RESUMEN
<p><b>AIM</b>The effect and possible mechanism of Melatonin (MEL) on firing rate of pain neurons in lateral habenular nucleus of rats were investigated in the experiment.</p><p><b>METHODS</b>Single extracellular firing were recorded to study the firing rate changes of pain neurons and sensitivity changes to pain stimulation induced by MEL in LHb of rats. Reverse effect of naloxone on the analgesia induced by melatonin was also observed.</p><p><b>RESULTS</b>Melatonin showed the effects on the firing of pain neurons in the LHb and decreased the sensitivity of pain neurons to pain stimulation, which could be reversed by naloxone.</p><p><b>CONCLUSION</b>Melatonin can change the responses of pain neurons to pain stimulation via opioid receptor in the LHb, which might be one of analgesic mechanisms by MEL.</p>