Pressure (≤4 ATA) increases membrane conductance and firing rate in the rat solitary complex

Abstract

Neuronal sensitivity to pressure, barosensitivity, is illustrated by high-pressure nervous syndrome, which manifests as increased central nervous system excitability when heliox or trimix is breathed at >15 atmospheres absolute (ATA). We have tested the hypothesis that smaller levels of pressure (<or=4 ATA) also increase neuronal excitability. The effect of hyperbaric helium, which mimics increased hydrostatic pressure, was determined on putative CO2/H+-chemoreceptor neurons in the solitary complex in rat brain stem slices by intracellular recording. Pressure stimulated firing rate in 31% of neurons (barosensitivity) and decreased input resistance. Barosensitivity was retained during synaptic blockade and was unaffected by antioxidants. Barosensitivity was distributed among CO2/H+-chemosensitive and -insensitive neurons; in CO2/H+-chemosensitive neurons, pressure did not significantly reduce neuronal chemosensitivity. We conclude that moderate pressure stimulates certain solitary complex neurons by a mechanism that possibly involves an increased cation conductance, but that does not involve free radicals. Neuronal barosensitivity to <or=4 ATA may represent a physiological adaptive response to increased pressure or a pathophysiological response that is the early manifestation of high-pressure nervous syndrome.

Hyperbaric oxygen and chemical oxidants stimulate CO2/H+-sensitive neurons in rat brain stem slices

“…As shown in Table 1, HBO2 is defined in the intact CNS as a PtiO 2 in excess of 200 Torr measured when breathing 100% O2 at Ͼ1 ATA. However, as stated above, the typical control PtiO 2 in the brain slice at normobaric pressure is very hyperoxic, compared with the intact animal breathing normobaric air, and mimics neural Pti O 2 during HBO2 (in vivo), only minus the physical pressure component (23,51,53). Therefore, in this study, HBO2 in the brain stem slice was defined as an aCSF PO2 Ͼ720 Torr at PB Ͼ1 ATA.…”

“…Therefore, in this study, HBO2 in the brain stem slice was defined as an aCSF PO2 Ͼ720 Torr at PB Ͼ1 ATA. Brain stem slices were exposed to two different levels of HBO 2, depending on PB (ϳ1,680 and ϳ2,470 Torr of O 2) (53). Acute exposure to HBO2 raised PO2 in the core of the brain stem slice to ϳ1,000-ϳ1,500 Torr (51), which is comparable to Pti O 2 measured in the intact CNS of rats breathing 95% O 2-5% CO2 at 4-5 ATA, or 100% O2 at 5-6 ATA (23,40).…”

“…As in previous studies (22,29,53), helium was used as the compression medium for testing neuronal barosensitivity. Helium is inert and of low solubility in water and lipid membranes at PB Ͻ 245 ATA (5,23); therefore, the effects of helium over the range of PB values used in this study (Յ4 ATA) are attributed to the effects of pressure per se rather than the narcotic actions of increased partial pressure of helium (5,23,53). Before compression, room air was purged from the chamber atmosphere and replaced with 100% helium.…”

“…Statistical difference between multiple groups was determined by one-way ANOVA and Tukey's multiple comparison test. Contingency tables were used to compare the incidence of HBO 2 responsiveness to CO2/H ϩ chemosensitivity and barosensitivity (53). The significance of associations between these tabulated parameters was determined by using the Fisher's exact test with the Yates continuity correction when appropriate (P Յ 0.05).…”

“…However, we observed in some SC neurons that PB Յ 4 ATA of helium increased firing rate and decreased R in (i.e., these neurons were barosensitive), but that there was not a strong relationship between neuronal sensitivity to either HBO 2 or CO 2 /H ϩ and hyperbaric helium. Details regarding barosensitivity of SC neurons to this range of hyperbaric helium and its potential physiological significance are reported in the companion paper (53).…”