Pharmacology of drug self-administration

Low doses of morphine reduce voluntary alcohol consumption in rhesus monkeys

Experimental opioid modulation has been found to influence the consumption of alcohol in animals. Whereas it has generally been agreed upon that opiate antagonists reduce alcohol consumption, the results with opiate agonists are less consistent. The present study reports on the effect of low doses of morphine in 8 adult male rhesus monkeys that had a free choice in drinking water, a 16% and a 32% ethanol/water solution, (a) during continuous ad libitum access (Experiment I), and (b) after 2 days of alcohol abstinence (Experiment II). In both experiments each monkey received a single morphine injection (i.m.) in 5 different doses (0.03, 0.06, 0.17, 0.50, 1.50 mg.kg-1); each morphine injection (i.m.) was placebo-controlled in a cross-over design. Consumption was measured from 16.00 h in the afternoon (30 min after injection) to 08.30 h the next morning. In Experiment I after 0.50 and 1.50 mg.kg-1 of morphine ethanol intake and water consumption were both reduced during the first hours after injection; only ethanol intake remained reduced during the subsequent night. Effects lasted not longer than 24 h. In Experiment II, morphine administered 30 min before reintroduction of ethanol solutions reduced ethanol intake at doses of 0.17, 0.50 and 1.50 mg.kg-1; water consumption was unaffected. The reduction lasted for the subsequent night after the 2 highest doses. Records obtained of various spontaneous behavioural activities made it unlikely that the used dose range had induced some aspecific sedation; monkeys remained alert and active. The results are contradictory with studies in which low doses of morphine stimulated alcohol drinking in rats. The present results seem to support the hypothesis that at least in monkeys morphine can compensate for some effects of alcohol.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

Molecular Mechanisms Associated with Cocaine Effects

Cocaine has been shown to be a highly addictive and toxic drug. It produces these effects and a variety of other physiological and behavioral effects through its interactions with several distinct central nervous system receptor sites. We present the results of a series of studies that utilized multiple site analyses to elucidate which cocaine binding sites influence the reinforcing and toxic effects of cocaine and with what proportion of influence. The nature of cocaine interactions with monoamine transporters is also discussed, especially with the dopamine transporter, which has been shown to be the cocaine binding site that is primarily associated with the reinforcing effects of cocaine. We also provide evidence that vulnerability to both the toxic and addictive effects of cocaine may be significantly influenced by genetic differences in both humans and animals. In view of the fact that cocaine is commonly abused in a polydrug situation, we present the results of both behavioral and biochemical experiments which suggest that common biochemical pathways may mediate the reinforcing or addictive properties of drugs of abuse. Finally, we discuss research on the biochemical mechanisms associated with effects of ethanol, particularly those which may also influence cocaine self-administration and speculate on pharmacotherapeutic strategies for concurrent abuse of cocaine and ethanol.

Effects of d-amphetamine injected into the nucleus accumbens on ethanol reinforced behavior

Rats, initiated to self-administer 10% (v/v) ethanol in an operant situation using the sucrose-fading procedure, received bilateral n. accumbens microinjections of d-amphetamine prior to operant sessions. Doses of 4 micrograms, 10 micrograms and 20 micrograms/brain were administered and some animals also received a 4 microgram/brain dose of LY171555. Three different effects were observed: increased, decreased and no change in total session responding. There was no clear relation between injection area in the n. accumbens and type of effect observed. For either an increase or decrease in total session responding, momentary response rates were decreased. Both d-amphetamine and LY171555 produced similar results. The data support the hypothesis that dopamine in the n. accumbens is involved with ethanol reinforced operant responding but in a complex manner.

Effect of naltrexone on alcohol consumption during chronic alcohol drinking and after a period of imposed abstinence in free-choice drinking rhesus monkeys

Relapse into problematic alcohol drinking is a serious problem in the treatment of alcoholism. Free-choice drinking rhesus monkeys show relapse-like behaviour after imposed abstinence of alcohol, by immediately reinitiating ethanol intake at an increased level. The relapse-like behaviour of the monkeys seems not induced by physical withdrawal, but rather argues for a resistance to extinction of ethanol-reinforced behaviour. It has been suggested that endogenous opioids play a role in the positive reinforcing effect of ethanol. In this study, the effect of the opiate antagonist naltrexone was investigated in eight adult male rhesus monkeys (Macaca mulatta) who had about 1 year experience with alcohol drinking, under two conditions: 1) (expt 1) during continuous and concurrent supply of drinking water and two ethanol/water solutions (16% and 32% (v/v], and 2) (expt 2) after 2 days of alcohol abstinence. In both experiments, each monkey received six doses of naltrexone (0.02, 0.06, 0.17, 0.5, 1.0, 1.5 mg.kg-1); each dose was paired with a placebo injection (im) in a cross-over design. Consumption was measured from 16.00 hours in the afternoon (30 min after injection) to 9.00 hours the next morning. In experiment 1 naltrexone reduced total net ethanol intake in a graded dose-dependent manner. The effect of naltrexone was apparent shortly after injection, and lasted until the following day. Consumption of drinking water was reduced only shortly after injection. In expt 2, reduction of net ethanol intake was largely restricted to the first few hours of reinitiation of alcohol drinking, i.e. the period in which the abstinence-induced increase was manifest. Consumption of drinking water was not affected by naltrexone. Naltrexone hardly influenced consumption of the non-preferred ethanol solution of 32%. It is postulated that the opioid modulation specifically interacted with positively reinforced behaviour. In expt 2 naltrexone reduced ethanol intake at a lower dose (0.17 mg.kg-1) compared to expt 1 (0.50 mg.kg-1), but net ethanol intakes however remained higher. It might be that alcohol abstinence resulted in altered opioid activity, leading to increased ethanol-seeking behaviour. The renewed presentation of ethanol solutions (also) might have stimulated reinitiation of alcohol drinking, representing conditioned incentive stimuli. The reported monkey model of relapse in alcohol drinking could be a useful tool to evaluate new hypotheses and experimental treatments with respect to human alcoholism.