Analgesic effects on endogenous melatonin secretion

Animal welfare concerns have led to increased demands for use of post-operative analgesia in association with experimental surgery. Such treatment is appropriate provided it has no effect on experimental parameters. In order to assess any effects of analgesics on endogenous melatonin production, groups of six ewes were bled at 30 min intervals for the first 4 hr of darkness a) without analgesic treatment and b) immediately after analgesic administration. Analgesics tested were ketoprofen, phenylbutazone (both non-steroidal anti-inflammatory drugs or NSAIDs) and buprenorphine (an opioid). Plasma melatonin was measured by radioimmunoassay and 4 hr secretion profiles computed for each animal. Ketoprofen and buprenorphine treatment reduced mean four hour melatonin secretion profiles by 50.6% (from 1,347.3 to 665.1 pg/ml.hr) and 38.6% (from 287.8 to 171.3 pg/ml.hr), respectively. Only in the case of phenylbutazone was the response not statistically significant; hence this drug has been selected as a post-operative analgesic for future experimental studies involving measurement of melatonin levels.