PKC and PKA inhibitors reinstate morphine-induced behaviors in morphine tolerant mice

Male Swiss Webster mice exhibited antinociception, hypothermia and Straub tail 3h following a 75mg morphine pellet implantation. These signs disappeared by 72h, and the morphine-pelleted mice were indistinguishable from placebo-pelleted ones, although brain morphine concentrations ranged from 200 to 400ng/gm. We previously demonstrated that chemical inhibitors of protein kinase C (PKC) and A (PKA) are able to reverse morphine tolerance in acutely morphine-challenged mice. However, it was not known whether the reversal of tolerance was due to the interaction of kinase inhibitors with the morphine released from the pellet, the acutely injected morphine to challenge tolerant mice, or both. The present study aimed at determining the interaction between the PKC and PKA inhibitors and the morphine released "solely" from the pellet to reinstate the morphine-induced behavioral and physiological effects, 72h after implantation of morphine pellets. Placebo or 75mg morphine pellets were surgically implanted, and testing was conducted 72h later. Our results showed that the intracerebroventricular (i.c.v.) administration of the PKC inhibitors, bisindolylmaleimide I and G?-6976 as well as the PKA inhibitors, 4-cyano-3-methylisoquinoline and KT-5720, restored the morphine-induced behaviors of antinociception, Straub tail and hypothermia in morphine-pelleted mice to the same extent observed 3h following the pellet implantation. The tail withdrawal and the hot plate reaction time expressed as percent maximum possible effect (%MPE) was increased to 80-100 and 41-90%, respectively, in PKC and PKA inhibitor-treated morphine tolerant mice compared to 2-10% in non-treated mice. Similarly, a significant hypothermia (1.3-4.0 degrees C decrease in body temperature) was detected in PKC and PKA inhibitor-treated morphine tolerant mice compared to an euthermic state in non-treated morphine tolerant mice. Finally, the Straub tail score was increased to 1.1-1.6 in PKC and PKA inhibitor-treated tolerant mice, whereas it was totally absent in non-treated animals. It is noticeably that the kinase inhibitors used in the study had no effect in placebo-pelleted mice. Our results provide the first evidence on the ability of PKC and PKA inhibitors to reinstate the behavioral and physiological effects of morphine in non-challenged morphine-tolerant animals.