Long-term exposure to opioid antagonists up-regulates prodynorphin gene expression in rat brain

Susceptibility to Pentylenetetrazole-Induced Seizures in Mice with Distinct Activity of the Endogenous Opioid System

Currently, pharmacotherapy provides successful seizure control in around 70% of patients with epilepsy; however, around 30% of cases are still resistant to available treatment. Therefore, effective anti-epileptic therapy still remains a challenge. In our study, we utilized two mouse lines selected for low (LA) and high (HA) endogenous opioid system activity to investigate the relationship between down- or upregulation of the opioid system and susceptibility to seizures. Pentylenetetrazole (PTZ) is a compound commonly used for kindling of generalized tonic-clonic convulsions in animal models. Our experiments revealed that in the LA mice, PTZ produced seizures of greater intensity and shorter latency than in HA mice. This observation suggests that proper opioid system tone is crucial for preventing the onset of generalized tonic-clonic seizures. Moreover, a combination of an opioid receptor antagonist-naloxone-and a GABA receptor agonist-diazepam (DZP)-facilitates a significant DZP-sparing effect. This is particularly important for the pharmacotherapy of neurological patients, since benzodiazepines display high addiction risk. In conclusion, our study shows a meaningful, protective role of the endogenous opioid system in the prevention of epileptic seizures and that disturbances in that balance may facilitate seizure occurrence.

The Role of Opioid Receptor Antagonists in Regulation of Blood Pressure and T-Cell Activation in Mice Selected for High Analgesia Induced by Swim Stress

Opioid peptides and their G protein-coupled receptors are important regulators within the cardiovascular system, implicated in the modulation of both heart and vascular functions. It is known that naloxone-an opioid antagonist-may exert a hypertensive effect. Recent experimental and clinical evidence supports the important role of inflammatory mechanisms in hypertension. Since opioids may play a role in the regulation of both blood pressure and immune response, we studied these two processes in our model. We aimed to evaluate the effect of selective and non-selective opioid receptor antagonists on blood pressure and T-cell activation in a mouse model of high swim stress-induced analgesia. Blood pressure was measured before and during the infusion of opioid receptor antagonists using a non-invasive tail-cuff measurement system. To assess the activation of T-cells, flow cytometry was used. We discovered that the non-selective antagonism of the opioid system by naloxone caused a significant elevation of blood pressure. The selective antagonism of μ and κ but not δ opioid receptors significantly increased systolic blood pressure. Subsequently, a brief characterization of T-cell subsets was performed. We found that the blockade of μ and δ receptors is associated with the increased expression of CD69 on CD4 T-cells. Moreover, we observed an increase in the central memory CD4 and central memory CD8 T-cell populations after the δ opioid receptor blockade. The antagonism of the μ opioid receptor increased the CD8 effector and central memory T-cell populations.

The standardized Withania somnifera Dunal root extract alters basal and morphine-induced opioid receptor gene expression changes in neuroblastoma cells

Background

Behavioral studies demonstrated that the administration of Withania somnifera Dunal roots extract (WSE), prolongs morphine-elicited analgesia and reduces the development of tolerance to the morphine’s analgesic effect; however, little is known about the underpinning molecular mechanism(s). In order to shed light on this issue in the present paper we explored whether WSE promotes alterations of μ (MOP) and nociceptin (NOP) opioid receptors gene expression in neuroblastoma SH-SY5Y cells.

Methods

A range of WSE concentrations was preliminarily tested to evaluate their effects on cell viability. Subsequently, the effects of 5 h exposure to WSE (0.25, 0.50 and 1.00 mg/ml), applied alone and in combination with morphine or naloxone, on MOP and NOP mRNA levels were investigated.

Results

Data analysis revealed that morphine decreased MOP and NOP receptor gene expression, whereas naloxone elicited their up-regulation. In addition, pre-treatment with naloxone prevented the morphine-elicited gene expression alterations. Interestingly, WSE was able to: a) alter MOP but not NOP gene expression; b) counteract, at its highest concentration, morphine-induced MOP down-regulation, and c) hamper naloxone-induced MOP and NOP up-regulation.

Conclusion

Present in-vitro data disclose novel evidence about the ability of WSE to influence MOP and NOP opioid receptors gene expression in SH-SY5Y cells. Moreover, our findings suggest that the in-vivo modulation of morphine-mediated analgesia by WSE could be related to the hindering of morphine-elicited opioid receptors down-regulation here observed following WSE pre-treatment at its highest concentration.

Opioid antagonists for pain

INTRODUCTION

Opioid receptor antagonists are well known for their ability to attenuate or reverse the effects of opioid agonists. This property has made them useful in mitigating opioid side effects, overdose and abuse. Paradoxically, opioid antagonists have been reported to produce analgesia or enhance analgesia of opioid agonists. The authors review the current state of the clinical use of opioid antagonists as analgesics.

AREAS COVERED

Published clinical trials, case reports and other sources were reviewed to determine the effectiveness and safety of opioid antagonists for use in relieving pain. The results are summarized. Postulated mechanisms for how opioid antagonists might exert an analgesic effect are also briefly summarized.

EXPERT OPINION

Since the comprehensive review by Leavitt in 2009, few new studies on the use of opioid antagonists for pain have been published. The few clinical trials generally consist of small populations. However, there does appear to be a trend of effectiveness of low doses (higher doses antagonize opioid agonist effects). How opioid antagonists can elicit an analgesic effect is still unclear, but a number of possibilities have been suggested. Although the data do not yet support recommendation of widespread application of this off-label use of opioid antagonists, further study appears worthwhile.

Pleiotropic opioid regulation of spinal endomorphin 2 release and its adaptations to opioid withdrawal are sexually dimorphic

We studied adaptations to acute precipitated opioid withdrawal of spinal μ-opioid receptor (MOR)-coupled regulation of the release of endomorphin 2 (EM2). The release of this highly MOR-selective endogenous opioid from opioid-naive spinal tissue of male rats is subjected to MOR-coupled positive as well as negative modulation via cholera toxin-sensitive G(s) and pertussis toxin-sensitive G(i)/G(o), respectively. The net effect of this concomitant bidirectional modulation is inhibitory. MOR-coupled pleiotropic regulation of EM2 release is retained in opioid-withdrawn spinal tissue of male rats, but the balance of MOR-coupled inhibitory and facilitatory regulation shifted such that facilitatory regulation predominates. Augmented coupling of MOR to G(s) is causally associated with this change. Strikingly, pleiotropic characteristics of MOR-coupled regulation of spinal EM2 release and adaptations thereof to opioid withdrawal are male-specific. In females, MOR-coupled regulation of EM2 release from opioid-naive and -withdrawn spinal tissue does not have a significant G(s)-coupled facilitatory component, and MOR-coupled inhibition of EM2 release persists unabated in withdrawn preparations. The male-specific adaptations to chronic morphine that shift the relative predominance of opposing dual G protein-coupled MOR pathways provides a mechanism for mitigating inhibitory MOR signaling without losing MOR-coupled feedback regulation. These adaptations enable using endogenous EM2 as a substitute for morphine that had been precipitously removed. The sexually dimorphic functionality and regulation of spinal EM2/MOR-coupled signaling suggest the clinical utility of using sex-specific treatments for addiction that harness the activity of endogenous opioids.

Effects of prolonged treatment with the opiate tramadol on prodynorphin gene expression in rat CNS

A low abuse liability is reported for tramadol, an analgesic drug centrally acting through either opioid or nonopioid mechanisms. In this paper, we evaluated the effects of the repeated administration (7 d) of different doses of tramadol (10, 20, and 80 mg/kg, intraperitoneally) on the opioid precursor prodynorphin biosynthesis, in comparison with morphine (10 mg/kg, intraperitoneally), in the rat central nervous system (CNS). Northern analysis showed that morphine and tramadol produced different effects. While morphine caused a downregulation of prodynorphin mRNA levels in all investigated areas (hypothalamus, hippocampus, and striatum), tramadol did not cause any significant change in the striatum, and did not decrease prodynorphin biosynthesis in the hypothalamus and in the hippocampus, at nontoxic doses (10 and 20 mg/kg). The highest dose of tramadol (80 mg/kg) decreased prodynorphin mRNA levels in the hypothalamus and the hippocampus but not in the striatum. These data give some information on tramadol effects at molecular level in the CNS. They indicate that the alterations of prodynorphin gene expression caused by tramadol and morphine show a different pattern that may be related to the different abuse potential of the two analgesic drugs.

Alterations in prodynorphin gene expression and dynorphin levels in different brain regions after chronic administration of 14-methoxymetopon and oxycodone-6-oxime

Previous studies showed that opioid drugs-oxycodone-6-oxime and 14-methoxy-5-methyl-dihydromorphinone (14-methoxymetopon)-produced less respiratory depressive effect and slower rate of tolerance and dependence, respectively. It was also reported that morphine decreased the prodynorphin gene expression in the rat hippocampus, striatum and hypothalamus. In this study, we determined the prodynorphin gene expression and dynorphin levels in selected brain regions of opioid tolerant rats. We found that in the striatum morphine decreased, while oxycodone-6-oxime increased and 14-methoxymetopon did not alter the prodynorphin gene expression. In the nucleus accumbens, morphine and oxycodone-6-oxime did not change, while 14-methoxymetopon increased the prodynorphin gene expression. In the hippocampus both oxycodone-6-oxime and 14-methoxymetopon enhanced, whereas morphine did not alter the prodynorphin gene expression. In the rat striatum only oxycodone-6-oxime increased dynorphin levels significantly in accordance with the prodynorphin mRNA changes. In the hippocampus both opioid agonists increased the dynorphin levels significantly similarly to the augmented prodynorphin gene expression. In ventral tegmental area only 14-methoxymetopon increased dynorphin levels significantly. In nucleus accumbens and the temporal-parietal cortex the changes in the prodynorphin gene expression and the dynorphin levels did not correlate. Since the endogenous prodynorphin system may play a modulatory role in the development of opioid tolerance, the elevated supraspinal dynorphin levels appear to be partly responsible for the reduced degree of tolerance induced by the investigated opioids.

Changes of smoking behavior and serum adrenocorticotropic hormone, cortisol, prolactin, and endogenous opioids levels in nicotine dependence after naltrexone treatment

This study was done to evaluate the therapeutic effects of naltrexone on smoking behaviors and to measure the changing of brain substances for elucidating the mode of action by naltrexone. Twenty-five voluntarily participated healthy male smokers were randomly assigned to naltrexone group or placebo group for 2 weeks. In this study, naltrexone group showed significant reduction in daily cigarette consumption amount, the expiratory CO levels, brief questionnaire for smoking urge (B-QSU) score, and FTQ score. However, only 2 subjects in naltrexone group quitted smoking completely at 4th week. Plasma levels of pituitary hormones (ACTH, cortisol, and prolactin) and endogenous opioids (beta-endorphin and dynorphin A) were checked weekly before and after the 'provocation and smoking coupled' stimulus once in a week for 3 weeks. In naltrexone group, pituitary hormones showed upward tendencies even though only the prolactin had statistical significance. However, beta-endorphin and dynorphin A were not significantly different between the two groups. It was suggested that naltrexone made effects on hypothalamo-pituitary-adrenocortical axis activity as well as smoking behavior. However, the meaning of these endocrinal changes by naltrexone is not conclusive, whether it is beneficial or aversive.

Local μ and δ opioid receptors regulate amphetamine-induced behavior and neuropeptide mRNA in the striatum

The purpose of this study was to investigate the role that mu and delta opioid receptor blockade has upon stimulant-induced behavior and neuropeptide gene expression in the striatum. Acute administration of amphetamine (2.5 mg/kg i.p.) caused an increase in behavioral activity and preprodynorphin, substance P, and preproenkephalin mRNA expression. Intrastriatal infusion of the mu opioid antagonist, H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), or the delta opioid antagonist, H-Tyr-Tic[CH(2)NH]-Phe-Phe-OH (TIPPpsi), significantly decreased amphetamine-induced vertical activity. However, only CTAP reduced amphetamine-induced distance traveled. Quantitative in situ hybridization histochemistry revealed that CTAP blocked amphetamine-induced preprodynorphin and substance P mRNA. However, preproenkephalin mRNA levels in the dorsal striatum were increased to the same extent by CTAP, amphetamine, or a combination of the two drugs. In contrast, TIPPpsi significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that both mu and delta receptor subtypes differentially regulate amphetamine-induced behavior and neuropeptide gene expression in the rat striatum.

Opioid receptor blockade in rat nucleus tractus solitarius alters amygdala dynorphin gene expression

It has been suggested that an opioidergic feeding pathway exists between the nucleus of the solitary tract (NTS) and the central nucleus of the amygdala. We studied the following three groups of rats: 1) artificial cerebrospinal fluid (CSF) infused in the NTS, 2) naltrexone (100 microg/day) infused for 13 days in the NTS, and 3) artificial CSF infused in the NTS of rats pair fed to the naltrexone-infused group. Naltrexone administration resulted in a decrease in body weight and food intake. Also, naltrexone infusion increased dynorphin, but not enkephalin, gene expression in the amygdala, independent of the naltrexone-induced reduction in food intake. Gene expression of neuropeptide Y in the arcuate nucleus and neuropeptide Y peptide levels in the paraventricular nucleus did not change because of naltrexone infusion. However, naltrexone induced an increase in serum leptin compared with pair-fed controls. Thus chronic administration of naltrexone in the NTS increased dynorphin gene expression in the amygdala, further supporting an opioidergic feeding pathway between these two brain sites.

The effect of paracetamol on nociception and dynorphin A levels in the rat brain

Male Wistar rats were administered with naloxone (1 mg/kg i.p.) or MR 2266 (5 mg/kg i.p) 15 min before paracetamol (400 mg/kg i.p.) treatment and the pain threshold was evaluated. Rats were subjected to the hot-plate and formalin tests and immunoreactive dynorphin A (ir-dynorphin A) levels were measured in the hypothalamus, hippocampus, striatum, brainstem, frontal and parietal-temporal cortex by radioimmunoassay. Pretreatment with naloxone abolished paracetamol antinociceptive activity both in hot-plate and in the first phase, but not in the second phase of the formalin test, while MR 2266 pretreatment was able to antagonise paracetamol effect either in the hot-plate test or in both phases of the formalin test. Among different brain areas investigated paracetamol significantly decreased ir-dynorphin A levels only in the frontal cortex. MR 2266 but not naloxone reversed the decrease in ir-dynorphin A levels elicited by paracetamol. Paracetamol seems to exert its antinociceptive effect also through the opioidergic system modulating dynorphin release in the central nervous system (CNS) of the rat, as suggested by the decrease in the peptide levels.

A heroin-, but not a cocaine-expecting, self-administration state preferentially alters endogenous brain peptides

The purpose of the current study was to assess neuropeptidergic alterations during a phase of the drug addiction cycle associated with drug craving as compared to a time period when the drug had been recently self-administered. Male Wistar rats were allowed to self-administer cocaine, heroin or saline for 6 h for 5 consecutive days. Immediately following the last self-administration session ('acute drug on board' state), and just before the next scheduled session ('drug expecting' state), the animals were decapitated and the levels of dynorphin A and B, [Met5]- and [Leu5]-enkephalin and substance P were measured in different brain areas. During the 'acute drug on board' state, peptide levels in animals that self-administered heroin or cocaine were not significantly changed. In contrast, during the 'drug expecting' state, heroin-treated animals had increased levels of dynorphin A, dynorphin B and [Met5]-enkephalin in the caudal striatum as compared to the cocaine- and saline-treated animals, and the level of [Leu5]-enkephalin was increased as compared to the cocaine-treated group. In the septum, an increase of [Met5]-enkephalin and substance P was observed in the animals expecting heroin as compared to the saline- and/or cocaine-treated animals. In the caudal striatum, substance P levels were elevated in the heroin- and cocaine-expecting animals. In conclusion, heroin, as compared to cocaine, appears to have a more pronounced effect on dynorphin, enkephalin and substance P levels in the caudal striatum and septum, especially during periods when self-administration of the drug is expected.

Methamphetamine alters prodynorphin gene expression and dynorphin A levels in rat hypothalamus

Chronic administration of morphine or cocaine affects opioid gene expression. To better understand the possible existence of common neuronal pathways shared by different classes of drugs of abuse, we studied the effects of methamphetamine on the gene expression of the opioid precursor prodynorphin and on the levels of peptide dynorphin A in the rat brain. Acute (6 mg/kg, intraperitoneally, i.p.) and chronic (6 mg/kg, i.p. for 15 days) methamphetamine markedly raised prodynorphin mRNA levels in the hypothalamus, whereas no effect was observed in the hippocampus. Dynorphin A levels increased after chronic treatment in the hypothalamus and in the striatum, whereas no significant changes were detected after acute treatment. These results indicate that methamphetamine affects prodynorphin gene expression in the hypothalamus, which may be an important site (also for its relevant neuroendocrine correlates) for opioidergic mechanisms activated by addictive drugs.

Pain tolerance in opioid addicts on and off naltrexone pharmacotherapy: a pilot study

Under certain experimental and clinical conditions, opioid antagonists have been demonstrated to have analgesic properties. In this open-label, nonrandomized, within-subject comparison, the effect of chronic treatment with the antagonist, naltrexone, on tolerance for experimental pain was evaluated in a small sample of male opioid addicts (N = 10) receiving naltrexone maintenance. Cold-pressor pain tolerance was measured during (> or = 6 weeks) and after discontinuation (> or = 1 week) of naltrexone treatment. Intra-subject comparison revealed that eight of the ten subjects were more pain tolerant (median + 20 sec) while receiving naltrexone. It is suggested that either midbrain opioid system upregulation in the presence of naltrexone or underlying individual differences in pain tolerance in persons with addictive disease provide potential explanations for these findings.

Naloxone-induced supersensitivity of oxytocin neurones to opioid antagonists

Here we report that a single administration of naloxone to conscious rats produces no significant increase in oxytocin release, but when repeated 3-4 days later results in a large release of oxytocin. Plasma oxytocin concentrations were measured in conscious and urethane-anaesthetized rats pretreated with naloxone or isotonic saline on Day 1. On Days 2, 3 or 4, a second dose of naloxone was given, producing an increase in oxytocin secretion in naloxone-pretreated groups (P < 0.05 vs. controls) on Day 3 and 4, but not on Day 2. The specificity of the opioid antagonist supersensitivity was determined by injection of the kappa-antagonist nor-binaltorphimine (nor-BNI). Pretreated rats (naloxone, saline or nor-BNI, Day 1) received an additional acute nor-BNI injection (Day 4) which increased plasma oxytocin concentration in the three groups. However, this increase was higher in naloxone-pretreated rats with no differences between the nor-BNI- and saline-pretreated animals. Measurements of electrical activity of single supraoptic nucleus oxytocin neurons and of plasma oxytocin concentration (Day 4) showed that naloxone modestly enhanced the responsiveness of oxytocin neurons to cholecystokinin (CCK) in naloxone-pretreated rats (by comparison with saline-pretreated rats), but had only a small effect on basal firing rate that did not differ between naloxone-pretreated rats and saline-pretreated rats. To investigate whether naloxone-pretreatment modified the effect of morphine on CCK-induced oxytocin release, on Day 4 CCK was injected i.v. with or without morphine. Morphine at a dose of 0.1 mg/kg did not affect CCK-induced oxytocin release, whereas 1 mg/kg of morphine blocked this release in both saline- and naloxone-pretreated rats. The results suggest that naloxone induces opioid antagonist supersensitivity on oxytocin secretion, mainly by up-regulating kappa-opioid mechanisms on oxytocin nerve terminals in the posterior pituitary.

Lactation decreases mRNA levels of opioid peptides in the arcuate nucleus of the rat

The state of lactation results in increased food intake to compensate for the increased energy expenditure to produce nutrients supplied to the offspring. In this study, Sprague-Dawley female rats lactating for 10-16 days, and rats 7 days post-lactation were implanted with osmotic minipumps infusing either naltrexone (NTX) (70 microg/h) or saline (0.9%) over a 48 h period. mRNA levels of pro-dynorphin (proDYN), pro-opiomelanocortin (POMC) and pro-enkephalin (proENK) were measured in the arcuate nucleus (ARC) and whole pituitary of both groups. In both saline- and NTX-treated lactating subjects, food intake was higher than in post-lactating subjects (P < 0.01). In post-lactating subjects, NTX decreased food intake by 27% during the infusion period (P < 0.05). There were no significant differences in body weight between the treatment groups; however, naltrexone decreased body weight gain in both lactating and post-lactating subjects. In both saline and NTX-treated lactating subjects, ARC mRNA levels of proDYN, POMC and proENK were significantly decreased compared with the saline or NTX-treated post-lactating subjects (P < 0.01). NTX did not significantly influence gene expression of opioid peptides in the ARC in either the lactating or the post-lactating subjects. Neither the lactation condition nor NTX administration significantly changed mRNA levels of proDYN, POMC or proENK in whole pituitary. Thus, as has been noted in energy-deprived rats, opioid peptide gene expression is decreased in the ARC of lactating rats, a period during which rats have increased energy requirements.

Dynorphin and epilepsy

Studies on dynorphin involvement in epilepsy are summarised in this review. Electrophysiological, biochemical and pharmacological data support the hypothesis that dynorphin is implicated in specific types of seizures. There is clear evidence that this is true for complex partial (limbic) seizures, i.e. those characteristic of temporal lobe epilepsy, because; (1) dynorphin is highly expressed in various parts of the limbic system, and particularly in the granule cells of the hippocampus; (2) dynorphin appears to be released in the hippocampus (and in other brain areas) during complex partial seizures; (3) released dynorphin inhibits excitatory neurotransmission at multiple synapses in the hippocampus via activation of kappa opioid receptors; (4) kappa opioid receptor agonists are highly effective against limbic seizures. Data on generalised tonic-clonic seizures are less straightforward. Dynorphin release appears to occur after ECS seizures and kappa agonists exert a clear anticonvulsant effect in this model. However, more uncertain biochemical data and lack of efficacy of kappa agonists in other generalised tonic-clonic seizure models argue that the involvement of dynorphin in this seizure type may not be paramount. Finally, an involvement of dynorphin in generalised absence seizures appears unlikely on the basis of available data. This may not be surprising, given the presumed origin of absence seizures in alterations of the thalamo-cortical circuit and the low representation of dynorphin in the thalamus. In conclusion, it may be suggested that dynorphin plays a role as an endogenous anticonvulsant in complex partial seizures and in some cases of tonic-clonic seizures, but most likely not in generalised absence. This pattern of effects may coincide with the antiseizure spectrum of selective kappa agonists.

Chronic intracerebroventricular cocaine differentially affects prodynorphin gene expression in rat hypothalamus and caudate-putamen

We investigated the effects of sustained administration of cocaine on the regulation of prodynorphin gene expression in rat brain. Intracerebroventricular (i.c.v.) infusion of cocaine hydrochloride (30 micrograms/day) for 7 days, by means of osmotic minipumps, elicited a significant 35% decrease of prodynorphin mRNA levels in rat hypothalamus and increase (22%) in caudate-putamen. At the same time and in the same animals, no significant changes were detected in the hippocampus or in the nucleus accumbens. These results indicate that continuously infused cocaine is able to modulate expression of the prodynorphin gene in opposite directions or has no effect on prodynorphin expression, depending on the brain region analysed. Cocaine, as well as opiates, might activate specific neuronal pathways, shared by different classes of drugs of abuse, involving, at least in part, the endogenous opioid system.

Prolonged antagonism of morphine-induced locomotor stimulation by kappa opioid agonists: enhancement by prior morphine exposure

The selective kappa agonists U50488 (10 mg/kg, i.p.) and spiradoline (1 mg/kg, i.p.) attenuated the locomotor activating effects of a morphine challenge (5 mg/kg, i.p.) administered 19 h later in rats. This antagonism of morphine by a kappa agonist was reversed by the selective kappa antagonist, norbinaltorphimine (10 mg/kg, s.c.). Furthermore, the kappa opioid antagonism of morphine was enhanced by prior morphine exposure (2 doses of 30 mg/kg, i.p. administered once a day for 2 days). The present data suggest that kappa-micro opioid interactions may occur over time periods that exceed the acute durations of drug actions.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. 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.