α-synuclein accumulation into dopaminergic neurons is a pathological hallmark of Parkinson’s disease. We previously demonstrated that fatty acid-binding protein 3 (FABP3) is critical for α-synuclein uptake and propagation to accumulate in dopaminergic neurons. FABP3 is abundant in dopaminergic neurons and interacts with dopamine D2 receptors, specifically the long type (D_2L). Here, we investigated the importance of dopamine D_2L receptors in the uptake of α-synuclein monomers and their fibrils. We employed mesencephalic neurons derived from dopamine D_2L^−/−, dopamine D2 receptor null (D2 null), FABP3^−/−, and wild type C57BL6 mice, and analyzed the uptake ability of fluorescence-conjugated α-synuclein monomers and fibrils. We found that D_2L receptors are co-localized with FABP3. Immunocytochemistry revealed that TH⁺ D2L^−/− or D2 null neurons do not take up α-synuclein monomers. The deletion of α-synuclein C-terminus completely abolished the uptake to dopamine neurons. Likewise, dynasore, a dynamin inhibitor, and caveolin-1 knockdown also abolished the uptake. D_2L and FABP3 were also critical for α-synuclein fibrils uptake. D_2L and accumulated α-synuclein fibrils were well co-localized. These data indicate that dopamine D_2L with a caveola structure coupled with FABP3 is critical for α-synuclein uptake by dopaminergic neurons, suggesting a novel pathogenic mechanism of synucleinopathies, including Parkinson’s disease.

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.

The striatum contains several types of neurons including medium spiny projection neurons (MSNs), cholinergic interneurons (ChIs), and fast-spiking interneurons (FSIs). Modulating the activity of these neurons by the dopamine D2 receptor (D2R) can greatly impact motor control and movement disorders. D2R exists in two isoforms: D2L and D2S. Here, we assessed whether alterations in the D2L and D2S expression levels affect neuronal excitability and synaptic function in striatal neurons. We observed that quinpirole inhibited the firing rate of all three types of striatal neurons in wild-type (WT) mice. However, in D2L knockout (KO) mice, quinpirole enhanced the excitability of ChIs, lost influence on spike firing of MSNs, and remained inhibitory effect on spike firing of FSIs. Additionally, we showed mIPSC frequency (but not mIPSC amplitude) was reduced in ChIs from D2L KO mice compared with WT mice, suggesting spontaneous GABA release is reduced at GABAergic terminals onto ChIs in D2L KO mice. Furthermore, we found D2L deficiency resulted in reduced dendritic spine density in ChIs, suggesting D2L activation plays a role in the formation/maintenance of dendritic spines of ChIs. These findings suggest new molecular and cellular mechanisms for causing ChIs abnormality seen in Parkinson’s disease or drug-induced dyskinesias.

Dopamine D2 receptor (D2R)‐mediated signalling is involved in reward, motivation and alcohol use disorder. Perturbation of the D2R system may influence an individual's response to alcohol. Alternative splicing of the D2R gene generates two isoforms: D2 long form (D2L) and D2 short form (D2S). It is unclear whether differences in the expression of D2L and D2S influence alcohol's effects. Here we examined if altered expression levels of either D2R isoform would influence alcohol effects on reward‐related behaviour and relevant signalling pathways using knockout (KO) mice expressing either D2R isoform. We found that D2L KO mice (expressing only D2S) displayed a strong alcohol conditioned place preference (CPP) compared to WT mice and D2S KO mice (expressing only D2L). Alcohol exposure caused a downregulation of cannabinoid 1 receptors (CB1R) expression but an upregulation of cannabinoid 2 receptors (CB2R) expression in the striatum of D2L KO mice but not in WT and D2S KO mice. In addition, alcohol exposure resulted in decreased Akt phosphorylation selectively in D2L KO mice. Furthermore, the gene expressions of tyrosine hydroxylase (TH), Arc and RETN were also selectively downregulated in D2L KO mice following chronic alcohol exposure. Our results indicated that an alteration in the expression of D2S vs. D2L had significant impacts on alcohol‐induced reward and gene expression changes in the striatum. These findings suggest that the increased expression level of D2S to D2L may be a pathophysiological mechanism for developing alcoholism, possibly through triggering a cascade of changes in the cannabinoid‐Akt signalling pathway and relevant signalling network.

Dopamine and dopamine D2R receptors (D2R) are involved in regulating eating behavior and endocrine and metabolic functions. D2R exists in two D2R isoforms: D2L (long form) and D2S (short form). Little is known if the changes in the expression levels of D2S and D2L would cause metabolic alterations. Here, we examined the role of these two D2R isoforms in obesity and glucose homeostasis. Mice of two genotypes were fed a higher fat diet (HFD). Body weight and food intake were monitored chronically, and various fat pads were dissected. Glucose tolerance and insulin tolerance tests were conducted. Energy expenditure and respiratory exchange ratio were measured via indirect calorimetry. We found when feeding with HFD, dopamine D2L knockout (D2L KO) mice (expressing purely D2S) of both female and male gained significantly more body weight than wild‐type (WT) mice (expressing predominantly D2L) of both sexes. In addition, when feeding HFD, D2L KO mice showed an increased food intake compared to WT mice. Furthermore, when feeding HFD, both female and male D2L KO mice displayed impaired glucose tolerance. There were no significant differences in energy expenditure, respiratory quotient, and insulin sensitivity between D2L KO and WT mice. These results suggest that an increased expression level of D2S to D2L makes mice prone to obesity and hyperglycemia. Our findings identify a new risk factor contributing to the development of metabolic syndrome and increase our understanding of pathophysiological mechanisms leading to weight gain and diabetes.