Dendritic modulation of burst-like firing in sensory neurons

This report describes the variability of spontaneous firing characteristics of sensory neurons, electrosensory lateral line lobe (ELL) pyramidal cells , within the electrosensory lateral line lobe of weakly electric fish in vi vo. We show that these cells' spontaneous firing frequency, measures of spi ke train regularity (interspike interval coefficient of variation), and the tendency of these cells to produce bursts of action potentials are correla ted with the size of the cell's apical dendritic arbor. We also show that b ursting behavior may be influenced or controlled by descending inputs from higher centers that provide excitatory and inhibitory inputs to the pyramid al cells' apical dendrites. Pyramidal cells were classified as "bursty" or "nonbursty" according to whether or not spike trains deviated significantly from the expected properties of random (Poisson) spike trains of the same average firing frequency, and, in the case of bursty cells, the maximum wit hin-burst interspike interval characteristic of bursts was determined. Each cell's probability of producing bursts above the level expected for a Pois son spike train was determined and related to spontaneous firing frequency and dendritic morphology. Pyramidal cells with large apical dendritic arbor s have lower rates of spontaneous activity and higher probabilities of prod ucing bursts above the expected level, while cells with smaller apical dend rites fire at higher frequencies and are less bursty. The effect of blockin g non-N-methyl-D-aspartate (non-NMDA) glutamatergic synaptic inputs to the apical dendrites of these cells, and to local inhibitory interneurons, sign ificantly reduced the spontaneous occurrence of spike bursts and intracellu lar injection of hyperpolarizing current mimicked this effect. The results suggest that bursty firing of ELL pyramidal cells may be under descending c ontrol allowing activity in electrosensory feedback pathways to influence t he firing properties of sensory neurons early in the processing hierarchy.