Orexin/hypocretin-1 receptor antagonism reduces ethanol self-administration and reinstatement selectively in highly-motivated rats

Abstract

The orexin/hypocretin (ORX) system regulates motivation for natural rewards and drugs of abuse such as alcohol. ORX receptor antagonists, most commonly OX1R antagonists including SB-334867 (SB), decrease alcohol drinking, self-administration and reinstatement in both genetically-bred alcohol-preferring and outbred strains of rats. Importantly, levels of alcohol seeking and drinking in outbred rats are variable, as they are in humans. We have shown that OX1R antagonism selectively decreases homecage alcohol drinking in high-, but not low-alcohol-preferring rats. It is unknown, however, whether this effect is selective to homecage drinking or whether it also applies to alcohol seeking paradigms such as self-administration and reinstatement following extinction, in which motivation is high in the absence of alcohol. Here we trained Sprague Dawley rats to self-administer 20% ethanol paired with a light-tone cue on an FR3 regimen. Rats were then extinguished and subjected to cue-induced reinstatement. Rats were segregated into high- and low-ethanol-responding groups (HR and LR) based on self-administration levels. During self-administration and cue-induced reinstatement, rats were given SB or vehicle prior to ethanol seeking. In both conditions, OX1R antagonism decreased responding selectively in HR, but not LR rats. There were no non-specific effects of SB treatment on arousal or general behavior. These data indicate that ORX signaling at the OX1R receptor specifically regulates high levels of motivation for alcohol, even in the absence of direct alcohol reinforcement. This implicates the ORX system in the pathological motivation underlying alcohol abuse and alcoholism and demonstrates that the OX1R may be an important target for treating alcohol abuse.

Keywords: Alcoholism; Individual differences; Lateral hypothalamus; Motivation; Neuropeptide; Reward.

Figure 1. Comparison of alcohol self-administration behavior in HR vs LR rats

(A) Although HR and LR animals showed similar patterns of active lever responding on FR1 and FR2 sessions, HR animals showed significantly greater active lever responses during FR3 sessions. (B) There were no differences between HR and LR animals in terms of inactive lever responses at any stage of self-administration training. (C) Similar to active lever responses, HR animals exhibited a significantly greater number of rewarded well entries on FR3 self-administration sessions. (D) HR and LR animals showed similar levels of non-rewarded well entries during self-administration training. ** p<0.01; *** p<0.001; **** p<0.0001.

Figure 2. SB-334867 attenuates alcohol self-administration behavior in HR, but not LR rats

(A) In HR animals, SB10 and SB20 treatment significantly reduced active (but not inactive) lever responses during a 2-hr self-administration session. (B) HR animals also showed a significant reduction in the number of rewarded (but not non-rewarded) well entries following SB treatment. (C, D) In contrast to HR rats, self-administration behavior in LR rats was unaffected by SB treatment. *p<0.05; ** p<0.01; *** p<0.001; n.s. not significant.

Figure 3. HR rats exhibited greater levels of responding than LR rats on the first day of extinction

(A) Active lever presses during 10 days of extinction in HR and LR rats. (B) HR rats responded significantly more than LR rats on extinction day 1. By the last day of extinction, active lever responses were equivalent between HR and LR groups. ** p<0.01.

Figure 4. SB-334867 attenuates cue-induced reinstatement of alcohol seeking behavior in HR but not LR

rats. (A) HR animals showed a significant reinstatement of active lever responding during cue-induced reinstatement tests. This reinstatement was blocked by SB20 treatment. (B) Similarly, SB20 significantly attenuated the number of ‘rewarded’ well entries during cue-induced reinstatement tests and also reduced the number of non-rewarded well entries made. (C) SB20 treatment had no effect on the number of active or inactive lever responses made during reinstatement tests. (D) SB20 also did not affect the number of ‘rewarded’ or non-rewarded well entries made during testing. *p<0.05; ** p<0.01.

Figure 5. Comparison of homecage alcohol intake in HR vs LR rats

(A) Ethanol intake during 3-hr 2-bottle choice tests was equivalent between HR and LR groups. (B) Ethanol preference was also similar between the two groups.