What kind of antihistamine are ceterizine and chlorphenamine
Ceterizine: second-generation H1 antihistamine
Chlorphen(ir)amine: first-generation H1 antihistamine
Describe the target, location and histamine effects of the H1 receptors.
Target: histamine H1 receptors
Location of H1 receptors
Smooth muscles (especially bronchial and nasopharyngeal lining)
Vascular endothelial cell surfaces
Central nervous system
Histamine effects on H1 receptors
↑ Capillary dilation and permeability → hypotension and edema
↑ Bronchiolar smooth muscle contraction (via IP3 and DAG release) → bronchoconstriction
↑ Nasal and bronchial mucus production
↑ Activation of peripheral nociceptive receptors → ↑ pain and pruritus
↓ Conduction in AV node
Describe the effect in more detail.
Competitive, reversible antagonism or inverse agonism of histamine H1 receptors (Gq protein-coupled receptor)
Inhibition of increased vascular permeability
Inhibition of allergic bronchial constriction
Central action: sedation (especially first-generation antihistamines due to crossing of blood-brain barrier)
List side effects.
Sedation (significantly less pronounced in second-generation antihistamines due to their limited CNS activity)
Anticholinergic effects, e.g., dry mouth and eyes, mydriasis, urinary retention, tachycardia, dizziness, tinnitus (mostly with first-generation antihistamines)
Anti-α-adrenergic effects, e.g, postural hypotension (can lead to falls), weight gain
List indications of fist-generation antihistamines.
Anaphylactic shock (e.g., clemastine, dimetindene)
Insomnia: used as sedatives (sleep aid)
Nausea: used as antiemetic agents (e.g., diphenhydramine in hyperemesis during pregnancy) because of their central effects
List indications of second-generation antihistamines.
Allergies: used as antiallergic agent (e.g., in cases of rhinitis, conjunctivitis, hypersensitivity to drugs, urticaria)
List contraindications of first- and second generation antihistamines.
Benign prostatic hyperplasia
Second-generation antihistamines: childhood and pregnancy