Pharmacological actions of antihistamines

PHARMACOLOGICAL ACTIONS OF ANTIHISTAMINES

Antagonism of histamine

  • These drugs block the histamine induced, bronchoconstriction, contraction of the intestinal and other smooth muscles and the triple response.
  • Fall of blood pressure by low doses of histamine can be blocked, but additional doses of H2 antagonists are required to block the fall completely.
  • Release of adrenaline from the adrenal medulla can be abolished by pretreatment with antihistaminics. Constriction of larger blood vessels is also antagonised.
  • Action of histamine on gastric secretion is singularly not affected by these drugs.

Antiallergic action

  • Many manifestations of immediate hypersensitivity are suppressed. Urticaria, itching and angioedema are well controlled.
  • Anaphylactic fall in blood pressure is only partially prevented. The drug of choice for anaphylactic shock is only epinephrine.

On central nervous system

  • The older (first generation) antihistamincs produce variable degree of CNS depression.
  • This is dependent on the compound’s ability to penetrate blood brain barrier and its affinity for the central H1 receptors.
  • Some individuals experience stimulant effects like restlessness and insomnia.
  • Excitement and convulsions are frequently seen at toxic doses. The second generation antihistaminics are practically non sedating.
  • Based on their central nervous system depression, some antihistaminics are used for preventing motion sickness, to reduce tremors, rigidity and sialorrhoea of Parkinsonism.
  • Some are also effective as antitussives.

Anticholinergic action

  • Antagonistic to muscarinic actions of acetylcholine. If used concurrently with atropine or its substitutes, phenothiazines, tricyclic antidepressants or disopyramide, the anticholinergic action adds up.
  • Second generation antihistaminics are not anticholinergic.

Local anaesthetic

  • Some drugs like mepyramine and antazoline have strong while others have weak membrane stabilizing property.
  • However, they are not used clinically as local anaesthetics because they cause irritation when injected.

On blood pressure

  • Most antihistaminics cause a fall in blood pressure on intra venous injection but not on oral administration.

Uptake of nor-adrenaline

  • Antihistamines inhibit the neuronal uptake of noradrenaline.
Last modified: Monday, 17 October 2011, 8:31 AM