Abstract
In a classical conditioning paradigm, postnatal day (PND)-12 rats show aversion to the odor that has been paired with foot shock in a 30-min training session on PND-11. Impaired olfactory learning, however, is observed in young rats with intrauterine growth restriction (IUGR) induced by bilateral occlusion of the utero-ovarian artery for 20 min on day 16 of pregnancy. Transient feto-placental ischemia evokes ischemia/reperfusion injury. It is well known that melatonin is a powerful scavenger of oxygen free radicals. Biochemical analyses demonstrated that the administration of melatonin to pregnant rats protects against ischemia/reperfusion-induced lipid and DNA damage in fetal brain as well as oxidative damage in the mitochondria of placentas. Therefore we examined whether melatonin administration to pregnant rats is capable of restoring aversive olfactory learning in young rats with IUGR. IUGR rats born of the mother rat that had received melatonin administration during the whole pregnancy period showed aversion to the conditioned odor. Aversive olfactory learning was normally observed in non-IUGR rats born of the sham-operated mother rat. Melatonin, when infused directly into the olfactory bulb during odor-shock stimulation, failed to restore olfactory learning in IUGR rats. These results suggest that melatonin administration to pregnant rats is not able to restore olfactory learning in young rats with IUGR, but can preserve learning ability from IUGR by protection against ischemia/reperfusion-induced oxidative damage. [Jpn J Physiol 54 Suppl:S218 (2004)]
