Veterinary Neuropharmacology

CLASSIFICATION

• Muscarinic receptor antagonists block the actions of acetylcholine at the parasympathetic end organs and at the sweat glands.
• Nicotinic receptor antagonists block the actions of acetylcholine at all the autonomic ganglia (both sympathetic and parasympathetic) and at the neuromuscular endplate of skeletal muscles.
• Muscarinic antagonists
  • Alkaloids obtained from the plants belonging to the family Solanaceae have muscarinic receptor blocking effect.
  • The important members in this group are Atropa belladonna (deadly nightshade), Datura stromonium and Hyosciamus niger.
  • Atropine and scopolamine are the alkaloids of interest in this group.
  • In addition to these naturally occurring compounds, synthetic agents like homatropine, tropicamide, ipratropium, benztropine etc., exhibit muscarinic receptor blocking activity.
  • These muscarinic receptor antagonists occupy the muscarinic receptors competitively and prevent acetylcholine from binding with the cholinergic receptors.
  • All the organs in the body are not equally affected by the muscarinic blockers. 
  • With extremely high doses, atropine can block the nicotinic receptors
  • Natural alkaloid- Atropine is the prototype of the family of muscarinic blockers. It is well absorbed after oral administration and parenteral administration. Most species meatabolise it readily, 50% is excreted unchanged in the urine in less than 12 hours. Duration of action is 4 to 6 hours in most species.
  • Synthetic analogs- Scopolamine, homatropine, methscopolamine and methylatropine differ primarily in their pharmacokinetics.
  • Scopolamine is shorter acting and has more CNS effects than atropine.
  • Homatropine is less potent and less toxic with a faster onset and shorter duration than atropine. When applied to the eye it produces less side effects and is therefore more useful than atropine for producing mydriasis.
  • Methscopolamine and methylatropine are quarternary amines and thus have minimal side effects but are poorly absorbed when given orally.

Glycopyrrolate

• It is a quaternary nitrogen compound and therefore is not well absorbed from the gastrointestinal tract and penetrates the CNS poorly.
• It produces less sedation than scopalamine and is less effective than atropine at blocking gastric secretions.
• At ususal doses glycopyrrolate has little effect on resting/basal respiration, blood pressure or heart rate.

Aminopentamide

• This drugs inhibition of motility and secretions of the gastrointestinal tract are more pronounced than atropine while its mydriatic effect and inhibition of salivary secretions are less than atropine.