Stress and corticosterone increase the readily releasable pool of glutamate vesicles in synaptic terminals of prefrontal and frontal cortex.

Stress and glucocorticoids alter glutamatergic transmission, and the outcome of stress may range from plasticity enhancing effects to noxious, maladaptive changes. We have previously demonstrated that acute stress rapidly increases glutamate release in prefrontal and frontal cortex via glucocorticoid receptor and accumulation of presynaptic SNARE complex. Here we compared the ex vivo effects of acute stress on glutamate release with those of in vitro application of corticosterone, to analyze whether acute effect of stress on glutamatergic transmission is mediated by local synaptic action of corticosterone. We found that acute stress increases both the readily releasable pool (RRP) of vesicles and depolarization-evoked glutamate release, while application in vitro of corticosterone rapidly increases the RRP, an effect dependent on synaptic receptors for the hormone, but does not induce glutamate release for up to 20 min. These findings indicate that corticosterone mediates the enhancement of glutamate release induced by acute stress, and the rapid non-genomic action of the hormone is necessary but not sufficient for this effect.

Quinolinic acid induced neurodegeneration in the striatum: a combined in vivo and in vitro analysis of receptor changes and microglia activation.

PURPOSE: Huntington's disease (HD) is a progressive neurodegenerative disorder, which is characterised by prominent neuronal cell loss in the basal ganglia with motor and cognitive disturbances. One of the most well-studied pharmacological models of HD is produced by local injection in the rat brain striatum of the excitotoxin quinolinic acid (QA), which produces many of the distinctive features of this human neurodegenerative disorder. Here, we report a detailed analysis, obtained both in vivo and in vitro of this pharmacological model of HD. MATERIALS AND METHODS: By combining emission tomography (PET) with autoradiographic and immunocytochemical confocal laser techniques, we quantified in the QA-injected striatum the temporal behavior (from 1 to 60 days from the excitotoxic insult) of neuronal cell density and receptor availability (adenosine A(2A) and dopamine D(2) receptors) together with the degree of microglia activation. RESULTS: Both approaches showed a loss of adenosine A(2A) and dopamine D(2) receptors paralleled by an increase of microglial activation. CONCLUSION: This combined longitudinal analysis of the disease progression, which suggested an impairment of neurotransmission, neuronal integrity and a reversible activation of brain inflammatory processes, might represent a more quantitative approach to compare the differential effects of treatments in slowing down or reversing HD in rodent models with potential applications to human patients.

Advanced tracer techniques to monitor synaptic activity.

The two approaches presented here bypass postsynaptic receptors as indicators of quantal release, and thus they can provide information which is clearly distinct from that obtained with standard electrophysiological techniques. Indeed, the inherently variable responsiveness of the postsynaptic membrane makes it an unreliable indicator of presynaptic activity and this has fueled a lot of controversy, particularly in the area of synaptic plasticity. A major advantage of these two methods is their ability to detect changes at the single bouton level. This offers a lot of advantages including the possibility to study the functional role for exo-endocytosis but also plasticity against a background of great variability among a large number of synapses. The spatial resolving power of FM1-43 and anti-synaptotagmin antibodies may be valuable in future studies of spread of LTP between neighboring synapses and in the mapping the pattern of neuronal activity in complex networks of neurons.

Hyperpolarization-activated cyclic nucleotide-gated channel 1 is a molecular determinant of the cardiac pacemaker current I(f).

The pacemaker current I(f) of the sinoatrial node (SAN) is a major determinant of cardiac diastolic depolarization and plays a key role in controlling heart rate and its modulation by neurotransmitters. Substantial expression of two different mRNAs (HCN4, HCN1) of the family of pacemaker channels (HCN) is found in rabbit SAN, suggesting that the native channels may be formed by different isoforms. Here we report the cloning and heterologous expression of HCN1 from rabbit SAN and its specific localization in pacemaker myocytes. rbHCN1 is an 822-amino acid protein that, in human embryonic kidney 293 cells, displayed electrophysiological properties similar to those of I(f), suggesting that HCN1 can form a pacemaker channel. The presence of HCN1 in the SAN myocytes but not in nearby heart regions, and the electrophysiological properties of the channels formed by it, suggest that HCN1 plays a central and specific role in the formation of SAN pacemaker currents.

Calcium: the common theme in vesicular cycling.

Synaptic vesicle release is known to depend on calcium. A new technique for separating endocytosis from exocytosis now shows that calcium regulates both processes.

Agrin controls synaptic differentiation in hippocampal neurons.

Agrin controls the formation of the neuromuscular junction. Whether it regulates the differentiation of other types of synapses remains unclear. Therefore, we have studied the role of agrin in cultured hippocampal neurons. Synaptogenesis was severely compromised when agrin expression or function was suppressed by antisense oligonucleotides and specific antibodies. The effects of antisense oligonucleotides were found to be highly specific because they were reversed by adding recombinant agrin and could not be detected in cultures from agrin-deficient animals. Interestingly, the few synapses formed in reduced agrin conditions displayed diminished vesicular turnover, despite a normal appearance at the EM level. Thus, our results demonstrate the necessity of agrin for synaptogenesis in hippocampal neurons.

Loose-patch recordings of single quanta at individual hippocampal synapses.

Synapses in the central nervous system are typically studied by recording electrical responses from the cell body of the postsynaptic cell. Because neurons are normally connected by multiple synaptic contacts, these postsynaptic responses reflect the combined activity of many thousands synapses, and it remains unclear to what extent the properties of individual synapses can be deduced from the population response. We have therefore developed a method for recording the activity of individual hippocampal synapses. By capturing an isolated presynaptic bouton inside a loose-patch pipette and recording from the associated patch of postsynaptic membrane, we were able to detect miniature excitatory postsynaptic currents ('minis') arising from spontaneous vesicle exocytosis at a single synaptic site, and to compare these with minis recorded simultaneously from the cell body. The average peak conductance at a single synapse was about 900 pS, corresponding roughly to the opening of 90 AMPA-type glutamate-receptor channels. The variability in this conductance was about 30%, matching the value reported for the neuromuscular junction. Given that our synapses displayed single postsynaptic densities (PSDs), this variability is larger than would be predicted from the random opening of receptor channels, suggesting that they are not saturated by the content of a single vesicle. Therefore the response to a quantum of neurotransmitter at these synapses is not limited by the number of available postsynaptic receptors.

Common and distinct fusion proteins in axonal growth and transmitter release.

We have used the proteolytic properties of botulinum and tetanus neurotoxins (BoNT, TeNT) to cleave three proteins of the membrane fusion machinery, SNAP-25, VAMP/synaptobrevin, and syntaxin, in developing and differentiated rat central neurons in vitro. Then, we have studied the capacity of neurons to extend neurites, make synapses, and release neurotransmitters. All the toxins showed the expected specificity with the exception that BoNT/C cleaved SNAP-25 in addition to syntaxin and induced rapid neuronal death. In developing neurons, cleavage of SNAP-25 with BoNT/A inhibited axonal growth and prevented synapse formation. In contrast, cleavage of VAMP with TeNT or BoNT/B had no effects on neurite extension and synaptogenesis. All the toxins tested inhibited transmitter release in differentiated neurons, and cleavage of VAMP resulted in the strongest inhibition. These data indicate that SNAP-25 is involved in vesicle fusion for membrane expansion and transmitter release, whereas VAMP is selectively involved in transmitter release. In addition, our results support the hypothesis that synaptic activity is not essential for synapse formation in vitro.

Presynaptic component of long-term potentiation visualized at individual hippocampal synapses.

Long-term potentiation has previously been studied with electrophysiological techniques that do not readily separate presynaptic and postsynaptic contributions. Changes in exocytotic-endocytotic cycling have now been monitored at synapses between cultured rat hippocampal neurons by measuring the differential uptake of antibodies that recognize the intraluminal domain of the synaptic vesicle protein synaptotagmin. Vesicular cycling increased markedly during glutamate-induced long-term potentiation. The degree of potentiation was heterogeneous, appearing greater at synapses at which the initial extent of vesicular turnover was low. Thus, changes in presynaptic activity were visualized directly and the spatial distribution of potentiation could be determined at the level of single synaptic boutons.

LTP expression: hanging like a yo-yo?

Until a few years ago long-term potentiation or LTP was not a very popular subject for a cell biology-oriented audience. Recently, however, the picture has completely changed, mainly because most of the LTP papers are now dealing with specific molecules, second messengers, biochemical pathways, gene regulation, Ca2+ homeostasis, and so forth. This very big transition, has not led yet to a clear-cut picture on the actual locus of change after LTP induction and on the 'memory' molecule that bears the permanent change. Perhaps the biggest limitation is still the very small knowledge we have on how central synapses work. For this reason, at the moment, the most popular and conservative view is that LTP expression involves both pre- and postsynaptic changes. The aim of this review is to briefly summarize the status of LTP expression and to describe some possible future developments based on new approaches and techniques.

Glutamate-induced long-term potentiation of the frequency of miniature synaptic currents in cultured hippocampal neurons.

Glutamate application at synapses between hippocampal neurons in culture produces long-term potentiation of the frequency of spontaneous miniature synaptic currents, together with long-term potentiation of evoked synaptic currents. The mini frequency potentiation is initiated postsynaptically and requires activity of NMDA receptors. Although the frequency of unitary quantal responses increases strongly, their amplitude remains little changed with potentiation. Tests of postsynaptic responsiveness rule out recruitment of latent glutamate receptor clusters. Thus, postsynaptic induction can lead to enhancement of presynaptic transmitter release. The sustained potentiation of mini frequency is expressed even in the absence of Ca2+ entry into presynaptic terminals.

Persistent signalling and changes in presynaptic function in long-term potentiation.

Long-term potentiation (LTP) is an example of a persistent change in synaptic function in the mammalian brain, thought to be essential for learning and memory. At the synapse between hippocampal CA3 and CA1 neurons LTP is induced by a Ca2+ influx through glutamate receptors of the NMDA (N-methyl-D-aspartate) type (see Collingridge et al 1992, this volume). How does a rise in [Ca2+]i lead to enhancement of synaptic function? We have tested the popular hypothesis that Ca2+ acts via a Ca(2+)-dependent protein kinase. We found that long-lasting synaptic enhancement was prevented by prior intracellular injection of potent and selective inhibitory peptide blockers of either protein kinase C (PKC) or Ca2+/calmodulin-dependent protein kinase II (CaMKII), such as PKC(19-31) or CaMKII(273-302), but not by control peptides. Evidently, activity of both PKC and CaMKII is somehow necessary for the postsynaptic induction of LTP. To determine if these kinases are also involved in the expression of LTP, we impaled cells with microelectrodes containing protein kinase inhibitors after LTP had already been induced. Strikingly, established LTP was not suppressed by a combination of PKC and CaMKII blocking peptides, or by intracellular postsynaptic H-7. However, established LTP remained sensitive to bath application of H-7. Thus, the persistent signal may be a persistent kinase, but if so, the kinase cannot be accessed within the postsynaptic cell. Evidence for a presynaptic locus of expression comes from our studies of quantal synaptic transmission under whole-cell voltage clamp. We find changes in synaptic variability expected to result from enhanced presynaptic transmitter release, but little or no increase in quantal size. Furthermore, miniature synaptic currents in hippocampal cultures are increased in frequency but not amplitude as a result of a glutamate-driven postsynaptic induction. The combination of postsynaptic induction and presynaptic expression necessitates a retrograde signal from the postsynaptic cell to the presynaptic terminal.

[Ca2+]i oscillations from internal stores sustain exocytic secretion from the chromaffin cells of the rat.

A large (65%) fraction of in vitro cultured rat chromaffin cells exhibit spontaneous [Ca2+]i oscillations, and the rest can be recruited to oscillate by appropriate stimulations. Based on fura-2 single cell [Ca2+]i measurements, evidence is provided that these oscillations originate, via the activation of Ca(2+)-induced Ca(2+)-release, from intracellular Ca2+ stores in rapid equilibrium with extracellular Ca2+. By combining [Ca2+]i measurements with a specific plaque secretion assay we demonstrate that oscillating cells exhibit a spontaneous exocytic secretory activity whereas the cells with stable [Ca2+]i do not. [Ca2+]i oscillations appear therefore to account for the high unstimulated catecholamine release, an activity typical of the chromaffin cells of the rat.

Cytosolic calcium ion and arachidonic acid release and metabolism in macrophages.

The aim of the present work was to elucidate the role of cytosolic calcium ions, [Ca2+]i, in the control of arachidonic acid release and metabolism. [Ca2+]i was measured in resident peritoneal rat macrophages loaded with Fura2, and compared with the release of leukotriene B4(LTB4) and prostaglandin l2 (PGL2, assayed through its hydrolysis product 6-keto-PGF1 alpha). The calcium ionophore A 23187 stimulated both an increase in [Ca2+]i and the release of LTB4 and 6-keto-PGF1 alpha. On the contrary, zymosan and opsonized zymosan, while stimulating eicosanoid release to an extent only slightly lower than A 23187, did not affect [Ca2+]i. Lipopolysaccharide stimulated 6-keto-PGF1 alpha, but not LTB4, release, without affecting [Ca2+]i. In parallel experiments, macrophages were prelabelled with [3H]arachidonic acid and the release of total 3H-products was assayed and taken as an index of phospholipase activity. A 23187, zymosan and opsonized zymosan increased the release of 3H-products in the presence of Ca2+. When extracellular Ca2+ was removed, the ionophore-induced 3H-products release was greatly blunted, while the release induced by zymosan was actually augmented. Our data indicate that a generalized [Ca2+]i increase is not necessary for arachidonic acid release and metabolism in rat peritoneal macrophages.

Spontaneous [Ca2+]i fluctuations in rat chromaffin cells do not require inositol 1,4,5-trisphosphate elevations but are generated by a caffeine- and ryanodine-sensitive intracellular Ca2+ store.

A considerable fraction (65%) of single rat chromaffin cells loaded with the fluorescent [Ca2+]i indicator fura-2 exhibited spontaneous rhythmic fluctuations with an average period of approximately 100 s. Parallel patch clamp experiments as well as fura-2 experiments carried out in Ca2(+)-free and other modified media in the presence of Ca2+ and Na+ channel blockers indicated an origin from intracellular stores. Appropriate concentrations of agonists (bradykinin and histamine) for receptors (B2 and H1) that trigger generation of inositol 1,4,5-trisphosphate induced increased fluctuation frequency, recruitment of silent cells, and large [Ca2+]i changes at high doses. These effects were blocked by cell pretreatment with neomycin, a drug that inhibits inositol 1,4,5-trisphosphate generation. In contrast, spontaneous fluctuations and the effects of another drug, caffeine, which also induced increased frequency and recruitment, were unaffected by neomycin. Ryanodine caused first a prolongation and then (approximately 10 min) a block of both spontaneous fluctuations and caffeine effects, where the single transients after bradykinin and histamine were maintained. Caffeine and ryanodine are known to affect selectively the process of calcium-induced Ca2+ release; this is the first demonstration of [Ca2+]i fluctuation activity arising from Ca2(+)-induced Ca2+ release in nonmuscle cells with no strict requirement for inositol 1,4,5-trisphosphate involvement.

Ca2+ channels and intracellular Ca2+ stores in neuronal and neuroendocrine cells.

Changes in [Ca2+]i are essential in modulating a variety of cellular functions. In no other cell type does the regulation of [Ca2+]i reach the level of sophistication observed in cells of neuronal origin. Because of its physicochemical characteristics, the fluorescent Ca2+ indicator Fura-2 has become extremely popular among neuroscientists. The use of this probe, however, has generated a number of problems, in particular, extracytosolic trapping and leakage from intact cells. In the first part of this contribution we briefly discuss the practical application of Fura-2 to the study of [Ca2+]i in primary cultures of neurons and astrocytes. In the second part, we review some recent data (mainly from our laboratories) obtained in neurons and neuroendocrine cells, concerning the regulation of different types of Ca2+ channels and the role and mechanism of intracellular Ca2+ mobilization. The experimental evidence supporting the existence of a previously unrecognised organelle, the calciosome, that we hypothesize represents the functional equivalent in non-muscle cells of sarcoplasmic reticulum, will also briefly be discussed.

Effects of protein kinase C stimulation and free Ca2+ rise in mammalian egg activation.

Protein phosphorylation activity, chromosome segregation, and cortical granule exocytosis (CGE) have been studied in mouse eggs activated parthenogenetically by specific PKC stimulators such as 4 beta-phorbol 12-myristate 13-acetate (PMA) and 1-oleyl-2-acetylglycerol (OAG), or by agents inducing an immediate increase in cytosolic calcium concentration ([Ca2+]i) such as ethanol and Ca-ionophore A23187. When protein phosphorylation activity of mouse eggs was analyzed 10 min after different activation treatments, the phosphorylation of a 32 kDa polypeptide was a feature common to all different parthenogenetic agents used. The appearance of such labeling was independent of an increasing [Ca2+]i, as indicated by direct measurements of 1) cytosolic Ca2+ concentration with fura-2 and 2) exogenous Ca2+ entrance into activated eggs. Emission of the second polar body was blocked in PMA-elicited parthenogenones, whereas it was apparently normal in OAG-treated eggs, unless the eggs were continuously exposed to OAG. CGE was almost immediate in ethanol-activated eggs, but in PMA-treated cells, it occurred significantly later, with a timing corresponding to that found for the appearance of sustained Ca2+ oscillations in this system. Here, we propose that in mammalian eggs 1) PKC stimulation represents an early regulatory step in egg activation; 2) this kinase activity is turned off before the second meiotic cleavage; and 3) cytosolic free Ca2+ rise is essential for CGE occurrence.