Activation of presynaptic cAMP-dependent protein kinase is required for induction of cerebellar long-term potentiation

cAMP-EPAC-PKCε-RIM1α signaling regulates presynaptic long-term potentiation and motor learning

The cerebellum is involved in learning of fine motor skills, yet whether presynaptic plasticity contributes to such learning remains elusive. Here, we report that the EPAC-PKCε module has a critical role in a presynaptic form of long-term potentiation in the cerebellum and motor behavior in mice. Presynaptic cAMP-EPAC-PKCε signaling cascade induces a previously unidentified threonine phosphorylation of RIM1α, and thereby initiates the assembly of the Rab3A-RIM1α-Munc13-1 tripartite complex that facilitates docking and release of synaptic vesicles. Granule cell-specific blocking of EPAC-PKCε signaling abolishes presynaptic long-term potentiation at the parallel fiber to Purkinje cell synapses and impairs basic performance and learning of cerebellar motor behavior. These results unveil a functional relevance of presynaptic plasticity that is regulated through a novel signaling cascade, thereby enriching the spectrum of cerebellar learning mechanisms.

NMDA receptor-independent LTP in mammalian nervous system

Long-term potentiation (LTP) of synaptic transmission is a form of activity-dependent synaptic plasticity that exists at most synapses in the nervous system. In the central nervous system (CNS), LTP has been recorded at numerous synapses and is a prime candidate mechanism associating activity-dependent plasticity with learning and memory. LTP involves long-lasting increase in synaptic strength with various underlying mechanisms. In the CNS, the predominant type of LTP is believed to be dependent on activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR), which is highly calcium-permeable. However, various forms of NMDAR-independent LTP have been identified in diverse areas of the nervous system. The NMDAR-independent LTP may require activation of glutamate metabotropic receptors (mGluR) or ionotropic receptors other than NMDAR such as nicotinic acetylcholine receptor (α7-nAChR), serotonin 5-HT3 receptor or calcium-permeable AMPA receptor (CP-AMPAR). In this review, NMDAR-independent LTP of various areas of the central and peripheral nervous systems are discussed.

Norepinephrine Facilitates Induction of Long-term Depression through β-Adrenergic Receptor at Parallel Fiber-to-Purkinje Cell Synapses in the Flocculus

The cerebellum is involved in motor learning, and long-term depression (LTD) at parallel fiber-to-Purkinje cell (PF-PC) synapses has been considered to be a primary cellular mechanism for motor learning. In addition, the contribution of norepinephrine (NE) to cerebellum-dependent learning paradigms has been reported. Thus, the roles of LTD and of NE in motor learning have been studied separately, and the relationship between the effects of NE and LTD remains unclear. Here, we examined effects of β-adrenergic receptor (β-AR) activity on the synaptic transmission and LTD at PF-PC synapses in the cerebellar flocculus. The flocculus regulates adaptation of oculomotor reflexes, and we previously reported the involvement of both LTD and β-AR in adaptation of an oculomotor reflex. Here we found that specific agonists for β-AR or NE did not directly change synaptic transmission, but lowered the threshold for LTD induction at PF-PC synapses in the flocculus. In addition, protein kinase A (PKA), which is activated downstream of β-AR, facilitated the LTD induction. Altogether, these results suggest that NE facilitates LTD induction at PF-PC synapses in the flocculus by activating PKA through β-AR.

ERK/MAPK signaling and autism spectrum disorders

The MAPK pathway is a prominent intracellular signaling pathway regulating various intracellular functions. Components of this pathway are mutated in a related collection of congenital syndromes collectively referred to as neuro-cardio-facio-cutaneous syndromes (NCFC) or Rasopathies. Recently, it has been appreciated that these disorders are associated with autism spectrum disorders (ASD). In addition, idiopathic ASD has also implicated the MAPK signaling cascade as a common pathway that is affected by many of the genetic variants that have been found to be linked to ASDs. This chapter describes the components of the MAPK pathway and how it is regulated. Furthermore, this chapter will highlight the various functions of the MAPK pathway during both embryonic development of the central nervous system (CNS) and its roles in neuronal physiology and ultimately, behavior. Finally, we will summarize the perturbations to MAPK signaling in various models of autism spectrum disorders and Rasopathies to highlight how dysregulation of this pivotal pathway may contribute to the pathogenesis of autism.

14-3-3 Proteins in Glutamatergic Synapses

The 14-3-3 proteins are a family of proteins that are highly expressed in the brain and particularly enriched at synapses. Evidence accumulated in the last two decades has implicated 14-3-3 proteins as an important regulator of synaptic transmission and plasticity. Here, we will review previous and more recent research that has helped us understand the roles of 14-3-3 proteins at glutamatergic synapses. A key challenge for the future is to delineate the 14-3-3-dependent molecular pathways involved in regulating synaptic functions.

Climbing fibers in spinocerebellar ataxia: A mechanism for the loss of motor control

The spinocerebellar ataxias (SCAs) form an ever-growing group of neurodegenerative disorders causing dysfunction of the cerebellum and loss of motor control in patients. Currently, 41 different genetic causes have been identified, with each mutation affecting a different gene. Interestingly, these diverse genetic causes all disrupt cerebellar function and produce similar symptoms in patients. In order to understand the disease better, and define possible therapeutic targets for multiple SCAs, the field has been searching for common ground among the SCAs. In this review, we discuss the physiology of climbing fibers and the possibility that climbing fiber dysfunction is a point of convergence for at least a subset of SCAs.

Localization of Presynaptic Plasticity Mechanisms Enables Functional Independence of Synaptic and Ectopic Transmission in the Cerebellum

In the cerebellar molecular layer parallel fibre terminals release glutamate from both the active zone and from extrasynaptic “ectopic” sites. Ectopic release mediates transmission to the Bergmann glia that ensheathe the synapse, activating Ca²⁺-permeable AMPA receptors and glutamate transporters. Parallel fibre terminals exhibit several forms of presynaptic plasticity, including cAMP-dependent long-term potentiation and endocannabinoid-dependent long-term depression, but it is not known whether these presynaptic forms of long-term plasticity also influence ectopic transmission to Bergmann glia. Stimulation of parallel fibre inputs at 16 Hz evoked LTP of synaptic transmission, but LTD of ectopic transmission. Pharmacological activation of adenylyl cyclase by forskolin caused LTP at Purkinje neurons, but only transient potentiation at Bergmann glia, reinforcing the concept that ectopic sites lack the capacity to express sustained cAMP-dependent potentiation. Activation of mGluR1 caused depression of synaptic transmission via retrograde endocannabinoid signalling but had no significant effect at ectopic sites. In contrast, activation of NMDA receptors suppressed both synaptic and ectopic transmission. The results suggest that the signalling mechanisms for presynaptic LTP and retrograde depression by endocannabinoids are restricted to the active zone at parallel fibre synapses, allowing independent modulation of synaptic transmission to Purkinje neurons and ectopic transmission to Bergmann glia.

Caffeine Modulates Vesicle Release and Recovery at Cerebellar Parallel Fibre Terminals, Independently of Calcium and Cyclic AMP Signalling

Background

Cerebellar parallel fibres release glutamate at both the synaptic active zone and at extrasynaptic sites—a process known as ectopic release. These sites exhibit different short-term and long-term plasticity, the basis of which is incompletely understood but depends on the efficiency of vesicle release and recycling. To investigate whether release of calcium from internal stores contributes to these differences in plasticity, we tested the effects of the ryanodine receptor agonist caffeine on both synaptic and ectopic transmission.

Methods

Whole cell patch clamp recordings from Purkinje neurons and Bergmann glia were carried out in transverse cerebellar slices from juvenile (P16-20) Wistar rats.

Key Results

Caffeine caused complex changes in transmission at both synaptic and ectopic sites. The amplitude of postsynaptic currents in Purkinje neurons and extrasynaptic currents in Bergmann glia were increased 2-fold and 4-fold respectively, but paired pulse ratio was substantially reduced, reversing the short-term facilitation observed under control conditions. Caffeine treatment also caused synaptic sites to depress during 1 Hz stimulation, consistent with inhibition of the usual mechanisms for replenishing vesicles at the active zone. Unexpectedly, pharmacological intervention at known targets for caffeine—intracellular calcium release, and cAMP signalling—had no impact on these effects.

Conclusions

We conclude that caffeine increases release probability and inhibits vesicle recovery at parallel fibre synapses, independently of known pharmacological targets. This complex effect would lead to potentiation of transmission at fibres firing at low frequencies, but depression of transmission at high frequency connections.

The organization of plasticity in the cerebellar cortex: from synapses to control

The cerebellum is thought to play a critical role in procedural learning, but the relationship between this function and the underlying cellular and synaptic mechanisms remains largely speculative. At present, at least nine forms of long-term synaptic and nonsynaptic plasticity (some of which are bidirectional) have been reported in the cerebellar cortex and deep cerebellar nuclei. These include long-term potentiation (LTP) and long-term depression at the mossy fiber-granule cell synapse, at the synapses formed by parallel fibers, climbing fibers, and molecular layer interneurons on Purkinje cells, and at the synapses formed by mossy fibers and Purkinje cells on deep cerebellar nuclear cells, as well as LTP of intrinsic excitability in granule cells, Purkinje cells, and deep cerebellar nuclear cells. It is suggested that the complex properties of cerebellar learning would emerge from the distribution of plasticity in the network and from its dynamic remodeling during the different phases of learning. Intrinsic and extrinsic factors may hold the key to explain how the different forms of plasticity cooperate to select specific transmission channels and to regulate the signal-to-noise ratio through the cerebellar cortex. These factors include regulation of neuronal excitation by local inhibitory networks, engagement of specific molecular mechanisms by spike bursts and theta-frequency oscillations, and gating by external neuromodulators. Therefore, a new and more complex view of cerebellar plasticity is emerging with respect to that predicted by the original "Motor Learning Theory," opening issues that will require experimental and computational testing.

Rab3B protein is required for long-term depression of hippocampal inhibitory synapses and for normal reversal learning

Rab3B, similar to other Rab3 isoforms, is a synaptic vesicle protein that interacts with the Rab3-interacting molecule (RIM) isoforms RIM1α and RIM2α as effector proteins in a GTP-dependent manner. Previous studies showed that at excitatory synapses, Rab3A and RIM1α are essential for presynaptically expressed long-term potentiation (LTP), whereas at inhibitory synapses RIM1α is required for endocannabinoid-dependent long-term depression (referred to as "i-LTD"). However, it remained unknown whether i-LTD also involves a Rab3 isoform and whether i-LTD, similar to other forms of long-term plasticity, is important for learning and memory. Here we show that Rab3B is highly enriched in inhibitory synapses in the CA1 region of the hippocampus. Using electrophysiological recordings in acute slices, we demonstrate that knockout (KO) of Rab3B does not alter the strength or short-term plasticity of excitatory or inhibitory synapses but does impair i-LTD significantly without changing classical NMDA receptor-dependent LTP. Behaviorally, we found that Rab3B KO mice exhibit no detectable changes in all basic parameters tested, including the initial phase of learning and memory. However, Rab3B KO mice did display a selective enhancement in reversal learning, as measured using Morris water-maze and fear-conditioning assays. Our data support the notion that presynaptic forms of long-term plasticity at excitatory and inhibitory synapses generally are mediated by a common Rab3/RIM-dependent pathway, with various types of synapses using distinct Rab3 isoforms. Moreover, our results suggest that i-LTD contributes to learning and memory, presumably by stabilizing circuits established in previous learning processes.

Are mushroom bodies cerebellum-like structures?

The mushroom bodies are distinctive neuropils in the protocerebral brain segments of many protostomes. A defining feature of mushroom bodies is their intrinsic neurons, masses of cytoplasm-poor globuli cells that form a system of lobes with their densely-packed, parallel-projecting axon-like processes. In insects, the role of the mushroom bodies in olfactory processing and associative learning and memory has been studied in depth, but several lines of evidence suggest that the function of these higher brain centers cannot be restricted to these roles. The present account considers whether insight into an underlying function of mushroom bodies may be provided by cerebellum-like structures in vertebrates, which are similarly defined by the presence of masses of tiny granule cells that emit thin parallel fibers forming a dense molecular layer. In vertebrates, the shared neuroarchitecture of cerebellum-like structures has been suggested to underlie a common functional role as adaptive filters for the removal of predictable sensory elements, such as those arising from reafference, from the total sensory input. Cerebellum-like structures include the vertebrate cerebellum, the electrosensory lateral line lobe, dorsal and medial octavolateral nuclei of fish, and the dorsal cochlear nucleus of mammals. The many architectural and physiological features that the insect mushroom bodies share with cerebellum-like structures suggest that it might be fruitful to consider mushroom body function in light of a possible role as adaptive sensory filters. The present account thus presents a detailed comparison of the insect mushroom bodies with vertebrate cerebellum-like structures.

An efficient method for the long-term and specific expression of exogenous cDNAs in cultured Purkinje neurons

We present a simple and efficient method for expressing cDNAs in Purkinje neurons (PNs) present in heterogeneous mouse cerebellar cultures. The method combines the transfection of freshly dissociated cerebellar cells via nucleofection with the use of novel expression plasmids containing a fragment of the L7 (Pcp2) gene that, within the cerebellum, drives PN-specific expression. The efficiency of PN transfection (determined 13 days post nucleofection) is approximately 70%. Double and triple transfections are routinely achieved at slightly lower efficiencies. Expression in PNs is obvious after one week in culture and still strong after three weeks, by which time these neurons are well-developed. Moreover, high-level expression is restricted almost exclusively to the PNs present in these mixed cultures, which greatly facilitates the characterization of PN-specific functions. As proof of principle, we used this method to visualize (1) the morphology of living PNs expressing mGFP, (2) the localization and dynamics of the dendritic spine proteins PSD-93 and Homer-3a tagged with mGFP and (3) the interaction of live PNs expressing mGFP with other cerebellar neurons expressing mCherry from a β-Actin promoter plasmid. Finally, we created a series of L7-plasmids containing different fluorescent protein cDNAs that are suited for the expression of cDNAs of interest as N- and C-terminally tagged fluorescent fusion proteins. In summary, this procedure allows for the highly efficient, long-term, and specific expression of multiple cDNAs in differentiated PNs, and provides a favorable alternative to two procedures (viral transduction, ballistic gene delivery) used previously to express genes in cultured PNs.

Activation of silent and weak synapses by cAMP-dependent protein kinase in cultured cerebellar granule neurons

Presynaptic long term potentiation of synaptic transmission activates silent synapses and potentiates existing active synapses. We sought to visualise these two processes by studying the cAMP-dependent protein kinase (PKA) potentiation of presynaptic vesicle cycling in cultured cerebellar granule neurons.Using FM dyes to label the pool of recycling synaptic vesicles,we found that trains of electrical stimulation which do not potentiate already active synapses are sufficient to rapidly activate a discrete population comprising silent and very low activity synapses. Silent synapse activation required PKA activity and conversely, active synapses could be silenced by PKA inhibition. Surprisingly, the recycling pool of synaptic vesicles in recently activated synapses was larger than in already active synapses and equivalent to synapses treated with forskolin. Imaging of synaptic vesicle cycling and cytosolic Ca(2+) in individual nerve terminals confirmed that silent synapses have evoked Ca(2+) transients comparable to those of active synapses. Furthermore, across populations of active synapses, changes in Ca(2+) influx did not correlate with changes in the size of the pool of recycling synaptic vesicles. Finally, we found that stimulation of synapsin phosphorylation, but not RIM1α, by PKA was frequency dependent and long lasting. These data are consistent with the idea that PKA regulates synaptic vesicle recycling downstream of Ca(2+) influx and that this pathway is highly active in recently activated synapses.

Parasagittally aligned, mGluR1-dependent patches are evoked at long latencies by parallel fiber stimulation in the mouse cerebellar cortex in vivo

The parallel fibers (PFs) in the cerebellar cortex extend several millimeters along a folium in the mediolateral direction. The PFs are orthogonal to and cross several parasagittal zones defined by the olivocerebellar and corticonuclear pathways and the expression of molecular markers on Purkinje cells (PCs). The functions of these two organizations remain unclear, including whether the bands respond similarly or differentially to PF input. By using flavoprotein imaging in the anesthetized mouse in vivo, this study demonstrates that high-frequency PF stimulation, which activates a beamlike response at short latency, also evokes patches of activation at long latencies. These patches consist of increased fluorescence along the beam at latencies of 20-25 s with peak activation at 35 s. The long-latency patches are completely blocked by the type 1 metabotropic glutamate receptor (mGluR(1)) antagonist LY367385. Conversely, the AMPA and NMDA glutamate receptor antagonists DNQX and APV have little effect. Organized in parasagittal bands, the long-latency patches align with zebrin II-positive PC stripes. Additional Ca(2+) imaging demonstrates that the patches reflect increases in intracellular Ca(2+). Both the PLCβ inhibitor U73122 and the ryanodine receptor inhibitor ryanodine completely block the long-latency patches, indicating that the patches are due to Ca(2+) release from intracellular stores. Robust, mGluR(1)-dependent long-term potentiation (LTP) of the patches is induced using a high-frequency PF stimulation conditioning paradigm that generates LTP of PF-PC synapses. Therefore, the parasagittal bands, as defined by the molecular compartmentalization of PCs, respond differentially to PF inputs via mGluR(1)-mediated release of internal Ca(2+).

Myosin-Va transports the endoplasmic reticulum into the dendritic spines of Purkinje neurons

Extension of the endoplasmic reticulum (ER) into dendritic spines of Purkinje neurons (PNs) is required for cerebellar synaptic plasticity and is disrupted in animals with null mutations in Myo5a, the gene encoding myosin-Va^1–3. Notably, the mechanism ensuring the ER's localization to spines has not been unraveled. While it has been proposed that animal class V myosins localize organelles by tethering them to the actin cytoskeleton^4–7, we demonstrate here that myosin-Va acts as a point-to-point organelle transporter to pull ER as cargo into PN spines. Specifically, the myosin accumulates at the ER tip as the organelle moves into spines, and the myosin's ability to hydrolyze ATP is required for spine ER targeting. Moreover, myosin-Va is responsible for the vast majority of spine ER insertional events. Finally, attenuation of the myosin's ability to move along actin filaments reduces the maximum velocity of ER movement into spines, providing direct evidence that myosin-Va drives ER motility. Thus, we establish that an actin-based motor moves ER within animal cells, and we uncover the mechanism that mediates ER localization to PN spines, a prerequisite for synaptic plasticity.

Simulating the shaping of the fastigial deep nuclear saccade command by cerebellar Purkinje cells

Early lesion and physiological studies established the key contributions of the cerebellar cortex and fastigial deep nuclei in maintaining the accuracy of saccades. Recent evidence has demonstrated that fastigial oculomotor region cells (FORCs) provide commands that are critical both for driving and braking saccades. Modeling studies have largely ignored the mechanisms by which the FORC activity patterns, and those of the Purkinje cells (PCs) that inhibit them, are produced by the mossy fiber (MF) inputs common to both. We have created a hybrid network of integrate-and-fire and summation units to model the circuitry between PCs, FORCs, and MFs that can account for all observed PC and FORC activity patterns. The model demonstrates that a crucial component of FORC activity may be due to the rebound depolarization intrinsic to FORC neurons that, like the MF-driven activity of FORCs, is also shaped by PC inhibition and disinhibition. The model further demonstrates that the shaping of the FORC saccade command by PCs can be adaptively modified through plausible learning rules based on cerebellar long-term depression (LTD) and long-term potentiation (LTP), which are guided by climbing fiber (CF) input to PCs that realistically indicates only the direction (but not the magnitude) of saccade error. These modeling results provide new insights into the adaptive control by the cerebellum of the deep nuclear saccade command.

Differential distribution of exchange proteins directly activated by cyclic AMP within the adult rat retina

The recently discovered exchange protein directly activated by cAMP (Epac), a guanine exchange factor for the G-protein RAP-1, is directly activated by cAMP independently of protein kinase A (PKA). While cAMP is known to be an important second messenger in the retina, the presence of Epac has not been investigated in this tissue. The goal of the present study was to determine if the Epac1 and Epac2 genes are present and to characterize their location within the retina. Western blot analysis revealed that Epac1 and Epac2 proteins are expressed within the retina, and the presence of mRNA was demonstrated with the aid of reverse transcriptase polymerase chain reaction (RT-PCR). Additionally, we used immunofluorescence and confocal microscopy to demonstrate that Epac1 and Epac2 have overlapping as well as unique distributions within the retina. Both are present within horizontal cells, rod and cone bipolar cells, cholinergic amacrine cells, retrograde labeled retinal ganglion cells, and Müller cells. Uniquely, Epac2 was expressed by cone photoreceptor inner and outer segments, cell bodies, and synaptic terminals. In contrast, Epac1 was expressed in vesicular glutamate transporter 1 (VGlut1) and C-terminal binding protein 2 (CtBP2) positive photoreceptor synaptic terminals. Together, these results provide evidence that Epac1 and Epac2 are differentially expressed within the retina and provide the framework for further functional studies of cAMP pathways within the retina.

Long-term potentiation of excitatory synapses on neocortical somatostatin-expressing interneurons

Synaptic plasticity has been extensively studied in principal neurons of the neocortex, but less work has been done on GABAergic interneurons. Interneurons consist of multiple subtypes and their synaptic properties vary between subtypes. In the present study, we have examined long-term potentiation (LTP) of excitatory synapses on somatostatin (SS)-expressing interneurons in neocortex using transgenic mice that express enhanced green fluorescent protein in these interneurons. We found that a strong theta burst stimulation was required to induce LTP in SS interneurons. LTP was associated with a reduction in paired-pulse facilitation and was not blocked by an N-methyl-d-aspartate receptor (NMDAR) antagonist. LTP was not affected by chelating postsynaptic Ca(2+) with BAPTA, a fast Ca(2+) chelator, and blocking L-type voltage-dependent Ca(2+) channels with nimodipine. Application of forskolin, an activator of adenylate cyclase that increases cyclic adenosine monophosphate (cAMP) concentration, enhanced synaptic transmission and occluded subsequent induction of LTP. Finally, we found that LTP was blocked by protein kinase A (PKA) inhibitors. Our results suggest that excitatory synapses on SS interneurons express a presynaptic form of LTP that is not dependent on NMDARs or postsynaptic Ca(2+) rise but is dependent on the cAMP-PKA signaling pathway.

The novel distribution of phosphodiesterase-4 subtypes within the rat retina

Phosphodiesterases (PDEs) are important regulators of signal transduction processes. While much is known about the function of cyclic GMP-specific PDEs in the retina, much less is known about the closely related, cyclic AMP-specific PDEs. The purpose of the present study is to characterize and localize PDE4 within the adult rat retina. We have used Western blotting, RT-PCR, and immunohistochemistry together with retrograde labeling to determine the presence and location of each PDE4 subtype. Western blot analysis revealed that multiple isoforms of PDE4A, B, and D subtypes are present within the retina, whereas the PDE4C subtype was absent. These data were confirmed by RT-PCR. Using immunohistochemistry we show that all three PDE4s are abundantly expressed within the retina where they all colocalize with retrograde-labeled retinal ganglion cells, as well as bipolar cells, horizontal cells, and cholinergic amacrine cells, whereas Müller cells lack PDE4 expression. Uniquely, PDE4B was expressed by the inner and outer segments of rod photoreceptors as well as their terminals within the outer plexiform layer. Collectively, our results demonstrate that PDE4s are abundantly expressed throughout the rodent retina and this study provides the framework for further functional studies.

Alterations in corticostriatal synaptic plasticity in mice overexpressing human alpha-synuclein

Most forms of Parkinson's disease (PD) are sporadic in nature, but some have genetic causes as first described for the alpha-synuclein gene. The alpha-synuclein protein also accumulates as insoluble aggregates in Lewy bodies in sporadic PD as well as in most inherited forms of PD. The focus of the present study is the modulation of synaptic plasticity in the corticostriatal pathway of transgenic (Tg) mice that overexpress the human alpha-synuclein protein throughout the brain (ASOTg). Paired-pulse facilitation was detected in vitro by activation of corticostriatal afferents in ASOTg mice, consistent with a presynaptic effect of elevated human alpha-synuclein. However basal synaptic transmission was unchanged in ASOTg, suggesting that human alpha-synuclein could impact paired-pulse facilitation via a presynaptic mechanism not directly related to the probability of neurotransmitter release. Mice lacking alpha-synuclein or those expressing normal and A53T human alpha-synuclein in tyrosine hydroxylase-containing neurons showed, instead, paired-pulse depression. High-frequency stimulation induced a presynaptic form of long-term depression solely in ASOTg striatum. A presynaptic, N-methyl-d-aspartate receptor-independent form of chemical long-term potentiation induced by forskolin (FSK) was enhanced in ASOTg striatum, while FSK-induced cAMP levels were reduced in ASOTg synaptoneurosome fractions. Overall the results suggest that elevated human alpha-synuclein alters presynaptic plasticity in the corticostriatal pathway, possibly reflecting a reduction in glutamate at corticostriatal synapses by modulation of adenylyl cyclase signaling pathways. ASOTg mice may recapitulate an early stage in PD during which overexpressed alpha-synuclein dampens corticostriatal synaptic transmission and reduces movement.

PKG and PKA signaling in LTP at GABAergic synapses

Drugs of abuse usurp the mechanisms underlying synaptic plasticity in areas of the brain, a process that may contribute to the development of addiction. We previously reported that GABAergic synapses onto dopaminergic neurons in the ventral tegmental area (VTA) exhibit long-term potentiation (LTP(GABA)) blocked by in vivo exposure to morphine. The presynaptically maintained LTP requires the retrogradely released nitric oxide (NO) to activate a presynaptic cGMP signaling cascade. Previous work reported that inhibitory GABA(A) receptor synapses in the VTA are also potentiated by cAMP. Here, we explored the interactions between cGMP-dependent (PKG) and cAMP-dependent (PKA) protein kinases in the regulation of these GABAergic synapses and LTP(GABA). Activation of PKG was required for NO-cGMP signaling and was also essential for the induction of synaptically elicited LTP(GABA), but not for its maintenance. Synapses containing GABA(A) receptors were potentiated by NO-cGMP signaling, whereas synapses containing GABA(B) receptors on the same cells were not potentiated. Moreover, although the cAMP-PKA system potentiated GABA(A) synapses, synaptically induced LTP(GABA) was independent of PKA activation. Surprisingly, however, raising cGMP levels saturated potentiation of these synapses, precluding further potentiation by cAMP and suggesting a convergent end point for both signaling pathways in the regulation of GABAergic release. We further found that persistent GABAergic synaptic modifications observed with in vivo morphine did not involve the presynaptic cAMP-PKA cascade. Taken together, our data suggest a synapse-specific role for NO-cGMP-PKG signaling pathway in opioid-induced plasticity of VTA GABA(A) synapses.

Long-term potentiation of the responses to parallel fiber stimulation in mouse cerebellar cortex in vivo

Long-term potentiation (LTP) of parallel fiber-Purkinje cell (PF-PC) synapses in the cerebellum has been suggested to underlie aspects of motor learning. Previous in vitro studies have primarily used low frequency PF stimulation conditioning paradigms to generate either presynaptic PF-PC LTP (4-8 Hz) or postsynaptic PF-PC LTP (1 Hz). Little is known about the conditions that evoke PF-PC LTP in vivo. High frequency stimulation in vivo increases PC responsiveness to peripheral stimuli; however, neither the site of action nor the signaling pathways involved have been examined. Using flavoprotein autofluorescence optical imaging in the FVB mouse in vivo, this report describes that a conditioning stimulation consisting of a high frequency burst of PF stimulation (100 Hz, 15 pulse trains every 3 s for 5 min) evokes a long-term increase in the response to PF stimulation. Following the conditioning stimulation, the response to PF stimulation increases over 20 min to approximately 130% above baseline and this potentiation persists for at least 2 h. Field potential recordings of the responses to PF stimulation show that the postsynaptic component is potentiated but the presynaptic, parallel fiber volley is not. Paired-pulse facilitation does not change after the conditioning stimulation, suggesting the potentiation occurs postsynaptically. Blocking non-NMDA (N-methyl-d-aspartic acid) ionotropic glutamate receptors with DNQX (6,7-dinitroquinoxaline-2,3-dione disodium salt, 50 muM, bath application) during the conditioning stimulation has no effect on the long-term increase in fluorescence. However, blocking subtype I metabotropic glutamate receptors (mGLuR(1)) with LY367385 (200 muM) during the conditioning stimulation abolishes the long-term increase in fluorescence. Blocking GABAergic neurotransmission is not required to evoke this long-term potentiation. Blocking GABA(A) receptors reduces but does not eliminate the long-term potentiation. Therefore, this study demonstrates that high frequency PF stimulation generates long-term potentiation of PF-PC synapses in vivo. This novel form of LTP is generated primarily postsynaptically and is mediated by mGluR(1) receptors.

Impairments in motor coordination without major changes in cerebellar plasticity in the Tc1 mouse model of Down syndrome

Down syndrome (DS) is a genetic disorder arising from the presence of a third copy of human chromosome 21 (Hsa21). Recently, O'Doherty et al. [An aneuploid mouse strain carrying human chromosome 21 with Down syndrome phenotypes. Science 309 (2005) 2033–2037] generated a trans-species aneuploid mouse line (Tc1) that carries an almost complete Hsa21. The Tc1 mouse is the most complete animal model for DS currently available. Tc1 mice show many features that relate to human DS, including alterations in memory, synaptic plasticity, cerebellar neuronal number, heart development and mandible size. Because motor deficits are one of the most frequently occurring features of DS, we have undertaken a detailed analysis of motor behaviour in cerebellum-dependent learning tasks that require high motor coordination and balance. In addition, basic electrophysiological properties of cerebellar circuitry and synaptic plasticity have been investigated. Our results reveal that, compared with controls, Tc1 mice exhibit a higher spontaneous locomotor activity, a reduced ability to habituate to their environments, a different gait and major deficits on several measures of motor coordination and balance in the rota rod and static rod tests. Moreover, cerebellar long-term depression is essentially normal in Tc1 mice, with only a slight difference in time course. Our observations provide further evidence that support the validity of the Tc1 mouse as a model for DS, which will help us to provide insights into the causal factors responsible for motor deficits observed in persons with DS.

RIM1alpha phosphorylation at serine-413 by protein kinase A is not required for presynaptic long-term plasticity or learning

Activation of presynaptic cAMP-dependent protein kinase A (PKA) triggers presynaptic long-term plasticity in synapses such as cerebellar parallel fiber and hippocampal mossy fiber synapses. RIM1alpha, a large multidomain protein that forms a scaffold at the presynaptic active zone, is essential for presynaptic long-term plasticity in these synapses and is phosphorylated by PKA at serine-413. Previous studies suggested that phosphorylation of RIM1alpha at serine-413 is required for presynaptic long-term potentiation in parallel fiber synapses formed in vitro by cultured cerebellar neurons and that this type of presynaptic long-term potentiation is mediated by binding of 14-3-3 proteins to phosphorylated serine-413. To test the role of serine-413 phosphorylation in vivo, we have now produced knockin mice in which serine-413 is mutated to alanine. Surprisingly, we find that in these mutant mice, three different forms of presynaptic PKA-dependent long-term plasticity are normal. Furthermore, we observed that in contrast to RIM1alpha KO mice, RIM1 knockin mice containing the serine-413 substitution exhibit normal learning capabilities. The lack of an effect of the serine-413 mutation of RIM1alpha is not due to compensation by RIM2alpha because mice carrying both the serine-413 substitution and a RIM2alpha deletion still exhibited normal long-term presynaptic plasticity. Thus, phosphorylation of serine-413 of RIM1alpha is not essential for PKA-dependent long-term presynaptic plasticity in vivo, suggesting that PKA operates by a different mechanism despite the dependence of long-term presynaptic plasticity on RIM1alpha.

Presynaptically expressed long-term depression at cerebellar parallel fiber synapses

Plasticity at synapses between parallel fiber (PF) and Purkinje neurons (PN) is widely accepted as a cellular model for certain forms of cerebellar learning. Although PF-PN synapses are known to express bidirectional long-term plasticity at the postsynaptic site, long-term plasticity at the presynaptic site is currently limited to potentiation of the synapses. In this paper, we report on presynaptically expressed PF long-term depression (preLTD) that is observed when presynaptically expressed PF long-term potentiation (preLTP) is pharmacologically prevented. PF preLTD is most efficiently induced by 4 Hz PF stimulation and requires activation of cannabinoid CB1 receptors. Our results indicate that, during preLTD induction, endocannabinoids are released in an NMDA receptor-dependent, but not mGlu1 receptor-dependent, fashion. We conclude that bidirectional plasticity mechanisms exist for both presynaptic and postsynaptic components of cerebellar learning.

Regulation of hippocampus-dependent memory by cyclic AMP-dependent protein kinase

The hippocampus is crucial for the consolidation of new declarative long-term memories. Genetic and behavioral experimentation have revealed that several protein kinases are critical for the formation of hippocampus-dependent long-term memories. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of hippocampus-dependent memory. We review evidence that PKA is required for hippocampus-dependent memory in mammals, and we highlight some of the proteins that have been implicated as targets of PKA. Future directions and open questions regarding the role of PKA in memory storage are also described.

Endocannabinoid-mediated long-term plasticity requires cAMP/PKA signaling and RIM1alpha

Endocannabinoids (eCBs) have emerged as key activity-dependent signals that, by activating presynaptic cannabinoid receptors (i.e., CB1) coupled to G(i/o) protein, can mediate short-term and long-term synaptic depression (LTD). While the presynaptic mechanisms underlying eCB-dependent short-term depression have been identified, the molecular events linking CB1 receptors to LTD are unknown. Here we show in the hippocampus that long-term, but not short-term, eCB-dependent depression of inhibitory transmission requires presynaptic cAMP/PKA signaling. We further identify the active zone protein RIM1alpha as a key mediator of both CB1 receptor effects on the release machinery and eCB-dependent LTD in the hippocampus. Moreover, we show that eCB-dependent LTD in the amygdala and hippocampus shares major mechanistic features. These findings reveal the signaling pathway by which CB1 receptors mediate long-term effects of eCBs in two crucial brain structures. Furthermore, our results highlight a conserved mechanism of presynaptic plasticity in the brain.

Synaptic signalling in cerebellar plasticity

A major goal of learning and memory research is to correlate the function of molecules with the behaviour of organisms. The beautiful laminar structure of the cerebellar cortex lends itself to the study of synaptic plasticity, because its clearly defined patterns of neurons and their synapses form circuits that have been implicated in simple motor behaviour paradigms. The best understood in terms of molecular mechanism is the parallel fibre-Purkinje cell synapse, where presynaptic long-term potentiation and postsynaptic long-term depression and potentiation finely tune cerebellar output. Our understanding of these forms of plasticity has mostly come from the electrophysiological and behavioural analysis of knockout mutant mice, but more recently the knock-in of synaptic molecules with mutated phosphorylation sites and binding domains has provided more detailed insights into the signalling events. The present review details the major forms of plasticity in the cerebellar cortex, with particular attention to the membrane trafficking and intracellular signalling responsible. This overview of the current literature suggests it will not be long before the involvement of the cerebellum in certain motor behaviours is fully explained in molecular terms.

Timing dependence of the induction of cerebellar LTD

Long-term depression (LTD) of the granule cell to Purkinje cell synapse is thought to contribute to motor learning. According to the Marr/Albus/Ito model, sensory inputs drive granule cells to fire, thereby exciting Purkinje cells and influencing motor output. Inappropriate motor output causes neurons in the inferior olive to fire and activate Purkinje cells via the powerful climbing fiber (CF) synapse. CF activity is an error signal and the association of CF and granule cell parallel fiber (PF) activity results in LTD at coactivated PF synapses. Here we examine the timing dependence of LTD by using an induction protocol consisting of a single CF activation paired with a PF burst, with the relative timing of CF and PF activation systematically varied. LTD was most prominent when PF activation occurred before CF activation. A plot of LTD magnitude as a function of PF and CF timing was well approximated by a fit in which LTD peaked for PF activity approximately 80 ms before CF activation and the half width was approximately 300 ms. This indicates that the timing dependence of LTD is well suited to allow a CF to depress preceding PF inputs that generated inappropriate motor outputs. We also find that LTD induction and endocannabinoid release have a similar dependence on PF and CF timing. This suggests that the properties of endocannabinoid release may underlie the timing dependence of some forms of motor learning.

Synaptic plasticity in myenteric neurons of the guinea-pig distal colon: presynaptic mechanisms of inflammation-induced synaptic facilitation

The purpose of this study was to investigate the pre- and postsynaptic mechanisms that contribute to synaptic facilitation in the myenteric plexus of the trinitrobenzene sulphonic acid-inflamed guinea-pig distal colon. Intracellular recordings of evoked fast excitatory postsynaptic potentials (fEPSPs) in myenteric S neurons were evaluated, and the density of synaptic terminals was morphometrically analysed by transmission electron microscopy. In inflamed tissue, fEPSPs were reduced to control levels by the protein kinase A (PKA) inhibitor, H89, but H89 did not affect the fEPSPs in control tissue. This PKA activation in inflamed tissue did not appear to involve 5-HT(4) receptors because the antagonist/inverse agonist, GR 125487, caused comparable decreases of fEPSPs in both tissues. Inhibition of BK channels with iberiotoxin did not alter the fEPSPs in inflamed tissue, but increased the fEPSPs in control tissue to the amplitude detected in inflamed tissue. During trains of stimuli, run-down of EPSPs was less extensive in inflamed tissue and there was a significant increase in the paired pulse ratio. Depolarizations in response to exogenous neurotransmitters were not altered in inflamed tissue. These inflammation-induced changes were not accompanied by alterations in the pharmacological profile of EPSPs, and no changes in synaptic density were detected by electron microscopy. Collectively, these data indicate that synaptic facilitation in the inflamed myenteric plexus involves a presynaptic increase in PKA activity, possibly involving an inhibition of BK channels, and an increase in the readily releasable pool of synaptic vesicles.

Short-term potentiation at the parallel fiber–Purkinje cell synapse

Changes in synaptic efficacy at the parallel fiber (PF)-Purkinje cell (PC) synapse are postulated to be a cellular basis for motor learning. Although long-term efficacy changes lasting more than an hour at this synapse, i.e., long-term potentiation and depression, have been extensively studied, relatively short lasting synaptic efficacy changes, namely short-term potentiation (STP) lasting for tens of minutes, have not been discussed to date. Here we report that this synapse shows an apparent STP reliably by a periodic burst pattern of homosynaptic stimulation. This STP is presynaptically expressed, since it accompanies with a reduced paired-pulse facilitation and is resistant to postsynaptic Ca(2+) reduction by BAPTA injection or in P/Q-type Ca channel knockout cerebella. This novel type of synaptic plasticity at the PF-PC synapse would be a clue for understanding the presynaptic mechanisms of plasticity at this synapse.

Differential regulation of proteasome activity in the nucleus and the synaptic terminals

Proteasome is a multi-subunit proteolytic complex that degrades proteins covalently linked to multiple molecules of ubiquitin. Earlier studies showed a role for the ubiquitin-proteasome pathway in several models of long-term memory and other forms of synaptic plasticity. In Aplysia, the ubiquitin-proteasome pathway has been shown to contribute to the induction of long-term facilitation. In other model systems, ubiquitin-proteasome-mediated proteolysis has also been shown to play a role in synapse development. Previous studies of synaptic plasticity focused on changes in components or the substrates of the ubiquitin-proteasome pathway in whole neurons. Modification of specific synapses would require precise spatial and temporal regulation of the components of the ubiquitin-proteasome pathway within the subcellular compartments of neurons during learning. As a first step towards testing the idea of local regulation of the ubiquitin-proteasome pathway in neurons, we investigated proteasome activity in nuclear and synaptosomal fractions. Here we show that proteasome activity in the synaptic terminals is higher compared to the activity in the nucleus in the Aplysia nervous system as well as in the mouse brain. Furthermore, the proteasome activity in the two neuronal compartments is differentially modulated by protein kinases. Differential regulation of proteasome activity in neuronal compartments such as the synaptic terminals is likely to be a key mechanism underlying synapse-specific plasticity.

RIM function in short- and long-term synaptic plasticity

RIM1alpha (Rab3-interacting molecule 1alpha) is a large multidomain protein that is localized to presynaptic active zones [Wang, Okamoto, Schmitz, Hofmann and Südhof (1997) Nature (London) 388, 593-598] and is the founding member of the RIM protein family that also includes RIM2alpha, 2beta, 2gamma, 3gamma and 4gamma [Wang and Südhof (2003) Genomics 81, 126-137]. In presynaptic nerve termini, RIM1alpha interacts with a series of presynaptic proteins, including the synaptic vesicle GTPase Rab3 and the active zone proteins Munc13, liprins and ELKS (a protein rich in glutamate, leucine, lysine and serine). Mouse KOs (knockouts) revealed that, in different types of synapses, RIM1alpha is essential for different forms of synaptic plasticity. In CA1-region Schaffer-collateral excitatory synapses and in GABAergic synapses (where GABA is gamma-aminobutyric acid), RIM1alpha is required for maintaining normal neurotransmitter release and short-term synaptic plasticity. In contrast, in excitatory CA3-region mossy fibre synapses and cerebellar parallel fibre synapses, RIM1alpha is necessary for presynaptic long-term, but not short-term, synaptic plasticity. In these synapses, the function of RIM1alpha in presynaptic long-term plasticity depends, at least in part, on phosphorylation of RIM1alpha at a single site, suggesting that RIM1alpha constitutes a 'phosphoswitch' that determines synaptic strength. However, in spite of the progress in understanding RIM1alpha function, the mechanisms by which RIM1alpha acts remain unknown. For example, how does phosphorylation regulate RIM1alpha, what is the relationship of the function of RIM1alpha in basic release to synaptic plasticity and what is the physiological significance of different forms of RIM-dependent plasticity? Moreover, the roles of other RIM isoforms are unclear. Addressing these important questions will contribute to our view of how neurotransmitter release is regulated at the presynaptic active zone.

Time and tide in cerebellar memory formation

The notion that the olivocerebellar system is crucial for motor learning is well established. In recent years, it has become evident that there can be many forms of both synaptic and non-synaptic plasticity within this system and that each might have a different role in developing and maintaining motor learning across a wide range of tasks. There are several possible molecular and cellular mechanisms that could underlie adaptation of the vestibulo-ocular reflex and eyeblink conditioning. Although causal relationships between particular cellular processes and individual components of a learned behaviour have not been demonstrated unequivocally, an overall picture is emerging that the different types and sites of cellular plasticity relate importantly to the stage of learning and/or its temporal specifics.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Phosphorylation of cysteine string protein in the brain: developmental, regional and synaptic specificity

Protein phosphorylation modulates regulated exocytosis in most cells, including neurons. Cysteine string protein (CSP) has been implicated in this process because its phosphorylation on Ser10 alters its interactions with syntaxin and synaptotagmin, and because the effect of CSP overexpression on exocytosis kinetics in chromaffin cells requires phosphorylatable Ser10. To characterize CSP phosphorylation in the brain, we raised phosphospecific antibodies to Ser10. Western blotting revealed that the proportion of phosphorylated CSP (P-CSP) varies between distinct brain regions and also exhibits developmental regulation, with P-CSP highest early in development. Immunohistochemical analysis of the cerebellar cortex revealed a novel pool of P-CSP that did not colocalize with synaptic vesicle markers during early development. Strikingly, in the adult cerebellar granular layer P-CSP was highly enriched in a subset of glutamatergic synapses but undetectable in neighbouring GABA-ergic synapses. In view of the functional consequences of CSP phosphorylation, such differences could contribute to the synapse-specific regulation of neurotransmitter release.

Active zones for presynaptic plasticity in the brain

Some of the most abundant synapses in the brain such as the synapses formed by the hippocampal mossy fibers, cerebellar parallel fibers and several types of cortical afferents express presynaptic forms of long-term potentiation (LTP), a putative cellular model for spatial, motor and fear learning. Those synapses often display presynaptic mechanisms of LTP induction, which are either NMDA receptor independent of dependent of presynaptic NMDA receptors. Recent investigations on the molecular mechanisms of neurotransmitter release modulation in short- and long-term synaptic plasticity in central synapses give a preponderant role to active zone proteins as Munc-13 and RIM1-alpha, and point toward the maturation process of synaptic vesicles prior to Ca(2+)-dependent fusion as a key regulatory step of presynaptic plasticity.

Adapter protein 14-3-3 is required for a presynaptic form of LTP in the cerebellum

Long-term potentiation (LTP) of granule cell-Purkinje cell synapses in the mouse cerebellum requires phosphorylation by protein kinase A of the active-zone protein RIM1alpha at Ser413. Here, we show that the adapter protein 14-3-3 readily binds phosphorylated Ser413 in RIM1alpha, and that presynaptic transfection with a dominant-negative 14-3-3eta mutant, or a RIM1alpha mutant with enhanced 14-3-3 binding, inhibits LTP. Thus, RIM1alpha phosphorylation triggers presynaptic LTP in part through recruitment of 14-3-3 to phospho-Ser413-RIM1alpha.

Biophysical alterations of hippocampal pyramidal neurons in learning, ageing and Alzheimer's disease

A series of behavioral, electrophysiological, and molecular biochemical experiments are reviewed indicating that when animals learn hippocampus-dependent tasks, output neurons in the CA1 and CA3 hippocampal subfields show reductions in the slow, post-burst afterhyperpolarization (AHP). The slow AHP is mediated by an apamin-insensitive calcium-activated potassium current. A reduction in the slow AHP makes hippocampal neurons more excitable and facilitates NMDA receptor-mediated response and temporal summation. During normal aging and in a mouse model of Alzheimer's disease (AD), the slow AHP is increased, making neurons less excitable and making learning more difficult. The subgroup of aging animals that are able to learn demonstrates the capacity to increase neuronal excitability by reducing the size of the slow AHP. Similarly, in a mouse model of AD, mice that are able to learn normally after a genetic alteration have a normal capacity for increasing hippocampal neuron excitability by reducing their slow AHP. We suggest that reduction in the slow AHP is basic to learning in young and aging animals. Inability to modulate the slow AHP contributes to learning deficits that occur during aging and early stages of AD.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Reversing cerebellar long-term depression

The discovery of a postsynaptically expressed form of cerebellar parallel fiber-Purkinje cell long-term potentiation (LTP) raises the question whether this is the long-sought resetting mechanism for long-term depression (LTD). Extracellular monitoring of PC spikes enables stable prolonged recordings of parallel fiber-Purkinje cell synaptic efficacy. LTD, saturated by repeated induction protocols, can be reversed by a single round of postsynaptic LTP or nitric oxide (NO), enabling LTD to be reinduced. Conversely, after postsynaptic LTP has been saturated, one round of LTD permits fresh postsynaptic LTP. By contrast, after saturation of LTD, induction of presynaptic LTP or application of forskolin leaves LTD still saturated. Likewise, presynaptic LTP cannot be reversed by LTD. Therefore postsynaptic LTP mediated by NO without postsynaptic Ca2+ elevation, unlike presynaptic LTP mediated by cAMP, is a true counterbalance to LTD mediated by coincidence of NO plus postsynaptic Ca2+

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Synapses between parallel fibres and stellate cells express long-term changes in synaptic efficacy in rat cerebellum

Various forms of synaptic plasticity underlying motor learning have already been well characterized at cerebellar parallel fibre (PF)-Purkinje cell (PC) synapses. Inhibitory interneurones play an important role in controlling the excitability and synchronization of PCs. We have therefore tested the possibility that excitatory synapses between PFs and stellate cells (SCs) are also able to exhibit long-term changes in synaptic efficacy. In the present study, we show that long-term potentiation (LTP) and long-term depression (LTD) were induced at these synapses by a low frequency stimulation protocol (2 Hz for 60 s) and that pairing this low frequency stimulation protocol with postsynaptic depolarization induced a marked shift of synaptic plasticity in favour of LTP. This LTP was cAMP independent, but required nitric oxide (NO) production from pre- and/or postsynaptic elements, depending on the stimulation or pairing protocol used, respectively. In contrast, LTD was not dependent on NO production but it required activation of postsynaptic group II and possibly of group I metabotropic glutamate receptors. Finally, stimulation of PFs at 8 Hz for 15 s also induced LTP at PF-SC synapses. But in this case, LTP was cAMP dependent, as was also observed at PF-PC synapses for presynaptic LTP induced in the same conditions. Thus, long-term changes in synaptic efficacy can be accomplished by PF-SCs synapses as well as by PF-PC synapses, suggesting that both types of plasticity might co-operate during cerebellar motor learning.

Phosphorylation of RIM1α by PKA Triggers Presynaptic Long-Term Potentiation at Cerebellar Parallel Fiber Synapses

Presynaptic activation of protein kinase A (PKA) induces LTP in cerebellar parallel fiber synapses. Presynaptic LTP is known to require the active zone protein RIM1alpha, but the underlying induction mechanism remains unclear. We now show that PKA directly phosphorylates RIM1alpha at two sites. Using paired recordings from cultured cerebellar granule and Purkinje neurons, we demonstrate that LTP is absent in neurons from RIM1alpha KO mice but is rescued by presynaptic expression of RIM1alpha. Mutant RIM1alpha lacking the N-terminal phosphorylation site is unable to rescue LTP in RIM1alpha knockout neurons but selectively suppresses LTP in wild-type neurons. Our findings suggest that PKA-mediated phosphorylation of the active zone protein RIM1alpha at a single N-terminal site induces presynaptic LTP.

Developmental distribution of calretinin in mouse barrel cortex

We describe the postnatal development of calretinin expression in the mouse barrel cortex by immunohistochemistry. A densely staining neuropil and numerous cell bodies appeared throughout layer V, but only within barrel septa of layer IV, at postnatal day 4. This staining pattern became most robust at postnatal day 8. Thereafter, calretinin expression became reduced until the third postnatal week when it attained its mature levels, and the barrel-specific staining was no longer apparent.

Protein kinase A mediates voltage-dependent facilitation of Ca2+ current in presynaptic hair cells in Hermissenda crassicornis

The simplest cellular model for classical conditioning in the nudibranch mollusk, Hermissenda crassicornis, involves the presynaptic hair cells and postsynaptic photoreceptors. Whereas the cellular mechanisms for postsynaptic photoreceptors have been studied extensively, the presynaptic mechanisms remain uncertain. Here, we determined the phenotype of the voltage-dependent Ca(2+) current in the presynaptic hair cells that may be directly involved in changes in synaptic efficacy during classical conditioning. The Ca(2+) current can be classified as a P-type current because its activation voltage under seawater recording conditions is approximately -30 mV, it showed slow inactivation, and it is reversibly blocked by omega-agatoxin-IVA. The steady-state activation and inactivation curves revealed a window current, and the single-channel conductance is approximately 20 pS. The P-type current was enhanced by cAMP analogs (approximately 1.3-fold), and by forskolin, an activator of adenylyl cyclase (approximately 1.25-fold). In addition, the P-type current showed voltage-dependent facilitation, which is mediated by protein kinase A (PKA). Specifically, the PKA inhibitor peptide [PKI(6-22)amide] blocked the enhancement of the Ca(2+) current produced by conditioning depolarization prepulses. Because neurotransmitter release is mediated by Ca(2+) influx via voltage-gated Ca(2+) channels, and because of the nonlinear relationship between the Ca(2+) influx and neurotransmitter release, we propose that voltage-dependent facilitation of the P-type current in hair cells would produce a robust change in synaptic efficacy.

Parallel fiber receptive fields: a key to understanding cerebellar operation and learning

In several theories of the function of the cerebellum in motor control, the mossy-fiber-parallel fiber input has been suggested to provide information used in the control of ongoing movements whereas the role of climbing fibers is to induce plastic changes of parallel fiber (PF) synapses on Purkinje cells. From studies of climbing fibers during the last few decades, we have gained detailed knowledge about the zonal and microzonal organization of the cerebellar cortex and the information carried by climbing fibers. However, properties of the PF input to Purkinje cells and inhibitory interneurones have been largely unknown. The present review, which focuses on the C3 zone of the cerebellar anterior lobe, will present and discuss recent data of the cutaneous PF input to Purkinje cells, interneurons and Golgi cells as well as novel forms of PF plasticity.

Protein kinase C: its role in activity-dependent Purkinje cell dendritic development and plasticity

The cerebellum is a central organ in the control of motor learning and performance. In this respect, the cellular plasticity model systems of multiple climbing fiber elimination and long-term depression have been intensively studied. The signalling pathways involved in these plastic changes are now well understood on a molecular level and protein kinase C (PKC) activity appears to be crucially involved in both processes. Furthermore, as shown in recent studies, Purkinje cell dendritic development also critically depends on the activity of PKC. Thereby, the Ca(2+)-dependent PKC subtypes, activated by synaptic inputs through metabotropic glutamate receptors, trigger functional changes as well as long-term anatomical maturation of the Purkinje cell dendritic tree during cerebellar development at different time levels. This review summarizes these findings and forwards the hypothesis of a link between the functional mechanisms underlying LTD and the differentiation of Purkinje cell dendrites.