Receptor Types

Overview of Neurotransmitter Receptors

  • Definition of a Neurotransmitter

    • A neurotransmitter is a chemical messenger released from the end of an axon terminal.

    • Released from vesicles at the axon terminal's end.

    • Diffuses across the synaptic cleft into the space between neurons.

    • Binds specifically to receptors on a dendrite, soma, or another axon, initiating a response.

Types of Receptors for Neurotransmitters

  • There are two primary types of receptors for neurotransmitters:

    • Channel-linked receptors (direct activation)

    • Second messenger systems (indirect activation)

Channel-Linked Receptors

  • Characteristics of Channel-Linked Receptors

    • Direct activation mechanism.

    • Provides faster signal transmission compared to second messenger systems.

  • Mechanism of Action

    1. A neurotransmitter (e.g., acetylcholine abbreviated as ACh) binds to its specific receptor.

    2. The binding induces a change in the shape of the receptor due to the receptor's specificity (the receptor will not respond to other neurotransmitters like serotonin or dopamine).

    3. This change in shape opens an ion channel linked to the receptor.

    4. Sodium ions () can move down their concentration gradients into the cell once the channel opens.

  • Specific Receptor Example

    • Nicotinic receptor for acetylcholine:

    • Embedded in the plasma membrane as transmembrane proteins.

    • Rapidly opens when acetylcholine binds, resulting in a graded potential that may lead to an action potential in a neuron.

Second Messenger Systems

  • Characteristics of Second Messenger Systems

    • Indirect activation mechanism.

    • Generally results in slower responses compared to channel-linked receptors.

  • Mechanism of Action

    1. A neurotransmitter (e.g., acetylcholine) binds to a G protein-coupled receptor (GCPR).

    2. The binding induces a conformational change in the receptor that activates G proteins.

    • G protein is named for its association with guanosine diphosphate (GDP).

    1. The activated G protein exchanges GDP for guanosine triphosphate (GTP) resulting in changes to the G protein's conformation.

    2. The activated G protein can then influence an effector enzyme, such as adenylate cyclase.

  • Specific Enzyme Example

    • Adenylate cyclase:

    • Activated by GTP from the G protein.

    • Converts adenosine triphosphate (ATP) into cyclic AMP (cAMP), the second messenger.

  • Role of cAMP

    • Activates kinases in the pathway, leading to the phosphorylation of proteins, which alters their function.

  • Signal Termination

    • Involves the enzyme phosphodiesterase, which degrades cAMP to stop the signaling.

Advantages of Second Messenger Systems

  • Amplification of Signal

    • Unlike channel-linked receptors which exhibit a 1:1 relationship (one signaling molecule opens one channel), the second messenger system has a cascading effect.

    • One acetylcholine molecule can activate multiple G proteins, leading to activation of numerous effector enzymes and generating many cAMP molecules, which can affect various cellular functions or channels.

  • Versatility of Effects

    • The same neurotransmitter (e.g., acetylcholine) can induce different responses in different cell types based on the specific apparatus present within those cells.

    • This means that a neurotransmitter can activate different signaling pathways depending on the cellular context, leading to divergent effects in various tissues.