Abstract
Stress-induced antinociception (SIA) which is well characterised in the adult rat, can also be observed in young rats and by varying swimming times can be dissociated into opioid and non-opioid forms. However, swimming ability in the rat does not fully develop until the third postnatal week and this has precluded the study of swim SIA in neonates. In this study an attempt was made to develop a harness device to aid swimming in young rats. This device was employed successfully for the study of swim-SIA down to the age of 2 days without distress to the animals. Ontogenetic study of the opioid form of swim-SIA in the neonate rats using this device indicated that swim-SIA was absent at day 2 and day 5 but at postnatal day 10 a small level of SIA was evident which was reversed by naloxone (10mg/kg). Swim-SIA develops rapidly thereafter and the adult profile observed by day 25. The effect of maternal diazepam exposure influencing the development of opioid system functions, particularly stress-induced antinociception, in the neonatal offspring has been studied. Diazepam (1 or 10 mg/kg) was administered daily to mothers from conception to the postnatal day of experiment when the neonates were tested for swim-SIA. No significant effect of diazepam treatment upon maternal or litter weight gain was observed in all the groups over a chronic treatment period. Pre- and postweanling rat pups were assessed for opioid mediated stress-induced antinociception by 3min. swimming stress and measuring nociception using the tail immersion test. In preweanling rats stress-induced antinociception was observed in both vehicle and diazepam treated animals, but in diazepam treated groups (1 and 10 mg/kg) this was insensitive to reversal by the opioid antagonist naloxone suggesting that non-opioid systems are operating this response. In postweanling rats a similar insensitivity to naloxone was observed in 1mg/kg diazepam treated groups and with 10mg/kg diazepam there was no significant antinociception. The results suggest that maternal diazepam treatment interferes with the development of stress-mediated responses and that part of this toxicity is due to actions on opioid systems in the CNS. The opioid type of swim-stress induced antinociception is mediated via μ-sites in preweanling rats and predominantly by δ-sites in postweanling animals. In this study the effect of delay of weaning on the receptor transition of this behaviour in the developing rat has been investigated. 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 (20C) swimming for 3min periods and antinociception assessed by the tail immersion test (50C). Naloxone (10mg/kg) partially reversed swim-SIA in both 25 day old weaned and non-weaned rats. Naltrindole (1 mg/kg) partially reversed swim-SIA in 25 day old weaned rats but had no effect in non-weaned animals. Naltrindole (5mg/kg) completely abolished swim-SIA in weaned rats but was without effect in non-weaned groups. Antinociceptive responses to the μ-agonist alfentanil (60mg/kg) were unaffected by naltrindole at 1mg/kg but were partially reversed at 5mg/kg. In 30 day old non-weaned rats naltrindole (5mg/kg) abolished the swim-SIA. These findings indicate that transition from μ- to δ-receptor control of swim-SIA in rat pups can be delayed by between 5 to 10 days by delay of weaning and the environmental stimulus of weaning can activate opioid receptor subtype operation of biological responses in the developing animal. The stimulus of weaning appeared to be the critical element necessary for the transition from μ- to δ-receptor control of swim-SIA in rat pups. To study whether weaning stimulates the development of a subtype of the δ-opioid receptor, the antinociceptive activity of putative agonists for δ1 (DELT I) and for δ2, (DSLET) receptors in weaned and non-weaned 25 day old rats was investigated. In weaned rats, DSLET produced antinociception which was reversed by the δ2-antagonist, naltriben, but in the non-weaned rats DSLET had no effect. In contrast to these findings, dose related antinociceptive responses to DELT I were evident in both weaned and non-weaned rats. These responses were not antagonised by naltriben. The results indicate that the δ1 and δ2- opioid receptor subtypes develop differentially and independently and suggests that weaning is the stimulus for the expression of the δ2-opioid receptor subtype.