Nicotine is a highly addictive drug that motivates tobacco consumption with catastrophic health consequences worldwide. To produce addiction, a drug must affect reward centers in the brain and encode memories that drive new behaviors. Nicotinic signaling is known to produce long-lasting changes in synaptic connections, i.e. those contacts between neurons that mediating direct signaling, and to do so both in brain reward centers and in a brain region called the hippocampus which is required for memory formation. Such changes are likely to require coordinated action of multiple gene products and may be mediated by a class of RNA molecules called microRNAs (miRs). The reason that miRs are likely to be involved is that they can act as regulatory “hubs” controlling the expression of large gene sets. Recently it was found that cocaine produces addictive effects via changes in specific miRs, and that manipulating those miRs can block the addictive response. In preliminary studies we have found that nicotine repeatedly administered in vivo alters a number of miRs in brain reward centers and in the hippocampus. This offers the real possibility that therapeutic manipulation of specific miRs in vivo may have the power to block the addictive properties of nicotine.
To test this hypothesis, we will first extend the analysis of miRs to identify those most likely to mediate nicotinic addiction. Selection criteria will include miRs (1) that are relatively abundant and yet are changed most radically by nicotine treatment in the reward centers or hippocampus of rodents displaying the greatest behavioral response to nicotine, (2) that display similar changes in two species, suggesting conserved mechanisms, (3) that are predicted to regulate key pathways likely to mediate addiction (e.g. regulate synaptic components) as indicated by current software programs, and (4) that have conserved sequences in human miRs, rendering them potentially relevant for nicotine addiction therapy. To narrow the list of miR candidates further, we will determine which mediates nicotine-induced synaptic changes on neurons that express the “reward” transmitter dopamine in appropriate brain regions or on neurons in the hippocampus. Synaptic effects will be assessed by using antibodies to stain tissue sections for the numbers and sizes of synapses. MiR candidates will be individually manipulated in vivo either by repeated infusion of agents that alter the effective level of the miR or by single injection of an appropriate agent.
A miR found capable of reversing or preventing a nicotine-induced synaptic change will then receive top priority for the more time consuming and expensive behavioral assays. These will include increased motion, preference for drug-associated locations, and nicotine intravenous self-administration. Manipulating a candidate miR in vivo this way prior to and during the nicotine treatment will indicate whether it offers a mechanism for abrogating nicotine-induced addictive behavior. Identifying such miRs would represent a major step forward both for uncovering new molecular pathways driving addiction and, more importantly, for suggesting molecular targets for therapeutic intervention to combat the addiction. Such an outcome could have enormous biomedical impact, given the human damage caused by nicotine addiction.
This proposal is highly appropriate for a TRDRP Exploratory/Developmental Research Award because it offers a new and credible approach for overcoming the addictive effects of nicotine. Preliminary data are in hand and critical tests will be carried out. If successful as expected, the results will provide a compelling basis for subsequent proposals to federal agencies and other funding sources, as encouraged by the TRDRP mandate.