ANIMAL MODELS OF NOCICEPTION (PAIN)

ANKIT GILANINIPERA1113PC03PC 611

INTRODUCTION• Pain is a perception, and as such, it is one of the

outputs of a system in more highly evolved animals—the nociceptive system—which itself is a component of the overall set of controls responsible for homeostasis.

• Pain constitutes an alarm that ultimately has the role of helping to protect the organism: it both triggers reactions and induces learned avoidance behaviors, which may decrease whatever is causing the pain and, as a result, may limit the (potentially) damaging consequences.

•At the beginning of the twentieth century, Sherrington (1910) developed this concept and introduced the term nociception (from the Latin nocere, “to harm”).

•The term nociceptive refers to the potential of a stimulus to produce a tissue lesion and a reaction from the organism.

•The study of pain in awake animals raises ethical, philosophical, and technical problems.

•There is the problem that pain cannot be monitored directly in animals but can only be estimated by examining their responses to nociceptive stimuli; however, such responses do not necessarily mean that there is a concomitant sensation.

Types of tests

• Thermal methods• Mechanical methods• Chemical methods• Electrical methods

Based on the type of stimulus :

•Tests based on use of short duration stimuli (“phasic pain”)

A. Tests based on the use of thermal stimuli 1. The tail-flick test a. The tail-flick test using radiant heat b. The tail-flick test using immersion of

the tail 2. The paw withdrawal test 3. The hot plate test 4. Tests using cold stimuli B. Tests based on the use of mechanical stimuli

•Tests based on the use of long duration stimuli (“tonic pain”)

A. Intradermal injectionsB. Intraperitoneal injections of irritant agents

(the “writhing test”). C. Stimulation of hollow organs

The tail-flick test (tail-immersion)

The paw withdrawal test

The hot plate test

“writhing test”• Intraperitoneal Injections of Irritant Agents

•The intraperitoneal administration of agents that irritate serous membranes provokes a very stereotyped behavior in the mouse and the rat which is characterized by abdominal contractions, movements of the body as a whole (particularly of the hind paws), twisting of dorsoabdominal muscles, and a reduction in motor activity and motor incoordination.

•Writhing is defined as a stretch, tension to one side, extension of hind legs, contraction of the abdomen so that the abdomen of mice touches the floor, turning of trunk (twist).

Irritant agents

•Acetic acid•Phenyl quinone•Nacl, etc.

• The test is sometimes called the abdominal contortion test, the abdominal constriction response, or the stretching test, but more commonly it is known as the “writhing test”.

• Generally the measurements are of the occurrence per unit of time of abdominal cramps resulting from the injection of the algogenic agent. These behaviors are considered to be reflexes and to be evidence of visceral pain ; however, it would probably be wiser to call it peritoneovisceral pain

protocol

•Sprague dawley rats were divided into two groups :

1. Test – 100 mg/kg aspirin p.o.2. Control – normal saline

•After 30 min. all animals were administered 10 ml/kg of 0.6 %v/v acetic acid solution i.p.

•In all the animals the number of “wriths” produced were counted for a period of 20 minutes.

•Analgesic activity was evaluated

% inhibition = Mc – Mt x 100(or % protection) Mc

Mc = mean of wriths produced in control group

Mt = mean of wriths produced in test group

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