Evidence that weaning stimulates the developmental expression of a δ-opioid receptor subtype in the rat

To study whether weaning stimulates the development of a subtype of the δ-opioid receptor we have measured antinociceptive activity of putative δ 1 (DELT 1) and δ 2 (DSLET) agonists in weaned and non-weaned 25-day-old rats. In weaned rats, DSLET produced antinociception reversed by the δ 2-antagonist naltriben, but in non-weaned rats DSLET had no effect. Responses to DELT 1 were evident in both weaned and non-weaned rats and were not antagonised by naltriben. This is the first evidence that δ-receptor subtypes develop independently and suggests that weaning is the stimulus for the expression of the δ 2-subtype.