A molecular genetic approach to uncovering the differential functions of dopamine D2 receptor isoforms

Differential roles of two isoforms of dopamine D2 receptors in l-dopa-induced abnormal involuntary movements in mice

l-dopa and dopamine D2 receptor (D2R) agonists are commonly used to relieve the motor deficits of Parkinson's disease. However, long-term treatment with l-dopa or D2R agonists can induce adverse effects such as abnormal involuntary movements (AIMs), which are major limiting factors in achieving long-term control of parkinsonian syndromes. The pathophysiological mechanisms involved in the development of dopaminergic agonist-induced adverse effects are not well understood. Here, we examined the role of two D2R isoforms, D2S and D2L, in l-dopa-induced AIMs using dopamine D2L knockout (D2L KO) mice (expressing purely D2S) and wild-type mice (expressing predominantly D2L). We found that D2L KO mice displayed markedly enhanced AIMs in response to chronic treatment of l-dopa compared to wild-type mice. The l-dopa-induced enhancement of AIMs in D2L KO mice was significantly reduced by the D2R antagonist eticlopride. D2L KO mice also displayed markedly enhanced AIMs in response to chronic treatment with quinpirole, a preferential D2R agonist. These results suggest that D2S contributes more than D2L to dopaminergic agonist-induced AIMs. Our findings may uncover a new factor that contributes to the pathophysiology of dopaminergic drug-induced AIMs, a characteristic manifestation of dyskinesia and also present in psychosis. There is a possibility that the increased ratio of D2S to D2L in the brain plays a significant role in the development of AIM side effects induced by l-dopa or D2R agonists. See Video Abstract, http://links.lww.com/WNR/A622.

Hypothesizing dopaminergic genetic antecedents in schizophrenia and substance seeking behavior

The dopamine system has been implicated in both substance use disorder (SUD) and schizophrenia. A recent meta-analysis suggests that A1 allele of the DRD2 gene imposes genetic risk for SUD, especially alcoholism and has been implicated in Reward Deficiency Syndrome (RDS). We hypothesize that dopamine D2 receptor (DRD2) gene Taq1 A2 allele is associated with a subtype of non-SUD schizophrenics and as such may act as a putative protective agent against the development of addiction to alcohol or other drugs of abuse. Schizophrenics with SUD may be carriers of the DRD2 Taq1 A1 allele, and/or other RDS reward polymorphisms and have hypodopaminergic reward function. One plausible mechanism for alcohol seeking in schizophrenics with SUD, based on previous research, may be a deficiency of gamma type endorphins that has been linked to schizophrenic type psychosis. We also propose that alcohol seeking behavior in schizophrenics, may serve as a physiological self-healing process linked to the increased function of the gamma endorphins, thereby reducing abnormal dopaminergic activity at the nucleus accumbens (NAc). These hypotheses warrant further investigation and cautious interpretation. We, therefore, encourage research involving neuroimaging, genome wide association studies (GWAS), and epigenetic investigation into the relationship between neurogenetics and systems biology to unravel the role of dopamine in psychiatric illness and SUD.

Swimming exercise alleviates the symptoms of attention-deficit hyperactivity disorder in spontaneous hypertensive rats

Attention-deficit hyperactivity disorder (ADHD) is a neurobehavioral disorder characterized by inattention, hyperactivity and impulsivity. In the present study, we investigated the effects of swimming exercise on the symptoms of ADHD in correlation with the expression levels of dopamine and the dopamine D2 receptor. Adult male spontaneous hypertensive rats (SHRs) were used as animal models of ADHD and Wistar-Kyoto rats were used as controls. The activity, impulsivity and levels of non-aggressive and aggressive behaviors in rats were measured. The short-term memory in the animal models of ADHD was assessed using an open-field test. The social interaction test, elevated plus maze test and step-through avoidance test were additionally performed. The expression levels of tyrosine hydroxylase (TH), which catalyzes the rate‑limiting step of dopamine synthesis, and the dopamine D2 receptor in the prefrontal cortex, substantia nigra and striatum were evaluated. The expression levels of TH and the dopamine D2 receptor were detected using immunohistochemistry and western blotting, respectively. In ADHD rats, the activity, impulsivity and levels of non-aggressive and aggressive behaviors were higher than that in control rats. By contrast, short-term memory in ADHD rats deteriorated. Swimming exercise suppressed hyperactivity, impulsivity and non-aggressive and aggressive behaviors, and alleviated the short-term memory impairment observed in ADHD rats. The expression levels of TH and the dopamine D2 receptor were decreased and increased in ADHD rats, respectively, when compared with control rats. Swimming exercise enhanced the expression of TH and suppressed the expression of the dopamine D2 receptor in ADHD rats. In the present study, swimming exercise improved the symptoms of ADHD by upregulating the expression of dopamine and downregulating the expression of the dopamine D2 receptor.