Morphine abstinence is associated with increased brain cyclic AMP

Opioid-Induced Reductions in Amygdala Lateral Paracapsular GABA Neuron Circuit Activity

Opioid use and withdrawal evokes behavioral adaptations such as drug seeking and anxiety, though the underlying neurocircuitry changes are unknown. The basolateral amygdala (BLA) regulates these behaviors through principal neuron activation. Excitatory BLA pyramidal neuron activity is controlled by feedforward inhibition provided, in part, by lateral paracapsular (LPC) GABAergic inhibitory neurons, residing along the BLA/external capsule border. LPC neurons express µ-opioid receptors (MORs) and are potential targets of opioids in the etiology of opioid-use disorders and anxiety-like behaviors. Here, we investigated the effects of opioid exposure on LPC neuron activity using immunohistochemical and electrophysiological approaches. We show that LPC neurons, and other nearby BLA GABA and non-GABA neurons, express MORs and δ-opioid receptors. Additionally, DAMGO, a selective MOR agonist, reduced GABA but not glutamate-mediated spontaneous postsynaptic currents in LPC neurons. Furthermore, in LPC neurons, abstinence from repeated morphine-exposure in vivo (10 mg/kg/day, 5 days, 2 days off) decrease the intrinsic membrane excitability, with a ~75% increase in afterhyperpolarization and ~40-50% enhanced adenylyl cyclase-dependent activity in LPC neurons. These data show that MORs in the BLA are a highly sensitive targets for opioid-induced inhibition and that repeated opioid exposure results in impaired LPC neuron excitability.

Reduced activity of adenylyl cyclase 1 attenuates morphine induced hyperalgesia and inflammatory pain in mice

Opioid tolerance, opioid-induced hyperalgesia during repeated opioid administration, and chronic pain are associated with upregulation of adenylyl cyclase activity. The objective of this study was to test the hypothesis that a reduction in adenylyl cyclase 1 (AC1) activity or expression would attenuate morphine tolerance and hypersensitivity, and inflammatory pain using murine models. To investigate opioid tolerance and opioid-induced hyperalgesia, mice were subjected to twice daily treatments of saline or morphine using either a static (15 mg/kg, 5 days) or an escalating tolerance paradigm (10–40 mg/kg, 4 days). Systemic treatment with an AC1 inhibitor, ST03437 (2.5–10 mg/kg, IP), reduced morphine-induced hyperalgesia in mice. Lumbar intrathecal administration of a viral vector incorporating a short-hairpin RNA targeting Adcy1 reduced morphine-induced hypersensitivity compared to control mice. In contrast, acute morphine antinociception, along with thermal paw withdrawal latencies, motor performance, exploration in an open field test, and burrowing behaviors were not affected by intrathecal Adcy1 knockdown. Knockdown of Adcy1 by intrathecal injection also decreased inflammatory mechanical hyperalgesia and increased burrowing and nesting activity after intraplantar administration of Complete Freund’s Adjuvant (CFA) one-week post-injection.

Opioid use disorder

Opioid use disorder (OUD) is a chronic relapsing disorder that, whilst initially driven by activation of brain reward neurocircuits, increasingly engages anti-reward neurocircuits that drive adverse emotional states and relapse. However, successful recovery is possible with appropriate treatment, although with a persisting propensity to relapse. The individual and public health burdens of OUD are immense; 26.8 million people were estimated to be living with OUD globally in 2016, with >100,000 opioid overdose deaths annually, including >47,000 in the USA in 2017. Well-conducted trials have demonstrated that long-term opioid agonist therapy with methadone and buprenorphine have great efficacy for OUD treatment and can save lives. New forms of the opioid receptor antagonist naltrexone are also being studied. Some frequently used approaches have less scientifically robust evidence but are nevertheless considered important, including community preventive strategies, harm reduction interventions to reduce adverse sequelae from ongoing use and mutual aid groups. Other commonly used approaches, such as detoxification alone, lack scientific evidence. Delivery of effective prevention and treatment responses is often complicated by coexisting comorbidities and inadequate support, as well as by conflicting public and political opinions. Science has a crucial role to play in informing public attitudes and developing fuller evidence to understand OUD and its associated harms, as well as in obtaining the evidence today that will improve the prevention and treatment interventions of tomorrow.

Evaluation of the CART peptide expression in morphine sensitization in male rats

The importance of Cocaine- and amphetamine-regulated transcript (CART) peptide in reinforcing effects of addictive drugs specially alcohol and psychostimulants has been stablished. Involvement of CART peptide in rewarding effects of opioids in brain has recently been reported. Here we have studied the expression of CART mRNA and peptide in the reward pathway in morphine-induced sensitization phenomenon and also evaluated the peptide level fluctuations in CSF and plasma. Male Wistar rats received 7-day morphine injection (20mg/kg) and then after a 7-day washout period, a challenge dose of 10mg/kg morphine was administered and locomotor activity and oral stereotypical behaviors were recorded. Besides, the expression level of CART mRNA and peptide in four important areas of the mesocorticolimbic reward pathway including nucleus accumbens, striatum, prefrontal cortex, and hippocampus were measured by real-time PCR and western blotting, respectively. The level of the peptide in CSF and plasma was measured by Elisa method. The expression level of CART mRNA and protein in brain regions and also the peptide level in CSF and plasma were significantly down-regulated after 7-day morphine administration. These reduced levels returned to nearly normal rates after 7-day wash-out period. Administration of morphine challenge dose led to significant upregulation of CART gene expression (both mRNA and peptide) in the brain, and elevation of peptide level in CSF and plasma in morphine-sensitized rats. It can be concluded that CART is released in the framework of reward pathway and may serve as an important neurotransmitter in the process of morphine dependence and sensitization.

CART peptide and opioid addiction: Expression changes in male rat brain

Previous studies have shown the prominence of cocaine- and amphetamine-regulated transcript (CART) peptide in rewarding and reinforcing effects of drugs of abuse specially psychostimulants. The data regarding the effects of different stages of opioid addiction on CART expression and the interconnection between CART and opioids are not much available. Here we have studied the changes in the expression level of CART mRNA and protein in various parts of the brain reward pathway in different stages of opioid addiction. Groups of male rats received acute low-dose (10mg/kg), acute high-dose (80mg/kg) and chronic escalating doses of morphine. In addition, withdrawal and abstinence states were evaluated after injection of naloxone (1mg/kg) and long-term maintenance of addicted animals, respectively. Expression of CART mRNA in the brain was measured by real-time PCR method. Western blotting was used to quantify the protein level. CART mRNA and protein were both up-regulated in high-dose morphine-administered animals and also in the withdrawal group in the nucleus accumbens (NAc), striatum and prefrontal cortex (PFC). In the addicted group, CART mRNA and protein were both down-regulated in NAc and striatum. In the abstinent group, CART mRNA was down-regulated in NAc. In the hippocampus, the only observed change was the up-regulation of CART mRNA in the withdrawal group. We suggest that the modulatory role of CART peptide in rewarding and reinforcing effects of opioids weakens when opioids are used for a long time and is stimulated when acute stress such as naloxone-induced withdrawal syndrome or acute high-dose administration of morphine occurs to the animal.

Effect of electromagnetic field on cyclic adenosine monophosphate (cAMP) in a human mu-opioid receptor cell model

During the cell communication process, endogenous and exogenous signaling affect normal as well as pathological developmental conditions. Exogenous influences such as extra-low-frequency electromagnetic field (EMF) have been shown to effect pain and inflammation by modulating G-protein receptors, down-regulating cyclooxygenase-2 activity, and affecting the calcium/calmodulin/nitric oxide pathway. Investigators have reported changes in opioid receptors and second messengers, such as cyclic adenosine monophosphate (cAMP), in opiate tolerance and dependence by showing how repeated exposure to morphine decreases adenylate cyclase activity causing cAMP to return to control levels in the tolerant state, and increase above control levels during withdrawal. Resonance responses to biological systems using exogenous EMF signals suggest that frequency response characteristics of the target can determine the EMF biological response. In our past research we found significant down regulation of inflammatory markers tumor necrosis factor alpha (TNF-α) and nuclear factor kappa B (NFκB) using 5 Hz EMF frequency. In this study cAMP was stimulated in Chinese Hamster Ovary (CHO) cells transfected with human mu-opioid receptors, then exposed to 5 Hz EMF, and outcomes were compared with morphine treatment. Results showed a 23% greater inhibition of cAMP-treating cells with EMF than with morphine. In order to test our results for frequency specific effects, we ran identical experiments using 13 Hz EMF, which produced results similar to controls. This study suggests the use of EMF as a complementary or alternative treatment to morphine that could both reduce pain and enhance patient quality of life without the side-effects of opiates.

Molecular Changes in Opioid Addiction: The Role of Adenylyl Cyclase and cAMP/PKA System

For centuries, opiate analgesics have had a considerable presence in the treatment of moderate to severe pain. While effective in providing analgesia, opiates are notorious in exerting many undesirable adverse reactions. The receptor targets and the intracellular effectors of opioids have largely been identified. Furthermore, much of the mechanisms underlying the development of tolerance, dependence, and withdrawal have been delineated. Thus, there is a focus on developing novel compounds or strategies in mitigating or avoiding the development of tolerance, dependence, and withdrawal. This review focuses on the adenylyl cyclase and cyclic adenosine 3,5-monophosphate (cAMP)/protein kinase A (AC/cAMP/PKA) system as the central player in mediating the acute and chronic effects of opioids. This chapter also reviews the neuronal adaptive changes in the locus coeruleus, amygdala, periaqueductal gray, and ventral tegmental area induced by acute and chronic actions of opioid because these neuronal adaptive changes in these regions may underlie the behavioral changes observed in opiate users and abusers.

The combination of mitragynine and morphine prevents the development of morphine tolerance in mice

Mitragynine (MG) is the major active alkaloid found in Mitragyna speciosa Korth. In the present study, we investigated the enhancement of analgesic action of MG when combined with morphine and the effect of the combination on the development of tolerance towards morphine. Mice were administered intraperitoneally with a dose of MG (15 and 25 mg/kg b.wt) combined with morphine (5 mg/kg b.wt) respectively for 9 days. The antinociceptive effect was evaluated by a hot plate test. The protein expression of cyclic adenosine monophosphate (cAMP) and cAMP response element binding (CREB) was analyzed by immunoblot. Toxicological parameters especially liver and kidney function tests were assessed after the combination treatment with MG and morphine. The concurrent administration of MG and morphine showed significant (p < 0.05) increase in latency time when compared to morphine alone group and the outstanding analgesic effects in the combination regimens were maintained until day 9. For the protein expression, there was a significant increment of cAMP and CREB levels (p < 0.05) in group treated with 5 mg/kg morphine but there was no significant change of these protein expressions when MG was combined with morphine. There was a significant changes in toxicological parameters of various treated groups. The combination treatment of MG and morphine effectively reduce the tolerance due to the chronic administration of morphine.

Adenosinergic system: an assorted approach to therapeutics for drug addiction

Adenosine is an endogenous purine nucleoside and it is extensively present in the brain. It exerts several metabolic and neuromodulatory roles in the body. Adenosine also acts as an important messenger molecule for extracellular signaling and shows a homeostatic neuromodulatory function at the synaptic level. Extracellular adenosine exerts a wide variety of biological actions through four cell surface G-protein-coupled receptor subtypes, namely A1, A2A, A2B and A3 adenosine receptors. The extracellular levels of adenosine have been found to be enhanced in several neuropathological conditions, including drug addiction, and thus a neuroprotective role of adenosine was perceived by various experimental studies. The aversive withdrawal symptoms emanating from drug discontinuation provokes rebound drug intake patterns. In addition, alteration of neurotransmitter(s) release and changes in receptor expression contribute to the behavioral changes of drug withdrawal. Furthermore, the abuse of major drugs such as alcohol and opioids are reported to modulate extracellular adenosine levels. In this context, the neuromodulatory functions of adenosine would be valuable if projected to the clinical applications and thus, an increasing attention is currently given to the functional role of adenosine in human addictive disorders. This review will focus on recent clinical and experimental studies that reveal the actions of adenosine and related ligands in drug addiction and various drug-withdrawal syndromes. The evidence and reports provided in this review highlight the looming therapeutic potential of purinergic drugs, with a hope that new therapeutic interventions based on the adenosinergic concept will emerge in the coming years for the management of drug withdrawal syndrome.

Optogenetics: potentials for addiction research

Research on the biology of addiction has advanced significantly over the last 50 years expanding our understanding of the brain mechanisms underlying reward, reinforcement and craving. Novel experimental approaches and techniques have provided an ever increasing armory of tools to dissect behavioral processes, neural networks and molecular mechanisms. The ultimate goal is to reintegrate this knowledge into a coherent, mechanistic framework of addiction to help identify new treatment. This can be greatly facilitated by using tools that allow, with great spatial and temporal specificity, to link molecular changes with altered activation of neural circuits and behavior. Such specificity can now be achieved by using optogenetic tools. Our review describes the general principles of optogenetics and its use to understand the links between neural activity and behavior. We also provide an overview of recent studies using optogenetic tools in addiction and consider some outstanding questions of addiction research that are particularly amenable for optogenetic approaches.

Paradoxical relationship between RAVE (relative activity versus endocytosis) values of several opioid receptor agonists and their liability to cause dependence

AIM

To examine the relationship between the RAVE (relative activity versus endocytosis) values of opiate agonists and their dependence liability by studying several potent analgesics with special profiles in the development of physical and psychological dependence.

METHODS

The effects of (-)-cis-(3R,4S,2'R) ohmefentanyl (F9202), (+)-cis-(3R,4S,2'S) ohmefentanyl (F9204), dihydroetorphine (DHE) and morphine on [(35)S]GTP gamma S binding, forskolin-stimulated cAMP accumulation, and receptor internalization were studied in CHO cells stably expressing HA-tagged mu-opioid receptors (CHO-HA-MOR). cAMP overshoot in response to the withdrawal of these compound treatments was also tested.

RESULTS

All four agonists exhibited the same rank order of activity in stimulation of [(35)S]GTP gamma S binding, inhibition of adenylyl cyclase (AC) and induction of receptor internalization: DHE>F9204>F9202>morphine. Based on these findings and the previous in vivo analgesic data obtained from our and other laboratories, the RAVE values of the four agonists were calculated. The rank order of RAVE values was morphine>F9202>F9204>DHE. For the induction of cAMP overshoot, the rank order was F9202>or=morphine>F9204>or=DHE.

CONCLUSION

Taken in combination with previous findings of these compounds' liability to develop dependence, the present study suggests that the agonist with the highest RAVE value seems to have a relatively greater liability to develop psychological dependence relative to the agonist with the lowest RAVE value. However, the RAVE values of these agonists are not correlated with their probability of developing physical dependence or inducing cAMP overshoot, a cellular hallmark of dependence.

Mu and kappa opioid receptor agonists antagonize icilin-induced wet-dog shaking in rats

Icilin is a cooling agent that precipitates vigorous wet-dog shakes in rats after acute i.p. administration. Recent research has emphasized the peripheral agonist properties (e.g. activation of transient receptor potential channels, TRPM8 and TRPA1) of icilin rather than its unusual and pronounced behavioral effects, often classified as quasi-morphine withdrawal. We tested selective opioid receptor agonists against icilin-induced wet-dog shakes in rats. Shaking was antagonized following s.c. pretreatment with the mu agonists, morphine (1, 2, 3 mg/kg) and buprenorphine (0.10 mg/kg) or the kappa agonists, nalfurafine (0.02, 0.04 mg/kg) and U50,488H (5 mg/kg). Pretreatment with ICI 204,448 (1, 5, 10 mg/kg), the peripherally directed kappa agonist, or the delta agonist, SNC 80 (0.30, 1, 3, 10 mg/kg), had no marked effect on the incidence of shaking. We conclude that (a) icilin can trigger shaking via interactions within the central nervous system and (b) mu and kappa opioid receptors are involved in suppressing this stimulant behavior.

Nuclear factor kappaB signaling in opioid functions and receptor gene expression

Opiates are the most powerful of all known analgesics. The prototype opiate morphine has been used as a painkiller for several thousand years. Chronic usage of opiates not only causes drug tolerance, dependence, and addiction, but also suppresses immune functions and affects cell proliferation and cell survival. The diverse functions of opiates underscore the complexity of opioid receptor signaling. Several downstream signaling effector systems, including adenylyl cyclase, mitogen-activated protein kinase, Ca2+ channels, K+ channels, and phosphatidylinositol 3-kinase/Akt, have been identified to be critical in opioid functions. Nuclear factor-kappaB (NF-kappaB), one of the most diverse and critical transcription factors, is one of the downstream molecules that may either directly or indirectly transmit the receptor-mediated upstream signals to the nucleus, resulting in the regulation of the NF-kappaB-dependent genes, which are critical for the opioid-induced biological responses of neuronal and immune cells. In this minireview, we focus on current understanding of the involvement of NF-kappaB signaling in opioid functions and receptor gene expression in cells.

Pharmacological action of Panax ginseng on the behavioral toxicities induced by psychotropic agents

Morphine-induced analgesia has been shown to be antagonized by ginseng total saponins (GTS), which also inhibit the development of analgesic tolerance to and physical dependence on morphine. GTS is involved in both of these processes by inhibiting morphine-6-dehydrogenase, which catalyzes the synthesis of morphinone from morphine, and by increasing the level of hepatic glutathione, which participates in the toxicity response. Thus, the dual actions of ginseng are associated with the detoxification of morphine. In addition, the inhibitory or facilitated effects of GTS on electrically evoked contractions in guinea pig ileum (mu-receptors) and mouse vas deferens (delta-receptors) are not mediated through opioid receptors, suggesting the involvement of non-opioid mechanisms. GTS also attenuates hyperactivity, reverse tolerance (behavioral sensitization), and conditioned place preference induced by psychotropic agents, such as methamphetamine, cocaine, and morphine. These effects of GTS may be attributed to complex pharmacological actions between dopamine receptors and a serotonergic/adenosine A2A/ delta-opioid receptor complex. Ginsenosides also attenuate the morphine-induced cAMP signaling pathway. Together, the results suggest that GTS may be useful in the prevention and therapy of the behavioral side effects induced by psychotropic agents.

Inhibitory Control of the Acute Mu-Withdrawal Response by Indirectly Activated Adenosine A1 and Kappa-Opioid Systems in the Guinea-Pig Ileum; Reversal by Cholecystokinin

In the isolated guinea-pig ileum (GPI), the acute mu-opioid withdrawal response is inhibited by the kappa-opioid system, indirectly activated by the opioid agonist; yet, other inhibitory mechanisms are probably operating. On the other hand, cholecystokinin (CCK-8) strongly enhances the withdrawal response. In this study, we have shown that the adenosine A1 antagonist 8-cyclopenthyl-1,3-dimethylxantine (CPT) increased the withdrawal response in dermorphin/naloxone (NLX) tests but lacked any effect if the withdrawal tests were carried out in presence of CCK-8. In tissue preparations coming from a same animal both CPT and the kappa-opioid antagonist, nor-binaltorphimine (BNI), increased the intensity of the withdrawal responses; the effects of the two antagonists were additive. The intensity of withdrawal contractile responses in presence of CCK-8 was similar to those obtained in presence of the two antagonists. Tissue preparations tested with dermorphin/CCK-8/NLX and then washed out yielded contractile responses when subsequently challenged with CPT, BNI or BNI+CPT, with a percentage markedly higher than the percentage of the response to NLX challenge. BNI+CPT also increased the intensity of the response to NLX challenge. These data suggest that acute exposure of GPI to dermorphin induces the activation of both the adenosine A1 and kappa-opioid systems, which in turns inhibit the mu-withdrawal response. CCK-8 antagonises the inhibitory effect of the indirectly activated systems.

Opioid abuse and brain gene expression

Opiate addiction is a central nervous system disorder of unknown mechanism. Neuronal basis of positive reinforcement, which is essential to the action of opioids, relies on activation of dopaminergic neurons resulting in an increased dopamine release in the mesolimbic brain structures. Certain aspects of opioid dependence and withdrawal syndrome are also related to the activity of noradrenergic and serotonergic systems, as well as to both excitatory and inhibitory amino acid and peptidergic systems. The latter pathways have been recently proven to be involved both in the development of dependence and in counteracting the states related to relapse. An important role in neurochemical mechanisms of opioid reward, dependence and vulnerability to addiction has been ascribed to endogenous opioid peptides, particularly those acting via the mu- and kappa-opioid receptors. Opiate abuse leads to adaptive reactions in the nervous system which occur at the cellular and molecular levels. Recent research indicates that intracellular mechanisms of signal transmission-from the receptor, through G proteins, cyclic AMP, MAP kinases to transcription factors--also play an important role in opioid tolerance and dependence. The latter link in this chain of reactions may modify synthesis of target genes and in this manner, it may be responsible for opiate-induced long-lasting neural plasticity.

RGS Proteins: New Players in the Field of Opioid Signaling and Tolerance Mechanisms

In this article we review recent advances in our understanding of the crucial role of the Regulator of G protein Signaling (RGS) proteins in opioid signaling mechanisms and opioid tolerance development. Opioids exert their physiologic effects via complex G protein-coupled receptor-signaling mechanisms, and RGS proteins are now known to tightly regulate the G protein signaling cycle. RGS proteins contain GTPase-accelerating protein activity within their characteristic RGS domain and various other receptor signaling-related properties of their other functional domains. There have been more than 20 RGS proteins reported in the literature, and multiple RGS proteins have been shown to negatively regulate G protein-mediated opioid signaling, facilitate opioid receptor desensitization and internalization, and affect the rate at which opioid tolerance develops. Using RGS proteins as targets for future drug therapy aimed at modulating opioid effectiveness in both acute and chronic pain settings may be an important advance in the treatment of pain.

Historical review: Molecular and cellular mechanisms of opiate and cocaine addiction

The National Institute on Drug Abuse was founded in 1974, and since that time there have been significant advances in understanding the processes by which drugs of abuse cause addiction. The initial protein targets for almost all drugs of abuse are now known. Animal models that replicate key features of addiction are available, and these models have made it possible to characterize the brain regions that are important for addiction and other drug effects, such as physical dependence. A large number of drug-induced changes at the molecular and cellular levels have been identified in these brain areas and rapid progress is being made in relating individual changes to specific behavioral abnormalities in animal models of addiction. The current challenges are to translate this increasingly impressive knowledge of the basic neurobiology of addiction to human addicts, and to identify the specific genes that make some individuals either particularly vulnerable or resistant to addiction. In this article, I present a historical review of basic research on opiate and cocaine addiction.

Molecular basis of opioid dependence: role of signal regulation by G-proteins

1. Morphine and opiate narcotics are potent analgesics that have a high propensity to induce tolerance and physical dependence following their repeated administration. 2. The molecular basis of opiate dependence has not been completely elucidated, although the participation of opioid receptors is a prerequisite. Cellular dependence on opioids is believed to result from the chronic stimulation of opioid-regulated signalling networks. 3. As G-protein-coupled receptors, the opioid receptors must rely on heterotrimeric G-proteins for signal transduction. Recent advances in our understanding of G-protein signalling have unveiled novel signalling molecules and mechanisms, some of which may be intricately involved in the manifestation of opiate dependence. 4. In the present review, we will attempt to trace chronic opioid signals along elaborate G-protein-regulated pathways.

Involvement of cyclic AMP systems in morphine physical dependence in mice: prevention of development of morphine dependence by rolipram, a phosphodiesterase 4 inhibitor

In this study, we examined whether morphine dependence was inhibited by rolipram, a cyclic AMP selective phosphodiesterase inhibitor in mice, since a role for the cyclic AMP systems in the development of morphine dependence has been reported. Mice, which received morphine (10 mg kg(-1) s.c.) twice a day for 5 days showed withdrawal syndromes such as jumping, rearing and forepaw tremor following naloxone challenge (5 mg kg(-1) i.p.) on the 6th day. Such mice exhibited a significant elevation of cyclic AMP levels in the thalamus compared to control mice. However, co-administration of rolipram (1 mg kg(-1) i.p.) with morphine for 5 days significantly attenuated the severity of the withdrawal syndrome and the increase in the cyclic AMP levels after the administration of naloxone. In naïve mice, acute morphine treatment (10 mg kg(-1) s.c.) decreased cyclic AMP levels in the thalamus and cerebral cortex 10 min later. The decrease of cyclic AMP levels induced by acute morphine treatment was blocked by co-administration of rolipram (1 mg kg(-1) i.p.). However, acute rolipram did not affect the naloxone-precipitated morphine withdrawal syndrome. These results suggest that the elevation of the cyclic AMP levels is involved in the development of morphine withdrawal syndrome and that blockade of the morphine-induced reduction of cyclic AMP levels by chronic rolipram inhibits the development of dependence and the behavioural and biochemical changes induced by naloxone. Furthermore, rolipram may be a useful drug for attenuating the development of morphine dependence.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

3-Isobutyl-1-methylxanthine inhibits basal mu-opioid receptor phosphorylation and reverses acute morphine tolerance and dependence in mice

Phosphorylation of the mu-opioid receptor may play a role in opioid tolerance and dependence. 3-Isobutyl-1-methylxanthine (IBMX) was found to inhibit basal mu-opioid receptor phosphorylation (IC50 < or = 10 microM) either upon acute treatment or after 8 h pre-treatment in HEK293 cells transfected with the mu-opioid receptor. In mice made acutely tolerant to and dependent on morphine, IBMX (30-100 nmol, i.c.v.) significantly attenuated the naloxone-induced withdrawal jumping and partially reversed morphine antinociceptive tolerance. IBMX also blocked changes to mu-opioid receptor signaling associated with chronic morphine treatment, specifically, the inverse agonist effect elicited by naloxone, in which naloxone paradoxically elevated the cAMP levels in cells previously exposed to morphine for > or = 12 h. These results suggest a new effect of IBMX in inhibiting basal mu-opioid receptor phosphorylation, and provide additional evidence for the involvement of receptor phosphorylation in the development of opioid tolerance and dependence.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Down-regulation of mu-opioid receptor by full but not partial agonists is independent of G protein coupling

In C6 glial cells stably expressing rat mu-opioid receptor, opioid agonist activation is negatively coupled to adenylyl cyclase through pertussis toxin-sensitive G proteins. In membranes, [D-Ala2, N-MePhe4,Gly-ol5]enkephalin (DAMGO) increases guanosine-5'-O-(3-[35S]thio)triphosphate (GTP[gamma-35S]) binding by 367% with an EC50 value of 28 nM. Prolonged exposure to agonists induced desensitization of the receptor as estimated by a reduction in the maximal stimulation of GTP[gamma-35S] binding by DAMGO and rightward shifts in the dose-response curves. In cells treated with 10 microM concentrations of etorphine, DAMGO, beta-endorphin, morphine, and butorphanol, DAMGO-stimulated GTP[gamma-35S] binding was 58%, 149%, 205%, 286%, and 325%, respectively. Guanine nucleotide regulation of agonist binding was correspondingly lower in membranes from tolerant cells. Furthermore, chronic opioid treatment increased forskolin-stimulated adenylyl cyclase activity, and potency of DAMGO to inhibit cAMP accumulation was lower in morphine- and DAMGO-tolerant cells (EC50 = 55 and 170 nM versus 18 nM for control). Chronic treatment with agonists reduced [3H]DAMGO binding in membranes with the rank order of etorphine > DAMGO = beta-endorphin > morphine > butorphanol, and the affinity of DAMGO in alkaloid- but not peptide-treated membranes was significantly lower in comparison with control. Pertussis toxin treatment of the cells before agonist treatment did not prevent the down-regulation by full agonists; DAMGO and etorphine exhibited approximately 80% internalization, whereas the ability of partial agonists was greatly impaired. In addition to establishing this cell line as a good model for further studies on the mechanisms of opioid tolerance, these results indicate important differences in the inactivation pathways of receptor triggered by full and partial agonists.

Effect of adenosine receptor agonists and antagonists on the expression of opiate withdrawal in rats

The effects of the selective A1 adenosine receptor agonist N6-cyclopentyladenosine (CPA) and the selective A2a agonist 2-[p-(2-carboxethyl)phenylethyl-ethylamino]-5'-ethylcarboxamidoade nosine (CGS 21680) (each at 0.03, 0.1 and 0.3 mg/kg, SC) as well as the selective A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), non-selective antagonists 3-isobutyl-1-methylxanthine (IBMX), aminophylline, 3,7-dimethyl-1-propargyl-xanthine (DMPX) and 8(p-sulfophenyl)-theophylline (8-SPT) were investigated (each at 5, 10 and 30 mg/kg, SC) for their ability to alter the naloxone-precipitated opiate withdrawal syndrome in morphine-dependent rats. Effects of CPA and CGS 21680 on opiate withdrawal in the presence of aminophylline were also investigated. Both CPA and CGS 21680, caused a significant reduction in the incidence of body shakes, teeth chatter and paw shakes and decreased the amount of faecal matter produced. DPCPX, IBMX, DMPX, 8-SPT and aminophylline significantly increased the incidence of jumps and decreased the amount of faecal matter produced. The incidence of body shakes was significantly increased by DMPX, 8-SPT and IBMX. Neither CPA nor CGS 21680 were able to reverse the significant increase in the incidence of jumps caused by aminophylline. These data suggest that there is a role for endogenous adenosine in the modulation of the opiate abstinence syndrome and both A1 and A2a adenosine receptors are involved in this phenomenon.

Suppression of naloxone-precipitated withdrawal jumps in morphine-dependent mice by stimulation of prostaglandin EP3 receptor

1. We have shown that intracisternal (i.c.) administration of interleukin-1 beta (IL-1 beta) attenuates naloxone-precipitated withdrawal jumps in morphine-dependent mice, and the effect was partly mediated by the corticotropin-releasing factor. To elucidate further other possible mechanisms involved in the inhibitory effect of IL-1 beta on morphine withdrawal jumping behaviour, in this study, we examined the involvement of the prostaglandin-synthesis pathway, because prostaglandins have been shown to mediate the several central effects of IL-1. Furthermore, we examined the effects of subtype-selective prostaglandin receptor agonists on morphine withdrawal jumping behaviour. 2. Mice were rendered morphine-dependent by subcutaneous implantation of a pellet containing 11.5 +/- 0.3 mg morphine hydrochloride for 48 h. Morphine withdrawal syndromes were precipitated by intraperitoneal (i.p.) injection of naloxone (10 mg kg-1). The degree of physical dependence on morphine was estimated by counting the number of jumps, one of the typical withdrawal signs in mice, for 40 min. 3. The inhibitory effect of IL-1 beta (1 ng/mouse) administered intracisternally 30 min before naloxone (10 mg kg-1, i.p.) was significantly blocked by pretreatment with sodium salicylate (a cyclo-oxygenase inhibitor, 10 ng or 30 ng/mouse) administered intracisternally 15 min before IL-1 beta, while i.c. administration of sodium salicylate alone (3 ng, 10 ng or 30 ng/mouse) followed by i.c. administration of vehicle instead of IL-1 beta did not significantly change the number of jumps precipitated by naloxone. 4. Intracisternal administration of M&B28,767 (an EP3-receptor agonist, 1 fg-30 ng/mouse) and sulprostone (an EP1/EP3-receptor agonist, 10 fg-100 ng/mouse) 30 min before naloxone (10 mg kg,-1 i.p.) attenuated withdrawal jumps with a U-shaped dose-response, reaching a peak at 10 pg/mouse and 100 pg/mouse, respectively. On the other hand, i.c. administration of iloprost (an EP1/IP-receptor agonist, 10 fg-100 ng/mouse), butaprost (an EP2-receptor agonist, 10 fg-100 ng/mouse) or prostaglandin F2 alpha (a FP-receptor agonist, 10 fg-100 ng/mouse) 30 min before naloxone (10 mg kg-1, i.p.) did not significantly change the number of jumps precipitated by naloxone. 5. These results indicate that the prostaglandin-synthesis pathway is, at least in part, involved in the inhibitory effect of IL-1 beta on naloxone-precipitated withdrawal jumps in morphine-dependent mice, and that the prostaglandin synthesized in the brain suppresses the morphine withdrawal jumping behaviour via the EP3-receptor, but not via the EP1-, EP2-, IP- or FP-receptor.

Effect of adenosine analogues on the expression of opiate withdrawal in rats

The aim of this study was to test whether convergent dependence occurs in vivo. The adenosine A1 receptor agonist N6-[(R)-1-methyl-2-phenylethyl]adenosine (R-PIA), the A2 agonist 2-(phenylamino)adenosine (CV-1808), the nonselective A1, A2 agonist (adenosine-5'-ethylcarboxamide (NECA), and the alpha 2-adrenoceptor agonist clonidine were screened (each at 30, 100, and 300 micrograms/kg, SC) for their ability to alter naloxine-precipitated withdrawal signs in morphine-dependent rats. The results indicate that there is convergent dependence involving opioid and adenosine A1 receptors on those effects expressed by withdrawal diarrhoea, paw-shakes, teeth-chattering, body-shakes, and jumping. Further, dependence expressed by body-shakes involves convergence involving A1 receptors, as well as alpha 2-adrenoceptors; while A1 receptors are involved in dependence expressed by jumping, stimulation of alpha 2-adrenoceptors augments this sign. Adenosine analogues may be of clinical value for detoxification of opiate addicts.

Regulation of cyclic AMP by the mu-opioid receptor in human neuroblastoma SH-SY5Y cells

The human neuroblastoma clonal cell line SH-SY5Y expresses both mu- and delta-opioid receptors (ratio approximately 4.5:1). Differentiation with retinoic acid (RA) was previously shown to enhance the inhibition of adenylyl cyclase (AC) by mu-opioid agonists. We tested here the inhibition of cyclic AMP (cAMP) accumulation by morphine under a variety of conditions: after stimulation with prostaglandin E1 (PGE1), forskolin, and vasoactive intestinal peptide (VIP), both in the presence and in the absence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Morphine inhibition of the forskolin cAMP response (approximately 65%) was largely unaffected by the presence of IBMX. In contrast, deletion of IBMX enhanced morphine's inhibition of the PGE1 and VIP cAMP response from approximately 50 to approximately 80%. The use of highly mu- and delta-selective agents confirmed previous results that inhibition of cAMP accumulation by opioids is mostly mu, and not delta, receptor mediated in SH-SY5Y cells, regardless of the presence or absence of IBMX. Because of the large morphine inhibition and the high cAMP levels even in the absence of IBMX, PGE1-stimulated, RA-differentiated SH-SY5Y cells were subsequently used to study narcotic analgesic tolerance and dependence in vitro. Upon pretreatment with morphine over greater than or equal to 12 h, a fourfold shift of the PGE1-morphine dose-response curve was observed, whether or not IBMX was added. However, mu-opioid receptor number and affinity to the mu-selective [D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin were largely unaffected, and Na(+)- and guanyl nucleotide-induced shifts of morphine-[3H]naloxone competition curves were unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

mu-Opioid receptors and not kappa-opioid receptors are coupled to the adenylate cyclase in the cerebellum

The putative regulatory effect of opioids on adenylate cyclase was investigated in two different preparations containing, respectively, two different populations of opioid receptors: the rabbit cerebellum (greater than 75% mu-opioid receptors) and the guinea pig cerebellum (greater than 80% kappa-opioid receptors). In the mu-preparation, but not in the kappa-preparation, opioids inhibited the basal and the forskolin-stimulated adenylate cyclase activity in a dose-dependent manner and stereospecifically. The inhibition was in the 20-30% range, required the presence in the assay medium of Mg2+ and of GTP, but was independent of the presence of Na+. Pharmacological characterization of the inhibitory response in the rabbit cerebellum clearly showed that it was under the control of a mu-opioid binding site, with the effect being elicited by non-selective (etorphine and morphine) and mu-selective (Tyr-D-Ala-Gly-Me-Phe-Gly-ol) agonists, whereas delta- and kappa-selective agonists were almost totally ineffective. ADP ribosylation of inhibitory GTP-binding protein by pertussis toxin failed to block the inhibitory effect of opioids, and data presented suggest that this failure is likely to be the consequence of a limited access of the toxin to its substrate in rabbit cerebellum membranes.

Intrathecal pertussis toxin attenuates the morphine withdrawal syndrome in normal but not in arthritic rats

The effect of intrathecal pertussis toxin on morphine dependence was studied in rats suffering from chronic pain (Freund's adjuvant-induced arthritis). Animals were rendered tolerant-dependent by subcutaneous implantation of 3 pellets of 75 mg morphine base each. In both, normal and arthritic animals, 1 microgram pertussis toxin reduced the analgesia induced by morphine in the tail-flick test. Naloxone (1 mg/kg, s.c.) precipitated a withdrawal syndrome in arthritic animals that was milder in respect to the one produced in normal rats. Pretreatment with pertussis toxin significantly diminished the incidence of withdrawal signs such as jumps, squeak on touch, chattering, ptosis, body shakes and diarrhoea in tolerant-dependent normal rats, while this effect could not be observed in animals suffering from chronic pain. This differential activity of the toxin could be due to the altered tonus of certain neurotransmitter systems that accompanies the chronic situation of pain.

Autoradiographic mapping of a selective cyclic adenosine monophosphate phosphodiesterase in rat brain with the antidepressant [3H]rolipram

Rolipram is a clinically effective antidepressant with selective cAMP phosphodiesterase (PDE) inhibiting properties. (+/-)-[3H]Rolipram binds with high affinity (Kd = 2.52 +/- 0.47 nM) to sections of rat brain (Hill number = 0.90 +/- 0.05). Binding is stereospecific. Association of (+/-) [3H]rolipram to sections is rapid (47% of specific binding in the first minute, kobs = 0.52 min-1). Dissociation of (+/-)-[3H]rolipram exhibits non first order kinetics (3 component model; t1/2 = 2.5 min, 50 min and 6 h, respectively). A number of PDE inhibitors reduce (+/-)-[3H]rolipram binding to the level of nonspecific binding ((-)-rolipram, IC50 = 0.9 nM; (+/-)-rolipram, IC50 = 1.5 nM; Ro 20-1724, IC50 = 11 nM; ICI 63.197, IC50 = 35 nM; medazepam, IC50 = 240 nM; diazepam, IC50 = 1200 nM; IBMX, IC50 = 3800 nM). In vitro autoradiography reveals high binding site densities in the cerebellum, olfactory bulb, lateral septal nucleus, frontal cortex, subiculum and CA1 of hippocampus. Most of the labeled structures are part of the limbic system. In vivo autoradiography of (+/-)-[3H]rolipram binding shows much more nonspecific binding than in vitro, nevertheless the distribution pattern of (+/-)-[3H]rolipram binding sites is similar. A comparison of the distribution pattern of (+/-)-[3H]rolipram binding sites with that of an antidepressant (monoamine oxidase inhibitor, monoamine uptake inhibitor) reveals no overlap. Limited, though significant correlations exist with the distribution of beta 1-adrenergic, adenosine1 and glutamate/quisqualate receptors as well as protein kinase C, but not with beta 2-adrenergic receptors and forskolin binding sites.

Phosphodiesterase inhibitors potentiate opiate-antagonist discrimination by morphine-dependent rats

This study was performed to examine the relevance of the quasi-withdrawal syndrome in nondependent rats to the syndrome precipitated by naltrexone in rats physically dependent upon morphine. Morphine-dependent rats trained to discriminate between SC injections of naltrexone (0.1 mg/kg) and saline were pretreated with 10 mg/kg of a phosphodiesterase inhibitor: 3-isobutyl-1-methylxanthine (IBMX), Ro 20-1724, or papaverine. The naltrexone stimulus-generalization curve and dose-response curve for loss of body weight were shifted to the left by IBMX and Ro 20-1724, which produce quasi-withdrawal, but not by papaverine, which does not. IBMX also potentiated the naltrexone-like discriminative effects and loss of body weight induced by cyclazocine, an opioid agonist-antagonist. Butorphanol, another agonist-antagonist, occasioned choice responding appropriate for saline when tested alone but engendered more than 50% naltrexone-appropriate choice responses in rats pretreated with IBMX. Thus, phosphodiesterase inhibitors that produce an opiate quasi-withdrawal syndrome potentiate interoceptive stimuli and weight loss associated with the withdrawal syndrome precipitated by naltrexone in morphine-dependent rats. Furthermore, they appear to enhance the opiate-antagonist activity of opioids with mixed agonist and antagonist properties.

Kappa-opiate agonists inhibit adenylate cyclase and produce heterologous desensitization in rat spinal cord

The nature of the opiate modulation of adenylate cyclase following acute and chronic agonist exposure has been investigated in rat spinal cord. Using membranes of both adult rat spinal cord and spinal cord-dorsal root ganglion cocultures, we found that kappa-opiate receptors are negatively coupled to adenylate cyclase. The kappa-opiate agonists (e.g., U50488) inhibit significantly and dose-dependently the basal and the forskolin-stimulated cyclase activities, whereas mu and delta agonists are ineffective. The regulatory action is stereospecific and requires the presence of GTP. EGTA treatment of the plasma membranes abolished the effect of kappa-opiate agonists on the basal cyclase activity, and this inhibitory effect could not be restored by subsequent addition of Ca2+. The EGTA treatment did not affect the kappa agonist inhibition of the forskolin-stimulated cyclase. The results also show that following chronic exposure of cultured cells to etorphine or U50488, there is a loss of kappa agonist inhibition of the cyclase. Moreover, this desensitization process appears to be heterologous, because alpha 2-adrenergic agonists (e.g., clonidine or norepinephrine) and the muscarinic agonist (carbachol) exhibited significantly lower potency for inhibiting cyclase activity when compared to untreated cultures. This pattern of heterologous desensitization suggests that chronic exposure to kappa opiates leads to alterations in postreceptor regulatory components, possibly GTP-binding proteins.

Effects of the desensitization by morphine of the opiate-dependent adenylate cyclase system in the rat striatum on the activity of the inhibitory regulatory G protein

Opiates act through a specific receptor to inhibit the striatal adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4,6.1.1] and stimulate a high-affinity GTPase (EC 3.6.1). The present study analyzes the functions of the striatal adenylate cyclase complex following chronic morphine treatment in the rat. The inhibitory effects of GTP on basal adenylate cyclase activity, between 10(-6) and 10(-4) M, were reduced. Moreover, the half-maximal inhibitory concentration of the opiate receptor agonist (D-Ala2-Met5)-enkephalinamide (DAME) on striatal adenylate cyclase activity was increased by about four times, whereas the maximal effect was reduced in membranes from treated rats. In parallel, the half-maximal stimulatory concentration of DAME on GTPase was increased by two times, and the maximal stimulation was reduced from 60 to 25%. Binding studies performed with [3,5-3H]DAME (saturation curves) and with [3H]naloxone (competition curves) did not show any change in opiate receptor numbers and affinity. Moreover, the kinetics of the activation of the inhibitory GTP binding protein (Gi) which transduces the opiate receptor effect on adenylate cyclase showed a small but significant delay. Therefore, hypofunction of Gi can be, at least in part, responsible for the observed desensitization by morphine of the opiate-dependent GTPase and adenylate cyclase.

Incidence of morphine withdrawal and quasi-abstinence syndrome in a model of chronic pain in the rat

The development of tolerance to and dependence on morphine was studied in a model of experimental chronic pain in the rat (Freund's adjuvant-induced arthritis). Animals were rendered tolerant by subcutaneous implantation of 3 pellets of 75 mg morphine base each. Those pain-suffering rats developed tolerance to the analgesic effect of the alkaloid at a slower rate than control animals. Moreover, upon treatment with naloxone, these morphine-tolerant-dependent rats from the model, showed a lower incidence of several withdrawal symptoms, particularly jumping, chattering, ptosis, writhing, body shakes and squeaking on touch. These findings suggest that animals suffering from chronic pain present an altered physiological response to the continuous inhibitory effect of exogenous opioids. These differences do not seem to involve cAMP mediated mechanisms since 3-isobutyl-1-methylxanthine (IBMX) administration did not distinguish between control and arthritic rats.

The effects of long-term treatment of NG108-15 cells with penta- and tetrapeptide enkephalin dimers on opioid receptor binding and cyclic AMP (cAMP) levels

1. The effects of chronic treatment with a dimeric or monomeric penta- or tetrapeptide enkephalin analogue on binding and cyclic AMP (cAMP) accumulation in NG108-15 cells have been studied. 2. When the cells were cultured in the presence of 1 mumol of a pentapeptide analogue (dimer or monomer) for up to 96 hr, binding was reduced by greater than or equal to 90%. 3. In contrast, in the presence of 1 mumol of a tetrapeptide analogue (dimer or monomer), binding was reduced by only less than or equal to 30%. 4. The analogues had varying effects on regulation of cAMP formation. Desensitization, indicated by impaired opioid-mediated inhibition of prostaglandin E1 (PGE1)-stimulated cAMP accumulation, was clearly apparent only for cells pretreated with [D-Ala2,D-Leu5]enkephalin (DADLE), while cells pretreated with [D-Ala2,Leu5-NH-CH2-]2 (DPE2) showed minor impairment. 5. Thus, ligand dimerization appeared to have a modulating effect on regulation of adenylate cyclase activity but not to affect opioid-induced down-regulation.

Opioid inhibition of adenylate cyclase in the striatum and vas deferens of the rat

The activity of adenylate cyclase in striatal membrane-enriched fractions (25,000 g) was inhibited by morphine, beta-endorphin, [D-Ala2-D-Leu5] enkephalin (DADLenk), fentanyl and bremazocine. Whereas guanosine triphosphate (GTP) appeared essential for the expression of this effect, sodium chloride seemed to enhance the degree of inhibition. Dopamine stimulation and sodium fluoride activation of the enzyme was also suppressed by morphine, beta-endorphin and DADLenk. beta-Endorphin and DADLenk inhibited adenylate cyclase activity in vasa deferentia membrane-enriched fractions (25,000 g); both opioids required GTP and NaCl and were inhibited by a delta-opioid receptor antagonist and by naloxone. Morphine, bremazocine and tifluadom did not significantly alter the activity of the vas deferens enzyme. Basal cyclic AMP values of striatal slices were not significantly altered by morphine, beta-endorphin or DADLenk. However, dopamine-induced elevation of cyclic AMP was reduced by morphine and this effect of the opiate was suppressed by naloxone. Only beta-endorphin lowered the basal cyclic AMP values in the vas deferens. The physiological relevance of adenylate cyclase coupling to opioid receptor subtypes is considered.

Cyclic AMP or forskolin rapidly attenuates the depressant effects of opioids on sensory-evoked dorsal-horn responses in mouse spinal cord-ganglion explants

Exposure of fetal mouse spinal cord-ganglion explants to morphine (greater than 0.1 microM) results in naloxone-reversible, dose-dependent depression of sensory-evoked dorsal-horn synaptic-network responses within a few minutes. After chronic opiate exposure (1 microM) for 2-3 days, these dorsal cord responses recover and can then occur even in greater than 10 microM morphine. In the present study, when naive explants were treated with forskolin (10-50 microM)--a selective activate activator of cyclase (AC)--for 10-30 min prior to and during exposure to morphine (0.1-0.3 microM) or D-Ala2-D-Leu5-enkephalin (0.03-0.1 microM), the usual opioid depressant effects on dorsal-horn responses generally failed to occur (10-30 min tests). Dibutyryl cyclic AMP (10 microM) or the more lipid-soluble analog, dioctanoyl cyclic AMP (0.1 mM), produced a similar degree of subsensitivity to opiates as 10 microM forskolin. With high levels of forskolin (50 microM), even concentrations of morphine up to 1-10 microM were far less effective in depressing cord responses. These effects of exogenous cAMP analogs and forskolin on cord-ganglion explants are probably both mediated by increases in intracellular cAMP. The marked decrease in opioid sensitivity of cAMP or forskolin-treated cord-ganglion explants provides significant electrophysiologic data compatible with the hypothesis that neurons may develop tolerance and/or dependence during chronic opioid exposure by a compensatory enhancement of their AC/cAMP system following initial opioid depression of AC activity. Previous evidence relied primarily on behavioral tests and biochemical analyses of cell cultures. It will be of interest to determine if dorsal-horn tissues of cord-ganglion explants do, in fact, develop increased AC/cAMP levels as they express physiologic signs of tolerance during chronic exposure to opioids.

Continuous infusion of naloxone: effects on behavior and oxygen consumption

Twenty-eight hours of endorphin receptor blockade by subcutaneous naloxone infusion produced behavioral and respiratory symptoms resembling opiate abstinence syndrome. Rats were implanted subcutaneously with two Alzet osmotic minipumps delivering 0.7 mg/kg per hour naloxone or with two control minipumps containing distilled water only. They were observed for 10 minutes under blind conditions at 16 and 28 hours post-implantation. The naloxone-infused rats showed significantly more wet dog shakes, abdominal writhes and overall abstinence-like symptoms than did the control rats. These symptoms decreased after 28 hours despite continued naloxone infusion. Acute administration of naloxone failed to produce abstinence-like symptoms, even when combined with the trauma of carrying two implanted water-filled minipumps for 28 hours. In another experiment, naloxone-infused rats showed a highly significant 53.4% elevation of O2 consumption over water-infused control rats in a pure O2 atmosphere at 28 hours after implantation. This difference disappeared at 48 hours post-implantation. In contrast to the effect of naloxone infusion, acute administration of three different doses of naloxone failed to significantly increase O2 consumption.

Effect of salbutamol and the PDE-inhibitor RA 642 on the clonidine withdrawal syndrome in rats

The effect of continuous subcutaneous infusion of clonidine and the influence of concomitant treatment with salbutamol as well as the acute effects of the PDE-inhibitor RA 642 on heart rate and blood pressure of conscious normotensive rats were studied. The severity of the cardiovascular clonidine withdrawal symptoms was positively related to the amount of clonidine infused during treatment. Concomitant infusion of salbutamol (12 mg/kg/day) and clonidine (300 microgram/kg/day) attenuated the clonidine withdrawal tachycardia in conscious normotensive rats. No difference existed in the isoprenaline induced tachycardia in pithed normotensive rats, 8-14 h after cessation of infusion with clonidine (300 microgram/kg/day) and saline, clonidine and salbutamol (12 mg/kg/day), or saline alone. The PDE-inhibitor RA 642 (12 mg/kg, i.p.) aggravated the clonidine withdrawal syndrome in conscious normotensive rats. These data may indicate that long-term treatment with clonidine induces a hyperactivation of the adenylate-cyclase/cAMP-system.

Species differences in behavioural effects of rolipram and other adenosine cyclic 3H, 5H-monophosphate phosphodiesterase inhibitors

The effect of the phosphodiesterase (PDE) inhibitors rolipram, Ro 20-1724 and isobutylmethylxanthine (IBMX) on motor behaviour and rectal temperature was studied in mice, rats and guinea pigs following intraperitoneal administration (0.39 to 25 mg/kg). The selective adenosine cyclic 3',5'-monophosphate (cAMP) PDE inhibitors rolipram and Ro 20-1724 in each species caused a dissimilar pattern of neurotropic effects: Hypothermia and hypokinesia in mice, hypothermia, hypokinesia and head twitches in rats, hypothermia, hyperkinesia and head twitches in guinea pigs. The head twitches were associated with forepaw shaking and increased grooming. Rolipram was the most potent compound in the three species. In guinea pigs it was less active than in rats or mice. Ro 20-1724 was approx. 15 to 30 times less potent in inducing the characteristic alterations in the various species. The alkylxanthine PDE inhibitor IBMX, 0.39 to 6.25 mg/kg, slightly stimulated the locomotor activity of mice and rats, most probably due to antagonism of central adenosine actions. IBMX, 6.25 to 25 mg/kg, caused a pattern of neurotropic effects identical to that produced by the selective cAMP PDE inhibitors, indicating the prevalence of the cAMP PDE inhibitory action over the adenosine antagonistic action at higher dosages. IBMX was approx. as potent as Ro 20-1724 in this respect. The species differences in the neurotropic responses to cAMP PDE inhibition in vivo presumably reflect similar differences in the extent of cAMP accumulation in brain tissue of the three species in vitro. Enhanced availability of brain cAMP in vivo in the various rodent species seems to be correlated with diverse patterns of more or less complex motor behavioural symptoms.

Clonidine prevents methylxanthine stimulation of norepinephrine metabolism in rat brain

Methylxanthines can produce behavior resembling opiate withdrawal in rats. Since previous studies have demonstrated the involvement of central noradrenergic systems during naloxone-precipitated withdrawal, the effects of 3-isobutyl-1-methylxanthine (IBMX) on norepinephrine metabolism in rat brain were studied. It was found that administration of IBMX elevated levels of the major norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) in areas innervated by the locus coeruleus. The increases in MHPG was noted 1 h after administration and was maximal (270% of control) after 3 h. Levels of another norepinephrine metabolite, 3,4-dihydroxyphenylglycol, followed a similar pattern and time course. Coadministration of naloxone with IBMX did not affect the IBMX-induced elevation in MHPG. Administration of the alpha-agonist clonidine, however, antagonized the effects of IBMX on MHPG levels. The effects of IBMX and clonidine were dose dependent; the lowest dose of IBMX needed to elevate MHPG was 30 mumol/kg (i.p.), and clonidine (180 nmol/kg) reduced the effect of IBMX (100 mumol/kg) by 50%. The data, discussed in terms of a methylxanthine-noradrenergic interaction, suggest that withdrawal behaviors in general may be subserved by hyperactive noradrenergic neurons.

Opioid dependence and cross-dependence in the isolated guinea-pig ileum

The development of opioid dependence and tolerance attributed to selective types of opiate receptors was studied in the isolated ileum of guinea pigs chronically exposed to specific opioids. These investigations were based on reports that in this preparation highly tolerant opiate receptors may coexist with opiate receptors of almost unchanged sensitivity. Thus, the ilea were set up in vitro and tested for tolerance and dependence. Apparently precipitation of the withdrawal contracture, indicating dependence, proved a more sensitive parameter than the phenomenon of tolerance. Maximal dependence was determined at rather low degrees of tolerance (5 to 10 fold). The intensity of the withdrawal contracture failed to increase as opiate tolerance did. Furthermore, the experiments failed to present evidence for the existence of selective dependence at specific opiate receptor types. These findings may suggest multiple adaptational mechanisms upon chronic activation of opiate receptors. One mechanism may be responsible for the development of dependence and a low degree of tolerance, whilst a further increase of tolerance may be associated with changes at the opiate binding site level.

Effect of morphine on 3H-thymidine incorporation in the subependyma of the rat: an autoradiographic study

Following morphine treatment, an autoradiographic study investigated the uptake of 3H-thymidine by the subependymal cells in the rat brain. 3H-thymidine was administered subcutaneously to adult, male Sprague-Dawley rats 30 minutes after saline or morphine (19 mg/kg) injection. The animals were sacrified 1 hour after 3H-thymidine administration. In some experiments the opioid antagonist, naloxone, was given alone 45 minutes before 3H-thymidine or 125 minutes before morphine treatment. Three areas of the subependyma were evaluated in terms of the percentage labeled cells and number of grains per nucleus, and a dorsal-to-ventral gradiant was described. Morphine treatment significantly increased the number of 3H-thymidine labeled subependymal cells and number of grains/nucleus within labeled cells. Examination of the distribution of grains/nucleus showed that morphine-treated animals had significantly more cells labeled with 30 or more grains than did saline-injected controls. Prior administration of naloxone blocked the increased 3H-thymidine uptake in morphine-treated animals but had no significant influence on cell proliferation when administered alone. The data are discussed in terms of morphine's possible dual influence on mechanisms which enhance cell transition from G to S phase and/or which accelerate DNA synthesis once these cells have entered the S phase of cell replication.