Probes for VGLUT2

Loss of midbrain dopamine neurons causes the cardinal symptoms of Parkinson’s disease. However, not all dopamine neurons are equally vulnerable and a better understanding of the cell-type specific properties relating to selective dopamine neuron degeneration is needed. Most midbrain dopamine neurons express the vesicular glutamate transporter VGLUT2 during development and a subset continue to express low levels of VGLUT2 in adulthood, enabling the co-release of glutamate. Moreover, VGLUT2 expression in dopamine neurons can be neuroprotective since its genetic disruption was shown to sensitize dopamine neurons to neurotoxins. Here, we show that in response to toxic insult, and in two distinct models of alpha-synuclein stress, VGLUT2 dopamine neurons were resilient to degeneration. Dopamine neurons expressing VGLUT2 were enriched whether or not insult induced dopamine neuron loss, suggesting that while VGLUT2 dopamine neurons are more resilient, VGLUT2 expression can also be transcriptionally upregulated by injury. Finally, we observed that VGLUT2 expression was enhanced in surviving DA neurons from postmortem Parkinson’s disease subjects. These data indicate that emergence of a glutamatergic identity in dopamine neurons may be part of a neuroprotective response in Parkinson’s disease.

Methamphetamine (METH) is a highly addictive psychostimulant. The continuous use of METH may lead to its abuse and neurotoxicity that have been associated with METH-induced increases in release of dopamine (DA) and glutamate in the brain. METH action in DA has been shown to be mediated by redistribution of DA from vesicles into cytoplasm via vesicular monoamine transporter 2 (VMAT2) and the subsequent reversal of membrane DA transporter (DAT), while little is known about the mechanisms underlying METH-induced glutamate release. Recent studies indicate that a subpopulation of midbrain DA neurons co-expresses VMAT2 and vesicular glutamate transporter 2 (VGLUT2). Therefore, we hypothesized that METH-induced glutamate release may in part originate from such a dual phenotype of DA neurons. To test this hypothesis, we used Cre-LoxP techniques to selectively delete VGLUT2 from midbrain DA neurons, and then examined nucleus accumbens (NAc) DA and glutamate responses to METH using in vivo brain microdialysis between DA-VGLUT2-KO mice and their VGLUT2-HET littermates. We found that selective deletion of VGLUT2 from DA neurons did not significantly alter basal levels of extracellular DA and glutamate, but attenuated METH-induced increases in extracellular levels of DA and glutamate. In addition, DA-VGLUT2-KO mice also displayed lower locomotor response to METH than VGLUT2-HET control mice. These findings, for the first time, suggest that cell-type specific VGLUT2 expression in DA neurons plays an important role in the behavioral and neurochemical effects of METH. Glutamate corelease from DA neurons may in part contributes to METH-induced increase in NAc glutamate release.

Whether glutamate or itch-selective neurotransmitters are used to confer itch specificity is still under debate. We focused on an itch-selective population of primary afferents expressing MRGPRA3, which highly expresses Vglut2 and the neuropeptide neuromedin B (Nmb), to investigate this question. Optogenetic stimulation of MRGPRA3+ afferents triggers scratching and other itch-related avoidance behaviors. Using a combination of optogenetics, spinal cord slice recordings, Vglut2 conditional knockout mice, and behavior assays, we showed that glutamate is essential for MRGPRA3+ afferents to transmit itch. We further demonstrated that MRGPRA3+ afferents form monosynaptic connections with both NMBR+ and NMBR- neurons and that NMB and glutamate together can enhance the activity of NMBR+ spinal DH neurons. Moreover, Nmb in MRGPRA3+ afferents and NMBR+ DH neurons are required for chloroquine-induced scratching. Together, our results establish a new model in which glutamate is an essential neurotransmitter in primary afferents for itch transmission, whereas NMB signaling enhances its activities.

Rodent primary somatosensory cortex (S1) is organized in defined layers, where layer IV serves as the main target for thalamocortical projections. Serotoninergic signaling is important for the organization of thalamocortical projections and consequently proper barrel field development in rodents, and the vesicular monoamine transporter 2 (VMAT2) can be detected locally in layer IV S1 cortical neurons in mice as old as P10, but the identity of the Vmat2-expressing neurons is unknown. We here show that Vmat2 mRNA and also Vmat2-Cre recombinase are still expressed in adult mice in a sub-population of the S1 cortical neurons in the barrel field. The Vmat2-Cre cells showed a homogenous intrinsically bursting firing pattern determined by whole-cell patch-clamp, localized radial densely spinous basal dendritic trees and almost exclusively lack of apical dendrite, indicative of layer IV spiny stellate cells. Single cell mRNA sequencing analysis showed that S1 cortical Vmat2-Cre;tdTomato cells express the layer IV marker Rorb and mainly cluster with layer IV neurons, and RNAscope analysis revealed that adult Vmat2-Cre neurons express Vmat2 and vesicular glutamate transporter 1 (Vglut1) and Vglut2 mRNA to a high extent. In conclusion, our analysis shows that cortical Vmat2 expression is mainly confined to layer IV neurons with morphological, electrophysiological and transcriptional characteristics indicative of spiny stellate cells.