Disruptions in circadian rhythms are not only a common symptom of addiction, but also contribute to the development of substance dependence. However, the mechanisms by which circadian dysregulation impacts addiction are largely unknown. Mutations and polymorphisms in circadian clock genes have been shown to augment addiction-related behaviors in rodents and are associated with vulnerability to substance dependence in humans. CLOCK and NPAS2 are key regulators of the molecular clock. NPAS2 is similar to CLOCK in structure and function, however, there are key differences in the expression patterns of these proteins. CLOCK is ubiquitously expressed in the brain, while NPAS2 is highly, rhythmically expressed in the nucleus accumbens (NAc) and is enriched in D1 expressing neurons. Our lab found that while a mutation in the Clock gene increases cocaine preference and self-administration in mice, Npas2 knockout (KO) mice have decreased cocaine preference. These results suggest that NPAS2 and CLOCK play unique roles in regulating reward-related behaviors. However, my recent data show that despite a reduction in cocaine preference, Npas2 KO mice have increased cocaine self-administration. Instead of measuring cocaine preference, which is based on the pharmacology of cocaine and its pleasurable effects, self-administration measures active, volitional, chronic drug intake, as well as the reinforcing and motivational properties of cocaine, and relapse-like behavior. Since preference and drug intake are fundamentally different measures, the mechanisms by which NPAS2 affects these reward-related behaviors are likely unique. However, further research is needed to understand how alterations in circadian genes might be exacerbating drug intake. This proposal will focus on identifying possible cellular and molecular mechanisms underlying increased drug intake in Npas2 KO mice. Recently our lab found that Npas2 knockdown in the NAc increases glutamatergic transmission and AMPA/NMDAR ratio specifically in D1 neurons. Cocaine exposure alters glutamatergic transmission in the NAc and this is known to regulate self- administration and reinstatement. In addition, Npas2 KO increases dendritic spine density in the NAc, which could contribute to this increase in transmission. These results suggest that changes in glutamatergic neurotransmission could underlie increased cocaine self-administration in Npas2 KO mice. In this proposal, I aim to determine how NPAS2 regulates D1 glutamatergic signaling and whether increased transmission in the NAc contributes to increased cocaine self-administration in Npas2 KO mice. In order to understand how glutamatergic signaling is altered, I will first determine how Npas2 KO affects cellular structure and targeted RNA expression in NAc D1 neurons. Subsequently, I will attempt to normalize self-administration in Npas2 KO mice by inhibiting D1 NAc neurons. Together, these aims will begin to identify potential cellular and molecular mechanisms underlying the complicated role of NPAS2 in reward.
This proposal will provide insight into the cellular and molecular mechanisms by which altered expression of Npas2 can lead to addiction vulnerability. In the future, we will apply these results by manipulating specific proteins cell-type specifically in the NAc in order to further define the specific mechanisms underlying increased self-administration in Npas2 KO mice. Understanding the mechanisms behind addiction vulnerability will ultimately lead to targeted treatments for addiction.
