Orexin A activates leptin-responsive neurons in the arcuate nucleus

Orexins, also named hypocretins, arte newly described neuropeptides, which are produced almost exclusively in neurons of the lateral hypothalamus and have been shown to increase food intake after intracerebroventricular injec tion. Leptin, the ob-gene product released from white adipocytes, is suspec ted to reduce food intake mainly by acting on neurons in the arcuate nucleu s of the hypothalamus. Application of orexin A activated 85%, (66 out of 78 ) of all neurons of the rat arcuate nucleus investigated electrophysiologic ally in an in vitro slice preparation, by a direct excitatory postsynaptic effect. Leptin inhibited electrical activity in 10 out of 22 orexin-sensiti ve neurons in this brain region and excited only 3 neurons. These data give the first indication as to where and how orexin might interact with the le ptin-responsive hypothalamic network.