Orexin signaling via the orexin 1 receptor mediates operant responding for food reinforcement

Abstract

Background: Orexin (hypocretin) signaling is implicated in drug addiction and reward, but its role in feeding and food-motivated behavior remains unclear.

Methods: We investigated orexin's contribution to food-reinforced instrumental responding using an orexin 1 receptor (Ox1r) antagonist, orexin -/- (OKO) and littermate wildtype (WT) mice, and RNAi-mediated knockdown of orexin. C57BL/6J (n = 76) and OKO (n = 39) mice were trained to nose poke for food under a variable ratio schedule of reinforcement. After responding stabilized, a progressive ratio schedule was initiated to evaluate motivation to obtain food reinforcement.

Results: Blockade of Ox1r in C57BL/6J mice impaired performance under both the variable ratio and progressive ratio schedules of reinforcement, indicating impaired motivational processes. In contrast, OKO mice initially demonstrated a delay in acquisition but eventually achieved levels of responding similar to those observed in WT animals. Moreover, OKO mice did not differ from WT mice under a progressive ratio schedule, indicating delayed learning processes but no motivational impairments. Considering the differences between pharmacologic blockade of Ox1r and the OKO mice, animals with RNAi mediated knockdown of orexin were then generated and analyzed to eliminate possible developmental effects of missing orexin. Orexin gene knockdown in the lateral hypothalamus in C57BL/6J mice resulted in blunted performance under both the variable ratio and progressive ratio schedules, resembling data obtained following Ox1r antagonism.

Conclusions: The behavior seen in OKO mice likely reflects developmental compensation often seen in mutant animals. These data suggest that activation of the Ox1r is a necessary component of food-reinforced responding, motivation, or both in normal mice.

Figure 1

Ox1r blockade results in decreased responding for food. (A) Mean number of reinforcers earned. (B) Mean number of active and inactive nose pokes. (C) Mean number of active nose pokes represented as percentage of total responses. Vertical lines represent the standard error of the mean (SEM).

Figure 2

Ox1r blockade results in decreased progressive ratio responding. (A) Mean number of active and (B) inactive nose pokes averaged across three PR4 sessions. (C) Mean BP averaged across three PR4 sessions. Vertical lines represent the standard error of the mean (SEM).

Figure 3. Orexin −/− mice display a delayed instrumental acquisition

(A) Mean number of reinforcers earned. (B) Mean number of active and inactive nose pokes. (C) Mean number of active nose pokes represented as percentage of total responses. Vertical lines represent the standard error of the mean (SEM).

Figure 4

Example of lateral hypothalamic infection of shCtrl (A) and shOrx (B) virus. Sections were processed for detection of orexin neuropeptide (red) and GFP from viral infection (green). Right panels in (A) and (B) show merged images. (C) Higher power analysis of infection sites showing persistent orexin expression in neurons infected with shCtrl in contrast to no orexin expression seen in neurons infected with shOrx.

Figure 5

RNAi mediated knockdown of the orexin gene results in decreased responding for food. (A) Mean number of reinforcers earned. (B) Mean number of active and inactive nose pokes. (C) Mean number of active nose pokes represented as percentage of total responses. Vertical lines represent the standard error of the mean (SEM).

Figure 6

RNAi mediated knockdown of the orexin gene results in decreased progressive ratio responding. (A) Mean number of active and (B) inactive nose pokes averaged across three PR4 sessions. (C) Mean BP averaged across three PR4 sessions. Vertical lines represent the standard error of the mean (SEM).