Fig. 6.

Chronic fentanyl exposure led to tolerance to acute heroin and fentanyl through shortened duration of effect on minute ventilation. Rats, under continuous exposure to saline (n = 10 females, 12 males) or fentanyl (n = 11 females, 10 males), received an i.v. injection of heroin (600 µg/kg) or fentanyl (25 µg/kg) during two tests 24 hour apart. Heroin elicited decreases in minute ventilation that were longer-lasting in the saline-exposed group, with no significant difference between males and females (A). Maximal decreases in minute ventilation was similar across chronic drug treatment and sex (B). Fentanyl elicited decreases in minute ventilation that were longer lasting in the saline-exposed group and were similar in female and male rats. (C). As with heroin, maximal decreases in minute ventilation from fentanyl were similar across chronic drug treatment and sex (D). Data are expressed as mean ± S.D. in percentage of baseline in 5-minute bins over −20 to 60 minutes postadministration (A and C) or as the lowest value within 25 minutes postadministration (B and E). Dashed line indicates average baseline measure. *P < 0.05, from saline-treated group. ♀, female; ♂, male.