All five classes of dopamine receptors are putative miRNA targets in neurological pathologies, although D1R and D2R are the most interrogated to date. In neurodegenerative Parkinson's Disease (PD), these patients classically display upregulated expression of D1R and D2R with hypersensitive response to dopamine (Seeman and Niznik, 1990; Kaasinen et al., 2000; Hurley et al., 2001; Gerfen et al., 2002; Zhu et al., 2002; Chu et al., 2004; Hisahara and Shimohama, 2011). Studies by Tan et al revealed that genetic knockout of miR-128 in D1-neurons of wild-type (WT) mice enhances D1R-activation by a dopamine agonist and provokes increased ERK2 expression (Tan et al., 2013). Parallel responses of D1R-hyperexcitablity/ERK2 in miR-128 deficient mice and PD patients suggests this miRNA is an active mechanism underlying PD pathology. In a separate study, a drosophila model which recapitulates PD through common genetic variants of alpha-synuclein uncovered a dysfunctional D2R-miR-137 pathway (Kong et al., 2015). Expression of miR-137 was elevated in the PD drosophila and anti-correlated with D2R protein levels, consistent with miRNA target regulation. Subsequently, miR-137 binding sites in the D2R gene were validated through in vitro analyses.

Targeting of dopamine receptor signaling by physiological stressors that trigger depression and related neuropsychiatric phenotypes is guided by select miRNAs (Zhang et al., 2013, 2015). Early life stress (ELS) can broadly impact normal development of the dopamine system, leading to inadequate coping methods for chronic-unpredictable stress (CUS) and depression in adults. Expression profiles of D1R, miR-504 (a D1R target), and D2R were explored in rats with ELS modeled through maternal deprivation (MD) coupled with exposure to CUS at mature age (MD/CUS) (Zhang et al., 2013). The MD/CUS rats indeed showed significant correlation between D1R, D2R, miR-504 as well as correlation of D2R and miR-504 with depression-related behaviors. In a separate study also conducted by Zhang et al. they subsequently identified several miRNAs (miR-9, miR-200a, miR-141, and miR-326) that target the 3′-UTR of the D2R (Zhang et al., 2015). In vitro analyses confirmed that miR-9 binds the D2R transcript (Zhang et al., 2015). They also found that miR-9, miR-326, and D2R correlated with depressive-like phenotypes in mature MD/CUS rats. In addition, aberrant expression of miR-326 in the MD/CUS rats was normalized with the anti-depressant escitalopram (Zhang et al., 2015). These results not only implicate dopamine-regulated miRNAs in depression, but also suggests these ncRNAs may mediate treatment response to anti-depressants.

Targeting of dopamine receptors by miRNAs was also uncovered in a model for addiction (Li J. et al., 2013). Post-administration of alcohol to rats, the NAc in the basal forebrain displayed repressed levels of miR-382 levels while its bioinformatic target, D1R, was upregulated. Cell-based studies as well as site-specific knockdown and overexpression of miR-382 in rat NAc confirmed it targets the D1R gene (Li J. et al., 2013). Moreover, miR-382 overexpression altered the neuronal excitability responses of D1R and diminished desire to consume alcohol. This report makes the direct connection between miRNA regulation of D1R and neuropsychiatric phenotypes in an animal model.

Disease-associated polymorphic regions in the dopamine receptor genes can impact miRNA binding to their target sites. One such reported example is the D1R rs686 polymorphism linked to nicotine dependence (Huang and Li, 2009). Two miRNA sites were bioinformatically identified within the rs686 region, miR-504 and miR-296. Further, miR-504 regulation of D1R was confirmed in vitro. Notably, miR-504 appears to exhibit stronger binding potential to the rs686 allele compared to the wild-type (WT) allele, which could alter the allelic representation of DR1.

The dopamine transporter gene (DAT/SLC6A3) is critical for removal of dopamine in the synapse to halt neurotransmission (German et al., 2015; McHugh and Buckley, 2015). Correspondingly, the disruption of this gene reportedly provokes numerous distinct neuronal pathologies (German et al., 2015; McHugh and Buckley, 2015). Two separate studies investigated miRNA binding in polymorphic regions of the DAT gene. One study examined a 40-base pair 3′UTR region within the DAT transcript linked to Attention Deficit Hyperactivity Disorder (ADHD) (Sery et al., 2015). Bioinformatic analysis of this 40-base pair segment identified putative sites for miRs-1972, 30b-5p, 1301, and 6070 (Sery et al., 2015). A second study focused on a DAT polymorphism linked to bipolar disease, rs27072 (Pinsonneault et al., 2011). The rs27072 analysis revealed binding sequences for miRs-762, 4259, 3192, 3127, and 1266 (Pinsonneault et al., 2011). Collectively, although these findings are only predictive at this point, they provide a framework for future investigation.

In addition to dopamine, DAT can also transport the neurotoxin 6-hydroxydopamine (6-OHDA), eliciting phenotypes which recapitulate PD (Westerlund et al., 2010). 6-OHDA kills dopaminergic neurons, providing a platform to identify and study dopamine-regulated ncRNAs. A subset of miRNAs are reported to mediate 6-OHDA neuronal damage, including miRs-124, 126, 668-3p, let-7d-3p, 3077-3p, 665-5p, 99b-3p, 323-3p, 875, 207, 425-5p, 19b-3p, and 338-3p (Li L. et al., 2013; Kim et al., 2014; Saraiva et al., 2016).

The processing of miRNAs involves several stages, and their disruption is pathogenic (Kocerha et al., 2009, 2015; Chan and Kocerha, 2012; Chun et al., 2014). The primary-miRNA is cleaved by Dgcr8 and Drosha, followed by second cleavage with Dicer into the mature transcript (Bartel, 2004, 2009; Macias et al., 2013). The mature miRNA gets assembled, along with argonaute proteins, into RISC for target gene regulation (Bartel, 2004, 2009). Studies by Schaefer et al found that deficiency of the RISC-associated Argonaute 2 (Ago2) protein in D2R-neurons significantly reduced self-administration of cocaine in mice (Schaefer et al., 2010). This Ago2 mouse model was then used to compile a list of D2R-localized miRNAs which putatively control cocaine addiction.

Ago2 expression is also reportedly regulated by the pain medication morphine (García-Pérez et al., 2013). Acute administration of morphine in rats led to increased Ago2 levels as well as the disruption of a panel of dopaminergic markers such as Nurr1, Pitx3, and TH activity. Pitx3 and TH are mentioned below as part of a conserved miRNA mechanism with miR-133b in neurological disease. Notably, pain relief is mediated by dopamine release in the midbrain, a region where miR-133b functions are reported (Kim et al., 2007; Navratilova et al., 2015). Indeed, a separate study revealed morphine alters miR133b-Pitx3 expression and differentiation of dopaminergic neurons in a zebrafish model (Sanchez-Simon et al., 2010). These results suggest ncRNAs which are regulated by morphine influence neurotransmission.

The gene for the core miRNA processor, Dgcr8, is located within the 22ql1 chromosomal locus, a region persistently associated with schizophrenia and other neurological anomalies (Merico et al., 2015; Zhao et al., 2015). Studies by Chun et al showed that a mouse model for the 22q11 deletion impedes Dgcr8 function, leading to enhanced expression of D2R, and disruption of corticothalamic synaptic transmission (Chun et al., 2014). Moreover, the aberrant D2R signaling provokes a hypersensitive response to antipsychotics used for schizophrenia treatment. Identifying the specific miRNAs impacted by haploinsufficient Dgcr8 processing in the 22q11 mice would facilitate future studies. Importantly, these data also suggest ncRNA mechanisms can influence drug efficacy and medical outcomes.

A shared characteristic of many neurological diagnoses is impaired synaptic transmission; thus, it is pertinent to uncover conserved dopamine-linked ncRNA mechanisms in brain pathologies (Henstridge et al., 2016). One conserved ncRNA pathway was identified through miRNA expression profiles of midbrain dopamine neurons (mDNs) in PD patients (Kim et al., 2007; Briggs et al., 2015). One of the dysregulated miRNAs, miR-133b, controls mDN differentiation through a feedback loop with Pitx3 (Kim et al., 2007). Pitx3 is a transcription factor essential for DN survival (Bergman et al., 2010). A separate study, however, reported that miR-133b expression is not impacted within the substantia nigra region of the midbrain (Schlaudraff et al., 2014). These results suggest the presence of site-specific miRNA mechanisms, providing an additional layer of dopaminergic influence.

The Pitx3-miR-133b pathway associated with PD is also implicated in the neuropsychiatric disorder schizophrenia. Song et al reported that Pitx3-miR-133b is dysregulated in mice with schizophrenia phenotypes provoked through overexpression of the stress protein heme-oxygenase-1 (HO-1) (Song et al., 2012). Moreover, these mice exhibit disrupted profiles of dopaminergic markers D1R, Nurr1 and tyrosine hydroxylase (TH), as well as neurotransmitters serotonin and dopamine. Parallel changes of dopamine and serotonin in this model is consistent with extensive cross-talk reported in synaptic signaling (Henstridge et al., 2016). Further, an independent study found this miRNA pathway is altered in another neuropsychiatric disorder, cocaine addiction (Barreto-Valer et al., 2012). Zebrafish embryos were exposed to cocaine followed by gene expression analysis at various developmental timepoints. In addition to dysregulated levels of miR-133b, Pitx3, D1R, TH in these embryos, there were also significant changes in D2R (Barreto-Valer et al., 2012). Collectively, these data suggest subsets of epigenetic mechanisms are conserved across neurological diseases and, further, may contribute to a general trend of synaptic dysfunction in these pathologies.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.