Receptors

Receptors

Overview of core receptors in nervous system

• Acetylcholine (muscarinic & nicotinic)- parasympathetic

• Alpha & Beta - sympathetic

• Serotonin

• Dopamine

• Histamine

Alpha and Beta Receptors

Function: part of sympathetic nervous system's fight, flight or freeze response, function dependent on location.

Location & function: 

• Alpha 1

  • • Smooth muscle (most)- contraction (raise BP), reduce blood flow to GI tract

    • Pupils- dilate

    • Kidneys- retain Na+

• Alpha 2

  • • CNS, adrenergic & cholinergic neurons

    • Vascular smooth muscle (few)- decrease peripheral resistance (after load),

    • Platelets- increase aggregation

    • Fat cells- inhibit lipolysis, 

    • Pancreas- decrease insulin

• Beta 1

  • • Heart- increase cardiac output (positive chronotropic)

    • Kidneys- increase RAAS via renin secretion

• Beta 2- 

  • • Lungs- bronchiole dilatation

    • Smooth muscle relaxation: GI tract (reduce motility)

    • Adipose tissue: lipolysis

    • Pancreatic: insulin secretion

    • Uptake of K+ intracellularly

• Beta 3

  • • Fat cells: increase lipolysis

    • Bladder: relax

Drugs

• Alpha

  • • Alpha agonists: adrenaline, noradrenaline, metaraminol

    • Alpha blockers: 

      • • Alpha 1- doxazocin, prazosin, tamsulosin 

        • Phenoxybenzamine- alpha 1 & 2

• Beta

  • • Beta agonists: 

      • • Noradrenaline, adrenaline, isoprenaline (B1-3)

        • Beta 2 (lung): salbutamol, salmeterol, formoterol, terbutaline

    • Beta blockers: 

      • • Non selective: propanolol, carvediol, labetolol, sotalol

        • B1 (heart)- bisoprolol, atenolol, nebivolol, (acronym BAN)

Indications

• Anaphylaxis- adrenaline

• Hypertension : doxazocin, prazosin

• Benign Prostatic Hypertrophy : tamsulosin

• Phaeochromocytoma : phenoxybenzamine

• Asthma , Chronic Obstructive Pulmonary Disease (COPD) :  salbutamol, salmeterol, formoterol, terbutaline

• Betablockers: Atrial Fibrillation , Congestive Heart Failure , Hypertrophic obstructive cardiomyopathy (HOCM)  , Long QT Syndrome , thyrotoxicosis

• Hyperkalaemia : salbutamol (B2 agonist)

• Mirabegron: B3 agonist- overactive bladder

Points

Salbutamol causes a fine tremor because B2 receptors are found on the lung and skeletal muscle

Phenoxybenzamine preferred for phaeochromocytoma because it blocks alpha 1 & 2 receptors. 

Acetylcholine (muscarinic & nicotinic)

Nicotinic receptors: 

Located: NMJ and centrally

Types: Nm, Nn

Nicotine acts on nicotinic receptors

Diseases: myasthenia Gravis

Muscarinic receptors

Types: M1, M2, M3

Drugs: antagonist- atropine

Serotonin and Histamine

Serotonin receptors drugs:

1 - triptan

2 - antipsychotic

3- ondansetron

Histamine Receptors

H1: affect allergic response, e.g. cetirizine

H2: affect gastric stomach acid release, e.g. ranitidine

Note ranitidine therefore is of no use in allergic reactions. 

Written in 2024