Effects of leptin and orexin-A on food intake and feeding related hypothalamic neurons.

The lateral hypothalamic area (LHA) and the ventromedial hypothalamic nucleus (VMH) have historically been implicated in ingestive behavior, energy balance and body mass regulation. The LHA is more closely associated with the initiation of eating; whereas the VMH mediates the cessation of eating. The parvocellular part of the paraventricular nucleus (pPVN) is also included in the suppressing mechanism. Recently, two hypothalamic peptides, orexin-A and orexin-B, localized in the posterior and lateral hypothalamic perifornical region were discovered in the rat brain and they increase food intake. Leptin, a protein encoded by an obesity gene, expressed in adipose tissue and released into the blood also affects food intake. Orexin and leptin receptors have been localized in the LHA, pPVN, and VMH. The purpose of this study was to measure food intake in the rat in response to leptin and orexin-A; and to determine their electrophysiological effects on feeding related hypothalamic neurons. Results clearly show that leptin suppresses food intake whereas orexin-A increases food intake. These differences are associated with leptin and orexin-A modulatory effects on LHA, pPVN, and VMH glucose responding neurons. In the LHA, leptin inhibits a larger proportion of both glucose-sensitive neurons (GSNs) and non-GSNs. In the pPVN, leptin increases more GSNs in comparison to non-GSNs. Whereas in the VMH, leptin increases the activity of glucoreceptor neurons (GRNs) in comparison to non-GRNs. Orexin-A had opposite effects: increases activity of GSNs more than the non-GSNs in the LHA and significantly suppresses GRNs in the VMH. In the pPVN, orexin-A had no observable effects on neurons that have a low density of orexin 2 receptors. Results are discussed in terms of hypothalamic neural circuits that are sensitive to endogenous food intake inducing and reducing substances.