Oxytocin Receptor Activation Rescues Opioid-Induced Respiratory Depression by Systemic Fentanyl in the Rat

Abstract

Opioid overdose intervention by naloxone, a high affinity receptor antagonist, reverses opioid-induced respiratory depression (OIRD) and analgesia by displacing opioids. Systemic naloxone stimulates release of the hypothalamic neuropeptide oxytocin, which has analgesic properties and participates in cardiorespiratory homeostasis. To test the hypothesis that oxytocin can reverse OIRD, we assessed the rescue potential of graded doses (0, 0.1, 2, 5, 10, 50 nmol/kg, i.v.) of oxytocin to counter fentanyl (60 nmol/kg, i.v.)-induced depression of neural inspiration indexed by recording phrenic nerve activity (PNA) in anesthetized (urethane/α-chloralose), vagotomized, and artificially ventilated rats. Oxytocin dose-dependently rescued fentanyl OIRD by almost immediately reversing PNA burst arrest (P = 0.0057) and restoring baseline burst frequency (P = 0.0016) and amplitude (P = 0.0025) at low but not high doses, resulting in inverted bell-shaped dose-response curves. Oxytocin receptor antagonism (40 nmol/kg, i.v.) prevented oxytocin reversal of OIRD (arrest: P = 0.0066, frequency: P = 0.0207, amplitude: P = 0.0022). Vasopressin 1A receptor (V1aR) antagonism restored high-dose oxytocin efficacy to rescue OIRD (P = 0.0170 to P < 0.0001), resulting in classic sigmoidal dose-response curves, and prevented (P = 0.0135) transient hypertension from V1aR cross-activation (P = 0.0275). Alone, vasopressin (5 nmol/kg, i.v.) failed to reverse fentanyl respiratory arrest (P = 0.6184). The nonpeptide oxytocin receptor agonist WAY-267464 (75 nmol/kg, i.v.), which has V1aR antagonist properties, quickly reversed fentanyl OIRD (P < 0.0001), with rapid recovery of PNA frequency (P = 0.0011) and amplitude (P = 0.0044) without adverse hemodynamic consequences (P = 0.9991). Findings indicate that peptide and nonpeptide agonist activation of oxytocin receptors without V1aR cross-activation rescues fentanyl OIRD. Oxytocin receptor agonists could be lifesaving resuscitation agents that enhance rather than interrupt opioid analgesia. SIGNIFICANCE STATEMENT: Oxytocin receptor activation produces analgesia. Here, we demonstrate that activation by the US Food and Drug Administration-approved agonist oxytocin and the nonpeptide partial agonist WAY-267464 can each reverse fentanyl cardiorespiratory depression. Selective targeting of oxytocin receptors for resuscitation from opioid overdose, alone or in combination with an opioid antagonist, could eliminate or attenuate negative side effects associated with traditional opioid receptor antagonism.

Graphical abstract

Fig. 1.

Low-dose oxytocin rescues respiratory depression by fentanyl. (A) Representative integrated PNA (top) and burst frequency (bottom) responses to vehicle (VEH) and graded systemic doses (0.1, 2, 5, 10, 50, nmol/kg) of oxytocin (OT; i.v., n = 5–6/dose; gray-black triangles) after fentanyl (FENT; 60 nmol/kg, i.v.; red triangles/trace). Boxed panel denotes OT dose effective for reversal of FENT respiratory depression. a.u., arbitrary units. (B–D) Summary data for reversal of FENT action on PNA burst arrest (B), frequency (C), and amplitude (D) post-OT (black circles). Black triangles on the abscissa and ordinate indicate ED₅₀ values and maximum effect of OT (E_max), respectively. Symbols denote significance versus VEH (one-way ANOVA with Sidak post hoc test). *P = 0.0255 in (B); †P = 0.0012 for 5 nmol/kg and P = 0.0076 for 10 nmol/kg in (C); *P = 0.0451 in (D). Data in (B), (C), and (D) are expressed as means ± SEM for clarity.

Fig. 2.

Oxytocin dose-dependently rescues cardiodepression by fentanyl. (A) Representative blood pressure (MAP; superimposed white line) and heart rate (HR) responses to systemic fentanyl (FENT; 60 nmol/kg, i.v.; red triangle/trace) followed by vehicle (VEH) or graded systemic (intravenous) doses (0.1, 2, 5, 10, 50 nmol/kg; n = 5–6/dose) of oxytocin (OT; gray-black triangles). (B) MAP values at baseline (BL; black circles) and after FENT (red X’s) prior to each dose of OT with treatment difference from BL (Δ, gray circles) plotted with 95% confidence intervals (CI; right). (C) Summary of MAP responses to OT (black circles) post-FENT. (D) Summary of rate of MAP recovery during first 10 seconds post-OT (black circles). (E) HR values at BL (black circles) and after FENT (red X’s) prior to each dose of OT with treatment difference from BL (Δ, gray circles) plotted with 95% CI (right). (F) Summary of HR responses to OT (black circles) post-FENT. (G) Summary of rate of HR recovery during first 10 seconds post-OT (black circles). In (B–G): n = 31 rats. §P < 0.0001 between BL and post-FENT (paired two-tailed t test) in (B), (E); *P = 0.0191, †P = 0.0068, ‡P = 0.0006 versus BL (repeated measures two-way ANOVA with Sidak post hoc tests) in (C); *P = 0.0181, §P < 0.0001 versus VEH (one-way ANOVA with Sidak post hoc test) in (D). In (C) and (F), gray and red horizontal lines reference MAP values at BL and post-FENT, respectively. In (C) and (D), black triangles on the abscissa and ordinate indicate values of ED₅₀ and E_max, respectively. For analysis of dose-response curves in (C), (D), (F), and (G), data expressed as means ± SEM for clarity. BPM, beats per min.

Fig. 3.

Oxytocin reversal of fentanyl OIRD requires oxytocin receptors. Representative blood pressure (MAP; superimposed white line), PNA, and PNA frequency responses to oxytocin (OT; 5 nmol/kg, i.v.; dark gray triangle/trace) in the absence (A) and presence (B) of OT receptor antagonist atosiban (40 nmol/kg, i.v.; n = 5; blue triangle/trace) post-fentanyl (FENT; 60 nmol/kg, i.v.; red triangle/trace). (C) Summary data for reversal of FENT action on PNA burst arrest, frequency, and amplitude in the absence (black circles) or presence (blue triangles) of atosiban. (D) Summary data for MAP response at baseline (BL; light gray) and post-atosiban (blue), post-FENT (red), and post-OT (dark gray) in the absence (left/circles) or presence (right/triangles) of OT receptor blockade. In (C), symbols denote significance of 5 nmol/kg OT alone versus 5 nmol/kg OT + atosiban (unpaired two-tailed t test) for time to recovery of PNA burst arrest (*P = 0.0207), frequency (†P = 0.0066) and amplitude (†P = 0.0022). In (D), red † denotes P = 0.0090 (−Atosiban) and P = 0.0074 (+Atosiban) versus FENT, blue ‡ denotes P = 0.0007 versus +Atosiban, black * denotes P = 0.0351 versus BL; ns, not significant (repeated measures two-way ANOVA with Sidak post hoc test). a.u., arbitrary units.

Fig. 4.

Vasopressin V1aR blockade restores rescuing actions of high-dose oxytocin. (A) Representative blood pressure (MAP; superimposed white line), PNA, and burst frequency responses to vasopressin (VP; 1 nmol/kg, i.v.; n = 4; black triangle/trace) before and during VP V1a receptor antagonism (V1aRX; 0.4 nmol/kg, i.v.; orange triangle/trace). (B, C) Representative MAP, PNA, and burst frequency responses to oxytocin (OT) doses of 5 nmol/kg (n = 6, gray triangle/trace) (B) and 50 nmol/kg (n = 4, black triangle/trace) (C) after fentanyl (FENT) in the presence of V1aRX (orange triangle/trace). (D–F) Summary data for time to reversal of FENT action on PNA burst arrest (D), frequency (E), and amplitude (F) post-OT in the absence (black circles) and presence of V1aRX (orange triangles). (G, H) Summary data for MAP response (G) and rate of MAP recovery (H) post-OT in the absence (black circles) or presence of V1aRX (orange triangles). In (D)–(F), black symbols denote significance versus vehicle (one-way ANOVA with Sidak post hoc test) for V1aRX + 5 nmol/kg OT (PNA burst arrest: *P = 0.0221, frequency: ‡P = 0.0002, amplitude: ‡P = 0.0004) and V1aRX + 50 nmol/kg OT (PNA burst arrest: *P = 0.0414, frequency: ‡P = 0.0005, amplitude: †P = 0.0011). Orange symbols denote significance (two-way ANOVA with Sidak post hoc tests) of 50 nmol/kg OT versus V1aRX + 50 nmol/kg OT (PNA burst arrest: §P < 0.0001, frequency: §P < 0.0001, amplitude: †P = 0.0011). In (G), gray and red horizontal lines reference MAP values at baseline and post-FENT, respectively. In (G) and (H), *P = 0.0275 (G) and P = 0.0204 (H) comparing 50 nmol/kg OT alone versus baseline. In (G) and (H), orange * and † denote P = 0.0135 and P = 0.0013, respectively, comparing 50 nmol/kg OT alone versus 50 nmol/kg OT with V1aRX. Data analyzed by repeated measures three-way (G) and two-way (H) ANOVA, respectively, each with Sidak post hoc tests. Red downward arrows indicate change in dose-response curve by V1aR blockade. Black triangles on the abscissa and ordinate in (D)–(G) indicate ED₅₀ values and maximum effect of V1aRX + OT (E_max), respectively. Dose-response data expressed as means ± SEM. a.u., arbitrary units.

Fig. 5.

Vasopressin does not reverse fentanyl OIRD. Representative integrated PNA and PNA frequency and blood pressure (MAP; superimposed white line) responses to vehicle (VEH, i.v., n = 5, gray triangle/trace) (A) and vasopressin (VP, 5 nmol/kg, i.v., n = 4, black triangle/trace) (B) post-fentanyl (FENT; 60 nmol/kg, i.v.; red triangle/trace). (C) Summary data for time to reversal of FENT action on PNA burst arrest, frequency, and amplitude post-VEH (gray circles) or VP (black squares). (D) Summary data for MAP response and rate of MAP recovery over 10 seconds after VEH (gray circles) and VP (black squares) given post-FENT. Gray and red horizontal lines (left) reference baseline and post-FENT values of MAP. In (C), ns, not significant (unpaired two-tailed t test). In (D), *P = 0.0156, ‡P = 0.0004 (left) versus baseline; §P < 0.0001 versus VEH (repeated measures two-way ANOVA with Sidak post hoc test). For rate of MAP recovery (right), ‡P = 0.0002 versus VEH (unpaired two-tailed t test). a.u., arbitrary units.

Fig. 6.

Nonpeptide WAY-267464 reversal of fentanyl OIRD. Representative blood pressure (MAP; superimposed white line), PNA, and PNA frequency responses to vehicle (VEH; i.v., n = 5; gray triangle/trace) (A) and WAY-267464 (WAY; 75 nmol/kg, i.v.; n = 5; black triangle/trace) (B) post-fentanyl (FENT; 60 nmol/kg, i.v., red triangle/trace). (C) Summary data for reversal of FENT action on PNA burst arrest, frequency, and amplitude post-VEH (gray circles) or WAY (black circles). (D) Summary data for MAP response and rate of MAP recovery over 10 seconds after FENT by VEH (gray circles) or WAY (black circles). Gray and red horizontal lines (left) reference baseline and post-FENT values of MAP. In (C), symbols denote significance versus VEH (unpaired two-tailed t test) for PNA burst arrest (§P < 0.0001), frequency (†P = 0.0011), and amplitude (†P = 0.0044). In (D), *P = 0.0399. a.u., arbitrary units.