Electrophysiological characterization of sodium channel types in the HCN-1A human cortical cell line

High sensitivity bioassay of paralytic (PSP) and amnesic (ASP) algal toxins based on the fluorimetric detection of [Ca(2+)](i) in rat cortical primary cultures

A high sensitivity bioassay able to recognise small amounts of paralytic and amnesic toxins in algal acetic extracts is described. The method is based on the measure of intracellular [Ca(2+)](i) in primary cultures of rat cortical neurones preloaded with Fura-2 and submitted to electrical field stimulation. Under normal conditions the basal [Ca(2+)](i) level was about 50-100 nM and was nearly doubled during the peaks induced by trains of electrical pulses at 10 Hz for 10 s. Saxitoxin (STX) 3.5 nM and tetrodotoxin (TTX) 24 nM halved the peaks height without affecting basal [Ca(2+)](i). Conversely, domoic acid increased the basal [Ca(2+)](i) (EC(50)=3. 7 microM) and decreased the calcium peaks (EC(50)=7.3 microM). CNQX (a competitive antagonist of AMPA/KA receptors) at 10 microM shifted to the right by a factor of 3 the concentration-response curves of domoic acid. The extracts of non-toxic algae were well tolerated by up to 10 microg protein/ml, whereas extracts of Alexandrium lusitanicum at 1-4 microg protein/ml reduced [Ca(2+)](i) peaks and increased basal calcium levels. This toxic effect of A. lusitanicum was unexpected since parallel HPLC analysis showed only the presence of gonyautoxins, known to act like saxitoxin. Therefore, the bioassay on rat cortical neurones revealed a complex composition of the toxins present in A. lusitanicum. The relevance of fluorimetric detection of [Ca(2+)](i) in primary neuronal cultures in the evaluation of algal risk is stressed.

The tetrodotoxin-insensitive sodium current in rat dorsal root ganglia is unlikely to involve the expression of the tetrodotoxin-resistant sodium channel, SkM2

Tetrodotoxin-insensitive (TTX-I) sodium currents have been recorded from newborn and adult rat sensory neurons, but the sodium channel gene(s) responsible for the TTX-I current are unknown. Because SkM2, one of six voltage-sensitive sodium channel genes cloned from rat, encodes the only cloned channel that is relatively resistant to tetrodotoxin, we sought to test whether the TTX-I current in rat sensory neurons is due to the SkM2 channel. We hypothesized that the TTX-I current might be generated from (1) an RNA splicing variant of SkM2, (2) post-translational modification of the SkM2 protein, or (3) interaction with alternate additional channel subunits. SkM2 mRNA expression was examined in newborn rat dorsal root ganglia (DRG) by RNase protection assay. No SkM2 expression was detected. Therefore, we conclude that the TTX-I sodium current in DRG is unlikely to result from the expression of the SkM2 gene.

Endogenous bursting due to altered sodium channel function in rat hippocampal CA1 neurons

Intracellular recordings were obtained from pyramidal neurons in the rat hippocampal CA1 area in order to investigate membrane mechanisms involved in veratridine-induced epileptiform activity. Veratridine (0.03-0.2 microM) caused no changes in the passive membrane parameters including the resting potential, input resistance, and time constant. In the presence of small doses (0.03-0.1 microM) of veratridine, a single stimulus caused a relatively slow, large, synaptic-independent potential called the slow depolarizing after-potential (SDAP). When the hippocampal slice was treated with higher doses of veratridine (over 0.1 microM), bursting, or seizure-like activity (SLA) occurred in response to a brief super threshold intracellular stimulation. The duration of SLA bursting could be as long as ten seconds depending on the amplitude of SDAP, and was independent of the stimulus strength or duration. The frequency and configuration of SLA were sensitive to changes in membrane potential caused by applied DC current. At 0.3 microM or higher, veratridine induced spontaneous rhythmic bursting that was also sensitive to membrane potential changes. The evoked or spontaneous bursting is characterized by being: (1) independent of synaptic transmission in that it persisted after complete blockade of evoked synaptic potential with kynurenic acid (0.5 mM), (2) sensitive to selective inhibition by low doses of the specific sodium channel blockers tetrodotoxin (TTX) or cocaine with no apparent influence on the evoked action potential. These results indicate that endogenous SLA bursting can be induced in hippocampal CA1 pyramidal neurons when certain properties of sodium channels are altered by veratridine.

Human cortical neuronal (HCN) cell lines: a model for amyloid beta neurotoxicity

Human cortical neuronal cell lines HCN-1A and HCN-2 are killed for following exposure of the differentiated cells to amyloid beta-peptide(1-40), a component of senile plaques and other amyloid deposits in brains from Alzheimer's patients. We present a model of A beta toxicity uncomplicated by the presence of other cell types that can be used to address the mechanism of A beta neurotoxicity. This model will be useful in the evaluation of neuroprotective compounds which may attenuate cortical neuronal loss in Alzheimer's disease.