Hypoxia- and hypercapnia-induced DC potential shifts in rat at the scalp and the skull are opposite in polarity to those at the cerebral cortex.

In anaesthetized and artificially ventilated rats DC (direct current) potential shifts induced by hypoxia or hypercapnia for 10 min were monitored at the surface of the skin, of the skull and of the cerebral cortex. Hypoxia was induced by decreasing the inspiratory O2 content from 20 to 10 or to 6% O2 in N2. Hypercapnia was induced by applying gas mixtures with CO2 contents from 0 to 5, 10, 20 or 30% CO2 in O2. DC potentials were recorded with non-polarizing electrodes filled with 150 mM NaCl solution. Hypercapnia evoked a negative DC shift (1.2-1.9 mV) epicranially and a large positive DC deflection (8.8-17.1 mV) epidurally. In contrast, hypoxia elicited a positive DC shift in skull recordings with a negative DC shift (2.6-3.1 mV) overshooting the baseline during recovery. DC shifts at the skin were positive, but smaller in amplitude (up to 3.1 mV). At the surface of the dura hypoxic DC shifts were negative and smaller (1.5-2.2 mV) than at the skull. The results show that a negative DC shift induced by hypercapnia recorded non-invasively from the skin or the skull is not always reflecting an increased cortical activation state and, vice versa, a positive DC shift does not always reflect a decreased state of neuronal activation. The electrogenesis of gas content-induced potentials could be due to the electrochemical diffusion of ions through the selective permeability barrier between the blood and tissue compartment. These observations are relevant for any application of non-invasive DC electroencephalography in the human cortex.