The unexpected finding that neurons can co-release two neurotransmitter molecules, dopamine and GABA, through a common mechanism provides a further advance in our understanding of the nervous system. See Letter p.262
The striatum, a part of the brain that regulates motivation, reward responses, feeding and movement, integrates signals from other brain regions such as the substantia nigra and the ventral tegmental area. The inputs from these two regions arrive through nerve fibres that release dopamine, a neurotransmitter molecule that modulates neuronal activity by means of a slow-acting process. However, on page 262 of this issue, Tritsch et al.1 report that these modulatory neurons can also induce a rapid, short-lasting inhibition of striatal neurons by releasing another neurotransmitter, γ-aminobutyric acid (GABA). Surprisingly, the authors found that this GABA-mediated effect was dependent on the protein VMAT2, which is required for dopamine secretion, rather than on VGAT, a protein that was thought to be needed for GABA release.
Proteins such as VMAT2 and VGAT are called vesicular transporters, because they pump neurotransmitters from the neuronal cytoplasm into vesicles that are then unloaded at the synaptic junction between a sender neuron and a receiver neuron (Fig. 1). The released neurotransmitters bind to specific receptor proteins on the surface of the receiver cell, leading to changes in the cell's activity — by, for example, altering the flow of ions that cross the cell membrane through proteins called ion channels. Neurotransmitters can also bind to receptors on the sender cell (autoreceptors) that typically modulate the release or synthesis of the same neurotransmitter.
Dopamine-releasing (dopaminergic) neurons that project from the substantia nigra and the ventral tegmental area (VTA) form a heterogeneous population; these cells differ in the ion channels that they express on their surface2, in their receptor proteins (such as G-protein-coupled receptors3) and in their vesicular transporters4. Some of the neurons can secrete glutamate5,6,7 (a neurotransmitter and a precursor in the synthesis of GABA), and dopamine and glutamate are stored and released from the same synaptic vesicles in a subset of dopaminergic neurons located mainly in the VTA. Glutamate transport into vesicles has been shown to be dependent on a specific vesicular transporter (VGLUT2)4.
To clarify how dopaminergic neurons in the substantia nigra affect striatal activity, Tritsch et al. used an optogenetic technique that allowed them to turn the neurons on and off in brain slices obtained from genetically engineered mice. In this experimental set-up, brief pulses of light induced the neurons to secrete dopamine. Surprisingly, the authors found that this selective activation of dopaminergic neurons produced large, GABA-dependent inhibitory currents and small, glutamate-dependent excitatory currents in certain striatal neurons. The GABA-dependent currents were large enough to inhibit the firing of the striatal neurons. Even more surprising was the finding that knocking out VGAT (the vesicular GABA transporter) did not block GABA's inhibitory effect, raising the question of where the released GABA was coming from. The researchers went on to show that GABA's inhibitory currents were blocked by inhibiting VMAT2, suggesting that GABA is concentrated with dopamine in the same synaptic vesicles.
Further support for this conclusion came from a remarkable series of experiments. The authors found, for example, that decreasing the release of dopamine by inhibiting its synthesis, or by knocking out the expression of VGLUT2 or VGAT, did not affect the inhibition by GABA. By contrast, VMAT inhibitors blocked the secretion of both dopamine and GABA. Taken together, these assays indicate that VMAT2 alone is responsible for the accumulation of GABA in dopamine-containing vesicles. How GABA is synthesized in these neurons remains to be determined, because only a small subset of dopaminergic neurons seems to express the gene Gad65 (also known as Gad2)8, which encodes a key enzyme involved in the synthesis of the neurotransmitter.
Tritsch and colleagues' work shows, therefore, that co-released dopamine and GABA modulate the activity of the striatal output neurons in temporally and mechanistically distinct ways. It also suggests that, in the striatum, GABA-mediated inhibition dominates over glutamate-mediated activation. Moreover, the authors' results indicate that GABA is probably co-released by other dopamine neurons, and so it will be important to determine the physiological impact of this process in the striatum and in other brain areas in vivo.
Although the optogenetic technique was instrumental in identifying this process, the method is known to produce synchronous stimulation of many neurons, and so it is not clear whether GABA-dependent inhibition will be observed in the absence of this massive stimulation. It is worth noting, nevertheless, that dopaminergic neurons in subregions of the substantia nigra and the VTA tend to burst in relative synchrony as a result of a common input9. In addition, the fibres of neurons in these subregions tend to project to the same brain areas2, and so it is possible that a phasic (intermittent) GABA-dependent inhibition occurs in selective areas.
As the authors point out, VMAT is expressed not only by dopaminergic neurons but also by neurons that release other monoamines (neurotransmitters that are synthesized from aromatic amino acids), such as serotonin, adrenaline and noradrenaline. These neurons would therefore also be expected to release GABA. If true, then this property can be added to the many similarities among monoaminergic neurons — for example, in their firing rate and pattern, in the co-release of glutamate and monoamines, and in the expression of their respective inhibitory autoreceptors. Transmission from monoaminergic neurons can thus induce not only slow, long-lasting modulation but also brief, phasic inhibition. In my opinion, this finding will change the way researchers think about the roles of monoaminergic neurons in the functioning of the brain.
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