VGLUT1/VGAT co-expression sustains glutamate-gaba co-release and is regulated by activity

Abstract

In adult neocortex, VGLUT1 (also known as SLC17A7), the main glutamate vesicular transporter, and VGAT (also known as SLC32A1), the c-aminobutyric acid (GABA) vesicular transporter, are co-expressed in a subset of axon terminals forming both symmetric and asymmetric synapses, where they are sorted into the same vesicles. However, the functional consequence of this colocalization in cortical neurons has not been clarified. Here, we tested the hypothesis that cortical axon terminals co-expressing VGLUT1 and VGAT can evoke simultaneously monosynaptic glutamate and GABA responses, and investigated whether the amount of terminals co-expressing VGLUT1 and VGAT is affected by perturbations of excitation-inhibition balance. In rat primary cortical neurons, we found that a proportion of synaptic and autaptic responses were indeed sensitive to consecutive application of selective glutamate and GABA A receptor blockers. These 'mixed' synapses exhibited paired-pulse depression. Notably, reducing the activity of the neuronal network by treatment with glutamate receptor antagonists decreased the amount of 'mixed' synapses, whereas reducing spontaneous inhibition by treatment with bicuculline increased them. These synapses might contribute to homeostatic regulation of excitation-inhibition balance.

Insufficient developmental excitatory neuronal activity fails to foster establishment of normal levels of inhibitory neuronal activity

“…Prevention of the GABA‐induced increase in signaling for networks stimulated for 1 day by AP5 and CNQX (both of which suppress excitatory signaling; Ho et al, 2002) provides further evidence that GABA treatment induced an increase in excitatory activity in networks stimulated for 1 day. These findings are consistent with the detection of synapses that release both excitatory and inhibitory neurotransmitters, the activity of which was reduced following inhibition of glutamatergic receptors (Gillespie et al, 2005; Fattorini et al, 2015; Akgül and McBain, 2016). They further suggest that those neurons that are initially excitatory but undergo a developmental transition to inhibitory neurons may release, and/or respond to, a combination of excitatory and inhibitory neurotransmitters during this developmental transition.…”

Multiplexed neurochemical signaling by neurons of the ventral tegmental area

“…As detailed above, the multiplexed neurotransmission of the VTA-VGluT2 neurons is an emerging factor involved in the complexity of VTA function. Based on observations that different combinations of neurotransmitters are multiplexed throughout the brain (Trudeau 2004; Gillespie et al 2005; Zhou et al, 2005; Gras et al, 2008; Noh et al, 2010; Higley et al, 2011; Tritsch et al 2012; Hnasko and Edwards, 2012 Münster-Wandowski et al, 2013; Nelson et al, 2014; Root et al, 2014a; Shabel et al, 2014; Qi et al, 2014; Zhang et al, 2015; Fattorini et al, 2015; Saunders et al, 2015), we suggest that multiplexed neurotransmission conveys distinct messages depending on the neurotransmitter content of each circuit, momentary singular or multiplexed signaling, and perhaps even the time scale of neurotransmitter function. Furthermore, we speculate that changes in the influence of one or more of the multiplexed neurotransmitters, by way of either presynaptic of postsynaptic changes, may result from and result in observable changes in behavior.…”

“…For instance, glutamate and GABA co-neurotransmission has been reported in epilepsy models within mossy fiber terminals (Gutiérrez et al, 2003, Gutiérrez 2003, 2005; Trudeau & Gutiérrez , 2007; Münster-Wandowski et al, 2013), developing medial trapezoid body terminals from the lateral superior olive (Gillespie et al 2005; Noh et al, 2010), entopeduncular nucleus projection to the LHb (Shabel et al, 2014), and cortex (Fattorini et al, 2015). In addition, there is evidence for GABA and DA co-transmission in by substantia nigra pars compacta neurons as well as retinal amacrine neurons (Tritsch et al 2012; Hirasawa et al, 2012).…”

Parallel Visual Circuitry in a Basal Chordate

“…From in situ hybridization we observed some PR-IIs exclusively express VGAT, while other coexpress VGAT and VGLUT. The significance of VGAT/VGLUT coexpression in the Ciona visuomotor pathway is not yet clear, although similar coexpression is widely observed in mammalian brains [45,46], and invertebrates [47]. It is speculated that co-release of GABA and GLUT may serve to tune excitatory/inhibitory balance.…”