Biomedical subjects

Yildirim Sara

Publications and source records attributed to Yildirim Sara.

10 recordsLinked to original sources

PubMed2007

Fast synaptic vesicle reuse slows the rate of synaptic depression in the CA1 region of hippocampus.

During short-term synaptic depression, neurotransmission rapidly decreases in response to repetitive action potential firing. Here, by blocking the vacuolar ATPase, alkalinizing the extracellular pH, or exposing hippocampal slices to pH buffers, we impaired neurotransmitter refilling, and electrophysiologically tested the role of vesicle reuse in synaptic depression. Under all conditions, synapses onto hippocampal CA1 pyramidal cells showed faster depression with increasing stimulation frequencies. At 20 Hz, compromising neurotransmitter refilling increased depression within 300 ms reaching completion within 2 s, suggesting a minimal contribution of reserve vesicles to neurotransmission. In contrast, at 1 Hz, depression emerged gradually and became significant within 100 s. Moreover, the depression induced by pH buffers was reversible with a similar frequency dependence, suggesting that the frequency-dependent increase in depression was caused by impairment of rapid synaptic vesicle reuse. These results indicate that synaptic vesicle trafficking impacts the kinetics of short-term synaptic plasticity at an extremely rapid time scale.

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PubMed2005

Reciprocal interaction of serotonin and neuronal activity in regulation of cAMP-responsive element-dependent gene expression.

Neuronal activity triggers multiple signal transduction pathways and potently regulates gene expression in the brain. In the central nervous system, in addition to the synaptic input, neurons are subject to neuromodulatory influences that can activate the same signaling elements. However, the principles that govern the interaction of neuromodulators and neuronal activity in the regulation of gene expression are unclear. Here, we examine how serotonergic neuromodulation interacts with neuronal activity in the regulation of gene expression in hippocampal neurons. We show that cAMP-response element binding protein (CREB) phosphorylation and gene expression were stimulated by serotonin (5-HT) in the absence of neuronal activity. In contrast, in the presence of neuronal activity, 5-HT inhibited gene expression down to the baseline, although neuronal activity alone was sufficient to maximally activate gene expression. The ability of 5-HT to stimulate CREB phosphorylation in the absence of neuronal activity or inhibit CREB phosphorylation during activity was due to a tight balance between protein kinases and phosphatases that could be physiologically tilted by different serotonergic receptors or exogenously influenced by phosphatase and kinase inhibitors. Taken together, these results suggest a reciprocal inhibitory interaction between neuronal activity and 5-HT in the regulation of cAMP response element-dependent gene expression in hippocampal neurons.

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PubMed2005

Phorbol esters target the activity-dependent recycling pool and spare spontaneous vesicle recycling.

Using electrophysiology and styryl dye imaging, we studied the effect of phorbol 12-myristate 13-acetate (PMA) on activity-dependent and spontaneous vesicle recycling. In electrophysiological experiments, we found that the PMA effect depended on the maturational state of the synapses. Spontaneous neurotransmitter release from nascent synapses without a functional readily releasable pool (RRP) was unresponsive to PMA application. In contrast, mature synapses responded robustly to PMA application, consistent with previous studies. Using styryl dye imaging, we found that there was a PMA-dependent increase in the size of the RRP when PMA was present before, during, or after activity-dependent dye uptake, suggesting that this effect involves an increase in the population of the RRP by vesicles recruited from the reserve pool. Additionally, we found that when PMA was present during spontaneous dye uptake, there was an increase in dye labeling, and these additional dye-loaded vesicles showed rapid destaining in response to strong stimulation and were also releasable by hypertonic sucrose. In contrast, these observations were not reproducible when PMA treatment was performed after spontaneous dye uptake and extracellular dye washout. Together, these findings suggest that the increased spontaneous neurotransmission in the presence of PMA was attributable to release of vesicles from the RRP rather than an effect of PMA on the spontaneously recycling pool. Thus, the phorbol esters selectively regulate the activity-dependent pool of vesicles, indicating that priming mechanisms that prepare vesicles for fusion, which are targeted by phorbol esters, are different for the spontaneous and evoked forms of fusion.

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PubMed2005

An isolated pool of vesicles recycles at rest and drives spontaneous neurotransmission.

Spontaneous synaptic vesicle fusion is a common property of all synapses. To trace the origin of spontaneously fused vesicles in hippocampal synapses, we tagged vesicles with fluorescent styryl dyes, antibodies against synaptotagmin-1, or horseradish peroxidase. We could show that synaptic vesicles recycle at rest, and after spontaneous exo-endocytosis, they populate a reluctantly releasable pool of limited size. Interestingly, vesicles in this spontaneously labeled pool were more likely to re-fuse spontaneously compared to vesicles labeled with activity. We found that blocking vesicle refilling at rest selectively depleted neurotransmitter from spontaneously fusing vesicles without significantly altering evoked transmission. Furthermore, in the absence of the vesicle SNARE protein synaptobrevin (VAMP), activity-dependent and spontaneously recycling vesicles could mix, suggesting a role for synaptobrevin in the separation of the two pools. Taken together these results suggest that spontaneously recycling vesicles and activity-dependent recycling vesicles originate from distinct pools with limited cross-talk with each other.

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PubMed2005

Selective capability of SynCAM and neuroligin for functional synapse assembly.

Synaptic cell adhesion is central for synapse formation and function. Recently, the synaptic cell adhesion molecules neuroligin 1 (NL1) and SynCAM were shown to induce presynaptic differentiation in cocultured neurons when expressed in a non-neuronal cell. However, it is uncertain how similar the resulting artificial synapses are to regular synapses. Are these molecules isofunctional, or do all neuronal cell adhesion molecules nonspecifically activate synapse formation? To address these questions, we analyzed the properties of artificial synapses induced by NL1 and SynCAM, compared the actions of these molecules with those of other neuronal cell adhesion molecules, and examined the functional effects of NL1 and SynCAM overexpression in neurons. We found that only NL1 and SynCAM specifically induced presynaptic differentiation in cocultured neurons. The induced nerve terminals were capable of both spontaneous and evoked neurotransmitter release, suggesting that a full secretory apparatus was assembled. By all measures, SynCAM- and NL1-induced artificial synapses were identical. Overexpression in neurons demonstrated that only SynCAM, but not NL1, increased synaptic function in immature developing excitatory neurons after 8 d in vitro. Tests of chimeric molecules revealed that the dominant-positive effect of SynCAM on synaptic function in developing neurons was mediated by its intracellular cytoplasmic tail. Interestingly, morphological analysis of neurons overexpressing SynCAM or NL1 showed the opposite of the predictions from electrophysiological results. In this case, only NL1 increased the synapse number, suggesting a role for NL1 in morphological synapse induction. These results suggest that both NL1 and SynCAM act similarly and specifically in artificial synapse induction but that this process does not reflect a shared physiological function of these molecules.

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PubMed2004

The role of nitric oxide on contractile impairment during endotoxemia in rat diaphragm muscle.

We examined the contribution of nitric oxide (NO) on the contractile impairment in diaphragm muscles of endotoxemic rats. Force-frequency relationship was depressed 24 h after lipopolysaccharide administration. 7-Nitroindazole, aminoguanidine and 1H-[1,2,4]Oxadiazole (4,3-a)quinoxalin-1-one (ODQ) partially restored the contractile impairment, Nomega-Nitro-L-Arginine (L-NNA) was ineffective. K+ contractions were reduced by 50% in endotoxemic muscles, 7-nitroindazole partially recovered, while aminoguanidine and L-NNA were ineffective. Verapamil reduced contractility to a greater extent in endotoxemic muscles. Caffeine and ryanodine contractions were augmented during endotoxemia without NOS contribution. L-NNA, 7-nitroindazole, ODQ and hemoglobin did not affect, but aminoguanidine completely restored partially inhibited neurotransmission by d-tubocurarine. Endotoxemia did not change membrane potentials and neurotransmitter release but slightly increased excitability. At this stage of endotoxemia, (1) constitutive NOS appears to be the dominant isoform, (2) NO does not have a major role on contractile dysfunction and (3) impairment could be explained by altered sensitivity of the voltage sensor. (4) NO does not substantially modulate neuromuscular transmission in normal and endotoxemic rats.

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PubMed2002

SynCAM, a synaptic adhesion molecule that drives synapse assembly.

Synapses, the junctions between nerve cells through which they communicate, are formed by the coordinated assembly and tight attachment of pre- and postsynaptic specializations. We now show that SynCAM is a brain-specific, immunoglobulin domain-containing protein that binds to intracellular PDZ-domain proteins and functions as a homophilic cell adhesion molecule at the synapse. Expression of the isolated cytoplasmic tail of SynCAM in neurons inhibited synapse assembly. Conversely, expression of full-length SynCAM in nonneuronal cells induced synapse formation by cocultured hippocampal neurons with normal release properties. Glutamatergic synaptic transmission was reconstituted in these nonneuronal cells by coexpressing glutamate receptors with SynCAM, which suggests that a single type of adhesion molecule and glutamate receptor are sufficient for a functional postsynaptic response.

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PubMed2002

Fast vesicle recycling supports neurotransmission during sustained stimulation at hippocampal synapses.

High-frequency induced short-term synaptic depression is a common feature of central synapses in which synaptic responses rapidly decrease to a sustained level. A limitation in the availability of release-ready vesicles is thought to be a major factor underlying this phenomenon. Here, we studied the kinetics of vesicle reavailability and reuse during synaptic depression at hippocampal synapses. High-intensity stimulation of neurotransmitter release was induced by hyperosmolarity, high potassium, or action potential firing at 30 Hz to produce synaptic depression. Under these conditions, synaptic transmission rapidly depressed to a plateau level that was typically 10-40% of the initial response and persisted at this level for at least 5 min regardless of the developmental stage of synapses. This nondeclining phase of transmission was partly sustained by fast recycling and reuse of synaptic vesicles even after minutes of stimulation. Simultaneous electrical recording of postsynaptic responses and styryl dye destaining showed that after an initial round of exocytosis, vesicles were available for reuse with a delay between 1 and 3 sec during 30 Hz action potential or hypertonicity-induced stimulation. During these stimulation paradigms, there was a limited mobilization of vesicles from the reserve pool. During 10 Hz stimulation, however, the extent of vesicle reuse was minimal during the first 20 sec. These results suggest a role for fast vesicle recycling as a functional homeostatic mechanism that prevents vesicle depletion and maintains synaptic responses in the face of intense stimulation.

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PubMed2002

Development of vesicle pools during maturation of hippocampal synapses.

We studied the emergence of vesicle pool organization at developing hippocampal synapses by monitoring vesicle recycling and neurotransmitter release as well as examining electron micrographs. Our analysis suggests that presynaptic boutons go through three distinct functional states to mature. At the onset the synapses lack readily releasable vesicles although they possess a pool of recycling vesicles that can release neurotransmitters under strong stimulation. In the next stage the majority of these recycling vesicles switches to a functionally docked state and forms the readily releasable pool (RRP). After assembly of the RRP, new vesicles build the reserve pool. At the mature state the size of the RRP increases linearly with increasing recycling pool size. Furthermore, this preferential filling of the RRP during early synapse maturation is reduced strikingly in synapses deficient in synapsin I and II. Taken together, these results expose a mechanism that ensures functionally effective allocation of a limited number of vesicles in a CNS synapse.

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PubMed2002

Persistent defect in transmitter release and synapsin phosphorylation in cerebral cortex after transient moderate ischemic injury.

BACKGROUND AND PURPOSE: Synaptic transmission is highly vulnerable to metabolic perturbations. However, the long-term consequences of transient metabolic perturbations on synapses are not clear. We studied the long-lasting changes in synaptic transmission and phosphorylation of presynaptic proteins in penumbral cortical neurons after transient moderate ischemia. METHODS: Rats were subjected to 1 hour of middle cerebral artery occlusion. After reperfusion, electric activity of neurons in the peri-infarct region was recorded intracellularly and extracellularly in situ. Phosphorylation of synapsin-I and tyrosine residues was studied by immunohistochemistry. RESULTS: Neurons in the penumbra displayed no postsynaptic potentials 1 to 3 hours after recirculation. However, these cells were able to generate action potentials and were responsive to glutamate, suggesting that postsynaptic excitability was preserved but the synaptic transmission was blocked because of a presynaptic defect. The synaptic transmission was still depressed 24 hours after recirculation in neurons in the peri-infarct area that survived ischemia. The amount of immunoreactive synapsin-I, synaptophysin, and synaptotagmin was not appreciably changed for 72 hours after reperfusion. However, phosphorylation of synapsin-l was significantly decreased, whereas phosphotyrosine immunoreactivity was increased, suggesting a selective defect in synapsin-I phosphorylation. CONCLUSIONS: These data demonstrate that synaptic transmission may be permanently impaired after transient moderate brain injury. Since postsynaptic excitability is preserved, the transmission failure is likely to be caused by presynaptic mechanisms, one of which may be impaired phosphorylation of presynaptic proteins.

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