Intermittent access to 20% ethanol induces high ethanol consumption in Long-Evans and Wistar rats

Abstract

Background: There has been some difficulty getting standard laboratory rats to voluntarily consume large amounts of ethanol without the use of initiation procedures. It has previously been shown that standard laboratory rats will voluntarily consume high levels of ethanol if given intermittent-access to 20% ethanol in a 2-bottle-choice setting [Wise, Psychopharmacologia 29 (1973), 203]. In this study, we have further characterized this drinking model.

Methods: Ethanol-naïve Long-Evans rats were given intermittent-access to 20% ethanol (three 24-hour sessions per week). No sucrose fading was needed and water was always available ad libitum. Ethanol consumption, preference, and long-term drinking behaviors were investigated. Furthermore, to pharmacologically validate the intermittent-access 20% ethanol drinking paradigm, the efficacy of acamprosate and naltrexone in decreasing ethanol consumption were compared with those of groups given continuous-access to 10 or 20% ethanol, respectively. Additionally, ethanol consumption was investigated in Wistar and out-bred alcohol preferring (P) rats following intermittent-access to 20% ethanol.

Results: The intermittent-access 20% ethanol 2-bottle-choice drinking paradigm led standard laboratory rats to escalate their ethanol intake over the first 5 to 6 drinking sessions, reaching stable baseline consumption of high amounts of ethanol (Long-Evans: 5.1 +/- 0.6; Wistar: 5.8 +/- 0.8 g/kg/24 h, respectively). Furthermore, the cycles of excessive drinking and abstinence led to an increase in ethanol preference and increased efficacy of both acamprosate and naltrexone in Long-Evans rats. P-rats initiate drinking at a higher level than both Long-Evans and Wistar rats using the intermittent-access 20% ethanol paradigm and showed a trend toward a further escalation in ethanol intake over time (mean ethanol intake: 6.3 +/- 0.8 g/kg/24 h).

Conclusion: Standard laboratory rats will voluntarily consume ethanol using the intermittent-access 20% ethanol drinking paradigm without the use of any initiation procedures. This model promises to be a valuable tool in the alcohol research field.

Fig. 1

The intermittent-access 20% ethanol drinking paradigm in Long–Evans rats induces a robust escalation in (A) ethanol intake and (B) preference for ethanol over water, reaching a baseline ethanol consumption of 5.1 ± 0.6 g/kg/24 h. There was a significant decrease in the water intake and a significant increase in ethanol intake over time on the ethanol days (C). There was no significant difference between the total fluid intake on the ethanol compared with the water days (D). The values are expressed as mean ethanol intake (g/kg/24 h), fluid intake (ml/24 h), or preference (ratio of ethanol over total fluid intake) ± SEM at each drinking session. **p < 0.01 and ***p < 0.001 compared with the first drinking session (one-way repeated measures ANOVA followed by Newman–Keuls post hoc test) n = 12.

Fig. 2

High levels of ethanol intake are maintained and ethanol preference is increased following a prolonged (40 days) period of abstinence from ethanol. Mean ethanol consumption (A) and preference for ethanol (B) were compared for the 10 drinking sessions before and after a 40-day-long ethanol deprivation period (paired Student’s t-test). The values are expressed as mean ethanol consumed (g/kg/24 h) and preference for ethanol (%) ± SEM, respectively, ***p < 0.001 compared as described in figure, n = 12.

Fig. 3

Acamprosate (200 mg/kg i.p.) decreases ethanol consumption using the intermittent-access 20% ethanol but not the continuous-access 10 or 20% ethanol drinking paradigms in Long–Evans rats. Data from the 3 drinking paradigms were analyzed separately (one-way repeated measures ANOVA within each group followed by Newman–Keuls post hoc test). The values are expressed as mean ethanol consumed (g/kg/24 h) ± SEM, **p < 0.01 compared with vehicle within treatment group, n = 7 to 12 per group. Acamp, Acamprosate.

Fig. 4

Low doses of naltrexone decrease ethanol consumption using the intermittent 20% ethanol drinking paradigm in Long–Evans rats when compared with continuous 10 or 20% ethanol access. Data from the 3 drinking paradigms were analyzed separately (one-way repeated measures ANOVA within each group followed by Newman–Keuls post hoc test). The values are expressed as mean ethanol consumed (g/kg/30 min) ± SEM; *p < 0.05, **p < 0.01, and ***p < 0.001 compared with vehicle within treatment group, n = 7 to 12 per group.

Fig. 5

High ethanol consumption in Wistar and P-rats using the intermittent-access 20% ethanol drinking paradigm. (A) Intermittent-access to 20% ethanol induced high levels of ethanol consumption in Wistar rats (n = 11), reaching a baseline consumption of 5.8 ± 0.8 g/kg/24 h and (B) P-rats (n = 6) initiate drinking at a high level and show a nonsignificant trend to increase their consumption of ethanol, baseline consumption: 6.3 ± 0.8 g/kg/24 h. The values are expressed as mean ethanol intake (g/kg/24 h) ± SEM at each drinking session. **p < 0.01 and ***p < 0.001 compared with the first drinking session (one-way repeated measures ANOVA followed by Newman–Keuls post hoc test).

Fig. 6

Blood ethanol concentrations (BECs; mg/dl) obtained following 30 minutes of voluntary oral ethanol consumption using the intermittent-access 20% ethanol drinking paradigm. The blood sample for BEC analysis was collected approximately 45 minutes into the dark cycle. The amount of ethanol consumed after 30 minutes of a drinking session significantly correlated with the measured BECs (linear regression): (A) Long–Evans: r² = 0.85, n = 10; (B) Wistar: r² = 0.63, n = 10; (C) P-rats: r² = 0.93, n = 6.