Previous reports suggested that some neurones located in the rostral ventrolateral medulla (RVL) can act as fast
oxygen sensors which enhance the sympathetic activity and blood pressure independent of peripheral chemoreceptors. The aim of this study was to compare hypoxic responses of different subpopulations of RVL neurones to ascertain whether the hypoxic sensitivity is restricted to one group of these neurones. Whole-cell patch-clamp recordings were made from acutely dissociated neurones obtained from RVL of P13-P19 rats. Short-lasting
hypoxia (1-2 min) was evoked by pressure injection of NaCN or lowering pO2. Cells projecting to the upper thoracic segments were retrogradely labelled with fluorescent beads. Catecholaminergic (CA) or non-catecholaminergic (non-CA) neurones were identified using single-cell reverse-transcription polymerase chain reaction (RT-PCR) or immunocytochemistry. Recordings were made from 38 neurones (26 spinally-projecting, 12 non-spinal) using Cs+/
TEA or K+-containing pipettes. In most of the cells tested with slow depolarising ramp commands (78%; including spinally-projecting and non-spinal neurones, as well as CA and non-CA neurones), NaCN or
hypoxia evoked a reversible increase of the sustained inward current. Extracellular application of 1 mM Co2+ or 25 nM TTX revealed three components of the
hypoxia-sensitive inward current which resembled the persistent
sodium (
INaP), low threshold
calcium (LVA Ca2+) and high threshold
calcium (HVA Ca2+) currents. The NaCN or
hypoxia induced increase of the current could also be observed during step commands. Recordings with K+-containing pipettes during similar depolarising ramps revealed, in addition, a reversible increase of IK in 78% of tested cells (in all four types of examined neurones). These results are consistent with the concepts that RVL neurones can act as a central
oxygen sensor. However, in contrast to the previously published data demonstrating that in
pentobarbital anaesthetised rats only the barosensitive and spinally projecting cells were affected by a short-lasting
hypoxia, our findings obtained with dissociated RVL neurones indicate that sensitivity to
hypoxia is widely distributed within this part of the medulla oblongata.