AMP-activated protein kinase slows D2 dopamine autoreceptor desensitization in substantia nigra neurons

Dopamine receptor D4 (DRD4) negatively regulates UCP1- and ATP-dependent thermogenesis in 3T3-L1 adipocytes and C2C12 muscle cells

The activation of beige fat and muscle tissues is an interesting and encouraging target for therapeutic intervention in obesity owing to their remarkable lipolytic activity and energy-consuming futile cycles. This study examined the effect of dopamine receptor D4 (DRD4) on lipid metabolisms as well as UCP1- and ATP-dependent thermogenesis in Drd4-silenced 3T3-L1 adipocytes and C2C12 muscle cells. Silencing of Drd4, followed by quantitative real-time PCR, immunoblot analysis, immunofluorescence, and staining methods, were applied to evaluate the effects of DRD4 on diverse target genes and proteins of both cells. The findings showed that DRD4 was expressed in the adipose and muscle tissues of normal and obese mice. Furthermore, the knockdown of Drd4 upregulated the expression of brown adipocyte-specific genes and proteins while downregulating lipogenesis and the adipogenesis marker proteins. Drd4 silencing also upregulated the expression of key signaling molecules involved in ATP-dependent thermogenesis in both cells. This was further elucidated by mechanistic studies showing that a Drd4 knockdown mediates UCP1-dependent thermogenesis via the cAMP/PKA/p38MAPK pathway in 3T3-L1 adipocytes and UCP1-independent thermogenesis via the cAMP/SLN/SERCA2a pathway in C2C12 muscle cells. In addition, siDrd4 also mediates myogenesis via the cAMP/PKA/ERK1/2/Cyclin D3 pathway in C2C12 muscle cells. Silencing of Drd4 promotes β3-AR-dependent browning in 3T3-L1 adipocytes and α1-AR/SERCA-based thermogenesis through an ATP-consuming futile process in C2C12 muscle cells. Understanding the novel functions of DRD4 on adipose and muscle tissues in terms of its ability to enhance energy expenditure and regulate whole-body energy metabolism will aid in developing novel obesity intervention techniques.

Effects of Chemogenetic Inhibition of D1 or D2 Receptor-Containing Neurons of the Substantia Nigra and Striatum in Mice With Tourette Syndrome

As tourette syndrome (TS) is a common neurobehavioral disorder, the primary symptoms of which include behavioral stereotypies. Dysfunction of the substantia nigra-striatum network could be the main pathogenesis of TS, which is closely associated with dopamine (DA) and its receptors. TS is often resistant to conventional treatments. Therefore, it is necessary to investigate the neurobiological mechanisms underlying its pathogenesis. In this study, we investigated whether chemogenetic activation or inhibition of dopaminergic D1 receptor (D1R)- or D2 receptor (D2R)-containing neurons in the substantia nigra pars compacta (SNpc) or dorsal striatum (dSTR) affected the stereotyped behavior and motor functions of TS mice. Intraperitoneal injection of 3,3'-iminodipropionitrile (IDPN) was used to induce TS in mice. Stereotyped behavior test and open-field, rotarod, and grip strength tests were performed to evaluate stereotyped behavior and motor functions, respectively. Immunofluorescence labeling was used to detect the co-labeling of virus fluorescence and D1R or D2R. We found that chemogenetic inhibition of D1R- or D2R-containing neurons in the SNpc and dSTR alleviated behavioral stereotypies and motor functions in TS mice. Chemogenetic activation of D1R-containing neurons in the dSTR aggravated behavioral stereotypies and motor functions in vehicle-treated mice, but neither was aggravated in TS mice. In conclusion, chemogenetic inhibition of D1R- or D2R-containing neurons in the SNpc and dSTR alleviated behavioral stereotypies of TS, providing a new treatment target for TS. Moreover, the activation of D1R-containing neurons in the dSTR may contribute to the pathogenesis of TS, which can be chosen as a more precise target for treatment.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Ethanol metabolism: The good, the bad, and the ugly

Throughout the world, ethanol is both an important commercial commodity and a source of major medical and social problems. Ethanol readily passes through biological membranes and distributes throughout the body. It is oxidized, first to acetaldehyde and then to acetate, and finally by the citric acid cycle in virtually all tissues. The oxidation of ethanol is irreversible and unregulated, making the rate dependent only on local concentration and enzyme activity. This unregulated input of reducing equivalents increases reduction of both cytoplasmic and intramitochondrial NAD and, through the latter, cellular energy state {[ATP]/([ADP][Pi])}. In brain, this increase in energy state stimulates dopaminergic neural activity signalling reward and a sense of well being, while suppressing glutamatergic neural activity signalling anxiety and unease. These positive responses to ethanol ingestion are important to social alcohol consumption. Importantly, decreased free [AMP] decreases AMP-dependent protein kinase (AMPK) activity, an important regulator of cellular energy metabolism. Oxidation of substrates used for energy metabolism in the absence of ethanol is down regulated to accommodate the input from ethanol. In liver, chronic ethanol metabolism results in fatty liver and general metabolic dysfunction. In brain, transport of other oxidizable metabolites through the blood-brain barrier and the enzymes for their oxidation are both down regulated. For exposures of short duration, ethanol induced regulatory changes are rapid and reversible, recovering completely when the concentrations of ethanol and acetate fall again. Longer periods of ethanol exposure and associated chronic suppression of AMPK activity activates regulatory mechanisms, including gene expression, that operate over longer time scales, both in onset and reversal. If chronic alcohol consumption is abruptly ended, metabolism is no longer able to respond rapidly enough to compensate. Glutamatergic neural activity adapts to chronic dysregulation of glutamate metabolism and suppression of glutamatergic neural activity by increasing excitatory and decreasing inhibitory amino acid receptors. A point is reached (ethanol dependence) where withdrawal of ethanol results in significant metabolic energy depletion in neurons and other brain cells as well as hyperexcitation of the glutamatergic system. The extent and regional specificity of energy depletion in the brain, combined with hyperactivity of the glutamatergic neuronal system, largely determines the severity of withdrawal symptoms.

Dopamine Evokes a Trace Amine Receptor-dependent Inward Current that is Regulated by AMP Kinase in Substantia Nigra Dopamine Neurons

We reported recently that activators of AMP-activated protein kinase (AMPK) slow the rundown of current evoked by the D2 autoreceptor agonist quinpirole in rat substantia nigra compacta (SNC) dopamine neurons. The present study examined the effect of AMPK on current generated by dopamine, which unlike quinpirole, is a substrate for the dopamine transporter (DAT). Using whole-cell patch-clamp, we constructed current-voltage (I-V) plots while superfusing brain slices with dopamine (100 μM) for 25 min. Two minutes after starting superfusion, dopamine evoked a peak current with an average slope conductance of 0.97 nS and an estimated reversal potential (E_rev) of -113 mV, which is near that expected for K⁺. But after 10 min of superfusion, dopamine-evoked currents had shifted to more depolarized values with a slope conductance of 0.64 nS and an E_rev of -83 mV. This inward shift in current was completely blocked by the DAT inhibitor GBR12935. However, an AMPK blocking agent (dorsomorphin) permitted the emergence of inward current despite the continued presence of the DAT inhibitor. When D2 autoreceptors were blocked by sulpiride, I-V plots showed that dopamine evoked an inward current with an estimated slope conductance of 0.45 nS with an E_rev of -57 mV. Moreover, this inward current was completely blocked by the trace amine-associated receptor 1 (TAAR1) antagonist EPPTB. These results suggest that dopamine activates a TAAR1-dependent non-selective cation current that is regulated by AMPK.