Male fruit flies unable to find sexual partners increase their intake of alcohol, according to a new study from the University of California, San Francisco. Sexual deprivation but not social isolation caused them to consume more alcohol. The California research team was interested in finding out how environmental factors can cause addictions, and to investigate how addiction begins in the molecular level in brain chemicals. "Reading this study is like looking back into time to see the very origins of the reward circuit that drives fundamental behaviors like sex, eating and sleeping," said Dr. Markus Heilig, clinical director of the National Institute on Alcohol Abuse and Alcoholism and the National Institute on Drug Abuse. The brain's reward system apparently has changed very little during its long evolutionary process. For the new study one group of male flies was allowed to mate freely with available female virgins. Another group had the opposite experience in that they were commingled with females who had already mated and spurned the advances of any new partners. Males who had been rejected drank a mixture spiked with alcohol 70% of the time, compared to males who had sex who drank alcohol only 50% of the time. For the second experiment both virgin and mated males were commingled. The virgin males again showed more preference for alcohol. For the third experiment, virgin males were put with dead females and again they preferred alcohol more than the males who had found mates. The flies' appetite for alcohol was dependent upon the levels of neuropeptide F in their brains. When these levels were low, they consumed more alcohol. Previous research concluded that this chemical is involved in eating, sleeping, responses to stress, and other activities. Humans have a similar chemical called neuropeptide Y. "What we did in order to establish a causal relationship was manipulate the system," said Dr. Ulrike Heberlein, co-author of the study. "We were able to prove it in both directions, so that is strong evidence of causality. We really wanted to find a molecular mechanism that links experiences to drug related behaviors." "The study implies that it is the system that goes haywire in addiction," said Dr. George Koob, a neurobiology professor at the Scripps research Institute in L Jolla, California, "and it is very sensitive to stress. For example, after you lose a loved one or a relationship has crashed, you get dysphoric, and your neuropeptide F level goes down, and this provides a strong urge to drink a lot -- whether you are a mammal or a fruit fly." This study was published in the journal Science.