One Neuron, one Transmitter? It’s complicated.

During graduate school I spent two years in a neuroscience lab studying synapse formation before ultimately switching to the study of cancer biology. During those two years I thought I’d developed a reasonable understanding of basic neuroscience and part of that understanding involved the idea that a particular neuron was defined by the type of neurotransmitter that it released.  Accordingly, certain neurons would be referred to as dopaminergic, or glutamatergic (excitatory), GABAergic (inhibitory) etc.  However, while attending a poster session at the recent Winter Brain Conference in Big Sky, Montana I visited a poster showing neurons from intact rat brain VTA (ventral tegmental area) that co-stained for both dopaminergic and glutamatergic markers within the same cell. Thinking that this was strange I asked some of the other scientists at the poster if this was a phenomenon that they were familiar with, since my understanding was that each neuron only produced a single neurotransmitter, and they seemed non-committal and unsure what to make of it.

Of course it turns out that many neurons do not make a single neurotransmitter. As much as we’d like to simplify neurotransmission the reality is perhaps more complicated with individual neurons capable of synthesizing a variety of neurotransmitters resulting in a variety of possible signaling outcomes at individual synapses

The Evidence

Both GABA and glutamate neurotransmitters are co-released from entopeduncular nucleus neurons.

The entopeduncular nucleus (EP) is a major basal ganglia output nucleus and sends a large projection to the lateral habenula (LHb) mediating risk/reward decisions. Specific neurons within the EP project onto the LHb neurons and release both GABA and glutamate from the same axon onto individual LHb neurons. These neurons contain GAD65 and GAD67 (GABA synthetic enzymes) as well as the vesicular transporters VGAT and vGLUT2 which transport GABA and glutamate respectively into synaptic vesicles, suggesting that all of the necessary machinery for synthesis and co-release of these neurotransmitters is present. Intriguingly GABA and Glutamate also appear to be simultaneously released from the same synaptic vesicles in these neurons [Shabel SJ et al. 2014]. The extent to which this GABA co-release impacts downstream signaling and behavior remains to be determined.

Midbrain dopaminergic neurons co-release both glutamate and GABA.

Dopaminergic neurons in the midbrain play an important role in addiction/reward behavior and motor planning. Several studies have demonstrated co-release of both glutamate and GABA from midbrain dopaminergic neurons onto post-synaptic neurons leading to increased excitatory and inhibitory output respectively. Consistent with a role for glutamate release in these dopaminergic neurons they express the glutamate transporter vGlut2 and genetic ablation of vGlut2 prevents glutamate release and severely reduces excitatory output in post-synaptic cells [Hnasko TS et al. 2010]. Related work, published just last month, furthered these observations by reporting that in midbrain VTA (ventral tegmental area) neurons co-releasing dopamine and glutamate, glutamate alone was sufficient to mediate reward-seeking behavior; demonstrating the importance of released glutamate in these neurons independent of dopamine [Zell V et al.]. Interestingly, while capable of releasing GABA, midbrain dopaminergic neurons were found to lack expression of the classic GABA transporters GAD65 and GAD67 and instead were found to take up GABA through non-canonical means [Tritsch, N. X et al 2012 and 2014].

Among the original evidence suggesting that individual neurons can co-release multiple neurotransmitters was the finding that hippocampal mossy fibers express both glutamate and GABA transporters [Ottersen OP 1984, Sandler R 1991] challenging the prevailing notion that these are purely excitatory neurons. Additional studies built on these observations, but these findings remain controversial and were more recently refuted [Motokazu Uchigashima 2007].  

While numerous CNS neurons appear to coexpress two or more neurotransmitters, the relevance and functional significance of this is just beginning to become better understood. As one might expect, it’s complicated.

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Relevant Citations