Effect of morphine, methadone, hydromorphone or oxymorphone on the thermal threshold, following intravenous or buccal administration to cats.

OBJECTIVE: To determine the effects of morphine, methadone, hydromorphone or oxymorphone on the thermal threshold in cats, following buccal and intravenous (IV) administration. STUDY DESIGN: Randomized crossover study. ANIMALS: Six healthy adult female ovariohysterectomized cats weighing 4.5 ± 0.4 kg. METHODS: Morphine sulfate (0.2 mg kg-1 IV or 0.5 mg kg-1 buccal), methadone hydrochloride (0.3 mg kg-1 IV or 0.75 mg kg-1 buccal), hydromorphone hydrochloride (0.1 mg kg-1 IV or 0.25 mg kg-1 buccal) or oxymorphone hydrochloride (0.1 mg kg-1 IV or 0.25 mg kg-1 buccal) were administered. All cats were administered all treatments. Skin temperature and thermal threshold were measured in duplicate prior to drug administration, and at various times up to 8 hours after drug administration. The difference between thermal threshold and skin temperature (ΔT) was analyzed. RESULTS: Administration of methadone and hydromorphone IV resulted in significant increases in ΔT at 40 minutes after drug administration. Buccal administration of methadone resulted in significant increases in thermal threshold, although no significant difference from baseline measurement was detected at any time point. IV administration of morphine and oxymorphone, and buccal administration of morphine, hydromorphone and oxymorphone did not cause significant thermal antinociception. CONCLUSION AND CLINICAL RELEVANCE: At the doses used in this study, IV administration of methadone and hydromorphone, and buccal administration of methadone resulted in transient thermal antinociception. The results of this study do not allow us to predict the usefulness of these drugs for providing analgesia in clinical patients.