Neurotransmitters and neuromodulators controlling the hypoxic respiratory response in anaesthetized cats

The contributions of neurotransmitters and neuromodulators to the responses of the respiratory network to acute hypoxia were analysed in anaesthetized cats.Samples of extracellular fluid were collected at 1–1.5 min time intervals by microdialysis in the medullary region of ventral respiratory group neurones and analysed for their content of glutamate, γ-aminobutyric acid (GABA), serotonin and adenosine by high performance liquid chromatography. Phrenic nerve activity was correlated with these measurements.Levels of glutamate and GABA increased transiently during early periods of hypoxia, coinciding with augmented phrenic nerve activity and then fell below control during central apnoea. Serotonin and adenosine increased slowly and steadily with onset of hypoxic depression of phrenic nerve activity.The possibility that serotonin contributes to hypoxic respiratory depression was tested by microinjecting the 5-HT-1A receptor agonist 8-OH-DPAT into the medullary region that is important for rhythmogenesis. Hypoxic activation of respiratory neurones and phrenic nerve activity were suppressed. Microinjections of NAN-190, a 5-HT-1A receptor blocker, enhanced hypoxic augmentation resulting in apneustic prolongation of inspiratory bursts.The results reveal a temporal sequence in the release of neurotransmitters and neuromodulators and suggest a specific role for each of them in the sequential development of hypoxic respiratory disturbances

Adenosine release in nucleus tractus solitarii does not appear to mediate hypoxia-induced respiratory depression in rats

The time course of adenosine release in the nucleus tractus solitarii (NTS) and ventrolateral medulla (VLM) during acute systemic hypoxia was investigated in the anaesthetised rat by means of amperometric enzymatic sensors. It was found that acute hypoxia induced a significant delayed increase in adenosine level (reaching levels as high as 5 μM) in the NTS and that hypoxia-induced release of adenosine was similar at various regions of the NTS along its rostro-caudal axis. Significantly smaller or no increases in adenosine levels at all in response to hypoxia were observed in the VLM. The increase in adenosine level in the NTS occurred during reoxygenation after the termination of the hypoxic challenge and was accompanied by a smaller increase in inosine concentration. At the dorsal surface of the brainstem, only release of inosine was detected following acute hypoxia. Addition of the ecto-5′-nucleotidase inhibitor α,β-methylene ADP (200 μM) to the dorsal surface of the brainstem completely abolished the signal evoked by hypoxia, suggesting that the inosine arose from adenosine that was produced in the extracellular space by the prior release of ATP. This study indicates that following systemic hypoxia, adenosine levels in the NTS increase to a significantly greater extent than in the VLM. However, the increase in adenosine concentration in the NTS occurs too late to be responsible for the hypoxia-induced depression of the respiratory activity

Effect of hypoxia on the hypopnoeic and apnoeic threshold for CO2 in sleeping humans

Rhythmic breathing during sleep requires that PCO2 be maintained above a sensitive hypocapnic apnoeic threshold. Hypoxia causes periodic breathing during sleep that can be prevented or eliminated with supplemental CO2. The purpose of this study was to determine the effect of hypoxia in changing the difference between the eupnoeic PCO2 and the PCO2 required to produce hypopnoea or apnoea (hypopnoea/apnoeic threshold) in sleeping humans.The effect of hypoxia on eupnoeic end-tidal partial pressure of CO2 (PET,CO2) and hypopnoea/apnoeic threshold PET,CO2 was examined in seven healthy, sleeping human subjects. A bilevel pressure support ventilator in a spontaneous mode was used to reduce PET,CO2 in small decrements by increasing the inspiratory pressure level by 2 cmH2O every 2 min until hypopnoea (failure to trigger the ventilator) or apnoea (no breathing effort) occurred. Multiple trials were performed during both normoxia and hypoxia (arterial O2 saturation, Sa,O2 = 80 %) in a random order. The hypopnoea/apnoeic threshold was determined by averaging PET,CO2 of the last three breaths prior to each hypopnoea or apnoea.Hypopnoeas and apnoeas were induced in all subjects during both normoxia and hypoxia. Hypoxia reduced the eupnoeic PET,CO2 compared to normoxia (42.4 ± 1.3 vs. 45.0 ± 1.1 mmHg, P < 0.001). However, no change was observed in either the hypopnoeic threshold PET,CO2 (42.1 ± 1.4 vs. 43.0 ± 1.2 mmHg, P > 0.05) or the apnoeic threshold PET,CO2 (41.3 ± 1.2 vs. 41.6 ± 1.0 mmHg, P > 0.05). Thus, the difference in PET,CO2 between the eupnoeic and threshold levels was much smaller during hypoxia than during normoxia (-0.2 ± 0.2 vs. -2.0 ± 0.3 mmHg, P < 0.01 for the hypopnoea threshold and -1.1 ± 0.2 vs. -3.4 ± 0.3 mmHg, P < 0.01 for the apnoeic threshold). We concluded that hypoxia causes a narrowing of the difference between the baseline PET,CO2 and the hypopnoea/apnoeic threshold PET,CO2, which could increase the likelihood of ventilatory instability