Excessive ethanol drinking following a history of dependence: animal model of allostasis. Neuropsychopharmacology 22(6

Excessive drinking of ethanol in animals can be produced by a number of factors including altering palatability, genetics, and history of consumption. There is evidence that certain symptoms of withdrawal can persist for a number of weeks or even months following chronic ethanol exposure in humans In a larger cohort of 312 abstinent alcoholics, 20-25% of them showed signs of anxiety and depression, as determined from the Symptom Check-List 90 (self-report inventory with coverage of areas of symptomology and psychopathology) six months to two years following withdrawal Potential rodent models of excessive ethanol drinking include the alcohol deprivation effect in nondependent animals, ethanol self-administration in dependent withdrawing animals, and ethanol self-administration in animals with a history of dependence following periods of abstinence. The alcohol deprivation effect is a transient increase in ethanol self-administration in animals following periods of abstinence from ethanol for several days to several weeks Animal studies directed at examining the effects of withdrawal on ethanol self-administration have produced varying results Allostasis is a form of physiological regulation first hypothesized to describe the fluctuations in blood pressure and immune system function that are not well explained by homeostasis The persistent affective withdrawal symptoms observed in human alcoholics also may be a reflection of a shift in reward set point such that reward system functioning is decreased below baseline in the drug-free state. It has been argued that global allostatic changes associated with chronic demands lead to an unpleasant withdrawal state when the demands are lifted and provides the basis for seeking conditions of high demand The purpose of the current experiments was to examine potential alterations in operant oral ethanol selfadministration following protracted abstinence in rats with a history of dependence. Based on the conceptualization of ethanol addiction as reflecting an enhancement in drug reward set point or threshold (Koob and Le Moal 1997), it was hypothesized that rats with a history of dependence would show a long lasting increase in baseline ethanol responding. Therefore, the model described herein has the potential to provide a means to examine long lasting alterations in the mechanisms mediating ethanol reward which may be partly responsible for the chronically relapsing nature of alcoholism in humans. A paradigm was used in which rats were first trained to lever press for ethanol and then made dependent on ethanol in vapor chambers MATERIALS AND METHODS General Methods Animals. Male Wistar rats obtained from Charles River Laboratory (Kingston, NY) were used in Experiments 1-3 ( n ϭ 48), whereas Wistar rats originally derived from Charles River Laboratory (Kingston, NY), but bred in the Beckman Laboratories of The Scripps Research Institute, were used in Experiment 4 ( n ϭ 12). These latter Wistar rats are bred using a circular-pair random system of breeding in order to maintain genetic heterogeneity and new breeders are obtained from Charles River as determined by our internal Genetics Advisory Board. Separate groups of rats were used for each of the four experiments. Body weight was 180-200 g at the start of the experiments. Rats were housed 2-3 per cage with food and water available ad libitum , except for three days of water restriction at the initiation of operant testing (see below). Lights were on a 12-hr light/dark cycle, with lights on at 6:00 AM. All procedures met the guidelines of the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Operant Ethanol Self-Administration. Ethanol dilutions (5, 8, and 10% w/v) were prepared with 95% ethyl alcohol and water. Saccharin (Sigma Chemical Co., St. Louis, MO) was added to water or the ethanol solutions to achieve a concentration of 0.2% w/v. Standard operant chambers (Coulbourn Instruments, Allentown, PA) housed in sound-attenuated, ventilated cubicles were used for ethanol self-administration. Syringe pumps dispensed ethanol and water into two stainless steel drinking cups mounted 4 cm above the grid floor in the middle of one side panel. Two retractable levers were located 4.5 cm to either side of the drinking cups. Fluid delivery and recording of operant responding were controlled by microcomputer. A continuous reinforcement (FR1) schedule was used, and responding resulted in delivery of 0.1 ml of fluid. During the 0.5 second in which the pumps are activated, lever presses are not recorded. Rats were trained to lever press for ethanol using an adaptation of Samson's sweetened solution fading procedure Operant sessions were 30 minutes during the training phase and were conducted five days per week between 9:00 AM and 12:00 PM. Extended operant self-administration test sessions (12 hr) were conducted in Experiments 1 and 2. These were initiated at 6:00 PM, the beginning of the rats' active phase (dark cycle). Food was 584 A.J. Roberts et al. N EUROPSYCHOPHARMACOLOGY 2000 -VOL . 22 , NO . 6 placed in each operant box and 10% ethanol and water were available contingent on lever pressing. Test sessions in Experiments 3 and 4 were conducted in a manner similar to those used during training (30 min). Ethanol Vapor Chamber Procedure. Ethanol vapor exposure is a reliable technique for inducing ethanol dependence in that blood alcohol levels can be tightly controlled and animals are free-moving and gain weight normally Target blood alcohol levels were 150-200 mg% across the 2-4 week exposure time. This paradigm has been shown to produce physical dependence as evidenced by the appearance of observable withdrawal symptoms upon removal from the chambers Experiment 1 The purpose of this experiment was to determine whether increased responding for ethanol persisted beyond the acute withdrawal phase. It was shown previously that rats trained to lever press for ethanol and then exposed to ethanol vapor for two weeks, selfadministered more ethanol in operant boxes than control rats exposed to air across an entire 12-hr withdrawal test session Eight rats were trained to lever press for 10% ethanol and exposed to ethanol vapor ( n ϭ 4) or control air ( n ϭ 4). Groups were matched based on the final three days of 30-min operant testing. Following two weeks of vapor exposure, all rats were removed and placed immediately in operant boxes for a 12-hr overnight (active phase of circadian cycle) ethanol self-administration session. The rats were then re-exposed to the vapor chambers for five days and retested in a second 12-hr overnight session. Following this test, the rats were left out of the vapor chambers and not disturbed, except for routine husbandry, for two weeks. At this time, another 12-hr operant ethanol self-administration session was conducted. The rats were retested in this manner at 4 and 8 weeks following removal from the vapor chambers with no exposure to the operant boxes between these retest sessions (see Experiment 2 The purpose of Experiment 2 was to determine whether prior exposure to ethanol vapor would lead to in- Ethanol Self-Administration in Protracted Abstinence 585 creased ethanol self-administration even if the rats were not tested previously during early withdrawal. In this case, rats would be tested in the absence of potential learned associations between lever pressing for ethanol and relief from early withdrawal symptoms. Again, the hypothesis was that rats previously exposed to ethanol vapor should have greater motivation to lever press for ethanol following protracted withdrawal. Twenty eight rats were trained to lever press for 10% ethanol. On the final day of training, a 12-hr overnight test in the operant boxes was conducted in order to control for the extra test given to the rats during withdrawal in Experiment 1, as well as to select groups showing indistinguishable patterns of responding. The rats were then exposed for two weeks to ethanol vapor ( n ϭ 13) or control air ( n ϭ 15). One rat in the ethanol vapor group was lost due to over-intoxication (BAL Ͼ 300 mg%). Following removal from the vapor chambers, the rats were not disturbed for two weeks at which time they were tested in a second 12-hr overnight session in the operant self-administration boxes. This was repeated four and eight weeks following removal from the vapor chambers (see Experiment 3 The purpose of this experiment was to examine the effect of prior exposure to chronic ethanol vapor and subsequent abstinence on the resumption of daily 30-min test sessions. The results of Experiment 2 suggested that the only difference between the groups following protracted abstinence (when rats were not tested during early withdrawal) was in the first hour of retests. Therefore, the purpose of this experiment was to examine the effects of chronic ethanol exposure and a two week period of abstinence on the alcohol deprivation effect and its recovery to stable baseline responding. In addition, these rats were scored for ethanol withdrawal severity in order to confirm that the ethanol vapor chamber protocol was producing physical dependence as had been found previously Twelve rats were trained to lever press for 10% ethanol and then half were exposed to ethanol vapor ( n ϭ 6) and half were exposed to air ( n ϭ 6) for two weeks. Eight hours following removal from the chambers, the rats were tested for ethanol withdrawal symptoms. Rats were scored from 0 (undetectable) to 2 (severe) in a test of ventromedial distal limb flexion and observed for tail stiffness and abnormal body posture. The sum of these three measures represented overall withdrawal severity, yielding a range from 0 to 6. More detail regarding this procedure has been previously published Experiment 4 The results of the previous experiments showed that there appears to be an enhancement of the alcohol deprivation effect and an increase to a new higher baseline of responding for ethanol in rats previously exposed to ethanol vapor, relatively to air exposed rats. Thus, the purpose of Experiment 4 was to further investigate this finding by allowing rats three cycles of abstinence and retesting following a single ethanol vapor exposure. In this experiment, rats were exposed to ethanol vapor or air for one month. Again, it was hypothesized that rats previously exposed to ethanol vapor would show enhanced alcohol deprivation effects and higher baseline levels of responding over several repeated tests. Twelve rats were trained to lever press for 10% ethanol and then half were exposed to ethanol vapor ( n ϭ 6), whereas the other half was exposed to air ( n ϭ 6) for four weeks. Following removal from the chambers, the rats were left undisturbed for two weeks and then daily 30-min operant ethanol self-administration sessions were resumed for seven days. Rats were deprived of ethanol (not exposed to operant self-administration boxes) for three 1-week intervals separated by seven daily 30-min operant sessions in which 10% ethanol and water were available (see Data Analysis Operant responding was analyzed by mixed factor 2-way analysis of variance. The between-subject factor was group (ethanol vapor versus air control) and the within-subject factor was time (hours in Experiments 1 and 2 and repeated daily sessions in Experiments 3 and 4). Each cycle of retest was analyzed separately. Interactions between group and either hour or session were investigated using simple main effects analyses. RESULTS Operant responding for ethanol by rats in the present experiments was similar to that found in previous experiments in which blood alcohol levels were determined. For example, at the completion of a 12-hr early withdrawal test session, similar to that used in Experiment 1, rats were found to have blood alcohol levels of approximately 100 mg% N EUROPSYCHOPHARMACOLOGY 2000 -VOL . 22 , NO . 6 have been associated with blood alcohol levels of 40-80 mg% Experiment 1 Baseline responding for ethanol was 21.1 Ϯ 2.4 responses in the final 30-min training session prior to ethanol vapor exposure. Blood alcohol levels achieved by rats in the ethanol vapor chambers averaged 157 Ϯ 19 mg% across the two weeks of exposure (time course data not shown). Body weights upon removal from the vapor chambers were 454 Ϯ 51 g in the ethanol vapor group and 436 Ϯ 21 g in the air control group. The results of this experiment are shown in These results suggest that the enhancement in operant responding for ethanol by rats exposed to chronic ethanol vapor which accompanies acute withdrawal is maintained over several weeks of protracted abstinence. Experiment 2 Baseline responding for ethanol was 27.2 Ϯ 3.8 responses in the final 30-min training session prior to ethanol vapor exposure. Blood alcohol levels of ethanol vapor-exposed rats averaged 149 Ϯ 15 mg%. Body weights upon removal from the vapor chambers were 483 Ϯ 20 g in the ethanol vapor group and 486 Ϯ 18 g in the air control group. The results of Experiment 2 are shown in Experiment 3 Baseline responding for ethanol was 26.3 Ϯ 3.7 responses in the final 30-min training session prior to ethanol vapor exposure. Blood alcohol levels averaged 150 Ϯ 6 mg% across the two weeks of ethanol vapor exposure. Body weights upon removal from the vapor chambers were 492 Ϯ 21 g in the ethanol vapor group and 481 Ϯ 16 g in the air control group. Withdrawal scores in ethanol vapor-treated rats averaged 3.9 Ϯ 0.4 and in air controls were 0.57 Ϯ 0.2. The scores of the ethanol vapor group are consistent with previous results The results of Experiment 3 are shown in NEUROPSYCHOPHARMACOLOGY 2000-VOL. 22, NO. 6 the first retest session as indicated by a significant main effect of group (F[1,12] ϭ 11.7, p Ͻ .01) and a significant group by time epoch interaction (F[5,60] Experiment 4 Baseline responding for ethanol was 19.2 Ϯ 3.1 responses in the final 30-min training session prior to ethanol vapor exposure. Blood alcohol levels of ethanol vapor rats averaged 143 Ϯ 15 mg% for the one-month NEUROPSYCHOPHARMACOLOGY 2000-VOL. 22, NO. 6 Ethanol Self-Administration in Protracted Abstinence 589 exposure time. Body weights upon removal from the vapor chambers were 506 Ϯ 17 g in the ethanol vapor group and 522 Ϯ 16 g in the air control group. The results of this experiment are shown in NEUROPSYCHOPHARMACOLOGY 2000-VOL. 22, NO. 6 ses revealing significant effects of group in the first three test sessions of the blocks (F[1,18] ϭ 8.6-9.1, p Ͻ .01). The results of Experiment 4 are in accordance with those of the previous three experiments in that prior exposure to chronic ethanol vapor is associated with an increase in operant ethanol self-administration. As in Experiment 2, when rats were not tested during withdrawal, this effect appeared to be statistically reliable through the 4th or 5th week following removal from the vapor chambers. DISCUSSION The principal result of these experiments was that rats with a history of chronic ethanol exposure, sufficient to produce signs of physical dependence, showed persistent increases in operant ethanol self-administration during withdrawal and protracted abstinence. This is an important finding as it suggests that increases in ethanol self-administration can persist for 4-8 weeks fol- NEUROPSYCHOPHARMACOLOGY 2000-VOL. 22, NO. 6 Ethanol Self-Administration in Protracted Abstinence 591 lowing a single chronic ethanol exposure sufficient to produce dependence. The increase was anywhere from 30% to 100% above baseline, and, based on previous data would result in an increase in blood alcohol levels from about 25-30 mg% to 40-80 mg% in 30-min sessions The differences between the findings of Experiments 1 and 2 are quite intriguing and suggest that experience with ethanol self-administration during the early withdrawal period leads to a more robust increase in ethanol self-administration under conditions of protracted abstinence. This may be due to the development of an association between ethanol drinking and alleviation of withdrawal signs which enhances the reinforcing efficacy of ethanol. If animals are allowed to self-administer ethanol during repeated withdrawals, they show more robust increases in preference for ethanol 592 A.J. Roberts et al