The effect of DAMGO injections on the respiratory pattern varies between subareas of the ventral respiratory column in adult rabbits

Abstract

We have previously described how sequential microinjections of high concentrations of the mu‐opioid agonist [D‐Ala2, N‐MePhe4, Gly‐ol]‐enkephalin (DAMGO) into the ventral respiratory column (VRC) depressed ventilation with rather variable changes in respiratory pattern in the adult, in vivo, decerebrate rabbit model (1). This study aimed to distinguish whether DAMGO injections into subareas of the VRC that were functionally identified by their differential respiratory pattern response to local microinjection of α‐Amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) resulted in different effects on the respiratory pattern. The study was approved by the local Animal Care Committee and conformed to NIH standards. Adult New Zealand White rabbits (3–4 kg) were anesthetized, tracheotomized, ventilated, decerebrated and vagotomized. Nerve activity was recorded from the c5 rootlet of the phrenic nerve, time averaged and used to calculate inspiratory and expiratory durations. Gridwise localized pressure injections of AMPA (50 μM, 70 nl) utilizing a multibarrel glass pipette with a tip diameter of 20μm into the VRC rostral of obex identified two areas with distinctly different pattern changes: An area (on average 2mm rostral from obex, 2.5mm lateral from midline, 4.5mm ventral to the dorsal surface) where AMPA injection caused significant tachycardia with decreased peak phrenic activity (PPA) and a second area (on average 1mm rostral, 2 mm lateral, 3.5mm ventral) where AMPA caused less tachycardia and an increase in PPA. In twelve animals, DAMGO injection (100μM, 700nl) into the more rostral area caused a decrease in inspiratory duration (from 1.2±0.1sec to 0.9±0.2sec, mean±SE) while injection into the more caudal area caused a small increase (from 1.3±0.1sec to 1.4±0.1sec). These effects were significantly different (p<0.001, 2‐way ANOVA, factors: location, drug). DAMGO injection increased expiratory duration similarly in both areas (from 1.1±0.2 sec to 1.9±0.4sec and from 1.5±0.3sec to 2.2±0.3sec, difference between areas not significant). This resulted in no change in respiratory rate in the rostral area but a decrease in rate in the caudal area (from 25±3 bpm to 20±3bpm, difference between areas p<0.001). PPA was similarly decreased in both areas (from 100% to 81±7% and from 100% to 95±2%, difference not significant). While the rostral area in location and AMPA response may match the preBötzinger area as described, e.g., by Marchenko (2), respiratory pattern can also be affected in more caudal subareas of the VRC. We suggest that glutamate agonists can elicit tachypnea in various subareas of the VRC and that injection of DAMGO into these individual subareas can have different effects on the respiratory pattern. This may be an explanation for diverging results of DAMGO injection between different studies using glutamate agonists for functional identification of their target.Support or Funding InformationNIH R01GM112960‐01A1 (Dr. Stucke) and VA merit grant I01BX000721 (Dr. Zuperku).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.