Graded boosting of synaptic signals by low threshold voltag e activated calcium conductance

MARTIN CARBO TANO; LIDIA SZCZUPAK

Chicago

Congreso; Neuroscience 2015; 2015

Society for Neuroscience

p { margin-bottom: 0.25cm; line-height: 120%; }Theneuritic tree is the cellular domain at which neurons integrate andprocess synaptic inputs. In the past decades it became clear thatdendrites do not rely purely on their passive membrane properties butare supplied with voltage-activated conductances that affect theirprocessing capabilities. Among these conductances, low thresholdvoltage-activated calcium conductances (LT-VACCs) play a substantialrole in shaping the biophysical attributes of neurites. However, itis not known to what extent the activation of these conductancesshapes the synaptic responses.Herewe have investigated how LT-VACCs affect synaptic integration in apremotor nonspiking (NS) neuron of the leech nervous system. Thesecells exhibit an extensive neuritic tree, do not fire Na+-dependentspikes but express a LT-VACC that was sensitive to 250 µM Ni2+and 100 µM NNC 55-0396 (NNC,state drug nature). Calcium imagingstudies showed that this LT-VACC is distributed throughout the mainbranches and is activated by synaptic responses evoked by stimulationof pressure sensitive (P) neurons. The resulting Ca2+signals were a graded function of the electrophysiological synapticresponses and spread throughout the neuritic tree with no majorattenuation. Thus in NS neurons, synaptically-evoked Ca2+signals are a graded function of synaptic inputs that spreadglobally. NNC decreased these synaptic responses and abolished theconcomitant widespread Ca2+signals. Coherent with the interpretation that the LT-VACC amplifiedsignals at the postsynaptic level, this conductance also amplifiedthe responses of NS neurons to direct injection of sinusoidalcurrent. Synaptic amplification thus is achieved via a positivefeedback in which depolarizing signals activate a LT-VACC that, inturn, boosts these signals.Theresults presented here indicate that LT-VACC amplified the responsesto synaptic inputs generating a ?graded boosting? where theamplified signal preserved a linear relationship with the firingfrequency of the presynaptic neuron. The wide distribution of LT-VACCcould support the active propagation of depolarizing signals, turningthe complex NS neuritic tree into a relatively compact electricalcompartment.