Reversal of morphine-induced respiratory depression with the µ1-opioid receptor antagonist naloxonazine engenders excitation and instability of breathing

Abstract

The administration of opioid receptor antagonists is believed to overcome ventilatory depressant effects of opioids. Here we show that many ventilatory depressant effects of morphine are converted to excitatory responses after µ₁-opioid receptor blockade, and that these responses are accompanied by ventilatory instability. In this study, we report 1) ventilatory responses elicited by morphine (10 mg/kg, iv) and 2) ventilatory responses elicited by a subsequent hypoxic-hypercapnic (HH) gas challenge and return to room air in male Sprague Dawley rats pretreated with 1) vehicle, 2) the centrally acting selective µ₁-opioid receptor antagonist, naloxonazine (1.5 mg/kg, iv), or 3) the centrally acting (delta 1,2) δ_1,2-opioid receptor antagonist, naltrindole (1.5 mg/kg, iv). The morphine-induced decreases in frequency of breathing, peak inspiratory flow, peak expiratory flow, expiratory flow at 50% expired TV, inspiratory drive, and expiratory drive in vehicle-treated rats were converted to profound increases in naloxonazine-treated rats. Additionally, the adverse effects of morphine on expiratory delay and apneic pause were augmented in naloxonazine-treated rats, and administration of morphine increased ventilatory instability (i.e., noneupneic breathing index) in naloxonazine-treated rats, which was not due to increases in ventilatory drive. Subsequent exposure to a HH gas challenge elicited qualitatively similar responses in both groups, whereas the responses upon return to room air (e.g., frequency of breathing, inspiratory time, expiratory time, end expiratory pause, relaxation time, expiratory delay, and noneupneic breathing index) were substantially different in naloxonazine-treated versus vehicle-treated rats. The above mentioned effects of morphine were only marginally affected in naltrindole-treated rats. These novel data highlight the complicated effects that µ₁-opioid receptor antagonism exerts on the ventilatory effects of morphine.NEW & NOTEWORTHY This study shows that the systemic injection of morphine elicits a pronounced overshoot in ventilation in freely-moving Sprague Dawley rats pretreated with the centrally-acting selective µ₁-opioid receptor antagonist, naloxonazine, but not with the centrally-acting δ_1,2-opioid receptor antagonist, naltrindole. This suggests that morphine can recruit a non-µ₁-opioid receptor system that promotes breathing.

Keywords: male Sprague Dawley rats; morphine; naloxonazine; ventilatory depression; ventilatory instability.

Figure 1.

Changes in frequency of breathing (Panel A), tidal volume (Panel B), and minute ventilation (Panel C) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels D, E and F, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 2.

Changes in frequency of breathing (Panel A), tidal volume (Panel B), and minute ventilation (Panel C) elicited by the injection of vehicle (VEH) or naltrindole (NLT, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of total changes in these parameters elicited by VEH or NLT, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels D, E and F, respectively. Data are shown as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. There were no between-group differences in the responses (P > 0.05, for all comparisons).

Figure 3.

Changes in inspiratory time (Ti) (Panel A), expiratory time (Panel B) and Te/Ti (Panel C) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels D, E and F, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 4.

Changes in end inspiratory pause (End Insp Pause, EIP) (Panel A) and end expiratory pause (End Exp Pause, EEP) (Panel B) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels C and D, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 5.

Changes in peak inspiratory flow (Peak Insp Flow, PIF) (Panel A), peak expiratory flow (Peak Exp Flow, PEF) (Panel B) and PEF/PIF (Panel C) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels D, E and F, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 6.

Changes in expiratory flow at 50% expired tidal volume (EF₅₀) (Panel A) and rate of achieving peak inspiratory flow (Rpef) (Panel B) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels C and D, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 7.

Changes in relaxation time (RT) (Panel A), expiratory delay (Te-RT) (Panel B) and apneic pause ((Te-RT) – 1, AP) (Panel C) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels D, E and F, respectively. The data are shown as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 8.

Changes in inspiratory drive (InspD) (Panel A) and expiratory drive (ExpD) (Panel B) elicited by the injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels C and D, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 9.

Changes in non-eupneic breathing index (NEBI) (Panel A) and NEBI corrected for frequency of breathing (NEBI/Freq) (Panel B) elicited by injection of vehicle (VEH) or naloxonazine (NLZ, 1.5 mg/kg, IV), followed by the injection of morphine (10 mg/kg, IV), and a subsequent hypoxic-hypercapnic challenge (air-flow OFF) and return to room-air (air-flow ON). Summaries of the total changes in these parameters elicited by VEH or NLZ, by morphine over the first 5 min (M5) and over the 75 min (M75) post-injection period, and by the hypoxic-hypercapnic (HH) challenge and return to room-air (RA), are shown in Panels C and D, respectively. The data are presented as mean ± SD. There were 6 rats in each group. The summary data were analyzed by repeated measures ANOVA followed by the Bonferroni test for multiple comparisons. *P < 0.05, significant response from Pre-values. ^†P < 0.05/5 comparisons, responses in NLZ-treated rats versus vehicle-treated rats.

Figure 10.

Naloxonazine (NLZ), the centrally-acting selective mu₁ (μ₁)-opioid receptor antagonist, binds to the mu₁-opioid receptor. However, NLZ alone does not alter ventilatory responses in Sprague Dawley male rats. Addition of morphine to the μ₁-opioid receptor bound with NLZ recruits a possible non-μ₁-opioid receptor protein signaling complex, which trigger downstream signaling events that promote breathing. Contrarily, when naltrindole (NLT), the centrally-acting delta_1,2 (δ_1,2)-opioid receptor antagonist, is bound to the δ_1,2-opioid receptor, the ventilatory depressant responses on breathing following the addition of morphine are observed and there is no recruitment of a non-μ₁-opioid receptor protein signaling complex that promotes breathing. Therefore, centrally-acting μ₁-opioid receptor antagonism with NLZ recruits a complicated protein signaling complex that exerts alterations on the ventilatory effects of morphine which are not seen with other selective opioid receptor antagonists, such as NLT.