Abstract
Newborn rats were injected (ip) with morphiceptin [72.7 micrograms/kg (a mu-type opioid receptor agonist)], D-alanine2-D-leucine5-enkephalin [79.4 micrograms/kg (a delta-receptor agonist)], or saline for 7 days after birth. Testing on a complex maze on Day 25 revealed a significant sex-dependent facilitation of performance by the opioid peptides. Peptide-treated females performed better than the males on the first day of training as measured by errors. Opioid treatment increased mortality, as three times as many peptide-treated animals died in comparison to the saline control group. Peptide treatment did not affect locomotor activity measured in an open field. Weight at Day 24 was also affected by the peptide treatment, females and males injected with the opioids being lighter and heavier, respectively, than the control group.
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