HARDWIRED HUNGER Two distinct but adjacent sets of cells in the same part of the brain respond in opposite ways to the same hormone: leptin, which, when lacking in the bloodstream, results in a voracious appetite. A group led by Joel K. Elmquist, neuroendocrinology researcher at Beth Israel Deaconess Medical Center and assistant professor of neurology and medicine at Harvard Medical School, set out to understand the leptin pathway by injecting the hormone into rats and following its progress to the lateral hypothalamus area, the part of the brain associated with appetite.In one cell type, leptin bound to neuropeptide Y (NPY), which releases peptides encouraging eating, and in the other cell type it bound to pro-opiomelanocortin (POMC), which releases peptides discouraging eating. Both neuronal cell types expressed SOCS-3, a gene that encodes a protein that acts as a negative feedback regulator of leptin receptor signaling, but only the...

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