Feline analgesia following central administration of opioids

Activity of opioids in cats was assessed by employing the tail-flick method. Microinjection of morphine (100 micrograms) or etorphine (2.5 micrograms) into the ventrolateral periaqueductal gray (VLPAG) region resulted in significant analgesia. Smaller doses of morphine (10 and 50 micrograms) or etorphine (0.62 and 1.25 micrograms) were without significant effect. Methadone likewise produced no significant analgesic action in doses as large as 360 micrograms in the ventrolateral periaqueductal gray. Rank order potency, i.e. etorphine greater than morphine much greater than methadone, was similar following systemic administration. Increased latency of tail-flick response after injection of etorphine was diminished by administration of naloxone, either systemically or centrally, thus indicating a specific opiate-mediated response.