Ultrastructural characterization of the mesostriatal dopamine innervation in mice, including two mouse lines of conditional VGLUT2 knockout in dopamine neurons

Abstract

Despite the increasing use of genetically modified mice to investigate the dopamine (DA) system, little is known about the ultrastructural features of the striatal DA innervation in the mouse. This issue is particularly relevant in view of recent evidence for expression of the vesicular glutamate transporter 2 (VGLUT2) by a subset of mesencephalic DA neurons in mouse as well as rat. We used immuno-electron microscopy to characterize tyrosine hydroxylase (TH)-labeled terminals in the core and shell of nucleus accumbens and the neostriatum of two mouse lines in which the Vglut2 gene was selectively disrupted in DA neurons (cKO), their control littermates, and C57BL/6/J wild-type mice, aged P15 or adult. The three regions were also examined in cKO mice and their controls of both ages after dual TH-VGLUT2 immunolabeling. Irrespective of the region, age and genotype, the TH-immunoreactive varicosities appeared similar in size, vesicular content, percentage with mitochondria, and exceedingly low frequency of synaptic membrane specialization. No dually labeled axon terminals were found at either age in control or in cKO mice. Unless TH and VGLUT2 are segregated in different axon terminals of the same neurons, these results favor the view that the glutamatergic cophenotype of mesencephalic DA neurons is more important during the early development of these neurons than for the establishment of their scarce synaptic connectivity. They also suggest that, in mouse even more than rat, the mesostriatal DA system operates mainly through non-targeted release of DA, diffuse transmission and the maintenance of an ambient DA level.

Ultrastructural localization of tyrosine hydroxylase in tree shrew nucleus accumbens core and shell

“…While the delineation of the core and shell boundary was distinct with calbindin staining in the tree shrew NAcc, heterogeneity within subregions was also observed. The present study examined TH organization at the level of the core and shell subregions as a goal of this study was to compare the staining patterns to that of other species, and these regions have been described previously across species, including rodent, non-human primate, and human (Rat: Záborsky et al, 1985; Bérubé-Carrière et al, 2012, Non-human primate: Meredith et al, 1996; Brauer et al, 2000, Human: Meredith et al, 1996; Voorn et al, 1996). Regions for the stereological analysis were selected randomly within the core and shell boundaries, thus the further heterogeneity was not selectively targeted for study.…”

“…The significance of these neurons forming both types of synapses has been widely discussed and often focuses on evidence that DA or TH colocalizes with markers for excitatory neurotransmitters in dopamine neurons (Sulzer et al, 1998; Kawano et al, 2006; Dal Bo et al, 2004; Yamaguchi et al, 2007) and releases these neurotransmitters synaptically (Sulzer et al, 1998; Joyce and Rayport, 2000; Chuhma et al, 2004; Stuber et al, 2010; Tecuapetla et al, 2010). While the co-release of glutamate from dopaminergic neurons has long been discussed (for reviews, see Sulzer and Rayport, 2000; Trudeau, 2004; Lapish et al, 2007), ultrastructural (Sulzer et al, 1998; Moss et al, 2011; Bérubé-Carrière et al, 2012) and anatomical (Hattori et al, 1991) studies using dual-labeling techniques have shown that the neurotransmitters likely do not colocalize within the same terminals in adult rats. It would therefore be unlikely that an asymmetric PSD of a TH-IR terminal is due to the co-release of glutamate from the same terminal.…”

Elevated Excitatory Input to the Nucleus Accumbens in Schizophrenia: A Postmortem Ultrastructural Study

“…[14][15][16][17][18][19] The elaborate multi-perforated synapses, however, have not been reported in other species or in other regions of the human brain. Increased spine volume, synapse length, and perforations are all associated with increased synaptic activity.…”

An update on the connections of the ventral mesencephalic dopaminergic complex

“…It is unclear if DA and GLU are packaged together in the same synaptic vesicles of VTA DA neurons or even occupy the same axon terminals. That is, one study in rat found that VGLUT2 co-immunoprecipitates with the vesicular monoamine transporter 2 (VMAT2) from striatal synaptic vesicles (Hnasko et al, 2010), but no double-labeling of TH and VGLUT2 was detected in axon terminals in the mouse Acb in another study (Bérubé-Carrière et al, 2012). …”