Why should there be a factory making powerful pain-killing drugs inside our heads? Since the discovery in 1975 that natural counterparts to morphine and other opiate drugs exist in the brain there has been much speculation about their role in the body. It has been suggested that they may be involved in the suppression of pain. the control of emotion and even the regulation of body fat. Geoff Watts examines these and other theories that try to explain our natural opium dens. Producer DEBORAH COHEN (Repeated: Fri 11.5 am)