Abnormal neural encoding of repeated speech stimuli in noise in children with learning problems

OBJECTIVES

This study investigated whether neurophysiologic responses to repeated speech stimuli, presented in quiet and noise, differed between normal children (NL) and children with learning problems (LP).

METHODS

Subjects were normal-hearing, school-age children. NL subjects scored significantly better than LP subjects on measures of reading, spelling and speech sound discrimination. Stimuli (40 ms /da/) were presented to the right ear at 80 dB SPL. Stimuli were presented in trains of four, separated within trains by 360 ms. The interval between trains was 1060 ms. Stimuli were presented in quiet and in white noise (S/N+15). Cortical responses were recorded from an electrode placed along the midline at Cz.

RESULTS

Correlations between the first and 4th responses were lower in noise than in quiet for LP subjects only. Response correlations in quiet were no different between groups. There were no root-mean-square (RMS) amplitude differences between groups.

CONCLUSIONS

Response correlation in noise suggested that the LP population consisted of two subgroups, one whose responses appeared relatively normal, and another whose responses were severely degraded by repetition in noise. Response correlations in noise were related to behavioral measures of auditory processing and spelling. These findings suggest that abnormal, asynchronous, auditory cortical encoding may underlie some language-based learning problems.

A discrete amino terminal domain of Kv1.5 and Kv1.4 potassium channels interacts with the spectrin repeats of alpha-actinin-2

The interaction between the amino terminus of Kv1-type potassium channels and alpha-actinin-2 has been investigated. Using a combination of yeast two-hybrid analysis and in vitro binding assays, alpha-actinin-2 was found to bind to the N-termini of both Kv1.4 and Kv1.5 but not to the equivalent segments of Kv1.1, Kv1.2 or Kv1.3. Deletion analysis in the in vitro binding assays delineated the actinin binding region of Kv1.5 to between amino acids 73 and 148 of the channel. The Kv1.5 binding sites in alpha-actinin-2 were found to lie within actinin's internal spectrin repeats. Unlike the reported interaction between actinin and the NMDA receptor, calmodulin was found to have no effect on actinin binding to Kv1.5.

herg encodes a K+ current highly conserved in tumors of different histogenesis: a selective advantage for cancer cells?

The human ether-a-go-go-related gene (herg) encodes a K+ current (IHERG) that plays a fundamental role in heart excitability by regulating the action potential repolarization (IKr); mutations of this gene are responsible for the chromosome 7-linked long QT syndrome (LQT2). In this report, we show that in a variety (n = 17) of tumor cell lines of different species (human and murine) and distinct histogenesis (neuroblastoma, rhabdomyosarcoma, adenocarcinoma, lung microcytoma, pituitary tumors, insulinoma beta-cells, and monoblastic leukemia), a novel K+ inward-rectifier current (IIR), which is biophysically and pharmacologically similar to IHERG, can be recorded with the patch-clamp technique. Northern blot experiments with a human herg cDNA probe revealed that both in human and murine clones the very high expression of herg transcripts can be quantified in at least three clearly identifiable bands, suggesting an alternative splicing of HERG mRNA. Moreover, we cloned a cDNA encoding for IIR from the SH-SY5Y human neuroblastoma. The sequence of this cDNA result was practically identical to that already reported for herg, indicating a high conservation of this gene in tumors. Consistently, the expression of this clone in Xenopus oocytes showed that the encoded K+ channel had substantially all of the biophysical and pharmacological properties of the native IIR described for tumor cells. In addition, in the tumor clones studied, IIR governs the resting potential, whereas it could not be detected either by the patch clamp or the Northern blot techniques in cells obtained from primary cell cultures of parental tissues (sensory neurons and myotubes), whose resting potential is controlled by the classical K+ anomalous rectifier current. This current substitution had a profound impact on the resting potential, which was markedly depolarized in tumors as compared with normal cells. These results suggest that IIR is normally only expressed during the early stages of cell differentiation frozen by neoplastic transformation, playing an important pathophysiological role in the regulatory mechanisms of neoplastic cell survival. In fact, because of its biophysical features, IIR, besides keeping the resting potential within the depolarized values required for unlimited tumor growth, could also appear suitable to afford a selective advantage in an ischemic environment.

Stable expression and characterization of the human brain potassium channel Kv2.1: blockade by antipsychotic agents

We have cloned the cDNA encoding the voltage-dependent K+ channel Kv2.1 from human brain (hKv2.1). RNase protection and RT-PCR (reverse transcriptase-PCR) experiments reveal abundant Kv2.1 transcripts in human brain with virtually no expression detectable in human heart. hKv2.1 has been stably transfected into a human glioblastoma cell line, and transformed cells display large, slowly activating outward currents. The kinetics, steady-state activation and inactivation parameters, and external tetraethylammonium sensitivity were all similar to those described previously for hKv2.1 channels transiently expressed in Xenopus oocytes or other mammalian cell lines. A number of dopamine receptor antagonist/antipsychotic agents were shown to block hKv2.1. Trifluoperizine, trifluperidol and pimozide produced time-dependent blockade of hKv2.1 with IC50 values of approx. 1-2 microM. The diphenylbutylpiperidine fluspirilene was shown to be 4-5-fold more potent than the other agents tested inhibiting hKv2.1 current with an IC50 value of 297 nM. The block produced by fluspirilene was both time- and frequency-dependent. Furthermore, fluspirilene (1 microM) shifted the midpotential of the hKv2.1 steady-state inactivation curve by approx. 15 mV in the hyperpolarizing direction. These results demonstrate the usefulness of this transfection system for the pharmacological characterization of hKv2. 1. Fluspirilene proved to be a relatively potent blocker of hKv2.1 and may provide a useful starting point for the development of more potent and selective agents active against this brain K+ channel.