In many principal brain neurons, the fast, all-or-none Na ϩ spike initiated at the proximal axon is followed by a slow, graded afterdepolarization (ADP). The spike ADP is critically important in determining the firing mode of many neurons; large ADPs cause neurons to fire bursts of spikes rather than solitary spikes. Nonetheless, not much is known about how and where spike ADPs are initiated. We addressed these questions in adult CA1 pyramidal cells, which manifest conspicuous somatic spike ADPs and an associated propensity for bursting, using sharp and patch microelectrode recordings in acutely isolated hippocampal slices and single neurons. Voltage-clamp commands mimicking spike waveforms evoked transient Na ϩ spike currents that declined quickly after the spike but were followed by substantial sustained Na ϩ spike aftercurrents. Drugs that blocked the persistent Na ϩ current (I NaP ), markedly suppressed the sustained Na ϩ spike aftercurrents, as well as spike ADPs and associated bursting. Ca 2ϩ spike aftercurrents were much smaller, and reducing them had no noticeable effect on the spike ADPs. Truncating the apical dendrites affected neither spike ADPs nor the firing modes of these neurons. Application of I NaP blockers to truncated neurons, or their focal application to the somatic region of intact neurons, suppressed spike ADPs and associated bursting, whereas their focal application to distal dendrites did not. We conclude that the somatic spike ADPs are generated predominantly by persistent Na ϩ channels located at or near the soma. Through this action, proximal I NaP critically determines the firing mode and spike output of adult CA1 pyramidal cells.

Post-mortem human brain tissue represents a vast potential source of neural progenitor cells for use in basic research as well as therapeutic applications. Here we describe five human neural progenitor cell cultures derived from cortical tissue harvested from premature infants. Time-lapse videomicrography of the passaged cultures revealed them to be highly dynamic, with high motility and extensive, evanescent intercellular contacts. Karyotyping revealed normal chromosomal complements. Prior to differentiation, most of the cells were nestin, Sox2, vimentin, and/or GFAP positive, and a subpopulation was doublecortin positive. Multilineage potential of these cells was demonstrated after differentiation, with some subpopulations of cells expressing the neuronal markers beta-tubulin, MAP2ab, NeuN, FMRP, and Tau and others expressing the oligodendroglial marker O1. Still other cells expressed the classic glial marker glial fibrillary acidic protein (GFAP). RT-PCR confirmed nestin, SOX2, GFAP, and doublecortin expression and also showed epidermal growth factor receptor and nucleostemin expression during the expansion phase. Flow cytometry showed high levels of the neural stem cell markers CD133, CD44, CD81, CD184, CD90, and CD29. CD133 markedly decreased in high-passage, lineage-restricted cultures. Electrophysiological analysis after differentiation demonstrated that the majority of cells with neuronal morphology expressed voltage-gated sodium and potassium currents. These data suggest that post-mortem human brain tissue is an important source of neural progenitor cells that will be useful for analysis of neural differentiation and for transplantation studies.

The generation of high-frequency spike bursts ("complex spikes"), either spontaneously or in response to depolarizing stimuli applied to the soma, is a notable feature in intracellular recordings from hippocampal CA1 pyramidal cells (PCs) in vivo. There is compelling evidence that the bursts are intrinsically generated by summation of large spike afterdepolarizations (ADPs). Using intracellular recordings in adult rat hippocampal slices, we show that intrinsic burst-firing in CA1 PCs is strongly dependent on the extracellular concentration of Ca 2ϩ ([Ca 2ϩ

Chronically epileptic rats, produced by prior injection of pilocarpine, were used to investigate whether changes in intrinsic neuronal excitability may contribute to the epileptogenicity of the hippocampus in experimental temporal lobe epilepsy (TLE). Paired extra‐/intracellular electrophysiological recordings were made in the CA1 pyramidal layer in acute hippocampal slices prepared from control and epileptic rats and perfused with artificial cerebrospinal fluid (ACSF). Whereas orthodromic activation of CA1 neurons evoked only a single, stimulus‐graded population spike in control slices, it produced an all‐or‐none burst of population spikes in epileptic slices. The intrinsic firing patterns of CA1 pyramidal cells were determined by intrasomatic positive current injection. In control slices, the vast majority (97 %) of the neurons were regular firing cells. In epileptic slices, only 53 % the pyramidal cells fired in a regular mode. The remaining 47 % of the pyramidal cells were intrinsic bursters. These neurons generated high‐frequency bursts of three to six spikes in response to threshold depolarizations. A subgroup of these neurons (10.1 % of all cells) also burst fired spontaneously even after suppression of synaptic activity. In epileptic slices, burst firing in most cases (ca 70 %) was completely blocked by adding the Ca2+ channel blocker Ni2+ (1 mm) to, or removing Ca2+ from, the ACSF, but not by intracellular application of the Ca2+ chelater 1,2‐bis(o‐aminophenoxy)ethane‐N,N,N′,N′‐tetra‐acetic acid (BAPTA), suggesting it was driven by a Ca2+ current. Spontaneously recurring population bursts were observed in a subset of epileptic slices. They were abolished by adding 2 μm 6‐cyano‐7‐nitro‐quinoxaline‐2,3‐dione (CNQX) to the ACSF, indicating that synaptic excitation is critical for the generation of these events. All sampled pyramidal cells fired repetitively during each population burst. The firing of spontaneously active bursters anteceded the population discharge, whereas most other pyramidal cells began to fire conjointly with the first population spike. Thus, spontaneous bursters are likely to be the initiators of spontaneous population bursts in epileptic slices. The dramatic up‐regulation of intrinsic bursting in CA1 pyramidal cells, particularly the de novo appearance of Ca2+‐dependent bursting, may contribute to the epileptogenicity of the hippocampus in the pilocarpine model of TLE. These findings have important implications for the pharmacological treatment of medically refractory human TLE.

A single episode of status epilepticus (SE) causes numerous structural and functional changes in the brain that can lead to the development of a chronic epileptic condition. Most studies of this plasticity have focused on changes in excitatory and inhibitory synaptic properties. However, the intrinsic firing properties that shape the output of the neuron to a given synaptic input may also be persistently affected by SE. Thus, 54% of CA1 pyramidal cells, which normally fire in a regular mode, are persistently converted to a bursting mode after an episode of SE induced by the convulsant pilocarpine. In this model, intrinsic bursts evoked by threshold-straddling depolarizations, and their underlying spike afterdepolarizations (ADPs), were resistant to antagonists of N-, P/Q-, or L-type Ca2+ channels but were readily suppressed by low (30-100 microm) concentrations of Ni2+ known to block T- and R-type Ca2+ channels. The density of T-type Ca2+ currents, but not of other pharmacologically isolated Ca2+ current types, was upregulated in CA1 pyramidal neurons after SE. The augmented T-type currents were sensitive to Ni2+ in the same concentration range that blocked the novel intrinsic bursting in these neurons (IC50 = 27 microm). These data suggest that SE may persistently convert regular firing cells to intrinsic bursters by selectively increasing the density of a Ni2+-sensitive T-type Ca2+ current. This nonsynaptic plasticity considerably amplifies the output of CA1 pyramidal neurons to synaptic inputs and most probably contributes to the development and expression of an epileptic condition after SE.