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A comparison of analgesia and suppression of oxytocin release by opiates
Author(s) -
Clarke G.,
Wright D.M.
Publication year - 1984
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1984.tb16235.x
Subject(s) - pentazocine , morphine , oxytocin , potency , (+) naloxone , anesthesia , medicine , nociception , buprenorphine , pethidine , nociceptin receptor , tail flick test , chemistry , pharmacology , analgesic , opioid , endocrinology , opioid peptide , receptor , in vitro , biochemistry
1 The potency of opiates for suppressing oxytocin release relative to their potency as analgesics was tested in lactating rats. 2 Oxytoxin release was evoked by the sucking of the young in urethane‐anaesthetized and unanaesthetized rats, and was detected by the characteristic behaviour of the young and milk yield respectively. 3 The tail‐flick test, using noxious radiant heat, was used to assess analgesia. 4 Intraperitoneal injection of morphine (1 mg kg −1 and 5 mg kg −1 ) significantly reduced milk yield in unanaesthetized rats. 5 Urethane‐anaesthetized rats displayed a pattern of reflex milk‐ejection responses similar to that found in conscious rats. This reflex was significantly inhibited in a dose‐related, naloxone‐reversible manner by buprenorphine (ED 50 0.18 mg kg −1 ), meptazinol (ED 50 : 14.0 mg kg −1 ), morphine (ED 50 : 0.67 mg kg −1 ), pentazocine (ED 50 : 15.0 mg kg −1 ) and pethidine (ED 50 : 7.9 mg kg −1 ). 6 Although intraperitoneal injection of morphine (5 mg kg −1 ) abolished the increase in intramammary pressure occurring at reflex milk‐ejection, that evoked by intravenous oxytocin (0.5−1 mu) was unaffected. 7 Each opiate also caused significant, dose‐related, naloxone‐reversible increases in tail‐flick latency. The ED 50 doses were buprenorphine (ED 50 : 0.14 mg kg −1 ), meptazinol (ED 50 : 12.5 mg kg −1 ), morphine (ED 50 : 5.0 mg kg −1 ), pentazocine (ED 50 : 12.5 mg kg −1 ) and pethidine (ED 50 : 6.1 mg kg −1 ). 8 The order of potency for analgesia and for suppression of oxytocin release were identical, namely: buprenorphine > morphine > pethidine > meptazinol > pentazocine. 9 The results obtained with lactating rats suggest that secretion of the hormone oxytocin is substantially reduced during opiate‐induced analgesia.