New research performed in rats suggests that orexin, a brain chemical involved in feeding behavior, arousal, and sleep, also plays a role in reward function and drug-seeking behavior.
Dr. Glenda Harris and her colleagues at the University of Pennsylvania showed that the activation of orexin-secreting brain cells in the hypothalamus, a brain region that controls many vital functions such as eating, body temperature, fat metabolism, etc. is strongly correlated with food- and drug-seeking behaviors. Past anatomical studies have shown that these cells in the lateral hypothalamus also project to adjacent reward-associated areas of the brain.
This study suggests that orexin may be a factor in modulating reward-seeking characteristic of substance abuse. The findings help to better identify neural pathways involved in drug abuse, craving and relapse, which may ultimately help scientists find more effective therapies.
This study is published online August 14, 2005 in the journal Nature.
"The brain cells that secrete this substance, orexin, are in an area of the brain called the lateral hypothalamus," says Dr. Nora D. Volkow, director of the National Institute on Drug Abuse (NIDA), National Institutes of Health, which supported the study. "This brain region has been implicated in reward function for many years, but no one was sure which brain chemicals, or neurotransmitters, were involved. For the first time, we now know exactly which substances are involved, which is a significant step forward in developing treatments."
The hypothalamus is a small area of the brain above and behind the roof of the mouth. It is involved with the involuntary nervous system and control of processes such as fluid maintenance, sugar balance, fat metabolism, regulation of body temperature, and control of hormonal secretion. The lateral hypothalamus is referred to as the brain's hunger center.
"We found that the more animals seek out cues associated with food or drug reward, the more activated these neurons become," says Dr. Harris. In rats that had their drug-seeking behavior extinguished, the preference for drug-associated cues was reinstated by chemically activating these cells and orexin production. These data suggest that this brain system may be involved in the development of drug craving that can perpetuate both addiction and relapse.
"This neural system may be activated by environmental cues that cause addicts to relapse back to drug-taking behavior even after successfully going through rehabilitation and achieving abstinence," says Dr. Volkow.