Modulation of GABAergic transmission by activity via postsynaptic Ca2+-dependent regulation of KCC2 function

Activity-induced modification of GABAergic transmission contributes to the plasticity of neural circuits. In the present work we found that prolonged postsynaptic spiking of hippocampal neurons led to a shift in the reversal potential of GABA-induced Cl- currents (E(Cl)) toward positive levels in a duration- and frequency-dependent manner. This effect was abolished by blocking cytosolic Ca2+ elevation and mimicked by releasing Ca2+ from internal stores. Activity- and Ca2+-induced E(Cl) shifts were larger in mature neurons, which express the K-Cl cotransporter KCC2 at high levels, and inhibition of KCC2 occluded the shifts. Overexpression of KCC2 in young cultured neurons, which express lower levels of KCC2 and have a more positive E(Cl), resulted in hyperpolarized E(Cl) similar to that of mature cells. Importantly, these young KCC2-expressing neurons became responsive to neuronal spiking and Ca2+ elevation by showing positive E(Cl) shifts. Thus, repetitive postsynaptic spiking reduces the inhibitory action of GABA through a Ca2+-dependent downregulation of KCC2 function.     

Myosin Vb mobilizes recycling endosomes and AMPA receptors for postsynaptic plasticity

Learning-related plasticity at excitatory synapses in the mammalian brain requires the trafficking of AMPA receptors and the growth of dendritic spines. However, the mechanisms that couple plasticity stimuli to the trafficking of postsynaptic cargo are poorly understood. Here we demonstrate that myosin Vb (MyoVb), a Ca2+-sensitive motor, conducts spine trafficking during long-term potentiation (LTP) of synaptic strength. Upon activation of NMDA receptors and corresponding Ca2+ influx, MyoVb associates with recycling endosomes (REs), triggering rapid spine recruitment of endosomes and local exocytosis in spines. Disruption of MyoVb or its interaction with the RE adaptor Rab11-FIP2 abolishes LTP-induced exocytosis from REs and prevents both AMPA receptor insertion and spine growth. Furthermore, induction of tight binding of MyoVb to actin using an acute chemical genetic strategy eradicates LTP in hippocampal slices. Thus, Ca2+-activated MyoVb captures and mobilizes REs for AMPA receptor insertion and spine growth, providing a mechanistic link between the induction and expression of postsynaptic plasticity.     

Pair recordings reveal all-silent synaptic connections and the postsynaptic expression of long-term potentiation

The activation of silent synapses is a proposed mechanism to account for rapid increases in synaptic efficacy such as long-term potentiation (LTP). Using simultaneous recordings from individual pre- and postsynaptic neurons in organotypic hippocampal slices, we show that two CA3 neurons can be connected entirely by silent synapses. Increasing release probability or application of cyclothiazide does not produce responses from these silent synapses. Direct measurement of NMDAR-mediated postsynaptic responses in all-silent synaptic connections before and after LTP induction show no change in failure rate, amplitude, or area. These data do not support hypotheses that synapse silent results from presynaptic factors or that LTP results from increases in presynaptic glutamate release. LTP is also associated with an increase in postsynaptic responsiveness to exogenous AMPA. We conclude that synapse silence, activation, and expression of LTP are postsynaptic.     

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus.

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.     

Ca2+ signalling in postsynaptic dendrites and spines of mammalian neurons in brain slice.

Postsynaptic Ca2+ changes are involved in control of cellular excitability and induction of synaptic long-term changes. We monitored Ca2+ changes in dendrites and spines during synaptic and direct stimulation using high resolution microfluorometry of fura-2 injected into CA3 pyramidal neurons in guinea pig hippocampal slice. When driven by current injection from an intracellular electrode or with synaptic stimulation, postsynaptic Ca2+ accumulations were highest in the proximal dendrites with a pronounced fall-off towards the soma and some fall-off towards more distal dendrites. Muscarinic activation by low concentrations of carbachol strongly increased intradendritic Ca2+ accumulation during directly-evoked repetitive firing. This enhancement occurred in large part because muscarinic activation suppressed the normal Ca(2+)-dependent activation of K-channels that mediates adaptation of firing. Repetitive firing of cholinergic fibers in the slice reproduced the effects of carbachol. Inhibition of acetylcholine-esterase activity by eserine enhanced the effects of repetitive stimulation of chlolinergic fibers. All effects were reversible and were blocked by the muscarinic antagonist atropine. Ca2+ accumulations in postsynaptic spines might be the basis of specificity of synaptic plasticity. With selective stimulation of few associative/comissural fibers, Ca2+ accumulated in single postsynaptic spines but not in the parent dendrite. With strong stimulation, dendrite levels also increased but spine levels were considerably higher. The NMDA-receptor antagonist AP-5 blocked Ca(2+)-peaks in spines, but left Ca2+ changes in dendrite shafts largely unaffected. Sustained steep Ca2+ gradients between single spines and the parent dendrite, often lasting several minutes, developed with repeated stimulation. Our results demonstrate a spine entity that can act independent from the dendrite with respect to Ca(2+)-dependent processes. Muscarinic augmentation of dendritic Ca2+ levels might reduce diffusional loss of Ca2+ from hot spines into the parent dendrite, thus supporting cooperativity and associativity of synaptic plasticity.     

Bidirectional modification of presynaptic neuronal excitability accompanying spike timing-dependent synaptic plasticity

Correlated pre- and postsynaptic activity that induces long-term potentiation is known to induce a persistent enhancement of the intrinsic excitability of the presynaptic neuron. Here we report that, associated with the induction of long-term depression in hippocampal cultures and in somatosensory cortical slices, there is also a persistent reduction in the excitability of the presynaptic neuron. This reduction requires postsynaptic Ca(2+) elevation and presynaptic PKA- and PKC-dependent modification of slow-inactivating K(+) channels. The bidirectional changes in neuronal excitability and synaptic efficacy exhibit identical requirements for the temporal order of pre- and postsynaptic activation but reflect two distinct aspects of activity-induced modification of neural circuits.     

Presynaptic specificity of endocannabinoid signaling in the hippocampus

Endocannabinoids are retrograde messengers released by neurons to modulate the strength of their synaptic inputs. Endocannabinoids are thought to mediate the suppression of GABA release that follows depolarization of a hippocampal CA1 pyramidal neuron-termed "depolarization-induced suppression of inhibition" (DSI). Here, we report that DSI is absent in mice which lack cannabinoid receptor-1 (CB1). Pharmacological and kinetic evidence suggests that CB1 activation inhibits presynaptic Ca2+ channels through direct G protein inhibition. Paired recordings show that endocannabinoids selectively inhibit a subclass of synapses distinguished by their fast kinetics and large unitary conductance. Furthermore, cannabinoid-sensitive inputs are unusual among central nervous system synapses in that they use N- but not P/Q-type Ca2+ channels for neurotransmitter release. These results indicate that endocannabinoids are highly selective, rapid modulators of hippocampal inhibition.     

NCAM promotes assembly and activity-dependent remodeling of the postsynaptic signaling complex

The neural cell adhesion molecule (NCAM) regulates synapse formation and synaptic strength via mechanisms that have remained unknown. We show that NCAM associates with the postsynaptic spectrin-based scaffold, cross-linking NCAM with the N-methyl-d-aspartate (NMDA) receptor and Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIalpha) in a manner not firmly or directly linked to PSD95 and alpha-actinin. Clustering of NCAM promotes formation of detergent-insoluble complexes enriched in postsynaptic proteins and resembling postsynaptic densities. Disruption of the NCAM-spectrin complex decreases the size of postsynaptic densities and reduces synaptic targeting of NCAM-spectrin-associated postsynaptic proteins, including spectrin, NMDA receptors, and CaMKIIalpha. Degeneration of the spectrin scaffold in NCAM-deficient neurons results in an inability to recruit CaMKIIalpha to synapses after NMDA receptor activation, which is a critical process in NMDA receptor-dependent long-term potentiation. The combined observations indicate that NCAM promotes assembly of the spectrin-based postsynaptic signaling complex, which is required for activity-associated, long-lasting changes in synaptic strength. Its abnormal function may contribute to the etiology of neuropsychiatric disorders associated with mutations in or abnormal expression of NCAM.     

Role of GABAA-Mediated Inhibition and Functional Assortment of Synapses onto Individual Layer 4 Neurons in Regulating Plasticity Expression in Visual Cortex

Layer 4 (L4) of primary visual cortex (V1) is the main recipient of thalamocortical fibers from the dorsal lateral geniculate nucleus (LGN_d). Thus, it is considered the main entry point of visual information into the neocortex and the first anatomical opportunity for intracortical visual processing before information leaves L4 and reaches supra- and infragranular cortical layers. The strength of monosynaptic connections from individual L4 excitatory cells onto adjacent L4 cells (unitary connections) is highly malleable, demonstrating that the initial stage of intracortical synaptic transmission of thalamocortical information can be altered by previous activity. However, the inhibitory network within L4 of V1 may act as an internal gate for induction of excitatory synaptic plasticity, thus providing either high fidelity throughput to supragranular layers or transmittal of a modified signal subject to recent activity-dependent plasticity. To evaluate this possibility, we compared the induction of synaptic plasticity using classical extracellular stimulation protocols that recruit a combination of excitatory and inhibitory synapses with stimulation of a single excitatory neuron onto a L4 cell. In order to induce plasticity, we paired pre- and postsynaptic activity (with the onset of postsynaptic spiking leading the presynaptic activation by 10ms) using extracellular stimulation (ECS) in acute slices of primary visual cortex and comparing the outcomes with our previously published results in which an identical protocol was used to induce synaptic plasticity between individual pre- and postsynaptic L4 excitatory neurons. Our results indicate that pairing of ECS with spiking in a L4 neuron fails to induce plasticity in L4-L4 connections if synaptic inhibition is intact. However, application of a similar pairing protocol under GABA_ARs inhibition by bath application of 2μM bicuculline does induce robust synaptic plasticity, long term potentiation (LTP) or long term depression (LTD), similar to our results with pairing of pre- and postsynaptic activation between individual excitatory L4 neurons in which inhibitory connections are not activated. These results are consistent with the well-established observation that inhibition limits the capacity for induction of plasticity at excitatory synapses and that pre- and postsynaptic activation at a fixed time interval can result in a variable range of plasticity outcomes. However, in the current study by virtue of having two sets of experimental data, we have provided a new insight into these processes. By randomly mixing the assorting of individual L4 neurons according to the frequency distribution of the experimentally determined plasticity outcome distribution based on the calculated convergence of multiple individual L4 neurons onto a single postsynaptic L4 neuron, we were able to compare then actual ECS plasticity outcomes to those predicted by randomly mixing individual pairs of neurons. Interestingly, the observed plasticity profiles with ECS cannot account for the random assortment of plasticity behaviors of synaptic connections between individual cell pairs. These results suggest that connections impinging onto a single postsynaptic cell may be grouped according to plasticity states.     

Silent synapses and the emergence of a postsynaptic mechanism for LTP

Silent synapses abound in the young brain, representing an early step in the pathway of experience-dependent synaptic development. Discovered amidst the debate over whether long-term potentiation reflects a presynaptic or a postsynaptic modification, silent synapses--which in the hippocampal CA1 subfield are characterized by the presence of NMDA receptors but not AMPA receptors--have stirred some mechanistic controversy of their own. Out of this literature has emerged a model for synapse unsilencing that highlights the central role for postsynaptic AMPA-receptor trafficking in the expression of excitatory synaptic plasticity.     

Synaptic targeting of the postsynaptic density protein PSD-95 mediated by lipid and protein motifs

During synaptic development, proteins aggregate at specialized pre- and postsynaptic structures. Mechanisms that mediate protein clustering at these sites remain unknown. To investigate this process, we analyzed synaptic targeting of a postsynaptic density protein, PSD-95, by expressing green fluorescent protein- (GFP-) tagged PSD-95 in cultured hippocampal neurons. We find that postsynaptic clustering relies on three elements of PSD-95: N-terminal palmitoylation, the first two PDZ domains, and a C-terminal targeting motif. In contrast, disruptions of PDZ3, SH3, or guanylate kinase (GK) domains do not affect synaptic targeting. Palmitoylation is sufficient to target the diffusely expressed SAP-97 to synapses, and palmitoylation cannot be replaced with alternative membrane association motifs, suggesting that a specialized synaptic lipid environment mediates postsynaptic clustering. The requirements for PDZ domains and a C-terminal domain of PSD-95 indicate that protein-protein interactions cooperate with lipid interactions in synaptic targeting.     

Cholinergic input uncouples Ca2+ changes from K+ conductance activation and amplifies intradendritic Ca2+ changes in hippocampal neurons

Muscarinic synaptic activation is known to be involved in cortical arousal as well as learning. Although simple increases in the electrical responsiveness of neurons might be the basis of arousal, the linkage of muscarinic transmission to the synaptic plasticity that might underlie learning is lacking. Most models of synaptic plasticity involve postsynaptic Ca2+ changes as a trigger for subsequent processes. We imaged muscarinic effects on free Ca2+ accumulation during intracellular recordings from CA3 pyramidal neurons in the guinea pig hippocampal slice. Muscarinic activation, either by repetitive stimulation of cholinergic fibers or by bath-applied carbachol, strongly increased intradendritic Ca2+ accumulation during directly evoked repetitive firing, in part by blocking a Ca(2+)-dependent K+ conductance. The effects of repetitive stimulation of cholinergic fibers were enhanced by the acetylcholine-esterase blocker eserine and blocked by the muscarinic antagonist atropine. These findings demonstrate a novel muscarinic reinforcement of Ca2+ changes during excitation, which are probably significant for synapse modification.     

Simulation of postsynaptic glutamate receptors reveals critical features of glutamatergic transmission

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors.     

Neurexins induce differentiation of GABA and glutamate postsynaptic specializations via neuroligins

Formation of synaptic connections requires alignment of neurotransmitter receptors on postsynaptic dendrites opposite matching transmitter release sites on presynaptic axons. beta-neurexins and neuroligins form a trans-synaptic link at glutamate synapses. We show here that neurexin alone is sufficient to induce glutamate postsynaptic differentiation in contacting dendrites. Surprisingly, neurexin also induces GABA postsynaptic differentiation. Conversely, neuroligins induce presynaptic differentiation in both glutamate and GABA axons. Whereas neuroligins-1, -3, and -4 localize to glutamate postsynaptic sites, neuroligin-2 localizes primarily to GABA synapses. Direct aggregation of neuroligins reveals a linkage of neuroligin-2 to GABA and glutamate postsynaptic proteins, but the other neuroligins only to glutamate postsynaptic proteins. Furthermore, mislocalized expression of neuroligin-2 disperses postsynaptic proteins and disrupts synaptic transmission. Our findings indicate that the neurexin-neuroligin link is a core component mediating both GABAergic and glutamatergic synaptogenesis, and differences in isoform localization and binding affinities may contribute to appropriate differentiation and specificity.     

Alternative splicing controls selective trans-synaptic interactions of the neuroligin-neurexin complex

Formation of synapses requires specific cellular interactions that organize pre- and postsynaptic compartments. The neuroligin-neurexin complex mediates heterophilic adhesion and can trigger assembly of glutamatergic and GABAergic synapses in cultured hippocampal neurons. Both neuroligins and neurexins are encoded by multiple genes. Alternative splicing generates large numbers of isoforms, which may engage in selective axo-dendritic interactions. We explored whether alternative splicing of the postsynaptic neuroligins modifies their activity toward glutamatergic and GABAergic axons. We find that small extracellular splice insertions restrict the function of neuroligin-1 and -2 to glutamatergic and GABAergic contacts and alter interaction with presynaptic neurexins. The neuroligin isoforms associated with GABAergic contacts bind to neurexin-1alpha and a subset of neurexin-1betas. In turn, these neurexin isoforms induce GABAergic but not glutamatergic postsynaptic differentiation. Our findings suggest that alternative splicing plays a central role in regulating selective extracellular interactions through the neuroligin-neurexin complex at glutamatergic and GABAergic synapses.     

Calmodulin activation by calcium transients in the postsynaptic density of dendritic spines

The entry of calcium into dendritic spines can trigger a sequence of biochemical reactions that begins with the activation of calmodulin (CaM) and ends with long-term changes to synaptic strengths. The degree of activation of CaM can depend on highly local elevations in the concentration of calcium and the duration of transient increases in calcium concentration. Accurate measurement of these local changes in calcium is difficult because the spaces are so small and the numbers of molecules are so low. We have therefore developed a Monte Carlo model of intracellular calcium dynamics within the spine that included calcium binding proteins, calcium transporters and ion channels activated by voltage and glutamate binding. The model reproduced optical recordings using calcium indicator dyes and showed that without the dye the free intracellular calcium concentration transient was much higher than predicted from the fluorescent signal. Excitatory postsynaptic potentials induced large, long-lasting calcium gradients across the postsynaptic density, which activated CaM. When glutamate was released at the synapse 10 ms before an action potential occurred, simulating activity patterns that strengthen hippocampal synapses, the calcium gradient and activation of CaM in the postsynaptic density were much greater than when the order was reversed, a condition that decreases synaptic strengths, suggesting a possible mechanism underlying the induction of long-term changes in synaptic strength. The spatial and temporal mechanisms for selectivity in CaM activation demonstrated here could be used in other signaling pathways.     

Synaptic strength as a function of post- versus presynaptic expression of the neural cell adhesion molecule NCAM

To evaluate the contributions of the pre- versus postsynaptic expression of NCAM in regulation of synaptic efficacy, we cultured dissociated hippocampal cells from NCAM-deficient and wild-type mice in homo- and heterogenotypic combinations. Double recordings from synaptically coupled neurons maintained in heterogenotypic cocultures showed that synaptic strength of excitatory but not inhibitory synapses depended on expression of NCAM post- but not presynaptically. This correlated with higher levels of potentiation and synaptic coverage of NCAM-expressing neurons compared to NCAM-deficient neurons in heterogenotypic cocultures. Synaptic density was the same in homogenotypic cultures of NCAM-deficient and wild-type neurons as well as in heterogenotypic cocultures in which glutamate receptors were blocked. These observations indicate that the relative levels of postsynaptic NCAM expression control synaptic strength in an activity-dependent manner by regulating the number of synapses.     

Ca2+ entry via postsynaptic voltage-sensitive Ca2+ channels can transiently potentiate excitatory synaptic transmission in the hippocampus.

We have studied the role of Ca2+ entry via voltage-sensitive Ca2+ channels in long-term potentiation (LTP) in the CA1 region of the hippocampus. Repeated depolarizing pulses, in the presence of the NMDA receptor antagonist D-APV and without synaptic stimulation, resulted in a potentiation of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs). This depolarization-induced potentiation was augmented in raised extracellular Ca2+ and was blocked by intracellular BAPTA, a Ca2+ chelator, or by nifedipine, a Ca2+ channel antagonist, indicating that the effect was mediated by Ca2+ entry via voltage-sensitive Ca2+ channels. Although the peak potentiation could be as large as 3-fold, the EPSP(C)s decayed back to baseline values within approximately 30 min. However, synaptic activation paired with depolarizing pulses in the presence of D-APV converted the transient potentiation into a sustained form. These results indicate that a rise in postsynaptic Ca2+ via voltage-sensitive Ca2+ channels can transiently potentiate synaptic transmission, but that another factor associated with synaptic transmission may be required for LTP.     

Ca(2+)-assisted receptor-driven endocannabinoid release: mechanisms that associate presynaptic and postsynaptic activities

Endogenous cannabinoids (endocannabinoids) serve as retrograde messengers at synapses in various regions of the brain. They are released from postsynaptic neurons and cause transient and long-lasting reduction of neurotransmitter release through activation of presynaptic cannabinoid receptors. Endocannabinoid release is induced either by increased postsynaptic Ca(2+) levels or by activation of G(q/11)-coupled receptors. When these two stimuli coincide, endocannabinoid release is markedly enhanced, which is attributed to the Ca(2+) dependency of phospholipase Cbeta (PLCbeta). This Ca(2+)-assisted receptor-driven endocannabinoid release is suggested to participate in various forms of synaptic plasticity, including short-term associative plasticity in the cerebellum and spike-timing-dependent long-term depression in the somatosensory cortex. In these forms of plasticity, PLCbeta seems to function as a coincident detector of presynaptic and postsynaptic activities.     

Extracellular Ca2+ depletion contributes to fast activity-dependent modulation of synaptic transmission in the brain

Synaptic activation is associated with rapid changes in intracellular Ca(2+), while the extracellular Ca(2+) level is generally assumed to be constant. Here, using a novel optical method to measure changes in extracellular Ca(2+) at high spatial and temporal resolution, we find that brief trains of synaptic transmission in hippocampal area CA1 induce transient depletion of extracellular Ca(2+). We show that this depletion, which depends on postsynaptic NMDA receptor activation, decreases the Ca(2+) available to enter individual presynaptic boutons of CA3 pyramidal cells. This in turn reduces the probability of consecutive synaptic releases at CA3-CA1 synapses and therefore contributes to short-term paired-pulse depression of minimal responses. This activity-dependent depletion of extracellular Ca(2+) represents a novel form of fast retrograde synaptic signaling that can modulate glutamatergic information transfer in the brain.