Effects of Reference Analgesics and Psychoactive Drugs on the Noxious Heat Threshold of Mice Measured by an Increasing‐Temperature Water Bath

The study aimed at validating an increasing‐temperature water bath suitable for determining the noxious heat threshold for use in mice. The noxious heat threshold was determined by immersing the tail of the gently held awake mouse into a water container whose temperature was near‐linearly increased at a rate of 24°C/min. until the animal withdrew its tail, that is, heating attained the noxious threshold. The effects of standard analgesic, neuroleptic and anxiolytic drugs were investigated in a parallel way on both the noxious heat threshold and the psychomotor activity assessed by the open field test. Morphine, diclofenac and metamizol (dipyrone) elevated the heat threshold of mice with minimum effective doses of 6, 30 and 1000 mg/kg i.p., respectively. These doses of morphine and diclofenac failed to induce any remarkable effect on psychomotor activity in the open field test while that of metamizol exerted a profound inhibition. The anxiolytic diazepam and the neuroleptic droperidol at doses evoking a mild and moderate, respectively, psychomotor inhibition failed to alter the heat threshold. Combination of a subliminal dose of morphine (regarding both antinociceptive and psychomotor inhibitory action) with diclofenac, metamizol, diazepam or droperidol at doses also subliminal regarding the thermal antinociceptive effect elevated the noxious heat threshold without major additional effects in the open field test. It is concluded that the increasing‐temperature water bath is suitable for studying the thermal antinociceptive effects of morphine and diclofenac as well as the morphine‐sparing action of diclofenac, metamizol, droperidol and diazepam. Behavioural testing is recommended when testing analgesics.