Intrinsic properties of rostral ventrolateral medulla pre-sympathetic and bulbospinal respiratory neurons of juvenile rats are not affected by chronic intermittent hypoxia.

Exp Physiol. 2014 Apr 11. [Epub ahead of print] Almado CE1, Leao RM, Machado BH. This group has previously shown that rats exposed to chronic intermittent hypoxia (CIH) will exhibit hypertension which correlates with increased sympathetic nerve activity (SNA) during the late expiratory period of breathing. This suggested activation of augmenting expiratory (aug-E) neurons in the RVLM and botzinger complex which project to the phrenic nucleus in the spinal cord. That could either come from changes in cell signaling, or changes in intrinsic cell properties. In this paper they examined the latter of the two possibilities. They used brainstem slices to do patch clamp recordings of the cells in the RVLM and the BotC. To retrogradely label the RVLM cells of interest, they injected a fluorescent tracer in to the T3-T4 IML. To label the RVLM/BotC aug-E cells that project to the phrenic nucleus, they injected rhodamine microbeads into the phrenic nucleus at C4-C5. 4 days later, after recovery, they began the CIH protocol (5 mins normoxia, 4 mins of pure N2 infusion to make 40s of hypoxia at 6% O2, protocol repeated every 9 minutes for 8hr/day, 10 days). The brains of rats were then removed and sectioned so that they could get slices containing the cells they wanted for testing. Cells were identified by fluorescence within the 3 200um slices they could get per brain ALL 18 presympathetic RVLM neurons they found were spontaneously active in slices, and only had a slight reduction in firing frequency after simultaneous blockade of multiple ionotropic receptors, suggesting that the cells have pacemaker activity. When they looked at the properties of these cells after CIH, they found no difference in membrane potential, firing frequency, capacitance, resistance, etc. When they looked at the RVLM/BotC neurons that project to the phrenic nucleus, they looked to be similar to the other cells in terms of cell properties, except that they had a higher input resistance and different, irregular, patern of spontaneous action potential firing. However, they once again found no differences between CIH and control groups. The take home message here is that CIH did NOT alter the intrinsic properties of cells, but that it most likely alters neuromodulation - but they can not say that other cells involved in breathing weren't altered. It would have been neat if they could have maybe drugged the ringer with assorted ionotropic recptor agonists as well as antagonists to see if they would have had differential changes in receptor activity, or if they could have labeled the cells through the patch pipette, fixed the slices, and then done a reconstruction... but I suppose you can only do so much at a time with your experiments. -DH