Mice lacking rhes show altered morphine analgesia, tolerance, and dependence

Abstract

Rhes, the Ras Homolog Enriched in Striatum, is an intermediate-size GTP binding protein. Although its full functions are not yet known, it has been shown to affect signaling and behaviors mediated by G protein-coupled receptors. Here we have tested whether Rhes affects behaviors mediated by opioid receptors. Wild type and rhes-deficient mice were administered morphine and tested for analgesia in formalin and tail flick tests. Rhes⁻/⁻ mice showed significantly enhanced analgesia in both tests relative to rhes+/+ mice. Furthermore, rhes⁻/⁻ mice did not display tolerance to repeated morphine administration and displayed significantly less withdrawal than rhes+/+ mice. These findings indicate that Rhes is involved in behaviors mediated by mu opioid receptors and in the adaptive response to repeated morphine administration.

Fig 1

Analgesia assessed with the formalin test. (A-C) Time spent licking (group mean ± SEM) in the early (1^st five minutes) or late (remaining 55 minutes) phases after 0 mg/kg (A), 3 mg/kg (B), or 10 mg/kg (C) morphine. (D,E) The data are shown as a time course over 60 minutes with group mean ± SEM for each 5 minute interval for rhes^+/+ (D) and rhes^−/− (E) mice. ** p < 0.01 compared with rhes^+/+ mice at the same phase. n = 9–10.

Fig 2

Analgesia assessed with the tail flick test. (A,B) Time course of latency to respond presented as group means ± SEM for rhes^+/+ (A) and rhes^−/− (B) mice. (C) Dose-response curves for analgesia assessed at 45 minutes post-morphine injection, showing a threefold leftward shift in the ED₅₀ value of rhes^+/+ mice relative to rhes^−/− mice. n = 5 for 0, 0.3, and 1 mg/kg, n= 9–11 for 3 and 10 mg/kg.

Fig 3

Attenuation of tolerance and withdrawal in rhes^−/− mice. (A,B) %MPE group means ± SEM of 3 mg/kg (A) or 10 mg/kg (B) morphine in rhes^+/+ and rhes^−/− mice on Day 1 and Day 5 of repeated drug administration. Percent MPE was significantly lower in rhes^+/+ mice on day five versus day one after 3 mg/kg and 10 mg/kg morphine, thus indicating tolerance, whereas rhes^−/− mice showed no such difference. n = 8–9. ***p<0.001 versus rhes^+/+ mice on the same day; # p<0.05, # # p<0.01 versus day 1 in the same genotype. (C) In naloxone-precipitated withdrawal, rhes ^−/− mice show significantly reduced global withdrawal scores as compared with rhes^+/+ mice. n = 8–11. *p<0.05, *** p<0.001 compared with rhes^+/+ mice. All bars represent group mean ± SEM.