Comprehensive Study Notes on GABAergic Transmission and Pharmacology and Physiology

Overview of GABAergic Transmission and Neuronal Distribution

• Relative Abundance: Approximately $20\%$ of neurons in the Central Nervous System (CNS) are GABAergic.

• Principal Function: GABA ($\gamma$-aminobutyric acid) serves as the primary inhibitory neurotransmitter in the CNS. Its main role is preventing excessive neuronal firing and maintaining balanced brain function through receptor binding, which leads to hyperpolarization.

• Neuronal Classification:

  • Type I Golgi (Long Projection Neurons): Located in the Substantia Nigra (SN), Striatum, Pallidum, Hippocampus, Limbic system (including the Nucleus Accumbens or NAc), Cerebellum, and Hypothalamus.

  • Interneurons: Found primarily in the Cortex (CTX) and the Spinal cord.

GABA Synthesis and the Glutamate Decarboxylase (GAD) Enzyme

• Synthesis Enzymes: Catalyzed by Glutamate Decarboxylase, specifically isoforms GAD2 or GAD65.

• Localization: CNS presynaptic terminals, glia, and various peripheral tissues including the pancreas, intestine, kidney, lung, and testicle.

• Cofactor: Vitamin $B_6$ is required for GAD activity.

• Autoantibodies (anti-GAD2): The presence of autoantibodies against GAD2 is associated with several pathologies:

  • Stiff Person Syndrome: Characterized by progressive muscle rigidity and spasms.

  • Spinocerebellar Ataxia Type 1: A genetic disorder affecting coordination and movement.

  • Type 1 Diabetes: Observed in approximately $80\%$ of cases.

  • LADA: Latent Autoimmune Diabetes in Adults.

• Genetic Polymorphisms: Polymorphisms in the GAD2 gene are linked to fluctuations (increase/decrease) in obesity.

GABA Transportation and Vesicular Storage

• Vesicular GABA Transporter (vGAT):

  • Mechanism: Entry of GABA into vesicles is dependent on an electrochemical gradient generated by a proton pump.

  • Structure: A $10$ transmembrane domain ($10$ DTM) structure.

  • Terminals: Both the $NH_2$ and $COOH$ terminals are located in the intracellular space.

• High Affinity GABA Transporters (GAT):

  • Function: Solute carrier transporters that are $Na^+\text{-dependent}$.

  • GAT1 and GAT2: Located predominantly in neurons. These are inhibited by the pharmacological agent Tiagabine.

  • GAT3: Located predominantly in glial cells.

  • Structure: A $12$ transmembrane domain ($12$ DTM) structure featuring $5$ intracellular loops and $6$ extracellular loops.

GABA Metabolism and the GABA Shunt

• Metabolic Pathway Sequence:

  1. $\text{Glucose} \xrightarrow{\text{Glycolysis}} \text{Pyruvate}$

  2. $\text{Pyruvate} \xrightarrow{\text{PDH (Pyruvate Dehydrogenase)}} \text{Acetyl-CoA}$

  3. $\text{Acetyl-CoA} \xrightarrow{\text{Krebs Cycle}} \alpha\text{-Ketoglutarate}$

  4. $\alpha\text{-Ketoglutarate} \xrightarrow{\text{GDH (Glutamate Dehydrogenase)}} \text{Glutamate}$

  5. $\text{Glutamate} \xrightarrow{\text{GAD (Glutamate Decarboxylase)}} \text{GABA}$

  6. $\text{GABA} \xrightarrow{\text{GABA-T (GABA Transaminase)}} \text{Succinic Semialdehyde}$

  7. $\text{Succinic Semialdehyde} \xrightarrow{\text{SSADH (Succinic Semialdehyde Dehydrogenase)}} \text{Succinate}$

  8. $\text{Succinate} \rightarrow \text{Re-enters Krebs Cycle}$

• Pharmacological Inhibition of GABA-T:

  • Vigabatrine: Irreversibly inhibits GABA Transaminase.

  • Valproic acid: Inhibits GABA Transaminase to increase GABA levels.

GABA Receptor Classification and Peripheral Presence

• Receptor Types:

  • GABA-A: Ionotropic receptor (chloride channel).

  • GABA-B1,2: Metabotropic receptors (G-protein coupled).

  • GABA-C: Ionotropic receptor.

• Peripheral Distribution: GABA receptors are found outside the CNS in the lung, liver, heart, spleen, pancreas, GI tract, sperm, testicles, prostate, mammary gland, and adrenal glands. They are also found in kidney, liver, and colon tumors.

Pharmacology of the GABA-A Receptor

• Allosteric Modulation Sites: The receptor contains distinct binding sites for:

  • Benzodiazepines (BDZ)

  • Barbiturates

  • Steroids

  • General Anesthetics

  • Ethanol

• Ligands:

  • Agonists: GABA (endogenous), Muscimol (derived from Amanita muscaria).

  • Competitive Antagonist: Bicuculline (derived from Dicentra cucullaria).

Specifics of Benzodiazepine (BDZ) Modulation

• Mechanism of Action: BDZs function as Positive Allosteric Modulators (PAMs). They require the presence of GABA to function ($GABA + BDZ \rightarrow \uparrow \text{GABA effect}$).

  • Channel Potentiation: They increase the likelihood of the chloride channel opening in the presence of $1$ or $2$ molecules of GABA.

  • Desensitization: They decrease receptor desensitization in the presence of $2$ molecules of GABA.

• Clinical Applications:

  • Anxiolytic effect: Used in acute therapy for $8-12$ weeks.

  • Sedative-hypnotic effect: For the treatment of insomnia.

  • Central muscle relaxant effect: Used in preanesthesia.

  • Antiepileptic effect: Treatment for myoclonic epilepsy, absence seizures, status epilepticus (delivered i.v.), and febrile seizures (delivered via microenemas).

Specifics of Barbiturate Modulation

• Binding Site: Located inside the chloride channel.

• Mechanism of Action: Function as PAMs.

  • They increase the duration (time) of channel opening.

  • GABA-Independence: At high dosages, barbiturates can enhance $Cl^-$ influx even in the absence of GABA.

• Clinical Pharmacology and Half-lives ($t_{1/2}$):

  • Anesthesiology: Thiopental ($t_{1/2} = 20\,\text{min}$, used for induction); Secobarbital and Pentobarbital ($t_{1/2} = 3-4\,\text{hrs}$); Amobarbital ($t_{1/2} = 4-6\,\text{hrs}$).

  • Epilepsy: Fenobarbital ($t_{1/2} > 12\,\text{hrs}$).

• Non-Clinical Uses:

  • War context: Thiopental (also known as Pentothal*).

  • Euthanasia (Medical Assistance in Suing - MAS): High doses of Thiopental or Pentobarbital (Nembutal*).

  • Death Penalty: Thiopental or Pentobarbital i.v., often in combination with pancuronium and $KCl$ (i.v.) or midazolam and hydromorphone (i.m.).

• Synergistic Interactions: Barbiturates promote the binding of GABA, BDZ, and ethanol. Conversely, BDZs and ethanol promote the binding of barbiturates, creating a dangerous synergistic effect.

Physiological Roles of GABAergic Transmission

• Muscle Tone Regulation: Neurons in the spinal cord govern muscle tone by inhibiting motor neurons, which prevents spasticity and excessive contraction.

• Anxiety and Stress Control: GABA reduces overactivity in neural circuits associated with stress, promoting emotional stability.

• Sleep Regulation: Essential for initiating/maintaining sleep by inhibiting arousal-promoting brain regions.

• Emotional Stability: Regulates overactive circuits to stabilize mood, addressing disorders like depression.

• Motor Control: Coordinates movement in motor control areas to ensure smooth voluntary execution and prevent involuntary contractions.

• Cognitive Function: Influences attention and focus by adapting neural network activity, preventing distractions and maintaining clarity.