Electrophysiological diversity of pyramidal‐shaped neurons at the granule cell layer/hilus border of the rat dentate gyrus recorded in vitro

Abstract

In the rat dentate gyrus, pyramidal-shaped cells located on the border of the granule cell layer and the hilus are one of the most common types of γ-aminobutyric acid (GABA)-immunoreactive neurons. This study describes their electrophysiological characteristics. Membrane properties, patterns of discharge, and synaptic responses were recorded intracellularly from these cells in hippocampal slices. Each cell was identified as pyramidal-shaped by injecting the marker Neurobiotin intracellularly (n = 17).In several respects the membrane properties of the sampled cells were similar to "fast-spiking" cells (putative inhibitory interneurons) that have been described in other areas of the hippocampus. For example, input resistance was high (mean 91.3 megohms), the membrane time constant was short (mean 7.7 ms), and there was a large afterhyperpolarization following a single action potential (mean 10.5 mV at resting potential). However, the action potentials of most pyramidalshaped cells were not as brief (mean 1.2 ms total duration) as those of most previously described fast-spiking cells. Many pyramidal-shaped neurons had strong spike frequency adaptation relative to other fast-spiking cells. Although these latter two characteristics were apparent in the majority of the sampled cells, there were exceptional pyramidal-shaped neurons with fast action potentials and weak adaptation, demonstrating the electrophysiological variability of pyramidal-shaped cells.Responses to outer molecular layer stimulation were composed primarily of excitatory postsynaptic potentials (EPSPs) rather than inhibitory postsynaptic potentials (IPSPs), and were usually small (EPSPs evoked at threshold were often less than 2 mV), and brief (less than 30 ms). There was variability, because in a few cells EPSPs evoked at threshold were much larger. However, regardless of EPSP amplitude, suprathreshold stimulation (up to 4 times the threshold stimulus strength) rarely evoked more than one action potential in any cell. The results suggest that stimulation of perforant path axons produces limited excitatory synaptic responses in pyramidal-shaped neurons. This may be one of the reasons why they are relatively resistant to prolonged perforant path stimulation.The pyramidal-shaped neurons located at the base of the granule cell layer have been associated historically with a basket plexus around granule cell somata, and have been called pyramidal "basket" cells. However, basket-like endings were rare and axon collaterals outside the granule cell layer were common. Many axon collaterals were as far from the granule cell layer as the outer molecular layer and the central hilus, and antidromic action potentials could be recorded in some

Cellular mechanisms underlying acquired epilepsy: The calcium hypothesis of the induction and maintainance of epilepsy

“…Since numerous changes in Ca 2+ dynamics in the interneurons as well as pyramidal neurons have been observed in the pilocarpine and in the SE and GET in vitro models of AE, it is possible that these changes in Ca 2+ dynamics may play a role in some of the many plasticity changes observed in pyramidal neurons, dentate granule neurons and interneurons that occur with the development of AE. These changes include changes in GABA receptor function (Coulter, 2001), the initiation of mossy fiber sprouting (Ben-Ari, 2001, Nadler, 2003, alterations in peptide expression (Wasterlain et al, 2002), neuronal reorganization (Sanabria et al, 2002), reorganization of cannabinoid receptor expression (Wallace et al, 2003), changes in the expression of BDNF (Scharfman, 1999;Scharfman et al, 1999;Binder et al, 2001;Scharfman, 2002b), the onset of neurogenesis (Parent et al, 1997(Parent et al, , 2002, and other cellular and functional changes (Scharfman, 1992(Scharfman, , 1995. It is important to understand the molecular basis for these long-term alterations.…”

Modulation of vascular endothelial growth factor (VEGF) expression in motor neurons and its electrophysiological effects

“…In some experiments, 4% Neurobiotin in 1 M potassium acetate was used to backfill electrodes; the shaft was filled with 1 M potassium acetate alone. Methods to inject Neurobiotin, and process the slice after Neurobiotin injection, are described elsewhere [18].…”

“…Terminology and measurements of intrinsic properties have been described elsewhere [18]. In brief, resting membrane potential (RMP) was defined as the difference between the intracellular potential and the potential recorded after the electrode was withdrawn from the cell.…”

Functional Implications of Seizure-Induced Neurogenesis

“…Granule cells also have an action potential duration that is consistent with a regular spiking neuron, the action potential has a high ratio of rate of rise to rate of decay (dv/dt ratio), and there is no significant inward rectification or "sag" in response to hyperpolarization from resting potential. 84,85,87,103,104,113 Other cell types may have one or another of these characteristics, but not all of them. Therefore, it was striking when many healthy hilar neurons were encountered that had electrophysiological characteristics of granule cells.…”