[Changes in the binding characteristics of central kappa-opioid receptor during the development of 100 Hz electroacupuncture (EA) tolerance in rats].

The effect of repetitive 100 Hz EA stimulation on the characteristics of binding of kappa opioid receptors in discrete brain regions and the spinal cord of rats was investigated by radioligand binding assay. 100 Hz EA was administered once a day (30 min) for 7 d. Changes in Bmax and Kd of kappa opioid receptors of 4 discrete brain regions (cortex, midbrain, pons-medulla and striatum) and spinal cord were observed at d 1, 3, 5 and 7, respectively. It was found that the numbers (Bmax) of kappa opioid receptors of all the brain regions observed were decreased during the development of 100 Hz EA tolerance with a differential time course. In rat cortex and pons-medulla, a marked down regulation of kappa opioid receptors occurred within 24 h and remained at a low level throughout the observation. The Bmax of kappa receptor in spinal cord showed a bell-shaped curve, i.e., up regulation in the first 5 days and down regulation at d 7. In contrast a slow and steady down regulation was found in the midbrain and striatum, matching the course of development of 100 Hz EA tolerance. No significant changes in Kd of [3H]-U69593 were found except for a decrease of Kd in midbrain. In conclusion, administration of 100 Hz EA 30 min per day for 7 consecutive days resulted in a significant down regulation of kappa opioid receptor in CNS, especially in midbrain and striatum.