The NIDA Mini-Convention, Frontiers in Addiction Research, held in New Orleans in October in connection with the annual Society for Neuroscience meeting, was an extraordinary event that showcased a wide range of exciting new research in areas related to drug abuse and addiction. In an earlier message I described some of the important new research on glia that was presented. Here I will highlight some of the other presentations I attended about research that has expanded our knowledge of the brain processes underlying motivation and addiction.
Linking Food and Drug Reward
One exciting area of research that was the subject of a session at the Mini-Conference is the close relationship between the mechanisms that govern the rewarding properties of drugs and those that govern food intake and reward. Research has revealed that peripheral hormones (produced outside the brain) that control appetite and food energy regulation also act on reward-related dopamine and other neurotransmitter mechanisms in the brain.
For example, ghrelin, which is produced by the stomach and increases appetite by stimulating the hypothalamus, also plays a role in the rewarding effects of drugs such as cocaine, alcohol, and methamphetamine. Suzanne L. Dickson (University of Gothenburg) opened the session by presenting research on ghrelin, including her findings that the hormone increases alcohol intake in mice and that suppressing ghrelin signaling decreases not only food reward but also alcohol reward and cocaine locomotor stimulation. Gina M. Leinninger (Michigan State University) then discussed her research on leptin, ghrelin’s hormonal counterpart in maintaining the homeostatic balance of food intake. Leptin is produced by fat tissue and signals satiety in the brain, where it also interacts with the brain’s dopamine reward system. By acting through another brain peptide, orexin, leptin can blunt dopamine signaling in the reward system and decrease motivated food intake as well as other forms of activity (and thereby could be involved in other mechanisms contributing to obesity than overeating).
Finally, Aurelio A. Galli (Vanderbilt University) discussed the relationship between insulin signaling and addiction. Insulin not only acts throughout the body to control glucose metabolism but also acts directly in the brain to regulate eating. Disorders that influence insulin levels or signaling, such as diabetes and anorexia, can degrade the system that clears dopamine from synapses, leading to eating disorders as well as a range of neurocognitive and psychiatric disorders including drug addiction.
Understanding how these peripheral hormones influence reward and motivation may help clarify how compulsive eating can lead to obesity and how compulsive drug use may progress to addiction. Such research could lead to new therapies for addictive disorders of all kinds, including eating disorders.
Remembering a NIDA Colleague
A special event at the Mini-Convention was a symposium dedicated to the memory of Toni Shippenberg, who until her death from cancer in June had worked for 20 years at NIDA, last serving as Chief of the Integrative Neuroscience Branch of NIDA’s Intramural Research Program. Colleagues and friends of Toni gave personal testimonials that highlighted her love of science, her exceptional qualities as a mentor, her spirit of adventure, and her generosity as a person, and Howard Fields (University of California, San Francisco) presented an overview of her research.
Toni’s work illuminated some of the seemingly paradoxical properties of opioids by clarifying the different roles of two types of opioid receptor in the brain’s mesolimbic reward system, the mu and kappa receptors. In conditioning studies using rats, Toni and her collaborators were able to show that activating the kappa receptor decreased dopamine transmission and produced conditioned aversion. They further went on to dissect the specific neural pathways responsible for these effects. Toni’s crucial research added immeasurably to our understanding of the role of brain opioid systems in motivation and how drugs of abuse can dysregulate these systems. Ultimately her work may result in novel therapeutic strategies targeting the kappa opioid system for treating disorders ranging from addiction to depression and even pain.