Single rodent mesohabenular axons release glutamate and GABA

Abstract

The lateral habenula (LHb) is involved in reward, aversion, addiction, and depression, through descending interactions with several brain structures, including the ventral tegmental area (VTA). VTA provides reciprocal inputs to LHb, but their actions are unclear. Here we show that the majority of rat and mouse VTA neurons innervating LHb co-express markers for both glutamate-signaling (vesicular glutamate transporter 2, VGluT2) and GABA-signaling (glutamate decarboxylase, GAD; and vesicular GABA transporter, VGaT). A single axon from these mesohabenular neurons co-expresses VGluT2-protein and VGaT-protein, and surprisingly establishes symmetric and asymmetric synapses on LHb neurons. In LHb slices, light activation of mesohabenular fibers expressing channelrhodopsin-2 (ChR2) driven by VGluT2 or VGaT promoters elicits release of both glutamate and GABA onto single LHb neurons. In vivo light-activation of mesohabenular terminals inhibits or excites LHb neurons. Our findings reveal an unanticipated type of VTA neuron that co-transmits glutamate and GABA, and provides the majority of mesohabenular inputs.

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

“…In addition, Glu and GABA co-release mediated by VGLUT2 and VGAT co-expression has been recently reported in terminals originating from basal ganglia (Shabel et al, 2014) and ventral tegmental area (Root et al, 2014), which form synapses on lateral habenular neurons. Although in a previous study, we have demonstrated that in the neocortex VGLUT1 and VGAT are co-expressed in a subset of axon terminals forming both symmetric and asymmetric synapses, that VGLUT1 and VGAT are sorted to the same vesicles and that, at synapses expressing the vesicular heterotransporters, these vesicles participate in the exo-endocytotic cycle (Fattorini et al, 2009), whether VGLUT1 and VGAT co-expression had functional consequences was still undefined.…”

“…By preventing systemic overexcitability by downregulating synaptic activity, this population of mixed synapses might play a role in regulating excitation-inhibition balance in cortical microcircuits. Interestingly, Glu and GABA co-release on habenular neurons has been associated with the pathophysiology of mood disorders (Root et al, 2014;Shabel et al, 2014). It is thus tempting to speculate that activity-dependent regulation of Glu and GABA co-release, induced by excitation-inhibition unbalance, might contribute to regulating mood and cognition in both normal and pathological conditions (e.g.…”

Glial and Neuroimmune Mechanisms as Critical Modulators of Drug Use and Abuse

“…Neurons within the VTA are not identical in the synaptic input they receive, nor do they project to identical targets (Beier et al, 2015). In addition, VTA neurons may release more than just dopamine, often co-releasing glutamate and GABA simultaneously (Hnasko et al, 2010;Root et al, 2014;Tritsch and Sabatini, 2012), which complicates the characterization of post-synaptic effects. An additional hindrance in translating addiction research involves the necessity of comparing results across experiments that can vary dramatically by a multitude of factors, including the drug concentration, method of delivery (eg, experimenter administered vs self-administration), timecourse of drug access (eg, hours of access per day, number of days of access), and the consequent neuroadaptations that occur from repeated drug exposure.…”

Contemporary approaches to neural circuit manipulation and mapping: focus on reward and addiction

“…Some neurons in the VTA that project to NAc transmit both dopamine and glutamate, but from spatially distinct synapses on the axon terminals [109]. Furthermore, some neurons projecting from the VTA to the LHb transmit both glutamate and GABA (but not dopamine), establishing an exception to the classic excitatory/inhibitory dichotomy of neuron classification [187]. These studies emphasize the importance of a fundamental question in neuroscience: what defines a neuron's phenotype?…”