Orexin-A/Hypocretin-1 Mediates Cocaine-Seeking Behavior in the Posterior Paraventricular Nucleus of the Thalamus via Orexin/Hypocretin Receptor-2

Abstract

Orexin/hypocretin (Orx/Hcrt) projections from the lateral hypothalamus to the paraventricular nucleus of the thalamus (PVT) are implicated in drug addiction. Specifically, the posterior section of the PVT (pPVT) innervates brain structures that modulate motivated behavior. This study investigated the role of pPVT-Orx/Hcrt transmission in cocaine-seeking behavior. Because the effects of Orx/Hcrt are mediated by two Orx/Hcrt receptors (Hcrt-r1 and Hcrt-r2), we examined the extent to which Hcrt-r1 and Hcrt-r2 are involved in Orx/Hcrt-induced cocaine seeking. Male Wistar rats were made cocaine dependent by self-administering cocaine 6 hours/day (long access) for 21 days. After self-administration training, the rats underwent daily extinction training, during which cocaine was withheld. After extinction, the rats were injected into the pPVT with Orx-A/Hcrt-1 (0-2 µg) alone or, using a single dose of 0.5 µg, in combination with an Hcrt-r1 antagonist (SB334867; 0-15 µg) or an Hcrt-r2 antagonist (TCSOX229; 0-15 µg). Orx-A/Hcrt-1 alone reinstated (primed) cocaine seeking. Unexpectedly, coadministration of Orx-A/Hcrt-1 with SB334867 did not have any effects on Orx-A/Hcrt-1-induced reinstatement, whereas when coadministered with Orx-A/Hcrt-1, TCSOX229 prevented cocaine-seeking behavior. These results indicate that Hcrt-r2 in the pPVT mediates the reinstating effect of Orx-A/Hcrt-1 in animals with a history of cocaine dependence and further identify Hcrt-r2 as a possible molecular target that can guide future therapeutic approaches for the prevention of drug-seeking behavior.

Fig. 1.

Behavioral procedure.

Fig. 2.

Injector placement in the pPVT (Paxinos and Watson, 1997). (A) Schematic representation. (B) Representative image of injector placement. Scale bar: 2000 µm.

Fig. 3.

Priming effect of Orx-A/Hcrt-1. (A) Time course of cocaine self-administration over the 21 days of training. Tukey post hoc tests *P < 0.05; **P < 0.01; ***P < 0.001 vs. respective previous day (data shown depict the mean ± S.E.M.; n = 38). Intra-pPVT administration of Orx-A/Hcrt-1 induces cocaine-seeking behavior. *P < 0.05; **P < 0.01; vs. 0 μg (saline). ∆, % increase in responding): *P < 0.05; **P < 0.01 vs. 0 μg (saline). Asterisks (*, **) are referring to both the responses and the % increase. EXT, extinction; SHAM, sham injection (data shown depict the mean ± S.E.M; n = 38 for EXT and SHAM; n = 7–9 for Orx-A/Hcrt-1 doses).

Fig. 4.

Effect of SB334867 Orx-A/Hcrt-1’s priming effect. (A) Time course of cocaine self-administration over the 21 days of training. Tukey post hoc tests *P < 0.05; **P < 0.01; ***P < 0.001 vs. respective previous day (data shown depict the mean ± S.E.M; n = 38). (B) Lack of effect of SB334867 on Orx-A/Hcrt-1 (0.5 µg) priming effect: *P < 0.05, vs. VEH (DMSO): ∆: % decrease in responding: *P < 0.05, vs. VEH (DMSO). Asterisks (*) refer to both the responses and the % decrease. EXT, extinction; SHAM, sham injection; VEH, vehicle (data shown depict the mean ± S.E.M; n = 38 for EXT and SHAM; n = 9 for VEH; n = 7–9 for SB334867 doses).

Fig. 5.

Effect of TCSOX229 Orx-A/Hcrt-1’s priming effect (A) Cocaine intake over the self-administration days. Tukey post hoc tests: *P < 0.05; **P < 0.01 vs. respective previous day (data shown depict the mean ± S.E.M; n = 34). (B) Reversal of Orx-A/Hcrt-1 (0.5 µg) priming effect by TCSOX229. *P < 0.05; **P < 0.01 vs. VEH (saline); ^#P < 0.05, vs. 0 μg (Orx-A/Hcrt-1 alone). ∆: % decrease responding: *P < 0.05, vs. VEH (saline); ^#P < 0.05, vs. 0 μg (Orx-A/Hcrt-1 alone). Symbols (*, #) refers to both the responses and the % decrease. EXT, extinction; SHAM, sham injection; VEH, vehicle (data shown depict the mean ± S.E.M; n = 34 for EXT and SHAM; n = 9 for VEH; n = 6 to 7 for TCSOX229 doses).