Astrocytes are implicated in BDNF-mediated enhancement of hippocampal long-term potentiation

It is known that astrocytes, by the Ca2+-dependent release of gliotransmitters, which then act in pre- and post-synaptic receptors, modulate neuronal transmission and plasticity. Thus, hippocampal θ-burst long-term potentiation (LTP), which is a form of synaptic plasticity, can be modulated by astrocytes, since these cells release gliotransmitters that are crucial for the maintenance of LTP. Therefore, in this study, we hypothesized that the facilitatory action of BDNF upon LTP would involve astrocytes. To address that possibility, fEPSP recordings were performed in CA3-CA1 area of hippocampal slices from three different experimental models: Wistar rats where astrocytic metabolism was selectively reduced by a gliotoxin, the DL-fluoricitric acid (FC), IP3R2-/- mice model, which lack IP3R2-mediated Ca2+-signaling in astrocytes and dn-SNARE transgenic mice, in which the SNARE-dependent release of gliotransmitters. For the three models we observed that the astrocytic impairment abolished the excitatory BDNF effect upon hippocampal LTP, only while inducing LTP with a mild θ-burst stimulation paradigm. The present data shows for the first time that astrocytes play an active role in the facilitatory action of BDNF upon LTP, depending on stimulation paradigm.

In VitroAntiophidian Mechanisms ofHypericum brasilienseChoisy Standardized Extract: Quercetin-Dependent Neuroprotection

The neuroprotection induced byHypericum brasilienseChoisy extract (HBE) and its main active polyphenol compound quercetin, againstCrotalus durissus terrificus(Cdt) venom and crotoxin and crotamine, was enquired at both central and peripheral mammal nervous system. Cdt venom (10 μg/mL) or crotoxin (1 μg/mL) incubated at mouse phrenic nerve-diaphragm preparation (PND) induced an irreversible and complete neuromuscular blockade, respectively. Crotamine (1 μg/mL) only induced an increase of muscle strength at PND preparations. At mouse brain slices, Cdt venom (1, 5, and 10 μg/mL) decreased cell viability. HBE (100 μg/mL) inhibited significantly the facilitatory action of crotamine (1 μg/mL) and was partially active against the neuromuscular blockade of crotoxin (1 μg/mL) (data not shown). Quercetin (10 μg/mL) mimicked the neuromuscular protection of HBE (100 μg/mL), by inhibiting almost completely the neurotoxic effect induced by crotoxin (1 μg/mL) and crotamine (1 μg/mL). HBE (100 μg/mL) and quercetin (10 μg/mL) also increased cell viability in mice brain slices. Quercetin (10 μg/mL) was more effective than HBE (100 μg/mL) in counteracting the cell lysis induced by Cdt venom (1 and 10 μg/mL, resp.). These results and a further phytochemical and toxicological investigations could open new perspectives towards therapeutic use ofHypericum brasiliensestandardized extract and quercetin, especially to counteract the neurotoxic effect induced by snake neurotoxic venoms.

Regulation of TRPM2 by Extra- and Intracellular Calcium

TRPM2 is a calcium-permeable nonselective cation channel that is opened by the binding of ADP-ribose (ADPR) to a C-terminal nudix domain. Channel activity is further regulated by several cytosolic factors, including cyclic ADPR (cADPR), nicotinamide adenine dinucleotide phosphate (NAADP), Ca2+ and calmodulin (CaM), and adenosine monophosphate (AMP). In addition, intracellular ions typically used in patch-clamp experiments such as Cs+ or Na+ can alter ADPR sensitivity and voltage dependence, complicating the evaluation of the roles of the various modulators in a physiological context. We investigated the roles of extra- and intracellular Ca2+ as well as CaM as modulators of ADPR-induced TRPM2 currents under more physiological conditions, using K+-based internal saline in patch-clamp experiments performed on human TRPM2 expressed in HEK293 cells. Our results show that in the absence of Ca2+, both internally and externally, ADPR alone cannot induce cation currents. In the absence of extracellular Ca2+, a minimum of 30 nM internal Ca2+ is required to cause partial TRPM2 activation with ADPR. However, 200 μM external Ca2+ is as efficient as 1 mM Ca2+ in TRPM2 activation, indicating an external Ca2+ binding site important for proper channel function. Ca2+ facilitates ADPR gating with a half-maximal effective concentration of 50 nM and this is independent of extracellular Ca2+. Furthermore, TRPM2 currents inactivate if intracellular Ca2+ levels fall below 100 nM irrespective of extracellular Ca2+. The facilitatory effect of intracellular Ca2+ is not mimicked by Mg2+, Ba2+, or Zn2+. Only Sr2+ facilitates TRPM2 as effectively as Ca2+, but this is due to Sr2+-induced Ca2+ release from internal stores rather than a direct effect of Sr2+ itself. Together, these data demonstrate that cytosolic Ca2+ regulates TRPM2 channel activation. Its facilitatory action likely occurs via CaM, since the addition of 100 μM CaM to the patch pipette significantly enhances ADPR-induced TRPM2 currents at fixed [Ca2+]i and this can be counteracted by calmidazolium. We conclude that ADPR is responsible for TRPM2 gating and Ca2+ facilitates activation via calmodulin.