In vivo electrophysiological analysis of lucifer yellow-coupled hippocampal pyramids

Abstract

Small transient all-or-none depolarizations (also termed in the literature fast prepotentials, spikelets, pseudospikes, d-spikes, or short latency depolarizations) and their association with lucifer yellow (LY) dye-coupling were analyzed in CA1-CA3 hippocampal pyramidal cells in urethane anesthetized rats. It was found that (a) 15 of the 24 LY-injected pyramidal neurons (63%) showed dye-coupling; (b) spontaneous, anti- and orthodromically evoked spikelets (3-7 ms in duration; 3- to 12-mV peak) were recorded in 40 of 95 cells (42%); (c) there was a significantly higher probability of dye-coupled neurons with spiklets and of uncoupled ones without spikelets; (d) spikelet waveform and amplitude were unaffected by spontaneous or imposed polarizations; (e) large hyperpolarizations could reduce the rate and even prevent spikelets; and (f) spikelets could precede or follow spikes, the latter were more frequent with large depolarizations. Electrophysiological findings, and the association of dye-coupling and spikelets, suggest strongly that at least some spikelets are coupling potentials. This implies that pyramidal cells may be electronically coupled under physiological conditions.