Local Changes in Water Balance as a Marker of Neuron Activity in the Somatosensory System in Neonatal Rat Pups

Recording of intrinsic optical signals (IOS) is widely used for functional mapping in the central nervous system. However, the effectiveness of the IOS method depends critically on the chromophore used. The most commonly used chromophores for detection of IOS in living organisms are the oxygenated and deoxygenated forms of hemoglobin, which provide for indirect assessment of neuron activity in terms of changes in oxygenated blood levels. However, in some cases these changes are weak, decreasing the effectiveness of the IOS method. We report here our studies of the potential for using water as the chromophore in the spectral recording of IOS. Stimulation of the sensory whiskers on the snouts of neonatal rat pups induced IOS in the corresponding parts of the animals’ sensorimotor cortex. Evoked IOS were recorded in the near infrared (IR) part of the spectrum. Shifts in the recorded spectral range were accompanied by decreases in the spontaneous noise level which, despite the low signal amplitude, led to a significant increase in the signal: noise ratio. These results provide evidence of the potential for using water as a marker for neural activity using the IOS recording method.