Sex during adolescence may have lasting negative effects on the body, most likely because it occurs when the nervous system is developing, U.S. researchers say.Study co-author John Morris, a doctoral student in psychology at Ohio State University, Zachary Weil, research assistant professor, and Randy Nelson, professor and chairman of the department of neuroscience, said their research used laboratory animals, but their findings provide information that might be applicable to understanding human sexual development.Researchers paired adult female hamsters with male hamsters when the males were 40 days old -- the equivalent of a human\'s mid-adolescence.The study found that male animals with an early-life sexual experience later showed more signs of depressive-like behaviors as well as lower body mass, smaller reproductive tissues and changes to cells in the brain than did hamsters first exposed to sex later in life or experienced no sex at all.Among the cell changes observed in the animals that had sex during adolescence were higher levels of expression of a gene associated with inflammation in their brain tissue and less complex cellular structures in key signaling areas of the brain. They also had stronger immune response, suggesting their immune systems were in a heightened state of readiness even without the presence of infection -- a potential sign of an autoimmune problem, the study said.The findings were presented at the Society for Neuroscience annual meeting in Washington.