Nicotine is responsible for more than four million smoking related deaths each year. Yet people still smoke. Why? One reason is the stranglehold of addiction, started when nicotine enhances the release of dopamine, a chemical neurotransmitter that induces a feeling of pleasure. Knowing specifically which receptor molecules are activated by nicotine in the dopamine-releasing cells would be a promising first step in developing a therapeutic drug to help people kick the habit. But many types of cell receptor proteins respond to nicotine, each in turn comprising a set of "subunit proteins" chosen from 15 or so related members of a gene “family”. This project has determined that when receptors with a specific subunit known as alpha4 are activated by nicotine, this is sufficient for some addiction-related events, such as reward behavior, sensitization, and tolerance. This research suggests that alpha4, and the molecules that are triggered in turn by alpha4, may prove to be useful targets for addiction intervention therapies.
Previous work in several laboratories in the 1990s had suggested that, of the many so-called nicotinic acetylcholine receptors, one consisting of both alpha4 and beta2 subunits was important for nicotine addiction. This had been determined by the use of so-called "knockout" mice. In this method, scientists identify a gene that may affect a behavior of interest, then interrupt or knock it out in a specially bred strain of mice. In this case, the effects of nicotine on the knockout mice and normal mice were compared, and the mice without the beta2 subunit lacked some responses to nicotine.
In work to identify nicotinic receptor subtypes that were sufficient to elicit nicotine dependence, we examined the “partner” alpha4 subunit suspected in addiction. But instead of experimenting with a knockout mouse, we developed "hypersensitive knock-in" mice. We replaced a naturally occurring bit of DNA with a mutation that changed only a single amino acid in only one of the mouse’s 30,000 genes; but that change made the alpha4 subunit highly sensitive to nicotine. We first selected the proper mutation in frog eggs and cultures, then bred a strain of mice. As hypothesized, the mice with the re-engineered alpha4 receptor proved to be highly sensitive even to very small doses of nicotine that didn't activate other nicotinic receptor types. This finding shows the alpha4 subunit is a prime candidate to be studied at the molecular and cellular level, and as a possible target to develop a medication that would reduce the nicotine-induced release of dopamine, hopefully reducing nicotine’s addictive grip on smokers.
Knockout mice leave us with nothing to measure. The hypersensitive strategy has the advantage that it isolates and amplifies the “downstream” chain of molecular signals within nerve cells that occur after nicotine activates its receptors—and these signals, when repeatedly stimulated by nicotine, eventually cause the as-yet unknown long-term changes in nerve cells that are presumably the biological basis of addiction. We are now tracking down those signals, molecules, and newly activated genes, using the hypersensitive mice. The eventual hope: one of those molecular signals might provide a molecular target that can be specifically blocked, providing a therapy against addiction. This strategy would resemble modern cancer drugs, such as Gleevec, which block only specific signaling molecules needed by proliferating cancer cells.
We are optimistic about ultimately defeating nicotine's addictive power. It's a complicated pathway that still must be broken down into individual steps before we can understand it fully. But we believe that nicotine addiction will be among the first addictions to be solved, because we have so many tools to study it. |