Existence and distinction of acid‐evoked currents in rat astrocytes

Astrocytes are vital structures that support and/or protect neighboring neurons from pathology. Although it is generally accepted that glutamate receptors mediate most astrocyte effects, acid‐evoked currents have recently attracted attention for their role in this regard. Here, we identified the existence and characteristics of acid‐sensing ion channels (ASICs) and the transient receptor potential vanilloid type 1 (TRPV1) in astrocytes. There were two types of currents recorded under the application of acidic solution (pH 6.0) in cultured rat astrocytes. Transient currents were exhibited by 10% of the astrocytes, and sustained currents were exhibited by the other 90%, consistent with the features of ASIC and TRPV1 currents, respectively. Western blotting and immunofluorescence confirmed the expression of ASIC1, ASIC2a, ASIC3, and TRPV1 in cultured and in situ astrocytes. Unlike the ASICs expressed in neurons, which were mainly distributed in the cell membrane/cytoplasm, most of the ASICs in astrocytes were expressed in the nucleus. TRPV1 was more permeable to Na⁺ in cultured astrocytes, which differed from the typical neuronal TRPV1 that was mainly permeable to Ca²⁺. This study demonstrates that there are two kinds of acid‐evoked currents in rat astrocytes, which may provide a new understanding about the functions of ligand‐gated ion channels in astrocytes. © 2010 Wiley‐Liss, Inc.     

Presynaptic pH and vesicle fusion in Drosophila larvae neurones

Both intracellular pH (pH_i) and synaptic cleft pH change during neuronal activity yet little is known about how these pH shifts might affect synaptic transmission by influencing vesicle fusion. To address this we imaged pH‐ and Ca²⁺‐sensitive fluorescent indicators (HPTS, Oregon green) in boutons at neuromuscular junctions. Electrical stimulation of motor nerves evoked presynaptic Ca²⁺_i rises and pH_i falls (～0.1 pH units) followed by recovery of both Ca²⁺_i and pH_i. The plasma‐membrane calcium ATPase (PMCA) inhibitor, 5(6)‐carboxyeosin diacetate, slowed both the calcium recovery and the acidification. To investigate a possible calcium‐independent role for the pH_i shifts in modulating vesicle fusion we recorded post‐synaptic miniature end‐plate potential (mEPP) and current (mEPC) frequency in Ca²⁺‐free solution. Acidification by propionate superfusion, NH₄⁺ withdrawal, or the inhibition of acid extrusion on the Na⁺/H⁺ exchanger (NHE) induced a rise in miniature frequency. Furthermore, the inhibition of acid extrusion enhanced the rise induced by propionate addition and NH₄⁺ removal. In the presence of NH₄⁺, 10 out of 23 cells showed, after a delay, one or more rises in miniature frequency. These findings suggest that Ca²⁺‐dependent pH_i shifts, caused by the PMCA and regulated by NHE, may stimulate vesicle release. Furthermore, in the presence of membrane permeant buffers, exocytosed acid or its equivalents may enhance release through positive feedback. This hitherto neglected pH signalling, and the potential feedback role of vesicular acid, could explain some important neuronal excitability changes associated with altered pH and its buffering. Synapse 67:729–740, 2013 . © 2013 Wiley Periodicals, Inc.     

Deletion of Cav2.1(α1A) subunit of Ca2+‐channels impairs synaptic GABA and glutamate release in the mouse cerebellar cortex in cultured slices

Deletion of both alleles of the P/Q‐type Ca²⁺‐channel Ca_v2.1(α_1A) subunit gene in mouse leads to severe ataxia and early death. Using cerebellar slices obtained from 10 to 15 postnatal days mice and cultured for at least 3 weeks in vitro, we have analysed the synaptic alterations produced by genetically ablating the P/Q‐type Ca²⁺‐channels, and compared them with the effect of pharmacological inhibition of the P/Q‐ or N‐type channels on wild‐type littermate mice. Analysis of spontaneous synaptic currents recorded in Purkinje cells (PCs) indicated that the P/Q‐type channels play a prominent role at the inhibitory synapses afferent onto the PCs, with the effect of deleting Ca_v2.1(α_1A) partially compensated. At the granule cell (GC) to PC synapses, both N‐ and P/Q‐type Ca²⁺‐channels were found playing a role in glutamate exocytosis, but with no significant phenotypic compensation of the Ca_v2.1(α_1A) deletion. We also found that the P/Q‐ but not N‐type Ca²⁺‐channel is indispensable at the autaptic contacts between PCs. Tuning of the GC activity implicates both synaptic and sustained extrasynaptic γ‐aminobutyric acid (GABA) release, only the former was greatly impaired in the absence of P/Q‐type Ca²⁺‐channels. Overall, our data demonstrate that both P/Q‐ and N‐type Ca²⁺‐channels play a role in glutamate release, while the P/Q‐type is essential in GABA exocytosis in the cerebellum. Contrary to the other regions of the CNS, the effect of deleting the Ca_v2.1(α_1A) subunit is partially or not compensated at the inhibitory synapses. This may explain why cerebellar ataxia is observed at the mice lacking functional P/Q‐type channels.     

Calcium Channel-Dependent Induction of Long-Term Synaptic Plasticity at Excitatory Golgi Cell Synapses of Cerebellum

Golgi cells, together with granule cells and mossy fibers, form a neuronal microcircuit regulating information transfer at the cerebellum input stage. Despite theoretical predictions, little was known about long-term synaptic plasticity at Golgi cell synapses. Here, we have used whole-cell patch-clamp recordings and calcium imaging to investigate long-term synaptic plasticity at excitatory synapses impinging on Golgi cells. In acute mouse cerebellar slices, mossy fiber theta-burst stimulation (TBS) could induce either long-term potentiation (LTP) or long-term depression (LTD) at mossy fiber-Golgi cell and granule cell-Golgi cell synapses. This synaptic plasticity showed a peculiar voltage dependence, with LTD or LTP being favored when TBS induction occurred at depolarized or hyperpolarized potentials, respectively. LTP required, in addition to NMDA channels, activation of T-type Ca²⁺ channels, while LTD required uniquely activation of L-type Ca²⁺ channels. Notably, the voltage dependence of plasticity at the mossy fiber-Golgi cell synapses was inverted with respect to pure NMDA receptor-dependent plasticity at the neighboring mossy fiber-granule cell synapse, implying that the mossy fiber presynaptic terminal can activate different induction mechanisms depending on the target cell. In aggregate, this result shows that Golgi cells show cell-specific forms of long-term plasticity at their excitatory synapses, that could play a crucial role in sculpting the response patterns of the cerebellar granular layer.SIGNIFICANCE STATEMENT This article shows for the first time a novel form of Ca²⁺ channel-dependent synaptic plasticity at the excitatory synapses impinging on cerebellar Golgi cells. This plasticity is bidirectional and inverted with respect to NMDA receptor-dependent paradigms, with long-term depression (LTD) and long-term potentiation (LTP) being favored at depolarized and hyperpolarized potentials, respectively. Furthermore, LTP and LTD induction requires differential involvement of T-type and L-type voltage-gated Ca²⁺ channels rather than the NMDA receptors alone. These results, along with recent computational predictions, support the idea that Golgi cell plasticity could play a crucial role in controlling information flow through the granular layer along with cerebellar learning and memory.     

Involvement of PKA and ERK pathways in ghrelin‐induced long‐lasting potentiation of excitatory synaptic transmission in the CA1 area of rat hippocampus

Acute effects of ghrelin on excitatory synaptic transmission were evaluated on hippocampal CA1 synapses. Ghrelin triggered an enduring enhancement of synaptic transmission independently of NMDA receptor activation and probably via postsynaptic modifications. This ghrelin‐mediated potentiation resulted from the activation of GHS‐R1a receptors as it was mimicked by the selective agonist JMV1843 and blocked by the selective antagonist JMV2959. This potentiation also required the activation of PKA and ERK pathways to occur as it was inhibited by KT5720 and U0126, respectively. Moreover it most probably involved Ca²⁺ influxes as both ghrelin and JMV1843 elicited intracellular Ca²⁺ increases, which were dependent on the presence of extracellular Ca²⁺ and mediated by L‐type Ca²⁺ channels opening. In addition, ghrelin potentiated AMPA receptor‐mediated [Ca²⁺]i increases while decreasing NMDA receptor‐mediated ones. Thus the potentiation of synaptic transmission by GHS‐R1a at hippocampal CA1 excitatory synapses probably results from postsynaptic mechanisms involving PKA and ERK activation, which are producing long‐lasting enhancement of AMPA receptor‐mediated responses.     

Copper signaling in the mammalian nervous system: Synaptic effects

Copper is an essential metal present at high levels in the CNS. Its role as a cofactor in mitochondrial ATP production and in essential cuproenzymes is well defined. Menkes and Wilson's diseases are severe neurodegenerative conditions that demonstrate the importance of Cu transport into the secretory pathway. In the brain, intracellular levels of Cu, which is almost entirely protein bound, exceed extracellular levels by more than 100‐fold. Cu stored in the secretory pathway is released in a Ca²⁺‐dependent manner and can transiently reach concentrations over 100 μM at synapses. The ability of low micromolar levels of Cu to bind to and modulate the function of γ‐aminobutyric acid type A (GABA_A) receptors, N‐methyl‐D‐aspartate (NMDA) receptors, and voltage‐gated Ca²⁺ channels contributes to its effects on synaptic transmission. Cu also binds to amyloid precursor protein and prion protein; both proteins are found at synapses and brain Cu homeostasis is disrupted in mice lacking either protein. Especially intriguing is the ability of Cu to affect AMP‐activated protein kinase (AMPK), a monitor of cellular energy status. Despite this, few investigators have examined the direct effects of Cu on synaptic transmission and plasticity. Although the variability of results demonstrates complex influences of Cu that are highly method sensitive, these studies nevertheless strongly support important roles for endogenous Cu and new roles for Cu‐binding proteins in synaptic function/plasticity and behavior. Further study of the many roles of Cu in nervous system function will reveal targets for intervention in other diseases in which Cu homeostasis is disrupted. © 2012 Wiley Periodicals, Inc.     

Endogenous calcium buffering at photoreceptor synaptic terminals in salamander retina

Calcium operates by several mechanisms to regulate glutamate release at rod and cone synaptic terminals. In addition to serving as the exocytotic trigger, Ca²⁺ accelerates replenishment of vesicles in cones and triggers Ca²⁺‐induced Ca²⁺ release (CICR) in rods. Ca²⁺ thereby amplifies sustained exocytosis, enabling photoreceptor synapses to encode constant and changing light. A complete picture of the role of Ca²⁺ in regulating synaptic transmission requires an understanding of the endogenous Ca²⁺ handling mechanisms at the synapse. We therefore used the “added buffer” approach to measure the endogenous Ca²⁺ binding ratio (κ_endo) and extrusion rate constant (γ) in synaptic terminals of photoreceptors in retinal slices from tiger salamander. We found that κ_endo was similar in both cell types—～25 and 50 in rods and cones, respectively. Using measurements of the decay time constants of Ca²⁺ transients, we found that γ was also similar, with values of ～100 s^?1 and 160 s^?1 in rods and cones, respectively. The measurements of κ_endo differ considerably from measurements in retinal bipolar cells, another ribbon‐bearing class of retinal neurons, but are comparable to similar measurements at other conventional synapses. The values of γ are slower than at other synapses, suggesting that Ca²⁺ ions linger longer in photoreceptor terminals, supporting sustained exocytosis, CICR, and Ca²⁺‐dependent ribbon replenishment. The mechanisms of endogenous Ca²⁺ handling in photoreceptors are thus well‐suited for supporting tonic neurotransmission. Similarities between rod and cone Ca²⁺ handling suggest that neither buffering nor extrusion underlie differences in synaptic transmission kinetics. Synapse 68:518–528, 2014 . © 2014 Wiley Periodicals, Inc.     

R‐type Ca2+ channels contribute to fast synaptic excitation and action potentials in subsets of myenteric neurons in the guinea pig intestine

Background R‐type Ca²⁺ channels are expressed by myenteric neurons in the guinea pig ileum but the specific function of these channels is unknown. Methods In the present study, we used intracellular electrophysiological techniques to determine the function of R‐type Ca²⁺ channels in myenteric neurons in the acutely isolated longitudinal muscle‐myenteric plexus. We used immunohistochemical methods to localize the Ca_V2.3 subunit of the R‐type Ca²⁺ channel in myenteric neurons. We also studied the effects of the non‐selective Ca²⁺ channel antagonist, CdCl₂ (100 μmol L^?1), the R‐type Ca²⁺ channel blockers NiCl₂ (50 μmol L^?1) and SNX‐482 (0.1 μmol L^?1), and the N‐type Ca²⁺ channel blocker ω‐conotoxin GVIA (CTX 0.1 μmol L^?1) on action potentials and fast and slow excitatory postsynaptic potentials (fEPSPs and sEPSPs) in S and AH neurons in vitro. Key Results Ca_V2.3 co‐localized with calretinin and calbindin in myenteric neurons. NiCl₂ and SNX‐482 reduced the duration and amplitude of action potentials in AH but not S neurons. NiCl₂ inhibited the afterhyperpolarization in AH neurons. ω‐conotoxin GVIA, but not NiCl₂, blocked sEPSPs in AH neurons. NiCl₂ and SNX‐482 inhibited cholinergic, but not cholinergic/purinergic, fEPSPs in S neurons. Conclusions and Inferences These data show that R‐type Ca²⁺ channels contribute to action potentials, but not slow synaptic transmission, in AH neurons. R‐type Ca²⁺ channels contribute to release of acetylcholine as the mediator of fEPSPs in some S neurons. These data indicate that R‐type Ca²⁺ channels may be a target for drugs that selectively modulate activity of AH neurons or could alter fast synaptic excitation in specific pathways in the myenteric plexus.     

Modulation of presynaptic Ca2+ currents in frog motor nerve terminals by high pressure

Presynaptic Ca²⁺‐dependent mechanisms have already been implicated in depression of evoked synaptic transmission by high pressure (HP). Therefore, pressure effects on terminal Ca²⁺ currents were studied in Rana pipiens peripheral motor nerves. The terminal currents, evoked by nerve or direct stimulation, were recorded under the nerve perineurial sheath with a loose macropatch clamp technique. The combined use of Na⁺ and K⁺ channel blockers, [Ca²⁺]_o changes, voltage‐dependent Ca²⁺ channel (VDCC) blocker treatments and HP perturbations revealed two components of presynaptic Ca²⁺ currents: an early fast Ca²⁺ current (I_CaF), possibly carried by N‐type (Ca_V2.2) Ca²⁺ channels, and a late slow Ca²⁺ current (I_CaS), possibly mediated by L‐type (Ca_V1) Ca²⁺ channels. HP reduced the amplitude and decreased the maximum (saturation level) of the Ca²⁺ currents, I_CaF being more sensitive to pressure, and may have slightly shifted the voltage dependence. HP also moderately diminished the Na⁺ action current, which contributed to the depression of VDCC currents. Computer‐based modeling was used to verify the interpretation of the currents and investigate the influence of HP on the presynaptic currents. The direct HP reduction of the VDCC currents and the indirect effect of the action potential decrease are probably the major cause of pressure depression of synaptic release.     

Two types of Ca2+ channel linked to two endocytic pathways coordinately maintain synaptic transmission at the Drosophila synapse

Endocytosis at the presynaptic terminal is initiated by Ca²⁺ influx through voltage‐gated Ca²⁺ channels. At the Drosophila neuromuscular junction, we demonstrated two components of endocytosis linked to distinct Ca²⁺ channels. A voltage‐gated Ca²⁺ channel blocker, (R)‐(+)‐Bay K8644 (R‐BayK), selectively blocked one component (R‐BayK‐sensitive component) without affecting exocytosis, while low concentrations of La³⁺ preferentially depressed the other component (La³⁺ ‐sensitive component). In a temperature‐sensitive mutant, shibire^ts, at non‐permissive temperatures, dynamin clusters were found immunohistochemically at the active zone (AZ) during the R‐BayK‐sensitive endocytosis, while they were detected at the non‐AZ during the La³⁺‐sensitive endocytosis. Immunostaining of the Ca²⁺ channel α₂δ subunit encoded by straightjacket (stj) was found within the AZ, and a mutation in stj depressed the R‐BayK‐sensitive component but enhanced the La³⁺ ‐sensitive one, indicating that the α₂δ subunit is associated with the R‐BayK‐sensitive Ca²⁺ channel. Filipin bound to the non‐AZ membrane and inhibited the La³⁺ ‐sensitive component, but not the R‐BayK‐sensitive one. We concluded that the R‐BayK‐sensitive component of endocytosis occurred at the AZ and termed this AZ endocytosis. We also concluded that the La³⁺ ‐sensitive component occurred at the non‐AZ and termed this non‐AZ endocytosis. These two types of endocytosis were modulated by various drugs towards opposite directions, indicating that they were differentially regulated. During high‐frequency stimulation, AZ endocytosis operated mainly in the early phase, whereas non‐AZ endocytosis operated in the late phase. Thus, intense synaptic transmission is coordinately maintained by synaptic vesicle recycling initiated by Ca²⁺ influx through the two types of Ca²⁺ channel.     

Roles of somatic A-type K~+ channels in the synaptic plasticity of hippocampal neurons

In the mammalian brain, information encoding and storage have been explained by revealing the cellular and molecular mechanisms of synaptic plasticity at various levels in the central nervous system, including the hippocampus and the cerebral cortices. The modulatory mechanisms of synaptic excitability that are correlated with neuronal tasks are fundamental factors for synaptic plasticity, and they are dependent on intracellular Ca²⁺-mediated signaling. In the present review, the A-type K⁺ (I _A) channel, one of the voltage-dependent cation channels, is considered as a key player in the modulation of Ca²⁺ influx through synaptic NMDA receptors and their correlated signaling pathways. The cellular functions of I _A channels indicate that they possibly play as integral parts of synaptic and somatic complexes, completing the initiation and stabilization of memory.     

Mechanisms,pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors

Photoreceptors have depolarized resting potentials that stimulate calcium‐dependent release continuously from a large vesicle pool but neurons can also release vesicles without stimulation. We characterized the Ca²⁺ dependence, vesicle pools, and release sites involved in spontaneous release at photoreceptor ribbon synapses. In whole‐cell recordings from light‐adapted horizontal cells (HCs) of tiger salamander retina, we detected miniature excitatory post‐synaptic currents (mEPSCs) when no stimulation was applied to promote exocytosis. Blocking Ca²⁺ influx by lowering extracellular Ca²⁺, by application of Cd²⁺ and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca²⁺. We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd²⁺ or by increased intracellular Ca²⁺ buffering in rods, but not in cones, that were voltage clamped at ?70 mV. By inhibiting the vesicle cycle with bafilomycin, we found the frequency of mEPSCs declined more rapidly than the amplitude of evoked excitatory post‐synaptic currents (EPSCs) suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca²⁺‐independent release using total internal reflectance fluorescence (TIRF) microscopy to visualize fusion of individual vesicles loaded with dextran‐conjugated pHrodo. Spontaneous release in rods occurred more frequently at non‐ribbon sites than evoked release events. The function of Ca²⁺‐independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.     

Calcium entry via TRPV1 but not ASICs induces neuropeptide release from sensory neurons

Inflammatory mediators induce neuropeptide release from nociceptive nerve endings and cell bodies, causing increased local blood flow and vascular leakage resulting in edema. Neuropeptide release from sensory neurons depends on an increase in intracellular Ca^2 + concentration. In this study we investigated the role of two types of pH sensors in acid-induced Ca^2 + entry and neuropeptide release from dorsal root ganglion (DRG) neurons. The transient receptor potential vanilloid 1 channel (TRPV1) and acid-sensing ion channels (ASICs) are both H⁺-activated ion channels present in these neurons, and are therefore potential pH sensors for this process. We demonstrate with in situ hybridization and immunocytochemistry that TRPV1 and several ASIC subunits are co-expressed with neuropeptides in DRG neurons. The activation of ASICs and of TRPV1 led to an increase in intracellular Ca^2 + concentration. While TRPV1 has a high Ca^2 + permeability and allows direct Ca^2 + entry when activated, we show here that ASICs of DRG neurons mediate Ca^2 + entry mostly by depolarization-induced activation of voltage-gated Ca^2 + channels and only to a small extent via the pore of Ca^2 +-permeable ASICs. Extracellular acidification led to the release of the neuropeptide calcitonin gene-related peptide from DRG neurons. The pH dependence and the pharmacological profile indicated that TRPV1, but not ASICs, induced neuropeptide secretion. In conclusion, this study shows that although both TRPV1 and ASICs mediate Ca^2 + influx, TRPV1 is the principal sensor for acid-induced neuropeptide secretion from sensory neurons.     

Synaptic vesicle glycoprotein 2A modulates vesicular release and calcium channel function at peripheral sympathetic synapses

Synaptic vesicle glycoprotein (SV)2A is a transmembrane protein found in secretory vesicles and is critical for Ca²⁺‐dependent exocytosis in central neurons, although its mechanism of action remains uncertain. Previous studies have proposed, variously, a role of SV2 in the maintenance and formation of the readily releasable pool (RRP) or in the regulation of Ca²⁺ responsiveness of primed vesicles. Such previous studies have typically used genetic approaches to ablate SV2 levels; here, we used a strategy involving small interference RNA (siRNA) injection to knockdown solely presynaptic SV2A levels in rat superior cervical ganglion (SCG) neuron synapses. Moreover, we investigated the effects of SV2A knockdown on voltage‐dependent Ca²⁺ channel (VDCC) function in SCG neurons. Thus, we extended the studies of SV2A mechanisms by investigating the effects on vesicular transmitter release and VDCC function in peripheral sympathetic neurons. We first demonstrated an siRNA‐mediated SV2A knockdown. We showed that this SV2A knockdown markedly affected presynaptic function, causing an attenuated RRP size, increased paired‐pulse depression and delayed RRP recovery after stimulus‐dependent depletion. We further demonstrated that the SV2A–siRNA‐mediated effects on vesicular release were accompanied by a reduction in VDCC current density in isolated SCG neurons. Together, our data showed that SV2A is required for correct transmitter release at sympathetic neurons. Mechanistically, we demonstrated that presynaptic SV2A: (i) acted to direct normal synaptic transmission by maintaining RRP size, (ii) had a facilitatory role in recovery from synaptic depression, and that (iii) SV2A deficits were associated with aberrant Ca²⁺ current density, which may contribute to the secretory phenotype in sympathetic peripheral neurons.     

Synaptic Contributions to Cochlear Outer Hair Cell Ca2+ Dynamics

For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca²⁺ levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca²⁺ occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca²⁺ influx have been poorly explored: voltage-gated L-type Ca²⁺ channels (VGCCs) at synapses with Type II afferent neurons, and α9α10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca²⁺ influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca²⁺ ions originated in MOC synapses, but not by VGCC activation, was confined by Ca²⁺-ATPases most likely present in nearby synaptic cisterns. Although Ca²⁺ currents in OHCs are small, VGCC Ca²⁺ signals were comparable in size to those elicited by α9α10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca²⁺ signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca²⁺ entry sites are closely positioned, they differ in their regulation by intracellular cisterns and/or organelles, suggesting the existence of well-tuned mechanisms to separate the two different OHC synaptic functions.SIGNIFICANCE STATEMENT Outer hair cells (OHCs) are sensory cells in the inner ear operating under very special constraints. Acoustic stimulation leads to fast changes both in membrane potential and in the intracellular concentration of metabolites such as Ca²⁺. Tight mechanisms for Ca²⁺ control in OHCs have been reported. Interestingly, Ca²⁺ is crucial for two important synaptic processes: inhibition by efferent cholinergic neurons, and glutamate release onto Type II afferent fibers. In the current study we functionally imaged Ca²⁺ at these two different synapses, showing close positioning within the basolateral compartment of OHCs. In addition, we show differential regulation of these two Ca²⁺ sources by synaptic cisterns and/or organelles, which could result crucial for functional segregation during normal hearing.     

Differential regulation of spontaneous and evoked inhibitory synaptic transmission in somatosensory cortex by retinoic acid

Retinoic acid (RA), a developmental morphogen, has emerged in recent studies as a novel synaptic signaling molecule that acts in mature hippocampal neurons to modulate excitatory and inhibitory synaptic transmission in the context of homeostatic synaptic plasticity. However, it is unclear whether RA is capable of modulating neural circuits outside of the hippocampus, and if so, whether the mode of RA's action at synapses is similar to that within the hippocampal network. Here we explore for the first time RA's synaptic function outside the hippocampus and uncover a novel function of all‐trans retinoic acid at inhibitory synapses. Acute RA treatment increases spontaneous inhibitory synaptic transmission in L2/3 pyramidal neurons of the somatosensory cortex, and this effect requires expression of RA's receptor RARα both pre‐ and post‐synaptically. Intriguingly, RA does not seem to affect evoked inhibitory transmission assayed with either extracellular stimulation or direct activation of action potentials in presynaptic interneurons at connected pairs of interneurons and pyramidal neurons. Taken together, these results suggest that RA's action at synapses is not monotonous, but is diverse depending on the type of synaptic connection (excitatory versus inhibitory) and circuit (hippocampal versus cortical). Thus, synaptic signaling of RA may mediate multi‐faceted regulation of synaptic plasticity. In addition to its classic roles in brain development, retinoic acid (RA) has recently been shown to regulate excitatory and inhibitory transmission in the adult brain. Here, the authors show that in layer 2/3 (L2/3) of the somatosensory cortex (S1), acute RA induces increases in spontaneous but not action‐potential evoked transmission, and that this requires retinoic acid receptor (RARα) both in presynaptic PV‐positive interneurons and postsynaptic pyramidal (PN) neurons.     

Functional Integration of Calcium Regulatory Mechanisms at Purkinje Neuron Synapses

Cerebellar Purkinje neurons receive synaptic inputs from three different sources: the excitatory parallel fibre and climbing fibre synapses as well as the inhibitory synapses from molecular layer stellate and basket cells. These three synaptic systems use distinct mechanisms in order to generate Ca²⁺ signals that are specialized for specific modes of neurotransmitter release and post-synaptic signal integration. In this review, we first describe the repertoire of Ca²⁺ regulatory mechanisms that generate and regulate the amplitude and timing of Ca²⁺ fluxes during synaptic transmission and then explore how these mechanisms interact to generate the unique functional properties of each of the Purkinje neuron synapses.     

Regional and subunit-specific downregulation of acid-sensing ion channels in the pilocarpine model of epilepsy

Acid-sensing ion channels (ASICs) constitute a recently discovered family of excitatory cation channels, structurally related to the superfamily of degenerin/epithelial sodium channels. ASIC1b and ASIC3 are highly expressed in primary sensory neurons and are thought to play a role in pain transmission related to acidosis. ASIC1a, ASIC2a, and ASIC2b are also distributed in the central nervous system where their function remains unclear. We investigated here the regulation of their expression during status epilepticus (SE), a condition in which neuronal overexcitation leads to acidosis. In animals treated with pilocarpine (380 mg/kg) to induce SE, we observed a marked decrease of ASIC2b mRNA levels in all hippocampal areas and of ASIC1a mRNA levels in the CA1-2 fields. These changes were also observed after protective treatment from neuronal cell death with diazepam (10 mg/kg) and pentobarbital (30 mg/kg). These findings suggest a key role of channels containing ASIC1a and ASIC2b subunits in both normal and pathological activity of hippocampus.