Ablation of neuromedin B (NMB)‐expressing neurons located within retrotrapezoid nucleus (RTN) reduces the central respiratory chemoreflex (CRC) selectively in conscious rats

RTN senses PCO 2 and regulates breathing in a CO 2 ‐dependent manner but the full extent of RTN's contribution to the CRC is not known. Lesions produced by microinjecting a substance‐P analog conjugated with saporin (SSP‐SAP) elicit small and/or transient CRC reductions in rats. Interpretative limitations are many: development of compensatory mechanisms, especially of carotid body origin, difficulty to identify the critical neurons and to lesion them sufficiently. As shown recently, neuromedin B ( Nmb ) seems to be a selective marker of the Phox2b +/ Vglut2 + RTN neurons that contribute to the CRC. In the present study, we microinjected saline or SSPSAP (3 injections per side; ng/injection: 0.6, 1.2 or 2.4 ng) to destroy RTN in ~250 g Sprague‐Dawley male rats. Physiological experiments were conducted three weeks later in unanesthetized animals, following which we counted the surviving Nmb+ neurons using fluorescent ISH (FISH) in a 1/6 series of sections. SSP‐lesioned rats were regrouped post hoc into two clusters according to the number of surviving RTN Nmb+ neurons: mild lesion group (ML rats; N= 13; > 55 Nmb + cells) and large lesion group (LL rats; N=11; 4–55 Nmb + cells remaining). Control rats (C rats; N= 11) had 324 ± 48 Nmb + neurons (these and all subsequent values represent mean ± SD). Under normoxia, arterial pH was virtually the same in all groups (LL rats: 7.46 ± 0.03; ML: 7.48 ± 0.01; C: 7.49 ± 0.01). PaO 2 was lower in LL rats than in C and ML animals (71 ± 7 vs. 80 ± 6 vs. 78 ± 6 mmHg) but PaCO 2 was higher (49 ± 7 vs. 38 ± 3 vs. 39 ± 5 mmHg). BP measured via arterial catheter was the same in all groups. Under normoxia, LL rats had a reduced V T relative to C and ML rats during quiet waking (0.39 ± 0.1 vs. 0.49 ± 0.07 vs. 0.54 ± 0.1 mL/100g), SW sleep (0.32 ± 0.08 vs. 0.45 ± 0.06 vs. 0.50 ± 0.1 mL/100g) and REM sleep (0.25 ± 0.06 vs. 0.35±0.05 vs. 0.36 ± 0.1 mL/100g). Short‐term hyperoxia (1 min of 65% FiO 2 , balance N 2 ) caused a larger decrease in V E in LL and in ML rats compared to C (ΔV E : −11 ± 3 vs. −13 ± 6 vs. −8 ± 3 mL/100g/min). Of note, the hypoventilation still present after 20 min of continuous hyperoxia and was larger in LL than in ML and C rats (ΔV E : −9 ± 6 vs. −5 ± 10 vs. 0.9 ± 4 mL/100g/min). When exposed to 6 % FiCO 2 in 65% FiO 2 LL rats had a greatly reduced breathing stimulation (hypercapnic ventilatory reflex, HCVR) compared to C and ML rats (ΔV E : 17.8 ± 9 vs. 63.6 ± 12 vs. 49.5 ± 21 mL/100g/min). The HCVR to 9 % FiCO 2 in 65% FiO 2 was reduced in similar proportion (ΔV E : 25.6 ± 9 vs. 85.9 ± 14 vs. 76.3 ± 27 mL/100g/min). Hypoxia (10% FiO 2 , balance N 2 ) activated V E similarly in all three groups (ΔV E : 34.1 ± 6 vs. 27.2 ± 8 vs. 28.2 ± 9 in mL/100g/min for LL, C and ML rats respectively) although ΔV T was enhanced and ΔF R reduced in LL rats. In conclusion, >80% RTN Nmb+ neurons need to be destroyed to produce a notable reduction of the HCVR. Large RTN lesions reduce the CRC by ≥ 71% without decreasing the hypoxic ventilatory reflex. Large RTN lesions raise the arterial PCO 2 homeostatic set‐point by ≥10 mmHg and lower PaO 2 by ~9 mmHg but have little influence on long‐term arterial pH. Finally, the loss of breathing stimulation contributed by RTN is largely compensated by a permanent state of hypoxia and increased carotid body activity. Support or Funding Information National Institutes of Health (Grants RO1 HL074011 and RO1 HL 028785 to P.G.G.) This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .