Tolerance and evidence of physical dependence to daily codeine injections in the rat

Core temperatures, measured by telemetry, and acquisition of food pellets on a continuous reinforcement schedule were recorded every 30 min in unrestrained male rats given saline for 5 days before and 5 days after 10 daily SC injections of codeine phosphate (200 mg/kg) at 08:00 hr. After the first codeine injection rats were immobile, slightly catatonic, breathed shallowly and had elevated core temperatures, loss of body weight and inhibition of feeding activity. As injections of codeine were repeated, the initial depressant signs decreased and the period of inhibited feeding was replaced by prolonged (greater than 8 hr) post-injection bouts of feeding activity (stimulated feeding) during daylight hours. Core temperatures remained elevated during this phase of drug-induced feeding activity. Mean body weight and 24-hr food intake remained below control levels over the 10-day codeine period as diurnal feeding patterns became reversed. On the first withdrawal day core temperatures declined and feeding patterns changed from those responses on the last codeine day as the rats lost body weight and were hyperirritable. As withdrawal continued core temperature and feeding patterns began to resemble those of the saline control period, body weights increased and hyperirritability subsided. In this study, tolerance and evidence of physical dependence to daily injections of codeine could be demonstrated in rats by continuous monitoring of their diurnal feeding and temperature responses.     

Discrete-trials heroin self-administration produces sensitization to the reinforcing effects of cocaine in rats

Rationale The prevalence of cocaine use in opioid-dependent individuals is reportedly high, and the associated negative health and social consequences are severe and well documented. Sensitization of the reinforcing effects of cocaine has been demonstrated following noncontingent opioid exposure in animals; however, no preclinical studies have investigated the impact of opioid self-administration on cocaine’s reinforcing effects. Objective Experiments were designed to investigate whether access to heroin self-administration altered the subsequent reinforcing effects of cocaine. Methods Baseline responding for cocaine under a progressive ratio schedule was first established. Heroin was then self-administered under a 24-h discrete-trials procedure (DT5; access to heroin five times per hour). Subsequently, cocaine-maintained responding was reassessed. Results Here we demonstrate that 10 days of DT5 heroin self-administration (50 μg/kg per infusion) resulted in an increase in cocaine’s reinforcing effects at several doses across the cocaine dose–effect curve (0.38–3.0 mg/kg per infusion). These increases were relatively long lasting, exceeding the time course of a mild withdrawal syndrome. Conclusions The DT5×10-day history of heroin self-administration resulted in an upward shift in the cocaine dose–effect curve, suggesting that DT5 heroin self-administration produced an increase in potency and sensitization of the maximal effectiveness with which cocaine functions as a reinforcer. The present experiments contribute to a growing amount of preclinical evidence suggesting an impact of opioid exposure on the reinforcing effects of cocaine, which may partially explain the high incidence of cocaine use in opioid-dependent individuals.     

The dissociation of heroin-seeking patterns induced by contextual,discriminative, or discrete conditioned cues in a model of relapse to heroin in rats

Rationale The role of heroin-related stimuli in motivating the resumption of heroin use is not fully understood. Objectives The objective was to characterize the relative importance of drug-related contextual stimuli, discriminative stimuli (DS), or discrete conditioned stimuli (CSs) on drug seeking when rats were reintroduced into the operant context after withdrawal. Methods Nose-poke responding by male rats was reinforced with intravenous heroin (0.05 mg/kg per infusion, 4-h session daily) under a progressive ratio schedule of reinforcement for 14 days. Each session began with the illumination of a green light in the active hole that served as DS. Each earned heroin injection was paired with a 5-s compound cue light and the sound of the infusion pump that served as the discrete CSs. Results Response rates of heroin seeking induced by the contextual stimuli were comparable to the average rates of responding during self-administration training, but rates induced by either DS or CSs were greater than those induced by the contextual stimuli alone (P<0.05). The responding induced by contingent presentations of CSs was higher than that of DS after extinction of instrumental behavior. The drug seeking induced by CSs can be maintained after 3 days extinction with DS in the original context, although the responding elicited by DS cannot be recovered after 3 days of extinction with CSs. Conclusions The relapse to drug seeking can be elicited separately by environmental cues, heroin-predictive DS, or discrete CSs in the same rat after withdrawal.     

Changes in patterns of drug injection concurrent with a sustained reduction in the availability of heroin in Australia

Between 1996 and 2000, heroin was the drug most frequently injected in Australia, and viable heroin markets existed in six of Australia's eight jurisdictions. In 2001, there was a dramatic and sustained reduction in the availability of heroin that was accompanied by a substantial increase in its price, and a 14% decline in the average purity of seizures analysed by forensic laboratories. The shortage of heroin constitutes a unique natural experiment within which to examine the impact of supply reduction. This paper reviews one important correlate of the shortage, namely changes in patterns of illicit drug injection. A number of studies have consistently suggested that between 2000 and 2001, there was a sizeable decrease in both prevalence and frequency of heroin injection among injecting drug users. These changes were accompanied by increased prevalence and frequency of stimulant injection. Cocaine was favoured in NSW, the sole jurisdiction in which a cocaine market was established prior to the heroin shortage; whereas methamphetamine predominated in other jurisdictions. Some data suggest that, at least in the short-term, some drug injectors left the market altogether subsequent to the reduced heroin availability. However, the findings that (1) some former heroin users switched their drug preference to a stimulant; and (2) subsequently attributed this change to the reduced availability of heroin, suggests that reducing the supply of one drug may serve to increase the use of others. Given the differential harms associated with the use of stimulants and opiates, this possibility has grave implications for Australia, where the intervention and treatment system is designed primarily to accommodate opiate use and dependence.     

Effects of ibogaine on responding maintained by food,cocaine and heroin reinforcement in rats

The effects of ibogaine (40 and 80 mg/kg, IP), an indole alkaloid proposed for the treatment of drug abuse, were determined in three different groups of rats responding under an FR10 schedule of food, cocaine or heroin reinforcement. Ibogaine (80 mg/kg, IP) given 60 min before the start of the session resulted in a 97% decrease in the number of ratios completed under the food reinforcement schedule and resulted in a decrease in responding the following day. Neither 40 mg/kg ibogaine given 60 min prior to the session nor 80 mg/kg given 24 h before the session suppressed responding maintained by cocaine infusions (0.33 mg/infusion). Pretreatment with 80 mg/kg ibogaine either 60 or 90 min prior to the session suppressed cocaine self-administration on the day it was administered and the longer pretreatment continued to suppress responding for 48 h. Responding maintained by heroin (18 µg/infusion) was the most sensitive to the effects of ibogaine. Both 40 and 80 mg/kg ibogaine resulted in an almost complete suppression of responding following a 60-min pretreatment period. Responding maintained by heroin returned to control levels the day following the administration of ibogaine.     

The effects of dose and repeated administration on the longer-term hypophagia produced by amphetamine in rats

Rats are hypophagic approximately 1-3 and 13-27 h after receiving amphetamine (2.0 mg/kg). This study examined how these short- and longer-term phases of hypophagia were affected by repeated administration of different amphetamine doses. Throughout eight five-day tests, the rats could lever press for food pellets for 1-hour periods beginning every three hours. On test day 1, the rats were treated with saline, and on test day 3, they were treated with a dose of amphetamine. Across tests, for one group, treatment on day 3 alternated between 0.0 (saline) and 0.5 mg/kg amphetamine; for a second, group treatment on day 3 alternated between 1.0 and 2.0 mg/kg amphetamine; and for a third group, treatment on day 3 was always 1.0 mg/kg amphetamine. The patterns of food intake following day 1 saline and day 3 treatment were compared. Short-term food intake was abolished by 0.5, 1.0, and 2.0 mg/kg amphetamine, and no tolerance was observed to this effect. Longer-term hypophagia was produced by 1.0 and 2.0 but not by 0.5 mg/kg. Tolerance to longer-term hypophagia was seen when 1.0 mg/kg alone was used as the day 3 treatment, but not when 1.0 and 2.0 mg/kg were alternated across tests as the day 3 treatment. Short- and longer-term hypophagia were dissociated by threshold doses for elicitation and by differential tolerance. Occasional receipt of a higher amphetamine dose may sometimes increase the longer-term hypophagia produced by a lower dose.     

Acamprosate suppresses the expression of morphine-induced sensitization in rats but does not affect heroin self-administration or relapse induced by heroin or stress

Acamprosate (calcium-acetyl homotaurinate) is a new compound used in the treatment of alcohol abuse. Because of the putative link between alcoholism and the endogenous opioid systems in both humans and laboratory animals, we tested in rats the effects of acamprosate on behavioral and neurochemical effects of opioid drugs related to their abuse potential. These included sensitization to the behavioral effects of morphine, morphine-induced dopamine (DA) release in the nucleus accumbens (NAS), intravenous (IV) heroin self-administration and relapse to heroin seeking in drug-free rats. In experiment 1, rats were injected daily with either morphine (10 mg/kg, SC) or saline for 14 days. Three days later in a test for the expression of sensitization, an injection of morphine (10 mg/kg) resulted in increased locomotor activity and enhanced DA release in the NAS in rats previously exposed to morphine. Acamprosate (two injections of 200 mg/kg; 12 h apart; IP) suppressed the expression of the sensitized responses, but did not alter the effects of morphine in drug-naive control rats. In experiment 2, it was found that acamprosate (two injections of 50–200 mg/ kg; IP) had no consistent effects on IV heroin self-administration (50–100 μg/kg per infusion) and, in experiment 3, that acamprosate (100–200 mg/ kg, IP) did not alter reinstatement of drug seeking induced by priming injections of heroin (0.25 mg/kg, SC) or a footshock stressor (15 min; 0.5 mA) after a 5- to 8-day period of extinction. Thus, although acamprosate attenuated the expression of sensitized locomotor activity and DA release in the NAS, it did not have any consistent effect on either the intake of heroin during the maintenance phase or the relapse to heroin seeking in a drug-free state. Thus, to the extent that the self-administration and the reinstatement procedures provide valid preclinical models for drug use and relapse in humans, our data suggest that acamprosate may not be effective in altering drug-taking behavior in heroin users. Received: 4 November 1997/Final version: 25 January 1998     

Effects of opioid and dopamine receptor antagonists on relapse induced by stress and re-exposure to heroin in rats

The effects of blockade of opioid and dopamine receptors on relapse to heroin-seeking induced by footshock stress and re-exposure to heroin were examined in a reinstatement procedure. Male rats were trained to self-administer heroin (100 µg/kg per infusion, IV; four 3-h sessions/day for 8–11 consecutive days). Extinction sessions were given for 5–7 days during which saline was substituted for heroin. In nine groups, the effects on relapse induced by footshock (10 min, 0.5 mA, 0.5 s on with a mean off period of 40 s), heroin priming (0.25 mg/kg), and saline priming were studied after pretreatment with either naltrexone (1 or 10 mg/kg, SC), the D₁-like receptor antagonist SCH 23390 (0.05 or 0.1 mg/kg, IP), the D₂-like receptor antagonist raclopride (0.25 or 0.5 mg/kg, IP), the mixed dopamine antagonist flupenthixol decanoate (3 or 6 mg/kg, IM), or IP injection of saline (control condition). Naltrexone, flupenthixol, raclopride, and the highest dose of SCH 23390 attenuated heroin-induced relapse: only the mixed DA receptor antagonist, flupenthixol, attenuated footshock-induced relapse. These results, and those from microdialysis showing that heroin elicits greater locomotor activity and DA release in the nucleus accumbens than footshock, suggest that the neurochemical events underlying stress- and heroin-induced relapse are not identical.     

Cocaine seeking over extended withdrawal periods in rats: time dependent increases of responding induced by heroin priming over the first 3 months

Rationale Using a rat relapse model, recent studies reported time dependent increases in cocaine seeking induced by re-exposure to cocaine cues, but not cocaine itself, over withdrawal periods of up to 3 months.Objectives In the present study, we explored the time course of cocaine seeking induced by priming injections of heroin over the first 3 months of withdrawal from cocaine.Methods Rats were trained to self-administer intravenous cocaine for 6 h/day over a period of 10 days. Cocaine seeking induced by heroin priming was then assessed in different groups of rats after 1 day, and 1 and 3 months of withdrawal from cocaine. During the test day, rats were first given six 1-h extinction sessions. Subsequently, reinstatement of cocaine seeking induced by non-contingent saline and heroin injections (0.125 and 0.25 mg/kg, SC) was assessed during three 1-h sessions.Results As in previous studies, extinction responding was substantially greater after 1 and 3 months of withdrawal than after 1 day. More importantly, we also found that the effect of heroin priming on reinstatement of cocaine seeking is time dependent, with higher responding occurring after 1 and 3 months than after 1 day.Conclusion The present results replicate previous findings on the time dependent increases in resistance to extinction after withdrawal from cocaine, and further indicate that the duration of the drug withdrawal period is a critical modulator of the effect of heroin priming on cocaine seeking. These data may have implications for the treatment of cocaine relapse induced by other drugs.     

Safety assessment of meat from transgenic cattle by 90-day feeding study in rats

The study was carried out to evaluate the subchronic toxicity of meat derived from human lactoferrin gene-modified cattle in male and female Wistar rats. Rats were fed 5% or 10% transgenic meat diet, 5% or 10% conventional meat diet, or AIN93G diet for 90 days. During the study, body weight and food consumption were weighed weekly and clinical observations were conducted daily. At the end of the study, urinary examination, hematology and blood biochemistry examination, macroscopic and microscopic examinations were performed. There were no biologically significant differences in these factors between the rat groups fed transgenic meat diet and conventional meat diet. Therefore, the present 90-day rodent feeding study suggests that meat derived from the transgenic cattle is equivalent to meat from conventional cattle in use as dietary supplements.     

Route of administration influences substitution patterns in rats trained to discriminate methadone vs. vehicle

Replacement therapy with the synthetic μ-opioid agonist methadone is an efficacious treatment for opioid abuse. While much is known about methadone's pharmacology, its discriminative stimulus properties remain largely unexplored. The present study sought to establish methadone discrimination in rats. Moreover, some research suggests that route of administration alters the discriminative stimulus of methadone. Thus, the present study also compared intraperitoneal (i.p.) and subcutaneous (s.c.) routes of administration. Male Sprague–Dawley rats were trained to discriminate 3.0 mg/kg methadone (i.p.) from vehicle in a two-lever discrimination procedure. Generalization tests were conducted with a variety of compounds administered i.p. and s.c. Methadone fully substituted for itself, yielding ED₅₀s of 1.5 mg/kg (i.p.) and 0.2 mg/kg (s.c.). Naltrexone (i.p.), an opioid antagonist produced a dose-dependent reduction in methadone-appropriate responding. The methadone stereoisomers fully substituted for methadone when given s.c.; however, when administered i.p., (+) and (−) methadone produced partial and no substitution, respectively. Heroin fully generalized to methadone regardless of administration route, while morphine fully substituted when given s.c., but not i.p. The kappa-agonist U50-488 failed to generalize to methadone with either route of administration. These results demonstrated that methadone's discriminative stimulus is mediated through μ-opioid receptor activity and is similar to that of commonly abused opioids (heroin, morphine). Additionally, route of administration produced differential results for many of the drugs tested, suggesting decreased drug bioavailability following i.p. administration due to hepatic first pass metabolism. Taken together, these results suggest that methadone's shared subjective effects with abused opioids, as well as its unique metabolic properties contribute to its efficacy in opioid maintenance therapy.     

Safety assessment of powdered Cistanche deserticola Y. C. Ma by a 90-day feeding test in Sprague-Dawley rats

Cistanche deserticola (C. deserticola), a holoparasitic plant widely distributed in arid or semi-arid areas in Eurasia and North Africa, has been used as an important tonic in traditional Eastern medicine for centuries. However, little information on the systemic toxicity and safety evaluation of it is available. The purpose of this study was to investigate the potential toxicity of powdered C. deserticola as a novel food ingredient by use of a subchronic toxicity study in Sprague-Dawley (SD) rats. A total of 80 male and female rats were fed with diets containing 8, 4, 2 and 0% (control) powdered C. deserticola for 90 days. A toxicological assessment was performed including mortality, body and organ weight, food consumption, blood biochemistry, hematology, gross necropsy and histopathological examinations. There were no signs of toxicity and treatment-related changes in rats treated with powdered C. deserticola. The no-observed-adverse-effect level (NOAEL) of powdered C. deserticola was 7.8?g?kg^?1 body weight for males and 8.0?g?kg^?1 body weight for females of rats under the experimental conditions of this study.     

Safety assessment of transgenic canola RF3 with bar and barstar gene on Sprague-Dawley (SD) rats by 90-day feeding test

Canola is one of the most important plant oilseed crops. To avoid the threat of herbicides, the RF3 line with bar gene and barstar gene was developed, which can act as glufosinate resistance resources and restore fertility in hybrid lines. To assess the food safety of transgenic canola RF3, 2.5%, 5% and 10% GM canola RF3 and its non-GM isogenic line Drakkar were formulated into diet to feed Spragure-Dawley (SD) rats for 90 days. The effects on the general growth and toxicological parameters, as well as gut microbiota of rats, were evaluated. Several significant differences on body weight, feed consumption, relative organ weight, hematology and serum biochemistry were observed among rats in the 90-day feeding test. However, these statistical differences were randomly observed among different groups and were not dose-related, which were not considered to be biologically significant. Furthermore, the results of bacterial 16S rRNA sequencing of fecal samples showed that the diets containing GM canola did not disturb the balance of gut microbiota. In conclusion, the canola RF3 is considered as safe and wholesome as the non-GM canola based on this 90-day feeding test and gut microbiota analysis.     

The production of skeletal muscle atrophy and mammary tumors in rats by feeding 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide

Chronic toxicity and carcinogenicity of a food additive, 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2) was examined by its oral administration to Wistar rats for 18 months. Male rats given 0.4% AF-2 diet developed ataxia of the hind foot around 4 months of feeding, and severe atrophy of skeletal muscles of the whole body was seen in 31 of 43 rats after 12 months. In female rats, mammary tumors developed in 78% of the animals given 0.4% AF-2 diet. Multiple papillomas were observed in the forestomach.