Activation of orexin neurones after noxious but not conditioned fear stimuli in rats.

In rats, noxious stimuli increase food intake while conditioned fear stimuli decrease it. Orexin neurones play a facilitative role in food intake. Here, we examined expression of Fos protein in orexin neurones after noxious or conditioned fear stimuli in rats. Noxious stimuli significantly induced Fos protein in orexin neurones. On the other hand, conditioned fear stimuli did not significantly change expression of Fos protein in orexin neurones. The results demonstrate selective activation of orexin neurones by noxious stimuli, suggesting that effects of stressful stimuli upon orexin neurones are dependent upon the stimuli used. This finding is consistent with an idea that orexin neurones are involved in stress-induced food intake.