Dendritic excitability during increased synaptic activity in rat neocortical L5 pyramidal neurons

Abstract

Recent years have seen increased study of dendritic integration, mostly in acute brain slices. However, due to the low background activity in brain slices the integration of synaptic input in slice preparations may not truly reflect conditions in vivo. To investigate dendritic integration, back-propagation of the action potential (AP) and initiation of the dendritic Ca(2+) spike we simultaneously recorded membrane potential at the soma and apical dendrite of layer 5 (L5) pyramidal neurons in quiescent and excited acute brain slices. After excitation of the brain slice the somatic input resistance decreased and the apparent passive space constant shortened. However, the back-propagating AP and dendritic Ca(2+) spike were robust during increased synaptic activity. The dendritic Ca(2+) spike was suppressed by the ionic composition of the bath solution required for slice excitation, suggesting that Ca(2+) spikes may be smaller in vivo than in the acute slice preparation. The results presented here suggest that, under the conditions of slice excitation examined in this study, the increased membrane conductance induced by activation of voltage-gated channels during back-propagation of the AP and dendritic Ca(2+) spike initiation is sufficiently larger than the membrane conductance at subthreshold potentials to allow these two regenerative dendritic events to remain robust over several levels of synaptic activity in the apical dendrite of L5 pyramidal neurons.

Patch-clamp recordings of rat neurons from acute brain slices of the somatosensory cortex during magnetic stimulation

“…To induce somatic depolarization we increased synaptic activity in the slice by bathing with ACSF 2 (ACSF with increased K + and reduced Ca 2+ concentration). We have previously shown that this modified ACSF increases synaptic input to cortical neurons leading to somatic depolarization (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). We have reported that the average membrane potential depolarized by 5–7 mV while the membrane potential variance increased almost 10-fold from 0.03 to 0.4 mV 2 (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008).…”

“…We have previously shown that this modified ACSF increases synaptic input to cortical neurons leading to somatic depolarization (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). We have reported that the average membrane potential depolarized by 5–7 mV while the membrane potential variance increased almost 10-fold from 0.03 to 0.4 mV 2 (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). We have also reported that the input resistance decreased by ~10 MΩ when ACSF was replaced with ACSF 2 and that the current threshold of the neuron decreased approximately by half from 280 to 130 pA (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008).…”

“…We have reported that the average membrane potential depolarized by 5–7 mV while the membrane potential variance increased almost 10-fold from 0.03 to 0.4 mV 2 (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). We have also reported that the input resistance decreased by ~10 MΩ when ACSF was replaced with ACSF 2 and that the current threshold of the neuron decreased approximately by half from 280 to 130 pA (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). Thus, this manipulation could be considered as a reasonable replacement of a current injection through the whole-cell electrode.…”

“…Next, the impact of network activity on action potential latency was tested. Synaptic activity in the slice was increased by replacing ACSF with ACSF 2 (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). Under these conditions the mean latency recorded for L5 pyramidal neurons was 0.22 ± 0.08 ms ( n = 19) and for interneurons 0.28 ± 0.13 ms ( n = 8, Figure 5F).…”

“…This procedure was approved by the national committee for experiments on laboratory animals at the Israeli Ministry of Health. Slices (sagittal, 300 μm thick) were prepared from the somatosensory cortex using previously described techniques (Bar-Yehuda and Korngreen, 2007; Bar-Yehuda et al, 2008). All experiments were carried out at room temperature (20–22°C).…”