Project A: Preclinical mouse model reveals a mechanism linking two known vulnerability factors for alcohol abuse: heightened alcohol stimulation and low striatal dopamine D2 receptors (Study accepted for publication and currently under press in Cell Reports). Alcohol produces both stimulant and sedative effects in humans and rodents. In humans, alcohol abuse disorder is associated with a higher stimulant and lower sedative responses to alcohol. Here we show this association is conserved in mice and demonstrate a causal link with another liability factor: low expression of striatal dopamine D2 receptors (D2Rs). Using transgenic mouse lines, we found that selective loss of D2Rs on striatal medium spiny neurons enhances sensitivity to ethanol stimulation and generates resilience to ethanol sedation. These mice also display higher preference and escalation of ethanol drinking, which continues despite aversive outcomes. We found that striatal D1R activation is required for ethanol stimulation and this signaling is enhanced in mice with low striatal D2Rs. Together, these data demonstrate a link between two vulnerability factors for alcohol abuse and offer evidence for a mechanism where low striatal D2Rs triggers D1R hypersensitivity, ultimately leading to compulsive-like drinking. Project B: Striatal cholinergic interneurons are a novel target of corticotropin releasing factor (CRF) (study published in the Journal of Neuroscience). While the presence of CRF receptors in the dorsal and ventral striatum has been acknowledged, the cellular identity and the functional consequences of the receptor activation is unknown. Here we report that striatal cholinergic interneurons express CRF-R1 receptors and are acutely activated by the neuropeptide CRF that is released in response to salient environmental stimuli. Cholinergic interneurons make less than 1 % of the cells in the striatum but are critical regulators of the striatal circuitry and its output. CRFs fast and potent activation of cholinergic interneurons could then have far reaching behavioral implications across motivated behaviors controlled by the striatum. Summary: Cholinergic interneurons (CINs) are critical regulators of striatal network activity and output. Changes in CIN activity are thought to encode salient changes in the environment and stimulus-response-outcome associations. Here we report that the stress-associated neuropeptide corticotropin releasing factor (CRF) produces a profound and reliable increase in the spontaneous firing of CINs in both dorsal striatum (DS) and nucleus accumbens (NAc) through activation of CRF type 1 receptors (CRF-R1), production of cAMP and reduction in spike accommodation in male mice. The increase of CIN firing by CRF results in the activation muscarinic acetylcholine receptors type 5, which mediate potentiation of dopamine transmission in the striatum. This study provides critical mechanistic insight into how CRF modulates striatal activity and dopamine transmission in the NAc to likely account for CRF facilitation of appetitive behaviors. Project C: Lack of LRRK2, a Parkinsons disease-related protein, promotes compulsive-like and high alcohol intake in mice (study to be submitted for publication). Chronic alcohol exposure alters striatal function and drives compulsive alcohol-seeking despite negative consequences, one of the hallmarks of alcohol use disorders. The striatum plays a central role in goal-directed behaviors and it is thought to undergo long-lasting changes that drive addictive behaviors. We previously found that the Lrrk2 gene is upregulated in the striatum of animals that show inflexible alcohol drinking as defined by high alcohol preference even after its taste-adulteration. The Lrrk2 gene product is an AKAP that regulates PKA availability at spines and it is involved in synaptic modulation in striatal neurons. We hypothesized that the Lrrk2 gene through its modulation of PKA signaling downstream of dopamine D1 receptors (D1R) is involved in facilitating compulsive alcohol taking. To prove this hypothesis, we first tested whether alcohol drinking can modulate Lrrk2 levels in C57BL/6 mice. Using qPCR and RNAscope, we found that alcohol drinking increased mRNA levels for Lrrk2 in the dorsal striatum, in both D1R and D2R-expressing neurons, the two classes of projection neurons in the striatum. Interestingly and contrary to our prediction, alcohol reduced total protein levels for Lrrk2 in the dorsolateral striatum. To assess whether a preexisting downregulation of Lrrk2 protein levels is sufficient to change alcohol drinking, we tested different Lrrk2 cell-specific knockout mice on alcohol drinking tasks and other alcohol-related behaviors. We found that mice lacking Lrrk2 constitutively show enhanced alcohol consumption. Similarly, when the Lrrk2 gene was specifically deleted in D1R neurons, mice showed an increased and persistent alcohol consumption even after quinine adulteration compared to littermate controls. Moreover, these D1-Lrrk2-KO mice consumed more alcohol in an operant self-administration task and showed higher breakpoint, an indication of higher motivation to consume alcohol. Additionally, D1-Lrrk2-KO mice showed enhanced alcohol-induced locomotion, a response that is mediated by dopamine D1R, as well as are more sensitive to a D1-like receptor agonist. These findings suggest that Lrrk2 regulation of PKA activity downstream of D1R in direct-pathway striatal neurons plays an important role in regulating alcohol consummatory behaviors.
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