Food deprivation reveals strain differences in opiate intake of Sprague-Dawley and Wistar rats

Molecular testing for adolescent and young adult central nervous system tumors: A Canadian guideline

The 2021 World Health Organization (WHO) classification of CNS tumors incorporates molecular signatures with histology and has highlighted differences across pediatric vs adult-type CNS tumors. However, adolescent and young adults (AYA; aged 15–39), can suffer from tumors across this spectrum and is a recognized orphan population that requires multidisciplinary, specialized care, and often through a transition phase. To advocate for a uniform testing strategy in AYAs, pediatric and adult specialists from neuro-oncology, radiation oncology, neuropathology, and neurosurgery helped develop this review and testing framework through the Canadian AYA Neuro-Oncology Consortium. We propose a comprehensive approach to molecular testing in this unique population, based on the recent tumor classification and within the clinical framework of the provincial health care systems in Canada.

Intravenous self-administration of etonitazene alone and combined with cocaine in rhesus monkeys: comparison with heroin and antagonism by naltrexone and naloxonazine

RATIONALE

In humans, micro opioid-cocaine combinations (speedballs) have been reported to heighten pleasurable effects and result in greater abuse potential compared to either drug individually. Emerging evidence in animals suggests that the ability of mu opioids to enhance the reinforcing effects of cocaine might be independent of their mu intrinsic efficacy even though mu agonist efficacy appears to be a determinant in the reinforcing effects of micro opioids themselves.

OBJECTIVES

This study examined the relationship between agonist efficacy, self-administration, and the enhancement of cocaine self-administration using the high-efficacy mu agonist etonitazene.

MATERIALS AND METHODS

Rhesus monkeys self-administered cocaine, heroin, etonitazene, and opioid-cocaine combinations under a progressive-ratio schedule of intravenous drug injection.

RESULTS

Unlike cocaine and heroin, etonitazene did not maintain consistent self-administration at any dose tested (0.001-1.0 microg/kg/injection). However, combining etonitazene (0.1-1.0 microg/kg/injection) with cocaine (0.01 and 0.03 mg/kg/injection) enhanced cocaine self-administration, and this enhancement was attenuated by naltrexone. These effects are similar to those obtained by combining non-reinforcing doses of heroin and cocaine. Antagonism of etonitazene-cocaine and heroin-cocaine self-administration by naloxonazine was short lasting and was not maintained after 24 h (when naloxonazine's purported micro(1) subtype antagonist effects are thought to predominate).

CONCLUSIONS

The results suggest that high micro agonist efficacy does not guarantee consistent drug self-administration and that the ability of mu agonists to enhance cocaine self-administration does not depend exclusively on reinforcing efficacy. Moreover, the results do not support a major role for micro(1) receptor mechanisms in either etonitazene- or heroin-induced enhancement of cocaine self-administration.

Equal sensitivity to cocaine reward in addiction-prone and addiction-resistant rat genotypes

Rat genotypes tentatively identified as addiction-prone or addiction-resistant on the basis of alcohol preference and locomotor responsiveness to novelty--Lewis versus Fischer strains and Nijmegen high versus low responder lines--differed in time to develop intravenous cocaine self-administration habits, but did not differ in sensitivity to the ability of cocaine reward to summate with lateral hypothalamic brain stimulation reward. Moreover, rats from the Nijmegen low-responder line that initiated self-administration came to do so compulsively and to the same degree as did the Nijmegen high-responder rats. Thus the differences between both sets of genotypes appeared to reflect differences in reactions to the testing situation more than differences in reaction to the reinforcing drug per se.

Dietary modulation of mu and kappa opioid receptor-mediated analgesia

Research has demonstrated that intake of palatable carbohydrates and fats enhanced morphine-induced analgesia (MIA) in Sprague-Dawley rats. To determine if the effects of palatable foods on nociceptive responses would generalize to other strains of animals and other opioid agonists, the present experiments investigated whether intake of palatable foods would: a) alter MIA in Long-Evans rats, and b) alter analgesia produced by drugs acting at kappa opioid receptors. In experiment 1, adult male Long-Evans rats were fed Purina chow alone or chow and either a 32% sucrose solution, a 0.15% saccharin solution, or hydrogenated vegetable fat. Using a tail-flick apparatus, nociceptive responses, measured as percent maximal possible effect (%MPE), were examined after morphine administration [0.0, 1.0, 3.0, and 6.0 mg/kg subcutaneously (SC)]. %MPEs varied directly as a function of dose and were significantly greater for rats fed chow and either sucrose or fat than for rats fed chow alone or chow and saccharin. Experiment 2 compared the analgesic effect of the kappa opioid receptor agonist U50,488H (0, 5.0, 10.0, and 20.0 mg/kg SC) in rats fed chow alone or chow and a 32% sucrose solution. Administration of U50,488H led to analgesia. However, %MPEs did not vary directly as a function of dose. %MPEs of rats fed chow and sucrose were significantly greater than those of rats fed chow alone after injections of 10.0 and 20.0 mg/kg U50,488H. Experiment 3 compared the analgesic effect of U50,488H (5.0, 10.0, 15.0, and 20.0 mg/kg SC) in rats fed chow alone or chow and either a 0.15% saccharin solution or hydrogenated vegetable fat. Administration of U50,488H led to analgesia. However, %MPEs did not vary directly as a function of dose or as a function of diet. %MPEs of rats fed chow and fat were significantly greater than those of rats fed chow alone after injection of 5.0 mg/kg U50,488H.

Alcohol as a food: a commentary on Richter

The present study examined the maintenance of voluntary alcohol intake in male Long-Evans rats. A microstructural analysis of consummatory behaviors (food, alcohol, water) was carried out using a computerized drinkometer system. In this sample of animals, there was no association (r = 0.07) between total food intake and total alcohol intake. There was no compensation for the extra calories ingested in the form of alcohol via a reduction in total food intake, or a reduction in food bout sizes associated with pre- or postprandial alcohol consumption. Further microstructural analyses determined that there were no significant difference between water and alcohol in terms of their distribution in relation to food (non-, pre-, or postprandial bouts). Of the total of 586 bouts of fluid intake analyzed, 45.6% were consumed postprandially, with a similar number (43.2%) consumed nonprandially. A comparison of the size of food bouts associated with different fluid bout types (pre- or postprandial) indicated that food bouts were the same size regardless of whether they were accompanied by water or alcohol. A final analysis determined that 55% of the total daily alcohol intake was consumed postprandially, and that the sizes of non-, pre-, or postprandial fluid bouts were significantly different for water vs. alcohol. Post hoc pairwise comparisons found that alcohol postprandial bouts were significantly larger than all types of water bouts. Alcohol and water bouts ranged in size from < 0.5 ml to > 5.5 ml. There was a significant difference in the distribution of bout sizes with more alcohol bouts at the high end of the distribution. Only 24% of the water bouts were > 2.5 ml compared to 48.4% of the alcohol bouts. The results of this study demonstrate that rats organize their consummatory behavior in many discrete, short bouts. There were considerable individual differences in alcohol preference, alcohol-bout frequency, duration, and size, as well as the prandial distribution of bouts. All of these variables together produce the "pattern" of alcohol intake in individual animals, and is likely to influence the level of intoxication achieved. Although rats do not dissociate their alcohol intake from normal feeding patterns, alcohol bouts occurring postprandially are significantly larger than other bouts of fluid consumption, suggesting that animals perceive the pharmacological effects of and are affected by the alcohol they consume. In animals with a preference for alcohol solutions, it is unlikely that alcohol is consumed as a food.

Intake of dietary sucrose or fat reduces amphetamine drinking in rats

The effects of intake of a palatable food source on oral amphetamine intake were assessed in adult male Long-Evans rats. In Experiment 1, six rats were given an amphetamine sulfate solution (0.1 mg/ml) and four rats were given water as their sole source of fluid. Rats were given a choice of chow and granulated sucrose for a week, alternated with weeks when only chow was fed. In Experiment 2, eight rats were given the amphetamine solution, and four rats water to drink. Rats were fed chow and hydrogenated vegetable fat for a week alternated with weeks when only chow was available. In both experiments, rats drank significantly less of the amphetamine solution when the palatable food choice was available than when given only chow to eat. Intake of palatable foods had a significantly smaller effect on water intake. In both experiments, rats drinking the amphetamine solution took in less fluid and less calories and gained less weight than rats drinking water. However, in Experiment 1, when sucrose was available, rats drinking amphetamine consumed a significantly greater proportion of their calories as sucrose than rats drinking water. Similarly, in Experiment 2, rats drinking the amphetamine solution chose a significantly greater percentage of their calories as fat than rats drinking water. These results demonstrate that intake of sucrose or fat leads to a significant reduction in amphetamine intake, and that the anorectic effects of amphetamine are not equivalent for different types of foods.

Melanin-concentrating hormone is a potent anorectic peptide regulated by food-deprivation and glucopenia in the rat

Melanin-concentrating hormone is a cyclic nonadecapeptide that is produced almost exclusively in neurons of the lateral hypothalamus and sub zona incerta areas while fibers are widespread in the rat brain. Such a localization strongly suggests that this peptide might participate as neurotransmitter/neuromodulator in the control of feeding behavior. In this study we examined first the influence of rat melanin-concentrating hormone on feeding behavior at different times either in the light or in the dark period (light off at 18.00 h) of the day in fed Wistar rats. Intracerebroventricular injection of rat melanin-concentrating hormone (1-100 ng per rat) at 18.00 h reduced food consumption as early as 2 h after injection and for the next 24 h. In addition, similar anorectic effect was noted after bilateral administration of 1 ng melanin-concentrating hormone into the lateral hypothalamic area at 11.30 h but not at 16.30 h. These findings strongly suggest that rat melanin-concentrating hormone may exert inhibitory control over food intake behavior depending on the circadian rhythm. Second, we investigated the modifications induced by food deprivation/refeeding on melanin-concentrating hormone messenger RNA levels in Wistar rats. Total RNA was isolated from whole hypothalamic dissections and the contents of melanin-concentrating hormone messenger RNA, beta-actin messenger RNA (taken as sample control) and neuropeptide Y messenger RNA (taken as control of food-deprivation paradigms) were assessed by using northern blotting. The time-course of messenger RNA expression was determined in groups of rats deprived for 24, 48 and 72 h and revealed a three-fold induction of melanin-concentrating hormone messenger RNA by 24 and 48 h, with reduced increase at 72 h. As expected, the same treatment led to a three-fold increase in neuropeptide Y messenger RNA content by 48 and 72 h. Refeeding groups of animals for up to 72 h after 24 h of food deprivation resulted in full restoration of melanin-concentrating hormone messenger RNA levels by 24 h. Strikingly, a large range of variations in melanin-concentrating hormone messenger RNA content between individuals was observed in food-deprived versus controls or refed rats suggesting that genetic or environmental factors may alter response in melanin-concentrating hormone gene activity after food deprivation. Finally, we investigated the effects of short-term glucoprivation induced by intraperitoneal administration of either 2-deoxy-D-glucose or insulin on melanin-concentrating hormone messenger RNA expression. A transitory increase in melanin-concentrating hormone messenger RNA content was noted 1 h after 2-deoxy-D-glucose injection while melanin-concentrating hormone messenger RNA levels rose two-fold only 5 h after insulin treatment. These results indicate that 2-deoxy-D-glucose and insulin activate melanin-concentrating hormone gene expression through likely distinct regulatory pathways.

Genetic models in the study of alcoholism and substance abuse mechanisms

1. Vulnerability to substance abuse is an important emerging issue. Some related factors are the relationship between propensity to self-administer a drug and neurosensitivity to that drug; similarities and differences between various models of drug seeking behavior; and the commonality of drug-seeking behavior across drugs and genotypes, that is, whether reinforcement from and abuse of alcohol and other drugs define variations within a single behavioral phenomenon, or whether reinforcement and abuse must be individually defined for each substance involved. 2. Findings related to these issues are now being obtained from the areas of pharmacogenetics and operant drug self-administration. 3. The results indicate that reinforcement from alcohol and other drugs is only moderately related to preference for alcohol and other drugs. In addition, neurosensitivity to drugs appears to have little influence on whether that drug will come to serve as a positive reinforcer for any given individual or animal. Indeed, the critical factor appears to be the individual organism's innate propensity to find a particular drug reinforcing. 4. Initial findings also show that genotypic patterns of reinforcement from ethanol appear to correlate highly with patterns of reinforcement from cocaine and opiates. 5. From these findings it is concluded that there exist important genetic determinants of drug reinforced behavior; reinforcement is an important and independent effect of several psychoactive drugs; and drug seeking behaviors maintained by ethanol, cocaine and opiates may have at least some common biological determinants.

Etonitazene delivered orally serves as a reinforcer for Lewis but not Fischer 344 rats

Oral etonitazene self-administration was systematically investigated in two inbred strains of rats, Lewis (LEW) and Fischer 344 (F344). For LEW rats, etonitazene maintained higher rates of lever pressing and was consumed in larger volumes than the water vehicle when the reinforcement schedule was fixed ratio (FR) 8. In contrast, with F344 rats responding did not systematically exceed water values at any etonitazene concentration. LEW rats also drank substantially more etonitazene than F344 rats, and at FR 8 only LEW rats showed the typical inverted U-shaped function between etonitazene concentration and number of responses. For the LEW strain, response rate increased as FR size increased from FR 1 to FR 2 and FR 4, but decreased at FR 8. For the F344 strain, as FR size increased response rate showed small increases, but the response rates were far lower than those of the LEW strain. The results support the conclusion that etonitazene was an effective reinforcer for LEW but not F344 rats. These findings demonstrate genetic differences in opioid reinforcement of operant behavior and indicate that genotype can be an important determinant of whether etonitazene serves as a reinforcer.

Reward and abuse of opiates

The dependence creating properties of drugs are mediated by structures in the brain. The mesolimbic system seems to play a crucial role in the behaviourally reinforcing effects of opiates and other drugs of abuse. The significance of dopamine in opiate reinforcement is still a matter of debate, in spite of the large number of studies on this subject. Dopamine appears to be involved in conditioning processes and in drug self-administration behaviour only once it has been established. Neuropeptides, centrally active fragments of hormones, may play a role in the individual vulnerability for the development of drug dependence. Administration of a number of wellknown neuropeptides attenuates the acquisition of drug self-administration behaviour. The virtues and flaws of some widely used animal models for drug dependence are discussed.

Genetic approaches to studying drug abuse: correlates of drug self-administration

Some important issues in substance abuse are the relationship between propensity to self-administer a drug and neurosensitivity to that drug; similarities and differences between various models of drug-seeking behavior; and the commonality of drug-seeking behavior across drugs and genotypes. Findings related to these issues are now emerging from the areas of pharmacogenetics and operant drug self-administration. Ethanol has been readily established as a positive reinforcer in AA (Alcohol Accepting), P (Preferring) and LEWIS rats, as well as C57BL/6J and LS/Ibg mice. In low ethanol preferring F344 and NP (Non-Preferring) rats, ethanol maintains significant but low levels of responding. Ethanol does not maintain lever-pressing behavior in BALB/cJ or SS/Ibg mice, and is avoided in DBA/2J mice. This pattern of reinforcement from ethanol is only moderately correlated with ethanol preference, and is not correlated with neurosensitivity to ethanol, at least as measured by duration of loss of the righting reflex (LORR). However, these genotypic patterns of reinforcement from ethanol do appear to correlate highly with patterns of reinforcement from cocaine and opiates. From these findings it is concluded that: 1) there exist important genetic determinants of drug reinforced behavior; 2) ethanol preference is not a highly accurate measure of reinforcement from ethanol; 3) sensitivity to ethanol as measured by LORR and self-administration of this drug are not highly genetically correlated; and 4) drug-seeking behaviors maintained by ethanol, cocaine and opiates may have at least some common biological determinants.

Atypical endogenous opioid systems in mice in relation to an auto-addiction opioid model of anorexia nervosa

We have proposed that the atypical opioid system in the mouse may be representative of that in the anorexia nervosa patient and may account for a biological predisposition to the disorder. This is in the context of our auto-addiction model of anorexia nervosa in which endogenous opioids play a critical role in its etiology. Morphine activation of the endogenous opioid systems increases food intake and causes sedation in most species, including normal humans and rats. In contrast in BALB/C mice, morphine causes anorexia and hyperactivity, which we suggest may be true in the anorexia nervosa patient. A variety of atypical opioid systems have been demonstrated in different mouse strains, based on other responses. The present study examines these strains with reference to the responses relevant to our anorexia nervosa model. Three patterns are described--anorexia with hyperactivity (BALB/C and C57BL/6J mice), anorexia without hyperactivity (DBA/J mice), and a biphasic curve with hyperphagia at low doses and anorexia and hyperactivity at higher doses (CF-1 mice). Only female mice were used. These atypical opioid systems may reflect a spectrum of biological predispositions to the disorder. These strain differences may also provide useful correlations of the genetic determinants of various opiate responses and provide useful comparisons in characterizing the essential features responsible for the atypical responses.

Taste vs. CNS effects in voluntary oral opiate intake: studies with a novel device and technique

An apparatus is described which negates the influence of rats' position preferences by presenting alternative solutions at the same location. The licks for both solutions were monitored over consecutive short intervals by lick detectors with computer capture of data. Rats given a choice between water and dilute solutions of the high-potency opiates etonitazene (1.0-5.0 micrograms/ml) or fentanyl (10-50 microgram/ml) either licked equally for the two solutions, or gradually developed a preference or aversion regarding the opiate over the course of several days. In contrast, preferential licking for solutions with a definite taste, saccharin or quinine, was established in hours. These data indicate that the taste per se of these opiates is not aversive to rats, and that preferences for or aversions to the opiates have some other base, presumably one or more actions on the central nervous system.

Genetic approaches to the analysis of addiction processes

Recent studies have shown that large genetic differences exist in the extent to which animals will work to obtain drugs abused by humans. These findings suggest that there may be human populations with elevated risk for developing drug addictions. Frank George and Steven Goldberg describe the behavioral genetic and self-administration methods used in these studies of addiction processes, review the findings obtained in genetic studies of drug addiction, and present hypotheses that can be explored in the attempt to better understand and prevent drug addiction.

Fixed-ratio schedules of oral ethanol self-administration in inbred mouse strains

Previous studies of ethanol reinforcement in BALB/cJ and C57BL/6J mice have shown that over a range of concentrations oral ethanol appeared to serve as a reinforcer only for the C57BL/6J mice. In the previous studies BALB/cJ mice maintained rates of responding for ethanol that only slightly exceeded the rates maintained by the vehicle, water. However, the quantity of ethanol consumed with the continuous reinforcement schedule (fixed ratio one) may have led to pharmacologically significant effects, given the high sensitivity to ethanol of this genotype. The present study tested whether and to what extent ethanol would maintain responding under increasing fixed ratio size in these two strains of mice at ethanol concentrations of 0%, 8%, and 16% (w/v). For the C57BL/6J mice, as fixed-ratio size increased from 1 to 2, 4, and 8, there were almost directly proportional increases in response rate at ethanol concentrations of 8% and 16% (w/v), but not at 0%. Post-session blood ethanol levels confirmed intake of pharmacologically significant quantities. The volume consumed per unit of body weight decreased as fixed-ratio size increased. For the BALB/cJ mice, at no condition did ethanol maintain responding at levels that significantly exceeded vehicle maintained responding. BALB/cJ mice did not differ from C57BL/6J mice as fixed-ratio size was increased during vehicle conditions. These results, along with earlier findings, demonstrate that ethanol can serve as a reinforcer for C57BL/6J mice but not in BALB/cJ mice over a range of schedule conditions. They further support the conclusion that genotype is an important determinant of ethanol reinforced behavior.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.

Differential concentration-response curves for oral ethanol self-administration in C57BL/6J and BALB/cJ mice

In an earlier study ethanol drinking was induced by reducing mice to 80% of their free-feeding weight and feeding them their daily food allotment prior to the experimental session. The mice were given an ascending series of ethanol concentrations (1 to 8%). The inducing condition was subsequently eliminated to determine if the drinking of 8% ethanol would persist in its absence. Eight percent ethanol served as a reinforcer for the C57BL/6J mice but not for the BALB/cJ mice. The purpose of the present study was to examine strain differences in ethanol maintained behavior over a range of concentrations from 1 to 32% (w/v). Ethanol served as a reinforcer for the C57BL/6J mice at concentrations of 4, 8 and 16%. Lever presses and volume of liquid consumed per unit of body weight were inverted U-shaped functions of ethanol concentration. Post-session blood ethanol levels confirmed intake of pharmacologically significant amounts of ethanol. Results with the BALB/cJ mice were very different from those with the C57BL/6J mice. The level of responding did not increase above baseline levels at any of the concentrations tested, and levels of responding decreased below baseline at 32%. Thus, ethanol did not serve as a reinforcer for the BALB/cJ mice at any of the concentrations tested. These results demonstrate that over a range of ethanol concentrations genotype is an important determinant of ethanol reinforced behavior.