Intercellular Communication Methods

• Intercellular communication involves cells sending signals to each other.

Direct Intercellular Communication

• Cells must be in physical contact.
• Advantage: very fast signaling.
• Disadvantage: limited signaling distance.

Indirect Intercellular Communication

• Cells are not physically touching, using chemical messengers.
• Target cell needs a receptor for the messenger.
• Secretory cell sends the signal.
• Examples of messengers: paracrines, neurotransmitters, hormones.

Paracrines & Neurotransmitters

• Advantage: faster speed compared to hormones.
• Disadvantage: shorter duration of effect.

Hormones (Endocrine Signaling)

• Advantage: long-distance signaling capability.
• Disadvantage: slower speed.

Ligands and Receptors

• Ligands are molecules that bind to receptors.
• Endocrine signaling involves polar (hydrophilic) or nonpolar (hydrophobic) messengers.
• Hydrophobic messengers require a carrier protein to travel in the blood.
• Receptor location on a target cell provides significant information about its corresponding chemical messenger.
• Specificity of binding is determined by shape, and affinity by attraction/charge.
• Receptor activation depends on: ligand concentration, receptor expression, and receptor-ligand affinity.
• Adding more ligand can only increase the response until 100% receptor saturation is achieved.
• The number of receptors can be adjusted via upregulation (increase) and downregulation (decrease).
• Ligands and receptors typically adapt to each other’s presence and concentration.

Drugs

• Drugs have a shape similar enough to a ligand to bind to its receptor.
• Drugs typically activate the mesolimbic region in the brain.
• Opiates are derived from the poppy plant and manage pain, and are considered nature’s pain relievers.
• Endorphins manage minor pain or stress, and are found in low concentrations.
• Pain is a physiological response.

Tolerance

• Pharmacokinetic tolerance: lower drug concentration reaching the receptor site.
• Pharmacodynamic tolerance: body becomes more efficient at breaking down the drug, or receptor downregulation occurs.
• Agonists activate receptors like a key opening a door.
• Antagonists block receptors like tape over a keyhole.
• Hydrophilic substances are lipophobic (repelled by fats).
• Hydrophobic substances are lipophilic (attracted to fats).
• Second messengers are not needed for hydrophobic messengers because they can cross the cell membrane directly.

Steroid Hormones

• A ligand that crosses the membrane is typically a steroid hormone.
• The most common response is a change in gene expression or protein synthesis.

Receptor Types

• The receptor itself can function as an ion channel.
• The receptor can be an enzyme-linked receptor, initiating a domino effect upon ligand binding.
• The receptor can be connected to a G-protein.
  • When a ligand binds, the G-protein breaks free and binds to an ion channel or enzyme.
  • G-proteins are often described as "molecular switches".
  • GTP (guanosine triphosphate) indicates the active state.
  • GTP is broken down into GDP (guanosine diphosphate) to deactivate the G-protein.

Ion Channels

• G-protein-regulated ion channels are slower but longer-lasting.
• These channels can be opened and closed by the G-protein.
• Fast channels are typically closed, and the ligand binding opens them.

Second Messenger Pathways

• G-proteins regulate 5 major second messenger pathways:
  • cAMP (cyclic adenosine monophosphate)
  • cGMP (cyclic guanosine monophosphate)
  • IP3 (inositol trisphosphate)
  • DAG (diacylglycerol)
  • Calcium
• Calcium is kept at a low concentration and can activate many pathways.

Explore intercellular communication: direct contact for speed, indirect signaling via messengers. Understand paracrines, neurotransmitters, and hormones. Learn about ligands, receptors, and hydrophobic/hydrophilic messengers in endocrine signaling.