This document summarizes screening methods for central and peripheral analgesics. For central analgesics, it describes in vivo methods like Haffner's tail clip, hot plate, tail immersion, and formalin tests that assess response to painful stimuli in mice and rats. For peripheral analgesics, it discusses writhing tests using acetic acid or phenylquinone in mice, Randall-Selitto testing in inflamed rat paws, and duodenum distension in rats to measure visceral pain responses. The document provides classifications and examples of different classes of central and peripheral analgesic agents and their mechanisms of action.

1. SCREENING METHODS FOR
ANALGESICS
DR. AMIT D. SHARMA
Third year resident,
Department of pharmacology
SBKS MI & RC.
Date : 09/05/2015

2. Contents
 Definitions
 Central analgesics : agents, mechanism, in vivo
methods
 Peripheral analgesics : agents, mechanism, in vivo
methods

3. Definitions
 Pain- an unpleasant sensory or emotional experience
associated with actual or potential tissue damage, or described
in terms of such damage (WHO).
 Features of inflammation
Pain (dolor)
rubor (redness)
tumor (swelling)
calor (heat)
functio laesa (loss of function).

4.  Analgesics – the drugs which possess significant pain-
relieving properties by acting in the central nervous
system or on peripheral pain receptors without
significantly affecting consciousness.
 Two groups :
I. Narcotic/opioid /morphine like analgesics(central)
II. Non-narcotic/ non steroidal anti-inflammatory-
analgesic-antipyretic agents(peripheral)

5. CENTRALANALGESIC
ACTIVITY

6. Classification
 Morphine analogues:
I. Agonists – morphine(prototype), diamorphine(heroin),
codeine, pholcodeine, levorphanol
II. Partial agonists – nalorphine, levallorphan
 Synthetic derivatives unrelated to morphine :
I. Agonists – Pethidine (meperidine), fentanyl, sufentanil,
alfentanil, remifentanil, methadone, oxycodone
II. Partial agonists – pentazocine, cyclazocine,
buprenorphine, nalbuphine, butorphanol
III. Mu receptor agonist with other mode of action –
tramadol

7. IN VIVO METHODS

8. Haffner’s tail clip method, 1929
 The test compounds are administered subcutaneously to
fed mice or orally to fasted animals.
 Then an artery clip is applied to the root of the tail
(approximately 1 cm from the body) of mice.
 The reaction time is noted.
 Response : biting the clip or the tail near the location of the
clip.

9. Hot plate method
 Woolfe and Mac Donald (1944)
 Mice are put on hot plate(temp 55° to 56 °C) and then
latency is noted following administration of test drug.
 Response – jumping or paw licking

10. Tail immersion method
 Restraining the young female wistar rat, lower 5 cm
portion of the tail is immersed in a cup of freshly filled
water of exactly 55°C.
 The reaction time is determined before and periodically
after either oral or subcutaneous administration of the
test substance.
 The cut off time of the immersion is 15 seconds.
 Response – withdrawal of tail or attempt to escape.

11. Grid shock test
 Blake et al(1963)
 Central and peripheral analgesics
 Stimulus is given in the form of square wave pulses to male
mice. The output of the stimulator is connected to the wires of
grid with fixed resistance , which is kept parallel to
oscilloscope.
 Pain thresholds are determined in each individual mouse
twice before and after administration of the test drug.
 Response – on oscilloscope, a startling reaction, increase
locomotion or attempt to jump.

12. Electrical stimulation of the tail
 Burn et al, 1950
 Central and peripheral analgesics
 First test compound is given and then rectangular wave
pulses from a constant voltage stimulator are applied to
the alligator clips attached to the tail of the mice.
 Response – spinal reflexes, complete vocalization or
behavior changes(escape, aggression). The normal
response time range of the stimuli is 3–4 sec.

13. Tooth pulp stimulation
 Kohl and Reffert (1938)
 Opioid agonists, pyrazolone derivatives
 After anaesthetizing the rabbit with thiopental, current is
applied through the electrodes passed in the dental drill
holes exposed to pulp chambers.
 Pain thresholds are determined in each individual rabbit
twice before and after administration of the test drug.
 Response - licking, biting, chewing or head flick.

14. Formalin test in rats
 Dubuisson and Dennis,1977
 Male wistar rats are administered 0.05 ml of 10%
formalin in dorsum of the front paw along with test drug
subcutaneously or orally.
 Response - elevation or favoring of the paw or
excessive licking and biting of the paw.

15. PERIPHERAL
ANALGESIC ACTIVITY

16. Classification
 Nonselective COX inhibitors (traditional NSAIDs)
I. Salicylates – aspirin
II. Propionic acid derivatives – ibuprofen, naproxen
III. Fenamide – mephenamic acid
IV. Enolic acid derivatives – piroxicam
V. Acetic acid derivatives – ketorolac, indomethacin
VI. Pyrazolone derivatives – phenylbutazone

17.  Preferential COX-2 inhibitors - nimesulide, diclofenac,
aceclofenac, etodolac.
 Selective COX-2 inhibitors – celecoxib, etoricoxib.
 Analgesics-antipyretics with poor antiinflammatory
action-
I. Paraaminophenol derivative – paracetamol(acetaminophen).
II. Pyrazolone derivatives – metamizol(dipyrone),
prophenazone.
III. Benzoxazocine derivative – nefopam.

18. IN VIVO METHODS

19. Writhing tests
 Mice are administered the test drug before intraperitoneal
injection of irritating agent like phenylquinone or acetic
acid.
 Pain sensation in acetic acid induced writhing paradigm is
elicited by producing localized inflammatory response due
to release of free arachidonic acid from tissue phospholipids
via cyclo-oxygenase (COX), and producing prostaglandin
specifically PGE2 and PGF2α, the level of lipoxygenase
products may also increases in peritoneal fluids.

20.  Response – writhe (stretching of the abdomen with
simultaneous stretching of at least one hind limb)
 Aconitine can also be used instead of above drugs.
 Rats – 4% sodium chloride.

21. Pain in inflamed tissue
(RANDALL-SELITTO TEST)
 Principle - inflammation increases the sensitivity to pain and
that this sensitivity is susceptible to modification by
analgesics.
 Male wistar rats are given the test drug orally,
subcutaneously or intraperitoneally.
 Inflammation is induced by subcutaneous injection of
Brewer’s yeast into the plantar surface of the left hind paw of
the rat .Then pressure is applied through a tip to the plantar
surface of the rat’s foot at a constant rate by a special
apparatus to the point at which the animal struggles, squeals
or attempts to bite.

22. Mechanical visceral pain model in the rat
 Coburn et al 1989,deLeo et al 1989.
 First test drug is administered into male Sprague Dawley
rats and anaesthetized with nitrous oxide and halothane.
 One piece balloon catheter is passed through
gastrostomy tube into first part of duodenum.
 Inflation of balloon distension of duodenum
writhing.

23. Behavioural responses score
0 : Normal behaviour defined as exploration, escape attempts
and resting
1 : Slightly modified behaviour defined as cessation of
exploration, focusing, wet-dog shake, excessive facial
grooming, teeth chattering and deep breathing
2 : Mildly to moderately modified behaviour defined as
retching-like activity, hunching, abdominal grooming or
nipping and immobility of the hind limbs (disappears with
removal of the stimulus).

24. 3 : Severely modified behaviour defined as stretching of
the hind limbs, arching and dorsoflection of the hind
paws.
4 : Intensive visceromotor activity defined as repetitive
stretching of the body, extension of the hind limbs, and
pelvis, frequent rotating sideward , i.e., writhing.

25. References
 H. Gerhard Vogel et al. Drug discovery and evaluation :
Pharmacological assays, third edition.
 K D Tripathi. Essentials of medical pharmacology,
seventh edition.
 H L Sharma, K K Sharma. Principles of pharmacology,
second edition.
 Harsh Mohan. Textbook of pathology, sixth edition.

26. THANK YOU