Stress-Induced Enhancement of Ethanol Intake in C57BL/6J Mice with a History of Chronic Ethanol Exposure: Involvement of Kappa Opioid Receptors

Abstract

Our laboratory has previously demonstrated that daily forced swim stress (FSS) prior to ethanol drinking sessions facilitates enhanced ethanol consumption in mice with a history of chronic intermittent ethanol (CIE) vapor exposure without altering ethanol intake in air-exposed controls. Because both stress and chronic ethanol exposure have been shown to activate the dynorphin/kappa opioid receptor (KOR) system, the present study was designed to explore a potential role for KORs in modulating stress effects on ethanol consumption in the CIE model of dependence and relapse drinking. After stable baseline ethanol intake was established in adult male C57BL/6J mice, subjects received chronic intermittent exposure (16 h/day × 4 days/week) to ethanol vapor (CIE group) or air (CTL group). Weekly cycles of inhalation exposure were alternated with 5-day limited access drinking tests (1 h access to 15% ethanol). Experiment 1 compared effects of daily FSS and KOR activation on ethanol consumption. CIE and CTL mice were either exposed to FSS (10 min), the KOR agonist U50,488 (5 mg/kg), or a vehicle injection (non-stressed condition) prior to each daily drinking session during test weeks. FSS selectively increased drinking in CIE mice. U50,488 mimicked this effect in CIE mice, but also increased drinking in CTL mice. Experiment 2 assessed effects of KOR blockade on stress-induced drinking in CIE and CTL mice. Stressed and non-stressed mice were administered the short-acting KOR antagonist LY2444296 (0 or 5 mg/kg) 30 min prior to each drinking session during test weeks. FSS selectively increased ethanol consumption in CIE mice, an effect that was abolished by LY2444296 pretreatment. In Experiment 3, CIE and CTL mice were administered one of four doses of U50,488 (0, 1.25, 2.5, 5.0 mg/kg) 1 h prior to each daily drinking test (in lieu of FSS). All doses of U50,488 increased ethanol consumption in both CIE and CTL mice. The U50,488-induced increase in drinking was blocked by LY2444296. Our results demonstrate that the KOR system contributes to the stress enhancement of ethanol intake in mice with a history of chronic ethanol exposure.

Keywords: dynorphin; ethanol; kappa opioid receptor; mice; stress.

Figure 1

Procedure overview. Each study consisted of a period of baseline ethanol consumption followed by repeated 1-week cycles of CIE (or air) exposure alternated with 1-week drinking test weeks. During test weeks in Experiment 1, CIE and CTL mice were subjected to one of three conditions: a 10-min forced swim stress (FSS) exposure 4 h before drinking (indicated by Δ), an injection of the KOR agonist U50,488 (5 mg/kg) 1 h before drinking (indicated by °), or a saline injection 1 h before drinking (non-stressed control group). During each test week in Experiment 2, CIE and CTL mice were assigned to one of four conditions defined by the 2 × 2 design: FSS or no stress (indicated by Δ) 4 h before drinking and an injection of LY2444296 (5 mg/kg) or vehicle 30 min before drinking (indicated by ×). During Tests 1–3 of Experiment 3, CIE and CTL mice received an injection of U50,488 (0, 1.25, 2.5, or 5 mg/kg) 1 h before drinking (indicated by °). Mice were not treated during Tests 4–5 (washout). During Test 6, CIE and CTL mice were assigned to one of four conditions: pretreatment with LY2444296 (5 mg/kg) or vehicle 1.5 h before drinking (indicated by ×) and administration of U50,488 (5 mg/kg) or vehicle 1 h before drinking (indicated by °).

Figure 2

Average weekly ethanol consumption (g/kg). Among air-exposed control (CTL) mice, administration of the KOR agonist U50,488 (5 mg/kg) resulted in elevated ethanol intake relative to baseline (indicated by ^∧), non-stressed (NS) mice (indicated by *), and forced swim stress-exposed (FSS) mice (indicated by ⁺) during all three test weeks. Among mice exposed to chronic intermittent ethanol (CIE), administration of U50,488 resulted in elevated ethanol intake relative to baseline (indicated by ^∧) and non-stressed mice (indicated by *) during Tests 1–3, and relative to FSS mice during Test 3 (indicated by ⁺). FSS mice demonstrated elevated ethanol consumption relative to baseline during all three test weeks (indicated by ^∧).

Figure 3

Average weekly ethanol consumption (g/kg). Among air-exposed control (CTL) mice, no effects of stress, chronic ethanol exposure (CIE), or KOR blockade were observed. Among CIE-exposed mice, FSS exposure (solid black bars) increased ethanol consumption relative to baseline consumption during all three test weeks (indicated by ^∧), relative to CTL-FSS mice during Tests 2–3 (indicated by ⁺), and relative to CIE-NS mice during Test 3 (indicated by *). Pretreatment with the KOR antagonist LY2444296 (striped black bars) blocked the FSS-induced increased drinking in CIE mice and resulted in a significant reduction from CIE-FSS vehicle mice during Test 3 (indicated by ^#).

Figure 4

Blood ethanol concentrations (BECs) following the final drinking session. Vehicle-treated CIE-FSS mice had higher BECs than CIE-NS mice (indicated by *) and CTL-FSS mice (indicated by ⁺). The FSS-induced increase in BEC was blocked by pretreatment with LY2444296, which resulted in significantly lower BECs than vehicle-treated CIE-FSS mice (indicated by ^#).

Figure 5

Average weekly ethanol consumption (g/kg). All doses of the KOR agonist U50,488 resulted in elevated ethanol consumption relative to baseline drinking (indicated by ^∧) and vehicle-injected controls (indicated by *) during Tests 1–3 for both CIE and CTL mice.

Figure 6

Average ethanol consumption during final test week (g/kg). For both CIE and CTL mice administered 5 mg/kg U50,488, pretreatment with the KOR antagonist LY2444296 blocked the increase in drinking observed in vehicle-treated mice (indicated by *).