Time course of the changes in central nitric oxide synthase activity following chronic treatment with morphine in the mouse: reversal by naltrexone

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Venlafaxine inhibits naloxone-precipitated morphine withdrawal symptoms: Role of inflammatory cytokines and nitric oxide

Opioid-induced neuroinflammation plays a role in the development of opioid physical dependence. Moreover, nitric oxide (NO) has been implicated in several oxidative and inflammatory pathologies. Here, we sought to determine whether treatment with venlafaxine during the development of morphine dependence could inhibit naloxone-precipitated withdrawal symptoms. The involvement of neuro-inflammation related cytokines, oxidative stress, and L-arginine (L-arg)-NO pathway in these effects were also investigated. Mice received morphine (50 mg/kg/daily; s.c.), plus venlafaxine (5 and 40 mg/kg, i.p.) once a day for 3 consecutive days. In order to evaluate the possible role of L-arg-NO on the effects caused by venlafaxine, animals received L-arg, L-NAME or aminoguanidine with venlafaxine (40 mg/kg, i.p.) 30 min before each morphine injection for 3 consecutive days. On 4^th day of experiment, behavioral signs of morphine-induced physical dependence were evaluated after i.p. naloxone injection. Then, brain levels of tissue necrosis factor-alpha (TNF-α), interleukin-1-beta (IL-1β), interleukin-6 (IL-6), interleukin-10 (IL-10), brain-derived neurotrophic factor (BDNF), NO and oxidative stress factors including; total thiol, malondialdehyde (MDA) contents and glutathione peroxidase (GPx) activity were determined. Co-administration of venlafaxine (40 mg/kg) with morphine not only inhibited the naloxone-precipitated withdrawal signs including jumping and weight loss, but also reduced the up-regulation of TNF-α, IL-1β, IL-6, NO and MDA contents in mice brain tissue. However, repeated administration of venlafaxine inhibited the decrease in the brain levels of BDNF, total thiol and GPx. Pre-administration of L-NAME and aminoguanidine improved, while L-arg antagonized the venlafaxine-induced effects. These results provide evidences that venlafaxine could be used as a candidate drug to inhibit morphine withdrawal through the involvement of inflammatory cytokines and l-arginine-NO in mice.

Methadone's effects on pentylenetetrazole-induced seizure threshold in mice: NMDA/opioid receptors and nitric oxide signaling

Methadone is a synthetic opioid used to treat opiate withdrawal and addiction. Studies have demonstrated the impact of methadone on seizure susceptibility. This study investigated the modulatory impacts of acute and subchronic (three times daily for 5 days) intraperitoneal methadone treatment on pentylenetetrazole-induced clonic seizure threshold (CST) in mice, as well as the involvement of the nitric oxide, N-methyl-d-aspartate (NMDA), and µ-opioid pathways. Acute administration of different doses of methadone (0.1, 0.3, 1, and 3 mg/kg) 45 min before CST significantly decreased the seizure threshold. Additionally, pretreatment with noneffective doses of an opioid receptor antagonist (naltrexone) and NMDA receptor antagonists (ketamine and MK-801) inhibited methadone's proconvulsive activity in the acute phase, while l-NAME (a nonspecific nitric oxide synthase (NOS) inhibitor) did not affect that activity. In the subchronic phase, methadone (3 mg/kg) demonstrated an anticonvulsive effect. Although subchronic pretreatment with noneffective doses of l-NAME and 7-nitroindazole (a specific neuronal NOS inhibitor) reversed methadone's anticonvulsive activity, aminoguanidine (a specific inducible NOS inhibitor), naltrexone, MK-801, and ketamine did not change methadone's anticonvulsive characteristic. Our results suggest that NMDA and µ-opioid receptors may be involved in methadone's proconvulsive activity in the acute phase, while methadone's anticonvulsive activity may be modulated by neuronal NOS in the subchronic phase.

Thalidomide attenuates development of morphine dependence in mice by inhibiting PI3K/Akt and nitric oxide signaling pathways

Morphine dependence and the subsequent withdrawal syndrome restrict its clinical use in management of chronic pain. The precise mechanism for the development of dependence is still elusive. Thalidomide is a glutamic acid derivative, recently has been reconsidered for its clinical use due to elucidation of different clinical effects. Phosphoinositide 3-kinase (PI3K) is an intracellular transducer enzyme which activates Akt which in turns increases the level of nitric oxide. It is well established that elevated levels of nitric oxide has a pivotal role in the development of morphine dependence. In the present study, we aimed to explore the effect of thalidomide on the development of morphine dependence targeting PI3K/Akt (PKB) and nitric oxide (NO) pathways. Male NMRI mice and human glioblastoma T98G cell line were used to study the effect of thalidomide on morphine dependence. In both models the consequent effect of thalidomide on PI3K/Akt and/or NO signaling in morphine dependence was determined. Thalidomide alone or in combination with PI3K inhibitor, Akt inhibitor or nitric oxide synthase (NOS) inhibitors significantly reduced naloxone induced withdrawal signs in morphine dependent mice. Also, the levels of nitrite in hippocampus of morphine dependent mice were significantly reduced by thalidomide in compared to vehicle treated morphine dependent mice. In T98G human glioblastoma cells, thalidomide alone or in combination with PI3K and Akt inhibitors significantly reduced iNOS expression in comparison to the morphine treated cells. Also, morphine-induced p-Akt was suppressed when T98G cells were pretreated with thalidomide. Our results suggest that morphine induces Akt, which has a crucial role in the induction of NOS activity, leading to morphine dependence. Moreover, these data indicate that thalidomide attenuates the development of morphine dependence in vivo and in vitro by inhibition of PI3K/Akt and nitric oxide signaling pathways.

Hyperbaric oxygen treatment suppresses withdrawal signs in morphine-dependent mice

Hyperbaric oxygen (HBO2) therapy reportedly reduces opiate withdrawal in human subjects. The purpose of this research was to determine whether HBO2 treatment could suppress physical signs of withdrawal in opiate-dependent mice. Male NIH Swiss mice were injected s.c. with morphine sulfate twice a day for 4 days, the daily dose gradually increasing from 50mg/kg on day 1 to 125mg/kg on day 4. On day 5, withdrawal was precipitated by i.p. injection of 5.0mg/kg naloxone. Mice were observed for physical withdrawal signs, including jumping, forepaw tremor, wet-dog shakes, rearing and defecation for 30min. Sixty min prior to the naloxone injection, different groups of mice received either a 30-min or 60-min HBO2 treatment at 3.5atm absolute. HBO2 treatment significantly reduced naloxone-precipitated jumping, forepaw tremor, wet-dog shakes, rearing and defecation. Based on these experimental findings, we concluded that treatment with HBO2 can suppress physical signs of withdrawal syndrome in morphine-dependent mice.

Nitroxidergic modulation of behavioural, cardiovascular and immune responses, and brain NADPH diaphorase activity upon morphine tolerance/dependence in rats

Opioid and non-opioid effects of acute and chronic morphine administration on behaviour, cardiovascular responses, cell proliferation and apoptosis and nitric-oxide synthase (NOS) activity were studied in rats. A novel score-point scale was introduced to quantify the signs of opioid withdrawal syndrome. NOS inhibitor L-NAME (N^G-nitro-L-arginine methyl ester) was applied to reveal the role of NOS/NO pathway in the modulation of morphine-induced in vivo and in vitro responses. The obtained data showed that chronic co-administration of L-NAME drastically attenuated naloxone-precipitated withdrawal syndrome and prevented the development of morphine tolerance to cardiovascular action of morphine. The apoptotic process was very much restricted by L-NAME supplementation of chronic morphine treatment, which resulted in few apoptotic cells, less low molecular weight genomic DNA and preservation of high molecular weight non-fragmented genomic DNA. The study provides new data for nitroxidergic modulation of opioid tolerance and dependence.

Involvement of nitridergic and opioidergic pathways in the antinociception of gabapentin in the orofacial formalin test in mice

BACKGROUND

Pain is one of the most common problems in clinical medicine. There is considerable evidence that pharmacologic approaches are the most widely used therapeutic options to ameliorate persistent or chronic pain. In this study it was evaluated the effect of l-NAME and naltrexone in the antinociception induced by administration of gabapentin in the orofacial formalin test of mice.

METHODS

The algesiometer assay was performed by the administration of 20 μl of 2% formalin solution injected into the upper right lip of each mouse.

RESULTS

The dose of gabapentin that produces the 50% of the maximum possible effect (ED50) was significantly increased by the pretreatment with l-NAME or naltrexone.

CONCLUSIONS

These results suggest that gabapentin produce antinociception partly via the activation nitridergic pathways and opioid system.

Dual effect of cAMP agonist on ameliorative function of PKA inhibitor in morphine-dependent mice

The present study shows interactive effects of bucladesine (db-cAMP) as a cyclic adenosine monophosphate (cAMP) agonist and H-89 as a protein kinase A (PKA) inhibitor on naloxone-induced withdrawal signs in morphine-dependent mice. Animals were treated subcutaneously with morphine thrice daily with doses progressively increased from 50 to 125 mg/kg. A last dose of morphine (50 mg/kg) was administered on the 4th day. Several withdrawal signs were precipitated by intraperitoneal (i.p.) administration of naloxone (5 mg/kg). Different doses of bucladesine (50, 100, 200 nm/mouse) and H-89 (0.05, 0.5, 1, 5 mg/kg) were administered (i.p.) 60 min before naloxone injection. In combination groups, bucladesine was injected 15 min before H-89 injection. Single administration of H-89 (0.5, 1, 5 mg/kg) and bucladesine (50, 100 nm/mouse) significantly attenuated prominent behavioral signs of morphine withdrawal. Lower doses of bucladesine (50, 100 nm/mouse) in combination with H-89 (0.05 mg/kg) increased the inhibitory effects of H-89 on withdrawal signs while in high dose (200 nm/mouse) decreased the ameliorative function of H-89 (0.05 mg/kg) in morphine-dependent animals. It is concluded that H-89 and bucladesine could affect morphine withdrawal syndrome via possible interaction with cyclic nucleotide messengering systems, protein kinase A signaling pathways, and modified related neurotransmitters.

Pioglitazone potentiates development of morphine-dependence in mice: possible role of NO/cGMP pathway

Peroxizome proliferator-activated receptor gamma (PPARγ) is highly expressed in the central nervous system where it modulates numerous gene transcriptions. Nitric oxide synthase (NOS) expression could be modified by simulation of PPARγ which in turn activates nitric oxide (NO)/soluble guanylyl-cyclase (sGC)/cyclic guanosine mono phosphate (cGMP) pathway. It is well known that NO/cGMP pathway possesses pivotal role in the development of opioid dependence and this study is aimed to investigate the effect of PPARγ stimulation on opioid dependence in mice as well as human glioblastoma cell line. Pioglitazone potentiated naloxone-induced withdrawal syndrome in morphine dependent mice in vivo. While selective inhibition of PPARγ, neuronal NOS or GC could reverse the pioglitazone-induced potentiation of morphine withdrawal signs; sildenafil, a phosphodiesterase-5 inhibitor amplified its effect. We also showed that nitrite levels in the hippocampus were significantly elevated in pioglitazone-treated morphine dependent mice. In the human glioblastoma (U87) cell line, rendered dependent to morphine, cAMP levels did not show any alteration after chronic pioglitazone administration while cGMP measurement revealed a significant rise. We were unable to show a significant alteration in neuronal NOS mRNA expressions by pioglitazone in mice hippocampus or U87 cells. Our results suggest that pioglitazone has the ability to enhance morphine-dependence and to augment morphine withdrawal signs. The possible pathway underlying this effect is through activation of NO/GC/cGMP pathway.

Changes in anxiety in abstinence correlate with the state of the nigrostriatal system in the rat hippocampus

Opiate dependence results from impairments of neuronal plasticity, i.e., so-called aberrant neuroplasticity, formation of which involves long-term structural-functional rearrangements persisting even during drug abstinence. Nitric oxide (NO) is involved both in mediating the effects of opiates and in the mechanisms of some types of neuroplasticity, so NO may potentially take part in the development of psychopathological processes on opiate withdrawal. The present study addressed measures of the nitrergic system (nitric oxide synthase (NOS) activity and nitrite and nitrate (NO (x) (-) ) concentrations) in areas of the rat brain; anxiety was also assessed, in terms of behavioral measures in the elevated plus maze, during morphine withdrawal. NOS activity was found to increase by day 3, while the NO (x) (-) concentration was increased by day 6 of withdrawal, these changes being seen only in the hippocampus. At six days after morphine withdrawal, rats showed more entries into the open arms of the elevated plus maze and remained in these arms longer. Correlations were found between measures of the NO system in the hippocampus and the behavior of the animals in the maze. These results suggest that changes in the activity of the nitrergic system in the hippocampus represent one of the molecular mechanisms impairing the behavior of animals in abstinence.

Cross talk between nitric oxide and ERK1/2 signaling pathway in the spinal cord mediates naloxone-precipitated withdrawal in morphine-dependent rats

Our recent study has shown activation of spinal extracellular signal-regulated kinase-1 and -2 (ERK1/2), a member of the mitogen-activated protein kinase (MAPK) family, contributes to naloxone-precipitated withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats. However, the mechanism and significance of the spinal ERK1/2 activation during morphine dependence and withdrawal remain unknown. In this study, we reported that intrathecal (i.t.) pretreatment with either the non-selective nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), neuronal NOS (nNOS) inhibitor 7-nitro indazole (7-NI), or the inducible NOS (iNOS) inhibitor aminoguanidine (AG), could reduce morphine withdrawal-induced increase of phospho-ERK1/2 (pERK1/2) expression in the rat spinal cord. On the other hand, attenuation of the spinal ERK phosphorylation by the MAPK kinase (MEK) inhibitor U0126 also could inhibit the increase of nNOS and iNOS expression in the spinal cord of morphine withdrawal rats. Inhibitory expression of pERK1/2 by i.t. NOS inhibitor L-NAME, 7-NI or AG and of nNOS and iNOS by i.t. U0126 in the spinal cord were accompanied by decreased scores of morphine withdrawal and the inhibited spinal Fos protein (a maker for neuronal excitation or activation) expression induced by morphine withdrawal. These findings suggest cross talk between nitric oxide (NO) and the ERK1/2 signaling pathway mediates morphine withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats.

Alterations of postsynaptic density proteins in the hippocampus of rat offspring from the morphine-addicted mother: Beneficial effect of dextromethorphan

Infants passively exposed to morphine or heroin through their addicted mothers usually develop characteristic withdrawal syndrome of morphine after birth. In such early life, the central nervous system exhibits significant plasticity and can be altered by various prenatal influences, including prenatal morphine exposure. Here we studied the effects of prenatal morphine exposure on postsynaptic density protein 95 (PSD-95), an important cytoskeletal specialization involved in the anchoring of the NMDAR and neuronal nitric oxide synthase (nNOS), of the hippocampal CA1 subregion from young offspring at postnatal day 14 (P14). We also evaluated the therapeutic efficacy of dextromethorphan, a widely used antitussive drug with noncompetitive antagonistic effects on NMDARs, for such offspring. The results revealed that prenatal morphine exposure caused a maximal decrease in PSD-95 expression at P14 followed by an age-dependent improvement. In addition, prenatal morphine exposure reduced not only the expression of nNOS and the phosphorylation of cAMP responsive element-binding protein at serine 133 (CREB(Serine-133)), but also the magnitude of long-term depression (LTD) at P14. Subsequently, the morphine-treated offspring exhibited impaired performance in long-term learning and memory at later ages (P28-29). Prenatal coadministration of dextromethorphan with morphine during pregnancy and throughout lactation could significantly attenuate the adverse effects as described above. Collectively, the study demonstrates that maternal exposure to morphine decreases the magnitude of PSD-95, nNOS, the phosphorylation of CREB(Serine-133), and LTD expression in hippocampal CA1 subregion of young offspring (e.g., P14). Such alterations within the developing brain may play a role for subsequent neurological impairments (e.g., impaired performance of long-term learning and memory). The results raise a possibility that postsynaptic density proteins could serve an important role, at least in part, for the neurobiological pathogenesis in offspring from the morphine-addicted mother and provide tentative therapeutic strategy.

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.

Knockdown of spinal cord postsynaptic density protein-95 prevents the development of morphine tolerance in rats

The activation of spinal cord N-methyl-D-aspartate (NMDA) receptors and subsequent intracellular cascades play a pivotal role in the development of opioid tolerance. Postsynaptic density protein-95 (PSD-95), a molecular scaffolding protein, assembles a specific set of signaling proteins around NMDA receptors at neuronal synapses. The current study investigated the possible involvement of PSD-95 in the development of opioid tolerance. Opioid tolerance was induced by intrathecal injection of morphine sulfate (20 microg/10 microl) twice a day for 4 consecutive days. Co-administration of morphine twice daily and PSD-95 antisense oligodeoxynucleotide (50 microg/10 microl) once daily for 4 days not only markedly reduced the PSD-95 expression and its binding to NMDA receptors in spinal cord but also significantly prevented the development of morphine tolerance. In contrast, co-administration of morphine twice daily and PSD-95 missense oligodeoxynucleotide (50 microg/10 microl) once daily for 4 days did not produce these effects. The PSD-95 antisense oligodeoxynucleotide at the doses we used did not affect baseline response to noxious thermal stimulation or locomotor function. The present study indicates that the deficiency of spinal cord PSD-95 attenuates the development of opioid tolerance. These results suggest that PSD-95 might be involved in the central mechanisms of opioid tolerance and provide a possible new target for prevention of development of opioid tolerance.

Spinal cord nitric oxide synthase and heme oxygenase limit morphine induced analgesia

Spinal cord tissue contains two enzyme systems capable of producing monoxide gases which in turn are linked to the stimulation of soluble guanylate cyclase, nitric oxide synthase (NOS) which produces NO and heme oxygenase (HO) which produces CO. Reports from several laboratories link these two enzyme systems to pain of inflammatory and neuropathic etiologies. Additional studies have demonstrated that the activation of the NOS system by morphine limits the spinal analgesic action of this drug. In this study we first employed the hot plate model of pain to demonstrate that the NOS inhibitor L-NAME and the HO inhibitor Sn-P potentiate the analgesic actions of intrathecally administered morphine while having no intrinsic analgesic action at the doses used. We then determined that L-NAME loses its ability to potentiate morphine in nNOS null-mutant mice, while Sn-P no longer potentiates morphine in mice lacking a functional HO-2 gene. The intrathecal injection of the cGMP analog 8-Br cGMP caused hyperalgesia in the hot plate assay. Focusing on the possible involvement of cGMP metabolism, we documented that morphine stimulates cGMP production in a spinal cord slice model in a concentration dependent and naloxone reversible manner. Both L-NAME and Sn-P were potent inhibitors of morphine-stimulated cGMP production. Buffer containing either CO or the NO donor compound SNAP stimulated cGMP production as well. In spinal cord slices from either nNOS or HO-2 null-mutant animals morphine did not stimulate cGMP production. Taken together our data suggest that spinal monoxide generation modifies the acute analgesic actions of morphine.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Effect of chronic administration of [D-Pen2, D-Pen5] enkephalin on the activity of nitric oxide synthase in brain regions and spinal cord of mice

The effect of multiple intracerebroventricular (i.c.v.) injections of [D-Pen2, D-Pen5]enkephalin (DPDPE), a delta-opioid receptor agonist, on the activity of nitric oxide synthase (NOS) was determined in the brain regions and spinal cord of the mouse. Male Swiss Webster mice were injected twice daily with DPDPE (20 micrograms/mouse, i.c.v.) or its vehicle for 4 days. This procedure has previously been shown to induce tolerance to the antinociceptive actions of DPDPE in mice. On day 5, the animals treated with DPDPE were either sacrificed 20 min after an i.c.v. injection of DPDPE (tolerant) or without any injection (abstinent i.e., 16 h after the last injection of DPDPE). NOS activity in brain regions (cortex, striatum, hippocampus, midbrain, pons/medulla, hypothalamus and cerebellum) and spinal cord was determined by the rate of conversion of arginine into citrulline. Tolerance to DPDPE was associated increases in NOS activity in midbrain (49%) and pons/medulla (32%) and decreases in cerebellum (28%) and spinal cord (44%). However, NOS activity was unchanged in the cortex, corpus striatum, hippocampus and hypothalamus. On the other hand, during abstinence from DPDPE, NOS activity increased in midbrain (84%) and hypothalamus (35%) but decreased in cerebral cortex (27%) cerebellum (27%) and spinal cord (20%). NOS activity was unchanged in the corpus striatum, hippocampus and pons/medulla. Previous studies from this laboratory had demonstrated that chronic administration of mu- and kappa-opioid receptor agonists results in increases NOS activity in certain brain regions and that tolerance to mu- and kappa-, but not to delta-opioid receptor agonists, is attenuated by NOS inhibitors. The present studies, for the first time, demonstrate decreases in NOS activity in certain brain regions and spinal cord of mice treated chronically with delta-opioid receptor agonist. Furthermore, these findings may explain the inability of NOS inhibitors to attenuate tolerance to DPDPE in mice.

Effect of multiple injections of U-50, 488H, a kappa-opioid receptor agonist, on the activity of nitric oxide synthase in brain regions and spinal cord of mice

1. The time course of the effects of multiple injections of U-50,488H, a kappa-opioid receptor agonist, and its subsequent termination on its analgesic action and nitric oxide synthase (NOS) activity was determined in the brain regions and spinal cord of the mouse. 2. Male Swiss-Webster mice were rendered tolerant to U-50,488H by twice-daily injections of the drug (25 mg/kg, IP) for 4 days. Vehicle-injected mice served as controls. 3. In tolerant mice, NOS activity was unchanged in brain regions and the spinal cord after treatment with U-50,488H. During abstinence from U-50,488H, NOS activity was found to be increased in the cortex and remainder of the brain, but no change was noted in the cerebellum, midbrain and spinal cord. 4. These studies demonstrate that withdrawal from the short-term treatment with U-50,488H in mice causes induction of NOS in certain brain regions. However, long-term treatment and withdrawal from U-50,488H are not associated with changes in the central NOS activity and indicate a possible adaptation in the NOS activity.

Effect of chronic administration of morphine, U-50, 488H and [D-Pen2, D-Pen5]enkephalin on the concentration of cGMP in brain regions and spinal cord of the mouse

The effects of chronic administration and subsequent withdrawal of mu-, kappa- and delta-opioid receptor agonists on the levels of cyclic GMP in several brain regions and spinal cord of mice were determined in an attempt to further study the role of NO cascade in opioid actions. The agonists at mu-, kappa- and delta-opioid receptor included morphine, U-50,488H and DPDPE, respectively. Tolerance to morphine was associated with highly significant increases in cGMP levels in corpus striatum (41%), cortex (36%), midbrain (73%) and cerebellum (51%) relative to controls. Abstinence caused increases in cGMP levels in corpus striatum (61%) and pons and medulla (45%). Tolerance to U-50,488H resulted in increases in cGMP levels in midbrain (52%) whereas abstinence from U-50,488H increased the cGMP levels in pons and medulla (76%). Tolerance to DPDPE was associated with increases in cGMP levels in hypothalamus (12%) and pons and medulla (33%) but decreases in cerebellum (66%) and spinal cord (58%). Abstinence from DPDPE produced increases in cGMP levels in pons and medulla (14%) but decreases in cerebellum (67%) and spinal cord (50%). Overall treatment with morphine and U-50,488H produced increases in cGMP levels in brain regions whereas DPDPE produced decreases in brain regions and spinal cord. Previous studies have shown that chronic administration of mu- and kappa-opioid receptor agonists induce NO synthase (NOS) in certain brain regions and that the inhibitors of NO synthase attenuate tolerance to mu- and kappa- but not to delta-opioid receptors agonists. Since activation of NO increases the production of cGMP, the present results demonstrating alterations of cGMP levels by mu-, kappa- and delta-opioid receptor agonists are consistent with the behavioral results with NOS inhibitors on tolerance to mu-, kappa- and delta-opioid receptor agonists.

Effect of 7-nitroindazole on tolerance to morphine, U-50,488H and [D-Pen2, D-Pen5] enkephalin in mice

The effects of 7-nitroindazole (7-NI), an inhibitor of the neuronal nitric oxide synthase (nNOS) which does not increase blood pressure, on tolerance to the antinociceptive activity of mu-(morphine), kappa-(U-50,488H) and delta-([D-Pen2, D-Pen5]enkephalin, DPDPE) opioid receptor agonists were determined in mice. Male Swiss-Webster mice were made tolerant by twice daily injections of morphine (20 mg/kg, s.c.), U-50,488H (25 mg/kg, i.p.) or DPDPE (20 micrograms/mouse, i.c.v.) for 4 days. When tested on day 5, tolerance to their antinociceptive activity was evidenced by decreased response in chronic drug treated mice in comparison to vehicle-injected mice. Concurrent administration of 7-NI (20, 40 or 80 mg/kg, i.p.) with DPDPE did not modify the development of tolerance to the antinociceptive action of DPDPE. However, 7-NI (40 or 80 mg/kg, i.p.) inhibited the development of tolerance to the antinociceptive activity of morphine and U-50,488H but the lower dose of 7-NI (20 mg/kg, i.p.) was not effective. Chronic administration of 7-NI by itself did not modify the acute response to morphine, U-50,488H or DPDPE. It is concluded that a specific inhibitor of nNOS can inhibit tolerance to the antinociceptive activity of mu- and kappa- but not of delta-opioid receptor agonists in mice.