Drugs of abuse share the ability to enhance dopamine (DA) release in the mesocorticolimbic system. This increase in DA is thought to drive persistent adaptations in the brain and behavior that contribute to the progression of addiction. One such adaptation is a cocaine exposure-induced suppression of G protein-dependent inhibitory signaling in DA neurons of the ventral tegmental area (VTA), a cell population important for reward-related behavior. This cocaine-induced adaptation involves the internalization of G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels, a key contributor to inhibitory G protein pathways that normally temper DA neurotransmission in the mesocorticolimbic system. Dopamine 2 receptor (D2R) activation mediates this adaptation. While methamphetamine, another psychostimulant, can induce a similar adaptation in inhibitory G protein signaling, other drugs of abuse, i.e. morphine, are unable to induce a GIRK channel adaptation. Thus, inhibitory G protein signaling in VTA DA neurons could be important for tempering the behavioral response to cocaine, and could represent an inhibitory “barrier” to addiction. The goal of this thesis research is to understand the impact of GIRK channel activity signaling on behavioral sensitivity to cocaine. The work in this thesis tests the hypothesis that the strength of inhibitory G protein signaling in VTA DA neurons is inversely related to behavioral sensitivity to cocaine. This hypothesis predicts decreasing GIRK channel activity in DA neurons will increase behavioral sensitivity to cocaine. To test this, a genetic strategy was employed, to ablate GIRK channels in DA neurons using DATCre(+):Girk2fl/fl mice. The strength of two significant G protein receptor-dependent signaling dependent on GIRK channels were significantly reduced in a dopamine neuron-specific manner. DATCre(+):Girk2fl/fl mice displayed increased locomotor responding to both acute and repeated cocaine, as well as increased responding for, and intake of, cocaine in intravenous self-administration. The DATCre(+):Girk2fl/fl manipulation parallels the cocaine-induced suppression of GIRK-dependent signaling in VTA DA neurons, and suggests the GIRK channel in DA neurons temper behavioral sensitivity to cocaine. This hypothesis was further tested in a VTA-specific manner using a Cre-dependent viral approach, overexpressing GIRK channels with opposing functional roles. The overexpression of GIRK2 increased inhibitory G protein signaling and decreased cocaine-induced locomotion, while conversely, overexpression of GIRK3 decreased inhibitory G protein signaling and increased cocaine-induced locomotion. Overall, this supports the hypothesis GIRK channel activity in VTA DA neurons tempers behavioral sensitivity to drugs of abuse. In addition to addiction, VTA DA neurons have been implicated in negative affective behaviors, notably following stress. Interestingly, manipulating GIRK channel activity did not alter depression- and anxiety-related behavior, suggests that inhibitory signaling in VTA DA neurons mediated by GIRK channels plays a minimal role in negative affective behaviors, at least in non-stress conditions. However, footshock, a more severe form of stress, elicited adaptations in GIRK channel activity in VTA DA neurons, suggesting that GIRK channel activity could influence behavior following stress. Taken together, the work in this thesis suggests the GIRK channel present in VTA DA neurons contributes to the behavioral effects of cocaine, and could represent a promising therapeutic target for psychostimulant addiction.
University of Minnesota Ph.D. dissertation. February 2019. Major: Neuroscience. Advisor: Kevin Wickman. 1 computer file (PDF); xi, 197 pages.
G protein-gated potassium channels in ventral tegmental area dopamine neurons temper behavioral sensitivity to cocaine.
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