Extrasynaptic Glutamate Spillover in the Hippocampus: Dependence on Temperature and the Role of Active Glutamate Uptake

AbstractAt excitatory synapses on CA1 pyramidal cells of the hippocampus, a larger quantal content is sensed by N-methyl-D-aspartic acid receptors (NMDARs) than by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). A novel explanation for this discrepancy is that glutamate released from terminals presynaptic to one cell can diffuse to and activate NMDARs, but not AMPARs, on a neighboring cell. If this occurs in the living brain, it could invalidate the view that glutamatergic synapses function as private communication channels between neurons. Here, we show that the discrepancy in quantal content mediated by the two receptors is greatly decreased at physiological temperature, compared with conventional recording conditions. This effect of temperature is not due to changes in release probability or uncovering of latent AMPARs. It is, however, partially reversed by the glutamate uptake inhibitor dihydrokainate. The results suggest that glutamate transporters play a critical role in limiting the extrasynaptic diffusion of glutamate, thereby minimizing cross-talk between neighboring excitatory synapses

Cardiac Safety Implications of hNav1.5 Blockade and a Framework for Pre-Clinical Evaluation

The human cardiac sodium channel (hNav1.5, encoded by the SCN5A gene) is critical for action potential generation and propagation in the heart. Drug-induced sodium channel inhibition decreases the rate of cardiomyocyte depolarization and consequently conduction velocity and can have serious implications for cardiac safety. Genetic mutations in hNav1.5 have also been linked to a number of cardiac diseases. Therefore, off-target hNav1.5 inhibition may be considered a risk marker for a drug candidate. Given the potential safety implications for patients and the costs of late stage drug development, detection, and mitigation of hNav1.5 liabilities early in drug discovery and development becomes important. In this review, we describe a pre-clinical strategy to identify hNav1.5 liabilities that incorporates in vitro, in vivo, and in silico techniques and the application of this information in the integrated risk assessment at different stages of drug discovery and development

On the action of the anti-absence drug ethosuximide in the rat and cat thalamus

The action of ethosuximide (ETX) on Na+, K+, and Ca2+ currents and on tonic and burst-firing patterns was investigated in rat and cat thalamic neurons in vitro by using patch and sharp microelectrode recordings. In thalamocortical (TC) neurons of the rat dorsal lateral geniculate nucleus (LGN), ETX (0.75-1 mM) decreased the noninactivating Na+ current, INaP, by 60% but had no effect on the transient Na+ current. In TC neurons of the rat and cat LGN, the whole-cell transient outward current was not affected by ETX (up to 1 mM), but the sustained outward current was decreased by 39% at 20 mV in the presence of ETX (0.25-0.5 mM): this reduction was not observed in a low Ca2+ (0.5 mM) and high Mg2+ (8 mM) medium or in the presence of Ni2+ (1 mM) and Cd2+ (100 µm). In addition, ETX (up to 1 mM) had no effect on the low-threshold Ca2+ current, I T, of TC neurons of the rat ventrobasal (VB) thalamus and LGN and in neurons of the rat nucleus reticularis thalami nor on the high-threshold Ca2+ current in TC neurons of the rat LGN. Sharp microelectrode recordings in TC neurons of the rat and cat LGN and VB showed that ETX did not change the resting membrane potential but increased the apparent input resistance at potentials greater than -60 mV, resulting in an increase in tonic firing. In contrast, ETX decreased the number of action potentials in the burst evoked by a low-threshold Ca2+ potential. The frequency of the remaining action potentials in a burst also was decreased, whereas the latency of the first action potential was increased. Similar effects were observed on the burst firing evoked during intrinsic δ oscillations. These results indicate an action of ETX on / NaP and on the Ca2+-activated K+ current, which explains the decrease in burst firing and the increase in tonic firing, and, together with the lack of action on low- and high-threshold Ca2+ currents, the results cast doubts on the hypothesis that a reduction of / τ in thalamic neurons underlies the therapeutic action of this anti-absence medicine

Englerin A Agonizes the TRPC4/C5 Cation Channels to Inhibit Tumor Cell Line Proliferation

<div><p>Englerin A is a structurally unique natural product reported to selectively inhibit growth of renal cell carcinoma cell lines. A large scale phenotypic cell profiling experiment (CLiP) of englerin A on ¬over 500 well characterized cancer cell lines showed that englerin A inhibits growth of a subset of tumor cell lines from many lineages, not just renal cell carcinomas. Expression of the TRPC4 cation channel was the cell line feature that best correlated with sensitivity to englerin A, suggesting the hypothesis that TRPC4 is the efficacy target for englerin A. Genetic experiments demonstrate that TRPC4 expression is both necessary and sufficient for englerin A induced growth inhibition. Englerin A induces calcium influx and membrane depolarization in cells expressing high levels of TRPC4 or its close ortholog TRPC5. Electrophysiology experiments confirmed that englerin A is a TRPC4 agonist. Both the englerin A induced current and the englerin A induced growth inhibition can be blocked by the TRPC4/C5 inhibitor ML204. These experiments confirm that activation of TRPC4/C5 channels inhibits tumor cell line proliferation and confirms the TRPC4 target hypothesis generated by the cell line profiling. In selectivity assays englerin A weakly inhibits TRPA1, TRPV3/V4, and TRPM8 which suggests that englerin A may bind a common feature of TRP ion channels. <i>In vivo</i> experiments show that englerin A is lethal in rodents near doses needed to activate the TRPC4 channel. This toxicity suggests that englerin A itself is probably unsuitable for further drug development. However, since englerin A can be synthesized in the laboratory, it may be a useful chemical starting point to identify novel modulators of other TRP family channels.</p></div

Englerin A affects proliferation of a subset of cancer cell lines across many cell lineages while englerin B is inactive.

<p>(<b>A</b>) Scatterplot of englerin A cell line profiling experiment. (<b>B</b>) Scatterplot of englerin B cell line profiling experiment. Each point represents effect of englerin A or B on growth of a single tumor cell line. Y-axis indicates maximal effect on growth and X-axis indicates potency. Tumor cell line lineage is indicated by color and the legend is in the figure. Englerin A sensitive (circles), englerin A refractory (squares) and englerin A intermediate (diamonds) cell line calls are indicated.</p

Englerin A agonizes the TRPC4/C5 ion channels and channel activation is needed for cell growth inhibition.

<p>(<b>A</b>) Calcium flux stimulated by englerin A in HEK293T cells overexpressing different TRPC proteins (mean +/- standard deviation): TRPC5 (closed diamonds), TRPC4beta (closed squares), TRPC4 (closed circles), TRPC6 (open squares), mock transfected cells (open circles). (<b>B</b>) Membrane depolarization stimulated by englerin A in HEK293T cells overexpressing different TRPC proteins (mean +/- standard deviation), markers as above. (<b>C</b>) TRPC4 current evoked by stimulation of 5 μM Englerin A, saline, or 5 μM Englerin A + 10 μM ML204 in 293T cells with Doxycyline-induced TRPC4. Currents were elicited by 200 ms voltage ramps from -100 to +100 mV, applied every 10 s from holding potential of 0 mV. (<b>D</b>) Summary of englerin A, englerin-B and ML-204 activity on membrane currents (mean +/- S.E.M.) (<b>E</b>) A-673 cell viability in the presence or absence of 50 nM englerin A and/or 50 μM ML204, a TRPC4/C5 channel blocker (mean +/- standard deviation).</p