Segregation of dopamine and glutamate release sites in dopamine neuron axons: regulation by striatal target cells

Abstract

Dopamine (DA) is a key regulator of circuits controlling movement and motivation. A subset of midbrain DA neurons has been shown to express the vesicular glutamate transporter (VGLUT)2, underlying their capacity for glutamate release. Glutamate release is found mainly by DA neurons of the ventral tegmental area (VTA) and can be detected at terminals contacting ventral, but not dorsal, striatal neurons, suggesting the possibility that target-derived signals regulate the neurotransmitter phenotype of DA neurons. Whether glutamate can be released from the same terminals that release DA or from a special subset of axon terminals is unclear. Here, we provide in vitro and in vivo data supporting the hypothesis that DA and glutamate-releasing terminals in mice are mostly segregated and that striatal neurons regulate the cophenotype of midbrain DA neurons and the segregation of release sites. Our work unveils a fundamental feature of dual neurotransmission and plasticity of the DA system.-Fortin, G. M., Ducrot, C., Giguère, N., Kouwenhoven, W. M., Bourque, M.-J., Pacelli, C., Varaschin, R. K., Brill, M., Singh, S., Wiseman, P. W., Trudeau, L.-E. Segregation of dopamine and glutamate release sites in dopamine neuron axons: regulation by striatal target cells.

Vglut2 expression in dopamine neurons contributes to post-lesional striatal reinnervation

“…To recapitulate, we find here that expression of Th and Vglut2 is reciprocally regulated in DA neurons in response to cellular stress. While previous work showed that the presence of striatal cells reinforces the Th phenotype of DA neurons (Fortin et al, 2019), a low dose of MPP+ results in the opposite and favors their glutamatergic phenotype, as revealed by an increase in Vglut2 mRNA copy number. Finally, our in vitro data suggest the possibility that upregulated GDNF expression in the post-lesional brain could elevate Vglut2 expression above detection level and as such could be involved in the observed enhanced prevalence of Vglut2 expression in DA neurons in the post-lesional brain in vivo .…”

“…Interestingly, it had previously been shown that the ionotropic NMDA glutamate receptor subunit NR1 is found on the growth cone of DA neurons and suggested that axonal outgrowth in primary DA neurons can be driven by an autocrine glutamate-mediated feedback loop (Schmitz et al, 2009). In addition, we have also shown that post-natal dorsal striatal cells repress Vglut2 expression in cultured DA neurons (Fortin et al, 2019).…”

“…At E11.5 DA neurons start extending their axons dorsally and rostrally, and at E14.5 the first growth cones reach the striatum (Kolk et al, 2009). We recently showed that dorsal striatal target cells exert a repressive effect on Vglut2 expression in DA neurons (Fortin et al, 2019). In order to test the role of VGLUT2 expression in DA neurons on axon outgrowth, we investigated the effect of moderate over-expression of VGLUT2, using a lentiviral Vglut2-Venus construct driven by a synapsin promoter, in a monoculture of primary postnatal DA neurons (Fig 2A).…”

“…The gradual decrease of Vglut2 expression by DA neurons during embryonic development suggests that intrinsic or extrinsic signals regulate expression of this gene in DA neurons. We previously showed that interaction of DA neurons with dorsal striatal cells in vitro appears to negatively regulate Vglut2 expression in these neurons (Fortin et al, 2019). Other work has shown that neurotoxic lesions increase the likelihood of Vglut2 expression in surviving DA neurons (Dal Bo et al, 2008; Shen et al, 2018; Steinkellner et al, 2018), but it is currently not yet clear whether this is caused by enhanced Vglut2 expression per DA neuron or enhanced survival of DA neurons that co-express Vglut2 and Th .…”

“…Moreover, it has been proposed that the presence of VGLUT2 on a synaptic vesicle could promote DA synaptic vesicle loading through altering the vesicular pH (Aguilar et al, 2017). It needs to be noted however, that several studies have shown that VGLUT2 is typically found to be segregated from TH, DAT and VMAT2 in dopaminergic axonal varicosities (Fortin et al, 2019; Zhang et al, 2015), minimizing the potential significance of this cellular function of VGLUT2 in adult DA axons.…”