Evidence for opiate tolerance in newborn rats

Area of Science:

• Pharmacology
• Neuroscience
• Developmental Biology

Background:

• Tolerance to morphine's antinociceptive effects is reportedly absent in rats under 15 days old.
• This phenomenon may be attributed to the proliferation of opiate receptors in early postnatal life.
• Rapidly proliferating opiate receptors might mask the development of tolerance.

Purpose of the Study:

• To investigate the onset of tolerance to morphine's antinociceptive effects in young rats.
• To determine if tolerance develops earlier than the previously reported 15 postnatal days.
• To examine the relationship between morphine administration and antinociception in early development.

Main Methods:

• Newborn rats were administered morphine (20 mg/kg) or saline daily from postnatal days 5 to 8.
• Antinociception was assessed daily using the tail-flick method.
• On day 9, a dose-response curve for morphine (0-20 mg/kg) was generated to assess antinociception.

Main Results:

• Antinociception remained consistent from days 5 to 8, indicating no diminished effect.
• On day 9, a significant rightward shift in the morphine dose-response curve was observed.
• This shift indicates the development of tolerance to morphine's antinociceptive effects.

Conclusions:

• Tolerance to morphine-induced antinociception can occur earlier than 15 postnatal days in rats.
• The findings challenge the established timeline for the development of morphine tolerance in early life.
• Further research is needed to elucidate the precise mechanisms underlying early-onset tolerance.