Protracted withdrawal from ethanol and enhanced responsiveness stress: regulation via the dynorphin/kappa opioid receptor system

Abstract

Although recent work suggests that the dynorphin/kappa opioid receptor (DYN/KOR) system may be a key mediator in the stress-related effects of alcohol, the regulation of long-term changes associated with protracted withdrawal from ethanol via the DYN/KOR system has yet to be explored. The objective of the present study was to determine the role of the DYN/KOR system in the regulation of anxiety-related behaviors during an extended period of abstinence from ethanol in animals with a history of ethanol dependence. Male Wistar rats (n = 94) were fed an ethanol or control liquid diet for 25-30 days. Six weeks after its removal, rats were exposed to 20 min of immobilization, and the ability of the KOR antagonist nor-binaltorphimine (nor-BNI) (0-20 mg/kg, intraperitoneal [i.p.]) to attenuate the enhanced responsiveness to stress observed in rats chronically exposed to ethanol was investigated using the elevated plus maze. In addition, the ability of U50,488 (0-10 mg/kg, i.p.) to prime anxiety-like behavior during protracted withdrawal was also examined. Rats with a history of ethanol dependence showed a significant decrease in open-arm exploration after exposure to restraint, indicating an anxiety-like state, compared to similarly treated controls, an effect that was blocked by nor-BNI. nor-BNI also selectively decreased center time and open-arm approaches in ethanol-exposed rats. The highest dose of U50,488 decreased open-arm exploration and the total number of arm entries in ethanol-exposed and control rats. Although lower doses of U50,488 did not affect open-arm exploration in either group, the 0.1 mg/kg dose selectively decreased motor activity in the ethanol-exposed rats when compared to similarly pretreated controls. These findings further support the hypothesis that behaviors associated with withdrawal from ethanol are in part regulated by the DYN/KOR system, and suggest that these effects may be long lasting in nature.

Figure 1

KOR regulation of the enhanced responsiveness to stress observed in rats with a history of ethanol dependence during protracted withdrawal from ethanol. Rats were fed an ethanol or control liquid diet for 25–30 days. Six weeks after its removal, rats were pretreated with nor-BNI or vehicle 24–30 hours prior to testing in the elevated plus maze. Immobilization stress (20 minutes) took place immediately prior to testing. A. nor-BNI reverses the anxiogenic-like effects of mild restraint in rats with a history of ethanol dependence. The percentage of time spent exploring the open arms, as indicated by the ratio of open-arm to total arm time and entries, has been proposed to relate inversely to anxiety. *p < .05 compared to all other groups, Fisher's test. B. Effects of nor-BNI on total number of arm entries. Arm entries have been proposed as an index of locomotor activity. C. Effect of nor-BNI on time spent in the center of the elevated plus maze in rats with a history of ethanol dependence. A decrease in time spent in the center has been proposed as a measure of decreased risk-assessment behavior. *p < .05 compared to vehicle-treated controls, Fisher's test. D. nor-BNI decreases open-arm approaches in rats with a history of ethanol dependence. A decrease in open-arm approaches has been proposed as a measure of decreased risk-assessment behavior. *p < .05 compared to controls, Fisher's test.

Figure 2

Effects of low dose U50,488 injections on behavior in the elevated plus maze in rats with a history of ethanol dependence during protracted withdrawal from ethanol. Rats were fed an ethanol or control liquid diet for 25–30 days. Six weeks after its removal, rats were pretreated with U50,488 (0.1–10 mg/kg, i.p.) or saline 10 minutes before testing in the elevated plus maze. A. Effects of U50,488 on open-arm exploration in the elevated plus maze. The percentage of time spent exploring the open arms, as indicated by the ratio of open-arm to total arm time and entries, has been proposed to relate inversely to anxiety. B. U50,488 decreases locomotor activity in rats with a history of ethanol dependence. Arm entries have been proposed as an index of locomotor activity. *p < .05 compared to controls treated with 0.1 or 1.0 mg/kg U50,488 and vehicle-treated ethanol rats, Fisher's test. C. Effects of U50,488 on center time in the elevated plus maze. Time spent in the center has been proposed as a measure of risk-assessment behavior. D. Effects of U50,488 on open-arm approaches in the elevated plus maze. A decrease in open-arm approaches has been proposed as a measure of decreased risk-assessment behavior. *p < .05 compared to controls, ^#p < .05 compared to controls treated with 0.1 or 1.0 mg/kg U50,488, Fisher's test.