Carbon dioxide sensitivity of dye coupling among glia and neurons of the neocortex

Among neocortical astrocytes and neurons, intracellular injection of the fluorescent dye Lucifer Yellow CH into single cells will often label multiple adjacent cells. It is possible that this intercellular dye movement occurs through gap junctions, which in several systems are sensitive to cytoplasmic acidification. In the present study we tested the effect of increased CO2 levels, a treatment expected to decrease intracellular pH (pHi), on the prevalence of glial and neuronal dye coupling in neocortical slices. When CO2 levels were raised to 40% or 50%, dye coupling among astrocytes was completely abolished. Under the same conditions, the prevalence of dye coupling among neocortical neurons of adult guinea pigs was significantly reduced to 18.7%, compared to the control level of 32.3%. Dye coupling among immature rat neocortical neurons, which normally occurs at relatively high rates, was not measurably affected by CO2 levels up to 50%. The results suggest that coupling between cortical glia is very sensitive to cytoplasmic acidification. The relative insensitivity of neuronal coupling to CO2 may indicate either that the conductance of neuronal gap junctions is only weakly affected by pHi or that neuron-to-neuron dye passage occurs via other types of intercytoplasmic pathways.