Abstract
Recently we showed that delay of weaning can prevent the transition from μ- to δ- opioid receptor control of stress-induced antinociception between postnatal day 20 and day 25 (1). We hypothesised that weaning might be the stimulus for the developmental expression of a subtype of the δ-receptor since 25% of δ-receptors develop in the fourth postnatal week (2) and have therefore studied the antinociceptive activity of putative selective δ 1 and δ 2 -agonists in weaned and non-weaned 25 day old rats. Antinociceptive responses were measured after i.p. administration of [D-Ser 2 , Leu 5 , Th 6 ]-enkephalin (DSLET) (δ 2 ) and [D-Ala 2 ]-deltorphin I (DELT I) (δ 1 )
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