Calcium electrogenesis in neocortical pyramidal neurons in vivo.
Much of what is known about Ca2+ electrogenesis in neocortical cells has been derived from in vitro studies. Since Ca2+ currents are controlled by various modulators, comparing these findings to in vivo data is essential. Here, we analysed tetrodotoxin (TTX)-resistant, presumably Ca2+-mediated potentials in intracellularly recorded neocortical neurons in vivo. TTX was applied locally to block Na+ channels. Its effectiveness was demonstrated by the elimination of fast spikes and orthodromic responses. In response to depolarizing current pulses bringing the membrane potential beyond approximately -33 mV, 71% of neurons generated high-threshold Ca2+ spikes averaging 17 mV. This is in contrast with in vitro findings, where high-threshold spikes could only be elicited following the blockade of K+ conductances. Consistent with this, neurons dialysed with K+ channel blockers in vivo generated high-threshold spikes that had a lower threshold (approximately -40 mV) and, with intracellular Cs+, a larger amplitude, indicating the presence of K+ currents opposing the activation of Ca2+ channels. Only 15% of cortical cells displayed low-threshold Ca2+ spikes. To compare high-threshold Ca2+ spikes evoked by synaptic stimuli or current injection, another group of cortical neurons was dialysed with QX-314 and Cs+, in the absence of extracellular TTX. Synaptic stimuli applied on a background of membrane depolarization elicited presumed Ca2+ spikes whose amplitude varied in a stepwise fashion. Thus, although there are numerous similarities between in vivo and in vitro data, some significant differences were found, which suggest that the high-voltage activated Ca2+ currents and/or the K+ conductances that oppose them are subjected to different modulatory influences in vivo than in vitro.
Paré
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Département de Physiologie, Faculté de Médecine, Université Laval, Québec, Canada. denis.pare@phs.ulaval.ca
Lang
E J
EJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D002415:Cats; D004566:Electrodes; D004569:Electroencephalography / Q000379:methods; D005260:Female; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D017551:Microdialysis; D019579:Neocortex / Q000502:physiology*; D009474:Neurons / Q000502:physiology*; D010477:Perfusion / Q000379:methods; D026902:Potassium Channel Blockers; D011189:Potassium Chloride / Q000737:chemistry; D017966:Pyramidal Cells / Q000502:physiology*; D013569:Synapses / Q000502:physiology; D013779:Tetrodotoxin / Q000008:administration & dosage / Q000494:pharmacology
D015220:Calcium Channels; D026902:Potassium Channel Blockers; D013779:Tetrodotoxin; D011189:Potassium Chloride; D002118:Calcium