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 depolrizations 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.