Involvement of dopamine D3 receptor in impulsive choice decision-making in male rats

Dopamine receptor D3 affects the expression of Period1 in mouse cells via DRD3-ERK-CREB signaling

Circadian rhythm alterations are related to the onset and severity of various diseases. The expression of the dopamine receptor D3 (DRD3) is regulated by clock genes, and DRD3 functional abnormalities are linked to various neurological diseases. However, the relationship between DRD3 function and circadian machinery is unclear. Here, we demonstrate the influence of DRD3 on the circadian machinery. Although the expression of DRD3 in mouse suprachiasmatic nucleus (SCN) did not show a circadian rhythm, the expression of Per1 mRNA was altered in the SCN of Drd3 knockout (Drd3-/-) mice compared to that in wild-type (WT) mice. These differences were caused by the upregulation of the DRD3-extracellular signal-regulated kinase-cAMP response element binding protein (DRD3-ERK-CREB) signaling pathway in cultured cells and SCN. In addition, Drd3-/- mice demonstrated increased period length of locomotor activity than WT mice only under constant dark conditions. Expression of clock genes in the liver, which does not express DRD3, was affected by the loss of DRD3 only under constant dark conditions, similar to that in the SCN. These results suggest that DRD3 expressed in the SCN regulates the central clock via endogenous ligands and affects peripheral organs. This may provide new evidence to unravel the relationship between dopamine neurotransmission and the circadian clock, which has not yet been fully elucidated.

Cocaine Self-Administration Increases Impulsive Decision-Making in Low-Impulsive Rats Associated with Impaired Functional Connectivity in the Mesocorticolimbic System

Impulsivity is often considered a risk factor for drug addiction; however, not all evidence supports this view. In the present study, we used a food reward delay-discounting task (DDT) to categorize rats as low-, middle-, and high-impulsive but failed to find any difference among these groups in the acquisition and maintenance of cocaine self-administration (SA), regardless of electrical footshock punishment. Additionally, there were no group differences in locomotor responses to acute cocaine in rats with or without a history of cocaine SA. Unexpectedly, chronic cocaine SA selectively increased impulsive choice in low-impulsive rats. Resting-state fMRI analysis revealed a positive correlation between impulsivity and cerebral blood volume in the midbrain, thalamus, and auditory cortex. Using these three regions as seeds, we observed a negative correlation between impulsivity and functional connectivity between the midbrain and frontal cortex, as well as between the thalamus and frontal cortex (including the orbitofrontal, primary, and parietal cortices) in low-impulsive rats. These correlations were attenuated following chronic cocaine SA. RNAscope in situ hybridization assays revealed a significant reduction in dopamine (DA) D1, D2, and D3 receptor mRNA expression in the corticostriatal regions of low-impulsive rats after cocaine SA. Our findings challenge the widely held view that impulsivity is a vulnerability factor for cocaine use disorder. Instead, chronic cocaine use appears to selectively increase impulsive choice decision-making in normally low-impulsive rats, associated with reduced functional connectivity and DA receptor expression in the mesocorticolimbic DA network.