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

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.