Modulation of Ca21/Calmodulin-Dependent Protein Kinase II Activity by Acute and Chronic Morphine Administration in Rat Hippocampus: Differential Regulation of a and b Isoforms

Calcium/calmodulin-dependent protein kinase II (CaMK II) has been shown to be involved in the regulation of opioid receptor signaling. The present study showed that acute morphine treat-ment significantly increased both Ca21/calmodulin-indepen-dent and Ca21/calmodulin-dependent activities of CaMK II in the rat hippocampus, with little alteration in the protein level of either a or b isoform of CaMK II. However, chronic morphine treatment, by which rats were observed to develop apparent tolerance to morphine, significantly down-regulated both Ca21/ calmodulin-independent and Ca21/calmodulin-dependent ac-tivities of CaMK II and differentially regulated the expression of a and b isoforms of CaMK II at protein and mRNA levels. Application of naloxone or discontinuation of morphine treat-ment after chronic morphine administration, which induced th