Cell Calcium 39 (2006) 455–466 Modulation of neuronal excitability by intracellular calcium buffering: From spiking to bursting C. Roussel a , T. Erneux b , S.N. Schiffmann a , D. Gall a, ∗ a Laboratoire de Neurophysiologie (CP601), Facult´ e de M´ edecine, Universit´ e Libre de Bruxelles, Route de Lennik 808, B-1070 Bruxelles, Belgium b Optique Nonlin´ eaire Th´ eorique (CP231), Facult´ e des Sciences, Universit´ e Libre de Bruxelles, Boulevard du Triomphe, B-1050 Bruxelles, Belgium Received 6 October 2005; received in revised form 1 December 2005; accepted 21 January 2006 Available online 13 March 2006 Abstract We have investigated the detailed regulation of neuronal firing pattern by the cytosolic calcium buffering capacity using a combination of mathematical modeling and patch–clamp recording in acute slice. Theoretical results show that a high calcium buffer concentration alters the characteristic regular firing of cerebellar granule cells and that a transition to various modes of oscillations occurs, including bursting. Using bifurcation analysis, we show that this transition from spiking to bursting is a consequence of the major slowdown of calcium dynamics. Patch–clamp recordings on cerebellar granule cells loaded with a high concentration of the fast calcium buffer BAPTA (15mM) reveal dramatic alterations in their excitability as compared to cells loaded with 0.15mM BAPTA. In high calcium buffering conditions, granule cells exhibit all bursting behaviors predicted by the model whereas bursting is never observed in low buffering. These results suggest that cytosolic calcium buffering capacity can tightly modulate neuronal firing patterns leading to generation of complex patterns and therefore that calcium-binding proteins may play a critical role in the non-synaptic plasticity and information processing in the central nervous system. © 2006 Elsevier Ltd All rights reserved. Keywords: Bursting; Excitability; Calcium buffering; Mathematical model; Patch–clamp; Cerebellar granule cell 1. Introduction Calcium ions (Ca 2+ ) regulate a great variety of cellular processes, including hormone and neurotransmitter release, ionic channel permeability, enzyme activity and gene tran- scription [1]. In particular, at the neuronal level, variations of cytosolic Ca 2+ concentration regulate synaptic plasticity which is involved in the establishment of memory. There- fore, to play this crucial modulatory role, the cytosolic cal- cium concentration must be tightly regulated. Several molec- ular mechanisms are at work but calcium-binding proteins are critical elements to modulate or mediate the actions of calcium. Many excitable cells contain large concentration of Ca 2+ buffering proteins [2,3]. However, their physiological functions remain poorly understood. ∗ Corresponding author. Tel.: +32 2 555 4103; fax: +32 2 555 4121. E-mail address: dgall@ulb.ac.be (D. Gall). The large family of calcium-binding proteins is classified in trigger or buffer proteins. While trigger proteins, such as calmodulin, change their conformation upon binding Ca 2+ , buffer proteins (e.g. calretinin or calbindin) simply bind Ca 2+ as its concentration increases within a cell [4]. Therefore, the actual consensus concerning the function of these calcium buffer proteins is that they act as passive modulators of the cytosolic calcium levels. A recent study [5] has demonstrated that a lack of calretinin, a calcium-binding protein highly expressed in cerebellar granule cells, leads to an increased neuronal excitability through a reduction in cytosolic calcium buffering capacity. This result suggests the existence of a link between the cellular Ca 2+ -binding protein expression and excitability. The aim of this work is to establish to which extent cal- cium buffering can regulate the firing pattern of excitable cells such as neurons. First, using a mathematical model, we have investigated the consequence of various calcium buffer 0143-4160/$ – see front matter © 2006 Elsevier Ltd All rights reserved. doi:10.1016/j.ceca.2006.01.004