Glial potentials in hippocampus.

In rats under urethane anaesthesia, intracellular recordings were made from 36 cells, mainly in CA1, that had all the characteristics of glia: unusually high and stable resting potentials (-79.6 +/- 6.0 mV, mean +/- SD) and total absence of spikes or synaptic potentials. They were exceptionally sensitive to surrounding neuronal activity, being readily depolarized by very low frequency stimulation (0.5-2 Hz) of the fimbria. In the range 0.5-2 Hz, the mean peak depolarizations increased linearly with frequency of fimbrial stimulation (9.1 +/- 0.53 mV/Hz). At frequencies of 5 Hz or more, the depolarizations were highly variable, sometimes reaching a maximum of 25-30 mV, but the overall mean was not significantly greater than for 2 Hz stimulation. The depolarizations decayed slowly, with a half-time of 4.2 +/- 1.22 s and were often followed by a prolonged undershoot (lasting over 1 min). Alvear and especially septal stimulation were much less effective in evoking glial depolarizations. One cell that initially had all the characteristics of a glia, during very prolonged stable recording, developed responses, such as synaptic potentials and 20-40 mV action potentials evoked by fimbrial or alvear stimulation, consistent with strong electrical coupling to at least one neighbouring neuron.