Orexin mediates morphine place preference, but not morphine-induced hyperactivity or sensitization

Abstract

Orexin (or hypocretin) has been implicated in mediating drug addiction and reward. Here, we investigated orexin's contribution to morphine-induced behavioral sensitization and place preference. Orexin-/- (OKO) mice and littermate wild-type (WT) controls (n=56) and C57BL/6J mice (n=67) were tested for chronic morphine-induced locomotor sensitization or for conditioned place preference (CPP) for a morphine- or a cocaine-paired environment. C57BL/6J mice received the orexin receptor 1 (Ox1r) antagonist, SB-334867, prior to test sessions. OKO mice did not significantly differ from WT controls in locomotor activity following acute- or chronic-morphine treatments. Similarly, mice treated with the Ox1r antagonist did not differ from vehicle controls in locomotor activity following acute- or chronic-morphine treatments. In contrast, while OKO mice did not differ from WT controls in preference for a morphine-paired environment, the Ox1r antagonist significantly attenuated place preference for a morphine-, but not a cocaine-paired, environment. These data suggest that orexin action is not required for locomotor responses to acute and chronic morphine, but Ox1r signaling can influence morphine-seeking in WT animals.

Figure 1. Orexin -/- mice (n=8) display normal acute locomotor and sensitization responses to morphine compared to WT controls (n=8)

(A) Activity measured in 5 minute intervals during habituation, saline, and morphine treatment sessions. (B) Activity following saline, first morphine, and fifth morphine treatment session. Vertical lines represent the standard error of the mean (SEM). * indicates significant differences from all other sessions.

Figure 2. Pharmacological blockade of Ox1r in mice treated with SB-334867 (n=7) fails to affect acute locomotor and sensitization responses to morphine (10 mg/kg, s.c.) compared to mice treated with vehicle (n=6)

(A) Activity measured in 5 minute intervals during habituation, saline, and morphine treatment sessions. (B) Activity following saline, first morphine, and fifth morphine treatment session. Vertical lines represent the standard error of the mean (SEM). * indicates significant differences from all other sessions.

Figure 3. Pharmacological blockade of Ox1r in mice treated with SB-334867 (n=6) fails to affect acute locomotor and sensitization responses to morphine (5 mg/kg, s.c.) compared to mice treated with vehicle (n=6)

(A) Activity measured in 5 minute intervals during habituation, saline, and morphine treatment sessions. (B) Activity following saline, first morphine, and fifth morphine treatment session. Vertical lines represent the standard error of the mean (SEM). * indicates significant differences from all other sessions. ** indicates significance from baseline only.

Figure 4. Genetic mutation of the orexin gene fails to affect preference for an environment previously paired with morphine

Difference in time spent in the drug-paired environment compared with time spent in the unpaired environment (paired minus unpaired) for (A) male OKO mice (n=14) and WT controls (n=14) and (B) female OKO mice (n=6) and WT controls (n=6) conditioned with morphine. Vertical lines represent the standard error of the mean (SEM).

Figure 5. Pharmacological blockade of Ox1r attenuates preference for an environment previously paired with morphine, but not cocaine

Difference in time spent in the drug-paired environment compared with time spent in the unpaired environment (paired minus unpaired) for (A) SB-334867 treated mice (n=13) and vehicle treated controls (n=13) conditioned with morphine, and (B) SB-334867 treated mice (n=8) and vehicle treated controls (n=8) conditioned with cocaine. Vertical lines represent the standard error of the mean (SEM). * represents a significant SB-334867 effect on preference for drug-paired side (P<0.03).