VTA dopamine connectivity and functional responses to drugs of abuse

Abstract Substance use and addiction pose significant and increasing challenges to public health. The distribution of dopamine efferents from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) and to the medial prefrontal cortex (mPFC) has been hypothesized to influence the predisposition to drug abuse. In addition, the continued maturation of mesocortical projection into early adulthood renders it more susceptible to risk factors such as exposure to drugs of abuse during adolescence. Netrin1/DCC-mediated signaling has emerged as a key regulator of VTA dopamine circuit formation and maturation. However, while reducing DCC activity in mice is shown to increase mesocortial projection, humans carrying monoallelic DCC mutations are found to have reduced mesocortial connectivity. This notable discrepancy reflects a significant gap in knowledge as to how DCC signaling directs VTA dopamine axon targeting and in turn influences addiction-related behaviors. Our recent study has identified novel genetic means to dampen and elevate DCC function by overexpressing distinct receptor variants. Combining the new genetic approach, advanced whole brain imaging, and real-time in vivo dopamine recording, we will test the central hypothesis that axon targeting in the mesocorticolimbic pathway influences the functional responses to drugs of abuse. Specifically in this pilot study, we will investigate how bidirectional changes in DCC signaling alters mesocortical connection, particularly during the circuit maturation through adolescence. We will determine both anatomical and functional changes and also correlate these changes with addition-related behaviors in mice. Aim 1: Determine how changes in DCC activity affects mesocortical axon connection. Using the Cre-loxP system, we will dampen or elevate DCC signaling by overexpressing distinct DCC isoforms specifically in VTA dopamine neurons and during adolescent development. We will follow the resulting VTA axon projection with a tdTomato reporter in the intact brain, using high-speed high-resolution ribbon scanning confocal microscopy after tissue clearing. Completion of this aim will allow us to compare how bidirectional changes in DCC signaling impact mesocortical circuit maturation. It will also help distinguish different possibilities that could contribute to the discrepancy between mice and humans regarding mesocortical connectivity. Aim 2. Determine how changes in DCC activity affects dopamine release in the mPFC during behavioral responses to drugs of abuse. Using the same genetic approach to induce bidirectional changes in DCC signaling as in Aim 1, we will also determine the corresponding alterations in dopamine release in the mPFC and the effects on animal behaviors. Using dLight1.3b fluorescent indicator and fiber photometry, we will follow real-time dopamine dynamics in the mPFC during conditioned place preference testing. This will help determine the functional outcomes of altered mesocortical circuit maturation and delineate the anatomical-structural relationship in the dopamine circuit.