Involvement of cAMP/cAMP-dependent protein kinase signaling pathway in regulation of Na+, K+-ATPase upon activation of opioid receptors by morphine

The depolarization of neurons induced by impairment of Na,K-ATPase activity after long-term opiate treatment has been shown to involve the development of opioid dependence. However, the mechanisms underlying changes in Na,KATPase activity after opioid treatment are unclear. The bestestablished molecular adaptation to long-term opioid exposure is up-regulation of the cAMP/cAMP-dependent protein kinase (PKA) signaling pathway; this study, therefore, was undertaken to investigate the role of up-regulation of cAMP/PKA signaling pathway in alteration of the mouse hippocampal Na,KATPase activity. The results demonstrated that short-term morphine treatment dose dependently stimulated Na,K-ATPase activity. This action could be significantly suppressed by adenylyl cyclase activator 7-acetoxy-8,13-epoxy-1,6,9-trihydroxylabd-14-en-11-one (forskolin), or the cAMP analog dibutyryl-cAMP. Contrary to short-term morphine treatment, long-term treatment significantly inhibited Na,K-ATPase activity. Moreover, an additional decrease in Na,K-ATPase activity was observed by naloxone precipitation. The effects of both short- and long-term morphine treatment on Na,KATPase activity were naltrexone-reversible. The regulation of Na,K-ATPase activity by morphine was inversely correlated with intracellular cAMP accumulation. N-[2-(4-Bromocinnamylamino)ethyl]-5-isoquinoline (H89), a specific PKA inhibitor, mimicked the stimulatory effect of short-term morphine but antagonized the inhibitory effect of long-term morphine treatment on Na,K-ATPase activity. However, okadaic acid, a protein phosphatase inhibitor, suppressed short-term morphine stimulation but potentiated long-term morphine inhibition of Na,K-ATPase activity. The regulation of Na,K-ATPase activity by morphine treatment seemed to associate with the alteration in phosphorylation level but not to be relevant to the change in abundance of Na,K-ATPase. These findings strongly demonstrate that cAMP/PKA signaling pathway involves regulation of Na,K-ATPase activity after activation of opioid receptors.