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 ≈–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 (≈–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.