The study of the effect of drugs of abuse on protein kinase A activity in mouse brain and spinal cord

Morphine and Delta9-THC are drugs that produce analgesia and rewarding effects. However, chronic treatment with morphine and Delta 9-THC produces problematic side-effects including tolerance and physical dependence. The cellular mechanisms underlying opioid and cannabinoid antinociceptive tolerance have been studied for years. Research has demonstrated that the expression of morphine and Delta9-THC antinociceptive tolerance may be mediated through intracellular signaling pathways, such as the adenylyl cyclase/Protein Kinase A (PKA) cascade. The present study investigated the role of PKA in the expression of morphine and Delta9-THC antinociceptive tolerance. Male Swiss Webster mice were treated chronically with morphine or Delta9-THC and the warm-water tail-flick test was used to assess antinociception. These studies revealed that the level and the duration of morphine antinociceptive tolerance both influenced whether PKA activity was increased in mouse brain and spinal cord. Cytosolic PKA activity was increased in the thalamus of 3-day morphine-tolerant mice expressing a 45-fold level of tolerance, but not in mice that expressed a 10-fold level of tolerance. In addition, cytosolic PKA activity was increased in the lumbar spinal cord (LSC) of 15-day morphine-tolerant mice. However, chronic treatment with Delta9-THC had no effect on neuronal PKA activity even in mice that expressed a high level of antinociceptive tolerance. The absence of an effect of chronic treatment with Delta9-THC on neuronal PKA activity was supported by the development of a positive control in which the PKA activator Sp-8-Br-cAMPS was administered intracerebroventricularly (i.c.v.) and intrathecally (i.t.) in drug-naive mice and increases in PKA activity were observed in several brain regions and LSC. Finally, the i.c.v. injection of two peptide fragments of native Protein Kinase A inhibitor (PKI) peptide, PKI-(6-22)-amide and PKI-(Myr-14-22)-amide, significantly reversed antinociceptive tolerance in mice treated chronically with morphine. PKI-(6-22)-amide (i.c.v.) also inhibited PKA activity in brain regions (thalamus, periaqueductal gray (PAG), and medulla) and LSC, which studies have shown play a role in morphine-induced analgesia. Moreover, PKI-(6-22)-amide reduced the increase in PKA activity in thalamus and LSC observed with chronic morphine treatment. Overall, these studies provide evidence that PKA plays a role in morphine tolerance, but not Delta9-THC tolerance at the doses and times tested.