Effects of α4β2 and α7 nicotinic acetylcholine receptor targeting agents on fentanyl-induced respiratory depression in newborn rats. (A–F) Representative whole-body plethysmographic recordings from six postnatal day 3 pups. All drugs tested were administered subcutaneously (sc) in the posterior neck region. Administration of fentanyl (35 μg/kg, subcutaneously, A–D) or coadministration with DHβE (6 mg/kg, subcutaneously, E and F) caused a marked depression of respiratory frequency and a mild depression of tidal volume within 7 min after fentanyl administration. Subsequent administration of saline had no effect on fentanyl-induced respiratory depression (A). Nicotine (0.6 mg/kg, subcutaneously, B) and A85380 (0.06 mg/kg, subcutaneously, C), but not PNU282987 (20 mg/kg, subcutaneously, D) reversed fentanyl-induced respiratory depression. However, neither nicotine (0.6 mg/kg, subcutaneously, E) nor A85380 (0.06 mg/kg, subcutaneously, F) had any effect on respiratory depression induced by fentanyl coadministrated with DHβE (6 mg/kg, subcutaneously). Respiratory variables were presented and measured before fentanyl administration (top traces), 7 min after fentanyl (middle traces), and 12 min after fentanyl administration (bottom traces). (G–I) Population data (mean and SD) showing respiratory frequency (f_R), tidal volume (V_T), and minute ventilation (V_E) at 12 min after fentanyl (4 to 5 min after administration of nicotinic acetylcholine receptor agonists and antagonists). Respiratory parameters were calculated over an average of 1 min of continuous recordings relative to control before fentanyl administration. *P < 0.05, **P < 0.01, ***P < 0.001, statistically significant difference; ns: P > 0.05, no significant difference in compared groups, using two-way repeated measures ANOVA (Holm–Sidak method). DHβE, Dihydro-beta-erythroidine hydrobromide; HPCD, 2-hydroxypropyl-β-cyclodextran; PNU, PNU282987.