Aim: To study the Analgesic effect of the given drug (morphine) in mice using the hot plate method. (Eddy and Leimbach)
What is the principle of the hot plate method?
Principle: Pain is an unpleasant feeling, which makes us uncomfortable and reduces physical as well as mental alertness. Although pain is also useful for us because it acts as a warning signal and it warns about something uncommon inside or outside our body. If the pain is minor it may be tolerated but if the pain becomes severe it has to be managed at the earliest.
Analgesia is defined as a state of reduced awareness of pain, and analgesics are substances that decrease pain sensation(pain–killers) by increasing the threshold to painful stimuli. The commonly used analgesics are aspirin, paracetamol(non-narcotic type), and morphine (narcotic type).
Painful reactions in experimental animals can be applying noxious (unpleasant)stimuli such as
- Thermal (radiant heat as a source of pain)
- Chemical (irritants such as acetic acid and bradykinin)
- Physical pressure(tail compression)
In the laboratory, commonly used procedures are the tail-flick (tail-withdrawal from the radiant heat) method using analgesiometer, the hot plate (jumping from the hot plate at 55 º C) method, and acetic acid-induced writhing.
Animal: Mice (20-25 g)
Equipment: Eddy’s hot plate
Drugs: Morphine sulphate(dose 5 mg/kg,sc.,prepare a stock solution containing 0.5 mg/ml and inject 1 ml/100 g of body weight of mouse)
- Weigh and number the mice.
- Take the basal reaction –time by observing hind paw licking or jump response (whichever appears first) in animals when placed on the hot plate maintained at constant temperature (55º C).normally animals show such response in 6-8 sec. A cut-off period of 15 sec is observed to avoid damage to the paws.
- Inject morphine into animals and note the reaction time of animals on the hot plate at 15,30,60 and 120 min after the drug administration. As the reaction time increases with morphine,15 sec is taken as maximum analgesia and the animals are removed from the hot plate to avoid injury to the paws.
- Calculate the percent increase in reaction time (as an index of analgesia ) at each time interval.
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