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

1. Acetylcholine is a regulatory cofactor for numerous activity-dependent p rocesses of central nervous system development and plasticity in which incr eases in cytosolic calcium ion concentration ([Ca2+](cyto)) couple membrane excitation to cellular changes. We examined how cholinergic receptor activ ation affects temporal and spatial aspects of increases in [Ca2+](cyto) dur ing short trains of action potentials in hippocampal CA1 pyramidal neurones . Membrane-impermeant Ca2+-sensitive dye was introduced into the cytosol du ring whole-cell recordings, and Ca2+-dependent fluorescence was recorded fr om somatic, nuclear and proximal dendrite regions with high temporal resolu tion. 2. In all neuronal compartments, the cholinergic agonist carbachol (5 mu M) increased resting [Ca2+](cyto) and the maximum [Ca2+](cyto) attained durin g a short action potential train. Carbachol also slowed the recovery of [Ca 2+](cyto) towards resting levels. The largest increases in peak cytosolic C a2+ concentration (Delta[Ca2+](cyto)) were seen in the dendrite and apical cell body, while relaxations of the carbachol-induced increase in Delta[Ca2 +](cyto) showed greater prolongation in the nucleus and basal cell body. 3. Most significantly, the difference between Ca2+ signals recorded before and during exposure to carbachol consistently showed a monotonic rise and s mooth, 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. 4. The effects of carbachol were partially reversed by simultaneous exposur e to atropine, or partially inhibited by inclusion of heparin in the intrac ellular solution, indicating the involvement of muscarinic acetylcholine re ceptors and InsP(3)-sensitive Ca2+-release channels. Our data indicate that carbachol-induced slowing of [Ca2+](cyto) relaxations after each action po tential results in enhanced accumulation of Ca2+ in the cytosol in the abse nce of changes in action potential-driven Ca2+ entry. By modulating the tim e course of Ca2+ signals, cholinergic stimulation may regulate the activati on of Ca2+-dependant intracellular processes dependent on patterns of [Ca2](cyto) changes.