Increased exocytotic capability of rat cerebellar granule neurons cultured under depolarizing conditions

Multifaceted analysis of nanotoxicity using primary cultured neurons

Various nanomaterials have been produced with the development of nanotechnology, some of which have been reported to have adverse effects on several types of cells, organs, and the environment. It has been suggested that some small nanoparticles can cross the blood-brain barrier and accumulate in the brain, which may be a potential cause of brain diseases. Neuronal cells are vulnerable to hypoxia, hypotrophy, and mechanical and oxidative stress. Therefore, it is essential to assess the toxicity of nanoparticles to neurons accurately. In this report, we describe a primary culture protocol to evaluate the toxicity of nanoparticles on neurons, a potential high-throughput method for assessing the cytotoxicity, and a method for evaluating the effect on neuronal maturation. This report assessed the toxicity of silicon dioxide, zinc oxide, and iron nanoparticles using rat hippocampal neurons, which are used frequently in pharmacological and physiological studies. Based on the methods and protocols we reported in this report, it may be possible to evaluate nanotoxicity to various neurons by using primary cultures of other brain regions (cerebral cortex, cerebellum, thalamus, etc.), spinal cord, and peripheral nerves.

Vesicular release of glutamate from hippocampal neurons in culture: an immunocytochemical assay

Glutamate, the main excitatory neurotransmitter in the brain, may cause excitotoxic damage through excessive release during a number of pathological conditions. We have developed an immunocytochemical assay to investigate the mechanisms and regulation of glutamate release from intact, cultured neurons. Our results indicate that cultured hippocampal neurons have a large surplus of glutamate available for release upon chemically induced depolarization. Long incubations with high K(+)-concentrations, and induction of repetitive action potentials with the K(+)-channel blocker 4-aminopyridine (4-AP), caused a significant reduction in glutamate labeling in a subset of boutons, demonstrating that transmitter release exceeded the capacity for replenishment. The number of boutons where release exceeded replenishment increased continuously with time of stimulation. This depletion was Ca(2+)-dependent and sensitive to bafilomycin A1 (baf), indicating that it was dominated by vesicular release mechanisms. The depletion of glutamate from cell bodies and dendrites was also Ca(2+)-dependent. Thus, under the present conditions, cytosolic glutamate is taken up in vesicles prior to release, and the main escape route for the amino acid is through vesicular exocytosis. Depolarization with lower concentrations of K(+) caused sustainable release of glutamate, i.e., without full depletion.

Long-lasting synapse formation in cultured rat hippocampal neurons after repeated PKA activation

Recently, we reported that the repeated activation of cyclic-AMP-dependent protein kinase (PKA) in the rat hippocampus under tissue culture conditions induced the enhancement of excitatory postsynaptic potential (EPSP), which lasted more than 2 weeks and was accompanied by the formation of morphologically identifiable synapses. Here we examined whether an equivalent synapse formation is induced in dissociated cell cultures of rat hippocampal neurons. Brief (15-min) application of Sp-cAMPS (a membrane-permeable analog of cyclic AMP) induced an increase in the number of synaptic sites (identified by the apposition of immunocytochemically labeled pre- and postsynaptic structures). There were two types of increase: a short-lasting one that lasted less than 24 h after a single application of Sp-cAMPS, and a long-lasting one that lasted more than 2 weeks after repeated applications. The long-lasting increase in synaptic sites was dependent on the time and interval of application and was suppressed by Rp-cAMPS (a PKA inhibitor). The synapses were judged to be active based on the endocytosis of FM1-43, a fluorescent dye. Electron microscopy confirmed the increase in the number of synaptic ultrastructures. The present results show that the synaptogenesis induced by repeated PKA activation is reproducible in a neuronal network that is reconstituted under dissociated cell culture conditions. This experimental system, together with the synaptogenesis in the slice culture system described previously, serves as a good in vitro model for the analysis of the process of conversion from short-lasting plasticity (lasting for hours) into a long-lasting one (lasting for days-weeks).

Survival promotion of rat cerebellar granule neurons by co-culture with pontine explant

Cultured cerebellar granule neuron (CGN) of the rat is the most frequently used model system for analysis of activity-dependent neuronal survival. CGNs do not survive longer than 2 weeks in a standard culture medium unless KCl (or other excitants such as glutamate) is added. It is assumed that KCl represents synaptic activity, but no tests have been made on whether the survival of CGNs really depends on the synaptic input. Here we co-cultured CGNs with an explant of the pons including the basilar pontine nucleus (BPN), which is one of the input sources of CGNs in vivo, to confirm if synaptic input is really a determinant for the survival of these cells. In this co-culture system, the viability of CGNs was significantly increased without the addition of KCl. The survival promotion was confined to the population of CGNs having contact with neurites of BPN and was cancelled by an application of tetrodotoxin or antagonists of glutamate receptors, indicating that the survival depended on synaptic activity. Explants of other glutamatergic tissues including the hippocampus failed to promote the survival, although neurites grew out from these explants as vigorously as from the BPN explants. Calcium and FM1-43 imaging examinations revealed that the CGNs had formed functional synapses with the BPN explant but not with the hippocampal explant. These results, confirming the assumption that synaptic activity determines neuronal survival, provide evidence for presynaptic contribution to the survival.

Non-synaptic exocytosis enhanced in rat cerebellar granule neurons cultured under survival-promoting conditions

Cultured rat cerebellar granule neurons (CGNs) are often used to analyze activity-dependent neuronal selection occurring during brain development. The CGNs survive long only when the culture medium contains a depolarizing agent. However, it is argued whether the depolarization critical for survival is of presynaptic or postsynaptic compartment. Since CGNs form no synapses among them, it is generally assumed that the latter would be the case. But it is possible that the depolarization would induce exocytosis of survival-promoting substances whether or not CGNs form synapses. Here we directly examined the exocytotic activities of CGNs under survival-promoting and survival-limiting conditions by electron microscopy to support this possibility. CGNs possessed clusters of synaptic vesicle-like vesicles (SVVs) in neuritic varicosities. CGNs cultured in high-KCl medium had significantly smaller SVV clusters than those cultured in low-KCl medium. The number of SVVs increased when the high KCl-cultured CGNs were transferred to low-KCl medium, indicating a sustained high rate of exocytosis in high-KCl medium. The majority of the varicosities containing SVVs were not apposed to definite postsynaptic structures, indicating that exocytosis occurs from a non-synaptic surface. Fluorescence Ca(2+) imaging revealed that the high KCl-cultured CGNs had spots of high Ca(2+) along their neurites, corresponding to the varicosities.

Brain-derived neurotrophic factor modulates cerebellar plasticity and synaptic ultrastructure

Neurotrophins are key regulators of neuronal survival and function. Here we show that TrkB, the receptor for brain-derived neurotrophic factor (BDNF), is located at parallel fiber to Purkinje cell (PF/PC) synapses of the cerebellum. To determine the effects of TrkB receptor activation on synapse formation and function, we examined the parallel fiber to Purkinje cell synapses of mice with a targeted deletion of the BDNF gene. Although Purkinje cell dendrites are abnormal in BDNF -/- mice, PF/PC synapses are still able to form. Immunohistochemical analysis of mutant animals revealed the formation of numerous PF/PC synapses with the appropriate apposition of presynaptic and postsynaptic proteins. These synapses are functional, and no differences were detected in the waveform of evoked EPSCs, the amplitude of spontaneous mini-EPSCs, or the response to prolonged 10 Hz stimulus trains. However, paired-pulse facilitation, a form of short-term plasticity, is significantly decreased in BDNF -/- mice. Detailed ultrastructural analysis of the presynaptic terminals demonstrated that this change in synaptic function is accompanied by an increase in the total number of synaptic vesicles in mutant mice and a decrease in the proportion of vesicles that are docked. These data suggest that BDNF regulates both the mechanisms that underlie short-term synaptic plasticity and the steady-state relationship between different vesicle pools within the terminal.

p38 mitogen-activated protein kinase regulates low potassium-induced c-Jun phosphorylation and apoptosis in cultured cerebellar granule neurons

Cultured rat cerebellar granule neurons are widely used as a model system for studying neuronal apoptosis. After maturation by culturing in medium containing 26 mm potassium (high K(+)), changing to medium containing 5 mm potassium (low K(+); LK) rapidly induces neuronal apoptosis. Then over 50% of granule cells die within 24 h. However, the molecular mechanisms by which the LK-induced apoptosis occurs in cultured cerebellar granule cells remain unclear. In the present study, we found that p38 MAP kinase (p38) was an important factor for LK-induced apoptosis. Three hours after changing to LK medium, p38 was markedly activated. In addition, SB203580, a specific inhibitor of p38, strongly inhibited the phosphorylation and expression of c-Jun in LK-induced apoptosis of cultured cerebellar granule cells. In vitro kinase assay using glutathione S-transferase-c-Jun as a substrate showed that p38 directly phosphorylated c-Jun. Furthermore, in the presence of SB203580, about 80% of neurons survived. These results indicate that p38 regulates LK-induced apoptosis of cerebellar granule neurons.