Depth profiles of pH and PO2 in the in vitro brainstem preparation of the tadpole Rana catesbeiana

Abstract

Extracellular pH and PO2 was recorded in the isolated in vitro brainstem of the metamorphic tadpole, Rana catesbeiana while the brainstem preparation was superfused with oxygenated mock cerebrospinal fluid of pH = 7.8, PCO2 = 17 Torr, PO2 = 600 Torr at 23 degrees C. Using pH and PO2 microelectrodes, the ventral medullary surface was penetrated at midline and lateral sites between cranial nerves V and X. Mean pH and PO2 gradients of 0.07 pH units/100 microns and 60 Torr/100 microns were detected in the superfusate, 100-200 microns above the ventral surface of the brainstem. These gradients remained virtually constant for the first 100-200 microns below the medullary surface. Beyond this level, pH and PO2 gradients decreased in a curvilinear fashion. For midline tracts, minimum values of pH and PO2 (7.58 +/- 0.05 and 323 +/- 31 Torr) were reached at a depth of 500-750 microns, whereas for lateral tracts, mean minimum values of pH and PO2 (7.34 +/- 0.12 and 240 +/- 68 Torr), were recorded at 850-900 microns. With further electrode advancement, pH and PO2 gradients in both midline and lateral tracts reversed as levels began to increase. Between CN V and X, lateral width was 4.34 +/- 0.57 mm, while dorsal-ventral thickness in midline and lateral regions was 0.92 +/- 0.21 and 1.31 +/- 0.22 mm, respectively. Overall, the in vitro tadpole brainstem provides a robust neural preparation which, although moderately acidic, is well oxygenated throughout all tissue layers.