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GABA is the major inhibitory neurotransmitter in the brain. In my laboratory, we are studying how GABA is released from neurons and glia, and how this release is affected by anticonvulsants. In addition to the well-characterized mechanism of vesicular release that occurs due to fusion of synaptic vesicles, many neuro-transmitters, including GABA, can be released when their uptake transporter operates in reverse. This carrier-mediated GABA release has been recognized to exist for many years, but the physiological relevance has been unclear. Since the GABA transporter is electrogenic and sodium-dependent (Figure 1), this nonvesicular form of GABA release can be induced by high intracellular sodium and depolarization. We have recently shown that GABA transporter reversal occurs surprisingly easily - in response to mild depolarization induced by an increase in extracellular K+ to as little as 6-12 mM (Figure 2).
There is normally an increase in extracellular K+ during a seizure. Along with the increase in intracellular Na+ and the depolarization that would occur, this suggests that carrier-mediated GABA release would be greatest during seizures, and would act to reduce excitability. Indeed, we found that the anticonvulsant gabapentin can enhance nonvesicular GABA release induced via heteroexchange release by the GABA analog nipecotic acid. Along with the recently described effect of gabapentin on calcium currents, the enhancement of nonvesicular GABA release could inhibit seizures. We have recently shown that another anticonvulsant, vigabatrin, also enhances reversal of the GABA transporter. The primary mechanism of vigabatrin is to inhibit GABA transaminase, the enzyme that metabolizes GABA. This leads to an increase in cytosolic [GABA], which would favor reversal of the GABA transporter.
In addition to the effect of vigabatrin in enhancing transporter reversal, this drug also induces a large tonic, GABA mediated inhibition of neurons (Figure 3). This effect of vigabatrin is very potent, with as little as 50 nM vigabatrin inducing significant tonic inhibition. It is also slow to develop (3-4 days), consistent with the slow accumulation of GABA in the cytosol. We have shown that the tonic inhibition can be due to continuous, spontaneous GABA transporter reversal under some conditions. However, more importantly than the fact that it can occur due to transporter reversal is the following.
An increase in cytosolic level would lead to a shift in the equilibrium for the transmembrane GABA gradient to a higher extracellular [GABA], as shown in Figure 4. With all else being equal, an increase in intracellular [GABA] would lead to an increase in extracellular [GABA] at steady state. If extracellular [GABA] is sufficiently high this would activate high affinity, extrasynaptic GABA receptors, leading to tonic inhibition. This type of tonic inhibition was only recently discovered, and is rapidly becoming recognized as playing an important role in regulation of brain excitability. Thus, the increase in tonic inhibition induced by vigabatrin may well be the major reason why this drug prevents seizures.
GABA transporter reversal also potentially plays an important role in Parkinson's Disease. Dopaminergic neurons of the substantia nigra receive a large GABAergic input that inhibits the output of these neurons to the basal ganglia. We have recently found that GABA transporter reversal also occurs in the substantia nigra, and are currently examining whether vigabatrin induces tonic inhibition in these dopaminergic neurons. Current experiments are aimed at determining whether high frequency firing can induce a sufficient amount of carrier-mediated GABA release to cause postsynaptic inhibition of neurons. We are also examining which cell type (neurons vs glia) and which GABA transporter (GAT1 vs GAT3) is primarily involved in nonvesicular GABA release. Defining the conditions under which reverse GABA transport occurs is important for defining normal and pathologic synaptic physiology. Determining how this form of GABA release can be modulated by anticonvulsants and other drugs may lead to more rational and effective treatment for patients with seizures and Parkinson's Disease. For more detailed information about the work described here, please see our published papers and reviews in our list of publications.
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