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Parasympatholytics

Parasympatholytics, also known as anticholinergics, are drugs that inhibit acetylcholine's actions at postganglionic nerve endings and cholinergic receptors, with atropine being the prototype. These drugs are classified into natural alkaloids, semisynthetic, and synthetic compounds, and have various therapeutic uses including treatment for CNS disorders, antispasmodics, antisecretory agents, and ophthalmic applications. Adverse effects may include dry mouth, increased body temperature, and urinary retention, with contraindications for patients with certain medical conditions.

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PARASYMPATHOLYTICS (Anticholinergics) Prof. Amol B. Deore Department of Pharmacology MVP’s Institute of Pharmaceutical Sciences, Nashik
Parasympatholytics •Parasympatholytics are the drugs which block or inhibit the actions of acetylcholine at postganglionic nerve endings and cholinergic receptors. •They are also referred as anticholinergics or cholinergic blocking agents or antispasmodics.
Anticholinergic drugs include atropine and related drugs- atropine is the prototype. Atropine is obtained from the plant Atropa belladonna. Atropine and scopolamine (hyoscine) are the belladonna alkaloids. They compete with acetylcholine for muscarinic receptors and block these receptors-they are muscarinic antagonists.
Classification of parasympatholytics Classification of parasympatholytics 1) Natural Alkaloids Ex. Atropine, Hyoscine, Scopolamine 2) Semisynthetic Compounds Ex. Atropine sulphate, Homatropine, Ipratropium bromide, Tiotropium bromide 3) Synthetic Compounds a) Mydriatics Ex. Cyclopentolate, Tropicamide, Eucatropine b) Antisecretory-antispasmodics Ex. Propantheline, dicyclomine, methantheline, glycopyrrolate, pipenzolate, pirenzepine, telenzepine, tolterodine, propiverine c) Antiparkinsonian Ex. Trihexylphenidyl, Procyclidine, Benzotropine, oxyphenadrine
ATROPINE Atropine and other synthetic anticholinergic agents competitively antagonise the muscarinic cholinergic receptors. Thus prevents the action of Ach on muscarinic receptors. These drugs block the muscarinic receptors on smooth muscles, cardiac muscles, exocrine glands and CNS.
Mechanism of action of Atropine Acetylcholine Atropine binds to the same receptor sites as Acetylcholine and prevents Acetylcholine from binding smooth muscles, cardiac muscles, exocrine glands and CNS. Muscarinic receptors
Tissue/ system Pharmacological effects Central nervous system M1 receptor In small doses, stimulates respiratory centre, and antiparkinsonism action; In large doses: Disorientation; incoordination, sedation, depression, stupor, coma Heart M2 receptor In low doses, produces bradycardia; In large doses, produces tachycardia; Increased cardiac output due vagus nerve stimulation GIT system M3 receptor Decreased salivation; Reduced GIT motility; Decreased gastric, pancreatic, intestinal and biliary secretions
Respiratory system M3 receptor Bronchial dilation; Reduced bronchial mucus secretion Urinary tract system M3 Relaxation of ureter; Constriction of urinary sphincter Eye M3 receptor Mydriasis (relaxation of circular muscles of iris leading to pupil dilation); Cycloplegia (far vision); Glaucoma due to increased intraocular pressure due to blockade in aqueous humour drainage; Skin M3 receptor Flushing (red colour skin on face and neck); Decreased sweating, reduced heat loss; Rise in boy temperature (hyperpyrexia)
EFFECT ON THE EYE
Iris Pupil
• The smooth muscles of the iris of the eye have a dual nerve supply (parasympathetic and sympathetic nerve supply). • The parasympathetic supply is carried out via oculomotor nerve and innervates circular muscles of iris. • The stimulation of these muscles causes miosis (pupil constriction). • The sympathetic supply is carried out via spinal nerve and innervates radial muscles of iris. • The stimulation of radial muscles causes mydriasis (pupil dilation).
When atropine is instilled in to eye (1-2% solution), a dilation of the pupil (mydriasis) and cycloplegia occurs. The lens becomes less convex and eye is fixed for distant vision (far vision). • The onset of action of atropine for mydriasis action is 10-20 min. • The duration of action of atropine for mydriasis is 2 hours to 2 days. • The cycloplegic effect of atropine begins within 30 min and last for 24- 48 hours.
Therapeutic uses • CNS disorders: they are used to reduce tremors and rigidity of Parkinsonism’s disease. • As antispasmodics: they relax the spasm of smooth muscles of the intestinal (as antidiarrheal action), urinary and biliary tracts. Ipratropium is used as bronchodilator in cases of bronchial asthma. • As anti-secretory agent: they are used to reduce gastric secretion in cases of peptic ulcer and to reduce sweating in tuberculosis patients with profuse midnight sweating.
• As preanesthetic medication: atropine is sued to reduce salivary and bronchial mucus secretions. Hence used as preanesthetic medication. • Ophthalmic uses: atropine is used to produce mydriasis and cycloplegia for measurement of refractive errors, and other diagnostic procedures. They are also used in treatment corneal ulcers, choroiditis and iridocyclitis. • Atropine is a cardiac stimulant hence used in heart block due to digitalis toxicity. • In urinary incontinence (enuresis) • Atropine sulphate is used as antidote in Organophospharus poisoning due to its antimuscarinic actions.
Dose: • Atropine sulphate (oral, SC, eye drops) 0.5 mg to 1 mg. • for eye drops 1-2% solution may be used.
Adverse drug reactions of atropine • Dryness of mouth and throat (xerostomia): due to reduction in salivation, leading to dysphagia (difficulty for swallowing). • The skin is dry, hot and red, especially in the region of the face and neck (flushing). • The body temperature may be raised due to decreased sweating and reduced heat loss leading to hyperpyrexia. • Increased in intraocular pressure leading to glaucoma. • Blurred vision due to paralysis of circular muscles and ciliary bodies of iris (pupil dilation i.e. mydriasis). Photophobia may result due to widely dilated pupils.
• Urinary retention may occur due to loss of bladder tone especially in elder male patients with prostate enlargement. (Urinary retention is defined as the inability to completely or partially empty the bladder. Suffering from urinary retention means you may be unable to start urination, or if you are able to start, you can't fully empty your bladder) • Palpitation and tachycardia may occur due to blockade of cardiac vagus nerve. • Constipation occurs due to reduced GIT motility and peristalsis.
Contraindications • Atropine is contraindicated in patients with narrow angle glaucoma, angina pectoris, congestive heart failure and prostate enlargement.
ACUTE BELLADONNA POISONING It may occur accidentally by ingestion of leaves/ roots of Atropa belladonna or seeds of Dhatura stromanium or overdose of atropine or scopolamine or anticholinergic drugs.
Symptoms M1 blockade actions: Hallucinations, mania, delirium, heavy breathing, stupor, respiratory collapse, coma and death M2 blockade actions: Tachycardia, palpitation M3 blockade actions: Severe dryness of mouth and throat Dysphagia (difficulty for swallowing) and extreme thirst Wide pupillary dilation (mydriasis) Blurred vision, photophobia Respiratory collapse Redness of skin, flushing, skin rash Rise in body temperature (hyperpyrexia) Urinary urgency, difficulty for micturition, urinary retention Muscle incoordination
Management of belladonna poisoning • The patient should be hospitalised immediately. • Artificial ventilator must be given. • The antidote of choice is Physostigmine salicylate. Intravenous injection of 1-4 mg of physostigmine salicylate controls delirium (reduced awareness of surroundings) and coma. • Hyperpyrexia may be treated by an ice cap and cold water sponging on whole body. • Respiratory stimulants like caffeine may be given to control respiratory collapse. • Diazepam may be given if mental symptoms are disturbance. • Other supportive measures are to be given.
Atropine produces photophobia. Why? • Atropine is anticholinergic (antimuscarinic) agent with wide range of actions. In eye, it blocks muscarinic M3 receptors on iris hence causes pupil dilation (mydriasis), and cycloplegia. • As atropine causes paralysis of circular muscles of iris and ciliary body, hence intensity of light entering in the eye is not adjusted. Photophobia is fear from light or irritation in bright light (blinking of eyelids). • Thus in bright light miosis could not occur, which makes individual to avoid bright light and prefer dim dark light. In this way, Atropine produces photophobia.
Atropine substitutes are preferred over plain atropine. Why? • Atropine is natural belladonna alkaloid which blocks all the muscarinic receptors (antimuscarinic agent) viz. M1, M2 and M3. Thus atropine gives generalized effects on the organs/system. In other words, it is non-selective and irrational. • Atropine substitutes are semisynthetic or synthetic drugs which chemically resemble to atropine. They produce more selective action i.e. they are selective block either M1 or M2 or M3 receptors. • In order to produce target specific effects, atropine substitutes are preferred over plain atropine.
Drugs acting on eye
MIOSIS • Decrease pupil size due to constriction of circular muscle and dilation of radial muscles by parasympathetic stimulation (M3) • Miotics: these drugs decrease the pupil size by pupil constriction. • For example-parasympathomimetics: pilocarpine, physostigmine, neostigmine • Uses: in open angle glaucoma • ADR: Spams of accommodation
MYDRIASIS • Increase pupil size due to dilation of circular muscle and constriction of radial muscles by sympathetic stimulation (alpha-1) • Mydriatic: these drugs increase the pupil size by pupil dilation. • For example- • Sympathomimetics: Ephedrine, phenylephrine • Parasympatholytics: atropine, homatropine, Cyclopentolate, Tropicamide. • Uses: For testing error of refraction, ophthalmic diagnosis, • ADR: Cycloplegia, Photophobia
THANKING YOU Prof. Amol B. Deore Department of Pharmacology MVP’s Institute of Pharmaceutical Sciences, Nashik