Ionotropic and Metabotropic Receptors

Overview

Function: Both types of receptors play crucial roles in synaptic transmission by affecting the opening or closing of postsynaptic ion channels though they operate differently.
Types of Receptors:
1. Ionotropic Receptors
2. Metabotropic Receptors

Structure: Multimers comprising 4-5 individual protein subunits.
Domains:
- Extracellular site: Binds neurotransmitters.
- Membrane-spanning domain: Forms an ion channel.
Function: Activation occurs when a neurotransmitter (ligand) binds, allowing ions to flow through the channel.
Speed of Action: Mediates rapid postsynaptic effects.
- Postsynaptic potentials arise within milliseconds of an action potential and last only a few tens of milliseconds.

Structure: Do not contain ion channels.
- Instead, these receptors affect channels by activating intermediate molecules called G-proteins.
Domains:
- Extracellular domain: Contains neurotransmitter binding site.
- Intracellular domain: Binds to G-proteins.
G-Proteins:
- Consist of three subunits: alpha, beta, and gamma.
- Alpha Subunit: Binds guanine nucleotides (GTP or GDP).
  - GDP Binding: Forms an inactive trimer with beta and gamma subunits.
  - GTP Binding: Activates G-protein, causing dissociation of alpha from beta-gamma complex.
- Activation Process:
  1. Extracellular signal activates receptor.
  2. GDP is replaced with GTP on alpha subunit.
  3. Activated G-protein interacts with downstream effectors.
Function:
- GTP-bound alpha and beta-gamma complex interact with effector molecules.
- Can directly affect ion channels or activate enzymes producing intracellular messengers.
Speed of Action: Produces slower responses compared to ionotropic receptors.
- Responses can range from hundreds of milliseconds to minutes or longer.

A single neurotransmitter may activate both receptor types at the same synapse, leading to both fast and slow postsynaptic potentials.