Most people who have known a drug addict, or have watched Trainspotting or ER, know that one of the more insidious parts of addiction is the need for more and more drug to achieve a “high.” This leads the addict into a spiral of drug-seeking behavior, and brain changes, which lead to the person just trying to feel normal again. Why is it that, over time, opiate drugs lose their rewarding effects? Until recently, the molecular mechanism behind “why” was unknown, but a new study published in Nature (subscription required) this month explains that has to do with the decrease of a certain signaling pathway in the dopamine reward system.
Consider this: In the 1950s, a scientist named Wikler conducted an experiment to measure the intake of morphine of a addict who was given free access over 100 days (clearly this could not be repeated today!). He found that addicts increased their drug intake from around 50mg in the beginning, to over 1400mg after 100 days! Animal studies which allow rats to press a lever to receive an opiate also increase their lever-pressing activity over time, often to the exclusion of everything else in their environment–food, water, sleep. Both humans and animals display a tolerance to the drug over time, they become less and less sensitive to the opiate’s actions and require more drug to achieve a baseline feeling.
In a model of chronic opiate exposure, just one day after withdrawl from morphine there is a 25% decrease in the ventral tegmental area (VTA) neurons but not other neurons. This effect can be reversed by the growth factor BDNF, which signals through 3 pathways: the protein insulin receptor substate 2 (IRS2), phospholipase C y, and the extracellular signal regulated kinase (ERK) pathway. So, it would follow that the decrease in dopaminergic neurons, brought about by opiate withdrawl, was induced by one of these pathways. But which one?
Through an impressive array of behavioral and molecular experiments, Russo et al discovered that the responsible party was the IRS2-Akt pathway. They even determined that injections into the VTA of chemical depressants to the IRS2 pathway resulted in decreased morphine reward just like chronic morphine administration, while chemical stimulants of the pathway enhanced reward. All this suggests that morphine abuse downregulates this pathway in the VTA neurons, resulting in decreases in VTA dopamine cell size which lead to tolerance.
This data is preliminary and awaits further clarification of the pathway, and how it generalizes to people. However, it is quite possible that a new method of treating addiction could be developed from these results.