Voltage-dependent currents prolong single-axon postsynaptic potentials in layer III pyramidal neurons in rat neocortical slices

Abstract

1. Using isolated slices of rat cingulate and sensorimotor cortex, intracellular recordings were obtained from pyramidal neurons in layer III. Simultaneous extracellular recordings were obtained from neurons in ventral layer III and layer IV. Spike-triggered averaging was employed to investigate synaptic connections from neurons in layers III/IV to pyramidal cells in layer III. 2. Of 701 simultaneously recorded pairs of neurons, comprising 699 extracellularly and 128 intracellularly recorded neurons, synaptic connections were demonstrated in 30 pairs. Of these, 29 were excitatory postsynaptic potentials (EPSPs) and 1, an inhibitory postsynaptic potential (IPSP). Single-axon EPSPs with a wide variety of amplitudes were recorded: the range recorded at membrane potentials between -68 and -72 mV was 0.079-2.3 mV. Comparing recordings obtained from different cells, EPSP amplitude was found to be independent of both the membrane resistance of the postsynaptic neuron and the EPSP time course; i.e., the largest EPSPs were not necessarily those recorded from neurons with the highest input resistance, nor those with the briefest time course. 3. Shape indices: width at half amplitude and rise-time, indicative of both proximal and distal synaptic locations were obtained. Normalized rise-times were between 0.1 and 2 times the membrane time constant and half-widths between 0.8 and 20 times. 4. The majority of postsynaptic neurons displayed nonlinear voltage relations typical of pyramidal neurons, and the contribution to EPSP shape of voltage-dependent currents was investigated. EPSP amplitude and duration were found to be dependent on membrane potential. The majority of single-axon EPSPs (26 of 29), increased in amplitude and duration with membrane depolarization over the range -95 - -50 mV, despite the significant decrease in driving force for the EPSP that would be expected to accompany such large depolarizations. This increase coincided with an increase in the amplitude of voltage responses to small injected current pulses. 5. It is concluded that the amplitude and time course of single-axon EPSPs recorded in cortical pyramidal somata are affected not only by the amplitude of the postsynaptic current and the location(s) of the synapse(s) relative to the soma, but also by voltage-dependent currents. The possibility that the increase in amplitude and duration of these EPSPs with membrane depolarization is due to N-methyl-D-aspartate receptor involvement is discussed.

Excitatory synaptic potentials in kainic acid-denervated rat CA1 pyramidal neurons

“…This fluctuation between EPSPs with different time components suggests the intermittent presence of two different types of postsynaptic potentials (Dale and Roberts, 1985). The delineation between an early response and the late response was also similar to intermixed fast and slow EPSP responses in neurons from the spinal cord (Mayer and Westbrook, 1987) and neocortex (Thomson et al, 1988). The histogram in Figure 3C shows the variability of the signal in comparison to the background noise, with some overlap of the two histograms, suggesting the occurrence of failures.…”

The role of negative conductances in neuronal subthreshold properties and synaptic integration

“…Blocking NMDA currents with AP5 did not affect substantially the EPSP (Deisz et al 1991;González-Burgos and Barrionuevo 2001;Hirsch and Gilbert 1991;Stuart and Sakmann 1995;Thomson et al 1988). However, the effect of NMDA currents in some other areas appears to be more significant.…”

“…It is known that the amplitude and duration of EPSPs are voltage dependent and that depolarization of the membrane potential to values close to action potential threshold amplifies the EPSP amplitude and prolongs its decay phase (Deisz et al 1991;Thomson et al 1988;Zsiros and Hestrin 2005). Experiments using sodium and calcium blockers in the perfusion bath could be performed in order to determine whether these postsynaptic conductances are responsible for this effect, but since these blockers also impair glutamate release by the presynaptic fiber it is complicated to assess their effect on the EPSPs.…”

“…The amplification of EPSPs by I NaP has been extensively observed in neurons from the neocortex (Andreasen and Lambert 1999;Carter et al 2012;Deisz et al 1991;Fricker and Miles 2000;González-Burgos and Barrionuevo 2001;Hirsch and Gilbert 1991;Lipowsky et al 1996;Rotaru et al 2007;Schwindt and Crill 1995;Stafstrom et al 1985;Stuart and Sakmann 1995;Thomson et al 1988;Zsiros and Hestrin 2005), hippocampus (Ceballos et al 2017;Urban et al 1998;Vervaeke et al 2006), entorhinal cortex (Economo et al 2014;Rosenkranz and Johnston 2007), dorsal cochlear nucleus (Hirsch and Oertel 1988), subthalamic nucleus (Farries et al 2010), hypothalamus (Branco et al 2016), olfactory bulb (Liu and Shipley 2008), dorsal horn of the spinal cord (Prescott and De Koninck 2005), medial superior olive and substantia nigra pars compacta (Yamashita and Isa 2004). In most of these experiments, blocking I NaP with TTX abolished the voltage dependence of the EPSP amplitude and area.…”

“…Later studies showed that I NaP can be activated during a single EPSP, slowing its decay phase (Hirsch and Oertel 1988;Stafstrom et al 1985). Thomson et al (1988) showed that the amplitude and duration of EPSPs are voltage dependent and that depolarization of the membrane potential to values close to the action potential threshold amplifies the EPSP amplitude and prolongs its decay phase. It was subsequently established that these effects are mediated mainly by I NaP (Andreasen and Lambert 1999;Deisz et al 1991;Lipowsky et al 1996;Schwindt and Crill 1995;Stuart and Sakmann 1995;Urban et al 1998).…”