Drug addiction is a devastating recurrent illness, which has substantial cost to both the individual and society. The transition from drug use to addiction is believed to be mediated by drug-induced alterations in neuronal intracellular signaling pathways and subsequent changes in gene expression. While many of these drug-induced alterations are transient, it is hypothesized that long-term alterations in gene expression must persist, which underlie long-term addiction-related behaviors such as craving and relapse. Activation of dopaminoceptive neurons in the nucleus accumbens (NAc), a brain region intricately involved in the rewarding and motivational aspects of drug addiction, has been shown to initiate a variety of neuronal, and ultimately, behavioral responses. For example, local activation of D1 or D2 dopamine receptors in the NAc has previously been shown to increase drug-seeking responses in withdrawal. Alternatively, brain-wide D1 receptor stimulation blocks both cocaine and cue-induced reinstatement. As cocaine addicted individuals frequently report that their recovery is hampered by increased craving for drug over time, the first aim of the proposed studies will be to determine if cocaine-seeking behaviors increase over extended periods of withdrawal in mice, similar to previous reports of "incubation" of cocaine seeking in rats. Demonstrating the incubation phenomenon in mice would allow the powerful use of transgenic mouse models in the study the neuroadaptations that coincide with incubation. Thus, the second aim of the proposed research will utilize fluorescence activated cell sorting (FACS) to obtain isolated populations of D1-containing or D2-containing neurons from the NAc of mice at withdrawal times that coincide with incubation of cocaine-seeking behavior. These isolated neuronal populations will be examined using microarray technology to identify differential changes in gene expression that persist or develop in D1 and D2 receptor-containing cell types from early to late cocaine withdrawal. Changes in mRNA and protein levels will be verified by qRT-PCR and Western blot, respectively.
The final aim of this project will determine whether extinction of cocaine seeking alters these long- term changes in gene expression, or produces new changes induced specifically by extinction training experience in D1 and D2 cell types. These studies will allow us to identify differential neuroadaptations in the direct (D1) versus indirect (D2) ventral striatal output pathways, which could potentially contribute to a propensity for relapse, as well as determine the effect of extinction training on gene expression in these pathways as a potential treatment strategy to normalize or alter changes related to long-term withdrawal from chronic cocaine self-administration.
The proposed work will provide insights into the fundamental alterations that persist even into long-term withdrawal in the brain, which may contribute to increases in drug craving over time and an increased propensity toward relapse. Application of this knowledge may provide insights into new treatment strategies during drug withdrawal thereby decreasing the relapse rate and improving treatment outcomes.