Opioid receptor upregulation and supersensitivity in mice: effect of morphine sensitivity

Behavioral pharmacology of methocinnamox: A potential new treatment for opioid overdose and opioid use disorder

Opioid overdose and opioid use disorder continue to be significant public health challenges despite the availability of effective medications and significant efforts at all levels of society. The emergence of highly potent and efficacious opioids such as fentanyl and its derivatives over the last decade has only exacerbated what was already a substantial problem. Behavioral pharmacology research has proven invaluable for understanding the effects of drugs as well as developing and evaluating pharmacotherapies for disorders involving the central nervous system, including substance abuse disorders. This paper describes a program of research characterizing a potent, selective, and long-lasting mu opioid receptor antagonist, methocinnamox, and evaluating its potential for treating opioid overdose and opioid use disorder. Studies in rodents and nonhuman primates demonstrate that methocinnamox prevents and reverses opioid-induced ventilatory depression and selectively blocks opioid self-administration. This work, taken together with rigorous in vitro and ex vivo studies investigating methocinnamox neuropharmacology, lays a solid foundation for the therapeutic utility of this potentially life-saving medication. Moreover, these studies demonstrate how rigorous behavioral pharmacological studies can be integrated in a broader drug discovery and development research program.

Genetic variation regulates opioid-induced respiratory depression in mice

In the U.S., opioid prescription for treatment of pain nearly quadrupled from 1999 to 2014. The diversion and misuse of prescription opioids along with increased use of drugs like heroin and fentanyl, has led to an epidemic in addiction and overdose deaths. The most common cause of opioid overdose and death is opioid-induced respiratory depression (OIRD), a life-threatening depression in respiratory rate thought to be caused by stimulation of opioid receptors in the inspiratory-generating regions of the brain. Studies in mice have revealed that variation in opiate lethality is associated with strain differences, suggesting that sensitivity to OIRD is genetically determined. We first tested the hypothesis that genetic variation in inbred strains of mice influences the innate variability in opioid-induced responses in respiratory depression, recovery time and survival time. Using the founders of the advanced, high-diversity mouse population, the Diversity Outbred (DO), we found substantial sex and genetic effects on respiratory sensitivity and opiate lethality. We used DO mice treated with morphine to map quantitative trait loci for respiratory depression, recovery time and survival time. Trait mapping and integrative functional genomic analysis in GeneWeaver has allowed us to implicate Galnt11, an N-acetylgalactosaminyltransferase, as a gene that regulates OIRD.

Heroin vaccine: Using titer, affinity, and antinociception as metrics when examining sex and strain differences

Anti-drug vaccines have potential as new interventions against substance use disorder (SUD). However, given the challenges seen with inter-individual variability in SUD vaccine trials to date, new interventions should ensure a robust immune response and safety profile among a diverse population. This requires accounting for sex and heritable genetic differences in response to both abused substances as well as the vaccination itself. To test response variability to our heroin-tetanus toxoid (Her-TT) immunoconjugate vaccine, we vaccinated male and female mice from several mouse strains including Swiss Webster (SW), BALB/c, and Jackson diversity mice (J:DO). Previous studies with vaccinated male SW mice demonstrated a rare hypersensitivity resulting in mice rapidly expiring with exposure to a low dose of heroin. Our results indicate that this response is limited to only male SW mice, and not to any other strain or female SW mice. Our data suggest that this hypersensitivity is not the result of an overactive cytokine or IgE response. Vaccination was similarly effective among the sexes for each strain and against repeated heroin challenge. Inbred BALB/c and J:DO mice were found to have the best vaccine response against heroin in antinociception behavioral assay. These results highlight the importance of incorporating both male and female subjects, along with different strains to mimic diverse human populations, as new SUD vaccines are being tested.

Continuous delivery of naltrexone and nalmefene leads to tolerance in reducing alcohol drinking and to supersensitivity of brain opioid receptors

Opioid antagonist treatments reduce alcohol drinking in rodent models and in alcohol-dependent patients, with variable efficacy across different studies. These treatments may suffer from the development of tolerance and opioid receptor supersensitivity, as suggested by preclinical models showing activation of these processes during and after subchronic high-dose administration of the short-acting opioid antagonist naloxone. In the present study, we compared equipotent low and moderate daily doses of naltrexone and nalmefene, two opioid antagonists in the clinical practice for treatment of alcoholism. The antagonists were given here subcutaneously for 7 days either as daily injections or continuous osmotic minipump-driven infusions to alcohol-preferring AA rats having trained to drink 10% alcohol in a limited access protocol. One day after stopping the antagonist treatment, [³⁵ S]GTPγS autoradiography on brain cryostat sections was carried out to examine the coupling of receptors to G protein activation. The results prove the efficacy of repeated injections over infused opioid antagonists in reducing alcohol drinking. Tolerance to the reducing effect on alcohol drinking and to the enhancement of G protein coupling to μ-opioid receptors in various brain regions were consistently detected only after infused antagonists. Supersensitivity of κ-opioid receptors was seen in the ventral and dorsal striatal regions especially by infused nalmefene. Nalmefene showed no clear agonistic activity in rat brain sections or at human recombinant κ-opioid receptors. The findings support the as-needed dosing practice, rather than the standard continual dosing, in the treatment of alcoholism with opioid receptor antagonists.

Naltrexone extended-release injection: an option for the management of opioid abuse

The United States Food and Drug Administration (FDA) approved naltrexone, a synthetic competitive antagonist at opioid receptors, in oral form in 1984 for use in the management of opioid abuse and addiction. Because naltrexone and its major metabolite, 6-β-naltrexone, are both competitive antagonists at opioid receptors - and thereby inhibit opioid agonist-induced effects including those desired by abusers - it was hypothesized that once maintained on naltrex-one, opioid-induced desirable effects would be diminished to the point that relapse to illicit use would decline because it was no longer rewarding. However, good medication compliance is a requisite for such a strategy to be effective and a systematic review of oral naltrexone concluded that this method of treatment was not superior for any outcomes measured (ie, retention, abstinence, or side effects) to placebo, psychotherapy, benzodiazepines, or buprenorphine treatment. In addition, the retention rate on oral naltrexone was very low (less than 30%). Recently, the FDA approved an extended-release formulation (intramuscular depot injection) of naltrexone for prevention of relapse to opioid dependence following opioid detoxification and to be used along with counseling and social support. Since it needs to be administered only monthly, as opposed to the daily administration required for the oral formulation, naltrexone injection has the potential for increasing adherence and retention rates. Concerns include liver damage at high doses (oral formulation) and possible opioid overdose if an attempt is made to surmount receptor antagonism by taking higher doses of an opioid agonist or if opioid receptors become "sensitized" under long-term antagonism. The focus of the present review is the current information regarding the safety and efficacy of naltrexone extended-release therapy.

Mu-opioid receptor up-regulation and functional supersensitivity are independent of antagonist efficacy

Chronic opioid antagonist treatment up-regulates opioid receptors and produces functional supersensitivity. Although opioid antagonists vary from neutral to inverse, the role of antagonist efficacy in mediating the chronic effects of opioid antagonists is not known. In this study, the effects of two putative inverse agonists (naltrexone, naloxone) and a putative neutral antagonist (6beta-naltrexol) were examined. Initially, peak effect (40 min, naltrexone and naloxone; 70 min, 6beta-naltrexol) and relative potency to antagonize morphine analgesia were determined (relative potencies = 1, 2, and 16, 6beta-naltrexol, naloxone, and naltrexone, respectively). Next, mice were infused for 7 days with naloxone (0.1-10 mg/kg/day), naltrexone (10 or 15 mg s.c. pellet), or 6beta-naltrexol (0.2-20 mg/kg/day), and spinal micro-opioid receptor density was examined, or morphine analgesia dose-response studies were conducted. All antagonists up-regulated mu-opioid receptors (60-122%) and induced supersensitivity (1.8-2.0-fold increase in morphine potency). There were no differences in antagonist potency to produce up-regulation or supersensitivity. These data suggest that opioid antagonist-induced mu-opioid receptor up-regulation and supersensitivity require occupancy of the receptor and that antagonist efficacy is not critical. Finally, the ED(50) to precipitate withdrawal jumping was examined in morphine-dependent mice. Naltrexone, naloxone, and 6beta-naltrexol produced withdrawal jumping, although potencies relative to 6beta-naltrexol were 211, 96, and 1, respectively. Thus, antagonist potency to precipitate opioid withdrawal was related to inverse agonist efficacy. Overall, the estimated relative potency of the opioid antagonists was a function of the outcome measured, and inverse agonist activity was not required for mu-opioid receptor up-regulation and supersensitivity.

Regulation of mu-opioid receptors, G-protein-coupled receptor kinases and beta-arrestin 2 in the rat brain after chronic opioid receptor antagonism

The aim of this study was to analyse the biochemical and behavioural consequences of chronic treatment with opioid receptor antagonists in rats. We have evaluated the respiratory depressant and antinociceptive effects of the mu-opioid agonist sufentanil, the density of brain mu-opioid receptors, and the expression of G-protein-coupled receptor kinases and beta-arrestin 2 in cerebral cortex and striatum, following sustained opioid receptor blockade. Our results demonstrate that 24 h after interruption of 7 days chronic infusion of naltrexone (120 microg/h), the respiratory depressant potency of the mu-opioid receptor agonist sufentanil was increased to a similar extent as the antinociceptive potency (about three-fold). This was accompanied by mu-opioid receptor up-regulation in several areas of the rat brain associated with opioid control of pain perception and breathing. Moreover, chronic treatment with either naltrexone (120 microg/h) or naloxone (120 microg/h) caused significant increases in the expression levels of G-protein-coupled receptor kinases types 2, 3, and 6, and of beta-arrestin 2 in brain cortex and striatum. Together our data suggest an increased constitutive receptor activity secondary to mu-opioid receptor up-regulation following chronic antagonist treatment.

Sex differences in thermal nociception and morphine antinociception in rodents depend on genotype

It has been appreciated for some time that the sexes can differ in their sensitivity to pain and its inhibition. Both the human and rodent literatures remain quite contentious, with many investigators failing to observe sex differences that others document clearly. Recent data from our laboratory have pointed to an interaction between sex and genotype in rodents, such that sex differences are observed in some strains but not others. However, these studies employed inbred mouse strains and are thus not directly relevant to existing data. We presently examined whether the observation of statistically significant sex differences in nociception and morphine antinociception might depend on the particular outbred rodent population chosen for study. Rats of both sexes and three common outbred strains were obtained from three suppliers (Long Evans, Simonsen; Sprague Dawley, Harlan; Wistar Kyoto, Taconic) and tested for nociceptive sensitivity on the 49 degrees C tail-withdrawal assay, and antinociception following morphine (1-10mg/kg, i.p.). In further studies, three outbred populations of mice (CD-1, Harlan; Swiss Webster, Harlan; Swiss Webster, Simonsen) were bred in our vivarium for several generations and tested for tail-withdrawal sensitivity and morphine antinociception (1-20male, and no significant difference. In a separate study in which the estrous cycle was tracked in female mice, we found evidence for an interaction between genotype and estrous phase relevant to morphine antinociception. However, estrous cyclicity did not explain the observed sex differences. These data are discussed with respect to the existing sex difference and pain literature, and also as they pertain to future investigations of these phenomena.

Analgesic responses to intrathecal morphine in relation to CSF concentrations of morphine-3,beta-glucuronide and morphine-6,beta-glucuronide

This study was performed to determine whether variations in analgesic responses to intrathecal morphine could be explained by cerebrospinal fluid (CSF) concentrations of morphine metabolites. Twenty-four CSF samples were collected at the beginning, middle and end of treatment periods in seven cancer patients with pain of malignant origin. CSF concentrations of morphine-3,beta-glucuronide (M3G) and morphine-6,beta-glucuronide (M6G) metabolites were measured by gas chromatography/mass spectrometry. Analgesic responses to morphine were estimated concurrent with CSF collection using a visual analog scale representing percentages of pain relief. Effective analgesia was defined as > or = 75% pain relief. CSF concentration of M3G and M6G in the 24 samples were 722 +/- 116 ng/ml and 699 +/- 158 ng/ml, respectively. CSF samples were categorized into two groups: (1) those collected during effective analgesia (N=14), and (2) those collected during ineffective analgesia (N=10). M6G levels detected in group 1 samples (effective analgesia) were significantly greater than those found in group 2 samples (ineffective analgesia) (978 +/- 243 ng/ml vs 309 +/- 68 ng/ml, P<0.05). Intergroup differences in CSF M3G concentrations and M3G/M6G ratios were not significant. It is concluded that CSF M6G may be indicative of effectiveness of analgesia in cancer patients subjected to intrathecal morphine.

Chronic morphine and naltrexone fail to modify mu-opioid receptor mRNA levels in the rat brain

Previous radioligand-binding studies have reported conflicting results concerning the effect of chronic morphine administration on the regulation of mu-opioid receptor (MOR) density. On the other hand, chronic administration of an opioid antagonist, such as naltrexone, has been shown to increase the density of the MOR. In order to determine if the changes in the MOR are associated with alterations in receptor mRNA levels, we investigated MOR gene expression following chronic treatment with morphine and/or naltrexone. MOR mRNA levels, determined by the ribonuclease protection assay (RPA), were unchanged with respect to control during chronic morphine treatment and morphine withdrawal in each of the analysed brain areas. Furthermore, chronic administration of naltrexone did not result in changes of MOR mRNA levels in rat striatum of naive and morphine-dependent rats, suggesting that the up-regulation of the MOR density, at least in this tissue, is not regulated at transcriptional level.

Magnitude of tolerance to fentanyl is independent of mu-opioid receptor density

The effect of a mu-opioid receptor irreversible antagonist on the development of tolerance to fentanyl was determined in mice. Mice were injected with saline or clocinnamox (3.2 mg/kg, i.p.) and 4 h later mice implanted s.c. with a placebo pellet or an osmotic minipump that infused fentanyl (0.165 mg/kg per day) for 3 days. Fentanyl pumps and placebo pellets were removed on the third day following implantation and 4 h later mu-opioid receptor saturation binding studies in whole brain ([3H][D-Ala2,MePhe4,Gly-ol5]enkephalin: DAMGO) or fentanyl analgesic dose-response studies (tailflick assay) were conducted. Fentanyl infusions and clocinnamox both significantly reduced the potency of fentanyl by 2.8- and 2.4-fold, respectively. When fentanyl and clocinnamox were administered together, a significant 5.0-fold reduction in fentanyl potency relative to the saline-placebo group was observed, which represents an additive effect of clocinnamox and fentanyl. The ED50 of fentanyl in clocinnamox-treated mice was shifted 2.1-fold by fentanyl infusion relative to the clocinnamox-placebo group. This is comparable to the 2.8-fold shift in the ED50 produced by fentanyl infusion in saline-treated mice. In binding studies, fentanyl produced a small (-9%) reduction in Bmax, while clocinnamox significantly reduced (-41%) mu-opioid receptor density without altering affinity (Kd). In the clocinnamox-fentanyl group, there was a 50% reduction in Bmax, which is similar to the additive effect observed in analgesia studies. These data indicate that changes in mu-opioid receptor density prior to the development of tolerance to fentanyl do not impact on the magnitude of tolerance.

The genetics of pain and pain inhibition

The present review summarizes the current state of knowledge about the genetics of pain-related phenomena and illustrates the scope and power of genetic approaches to the study of pain. We focus on work performed in our laboratories in Jastrzebiec, Poland; Portland, OR; and Los Angeles, which we feel demonstrates the continuing usefulness of classical genetic approaches, especially when used in combination with newly available molecular genetic techniques.

Morphine analgesia and tolerance in mice selectively bred for divergent swim stress-induced analgesia

Morphine-induced analgesia and tolerance were examined in Swiss Webster mice selectively bred for high and low swim stress-induced analgesia. Morphine produced a dose-dependent analgesia in both lines; it was 4-fold more potent in the high analgesia line than in the low analgesia line. Despite the differences in morphine-induced analgesia, the degree of tolerance was the same in both lines. Together, these data suggest that selective breeding of mice for high and low swim stress-induced analgesia produced a striking difference in morphine-induced analgesia without affecting the degree of tolerance. Thus, while there is a common genetic determination in swim stress-induced and morphine-induced analgesia, the development of tolerance to morphine possibly relies on a different genetic background.

Naloxone ameliorates the learning deficit induced by pentylenetetrazol kindling in rats

Endogenous opioid peptides modulate and regulate processes of central excitability. Furthermore, opioids are thought to interfere with processes of learning and memory storage. In order to study the effects of endogenous opioids on both processes we injected in the course of development of pentylenetetrazol kindling the opiate receptor antagonist naloxone, and tested the animals afterwards in a shuttle-box task. It was found that naloxone pretreatment had dissociative effects. There was no effect on seizure outcome, whereas the learning deficit was ameliorated in the kindled group. The data suggest that endogenous opioid peptides contribute to the learning deficit found in pentylenetetrazol-kindled rats.

Impaired supersensitivity to morphine following chronic naltrexone treatment in senescent rats

Chronic administration of opiate antagonists produces an increase in the density of opiate receptors, as well as an enhanced sensitivity to the analgesic and locomotor depressant effects of morphine. The present study assessed whether aging alters these regulatory processes. Young (3-4 months), middle-aged (10-11 months), and senescent (25-30 months) rats were implanted subdermally with slow-release naltrexone pellets or were given sham surgery. The pellets were removed 10 days later. Twenty-four hours after pellet removal, morphine-induced (5 mg/kg, SC) analgesia and locomotor activity were assessed. Young and middle-aged rats treated with naltrexone showed enhanced sensitivity to the analgesic and locomotor activity depressant effects of morphine relative to age-matched controls. In contrast, senescent rats treated with naltrexone did not differ from age-matched controls in their response to morphine. The density of opiate receptors labeled with 3H-naloxone was measured in the anterior striatum. Both young and senescent rats treated with naltrexone exhibited an increase in opiate receptor density relative to age-matched controls. The results indicate that senescent rats are capable of up-regulating opiate receptors following chronic naltrexone treatment but do not exhibit the corresponding functional supersensitivity to morphine.

Evidence for an opioid inhibitory effect on T cell proliferation

The proliferative response of human or rat T lymphocytes to phytohemagglutinin (PHA) or concanavalin A (ConA) was measured after acute (30 min) or chronic (8 days) treatment with the opiate receptor antagonists naloxone or naltrexone. Both in the rat and in the human, proliferation was significantly enhanced by acute treatment with the opiate receptor antagonists. In contrast, after chronic treatment proliferation always decreased. The sudden removal of an opioid inhibitory tone might be the basis for the increased proliferative responses observed after acute treatment. The decrease after chronic treatment could be ascribed to the amplification of the inhibitory effect of endogenous opioids due to the up-regulation of opiate receptors that follows chronic antagonist administration. Receptor binding studies of beta-endorphin receptors on splenocytes of chronically naloxone treated rats confirmed this hypothesis: a higher number of beta-endorphin receptors were expressed on splenocytes of naloxone-treated rats compared to controls (Bmax = 9.8 x 10(-12) vs. 1.16 x 10(-12), respectively).

Characterization of mu opioid receptor binding during amygdala kindling in rats and effects of chronic naloxone pretreatment: an autoradiographic study

Using in vitro autoradiography, mu receptor binding in rat brain was characterized at different amygdala kindling stages and in amygdaloid kindled animals pretreated chronically with naloxone. Male Sprague-Dawley rats implanted with bipolar electrodes in the right amygdala received one of the following pretreatments s.c. for 14 days via osmotic minipumps: normal saline solution, 0.5 microliters/h, or naloxone HCl, 75 micrograms/h. Two days after treatments were accomplished animals were stimulated daily. Our data showed different patterns of mu receptor binding during the normal kindling process: during stage II-III, pronounced binding increase was detected in cingulate, temporal and entorhinal cortices, anterior amygdala, caudate putamen, thalamic nuclei, ventrolateral and dorsolateral portions of central gray, substantia nigra pars compacta and pars reticulata. Twenty-four hours after the last stage V kindled seizure, enhanced binding was observed in cingulate and frontoparietal cortices, anterior amygdala, caudate putamen and ventromedial thalamic nucleus. Twenty-eight days after the last stage V kindled seizure, binding augmentation was noticed in cingulate and frontoparietal cortices, whereas decreased binding was detected in amygdala complex, substantia nigra pars reticulata, piriform, perirhinal, parietal, temporal and entorhinal cortices. Mu receptor binding in kindled rats chronically pretreated with naloxone was significantly higher in several structures when compared with control and normal kindled groups. Our data indicate different regional selective patterns of mu receptor binding during amygdala kindling which may depend on epileptogenesis and long-term changes induced by this process. In addition, even higher mu receptor binding results from chronic naloxone administration prior to kindling.

Pharmacological specificity of enhanced sensitivity to naltrexone in rats

Rats treated weekly with cumulative doses (1-100 mg/kg, IP) of naltrexone develop an enhanced sensitivity to the operant response-rate decreasing effect of naltrexone. In the present experiment the pharmacological specificity of that enhanced sensitivity was determined by testing a variety of drugs for cross-sensitivity to naltrexone. Cross-sensitivity was evaluated with two procedures. In one, dose-effect functions were determined for each of the test compounds before and after the development of enhanced sensitivity to naltrexone in a single group of rats. In the second procedure, one group of rats was made sensitive to naltrexone, while a second was not. Test compounds were then evaluated in both groups. For both procedures, a shift to the left in the dose-effect functions similar to naltrexone was considered evidence of cross-sensitivity. Of the opioid antagonists tested, only naloxone showed clear cross-sensitivity to naltrexone, although MR 2266 and diprenorphine also showed evidence of cross-sensitivity. The opioid antagonist quadazocine did not show cross-sensitivity to naltrexone on the day of testing, although some evidence of cross-sensitivity was evident 24 h later. In addition, the dose-effect function for d-amphetamine was significantly changed following naltrexone treatment. No evidence of cross-sensitivity was observed for the optical isomer of naloxone, d-naloxone, or for naloxone's quaternary derivative, naloxone methiodide. None of the opioid agonists or agonist-antagonists tested showed cross-sensitivity to naltrexone (i.e. morphine, U-50, 488H, ethylketocyclazocine, N-allylnormetazocine and pentazocine). The non-opioid drugs chlordiazepoxide and phencyclidine also failed to show evidence of cross-sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic naloxone pretreatment on amygdaloid kindling in rats

Effects of chronic naloxone pretreatment (75 or 270 micrograms/h for 14 days) on the development of amygdaloid kindling in rats were evaluated. The acquisition of seizure activity was modified in the naloxone pretreated animals, depending on the nucleus stimulated: facilitation of stages IV and V occurred in 37%, variability of electrographic and behavioral responses to electrical stimulation during the kindling development in 33%, and facilitation of stages IV and V followed by long periods of seizure suppression in 29%. Enhancement of postictal seizure suppression during a recycling paradigm was observed in all the naloxone pretreated rats. It was concluded that the chronic administration of naloxone (known to induce opioid binding upregulation and supersensitivity), in association with the enduring changes in opioid mechanisms provoked by kindled seizures, were responsible for the facilitation and suppression of epileptic activity. These findings support bidirectional modulatory effects of opioid peptides on epileptic seizures as well as the view that epileptic seizures can induce enduring alterations in opioid mechanisms.

Species differences in mu- and delta-opioid receptors

The mu opioid receptor ligand [D-Ala2, NMePhe4, Gly-ol5]enkephalin (DAGO) and delta opioid receptor ligand [D-Pen2,D-Pen5]enkephalin (DPDPE) show similar specificity in competition binding studies in whole brain homogenate in rat and mouse. However, in saturation studies, the density and affinity of DPDPE binding sites were substantially greater in the mouse. There was no difference between the mouse and rat in the density and affinity of DAGO sites. Results from dose-response studies for analgesia using the same ligands administered i.c.v. in both species paralleled the binding studies. DAGO was approximately 2 times more potent in the mouse compared to the rat; while DPDPE was more than 15 times more potent in the mouse. Thus, binding capacity and affinity differences appear to be related to the functional potency of the mu and delta ligands in the two species. These results suggest that the difference in potency of DPDPE between rat and mouse is related to the differences in brain delta opioid receptors.

Differences in the bimodal effects of morphine in the vas deferens of two strains of rats

1. The effect of morphine on the smooth muscle of the isolated vas deferens of Sprague-Dawley and Wistar rats was studied. 2. Morphine produces pre- and post-synaptic effects on the contractile activity of the vasa deferentia. 3. The pre-synaptic action is an increase in the basal tension of the vas deferens. 4. The post-synaptic effect is a bimodal change in the height of the muscular twitch induced by neurogenic transmural stimulation. 5. Sprague-Dawley rats are more sensitive than Wistar rats to the actions of morphine. 6. The morphine induced responses were antagonized by naloxone better in Wistar than Sprague-Dawley rats. 7. The findings obtained suggest a genetic differences in the synthesis of opioid subtype receptors.

The nature of opioid responsiveness and its implications for neuropathic pain: new hypotheses derived from studies of opioid infusions

In recent years, the observation that the response of patients to opioid drugs may be influenced by properties inherent in the pain or pain syndrome, such as its pathophysiology, has evolved into the belief that certain types of pain, e.g., neuropathic pains, may be unresponsive to these drugs. This concept has important implications for both clinical practice and basic understanding of opioid mechanisms. We critically evaluate opioid responsiveness, particularly as it relates to neuropathic pain, and propose a clinically relevant definition and a paradigm for its investigation. The paradigm is illustrated by analgesic responses to opioid infusion in 28 patients with neuropathic pains and by a detailed presentation of the pharmacokinetic and pharmacodynamic relationships in one of these patients, whose central pain responded promptly to an infusion of hydromorphone. From this analysis, we hypothesize that (1) opioid responsiveness in man can be defined by the degree of analgesia achieved during dose escalation to either intolerable side effects or the occurrence of 'complete' or 'adequate' analgesia; (2) opioid responsiveness is a continuum, rather than a quantal phenomenon; (3) opioid responsiveness is determined by a diverse group of patient characteristics and pain-related factors, as well as drug-selective effects; and (4) a neuropathic mechanism may reduce opioid responsiveness, but does not result in an inherent resistance to these drugs. Given the complexity of factors contributing to opioid responsiveness and the observation that outcome cannot be reliably predicted, opioids should not be withheld on the assumption that pain mechanism, or any other factor, precludes a favorable response. Both the clinical use of opioids and paradigms to investigate opioid responsiveness should include dose escalation to maximally tolerated levels and repeated monitoring of analgesia and other effects.

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.

Simultaneous development of opioid tolerance and opioid antagonist-induced receptor upregulation

Mice treated chronically with opioid antagonists have increased receptor density in brain and are supersensitive to the pharmacodynamic action of morphine. In the present study mice were implanted subcutaneously with naltrexone or placebo pellets for 8 days. During implantation mice received daily injections of morphine (100 or 250 mg/kg) or saline. Morphine analgesia was completely blocked in mice that were implanted with naltrexone at the low dose of morphine; while some analgesic action was observed at the higher dose. Mice implanted with placebo were analgesic following the daily morphine treatment. At the end of 8 days the pellets were removed and 24 h later some mice were tested for morphine analgesia while others were examined in binding studies. Naltrexone treatment increased [3H]naloxone, 3H[D-Ala2-D-Leu5]enkephalin (DADLE) and 3H[D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO) binding compared to controls and increased the analgesic potency of morphine. Daily treatment with morphine did not alter brain opioid binding or naltrexone-induced receptor upregulation. Mice injected daily with morphine were significantly less sensitive to morphine (tolerant) than their respective saline control group for both the placebo and the naltrexone-treated groups. However, naltrexone-treated mice were more sensitive to morphine than placebo controls regardless of whether they were injected daily with morphine or not. These results indicate that if naltrexone-induced opioid receptor upregulation occurs in the presence of repeated agonist administration, the new binding sites mediate tolerance via desensitization to morphine.

Codeine analgesic and morphine hyperalgesic effects on thermal nociception in domestic fowl

The effects of codeine phosphate and morphine sulfate (2.5, 15.0, and 30 mg/ml/kg; IM) on latency of a jump response elicited by a noxious (61 degrees C) thermal stimulus were studied in White Leghorn cockerels at 15-16 days posthatch. Codeine induced a significant dose-dependent increase in jump response latency (analgesic effect), whereas morphine at each dose induced a significant decrease in jump response latency (hyperalgesic effect). Naloxone (5 mg/ml/kg) reversed the hyperalgesic effect of morphine (30 mg/ml/kg) and potentiated codeine analgesic effects. It is unlikely that codeine analgesic effects in domestic fowl reflect demethylation of codeine to morphine. These opposite codeine and morphine effects may reflect the interaction of these opiates at different populations of opioid receptors or at different substrates.

Chronic opioid antagonist treatment increases mu and delta receptor mediated spinal opioid analgesia

Chronic systemic treatment with the opioid antagonist, naltrexone, increased the analgesic potency of intrathecally administered opioid agonists at spinal mu and delta receptors, but not at kappa receptors. These results indicate that, contrary to a previous report, opioid receptors in the spinal cord display up-regulation and supersensitivity and are no different in this regard from supraspinal receptors.