SCREENING OF ANTITUSSIVES

AND EXPECTORANTS

CONTENTS• Cough• Causes• Cough reflex• Cough receptors• Mechanical events in cough

• Drugs used in cough

• Screening of Antitussive activity

• Screening of Expectorants.

COUGH

• An essential, protective function for human airways & lungs.

• Cough reflex involves a highly orchestrated series of involuntary muscular actions with potential for input from cortical pathways as well.

• Ineffective cough reflex??

• Excessive cough??

COUGH REFLEX

• Larynx, Trachea, Carina, Bronchi, Terminal bronchioles, Alveoli

sensitive to foreign matter; corrosive chemical stimuli.

• Afferent nerve impulse respiratory passage vagus nerves medulla

of brain.

• Afferent pathway; Central pathway; Efferent pathway.

COUGH REFLEX

• AFFERENT PATHWAY: Sensory nerve fibres located in ciliated

epithelium of upper airways, cardiac and esophageal branches of

diaphragm.

• CENTRAL PATHWAY: Upper brainstem; Pons.

COUGH REFLEX• EFFERENT PATHWAY:

• Impulse from cough centre vagus, phrenic, spinal motor nerve to diaphragm,

abdominal wall & muscles.

• Nucleus retroambigualis phrenic, spinal motor nerve inspiratory &

expiratory muscles

• Nucleus ambiguous laryngeal branches of vagus larynx.

COUGH REFLEX

• Termination of vagal afferents abundant in airway mucosa from

upper airways to terminal bronchioles and lung parenchyma.

• 3 broad classes of afferent nerves:

• Rapidly Adapting Receptors (RAR)

• Slowly Adapting Stretch Receptors (SARs)

• C- fibres.

RAPIDLY ADAPTING RECEPTORS• Dynamic receptors respond to changes in mechanical properties.

• Sporadically active throughout respiratory cycle

• Activation• Stimuli evoking bronchospasm/obstruction resulting from mucus

secretion/edema.

• Histamine, Capsaicin, Substance P, Bradykinin

• Stimuli evoking cough

SLOWLY ADAPTING STRETCH RECEPTORS (SARs)

• Highly sensitive to mechanical forces put on lung during breathing.

• Activity increases during inspiration, peaks just prior to initiation of expiration.

• Activation• Central inhibition of cholinergic drive to airways.

• Decreases phrenic nerve activity

• Decreases airway smooth muscle tone.

C- FIBRES• Majority of afferent nerves innervating airways and lungs are

unmyelinated C-fibres.

• PG-E2, Adrenaline, Adenosine, Capsaicin, Bradykinin sensitise C-fibres.

• Activation• Increased airway parasympathetic nerve activity

• Chemoreflex characterized by apnea, bradycardia and hypotension.

MECHANICAL EVENTS OF COUGH

• INSPIRATORY PHASE: Inhalation; Generates the volume necessary

for effective cough.

• COMPRESSION PHASE: Closure of larynx combined with

contraction of muscles of chest wall, diaphragm and abdominal wall

increase in intrathoracic pressure

MECHANICAL EVENTS OF COUGH

• EXPIRATORY PHASE:

• Glottis opens high expiratory airflow; Coughing

• Large airway compression

• High flows- dislodge mucus from airways- allows removal from

tracheobronchial tree.

DRUGS FOR COUGH

1. PHARYNGEAL DEMULCENTS Lozenges, Cough drops, Linctuses containing syrup, Glycerine.

2. EXPECTORANTSa. BRONCHIAL SECRETION ENHANCERS Sodium/Potassium

citrate, Potassium iodide, Guaiphenesin, Ammonium chloride.

b. MUCOLYTICS Bromhexine, Ambroxol, Acetylcysteine

DRUGS FOR COUGH

3. ANTITUSSIVES (COUGH CENTRE SUPPRESSANTS)

a. OPIOIDS Codeine, Pholcodeine, Ethylmorphine

b. NON OPIOIDS Noscapine, Dextromethorphan

c. ANTIHISTAMINES Cholrpheniramine maleate, Diphenhydramine,

Promethazine

SCREENING OF ANTITUSSIVE ACTIVITY

• Antitussive activity after irritant inhalation in guinea pigs

• Cough induced by mechanical stimulation

• Cough induced by stimulation of the nervus laryngicus superior

• Cough induced by ammonia in cats

• Cough induced by sulfuric acid in dogs

ANTITUSSIVE ACTIVITY AFTER IRRITANT INHALATION IN GUINEA PIGS

• PURPOSE AND RATIONALE

• Chemical activation of cough centres in brain

• Several animal species and several irritants have been used.

• Most frequent - citric acid induced cough in guinea pigs

ANTITUSSIVE ACTIVITY AFTER IRRITANT INHALATION IN GUINEA PIGS• PROCEDURE• Guinea pigs – either gender – 300-400g

• Placed in cylindrical glass vessel with 2 tubes at either ends for influx and efflux of aerosol.

• Latter tube side arm connecting to a tambour changes in pressure registered

• Pinch clamp with variable screw permits regulation of sensitivity of system

ANTITUSSIVE ACTIVITY AFTER IRRITANT INHALATION IN GUINEA PIGS

• Animal exposed to aerosol of 7.5% citric acid in water for 10 min

• Number of tussive responses registered

• One hour later test substance orally/ SC

• 30 min later aerosol exposure

• Number of coughs during 10 min recorded.

ANTITUSSIVE ACTIVITY AFTER IRRITANT INHALATION IN GUINEA PIGS• EVALUATION

• Number of coughs after treatment is expressed as percentage of control

period. Using various doses, ED50 values can be calculated.

• CRITICAL ASSESSMENT• Proven to be an effective method

MODIFICATIONS

• Citric cough model codeine, morphine, selective neurokinin 3 receptor

antagonist, sigma-1 receptor agonists.

• Capsaicin inhalation guinea pigs GABA B agonists, NK1, NK2,

tachykinin receptor antagonist.

• Other irritants Ammonia, Nebulised sulphuric acid, Sulphur dioxide.

COUGH INDUCED BY MECHANICAL STIMULATION

• RATIONALE-

• Cough can be induced by mechanical stimulation of trachea in

anaesthetized guinea pigs.

• INCLUSION CRITERIA-

• Male guinea pigs- 350-400g- maintained at temperature 21 ± 2 ͦ C; relative

humidity 55 ± 10 %; 12:12 light dark cycle; food and water ad libitum 1

week before use.

COUGH INDUCED BY MECHANICAL STIMULATION- PROCEDURE

• Overnight fasting anesthesia with 25% urethane (4ml/kg IP) induces surgical levels of analgesia without depressant effects on respiratory function.

• Monitoring analgesia disappearance of head shaking in response to ear pinch.

• Animals maintained at constant body temperature 37 ͦ C by means of heated plate.

COUGH INDUCED BY MECHANICAL STIMULATION- PROCEDURE

• Thin steel wire inserted into trachea through small incision near cricoid cartilage

• Steel wire is pushed to reach bifurcation of trachea 5 min before and 30, 60 and 120 minutes after oral drug administration.

• One violent cough occurs upon each stimulation.

• Animals which respond to stimulus before dosing are selected

• Randomly assigned to receive the test drug at various doses or the standard drug.

COUGH INDUCED BY MECHANICAL STIMULATION- EVALUATION

• Evaluation of the statistical significance of the results is performed

with Student’s t-test for paired data.

• ED50 values are determined by logit transformation.

OTHER METHODS

• Nylon bristled stimulator thrust into the trachea in dogs

• Silver thread in decerebrated guinea pigs

• Vibration of an iron slung in trachea of a dog induced by an

electromagnet

• Electrical stimulation of trachea via bronchoscope

• Through implanted copper electrodes.

COUGH INDUCED BY STIMULATION OF NERVUS LARYNGICUS SUPERIOR

• PURPOSE

• Stimulation of nervus laryngicus superior induces coughing

• Antitussive agents with predominant central action suppress coughing reflex.

• INCLUSION CRITERIA

• Cats either gender 2-3 kg

COUGH INDUCED BY STIMULATION OF NERVUS LARYNGICUS SUPERIOR- PROCEDURE

• Anaesthetised 40 mg/kg IP; Pentobarbital

• Placed on a heated operating table; Extremities secure.

• Fur is shaved ventrally at neck

• Small incisions made at both sides of larynx

• Superior laryngeal nerves prepared carefully.

COUGH INDUCED BY STIMULATION OF NERVUS LARYNGICUS SUPERIOR- PROCEDURE

• Trachea is exposed and cannulated after a median incision

• Connected to Fleisch tube (Size 00)

• Femoral artery cannulated via Statham pressure transducer for registration of BP.

• Femoral vein cannulated for intravenous application of test substances.

COUGH INDUCED BY STIMULATION OF NERVUS LARYNGICUS SUPERIOR- PROCEDURE

• Small hook electrodes- attached to each laryngeal nerve

• Wave impulse Frequency 50 Hz; Impulse width 0.5 ms; Amplitude 0.2 to 1 Volt; Duration 1-10s every 5 min

• The intensity of the forced expiration is measured by the Fleisch pneumotachograph and recorded simultaneously with blood pressure on a polygraph.

COUGH INDUCED BY STIMULATION OF NERVUS LARYNGICUS SUPERIOR- PROCEDURE

• Control response to 3 stimuli recorded before IV application of test compound.

• Stimuli repeated every 5 min after test compound/ standard is injected

• Suppression of forced expiration is recorded over 1 hr

• Then, next dose/ standard drug is given.

COUGH INDUCED BY STIMULATION OF NERVUS LARYNGICUS SUPERIOR- EVALUATION

• Total or partial suppression of the forced expiration are recorded over

time and expressed as percentage of control.

• Intensity and duration of the effect are compared with the standard.

OTHER METHODS

• Electrical stimulation of the dorsolateral region of the medulla with

bipolar needle electrodes oriented by means of a stereotactic

instrument.

• Cough in conscious dogs by stimulation of the vagus nerve in a

surgically prepared skin loop.

SCREENING OF EXPECTORANTS

SCREENING OF EXPECTORANTS• In vitro studies of mucus secretion

• Acute studies of mucus secretion

• Studies of mucus secretion with chronic cannulation

• Broncheoalveolar lavage

• Ciliary Activity

• Studies of mucociliary transport

• Culture of tracheal epithelial cells

• Alveolar macrophages.

IN VITRO STUDIES OF MUCUS SECRETION

• PURPOSE

• Mucus secretion has been studied in isolated tracheas from ferrets and

dogs.

• PROCEDURE

• Ferrets- either gender- 0.6-1.5kg

• Anaesthetized with sodium barbital IP.

IN VITRO STUDIES OF MUCUS SECRETION- PROCEDURE

• Trachea exposed, cannulated with special Perspex cannula, 5 mm below larynx.

• Animal sacrificed chest opened along midline.

• Trachea exposed to carina; Adjacent tissues removed.

IN VITRO STUDIES OF MUCUS SECRETION- PROCEDURE

• Trachea- laryngeal end down- mounted in water jacketed organ bath.

• Bathed on its submucosal site with Krebs-Henseleit solution+ 0.1% glucose at 37 ͦ C; Bubbled with 95% O2 and 5% CO2

• Lumen of trachea- air-filled.

• Plastic catheter inserted into lower cannula forms airtight seal secretions withdrawn and collected periodically.

IN VITRO STUDIES OF MUCUS SECRETION- PROCEDURE

• Volume of secretions weight difference of catheter lengths with/ without secretions.

• Portions of Ferret trachea cut longitudinally along posterior wall; flattened; pinned to Perspex chamber simultaneous measurement of mucus secretion and changes in tissue volume.

• Promotion of mucus secretion electrical field stimulation at 50-100V 20 Hz, 1-2ms through pins holding tissue.

IN VITRO STUDIES OF MUCUS SECRETION- PROCEDURE

• Epithelium coated with powdered tantalum dust mucus secretion from submucosal glands through gland ducts tantalum traps mucus.

• Hemispherical hillocks formed.

• Surface photographed through dissecting microscope; hillock diameters measured

• Secretion volume per unit area calculated;

• Drugs can be added to submucosal bath.

• EVALUATION

• Secretory response after electrical stimulation in the presence/ absence

of drugs is recorded after 45, 90 & 135 minutes.

ACUTE STUDIES OF MUCUS SECRETION

• PURPOSE• To study the influence of drugs, methods of collecting bronchial mucus.

• PROCEDURE• Rabbits 2.5- 3.5kg anaesthetized IP 1.1 to 1.4 g/kg urethane.

• Trachea exposed; Half opened 2 cm below cricoid cartilage

• T-cannula one arm- inserted into trachea.• Perpendicular arm connected to air outlet of humidifier (temp 35-38 ͦ

C; humidity 80% )

ACUTE STUDIES OF MUCUS SECRETION- PROCEDURE

• Other arm collection tube

• Rabbit restrained in supine position 60 ͦ inclined board with head downwards.

• Mucus secretion vagal stimulation/ ammonium chloride given by stomach tube/ pilocarpine IP.

EVALUATION

• Time response curves after stimulants of mucus secretion are

compared with data from untreated animals.

STUDIES OF MUCUS SECRETION WITH CHRONIC CANNULATION

• PURPOSE

• Several techniques have been developed for chronic collection of mucus

(Wardell et al 1970; Yankell et al 1970; Scuri et al 1980)

• PROCEDURE

• Beagle dogs 9-11kg

• Anaesthetised IV injection 35-40 mg/kg pentobarbital sodium.

STUDIES OF MUCUS SECRETION WITH CHRONIC CANNULATION- PROCEDURE

• Cervical trachea exposed midline skin incision; blunt dissection of muscles.

• Transection of segment of 10 rings length with intact blood and nerve supply.

• Cephalic & caudal parts of trachea- anastomosed with interrupted gut sutures to re-establish patent airway.

STUDIES OF MUCUS SECRETION WITH CHRONIC CANNULATION- PROCEDURE

• Isolated segment- loosened slightly- turned 180 ͦ to reverse cilia movement

• Funnel shaped silicone canula attached to outer surface of proximal end of tracheal segment with surgical mesh and sutured in place

• After cannulation segment placed in pocket below sternohyoid muscle; cannula brought to surface and exteriorized.

STUDIES OF MUCUS SECRETION WITH CHRONIC CANNULATION- PROCEDURE

• Isolated segment closed at caudal end with interrupted gut sutures.

• Mucosal surface of cervical end is sutured; Interrupted skin sutures.

• Muscles, Skin normally sutured.

• In 2-3 weeks, skin heals over small stoma subcutaneous pouch of functioning tracheal tissue

STUDIES OF MUCUS SECRETION WITH CHRONIC CANNULATION- PROCEDURE

• A balloon may be placed into the pouch

• Pressure changes in balloon contraction of smooth tracheal muscles

physostigmine injection/ vagal stimulation/ relaxation after atropine

injection recorded demonstrating parasympathetic innervation

STUDIES OF MUCUS SECRETION WITH CHRONIC CANNULATION- EVALUATION

• Parasympathomimetic stimulation (0.5mg/kg pilocarpine SC)

increases flow rate of tracheal fluids.

• Pressure changes in balloon after injection of parasympathomimetic/

sympathomimetic drugs are compared with baseline values.

REFERENCES

• Textbook of Medical Physiology, Guyton, 11th edition

• Davidson’s Principles and Practice of Medicine, 21st edition.

• Ganong’s review of medical physiology, 23rd edition.

• Essentials of Medical Pharmacology, K. D. Tripathi, 6th edition• Drug Discovery and Evaluation of Pharmacological Assays- H.

Gerhard. Vogel- 3rd edition

• Polverino et al.: Anatomy and neuro-pathophysiology of the cough reflex arc. Multidisciplinary Respiratory Medicine 2012 7:5.

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