Origin and modulation of action potential evoked calcium signals in hippocampal CA1 pyramidal neurons
Creators: name: Sandler, Vladislav Michael
Publisher: name: University of British Columbia
Year: 2009

description: Calcium is an important second messenger that participates in triggering and regulating numerous neuronal processes. Action potentials (APs or AP) initiate rapid changes of intracellular Ca2+-concentration ([Ca2+];) in both soma and dendrites of central neurons. We addressed two major questions about the origin and modulation of these changes in [Ca2 +]i. Firstly, how does a neurotransmitter, serotonin (5-HT), modulate the backpropagation of APs and associated changes in the [Ca2+]; in CA1 hippocampal pyramidal neurons? Secondly, do APs trigger Ca2+-induced Ca2+-release (CICR) from internal stores in these neurons? We used whole-cell somatic or dendritic patch-clamp recordings combined with high-speed imaging of [Ca2+]; and analyses of the responses to applications of pharmacological agents, studying the changes in electrical membrane properties and [Ca2+]i. The experiments were conducted in the CA1 pyramidal neurons of in vitro slices from the rat hippocampus (11 day- to 5 week-old). Changes in [Ca2+]; were measured in neurons filled with bisfura- 2, Calcium Green-1 or fura-2-AM. Bath applications of 5-HT increased membrane conductance and hyperpolarized both soma and apical dendrites. They also lowered peak potentials of antidromically-activated, backpropagating APs in the dendrites. In the soma, 5-HT applications increased the absolute AP-amplitude while slightly decreasing peak potentials. 5-HT reduced the amplitude of the AP-evoked changes in [Ca2+]i at all locations along the apical dendrites and soma. The application of 5-HT and antidromically evoked APs generated, through synergistic actions, increases in [Ca2+]j that propagated along dendrites (Ca2 + waves). Such waves originated in the proximal or middle apical dendrites and were not accompanied by a significant change in somatic membrane potential. A minimum of five APs was required to evoke the waves. According to these new observations and supporting literature, the waves are a likely consequence of Ca2+-induced Ca2+-release (CICR) from internal stores through (inositol-1,4,5- triphosphate) IP3-sensitive channels. In the absence of 5-HT, APs evoked CICR. Caffeine application increased the amplitude of AP-induced changes in [Ca2+]i. During simultaneous calcium imaging, the whole-cell recordings showed that caffeine application did not significantly change either the resting membrane potential or amplitude and shape of APs. The enhancement of AP-evoked Ca2+-transients due to caffeine application could not be attributed to protein phosphorylation or modulation of high-threshold Ca2+-channels. Applications of IBMX, a non-specific inhibitor of phosphodiesterases, forskolin, an activator of adenylyl cyclase, H-89, an inhibitor of PKA and PKG, or nifedipine, a blocker of high-threshold Ca2 + channels, did not mimic or prevent the caffeine effect. Pretreatment of neurons with thapsigargin or cyclopiazonic acid (CPA) -- substances that facilitate depletion of intracellular Ca2+-stores by blocking endoplasmic reticulum specific Ca2+-ATPases -- precluded this effect. Similar pretreatment with ryanodine, a blocker of 'ryanodine-sensitive' channels, also precluded the caffeine effect. Despite a presence or absence of caffeine, applications of thapsigargin, ryanodine or CPA reduced the AP-evoked changes in [Ca2+]i. From these new experimental observations, we can conclude that the CICR through ryanodine-sensitive channels contributes to the AP-induced changes of [Ca2+]; in hippocampal CA1 pyramidal neurons. [Scientific formulae used in this abstract could not be reproduced.]