Cholinergic stimulation enhances cytosolic calcium ion accumulation in mouse hippocampal CA1 pyramidal neurones during short action potential trains

Abstract

Acetylcholine is a regulatory cofactor for numerous activity‐dependent processes of central nervous system development and plasticity in which increases in cytosolic calcium ion concentration ([Ca2+]cyto) couple membrane excitation to cellular changes. We examined how cholinergic receptor activation affects temporal and spatial aspects of increases in [Ca2+]cyto during short trains of action potentials in hippocampal CA1 pyramidal neurones. Membrane‐impermeant Ca2+‐sensitive dye was introduced into the cytosol during whole‐cell recordings, and Ca2+‐dependent fluorescence was recorded from somatic, nuclear and proximal dendrite regions with high temporal resolution. In all neuronal compartments, the cholinergic agonist carbachol (5 μM) increased resting [Ca2+]cyto and the maximum [Ca2+]cyto attained during a short action potential train. Carbachol also slowed the recovery of [Ca2+]cyto towards resting levels. The largest increases in peak cytosolic Ca2+ concentration (Δ[Ca2+]cyto) were seen in the dendrite and apical cell body, while relaxations of the carbachol‐induced increase in Δ[Ca2+]cyto showed greater prolongation in the nucleus and basal cell body. Most significantly, the difference between Ca2+ signals recorded before and during exposure to carbachol consistently showed a monotonic rise and smooth fall in all cell compartments, suggesting that the increase in [Ca2+]cyto associated with each action potential was not altered by carbachol. Consistent with this view, changes in Ca2+ signalling were not accompanied by changes in action potential waveforms. The effects of carbachol were partially reversed by simultaneous exposure to atropine, or partially inhibited by inclusion of heparin in the intracellular solution, indicating the involvement of muscarinic acetylcholine receptors and InsP3‐sensitive Ca2+‐release channels. Our data indicate that carbachol‐induced slowing of [Ca2+]cyto relaxations after each action potential results in enhanced accumulation of Ca2+ in the cytosol in the absence of changes in action potential‐driven Ca2+ entry. By modulating the time course of Ca2+ signals, cholinergic stimulation may regulate the activation of Ca2+‐dependent intracellular processes dependent on patterns of [Ca2+]cyto changes.

Calcium transients in subcompartments of the leech Retzius neuron as induced by single action potentials

“…A prominent increase in the Ca 2ϩ concentration occurred in the distal parts of the axon, which equaled or even exceeded the Ca 2ϩ increase in all other cell compartments, including the dendrites. This is in contrast to vertebrate cells, where the largest Ca 2ϩ increases have been found in dendrites and in presynaptic terminals (Jaffe et al, 1992;Schiller et al, 1995;Eilers et al, 1995;Callewaert et al, 1996;Helmchen et al, 1997;Beier and Barish, 2000). The Ca 2ϩ -dependent fluorescent signals induced by APs in rat Purkinje neurons were hardly distinguishable from the noise, indicating a very small Ca 2ϩ influx into the axon (Callewaert et al, 1996).…”

“…Hence, the spatial distribution of the Ca 2ϩ influx during AP firing can play an important role in the modulation of information processing (for reviews see Augustine and Neher, 1992;Regehr and Tank, 1994;. Ca 2ϩ transients induced by single APs have been studied in detail in mammalian pyramidal neurons, showing Ca 2ϩ signals in the soma, including nuclear calcium signals, in dendrites, and in particular in presynaptic terminals (calyx of Held) (Schiller et al, 1995;Helmchen et al, 1997;Beier and Barish, 2000). Axonal Ca 2ϩ signaling due to single APs has been studied in rat cerebellar Purkinje neurons, showing only very small Ca 2ϩ transients in the axon, while Ca 2ϩ influx of much larger amplitude was found in the dendrites and the submembrane region of the cell body (Eilers et al, 1995;Callewaert et al, 1996).…”

Muscarinic Enhancement of R-Type Calcium Currents in Hippocampal CA1 Pyramidal Neurons

“…K ϩ conductance blockade and the IP 3 -mediated release of Ca 2ϩ from intracellular stores are thought to contribute to this accumulation Connor, 1991, 1992;Tsubokawa and Ross, 1997;Beier and Barish, 2000). However, our results suggest that the stimulation of R-type Ca 2ϩ currents also might contribute to the muscarinicmediated intracellular Ca 2ϩ accumulations.…”

“…Despite the general suppression of Ca 2ϩ currents, muscarinic activation paradoxically increases intracellular Ca 2ϩ accumulations in the dendrites and spines from depolarization or synaptic stimulation Connor, 1991, 1992;Tsubokawa and Ross, 1997;Beier and Barish, 2000). K ϩ conductance blockade and the IP 3 -mediated release of Ca 2ϩ from intracellular stores are thought to contribute to this accumulation Connor, 1991, 1992;Tsubokawa and Ross, 1997;Beier and Barish, 2000).…”

Calcium-Dependent But Action Potential-Independent BCM-Like Metaplasticity in the Hippocampus

“…On the other hand, this form of heterosynaptic metaplasticity diverges from the computational models in (1) being compartmentally restricted when priming stimulation is delivered to afferents in stratum radiatum, (2) being independent of postsynaptic action potentials in the affected cell, and (3) being driven by muscarinic rather than glutamatergic receptor activation. Activation of muscarinic receptors elevates intracellular concentrations of calcium in both neurons and glia in the hippocampus (Egorov and Mül-ler, 1999;Beier and Barish, 2000;Araque et al, 2002;Perea and Araque, 2005), and it will require additional approaches to determine which cell types and signaling pathways mediate the metaplasticity effects. Regardless, these experiments raise the question whether BCM-like metaplasticity in other brain regions such as visual cortex involve mechanisms similar to those shown here for the hippocampus.…”