Glutamate Neurotransmission

Definition and Functional Similarity

Glutamate is the ionized form of glutamic acid and serves as a major excitatory neurotransmitter in the central nervous system. It is functionally similar to glycine in certain contexts.

• Key Point: All neurons and glial cells contain significant amounts of glutamate.

• Example: Glutamate is essential for synaptic transmission and neural communication.

Glutamate Synthesis

Glutamate is synthesized from glutamine via enzymatic reactions.

• Reaction:

• Enzyme: Glutaminase catalyzes the conversion.

VGLUT Transporters

Vesicular glutamate transporters (VGLUTs) are specific to glutamatergic neurons and serve as markers for these cells.

Additional info: Hair cells of the cochlea use glutamate as a neurotransmitter., VGLUT3 knockout: deaf.

Glutamate Storage and Release

• Key Point: Glutamate can be stored and released as a co-transmitter with other neurotransmitters.

Glutamate Removal and Recycling

Glutamate and aspartate are removed from the synaptic cleft by Excitatory Amino Acid Transporters (EAATs).

• Astrocytes convert glutamate to glutamine via glutamine synthase for recycling.

Glutamate Receptors

Glutamate acts on both ionotropic and metabotropic receptors.

• Ionotropic: NMDA, AMPA, Kainate (mediate fast synaptic transmission)

• Metabotropic: Group I, Group II, Group III (mediate slow synaptic transmission)

NMDA Receptor Function

NMDA receptors require both glutamate and glycine (or D-serine) to bind simultaneously for channel opening, allowing Na+ and Ca2+ influx.

• Blockers: Drugs like PCP, ketamine, and Mg2+ can block NMDA channels.

Metabotropic Glutamate Receptors (mGluRs)

There are 8 metabotropic glutamate receptors (mGluR1 to mGluR8) involved in diverse brain functions.

• Presynaptic autoreceptors can inhibit glutamate release (e.g., L-AP4 is a selective agonist).

• Functions: Locomotor activity, coordination, mood, pain perception.

Glutamate in Learning and Memory

• AMPA and NMDA receptors are crucial for learning and memory processes.

• Ampakines enhance AMPA receptor action by reducing desensitization.

Long-Term Potentiation (LTP)

LTP is a persistent increase in synaptic strength following high-frequency stimulation, often involving NMDA receptor activation.

• Key Point: The hippocampus has a high density of NMDA receptors; antagonists impair spatial learning.

• Induction: LTP can be induced by mechanical stimulation that triggers glutamate release.

Long-Term Depression (LTD)

LTD is a persistent decrease in synaptic strength, opposite to LTP.

• Location: Studied in hippocampus and cerebellum (parallel fibers and Purkinje cells).

• Mechanism: In hippocampal LTD, low-frequency stimulation leads to AMPA receptor removal from the postsynaptic membrane.

• Metabotropic receptors: In cerebellar LTD, mGluRs activate intracellular signaling cascades.

• Function: Involved in forgetting, motor learning, and neural circuit fine-tuning.

Excitotoxicity and Neurodegeneration

Excessive glutamate stimulation can cause neuronal death (excitotoxicity).

• Direct injection: Causes excitotoxic lesions due to overstimulation of NMDA and AMPA receptors, leading to Ca2+ overload.

• Excitotoxicity hypothesis: Supported by experiments showing glutamate/NMDA injection causes neuronal death; ischemia leads to glutamate accumulation and injury.

Cell Death Mechanisms

• Necrosis: Lysis due to osmotic swelling.

• Apoptosis: Programmed cell death.

GABA and Glycine Neurotransmission

Inhibitory Amino Acid Neurotransmitters

GABA and glycine are the major inhibitory neurotransmitters in the CNS.

• Key Point: GABA is synthesized only by GABAergic neurons and functions solely as a neurotransmitter.

GABA Synthesis

• Reaction:

• Inhibitors: Allylglycine and 3-mercaptopropionic acid block GABA synthesis (used in vitro).

GABA Storage and Transport

• VGAT: GABA is loaded into vesicles via the vesicular GABA transporter (VGAT).

• GABA and glycine can be transported by the same proteins (GAT-1, GAT-2, GAT-3).

GABA Metabolism and Recycling

• GABA aminotransferase (GABA-T): Metabolizes GABA to glutamate and succinate in neurons and astrocytes.

• Glutamine cycle: Astrocytes release glutamine, which is taken up by neurons, converted to glutamate, and used to remake GABA.

GABAergic Dysfunction and Epilepsy

• Epilepsy: Linked to mutations in GABA-A (ionotropic Cl- receptor) subunits, not GABA-B.

• Effect: Disrupted inhibitory neurotransmission leads to neuronal hyperexcitability and seizures.

GABAergic Drugs

• Vigabatrin: Irreversible inhibitor of GABA-T, prevents GABA metabolism, used as an anticonvulsant (can affect vision by impacting GABAergic interneurons in retina).

GABA Receptors

• GABA-A: Ionotropic receptor, composed of 5 subunits (combinations of α, β, γ, δ), allows Cl- influx, hyperpolarizes postsynaptic cell.

• Ligands: GABA, barbiturates, picrotoxin (blocks GABA receptors), benzodiazepines, neurosteroids (secreted from pineal gland to prevent apoptosis in cerebellum).

• GABA-B: Metabotropic receptor, two subunits, inhibits postsynaptic cells by reducing cAMP and opening K+ channels (causing hyperpolarization).

GABAergic System and Motor Control

• Key Point: Impaired dopamine input in the striatum (as in Parkinson's disease) leads to abnormal striatal GABAergic firing and motor abnormalities.

Summary Table: Glutamate vs. GABA