The effects of morphine at different embryonic ages on memory consolidation and rewarding properties of morphine in day-old chicks

Prenatal exposure to morphine can alter the capacities for learning and memory and the sensitivity to drugs of abuse in progeny. In the present study, we examined the effects of morphine during chick embryonic period of 5-8, 9-12, 13-16 and 17-20 on cognitive function and the sensitivities to morphine reward in the post-hatch chick, using the one-trial passive avoidance learning task and the conditioned place preference paradigm. It was observed that the injection of morphine (1 mg/kg of egg weight) during E5-8, but not in other three periods, significantly impaired intermediate- and long-term memory in one-day-old chicks. On the other hand, the chicks prenatally exposed to morphine during E17-20 remarkably not only acquired but also maintained the conditioned place preference induced by morphine. The present results suggest that there are two time-windows during development, which in the chick are around E5-8 and E17-20, when prenatal morphine exposure is likely to confer maximal risks for vulnerabilities to breakdown of memory consolidation and to morphine-induced reward in day-old chicks respectively.