Pharmacology of 5-HT, Substance P, PAF, and Eicosanoids

Nausea and Vomiting
- Mediated by the release of $5-HT$ .
- This response is especially evoked by cytotoxic drugs or radiotherapy.
Migraine Pathophysiology
- $5-HT$ is theorized to initiate the initial vasoconstrictor phase of migraine headaches.
- It is involved in the neurogenic inflammation of the affected blood vessels.
Haemostasis and Platelet Function
- Platelets release $5-HT$ during the process of aggregation at the site of blood vessel injury.
- Mechanism: Working in concert with collagen and other biochemical mediators, $5-HT$ accelerates the processes of platelet aggregation and clot formation.
Intestinal Motility and Regulation
- Enterochromaffin cells and neurons containing $5-HT$ play a role in regulating the digestive system.
- They are responsible for the regulation of peristalsis and local reflexes within the gut.

Definition and Core Function
- Substance P is a neuropeptide that serves as a critical mediator of pain transmission within the Central Nervous System ( $CNS$ ).
- It acts as a mediator during neurogenic inflammation in the periphery.
Biological and Cellular Activities
- Enhances vascular permeability.
- Acts as a chemoattractant for white blood cells ( $WBCs$ ).
- Activates phagocytic activity in relevant cells.
- Increases the production and subsequent release of inflammatory mediators by neutrophils and macrophages.
Biosynthesis Process
- Synthesis begins in the ribosome, where it is produced as a larger precursor protein.
- It is subsequently converted through enzymatic activity into its active form, an undecapeptide (an 11-amino acid peptide).

Chemical Nature
- $PAF$ is a polar lipid derived from the cell membrane.
- It is characterized by intense biological activity at low concentrations.
Cellular Sources of PAF
- $PAF$ is synthesized by a wide variety of cell types, including:
  - Platelets
  - Neutrophils
  - Monocytes
  - Mast cells
  - Eosinophils
  - Renal mesangial cells
  - Renal medullary cells
  - Vascular endothelial cells

Vascular and Cardiovascular Effects
- It acts as a potent dilator in the majority of vascular beds.
- Intravenous ( $IV$ ) administration results in hypotension.
- $IV$ administration causes the formation of intravascular platelet aggregates and induces thrombocytopenia.
Leukocyte Activation
- Stimulates polymorphonuclear leukocytes ( $PMNs$ ) to aggregate.
- Triggers the release of leukotrienes ( $LTs$ ) and lysosomal enzymes.
- Stimulates the generation of superoxide radicals.
Smooth Muscle Effects
- Generally induces contraction in the gastrointestinal, uterine, and pulmonary smooth muscles.
Gastric and Ulcerogenic Effects
- Recognized as the most potent known ulcerogen.
- $IV$ administration causes hemorrhagic erosions of the gastric mucosa.
- These erosions are severe enough to extend into the submucosa.
Renal Effects
- Induces a decrease in renal blood flow.
- Reduces the Glomerular Filtration Rate ( $GFR$ ).
- Decreases total urine volume.
- Reduces the excretion of sodium ions ( $Na^+$ ).

Involvement in Inflammatory Disorders
- Anaphylaxis.
- Bronchial asthma.
- Endotoxic shock.
- Various skin diseases.
Severe Allergic Reactions
- Directly involved in anaphylactic shock.
- Contributes to bronchial hyperreactivity and increased lethality in respiratory distress scenarios.

Definition
- Eicosanoids are the oxygenation products of polyunsaturated long-chain fatty acids.
General Characteristics
- They are highly potent molecules.
- They exhibit a wide range of biological activities across various organ systems.
- They possess significant therapeutic potential.
Arachidonic Acid
- A primary precursor for eicosanoids.
- Features a carboxylic acid group ( $COOH$ ) and a hydrocarbon chain ending in a methyl group ( $CH_3$ ).