Ml206

All cardiorespiratory variables were sampled continuously at 1 kHz via an analog‐to‐digital converter (Powerlab 16/30, ADInstruments Ltd, Oxford, UK) during all resting gCBF and CVRCO2 ultrasound scans. Mean arterial blood pressure (MAP) and heartrate (HR) were measured by finger photoplethysmography (Finometer PRO, Finapres Medical Systems, Amsterdam, The Netherlands). The partial pressure of end‐tidal carbon dioxide (PETCO2) and oxygen (PETO2) were sampled via insertion of a sample line into a mouthpiece worn by the participant that connected in series to a bacteriological filter and a calibrated gas analyser (ML206, ADInstruments). All data were interfaced with LabChart (version 8, ADInstruments).

Whole-body plethysmography was used to record respiratory activity in unrestrained conscious adult rats^{44 (link)–46 (link)}. Briefly, 5–7 days after the injections of viral vectors the rats were placed in a Plexiglas recording chamber (1 l) that was flushed continuously with humidified air (21% O₂, 79% N₂; temperature 22–24 °C), at a rate of 1.2 l min⁻¹. In order to take into the account circadian variations of the physiological parameters, respiratory activity in all the animals was assessed at the same time of the day (between 1100 and 1500 hours). The animals were allowed to acclimatize to the chamber environment for ~60 min followed by 30 min recording period of resting respiratory activity. For the experiments involving hypoxic challenges, the O₂ concentration in the inspired air was reduced to 10% (balanced with N₂) for 10 min. In a separate series of experiments, hypercapnia was induced by stepwise increases in CO₂ concentration in the respiratory gas mixture to 3% and 6% in hyperoxic environment (>50% O₂, balanced with N₂) to reduce the drive from the peripheral chemoreceptors. Each CO₂ concentration was maintained for 5 min. Concentrations of O₂ and CO₂ in the plethysmography chamber were monitored online using a fast-response O₂/CO₂ analyzer (ML206, AD Instruments). Data were acquired using Power1401 interface and analyzed offline using Spike2 software (CED).

Participants were equipped with a three-lead electrocardiogram for measurement of R–R interval and heart rate. CBF was indexed using transcranial Doppler ultrasound (ST3, Spencer Technologies, Seattle, WA, USA) to record blood velocity in the middle (MCAv) cerebral artery on either the right or the left side. After the vessel was identified and signals optimized according to depth, waveform, and velocity, the ultrasound probes were locked in place with a fitted head frame (Spencer Technologies, Seattle, WA, USA). Beat-to-beat BP was recorded with finger photoplethysmography (Finometer PRO, Finapres Medical Systems, Amsterdam, The Netherlands), and partial pressure of expired carbon dioxide (P_ETCO2) was monitored using an online gas analyzer (ML206, AD Instruments, Colorado Springs, CO, USA). All data were sampled at 1,000 Hz (PowerLab 8/30 ML880, AD Instruments) and stored for offline analysis using commercially available software (LabChart version 7.1, AD Instruments).