Antihistamines Flashcards
Autocoid =
‘self-remedy’
Local hormone =
Chemical messenger formed by tissues on which it acts.
Where is histamine found?
High concentration in lungs, skin and smooth muscle (blood vessels)
Stored in basophils and mast cells
In granuoles
Rapidly inactivated by amine oxidase if not stored
Histamine in the stomach
Histamine found un histaminocytes
Histamine in the CNS
Histamine found in histaminergic neurones
Histamine in venom
Histamine found in toxics of venom
EG Insect stings and plant stings
Synthesis of Histamine
Synthesised from histidine (amino acid)
Release of Histamine from Mast cells: Acute inflammatory response
One of several mediators released in response to a pathogen. EG Bacteria or toxin.
Complement components -> C3a and C5a receptors -> Mast cell.
Pathogen -> Pathogen pattern receptors -> Mast cell
Release of Histamine from Mast cells: Hypersensitivity
Allergic immune system reaction to typical harmless substances. Harmless antigen (EG Pollen) -> Immunoglobulin (Ig) E Cell fixed antibody -> Mast cell
Release of Histamine from Mast cells: Physical destruction
Mast cell
Release of Histamine from CNS: Arousal pathways
Histaminergic neurones in TMN.
Spontaneously active and release histamine during wakefulness.
Release of Histamine from CNS: Emetic centre
Histaminergic neurones in TMN also receive input from vestibular centre in inner ear (spatial orientation) and other sensory inputs.
If mismatch between inputs = Activate histaminergic neurones = Activate emetic (Vomiting) centre in medulla.
Histamine receptors
G-protein coupled receptors
H1: Myosin phosphorylation, vascular permeability, NFkB activation. Inside Ca2+ increases. H1 coupled to phospholipase C
H2: Proton pump function, gastric acid secretion. Increase in inside cAMP. Coupled to adenylate cyclase.
H3/H4: Proton pump function, gastric acid secretion. decrease in inside cAMP. Coupled to adenylate cyclase.
Histamine receptor functions (H1)
CNS neuronal activation Nasal and bronchial mucus secretion Smooth muscle contraction Sensory nerve endings Proinflammatory cytokine secretion
Histamine receptor functions (H2)
Stimulation of gastric acid secretion
Cardiac stimulation - Chronotropic (time change), increase HR
Histamine receptor functions (H1 and H2)
Capillary permeability and dilation
Cardiac stimulation - Inotropic (time change), increase heart contraction
Histamine receptor functions - Pathologies
Allergic reaction: mild to severe. Immune system hypersensitive response to typically harmless substance in environment. EG Pollen or food Release of histamine from mast cells Can be localised
Allergic rhinitis
Inflammation inside nose in response to allergen EG Pollen or dust Very common (1 in 5) Can be localised immune response Sneezing, itchiness, runny nose Sensory nerve ending stimulation Increase nasal secretions Capillary permeability and dilation
Urticaria (hives)
Skin rash in response to allergen or toxic substance. EG Nettle sting, latex Itchy, raised bump, redness, pain Sensory nerve ending stimulation Capillary permeability and dilation Can be localised response
Moderate allergic reaction to allergen. EG food
1. Rhinitis conjunctivitis Sensory nerve ending stimulation Increase secretion Capillary permeability and dilation 2. Hives Sensory nerve ending stimualtion Capillary permeability and dilation 3. Tummy pain/diarrhoea Intestinal muscle contraction 4. Wheezing/chest tightness/shortness of breath Bronchiole contraction
Severe allergic reactions (Anaphylaxis)
Reapid onset
Breathing difficulties
Rapid HR
Loss of conciousness
Motion sickness
Sensory input (visual)
Spatial orientation (vestibular system in inner ear)
Input mismatch at Tuberomamillary nucleus
Histamine released to Emetic centre
Contraction of stomach muscles
Motion sickness
Antihistamines: 1st generation - Prophylaxis
Much more effective if given before histamine receptor stimulation to prevent development of symptoms
Antihistamines: 1st generation - Receptors
Typically H1 receptor antagonists
Can be muscarinic cholinergic receptor antagonists
Particularly diphenhydramine and promethazine
Can be α1-adrenoceptor antagonists
Particularly promethazine