Nitric oxide within the ventral tegmental area is involved in mediating morphine reward

In the present study, the effects of intra-ventral tegmental area injection of L-arginine, a nitric oxide (NO) precursor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, on morphine-induced conditioned place preference in male Wistar rats were investigated. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (0.5-10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intra-ventral tegmental area administration of a low dose of L-arginine (0.05 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited significant conditioned place preference; however, a higher dose of L-arginine (0.1 microg/rat) reduced the morphine response. Intra-ventral tegmental area administration of L-NAME (0.03 and 0.1 microg/rat) decreased the acquisition of morphine (7.5 mg/kg)-induced place preference. The response to different doses of L-arginine was decreased by L-NAME (0.03 microg/rat). L-Arginine and L-NAME by themselves did not elicit any effect on place conditioning; however, intra-ventral tegmental area administration of L-arginine (0.01-0.1 microg/rat) and a higher dose of L-NAME (0.1 microg/rat) significantly decreased the expression of morphine (7.5 mg/kg)-induced place preference. The attenuation of already established morphine-induced place preference on the test day by L-arginine was inhibited by L-NAME (0.03 microg/rat). The results indicate that NO may be involved in the acquisition and expression of morphine-induced place preference.     

Morphine state-dependent learning sensitization and interaction with nitric oxide

In the present study, the effects of nitric oxide (NO) precursor L-arginine and L-NAME, a potent inhibitor of NO synthase (NOS), on the expression of sensitization of morphine were investigated. Pre-training administration of morphine (5 mg/kg) impaired memory retrieval compared to pre-training saline-treated animals. Amnesia due to pre-training morphine (5 mg/kg) was restored by pre-test morphine (5 mg/kg).The retrieval impairment was also inhibited in mice which had received once-daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days, followed by 5 days of no drug treatment before training (in order to induce morphine sensitization). Administration of L-arginine (60 mg/kg/day - 3 days) or L-NAME (20 mg/kg/day - 3 days) before training did not alter morphine state dependency. During acquisition of sensitization, administration of L-arginine (60 mg/kg) 20 min before morphine (10 mg/kg/day, for 3 days) increased, while injection of L-NAME (20 mg/kg) 20 min before morphine (30 mg/kg/day, for 3 days) decreased morphine state dependency. It is concluded that NO is involved in the morphine-induced sensitization.     

Fenamate-induced enhancement of heterologously expressed HERG currents in Xenopus oocytes

In the present study, the effects of intra-nucleus accumbens injection of L-arginine, a nitric oxide (NO) precursor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, on morphine-induced conditioned place preference in male Wistar rats were investigated. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (0.5-10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intra-accumbens administration of L-arginine (0.03 and 0.05 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited significant conditioned place preference, while intra-accumbens administration of L-NAME (0.3, 0.1 and 1 microg/rat) decreased the acquisition of conditioned place preference induced by morphine (7.5 mg/kg). The response to different doses of L-arginine was decreased by L-NAME (0.03 microg/rat). L-Arginine and L-NAME by themselves did not elicit any effect on place conditioning. Intra-accumbens administration of L-arginine but not L-NAME significantly decreased the expression of morphine (7.5 mg/kg)-induced place preference. The attenuation of already established morphine-induced place preference on the test day by L-arginine was inhibited by L-NAME. The results indicate that NO may be involved in the acquisition and expression of morphine-induced place preference.     

The effect of cyclosporin A on morphine tolerance and dependence: involvement of L-arginine/nitric oxide pathway

Cyclosporin A is known to decrease nitric oxide (NO) production in nervous tissues. The effects of systemic cyclosporine A on the induction and expression of morphine tolerance and dependence, acute morphine-induced antinociception, and the probable involvement of the L-arginine/nitric oxide pathway in these effects were assessed in mice. Cyclosporin A (20 mg/kg), N(G)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg) and a combination of the two at lower and per se non-effective doses (5 and 3 mg/kg, respectively) showed a similar pattern of action, inhibiting the induction of tolerance to morphine-induced antinociception and increasing the antinociception threshold in the expression phase of morphine tolerance. These agents also inhibited the expression of morphine dependence as assessed by naloxone-precipitated withdrawal signs, while having no effect on the induction of morphine dependence. L-Arginine, at a per se non-effective dose (60 mg/kg), inhibited the effects of Cyclosporin A. Moreover, acute administration of Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) enhanced the antinociception induced by acute administration of morphine (5 mg/kg), while chronic pretreatment with Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) for 2 days (twice daily) did not affect morphine-induced antinociception. The inducible nitric oxide synthase inhibitor, aminoguanidine (100 mg/kg), did not alter morphine antinociception, tolerance or dependence. In conclusion, decreasing NO production through constitutive nitric oxide synthase may be a mechanism through which cyclosporin A differentially modulates morphine tolerance, dependence and antinociception.     

Role of nitric oxide in the acquisition and expression of apomorphine- or morphine-induced locomotor sensitization

In the present study, the effects of L-arginine, a nitric oxide (NO) precursor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, on apomorphine- or morphine-induced locomotor sensitization in male albino mice were investigated. Our data showed that subcutaneous (s.c.) injection of apomorphine (2-10 mg/kg) or morphine sulphate (5-50 mg/kg) significantly increased locomotor behaviour in a dose-dependent manner. Intraperitoneal (i.p.) administration of L-arginine (100 mg/kg) increased locomotor activity, whereas L-NAME (20 mg/kg) decreased it. L-Arginine and L-NAME increased and decreased apomorphine- or morphine-induced locomotions, respectively. The locomotor behavioural response was enhanced in mice pretreated with apomorphine (2 mg/kg, daily x3 days) or morphine (10 mg/kg, daily x3 days) alone, indicating that sensitization had developed. Administration of L-arginine 30 min before each of three daily doses of apomorphine or morphine increased the development of sensitization, while administration of L-NAME 30 min before each of three daily doses of apomorphine or morphine decreased the acquisition of sensitization induced by apomorphine or morphine. Administration of L-arginine significantly increased and L-NAME significantly and dose-dependently decreased the expression of both apomorphine- and morphine-induced sensitization. The results indicate that NO may be involved in the acquisition and expression of apomorphine- or morphine-induced sensitization.     

Effect of acute and chronic administration of L-arginine on morphine induced inhibition of gastrointestinal motility

Effect of acute and chronic administration of L-arginine on morphine induced gastrointestinal inhibition was tested in rats. In the test for acute effect, L-arginine (200 mg/kg, i.v.) was given 10 minutes before the charcoal meal test. In the test for chronic effects, L-arginine (200 mg/kg, i.v.) was given twice a day for 4 days. Charcoal meal test was done on the fifth day. Morphine was administered 45 minutes before the charcoal meal test. Results showed that acute administration of L-arginine did not affect the morphine's action on the GIT. In contrast, chronic administration of L-arginine reversed the morphine induced decrease in gastrointestinal motility. The reversal was however, not complete. This data suggests that inhibition of NO may be one of the mechanism of morphine induced constipation.     

Effects of 7-OH-DPAT on cocaine-seeking behavior and on re-establishment of cocaine self-administration

Effects of the D2-like dopamine agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT), on cocaine-seeking behavior and re-establishment of cocaine self-administration were examined. Rats were trained to lever press for cocaine infusions (0.25 mg/kg iv). Some were then tested for cocaine-seeking behavior (i.e., lever presses in the absence of cocaine re-inforcement) immediately following acute 7-OH-DPAT (0.001, 0.01, 0.1, or 1.0 mg/kg sc) or saline administration. Others were tested immediately or 2-23 h following repeated daily 7-OH-DPAT (1.0 mg/kg sc) or saline administration for extinction of cocaine-seeking behavior, cocaine re-instatement of cocaine-seeking behavior, and re-establishment of cocaine self-administration following extinction. 7-OH-DPAT-induced changes in locomotion were also assessed. Cocaine-experienced animals exhibited cross-tolerance to the transient hypoactivity produced by acute 7-OH-DPAT administration. Acute administration of low doses (0.01-0.1 mg/kg) of 7-OH-DPAT attenuated cocaine-seeking behavior, whereas the highest dose (1.0 mg/kg) initially attenuated, then increased, cocaine-seeking behavior. In animals tested immediately following one of the repeated administrations, 7-OH-DPAT did not alter cocaine self-administration, but sensitized locomotion. Repeated 7-OH-DPAT administration also increased cocaine-seeking behavior when administered 0 h, but not 2 or 4 h, before cocaine priming (15 mg/kg ip) and testing. In animals tested 17-23 h following one of the repeated administrations, cocaine-seeking behavior and re-establishment of cocaine self-administration were attenuated, but maintenance of self-administration following re-establishment, cocaine re-instatement of extinguished cocaine-seeking behavior, and spontaneous locomotion were unaltered. The findings suggest that following repeated administration, 7-OH-DPAT produces a transient increase (<2 h) in incentive motivation for cocaine that is followed by a protracted decrease in incentive motivation for cocaine.     

Comparison of simultaneous administration of lithium with L-NAME or L-arginine on morphine withdrawal syndrome in mice

Due to the claim that chronic administration of lithium or L-N(G)-nitroarginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor reduces morphine withdrawal syndrome, the effects of chronic administration of lithium, L-NAME, or L-arginine (L-Arg), a precursor of NO, alone or co-administration of lithium with L-Arg or L-NAME, on naloxone-precipitated withdrawal syndrome and physical dependence development to morphine in mice chronically treated with morphine, were evaluated. Morphine dependency was induced by the intraperitoneal injection (i.p.) of morphine (10 mg/kg), once daily for 7 days. Physical dependence to morphine was observed by precipitating an abstinence syndrome with naloxone (2 mg/kg, i.p.). Chronic administration of L-NAME (10 mg/kg, i.p., once daily, for 7 days after 10 days of receiving only tap water and food prior to naloxone), decreased all withdrawal signs significantly, while L-Arg (200 mg/kg, as above) increased only some withdrawal signs significantly in morphine-dependent mice. Chronic administration of lithium (600 mg/kg, in drinking water) alone or co-administration of lithium (as above) with L-NAME (10 mg/kg) or L-Arg (200 mg/kg, i.p., once daily) for 7 days after 10 days of receiving only lithium (as above) and food, decreased all withdrawal signs and physical dependence significantly in morphine-dependent mice. The results obtained indicate that co-administration of L-NAME with lithium increases the effect of lithium or L-NAME alone, on withdrawal signs, but this increase is not significantly different as compared to chronic lithium or L-NAME administration alone; while co-administration of L-Arg with lithium decreases the effects of lithium on withdrawal signs and this decrease is not significant as compared to chronic lithium administration alone. These findings indicate that nitric oxide may be involved in modulation of naloxone-induced withdrawal syndrome, and treatment with lithium could have some effect on this system. Copyright 2000 John Wiley & Sons, Ltd.     

Effects of levo-alpha-acetylmethadol (LAAM) on morphine self-administration in the rat

Rats bearing cerebrocortical electrodes for recording the electroencephalogram (EEG) were rendered tolerant to and physically dependent on morphine and subsequently trained to self-administer morphine (10 mg/kg/injection) through a chronic intravenous cannula. Morphine was available for selfadministration 24 h/day. Once morphine intake had stabilized (10–12 injections/day), levo-alpha-acetylmethadol (LAAM) was administered noncontingently via a chronic intragastric (IG) cannula as a single daily dose of either 1 or 4 mg/kg. These morphine self-administering rats were maintained on daily LAAM treatment for 12 consecutive days. Analysis of the patterns of lever pressing, morphine self-injections, and sleepawake behavior revealed that daily IG administration of LAAM effectively suppressed morphine self-administration. The 1 mg/kg dose of LAAM reduced morphine intake by 30%–50%, while 4 mg/kg produced an 80%–90% decrease. The reduction in morphine self-administration occurred in the absence of overt signs of narcotic withdrawal, behavioral toxicity, or disruption of sleep-awake behavior in these rats. Termination of LAAM maintenance resulted in a gradual return of lever pressing and morphine intake to pre-LAAM levels.Preliminary report of this study appeared in The Pharmacologist 21, 226 (1979)     

The effects of simultaneous administration of alpha(2) -adrenergic agents with L-NAME or L-arginine on the development and expression of morphine dependence in mice

Both alpha(2)-adrenoceptors and the L-arginine/nitric oxide (NO) pathway have been implicated in the modulation of morphine dependence. This study examined the effects of simultaneous administration of the alpha(2)-adrenoceptor agonist clonidine or the antagonist yohimbine together with the NO precursor L-arginine or the NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the induction and expression of morphine dependence as assessed by naloxone-precipitated withdrawal jumping and diarrhoea. Male NMRI mice weighing 20-30 g were used. In the induction phase, clonidine (0.01-0.1 mg/kg) intensified and yohimbine (0.5-2 mg/kg) attenuated the degree of morphine dependence. Yohimbine reversed the effect of clonidine. L-NAME (5 and 10 mg/kg) did not affect the development of morphine dependence, but significantly potentiated the effects of both subeffective (0.01 mg/kg) and effective (0.03 mg/kg) doses of clonidine. L-Arginine did not alter morphine dependence but inhibited the effect of clonidine. The effects of yohimbine in the induction phase were attenuated by L-NAME, but were not significantly affected by L-arginine. In the expression phase, clonidine attenuated and yohimbine intensified the signs of dependence. The effect of clonidine was inhibited by yohimbine. In the expression phase, L-NAME attenuated the withdrawal syndrome at 10 mg/kg and showed potentiation with clonidine in suppressing withdrawal signs. L-Arginine did not alter morphine dependence, but at 20 mg/kg inhibited and at 100 mg/kg potentiated the attenuating effect of clonidine on the expression of withdrawal syndrome. The effect of yohimbine on the expression phase was also attenuated by L-NAME, but was not significantly affected by L-arginine. In conclusion, alpha(2)-adrenergic and NO pathways seem to be functionally linked in the modulation of opioid dependence.     

Role of nitric oxide in the rat hippocampal CA1 area on morphine-induced conditioned place preference

Effects of intrahippocampal CA1 injections of L-arginine, a nitric oxide (NO) precursor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, on morphine-induced conditioned place preference in male Wistar rats were investigated. Animals received subcutaneous (s.c.) injections of saline (1.0 ml/kg) or morphine (0.5-7.5 mg/kg) once daily for 3 days to induce conditioned place preference. The administration of L-arginine (0.3, 1.0, and 3.0 microg/rat), but not L-NAME (0.3, 1.0, and 3.0, microg/rat), prior to administration of morphine (5.0 mg/kg) during acquisition of morphine-induced conditioned place preference increased morphine-induced conditioned place preference, but the interaction between the response to morphine and/or L-arginine was not statistically significant. The response to L-arginine was blocked by L-NAME pre-administration. L-Arginine or L-NAME by itself did not induce conditioned place preference. The administration of L-arginine but not L-NAME, 1 min before conditioned place preference testing, increased the expression of morphine-induced conditioned place preference. Pre-administration of L-NAME blocked the L-arginine response. It is concluded that NO in the rat hippocampal CA1 area may be involved in morphine-induced conditioned place preference.     

The spinal nitric oxide involved in the inhibitory effect of midazolam on morphine-induced analgesia tolerance

Previous studies had shown that pretreatment with midazolam inhibited morphine-induced tolerance and dependence. The present study was to investigate the role of spinal nitric oxide (NO) in the inhibitory effect of midazolam on the development of morphine-induced analgesia tolerance. Subcutaneous injection of 100 mg/kg morphine to mice caused an acute morphine-induced analgesia tolerance model. To develop chronic morphine tolerance in mice, morphine was injected for three consecutive days (10, 20, 50 mg/kg sc on Day 1, 2, 3, respectively). In order to develop chronic tolerance model in rats, 10 mg/kg of morphine was given twice daily at 12 h intervals for 10 days. Midazolam was intraperitoneally injected 30 min prior to administration of morphine. Tail-flick test, hot-plate and formalin test were conducted to assess the nociceptive response. Immunocytochemistry, histochemistry and western blot were performed to determine the effect of midazolam on formalin-induced expression of Fos protein, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and nitric oxide synthase (NOS) in chronic morphine-tolerant rats, respectively. The results showed that pretreatment with midazolam significantly inhibited the development of acute and chronic morphine tolerance in mice, which could be partially reversed by intrathecal injection of NO precursor L-arginine (L-Arg). In chronic morphine-tolerant rats, pretreatment with midazolam significantly decreased the formalin-induced expression of Fos and Fos/NADPH-d double-labeled neurons in the contralateral spinal cord and NADPH-d positive neurons in the bilateral spinal cord. Both inducible NOS (iNOS) and neuronal NOS (nNOS) protein levels in the spinal cord were significantly increased after injection of formalin, which could be inhibited by pretreatment with midazolam. The above results suggested that the decrease of the activity and expression of NOS contributed to the inhibitory effect of midazolam on the development of morphine tolerance.     

Mediation of nitric oxide in inhibitory effect of morphine against electroshock-induced convulsions in mice

Nitric oxide (NO) and morphine have been coupled in many physiological as well as pathological processes. The present study examined the involvement of the L-arginine/NO pathway in the anticonvulsant properties of systemic morphine (2-30 mg/kg) against electroshock seizures (ECS) in mice. Morphine decreased the intensity of maximal electroshock seizures (MES) and increased the threshold for ECS. Neither the NOS substrate L-arginine (30, 60, and 100 mg/kg), the reversible nonspecific NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 3, 10, and 30 mg/kg), the irreversible specific inducible NOS inhibitor aminoguanidine (20, 50, and 100 mg/kg), nor the opioid receptor antagonist naloxone (0.1, 0.3, and 1 mg/kg) did alter per se the ECS threshold or the intensity of MES at doses used. However, both naloxone and L-NAME, but not aminoguanidine, inhibited the anticonvulsant effects of morphine (30 mg/kg) against ECS, while L-arginine potentiated the anticonvulsant effects of lower doses of morphine (2 or 10 mg/kg). Low doses of naloxone (0.1 or 0.3 mg/kg) or L-NAME (3 mg/kg), which did not alter morphine effect per se, showed additive anticonvulsant effects against MES. Thus, the L-arginine/NO pathway seems to play a role in the anticonvulsant properties of morphine against ECS and this mediation involves the constitutive, but not the inducible, form of nitric oxide synthase.     

Fluoxetine suppresses morphine tolerance and dependence: modulation of NO-cGMP/DA/serotoninergic pathways

Although the phenomenon of opioid tolerance and dependence has been widely investigated, neither opioid nor non-opioid mechanisms are completely understood. In view of the modulation of 5-HT transport into presynaptic terminals in the brain by nitric oxide (NO) via cGMP, and the existence of a tonic 5-HTergic inhibition of dopamine release, the present study investigated the effect of fluoxetine, a selective serotonin reuptake inhibitor, and NO modulators L-N(G)-nitroarginine methyl ester (L-NAME; NO synthase inhibitor) and L-Arginine (substrate for nitric oxide synthase) alone or in combination against morphine tolerance and dependence. Animals developed tolerance to the antinociceptive effect of morphine (10 mg/kg s.c. twice daily) on day 3 and the degree of tolerance was further enhanced on days 9 and 10. The development of tolerance to the antinociceptive effect of morphine was delayed by prior administration of fluoxetine (10 mg/kg i.p, twice daily for 9 days) and L-NAME (10 mg/kg i.p. twice daily for 9 days) alone or in combination. It was accentuated by L-Arginine (50 mg/kg i.p. twice daily for 9 days) alone or in combination with fluoxetine (10 mg/kg i.p. twice daily for 9 days). Similarly, fluoxetine (10 mg/kg i.p.) or L-NAME (10 mg/kg i.p.), when administered acutely on day 10, reversed morphine-induced tolerance. L-Arginine (50 mg/kg i.p.) however, when administered acutely on day 10, accentuated morphine tolerance. Fluoxetine (10 mg/kg i.p. twice daily for 9 days) suppressed the development of morphine dependence as assessed by naloxone (2 mg/kg i.p.)-precipitated withdrawal jumps. This suppression of dependence was potentiated by L-NAME (10 mg/kg i.p. twice daily for 9 days) and reversed by L-Arginine (50 mg/kg i.p. twice daily for 9 days), respectively. Acute administration of the respective drugs on day 10 modulated morphine dependence in a similar fashion. L-Arginine also reversed fluoxetine-induced weight loss in morphine-dependent animals. The present study demonstrated that fluoxetine suppressed the dependence and development of tolerance to the antinociceptive effect of morphine. Fluoxetine-induced suppression was potentiated by L-NAME and accentuated by L-Arginine. The results therefore suggest that a complex phenomenon such as morphine tolerance and dependence might involve close interplay of the NO-c GMP/5-HT/DA receptor system. To the best of the authors' knowledge, this is the first report to suggest targeting this cascade for amelioration of opioid tolerance and withdrawal syndrome.     

Role of nitric oxide in the induction and expression of morphine tolerance and dependence in mice.

1. The possible involvement of nitric oxide (NO) in the induction and expression of morphine tolerance and dependence was studied in mice. A two-day repeated injection regimen was used to induce morphine tolerance and dependence. Tolerance was assessed by the tail flick test and physical dependence by naloxone challenge, on the third day. 2. Two days pretreatment with L-arginine (20 mg kg-1, twice daily) or D-NG-nitro arginine methyl ester (D-NAME, 20 mg kg-1, twice daily) alone had no effect on subsequent morphine antinociception. L-NG-monomethyl arginine (L-NMMA, 10 mg kg-1, twice daily) for two days led to a slight increase (not statistically significant) in morphine antinociception; while L-NG-nitro arginine methyl ester (L-NAME, 10 mg kg-1, twice daily) for two days led to attenuation of morphine analgesia. None of the animals treated with these drugs alone showed signs characteristic of the opioid withdrawal syndrome upon naloxone challenge. 3. Induction phase L-arginine slowed the development of opioid tolerance and physical dependence, while L-NAME and L-NMMA led to a higher degree of tolerance but had no effect on the development of physical dependence. 4. L-Arginine and D-NAME had no effect on the expression of morphine tolerance and physical dependence. Expression phase L-NAME and L-NMMA, on the other hand, attenuated morphine tolerance and reduced the incidence of withdrawal signs. 5. NO may, therefore, play a role in both phases of morphine tolerance and dependence: elevation of NO levels during the induction phase delays the development of opioid tolerance/dependence, while inhibition of NO synthase accelerates the development of tolerance. Inhibition of NO attenuates the expression of both tolerance and physical dependence.     

Interoceptive stimuli produced by cocaine are blocked during diazepam withdrawal

Rats were trained to select one lever after an intraperitoneal (ip) injection of cocaine (10 mg/kg), and an alternate lever after saline injection under a FR10 schedule of food reinforcement. Following chronic administration of cocaine (20 mg/kg/8hr), but not diazepam (80 mg/kg/8hr) for 7 days, the dose-effect curve for the generalization of cocaine shifted twofold to the right. Subsequently, in the rats receiving chronic diazepam, RO 15-1788, a drug known to precipitate diazepam withdrawal, failed to substitute for cocaine: however, the discriminability of cocaine (5 mg/kg) was reduced from 83% to 13% selection of cocaine-appropriate lever after 72 hr following the last diazepam, injection. These data suggest that chronic administration of cocaine, but not diazepam, produced tolerance to the discriminative stimulus properties of cocaine, and following chronic diazepam administration and withdrawal, the stimulus properties of cocaine are antagonized.     

d-ala2-methionine-enkephalinamide self-administration in the morphine-dependent rat

The self-administration of d-enkephalin was studied in the dependent rat self-administering morphine. The rats were prepared with chronic IV and bilateral intraventricular (IVT) injection cannulae. They were made physically dependent on morphine and trained to lever press for IV morphine self-injections (inj) (10 mg/kg) on a fixed ratio (20) (FR20) schedule of reinforcement. Substitution of d-enkephalin either IVT. (40 g/inj) or IV (10 mg/kg/inj) in the morphinedependent rat maintained consistent lever pressing and self-administration.behavior similar to morphine self-administration. No signs of abstinence were observed during the d-enkephalin substitution. However, saline substitution (0,05 ml/inj IV) for morphine in the self-administering rat produced an abstinence syndrome characterized by extinction of responding, wet-dog shakes, writhes, and diarrhea, which were reversed for 1 h by a single IVT injection of d-enkephalin (40 or 80 g). These results indicate that d-enkephalin will serve as a reinforcer to maintain opiate-seeking behavior and support physical dependence in the rat.     

The role of nitric oxide within the nucleus accumbens on the acquisition and expression of morphine-induced place preference in morphine sensitized rats

In the present study, the effects of intra-accumbal administration of L-arginine, a nitric oxide precursor, and N(G)-nitro-L-arginine methyl-ester (L-NAME), a nitric oxide synthase inhibitor, on the acquisition and expression of morphine-induced place conditioning in morphine-sensitized rats were studied. Subcutaneous (s.c.) administration of morphine (2.5, 5 and 7.5 mg/kg) induced conditioned place preference. Repeated pretreatment of morphine (5 mg/kg, i.p.) followed by 5 days without drug treatment, increased conditioning response induced by morphine (0.25, 0.5 and 0.75 mg/kg). Intra-accumbal (intra-nucleus accumbens; 1 microg/rat) administration of L-arginine (0.3, 1 and 3 microg/rat) significantly increased or reduced the acquisition of morphine place conditioning in non-sensitized and sensitized rats respectively. However, the drug reduced expression of place conditioning by morphine in sensitized animals. Intra-nucleus accumbens injections of L-NAME (0.3, 1 and 3 microg/rat) reduced the acquisition and expression of morphine place conditioning in the sensitized animals. The results indicate that nitric oxide (NO) within the nucleus accumbens is involved in the acquisition and expression of morphine place conditioning in morphine-sensitized rats.     

Pre-exposure to the cannabinoid receptor agonist CP 55940 enhances morphine behavioral sensitization and alters morphine self-administration in Lewis rats

Three experiments examined the influence of pre-exposure to the cannabinoid receptor agonist CP 55940 ((-)-cis-3-(2-hydroxy-4-(1,1-dimethylheptyl)phenyl)-trans-4-(3-hydroxypropyl)cyclohexanol) on the sensitization of morphine-induced locomotor hyperactivity and self-administration in Lewis rats. In Experiment 1, rats received daily injections of vehicle or CP 55940 (0.1 mg/kg for 7 days then 0.2 mg/kg for a further 7 days). Four weeks later, the locomotor response to morphine (10 mg/kg s.c.) was tested once per day over a 3-h period for 14 consecutive days. Rats given morphine showed hypoactivity during the first hour following morphine but hyperactivity during the second and third hours. A progressive increase in hyperactivity to morphine was seen over the 14 days of administration, which was significantly greater in rats pre-treated with CP 55940. In Experiment 2, rats were given morphine (10 mg/kg) once a day for 14 days in combination with either vehicle, CP 55940 (0.1 mg/kg) or the cannabinoid CB(1) receptor antagonist SR 141716 (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride) (3 mg/kg). Both CP 55940 and SR 141716 initially inhibited the hyperactive response to morphine, but these effects gradually wore off and by the end of 14 days, hyperactivity was similar in all morphine-treated groups. When tested 3 weeks later for their response to morphine (10 mg/kg) given alone, rats previously given the morphine/CP 55940 combination, but not the SR 141716/morphine combination, showed a greater locomotor stimulation than those previously exposed to morphine only. In Experiment 3, rats were pre-exposed to CP 55940 or vehicle for 14 days and were subsequently trained to self-administer morphine intravenously (1 mg/kg per lever press) for 14 days. Rats pre-exposed to CP 55940 self-administered a significantly greater number of morphine infusions than vehicle pre-exposed rats. However, both active and inactive ('dummy') lever presses were increased by cannabinoid pre-treatment. Overall, these results suggest that cannabinoid pre-exposure can lead to an exaggeration of morphine-induced hyperactivity and may alter the reinforcing effects of morphine in Lewis rats. The implications for 'gateway' theories of cannabinoid effects in humans are discussed.     

Effect of loperamide,haloperidol and methadone in rats trained to discriminate morphine from saline

In an operant procedure of lever pressing at FR10 schedule of food reinforcement, rats were trained to respond differentially in order to discriminate the effects of morphine (10 mg/kg) injection from those of saline injection. These rats learned to press a lever on one side after morphine injection and a lever on the opposite side after saline injection. In subsequent testing, these rats reliably emitted responses on the morphine lever after 10 or 20 mg/kg of morphine IP, 50 mg/kg of morphine given orally or 2 mg/kg methadone. Two mg/kg of morphine (or 10 or 20 mg/kg given orally) was recognized as saline. In contrast, after either loperamide (an antidiarrheal drug) given in doses up to 10 mg/kg or haloperidol (a neuroleptic) given in doses up to 0.32 mg/kg, all responses were made on the saline lever. Higher doses suppressed responding. Since neither the antidiarrheal activity nor the neuroleptic activity was sufficient to provide the discriminable cue associated with morphine, it is suggested that specific central effects produced only by narcotic analgesics are the basis for these morphine cues.