Fig. 2. Preadministration of CX717 (intraperitoneal [IP]) prevents respiratory depression induced by fentanyl in postnatal day (P17)–18 rats. (  A ) Preadministration of the vehicle HPCD (2-hydroxypropyl-b-cyclodextrin) solution (intraperitoneal) had no effect on the fentanyl-induced suppression of respiratory frequency in a P17 rat. (  B ) In another P17 rat, administration of CX717 (15 mg/kg, intraperitoneal) 2 min before the fentanyl administration markedly alleviated depression of respiratory frequency. (  C ) Population data showing the time course of changes of respiratory frequency in response to fentanyl administration after preadministration of vehicle or CX717. *  P < 0.05 relative to vehicle control respiratory frequency (n = 9 for each group).