Wytse J. Wadman

Health Council of the Netherlands: No need to change from SAR to time-temperature relation in electromagnetic fields exposure limits

The Health Council of the Netherlands (HCN) and other organisations hold the basic assumption that induced electric current and the generation and absorption of heat in biological material caused by radiofrequency electromagnetic fields are the only causal effects with possible adverse consequences for human health that have been scientifically established to date. Hence, the exposure guidelines for the 10 MHz–10 GHz frequency range are based on avoiding adverse effects of increased temperatures that may occur of the entire human body at a specific absorption rate (SAR) level above 4 W/kg. During the workshop on Thermal Aspects of Radio Frequency Exposure on 11–12 January 2010 in Gaithersburg, Maryland, USA, the question was raised whether there would be a practical advantage in shifting from expressing the exposure limits in SAR to expressing them in terms of a maximum allowable temperature increase. This would mean defining adverse time–temperature thresholds. In this paper, the HCN discusses the need for this, considering six points: consistency, applicability, quantification, causality, comprehensibility and acceptability. The HCN concludes that it seems unlikely that a change of dosimetric quantity will help us forward in the discussion on the scientific controversies regarding the existence or non-existence of non-thermal effects in humans following long duration, low intensity exposure to electromagnetic fields. Therefore, the HCN favours maintaining the current approach of basic restrictions and reference levels being expressed as SAR and in V/m or µT, respectively.

Deep Brain Stimulation in Patients with Refractory Temporal Lobe Epilepsy

Summary:  Purpose: This pilot study prospectively evaluated the efficacy of long‐term deep brain stimulation (DBS) in medial temporal lobe (MTL) structures in patients with MTL epilepsy.

Potential new antiepileptogenic targets indicated by microarray analysis in a rat model for temporal lobe epilepsy.

To get insight into the mechanisms that may lead to progression of temporal lobe epilepsy, we investigated gene expression during epileptogenesis in the rat. RNA was obtained from three different brain regions [CA3, entorhinal cortex (EC), and cerebellum (CB)] at three different time points after electrically induced status epilepticus (SE): acute phase [group D (1 d)], latent period [group W (1 week)], and chronic epileptic period [group M (3–4 months)]. A group that was stimulated but that had not experienced SE and later epilepsy was also included (group nS). Gene expression analysis was performed using the Affymetrix Gene Chip System (RAE230A). We used GENMAPP and Gene Ontology to identify global biological trends in gene expression data. The immune response was the most prominent process changed during all three phases of epileptogenesis. Synaptic transmission was a downregulated process during the acute and latent phases. GABA receptor subunits involved in tonic inhibition were persistently downregulated. These changes were observed mostly in both CA3 and EC but not in CB. Rats that were stimulated but that did not develop spontaneous seizures later on had also some changes in gene expression, but this was not reflected in a significant change of a biological process. These data suggest that the targeting of specific genes that are involved in these biological processes may be a promising strategy to slow down or prevent the progression of epilepsy. Especially genes related to the immune response, such as complement factors, interleukins, and genes related to prostaglandin synthesis and coagulation pathway may be interesting targets.

Inhibition of the multidrug transporter P-glycoprotein improves seizure control in phenytoin-treated chronic epileptic rats

Summary:  Purpose: Overexpression of multidrug transporters such as P‐glycoprotein (P‐gp) may play a significant role in pharmacoresistance, by preventing antiepileptic drugs (AEDs) from reaching their targets in the brain. Until now, many studies have described increased P‐gp expression in epileptic tissue or have shown that several AEDs act as substrates for P‐gp. However, definitive proof showing the functional involvement of P‐gp in pharmacoresistance is still lacking. Here we tested whether P‐gp contributes to pharmacoresistance to phenytoin (PHT) by using a specific P‐gp inhibitor in a model of spontaneous seizures in rats.

Corticosteroid receptor-dependent modulation of calcium currents in rat hippocampal CA1 neurons

Pyramidal CA1 neurons in the rat hippocampus contain mineralocorticoid (MRs) and glucocorticoid receptors (GRs) for corticosterone, which, in activated form, act as transcription factors of the genome. The relative MR and GR occupation changes throughout the day, with predominant MR occupation under rest in the morning and additional GR occupation in the evening and after stress. We examined the effect of MR and GR activation on Ca currents in hippocampal slices from adrenalectomized (ADX) rats under whole-cell voltage-clamp conditions. In slices from ADX rats, where MRs and GRs are unoccupied, Ca currents (particularly in the low-voltage range) were larger than in neurons from the sham-operated controls; these effects became apparent with a delay of > or = 3 days after ADX. Selective occupation of MRs in tissue from ADX rats greatly (by 70%) and persistently (up to 3 h) reduced transient but also sustained Ca conductances. Voltage dependency and kinetic properties of the currents were not affected. Occupation of GRs as well as MRs by corticosterone (30 nM) resulted in relatively large Ca currents, comparable to those recorded in tissue from mildly stressed sham-operated control animals. Interestingly, exclusive occupation of GRs with 30 nM RU 28362 was not sufficient to induce large Ca currents. The data suggest that the changes in MR and GR occupation throughout the day, related to circadian and stress-induced corticosterone release, are linked to marked alterations in Ca currents, with small Ca currents in the morning and large currents in the evening or after stress.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitability of prefrontal cortical pyramidal neurons is modulated by activation of intracellular type-2 cannabinoid receptors.

The endocannabinoid (eCB) system is widely expressed throughout the central nervous system (CNS) and the functionality of type-1 cannabinoid receptors in neurons is well documented. In contrast, there is little knowledge about type-2 cannabinoid receptors (CB2Rs) in the CNS. Here, we show that CB2Rs are located intracellularly in layer II/III pyramidal cells of the rodent medial prefrontal cortex (mPFC) and that their activation results in IP3R-dependent opening of Ca2+-activated Cl− channels. To investigate the functional role of CB2R activation, we induced neuronal firing and observed a CB2R-mediated reduction in firing frequency. The description of this unique CB2R-mediated signaling pathway, controlling neuronal excitability, broadens our knowledge of the influence of the eCB system on brain function.

Alternatively Spliced Isoforms of TRIP8b Differentially Control h Channel Trafficking and Function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (h channels) are the molecular basis for the current, Ih, which contributes crucially to intrinsic neuronal excitability. The subcellular localization and biophysical properties of h channels govern their function, but the mechanisms controlling these characteristics, and especially the potential role of auxiliary subunits or other binding proteins, remain unclear. We focused on TRIP8b, an h channel-interacting protein that colocalizes with HCN1 in cortical and hippocampal pyramidal neuron dendrites, and found that it exists in multiple alternative splice variants with distinct effects on h channel trafficking and function. The developmentally regulated splice variants of TRIP8b all shared dual, C terminus-located interaction sites with HCN1. When coexpressed with HCN1 in heterologous cells individual TRIP8b isoforms similarly modulated gating of Ih, causing a hyperpolarizing shift in voltage dependence of channel activation, but differentially upregulated or downregulated Ih current density and HCN1 surface expression. In hippocampal neurons, coexpression of TRIP8b isoforms with HCN1 produced isoform-specific changes of HCN1 localization. Interestingly, the TRIP8b isoforms most abundant in the brain are those predicted to enhance h channel surface expression. Indeed, shRNA knockdown of TRIP8b in hippocampal neurons significantly reduced native Ih. Thus, although TRIP8b exists in multiple splice isoforms, our data suggest that the predominant role of this protein in brain is to promote h channel surface expression and enhance Ih. Because Ih expression is altered in models of several diseases, including temporal lobe epilepsy, TRIP8b may play a role in both normal neuronal function and in aberrant neuronal excitability associated with neurological disease.

Increased hippocampal noradrenaline is a biomarker for efficacy of vagus nerve stimulation in a limbic seizure model

J. Neurochem. (2011) 117, 461–469.

Kindling of the hippocampus induces spatial memory deficits in the rat

Since kindling produces electrophysiological and morphological changes in the brain area stimulated, it may well affect behavioural functions dependent on the kindled area. Using an 8-arm maze, it was found that hippocampal kindling can induce specific memory deficits in spatial tasks. Reference (long-term) memory as well as working (short-term) memory were impaired. The largest impairment was observed during the period in which generalized convulsions occurred. Working memory but not reference memory impairment was reversible. Hippocampal kindling may be a useful experimental model for investigating behavioural deficits correlated with epileptogenesis.

CALCIUM CURRENTS IN PYRAMIDAL CA1 NEURONS IN VITRO AFTER KINDLING EPILEPTOGENESIS IN THE HIPPOCAMPUS OF THE RAT

Calcium is an important second messenger which plays a role in the regulation of neuronal excitability and in many forms of synaptic plasticity. In kindling epileptogenesis, a model of focal epilepsy, calcium plays an important role. The in situ patch-clamp technique was used to record calcium currents in slices obtained from kindled rats and controls. We found that low-voltage-activated calcium currents, probably of dendritic origin, were larger after kindling (80%). The transient high-voltage-activated calcium currents were also enhanced after kindling (50% higher). The increase of the current is accompanied by a decrease in the time constant of inactivation. The change was still present six weeks after the kindling stimulations were stopped. These data demonstrate that low-voltage-activated calcium currents are involved in epileptogenesis. Their enhancement in the dendrites will boost synaptic depolarization and result in enhanced calcium influx, which is critically dependent on the specific activation pattern.