Inhibition in the brain

What is the most abundant inhibitory neurotransmitter in the brain?

GABA (y-Aminobutyric acid)

How is GABA synthesised?

• GABA is synthesized from glutamate

• Enzyme involved: Glutamic Acid Decarboxylase (GAD)

Neurones that release GABA are called..

GABAergic neurones

Types of GABAergic Neurones

1. GABAergic Interneurones

• Make synaptic connections with nearby neurones = control the activity of large groups of neurones = synchronise neuronal firing

• Very diverse (~20 types) with different shapes and locations

2. GABAergic Projection Neurones

• Make synaptic connections with neurones located outside of the region

• Example: Medium spiny neurones of the striatum

GABAergic Synaptic Transmission

What is the presynaptic element of GABAergic synapses?

The ends of axons of GABAergic neurones

GABAergic Synaptic Transmission

How is GABA released into the synaptic cleft?

• Action potential arrives at GABAergic axon terminal

• Terminal membrane depolarises

• Voltage-gated Ca²⁺ channels open

• Ca²⁺ enters the terminal

• Ca²⁺ binds to specific proteins which triggers fusion of GABA-containing vesicles with the plasma membrane

• GABA is released into the synaptic cleft

GABAergic Synaptic Transmission

What happens after GABA is released into the synaptic cleft?

• GABA binds to GABAᴀ receptors

• GABAᴀ receptors are ligand-gated ion channels. They are permeable to mainly: Cl⁻ and HCO₃⁻

• Cl⁻ enters the postsynaptic neuron

• Membrane becomes more negative (hyperpolarisation)

• Membrane potential drops below resting potential (−70 mV). This change is called an IPSP

• IPSPs close together in time and space summate, resulting in further hyperpolarisation

• Leads to stronger inhibition

Where are GABAergic synapses found?

• Dendrites

• Soma

• Axonal Initial Segment (AIS)

In this electron micrograph, what do the black dots represent?

Specific antibodies labelled with gold particles used to detect the presence of GABA

Why so inhibition at the Axon Initial Segment (AIS) especially powerful?

Can strongly inhibit the initiation of action potentials

Bicuculline Experiment

What is bicuculline?

• A competitive antagonist of GABA_A receptor

• Blocks the binding of GABA to GABA_A receptors

Bicuculline Experiment

What does this experiment prove?

• Fast IPSPs are inhibited by Bicuculline

  » Shows that fast IPSPs are mediated by GABA_A receptors

• However, a slow hyperpolarising response is still detected

  » Caused by activation of GABA_B receptors which are not blocked by bicuculline

  » Produce slow IPSPs

2 types of GABA receptors on post-synaptic neurones

• GABA_A receptors

• GABA_B receptors

GABA_A Receptors

• Ionotropic (ligand-gated)

• Fast inhibition

• Blocked by bicuculline

• Produce fast IPSPs

GABA_B Receptors

Metabotropic (G-protein coupled):

• Activate second messenger cascades

• Open voltage-gated K⁺ channels

• K⁺ exits the neurone → hyperpolarisation

• Produce slow IPSPs

• Not blocked by bicuculline

How is the Na+/K+ concentration gradient maintained across the neuronal plasma membrane?

Na+/K+ ATPase

• Maintains ion gradients using ATP

• Pumps: Na⁺ out, K⁺ in

• Results in:

  • High Na⁺ outside (150 mM)

  • High K⁺ inside (110 mM)

How is the Cl- concentration gradient maintained across the neuronal plasma membrane?

Why is this gradient essential for GABA_A-mediated inhibition?

• Maintained by K⁺–Cl⁻ cotransporter (KCC2)

• Pumps Cl⁻ out of the neuron

• Results in:

  • High Cl⁻ outside (~130 mM)

  • Low Cl⁻ inside (~8–10 mM)

Without it, Cl⁻ would not enter the neurone when GABA_A receptors are activated by GABA and IPSPs would not be generated

Prevalence of GABA receptors in the brain

Second most abundant neurotransmitter receptors in the brain after glutamate receptors

2 main classes of GABA receptors

• Ionotropic GABA receptors - fast inhibition

• Metabotropic GABA receptors - slow inhibition

Ionotropic GABA Receptors

What are they?

• These receptors are ligand-gated ion channels.

• When GABA binds, the channel opens and allows:

  • Cl⁻ ions to enter

  • HCO₃⁻ ions to leave

• This makes the inside of the neurone more negative (hyperpolarised) → neurone is less likely to fire an action potential

2 types

• GABA_A Receptor

• GABA_C Receptor

Ionotropic GABA Receptors

GABA_A Receptors

• Where are they found?

• Speed of synaptic transmission

• Which disorders are these receptors involved in?

• Drugs which target these receptors?

• Controlled by?

• Expressed in all neurones the brain

• Fast synaptic inhibition

Involved in disorders such as:

• Anxiety

• Epilepsy

• Panic disorders

• Insomnia

Drugs which target GABA_A

• Benzodiazepines

• Barbiturates

• Anaesthetics

• Alcohol

• Also modulated by stress hormones and neurosteroids

Ionotropic GABA Receptors

GABA_A Receptors

Structure

• Pentamer (made up of 5 subunits)

• There are 16 different genes which code for the GABA_A receptor subunits

• Based on similarity in amino acid sequence, these subunits are further classified into 6 different groups: α, β, γ, δ, ε, θ, π

• Typical GABA_A receptor composition:

  2 α + 2 β + 1 γ (or δ/ε/θ/π)

Ionotropic GABA Receptors

GABA_A Receptors

What does subunit composition determine?

• GABA affinity

• Channel properties

• Drug sensitivity

• Where the receptor is expressed

• Subcellular localisation

Ionotropic GABA Receptors

GABA_C Receptors

Where are they expressed?

• Mainly expressed in the retina

• Less widespread than GABA_A

Ionotropic GABA Receptors

GABA_C Receptors

Structure

• Pentamer (made of 5 subunits)

• Forms from ρ subunits (ρ1–3)

Metabotropic GABA Receptors

What are they?

• G-protein-coupled receptors

• Main type are GABA_B receptors

• Produce slow inhibitory responses (slow IPSPs).

• Work via Gi/o proteins:

  • α subunit → inhibits adenylyl cyclase → ↓ cAMP → ↓ PKA activity

  • βγ subunits → open K⁺ channels → hyperpolarisation

  • Also inhibit Ca²⁺ channels → ↓ neurotransmitter release

Metabotropic GABA Receptors

GABA_B Receptors

Specific agonist of GABA_B receptors

Baclofen

Metabotropic GABA Receptors

GABA_B Receptors

Structure

GABA_B receptors are dimers:

• GABAB1 → GABA binds to the extracellular domains of the GABAB1 subunit

• GABAB2 → G-proteins bind to the intracellular domains of the GABAB2 subunit

Metabotropic GABA Receptors

GABA_B Receptors

Where are they located?

• Postsynaptic membrane → slow IPSPs

• Presynaptic GABA terminals → inhibit GABA release (presynaptic autoreceptors)

• Presynaptic glutamate terminals → inhibit glutamate release (presynaptic heteroreceptors)

What is the main inhibitory neurotransmitter in the spinal cord and brainstem

Glycine

What are glycine receptors (GlyRs)

Ligand-gated Cl- channels

Structure of glycine receptors

• Heteropentamer of α and β subunits

• 4 α isoforms and 1 β isoform

What happens when glycine receptors are activated?

• Influx of Cl-

• Postsynaptic membrane becomes hyperpolarised

• Reduced firing of action potentials

Competitive antagonist for the glycine receptor

Strychnine

• Causes over-excitation

• Leads to pain, muscle cramps, exaggerated startle

Apart from in the brain and spinal cord, where else does glycine mediate inhibitory neurotransmission?

Via glycinergic amacrine cells in the retina