Endogenous neurotrophin-3 regulates short-term plasticity at lateral perforant path-granule cell synapses

RhoGEF Tiam2 Regulates Glutamatergic Synaptic Transmission in Hippocampal CA1 Pyramidal Neurons

Glutamatergic synapses exhibit significant molecular diversity, but circuit-specific mechanisms that underlie synaptic regulation are not well characterized. Prior reports show that Rho-guanine nucleotide exchange factor (RhoGEF) Tiam1 regulates perforant path→dentate gyrus granule neuron synapses. In the present study, we report Tiam1's homolog Tiam2 is implicated in glutamatergic neurotransmission in CA1 pyramidal neurons. We find that Tiam2 regulates evoked excitatory glutamatergic currents via a postsynaptic mechanism mediated by the catalytic Dbl-homology domain. Overall, we present evidence for RhoGEF Tiam2's role in glutamatergic synapse function at Schaffer collateral→CA1 pyramidal neuron synapses.

Is neurotrophin-3 (NT-3): a potential therapeutic target for depression and anxiety?

Introduction: Neurotrophin-3 (NT-3) is thought to play a role in the neurobiological processes implicated in mood and anxiety disorders. NT-3 is a potential pharmacological target for mood disorders because of its effects on monoamine neurotransmitters, regulation of synaptic plasticity and neurogenesis, brain-derived neurotrophic factor (BDNF) signaling boosting, and modulation of the hypothalamic-pituitary-adrenal (HPA) axis. The mechanisms underlying NT-3 anxiolytic properties are less clear and require further exploration and definition. Areas covered: The evidence that supports NT-3 as a pharmacological target for anxiety and mood disorders is presented and this is followed by a reflection on the quandaries, stumbling blocks, and future perspectives for this novel target. Expert opinion: There is evidence for miRNAs being key post-transcriptional regulators of neurotrophin-3 receptor gene (NTRK3) in anxiety disorders; however, the anxiolytic properties of NT-3 need further examination and delineation. Moreover, NT-3 expression by non-neuronal cells and its role in brain circuits that participate in anxiety and mood disorders require further scrutiny. Further work is vital before progression into clinical trials can be realized.

Neurotrophin-3 modulates synaptic transmission

Neurotrophin-3 (NT-3) belongs to a family of growth factors called neurotrophins whose actions are centered in the nervous system. NT-3 is structurally related to other neurotrophins like brain-derived neurotrophic factor. The expression of NT-3 starts with the onset of neurogenesis and continues throughout life. A wealth of information links NT-3 to the growth, differentiation, and survival of hippocampal cells as well as sympathetic and sensory neurons. These studies have described the distribution of NT-3 and its receptors throughout development and in the mature nervous system. Prior works has begun to cell-type specific impact of NT-3 as well as identify the signaling pathways involved. However, much less is known about how NT-3 regulates synaptic transmission. This chapter focuses role of NT-3 in the modulation of synaptic transmission.

Klotho regulates CA1 hippocampal synaptic plasticity

Global klotho overexpression extends lifespan while global klotho-deficiency shortens it. As well, klotho protein manipulations inversely regulate cognitive function. Mice without klotho develop rapid onset cognitive impairment before they are 2months old. Meanwhile, adult mice overexpressing klotho show enhanced cognitive function, particularly in hippocampal-dependent tasks. The cognitive enhancing effects of klotho extend to humans with a klotho polymorphism that increases circulating klotho and executive function. To affect cognitive function, klotho could act in or on the synapse to modulate synaptic transmission or plasticity. However, it is not yet known if klotho is located at synapses, and little is known about its effects on synaptic function. To test this, we fractionated hippocampi and detected klotho expression in both pre and post-synaptic compartments. We find that loss of klotho enhances both pre and post-synaptic measures of CA1 hippocampal synaptic plasticity at 5weeks of age. However, a rapid loss of synaptic enhancement occurs such that by 7weeks, when mice are cognitively impaired, there is no difference from wild-type controls. Klotho overexpressing mice show no early life effects on synaptic plasticity, but decreased CA1 hippocampal long-term potentiation was measured at 6months of age. Together these data suggest that klotho affects cognition, at least in part, by regulating hippocampal synaptic plasticity.

Leukemia inhibitory factor impairs structural and neurochemical development of rat visual cortex in vivo

Minipump infusions into visual cortex in vivo at the onset of the critical period have revealed that the proinflammatory cytokine leukemia inhibitory factor (LIF) delays the maturation of thalamocortical projection neurons of the lateral geniculate nucleus, and tecto-thalamic projection neurons of the superior colliculus, and cortical layer IV spiny stellates and layer VI pyramidal neurons. Here, we report that P12-20 LIF infusion inhibits somatic maturation of pyramidal neurons and of all interneuron types in vivo. Likewise, DIV 12-20 LIF treatment in organotypic cultures prevents somatic growth GABA-ergic neurons. Further, while NPY expression is increased in the LIF-infused hemispheres, the expression of parvalbumin mRNA and protein, Kv3.1 mRNA, calbindin D-28k protein, and GAD-65 mRNA, but not of GAD-67 mRNA or calretinin protein is substantially reduced. Also, LIF treatment decreases parvalbumin, Kv3.1, Kv3.2 and GAD-65, but not GAD-67 mRNA expression in OTC. Developing cortical neurons are known to depend on neurotrophins. Indeed, LIF alters neurotrophin mRNA expression, and prevents the growth promoting action of neurotophin-4 in GABA-ergic neurons. The results imply that LIF, by altering neurotrophin expression and/or signaling, could counteract neurotrophin-dependent growth and neurochemical differentiation of cortical neurons.

Sphingosine 1-phosphate lyase ablation disrupts presynaptic architecture and function via an ubiquitin- proteasome mediated mechanism

The bioactive lipid sphingosine 1-phosphate (S1P) is a degradation product of sphingolipids that are particularly abundant in neurons. We have shown previously that neuronal S1P accumulation is toxic leading to ER-stress and an increase in intracellular calcium. To clarify the neuronal function of S1P, we generated brain-specific knockout mouse models in which S1P-lyase (SPL), the enzyme responsible for irreversible S1P cleavage was inactivated. Constitutive ablation of SPL in the brain (SPL^fl/fl/Nes) but not postnatal neuronal forebrain-restricted SPL deletion (SPL^fl/fl/CaMK) caused marked accumulation of S1P. Hence, altered presynaptic architecture including a significant decrease in number and density of synaptic vesicles, decreased expression of several presynaptic proteins, and impaired synaptic short term plasticity were observed in hippocampal neurons from SPL^fl/fl/Nes mice. Accordingly, these mice displayed cognitive deficits. At the molecular level, an activation of the ubiquitin-proteasome system (UPS) was detected which resulted in a decreased expression of the deubiquitinating enzyme USP14 and several presynaptic proteins. Upon inhibition of proteasomal activity, USP14 levels, expression of presynaptic proteins and synaptic function were restored. These findings identify S1P metabolism as a novel player in modulating synaptic architecture and plasticity.

Presynaptic size of associational/commissural CA3 synapses is controlled by fibroblast growth factor 22 in adult mice

Associational/commissural CA3-CA3 synapses define the recurrent CA3 network that generates the input to CA1 pyramidal neurons. We quantified the fine structure of excitatory synapses in the stratum radiatum of the CA3d area in adult wild type (WT) and fibroblast growth factor 22 knock-out (FGF22KO) mice by using serial 3D electron microscopy. WT excitatory CA3 synapses are rather small yet range 10 fold in size. Spine size, however, was small and uniform and did not correlate with the size of the synaptic junction. To reveal mechanisms that regulate presynaptic structure, we investigated the role of FGF22, a target-derived signal specific for the distal part of area CA3 (CA3d). In adult FGF22KO mice, postsynaptic properties of associational CA3 synapses were unaltered. Presynaptically, the number of synaptic vesicles (SVs), the bouton volume, and the number of vesicles in axonal regions (the super pool) were reduced. This concurrent decrease suggests concerted control by FGF22 of presynaptic size. This hypothesis is supported by the finding that WT presynapses in the proximal part of area CA3 (CA3p) that do not receive FGF22 signaling in WT mice were smaller than presynapses in CA3d in WT but of comparable size in CA3d of FGF22KO mice. Docked SV density was decreased in CA1, CA3d, and CA3p in FGF22KO mice. Because CA1 and CA3p are not directly affected by the loss of FGF22, the smaller docked SV density may be an adaptation to activity changes in the CA3 network. Thus, docked SV density potentially is a long-term regulator for the synaptic release probability and/or the strength of short-term depression in vivo.

A catalytic independent function of the deubiquitinating enzyme USP14 regulates hippocampal synaptic short-term plasticity and vesicle number

The ubiquitin proteasome system is required for the rapid and precise control of protein abundance that is essential for synaptic function. USP14 is a proteasome-bound deubiquitinating enzyme that recycles ubiquitin and regulates synaptic short-term synaptic plasticity. We previously reported that loss of USP14 in ax(J) mice causes a deficit in paired pulse facilitation (PPF) at hippocampal synapses. Here we report that USP14 regulates synaptic function through a novel, deubiquitination-independent mechanism. Although PPF is usually inversely related to release probability, USP14 deficiency impairs PPF without altering basal release probability. Instead, the loss of USP14 causes a large reduction in the number of synaptic vesicles. Over-expression of a catalytically inactive form of USP14 rescues the PPF deficit and restores synaptic vesicle number, indicating that USP14 regulates presynaptic structure and function independently of its role in deubiquitination. Finally, the PPF deficit caused by loss of USP14 can be rescued by pharmacological inhibition of proteasome activity, suggesting that inappropriate protein degradation underlies the PPF impairment. Overall, we demonstrate a novel, deubiquitination-independent function for USP14 in influencing synaptic architecture and plasticity.

Altered profile of basket cell afferent synapses in hyper-excitable dentate gyrus revealed by optogenetic and two-pathway stimulations

Cholecystokinin (CCK-) positive basket cells form a distinct class of inhibitory GABAergic interneurons, proposed to act as fine-tuning devices of hippocampal gamma-frequency (30-90 Hz) oscillations, which can convert into higher frequency seizure activity. Therefore, CCK-basket cells may play an important role in regulation of hyper-excitability and seizures in the hippocampus. In normal conditions, the endogenous excitability regulator neuropeptide Y (NPY) has been shown to modulate afferent inputs onto dentate gyrus CCK-basket cells, providing a possible novel mechanism for excitability control in the hippocampus. Using GAD65-GFP mice for CCK-basket cell identification, and whole-cell patch-clamp recordings, we explored whether the effect of NPY on afferent synapses to CCK-basket cells is modified in the hyper-excitable dentate gyrus. To induce a hyper-excitable state, recurrent seizures were evoked by electrical stimulation of the hippocampus using the well-characterized rapid kindling protocol. The frequency of spontaneous and miniature excitatory and inhibitory post-synaptic currents recorded in CCK-basket cells was decreased by NPY. The excitatory post-synaptic currents evoked in CCK-basket cells by optogenetic activation of principal neurons were also decreased in amplitude. Interestingly, we observed an increased proportion of spontaneous inhibitory post-synaptic currents with slower rise times, indicating that NPY may inhibit gamma aminobutyric acid release preferentially in peri-somatic synapses. These findings indicate that increased levels and release of NPY observed after seizures can modulate afferent inputs to CCK-basket cells, and therefore alter their impact on the oscillatory network activity and excitability in the hippocampus.

The mechanisms of the strong inhibitory modulation of long-term potentiation in the rat dentate gyrus

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.

HCN1 channels constrain DHPG-induced LTD at hippocampal Schaffer collateral-CA1 synapses

HCN channels play a fundamental role in determining resting membrane potential and regulating synaptic function. Here, we investigated the involvement of HCN channels in basal synaptic transmission and long-term depression (LTD) at the Schaffer collateral-CA1 synapse. Bath application of the HCN channel blocker ZD7288 (10 microM) caused a significant increase in synaptic transmission that was due to an enhancement in AMPA receptor-mediated excitatory postsynaptic potentials. This enhancement was accompanied by a significant decrease in the paired-pulse ratio (PPR), suggesting a presynaptic mechanism. Experiments with the irreversible use-dependent NMDA receptor blocker MK-801 showed that ZD7288 led to an increase in glutamate release probability. LTD induced by brief application of (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 microM, 10 min) was significantly enhanced when HCN channels were blocked by ZD7288 (10 microM) prior to DHPG application. Moreover, the concomitant increase in PPR after DHPG-induced LTD was significantly larger than without ZD7288 bath application. Conversely, ZD7288 application after DHPG washout did not alter DHPG-LTD. A significant enhancement of DHPG-LTD was also observed in HCN1-deficient mice as compared with wild types. However, LTD induced by low-frequency stimulation (LFS) remained unaltered in HCN1-deficient mice, suggesting a differential effect of HCN1 channels on synaptic plasticity constraining DHPG-LTD, but not LFS-LTD.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Developmental changes in short-term facilitation are opposite at temporoammonic synapses compared to Schaffer collateral synapses onto CA1 pyramidal cells

CA1 pyramidal neurons receive two distinct excitatory inputs that are each capable of influencing hippocampal output and learning and memory. The Schaffer collateral (SC) input from CA3 axons onto the more proximal dendrites of CA1 is part of the trisynaptic circuit, which originates in Layer II of the entorhinal cortex (EC). The temporoammonic (TA) pathway to CA1 provides input directly from Layer III of the EC onto the most distal dendrites of CA1 pyramidal cells, and is involved in spatial memory and memory consolidation. We have previously described a developmental decrease in short-term facilitation from juvenile (P13-18) to young adult (P28-42) rats at SC synapses that is due to feedback inhibition via synaptically activated mGluR1 on CA1 interneurons. It is not known how short-term changes in synaptic strength are regulated at TA synapses, nor is it known how short-term plasticity is balanced at SC and TA inputs during development. Here we describe a novel developmental increase in short-term facilitation at TA synapses, which is the opposite of the decrease in facilitation occurring at SC synapses. Although short-term facilitation is much lower at TA synapses when compared with SC synapses in juveniles, short-term plasticity at SC and TA synapses converges at similar levels of paired-pulse facilitation in the young adult rat. However, in young adults CA3-CA1 synapses still exhibit more facilitation than TA-CA1 synapses during physiologically-relevant activity, suggesting that the two pathways are each poised to uniquely modulate CA1 output in an activity-dependent manner. Finally, we show that there is a developmental decrease in the initial release probability at TA synapses that underlies their developmental decrease in facilitation, but no developmental change in release probability at SC synapses. This represents a fundamental difference in the presynaptic function of the two major inputs to CA1, which could alter the flow of information in hippocampus during development.

Paired-pulse ratio of synaptically induced transporter currents at hippocampal CA1 synapses is not related to release probability

When a synapse is stimulated in rapid succession, the second post-synaptic response can be larger than the first and termed paired-pulse facilitation. It has been reported that the paired-pulse ratio (PPR), which is the ratio of the amplitude of the second response to that of the first, depends on the probability of vesicular release at the synapse, and PPR has been used as an easy measure of the release probability. To re-examine the relation of PPR with transmitter release probability, we made whole-cell recordings from astrocytes and pyramidal neurons in the CA1 area of rat hippocampal slices, and studied responses evoked by paired-pulse stimulus of the Schaffer collaterals. In a control condition in which blockers for ionotropic glutamate receptors were added to the artificial cerebrospinal fluid, synaptically induced transporter currents (STCs) recorded from astrocytes showed PPF with similar dependency on stimulus interval as the AMPA-receptor-mediated excitatory post-synaptic currents (AMPA-EPSCs) recorded from pyramidal neurons. When the transmitter release was enhanced by raising Ca2+ concentration in the bathing medium or by applying 8-CPT, an adenosine A1 receptor antagonist, the PPR of the neuronal AMPA-EPSCs decreased significantly. In the same condition, although the amplitude of STCs was significantly increased, the PPR of STCs did not show significant change. The PPR of AMPA-EPSCs, however, recovered by lowering the stimulus intensity or by applying low concentration of NBQX, a competitive antagonist for AMPA-receptor. These results imply that the PPR of transmitter release at Schaffer collateral synapses stays constant as the release probability was altered.

Seizure suppression by GDNF gene therapy in animal models of epilepsy

Temporal lobe epilepsy patients remain refractory to available anti-epileptic drugs in 30% of cases, indicating a need for novel therapeutic strategies. In this context, glial cell line-derived neurotrophic factor (GDNF) emerges as a possible new agent for epilepsy treatment. However, a limited number of studies, use of different epilepsy models, and different methods of GDNF delivery preclude understanding of the mechanisms for the seizure-suppressant action of GDNF. Here we show that recombinant adeno-associated viral (rAAV) vector-based GDNF overexpression in the rat hippocampus suppresses seizures in two models of temporal lobe epilepsy. First, when rAAV-GDNF was injected before hippocampal kindling, the number of generalized seizures decreased, and the prolongation of behavioral convulsions in fully kindled animals was prevented. Second, injection of rAAV-GDNF after kindling increased the seizure induction threshold. Third, rAAV-GDNF decreased the frequency of generalized seizures during the self-sustained phase of status epilepticus. Our data demonstrate the complexity of mechanisms and the beneficial action of GDNF in epilepsy. Furthermore, we show that ectopic rAAV-mediated GDNF gene expression in the seizure focus is a feasible way to mitigate seizures and provides proof of principle that the neurotrophic factor-based gene therapy approach has the potential to be developed as alternative strategy for epilepsy treatment.

Environment matters: synaptic properties of neurons born in the epileptic adult brain develop to reduce excitability

Neural progenitors in the adult dentate gyrus continuously produce new functional granule cells. Here we used whole-cell patch-clamp recordings to explore whether a pathological environment influences synaptic properties of new granule cells labeled with a GFP-retroviral vector. Rats were exposed to a physiological stimulus, i.e., running, or a brain insult, i.e., status epilepticus, which gave rise to neuronal death, inflammation, and chronic seizures. Granule cells formed after these stimuli exhibited similar intrinsic membrane properties. However, the new neurons born into the pathological environment differed with respect to synaptic drive and short-term plasticity of both excitatory and inhibitory afferents. The new granule cells formed in the epileptic brain exhibited functional connectivity consistent with reduced excitability. We demonstrate a high degree of plasticity in synaptic inputs to adult-born new neurons, which could act to mitigate pathological brain function.

Control of synaptic consolidation in the dentate gyrus: mechanisms, functions, and therapeutic implications

Synaptic consolidation refers to the development and stabilization of protein synthesis-dependent modifications of synaptic strength as observed during long-term potentiation (LTP) and long-term depression (LTD). Activity-dependent changes in synaptic strength are thought to underlie memory storage and other adaptive responses of the nervous systems of importance in mood stability, reward behavior, and pain control. This chapter focuses on the mechanisms and functions of synaptic consolidation in the dentate gyrus, a critical structure not only in hippocampal memory function, but also in regulation of stress responses and cognitive aspects of depression. Recent evidence suggests that synaptic consolidation at excitatory medial perforant path-granule cell synapses requires brain-derived neurotrophic factor (BDNF) signaling and induction of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is strongly induced and transported to dendritic processes following high-frequency stimulation (HFS) that induces LTP in the rat dentate gyrus in vivo. Sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Furthermore, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has revealed a panel of BDNF-regulated genes that may cooperate with Arc during synaptic consolidation. In addition to regulating gene expression, BDNF signaling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively-expressed mRNA in dendrites, BDNF may effectively control the window of synaptic consolidation. Dysregulation of BDNF synthesis and Arc function, specifically within the dentate gyrus, is linked to behavioral symptoms and cognitive deficits in animal models of depression and Alzheimer's disease. Therapeutics strategies targeting synaptic consolidation hold promise for the future.

Neurotrophins in the dentate gyrus

Since the discovery of nerve growth factor (NGF) in the 1950s and brain-derived neurotrophic factor (BDNF) in the 1980s, a great deal of evidence has mounted for the roles of neurotrophins (NGF; BDNF; neurotrophin-3, NT-3; and neurotrophin-4/5, NT-4/5) in development, physiology, and pathology. BDNF in particular has important roles in neural development and cell survival, as well as appearing essential to molecular mechanisms of synaptic plasticity and larger scale structural rearrangements of axons and dendrites. Basic activity-related changes in the central nervous system (CNS) are thought to depend on BDNF modulation of synaptic transmission. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders. In this chapter, I review neurotrophin structure, signal transduction mechanisms, localization and regulation within the nervous system, and various potential roles in disease. Modulation of neurotrophin action holds significant potential for novel therapies for a variety of neurological and psychiatric disorders.

NT-3 facilitates hippocampal plasticity and learning and memory by regulating neurogenesis

In the adult brain, the expression of NT-3 is largely confined to the hippocampal dentate gyrus (DG), an area exhibiting significant neurogenesis. Using a conditional mutant line in which the NT-3 gene is deleted in the brain, we investigated the role of NT-3 in adult neurogenesis, hippocampal plasticity, and memory. Bromodeoxyuridine (BrdU)-labeling experiments demonstrated that differentiation, rather than proliferation, of the neuronal precursor cells (NPCs) was significantly impaired in DG lacking NT-3. Triple labeling for BrdU, the neuronal marker NeuN, and the glial marker GFAP indicated that NT-3 affects the number of newly differentiated neurons, but not glia, in DG. Field recordings revealed a selective impairment in long-term potentiation (LTP) in the lateral, but not medial perforant path-granule neuron synapses. In parallel, the NT-3 mutant mice exhibited deficits in spatial memory tasks. In addition to identifying a novel role for NT-3 in adult NPC differentiation in vivo, our study provides a potential link between neurogenesis, dentate LTP, and spatial memory.

Responses of excitatory hippocampal synapses to natural stimulus patterns reveal a decrease in short-term facilitation and increase in short-term depression during postnatal development

Schaffer collateral excitatory synapses onto CA1 pyramidal cells are subject to significant modulation by short-term plasticity. This presynaptic, history-dependent modulation of neurotransmitter release causes synaptic transmission to be sensitive to the frequency of the input. As a result, temporally irregular input patterns, such as those observed in vivo, produce synaptic responses over a very wide dynamic range that reflect a balance of short-term facilitation and short-term depression. The neonatal period is an important developmental period in the hippocampus, when functional representations of an animal's environment are being established through exploratory behavior. The strength of excitatory synapses and their modulation by short-term plasticity are critical to this process. One form of short-term plasticity, paired-pulse facilitation, has been shown to decrease as juvenile rats mature into young adults. However, little is known about the neonatal modulation of other forms of short-term plasticity, including the responses to temporally complex stimuli. We examined developmental modulation of the short-term dynamics of Schaffer collateral excitatory synapses onto CA1 pyramidal cells in acute hippocampal slices, using both constant frequency stimuli and natural stimulus patterns that were taken from in vivo recording of spike patterns of hippocampal cells. In response to constant frequency stimulation, synapses in slices from young adult rats (P28-P35) showed less short-term depression than did those in slices from juveniles (P12-P18). However, when the natural stimulus pattern (containing a wide mix of frequencies) was used, synapses from young adults instead showed more short-term depression and less short-term facilitation than did juveniles. Comparing the natural stimulus pattern responses with constant frequency stimulation of a similar frequency, we found that the average responses were similar in young adults (both showed modest depression). However, in juveniles, the natural pattern produced robust facilitation while constant frequency stimulation caused a large short-term depression. Our results reveal that there are developmental changes both in individual forms of short-term plasticity and in the relative balance between short-term facilitation and short-term depression that will alter the signal transfer characteristics of these synapses.

Mechanisms of target-cell specific short-term plasticity at Schaffer collateral synapses onto interneurones versus pyramidal cells in juvenile rats

Although it is presynaptic, short-term plasticity has been shown at some synapses to depend upon the postsynaptic cell type. Previous studies have reported conflicting results as to whether Schaffer collateral axons have target-cell specific short-term plasticity. Here we investigate in detail the short-term dynamics of Schaffer collateral excitatory synapses onto CA1 stratum radiatum interneurones versus pyramidal cells in acute hippocampal slices from juvenile rats. In response to three stimulus protocols that invoke different forms of short-term plasticity, we find differences in some but not all forms of presynaptic short-term plasticity, and heterogeneity in the short term plasticity of synapses onto interneurones. Excitatory synapses onto the majority of interneurones had less paired-pulse facilitation than synapses onto pyramidal cells across a range of interpulse intervals (20-200 ms). Unlike synapses onto pyramidal cells, synapses onto most interneurones had very little facilitation in response to short high-frequency trains of five pulses at 5, 10 and 20 Hz, and depressed during trains at 50 Hz. However, the amount of high-frequency depression was not different between synapses onto pyramidal cells versus the majority of interneurones at steady state during 2-10 Hz trains. In addition, a small subset of interneurones (approximately 15%) had paired-pulse depression rather than paired-pulse facilitation, showed only depression in response to the high-frequency five pulse trains, and had more steady-state high-frequency depression than synapses onto pyramidal cells or the majority of interneurones. To investigate possible mechanisms for these differences in short-term plasticity, we developed a mechanistic mathematical model of neurotransmitter release that explicitly explores the contributions to different forms of short-term plasticity of the readily releasable vesicle pool size, release probability per vesicle, calcium-dependent facilitation, synapse inactivation following release, and calcium-dependent recovery from inactivation. Our model fits the responses of each of the three cell groups to the three different stimulus protocols with only two parameters that differ with cell group. The model predicts that the differences in short-term plasticity between synapses onto CA1 pyramidal cells and stratum radiatum interneurones are due to a higher initial release probability per vesicle and larger readily releasable vesicle pool size at synapses onto interneurones, resulting in a higher initial release probability. By measuring the rate of block of NMDA receptors by the open channel blocker MK-801, we confirmed that the initial release probability is greater at synapses onto interneurones versus pyramidal cells. This provides a mechanism by which both the initial strength and the short-term dynamics of Schaffer collateral excitatory synapses are regulated by their postsynaptic target cell.

Differential suppression of seizures via Y2 and Y5 neuropeptide Y receptors

Neuropeptide Y (NPY) prominently inhibits epileptic seizures in different animal models. The NPY receptors mediating this effect remain controversial partially due to lack of highly selective agonists and antagonists. To circumvent this problem, we used various NPY receptor knockout mice with the same genetic background and explored anti-epileptic action of NPY in vitro and in vivo. In Y2 (Y2-/-) and Y5 (Y5-/-) receptor knockouts, NPY partially inhibited 0 Mg2+-induced epileptiform activity in hippocampal slices. In contrast, in double knockouts (Y2Y5-/-), NPY had no effect, suggesting that in the hippocampus in vitro both receptors mediate anti-epileptiform action of NPY in an additive manner. Systemic kainate induced more severe seizures in Y5-/- and Y2Y5-/-, but not in Y2-/- mice, as compared to wild-type mice. Moreover, kainate seizures were aggravated by administration of the Y5 antagonist L-152,804 in wild-type mice. In Y5-/- mice, hippocampal kindling progressed faster, and afterdischarge durations were longer in amygdala, but not in hippocampus, as compared to wild-type controls. Taken together, these data suggest that, in mice, both Y2 and Y5 receptors regulate hippocampal seizures in vitro, while activation of Y5 receptors in extra-hippocampal regions reduces generalized seizures in vivo.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.

Increased neurogenesis and the ectopic granule cells after intrahippocampal BDNF infusion in adult rats

There is evidence that BDNF influences the birth of granule cells in the dentate gyrus, which is one of the few areas of the brain that demonstrates neurogenesis throughout life. However, studies to date have not examined this issue directly. To do so, we compared the effects of BDNF, phosphate-buffered saline (PBS), or bovine serum albumin (BSA) on neurogenesis after infusion into the hippocampus of the normal adult rat, using osmotic pumps that were implanted unilaterally in the dorsal hilus. BDNF, PBS, and BSA were infused for 2 weeks. The mitotic marker bromodeoxyuridine (BrdU) was administered twice daily during the 2-week infusion period. At least 1 month after infusion ended, brains were processed immunocytochemically using antibodies to BrdU, a neuronal nuclear protein (NeuN), or calbindin D28K (CaBP), which labels mature granule cells. Stereology was used to quantify BrdU-labeled cells in the dorsal hippocampus that were double-labeled with NeuN or CaBP. There was a statistically significant increase in BrdU(+)/NeuN(+) double-labeled cells in the granule cell layer after BDNF infusion relative to controls. The values for BrdU(+)/NeuN(+) cells were similar to BrdU(+)/CaBP(+) cells, indicating that most new neurons were likely to be granule cells. In addition, BrdU(+)/NeuN(+)-labeled cells developed in the hilar region after BDNF infusion, which have previously only been identified after severe continuous seizures (status epilepticus) and associated pathological changes. Remarkably, neurogenesis was also increased contralaterally, but BDNF did not appear to spread to the opposite hemisphere. Thus, infusion of BDNF to a local area can have widespread effects on hippocampal neurogenesis. The results demonstrate that BDNF administration to the dentate gyrus leads to increased neurogenesis of granule cells. They also show that ectopic granule cells develop after BDNF infusion, which suggests that ectopic migration is not necessarily confined to pathological conditions. These results are discussed in light of the evidence that BDNF increases neuronal activity in hippocampus. Thus, the mechanisms underlying neurogenesis following BDNF infusion could be due to altered activity as well as direct effects of BDNF itself, and this is relevant to studies of other growth factors because many of them have effects on neuronal excitability that are often not considered.

Chronic BDNF deficiency permanently modifies excitatory synapses in the piriform cortex

Brain-derived neurotrophic factor (BDNF), aside from its classic neurotrophic role in development and survival of neurons, has been shown to be involved in modification and plasticity of central synapses. In mice with BDNF gene deletion (BDNF+/-), deficits in synaptic transmission are often observed but are reversed readily by administration of BDNF, suggesting its acute effect. In support, blockade of BDNF signaling in wild-type hippocampal slices by TrkB-IgG closely reproduces synaptic alterations observed in BDNF+/- mice. We demonstrate that in BDNF+/- mice, lateral olfactory tract (LOT) synapses exhibit decreased release probability of glutamate, suggested by increased paired-pulse facilitation (PPF) of field excitatory postsynaptic potentials (fEPSPs), as well as by slower blocking rate of N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) by MK-801 in the pyramidal neurons of the piriform cortex. The changes in PPF were not mimicked in wild-type mice by acute blockade of BDNF signaling by TkrB-IgG. These data imply that BDNF deficit during development might lead to chronic changes of excitatory transmission in LOT synapses. Modification of the LOT synapses in BDNF+/- mice was associated with altered inhibitory drive onto the mitral cells from the granule and glomerular neurons, which in turn exhibited decreased renewal rate compared to that in wild-type mice. Taken together, these data suggest that BDNF deficiency can have both acute and more permanent effects on synaptic function, particularly when BDNF signaling is compromised during the early stages of brain development. In the latter case, altered synaptic properties in BDNF+/- mice could be secondary to other complex changes in the brain, e.g., cell survival/proliferation.

Differential presynaptic modulation of excitatory and inhibitory autaptic currents in cultured hippocampal neurons

Short-term synaptic plasticity has an important role in higher cortical function. Hyperpolarization may effect a form of short-term plasticity by promoting recovery from sodium channel inactivation or by activating axonal A-type potassium channels. To determine whether one or both processes occur, we examined the effect of hyperpolarizing prepulses on autaptic currents in cultured postnatal rat hippocampal neurons. As expected of enhanced recovery from sodium channel inactivation, hyperpolarizing prepulses reversibly increased fast excitatory autaptic currents (eacs) mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), slow eacs mediated by N-methyl-D-aspartate receptors (NMDARs), and inhibitory autaptic currents (iacs) mediated by gamma-aminobutyric acidA receptors (GABAARs). Hyperpolarizing prepulses augmented nearly all fast and slow eacs but only half of the iacs. This change occurred without a change in autaptic current kinetics. Of note, hyperpolarizing prepulses did not significantly reduce autaptic currents in any neuron studied. The rapidly dissociating competitive antagonists kynurenate and L-2-amino-5-phosphonovaleric acid (LAPV) inhibited fast and slow eacs, respectively, to the same extent with and without a hyperpolarizing prepulse. In addition, hyperpolarizing prepulses revealed a slow eac even after the slow eac evoked without a prepulse was completely blocked by the open channel blocker, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). Finally, hyperpolarizing prepulses did not alter currents evoked by exogenous applications of glutamate and GABA. These findings suggest that hyperpolarizing prepulses preferentially enhance eacs over iacs, and that they do so, in part, by overcoming conduction block or by activating silent synapses.

Neuropeptide Y Y5 receptors suppress in vitro spontaneous epileptiform bursting in the rat hippocampus

Neuropeptide Y (NPY) has been implicated in antiepileptic action in different in vivo and in vitro epilepsy models in rats and mice. Both Y2 and Y5 receptors could mediate the seizure-suppressant effect of NPY. However, lack of selective ligands precluded previous studies from conclusively evaluating the role of Y5 receptors in anti-epileptiform action of NPY. In the present study, using the new highly selective Y5 receptor antagonist, CGP71683A, and agonist, [cPP]hPP, we show that the Y5 receptor subtype is centrally involved in NPY-induced suppression of spontaneous epileptiform (interictaform) bursting in the CA3 area of rat hippocampal slices. This novel finding underscores the importance of Y5 receptors as a potential target for future antiepileptic therapy, particularly, for interictal components of temporal lobe epilepsy.

Increased excitability in cortico-striatal synaptic pathway in a model of paroxysmal dystonia

Dystonias are movement disorders whose pathomechanism is largely unknown. Dystonic dt(sz) hamsters represent a model of primary dystonias, where alterations of striatal interneuron density and sodium channel function in projection neurones were described. Here, using cortico-striatal slices, we explore whether also the communication between neocortex and striatum is altered in dt(sz) hamsters. Field and intracellular recordings were done in dorsomedial striatum. Electrical stimulation was used to mimic neocortical afferents. Neuronal characteristics, synaptic connections, input-output relations and short- and long-term plasticity were analysed. Regarding cellular properties, striatal neurons of affected animals showed no alterations. Concerning network properties, evoked responses at threshold stimulation were mediated by (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors. In dt(sz) slices, field responses, paired-pulse accentuation and LTP were larger than in control, possibly by an increase in presynaptic release probability at glutamatergic synapses. In summary, the study indicates that a change of cortico-striatal communication is involved in the manifestation of paroxysmal dystonia in the dt(sz) mutant.

Presynaptic neurotrophin-3 increases the number of tectal synapses, vesicle density, and number of docked vesicles in chick embryos

To determine whether presynaptically derived neurotrophins may contribute to synaptic plasticity, we examined whether neurotrophin-3 (NT-3) changed the number, size, vesicle content, or vesicle distribution of synapses within the retinorecipient layers of the chick optic tectum. In this system, endogenous NT-3 derives presynaptically from retinal ganglion cell axons. Retinotectal synapses comprise the majority of synapses in superficial tectal layers, as demonstrated by destruction of retinotectal input by intraocular application of the drug monensin. To examine the effect of increased or decreased levels of NT-3, either exogenous NT-3 or monoclonal NT-3 blocking antibodies were injected into the optic tectum of 19-day-old chick embryos, spiked with radiolabeled protein to verify the success of injections and estimate effective concentrations. After 48 hours, the ultrastructure of superficial tectal layers was analyzed and compared with samples from control tecta injected with cytochrome C. NT-3 increased the number of synapses, synaptic vesicles/profile, synaptic vesicle densities, the number of docked vesicles, and the length of the synaptic profile. Deprivation of anterogradely transported endogenous NT-3 with NT-3 antibodies resulted in the opposite effect: decreased numbers of synapses, decreased vesicle densities, and decreased numbers of docked vesicles. Brain-derived neurotrophic factor (BDNF) had a largely different effect than NT-3. BDNF increased the density of vesicles and deprivation of endogenous TrkB ligands with TrkB fusion protein reduced the density of vesicles in the synapses, without effects on synapse number or docked vesicles. We conclude that anterogradely transported NT-3 affects synapse strength in a way that differs from that of presumably postsynaptic-derived BDNF.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

Neuropeptide Y inhibits in vitro epileptiform activity in the entorhinal cortex of mice

Previous studies show that neuropeptide Y (NPY) inhibits in vitro seizures in rodent hippocampus. Here, we explored the effect of NPY application on epileptiform discharges induced by perfusion with magnesium-free solution in slices of entorhinal cortex from two different mouse strains. NPY significantly reduced the duration of epileptiform discharges with a peak effect of 36-50%. This is the first study showing anti-epileptiform effect of NPY in the entorhinal cortex and also the first evidence that NPY inhibits seizures in a cortical region in mice. The entorhinal cortex has a central role in transferring information between the hippocampus and the rest of the brain. Therefore our data further strengthen the concept of NPY and its receptors as widespread regulators of epileptiform activity and as a potential future target for antiepileptic therapy.

Neurotrophin-3 is required for appropriate establishment of thalamocortical connections

In the vertebrate brain, the thalamus serves as a relay and integration station for diverse neuronal information en route from the periphery to the cortex. Formation of the thalamocortical tract occurs during pre- and postnatal development, with distinct thalamic nuclei projecting to specific cortical regions. The molecular forces that underlie the invasion by axons into specific cortical layers followed by activity-dependent maturation of synapses are poorly understood. We show that genetic ablation of neurotrophin-3 (NT-3) in the mouse neocortex results in reduction of a set of anatomically distinct axonal bundles projecting from thalamus through cortical white matter. These bundles include thalamocortical axons that normally establish connections with retrosplenial and visual cortex, sites of early postnatal NT-3 expression. These results implicate neurotrophins in the critical stage of precise thalamocortical connections.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Mechanisms of the release of anterogradely transported neurotrophin-3 from axon terminals

Neurotrophins have profound effects on synaptic function and structure. They can be derived from presynaptic, as well as postsynaptic, sites. To date, it has not been possible to measure the release of neurotrophins from axon terminals in intact tissue. We implemented a novel, extremely sensitive assay for the release and transfer of anterogradely transported neurotrophin-3 (NT-3) from a presynaptic to a postsynaptic location that uses synaptosomal fractionation after introduction of radiolabeled NT-3 into the retinotectal projection of chick embryos. Release of the anterogradely transported NT-3 in intact tissue was assessed by measuring the amount remaining in synaptosomal preparations after treatment of whole tecta with pharmacological agents. Use of this assay reveals that release of NT-3 from axon terminals is increased by depolarization, calcium influx via N-type calcium channels, and cAMP analogs, and release is most profoundly increased by excitation with kainic acid or mobilization of calcium from intracellular stores. NT-3 release depends on extracellular sodium, CaM kinase II activity, and requires intact microtubules and microfilaments. Dantrolene inhibits the high potassium-induced release of NT-3, indicating that release of calcium from intracellular stores is required. Tetanus toxin also inhibits NT-3 release, suggesting that intact synaptobrevin or synaptobrevin-like molecules are required for exocytosis. Ultrastructural autoradiography and immunolabel indicate that NT-3 is packaged in presumptive large dense-core vesicles. These data show that release of NT-3 from axon terminals depends on multiple regulatory proteins and ions, including the mobilization of local calcium. The data provide insight in the mechanisms of anterograde neurotrophins as synaptic modulators.

Suppressed kindling epileptogenesis in mice with ectopic overexpression of galanin

The neuropeptide galanin has been shown to suppress epileptic seizures. In cortical and hippocampal areas, galanin is normally mainly expressed in noradrenergic afferents. We have generated a mouse overexpressing galanin in neurons under the platelet-derived growth factor B promoter. RIA and HPLC analysis revealed up to 8-fold higher levels of galanin in transgenic as compared with wild-type mice. Ectopic galanin overexpression was detected especially in dentate granule cells and hippocampal and cortical pyramidal neurons. Galanin-overexpressing mice showed retardation of seizure generalization during hippocampal kindling, a model for human complex partial epilepsy. The high levels of galanin in mossy fibers found in the transgenic mice were further increased after seizures. Frequency facilitation of field excitatory postsynaptic potentials, a form of short-term synaptic plasticity assessed in hippocampal slices, was reduced in mossy fiber-CA3 cell synapses of galanin-overexpressing mice, indicating suppressed glutamate release. This effect was reversed by application of the putative galanin receptor antagonist M35. These data provide evidence that ectopically overexpressed galanin can be released and dampen the development of epilepsy by means of receptor-mediated action, at least partly by reducing glutamate release from mossy fibers.

Neurotrophic factors and gene therapy in spinal cord injury

Although it was once thought that the central nervous system (CNS) of mammals was incapable of substantial recovery from injury, it is now clear that the adult CNS remains responsive to various substances that can promote cell survival and stimulate axonal growth. Among these substances are growth factors, including the neurotrophins and cytokines, and growth-supportive cells such as Schwann cells, olfactory ensheathing glia, and stem cells. We review the effects of these substances on promoting axonal growth after spinal cord injury, placing particular emphasis on the genetic delivery of nervous system growth factors to specific sites of injury as a means of promoting axonal growth and, in limited instances, functional recovery.

Paired-pulse plasticity at the single release site level: an experimental and computational study

CA3-CA1 glutamatergic synapses in the hippocampus exhibit a large heterogeneity in release probability (p) and paired-pulse (PP) plasticity, established already in the early neonatal period when the CA3-CA1 connections consist of only a single release site. At such a site two factors decide initial release probability: the number of immediately releasable vesicles (preprimed pool) and the vesicle release probability (P(ves1)). Depletion and replenishment of this pool, an alteration in P(ves), and desensitization of postsynaptic receptors may contribute to PP plasticity. A model based on data from single neonatal CA3-CA1 synapses has been used to address the relative importance of these factors for the heterogeneity in PP plasticity. At a 20 msec PP interval, the PP ratio (P(2)/P(1)) varied from 0.1 to 4.5 among the synapses. At this interval desensitization and replenishment were of little importance. The heterogeneity was explained mostly by the variation in P(ves1), whereas the preprimed pool size was of minor importance. P(ves) altered from the first to the second stimulus such that P(ves2) was rather uniform among the synapses. Its variation thus contributed little to the heterogeneity in PP ratio. The model also shows that the relationship between alterations in release probability and PP ratio is complex. Thus, an increase in release probability can be associated with an increase, a decrease, or no change at all in PP ratio, depending on the original values of P(ves1) and the preprimed pool and on which one of these factors is altered to produce the increase in release probability.

Long-term potentiation of single subicular neurons in mice

Subicular neurons receive direct afferent connections from the vast majority of CA1 pyramidal cells and send their axons to the various brain areas. Because of this strategic position, subicular cells can modulate output of the hippocampus and, thus, play a significant part in memory, spatial processing, and seizure amplification and propagation from the hippocampus. Despite its important role as a hippocampal interface with different brain regions, present knowledge of the subiculum and the plastic properties of the synapses on the subicular neurons is rather limited. By using IR-DIC videomicroscopy and whole-cell patch-clamp recordings in mouse hippocampal slices, I demonstrated that long-term potentiation (LTP) in CA1-subicular cell synapses can be readily induced by high-frequency stimulation (HFS) of the afferents, but not by pairing of low-frequency stimulation with depolarization of postsynaptic cells. This tetanus-induced LTP is input specific, insensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist 3-[(R)-2Carboxipiperazin-4-yl]-propyl-1-phosphonic acid (R-CPP), and reduces paired-pulse facilitation in potentiated synapses. Subsequent morphologic analysis of the recorded cells, which were filled either with Lucifer Yellow or Biocytin, revealed pyramidal-shaped neurons localized predominantly in the deep layer of the subiculum, close to the CA1 border. Axons of the majority of these neurons extended to the alveus and on toward the hippocampus, probably exiting it via the fornix. These data indicate that CA1-subicular cell synapses in mice exhibit LTP, which can be expressed presynaptically, and its induction does not require NMDA-receptor activation. The observed activity-dependent plasticity might play an important role in the integrative mechanisms of the subiculum and may influence transfer of information from the hippocampus to subcortical and cortical brain areas.

Increased synaptic inhibition in dentate gyrus of mice with reduced levels of endogenous brain-derived neurotrophic factor

The aim of this study was to explore the role of endogenous neurotrophins for inhibitory synaptic transmission in the dentate gyrus of adult mice. Heterozygous knockout (+/-) mice or neurotrophin scavenging proteins were used to reduce the levels of endogenous brain-derived neurotrophic factor and neurotrophin-3. Patch-clamp recordings from dentate granule cells in brain slices showed that the frequency, but not the kinetics or amplitude, of miniature inhibitory postsynaptic currents was modulated in brain-derived neurotrophic factor +/- compared to wild-type (+/+) mice. Furthermore, paired-pulse depression of evoked inhibitory synaptic responses was increased in brain-derived neurotrophic factor +/- mice. Similar results were obtained in brain slices from brain-derived neurotrophic factor +/+ mice incubated with tyrosine receptor kinase B-immunoglobulin G, which scavenges endogenous brain-derived neurotrophic factor. The increased inhibitory synaptic activity in brain-derived neurotrophic factor +/- mice was accompanied by decreased excitability of the granule cells. No differences in the frequency, amplitude or kinetics of miniature inhibitory postsynaptic currents were seen between neurotrophin-3 +/- and +/+ mice. From these results we suggest that endogenous brain-derived neurotrophic factor, but not neurotrophin-3, has acute modulatory effects on synaptic inhibition onto dentate granule cells. The site of action seems to be located presynaptically, i.e. brain-derived neurotrophic factor regulates the properties of inhibitory interneurons, leading to increased excitability of dentate granule cells. We propose that through this mechanism, brain-derived neurotrophic factor can change the gating/filtering properties of the dentate gyrus for incoming information from the entorhinal cortex to hippocampus. This will have consequences for the recruitment of hippocampal neural circuitries both under physiological and pathological conditions, such as epileptogenesis.

PI-3 kinase and IP3 are both necessary and sufficient to mediate NT3-induced synaptic potentiation

Signaling mechanisms underlying neurotrophic regulation of synaptic transmission are not fully understood. Here we show that neurotrophin-3 (NT3)-induced potentiation of synaptic transmission at the neuromuscular synapses is blocked by inhibition of phosphoinositide-3 kinase, phospholipase C-gamma or the downstream IP3 receptors of phospholipase C-gamma, but not by inhibition of MAP kinase. However, neither stimulation of Ca2+ release from intracellular stores by photolysis of caged IP3, nor expression of a constitutively active phosphoinositide-3 kinase (PI3K*) in presynaptic motoneurons alone is sufficient to enhance transmission. Photo-uncaging of IP3 in neurons expressing PI3K* elicits a marked synaptic potentiation, mimicking the NT3 effect. These results reveal an involvement of PI3 kinase in transmitter release, and suggest that concomitant activation of PI3 kinase and IP3 receptors is both necessary and sufficient to mediate the NT3-induced synaptic potentiation.

Development and persistence of kindling epilepsy are impaired in mice lacking glial cell line-derived neurotrophic factor family receptor alpha 2

Seizure activity regulates gene expression for glial cell line-derived neurotrophic factor (GDNF) and neurturin (NRTN), and their receptor components, the transmembrane c-Ret tyrosine kinase and the glycosylphosphatidylinositol-anchored GDNF family receptor (GFR) alpha 1 and alpha 2 in limbic structures. We demonstrate here that epileptogenesis, as assessed in the hippocampal kindling model, is markedly suppressed in mice lacking GFR alpha 2. Moreover, at 6 to 8 wk after having reached the epileptic state, the hyperexcitability is lower in GFR alpha 2 knock-out mice as compared with wild-type mice. These results provide evidence that signaling through GFR alpha 2 is involved in mechanisms regulating the development and persistence of kindling epilepsy. Our data suggest that GDNF and NRTN may modulate seizure susceptibility by altering the function of hilar neuropeptide Y-containing interneurons and entorhinal cortical afferents at dentate granule cell synapses.

Deficient long-term memory and long-lasting long-term potentiation in mice with a targeted deletion of neurotrophin-4 gene

We examined the learning and memory of neurotrophin-4 (NT4)-/- mice by using fear conditioning. In both cue and context conditioning, we found significant deficits in the NT4 mutants at 2 and 24 h after training but not at 30 min. Hippocampal slices from the mutant mice showed normal basal synaptic transmission, short-term plasticity, and decremental long-term potentiation (LTP) at the Schaffer collateral-CA1 synapses. These findings, together with the normal short-term memory, suggest that the hippocampal development of NT4-/- mice is largely unaffected. However, consistent with the long-term memory defects, the long-lasting LTP at the same synapses was attenuated significantly in the mutant mice. Our results suggest that NT4 plays a physiological role essential for hippocampus- and amygdala-dependent long-term memory and hippocampal long-lasting LTP and that NT4 may be useful in the therapy of acquired disorders of learning and memory.

Intracellular Ca(2+) and Ca(2+)/calmodulin-dependent kinase II mediate acute potentiation of neurotransmitter release by neurotrophin-3

Neurotrophins have been shown to acutely modulate synaptic transmission in a variety of systems, but the underlying signaling mechanisms remain unclear. Here we provide evidence for an unusual mechanism that mediates synaptic potentiation at the neuromuscular junction (NMJ) induced by neurotrophin-3 (NT3), using Xenopus nerve-muscle co-culture. Unlike brain-derived neurotrophic factor (BDNF), which requires Ca(2+) influx for its acute effect, NT3 rapidly enhances spontaneous transmitter release at the developing NMJ even when Ca(2+) influx is completely blocked, suggesting that the NT3 effect is independent of extracellular Ca(2+). Depletion of intracellular Ca(2+) stores, or blockade of inositol 1, 4, 5-trisphosphate (IP3) or ryanodine receptors, prevents the NT3-induced synaptic potentiation. Blockade of IP3 receptors can not prevent BDNF-induced potentiation, suggesting that BDNF and NT3 use different mechanisms to potentiate transmitter release. Inhibition of Ca(2+)/calmodulin-dependent kinase II (CaMKII) completely blocks the acute effect of NT3. Furthermore, the NT3-induced potentiation requires a continuous activation of CaMKII, because application of the CaMKII inhibitor KN62 reverses the previously established NT3 effect. Thus, NT3 potentiates neurotransmitter secretion by stimulating Ca(2+) release from intracellular stores through IP3 and/or ryanodine receptors, leading to an activation of CaMKII.

Neurotrophin 3 potentiates glutamatergic responses of IHC afferents in the cochlea in vivo

Neurotrophins have traditionally been regarded as slow-acting signals essential for neuronal survival and differentiation. Recent studies with neuronal slices, cultures and nerve ending preparations have shown that neurotrophins generate acute changes in nerve activity. Among the secondary sensory cells are the inner hair cells (IHC) and taste buds, cells which express the neurotrophic factors necessary for the survival of their innervating neurons. If in these cells neurotrophins acutely affect the nerve activity of their afferent neurons, as in the central nervous system (CNS), this may have important functional implications for the corresponding sensory transduction processes. The neurotrophin NT-3 has been reported to be expressed in IHCs. We chose an in vivo application system for the microiontophoretic supply of NT-3 in the subsynaptic region of the IHC. The effect of NT-3 on spontaneous and evoked afferent cochlear nerve activities in adult guinea pig inner ear was studied. We observed that NT-3 rapidly increases the spontaneous and glutamate-evoked firing rate of IHC afferents. Moreover, firing induced by both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) were specifically enhanced during the presence of NT-3, a process which was selectively blocked by the tyrosine kinase receptor inhibitor K252a. Because we localized NT-3 mRNA not only in IHCs but also in the spiral ganglion, we propose that similar to other sensory systems, afferent and autocrine neurotrophin activities may be responsible for survival of cochlear neurons. In addition, NT-3 in IHCs may operate as a signal-dependent, intrinsic neuromodulator and/or neuroprotector.

Afferent-specific modulation of short-term synaptic plasticity by neurotrophins in dentate gyrus

Neurotrophins modulate synaptic transmission and plasticity in the adult brain. We here show a novel feature of this synaptic modulation, i.e. that two populations of excitatory synaptic connections to granule cells in the dentate gyrus, lateral perforant path (LPP) and medial perforant path (MPP), are differentially influenced by the neurotrophins BDNF and NT-3. Using field recordings and whole-cell patch-clamp recordings in hippocampal slices, we found that paired-pulse (PP) depression at MPP-granule cell synapses was impaired in BDNF knock-out (+/-) mice, but PP facilitation at LPP synapses to the same cells was not impaired. In accordance, scavenging of endogenous BDNF with TrkB-IgG fusion protein also impaired PP depression at MPP-granule cell synapses, but not PP facilitation at LPP-granule cell synapses. Conversely, in NT-3+/- mice, PP facilitation was impaired at LPP-granule cell synapses whilst PP depression at MPP-granule cell synapses was unaffected. These deficits could be reversed by application of exogenous neurotrophins in an afferent-specific manner. Our data suggest that BDNF and NT-3 differentially regulate the synaptic impact of different afferent inputs onto single target neurons in the CNS.

Mild experimental brain injury differentially alters the expression of neurotrophin and neurotrophin receptor mRNAs in the hippocampus

The molecular events responsible for impairments in cognition following mild traumatic brain injury are poorly understood. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), have been identified as having a role in learning and memory. We have previously demonstrated that following experimental brain trauma of moderate severity (2.0-2.1 atm), mRNA levels of BDNF and its high-affinity receptor, trkB, are increased bilaterally in the hippocampus for several hours, whereas NT-3 mRNA expression is decreased. In the present study, we used in situ hybridization to compare BDNF, trkB, NT-3, and trkC mRNA expression in rat hippocampus at 3 or 6 h after a lateral fluid percussion brain injury (FPI) of mild severity (1.0 atm) to sham-injured controls at equivalent time points. Mild FPI induced significant increases in hybridization levels for BDNF and trkB mRNAs, and a decrease in NT-3 mRNA in the hippocampus. However, in contrast to the bilateral effects of moderate experimental brain injury, the present changes with mild injury were restricted to the injured side. These findings demonstrate that even a mild traumatic brain injury differentially alters neurotrophin and neurotrophin receptor levels in the hippocampus. Such alterations may have important implications for neural plasticity and recovery of function in people who sustain a mild head injury.

Relative contribution of endogenous neurotrophins in hippocampal long-term potentiation

Recent evidence has shown that brain-derived neurotrophic factor (BDNF) is involved in hippocampal long-term potentiation (LTP). Because the reagents used in acute experiments react not only with BDNF but also with neurotrophin-4/5 (NT4/5) and neurotrophin-3 (NT3), we examined the involvement of these neurotrophins in LTP using two highly specific, function-blocking monoclonal antibodies against BDNF and NT3, as well as a TrkB-IgG fusion protein. Our results show that NT3 antibodies did not have any effects on LTP. However, both TrkB-IgG fusion proteins and BDNF antibody similarly reduced LTP, suggesting that only BDNF but no other ligands of the TrkB-receptor are likely to be involved in LTP induction. The reduction in LTP depended on the inducing stimuli and was only observed with theta-burst stimulation (TBS) but not with tetanic stimulation. We further observed that LTP was only reduced if BDNF was blocked before and during TBS stimulation, and BDNF antibodies did not affect early or late stages of LTP if they were applied 10, 30, or 60 min after TBS stimulation. These results point toward a specific and unique role of endogenous BDNF but not of other neurotrophins in the process of TBS-induced hippocampal LTP. Additionally, they suggest that endogenous BDNF is required for a limited time period only shortly before or around LTP induction but not during the whole process of LTP.