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Henrique von Gersdorff

Publications and source records attributed to Henrique von Gersdorff.

At least 19 recordsLinked to original sources
PubMed2007

Presynaptic Ca2+ buffers control the strength of a fast post-tetanic hyperpolarization mediated by the alpha3 Na(+)/K(+)-ATPase.

The excitability of CNS presynaptic terminals after a tetanic burst of action potentials is important for synaptic plasticity. The mechanisms that regulate excitability, however, are not well understood. Using direct recordings from the rat calyx of Held terminal, we found that a fast Na(+)/K(+)-ATPase (NKA)-mediated post-tetanic hyperpolarization (PTH) controls the probability and precision of subsequent firing. Notably, increasing the concentration of internal Ca(2+) buffers or decreasing Ca(2+) influx led to larger PTH amplitudes, indicating that an increase in [Ca(2+)](i) regulates PTH via inhibition of NKAs. The characterization for the first time of a presynaptic NKA pump current, combined with immunofluorescence staining, identified the alpha3-NKA isoform on calyx terminals. Accordingly, the increased ability of the calyx to faithfully fire during a high-frequency train as it matures is paralleled by a larger expression of alpha3-NKA during development. We propose that this newly discovered Ca(2+) dependence of PTH is important in the post-burst excitability of nerve terminals.

Action Potentials↗
PubMed2006

GABA transporters regulate a standing GABAC receptor-mediated current at a retinal presynaptic terminal.

At the axon terminal of goldfish retinal bipolar cells, GABA(C) receptors have been shown to mediate inhibitory reciprocal synaptic currents. Here, we demonstrate a novel standing GABAergic current mediated exclusively by GABA(C) receptors. Selective inhibition of GAT-1 GABA transporters on amacrine cells increases this tonic current and reveals a specific functional coupling between GAT-1 transporters and GABA(C) receptors. We propose that this GABA(C) receptor-mediated standing current serves to regulate synaptic gain by shunting depolarizing potentials that can produce Ca2+-dependent action potentials at the bipolar cell terminal. Furthermore, we find that the amount of GABA(C) receptor-mediated reciprocal feedback between bipolar cell terminals and amacrine cells is greatly increased when GAT-1 transporters are specifically blocked by NO-711 (1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride). The involvement of GAT-1 transporters in regulating this standing (or tonic) GABA(C) current implicates them in a novel role as major determinants of presynaptic excitability.

Action Potentials↗
PubMed2006

The great escape of glutamate from the depth of presynaptic invaginations.

The basal pole of a cone photoreceptor is in close contact with hundreds of bipolar cell dendrites. The function and properties of these unconventional junctions are a long-standing mystery. In this issue of Neuron, DeVries and colleagues provide compelling evidence that glutamate release from a single quanta can diffuse to distant AMPA/KA receptors on these basal junctions to generate slow mEPSCs.

Animals↗
PubMed2006

Diurnal changes in exocytosis and the number of synaptic ribbons at active zones of an ON-type bipolar cell terminal.

The number and morphology of synaptic ribbons at photoreceptor and bipolar cell terminals has been reported to change on a circadian cycle. Here we sought to determine whether this phenomenon exists at goldfish Mb-type bipolar cell terminals with the aim of exploring the role of ribbons in transmitter release. We examined the physiology and ultrastructure of this terminal around two time points: midday and midnight. Nystatin perforated-patch recordings of membrane capacitance (C(m)) revealed that synaptic vesicle exocytosis evoked by short depolarizations was reduced at night, even though Ca(2+) currents were larger. The efficiency of exocytosis (measured as the DeltaC(m) jump per total Ca(2+) charge influx) was thus significantly lower at night. The paired-pulse ratio remained unchanged, however, suggesting that release probability was not altered. Hence the decreased exocytosis likely reflects a smaller readily releasable vesicle pool at night. Electron microscopy of single sections from intact retinas averaged 65% fewer ribbons at night. Interestingly, the number of active zones did not change from day to night, only the probability of finding a ribbon at an active zone. Additionally, synaptic vesicle halos surrounding the ribbons were more completely filled at night when these on-type bipolar cells are more hyperpolarized. There was no change, however, in the physical dimensions of synaptic ribbons from day to night. These results suggest that the size of the readily releasable vesicle pool and the efficiency of exocytosis are reduced at night when fewer ribbons are present at bipolar cell terminal active zones.

Animals↗
PubMed2006

Physiological temperatures reduce the rate of vesicle pool depletion and short-term depression via an acceleration of vesicle recruitment.

The timing and strength of synaptic transmission is profoundly dependent on temperature. However, the temperature dependence of the multiple mechanisms that contribute to short-term synaptic plasticity is poorly understood. Here, we use voltage-clamp recordings to quantify the temperature dependence of exocytosis at the calyx of Held synapse. EPSC and miniature EPSC amplitudes were larger at physiological temperature, but quantal content during low-frequency (0.05 Hz) stimulation was constant after temperature jumps from 22-24 degrees C to 35-37 degrees C. The initial degree of EPSC depression during 100 Hz stimuli trains was unchanged with temperature, as were estimates of release probability and vesicle pool size. In contrast, physiological temperatures dramatically relieved depression measured after 40 stimuli at 100 Hz by increasing twofold the rate of recovery from depression. Presynaptic calyx recordings revealed that physiological temperature increased capacitance jumps resulting from 0.5 and 1 ms depolarizations by increasing Ca2+ influx. When Ca2+ entry was equalized at the two temperatures, exocytosis exhibited little temperature dependence for brief depolarizations. However, in response to longer depolarizations, raising temperature increased a slow phase of exocytosis, without affecting overall Ca2+ entry or the size of the readily releasable pool of vesicles. Higher temperatures also increased the rate of presynaptic Ca2+ current inactivation; nevertheless, the degree of steady-state EPSC depression was greatly reduced. Our results thus suggest that changes in steady-state EPSCs during stimulus trains at physiological temperature reflect larger quantal amplitudes and faster refilling of synaptic vesicle pools, leading to reduced short-term depression during prolonged high-frequency firing.

Animals↗
PubMed2005

Prolonged reciprocal signaling via NMDA and GABA receptors at a retinal ribbon synapse.

AMPA and GABAA receptors mediate most of the fast signaling in the CNS. However, the retina must, in addition, also convey slow and sustained signals. Given that AMPA and GABAA receptors desensitize quickly in the continuous presence of agonist, how are sustained excitatory and inhibitory signals transmitted reliably across retinal synapses? Reciprocal synapses between bipolar and amacrine cells in the retina are thought to play a fundamental role in tuning the bipolar cell output to the dynamic range of ganglion cells. Here, we report that glutamate release from goldfish bipolar cell terminals activates first AMPA receptors, followed by fast and transient GABAA-mediated feedback. Subsequently, prolonged NMDA receptor activation triggers GABAA and a slow, sustained GABAC-mediated reciprocal inhibition. The synaptic delay of the NMDA/GABAC-mediated feedback showed stronger dependence on the depolarization of the bipolar cell terminal than the fast AMPA/GABAA-mediated response. Although the initial depolarization mediated by AMPA receptors was important to prime the NMDA action, NMDA receptors could trigger feedback by themselves in most of the bipolar terminals tested. This AMPA-independent feedback (delay approximately 10 ms) was eliminated in 2 mm external Mg2+ and reduced in some terminals, but not eliminated, by TTX. NMDA receptors on amacrine cells with depolarized resting membrane potentials therefore can mediate the late reciprocal feedback triggered by continuous glutamate release. Our findings suggest that the characteristics of NMDA receptors (high agonist affinity, slow desensitization, and activation/deactivation kinetics) are well suited to match the properties of GABAC receptors, which thus provide part of the prolonged inhibition to bipolar cell terminals.

Animals↗
PubMed2005

Glutamate transporter studies reveal the pruning of metabotropic glutamate receptors and absence of AMPA receptor desensitization at mature calyx of Held synapses.

We examined the effect of glutamate transporter blockade at the calyx of Held synapse. In immature synapses [defined as postnatal day 8 (P8) to P10 rats], transporter blockade causes tonic activation of NMDA receptors and strong inhibition of the AMPA receptor-mediated EPSC amplitude. EPSC inhibition was blocked with a metabotropic glutamate receptor (mGluR) antagonist [1 microm LY341495 (2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid)], suggesting that elevated resting glutamate concentration specifically activates group II and group III mGluRs. Using mGluR subtype-specific agonists and antagonists, we determined that increased glutamate activates presynaptic mGluR2/3 and mGluR8 receptors but not mGluR4, although this receptor is present. Surprisingly, in older animals (P16-P18), transporter blockade had no effect on EPSC amplitude because of a developmental downregulation of group II/III mGluR activation in rats and mice. In contrast to other CNS synapses, we observed no effect of transporter blockade on EPSC decay kinetics, although expression of glutamate transporters was strong in nearby glial processes at both P9 and P17. Finally, using a low-affinity AMPA receptor antagonist (gamma-D-glutamylglycine), we show that desensitization occurs at P8-P10 but is absent at P16-P18, even during trains of high-frequency (100-300 Hz) stimulation. We suggest that diffusion and transporter activation are insufficient to clear synaptically released glutamate at immature calyces, resulting in significant desensitization. Thus, mGluRs may be expressed in the immature calyx to help limit glutamate release. In the more mature calyx, there is a far smaller diffusional barrier attributable to the highly fenestrated synaptic terminal morphology, so AMPA receptor desensitization is avoided and mGluR-mediated inhibition is not necessary.

Animals↗
PubMed2005

Long-term plasticity mediated by mGluR1 at a retinal reciprocal synapse.

The flow of information across the retina is controlled by reciprocal synapses between bipolar cell terminals and amacrine cells. However, the synaptic delays and properties of plasticity at these synapses are not known. Here we report that glutamate release from goldfish Mb-type bipolar cell terminals can trigger fast (delay of 2-3 ms) and transient GABA(A) IPSCs and a much slower and more sustained GABA(C) feedback. Synaptically released glutamate activated mGluR1 receptors on amacrine cells and, depending on the strength of presynaptic activity, potentiated subsequent feedback. This poststimulus enhancement of GABAergic feedback lasted for up to 10 min. This form of mGluR1-mediated long-term synaptic plasticity may provide retinal reciprocal synapses with adaptive capabilities.

Animals↗
PubMed2005

Presynaptic Na+ channels: locus, development, and recovery from inactivation at a high-fidelity synapse.

Na+ channel recovery from inactivation limits the maximal rate of neuronal firing. However, the properties of presynaptic Na+ channels are not well established because of the small size of most CNS boutons. Here we study the Na+ currents of the rat calyx of Held terminal and compare them with those of postsynaptic cells. We find that presynaptic Na+ currents recover from inactivation with a fast, single-exponential time constant (24 degrees C, tau of 1.4-1.8 ms; 35 degrees C, tau of 0.5 ms), and their inactivation rate accelerates twofold during development, which may contribute to the shortening of the action potential as the terminal matures. In contrast, recordings from postsynaptic cells in brainstem slices, and acutely dissociated, reveal that their Na+ currents recover from inactivation with a double-exponential time course (tau(fast) of 1.2-1.6 ms; tau(slow) of 80-125 ms; 24 degrees C). Surprisingly, confocal immunofluorescence revealed that Na+ channels are mostly absent from the calyx terminal but are instead highly concentrated in an unusually long (approximately 20-40 microm) unmyelinated axonal heminode. Outside-out patch recordings confirmed this segregation. Expression of Na(v)1.6 alpha-subunit increased during development, whereas the Na(v)1.2alpha-subunit was not present. Serial EM reconstructions also revealed a long pre-calyx heminode, and biophysical modeling showed that exclusion of Na+ channels from the calyx terminal produces an action potential waveform with a shorter half-width. We propose that the high density and polarized locus of Na+ channels on a long heminode are critical design features that allow the mature calyx of Held terminal to fire reliably at frequencies near 1 kHz.

Action Potentials↗
PubMed2004

Fast endocytosis is inhibited by GABA-mediated chloride influx at a presynaptic terminal.

Although multiple kinetic components of synaptic vesicle endocytosis have been identified, it has remained unclear whether neurons can differentially modulate these components. Using membrane capacitance measurements from isolated goldfish bipolar cell terminals, we found that the kinetics of endocytosis in retinal slices (single exponential decay; tau > 10 s) were significantly slower than those in acutely dissociated terminals (double exponential decay; tau(fast) approximately 1-2 s; tau(slow) > 10 s). Surprisingly, GABA(A) and/or GABA(C) receptor antagonists restored the fast component of endocytosis to terminals in retinal slices. Blocking GABAergic feedback from reciprocal synapses or removing external Cl(-) ions also allowed for fast endocytosis. Elevating internal Cl(-) via the patch pipette invariably slowed endocytosis, even in terminals dialyzed with increased Ca(2+) buffer. These results suggest a new role for GABA and Cl(-) ions in blocking the trigger for fast endocytosis at this ribbon-type synapse.

Animals↗
PubMed2004

Retroinhibition of presynaptic Ca2+ currents by endocannabinoids released via postsynaptic mGluR activation at a calyx synapse.

We investigated the mechanisms by which activation of group I metabotropic glutamate receptors (mGluRs) and CB1 cannabinoid receptors (CB1Rs) leads to inhibition of synaptic currents at the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) of the rat auditory brainstem. In approximately 50% of the MNTB neurons tested, activation of group I mGluRs by the specific agonist (s)-3,5-dihydroxyphenylglycine (DHPG) reversibly inhibited AMPA receptor- and NMDA receptor-mediated EPSCs to a similar extent and reduced paired-pulse depression, suggestive of an inhibition of glutamate release. Presynaptic voltage-clamp experiments revealed a reversible reduction of Ca2+ currents by DHPG, with no significant modification of the presynaptic action potential waveform. Likewise, in approximately 50% of the tested cells, the CB1 receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN) reversibly inhibited EPSCs, presynaptic Ca2+ currents, and exocytosis. For a given cell, the amount of inhibition by DHPG correlated with that by WIN. Moreover, the inhibitory action of DHPG was blocked by the CB1R antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) and occluded by WIN, indicating that DHPG and WIN operate via a common pathway. The inhibition of EPSCs by DHPG, but not by WIN, was abolished after dialyzing 40 mm BAPTA into the postsynaptic cell, suggesting that DHPG activated postsynaptic mGluRs. Light and electron microscopy immunolabeling indicated a presynaptic expression of CB1Rs and postsynaptic localization of mGluR1a. Our data suggest that activation of postsynaptic mGluRs triggers the Ca2+-dependent release of endocannabinoids that activate CB1 receptors on the calyx terminal, which leads to a reduction of presynaptic Ca2+ current and glutamate release.

2-Amino-5-phosphonovalerate↗
PubMed2004

GABAergic neurons participate in the brain's response to birdsong auditory stimulation.

Birdsong is a learned vocal behaviour that requires intact hearing for its development in juveniles and for its maintenance during adulthood. However, the functional organization of the brain circuits involved in the perceptual processing of song has remained obscure. Here we provide evidence that GABAergic mechanisms are an important component of these circuits and participate in the auditory processing of birdsong. We first cloned a zebra finch homologue of the gene encoding the 65-kDa isoform of glutamic acid decarboxylase (zGAD-65), a specific GABAergic marker, and conducted an expression analysis by in situ hybridization to identify GABAergic cells and to map their distribution throughout auditory telencephalic areas. The results showed that field L2, the caudomedial nidopallium (NCM) and the caudomedial mesopallium (CMM) contain a high number of GABAergic cells. Using patch-clamp brain slice recordings, we found abundant GABAergic mIPSCs in NCM. Pharmacological antagonism of mIPSCs induced large EPSC bursts, suggesting that tonic inhibition helps to stabilize NCM against runaway excitation via activation of GABA-A receptors. Next, using double fluorescence in situ hybridization and double immunocytochemical labelling, we demonstrated that large numbers of GABAergic cells in NCM and CMM show inducible expression of the transcriptional regulator ZENK in response to song auditory stimulation. These data provide direct evidence that GABAergic neurons in auditory brain regions are activated by song stimulation. Altogether, our results suggest that GABAergic mechanisms participate in auditory processing and perception, and might contribute to the memorization of birdsong.

Acoustic Stimulation↗
PubMed2003

Synaptic cleft acidification and modulation of short-term depression by exocytosed protons in retinal bipolar cells.

The release of vesicular protons during exocytosis causes a feedback inhibition of Ca2+ channels in photoreceptor terminals; however, the effect of this inhibition on subsequent exocytosis has not been studied. Here we show that a similar L-type Ca2+ channel inhibition occurs in bipolar cell terminals in slices of goldfish retina, and we investigate the effect that this has on subsequent exocytosis with membrane capacitance measurements. We find that transient Ca2+ current inhibition is correlated with exocytosis and modulated by the concentration of extracellular pH buffer. Ca2+ current inhibition is negligible in acutely dissociated terminals, demonstrating the importance of an intact synaptic cleft. The sensitivity of bipolar cell Ca2+ currents to extracellular pH was assessed: channel conductance is reduced and activation is shifted to more positive potentials by acidification. The effect of Ca2+ current inhibition on subsequent exocytosis was investigated by measuring paired-pulse depression. Under conditions in which there is a large amount of inhibition of Ca2+ influx, the degree of paired-pulse depression is significantly reduced. Finally, we show that under physiological (bicarbonate) buffering conditions, pronounced Ca2+ current inhibition occurs after exocytosis ( approximately 60% peak inhibition), which can decrease subsequent exocytosis during single depolarizations. We estimate that exocytosis is accompanied by a transient change in synaptic cleft pH from 7.5 to approximately 6.9. We suggest that this effect serves as an activity-dependent modulator of exocytosis at ribbon-type synapses where a large and compact coterie of vesicles can fuse at each active zone.

Animals↗
PubMed2003

Synaptic activation of presynaptic glutamate transporter currents in nerve terminals.

Glutamate uptake by high-affinity transporters is responsible for limiting the activation of postsynaptic receptors and maintaining low levels of ambient glutamate. The reuptake process generates membrane currents, which can be activated by synaptically released glutamate in glial cells and some postsynaptic neurons. However, less is known about presynaptic transporter currents because the small size of synaptic boutons precludes direct recordings. Here, we have recorded from two giant nerve terminals: bipolar cell synaptic terminals in goldfish retina and the calyx of Held in rat auditory brainstem. Exocytosis was evoked by brief depolarizations and measured as an increase in membrane capacitance. In isolated bipolar cell terminals, exocytosis was associated with an anion (NO3- or Cl-) current. The current peaked 2.8 msec after the start of the depolarization and decayed with a mean time constant of 8.5 msec. It was inhibited by the nontransportable glutamate transporter antagonist sc-threo-beta-benzyloxyaspartate (TBOA) but was insensitive to the GLT1/EAAT2 subtype-selective antagonist dihydrokainate and was affected by extracellular pH buffering. A TBOA-sensitive anion current was also evoked by application of exogenous glutamate to bipolar cell terminals. The large single-channel conductance, derived from noise analysis, and previous immunolocalization studies suggest that synaptically released glutamate activates EAAT5-type transporters in bipolar cell terminals. In contrast, neither exocytosis nor exogenous glutamate evoked a transporter current in the calyx of Held. Glutamate transporter currents with rapid kinetics are therefore identified and characterized in bipolar cell terminals, providing a valuable system for investigating the function and modulation of presynaptic glutamate transporters.

Amino Acid Transport System X-AG↗