PASSIVE AND ACTIVE MEMBRANE-PROPERTIES CONTRIBUTE TO THE TEMPORAL FILTERING PROPERTIES OF MIDBRAIN NEURONS IN-VIVO

This study examined the contributions of passive and active membrane p roperties to the temporal selectivities of electrosensory neurons in v ivo. The intracellular responses to time-varying (2-30 Hz) electrosens ory stimulation and current injection of 27 neurons in the midbrain of the weakly electric fish Eigenmannia were recorded. Each neuron was f illed with biocytin to reveal its anatomy. Neurons were divided into t wo biophysically distinct groups based on their frequency-dependent re sponses to sinusoidal current injection over the range 2-30 Hz. Fourte en neurons showed low-pass filtering, with a maximum decline in the am plitude of voltage responses of >2.6 dB (X = 4.30 dB, s = 1.10 dB) to sinusoidal current injection, These neurons also showed low-pass filte ring of electrosensory information but with larger maximum declines in postsynaptic potential amplitude (X = 9.53 dB, s = 3.34 dB; n = 10). These neurons had broad dendritic arbors and relatively spiny dendrite s. Five neurons showed all-pass filtering, having maximum decline in t he amplitude of voltage responses of <2.0 dB (X = 1,16 dB, s = 0.61 dB ). For electrosensory stimuli, however, these neurons showed low-, ban d-, or high-pass filtering. These neurons had small dendritic arbors a nd few or no spines. Voltage-dependent ''active'' conductances were re vealed in eight neurons by using several levels of current clamp. In f our of these neurons, the duration of the voltage-dependent conductanc es decreased in concert with the period of the electrosensory stimulus , whereas in the other four neurons the duration of the voltage-depend ent conductances was relatively short (<30 msec) and nearly constant a cross sensory stimulation frequencies. These conductances enhanced the temporal filtering properties of neurons.