GABA and Glycine Neurotransmitters PDF

**GABA and Glycine.** Ironically, the major **excitatory neurotransmitter, glutamate**, serves as the **precursor** to the main **inhibitory neurotransmitter, GABA**. GABA is synthesized by the enzyme **glutamic acid decarboxylase (GAD)**, which exists in two forms, **GAD-65 and GAD-67**, and is sp...

**GABA and Glycine.** Ironically, the major **excitatory neurotransmitter, glutamate**, serves as the **precursor** to the main **inhibitory neurotransmitter, GABA**. GABA is synthesized by the enzyme **glutamic acid decarboxylase (GAD)**, which exists in two forms, **GAD-65 and GAD-67**, and is specifically localized to **GABAergic neurons**. These neurons include highly diverse **inhibitory interneurons** (e.g., **basket cells** and **stellate cells**) and **projection neurons** (e.g., **Purkinje cells**) throughout the brain. The enzymatic process involves the **removal of a carboxyl group** from glutamate by GAD, converting it into GABA. Both have **different expression patterns**, GAD-65 is mostly expressed on the nerve axon terminals whilst GAD-67 is cytosolic and is expressed everywhere on cell body, dendrites and spines. GAD-65 is connected to the interneurons that guide promoters, and the immunostaining is used for identifying interneurons. **GABAergic neurotransmission:** **Glutamate**, the precursor molecule for **GABA**, is converted into GABA by the enzyme **glutamate acid decarboxylase (GAD)**. Once synthesized, GABA is **packed into vesicles** and released into the **synaptic cleft** during neurotransmission. Upon release, some GABA binds to **GABA receptors** on the **postsynaptic cell**, mediating inhibitory signals. However, most GABA **spills over** into the surrounding area, where it is taken up by **astrocytes**. Within astrocytes, **GABA transaminase enzymes** convert GABA back into **glutamate**, which is then returned to the **interneuron** for reuse in GABA synthesis. Additionally, some GABA is directly reabsorbed by the **interneuron** through **GABA transporters**, where it is either converted into **glutamate** or **succinate**, allowing it to re-enter the metabolic cycle. This tightly regulated process ensures efficient recycling and maintenance of GABA and glutamate levels in the synapse. In the **postsynaptic cell**, there are two primary types of **GABA receptors**: **GABA-A** and **GABA-B**, along with **GABA-C**, which is less common. **GABA-A**: An **ionotropic receptor** that functions as a **chloride ion channel**. When activated by neurotransmitters like **GABA** or the agonist **muscimol**, it opens, allowing **chloride ions (Cl⁻)** to pass into the cell along their concentration gradient. This influx of negatively charged chloride ions causes **hyperpolarization**, making the cell less likely to fire. Antagonists for GABA-A include **bicuculline** and **picrotoxin**. **GABA-C**: Also **ionotropic**, like GABA-A, but has a **different subunit composition** and pharmacological profile. The key difference between **GABA-A** and **GABA-C** is their sensitivity to **agonists and antagonists**. GABA-C responds to the antagonist **CACA** but not **bicuculline**, which selectively blocks GABA-A. **GABA-B**: A **metabotropic receptor** that works through **GPCR (G-protein coupled receptor) signalling** rather than an ion channel. GABA-B receptors mediate slower, longer-lasting inhibitory effects. Common agonists include **GABA** and **baclofen**, while antagonists are **phaclofen** and **5-OG-saclofen**. **Composition of the GABA-A & distribution****:** **GABA-A** is formed by **five subunits**, which can vary to create functional diversity. **GABA-C**, on the other hand, is made up of only **rho subunits**, with three types (**rho1, rho2, rho3**) combining to form a functional pore. **GABA-B** is a **metabotropic receptor** formed by the dimerization of two isoforms, **GABA-B1** and **GABA-B2**, which work together to create a functional receptor. Has two Alpha, two Beta and one Gamma. Different subunits have different binding sites for different compounds. GABA-C is also found in the retina and is a homomeric complex of rho subunits and this is GABA- C is resistant to both bicuculline and baclofen. **𝜋 family** is a subfamily of GABA receptors and is expressed in reproductive organs. **Multiple binding sites of the GABA-A receptor:** **The GABAB receptor is metabotropic:** **GABA-B** receptors do not directly allow ion flow but instead use **complex signalling pathways** to indirectly affect ion channels. These receptors are made up of two subunits, **GABA-B1** and **GABA-B2**, which must work together for functionality. When GABA binds to the receptor, it activates signalling pathways that: 1. **Open potassium channels (GIRK)**, allowing **potassium ions to flow out**, causing **hyperpolarization** (decrease in membrane potential), which silences the cell. 2. **Inhibit voltage-gated calcium channels**, reducing **calcium influx**, further contributing to hyperpolarization and cell silencing. 3. **Reduce calcium permeability in NMDA receptors**, thereby decreasing the likelihood of **long-term potentiation (LTP)** induction. Overall, **GABA-B receptor activation silences the cell** by hyperpolarizing the membrane and reducing excitatory signalling. **GABA-B in synaptic transmission:** **GABA-B** receptors are found in both **postsynaptic** and **presynaptic compartments**. When activated, these receptors **close voltage-gated calcium channels**, which are essential for the fusion of vesicles to the membrane and subsequent neurotransmitter release. In the **presynaptic terminal**, GABA can act on neighbouring terminals (e.g., glutamate terminals) or on its own terminal by activating presynaptic **GABA-B receptors**. This reduces **calcium inflow**, **depolarizes the terminal**, and decreases the release of neurotransmitters, such as **GABA** or **glutamate**. Thus, excess GABA release triggers a feedback mechanism that reduces the further release of GABA or glutamate, maintaining **neurotransmitter balance** in the system. **GABA-A&B receptors in synaptic plasticity:** In **hippocampal circuitry**, stimulation of **presynaptic axon terminals** releases **glutamate**, which binds to **AMPA** and **NMDA receptors** on the postsynaptic cell, generating **excitatory postsynaptic potentials (EPSPs)**. Simultaneously, stimulation of **neighbouring inhibitory terminals** releases **GABA**, activating **GABA-A** and **GABA-B receptors**, causing **inhibitory postsynaptic potentials (IPSPs)** through hyperpolarization. During **high-frequency activation**, the large amount of **glutamate released** evokes **multiple EPSPs** by activating AMPA and NMDA receptors. Concurrently, repeated stimulation of GABA axon terminals results in significant GABA release. This GABA activates **postsynaptic GABA receptors** and **presynaptic GABA-B receptors**. The activation of **presynaptic GABA-B receptors** provides a **feedback mechanism** that **silences the GABA terminals**, reducing further GABA release. By limiting GABA-mediated inhibition, this feedback mechanism **reduces inhibitory interference** during high-frequency stimulation, thus facilitating the induction of **long-term potentiation (LTP)**, a key process in synaptic plasticity and memory formation. **Drugs affecting GABAergic transmission:** **Red= Inhibitors Green= Activators.** **Convulsants** like **Bicuculline** inhibit **GABA-A receptors**, increasing neuronal activity and causing **convulsions**. In contrast, **skeletal muscle relaxants** like **Baclofen** activate **GABA-B receptors**, reducing neuronal activity and leading to **muscle relaxation.** **Ethanol In The Body:** **Ethanol in the body** follows a dose-dependent course of action. Initially, its concentration in **blood plasma halves** before crossing the **blood-brain barrier**, where it acts on **GABA-A receptors**. Depending on the dose, ethanol can produce effects ranging from **excitement**, **relaxation**, and **euphoria** to more severe outcomes like **unconsciousness**, **coma**, or **death**. **How ethanol works on GABA receptors:** In this experiment, **GABA-A IPSCs** are recorded. When the slice is treated with **80 mM ethanol**, there is a **slight increase in GABA IPSC**. By **silencing GABA-B receptors**, the **inhibition on GABA release** mediated by GABA-B is removed, leading to a more **pronounced effect of ethanol**. Ethanol directly binds to **GABA-A receptors**, enhancing their activity, and the involvement of **GABA-B receptors** further amplifies these effects. This demonstrates that both receptor types contribute to ethanol's impact on inhibitory signalling. In a normal, increasing conc of GABA, the GABA current goes up, but if you add ethanol the increase goes up even more. GABA also **modulates spontaneous neuronal activity** in a different disease system. Bicuculline silences the activity and so does NBQX and AP5 which are both antagonists of AMPA and NMDA receptors, so they block the activity. If any of this is disrupted there is disease conditions like Huntington's disease. **GABA transaminase (GABA-T) deficiency:** **GABA is excitatory neurotransmitter in embryonic nervous system:** During early embryonic development there is more chloride inside the cell than outside, so when GABA channels open the chloride will travel outside the cell and remove negative ions depolarising the cell, leading to increasing activity. As the development happens chloride is mostly extracellular. **Glycine:** Is a co-agonist of NMDA receptors, so required to open NMDA receptors. **Performance Enhancement:**  **Glycine** is also an inhibitory neurotransmitter that silences the cell. **Synthesis, release, re-uptake and degradation of glycine:** **Vesicular glycine transporter:** The **vesicular GABA transporter (VIAAT)** is capable of transporting both **GABA** and **glycine**, leading to its designation as a **vesicular inhibitory amino acid transporter**. Its function is regulated by the **extra-vesicular concentrations** of these two amino acids, with **glycine inhibiting GABA uptake** and vice versa. However, in some regions of the **CNS** rich in GABA and/or glycine, **VIAAT is absent**, suggesting the existence of **alternative transporters**. Additionally, some vesicles contain both **GABA and glycine**, and their **co-release** has been shown to activate specific **postsynaptic receptors**, highlighting a coordinated role in inhibitory neurotransmission. **Glycine receptors:** Composed of **α subunits** (four types) and a single type of **β subunit**, with **glycine binding sites** present only on the α subunits. Receptors may consist of α subunits alone or a combination of α and β subunits. Activated in the order: **glycine \> β-alanine \> taurine \> L- and D-alanine \> L-serine \>\> D-serine**. Inhibited: - **Competitively** by **strychnine**. - **Non-competitively** by **picrotoxin**, but receptors containing **β subunits** are **insensitive to picrotoxinin**. **Glycine is excitatory neurotransmitter in embryonic nervous system:** In the embryonic nervous system, **glycine acts as an excitatory neurotransmitter** due to the **NKCC1 transporter**, which increases intracellular **Cl⁻**, leading to depolarization. After **10 days postnatally**, the **KCC2 transporter** begins extruding **Cl⁻**, shifting glycine\'s effect to **inhibitory**. This developmental change is crucial for the maturation of neuronal circuits. **Glycine receptors are modulated by alcohols and other drugs:** **Glycine Receptors** are allosterically modulated by substances like **alcohols** (e.g., ethanol) and **anaesthetics** (e.g., enflurane and isoflurane). This means these substances can enhance the receptor\'s function when bound. The graph demonstrates how ethanol increases the glycine-induced currents, amplifying the inhibitory effects of glycine. Other modulators include **cocaine** and ligands of 5HT3 and NMDA receptors, indicating diverse mechanisms impacting glycine receptor activity. **Mechanism of Action for Alcohol:** **Alcohol enhances inhibitory neurotransmission by potentiating the activity of both GABA receptors and glycine receptors.** **Panels A and B:** Ethanol increases the amplitude of currents induced by GABA and glycine, amplifying their inhibitory effects. **Panels C and D:** Ethanol directly enhances glycine receptor currents, shown by larger current traces. **Panel E:** Demonstrates a dose-dependent effect of ethanol, with greater glycine receptor activation as ethanol concentration increases. **Conclusion:** Alcohol acts by boosting inhibitory signalling, which contributes to its sedative and depressant effects on the central nervous system.