RESUMEN
<p><b>OBJECTIVE</b>To examine the antagonization of phentolamine against the effects of norepinephrine (NE) on the activity of pain-related neurons in the parafascicular nucleus of morphine-dependent rats.</p><p><b>METHODS</b>Electric impulses were applied as nociceptive stimulus to the right sciatic nerve of morphine-dependent rats, and the discharges of the pain-related neurons in the parafascicular nucleus were recorded by extracellular recording method with glass microelectrodes.</p><p><b>RESULTS</b>Intracerebroventricular injection of norepinephrine resulted in the inhibition of evoked response of the pain-excited neurons as well as the excitation of evoked response of the pain-inhibiting neurons. Both the inhibitory effect on the electric discharges of the pain-excited neurons and the excitatory effect on the pain-inhibiting neurons of norepinephrine were almost completely blocked by intracerebroventricular administration of phentolamine.</p><p><b>CONCLUSION</b>Phentolamine antagonizes the inhibitory effect of norepinephrine on the activity of pain-related neurons in the parafascicular nucleus in morphine-dependent rats, and norepinephrine may play an important role in the integration of the pain signal through the alpha-receptors.</p>