Change of β-Endorphin Concentration in Rat Brain after Administration of Indomethacin or Carrageenin

This study aimed to investigate the behavior of an endogenous β-endorphin (β-EP) in the brain after subcutaneous (s.c.) injection of carrageenin or intravenous (i.v.) injection of indomethacin (IDM). The carrageenin was injected into rat hind paw subcutaneously in order to evoke only a local nociceptive stimulus. The β-EP concentration in the brain region was determined by radioimmunoassay at designated sampling times after the injection. It was observed that the β-EP concentration in the midbrain declined from 2.8±0.3 at 1 h to 1.3±0.02 ng/mg protein at 9h. After the s.c. injecction of carrageenin, the β-EP concentrations in the midbrain were found to be closely related to the nociceptive sensitivity which was determined by the Randall-Selitto test. On the other hand, a significant elevation of the β-EP concentration was observed in the hypothalamus from 3 h until 5 h compared with that of control. IDM was injected into rats at doses of 2.9, 5.8 and 8.6 mg/kg via the femoral vein. After i.v. administration of IDM, the β-EP increased in the hypothalamus, medulla oblongata, and midbrain, depending on the doses used. The value of hypothalamic β-EP concentration was two times higher than that of carrageenin. We found that nociceptive stimuli and IDM brought a change in the β-EP concentration in the brain of rats.