Opioids and behavior: genetic aspects

Predictive Value of Grooming Behavior for Development of Dermatitis in Selectively Bred P Rats as a Model of Trichotillomania Hair Pulling Disorder

Trichotillomania (TTM) is a body-focused repetitive disorder affecting as much as 0.5 to 2% of the population, with women four times more likely to be affected than men. This disorder causes impairment in daily function and significant distress. A potential animal model for this disorder is the inbred C57BL/6J mouse which displays clinical signs and behavioral characteristics similar to those described for people affected by this disorder. Because alcohol-preferring P rats also display similar clinical signs and behavioral characteristics, it was hypothesized that this selectively bred stock could be an additional animal model. In this study, 112 female P rats were recorded on digital media for 15 min after being sprayed with a mist of water and assessed for grooming patterns—oral, manual, and scratching. Significant elevations in scratching and oral grooming behavior were predictive of the future development of skin lesions. These findings suggest that P rats may be an additional model to study TTM, with the advantage of increased genetic variation (i.e., non-inbred) which mirrors the human population. The use of this model may help to identify preventative and therapeutic interventions for humans and other animals with similar body-focused repetitive disorders.

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.

Effects of Morphine and Time of Day on Pain and Beta-Endorphin

Clients report more pain at some times of day than at others due, in part, to the temporal variation of the body's inhibitory pain response. The analgesic effectiveness of morphine varies with the time of day, perhaps due to the inhibiting or enhancing effects of the drug on plasma beta-endorphin (BE). This experiment was designed to examine the timed effects of morphine on the pain-induced BE response. Six groups of treatment mice (injected with morphine sulfate) and 6 groups of control mice (injected with saline) were exposed to an acute pain stimulus at 4-h intervals, and blood was collected. Plasma BE was analyzed using radioimmunoassay. Control mice showed a robust cir-cadian BE-response rhythm with a peak at 0000 and a nadir at 1200, whereas the BE response of mice that received morphine was arrhythmic. Animals that received morphine tolerated the noxious stimulus longer, but the analgesia varied with time of day. These results indicate that morphine abolishes the rhythmic BE response to pain and does not inhibit pain equally at all times of day. Morphine doses should be titrated to maximize the endogenous pain control system while achieving analgesia with decreased dosages.

Attenuated pain response of obese mice (B6.Cg-lep(ob)) is affected by aging and leptin but not sex

Genetically obese mice (B6.Cg-lep(ob)) manifest decreased responses to noxious thermal stimuli (hotplate test) suggesting endogenous analgesia (Roy et al., 1981). To examine further the analgesic response of these mice, we conducted 4 experiments. Experiment 1 assessed the response of ob/ob mice to tail flick, another noxious thermal test. Tail-flick testing was performed on B6.Cg-lep(ob) mice (n=14) and B6.Cg-lep(OB/?) (n=12) across a range of temperatures. Ob/ob mice exhibited longer latencies than control mice at all temperatures tested. In Experiment 2, potential sex differences were examined. Tail-flick latencies in male and female ob/ob mice (n=6/group) did not differ. The final 2 experiments examined factors that could modulate endogenous analgesia. Experiment 3 assessed the effects of aging in ob/ob mice (n=10/group). Older mice displayed longer tail-flick latencies than did younger mice. Experiment 4 examined the effect of leptin administration in the leptin-deficient ob/ob mice. Two groups (n=10/group) of ob/ob mice received osmotic pump implants filled with either leptin or vehicle, and were tail-flick tested at days 7 and 14 post-implantation. Ob/ob mice receiving leptin showed shorter latencies than did vehicle-receiving ob/ob mice. Taken together, these results support earlier reports of heightened analgesia in ob/ob mice and suggest that aging further reduces the already impaired pain response. Furthermore, leptin deficiency partially contributes to decreased pain sensation of ob/ob mice.

Epidural pain management after open lateral thoracotomy: Female patients have better pain relief and need smaller amounts of analgesics than males

Background and aims

There is an ongoing dispute whether or not there is a gender difference in epidural drug requirements. The objective of this study was to compare the effects of a triple drug epidural mixture used for postoperative pain relief on male and female patients undergoing major surgery.

Materials and methods

To avoid possible influence of different age and type of surgery only 50–70 year old patients undergoing open lateral thoracotomy were included. 253 patients were enrolled: 116 males and 137 females. All patients received a mixture of bupivacaine 0.1 mg/ml, fentanyl 2 μg/ml, and adrenaline 2 μg/ml (BFA) by continuous infusion into the thoracic epidural space for postoperative pain relief. Infusion rate of the BFA solution was recorded and pain score was evaluated by numeric rating scale (NRS, 1–10) for 48 h post operatively.

Results

Adequate postoperative pain relief (NRS ≤ 3) at rest was accomplished in 91% of male patients on day one and 92% on day two and by 94% and 100% of female patients, respectively. Females had significantly lower median pain scores than males on day two both at rest (P < 0.011) and by movement (P < 0.012). In addition females required significantly smaller amounts of BFA mixture (P < 0.01) and less frequently rescue opioids (P < 0.025) than males.

Conclusion

Female patients had significantly better pain relief both at rest and by movement, needed smaller amounts of the epidural bupivacaine, fentanyl, adrenaline mixture for postoperative pain relief and received less frequently rescue opioids than males.

Implications

The dose of thoracic epidural infusion of low-concentration bupivacaine-, fentanyl-, and adrenaline-solution should routinely be set lower for postmenopausal women than for elderly male patients during and after thoracotomy.

Sex-specific responses to opiates: animal and human studies

It is widely reported that analgesic drugs acting at mu, kappa, and delta opioid-receptors display quantitative and qualitative differences in effect in males and females. These sex-related differences are not restricted to the analgesic/antinociceptive properties of opioids, but are also present in opioid-induced side effects, such as changes in respiration, locomotor activity, learning/memory, addiction, and changes in the cardiovascular system. An increasing number of well-controlled animal and human studies directly examining the issue of sex in the potency of opioids show that, although sex may affect opioid analgesia, the direction and magnitude of sex differences depend on many interacting variables. These include those specific to the drug itself, such as dose, pharmacology, and route and time of administration, and those particular to the subject, such as species, type of pain, genetics, age, and gonadal/hormonal status. In the current review, we systematically present these animal and human studies and discuss the data in relation to the depending variables. Although the observed sex differences in opioid effect may be clinically relevant, lack of knowledge on other factors involved in the large variability in patient opioid analgesic sensitivity should compel practitioners to customize their dosing regimens based on individual requirements.

Automated evaluation of sensitivity to foot shock in mice: inbred strain differences and pharmacological validation

Assessing foot shock sensitivity in rodents can be useful in identifying analgesic or hyperalgesic drugs, and phenotyping inbred or genetically altered mice. Furthermore, as foot shock is an integral part of several rodent behavioral models, sensitivity should also be assessed to accurately interpret behavioral measures from these models. To eliminate variability and increase the efficiency of manually scored shock sensitivity paradigms, we utilized a startle reflex system to automatically quantify responses to varying levels of foot shock. Eight inbred mouse strains were tested for reactivity to foot shock in this system, as well as inherent startle response activity to loud noise bursts. Strain rank order for shock reactivity differed from that for acoustic startle, suggesting that pathways activated in response to each differed. Analgesic doses of morphine and acetaminophen specifically reduced foot shock responses without affecting motor reflexive responses to loud noises in each strain tested. We also tested diazepam and scopolamine, which are often used to disrupt behavior in shock-related paradigms to illustrate the usefulness of this assay. Overall, these results demonstrate that our automated method is a quick and simple way to accurately assess potential foot shock sensitivity differences owing to strain, genotype or drug treatments.

Effects of morphine on circadian rhythms of motor activity and body temperature in pig-tailed macaques

Previous studies of the effects of opiates on motor activity and body temperature in nonhuman primates have been limited in scope and typically only conducted with restrained animals. The present study used radio-telemetry devices to continuously measure activity and temperature in unrestrained pig-tailed macaques for 24 h following morphine administration. Two dose-response functions (0.56 to 5.6 mg/kg, i.m.) were determined, one with morphine administered at 9 a.m. and one with morphine administrated at 3 p.m. Under both the 9 a.m. or 3 p.m. administration schedules, body temperature and activity were increased acutely. Activity was also reduced the following morning after morphine administered at either time. In other regards, morphine's effects on both temperature and activity differed between 9 a.m. and 3 p.m. injection, including periods of decreased activity immediately after the acute increases after 9 a.m. but not 3 p.m. administration. Surprisingly, motor activity also increased 9-12 h post-injection following morphine administered at 9 a.m., but not at 3 p.m. These results clearly show an interaction between timing of morphine administration and effects on temperature and activity. These results also underscore the fact that single injections of drugs may have multiple and delayed effects on circadian rhythms in macaques.

Dopamine and serotonin release in dorsal striatum and nucleus accumbens is differentially modulated by morphine in DBA/2J and C57BL/6J mice

Numerous studies have demonstrated that genetic factors significantly influence opioid ability to induce behavioral modification in mice. This differential sensitivity has been extensively studied, particularly in the DBA/2J and C57BL/6J strains. In the present study, using the "in vivo" microdialysis technique in these strains, we investigated the effect of morphine administration on the extracellular levels of dopamine (DA), serotonin (5-HT), and their metabolites in the nucleus accumbens and dorsal striatum--areas thought to be involved in morphine-induced locomotor hyperactivity. In the nucleus accumbens, morphine (20 mg/kg) significantly increased extracellular levels of DA in both strains. However, in dorsal striatum the morphine-induced increase of extracellular DA was lower in DBA/2J mice than in C57BL/6J. Moreover, morphine significantly stimulated 5-HT and 5-hydroxyindolacetic acid (5-HIAA) release both in nucleus accumbens and dorsal striatum of C57BL/6J mice, whereas it decreased 5-HT release without modifying 5-HIAA levels in DBA/2J mice. These results suggest that the different behavioral and biochemical responses to acute morphine described in these two strains could be mediated by different sensitivity of both the dopaminergic and the serotonergic systems.

Assessment of efficacy of long-term opioid therapy in pain patients with substance abuse potential

Clinical experience supports the notion that opioids can be used successfully to treat many chronic pain conditions. Unfortunately, few controlled trials have assessed which individuals benefit from long-term opioid therapy, and there is concern about the use of long-term opioid therapy in individuals with a substance-abuse history. This article contains three sections relevant to the assessment of individuals with chronic pain and a substance-abuse history who are receiving long-term opioid therapy. The first reviews the literature on opioid therapy, with a critique of biologic and environmental susceptibility factors for addiction. The second briefly reviews uncontrolled and controlled trials of opioid therapy for pain. The third reviews areas critical in assessing treatment efficacy and substance abuse in patients with chronic pain, both in terms of documentation of past behaviors and as a measure of outcome of opioid therapy. Potential guidelines for use of opioids in patients with chronic noncancer pain are outlined. Finally, questions are posed for future investigations of the efficacy of opioid therapy for patients with chronic pain and a substance-abuse history.

The pharmacogenetics of analgesia: toward a genetically-based approach to pain management

Interindividual differences in the experience of pain have been appreciated clinically for over a century. More recently, there has been a growing body of evidence demonstrating differences in analgesic response to various pharmacotherapies, although the source of this variability largely remains to be explained. To this end, basic science research is beginning to identify the allelic variants that underlie such antinociceptive variability using a multiplicity of animal models, and powerful genetic approaches are being exploited to accelerate this process. Although the vast majority of these studies have focused on the pharmacogenetics of opioids, owing to their prominent status as analgesics, the number of pharmacotherapies evincing genetically-based variability is rapidly expanding. In addition, analogous studies have been undertaken in humans, as a small but growing number of clinical trials have begun to evaluate prospectively the existence, if oftentimes not the origin, of interindividual differences in analgesic drug response. Importantly, with a few notable exceptions, such efforts have primarily identified differences in analgesic efficacy and/or potency between male and female human subjects. Looking toward the future development of one or more widely utilised, pharmacogenetic screens that would lead to modifications in treatment planning, at least with respect to the pharmacologic management of pain, this review will document the breadth of genetically-based variability in drug-mediated antinociception in animals. Specific examples in which the gene or genes underlying such variability have been postulated or identified will be given, while highlighting the effect of sex and its interactions with other genetic backgrounds. Finally, we will summarise and evaluate the literature on pharmacogenetic differences in human analgesic drug response, for which the influence of sex has served as one of the better studied and heuristically insightful examples.

Pain intolerance in opioid-maintained former opiate addicts: effect of long-acting maintenance agent

Patients on methadone maintenance therapy are relatively intolerant of pain, a finding hypothesized to reflect a hyperalgesic state induced by chronic opioid administration. To explore if the intrinsic activity of the opioid maintenance agent might affect expression of hyperalgesia in this population, withdrawal latency for cold-pressor (CP) pain was compared between small groups of methadone-maintained (n = 18), buprenorphine-maintained (n = 18), and matched control (n = 18) subjects. The opioid-maintained groups had equal and significantly shorter withdrawal latencies than controls, however it is possible that high rates of continued illicit opioid use precluded finding differences between methadone and buprenorphine groups. Differential effects of maintenance agent were found for the few subjects without illicit opioid use, such that withdrawal latencies for methadone-maintained (n = 5) were less than for buprenorphine-maintained (n = 7) which were less than controls (n = 18). Diminished pain tolerance in patients receiving opioid maintenance treatment has significant clinical implications. More research is needed to determine if buprenorphine offers advantages over methadone in this regard.

Pain responses in methadone-maintained opioid abusers

Providing pain management for known opioid abusers is a challenging clinical task, in part because little is known about their pain experience and analgesic requirements. This study was designed to describe pain tolerance and analgesic response in a sample of opioid addicts stabilized in methadone-maintenance (MM) treatment (n = 60) in comparison to matched nondependent control subjects (n = 60). By using a placebo-controlled, two-way factorial design, tolerance to cold-pressor (CP) pain was examined, both before and after oral administration of therapeutic doses of common opioid (hydromorphone 2 mg) and nonsteroidal anti-inflammatory (ketorolac 10 mg) analgesic agents. Results showed that MM individuals were significantly less tolerant of CP pain than control subjects, replicating previous work. Analgesic effects were significant neither for medication nor group. These data indicate that MM opioid abusers represent a pain-intolerant subset of clinical patients. Their complaints of pain should be evaluated seriously and managed aggressively.

Brain opioid receptor density relates to stereotypies in chronically stressed pigs

Opioid receptor densities were measured in the hippocampus of chronically stressed (tethered) pigs to study the involvement of endogenous opioid systems in stereotypy performance. Three groups of animals were housed tethered for 2 (n=12), 5.5 (n=12) and 8-9 months (n=8), respectively, and the intensity of stereotypy performance was determined. Opioid receptor densities were measured post mortem using membrane binding assays with [(3)H]naloxone as a ligand. A negative correlation was found between the density of opioid receptors and the intensity of stereotypy performance in the animals that had been housed tethered for 2 months. This correlation seemed to disappear with increasing duration of tethered housing. The data further suggest that, associated with the duration of tediered housing, there was a gradual decrease in the density of opioid receptors in the left hippocampal lobe of the low-stereotyping animals, but not in the right lobe, nor in the left and right lobes of the high-stereotypers. This suggests that chronic stress leads to a (asymmetrically expressed) progressive loss of opioid receptors in the hippocampus, and that stereotypies exert a mitigating effect on stress-induced changes in opioid receptor densities, supporting the hypothesis that stereotypies help the animals cope wife the adverse effects of chronic stress.

The genetic mediation of individual differences in sensitivity to pain and its inhibition

The underlying bases of the considerable interindividual variability in pain-related traits are starting to be revealed. Although the relative importance of genes versus experience in human pain perception remains unclear, rodent populations display large and heritable differences in both nociceptive and analgesic sensitivity. The identification and characterization of particularly divergent populations provides a powerful initial step in the genetic analysis of pain, because these models can be exploited to identify genes contributing to the behavior-level variability. Ultimately, DNA sequence differences representing the differential alleles at pain-relevant genes can be identified. Thus, by using a combination of "top-down" and "bottom-up" strategies, we are now able to genetically dissect even complex biological traits like pain. The present review summarizes the current progress toward these ends in both humans and rodents.

The eating disorders as addiction: a psychobiological perspective

A number of theories have been proposed to explain the substantial comorbidity between the eating disorders and the substance-related disorders. Among them is the claim that self-starvation--exacerbated by excessive exercising--is itself an addiction to the body's endogenous opioids. While efforts have also been made to identify an "addictive personality," attempts to establish whether eating-disordered patients share these characteristics have met with mixed success. The present study was designed to determine the degree to which anorexic and bulimic patients display addictive personality characteristics, and whether these traits are useful in predicting the severity of the patient's weight preoccupation and their degree of excessive exercising. Results confirmed that both anorexic and bulimic patients had high scores on the Addiction Scale of the Eysenck Personality Questionnaire, and that addictiveness and obsessive-compulsiveness were related simultaneously to weight preoccupation and excessive exercising in both patient groups. Findings are discussed within the framework of the auto-addiction opioid theory, and they highlight the similarities and differences in the personality structure of the eating-disorder subtypes.

High intensity social conflict in the Swiss albino mouse induces analgesia modulated by 5-HT1A receptors

Social conflict between mice produces analgesia in the attacked mouse. Both the magnitude and type (opioid or nonopioid) of this analgesia have been related to attack intensity and strain of mouse. In the present study low intensity social conflict (7 bites) did not produce analgesia, whereas high intensity - 30 and 60 bites - interactions produced, respectively, short-lasting (5 min) and very short-lasting (1 min) analgesia in Swiss albino mice, when compared with nonaggressive interaction (0 bite). The 30 bites aggressive interaction induced analgesia (AIIA) was not affected by IP injection of either naloxone (5.0 and 7.5 mg/kg) or diazepam (0.5, 1.0, 2.0 and 4.0 mg/kg). However, this attack-induced analgesia was reduced after IP administration of the 5-HT1A agonists, gepirone (0.3 and 3.0 mg/kg) and BAY R 1531 (0.01 mg/kg). These results indicate that the analgesia induced by 30 bites social conflict in Swiss albino mice does not involve opioid and GABA-benzodiazepine (GABA-BZD) mechanisms. In addition, they suggest that high-intensity social conflict activates serotonergic pain modulatory systems that act through 5-HT1A receptors.

Strain differences in mice antinociception: relationship between alprazolam and opioid receptor subtypes

This study was designed to investigate the antinociceptive effects of one of the most prescribed benzodiazepines (BZ) -i.e., alprazolam. Groups of CD-1, SWISS, BALB/c and C57BL mice were treated with alprazolam. Analgesia was assayed, using the radiant heat tailflick assay. Alprazolam given i.p. elicited analgesia in a dose-dependent manner only in the BALB/c mice (ED50 1.1 mg/kg). No analgesia was observed in CD-1 or C57BL mice. The sensitivity of SWISS mice was intermediate, but still very low. Intrathecally administered alprazolam elicited analgesia in BALB/c, Swiss and CD-1 mice with ED50 values of 10, 22.8 and 34.6 micrograms, respectively. No analgesia was observed in C57BL mice. Intracerebroventricular injections did not induce analgesia in any of the strains. In other sets of experiments with BALB/c mice, we found a supra-additivity increase in analgesia when a subthreshold dose of alprazolam was given with morphine (mu-subtype agonist). This interaction was antagonized by naloxone and less so by flumazenil. No effect was found when alprazolam was co-administered with other specific opioid agonists (delta and kappa). Our results demonstrate that injections of alprazolam can produce analgesia in different genetic subjects and can modify morphine-induced antinociception. The fact that the interaction between morphine and alprazolam analgesia was sensitive to naloxone but less to flumazenil indicates that the analgesic effects of alprazolam are mediated primarily by an opioid mechanism of action but less by benzodiazepines.

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.

Enhancement of morphine self-administration in drug naive, inbred strains of mice by acute emotional stress

The primary reinforcing effect of morphine was compared in two genetically inbred strains of mice (C57BL/6 and DBA/2) using the intravenous self-administration procedure in drug naive animals. The morphine self-administration differed between the mouse strains. DBA/2 but not C57BL/6 acquired self-administration of morphine with a bell-shaped unit dose-response curve. Acute physical stress induced by electrical footshocks did not significantly affect the self-administration in both strains. Acute emotional stress induced by forcing mice to witness another mouse being subjected to acute physical stress caused a shift of the bell-shaped unit dose-response curve of morphine self-administration to the left in the DBA/2 mice. The C57BL/6 mice, which initially failed to demonstrate stable self-administration, started to self-administer morphine after emotional but not physical stress. Emotional distress may increase the individual sensitivity to the rewarding effects of morphine and may render an individual more susceptible to development of drug dependence.

Opioid and nonopioid swim stress-induced analgesia: a parametric analysis in mice

Environmental stress causes the activation of two types of endogenous pain inhibitory systems in animals: opioid analgesia is antagonized by opiate receptor blockers (e.g., naloxone and naltrexone), whereas analgesia produced by nonopioid systems is insensitive to such antagonism. A large literature documents that the parameters of the laboratory stressor will determine the neurochemical identity of the resultant analgesia. In rats, low severity stressors produce opioid analgesia and higher severity stressors produce nonopioid analgesia. A recent parametric analysis of swim stress-induced analgesia (SSIA) in the female Quackenbush mouse, however, observed the opposite pattern. The present study is a parametric analysis of SSIA using a range of swim temperatures (15-38 degrees C), swim durations (45 s to 7 min), and genetic models [male Swiss-Webster mice, and mice selectively bred from this outbred strain for high (HA), low (LA), or control SSIA]. We find that in nonselected mice low severity swims (i.e., warm temperature, short duration) produce naloxone-sensitive opioid SSIA, whereas high severity swims (i.e., cold temperature, long duration) produce nonopioid SSIA. This pattern is also seen in HA mice displaying very high analgesic magnitudes, but not in LA mice displaying minimal SSIA. In the selectively bred mice, analgesia and hypothermia from forced swimming are positively correlated, but can be dissociated both genetically and neurochemically. Furthermore, swimming in body temperature (38 degrees C) water produces analgesia without concommitant hypothermia, and the increased magnitude of 38 degrees C SSIA displayed by HA mice over control levels is entirely opioid.

Ethical concerns in the research and treatment of complex disease

Research on and treatment of complex diseases raise familiar ethical issues concerning informed consent, privacy, confidentially, insurability, employability and social stigma. Consideration of the family as the unit of study, or point of medical intervention, presents some additional twists to these common ethical concerns. In addition, complex diseases present particular ethical challenges because different social and political incentives accompany placing emphasis on either the genetic or the environmental components of the diseases (e.g. in allocating research funds or ascribing responsibility for illness).

Oligogenic determination of morphine analgesic magnitude: a genetic analysis of selectively bred mouse lines

Two ongoing selective breeding projects have produced mice that display divergent analgesic responses to morphine. These two projects have selected for similar phenotypes: high and low levorphanol analgesia (HAR/LAR lines; Portland, OR) and high and low swim stress-induced analgesia (HA/LA lines; Jastrzebiec, Poland). Evidence suggests genetic commonalities between mice of the two projects. Using a Mendelian breeding protocol, we have recently found that one or two genetic loci predominantly determine the high morphine analgesia exhibited by HA mice. In the present study we demonstrate that the differential morphine analgesia (5 mg/kg, i.p.) displayed by HAR and LAR mice is similarly oligogenic, predominantly determined by two unlinked loci. A complementation analysis, in which the analgesic responses to morphine of the recessive homozygotes of each project (HAR and HA) were compared to those of their hybrid offspring (HAR x HA), revealed that different genetic loci have been fixed in each project. An intriguing bimodal distribution was observed in the HAR x HA population: Some HAR x HA hybrids displayed greater morphine analgesia than either HAR or HA mice, whereas others displayed minimal analgesia. LAR x LA hybrids displayed less analgesia than either LAR or LA mice. The analgesic responses of HAR x LA and LAR x HA mice were comparable to those of their low-line parents. These findings indicate not only that different loci were responsible for producing high morphine responders in each selection project but that these distinct loci can interact synergistically to produce "superhigh" and "superlow" responders.

One or two genetic loci mediate high opiate analgesia in selectively bred mice

The analgesic responses of humans and laboratory animals are characterized by substantial individual differences. The genetic basis of this variability can be studied experimentally in rodents using a program of selective breeding. One such program selected for high (HA) and low (LA) swim stress-induced analgesia (SSIA) on the hot-plate (56 degrees C) test in Swiss-Webster mice. These lines, which have been selectively bred for more than 25 generations, display markedly divergent opioid-mediated SSIA (3-min swims in 38 degrees C water), morphine analgesia (10 mg/kg, i.p.), and analgesia to the kappa-receptor agonist, U-50,488H (30 mg/kg, i.p.). The present study investigated the mode of inheritance of these opioid analgesias in HA and LA mice, using Mendelian genetic analyses. We report that the differential sensitivity of HA and LA mice to each of these analgesic manipulations appears to be determined oligogenically, by one or at the most two major genetic loci. The loci associated with each type of analgesia do not co-segregate, however, indicating that three distinct oligogenic effects have been identified. These findings suggest that the genetic determination of analgesic mechanisms may have simple components and as such may be amenable to further analysis using molecular genetic techniques.

Strain differences in the analgesic and reinforcing action of morphine in mice

The analgesic and reinforcing effects of morphine were compared in four strains of mice (C57BL/6, BALB/c, DBA, CBA). The analgesic action of morphine was measured in tail immersion (49 degrees C), hot plate (60 degrees C), and tail clip (four-point scale of nociceptive response) tests. The reinforcing action of morphine was studied in i.v. self-administration and conditioned place preference techniques. The results demonstrate strain differences in the analgesic and reinforcing action of morphine in mice. The relative rank order of the strains varied for the several tests as well as for the morphine effects. The lack of correlation between analgesic and reinforcing action of morphine in inbred strains supports the conclusion that analgesia and reinforcement are separate processes with different genetic control.

Chromosomal mapping of loci influencing sensitivity to cocaine-induced seizures in BXD recombinant inbred strains of mice

Among inbred mice, genetic factors mediate differences in sensitivity to the convulsant properties of cocaine; however, the gene(s) underlying cocaine's effects have not been identified. To help elucidate the gene(s) responsible for cocaine seizure susceptibility, we used recombinant inbred-quantitative trait loci (RI-QTL) analyses to identify chromosomal loci associated with cocaine-induced seizures. RI-QTL analyses seek to identify associations between a quantitative measure of a particular phenotype and one or more previously mapped marker genes across a panel of RI strains. This report describes an RI-QTL analysis of cocaine seizure susceptibility among 26 BXD RI strains. These strains showed a skewed, bimodal range of seizure susceptibility which could be the result of one or more modifying genes acting in concert with a major gene to influence cocaine sensitivity. Correlating the percent seizures displayed by each strain following 60 mg/kg cocaine with chromosomal marker data for these strains revealed a number of significant correlations clustered in two regions on chromosomes 12 and 6. This is the first identification of putative chromosomal loci associated with a cocaine-related phenotype and should facilitate identification of the gene(s) underlying cocaine toxicity and other cocaine-related phenotypes.

Opiate withdrawal intensity correlates with the presence of DSLET high-affinity binding

The goal of this study was to compare the characteristics of mu- and delta-opioid receptors in the cortex of DBA/2 and C57BL/6 mice, which differ in sensitivity to the long- and short-term effects of morphine. The characteristics of mu-opiate receptors were not different in the cortex of both strains. Both high- and low-affinity binding sites of DSLET, a specific ligand of delta-opiate receptors, were present in the cortex of C57BL/6 mice, whereas the high-affinity binding sites were not found in the cortex of DBA/2 mice. The absence of high-affinity DSLET binding sites, which are similar to the delta 2 type of opioid receptors, may explain the less intensive naloxone-precipitated withdrawal reaction of DBA/2 as compared with C57BL/6 mice.

Verticalization of behavior elicited by dopaminergic mobilization is qualitatively different between C57BL/6J and DBA/2J mice

Behavioral effects of dopaminergic stimulation were evaluated in C57BL/6J mice and compared to the effects occurring in DBA/2J mice, an inbred strain with reduced densities of striatal dopamine receptors. Effects of apomorphine (0.5-64 mg/kg) alone and in combination with cocaine (30 mg/kg) were assessed using a time-sampling technique that classified climbing and leaning in separate categories. Locomotion was also assessed in a separate experiment. Climbing occurred in DBA/2J mice only at doses of apomorphine that were 16 times higher than the smallest effective dose in C57BL/6J mice; nevertheless, relative to baseline values, effects were fairly comparable. By contrast, whereas DBA/2J mice showed dose-dependent leaning under apomorphine, C57BL/6J mice exhibited little leaning even at doses not producing climbing, and only after the highest apomorphine dose was leaning significantly increased. Apomorphine was equipotent in inducing gnawing across strains, although somewhat less efficacious in DBA/2J mice. When given alone, cocaine produced significant climbing, but not leaning or gnawing, in either strain. Whereas cocaine potentiated apomorphine-induced climbing and gnawing in both strains, apomorphine-induced leaning was not consistently changed by cocaine in either strain. These effects were not indirectly due to hyperkinesia, since neither apomorphine alone nor apomorphine and cocaine in combination was stimulant; apomorphine alone reduced locomotor activity and attenuated cocaine-induced hyperkinesia. The present data do not support a unitary, purely quantitative, account of insensitivity to dopaminergic stimulation based upon low densities of striatal dopamine receptors in DBA/2J mice. Rather, this constellation of results is suggestive of qualitative interstrain dissimilarities in dopaminergic responsiveness that could reflect organizational differences in receptor populations.

Mu-opiate receptor binding is up-regulated in mice selectively bred for high stress-induced analgesia

Pain perception and sensitivity to opiate analgesics strongly depend on genotype. Mice selectively bred for high (HA) and low (LA) swim stress-induced analgesia display markedly divergent morphine analgesia, a difference that appears to be determined by one or at the most two major genes. In an attempt to provide candidate genes mediating the supranormal analgesia displayed by HA mice, we performed mu-opiate receptor binding on 27th generation HA, LA, and control (C) mice using [3H]naloxone. HA mice were found to have significantly higher whole-brain receptor density (Bmax) than LA mice in whole brain homogenates; no significant difference in affinity (Kd) was observed. Quantitative autoradiography confirmed the line difference in whole-brain receptor binding. In the medial thalamus, a brain area implicated in ascending pathways of pain inhibition, HA mice were found to display significantly higher [3H]naloxone binding than C mice (a 64% increase) and LA mice (a 128% increase). No significant line differences were observed in any other brain locus. Thalamic mu receptors may therefore play an important role in a central 'volume control' mechanism of pain inhibition, and underlie individual differences in the responses of mice to opiate analgesic drugs.

Genetic animal models of alcohol and drug abuse

Behavioral and pharmacological responses of selectively bred and inbred rodent lines have been analyzed to elucidate many features of drug sensitivity and the adverse effects of drugs, the underlying mechanisms of drug tolerance and dependence, and the motivational states underlying drug reward and aversion. Genetic mapping of quantitative trait loci (QTLs) has been used to identify provisional chromosomal locations of genes influencing such pharmacological responses. Recent advances in transgenic technology, representational difference analysis, and other molecular methods now make feasible the positional cloning of QTLs that influence sensitivity to drugs of abuse. This marks a new period of synthesis in pharmacogenetic research, in which networks of drug-related behaviors, their underlying pharmacological, physiological, and biochemical mechanisms, and particular genomic regions of interest are being identified.

Strain-dependent differences in responsiveness of mice to nitrous oxide (N2O) antinociception

N2O antinociception was assessed in eight inbred and two outbred mouse strains. Results indicated the following order of responsiveness among the 10 strains: A/J (most sensitive), C57BL/6ByJ, C57BL/6J, BALB/cByJ, C3H/HeJ, Swiss-Webster, CXBK/ByJ, ICR, CBA/J and DBA/2J (least sensitive). These results demonstrate significant strain-dependent differences in antinociceptive responsiveness to N2O. The weak antinociceptive response to N2O in the DBA/2J strain, which is sensitive to morphine and U-50, 488H, indicates some underlying neurobiological difference in the DBA/2J mouse that imparts resistance to N2O. The responsiveness of CXBK/ByJ mice to N2O indicates that mu-opioid receptors may not play an important role in N2O antinociception in mice.

Quantitative trait loci (QTL) applications to substances of abuse: physical dependence studies with nitrous oxide and ethanol in BXD mice

Recombinant inbred (RI) mouse strains were developed primarily as a tool to detect and provisionally map major gene loci--those with effects large enough to cause a bimodal distribution in the trait of interest. This implied that progress toward gene mapping was possible only for gene loci accounting for at least half of the genetic variance. More recently, QTL (quantitative trait loci) approaches have been advanced that do not require bimodal distributions and are thus applicable to a much wider range of phenotypes. They offer the prospect of meaningful progress toward detecting and mapping minor as well as major gene loci affecting any trait of interest, provided there is a significant degree of genetic determination among the RI strains. This paper presents a review of RI gene mapping efforts concerning phenotypes related to drug abuse and presents new data for studies now in progress for nitrous oxide and acute ethanol withdrawal intensity. These two studies exemplify several strengths and limitations of the RI QTL approach.

Differences in one-way active avoidance learning in mice of three inbred strains

DBA/2J, C57BL/6J, and C3H/HeJ mice were given 10 one-way avoidance training trials per day, using an unconditioned stimulus intensity that provided equivalent motivation for learning to mice of all three strains, and were found to differ in their abilities to learn and retain the response. DBA/2J mice acquired the response in fewer days than did the mice of the other two strains, although C57BL/6J mice eventually reached a level of performance similar to that of DBA/2J mice. Both the rate of acquisition and the level at which avoidance performance stabilized were significantly lower in C3H/HeJ, than in DBA/2J or C57BL/6J, mice. In addition, DBA/2J mice showed a significantly greater task retention from one testing day to the next than did C57BL/6J or C3H/HeJ mice.

Genetic factors in neurotoxicology and neuropharmacology: a critical evaluation of the use of genetics as a research tool

Animals have evolved a detoxication system to enable them to survive in a hostile chemical environment in which foods contain many non-nutrient chemicals. Detoxication depends on enzymes which are often genetically polymorphic. As a result, inter-individual variation is common, and in humans several Mendelian loci have been identified. However, most variation in response is probably due to the action of several genes. Genetic variation in response to the neurotoxin MPTP and to chemically and physically-induced seizures is reviewed. In the former case, differences between pigmented and white mouse strains have been noted which are consistent with the hypothesis that humans are more sensitive than mice or rats because of the presence of melanin in human brains. However, variation in sensitivity probably also depends on other genes. In the case of audiogenic seizures, a single locus has been identified and mapped, but its relationship with seizures induced by other agents is not clear. Genetic variation in response to alcohol is also discussed. The failure of most toxicologists to consider genetic variation as a potentially confounding variable, and as a powerful research tool, is discussed critically in relation to non-repeatability of research on the neurotoxic effects of lead, and in relation to the genetic variation in MPTP, seizures, and alcohol response already noted. It seems clear that genetic methods provide a powerful research tool which is largely being ignored by toxicologists.

Cerebrospinal fluid beta-endorphin and cortisol study in Rett syndrome

An evaluation of cerebrospinal fluid and plasma beta-endorphin and cortisol levels was performed in 15 girls affected with classic Rett syndrome. There were no differences between the patient group and the control group in plasma cortisol and beta-endorphin levels. But in Rett syndrome, a significant increase in beta-endorphin was noted in the cerebrospinal fluid, with an elevation of the cerebrospinal fluid/plasma beta-endorphin ratio and a decrease in cerebrospinal fluid cortisol. A substantial overlap between patients and control group diminishes the diagnostic value of cerebrospinal fluid beta-endorphin assay in girls suspected of having Rett syndrome.

Kinetic parameters for plasma beta-endorphin in lean and obese Zucker rats

To determine plasma clearance kinetics for beta-endorphin (BE) by empirical compartmental analysis, a bolus of radioactive labeled 125I-BE was rapidly injected into a carotid artery catheter of unanesthetized lean (L) and obese (O) Zucker rats. The plasma disappearance of 125I was followed over a 3-h period. A 3-component exponential equation provided the best fit for plasma data. Plasma transit times were very short (10 s); however, plasma fractional catabolic rate was much slower. Plasma mean residence time was similar for both groups (50 min) as was recycle time (1.3 min). These data suggest that BE plasma disappearance kinetics are similar in L and O rats.

Strain-specific enhancement of splenic T cell mitogenesis and macrophage phagocytosis following peripheral axotomy

The inbred mouse strains C57BL/6 and DBA/2 were subjected to the selective depletion of peripheral nervous system norepinephrine (NE) by the administration of the neurotoxin 6-hydroxydopamine (6-OH-DA). Measurement of mitogen-induced lymphocyte proliferation following peripheral axotomy with 6-OH-DA revealed that significant enhancement (up to 200% of control values) of T, but not B, cell proliferative responses occurred in DBA/2, while no effect was observed in C57BL/6. Enhancement (up to 196% of control values) of luminol-enhanced chemiluminescence resulting from the phagocytosis of opsonized zymosan was also observed following 6-OH-DA treatment in DBA/2, but not C57BL/6. This strain-specific enhancement of immune responses occurred even though the administration of 6-OH-DA resulted in a similar depletion of splenic NE content (greater than 97% reduction of control values) in both strains as determined by high-pressure liquid chromatography-electrochemical detection. Blockage of the neurotoxic effects of 6-OH-DA was achieved by the use of the catecholamine uptake blocker desipramine-HCl. Administration of desipramine prior to 6-OH-DA blocked the reduction of splenic NE content and abrogated the enhancement of mitogen-induced T cell proliferation. The enhancement of phagocytosis by 6-OH-DA was not, however, altered by the prior administration of desipramine.

An ethological model for the study of activation and interaction of pain, memory and defensive systems in the attacked mouse. Role of endogenous opioids

The present work reviews neurochemical, physiological and behavioral data recorded from the attacked mouse and integrates them into a model of coping mechanisms during social conflict. More specifically, the possible relationships between systems of pain, memory and defense are presented, with special emphasis on the role of endogenous opioid peptides (EOPs). In recipients of attack, decreased beta-endorphin-like immunoreactivity and changes in opiate and benzodiazepine binding characteristics are found in structures of the brain defensive system. EOPs mediate the social conflict-induced increase of dopamine synthesis in the periaqueductal grey and frontal cortex. Social conflict analgesia in attacked mice is under the control of central opioid and nonopioid (e.g., benzodiazepine, glutamate) mechanisms, and is modified by experience (e.g., long-term analgesic reaction; tolerance). EOPs and pain-inhibitory mechanisms participate in the organization of behavioral defense, recuperative behavior and the memory of attack experience. The data are considered in relation to the perceptual-defensive-recuperative model of fear and pain, forwarded by Bolles and Fanselow.

Examination of the neuroendocrine basis for the social conflict-induced enhancement of immunity in mice

Intruder DBA/2 and C57BL/6 mice, which display different neuroendocrine responses to social conflict, evidence a pronounced increase in splenocyte phagocytosis of opsonized zymosan particles as a consequence of social conflict-induced stress. Interruption of the hypothalamic-pituitary-adrenal axis prior to social conflict results in an abrogation of the stress-induced enhancement of phagocytosis in DBA/2, but not in C57BL/6, mice. Administration of the opiate antagonists naloxone and naltrexone resulted in a potentiation of the stress-induced enhancement of phagocytosis in both strains. Similarly, administration of the alkylating antagonist beta-chlornaltrexamine which irreversibly blocks opioid binding sites potentiated the immune-enhancing effects of social conflict stress. Mitogen-induced T and B lymphocyte proliferation was unaffected by any of the experimental procedures with the exception of beta-chlornaltrexamine which suppressed activity equally in stressed and nonstressed groups. These results demonstrate the necessity of employing inbred murine strains in the dissection of the neuroendocrine pathways which govern stress-induced modulation of the immune system.

Developmental changes in opiate-induced analgesia in deer mice: sex and population differences

We examined developmental changes in nociception and mu (morphine) and kappa (U-50,488) opiate-induced analgesia in male and female deer mice of two different populations; Peromyscus maniculatus artemisiae from a mainland region and P. m. angustus from a small island. Both populations displayed significant developmental changes in nociception and morphine (10 mg/kg) and U-50,488 (10 mg/kg)-induced analgesia. Basal thermal response latencies (nociceptive responses) and the levels of mu and kappa opiate-induced analgesia increased over 14-35 days of age, with maximum analgesic responses in adults (35+ days of age). In both of the populations, young (neonatal-weaning) male mice displayed significantly higher thermal response latencies and greater levels of naloxone (1.0 mg/kg) antagonized opiate-induced analgesia than young females. There were also population differences in the levels of analgesia, the insular mice displaying greater mu and lower kappa opiate-induced analgesic responses than the mainland animals. The population differences in mu and kappa opiate-induced analgesia were evident in young and adult mice of both sexes. These results show that there are significant sex and population differences in nociception and opiate-induced analgesia in young (neonatal-weaning) and adult deer mice.

Genetics of responses to drugs of abuse

Genetic differences in drug metabolism, in the number of drug receptors in the brain, and in drug-seeking behavior may contribute to the variability of individual responses to drugs of abuse. Genetic models include inbred strains, mutants, sublines, and selectively bred mice and rats. They have been used to examine acute and chronic effects of narcotics, stimulants, and alcohol as well as drug-seeking behavior, withdrawal syndromes, and the stress-induced release of endogenous opioids. These genetic models should prove helpful in defining individual differences in susceptibility to addiction.

Ventilatory responses of dystrophic and control hamsters to naloxone

In this study we determined if endogenous opioid peptides may contribute to the depression of ventilation seen in dystrophic hamsters. Ventilation of control and dystrophic awake hamsters was determined prior to either naloxone (1 mg/kg) or saline administration and then 5, 15 and 30 minutes postinjection. Subsequently, animals were exposed to a hypercapnic challenge (7% CO2 in O2). Control hamsters increased ventilation significantly (p less than 0.01) after naloxone compared to saline treatment. In contrast, dystrophic hamsters showed no difference in ventilation when they received either naloxone or saline. Both groups increased ventilation significantly (p less than 0.05) after hypercapnic challenge, whether they had received naloxone or saline. Although dystrophic hamsters can respond to a ventilatory stimulant (CO2), naloxone did not increase ventilation, possibly indicating that endogenous opioids are not responsible for their depressed ventilation.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Genetic control of hippocampal cholinergic and dynorphinergic mechanisms regulating novelty-induced exploratory behavior in house mice

Neurobehavioral genetics endeavors to trace the pathways from genetic and environmental determinants to neuroanatomical and neurophysiological systems and, thence, to behavior. Exploiting genetic variation as a tool, the behavioral sequelae of manipulating these neuronal systems by drugs and antisera are analyzed. Apart from research in rats, this paper deals mainly with the genetically-influenced regulation in mice of exploratory behaviors that are adaptive in novel surroundings and are hippocampally-mediated. Special attention is paid to neuropeptidergic, GABAergic, and cholinergic synaptic functions in the mouse hippocampus. The behaviorally different inbred mouse strains C57BL/6 and DBA/2 show opposite reactions (reductions and increases, respectively, in exploration rates) to peripheral and intrahippocampal injections with agents that interfere with peptidergic, cholinergic, and GABAergic neurotransmission. These findings can be explained by an interdependent over-release of opioids, arrested GABA release, and excess acetylcholine in the hippocampal neuronal network of DBA/2 mice, as compared to C57BL/6 mice where these systems are functionally well balanced. Very similar results have been obtained with the lines SRH and SRL, derived from C57BL/6 and DBA/2, and genetically selected for rearing behavior. Most probably, the opioids act to disinhibit exploratory responses. An additional genetic approach is mentioned, in which four inbred mouse strains and one derived heterogeneous stock are used for estimating genetic correlations between structural properties of the hippocampal mossy fibers and levels of hippocampal dynorphin B, on the one hand, and frequencies of exploratory responses to environmental novelty, on the other.