Methylmercury reduces synaptic transmission and neuronal excitability in rat hippocampal slices.

In a previous study, we pointed out that the neurotoxic action evoked by methylmercury (MeHg), a potent environmental pollutant responsible for fatal food poisoning, is associated with alterations of cellular excitability by irreversible blockade of sodium and calcium currents. Here, we investigated the MeHg effects on synaptic transmission and neuronal plasticity using extracellular field recording in CA1 area of rat hippocampal slices. MeHg caused a fast and drastic depression of evoked field excitatory postsynaptic potentials (fEPSPs) in a concentration-dependent manner with an IC50 of 25.7 µM. This depression was partially caused by the irreversible reduction of axon recruitment deduced from the decrement of the fiber volley (FV) amplitude. Nevertheless, this MeHg-induced synaptic depression represents a true reduction of synaptic efficacy, as judged by input/output curves. In addition, a reduction on presynaptic release of glutamate was detected with the paradigm of paired-pulse facilitation during MeHg application. Moreover, MeHg also reduced population spike (PS) ampxlitude, and this effect was more prominent when the PS was evoked by ortodromic stimulation than by antidromic stimulation. Interestingly, despite these strong effects of MeHg on synaptic transmission and excitability, this compound did not modify the induction of long-term synaptic potentiation (LTP). The effects described here for MeHg were irreversible or very slowly reversible after drug wash-out. In summary, the blockade of sodium and calcium channels by MeHg affects synaptic transmission and cellular excitability but not synaptic plasticity.

Choline induces opposite changes in pyramidal neuron excitability and synaptic transmission through a nicotinic receptor-independent process in hippocampal slices.

Choline is present at cholinergic synapses as a product of acetylcholine degradation. In addition, it is considered a selective agonist for α5 and α7 nicotinic acetylcholine receptors (nAChRs). In this study, we determined how choline affects action potentials and excitatory synaptic transmission using extracellular and intracellular recording techniques in CA1 area of hippocampal slices obtained from both mice and rats. Choline caused a reversible depression of evoked field excitatory postsynaptic potentials (fEPSPs) in a concentration-dependent manner that was not affected by α7 nAChR antagonists. Moreover, this choline-induced effect was not mimicked by either selective agonists or allosteric modulators of α7 nAChRs. Additionally, this choline-mediated effect was not prevented by either selective antagonists of GABA receptors or hemicholinium, a choline uptake inhibitor. The paired pulse facilitation paradigm, which detects whether a substance affects presynaptic release of glutamate, was not modified by choline. On the other hand, choline induced a robust increase of population spike evoked by orthodromic stimulation but did not modify that evoked by antidromic stimulation. We also found that choline impaired recurrent inhibition recorded in the pyramidal cell layer through a mechanism independent of α7 nAChR activation. These choline-mediated effects on fEPSP and population spike observed in rat slices were completely reproduced in slices obtained from α7 nAChR knockout mice, which reinforces our conclusion that choline modulates synaptic transmission and neuronal excitability by a mechanism independent of nicotinic receptor activation.

Presynaptic muscarinic receptor subtypes involved in the enhancement of spontaneous GABAergic postsynaptic currents in hippocampal neurons.

We investigated the effects of muscarinic acetylcholine receptor (mAChR) activation on GABAergic synaptic transmission in rat hippocampal neurons. Current-clamp recordings revealed that methacholine produced membrane depolarization and action potential firing. Methacholine augmented the bicuculline-sensitive and GABA(A) -mediated frequency of spontaneous inhibitory postsynaptic currents (sIPSCs); the action of methacholine had a slow onset and longer duration. The increase in methacholine-evoked sIPSCs was completely inhibited by atropine and was insensitive to glutamatergic receptor blockers. Interestingly, methacholine action was not inhibited by intracellular perfusion with GDP-ß-S, suggesting that muscarinic effects on membrane excitability and sIPSC frequency are mainly presynaptic. McN-A-343 and pirenzepine, selective agonist and antagonist of the m1 mAChR subtype, respectively, neither enhanced sIPSCs nor inhibited the methacholine effect. However, the m3-m5 mAChR antagonist 4-DAMP, and the m2-m4 mAChR antagonist himbacine inhibited the methacholine effect. U73122, an IP(3) production inhibitor, and 2APB, an IP(3) receptor blocker, drastically decreased the methacholine effect. Recording of miniature events revealed that besides the effect exerted by methacholine on membrane firing properties and sIPSC frequency, muscarinic receptors also enhanced the frequency of mIPSCs with no effect on their amplitude, possibly modulating the molecular machinery subserving vesicle docking and fusion and suggesting a tight colocalization at the active zone of the presynaptic terminals. These data strongly suggest that by activating presynaptic m2, m3, m4 and m5 mAChRs, methacholine can increase membrane excitability and enhance efficiency in the GABA release machinery, perhaps through a mechanism involving the release of calcium from the endoplasmic reticulum.

Down-modulation of Ca2+ channels by endogenously released ATP and opioids: from the isolated chromaffin cell to the slice of adrenal medullae.

Modifications in Ca(2+) influx may lead to profound changes in the cell activity associated with Ca(2+)-dependent processes, from muscle contraction and neurotransmitter release to calcium-mediated cell death. Therefore, calcium entry into the cell requires fine regulation. In this context, understanding of the modulation of voltage-dependent Ca(2+) channels seems to be critical. The modulatory process results in the enhancement or decrement of calcium influx that may regulate the local and global cytosolic Ca(2+) concentrations. Here, we summarize the well-established data on this matter described in isolated chromaffin cells by our laboratory and others, and the new results we have obtained in a more physiological preparation: freshly isolated slices of mouse adrenal medullae.

Modulation of calcium channels by taurine acting via a metabotropic-like glycine receptor.

Taurine is one of the most abundant free amino acids in the central nervous system, where it displays several functions. However, its molecular targets remain unknown. It is well known that taurine can activate GABA-A and strychnine-sensitive glycine receptors, which increases a chloride conductance. In this study, we describe that acute application of taurine induces a dose-dependent inhibition of voltage-dependent calcium channels in chromaffin cells from bovine adrenal medullae. This taurine effect was not explained by the activation of either GABA-A, GABA-B or strychnine-sensitive glycine receptors. Interestingly, glycine mimicked the modulatory action exerted by taurine on calcium channels, although the acute application of glycine did not elicit any ionic current in these cells. Additionally, the modulation of calcium channels exerted by both taurine and glycine was prevented by the intracellular dialysis of GDP-ß-S. Thus, the modulation of voltage-dependent calcium channels by taurine seems to be mediated by a metabotropic-like glycinergic receptor coupled to G-protein activation in a membrane delimited pathway.