Glutamate and GABA Synthesis, Metabolism, and Neurotransmission

The metabolic pathway follows these steps:

1. GABA-T converts GABA into succinic semialdehyde (SSA).
2. The enzyme SSA dehydrogenase (SSADH) then oxidizes SSA into succinic acid.
3. Succinic acid enters the Krebs cycle, where it can eventually be converted back into $\alpha$-ketoglutarate.

Interestingly, GABA-T also facilitates the regeneration of glutamate from $\alpha$-ketoglutarate, creating a metabolic link between the two neurotransmitter systems.

Clinical Correlation: Vigabatrin In certain CNS disorders, such as epilepsy, increasing the available amount of GABA is therapeutic. The drug Vigabatrin acts as an irreversible inhibitor of GABA-T. By blocking the conversion of GABA to SSA, Vigabatrin prevents GABA degradation, thereby increasing the concentration of GABA available for release at inhibitory synapses.

The Glutamate-Glutamine Cycle

To prevent excitotoxicity—a condition where excess glutamate overstimulates neurons and leads to cell death—the brain employs a sophisticated recycling mechanism known as the Glutamate-Glutamine Cycle. This cycle relies on a coordinated partnership between neurons and glial cells.

1. Sequestration and Glial Conversion

When glutamate levels rise in the synaptic cleft, glutamate transporters located on both neuronal membranes $[Gt(n)]$ and glial membranes $[Gt(g)]$ sequester the glutamate from the synapse. Once inside the glial cell, the enzyme Glutamine Synthetase transforms the glutamate into the non-excitatory amino acid Glutamine.

2. Transport and Neuronal Reconversion

The glutamine is then transported out of the glial cell and back into the neuron. Once inside the neuron, a mitochondria-associated enzyme called Glutaminase converts the glutamine back into glutamate, making it ready for subsequent neurotransmission.

This cyclical relationship ensures that the brain maintains a continuous supply of neurotransmitters while efficiently removing excess glutamate to protect neural integrity.

Summary of Neurotransmitter Dynamics

The following table summarizes the fundamental differences between the two systems:

Exam Tip: Imbalance Consequences

• Reduced GABA Signaling: Leads to increased excitability, potentially causing anxiety or seizures.
• Increased GABA Signaling: Leads to reduced excitability, potentially causing sedation, drowsiness, or even coma in severe cases.
• Excess Glutamate: Can lead to NMDA receptor overactivation and calcium influx, resulting in neurotoxicity.