The Antinociceptive Effects of Tramadol on the Thermal Threshold Response in Cats

The study is of the antinociceptive effects of tramadol on the thermal threshold response in eight cats using a thermal threshold-testing device which contained a heater element and a temperature sensor placed on the shaved lateral thoracic area. Each cat randomly received an intramuscular administration of tramadol 2 mg/kg, morphine 0.2 mg/kg and saline 0.04 ml/kg (placebo) at a weekly interval from an observer unaware of the treatment. The thermal thresholds were measured and recorded by activation of the heater until the cat showed a positive response (e.g. skin flicks, turning and looking at the probe and jumping forwards). Three baseline measurements were made at 15 min intervals before the treatment. The thermal thresholds were measured at 15, 30, 45, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, 360, 480 and 720 min after the drug administration. All cats tolerated well with repeated thermal stimuli and continued normal activities (e.g. eating, drinking, defecation, urination, playing, grooming, and responded to human contact) throughout the testing period. The mean thermal thresholds±SD over 12 hours were 42.0±0.5°C (placebo), 43.2±0.9°C (morphine) and 44.2±1.6°C (tramadol). There were significant differences (ρ<0.05) of the mean thermal thresholds over 12 hours between the experimental and control groups. When compared with the pre-injection baseline, thermal threshold was significantly increased (ρ<0.05) between 15-270 min and 330-360 min after morphine and between 45-90 min, 180-210 min and 270-300 min after tramadol. In conclusion, tramadol had an analgesic efficacy on thermal stimuli in cats by increasing the thresholds significantly above the pre-injection basal thermal thresholds.