Analysis of Opioid-Induced Kinase Activation

Phosphorylation is the most important and common way of regulation of protein functions. It offers rapid and reversible regulation. Protein kinases catalyze phosphorylation of a protein and transfer the γ-phosphate of adenosine triphosphate (ATP) onto the serine, threonine, or tyrosine residue. It has been shown that stimulation of opioid receptors regulates activities of numerous protein kinases including protein kinase C (PKC), cAMP-dependent protein kinase (PKA), Ca²⁺ /calmodulin-dependent protein kinase II (CamK II), mitogin-activated protein kinases (MAPKs), and G protein coupled receptor kinases (1 –7 ). Kinases activated by opioids play an important role in regulation of opioid signaling, including homologous desensitization of opioid receptors. Studies have demonstrated that activation of these kinases that are key players in opioid signaling cascades also results in crosstalk of opioid signaling to other signal pathways. Furthermore, protein kinases activated by nonopioid signal pathways play important roles in heterologous regulation of opioid functions. Therefore, opioid researchers often face the challenge of determining changes in the activities of protein kinases in study of opioid signal transduction. Kinase assays have become a very common and useful tool in opioid research. This chapter describes practical protocols for measuring activities of CamKII (6 ,8 ,9 ), PKC (2 ,3 ,10 ), PKA (2 ,11 –13 ), and MAPK (14 —16 ) using radioactive or nonradioactive methods.