Effect of electroacupuncture on brain enkephalins content at different times in rats.

The basic level of methionin-enkephalin (MEK) in rat medulla oblongata plus pons was the highest at 05:00 o'clock and the lowest at 23:00. In both hypothalamus and striatum at 05:00 it was higher than at 11:00, 17:00 and 23:00. In both hippocampus and midbrain at 05:00 and 23:00 it was higher than those at 11:00 and 17:00. In the cortex it was higher at 05:00 than at 11:00 and 17:00. The basic level of leucine enkephalin (LEK) in the medulla oblongata plus pons was the highest at 11:00 and the lowest at 17:00 and 23:00. Electroacupuncture (EA) at 05:00 increased the MEK content in the medulla oblongata plus pons and the LEK level in the hypothalamus. It decreased the MEK content in the hypothalamus and the LEK level in the cortex; EA at 11:00 produced decreases of MEK concentrations in the medulla oblongata plus pons and the midbrain and increases of MEK content in the cortex, hypothalamus and striatum. At 17:00 it increased the MEK level in the hypothalamus and the LEK content in the hippocampus. At 23:00 it reduced the levels of MEK and LEK in the hippocampus. The results suggest that there are very marked circadian rhythms in the MEK basic levels of the six discrete brain regions observed and the LEK of the medulla oblongata plus pons; EA at different times produces different effects on brain MEK and LEK levels.