Breathing and temperature control disrupted by morphine and stabilized by clonidine in neonatal rats

Abstract

Background: Sedative-analgesics are often given to newborn infants and are known to affect many components of the autonomic nervous system. While morphine is most frequently used, α-2 adrenergic receptor agonists are being increasingly used in this population. Alpha-2 adrenergic receptors agonists also have anti-shivering properties which may make it a desirable drug to give to infants undergoing therapeutic hypothermia. The aim of this study was to systematically compare two different classes of sedative-analgesics, morphine, a μ-opioid receptor agonist, and clonidine an α-2 adrenergic receptor agonist on breathing, metabolism and core body temperature (CBT) in neonatal rodents.

Methods: Breathing parameters, oxygen consumption (VO2) and carbon dioxide production (VCO2), were measured prior to, 10 and 90 min after intraperitoneal (IP) administration of morphine (2, 10 or 20 mg/kg), clonidine (40, 200 or 400 μg/kg), or saline in Sprague-Dawley rat pups at postnatal day 7 (p7) while continuously monitoring CBT.

Results: Morphine reduced the respiratory rate, VO2 and VCO2 greater than clonidine at all dosages used (p<0.05, morphine vs. clonidine, for all metabolic and respiratory parameters). Furthermore, morphine induced prolonged respiratory pauses, which were not observed in animals treated with clonidine or saline. Morphine caused hypothermia which was dose dependent, while clonidine stabilized CBT in comparison to saline treated animals (p<0.0001).

Conclusion: In the newborn rat, morphine causes profound respiratory depression and hypothermia while clonidine causes minimal respiratory depression and stabilizes CBT. All together, we suggest that clonidine promotes autonomic stability and may be a desirable agent to use in infants being treated with therapeutic hypothermia.

Keywords: Clonidine; Hypothermia; Metabolism; Morphine; Respiratory depression; Sedative-analgesics.

Figure 1. Representative breathing tracings of animals treated with saline, morphine and clonidine measured at baseline, 10 and 90 mins after drug administration

Both morphine (2 mg/kg) and clonidine (40 μg/kg) decreased the RR when compared to animals treated with saline at similar time points. Morphine markedly affected the pattern of breathing with intermittent periods of prolonged pauses. Representative traces from 3 animals. RR, Respiratory Rate.

Figure 2. Bar graphs representing the RR

at (a) 10 minutes and (b) at 90 minutes after drug administration as a percent change from baseline. MOR_{2, 10, 20} (mg/kg) significantly reduced RR by 45% (77 ±17 breaths/min), 85% (28 ±1.2 breaths/min) and 90% (23±1.9 breaths/min), respectively at 90 mins. In contrast CLON_{200, 400} (μg/kg) reduced RR by only 35% (120 ±9 breaths/min) and 45% (91 ±2.6 breaths/min) respectively at 90 mins. p<0.05, 2-way-ANOVA. (n=20/treatment group). RR, Respiratory Rate.

Figure 3. Bar graphs showing the effect of different dosages of treatment on (a) T_i and (b) IBI

in p7 rat pups with MOR_{2, 10} and CLON_{40, 200}. IBI was prolonged in MOR₂ pups by 90 mins and in MOR₁₀ pups at both10 and 90 mins, p<0.001, vs. saline pups. CLON₄₀ pups only transiently increased IBI at 10 mins, p<0.01. Ti, inspiratory time; IBI, inter-breath interval.

Figure 4. Box and whisker plots showing the variability of T_i and IBI

at 10 and 90 mins after drug administration at low and moderate doses. IBI is more significantly affected than T_i. (a) & (b) T_i is prolonged from baseline in MOR₂ & MOR₁₀, p< 0.01 vs. baseline; only transiently prolonged in CLON₂₀₀ pups, p<0.01 vs. baseline. (c) & (d) IBI is prolonged in morphine treated pups, the greatest variability seen in MOR₁₀ pups with a median of 2.5s and range of 0.3–11s. The values for MOR₂ (median 0.45, range 0.1–5s) CLON₄₀ (median 0.3, range 0.1–1.0s) and CLON₂₀₀ (median 0.4, range 0.2–1.5s) were much less variable. p< 0.01 vs. baseline. Ti, inspiratory time; IBI, inter-breath interval.

Figure 5. Breathing traces of p7 rat pup treated with morphine and p7 rat pup exposed to hypoxia

(a) Normal breathing pattern prior to morphine administration (b) 10 minutes after morphine administration demonstrating prolonged IBI with characteristic respiratory pauses (c) Eupneic breathing pattern prior to hypoxia and (d) gasping pattern (rapid inspiratory rise with a retarded expiratory phase) induced by hypoxia.

Figure 6. Line graphs showing the effects of MOR and CLON on CBT

in p7 rat pups at (a) higher doses – MOR_{10, 20}, CLON _{200, 400}. Saline controls increased their CBT in comparison to treatment, p<0.0001, 2-way-ANOVA. Saline animals get progressively warmer by 2°C from baseline, p<0.0008, 1-way-ANOVA. MOR₂₀ induced hypothermia compared to CLON_{200, 400}, p<0.005, Bonferroni. (b) Bar graphs representing the change in CBT at 90 mins from baseline. At 90 mins saline controls are warmer than MOR_{2, 10, 20} & CLON_{40, 400}, p<0.05, 2-way-ANOVA. MOR₂₀ animals are cooler by 3°C from baseline, p<0.0001, 1-way ANOVA. (n=20/treatment group). CBT, Core Body Temperature.

Figure 7. Line graphs showing the effects of MOR and CLON on (a) VO₂ and (b) VCO₂

in p7 rat pups. MOR and CLON animals vs. saline controls have lower VO₂ and VCO₂, p<0.0001, 2-way-ANOVA. Saline pups increase VO₂ overtime, p<0.01. MOR₂₀ reduced VO₂ and VCO₂ in comparison to CLON₂₀₀, p<0.01, Bonferroni. Bar graphs showing the comparison of (c) VO₂ and (d) VCO₂ in the treatment groups at 90 mins of the experiment. (c) VO₂ in CLON₄₀₀, MOR₁₀ and MOR₂₀ treated pups were 50%, 80% and 90% lower than saline controls respectively (d) VCO₂ in CLON₄₀₀, MOR₁₀ and MOR₂₀ pups were 45%, 65% and 80% lower than in saline pups respectively. p<0.001, 2-way-ANOVA. (n=20/treatment group). VO₂, Oxygen consumption; VCO₂, carbon dioxide production.