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Calcium dynamics associated with action potentials in single nerve terminals of pyramidal cells in layer 2/3 of the young rat neocortex
Author(s) -
Koester Helmut J.,
Sakmann Bert
Publication year - 2000
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2000.00625.x
Subject(s) - neocortex , axon , soma , biophysics , neuroscience , pyramidal cell , electrophysiology , chemistry , schaffer collateral , voltage dependent calcium channel , calcium , anatomy , excitatory postsynaptic potential , inhibitory postsynaptic potential , biology , hippocampal formation , organic chemistry
1 Calcium dynamics associated with a single action potential (AP) were studied in single boutons of the axonal arbor of layer 2/3 pyramidal cells in the neocortex of young (P14‐16) rats. We used fluorescence imaging with two‐photon excitation and Ca 2+ ‐selective fluorescence indicators to measure volume‐averaged Ca 2+ signals. These rapidly reached a peak (in about 1 ms) and then decayed more slowly (tens to hundreds of milliseconds). 2 Single APs and trains of APs reliably evoked Ca 2+ transients in en passant boutons located on axon collaterals in cortical layers 2/3, 4 and 5, indicating that APs propagate actively and reliably throughout the axonal arbor. Branch point failures are unlikely to contribute to differences in synaptic efficacy and reliability in the connections made by layer 2/3 pyramidal cells. 3 AP‐evoked Ca 2+ transients in boutons were mediated by voltage‐dependent Ca 2+ channels (VDCCs), predominantly by the P/Q‐ and N‐subtypes. 4 Ca 2+ transients were, on average, of significantly larger amplitude in boutons than in the flanking segments of the axon collateral. Large amplitude Ca 2+ transients in boutons were spatially restricted to within ≤ 3 m of axonal length. 5 Single AP‐evoked Ca 2+ transients varied up to 10‐fold across different boutons even if they were located on the same axon collateral. In contrast, variation of Ca 2+ transients evoked by successive APs in a given single bouton was small (coefficient of variation, c.v. ≤ 0.21). 6 Amplitudes of AP‐evoked Ca 2+ signals did not correlate with the distance of boutons from the soma. In contrast, AP‐evoked Ca 2+ signals in spines of basal dendrites decreased slightly (correlation coefficient, r 2 = ‐0.27) with distance from the soma. 7 Measurements with the low‐affinity Ca 2+ indicator Magnesium Green suggest that the volume‐averaged residual free [Ca 2+ ] i in a bouton increases on average by 500 nM following a single AP. Higher concentrations of indicator caused, on average, a decrease in the amplitude and an increase in the decay time constant of Ca 2+ transients. Assuming a single‐compartment model the concentration dependence of decay time constants suggests a low endogenous Ca 2+ binding ratio close to 140, indicating that of the total Ca 2+ influx (≈2 fC) less than 1 % remained free. 8 The indicator concentration dependence of decay time constants further suggests that the residual free Δ[Ca 2+ ] i associated with an AP decays with a time constant of about 60 ms (35°C) reflecting a high Ca 2+ extrusion rate of about 2600 s −1 . 9 The results show that AP‐evoked volume‐averaged Ca 2+ transients in single boutons are evoked reliably and, on average, have larger amplitudes than Ca 2+ transients in other subcellular compartments of layer 2/3 pyramidal cells. A major functional signature is the large variation in the amplitude of Ca 2+ transients between different boutons. This could indicate that local interactions between boutons and different target cells modify the spatiotemporal Ca 2+ dynamics in boutons and cause target cell‐specific differences in their transmitter release properties.