symthesis: by mast and basophils

located in: gastric and mucosal cells and neurons in CNS

Action:inflammatory mediator, regulate acid transmission in gastric mucosa, regulate neurotransmission

located: smooth muscle, vascular endothelium, brain

Action: Lungs- bronchocontriction

-Vascular smooth muscle: postcapillary venule dilation (erythema)

-Vascular endothelium: Cellular contraction, edema

-peripheral nerves: sensitization (itching pain)

locations: gastric parietal cells, cardiac muscle, mast cells, brain

Effects: Heart-minor increase in hr and contractility

stomach-gastric acid secretion (PUD GERD)

MOA: inverse agonists- stabilizes the inactive H1 rc conformation

Receptor: 7-transmembrane G protein-coupled 

Metabolized: hepatic Cyp450

absorbed well, cmax 2-3 hrs

Lipophilic--> cross BBB

neutral at physiologic pH

(benadryl, hydroxyzine, chlorpheniramine, promethazine, doxepin)

bound to albumin and ionized at physiologic pH--> don't cross BBB

(Loratidine, cetirizine, desloratidine, levocetirizine)

rhinitis, conjunctivitis, urticaria, pruritis

NOT effective for systemic anaphylaxis or asthma because they only target histamine and not other local mediators (use epinephrine)

Off-use: motion sickness, chemotherapy n/v, insomnia(those that cross BBB)