Resumption of morphine self-administration by ex-addict rats: an attempt to modify tendencies to relapse

The effect of heroin dependence on resumption of heroin self-administration in rats

BACKGROUND

It has been proposed that relapse vulnerability in previously dependent individuals results from augmentation of drug-induced reinforcement due to repeated associations between the interoceptive properties of the drug and reduction of acute withdrawal distress.

METHODS

To test this hypothesis, male Sprague-Dawley rats self-administered 0.05 mg/kg/inf heroin on continuous reinforcement (CR) and progressive ratio (PR) schedules. During this period, they also received injections of vehicle or escalating doses of heroin. Following tests of naloxone-precipitated withdrawal, as well as a drug-free period (4 days), and extinction (9 sessions), they were pre-treated with vehicle or yohimbine (0.5mg/kg, IV) and tested for resumption of heroin self-administration (0.05 mg/kg/inf) on CR and PR schedules, or tested for reinstatement in extinction conditions.

RESULTS

Increased self-administration on the CR schedule was observed in the heroin-injected rats, but no group differences were observed on the PR schedule, in spite of greater signs of withdrawal precipitated by naloxone in the heroin-injected rats. More importantly, there were no group differences in resumption of heroin self-administration, and this was not altered by yohimbine.

CONCLUSIONS

These results suggest that relapse vulnerability cannot be uniquely ascribed to enhanced reinforcing action of drugs; contextual and other conditioning factors must play a role in modulating resumption of drug intake after abstinence.

Heroin self-administration by rats: influence of dose and physical dependence

Lever-pressing behavior reinforced by intravenous infusion of various concentrations of heroin, and consequent development of physical dependence, were examined in rats. In addition, the influence of opiate dependence, and of its disappearance following withdrawal, on heroin self-administration were investigated. It was found that intravenous self-administration of heroin at 0.03 mg/kg/infusion maintained self-administration behavior without producing physical dependence. Total responses per session decreased with increasing unit dose of heroin, whereas the total amount of drug self-administered was directly related to unit dose. Significantly greater numbers of withdrawal signs and percentage body weight losses in response to naloxone injections were observed following self-administration of heroin at 0.1, 0.3 or 0.6 mg/kg/infusion. Intake of heroin at 0.03 mg/kg/infusion, but not at 0.1, 0.3 or 0.6 mg/kg/infusion, was found to increase significantly in opiate-dependent and postdependent animals. These findings support the previous use of 0.03 mg/kg/infusion as a suitable dose for illustrating the reinforcing effect of heroin without the influence of physical dependence.

A new ingestion method for long-term morphine intoxication in rat

A new method for long-term morphine intoxication in rat was developed. It was designed to deal with the nutritional imbalance and body weight loss that generally occurs using conventional techniques for morphine treatment. The morphine is administered in a nutritionally complete diet. Also pair-feeding is used to deal with intoxicated rats that do not eat the same amount of food as controls. The technique was validated during the study of different intoxication conditions, using different initial doses, dose increments and final doses. An initial dose of 25 mg morphine/kg b.w., raised exponentially up to 340 mg/kg b.w. in 8 days, made the rats dependent, as tested by withdrawal signs, precipitated by excluding morphine from the diet, or by administration of antagonists. A final dose of up to 715 mg morphine/kg b.w. was reached in 13 days without decreased food intake. However, initial doses of 340 or 715 mg/kg led to impaired weight gain and diet consumption. Plasma morphine levels of 3 micrograms/ml serum were reached on a dose of 340 mg/kg b.w. Also, preference for morphine diet over control diet was evaluated by choice tests. The technique is simple, time-saving and inexpensive, allowing the treatment of numerous animals for long periods under standardized intoxication conditions. No animals get ill or die.

Behavioral effects of chronic oral administration of levo-alpha-acetylmethadol in the rat

levo-alpha-Acetylmethadol was orally administered via a sucrose solution to rats in their drinking water for 24 days. A control group received only sucrose. Bodyweight and fluid consumption were monitored daily. The behavioral effects during chronic drug administration and during eight days of withdrawal were studied using behavior controlled by a fixed-interval schedule of food reinforcement. Body weights of treated animals remained stable during drug administration but decreased by approximately 25% during withdrawal. There were no significant differences in volume of fluid consumed by the two groups. Response rate and number of reinforcements were decreased during drug administration. During withdrawal, response rates were greater than pre-drug control rates.

Drug-seeking behavior in the dog: lack of effect of prior passive dependence on morphine

Twelve dogs were made dependent on morphine (20 mg/kg per 24 hours) by intravenous passive administration of the drug. Six were gradually withdrawn and six were withdrawn abruptly. Subsequent tests for self-administration of morphine began within 8 to 18 weeks after morphine was last administered passively. Morphine was available for self-administration at several unit dose levels for 8 weeks. In comparison with control dogs administered with saline, there was no evidence that prior dependence on morphine influences subsequent self-administration of morphine, or that morphine is a primary reinforcer for dogs. Possible implications of these and related observations are discussed.

Methadone dependence in the rat

Physical dependence on methadone was induced in rats by an initial "forced drinking" procedure and subsequently by i.p. administration of the drug. In a subsequent Experimental Phase of the study the physical dependence of one group was sustained by a "methadone maintenance" treatment, while two other groups were withdrawn from the drug, one gradually and one abruptly. When relapse trials were carried out during a Readdiction Phase it was found that the maintained group voluntarily consumed significantly greater amounts of methadone than did the two withdrawal groups. These groups did not differ between themselves but did in turn ingest significantly more methadone than a control group with no prior exposure to the drug. The characteristic loss of body weight reliably found during withdrawal from morphine was not demonstrated. This may have been due to the unexpected weight loss which occurred during the last stage of the initial Addiction Phase. The dependent variables of amount of methadone solution and the percentage of fluid consumed as methadone solution correlated highly. However the amount of methadone solution ingested was a better indicator of addiction liability as it was not influenced by fluctuations in the amount of water consumed by the animals.