7th ISN special neurochemistry conference 'Synaptic function and dysfunction in brain diseases'.

Hippocampal synaptic plasticity, in the form of long-term potentiation (LTP) and long-term depression (LTD), enables spatial memory formation, whereby LTP and LTD are likely to contribute different elements to the resulting spatial representation. Dopamine, released from the ventral tegmental area particularly under conditions of reward, acts on the hippocampus, and may specifically influence the encoding of information into long-term memory. The dentate gyrus (DG), as the "gateway" to the hippocampus is likely to play an important role in this process. D1/D5 dopamine receptors are importantly involved in the regulation of synaptic plasticity thresholds in the CA1 region of the hippocampus and determine the direction of change in synaptic strength that occurs during novel spatial learning. Here, we explored whether D1/D5-receptors influence LTD that is induced in the DG following patterned afferent stimulation of the perforant path of freely behaving adult rats, or influence LTD that occurs in association with spatial learning. We found that LTD that is induced by afferent stimulation, and LTD that is facilitated by learning about novel landmark configurations, were both prevented by D1/D5-receptor antagonism, whereas agonist activation of the D1/D5-receptor had no effect on basal tonus or short-term depression. Other studies have reported that in the DG, D1/D5-receptor agonism or antagonism do not affect LTP, but agonism prevents depotentiation. These findings suggest that the dopaminergic system, acting via D1/D5-receptors, influences information gating by the DG and modulates the direction of change in synaptic strength that underlies information storage in this hippocampal substructure. Information encoded by robust forms of LTD is especially dependent on D1/D5-receptor activation. Thus, dopamine acting on D1/D5-receptors is likely to support specific experience-dependent encoding, and may influence the content of hippocampal representations of experience.

The novelty, saliency, and valency of ongoing experiences potently influence the firing rate of the ventral tegmental area (VTA) and the locus coeruleus (LC). Associative experience, in turn, is recorded into memory by means of hippocampal synaptic plasticity that is regulated by noradrenaline sourced from the LC, and dopamine, sourced from both the VTA and LC. Two persistent forms of synaptic plasticity, long-term potentiation (LTP), and long-term depression (LTD) support the encoding of different kinds of spatial experience. To what extent the VTA and the LC influence the direction of change of synaptic plasticity and therefore the content of stored experience is not clear. Here, we report that test-pulse activation of Schaffer-collateral-CA1 synapses of freely behaving male rats, in conjunction with VTA stimulation, results in LTP (>24 h), whereas concomitant hippocampal afferent and LC stimulation results in LTD (>24 h). Effects are frequency-dependent (1 to 50 Hz) and competitive: high-frequency (25 Hz), but not low-frequency (5 Hz) optogenetic activation of tyrosine hydroxylase-positive (TH+) neurons in the VTA, results in D1/D5R-dependent LTP, whereas 5 Hz (but not 1, or 25 Hz) activation of TH+ neurons in the LC results in hippocampal LTD that is both D1/D5 and β-AR-dependent. These results suggest that the VTA and LC do not work in synergy, but rather function in a competing fashion to drive different forms of information encoding through synaptic plasticity. Our findings indicate that information transmitted by the VTA and LC is likely to play a decisive role in the shaping of hippocampal information storage and the nature of learned experience.

Storage or processing of information underlies a mechanism able to change the efficacy of certain synapses depending on its activity. Long-term potentiation (LTP) and long-term depression (LTD) are two types of experimentally induced activity dependent change in synaptic efficacy (plasticity). The aim of this study was to determine the subtype specific involvement of group I metabotropic glutamate receptors (mGlus) in both forms of synaptic plasticity, namely LTP and LTD. The mGlu activation underlying pathological processes was also investigated. For all investigations, rats were chronically implanted with recording and stimulating electrodes to enable measurement of evoked potentials from medial perforant path -- dentate gyrus granule cell synapses. An injection cannula was inserted into the ipsilateral cerebral ventricle to enable drug application. Experiments were begun 10d subsequent to the implantation procedure. Histological investigations of hippocampal tissue was used to determine possible changes in cell viability after the induction of chemical LTD. Robust LTP which lasted for over 25h was generated using 200Hz tetanisation (HFT), whereas robust LTD was induced either by 1Hz low-frequency stimulation (LFS) or by pharmacological mGlu activation. The potent mGlu5 receptor agonist MPEP (2-methyl-6-(phenylethynyl)pyridine), applied in concentrations which did not affect basal synaptic transmission, dose-dependently impaired the induction and expression of LTP. Application of MPEP 5 min after tetanisation inhibited late LTP (> 24h). Injections of the potent mGlu1 agonist LY367385 ((S)-(+)-α-Amino-4-carboxy-2-methylbenzenacetic acid), in the concentrations investigated, did not effect basal synaptic transmission. In contrast, a dose- dependent impairment of LTP expression was observed. The lowest concentration used, had no effect on LTP induction, whereas higher concentrations reduced both LTP induction and expression. In contrast to MPEP application, the injection of LY367385 5min after LTP induction did not effect LTP expression. The results strongly implicate an involvement of mGlu1 in LTP induction, whereas the group I mGlu subtype 5 seems to be more involved in LTP expression. Investigating the subtype specific group I mGlu involvement in LTD afford differential results. Whereas LFS-induced LTD strongly required mGlu1 activation during LFS, chemical LTD is induced by activation of mGlu5. Induction of chemical LTD with the group I mGlu receptor agonist, (RS)-3,5-Dihydroxyphenylglycine (DHPG) occluded subsequent induction of LFS- induced LTD. Application of LFS following selective pharmacological activation of mGlu5 by (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) led to de-depression. Further analysis revealed that concurrent induction of chemically-induced and LFS-induced LTD led to an increase of intracellular calcium sufficient for the induction of LTP by LFS. Whereas LFS-induced LTD required mGlu1 activation during LFS, activation of mGlu5-mediated chemical LTD did not influence responses during LFS. In a next step, it was possible to induce stable LTD by application of the potent group III mGlu agonist AP4 (L-(+)-2-Amino-4-phosphonobutyric acid). The persistent LTD induced by the group I mGlu receptor agonist, DHPG, or the group III mGlu receptor agonist, AP4 were compared. Application of the protein synthesis inhibitor Anisomycin 2h prior to DHPG-injection inhibited the expression of LTD from 6h following DHPG injection. In contrast, Anisomycin did not affect LTD induced by AP4. A paired pulse paradigm determined that AP4-LTD is presynaptic whereas DHPG-LTD has both pre-and postsynaptic components. Histological evaluation at either 4h or 7d following AP4-injection showed that AP4-mediated LTD was ... [abstract truncated]

CaMKII: the Swiss army knife of synaptic plasticity

Estrogenic and androgenic steroids can be synthesised in the brain and rapidly modulate synaptic transmission and plasticity through direct interaction with membrane receptors for estrogens (ERs) and androgens (ARs). We used whole cell patch clamp recordings in brainstem slices of male rats to explore the influence of ER and AR activation and local synthesis of 17β-estradiol (E2) and 5α-dihydrotestosterone (DHT) on the long-term synaptic changes induced in the neurons of the medial vestibular nucleus (MVN). Long-term depression (LTD) and long-term potentiation (LTP) caused by different patterns of high frequency stimulation (HFS) of the primary vestibular afferents were assayed under the blockade of ARs and ERs or in the presence of inhibitors for enzymes synthesizing DHT (5α-reductase) and E2 (P450-aromatase) from testosterone (T). We found that LTD is mediated by interaction of locally produced androgens with ARs and LTP by interaction of locally synthesized E2 with ERs. In fact, the AR block with flutamide prevented LTD while did not affect LTP, and the blockade of ERs with ICI 182,780 abolished LTP without influencing LTD. Moreover, the block of P450-aromatase with letrozole not only prevented the LTP induction, but inverted LTP into LTD. This LTD is likely due to the local activation of androgens, since it was abolished under blockade of ARs. Conversely, LTD was still induced in the presence of finasteride the inhibitor of 5α-reductase demonstrating that T is able to activate ARs and induce LTD even when DHT is not synthesized. This study demonstrates a key and opposite role of sex neurosteroids in the long-term synaptic changes of the MVN with a specific role of T-DHT for LTD and of E2 for LTP. Moreover, it suggests that different stimulation patterns can lead to LTD or LTP by specifically activating the enzymes involved in the synthesis of androgenic or estrogenic neurosteroids.

Postsynaptic AMPA-type glutamate receptors (AMPARs) are among the major determinants of synaptic strength and can be trafficked into and out of synapses. Neuronal activity regulates AMPAR trafficking during synaptic plasticity to induce long-term changes in synaptic strength, including long-term potentiation (LTP) and long-term depression (LTD). Rab family GTPases regulate most membrane trafficking in eukaryotic cells; particularly, Rab11 and its effectors are implicated in mediating postsynaptic AMPAR insertion during LTP. To explore the synaptic function of Rab11Fip5, a neuronal Rab11 effector and a candidate autism-spectrum disorder gene, we performed shRNA-mediated knock-down and genetic knock-out (KO) studies. Surprisingly, we observed robust shRNA-induced synaptic phenotypes that were rescued by a Rab11Fip5 cDNA but that were nevertheless not observed in conditional KO neurons. Both in cultured neurons and acute slices, KO of Rab11Fip5 had no significant effect on basic parameters of synaptic transmission, indicating that Rab11Fip5 is not required for fundamental synaptic operations, such as neurotransmitter release or postsynaptic AMPAR insertion. KO of Rab11Fip5 did, however, abolish hippocampal LTD as measured both in acute slices or using a chemical LTD protocol in cultured neurons but did not affect hippocampal LTP. The Rab11Fip5 KO mice performed normally in several behavioral tasks, including fear conditioning, but showed enhanced contextual fear extinction. These are the first findings to suggest a requirement for Rab11Fip5, and presumably Rab11, during LTD.

In the present report, we focused our attention on the role played by the muscarinic acetylcholine receptors (mAChRs) in different forms of long-term synaptic plasticity. Specifically, we investigated long-term potentiation (LTP) and long-term depression (LTD) expression elicited by theta-burst stimulation (TBS) and low-frequency stimulation (LFS), respectively, in visual cortical slices obtained from different mAChR knockout (KO) mice. A normal LTP was evoked in M(1)/M(3) double KO mice, while LTP was impaired in the M(2)/M(4) double KO animals. On the other hand, LFS induced LTD in M(2)/M(4) double KO mice, but failed to do so in M(1)/M(3) KO mice. Interestingly, LFS produced LTP instead of LTD in M(1)/M(3) KO mice. Analysis of mAChR single KO mice revealed that LTP was affected only by the simultaneous absence of both M(2) and M(4) receptors. A LFS-dependent shift from LTD to LTP was also observed in slices from M(1) KO mice, while LTD was simply abolished in slices from M(3) KO mice. Using pharmacological tools, we showed that LTP in control mice was blocked by pertussis toxin, an inhibitor of G(i/o) proteins, but not by raising intracellular cAMP levels. In addition, the inhibition of phospholipase C by U73122 induced the same shift from LTD to LTP after LFS observed in M(1) single KO and M(1)/M(3) double KO mice. Our results indicate that different mAChR subtypes regulate different forms of long-term synaptic plasticity in the mouse visual cortex, activating specific G proteins and downstream intracellular mechanisms.

Transsynaptic Signaling by NO during Learning-Related Synaptic Plasticity

Stress and Cocaine Trigger Divergent and Cell Type–Specific Regulation of Synaptic Transmission at Single Spines in Nucleus Accumbens

Synaptic transmission is a complex process that can be fine-tuned and tweaked by numerous mechanisms. Nowhere is that more true than at the mossy fibre to CA3 pyramidal cell synapse in the hippocampus. This specialized synapse, formed by the granule cell axons making en passant connections onto large multiheaded postsynaptic spines (named thorny excrescences), has a number of curious properties and is functionally regulated by several well-characterized mechanisms. Mossy fibre synapses are fundamentally different to other hippocampal synapses because the expression of long-term plasticity (both potentiation and depression) is presynaptic, requiring a long-lasting change in glutamate release probability. The almost universal acceptance that synaptic plasticity is, at least in part, a cellular correlate for learning and memory has provoked a slew of studies describing the molecules that play a role in both mediating and modulating long-term changes in synaptic efficacy. One of the primary modes by which mossy fibre synaptic transmission can be modulated is through the activation of presynaptic glutamate-activated autoreceptors. Two classes of glutamate receptor are known to be important for modulating mossy fibre synaptic transmission. Activation of the group II metabotropic glutamate receptor, mGluR2, depresses synaptic transmission and is required for the induction of mossy fibre long-term depression (LTD) (Yokoi et al. 1996). In contrast, activation of presynaptic ionotropic glutamate receptors of the kainate receptor subfamily can both facilitate and depress transmission, and contribute to mossy fibre long-term potentiation (LTP) (Contractor et al. 2001). These autoreceptors therefore represent convenient targets for modulating mossy fibre synaptic transmission and could be important for targeting hippocampal function in neurological diseases. In this issue of The Journal of Physiology Omrani and colleagues (Omrani et al. 2009) present evidence that regulating the amount of glutamate uptake can have a dramatic impact on the ability of mossy fibre synapses to express long-term depression (LTD). The authors make use of an interesting observation that some β-lactam antibiotics can transcriptionally up-regulate the expression of the glutamate transporter GLT-1, a process that might hold some promise for reducing excitotoxicity in certain neuropathologies (Rothstein et al. 2005). The authors demonstrate that the expression of GLT-1 is increased in the brains of rats which are treated with the β-lactam antibiotic ceftriaxone. GLT-1 is generally thought to be an astroglial transporter; however, more recently it has been demonstrated that it may also be expressed in neurons, and more importantly, in excitatory synaptic terminals in the hippocampus (Chen et al. 2004). In agreement with this, Omrani et al. found that immunogold particles labelled both astrocyte processes and mossy fibre terminals in the stratum lucidum. In animals that were treated with ceftriaxone the density of both cytoplasmic and membrane-associated particles was increased. The authors hypothesized that the elevated expression of glutamate transporters close to the active zones (where glutamate is released) could potentially alter the glutamate transient in the synaptic cleft during induction of synaptic plasticity and fail to activate presynaptic autoreceptors that are required for LTP or LTD. In particular, the authors focused on mossy fibre LTD and the activation of mGluR2. Indeed, the authors found that in recordings from slices from ceftriaxone-treated animals, a low-frequency stimulation protocol failed to induce LTD; however, LTD could be rescued by application of the GLT-1-selective antagonist dihydrokainate (DHK). These results indicate that it is likely that presynaptic mGluRs, which are thought to be localized somewhat distant from mossy fibre release sites, are not effectively engaged by low-frequency LTD induction protocols in ceftriaxone-treated slices probably because glutamate is more efficiently transported from the synaptic cleft (Fig. 1). Figure 1 Model of GLT-1 modulation of mossy fibre synaptic plasticity The present study describes an intriguing observation, and furthers our knowledge of mechanisms that regulate mossy fibre synaptic transmission. In particular, the novel demonstration of the contribution of glutamate transporters to mossy fibre synaptic plasticity provides a potential target for enhancing (or decreasing) synaptic plasticity. The study also raises several questions which the authors did not fully explore here. For instance, there are deficits in both frequency facilitation and mossy fibre LTP in ceftriaxone-treated animals. As stated above, ionotropic kainate receptors contribute to these forms of plasticity, so it would have been interesting to determine if, in fact, a reduction in the activation of kainate receptors underlay deficits in these forms of plasticity when GLT-1 is up-regulated. A broader question about the implication of these findings might be answered by performing behavioural studies in ceftriaxone-treated animals. Ceftriaxone is in clinical use as an antibiotic and has been proposed to be neuroprotective against excitotoxity in stroke. However, if synaptic plasticity mechanisms are also impaired by ceftriaxone one might potentially expect some impact on learning and memory. It will take future studies to work out these details, but the current cellular studies provide fascinating insight into glutamate transporters and their role in regulating synaptic transmission.

NMDA-type glutamate receptors (NMDARs) contribute to many forms of long-term potentiation (LTP) and long-term depression (LTD). NMDARs are heteromers containing calcium-permeating neuronal receptor 1 (NR1) subunits and a variety of NR2 subunits. Evidence suggests that, in the CA1 region of the hippocampus, NR2A-containing NMDARs promote LTP whereas NR2B-containing receptors promote LTD. However, the calcium sensors that distinguish between these signals to promote the appropriate form of synaptic plasticity are not known. Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1) and Ras-GRF2 are highly similar calcium-stimulated exchange factors that activate Ras and Rac GTPases. Here, using a set of Ras-GRF knock-out mice, we show that Ras-GRF2 contributes predominantly to the induction of NMDAR-dependent LTP, whereas Ras-GRF1 contributes predominantly to the induction of NMDAR-dependent LTD in the CA1 region of the hippocampus of postpubescent mice (postnatal days 25-36). In contrast, neither Ras-GRF protein influences synaptic plasticity in prepubescent mice (postnatal days 14-18). Ras-GRF2 mediates signaling from (R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl-phosphonic acid-sensitive (NVP-AAM077-sensitive) (NR2A-containing) NMDARs to the Ras effector extracellular signal-related protein kinase 1/2 (Erk1/2) mitogen-activated protein (MAP) kinase, a promoter of NMDAR-induced LTP at this site. In contrast, Ras-GRF1 mediates signaling from ifenprodil-sensitive (NR2B-containing) NMDARs to the Rac effector p38 MAP kinase, a promoter of LTD. These findings show that, despite their similar functional domain organization, Ras-GRF1 and Ras-GRF2 mediate opposing forms of synaptic plasticity by coupling different classes of NMDARs to distinct MAP kinase pathways. Moreover, the postnatal appearance of Ras-GRF-dependent LTP and LTD coincides with the emergence of hippocampal-dependent behavior, implying that Ras-GRF proteins contribute to forms of synaptic plasticity that are required specifically for mature hippocampal function.

One of the most intriguing features of the brain is its ability to be malleable, allowing it to adapt continually to changes in the environment. Specific neuronal activity patterns drive long-lasting increases or decreases in the strength of synaptic connections, referred to as long-term potentiation and long-term depression, respectively. Such phenomena have been described in a variety of model organisms, which are used to study molecular, structural, and functional aspects of synaptic plasticity. This review originated from the first International Society for Neurochemistry (ISN) and Journal of Neurochemistry (JNC) Flagship School held in Alpbach, Austria (Sep 2016), and will use its curriculum and discussions as a framework to review some of the current knowledge in the field of synaptic plasticity. First, we describe the role of plasticity during development and the persistent changes of neural circuitry occurring when sensory input is altered during critical developmental stages. We then outline the signaling cascades resulting in the synthesis of new plasticity-related proteins, which ultimately enable sustained changes in synaptic strength. Going beyond the traditional understanding of synaptic plasticity conceptualized by long-term potentiation and long-term depression, we discuss system-wide modifications and recently unveiled homeostatic mechanisms, such as synaptic scaling. Finally, we describe the neural circuits and synaptic plasticity mechanisms driving associative memory and motor learning. Evidence summarized in this review provides a current view of synaptic plasticity in its various forms, offers new insights into the underlying mechanisms and behavioral relevance, and provides directions for future research in the field of synaptic plasticity. Read the Editorial Highlight for this article on page 788. Cover Image for this issue: doi: 10.1111/jnc.13815.

Activation of the cAMP pathway is one of the common mechanisms underlying long-term potentiation (LTP). In the Drosophila mushroom body, simultaneous activation of odour-coding Kenyon cells (KCs) and reinforcement-coding dopaminergic neurons activates adenylyl cyclase in KC presynaptic terminals, which is believed to trigger synaptic plasticity underlying olfactory associative learning. However, learning induces long-term depression (LTD) at these synapses, contradicting the universal role of cAMP as a facilitator of transmission. Here, we developed a system to electrophysiologically monitor both short-term and long-term synaptic plasticity at KC output synapses and demonstrated that they are indeed an exception in which activation of the cAMP-protein kinase A pathway induces LTD. Contrary to the prevailing model, our cAMP imaging found no evidence for synergistic action of dopamine and KC activity on cAMP synthesis. Furthermore, we found that forskolin-induced cAMP increase alone was insufficient for plasticity induction; it additionally required simultaneous KC activation to replicate the presynaptic LTD induced by pairing with dopamine. On the other hand, activation of the cGMP pathway paired with KC activation induced slowly developing LTP, proving antagonistic actions of the two second-messenger pathways predicted by behavioural study. Finally, KC subtype-specific interrogation of synapses revealed that different KC subtypes exhibit distinct plasticity duration even among synapses on the same postsynaptic neuron. Thus, our work not only revises the role of cAMP in synaptic plasticity by uncovering the unexpected convergence point of the cAMP pathway and neuronal activity, but also establishes the methods to address physiological mechanisms of synaptic plasticity in this important model. KEY POINTS: Although presynaptic cAMP increase generally facilitates synapses, olfactory associative learning in Drosophila, which depends on dopamine and cAMP signalling genes, induces long-term depression (LTD) at the mushroom body output synapses. By combining electrophysiology, pharmacology and optogenetics, we directly demonstrate that these synapses are an exception where activation of the cAMP-protein kinase A pathway leads to presynaptic LTD. Dopamine- or forskolin-induced cAMP increase alone is not sufficient for LTD induction; neuronal activity, which has been believed to trigger cAMP synthesis in synergy with dopamine input, is required in the downstream pathway of cAMP. In contrast to cAMP, activation of the cGMP pathway paired with neuronal activity induces presynaptic long-term potentiation, which explains behaviourally observed opposing actions of transmitters co-released by dopaminergic neurons. Our work not only revises the role of cAMP in synaptic plasticity, but also provides essential methods to address physiological mechanisms of synaptic plasticity in this important model system.

Cannabinoids, the active constituents of marijuana, are known to impair learning and memory. Receptors for cannabinoids are highly expressed in the hippocampus, a brain region that is believed to play an important role in certain forms of learning and memory. To investigate the possible contribution of cannabinoid receptor-mediated deficits in hippocampal function to the learning and memory impairments produced by marijuana, we studied the effects of cannabinoid receptor activation on two models of learning and memory, long-term potentiation (LTP) and long-term depression (LTD), in hippocampal slices. Although LTP and LTD of CA1 field potentials were blocked by cannabinoid receptor activation in the presence of Mg(2+), they could be induced after Mg(2+) was removed. Similarly, LTP and LTD of whole-cell EPSCs were unimpaired in the presence of cannabinoid receptor agonist when the postsynaptic membrane was depolarized during the LTP or LTD induction protocol. Cannabinoid receptor activation also reduced EPSCs and enhanced paired-pulse facilitation, while having no effect on the amplitude of spontaneous miniature EPSCs. Finally, as with cannabinoid receptor activation, inhibition of LTP by adenosine receptor activation could be overcome by removal of Mg(2+) or depolarization of the postsynaptic membrane during tetanus. Our results indicate that cannabinoid receptor activation does not directly inhibit the molecular mechanisms responsible for long-term synaptic plasticity but instead impairs LTP and LTD by reducing presynaptic neurotransmitter release to a level below that required to depolarize the postsynaptic membrane to relieve Mg(2+) blockade of NMDA receptors.

A Balance of Protein Synthesis and Proteasome-Dependent Degradation Determines the Maintenance of LTP