ANAESTHETICS USED IN EXPERIMENTAL ANIMALS

Anaesthesia used in the experimental study is to ensure analgesia, amnesia and immobilization of animals during the procedure. Unless contrary to the achievement of the results of the study, different anaesthetics should be used to control pain or distress during the experiment, making the study simpler, reliable and reproducible. The painful procedures are conducted under appropriate anaesthesia as recommended for each species of animal. It must also be ensured that the anaesthesia is given for the full duration of the experiment, and at no stage is the animal conscious to perceive pain.

Local or general anaesthesia may be used, depending on the type of surgical procedure. Local anaesthetics are used to block the nerve supply to a limited area and are used only for minor and rapid procedures. This should be carried out under expert supervision for regional infiltration of the surgical site, nerve blocks, and for epidural and spinal anaesthesia. The most common compounds for surface anaesthesia are tetracaine, procaine, lidocaine, mepivacaine, etc. But for most laboratory animals, general anaesthesia is the method of choice. General anaesthetics are drugs which produce a reversible loss of all sensation and consciousness, which are used for surgical procedures to render the animals unaware of the painful stimuli. General anaesthetics are used in the form of injectable or inhalants or a combination of both routes. The following anaesthetics are commonly used for laboratory animals.

INJECTABLE ANAESTETICS: Injectable anaesthetics commonly used for laboratory animals are:

Barbiturates: Barbiturates interfere with nerve impulse transmission both in the CNS and in the ganglia, producing depression of cardiovascular and spinal cord reflexes. Phenobarbitone sodium is used for prolonged experiments. Pentobarbitone sodium is used for inducing rapid anaesthetic effects. Thiopentone sodium induces rapid anaesthetic effects and is used for surgical operations of short duration. For laboratory animals, short-and very short-acting barbiturates are used. Marked slowing of respiration is produced by barbiturates. It can be used for all species.

Chloralose: It is prepared by heating equal parts of anhydrous glucose and chloral. α-chloralose is the active form. It induces surgical anaesthesia for 3-4 hours and has the advantage of greater constancy of the depth of anaesthesia. It is a suitable anaesthetic for dogs, cats and rats, but it is not suitable for rabbits as they are narcotised rather than anaesthetized.

Urethane: Urethane or ethyl carbamate is suitable only for acute experiments since it has delayed toxic effects on the liver and may cause agranulocytosis; it has little or no effect on nerve transmission and produces little reflex depression. Duration of anaesthesia is 3-4 hours. It is a suitable anaesthetic for rabbits and rats. Frogs can be anaesthetized by placing them in a covered beaker containing 5-10% urethane solution. 

Paraldehyde: It depresses only the cerebrum and not the medullary centres; hence, it has a wide margin of safety. Intravenous injection may produce cardiac dilatation, pulmonary congestion and oedema. It is used for dogs and cats.  

Ketamine: This is a unique anaesthetic characterized by analgesia, immobility, and amnesia with light sleep. It is a neuroleptic compound with a very fast onset of action after intramuscular injection. It can be used for all species.

Xylazine: It is frequently used for anaesthesia in combination with other substances. Alone, this compound is used to produce anaesthesia in cattle.

INHALATIONAL ANAESTHETICS: Inhalational anaesthetics play a minor role in small laboratory animals such as rodents, but are more commonly used in larger laboratory animals such as dogs, cats, monkeys, etc. The advantages of inhalational anaesthetics are controlling the depth of anaesthesia. Ether is used in a fume hood, but the use of ether as an anaesthetic agent is prohibited. Other inhalational anaesthetics widely used in animal studies are Halothane, Isoflurane, Enflurane, Desflurane and Sevoflurane.

Preanaesthetic medication: Before using anaesthetics, the animal is prepared for anaesthesia by overnight fasting and the administration of pre-anaesthetics, which block parasympathetic stimulation of the cardio-pulmonary system and reduce salivary secretion. Atropine (an anticholinergic agent) is the most commonly used as a preanaesthetic medication. Atropine (0.02-0.05 mg/kg by s.c. or i.m., or i.v. routes) is used for all species to reduce salivary and bronchial secretions and protect the heart from vagal inhibition, given before anaesthesia.

a) Inhalational anaesthetics in Guineapig	b) Inhalational anaesthetics in rat

Figure: Induction of inhalational anaesthesia in animals. 

Table: Commonly used anaesthetic drugs for laboratory animals.

Drugs (route)	Dose (mg/kg) in
	Mouse	Rat	Hamster	G. pig	Rabbit	Cat	Dog	Monkey
Ketamine HCl (i.m.)	22-24	22-24	-	22-24	22-24	30	30	15-40
Pentobarbitone sodium (i.v.)	35	25	35	30	30	25	20-30	35
Pentobarbitone sodium (i.p.)	50	50	-	40	40	-	-	-
Thiopentone sodium (i.v.)	25	20	20	20	20	25	25	25
Thiopentone sodium (i.p.)	50	40	40	55	-	-	-	60
Urethane (i.v.)	-		-	-	1.0	1.25	1.0	1.0
Urethane (i.p.)	-	0.75	-	1.5	1.0	1.50	-	-

 

PRECAUTIONS:

1. Species characteristics and variation must be kept in mind while using an anaesthetic.

2. Side effects such as excessive salivation, convulsions, excitement and disorientation should be suitably prevented and controlled.

3. The animal should remain under veterinary care till it completely recovers from anaesthesia and postoperative stress. 

REFERENCES

1.   Panigrahi G., Patra A., 2019. Experimental Pharmacology- I: bridges the gap between animal models and computer simulation models. 1^st edition, Nirali Prakashan, Pune, India.

2.     Medhi, B., Prakash, A., 2010. Practical Manual of Experimental and Clinical Pharmacology. 1^st edition, Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India.