Histaminics and Antihistaminics

Histamine-

• Main mediator of allergic,hypersensitivity and tissue injury reactions.
• Histamine is found in two forms in body-Mast cell and non mast cell
• Mast cells contain histamine and heparin.
• Tissues rich in histamine are skin,liver,lungs, gastric glands
• There are two receptors for histamine-H1 & H2
• H1 receptors are present on bronchioles,blood vessels,uterus,adrenal medulla.
• H2 receptors are present on heart,blood vessels & gastric glands.
• Histamine causes vasodilatation of small blood vessels.Vasodilatation caused by H1 receptor is due to EDRF release and vasodilatation caused by H2 receptors is due to dilatation of vessel muscles.
• Histamine show less effect on heart rate.But talking casually it tends to increase the heart rate.
• Histamine also increases capillary permeability.
• Histamine are bronchoconstrictor.
• Histamine increases gastric secretion.
• Histamine is not involved in delayed or retarded type of allergic reactions.
• Histamine act as an afferent transmitter and supposed to cause itching and pain at sensory nerve endings
• Histamines don't show much effect on human uterus.
• Histamine provokes adrenal medulla to release adrenaline.

Antihistaminics-

H1 antagonist-

• Antagonise H1 receptors.
• Control allergic and immediate hypersensitivity reactions.Cause bronchodilatation,vasoconstriction and they also control vascular permeability.Fall in BP produced by low doses of histamine is corrected by H1 antagonism but additional doses of H2 antagonism is prescribed to complete relieve from falling BP
• Block adrenaline release from adrenal medulla
• Control itching,urticaria and pain during allergic reactions.
• Antagonise muscuranic actions of Ach.Causes dry mouth.
• Causes CNS depression.Causes sedation.
• Recently newer generation of antihistaminics have been developed.The characteristic features of this newer generation of antihistaminics are as follows-

1. They doesn't produce drowsiness 
2. Anticholinergic action is less marked
3. They are more potent against inflammation and late phase of allergic reactions as they inhibit leukotrienes. 

• Some common antihistaminics- Pheniramine: Dose- 20-50mg oral or I.M

                                                                             Brand name- Avil 25/50 mg tab.22.5 mg/ml inj.

                                                         Promethazine: 25-50 mg oral or I.M

                                                                             Brand name:Phenergan 10,25mg tab. 25 mg/ml inj.