a rapid and a subsequent slowly developing rise in BP and tachycardia, and increased plasma corticosterone concentration. Pretreatment with yohimbine did not block the effect of morphine on the CV system nor alter the stimulatory effect of morphine on the secretion of corticosterone. In the intraperitoneal experiment, morphine (2 mg/100 g BW) stimulated the secretion of adrenocorticotropic hormone (ACTH) and corticosterone and prazosin (1 mg/kg BW) stimulated the secretion of corticosterone, but pretreatment with prazosin reduced the morphine-induced increment in plasma corticosterone concentration in unstressed rats. In stressed rats, morphine reduced the stress-induced increment in plasma ACTH and corticosterone concentrations and prazosin also reduced the stress-induced increment in plasma corticosterone concentration. Pretreatment with prazosin did not alter the inhibitory effect of morphine...     

Melatonin enhances the rewarding properties of morphine: involvement of the nitric oxidergic pathway

Melatonin has different interactions with opioids including the enhancement of the analgesic effects of morphine and also reversal of tolerance and dependence to morphine. The present study assessed the effect of melatonin on morphine reward in mice using a conditioned place preference (CPP) paradigm. Our data showed that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal (i.p.) administration of melatonin (1-40 mg/kg) alone did not induce either CPP or conditioned place aversion (CPA), while the combination of melatonin (5-20 mg/kg) and sub-effective dose of morphine (0.5 mg/kg) led to rewarding effect. We further investigated the involvement of the nitric oxidergic pathway in the enhancing effect of melatonin on morphine CPP, by a general nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). L-NAME (1 and 5 mg/kg, i.p.) alone or in combination with morphine (0.5 mg/kg) did not show any significant CPP or CPA. Co-administration of L-NAME (5 mg/kg) with an ineffective combination of melatonin (1 mg/kg) plus morphine (0.5 mg/kg) produced significant CPP that may imply the similarity of action of melatonin and L-NAME and involvement of the nitric oxidergic pathway in this regard. Our results indicate that pretreatment of animals with melatonin enhances the rewarding properties of morphine via a mechanism which may involve the nitric oxidergic pathway.     

Morphine-induced stimulation of pituitary-adrenocortical activity is mediated by activation of nitric oxide in the early stages of postnatal life in the rat

OBJECTIVE: The first aim of the present study was to determine if morphine, a prototypic mu-opioid agonist drug, affects pituitary-adrenocortical activity in developing rat pups (first and second weeks of postnatal life). The second aim of this study was to explore, in vivo, if nitric oxide (NO) could be involved in the neurohormonal response to morphine in the early stages of postnatal life. METHODS: Plasma ACTH and corticosterone concentrations were determined by RIA in rat pups (n=5-14 rats/experimental group) after they had been killed by decapitation. In a first experiment, 1-day and 1- and 2-week-old rats were treated s.c. with morphine (20 mg/kg) or with vehicle (0.9% NaCl) and killed 5-90 min later. In a second experiment, 2-week-old pups were pretreated s.c. with naltrexone (NAL; 0.4 mg/kg or 10 mg/kg), and injected 1 h later with either morphine (20 mg/kg) or vehicle, and killed 30 min later. Some pups injected with only NAL were killed 60 or 90 min later. On the other hand, pups injected with NAL (10 mg/kg) or NAL and morphine were killed 30 min later. In a third experiment, 2-week-old pups were pretreated s.c. with N-omega-nitro l arginine methyl-ester (L-NAME; 30 mg/kg or 100 mg/kg), and injected 1 h later with either morphine (20 mg/kg) or vehicle, and killed 30 min later. Moreover, some pups injected with L-NAME (100 mg/kg) or L-NAME with morphine were killed 30 min later. In a final experiment, pups were injected s.c. with either S-nitroso-N-acetylpenicillamine (SNAP; 5 mg/kg) or vehicle, and killed 60 or 90 min later. RESULTS: Morphine administered to rat pups elicited marked rises in both ACTH and corticosterone secretion. Moreover, these responses increased with advancing postnatal age. In 2-week-old rat pups, NAL, a competitive antagonist at mu-opioid receptors, administered alone increased both plasma ACTH and corticosterone concentrations 30 min later. L-NAME, a specific NO synthase inhibitor, did not affect plasma ACTH and corticosterone concentrations 30 min later when administered alone. NAL, when concomitantly administered with morphine, was unable to block morphine responses. In contrast, morphine responses were blocked by pretreatment (60 min before) with NAL or with L-NAME. Acute injection of SNAP increased both ACTH and corticosterone release. CONCLUSION: Our results suggest that opioids have controversial effects on pituitary-adrenocortical activity in the early postnatal period in the rat, and that endogenous NO is one of the major factors in the response of the pituitary-adrenocortical axis to morphine.     

Long-lasting glucocorticoid suppression of opioid-induced antinociception

The antinociceptive effect of morphine (5 mg/kg body weight i.p.) in rats subjected to various experimental manipulations of the pituitary-adrenocortical system was studied. The absence of adrenal steroids increased the sensitivity to morphine. The following findings suggest that glucocorticosteroids have a long-lasting influence on opioid-induced antinociception, even when the steroids have been removed by adrenalectomy. First, when rats were adrenalectomized in the morning under basal conditions of pituitary-adrenocortical activity (plasma corticosterone level less than 1 microgram %), the subsequent hypersensitivity to morphine-induced antinociception following adrenalectomy either in the morning or in the evening persisted for at least 2 weeks. Second, exposure to a novel environmental (stress of a new cage) or administration of corticosterone (10 mg/kg body weight s.c.) prior to morning adrenalectomy decreased the sensitivity to morphine measured 1 week later. Third, RU 38486, a glucocorticoid antagonist, injected in the lateral cerebral ventricle prior to the evening adrenalectomy increased subsequent morphine antinociception. In attempts to understand the long-term effect on morphine antinociception, the opioid receptor sites were quantified by an in vivo procedure. Quantitative autoradiography of binding sites labeled after intravenous administration of a tracer dose of [3H]-diprenorphine showed a decrease in retention of the labeled opioid in cortical and midbrain regions of rats adrenalectomized in the evening when compared with rats operated in the morning.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypothalamic-pituitary-adrenal activity and pro-opiomelanocortin mRNA levels in the hypothalamus and pituitary of the rat are differentially modulated by acute intermittent morphine with or without water restriction stress

Acute administration of morphine stimulates the secretion of hypothalamic-pituitary-adrenal (HPA) hormones, ACTH, beta-endorphin and corticosterone in the rat. In this study we investigated the effects of repeated multiple-dose morphine on HPA activity under two different conditions: without or with water restriction stress. Rats received six intermittent injections of morphine (6.25 mg/kg per injection, s.c.) every 2 h and were killed 30 min after the last injection. The results were as follows. (1) Morphine significantly elevated plasma ACTH and corticosterone levels; water restriction also significantly increased ACTH secretion, but with no significant increase of plasma corticosterone levels. In contrast, rats treated with morphine under the water restriction condition failed to show any increases of either ACTH or corticosterone levels. (2) Morphine did not change pro-opiomelanocortin (POMC) mRNA levels in the anterior pituitary; whereas water restriction significantly increased the POMC mRNA levels. The water restriction-induced increases of POMC mRNA in the anterior pituitary were absent in the rats which received morphine. (3) Morphine significantly increased POMC mRNA levels in the hypothalamus; water restriction had no effect. The morphine-induced increases in POMC mRNA in the hypothalamus were absent in the rat under the water restriction condition. These findings, that the effects of morphine on HPA activation or POMC mRNA expression depend on the presence of stress, suggest a counter-regulatory role of opiates on a stress response and opioid gene expression.     

Pharmacological studies on the serotoninergic and nonserotonin-mediated stimulation of prolactin and corticosterone secretion by fenfluramine. Effects of pretreatment with fluoxetine, indalpine, PCPA, and L-tryptophan

Administration of the serotonin-releasing drug fenfluramine to male rats caused a dose-dependent increase in both plasma prolactin and corticosterone levels. The effect of fenfluramine on prolactin was maximal at 30 min after injection, whereas the effect on plasma corticosterone levels reached a maximum 2 h after injection. In order to determine if the effect of fenfluramine on both hormones was mediated via serotonin release, rats were pretreated with the serotonin uptake inhibitors fluoxetine (10 mg/kg i.p.) or indalpine (10 mg/kg i.p.) 30 min prior to administration of fenfluramine (5 mg/kg i.p.). Both fluoxetine and indalpine inhibited the effect of fenfluramine on plasma prolactin levels, but did not modify the effect of fenfluramine on plasma corticosterone levels. Pretreatment of rats with the serotonin precursor L-tryptophan (100 mg/kg i.p.) potentiated the effect of a submaximal dose of fenfluramine (2 mg/kg i.p.) on plasma prolactin levels, but did not affect the corticosterone response. Depletion of serotonin stores by pretreatment with the serotonin inhibitor p-chlorophenylalanine (300 mg/kg i.p.; 72 h) did not significantly prevent the effect of fenfluramine on either hormone. There was a 34% inhibition of the effect of fenfluramine on plasma prolactin levels, but this effect was not statistically significant. The results of the experiments suggest that the effect of fenfluramine on prolactin secretion is mediated, at least in part, by a serotoninergic mechanism, but the effect on corticosterone secretion is not mediated via serotonin release.     

Nitric oxide synthase inhibitors protect against brain and liver damage caused by acute malathion intoxication

Objective: To investigate the effect of NG-nitro-L-arginine methyl ester(L-NAME), a nonselective nitric oxide synthase(NOS) inhibitor, and 7-nitroindazole(7-NI), a selective neuronal NOS inhibitor, on oxidative stress and tissue damage in brain and liver and on DNA damage of peripheral blood lymphocytes in malathion intoxicated rats. Methods: Malathion(150 mg/kg) was given intraperitoneally(i.p.) along with L-NAME or 7-NI(10 or 20 mg/kg, i.p.) and rats were euthanized 4 h later. The lipid peroxidation product malondialdehyde(MDA), nitric oxide(nitrite), reduced glutathione(GSH) concentrations and paraoxonase-1(PON-1) activity were measured in both brain and liver. Moreover, the activities of glutathione peroxidase(GPx) acetylcholin?esterase(ACh E), and butyrylcholinesterase(BChE), total antioxidant capacity(TAC), glucose concentrations were determined in brain. Liver enzyme determination, Comet assay, histopathological examination of brain and liver sections and inducible nitric oxide synthase(iNOS) immunohistochemistry were also performed. Results:(i) Rats treated with only malathion exhibited increased nitric oxide and lipid peroxidation(malondialdehyde) accompanied with a decrease in GSH content, and PON-1 activity in brain and liver. Glutathione peroxidase activity, TAC, glucose concentrations, ACh E and BCh E activities were decreased in brain. There were also raised liver aspartate aminotransferase(AST) and alanine aminotransferase(ALT) activities and increased DNA damage of peripheral blood lymphocytes(Comet assay). Malathion caused marked histopathological changes and increased the expression of iNOS in brain and liver tissues.(ii) In brain of malathion-intoxicated rats, L-NAME or 7-NI resulted in decreased nitrite and MDA contents while increasing TAC and PON1 activity. Reduced GSH and GPx activity showed an increase by L-NAME. ACh E activity increased by 20 mg/kg L-NAME and 10 mg/kg 7-NI. ACh E activity decreased by the higher dose of 7-NI while either dose of 7-NI resulted in decreased BCh E activity.(iii) In liver of malathion-intoxicated rats, decreased MDA content was observed after L-NAME or 7-NI. Nitrite level was unchanged by L-NAME but increased after 7-NI which also resulted in decreased GSH concentration and PON1 activity. Either inhibitor resulted in decreased liver ALT activity.(iv) DNA damage of peripheral blood lymphocytes was markedly inhibited by L-NAME or 7-NI treatment.(v) iNOS expression in brain and liver decreased by L-NAME or 7-NI.(vi) More marked improvement of the histopathological alterations induced by malathion in brain and liver was observed after 7-NI compared with L-NAME. Conclusions: In malathion intoxicated rats, the neuronal NOS inhibitor 7-NI and to much less extent L-NAME were able to protect the brain and liver tissue integrity along with improvement in oxidative stress parameters. The decrease in DNA damage of peripheral blood lymphocytes by NOS inhibitors also suggests the involvement of nitric oxide in this process.     

L-NAME, a nitric oxide synthase inhibitor, as a potential countermeasure to post-suspension hypotension in rats.

A large number of astronauts returning from spaceflight experience orthostatic hypotension. This hypotension may be due to overproduction of vasodilatory mediators, such as nitric oxide (NO) and prostaglandins. To evaluate the role of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) as a countermeasure against the post-suspension reduction in mean arterial pressure (MAP), we assessed the cardiovascular responses and vascular reactivity to 7-day 30 degrees tail-suspension and a subsequent 6 hr post-suspension period in conscious rats. After a pre-suspension reading, direct MAP and heart rate (HR) were measured daily and every 2 hrs post-suspension. The NO synthase inhibitor L-NAME (20 mg/kg, i.v.), or saline, were administered after the 7th day reading prior to release from suspension and at 2 and 4 hrs post-suspension. At 6 hrs post-suspension, vascular reactivity was assessed. While MAP did not change during the suspension period, it was reduced post-suspension. Heart rate was not significantly altered. L-NAME administration reversed the post-suspension reduction in MAP. In addition, the baroreflex sensitivity for heart rate was modified by L-NAME. Thus, the post-suspension reduction in MAP may be due to overproduction of NO and altered baroreflex activity.     

Serum biochemical responses under oxidative stress of aspartame in wistar albino rats

ObjectiveTo study whether the oral administration of aspartame (40 mg/kg body weight) for 15 d, 30 d and 90 d have any effect on marker enzymes, some selective liver and kidney function parameter, lipid peroxidation and antioxidant status in serum. To mimic human methanol metabolism, folate deficient animals were used.MethodAnimal weight, complete hemogram, marker enzyme in serum, some selected serum profile reflect liver and kidney function, plasma corticosterone level, and in serum, lipid peroxidation, nitric oxide, enzymatic and non-enzymatic antioxidant level was measured .ResultAfter 15 d of aspartame administration animals showed a significant change in marker enzymes, and antioxidant level. However, after repeated long term administration (30 d and 90 d) showed a significant change in some selected serum profile reflects liver and kidney function, along with marker enzymes, and antioxidant level.ConclusionsThis study concludes that oral administration of aspartame (40 mg/kg body weight) causes oxidative stress in Wistar albino rats by altering their oxidant/antioxidant balance.     

Reevaluation of the role of alpha 2-adrenoreceptors in morphine-stimulated release of growth hormone.

The relationship between opioidergic and alpha 2-adrenergic system in the regulation of GH secretion was studied using a novel alpha 2-antagonist, CH-38083, and chronic treatment with yohimbine or clonidine. In male Wistar rats morphine (3 mg/kg s.c.), and clonidine (31 micrograms/kg i.p.) induced a significant increase in plasma GH levels. The pretreatment with the alpha 2-antagonist yohimbine (1 and 3 mg/kg) effectively inhibited the GH releasing effect or morphine and clonidine. CH-38083 at the dose of 1 mg/kg did not interfere with the morphine-induced GH secretion, while it fully antagonized the GH-releasing effect of clonidine. Higher doses (3 and 5 mg/kg) of CH-38083 only partly inhibited GH secretion induced by morphine. In rats chronically treated with clonidine (2 micrograms/ml in the drinking water for 14 days) the GH response to an injection of clonidine was blocked, while the effect of morphine on the GH secretion remained unchanged. In long-term castrated rats the effect of clonidine (15, 31 and 250 micrograms/kg i.p.) on the GH secretion was significantly blunted, while the GH-releasing effect of morphine (1, 3 and 5 mg/kg s.c.) remained unchanged. The replacement of testosterone (10 mg/kg for 4 days) in castrates restored the effect of clonidine, whereas it decreased the stimulatory action of morphine on the GH secretion. In rats chronically treated with yohimbine (2 mg/kg i.p. 2-3 times daily for 14 days until sacrifice), the GH response to a high dose of clonidine (0.5 mg/kg i.p.) was blocked, while the effect of morphine (5 mg/kg s.c.) was significantly enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beneficial effects and improved survival in rodent models of septic shock with S-methylisothiourea sulfate, a potent and selective inhibitor of inducible nitric oxide synthase.

Enhanced formation of nitric oxide (NO) by both the constitutive and the inducible isoforms of NO synthase (NOS) has been implicated in the pathophysiology of a variety of diseases, including circulatory shock. Non-isoform-selective inhibition of NO formation, however, may lead to side effects by inhibiting the constitutive isoform of NOS and, thus, the various physiological actions of NO. S-Methylisothiourea sulfate (SMT) is at least 10- to 30-fold more potent as an inhibitor of inducible NOS (iNOS) in immunostimulated cultured macrophages (EC50, 6 microM) and vascular smooth muscle cells (EC50, 2 microM) than NG-methyl-L-arginine (MeArg) or any other NOS inhibitor yet known. The effect of SMT on iNOS activity can be reversed by excess L-arginine in a concentration-dependent manner. SMT (up to 1 mM) does not inhibit the activity of xanthine oxidase, diaphorase, lactate dehydrogenase, monoamine oxidase, catalase, cytochrome P450, or superoxide dismutase. SMT is equipotent with MeArg in inhibiting the endothelial, constitutive isoform of NOS in vitro and causes increases in blood pressure similar to those produced by MeArg in normal rats. SMT, however, dose-dependently reverses (0.01-3 mg/kg) the hypotension and the vascular hyporeactivity to vasoconstrictor agents caused by endotoxin [bacterial lipopolysaccharide (LPS), 10 mg/kg, i.v.] in anesthetized rats. Moreover, therapeutic administration of SMT (5 mg/kg, i.p., given 2 hr after LPS, 10 mg/kg, i.p.) attenuates the rises in plasma alanine and aspartate aminotransferases, bilirubin, and creatinine and also prevents hypocalcaemia when measured 6 hr after administration of LPS. SMT (1 mg/kg, i.p.) improves 24-hr survival of mice treated with a high dose of LPS (60 mg/kg, i.p.). Thus, SMT is a potent and selective inhibitor of iNOS and exerts beneficial effects in rodent models of septic shock. SMT, therefore, may have considerable value in the therapy of circulatory shock of various etiologies and other pathophysiological conditions associated with induction of iNOS.     

Brain catalase activity inhibition as well as opioid receptor antagonism increases ethanol-induced HPA axis activation

BACKGROUND: Growing evidence indicates that brain catalase activity is involved in the psychopharmacological actions of ethanol. Recent data suggest that participation of this enzymatic system in some ethanol effects could be mediated by the endogenous opioid system. The present study assessed whether brain catalase has a role in ethanol-induced activation of the HPA axis, a neuroendocrine system modulated by the endogenous opioid neurotransmission. METHODS: Swiss male mice received an intraperitoneal injection of the catalase inhibitor 3-amino-1,2,4-triazole (AT; 0-1 g/kg), and 0 to 20 hr after this administration, animals received an ethanol (0-4 g/kg; intraperitoneally) challenge. Thirty, 60, or 120 min after ethanol administration, plasma corticosterone levels were determined immunoenzymatically. In addition, we tested the effects of 45 mg/kg of cyanamide (another catalase inhibitor) and 0 to 2 mg/kg of naltrexone (nonselective opioid receptor antagonist) on ethanol-induced enhancement in plasma corticosterone values. RESULTS: The present study revealed that AT boosts ethanol-induced increase in plasma corticosterone levels in a dose- and time-dependent manner. However, it did not affect corticosterone values when measured after administration of saline, cocaine (4 mg/kg, intraperitoneally), or morphine (30 mg/kg, intraperitoneally). The catalase inhibitor cyanamide (45 mg/kg, intraperitoneally) also increased ethanol-related plasma corticosterone levels. These effects of AT and cyanamide on ethanol-induced corticosterone values were observed under treatment conditions that decreased significantly brain catalase activity. Indeed, a significant correlation between effects of catalase manipulations on both variables was found. Finally, we found that the administration of naltrexone enhanced the levels of plasma corticosterone after the administration of saline or ethanol. CONCLUSIONS: This study shows that the inhibition of brain catalase increases ethanol-induced plasma corticosterone levels. Results are discussed together with previous findings suggesting a putative linkage between brain ethanol metabolism and the endogenous opioid system to explain some of the neuroendocrine effects of ethanol.     

Mirtazapine, a noradrenergic and specific serotonergic antidepressant, attenuates morphine dependence and withdrawal in Sprague-Dawley rats

The effects of mirtazapine, a noradrenergic and specific serotonergic antidepressant, on morphine withdrawal and morphine conditioned place preference (CPP) were investigated in rats. Our results showed that some morphine withdrawal signs, including teeth chattering, grooming, chewing, and escape attendance, were attenuated by single pretreatments with 3, 10, or 30 mg/kg mirtazapine. Wet-dog shakes, rearing, and grooming were inhibited by daily pretreatment with 1, 3, or 10 mg/kg mirtazapine. The expression of morphine-induced CPP was significantly blocked by mirtazapine (10 or 30 mg/kg, i.p.), while chronic treatment with mirtazapine (1 or 10 mg/kg, i.p. once, daily, for six consecutive days) significantly attenuated the acquisition of morphine CPP. Our results demonstrated that mirtazapine attenuates morphine withdrawal and morphine-induced CPP in rats and suggest that mirtazapine may have therapeutic potential in the treatment of opiate dependence.     

Reduced susceptibility to epinephrine-induced arrhythmias in cirrhotic rats: the roles of nitric oxide and endogenous opioid peptides

BACKGROUND/AIMS: The clinical relevance of QT prolongation, the most widely recognized cardiac electrophysiological abnormality of cirrhosis, is still undefined. The aim of this study is to examine the susceptibility of chronic (4-week) bile duct-ligated rats to epinephrine-induced arrhythmias. The roles of nitric oxide and endogenous opioids were also evaluated. METHODS: Sham-operated and cirrhotic rats were treated with daily subcutaneous administrations of normal saline (1 ml/kg/day), L-NAME (a non-selective nitric oxide synthase inhibitor, 3mg/kg/day), and naltrexone (20mg/kg/day) during the fourth week after operation. In order to evaluate the effects of acute nitric oxide synthesis inhibition, additional groups of animals were treated by acute intraperitoneal L-NAME injections (3mg/kg). Arrhythmias were induced by intravenous injections of 10 microg/kg epinephrine. RESULTS: Despite QT prolongation (P<0.001), epinephrine induced fewer arrhythmias in cirrhotic rats compared to sham-operated animals (P<0.05). Chronic, but not acute, L-NAME administration corrected the QT prolongation in cirrhotic rats (P<0.001), and restored the susceptibility of cirrhotic rats to arrhythmias (P<0.05). Naltrexone injection without a significant effect on epinephrine-induced arrhythmias corrected QT interval in cirrhotic rats (P<0.001). CONCLUSIONS: This study shows that despite QT prolongation, cirrhotic animals are resistant against epinephrine-induced arrhythmias. This resistance is mediated by chronic nitric oxide overproduction.     

Mirtazapine,a Noradrenergic and Specific Serotonergic Antidepressant,Attenuates Morphine Dependence and Withdrawal in Sprague-Dawley Rats

The effects of mirtazapine, a noradrenergic and specific serotonergic antidepressant, on morphine withdrawal and morphine conditioned place preference (CPP) were investigated in rats. Our results showed that some morphine withdrawal signs, including teeth chattering, grooming, chewing, and escape attendance, were attenuated by single pretreatments with 3, 10, or 30 mg/kg mirtazapine. Wet-dog shakes, rearing, and grooming were inhibited by daily pretreatment with 1, 3, or 10 mg/kg mirtazapine. The expression of morphine-induced CPP was significantly blocked by mirtazapine (10 or 30 mg/kg, i.p.), while chronic treatment with mirtazapine (1 or 10 mg/kg, i.p. once, daily, for six consecutive days) significantly attenuated the acquisition of morphine CPP. Our results demonstrated that mirtazapine attenuates morphine withdrawal and morphine-induced CPP in rats and suggest that mirtazapine may have therapeutic potential in the treatment of opiate dependence.     

Involvement of bradykinin and nitric oxide in leptin-mediated glucose uptake in skeletal muscle

Regulation of glucose metabolism in peripheral tissues by leptin has been highlighted recently, although its mechanism is unclear. In this study, we postulated that bradykinin and nitric oxide (NO) are involved in the effect of leptin-mediated glucose uptake in peripheral tissues and examined these possibilities. Injection of leptin (200 pg/mouse) into the ventromedial hypothalamus-enhanced glucose uptake in skeletal muscle and brown adipose tissue, but not in white adipose tissue. Treatment with Hoe140 (0.1 mg/kg), bradykinin B2 receptor antagonist, or L-NAME (N:(G)-nitro-L-arginine methyl ester) (30 mg/kg), nitric oxide synthase inhibitor, did not influence the basal level of glucose uptake in skeletal muscle and the adipose tissue, whereas Hoe140 and L-NAME inhibited leptin-mediated glucose uptake in skeletal muscles, but had no effect in adipose tissue. However, Hoe140 and L-NAME did not inhibit insulin (1.0 U/kg)-mediated glucose uptake in all tissues examined. Taken together, these results suggest that leptin enhances bradykinin and/or the NO system, which contributes at least partially to the enhanced glucose uptake in skeletal muscles.     

L-NAME,A NITRIC OXIDE SYNTHASE INHIBITOR,AS A POTENTIAL COUNTERMEASURE TO POST-SUSPENSION HYPOTENSION IN RATS

A large number of astronauts returning from spaceflight experience orthostatic hypotension. This hypotension may be due to overproduction of vasodilatory mediators, such as nitric oxide (NO) and prostaglandins. To evaluate the role of the NO synthase inhibitor N^G-nitro-L-arginine methyl ester (L-NAME) as a countermeasure against the post-suspension reduction in mean arterial pressure (MAP), we assessed the cardiovascular responses and vascular reactivity to 7-day 30° tail-suspension and a subsequent 6 hr post-suspension period in conscious rats. After a pre-suspension reading, direct MAP and heart rate (HR) were measured daily and every 2 hrs post-suspension. The NO synthase inhibitor L-NAME (20 mg/kg, i.v.), or saline, were administered after the 7^th day reading prior to release from suspension and at 2 and 4 hrs post-suspension. At 6 hrs post-suspension, vascular reactivity was assessed. While MAP did not change during the suspension period, it was reduced post-suspension. Heart rate was not significantly altered. L-NAME administration reversed the post-suspension reduction in MAP. In addition, the baroreflex sensitivity for heart rate was modified by L-NAME. Thus, the post-suspension reduction in MAP may be due to overproduction of NO and altered baroreflex activity.     

Angiotensin II type 1 receptor antagonism improves endothelial vasodilator function in L-NAME-induced hypertensive rats by a kinin-dependent mechanism

OBJECTIVE: This study was designed to investigate the ability of a chronic blockade of angiotensin II type 1 receptors with losartan to reverse the endothelial dysfunction present in N-nitro-L-arginine methyl ester (L-NAME)-treated hypertensive rats and the possible dependence of this effect on bradykinin B2-receptor activation. METHODS: Rats treated with L-NAME alone (60 mg/kg per day for 8 weeks) or with L-NAME + losartan, L-NAME + icatibant (a bradykinin B2-receptor antagonist) and L-NAME + losartan + icatibant were studied. Losartan, icatibant or losartan + icatibant were co-administered with L-NAME during the last 4 weeks of the experiment. Endothelial nitric oxide synthase gene expression in aortic tissues, plasma nitrite/nitrate concentrations, the relaxant effect of acetylcholine on norepinephrine-precontracted aortic rings and 6-keto-PGF1alpha release from aortic rings were used as markers of the endothelial function. RESULTS: Rats treated with L-NAME alone and L-NAME + icatibant showed, as compared with untreated animals, a clear-cut increase in systolic blood pressure and a decrease of all the markers of endothelial function evaluated. In L-NAME-rats, administration of losartan reduced the systolic blood pressure and restored endothelial nitric oxide synthase gene expression, plasma nitrite/nitrate levels, the relaxant activity of acetylcholine on aortic rings and the generation of 6-keto-PGF1alpha from the aortic tissues. Co-administration of icatibant with losartan blunted the stimulatory effect of losartan on the markers of endothelial function evaluated. CONCLUSION: These results demonstrated that losartan is capable of reversing the endothelial vasodilator dysfunction in L-NAME-induced hypertensive rats, and that the beneficial effect of losartan is mediated by bradykinin B2-receptor activation.     

Effects of morphine and naloxone on plasma levels of LH, FSH, prolactin and growth hormone in the immature male pig

The effects of acute i.v. administration of gonadotrophin-releasing hormone (GnRH; 0.1 micrograms/kg), morphine (3 mg/kg) and/or naloxone (0.5 mg/kg) on LH and FSH secretion was evaluated in young male pigs (approximately 6 weeks old) with venous brachiocephalic cannulae. The effects of morphine and/or naloxone treatments on prolactin and GH were also evaluated. The influence of morphine on hypophysial hormone secretion was also examined 2 days after castration. Animals treated with morphine and/or naloxone were compared with saline-injected control animals. Injection of GnRH induced 400 and 50% increases in LH and FSH respectively. Morphine and/or naloxone did not influence LH secretion in intact or castrated animals. Morphine suppressed (P less than 0.01) FSH levels 40-60 min after injection whereas naloxone had no effect. Castration eliminated morphine-induced suppression of FSH. Injection of morphine followed by naloxone resulted in acutely raised (P less than 0.05) FSH concentrations. Morphine induced a threefold increase (P less than 0.01) in prolactin within 30 min of injection and naloxone inhibited the effect of morphine. Levels of GH were increased (P less than 0.01) 20 min after morphine treatment and this increase was delayed when naloxone was given immediately after morphine. Naloxone alone did not affect prolactin or GH secretion. Castration caused increases in LH (P less than 0.05) and FSH (P less than 0.01), did not influence prolactin or GH, and reduced plasma testosterone to undetectable (less than 1.0 nmol/l) levels. These results suggest that in young male pigs the hypothalamic-hypophysial axis is responsive to GnRH and gonadal negative feedback.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of adenosine analogues on plasma corticosterone concentration in rats.

In this study we examined the effect of the adenosine analogues: N6-cyclohexyladenosine, L-N6-phenylisopropyladenosine and 5'-N-ethylcarboxamidoadenosine on the plasma corticosterone concentration in rats. It was found that N6-cyclohexyladenosine (0.1-3.0 mg/kg), L-N6-phenylisopropyladenosine (0.1-1.0 mg/kg) and 5'-N-ethylcarboxamidoadenosine (0.01-1.0 mg/kg) dose-dependently increased the plasma corticosterone level. The effects of N6-cyclohexyladenosine (0.1 mg/kg) and L-N6-phenylisopropyladenosine (0.1 mg/kg) were completely blocked in animals pretreated with dexamethasone (3 x 1 mg/kg), as well as in animals with a pharmacological blockade of the release of hypothalamic corticotropin-releasing factor induced by chloropromazine (10 mg/kg), morphine (20 mg/kg) and nembutal (25 mg/kg), whereas the corticosterone response to 5'-N-ethylcarboxamidoadenosine (0.01 mg/kg) was blocked in dexamethasone-pretreated rats only. On the other hand, the adenosine receptor antagonists: 8-(p-sulfophenyl)-theophylline (30 mg/kg), 8-phenyltheophylline (10 and 30 mg/kg), 1,3-dipropyl-8-(2-amino-4-chloro)-phenylxanthine (1 and 3 mg/kg) and 1,3-dipropyl-7-methylxanthine (1 mg/kg) did not affect the corticosterone response to N6-cyclohexyladenosine, L-N6-phenylisopropyladenosine or 5'-N-ethylcarboxamidoadenosine. The obtained results indicate that N6-cyclohexyladenosine and L-N6-phenylisopropyladenosine stimulate the corticosterone secretion at the hypothalamic level, whereas 5'-N-ethylcarboxamidoadenosine is likely to act at the pituitary level. Although the effects produced by the adenosine analogues show that both A1 and A2 receptors are involved in the corticosterone response, negative results of the interaction studies with adenosine receptor antagonists indicate that further experiments are necessary to elucidate this problem.