Objective     To study the effect of orexin A on the spontaneous unit discharges of the prefrontal cortical neurons in anesthetized rats.      Methods     The unit discharges of neurons in the prefrontal cortex were recorded by glass microelectrode technique before and after intracerebroventricular injection of orexin A

pyrilamine (a histamine H

1

receptor antagonist)

or pyrilamine+orexin A.      Results     Extracellular single-unit recordings were obtained from a total of 55 prefrontal cortical neurons. Orexin A significantly induced the increase of the discharge rate. But no changes of discharge rate were observed after administration of pyrilamine. In contrast

orexin A failed to exert an excitatory action on unit di

scharges of cortical neurons following pretreatment with pyrilamine in the lateral ventricle.      Conclusion     Orexin A exerts an excitatory action on prefrontal cortical neurons

which probably results from activation of histamine H

1

receptor pathways.