COUPLING POTENTIALS IN CA1 NEURONS DURING CALCIUM-FREE-INDUCED FIELD BURST ACTIVITY

Small amplitude depolarizations (fast prepotentials, spikelets) record ed in mammalian neurons are thought to represent either dendritic acti on potentials or presynaptic action potentials attenuated by gap junct ions. We have used whole-cell recordings in an in vitro calcium-free m odel of epilepsy to record spikelets from CA1 neurons of the rat hippo campus. It was found that spikelet appearance was closely correlated w ith the occurrence of dye coupling between pyramidal neurons, indicati ng that both phenomena share a common substrate. Spikelets were charac terized according to waveform (amplitude and shape) and temporal occur rence. Spikelet amplitudes were found to be invariant with neuronal me mbrane potential, and their pattern of occurrence was indistinguishabl e from patterns of action potential firing in these cells. Voltage and current recordings revealed a spikelet waveform that was usually biph asic, comprised of a rapid depolarization followed by a slower hyperpo larization. Numerical differentiation of spike bursts resulted in wave forms similar to recorded spikelet sequences, while numerical integrat ion of spikelets yielded waveforms that were indistinguishable from ac tion potentials. Modification of spikelet waveforms by the potassium c hannel blocker tetraethylammonium chloride suggests that spikelets may arise from both resistive and capacitive transmission of presynaptic action potentials. Intracellular alkalinization and acidification brou ght about by perfusion with NH4Cl caused changes in spikelet frequency , consistent with reported alterations of field burst activity in this model of epilepsy. These results suggest that spikelets result from g ap junctional communication, and may be important determinants of neur onal activity during seizure-like activity.