BDNF and NT-4/5 enhance glutamatergic synaptic transmission in cultured hippocampal neurones

The effects of BDNF and NT-4/5 on AMPA receptor-mediated synaptic transmission were investigated with the patch clamp technique applied to embryonic and post-natal rat hippocampal neurones, cultured in serum-free medium. Spatially restricted application of neurotrophin-containing solution on to the recorded cells was performed and evoked as well as miniature excitatory postsynaptic currents (mepscs) were monitored. In approximately 25% of neurones tested a transient augmentation of evoked synaptic currents and a transient increase in the frequency of mepscs occurred with a delay of 0.5-5 min after the onset of BDNF or NT-4/5 application. The amplitudes of the AMPA receptor mediated mepscs were unaffected, suggesting a presynaptic action of BDNF and NT-4/5.

Neurotrophin-dependent modulation of glutamatergic synaptic transmission in the mammalian CNS

1. The protein family of the neurotrophins, consisting of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and Neurotrophin-3, -4/5, and -6 (NT-3; NT-4/5; NT-6) is well known to enhance the survival and to stabilize the phenotype of different populations of neurons in the central and the peripheral nervous system. These effects are mediated via binding to specific tyrosine kinase receptors (Trks) and to the low-affinity p75 neurotrophin receptor. 2. The neurotrophins NGF, BDNF, and NT-3 and the BDNF and NT-3 selective receptors (TrkB, TrkC) are expressed at high levels in neurons of the basal forebrain, the hippocampus, and the neocortex of the mammalian brain. The expression and secretion of NGF and BDNF in these brain areas is regulated by (physiological levels of) neuronal activity. 3. Exogenous application of the neurotrophins to hippocampal and neocortical neurons can enhance excitatory glutamatergic synaptic transmission via activation of Trk receptors. In addition, long-term potentiation (a potential cellular correlate for learning and memory formation in mammals) in the rodent hippocampus depends on endogenous supply of neurons with BDNF. 4. Judged by the analysis of electrophysiological data, the BDNF- and NT-3-induced enhancement of glutamatergic synapses is mediated by increasing the efficacy of glutamate release from the presynaptic neuron. However, neurotrophin-dependent postsynaptic enhancement of NMDA (but not AMPA) receptor responsiveness has also been shown. 5. On the molecular level, neither the pre- nor the postsynaptic modulation of glutamatergic synapses by neurotrophins is well understood. However, neurotrophins were shown to acutely affect intraneuronal Ca2+ levels and to influence molecular components of the transmitter release machinery, which could underly the presynaptic modifications, whereas BDNF-induced phosphorylation of NMDA-type glutamate receptors could account for the postsynaptic effects. 6. Taken together, these results suggest that the activity-dependent release of neurotrophins at frequently used synapses could modulate the synaptic efficacy at these junctions. Thus, neurotrophins might operate as locally released feedback modulators of synaptic transmission, and this could be a cellular correlate for certain aspects of information processing in the mammalian brain.

Modulation of unitary glutamatergic synapses by neurotrophin-4/5 or brain-derived neurotrophic factor in hippocampal microcultures: presynaptic enhancement depends on pre-established paired-pulse facilitation

The neurotrophins, nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5, have--in addition to their known effects as neuronal survival factors--recently been found to modulate synaptic transmission in the rat hippocampus and neocortex. Using standard whole-cell patch-clamp recordings, we have now investigated the acute effects of brain-derived neurotrophic factor and neurotrophin-4/5 on unitary (i.e. single cell activated) glutamatergic synaptic connections in microcultures of postnatal rat hippocampal neurons. We show that, in approximately 30% of the cells, glutamatergic synaptic transmission is enhanced to 170 +/- 52% (neurotrophin-4/5, 100 ng/ml) and 143 +/- 35% (brain-derived neurotrophic factor, 100 ng/ml) of control values, respectively. The enhancement is abolished in the presence of the specific Trk tyrosine kinase inhibitor k252a (200 nM). Depending on the particular cell investigated, the enhancement consisted of transient and sustained components in varying quantities. A minority of neurons (10%) showed a depression of glutamatergic synaptic transmission to 64 +/- 14% (brain-derived neurotrophic factor) and 61 +/- 11% of control (neurotrophin-4/5). The enhancement of unitary glutamatergic synaptic transmission is mediated predominantly by presynaptic modifications, as is evident from (i) the concomitant decrease in paired-pulse facilitation, (ii) the concomitant increase in the variance of the evoked unitary synaptic currents and (iii) the enhanced miniature excitatory postsynaptic/autaptic current frequencies that could be observed in the absence of an effect on miniature excitatory postsynaptic/autaptic current amplitudes. Finally, we show that the successful enhancement of synaptic transmission by neurotrophin-4/5 critically depends on the degree of paired-pulse facilitation prior to the start of neurotrophin application, with autapses/synapses initially showing a higher degree of paired-pulse facilitation being enhanced more effectively. Taken together, these results suggest that the brain-derived neurotrophic factor- and neurotrophin-4/5-mediated enhancement of unitary glutamatergic synaptic transmission in hippocampal cultures results predominantly from a presynaptic modulation of transmitter release, and this modulation could participate in the neurotrophin-dependent modification of glutamatergic synaptic transmission in the hippocampus in situ.

BDNF-GFP containing secretory granules are localized in the vicinity of synaptic junctions of cultured cortical neurons

The protein family of mammalian neurotrophins, comprising nerve-growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 and -4/5 (NT-3, NT-4/5), supports the survival and the phenotype of neurons from the central as well as the peripheral nervous system (CNS, PNS). In addition, exogenous application of neurotrophins has recently been found to modulate synaptic transmission in the rodent CNS. However, to provide evidence for a role of neurotophins as endogenous fast acting modulators of synaptic transmission, the synaptic localization and secretion of neurotrophins needs to be shown. We have now constructed a fusion protein consisting of N-terminal BDNF (the most abundant neurotrophin in the rodent hippocampus and neocortex) and C-terminal green fluorescent protein (GFP) to elucidate the cellular localization of BDNF in cortical neurons. Transient expression of BDNF-GFP in COS-7 cells revealed that the cellular localization in the trans-Golgi network (TGN), the processing of precursor proteins and the secretion of mature BDNF-GFP is indistinguishable from the properties of untagged BDNF. Upon transient transfection of primary rat cortical neurons, BDNF-GFP was found in secretory granules of the regulated pathway of secretion, as indicated by colocalization with the secretory granule marker secretogranin II. BDNF-GFP vesicles were found in the neurites of transfected neurons with a pattern reminiscent of the localization of endogenous BDNF in untransfected cortical neurons. BDNF-GFP vesicles were found predominantly in the somatodendritic compartment of the neurons, whereas additional axonal localization was found less frequently. Immunocytochemical staining of synaptic terminals with synapsin I antibodies revealed that the density of BDNF-GFP vesicles is elevated in the vicinity of synaptic junctions, indicating that BDNF is localized appropriately to function as an acute modulator of synaptic transmission. These data suggest that BDNF-GFP will be a useful tool to investigate synaptic release of BDNF during physiological synaptic stimulation, and will thereby allow us to elucidate the participation of neurotrophin release in activity dependent synaptic plasticity.

Postsynaptic BDNF signalling regulates long-term potentiation at thalamo-amygdala afferents

The neurotrophin brain-derived neurotrophic factor (BDNF) is known to regulate synaptic plasticity and memory formation in the hippocampus and the neocortex of the mammalian brain. In contrast, a role of BDNF in mediating synaptic plasticity and fear learning in the amygdala is just beginning to evolve. Using patch clamp recordings from projection neurons of the dorsal lateral amygdala (LA) in acute slices of mice, we now investigated the cellular mechanism of BDNF-mediated long-term potentiation (LTP) of excitatory postsynaptic currents (EPSCs) in the amygdala. LTP was elicited in cortical and thalamic synaptic inputs by pairing postsynaptic depolarisation with presynaptic stimulation. LTP in the cortico-amygdala pathway was not changed in heterozygous BDNF-knockout (BDNF(+/-)) mice. In contrast, pairing induced LTP in the thalamic input was abolished in BDNF(+/-) mice (BDNF(+/-): 104.0 ± 5.7% of initial EPSC values; WT: 132.5 ± 7.3%). Likewise, inhibition of BDNF/TrkB signalling with TrkB-IgGs as scavenger molecules for endogenous BDNF blocked LTP in wild-type mice in this pathway (TrkB-IgG: 102.7 ± 6.9% of initial EPSC values; control: 132.5 ± 8.7%). Inclusion of the tyrosine kinase inhibitor K252a in the pipette solution also prevented the induction of LTP in the thalamic pathway, indicating a postsynaptic site of action of BDNF in regulating LTP. Reduced BDNF levels in BDNF(+/-) mice did not affect intrinsic membrane properties of LA projection neurons. Likewise, presynaptic glutamate release, and postsynaptic membrane properties also remained unaffected in BDNF(+/-) mice. These data suggest a postsynaptic site of action of BDNF in mediating LTP selectively in the thalamic fear conditioning pathway.

The Adenomatous Polyposis Coli-protein (APC) interacts with the protein tyrosine phosphatase PTP-BL via an alternatively spliced PDZ domain

Mutations of the tumor suppressor protein APC (Adenomatous Polyposis Coli) are linked to familiar and sporadic human colon cancer. Here we describe a novel interaction between the APC protein and the protein tyrosine phosphatase PTP-BL carrying five PDZ protein-protein interaction domains. Exclusively, the second PDZ domain (PDZ2) of PTP-BL is binding to the extreme C-terminus of the APC protein, as determined by yeast two-hybrid studies. Using surface plasmon resonance analysis we established a dissociation constant (K(D)) of 8.1 x 10(-9) M. We find that a naturally occurring splice insertion of five amino acids (PDZ2b) abolishes its binding affinity to the APC protein. The in vivo interaction between PTP-BL and the APC protein was shown by coprecipitation experiments in transfected COS cells. Furthermore, in cultured epithelial Madine Carnine Kidney cells the subcellular colocalization was demonstrated for the nucleus and also for the tips of cellular extensions. The interaction of the APC protein with a protein tyrosine phosphatase may indirectly modulate the steady state levels of tyrosine phosphorylations of associated proteins, such as beta-catenin playing a major role in the regulation of cell division, migration and cell adhesion.

Developmental maturation of synaptic vesicle cycling as a distinctive feature of central glutamatergic synapses

The formation of chemical synapses in the mammalian brain involves complex pre- and postsynaptic differentiation processes. Presynaptically, the progressive accumulation of synaptic vesicles is a hallmark of synapse maturation in the neocortex [J Neurocytol 12 (1983b) 697]. In this study, we analyzed the functional consequences of presynaptic vesicle-pool maturation at central glutamatergic and GABAergic synapses. Using (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide (FM1-43) staining of recycling synaptic vesicles, we demonstrate a pronounced developmental increase in presynaptic vesicle accumulation during differentiation of neocortical neurons in culture. Using electrophysiological methods to study functional synaptic maturation, we found an improved recovery from hypertonic solution-induced depletion. As supported by the FM1-43 staining results, this change is most likely caused by a developmental increase in the number of reserve-pool vesicles. In addition, assuming a rapid reuse of freshly recycled vesicles, a developmental maturation of the endocytosis process may also contribute. The observed presynaptic maturation process occurred selectively at glutamatergic synapses, while GABAergic synapses did not show similar developmental alterations. Furthermore, we used high-frequency stimulation (HFS) of glutamatergic and GABAergic synapses to reveal the physiological consequences of reserve-pool maturation. As expected, recovery from HFS-induced depletion was incomplete at immature glutamatergic synapses and strongly improved during synapse maturation. Again, GABAergic synapses did not show similar developmental changes. Taken together, our study characterizes the functional consequences of a pronounced accumulation of reserve-pool vesicles occurring selectively at glutamatergic synapses.

Semaphorin4F interacts with the synapse-associated protein SAP90/PSD-95

Semaphorins are a family of secreted and membrane-associated proteins involved in growth cone guidance during development. Here, we describe the interaction of Semaphorin4F (Sema4F) with the post-synaptic density protein SAP90/PSD-95. Using the yeast two-hybrid system and coprecipitation assays we were able to show an interaction between the extreme C-terminus of Sema4F and the PDZ domains of SAP90/PSD-95. Heterologous coexpression of a chimeric EphrinB1/Semaphorin4F protein with SAP90/PSD-95 in COS cells leads to translocation of SAP90/PSD-95 from the cytosol to the membrane. Deletion analysis shows that this translocation activity of Sema4F is completely dependent on the presence of the last three C-terminal amino acids. In addition, Sema4F immunoreactivity is present in synaptosome fractions and enriched in post-synaptic density fractions. Consistently, in cultured hippocampal neurons, we demonstrate punctate colocalization of Sema4F and SAP90/PSD-95 in dendrites, furthermore we found colocalization of Sema4F with synapsin1 suggesting a synaptic localization. Our data implicate a new functional context for semaphorins at glutamatergic synapses.

NT-3 regulates BDNF-induced modulation of synaptic transmission in cultured hippocampal neurons

BDNF and NT-3 can modulate the development and plasticity of central synaptic transmission. Although the expression of NT-3 and BDNF in the rodent hippocampus coincides during perinatal development, little is known about possible functional interactions between both neurotrophins in synaptic development. Here, we have investigated the effects of combined long-term application of NT-3 and BDNF on excitatory glutamatergic (mEPSC) and inhibitory GABAergic miniature synaptic currents (mIPSC) in cultured embryonic hippocampal neurons. Our results show that the BDNF-induced twofold increase in mEPSC frequency is abolished by pre-treatment with NT-3. In addition, the NT-3-induced twofold downregulation of mIPSC frequency is reversed by BDNF. Finally, the BDNF-induced increase in c-fos expression is reduced by 50% after pre-treatment with NT-3. In summary, these data suggest an NT-3 controlled modulation of BDNF signalling in differentiating hippocampal neurons.

Cyclic AMP endogenously enhances synaptic strength of developing glutamatergic synapses in serum-free microcultures of rat hippocampal neurons

The time course of development of autaptic and synaptic connections and the contribution of endogenously activated cAMP signaling to the regulation of AMPA/kainate receptor-mediated synaptic transmission were studied in microcultures of isolated single hippocampal neurons or of pairs of neurons grown on astrocytic islands in serum-free culture medium. Standard whole cell patch clamp techniques were employed to monitor evoked and spontaneous autaptic and synaptic currents. Glutamatergic synaptic transmission became detectable after 4 days in vitro (DIV). After 9-10 DIV more than 80% of the neurons had developed glutamatergic autaptic and synaptic connections. Elevation of intracellular cAMP levels by application of forskolin (20 microM) or IBMX (200 microM) to autaptic neurons resulted in enhanced autaptic current amplitudes (forskolin: 146 +/- 9%, IBMX: 177 +/- 21% of control) and impaired paired pulse facilitation (PPF). Likewise, intracellular application of cAMP via the patch pipette into autaptic neurons or into the presynaptic neuron of a synaptically connected pair also resulted in enhanced autaptic/synaptic current amplitudes (170 +/- 16% of control). In contrast, injection of cAMP into the postsynaptic neuron of a synaptic pair failed to significantly enhance the synaptic responses. The magnitude of the cAMP-mediated enhancement depended on the initial autaptic/synaptic strength observed in an individual cell, with small autapses/synapses being enhanced more effectively. Application of an inhibitor of cAMP-mediated processes (Rp-cAMPS) reversibly reduced autaptic/synaptic current amplitudes (to 75 +/- 5% of control). Taken together, these results suggest that cAMP-mediated processes endogenously enhance the efficacy of developing glutamatergic autaptic and synaptic connections in serum-free microcultures of isolated hippocampal neurons.

Evans blue reduces macroscopic desensitization of non-NMDA receptor mediated currents and prolongs excitatory postsynaptic currents in cultured rat thalamic neurons

Fast application of L-glutamate, AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) or kainate to cultured rat thalamic neurons revealed properties of non-NMDA (N-methyl-D-aspartate) receptors similar to those described in hippocampal neurons. The kinetics of non-NMDA receptor-mediated currents were altered by the addition of the dye Evans Blue (EB). Macroscopic desensitization was reduced and activation and deactivation kinetics were slowed. Delayed addition of EB, after desensitization of non-NMDA receptors, resulted in reactivation of desensitized receptors. Thus, both ion channel gating and entry into the desensitized state were affected. Evans blue also slowed the activation and the decay of glutamatergic miniature EPSCs (excitatory postsynaptic currents), demonstrating that receptor kinetics determine the time course of the synaptic response.

Reduced number of functional glutamatergic synapses in hippocampal neurons overexpressing full-length TrkB receptors

Brain-derived neurotrophic factor (BDNF) acutely modulates the efficacy of central glutamatergic synapses via activation of the receptor tyrosine kinase TrkB. On a longer time scale, recent evidence suggests an additional role of TrkB signaling in the formation of excitatory synaptic connections. Here, we have overexpressed full-length TrkB receptors (fl-TrkB) in hippocampal neurons, to investigate the contribution of BDNF signaling to the maturation of glutamatergic synapses. Using patch clamp recordings, we show a three-fold reduction in glutamatergic excitatory autaptic and synaptic current amplitudes in neurons overexpressing fl-TrkB, and application of saturating concentrations of BDNF and NT-4/5 completely reverses this effect. Compatible with these overexpression data, in untransfected neurons, scavenging of endogenous BDNF and NT-4/5 by TrkB-IgGs reduces excitatory autaptic current (EAC) amplitudes. By overexpression of truncated TrkB receptors (TrkB.T1, TrkB.T2) and a chimeric receptor containing only the intracellular domain of fl-TrkB, we show that intra- and extracellular domains of fl-TrkB are necessary to observe the EAC reduction. Labeling of presynaptic terminals with FM 4-64 revealed, that the reduced EAC amplitudes in fl-TrkB overexpressing neurons are accompanied by a two-fold reduction in synapse number. These results suggest, that ligand-independent signaling through fl-TrkB receptors can decrease glutamatergic synaptic strength, if sufficient amounts of BDNF or NT-4/5 are not available.

The Relation Between Long-Term Synaptic Plasticity at Glutamatergic Synapses in the Amygdala and Fear Learning in Adult Heterozygous BDNF-Knockout Mice

Brain-derived neurotrophic factor (BDNF) heterozygous knockout mice (BDNF+/- mice) show fear learning deficits from 3 months of age onwards. Here, we addressed the question how this learning deficit correlates with altered long-term potentiation (LTP) in the cortical synaptic input to the lateral amygdala (LA) and at downstream intra-amygdala synapses in BDNF+/- mice. Our results reveal that the fear learning deficit in BDNF+/- mice was not paralleled by a loss of LTP, neither at cortical inputs to the LA nor at downstream intra-amygdala glutamatergic synapses. As we did observe early fear memory (30 min after training) in BDNF+/- mice while long-term memory (24 h post-training) was absent, the stable LTP in cortico-LA and downstream synapses is in line with the intact acquisition of fear memories. Ex vivo recordings in acute slices of fear-conditioned wildtype (WT) mice revealed that fear learning induces long-lasting changes at cortico-LA synapses that occluded generation of LTP 4 and 24 h after training. Overall, our data show that the intact LTP in the tested amygdala circuits is consistent with intact acquisition of fear memories in both WT and BDNF+/- mice. In addition, the lack of learning-induced long-term changes at cortico-LA synapses in BDNF+/- mice parallels the observed deficit in fear memory consolidation.

Differential modulation of AMPA receptor mediated currents by evans blue in postnatal rat hippocampal neurones

1. The modulation of non-N-methyl-D-aspartate (NMDA) receptor-mediated whole cell currents and of glutamatergic synaptic transmission by purified Evans Blue (EB) was investigated in rat cultured postnatal hippocampal neurones by use of patch clamp recordings and a fast drug application system. 2. Three different groups of neurones could be distinguished with respect to the type of modulation obtained with 10 microM EB: EB was either a predominant inhibitor of desensitization (13% of the neurones), a predominant inhibitor of current amplitudes (42%) or a mixed inhibitor of both properties (45%). Both effects were not use-dependent and reached maximal levels after 30 s of pre-equilibration with the diazo dye. 3. Dose-response curves obtained from glutamate activated whole cell currents yielded an IC50 value for EB of 13.3 microM (Hill coefficient: 1.3) for the inhibition of desensitization, and an IC50 value of 10.7 microM (Hill coefficient: 1.2) for the inhibition of current amplitudes. 4. Chicago acid SS (100 microM) which is one of the synthesis precursors of EB had no effect on current amplitudes of glutamate activated whole cell currents but was a weak inhibitor of desensitization in all hippocampal neurones investigated, irrespective of the type of modulation obtained with EB in the same neurone. 5. Oxidatively modified EB (the so-called VIMP (10 microM)) had no effect on the kinetics but was a partial inhibitor of glutamate-activated whole cell currents in all hippocampal neurones investigated. 6. EB (10 microM) inhibited the amplitudes of non-NMDA receptor mediated autaptic currents to the same extent (to 39 +/- 19% of control) as observed for glutamate activated whole cell currents (to 41 +/- 17% and 56 +/- 20%). However, the decay of the autaptic responses remained uninfluenced upon EB application, indicating that either receptor desensitization does not dominate the time course of the synaptic response or that the non-NMDA receptors sensitive to modulation of desensitization by EB are not present in the postsynaptic membrane. 7. In conclusion, EB differentially modulates alpha-amino-3-hydroxy-5-methyl -4-isoxazole propionic acid (AMPA) receptor gating in different subsets of neurones. Upon identification of the cellular determinants for the differential modulation (e.g. AMPA receptor subunit composition) EB could become a useful tool to investigate receptor subtypes during electrophysiological recordings.

Protein expression patterns of identified neurons and of sprouting cells from the leech central nervous system

It has previously been shown that cephalic, segmental, and caudal ganglia from the medicinal leech show differences in their protein composition. Here we studied whether the neuronal reorganization that occurs in cultured segmental ganglia from the medicinal leech is accompanied by detectable changes in the protein expression pattern. Using silver-stained two-dimensional gels we showed that after 5 and 12 days in culture changes in the protein patterns can be detected in isolated ganglia. The changes observed in the two-dimensional gels occurred concomitantly with a sprouting of serotoninergic neurites and a decreased transmitter content of dopaminergic neurites as shown by using the glyoxylic acid condensation reaction. In addition, we present evidence that Retzius cells, which can be identified by their characteristic morphology and action potential waveform, exhibit biochemically unique properties with respect to their protein expression pattern.

Fast desensitization of glutamate activated AMPA/kainate receptors in rat thalamic neurones

An essential part of the excitatory afferent input to the thalamus is mediated by glutamate receptors of the AMPA/kainate type. In contrast to other regions of the mammalian CNS, the biophysical properties of these receptors have not been investigated in thalamic neurones. Using a fast transmitter application system we studied L-glutamate activated currents of cultured neurones in the whole cell and outside-out patch configuration. Current-voltage relationships and dose-response curves of whole cell recordings were in close correspondence to results obtained from other brain areas. Analysis of outside-out patch currents revealed two types of desensitization time constants of 3.0 and 10.2 ms, with the former close to the time constant of decay of miniature glutamatergic synaptic currents.

Presynaptic Regulation of Tonic Inhibition by Neuromodulatory Transmitters in the Basal Amygdala

Tonic inhibition mediated by ambient levels of GABA that activate extrasynaptic GABA_A receptors emerges as an essential factor that tunes neuronal network excitability in vitro and shapes behavioral responses in vivo. To address the role of neuromodulatory transmitter systems on this type of inhibition, we employed patch clamp recordings in mouse amygdala slice preparations. Our results show that the current amplitude of tonic inhibition (I_tonic) in projection neurons of the basal amygdala (BA) is increased by preincubation with the neurosteroid THDOC, while the benzodiazepine diazepam is ineffective. This suggests involvement of THDOC sensitive δ subunit containing GABA_A receptors in mediating tonic inhibition. Moreover, we provide evidence that the neuromodulatory transmitters NE, 5HT, and ACh strongly enhance spontaneous IPSCs as well as I_tonic in the BA. As the increase in frequency, amplitude, and charge of sIPSCs by these neuromodulatory transmitters strongly correlated with the amplitude of I_tonic, we conclude that spill-over of synaptic GABA leads to activation of I_tonic and thereby to dampening of amygdala excitability. Since local injection of THDOC, as a positive modulator of tonic inhibition, into the BA interfered with the expression of contextual fear memory, our results point to a prominent role of I_tonic in fear learning.

Enhancement of postsynaptic serotonin-activated Cl- currents by depolarization-induced Ca2+ entry into leech neurons

Postsynaptic Ca2+ signals are a precondition for many types of synapse modification, including long-term potentiation. Here we describe a postsynaptic mechanism that augments synaptic currents at the serotonergic Retzius-P cell synapse of the leech. Depolarizations large enough to elicit Ca2+ currents evoked an increase in the amplitude of serotonin-activated Cl- currents in isolated embryonic neurons as well as at the Retzius-P cell synapse. The effect on embryonic whole cell currents lasted for up to one hour and was abolished in solutions containing Cd2+ as a Ca2+ channel blocker. Hence, Ca2+ influx into the postsynaptic neuron may be a common step regulating the plasticity of various vertebrate as well as invertebrate synapses.

[Aspects of vascular physiology in clinical and vascular surgical practice: basic principles of vascular mechanics]

BACKGROUND

To be able to evaluate properly a vascular problem, basic concepts of vascular physiology need to be considered, as they have been taught in physiology for a long time. This article deals with selected definitions and laws of passive vascular mechanics, subdivided into parameters of vascular filling and parameters of vascular flow. PARAMETERS OF VASCULAR FILLING: During vascular filling the transmural pressure distends the vascular wall until it is balanced by the wall tension. The extent of this distension up to the point of balance depends on the elasticity of the wall. Transmural pressure, wall tension and elasticity are defined, and their respective importance is described by clinical examples, e.g. aneurysm and varix. PARAMETERS OF VASCULAR FLOW: The vascular flow can be divided into stationary and pulsating components. Both components are relevant for the bloodstream. Since the blood flow is directed in the circuit, it can be understood in first approximation as stationary ("direct current").The direct current model uses only the average values of the pulsating variables. The great advantage of the direct current model is that it can be described with simple laws, which are not valid without reservation, but often allow a first theoretical approach to a vascular problem: Ohm's law, driving pressure, flow resistance, Hagen-Poiseuille law, wall shear stress, law of continuity, Bernoulli's equation and Reynold's number are described and associated with clinical examples.The heart is a pressure-suction pump and produces a pulsating flow, the pulse. The pulse runs with pulse wave velocity, which is much larger than the blood flow velocity, through the arterial vascular system. During propagation, the pulse has to overcome the wave resistance (impedance). Wherever the wave resistance changes, e.g., at vascular bifurcations and in the periphery, it comes to reflections. The incident (forward) and reflected (backward) waves are superimposed to yield the resulting pulse wave. This pulse wave allows one to distinguish pressure and flow pulse by measurement. Both are described separately, and their respective clinical meaning is illustrated by appropriate examples, e.g., arterial stiffness and pre-/postocclusive high/low resistance flow, respectively.

Development of serotonin-induced ion currents in identified embryonic Retzius cells from the medicinal leech (Hirudo medicinalis)

Retzius cells and the cutaneous baroreceptive P-cells of adult medicinal leeches (Hirudo medicinalis) respond to 5-HT by Cl- as well as by monovalent cation conductances (Drapeau et al., 1989). However, chemical synaptic connections between Retzius cells (Liu and Nicholls, 1989) as well as Retzius cells and P-cells in culture (Drapeau and Sanchez-Armass, 1988) are mediated by postsynaptic Cl- currents in response to 5-HT release from the presynaptic cell. It was the aim of the present study to find out whether the response, which has been shown to be involved in synaptic transmission or whether the cation current, which has so far only been observed upon extrasynaptic 5-HT application, dominates during embryogenesis. Currents induced by transmitter application with a fast perfusion system were measured with a single-electrode voltage clamp and patch pipettes in whole-cell configuration. Control experiments, performed on Retzius cells from adult leeches after 1 d in culture, showed Cl- currents in response to 5-HT application in all cells investigated. Ninety percent of the cells tested showed additional 5-HT-elicited Na+, K+, or both currents. However, the conductance of cation currents amounted only to 10% of the Cl- currents. Embryonic Retzius cells were identified in dissociated cultures by their violet-blue fluorescence acquired by preincubation of intact ganglia or embryos in culture medium containing the autofluorescent 5-HT analog 5,7-dihydroxytryptamine. The first responses to 5-HT were measured at embryonic day 10 (E10), after voltage-activated Ca2+ and Na+ currents had already developed. 5-HT application induced exclusively Cl- currents until E16. This suggests that the receptor-channel complex, which also mediates synaptic responses, is expressed first in measurable quantities. Hence, there is currently no indication that extrasynaptic receptors dominate in immature cells in this preparation.

Alterations in glycosylation and lectin pattern during phorbol ester-induced differentiation of U937 cells

Incubation of the human monoblastoid tumor cell line U937 with 5 x 10(-9) M 12-O-tetradecanoylphorbol-13-acetate (TPA) for 3 days was associated with marked morphological and functional changes including adherence of the cells and cessation of proliferation. While growth arrest and the viability of the cells were not influenced, the TPA-induced adherence of U937 cells could be totally blocked by incubation with tunicamycin, suggesting an involvement of N-linked carbohydrates in these cell attachment processes. The isolation and characterization of endogenous lectins with specificities for lactose, L-fucose, and D-mannose from U937 controls and TPA-differentiated U937 cells demonstrated marked differences in either the pattern and the distribution of these sugar-specific carbohydrate-binding proteins. Our results indicate that alterations in cell-surface carbohydrates are of central importance for the attachment of the differentiating U937 tumor cells.

Two kinetically distinct 5-hydroxytryptamine-activated Cl- conductances at Retzius P-cell synapses of the medicinal leech

The properties of serotonin (5-HT)-activated Cl- receptor/ion channel complexes in neurons of the CNS of the medicinal leech were analyzed. These channels mediate the postsynaptic response at the serotoninergic Retzius P-cell as well as Retzius-Retzius cell synapses. 5-HT-induced Cl- currents were activated by fast superfusion of transmitter on cells dissociated from embryonic leeches including histochemically identified Retzius cells. Whole-cell currents elicited by partial superfusion of the membrane and summated single-channel currents of outside-out patches showed times to peak of 18 +/- 4 msec and 10 +/- 5 msec, respectively, and desensitized with time constants of 28 +/- 3 msec and 20 +/- 11 msec. Persistence of single-channel openings in the outside-out configuration as well as lack of effect of dialysis of the whole cell with AMP-PNP or GDP-beta-S indicated that 5-HT directly gates the Cl- channels without involving second messenger cascades. In outside-out patches, two single-channel conductances of 13 pS and 32 pS were identified. While the 13 pS conductance desensitized, the 32 pS conductance activated within several tens of msec and showed no desensitization. We postulate that two subtypes of channels are coactivated by 5-HT and that the activation of the fast desensitizing channel could be responsible for the fast decaying component of the post-synaptic response. The slow conductance explains the second slower decay time constant of the postsynaptic response and could account for the tonic component sometimes observed at Retzius P-cell synapses.