[Effect of melatonin on release of beta-endorphin, norepinephrine and 5-hydroxytryptamine in rat brain].
OBJECTIVE
In order to explore the mechanism of action of melatonin to induce analgesia, the present study was undertaken to observe the effects of melatonin on the release of beta-endorphin (beta-Ep), norepinephrine (NE) and 5-hydroxytryptamine in rat brain.


METHODS
With the measurement of pain threshold, push-pull perfusion technique and radioimmunoassay were used to determine the immunoreactive beta-Ep content in the perfusate from the third ventricle of rat brain. The contents of 3-methoxy-4-hydroxyphenyl glycol (MHPG) and 5-hydroxyindole acetic acid in the microdialysate from rat brain were measured by techniques of in vivo microdialysis and high performance liquid chromatography with electrochemical detection.


RESULTS
The immunoreactive beta-Ep content in the perfusate from the third ventricle of rat brain was increased significantly (P < 0.05) following an intraperitoneal administration of 110 mg.kg-1 of melatonin, with the increase of pain threshold. The MHPG and 5-HIAA contents in the microdialysate from the periaqueductal gray (PAG) or the hypothalamus were not changed after the administration of melatonin.


CONCLUSIONS
Melatonin may promote the release of beta-Ep in brain, which may be one of the mechanisms of the analgesic action of melatonin. The analgesic action of melatonin may not be related to the release of NE and 5-HT in the PAG or the hypothalamus.
Yu
C X
CX
State Key Laboratory of Medical Neurobiology, Department of Neurobiology, Shanghai Medical University, Shanghai 200032, China. zjyucx@pub5.fz.fj.cn
Wu
G C
GC
Xu
S F
SF
Chen
C H
CH
D000700:Analgesics / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000378:metabolism*; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D009638:Norepinephrine / Q000378:metabolism*; D017288:Pain Threshold / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism*; D001615:beta-Endorphin / Q000378:metabolism*
D000700:Analgesics; D012701:Serotonin; D001615:beta-Endorphin; D008550:Melatonin; D009638:Norepinephrine