Evidence for opiate tolerance in newborn rats

Henriette Van Praag, Hanan Frenk*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


It has been reported that tolerance to the antinociceptive effect of morphine does not develop in rats younger than 15 days of age. This may be due to a masking effect of rapidly proliferating opiate receptors during the first 2 postnatal weeks. Newborn rats received morphine (20 mg/kg) or equivolume saline on postnatal days 5, 6, 7, 8 and antinociception was assessed on each day. On day 9, animals of both groups were injected with 0, 0.25, 0.50, 1, 2, 4, 8 or 20 mg/kg of morphine. Antinociception, tested by the tail-flick method, did not diminish over days 5-8, yet on day 9 a rightward shift in the dose-response curve occurred. Thus, tolerance in rats occurs to morphine induced antinociception earlier than 15 days postnatally.

Original languageEnglish
Pages (from-to)99-102
Number of pages4
JournalDevelopmental Brain Research
Issue number1
StatePublished - 20 May 1991


  • Antinociception
  • Morphine
  • Neonatal rat
  • Opiate receptor
  • Tail-flick test
  • Tolerance


Dive into the research topics of 'Evidence for opiate tolerance in newborn rats'. Together they form a unique fingerprint.

Cite this