Vagal Feedback Obscures the Effects of Systemic Opioids on Respiratory Rate in the preBötzinger Complex in Young Rabbits in vivo
Authors: Stucke Astrid G. , Palkovic Barbara , Callison Jennifer J. , Marchenko Vitaliy , Stuth Eckehard A.E. , Zuperku Edward J. 
Published: April 2020

Introduction

We have previously shown in vagus‐intact young rabbits that respiratory rate depression from clinical dose‐rates of remifentanil was partially reversed by naloxone injection into the Parabrachial Nucleus (PBN)
(1). On the contrary, naloxone injection into the preBötzinger Complex (preBötC) changed the ratio between inspiratory (TI) and expiratory (TE) duration but did not reverse respiratory rate depression
(2). To determine whether the entrainment of the respiratory rhythm to the ventilator contributed to the observed lack of respiratory rate reversal, we repeated the study in vagotomized animals.

Methods

The study was approved by the local Animal Care Committee and conformed to NIH standards. Young New Zealand White rabbits (16–39d, 155–547g) were anesthetized, tracheotomized, ventilated, decerebrated and vagotomized. Phrenic nerve activity was recorded from the c5 rootlet, time averaged and used to calculate TI, TE and peak phrenic activity (PPA). Gridwise localized pressure microinjections of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA, 50 μM, 35 nl) utilizing a multibarrel glass pipette with a tip diameter of 20μm into the ventral respiratory column rostral to obex identified the preBötC where AMPA injection caused significant tachypnea. We infused remifentanil until respiratory rate was depressed to 50% of control. After reaching steady‐state, we microinjected naloxone (1mM, 350nl) into the bilateral preBötC and, once the effect had reached steady‐state, we injected naloxone (100mcM) intravenously. Data are provided as mean±SD. Comparison of drug effects within one group used 1‐way repeated‐measures ANOVA. Comparison of drug effects between the current study and an earlier study in non‐vagotomized animals used 2‐way ANOVA.

Results

In seven rabbits, remifentanil infusion reduced respiratory rate from 29±14 breaths per minute (bpm) to 15±8 bpm, which was due to an increase in TI from 0.9±0.2s to 1.4±0.4s and an increase in TE from 1.6±1.1s to 3.4±1.8s (all p<0.001). Peak phrenic activity (PPA) was not significantly decreased (78±23%). Naloxone injection into the preBötC increased respiratory rate to 21±11bpm (p=0.013) due to a decrease in TI to 1.1±0.4s (p=0.028) and TE to 2.3±1.4s (p=0.009). PPA did not change. The difference from our previous study where we did not observe a reversal of respiratory rate depression (11±6 to 15±9bpm, n=12, p=0.198) was not significant. However, there was a difference between the change in TI:TE ratio, which increased more in vagus‐intact animals (0.5 to 1.1 vs. 0.5 to 0.5, p=0.04).

Conclusion

Systemic opioids affect respiratory phase duration through action in the preBötC in young rabbits, however, the effect is obscured when respiratory rhythm is entrained to the ventilator in vagus‐intact animals.

Support or Funding Information

R01GM112960‐01A1 (Dr. Stucke) and VA merit grant I01BX000721 (Dr. Zuperku)