Histamine and Antihistamines

Question 1

What are the main histamine receptors?

Answer 1

Histamine exerts its effect by acting on the agonists on. four different types of histamine receptors

Receptors: H1, H2, H3, H4 (all G-proten coupled receptors)

Question 2

what is the main pharmacological actions of histamine?

Answer 2

= histamine stimulates gastric gland secretion –> increase secretion of gastric juice high acidity

Question 3

Where is histamine present?

Answer 3

present in all tissues

• Skin, GI, lungs mainly

Question 4

What is the stimulation of H1 receptors by histamine produce

Answer 4

• Smooth muscle contraction = bronchostriction
• Vasodilation (Circulating histamine acts on H1 and
  cause release of nitric oxide which acts on vascular
  smooth muscle - relax)
• itch
• arousal and wakefulness (central affects)

Question 5

what is the “tripple response” to histamine and the role of histmine in allergy?

Answer 5

When injected intra-dermally histamine causes = reddening of skin, surrounding skin wheal and flare (accompanied by an itch)

• Reddening caused by vasodilation of small arterioles
• flare caused by axon reflex which dilates surrounding vessels
• weal caused by increase per ability of post-capillary venules

Question 6

When histamine is released from mast cells stimulates H1 recepetors and produces

Answer 6

• vasodilation
• increased capillary per ability
• itchy and swollen red eyes and itchy runny nose
• sneezing

Question 7

MEdications for histamine?

Answer 7

Antihistamines

• H1 receptor blockng agents –> antihistamines counteract the action of histamine
• non-sedating or sedating

Question 8

MOA of Sedating antihistamines and examples?

Answer 8

MOA = Block/inhibit the actions of histamine at H1 receptors

e.g. Promethazine (phenergan)
Doxylamine (sleep aid)

Question 9

Difference between Non-sedating and sedating antihistamine?

Answer 9

non-sedating

• DO NOT cross the BBB
• specific for histamine at H1 receptors e.g. do not block Muscarinic receptors

Sedating

• cross the BBB and cause sedation
• can exert effects on other receptors e.g. inhabit acetylcholine at muscarinic receptors

Question 10

MOA of non-sedating antihistamines and examples?

Answer 10

= Block/inhibit the actions of histamine at H1 receptors

e.g. Fexofenadine (telfast)
Loratadne (claratine)