Effect of delayed weaning on opioid receptor control of swim stress‐induced antinociception in the developing rat

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Abstract
1 The opioid type of swim-stress induced antinociception (SIA) is mediated via μ-sites in preweanling rats and predominantly by δ-sites in postweanling animals. We have studied the effect of delay of weaning on the receptor transition of this behaviour in the developing rat. 2 Litters were weaned normally at day 21 or allowed to remain with their mothers until assessment of swim SIA. Animals were stressed by warm water (20°C) swimming for 3 min periods and antinociception assessed by the tail immersion test (50°C). 3 Naloxone (10 mg kg−1) partially reversed swim SIA in both 25 day old weaned and non-weaned rats. 4 Naltrindole (1 mg kg−1) partially reversed swim SIA in 25 day old weaned rats but had no effect in non-weaned animals. Naltrindole (5 mg kg−1) completely abolished swim SIA in weaned rats but was without effect in non-weaned groups. Antinociceptive responses to the μ-agonist, alfentanil (60 μg kg−1) were unaffected by naltrindole at 1 mg kg−1 but were partially reversed at 5 mg kg−1. 5 In 30 day old non-weaned rats, naltrindole (5 mg kg−1) abolished the swim SIA. 6 In conclusion, transition from μ- to δ-receptor control of swim SIA in rat pups can be delayed by between 5 and 10 days by delay of weaning. The environmental stimulus of weaning can activate opioid receptor subtype operation of biological responses in the developing animal.