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GABAergic Synapse

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Chemical synapses can be classified according to the neurotransmitter that the presynaptic neuron is utilizing. GABAergic presynaptic axon terminals release Gamma aminobutyric acid ​, which acts on postsynaptic GABAA receptors , opening an intrinsic chloride channel. This action usually inhibits the postsynaptic neuron by hyperpolarizing the cell membrane. [16]

Neurons ​ are incapable of de novo synthesis of Glutamate ​ or Gamma aminobutyric acid [2] ​. Instead, they take up glutamine released from Astrocytes ​, which is then converted to glutamate by Glutaminase ​. GABAergic neurons and their axon terminals can be identified by visualising Glutamic acid decarboxylase ​. This enzyme catalyses the subsequent Decarboxylation ​ of glutamate to GABA. GABA is then taken up into Synaptic vesicles ​ by the Vesicular GABA transporter ​ (VGAT). Action potentials invading the axon terminal trigger the release machinery, causing synaptic GABA concentration to rise transiently to millimolar concentrations [4] ​. Released GABA is rapidly cleared by GABA transporters ​ (GAT 1 to 3) [5] [6] ​, of which GAT1 is mainly expressed presynaptically, and GAT3 on astrocytes [7] [8] ​. GAT1 and GAT3 can also operate in reverse mode and thus regulate GABAergic tone [9] ​.

Main components

The presynaptic terminal of a GABAergic neuron takes up Glutamine ​ released from neighbouring astrocytes, where it was converted from glutamate by Glutamate synthetase ​. Glutamine is then metabolised into Gamma aminobutyric acid ​ through Glutaminase ​ and Glutamic acid decarboxylase ​ (GAD). The Vesicular GABA transporter ​ (VGAT) translocates GABA into synaptic vesicles, from where it is released into the Synaptic cleft ​. Here, GABA can act on synaptic GABAA receptors ​ or Metabotropic GABAB receptors ​, and can also diffuse away to reach peri- and extrasynaptic GABAA receptors. GABA reuptake is mediated mainly by the transporters GAT1 on presynaptic terminals and GAT3 on astrocytes.

Within astrocytes, degradation of GABA is catalysed by GABA transaminase ​ to succinate semialdehyde (SSA), and further by SSA dehydrogenase to Succinic acid ​, which is fed into the Tricarboxylic acid cycle [2] ​. Synaptic GABAA receptors bind to the scaffolding protein Gephyrin ​, but can also diffuse out of synaptic sites, where exo-/endocytosis takes place.

Comparison to excitatory synapses

Basic numbers on GABAergic transmission are most readily available for the Hippocampus ​. In this region, about 15-20 % of all neurons use GABA as their main neurotransmitter [12] [13] ​. Similarly, the majority of hippocampal synapses are glutamatergic and the relative number of inhibitory synapses remains low: Pyramidal neurons ​ and Basket cells ​ receive about 6 % inhibitory versus 94 % excitatory inputs, while other interneuron types can have 20-30 % inhibitory inputs [14] [15] ​. Because of architectonic similarities, these estimates are likely to be in the same range also for the cortex [16] [17] ​. Two factors help explain why, despite these numbers, excitation and inhibition can still be at balance in the brain, allowing network operations to be under interneuron control: (i) several types of interneurons including basket and chandelier cells are characterised by considerably faster spiking rates when compared to principal neurons (in the order of 15 Hz versus 1 Hz [13] ​, and (ii) tonic inhibition mediated by extrasynaptic GABAARs is quantitatively important, with a three to four times larger total charge transfer compared to synaptic inhibition [18] [6] [19] ​.

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