Heterogeneous firing rate response of mouse layer V pyramidal neurons in the fluctuation-driven regime.

KEY POINTS: We recreated in vitro the fluctuation-driven regime observed at the soma during asynchronous network activity in vivo and we studied the firing rate response as a function of the properties of the membrane potential fluctuations. We provide a simple analytical template that captures the firing response of both pyramidal neurons and various theoretical models. We found a strong heterogeneity in the firing rate response of layer V pyramidal neurons: in particular, individual neurons differ not only in their mean excitability level, but also in their sensitivity to fluctuations. Theoretical modelling suggest that this observed heterogeneity might arise from various expression levels of the following biophysical properties: sodium inactivation, density of sodium channels and spike frequency adaptation. ABSTRACT: Characterizing the input-output properties of neocortical neurons is of crucial importance for understanding the properties emerging at the network level. In the regime of low-rate irregular firing (such as in the awake state), determining those properties for neocortical cells remains, however, both experimentally and theoretically challenging. Here, we studied this problem using a combination of theoretical modelling and in vitro experiments. We first identified, theoretically, three somatic variables that describe the dynamical state at the soma in this fluctuation-driven regime: the mean, standard deviation and time constant of the membrane potential fluctuations. Next, we characterized the firing rate response of individual layer V pyramidal cells in this three-dimensional space by means of perforated-patch recordings and dynamic clamp in the visual cortex of juvenile mice in vitro. We found that individual neurons strongly differ not only in terms of their excitability, but also, and unexpectedly, in their sensitivities to fluctuations. Finally, using theoretical modelling, we attempted to reproduce these results. The model predicts that heterogeneous levels of biophysical properties such as sodium inactivation, sharpness of sodium activation and spike frequency adaptation account for the observed diversity of firing rate responses. Because the firing rate response will determine population rate dynamics during asynchronous neocortical activity, our results show that cortical populations are functionally strongly inhomogeneous in young mouse visual cortex, which should have important consequences on the strategies of cortical computation at early stages of sensory processing.

Ca(2+) and Na(+) contribute to the swelling of differentiated neuroblastoma cells induced by equinatoxin-II.

Equinatoxin-II (EqTx-II), a cytotoxic protein (mol.wt 20 kDa) isolated from the sea anemone Actinia equina, was found to consistently increase the three-dimensional projected area of differentiated neuroblastoma (NG108-15) cells provided Ca(2+) was present in the medium. No swelling was detected when external NaCl was replaced by sucrose, but replacement of NaCl by Na-isethionate did not prevent the swelling, as revealed by confocal laser scanning microscopy. In addition, microspectrofluorometric measurements in cells preloaded with the Ca(2+) indicator fura-2/AM revealed that EqTx-II (100 nM) markedly increased the fluorescence (F(340)/F(380)) ratio indicating a rise of intracellular Ca(2+) concentration ([Ca(2+)](i)). The elevation of [Ca(2+)](i) exhibited two components that seem to be related to the kinetics of EqTx-II-induced Ca(2+) entry since pretreatment of cells with Ca(2+)-ATPase inhibitors (thapsigargin), Ca(2+) channel blockers (nifedipine and Gd(3+)) or prolonged exposure to a high K(+) (75 mM) medium did not alter EqTx-II-induced Ca(2+) signals. As far as we know, this is the first demonstration that EqTx-II causes swelling of neuroblastoma cells and that this effect is correlated both with an increase of [Ca(2+)](i) and needs the presence of extracellular Na(+). It is suggested that EqTx-II has the ability to insert into the plasma membrane of neuroblastoma cells and to form pores altering the membrane permeability and the intracellular osmolality, inducing a marked influx of water into the cells.