Dopamine neurons in culture express VGLUT2 explaining their capacity to release glutamate at synapses in addition to dopamine

Abstract

Dopamine neurons have been suggested to use glutamate as a cotransmitter. To identify the basis of such a phenotype, we have examined the expression of the three recently identified vesicular glutamate transporters (VGLUT1-3) in postnatal rat dopamine neurons in culture. We found that the majority of isolated dopamine neurons express VGLUT2, but not VGLUT1 or 3. In comparison, serotonin neurons express only VGLUT3. Single-cell RT-PCR experiments confirmed the presence of VGLUT2 mRNA in dopamine neurons. Arguing for phenotypic heterogeneity among axon terminals, we find that only a proportion of terminals established by dopamine neurons are VGLUT2-positive. Taken together, our results provide a basis for the ability of dopamine neurons to release glutamate as a cotransmitter. A detailed analysis of the conditions under which DA neurons gain or loose a glutamatergic phenotype may provide novel insight into pathophysiological processes that underlie diseases such as schizophrenia, Parkinson's disease and drug dependence.

Normal Biogenesis and Cycling of Empty Synaptic Vesicles in Dopamine Neurons of Vesicular Monoamine Transporter 2 Knockout Mice

“…However, despite the detection of low levels of VGLUT3 mRNA by in situ hybridization in one study (Fremeau et al, 2002), none of the VGLUTs have been detected in vivo in DA neurons at the protein level (Fremeau et al, 2001(Fremeau et al, , 2002Herzog et al, 2001;Gras et al, 2002;Schafer et al, 2002;Varoqui et al, 2002;Herzog et al, 2004). Further, even in cultured DA neurons, it appears that glutamate and DA are released from different terminals (Sulzer et al, 1998;Dal Bo et al, 2004). Consistent with this, we did not detect VGLUT immunoreactivity in TH-positive varicosities in our cultures.…”

“…This finding fits well with our recent work showing that the ability of cultured DA neurons to express VGLUT2 is partially repressed when DA neurons are in contact with other neurons (Dal Bo and Trudeau, unpublished results), as is the case in our mixed cultures. Moreover, even in isolated DA neuron cultures, TH-positive and VGLUT2-positive terminals are partially segregated (Dal Bo et al, 2004). In contrast to VGLUT2, cultured midbrain DA neurons have not been shown previously to express other VGLUT isoforms.…”

“…Under certain circumstances, cultured midbrain neurons have been reported to both express VGLUT2 and corelease glutamate along with DA (Sulzer et al, 1998;Dal Bo et al, 2004). In addition, glutamate-mediated synaptic responses have been detected in striatal neurons after extracellular stimulation of DA neurons in a mouse brain slice preparation (Chuhma et al, 2004).…”

“…When cultured in isolation, midbrain DA neurons have been reported to both express VGLUT2 and corelease the neurotransmitter glutamate in addition to DA (Sulzer et al, 1998;Dal Bo et al, 2004). We therefore examined whether the TH-positive varicosities in DA neurons in our cultures also expressed VGLUT2.…”

Dual-transmitter neurons: functional implications of co-release and co-transmission

“…The first evidence of co-transmission was noted in cultured dopaminergic neurons, which express the vesicular glutamate transporter (VGLUT2) and upon stimulation elicit fast glutamatergic EPSCs both in autaptic cultures and co-cultures [46–48]. The co-transmitter phenotype was later verified in vivo , as VTA afferents to the nucleus accumbens co-express tyrosine hydroxylase (TH) and VGLUT2 [49–51], however VGLUT2 expression declines with age [52–54].…”