Nicotine lessens the amount mice eat by activating specific neurons in the brain, perhaps explaining why people who stop smoking often gain weight.

Marina Picciotto at Yale University in New Haven, Connecticut, and her colleagues found that mice given nicotine daily for 30 days ate less and had lower body-fat levels than untreated mice. Nicotine increased the firing of brain neurons that produce a hormone precursor called pro-opiomelanocortin (POMC). When the POMC neurons fire, they release the hormone melanocortin. Mice in which the Pomc gene had been deleted ate the same amount whether or not they received nicotine. Those with a major melanocortin receptor gene silenced ate more when given nicotine than normal mice on nicotine.

The melanocortin hormone pathway regulates both energy use and food intake, so the authors think that nicotine has a two-pronged influence on body weight.

Science 332, 1330–1332 (2011)