Increase in diazepam binding inhibitor expression by sustained morphine exposure is mediated via μ-opioid receptors in primary cultures of mouse cerebral cortical neurons

Our previous in vivo experiment demonstrates that chronic morphine treatment up‐regulates diazepam binding inhibitor (DBI) transcripts in mouse cerebral cortex, although detailed mechanisms were unclear (Katsura et al. [1998] J. Neurochem. 71:2638–2641). This study sought to elucidate the precise mechanisms of DBI mRNA up‐regulation by long‐term treatment with morphine using primary cultures of mouse cerebral cortical neurons. A significant increase in DBI mRNA was observed after sustained exposure to 0.3 μM morphine for 2 days, and the maximal expression occurred after 2 days of exposure, whereas transient exposure to 0.3 μM morphine for 15 min, 1 hr, and 3 hr produced no changes in the expression. Continuous exposure to DAMGO also significantly increased DBI mRNA expression, which was completely abolished by a selective antagonist of μ‐opioid receptors, β‐funaltrexamine (β‐FNA). The morphine‐induced increase in DBI mRNA expression and its content were completely inhibited by naloxone and β‐FNA, and the inhibitory potential of naloxonazine was about half that of β‐FNA. On the other hand, κ‐ and δ‐opioid receptor antagonists showed no effects on the morphine‐induced increase in DBI mRNA. In addition, both a calmodulin antagonist and a CaM II kinase inhibitor significantly suppressed the morphine‐induced increase in DBI mRNA. These results indicate that the increase in DBI expression is induced by continuous activation of μ‐opioid receptors but not of κ‐ and δ‐opioid receptors and is regulated by the calcium/calmodulin‐related phosphorylation system. © 2007 Wiley‐Liss, Inc.