Role of orexin/hypocretin in conditioned sucrose-seeking in rats

Abstract

Rationale: The orexin/hypocretin system has recently been implicated in reward-seeking, especially for highly salient food and drug rewards. We reasoned that this system may be strongly engaged during periods of reward restriction, including food restriction.

Objectives: This study examined the involvement of the orexin (Orx) system in responding for sucrose, and in cue-induced reinstatement of extinguished sucrose-seeking, in ad libitum fed versus food-restricted male subjects.

Methods: Sprague-Dawley rats (n = 108) were trained to self-administer sucrose, and we determined the effects of pretreatment with the OxR1 receptor antagonist SB-334867 (SB; 10-30 mg/kg) on fixed ratio (FR) or progressive ratio (PR) sucrose self-administration, as well as on cue-induced reinstatement of sucrose-seeking. Finally, expression of the immediate early gene c-fos in Orx neurons was examined after self-administration, late extinction or cue-induced reinstatement of sucrose seeking.

Results: SB decreased lever responding (by about 1/3) and the number of reinforcers earned during FR, and less so during PR, schedules and decreased cue-induced reinstatement to sucrose-seeking to extinction levels, predominately in food-restricted rats. Additionally, Fos expression in Orx neurons in perifornical and dorsomedial hypothalamus was increased during extinction.

Conclusions: These results indicate that signaling at the OxR1 receptor is involved in pronounced sucrose reinforcement, and reinstatement of sucrose-seeking elicited by sucrose-paired cues, in food-restricted subjects. These findings lead us to conclude that conditioned activation of Orx neurons increases motivation for food reward during food restriction.

Figure 1

Operant responding for sucrose in food-restricted and ad libitum-fed rats. Food-restricted rats (n = 40) exhibited more active presses (p < 0.001) and obtained more sucrose pellets (p < 0.001) than ad libitum rats (n = 52) during FR1 self-administration. There was no difference in the number of inactive presses between groups.

Figure 2

Attenuation of fixed ratio responding for sucrose in food restricted rats by the OxR1 antagonist SB-334867 (SB). Food-restricted (n = 11–13, per dose) and ad libitum (n = 9–11, per dose) rats were pretreated with SB or vehicle 30 min prior to the self-administration session. * p < 0.05 versus vehicle injection.

Figure 3

Attenuation of progressive ratio responding for sucrose by the OxR1 antagonist SB-334867 (SB). Rats were pretreated with SB or vehicle 30 min prior to the progressive ratio self-administration session. * p < 0.05 30 mg/kg SB (ad libitum and restricted combined) versus vehicle.

Figure 4

Attenuation of cue-induced reinstatement of sucrose-seeking in food-restricted rats by the OxR1 antagonist SB-334867 (SB). In a within-subject design, rats were pretreated with SB (10 or 30 mg/kg) or vehicle 30 min prior to late extinction sessions (no cues or sucrose) or cue-induced reinstatement (tone + light cues). * indicates p < 0.01 versus vehicle + cue.

Figure 5

Percentages of Orx neurons that were Fos+ (% Orx Fos+) following self-administration, late extinction or cue-induced reinstatement of sucrose-seeking in (A) lateral hypothalamus, LH, (B) perifornical area, PFA or (C) dorsomedial hypothalamus, DMH. * = p < 0.001, late extinction versus self-administration or cue-induced reinstatement; # = p < 0.05, self-administration versus cue-induced reinstatement. Self-administration: n=5 ad libitum, n=5 food-restricted; Extinction: n=7 ad libitum, n=4 food-restricted; Cue-induced Reinstatement: n=10 ad libitum, n=6 food-restricted. Panels D and E are representative images of immunohistochemical double-labeling of orexin expression (brown) and c-Fos (dark purple) in D) LH and in E) PFA and DMH, respectively, during late extinction.