Modification of anxiety-like behaviors by nociceptin/orphanin FQ (N/OFQ) and time-dependent changes in N/OFQ-NOP gene expression following ethanol withdrawal

Abstract

Anxiety is a key consequence of ethanol withdrawal and important risk factor for relapse. The neuropeptide nociceptin/orphanin FQ (N/OFQ) or agonists at this peptide's receptor (NOP) exert anxiolytic-like and antistress actions. N/OFQ dysfunction has been linked to both a high-anxiety behavioral phenotype and excessive ethanol intake. Recent studies suggest a possible link between genetic polymorphisms of the NOP transcript and alcoholism. Thus, in the present study, the effects of intracerebroventricularly administered N/OFQ were tested for modification of anxiety-like behaviors, using the shock-probe defensive burying and elevated plus-maze tests, in ethanol-dependent versus non-dependent rats, 1 and 3 weeks following termination of ethanol exposure. Additionally, prepro-N/OFQ (ppN/OFQ) and NOP receptor gene expression was measured in the central nucleus of the amygdala, in the bed nucleus of the stria terminalis and in the lateral hypothalamus at the same timepoints in separate subjects. One week post-ethanol, N/OFQ dose-dependently attenuated elevated anxiety-like behavior in ethanol-dependent rats and produced anxiolytic-like effects in non-dependent controls in both behavioral tests. However, 3 weeks post-ethanol, N/OFQ altered behavior consistent with anxiogenic-like actions in ethanol-dependent rats but continued to exert anxiolytic-like actions in non-dependent controls. These findings were paralleled by ethanol history-dependent changes of ppN/OFQ and NOP gene expression that showed a distinctive time course in the examined brain structures. The results demonstrate that ethanol dependence and withdrawal are associated with neuroadaptive changes in the N/OFQ-NOP system, suggesting a role of this neuropeptidergic pathway as a therapeutic target for the treatment of alcohol abuse.

Figure 1

Effects of N/OFQ on anxiety-like behavior as determined 1 week following termination of chronic ethanol (ethanol-dependent) or vehicle (nondependent) administration in (A) the elevated plus maze and (B) defensive burying tests. Ethanol-dependent rats showed significantly increased behavioral manifestations of anxiety in both the elevated plus maze (reduced open arm time) and defensive burying (longer burying duration) models. N/OFQ dose-dependently reversed anxiety-like behavior in ethanol-dependent rats. In nondependent rats, N/OFQ produced anxiolytic-like effects, but only at the highest (2 µg) dose. * p < 0.05 vs. vehicle (0 dose) within the same group; ^§ p < 0.05 vs. nondependent.

Figure 2

Effects of N/OFQ on anxiety-like behavior measured 3 weeks following termination of chronic ethanol (ethanol-dependent) or vehicle (nondependent) administration in (A) the elevated plus maze and (B) defensive burying tests. Overt anxiety-like behavior in ethanol-dependent rats no longer differed from that of nondependent rats in terms of either elevated plus maze or defensive burying performance (see zero dose, vehicle condition). As during the 1-week post-withdrawal test (Figure 1), N/OFQ produced anxiolytic-like effects at the high (2µg) dose in nondependent rats. In contrast, N/OFQ produced anxiogenic-like effects at doses of 1.0 µg (elevated plus maze, defensive burying) and 2 µg (elevated plus maze) in ethanol-dependent rats. * p < 0.05 vs. vehicle (0 dose) within the same group; ^§ p < 0.05 vs. nondependent.

Figure 3

Relative gene expression levels of (A) NOP or (B) ppN/OFQ, in the BNST of rats measured 1 week or 3 weeks following termination of chronic ethanol (ethanol-dependent) or vehicle (nondependent) administration. Data are expressed as mean (±SEM) values relative to mRNA levels in experimentally-naïve age-matched controls. * p < 0.05, ** p < 0.01, *** p < 0.001.

Figure 4

Relative gene expression levels of (A) NOP or (B) ppN/OFQ, in the CeA of rats measured 1 week or 3 weeks following termination of chronic ethanol (ethanol-dependent) or vehicle (nondependent) administration. Data are expressed as mean (±SEM) values relative to mRNA levels in experimentally-naïve age-matched controls. *** p < 0.001.

Figure 5

Relative gene expression levels of (A) NOP or (B) ppN/OFQ, in the LH of rats measured 1 week or 3 weeks following termination of chronic ethanol (ethanol-dependent) or vehicle (nondependent) administration. Data are expressed as mean (±SEM) values relative to mRNA levels in experimentally-naïve age-matched controls. ** p < 0.01.