2. 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.
3. • 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.
4. • 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.
5. Types of tests
Based on the type of
stimulus :
• Thermal methods
• Mechanical methods
• Chemical methods
• Electrical methods
6. • 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
7. • Tests based on the use of long duration stimuli
(―tonic pain‖)
A. Intradermal injections
B. Intraperitoneal injections of irritant agents (the
―writhing test‖).
C. Stimulation of hollow organs
11. “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.
12. • 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).
15. • 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
16. protocol
• Sprague dawley rats were divided into two
groups :
1. Test – 100 mg/kg aspirin p.o.
2. Control – normal saline
17. • After 30 min. all animals were administered 10
ml/kg of 0.6 %v/v acetic acid solution i.p.
18. • In all the animals the number of ―wriths‖
produced were counted for a period of 20
minutes.
19. • 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
