l-tetrahydropalmatine increases leucine enkephalin levels in corpus striatum of rats

The effect of chronic l-tetrahydropalmatine (l-THP) administration on the level of leucine enkephalin (Leu-Enk) in rat corpus striatum was studied. After l-THP sc injection once daily for 2 wk, the striatal Leu-Enk level was elevated dose-dependently. However, a single injection of l-THP failed to change the Leu-Enk level. When rats received sc Sch-23390, a selective D1 antagonist, 15 nmol.kg-1 tid for 2 wk, striatal content of Leu-Enk increased from 0.17 +/- SD 0.03 ng.mg-1 tissue in control group to 0.23 +/- SD 0.05 ng.mg-1 tissue in Sch-23390 group (n = 8, P less than 0.05). Sulpiride (Sul), a selective D2 antagonist, 140 mumol.kg-1 sc given bid for 2 wk had no significant effect on the striatal Leu-Enk content. The results suggested that the blockade of D1 receptors by l-THP might be responsible for the increase of the striatal Leu-Enk content in rat.