Tolerance development to morphine in prenatally morphine-exposed male rats. (A) The latency and AUC of rats after receiving first injection of morphine. (B) The latency and AUC of rats after receiving 7th injection of morphine. (C) Rate of tolerance development to morphine in morphine or saline prenatally exposed rats. The animals more quickly developed a tolerance to morphine than the prenatally saline-exposed controls (F(1, 114) = 4.333, p < 0.05). All data are expressed as mean ± S.E.M, (N = 19 per group), *p < 0.05, ***p < 0.001 compared to saline control.