Effects of Different Systemic Opioid Doses on Subareas of the Ventral Respiratory Column
Authors: Palkovic Barbara , Callison Jennifer J. , Marchenko Vitaly , Stuth Eckehard A. E. , Zuperku Edward J. , Stucke Astrid G. 
Published: April 2020

Background

The Parabrachial Nucleus (PBN)/Kölliker‐Fuse (KF) complex are important subareas of the ventral respiratory column (VRC) that regulate inspiratory on‐ (
1) and off‐switch (
2). We have shown in vagus‐intact rabbits that respiratory rate depression caused by clinical opioid concentrations was in part mediated by the PBN (
3), and that systemic opioid effects on respiratory phase timing could be partially reversed in the preBötzinger complex (preBC) albeit without reversing respiratory rate depression (
4). The present study was to determine how much the combination of local microinjections of naloxone into the KF, PBN and preBC could reverse respiratory depression from intravenous remifentanil (REMI).

Methods

The study was approved by the local Animal Care Committee and conformed to NIH standards. Adult New Zealand White rabbits (3–4kg) were anesthetized, tracheotomized, ventilated, decerebrated and vagotomized. Phrenic nerve activity was recorded from the c5 rootlet, time averaged and used to calculate inspiratory and expiratory duration and peak phrenic activity (PPA). Gridwise localized pressure microinjections of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA, 50 μM, 70 nl) utilizing a multibarrel glass pipette into the VRC caudal to the inferior collicle identified the PBN where AMPA injection caused significant tachypnea and the KF where AMPA caused transient bradypnea and a decrease in PPA. The tachypneic response to AMPA injection in the medulla was used to identify the preBC area. After functional identification of the PBN, KF and preBC areas, we injected a bolus of 10mcg REMI IV, which resulted in apnea. On return of the breathing pattern, a REMI infusion was started to achieve a steady‐state respiratory rate depression of 50% of control rate. After reaching steady‐state, we injected naloxone (1mM, 700nl) into the bilateral KF, PBN and preBC. We then repeated the REMI bolus injection with the apneic dose of 10mcg. In a subset of animals, the 10 mcg REMI bolus was repeated after naloxone injection into the KF/PBN and after injection into the preBC. Residual respiratory depression was reversed at the end of the protocol with IV naloxone (300μM). Drug effects were compared with one‐way RM ANOVA. Data are expressed as mean±SD.

Results

In 11 animals, IV REMI depressed respiratory rate from 39±7 to 18±6 breaths per minute (BPM). Naloxone injection recovered respiratory rate to 20±9 in the KF (p=0.36), to 26±9 after additional injection into the PBN (p<0.001), and to 28±7 bpm after injection into the preBC area (p=0.54). The IV REMI bolus caused apnea >30s at control but only depressed respiratory rate to 12±6 bpm after naloxone injection into the KF/PBN (p<0.001) and to 16±7 after additional naloxone reversal in the preBC area (p=0.21, n=5).

Conclusion

Respiratory rate depression from IV opioids is mostly mediated by the PBN, however, higher opioid doses may also affect other brainstem areas.

Support or Funding Information

Supported by NIH R01GM112960701A1 (Dr. Stucke) and VA merit grant I01BX000721 (Dr. Zuperku)

Figure 1