Gene Expression Profile of Calcium/Calmodulin-Dependent Protein Kinase II alpha in Rat's Hippocampus during Morphine Withdrawal

Introduction: Calcium/calmodulin-dependent protein kinase II (CaMKII) which is highly expressed in the hippocampus is known to play a pivotal role in reward-related memories and morphine dependence. Methods: In the present study, repeated morphine injections once daily for 7 days was done to induce morphine tolerance in male Wistar rats, after which gene expression profile of alpha-isoform of CaMKII (CaMKII alpha) in the hippocampus was evaluated upon discontinuation of morphine injection over 21 days of morphine withdrawal. Control groups received saline for 7 consecutive days. For gene expression study, rats' brains were removed and the hippocampus was dissected in separate groups on days 1, 3, 7, 14, and 21 since discontinuation of of morphine injection. A semi-quantitative RT-PCR method was used to evaluate the gene expression profile. Results: Tolerance to morphine was verified by a significant decrease in morphine analgesia in a hotplate test on day 8 (one day after the final repeated morphine injections). Results showed that gene expression of CaMKIIa at mRNA level on day 1, 3, 7, 14 and 21 of morphine withdrawal was significantly altered as compared to the saline control group. Post hoc Tukey's test revealed a significantly enhanced CaMKIIa gene expression on day 14. Discussion: It can be concluded that CaMKIIa gene expression during repeated injections of morphine is increased and this increase continues up to 14 days of withdrawal then settles at a new set point. Therefore, the strong morphine reward-related memory in morphine abstinent animals may, at least partly be attributed to, the up-regulation of CaMKIIa in the hippocampus over 14 days of morphine withdrawal.