Differential effects of prenatal morphine exposure on analgesia produced by vaginocervical stimulation or systemic morphine administration in adult rats

The present study investigated the effects of prenatal morphine exposure on the magnitude of analgesia produced by vaginocervical stimulation (VS) or systemic morphine injection in adult rats. In Experiment 1, an acute subcutaneous morphine (1 mg/kg) injection induced a 124% greater increase in tail-flick latency (TFL) in adult rats exposed prenatally to saline than to morphine. By contrast, in Experiment 2, VS induced a 196% greater increase in TFL in adult rats exposed prenatally to morphine than to saline. Female rats exposed prenatally to morphine also had a greater VS-produced increase in vocalization threshold (VOC-T) to tail shock than those exposed prenatally to saline. Thus, the present study demonstrates that prenatal morphine exposure exerts diametrically opposite effects on analgesia that is produced in adulthood by morphine or VS, attenuating the former while potentiating the latter. These findings provide evidence that the mechanisms underlying the two types of analgesia differ fundamentally.