On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Increased NR2A expression and prolonged decay of NMDA-induced calcium transient in cerebellum of TgDyrk1A mice, a mouse model of Down syndrome

Transgenic mice overexpressing Dyrk1A (TgDyrk1A), a Down syndrome (DS) candidate gene, exhibit motor and cognitive alterations similar to those observed in DS individuals. To gain new insights into the molecular consequences of Dyrk1A overexpression underlying TgDyrk1A and possibly DS motor phenotypes, microarray studies were performed. Transcriptome analysis showed an upregulation of the NR2A subunit of the NMDA type of glutamate receptors in TgDyrk1A cerebellum. NR2A protein overexpression was also detected in TgDyrk1A cerebellar homogenates, in the synaptosome-enriched fraction and in TgDyrk1A primary cerebellar granular neuronal cultures (CGNs). In TgDyrk1A synaptosomes, calcium-imaging experiments showed a higher calcium uptake after NMDA stimulation. Similarly, NMDA administration promoted longer calcium transients in TgDyrk1A CGNs. Taken together, these results show that NMDA-induced calcium rise is altered in TgDyrk1A cerebellar neurons and indicate that calcium signaling is dysregulated in TgDyrk1A mice cerebella. These findings suggest that DYRK1A overexpression might contribute to the dysbalance in the excitatory transmission found in the cerebellum of DS individuals and DS mouse models.

Ca(2+)-evoked synaptic transmission and neurotransmitter receptor levels are impaired in the forebrain of trkb (-/-) mice

To determine the in vivo targets of long-lasting actions of TrkB signaling on synaptic function we analyze synaptic components of excitatory and inhibitory circuits in the cerebral cortex of trkB (-/-) mice. First, we show that K(+)-evoked glutamate and GABA release from forebrain mutant synaptosomes was decreased. Moreover, the dependence of regulated exocytosis on the SNARE SNAP-25 and the Ca(2+)-dependent neurotransmitter release were also impaired in trkB (-/-) mice. We also analyzed postsynaptic glutamate and GABA(A) ionotropic receptors in cortical areas of trkB mutant mice. By using Western blot we observed decreased levels of the AMPA receptor subunits GluR2/3 and GluR4 in trkB (-/-) forebrains. In contrast, the forebrain of mutant mice exhibited increased levels of the GABA(A) receptor subunit alpha3 and alpha5 and a reduction of the gamma2 subunit. Immunocytochemical analysis showed that the hippocampus and neocortex of mutant mice exhibited decreased numbers of interneurons positive for distinct AMPA and GABA(A) receptor subunits. Furthermore, alteration of inhibitory circuits in trkB (-/-) mice was also shown by the low expression of the GABA-synthesizing enzyme glutamic acid decarboxylase in mutant cortical areas. The present results indicate that long-lasting TrkB signaling is required for the precise adjustment of neurotransmitter release and for the correct composition of the fast glutamatergic and GABAergic receptor subunits in vivo.

Investigation of the modulation of glutamate release by sodium channels using neurotoxins

The modulation of neurotransmitter release by calcium channels is well established, yet, sodium channels were regarded mainly as charge carriers. Many lines of evidence suggest a more fine-tuning role played by sodium channels. Using rat cerebrocortical isolated nerve endings (synaptosomes) and two toxins that have separate sites of action on sodium channels and provoke distinct changes in channel kinetics, we were able to show that depending on the rate of increase in channel conductance, the outcome in terms of neurotransmitter release and calcium channel types coupled to that event are different. Mainly, our study focused on veratridine, an alkaloid from lilaceous plants that binds to sodium channel toxin site 2, and tityustoxin, a toxin purified from the venom of the Brazilian yellow scorpion Tityus serrulatus that binds to site 3. Veratridine induces a slower increase in intrasynaptosomal sodium and calcium concentrations, slower depolarization, delayed exocytosis and a slower and predominantly calcium-independent glutamate release, when compared to tityustoxin.Thus, we have used these two toxins to investigate the events that start with sodium entry and culminate with the release of glutamate in isolated nerve endings (synaptosomes) from rat cerebral cortex. With that in mind we measured intrasynaptosomal free sodium concentration [Na(+)](i), intrasynaptosomal free calcium concentration [Ca(2+)](i), membrane potential, exocytosis and glutamate release using fluorescent probes.

The role of ceramide in the modulation of intracellular Ca2+ levels by interleukin 1 beta in rat cortical synaptosomes

We recently demonstrated that the pro-inflammatory cytokine, interleukin 1beta (IL-1beta) elevates intracellular free Ca2+ levels ([Ca2+]i) in rat cortical synaptosomes in a manner involving activation of the IL-1 receptor and stimulation of p42 mitogen-activated protein (MAP) kinase. We now report that the effects of IL-1beta on [Ca2+]i are mimicked by the sphingolipid metabolite ceramide. In cortical synaptosomes ceramide elevates [Ca2+]i in a p42 MAP kinase-dependent manner, and we conclude that the effects of IL-1beta on Ca2+ homeostasis involve ceramide as an upstream component of the p42 MAP kinase pathway.

Enhancement of (45)Ca(2+) influx and voltage-dependent Ca(2+) channel activity by beta-amyloid-(1-40) in rat cortical synaptosomes and cultured cortical neurons. Modulation by the proinflammatory cytokine interleukin-1beta

Beta-amyloid protein is thought to underlie the neurodegeneration associated with Alzheimer's disease by inducing Ca(2+)-dependent apoptosis. Elevated neuronal expression of the proinflammatory cytokine interleukin-1beta is an additional feature of neurodegeneration, and in this study we demonstrate that interleukin-1beta modulates the effects of beta-amyloid on Ca(2+) homeostasis in the rat cortex. beta-Amyloid-(1-40) (1 microM) caused a significant increase in (45)Ca(2+) influx into rat cortical synaptosomes via activation of L- and N-type voltage-dependent Ca(2+) channels and also increased the amplitude of N- and P-type Ca(2+) channel currents recorded from cultured cortical neurons. In contrast, interleukin-1beta (5 ng/ml) reduced the (45)Ca(2+) influx into cortical synaptosomes and inhibited Ca(2+) channel activity in cultured cortical neurons. Furthermore, the stimulatory effects of beta-amyloid protein on Ca(2+) influx were blocked following exposure to interleukin-1beta, suggesting that interleukin-1beta may govern neuronal responses to beta-amyloid by regulating Ca(2+) homeostasis.

Regulation of intracellular Ca2+ concentration by interleukin-1beta in rat cortical synaptosomes: an age-related study

The pro-inflammatory cytokine interleukin-1beta (IL-1beta) is released by cells during injury and stress, and increased neuronal expression of IL-1beta is a feature of age-related neurodegeneration. We have recently reported that IL-1beta has a biphasic effect on the K+-induced rise in intracellular Ca2+ concentration ([Ca2+]i) in cortical synaptosomes, exerting an inhibitory effect on the K+-induced rise in [Ca2+]i at lower (3.5 ng/mL) concentrations and a stimulatory effect on the K+-induced rise in [Ca2+]i at higher (100 ng/mL) concentrations. In the present study, we observed that the K+-induced rise in [Ca2+]i was inhibited to a similar extent by the lower concentration of IL-1beta in cortical synaptosomes prepared from young (3-month-old), middle-aged (12-month-old) and aged (24-month-old) rats. In contrast, cortical synaptosomes prepared from the aged rats exhibited an increased susceptibility to the higher concentration of IL-1beta, resulting in a marked elevation in [Ca2+]i. We propose that the age-related increase in neuronal concentration of IL-1beta promotes a dramatic elevation in [Ca2+]i following membrane depolarization, thereby altering Ca2+ homeostasis and exacerbating neuronal vulnerability to excitotoxicity.

Halothane and isoflurane alter calcium dynamics in rat cerebrocortical synaptosomes

An increase in synaptosomal Ca2+ triggers neurotransmitter release and volatile anesthetics have been shown to inhibit neurotransmitter release by inhibition of Ca2+ entry. We have examined the effect of isoflurane and halothane on the kinetics of increase and decrease of Ca2+ in rat cerebrocortical synaptosomes ([Ca2+]in). We have also used specific Ca2+ antagonists to examine the role of L-, N-, and P-type Ca2+ channels. Synaptosomal [Ca2+]in was measured spectrofluorometrically using fura-2 as a Ca2+ reporter; Ca2+ transients were initiated by depolarization with 40 mM KCl. We found that < or = 1 minimum alveolar anesthetic concentration halothane and isoflurane decreased peak [Ca2+]in by approximately 40%, that both anesthetics decreased the rate of [Ca2+]in increase and decrease, that specific voltage-dependent calcium channel antagonists had little effect on peak or plateau [Ca2+]in, and that the volatile anesthetics increased the permeability of synaptosomal membranes to Ca2+. These results suggest that the volatile anesthetics, at clinically relevant concentrations, can alter Ca2+ homeostasis in the synapse.

IMPLICATIONS

Clinically relevant concentrations of halothane and isoflurane markedly depress K+-evoked increases in rat cerebrocortical synaptosomal calcium (Ca2+) unrelated to L-, N-, and P-type voltage-dependent calcium channels and increase the Ca2+ permeability of the synaptosomal membrane. These changes in Ca2+ dynamics could have profound effects on Ca2+ signaling in the synapse.

Biphasic modulation of intracellular Ca2+ concentration by interleukin-1beta in cortical synaptosomes: involvement of a pertussis toxin-sensitive G-protein and mitogen-activated protein kinase

The modulation of intracellular Ca2+ concentration ([Ca2+]i) by the pro-inflammatory cytokine interleukin-1beta (IL-1beta) was assessed in synaptosomes loaded with the Ca2+-sensitive dye, Fura-2AM. IL-1beta was found to exert a biphasic effect on the KCl-induced rise in [Ca2+]i, extending an inhibitory effect at lower (3.5 ng/ml) concentrations, and a stimulatory effect at high (100 ng/ml) concentrations. The inhibitory action of IL-1beta on [Ca2+]i was sensitive to pertussis toxin (PTX; 2 microg/ml), indicating a role for a PTX-sensitive G-protein, but was unaffected by the p42 mitogen-activated protein kinase kinase (MAP kinase kinase) inhibitor, PD 098059 (2 microM). In contrast, the stimulatory action of higher concentrations of IL-1beta on [Ca2+]i was blocked by PD 098059 and unaffected by PTX. We conclude that the biphasic actions of IL-1beta on the KCl-induced rise in [Ca2+]i are mediated through activation of alternative second messenger pathways.