Mu- and delta-opioid receptor antagonists precipitate similar withdrawal phenomena in butorphanol and morphine dependence

Development of antinociceptive tolerance and physical dependence following morphine i.c.v. infusion in mice

The chronic i.c.v. infusion of morphine has been reported for rats but not for mice. In the current report, the antinociceptive tolerance to both i.c.v. morphine infusion and s.c. implantation of morphine pellets in mice was compared. Physical dependence after i.c.v. morphine infusion was also evaluated. Osmotic minipumps were filled with morphine (50 mM), connected to i.c.v. cannulae, and implanted s.c. to deliver 50 nmol/h for 3 days (i.e., 3.6 micromol total). Robust jumping precipitated by naloxone (1 mg/kg, s.c.) indicated the development of physical dependence. Tolerance to i.c.v., i.t., and i.v. morphine (6.3-, 2.0-, and 4.4-fold, respectively) was observed using the tail flick test. Mice implanted with pellets containing 75 mg morphine for 3 days (i.e., approximately 260 micromol total) were also tolerant to morphine (6.5-, 7.5- and 18-fold, respectively). Thus, the tolerance developed using the two methods was not identical. These results allow comparison of morphine tested by 3 different routes (i.c.v., i.t., and i.v.) after chronic morphine treatment by two routes (i.c.v. and s.c.) in a single study.

Morphine withdrawal precipitated by specific mu, delta or kappa opioid receptor antagonists: a c-Fos protein study in the rat central nervous system

We have recently shown concurrent changes in behavioural responses and c-Fos protein expression in the central nervous system in both naive and morphine-dependent rats after systemic administration of the opioid antagonist naloxone. However, because naloxone acts on the three major types of opioid receptors, the present study aimed at determining, in the same animals, both changes in behaviour and c-Fos-like immunoreactivity after intravenous injection of selective opioid antagonists, such as mu (beta-funaltrexamine, 10 mg/kg), delta (naltrindole, 4 mg/kg) or kappa (nor-binaltorphimine, 5 mg/kg) opioid receptor antagonists, in naive or morphine-dependent rats. In a first experimental series, only beta-funaltrexamine increased c-Fos expression in the eight central nervous system structures examined, whereas no effect was seen after naltrindole or nor-binaltorphimine administration in naive rats. These results suggest a tonic activity in the endogenous opioid peptides acting on mu opioid receptors in normal rats. A second experimental series in morphine-dependent rats showed that beta-funaltrexamine had the highest potency in the induction of classical signs of morphine withdrawal syndrome, as well as the increase in c-Fos expression in the 22 central nervous system structures studied, suggesting a major role of mu opioid receptors in opioid dependence. However, our results also demonstrated that naltrindole and, to a lesser extent, nor-binaltorphimine were able to induce moderate signs of morphine withdrawal and relatively weak c-Fos protein expression in restricted central nervous system structures. Therefore, delta and kappa opioid receptors may also contribute slightly to opioid dependence.

Focal kappa-opioid receptor-mediated dependence and withdrawal in the nucleus paragigantocellularis

The nucleus paragigantocellularis (PGi) has been hypothesized to play an important role in the development of physical dependence on opioids, including the prototype mu-opioid receptor agonist, morphine, and the mixed agonist/antagonist, butorphanol, which shows selective kappa-opioid receptor agonist activity, in rats. In confirmation of previous work, electrical stimulation of the PGi in opioid-nai;ve rats induced stimulus-intensity-related, withdrawal-like behaviors similar to those observed during naloxone-precipitated withdrawal from dependence upon butorphanol. Novel findings were made in rats surgically implanted with cannulae aimed at the lateral ventricle and the right PGi and made physically dependent by intracerebroventricular infusion of either morphine (26 nmol/microl/h) or butorphanol (26 nmol/microl/h) through an osmotic minipump for 3 days. Two hours following termination of the opioid infusion, microinjections of naloxone (11 nmol/400 nl), a nonselective opioid receptor antagonist, or nor-binaltorphimine (nor-BNI) (3.84 nmol/400 nl), a selective kappa-opioid receptor antagonist, were made into the PGi of morphine-dependent and butorphanol-dependent rats. Discrete PGi injections precipitated withdrawal behaviors, with significant (P<.05) increases noted in the incidence of teeth chattering, wet-dog shakes, and scratching. Composite scores for behavioral withdrawal were significantly higher in nor-BNI-precipitated, butorphanol-dependent rats (score=6.8+/-0.6), in naloxone-precipitated, butorphanol-dependent rats (8.9+/-0.8), and in naloxone-precipitated, morphine-dependent rats (11.5+/-0.9) than in all other groups. Both kappa- and mu-opioid receptor mediated dependence can be demonstrated at the level of a discrete medullary site, the PGi, which further supports a specific role for this nucleus in elicitation of behavioral responses during opioid withdrawal.

Lack of mu-opioid receptor leads to an increase in the NMDA receptor subunit mRNA expression and NMDA-induced convulsion

The present study investigated in situ hybridization of N-methyl-D-aspartate (NMDA) receptor (NR) subunit mRNA and convulsion induced by intracerebroventricular injection of NMDA, in order to examine changes in NMDA receptor function in mu-opioid receptor gene knockout mice. Levels of NR1 and NR2A subunit mRNA were significantly increased in the parietal cortex (8.4 and 10.6%, respectively) and hypothalamus (8.7 and 15.2%, respectively) in mu-opioid receptor knockout mice. Levels of NR2B subunit mRNA were noted to be increased in the parietal cortex (9.1%), thalamus (7.7%), and hypothalamus (10.4%) in mu-opioid receptor knockout mice. The ED(50) for NMDA-induced convulsion in wild-type mice was 0.20 microg/10 microl/mouse. The ED(50) in mu-opioid receptor knockout mice was 0.14 microg/10 microl/mouse. There is a significant difference in the potency ratio of wild-type mice versus knockout mice (potency ratio: 1.44, P < 0.05). These results indicate that mu-opioid receptor knockout mice are more sensitive to NMDA-induced convulsion. Therefore, these results suggest that absence of mu-opioid receptor gene is accompanied by changes in the NMDA receptor system which can modulate the synaptic excitability in the process such as convulsion or epilepsy.

A murine model of opioid-induced hyperalgesia

Controversies surround the possible long-term physiological and psychological consequences of opioid use. Analgesic tolerance and addiction are commonly at the center of these controversies, but other concerns exist as well. A growing body of evidence suggests that hyperalgesia caused by the chronic administration of opioids can occur in laboratory animals and in humans. In these studies we describe a murine model of opioid-induced hyperalgesia (OIH). After the treatment of mice for 6 days with implanted morphine pellets followed by their removal, both thermal hyperalgesia and mechanical allodynia were documented. Additional experiments demonstrated that prior morphine treatment also increased formalin-induced licking behavior. These effects were intensified by intermittent abstinence accomplished through administration of naloxone during morphine treatment. Experiments designed to determine if the mu-opioid receptor mediated OLH in our model revealed that the relatively-selective mu-opioid receptor agonist fentanyl induced the thermal hyperalgesia and mechanical allodynia characteristic of OIH when administered in intermittent boluses over 6 days. In complimentary experiments we found that CXBK mice which have reduced mu-opioid receptor binding displayed no significant OIH after morphine treatment. Finally, we explored the pharmacological sensitivities of OIH. We found that the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) and the heme oxygenase (HO) inhibitor tin protoporphyrin (Sn-P) dose-dependently reduced OIH in this model while the NSAID indomethacin had no effect. Thus we have characterized a murine model of OIH which will be useful in the pursuit of the molecular mechanisms underlying this phenomenon.

The role of glutamate in the locus coeruleus during opioid withdrawal and effects of H-7, a protein kinase inhibitor, on the action of glutamate in rats

To investigate the role of glutamate in the locus coeruleus (LC) during opioid withdrawal, rats were continuously infused with morphine (a mu-opioid receptor agonist, 26 nmol/microl/h) or butorphanol (a mu/delta/kappa-mixed opioid receptor agonist, 26 nmol/microl/h) intracerebroventricularly (i.c.v.) via osmotic minipumps for 3 days. A direct LC injection of glutamate (1 or 10 nmol/5 microl) or naloxone (an opioid receptor antagonist, 24 nmol/5 microl) induced withdrawal signs in morphine- or butorphanol-dependent animals. However, these agents failed to precipitate any withdrawal signs in saline-treated control animals. On the other hand, the expression of withdrawal signs precipitated by the administration of glutamate or naloxone in opioid-dependent animals was completely blocked by concomitant infusion with 1 or 10 nmol/microl/h of an inhibitor of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase and protein kinase C, H-7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine]. In animals that had been infused with opioids in the same manner, i.c.v. injection of naloxone (48 nmol/5 microl) precipitated withdrawal signs and increased extracellular fluid levels of glutamate in the LC of morphine- or butorphanol-dependent rats measured by in vivo microdialysis method. However, concomitant infusion with H-7 inhibited the increases of glutamate levels in the LC. These results strongly suggest that an expeditious release of glutamate in the LC region plays an important role in the expression of physical dependence on opioids. Furthermore, the action on glutamate release might be increased by the enhancement of cAMP-dependent protein kinase and/or protein kinase C activity.

The nucleus paragigantocellularis and opioid withdrawal-like behavior

Participation of the nucleus paragigantocellularis (PGi) in mediation of opioid withdrawal was examined in conscious, unrestrained, non-opioid-dependent rats, using electrical stimulation of the PGi. A characteristic series of behaviors, which resembled those seen during naloxone-precipitated withdrawal from dependence on the opioid agonist, butorphanol, was elicited during 30 min of PGi stimulation. Thus, the behavioral syndrome has been termed opioid withdrawal-like. Simultaneous microdialysis measurement of glutamate within the locus ceruleus indicated a positive correlation between extracellular glutamate concentrations and behavioral responses. Behavioral responses were inhibited by 50% during reverse dialysis perfusion of the locus ceruleus with the glutamate receptor antagonist, kynurenic acid, without any effect on glutamate concentrations. Thus, increases in locus ceruleus glutamate partially mediate opioid withdrawal-like behavior. Intracerebroventricular (i.c.v.) injections of the opioid antagonist, naloxone, or of the mu-selective (beta-funaltrexamine) or the delta-selective (naltrindole) opioid antagonists decreased, but did not abolish, stimulation-induced behavioral responses. Similar i.c.v. injections of the kappa-selective antagonist, nor-binaltorphimine, had no effect on behavioral responses to PGi stimulation. Activation of the PGi by electrical stimulation can elicit behaviors similar to those observed during opioid withdrawal. Moreover, additional levels of complexity are evident in the neuropharmacology of PGi stimulation-induced opioid withdrawal-like behavior.

Contribution of glutamatergic systems in locus coeruleus to nucleus paragigantocellularis stimulation-evoked behavior

The role of extracellular glutamate, within the locus coeruleus, in mediation of the behavioral signs elicited by electrical stimulation of the nucleus paragigantocellularis (PGi) was investigated in conscious, opioid-naive rats. Each rat was prepared with a chronically implanted unilateral electrode within the PGi and a microdialysis guide cannula directed at the ipsilateral locus coeruleus. Opioid withdrawal-like behaviors (rearing, teeth-chattering, wet-dog shakes, etc.) and increases in extracellular glutamate concentrations within the locus coeruleus were evoked, in a frequency-dependent (0.5-50 Hz) manner, during PGi stimulation. Reverse dialysis perfusion of the locus coeruleus with the nonspecific glutamate receptor antagonist, kynurenic acid (0.1, 1 mM), reduced the intensity of stimulation-induced behaviors by roughly 50%, but had no effect on the corresponding increases in glutamate concentrations. Perfusion of the locus coeruleus with the glutamate transporter inhibitor, L-trans-pyrrolidine dicarboxylic acid, at 1, but not at 0.1, mM significantly increased glutamate levels in dialysates. Neither concentration of the transporter inhibitor altered the behavioral score. The results indicate that the opioid withdrawal-like behaviors elicited by electrical stimulation of the brainstem at the site of the PGi are positively correlated with locus coeruleus levels of glutamate, and suggest further that the behaviors are partially mediated by release of glutamate within the locus coeruleus or its immediate vicinity.

Electrical stimulation of nucleus paragigantocellularis induces opioid withdrawal-like behaviors in the rat

To examine a role for the medullary nucleus paragigantocellularis (PGi) in mediation of the symptomatology of opioid withdrawal, bilateral electrical stimulation of the PGi was performed in conscious, unrestrained, opioid naive (nondependent) rats. A characteristic series of behaviors was elicited during each 30-min session of PGi stimulation. The profile of these behaviors resembled qualitatively, but was not quantitatively identical with those seen during precipitated withdrawal from opioid dependence. This behavioral syndrome has been termed, opioid withdrawal-like behavior. The opioid withdrawal-like behaviors were voltage-, but not frequency-, dependent. Tolerance to repeated stimulation of the PGi did not develop following a series of 30-min runs of stimulation over 3.5 h. Intracerebroventricular (i.c.v.) injections of the nonselective opioid antagonist, naloxone, significantly decreased (by 40-50%) the intensity of stimulation-induced behavioral responses, as did injections of either the mu-selective (beta-funaltrexamine, beta-FNA) or the delta-selective (naltrindole, NTI) opioid antagonists. In contrast, similar i.c.v. injections of the kappa-selective antagonist, nor-binaltorphimine (nor-BNI), did not block behavioral responses to PGi stimulation. The results indicate that activation of the PGi by electrical stimulation can elicit behaviors similar to those observed during opioid withdrawal. Endogenous opioids, acting through mu- and delta-, but not kappa-opioid receptors, participate in mediating opioid withdrawal-like behaviors induced by PGi stimulation.

Supersensitivity of spinal opioid receptors to antagonists in intrathecal butorphanol and morphine dependence

The purpose of this investigation was to evaluate changes in the sensitivity of spinal opioid receptors to selective antagonists in rats rendered dependent on intrathecal (i.t.) butorphanol and morphine. Using quantitative autoradiography, competitive binding assays with selective opioid antagonists were performed in the spinal cord sections of i.t. butorphanol- and morphine-dependent rats in which withdrawal was precipitated by i.t. naloxone. In butorphanol-dependent rats, the spinal kappa-opioid receptor developed a greater degree of antagonist supersensitivity than the spinal delta- and mu-opioid receptors did. In contrast, the spinal mu-opioid receptor became more sensitive than the delta-opioid receptor in morphine-dependent rats. These results indicate that differential supersensitivity of spinal opioid receptors was induced after chronic i.t. infusions of butorphanol and morphine.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting 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; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Inhibitory effects of diltiazem, an L-type Ca2+ channel blocker, on naloxone-increased glutamate levels in the locus coeruleus of opioid-dependent rats

To investigate the effects of diltiazem, an L-type Ca2+ channel blocker, on naloxone-precipitated withdrawal and elevations of glutamate levels in the locus coeruleus (LC) of opioid-dependent rats, animals were continuously infused with 26 nmol/microliter/h of morphine or butorphanol intracerebroventricularly (i.c.v.) via osmotic minipumps for 3 days. I.c.v. injection of naloxone (an opioid-receptor antagonist, 48 nmol/5 microliters) precipitated withdrawal signs and increased extracellular fluid levels of glutamate in the LC of morphine- or butorphanol-dependent rats measured by in vivo microdialysis. Meanwhile, concomitant infusion of opioids with diltiazem (10 or 100 nmol/microliter/h) inhibited the withdrawal signs and prevented the elevations of glutamate levels in the LC. These results suggest that an expeditious release of glutamate in the LC region regulated by L-type Ca2+ channels mediated system plays a role in the expression of withdrawal signs from opioids.