Bilateral naloxone injection into the “tachypneic area” of the parabrachial nucleus (tPBN) significantly reversed IV remifentanil-induced respiratory rate depression in young and adult rabbits. Residual changes from control were completely reversed with IV naloxone injections. (A) Phrenic neurogram (PNG; a.u. [arbitrary units]) tracing during control conditions and after drug injections in an individual adult rabbit. (B) Summary data for changes in respiratory rate. (C) Summary data for changes in other respiratory parameters. Gray bars: young (n = 12). White bars: adult (n = 12). Data show mean ± SD. *P < 0.05 and ***P < 0.001 indicate significant differences between drug application conditions (linear mixed model, factors: drug application and developmental age). There were no differences in drug effects between young and adult animals.