Evaluation of endomorphin-1 on the activity of Na+,K+-ATPase using in vitro and in vivo studies

Abstract The goal of this study was to investigate the effects of endomorphin-1 on Na + ,K + -ATPase activity in mouse brain synaptosome in vitro, and its antinociceptive interaction with the Na + ,K + -ATPase inhibitor ouabain. Endomorphin-1 (0.1 nM–10 μM) produced a concentration-dependent (EC 50 : 43.19 nM, CI: 23.38–65.71 nM, E max : 25.86%, CI: 24.53–27.20%), naloxone-reversible increase of the synaptosomal Na + ,K + -ATPase activity. The intrathecally (i.t.) administered endomorphin-1 (2–20 μg) produced a dose-dependent short-lasting increase in the tail-flick latency. Ouabain itself (1–1000 ng, i.t.) did not cause antinociception. Treatment with 10 ng ouabain significantly decreased the antinociceptive effect of 2 μg endomorphin-1, but none of the other combinations did significantly differ from the endomorhin-1-treated groups. These data indicate that endomorphin-1 increases the activity of Na + ,K + -ATPase in vitro but this effect may play a weak role in the antinociception induced by intrathecal endomorphin-1.