CO₂ induced pH_i changes in the brain of polar fish: a TauCEST application

Chemical exchange saturation transfer (CEST) from taurine to water (TauCEST) can be used for in vivo mapping of taurine concentrations as well as for measurements of relative changes in intracellular pH (pH_i ) at temperatures below 37°C. Therefore, TauCEST offers the opportunity to investigate acid-base regulation and neurological disturbances of ectothermic animals living at low temperatures, and in particular to study the impact of ocean acidification (OA) on neurophysiological changes of fish. Here, we report the first in vivo application of TauCEST imaging. Thus, the study aimed to investigate the TauCEST effect in a broad range of temperatures (1-37°C) and pH (5.5-8.0), motivated by the high taurine concentration measured in the brains of polar fish. The in vitro data show that the TauCEST effect is especially detectable in the low temperature range and strictly monotonic for the relevant pH range (6.8-7.5). To investigate the specificity of TauCEST imaging for the brain of polar cod (Boreogadus saida) at 1.5°C simulations were carried out, indicating a taurine contribution of about 65% to the in vivo expected CEST effect, if experimental parameters are optimized. B. saida was acutely exposed to three different CO₂ concentrations in the sea water (control normocapnia; comparatively moderate hypercapnia OA_m  = 3300 μatm; high hypercapnia OA_h  = 4900 μatm). TauCEST imaging of the brain showed a significant increase in the TauCEST effect under the different CO₂ concentrations of about 1.5-3% in comparison with control measurements, indicative of changes in pH_i or metabolite concentration. Consecutive recordings of ¹ H MR spectra gave no support for a concentration induced change of the in vivo observed TauCEST effect. Thus, the in vivo application of TauCEST offers the possibility of mapping relative changes in pH_i in the brain of polar cod during exposure to CO₂ .