1. 451 PHL
3 rd lecture
Analgesics
Awatif B. Al-Backer

2. Drugs that selectively inhibit the perception (sensation) of the pain
Analgesics
Definition
Drugs that selectively inhibit the perception (sensation) of the pain
Classification
1- Peripheral (miscellaneous):
- Causal
- Non-causal
2- Central:
- Narcotic
- Non-narcotic
Awatif B. Al-Backer

3. - Stimulation of skin & mucous membranes - Fast response Deep:
Types of Pain
Superficial:
- Stimulation of skin & mucous membranes
- Fast response
Deep:
- Arises from muscles, joints, tendons, heart ..etc.
- Slow response causing sweating, nausea, &
vomiting
Awatif B. Al-Backer

4. Peripheral Analgesics
Causal
-Treat the cause
Example:
1. Atropine relief colic (antispasmodic)
2. Ergotamine treat migraine
3. Colchicines treat gout
Non-causal
- Does not treat the cause
Examples:
1- Local anaesthetics (for superficial tumor)
2- Counter-irritant (apply pain that counteract or mask the original one e.g. acupuncture)
Awatif B. Al-Backer

5. Narcotics Non-narcotic Central Analgesics - NSAID 1- Aspirin
The class - Opioids (morphine & morphine like drugs)
Examples
1- Natural (as codeine)
2- Semi synthetic e.g.
dihydromorphine&
diacetylmorphine (heroin)
3- Synthetic e.g. pethidine
4- Endogenous opiates as endorphins & encephalins
Non-narcotic
- NSAID
1- Aspirin
2- Paracetamol
3- Diclofenac
4- Piroxicam
5- Ibuprofin
6- Ketoprofen
Awatif B. Al-Backer

6. Central Analgesics Awatif B. Al-Backer Non-narcotic
Subcortical “thalamus”
No antagonist
Dull pain e.g. headache, toothache & backache
Low
No addiction. ↑in bleeding tendency & ulcer
Inhibits prostaglandin synthesis
by inhibition of cycloxygenase
enzyme
Narcotics
Site of Cortex & thalamus
action
Antagonist Naloxone, nalorphine
& levallorphan
Uses Sever & deep pain e.g.
cancer, MI, & anginal pain
Potency High
Side effect Addiction
MOA ‡ of opiatereceptors
(m, k ,s, d) and relief
the pain through the
release of
endorphine & encephalins
Awatif B. Al-Backer

7. Screening Methods For Analgesics
Principle:-
Pain is induced to a suitable animal and the response of the animal to the painful stimuli is recorded before and after administration
Awatif B. Al-Backer

8. 2- Chemical ( Writhing method)
Screening methods
Narcotics:
1- Thermal method
a- Hot plate
b- Tail flick
2- Mechanical method
Non-narcotic:
1- Electrical method
2- Chemical ( Writhing method)
Awatif B. Al-Backer

9. Instrument: Hot plate analgesiometer Painful stimulus: Heat (55°C)
Material
Animal: Mouse
Instrument: Hot plate analgesiometer
Painful stimulus: Heat (55°C)
Drug used: Morphine (0.5 % - 25 mg/kg)
Onset of action: licking of the feet, or jump out of the beaker ( normally occur within 20 sec.)
Awatif B. Al-Backer

10. Instrument: Tail-flick analgesiometer
Material:-
Animal: Mouse
Instrument: Tail-flick analgesiometer
Painful stimulus: Heat (by apply a beam of light 130°C)
Drug used: Morphine
Onset of action: flicking of the tail (Cut off- point_ should be less than or equal to 6 sec.)
Awatif B. Al-Backer

11. Writhing Method Principle:
The painful stimulus is induced by IP injection of an irritant substance (e.g. acetic acid)
Writhing:
Stretching of the body, withdrawing of the limb, retraction of the abdomen & the stomach touches the ground
Awatif B. Al-Backer

12. Painful stimulus: Chemicals e.g. acetic acid
Continue
Material
Animal: Mouse
Painful stimulus: Chemicals e.g. acetic acid
Drug used: NSAID e.g. Na salicylate
Drug
Onset of writhing
No. of writhing
Acetic acid (control)
Na salicylate (100mg/kg)
Na salicylate (200mg/kg)
Awatif B. Al-Backer

13. The no. of writhing measured within 10 min. after the onset time.
Continue
N.B:-
After treatment with Na. Salicylate, the onset of writhing will be delayed & the no. of writhing will decrease which explain the analgesic property of NSAIDs.
The no. of writhing measured within 10 min. after the onset time.
Awatif B. Al-Backer

14. Awatif B. Al-Backer