Met-enkephalin effects on associations formed in utero.

Rat fetuses exposed to an odor stimulus and an aversive stimulus in utero showed an aversion to the odor when tested 16 days postnatally. Fetuses that also received 80 micrograms/kg Met-enkephalin showed a greater aversion to the odor stimulus than those subjects that did not receive the peptide. The difference between these groups was marginally significant. Control subjects did not show an aversion. But, subjects exposed to the odor and Met-enkephalin without the aversive stimulus, when tested, showed a significant preference for the odor over other control groups. These data show that associative learning in rat fetuses at 20 days of gestation may be enhanced by administration of Met-enkephalin.