Temporal filtering properties of midbrain neurons in an electric fish: implications for the function of dendritic spines.

Electrosensory neurons in the torus semicircularis (midbrain) of the weakly electric fish Eigenmannia vary considerably in their dendritic structure and responses to modulations of the amplitude of electric organ discharges. We investigated possible relations between these properties by recording intracellularly and labeling individual neurons while modulating stimulus amplitude over rates of approximately 2-20 Hz. Morphologically distinct cell types generally differed in their responses to these stimuli. The amplitude envelope of the stimulus was nicely reflected in fluctuations of the membrane potential of heavily spined neurons. The amplitude of these stimulus-related depolarizations decreased markedly as the stimulus modulation rate was increased. For aspiny or sparsely spined neurons, however, the amplitude of stimulus-related depolarizations either increased or remained constant over this range of modulation rates. In these cells, the amplitude envelope of the stimulus was not well represented in the membrane potential. Instead, fast EPSPs were observed that varied in number over time in accordance with the amplitude envelope of the stimulus. Aspiny neurons in the tectum also coded the amplitude envelope of stimuli with poor fidelity. The amplitude of stimulus-related depolarizations, however, decreased as the rate of modulation of stimulus amplitude was increased, consistent with the notion that tectal neurons receive afferent input from the spiny toral neurons. Spiny neurons appear, therefore, to act as low-pass filters of temporal information in sensory signals. Aspiny cells, however, code high temporal frequencies. These data support the hypothesis that dendritic spines contribute to the low-pass filtering of inputs to neurons.