Study of local anesthetics by conduction anaesthesia in frogs

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BACKGROUND

Local anaesthesia is a technique to create loss of sensation in specific region of the body.¹ Usually they are used in minor surgeries, dental procedures, cesarean section with reduced pain and distress. This technique is safer and superior to general anaesthesia.²

Local anaesthetic drugs produce reversible effect and a loss of nociception. When these drugs are used on specific nerve pathways they show effects such as analgesia and paralysis. Local anaesthetics differ in their pharmacological properties and are used in various techniques such as surface (topical) anaesthesia, infiltration, epidural block, plexus block and spinal anaesthesia.²

Conduction anaesthesia is thought to be involved in penetration of nerve sheaths by anaesthetic agents, causing a reversible paralysis of the nerve. The isolated frog sciatic nerve experiment was the widely accepted model for deteremination of conduction anaesthesia. By observing the extent of changes reflected in potentials by local anaesthetics, the relationship between concentration of the drug used and the extent of conduction anaesthesia produced is determined.³

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REQUIREMENTS

Animals required: Frogs of either sex

Drugs: Lignocaine 0.2%, test drug (0.05% and 0.1%)

Reagents used: 0.65% NaCl solution,

Instruments used: Frog board, cotton balls, dissection instruments

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PROCEDURE

Select frogs of either sex. Decapitate them by using pair of scissors. Incise the skin in the thigh region at both sides and carefully expose the sciatic nerves in the thigh. Avoid stretching and injury to the nerve. Then suspend the frog on a vertical board. Soak small pieces of cotton in different test preparations (0.05% and 0.1%) and in standard drug (lignocaine 0.2%). Place the pellet dipped in lignocaine solution around sciatic nerve of one thigh and consider it as control. In other thigh place the cotton ball dipped in one of test concentration for 1 minute. Then remove cotton balls from both sides and place the frog legs in a bath containing 0.65% NaCl solution. This procedure allows for testing duration and reversibility of local anaesthesia. After 3 min remove the legs of the frog from the bath and pinch the toes/ankles with small forceps. If conduction anaesthesia is effective then the reflex action is abolished. This process is to be continued for every 3 minutes till the reflex action vanishes.⁴

OBSERVATION

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CONCLUSION

Time of onset and duration of anaesthesia are recorded for each concentration of test drug and time response or dose response curve can be determined.

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REFERENCES

1. William A, Catterall A, Kenneth M. Local Anesthetics. In: Laurence Bruton L,Goodman and Gilmans The Pharmacological Basis of Therapeutics, 12 th ed. New Delhi: Mc Graw Hill Medical; 2011: 564.
   
2. Local anesthesia. Available at: https://en.wikipedia.org/wiki/Local_anesthesia. Accessed on 17 October 2017.
   
3. Jefferson GC. The assessment of conduction anaesthesia. JPP. 1963;15(1):92-9.
   
4. Vogel GH. Study of conduction anaesthesia in the sciatic nerve of the frog. Drug Discovery and evaluation-Pharmacological assays. 2nd edition. Newyork: Springer-Verlag; 2002: 646.