Department of Pharmacology, Umeå University, S-901 87 Umeå, Sweden
Received 29 July 1999; in revised form 27 April 2000; accepted 12 May 2000
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ABSTRACT |
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INTRODUCTION |
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In our laboratory, a rat model of psychological dependence has been developed (Wahlström, 1994a). This model starts with a 1-year treatment period during which the rats are intermittently (once a week) exposed to intoxicating amounts of ethanol. The result of this treatment is that the rats take the same dose of ethanol independently of the offered ethanol concentration, indicating a need for a certain pharmacological effect of ethanol. The present experiment evaluates how this very stable ethanol intake is affected by pretreatments with different doses of ethanol.
Parts of this article have previously been presented at the 1999 Congress of the European Society for Biomedical Research on Alcoholism (Hedlund and Wahlström, 1999).
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MATERIALS AND METHODS |
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The effect of this treatment is that, during an evaluation period, rats treated according to these prerequisites take the same daily dose of ethanol in a continuous voluntary choice situation independently of the concentration of the offered ethanol solution in the range of 5 to 25% (Wahlström, 1987). This indicates a central nervous system mechanism that closely regulates ethanol intake, to reach a certain pharmacological effect. In our opinion, this defined need for a specific dose of the drug is an important part of both psychological dependence and craving. This stability of the ethanol intake independently of concentration is not seen in other rats, e.g. rats that have received, during the treatment period, saline injections instead of ethanol injections together with a continuous choice between 10% ethanol and water (Wahlström, 1987
), or rats recently introduced to ethanol (Myers and Oblinger, 1977
).
Animals
Male SpragueDawley rats (Mol:SPRD Han) were purchased from Möllegaards Breeding Centre Ltd, Li Skensved, Denmark. They were kept in individual cages for 10 days prior to the start of experiments. Each cage was equipped with two drinking bottles. One of the bottles always contained tap water and the other contained either water or an ethanol solution (10 or 20% w/v). Food [commercial rat pellets, R34, Lactamin, Stockholm, Sweden, consisting of 16.5% raw protein, 4.0% raw fat and 58.0% NFE (carbohydrates), 1255 kJ/100 g] was available ad libitum. The room where the rats were housed had a reversed light/dark schedule (lights on 19:0007:00) and a room temperature around 23°C. The age of the animals was determined from the age/weight curve (supplied by the breeder) and was approximately 5 weeks at the start of the experiment. The experiment was approved by the regional ethical committee for animal research (Umeå djurförsöksetiska nämnd).
Drugs
Ethanol (AB Svensk Sprit, Sweden) as a drinking fluid was mixed with tap water at concentrations of 10 and 20% (w/v). Ethanol for injection was dissolved in a 0.9% (w/v) NaCl solution and administered i.p. Control animals were given saline injections at a corresponding volume.
Treatment of psychologically dependent rats
Forty-eight rats were assigned randomly to four groups. Group 1 was a control group that received saline injections. Groups 2, 3, and 4 received ethanol injections to study the effect on voluntary ethanol intake. The groups are described in more detail below. As seen in the Results section, a few rats in each group died during the experiment, but no deaths occurred during the two tests with ethanol. The experimental design used in this experiment is illustrated in Fig. 1. Details of the model and the basic design have been described in earlier publications (Wahlström, 1987
, 1994a
,b
). The present experiment consisted of two periods. The first was a 1-year treatment period during which psychological dependence was induced. In the following evaluation period, the psychological dependence of the rats was tested according to our empirical criterion, and the pretreatments with alcohol injections were given.
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During the evaluation period (weeks 5574), the rats had a voluntary and continuous oral choice between ethanol and water, with random placement of the ethanol bottle each week. No ethanol injections were given, except for the two ethanol treatments (see Fig. 1). The basic ethanol concentration was 10% (v/v). The ethanol concentration was changed prior to the treatments to 20% for 3 weeks (weeks 60, 61, and 62) to test the psychological dependence of the rats. In this test, the average individual intake on 10% in the 2 weeks before and the 2 weeks after the period with 20% ethanol was compared by regression analysis to the average individual intake on 20% ethanol. Our main criterion of psychological dependence was that the individual rats must have the same ethanol intake in g/kg/day for both concentrations. The statistical criteria consisted of: (1) there had to be a significant correlation between the intakes of the two concentrations; (2) the slope of the regression line did not deviate from 1.0; (3) the regression line did not deviate from origin.
The first ethanol treatment was given on the first day of week 69. Group 2 was injected with 20%, group 3 was injected with 40% and group 4 was injected with 60% of the mean daily intake on the first 6 days of the previous week. The control group (group 1) was injected with saline. The ethanol was mixed with saline at concentrations of 4%, 8%, and 12% respectively. This procedure gave all rats a volume in ml determined by: (ethanol intake in g/kg/day) x5. No rat was injected with more than 12 ml.
A second ethanol treatment was given on the first day of week 71. This time, the rats received predetermined doses. Group 2 was injected with 0.5 g/kg, group 3 with 1.0 g/kg, group 4 with 2.0 g/kg, and group 1 with saline. The volume was 2.0 ml/kg body wt.
Statistical methods
Conventional parametric statistical methods were used. Differences between two groups were tested with Student's t-test. A P < 0.05 was used as the basic level of significance. No significance was denoted by NS. n denotes number of observations. Error bars denote 1 standard error of the mean (SEM). Correlation (r) and regression (b) coefficients were tested against 0. If the regression coefficient differed from 0, it was also tested against 1.0. In this case, NSD1 denotes that the slope of the regression line was not significantly different from 1.0.
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RESULTS |
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DISCUSSION |
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Another important consideration is the pharmacokinetic situation after the ethanol injections compared to the ensuing voluntary intake. The intoxicating effect of the ethanol injection subsides within a few hours, while other effects might be more long lasting. During which part of the 24-h period the ethanol intake is most influenced cannot be ascertained from the present experiment. In an earlier experiment, Gauvin et al. (1993) administered ethanol 15 min before a 30-min ethanol choice period. In that experiment, the intake of ethanol was increased after the dose of 0.25 g/kg, and decreased after 1.0, 1.5 and 2.0 g/kg. The 2.0 g/kg dose in that experiment reduced ethanol intake to 510% of the pretreatment intake, whereas in the present experiment only 2.0 g/kg reduced ethanol intake to 39% of the pretreatment intake (Table 1). One explanation of this difference could be that most of the voluntary intake of ethanol in the present experiment occurred late during the 24-h period.
The method of ethanol administration is of course important in relation to the results. In this experiment, the effect on voluntary intake was studied, whereas other investigators have found differences in the effects of intragastric vs i.p. injections on conditioned place preference (CPP; Ciccocioppo et al., 1999a) and conditioned taste aversion (CTA; Ciccocioppo et al., 1999b), where CTA developed with lower doses of ethanol when administered i.p. Furthermore, it is possible that different behavioural effects of ethanol are seen at different doses. For example, the discriminative stimulus effects of ethanol can be learned easily at doses above 1 g/kg (Kostowski and Bienkowski, 1999), whereas the increase in ethanol intake mentioned above was seen at 0.25 g/kg, and CPP as well as CTA are seen with doses between 0.25 g/kg and 1 g/kg. However, we believe that the behavioural measure of most interest in relation to alcoholism in humans is the voluntary oral intake of ethanol.
The concept of loss of control drinking is included in the third criterion of alcohol dependence according to the diagnostic criteria (DSM-IV) of the American Psychiatric Association (1994), which is fulfilled if the patient drinks alcohol in larger amounts or over a longer time period than was initially intended. In the present experiment, an increased intake of ethanol after ethanol pretreatment was only seen in rats with a low pretreatment intake. This suggests that ethanol exposure does not increase the craving for more ethanol in all individuals. Thus, if the present model involves a system regulating ethanol intake, also present in humans, ethanol exposure does not immediately trigger the system that increases the craving for ethanol. This suggests that a direct effect of ethanol on a system regulating ethanol intake could not be the mechanism behind the concept of loss of control drinking in humans.
The theory of loss of control drinking has also been tested in humans. Marlatt et al. (1973) gave alcoholics either tonic water with or without vodka, and told half of each group that they were drinking tonic and the other half that they were drinking tonic and vodka. The consumption of alcohol in this experiment was independent of the actual alcohol content, but strongly dependent of what the subjects were told they were drinking. Those who were told that they were drinking alcohol consumed more than twice the amount of those who were told they were drinking just tonic. Paredes et al. (1973) gave abstinent alcoholics access to alcohol on two consecutive days between 13:30 and 22:00, with the size of the drinks adjusted to keep BAC below 140 mg/dl. The authors observed the behaviour and concluded that alcohol did not trigger alcohol-seeking behaviour or make the patients lose control of their behaviour. In a study by Engle and Williams (1972) alcoholics received a strongly flavoured vitamin drink, half with and half without vodka. Half of each group were told they were drinking alcohol and the other half that they were not. The alcoholics then rated their desire for alcohol. The conclusion of the authors was that the desire for alcohol was related to what the alcoholics were told they were drinking and not what they actually had drunk. Thus, also in humans ethanol exposure does not directly trigger craving for more alcohol.
Robinson and Berridge (1993), in their incentive-sensitization theory of addiction, stated that the fact that drug craving remains high or is even elevated after drug administration is not consistent with either a negative reinforcement view of craving or a pleasure-seeking view. However, these facts are consistent with their incentive-sensitization theory of addiction where craving is the subjective experience associated with incentive salience attribution. When ethanol intake increases dopamine activity, which in this sensitized system produces incentive salience, drug wanting increases. The main foundation for their theories are reports of drug-induced drug craving for cocaine, heroin, and hydromorphone. However, the results of the present experiment and the clinical studies mentioned above indicate that small doses of alcohol do not result in increased craving or loss-of-control drinking in alcoholics. Thus, alcohol dependence does not seem to fit into this theory. Indeed, Robinson and Berridge (1993, p. 276) caution that whether the addictive potential of alcohol can be accounted for by incentive-sensitization remains to be seen.
However, other theories exist of why a relapse occurs and what role the first drink has in causing a relapse. Tiffany (1990) proposed that drug use in the addict is controlled by automatized action schemata, and that there exist in the abstinent addict non-automatic cognitive processes that counteract the urge to use the drug. These non-automatic processes (working against alcohol use) have always to be stronger than the automatic processes (working for alcohol use) if the alcoholic is to stay abstinent. The relapse then occurs if their nonautomatic processing is devoted to some other task (e.g. they are distracted) and the current environmental circumstances are completely supportive of the drug-use action plan. Furthermore, apart from the fact that alcohol reduces the efficiency of the non-automatic processes that counteract the drug-use plans, the presence of a first drink shows that stimuli that can trigger the drug-use plan are present.
The effect on ethanol intake of the ethanol injections in the rats participating in this experiment, that have had continuous access to ethanol, indicate that rats with a low intake of ethanol increase their intake when treated with low doses of ethanol, whereas rats with a high voluntary intake of ethanol decrease their ethanol intake regardless of dose. This could be consistent with the situation in humans (Engle and Williams, 1972; Marlatt et al., 1973
; Paredes et al., 1973
). It could also be consistent with the theories of Tiffany (1990) if it is assumed that the rats participating in this model do not have any non-automatic processes that work against ethanol intake. However, it is not certain if it is consistent with the incentive-sensitization theory of addiction, since independently of the administered dose, only rats with a low ethanol intake increased their intake.
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ACKNOWLEDGEMENTS |
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FOOTNOTES |
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