Prenatal stress interferes with the expression of opioid systems in rats. The present study determined the effect of prenatal stress on the opioid-influenced component of exploratory behavior, defined as the difference between the behavior of vehicle-treated and naloxone-treated rats, in three novel situations previously shown to cause different degrees of arousal. Pregnant rats were stressed three times weekly on a random basis by noise and flashing lights. Experiments were performed on 60-70-day-old offspring (male and female) of control and stressed dams. Fifteen minutes after injection of vehicle or naloxone (1 mg/kg), the proportion of time spent in eight different behavioral parameters, including locomotion, rearing, sniffing, hole poking, pivoting, and grooming, was assessed during 4 min of exposure to an open field, either with or without prior exposure to a hole box. The magnitude of the depressant effect of naloxone on exploration depended on the nature of the environment, previous experienceof the animal in another situation, and the parameter of exploration assessed. The opioid-influenced component of locomotion and rearing was significantly reduced by prenatal stress, particularly in female rats. Further studies using a cross-fostering design are needed to assess the relative contributions of pre- and postnatal factors to the reduction of opioid activity in prenatally stressed rats. More specific opioid antagonists could be used to determine the nature of the opioid receptors involved.
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