Compare and contrast the properties of (a) AMPA and NMDA receptors and (b) GABAA and GABAB receptors.

(a) AMPA vs. NMDA Receptors

AMPA Receptors:

• Ion Permeability: AMPA receptors are primarily permeable to sodium (Na⁺) and potassium (K⁺) ions. They are generally not permeable to calcium (Ca²⁺).
• Function: They mediate fast excitatory synaptic transmission in the central nervous system (CNS). Activation of AMPA receptors leads to rapid depolarization of the postsynaptic neuron.
• Kinetics: AMPA receptors have fast activation and desensitization kinetics, meaning they respond quickly to glutamate and also quickly become inactive.
• Role in Synaptic Plasticity: While they play a role in synaptic transmission, they are not directly involved in long-term potentiation (LTP) mechanisms.

NMDA Receptors:

• Ion Permeability: NMDA receptors are permeable to Na⁺, K⁺, and Ca²⁺ ions, allowing for calcium influx, which is crucial for various signaling pathways.
• Function: They mediate slower excitatory synaptic transmission and are involved in synaptic plasticity, including LTP and long-term depression (LTD).
• Kinetics: NMDA receptors have slower activation and desensitization kinetics compared to AMPA receptors. They require both ligand binding and depolarization to remove a magnesium (Mg²⁺) block from the channel.
• Role in Synaptic Plasticity: NMDA receptors are critical for synaptic plasticity and learning processes due to their calcium permeability and voltage-dependent properties.

(b) GABAA vs. GABAB Receptors

GABAA Receptors:

• Type: GABAA receptors are ionotropic receptors, meaning they are ligand-gated ion channels.
• Ion Permeability: They are primarily permeable to chloride ions (Cl⁻), leading to hyperpolarization of the postsynaptic neuron when activated by GABA.
• Function: They mediate fast inhibitory synaptic transmission in the CNS, contributing to the overall inhibitory tone of the brain.
• Kinetics: GABAA receptors have rapid activation and desensitization kinetics, allowing for quick inhibitory responses.

GABAB Receptors:

• Type: GABAB receptors are metabotropic receptors, meaning they are G-protein coupled receptors (GPCRs).
• Ion Permeability: They do not form ion channels directly but can influence ion channels indirectly through second messenger systems, typically leading to the opening of K⁺ channels and closing of Ca²⁺ channels.
• Function: They mediate slower, longer-lasting inhibitory effects compared to GABAA receptors. They play a role in modulating neurotransmitter release and neuronal excitability.
• Kinetics: GABAB receptors have slower kinetics compared to GABAA receptors, resulting in prolonged inhibitory effects.