Graded boosting of synaptic signals by low-threshold voltage-activated calcium conductance

Abstract

Low-threshold voltage-activated calcium conductances (LT-VACCs) play a substantial role in shaping the electrophysiological attributes of neurites. We have investigated how these conductances affect synaptic integration in a premotor nonspiking (NS) neuron of the leech nervous system. These cells exhibit an extensive neuritic tree, do not fire Na(+)-dependent spikes, but express an LT-VACC that was sensitive to 250 μM Ni(2+) and 100 μM NNC 55-0396 (NNC). NS neurons responded to excitation of mechanosensory pressure neurons with depolarizing responses for which amplitude was a linear function of the presynaptic firing frequency. NNC decreased these synaptic responses and abolished the concomitant widespread Ca(2+) signals. Coherent with the interpretation that the LT-VACC amplified signals at the postsynaptic level, this conductance also amplified the responses of NS neurons to direct injection of sinusoidal current. Synaptic amplification thus is achieved via a positive feedback in which depolarizing signals activate an LT-VACC that, in turn, boosts these signals. The wide distribution of LT-VACC could support the active propagation of depolarizing signals, turning the complex NS neuritic tree into a relatively compact electrical compartment.

A tale of two leeches: Toward the understanding of the evolution and development of behavioral neural circuits

“…By this approach, a functional map of identified neurons in the segmental ganglia of Hirudo was first compiled in the mid‐1970s (Ort et al, 1974) and was appended over the years (Muller et al, 1981). In addition, imaging experiments can be implemented to analyze a three‐dimensional profile of neuronal activity (Carbó Tano et al, 2015; Yang et al, 2013). A typical midbody ganglion of Hirudo contains ∼400 cells, the majority of which are bilaterally paired homologs (Figure 1b).…”