3. CNS 4 Inhibition in the Brain

What is GABA & how is it synthesized?

• Most abundant inhibitory neurotransmitter in the brain

• Synthesized from glutamate via glutamic acid decarboxylase (GAD)

• Neurones that synthesise GABA are called inhibitory GABAergic neurones

• Acts at inhibitory synapses to suppress neuronal activity

What are the two main types of inhibitory GABAergic neurones in the brain?

• Interneurones

  • Innervate nearby neurones (excitatory pyramidal or other inhibitory interneurones)

  • Control activity of large groups of neurones via widespread synapses

  • Mediate strong synchronisation of activity

  • ~20 different types with varied morphology & brain location

• Projection neurones

  • Innervate neurones outside their region (e.g. medium spiny neurones of striatum)

How does GABA cause inhibition at the synapse?

• GABA is stored in vesicles in presynaptic terminal

• AP arrival → vesicle fusion with membrane → GABA released into synaptic cleft

• GABA binds to GABA receptors (Cl⁻ channels) on postsynaptic membrane

• Cl⁻ flows into cell (high [Cl⁻] outside → low [Cl⁻] inside)

• Influx of negatively charged Cl⁻ → membrane becomes more negative (hyperpolarisation)

• This is called an inhibitory postsynaptic potential (IPSP)

• Hyperpolarisation ↓ probability of excitation by making it harder to reach threshold

• If excitatory & inhibitory inputs occur together, they may cancel out → no net change

How are IPSPs recorded & what does bicuculline show?

• Presynaptic GABAergic neurone fires APs → causes IPSPs in postsynaptic neurone

• IPSPs = small transient hyperpolarisations

• Bicuculline (GABA_A antagonist) blocks fast IPSPs

• Blocking GABA_A​ reveals slower GABA_B-mediated inhibition

How does KCC2 maintain GABAergic inhibition in the brain?

• KCC2 pumps Cl⁻ out of the cell, maintaining low intracellular Cl⁻

• This creates a Cl⁻ gradient that allows Cl⁻ to enter via GABA_A receptors

• Cl⁻ influx causes hyperpolarisation = inhibition

• Without KCC2, Cl⁻ builds up = ↓ inhibition = risk of seizures & death

What are the two types of GABA receptors & how do they function?

• Ionotropic GABA receptors:

  • Ligand-gated ion channels

  • GABA binding opens channel → ion influx (e.g. Cl⁻)

• Metabotropic GABA receptors:

  • G-protein coupled receptors

  • GABA binding → activates G-proteins & intracellular signalling (= longer-lasting inhibition of postsynaptic neurones

• Both types can be presynaptic or postsynaptic

What are the main types & functions of ionotropic GABA receptors?

• GABAA receptors: found in CNS

• GABAC receptors: found in retina

• Mediate Cl⁻ influx (minor HCO₃⁻ outflow)

• Cause membrane hyperpolarisation = inhibition of EPSPs

• GABAA agonists/allosteric modulators ↓ seizures & GABAA antagonists ↑ seizures

What are key features of GABA_A receptors?

• Expressed in all brain neurons

• Involved in anxiety, epilepsy, panic disorders & insomnia

• Targeted by benzodiazepines, barbiturates, anaesthetics & alcohol

• Modulated by stress hormones & neurosteroids

• Pentameric ligand-gated ion channels

• Pentameric: 2 α, 2 β, 1 γ/δ/ε/π/θ subunit

• GABA binds at α-β interface

• Benzodiazepines bind at α-γ interface

• Barbiturates bind intracellularly

• Subunit composition affects:

  • GABA affinity

  • Channel properties

  • Drug sensitivity

  • Cell type-specific expression

  • Subcellular localisation (e.g., γ needed for synaptic, δ for extrasynaptic → tonic inhibition)

How do GABAᵦ receptors function & what are their effects?

• GABAᵦ receptors are metabotropic, G-protein coupled (R1 & R2 subunits)

• Agonist = baclofen

• Gαᵢ/ₒ subunit = inhibits adenylyl cyclase = ↓ protein kinase A (PKA) activation

• PKA = enzyme that phosphorylates proteins to regulate cell activity

• β & γ subunits:

  • Activate K⁺ channels = hyperpolarisation

  • Inhibit Ca²⁺ channels = ↓ neurotransmitter release

What does the diagram show about GABAᴮ receptor localisation & function at synapses?

• GABAᴮ receptors act at both pre- & postsynaptic sites, with different effects depending on location & cell type

• Presynaptic (autoreceptor) on GABAergic terminal:

  • Activated by GABA it releases itself

  • Inhibits Cav2 Ca²⁺ channels = ↓ Ca²⁺ influx = ↓ GABA release

• Presynaptic (heteroreceptor) on glutamatergic terminal:

  • Activated by GABA from nearby GABAergic terminal

  • Inhibits Cav2 channels = ↓ Ca²⁺ influx = ↓ glutamate release

• Postsynaptic (on dendritic shaft/spine):

  • Activates Kir3 K⁺ channels = K⁺ efflux = hyperpolarisation

  • Produces a slow IPSP

• Also: Gαᵢ/o inhibits adenylyl cyclase = ↓ cAMP = ↓ PKA activation

• Astrocytes (GAT1 & GAT3) remove excess GABA

• Net effect depends on receptor location (pre/post) & cell type (GABAergic/glutamatergic)

What are the key features of glycine as an inhibitory neurotransmitter?

• Main inhibitory neurotransmitter in spinal cord & brainstem

• Glycine = simple amino acid

• Glycine receptors (GlyRs) = ligand-gated Cl⁻ channels with α & β subunits

• Activation = Cl⁻ influx = hyperpolarisation of postsynaptic membrane = ↓ neuronal firing

• Blocked by strychnine (competitive antagonist) = overexcitation: pain, cramps, startle response

• Also mediates inhibition in retina via glycinergic amacrine cells