-
[show abstract]
[hide abstract]
ABSTRACT: Signaling pathways involving phospholipase C (PLC) are involved in various neural functions. Understanding how these pathways are regulated will lead to a better understanding of their roles in neural functions. Previous studies demonstrated that receptor-driven PLCβ activation depends on intracellular Ca(2+) concentration ([Ca(2+)]i), suggesting the possibility that PLCβ-dependent cellular responses are basically Ca(2+) dependent. To test this possibility, we examined whether modulations of ion channels driven by PLC-coupled metabotropic receptors are sensitive to [Ca(2+)]i using cultured hippocampal neurons. Muscarinic activation triggered an inward current at -100mV (the equilibrium potential for K(+)) in a subpopulation of neurons. This current response was suppressed by pirenzepine (an M1-preferring antagonist), PLC inhibitor, non-selective cation channel blocker, and lowering [Ca(2+)]i. Using the neurons showing no response at -100mV, effects of muscarinic activation on K(+) channels were examined at -40mV. Muscarinic activation induced a transient decrease of the holding outward current. This current response was mimicked and occluded by XE991, an M-current K(+) channel blocker, suppressed by pirenzepine, PLC inhibitor and lowering [Ca(2+)]i, and enhanced by elevating [Ca(2+)]i. Similar results were obtained when group I metabotropic glutamate receptors were activated instead of muscarinic receptors. These results clearly show that ion channel modulations driven by PLC-coupled metabotropic receptors are dependent on [Ca(2+)]i, supporting the hypothesis that cellular responses induced by receptor-driven PLCβ activation are basically Ca(2+) dependent.
Brain research 03/2013; · 2.46 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Nuclear factor (NF)-κB is the key transcription factor involved in the inflammatory responses, and its activation aggravates tumors. Peptidoglycan (PGN), a main cell wall component of Gram-positive bacteria, stimulates Toll-like receptor 2 (TLR-2) and activates a number of inflammatory pathways, including NF-κB. Cannabinoids have been reported to exert anti-inflammatory and antitumor effects. The mechanisms underlying these actions, however, are largely unknown. The purpose of this study was to investigate whether cannabinoids can suppress the PGN-induced activation of NF-κB and cell growth via cannabinoid receptors in U87MG human malignant glioma cells. PGN treatment induced the phosphorylation of NF-κB and cell proliferation in a concentration-dependent manner. The main endocannabinoid, 2-arachidonoylglycerol, prevented the PGN-induced phosphorylation of NF-κB, which was reversed by the CB1 cannabinoid receptor antagonist, AM281. The synthetic cannabinoid, WIN55,212-2, abolished the PGN-activated cell growth, and this effect was reversed by AM281. The preferential expression of CB1 rather than CB2 receptors in these cells was confirmed by reverse transcription-mediated polymerase chain reaction experiments and the observation that the WIN55,212-2-induced morphological changes were completely reversed by AM281 but not by the CB2 antagonist, AM630. Our finding that cannabinoids suppress the NF-κB inflammatory pathway and cell growth via CB1 receptors in glioma cells provides evidence for the therapeutic potential of targeting cannabinoid receptors for the treatment of inflammation-dependent tumor progression.
Oncology Reports 07/2012; 28(4):1176-80. · 1.84 Impact Factor
-
Naotoshi Sugimoto,
Osamu Shido,
Kentaro Matsuzaki, Takako Ohno-Shosaku,
Yoshiaki Hitomi,
Masao Tanaka,
Toshioki Sawaki,
Yoshimasa Fujita,
Takanori Kawanami,
Yasushi Masaki,
Toshiro Okazaki,
Hiroyuki Nakamura,
Shoichi Koizumi,
Akihiro Yachie,
Hisanori Umehara
[show abstract]
[hide abstract]
ABSTRACT: The heat shock response has been extensively studied by a number of investigators to understand the molecular mechanism underlying the cellular response to severe heat stress (higher than 42°C). But, body or tissue temperature increases by only a few degrees Celsius during physiological events. Therefore, the physiological cellular response to mild heat stress rather than severe heat stress is likely to be more important. Repeated exposure to hyperthermia for consecutive 5 days induces heat acclimation which is an adaptive physiological process in humans and animals. However, thus far, the effect of continuous exposure to heat stress on cells has not been fully evaluated. In this study, we investigated an adaptive physiological process that is induced in culture cells by continuous exposure to mild heat stress for 5 days. Exposure to heat activated p38-mitogen-activated protein kinase; inhibited cell growth without apoptosis; and increased the levels of HSPs and HSF-1 in mouse fibroblast cells. Interestingly, exposure to heat regulated the expression of aquaporins and induced morphological change. In a physiological sense, these results suggested that continuous exposure to mild heat stress for 5 days, in which heat acclimation is attained in humans and animals, might induce molecular adaptation to heat in cells.
Cellular Physiology and Biochemistry 01/2012; 30(2):450-7. · 2.86 Impact Factor
-
Takako Ohno-Shosaku,
Yuto Sugawara,
Chiho Muranishi,
Keisuke Nagasawa,
Kozue Kubono,
Nami Aoki,
Mitsuki Taguchi,
Ryousuke Echigo,
Naotoshi Sugimoto,
Yui Kikuchi,
Ryoko Watanabe,
Mitsugu Yoneda
[show abstract]
[hide abstract]
ABSTRACT: Clozapine is the first atypical antipsychotic, and improves positive and negative symptoms of many patients with schizophrenia resistant to treatment with other antipsychotic agents. Clozapine induces minimal extrapyramidal side effects, but is more often associated with seizures. A large number of studies have been conducted to elucidate pharmacological profiles of clozapine and its major active metabolite, N-desmethylclozapine (NDMC). However, there are only a limited number of electrophysiological studies examining their effects on synaptic transmission. In this study, we examined effects of clozapine and NDMC on synaptic transmission by measuring inhibitory and excitatory postsynaptic currents in rat cultured hippocampal neurons. We found that clozapine and NDMC have qualitatively similar actions. They depressed the inhibitory transmission at 1-30 μM, and the excitatory transmission at 30 μM, the former being much more sensitive. The depression of IPSCs by 30 μM of these drugs was associated with an increase in the paired-pulse ratio. The GABA-induced currents were suppressed by these drugs, but less sensitive than IPSCs. The AMPA-induced currents were slightly potentiated by these drugs at 30 μM. At 30 μM, clozapine and NDMC slightly suppressed Ca(2+) and Na(+) channels. These results strongly suggest that clozapine and NMDC depress the inhibitory synaptic transmission mainly by antagonizing postsynaptic GABA(A) receptors, but at higher concentrations additionally by acting on presynaptic site, possibly in part through inhibition of presynaptic Ca(2+) and Na(+) channels. Preferential depression of inhibitory synaptic transmission by clozapine and NDMC might contribute to therapeutic actions and/or side-effects of clozapine.
Brain research 09/2011; 1421:66-77. · 2.46 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: During brain development, cAMP induces morphological changes and inhibits growth effects in several cell types. However, the molecular mechanisms underlying the growth inhibition remain unknown. Tumor suppressor protein phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a lipid phosphatase that inhibits the phosphoinositide 3-kinase (PI3K) pathway. The phosphorylation of Akt, which is one of the key molecules downstream of PI3K, inhibits apoptosis. In this study, we investigated the role of PTEN in cAMP-mediated growth inhibition. B92 rat glial cells were treated with 2 different cAMP stimulatory agents, a phosphodiesterase (PDE) inhibitor and a β-adrenoceptor agonist. Both cAMP stimulatory agents induced marked morphological changes in the cells, decreased cell number, decreased Akt phosphorylation, activated PTEN, cleaved caspase-3, and induced the condensation and fragmentation of nuclei. These results indicate that the cAMP stimulatory agents induced apoptosis. Protein phosphatase inhibitor prevented cAMP-induced dephosphorylation of PTEN and Akt. In addition, cAMP analogs and Epac-selective agonists affected PTEN and Akt activities. These results suggested that cAMP-induced apoptosis may be mediated by PTEN activation and Akt inhibition through protein phosphatase in B92 cells. Our results provide new insight into the role of PTEN in cAMP-induced apoptosis in glial cells.
Neuroscience Letters 06/2011; 497(1):55-9. · 2.11 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Since the first reports of endocannabinoid-mediated retrograde signaling in 2001, great advances have been made toward understanding the molecular basis and functions of the endocannabinoid system. Electrophysiological studies have revealed that the endocannabinoid system is functional at various types of synapses throughout the brain. Basic mechanisms have been clarified as to how endocannabinoids are produced and released from postsynaptic neurons and regulate neurotransmitter release through activating presynaptic cannabinoid CB(1) receptors, although there remain unsolved questions and some discrepancies. In addition to this major function, recent studies suggest diverse functions of endocannabinoids, including control of other endocannabinoid-independent forms of synaptic plasticity, regulation of neuronal excitability, stimulation of glia-neuron interaction, and induction of CB(1)R-independent plasticity. Using recently developed pharmacological and genetic tools, behavioral studies have elucidated the roles of the endocannabinoid system in various aspects of neural functions. In this review, we make a brief overview of molecular mechanisms underlying the endocannabinoid-mediated synaptic modulation and also summarize recent findings, which shed new light on a diversity of functional roles of endocannabinoids.
The Neuroscientist 04/2011; 18(2):119-32. · 4.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Protease-activated receptor 1 (PAR1) is a member of the G-protein coupled receptors that are proteolytically activated by serine proteases. Recent studies suggest a definite contribution of PAR1 to brain functions, including learning and memory. However, cellular mechanisms by which PAR1 activation influences neuronal activity are not well understood. Here we show that PAR1 activation drives retrograde endocannabinoid signaling and thereby regulates synaptic transmission. In cultured hippocampal neurons from rat, PAR1 activation by thrombin or PAR1-specific peptide agonists transiently suppressed inhibitory transmission at cannabinoid-sensitive, but not cannabinoid-insensitive, synapses. The PAR1-induced suppression of synaptic transmission was accompanied by an increase in paired-pulse ratio, and was blocked by a cannabinoid CB(1) receptor antagonist. The PAR1-induced suppression was blocked by pharmacological inhibition of postsynaptic diacylglycerol lipase (DGL), a key enzyme for biosynthesis of the major endocannabinoid 2-arachidonoylglycerol (2-AG), and was absent in knock-out mice lacking the α isoform of DGL. The PAR1-induced IPSC suppression remained intact under the blockade of metabotropic glutamate receptors and was largely resistant to the treatment that blocked Ca(2+) elevation in glial cells following PAR1 activation, which excludes the major contribution of glial PAR1 in IPSC suppression. We conclude that activation of neuronal PAR1 triggers retrograde signaling mediated by 2-AG, which activates presynaptic CB(1) receptors and suppresses transmitter release at hippocampal inhibitory synapses.
Journal of Neuroscience 02/2011; 31(8):3104-9. · 7.11 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The discovery of cannabinoid receptors and subsequent identification of their endogenous ligands (endocannabinoids) in early 1990s have greatly accelerated research on cannabinoid actions in the brain. Then, the discovery in 2001 that endocannabinoids mediate retrograde synaptic signaling has opened up a new era for cannabinoid research and also established a new concept how diffusible messengers modulate synaptic efficacy and neural activity. The last 7 years have witnessed remarkable advances in our understanding of the endocannabinoid system. It is now well accepted that endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid CB(1) receptors, and cause transient and long-lasting reduction of neurotransmitter release. In this review, we aim to integrate our current understanding of functions of the endocannabinoid system, especially focusing on the control of synaptic transmission in the brain. We summarize recent electrophysiological studies carried out on synapses of various brain regions and discuss how synaptic transmission is regulated by endocannabinoid signaling. Then we refer to recent anatomical studies on subcellular distribution of the molecules involved in endocannabinoid signaling and discuss how these signaling molecules are arranged around synapses. In addition, we make a brief overview of studies on cannabinoid receptors and their intracellular signaling, biochemical studies on endocannabinoid metabolism, and behavioral studies on the roles of the endocannabinoid system in various aspects of neural functions.
Physiological Reviews 02/2009; 89(1):309-80. · 26.87 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Depolarization-induced suppression of inhibition (DSI) or excitation (DSE) is a well-known form of endocannabinoid-mediated short-term plasticity that is induced by postsynaptic depolarization. It is generally accepted that DSI/DSE is triggered by Ca(2+) influx through voltage-gated Ca(2+) channels. It is also demonstrated that DSI/DSE is mediated by 2-arachidonoylglycerol (2-AG). However, how Ca(2+) induces 2-AG production is still unclear. In the present study, we investigated molecular mechanisms underlying the Ca(2+)-driven 2-AG production. Using cannabinoid-sensitive inhibitory synapses of cultured hippocampal neurons, we tested several inhibitors for enzymes that are supposed to be involved in 2-AG metabolism. The chemicals we tested include inhibitors for phospholipase C (U73122 and ET-18), diacylglycerol kinase (DGK inhibitor 1), phosphatidic acid phosphohydrolase (propranolol), and diacylglycerol lipase (DGL; RHC-80267 and tetrahydrolipstatin (THL)). However, unfavorable side effects were observed with these inhibitors, except for THL. Furthermore, we found that RHC-80267 hardly inhibited the endocannabinoid release driven by G(q/11)-coupled receptors, which is thought to be DGL-dependent. By contrast, THL exhibited no side effects as long as we tested, and was confirmed to inhibit the DGL-dependent process. Using THL as a DGL inhibitor, we demonstrated that DGL is involved in both hippocampal DSI and cerebellar DSE. To test a possible involvement of PLCdelta in DSI, we examined hippocampal DSI in PLCdelta1, delta3 and delta4-knockout mice. However, there was no significant difference in the DSI magnitude between these knockout mice and wild-type mice. The present study clearly shows that DGL is a prerequisite for DSI/DSE. The enzymes yielding DG remain to be determined.
Neuropharmacology 02/2008; 54(1):58-67. · 4.81 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Endocannabinoids are released from neurons in activity-dependent manners, act retrogradely on presynaptic CB(1) cannabinoid receptors, and induce short-term or long-term suppression of transmitter release. The endocannabinoid release is triggered by postsynaptic activation of voltage-gated Ca(2+) channels and/or G(q)-coupled receptors such as group I metabotropic glutamate receptors (I-mGluRs) and M(1)/M(3) muscarinic receptors. However, the roles of NMDA receptors, which provide another pathway for Ca(2+) entry into neurons, in endocannabinoid signalling have been poorly understood. In the present study, we investigated the possible contribution of NMDA receptors in endocannabinoid production by recording IPSCs in cultured hippocampal neurons. Under the conditions minimizing the activation of voltage-gated Ca(2+) channels, local application of NMDA (200 microm) transiently suppressed cannabinoid-sensitive IPSCs, but not cannabinoid-insensitive IPSCs. This NMDA-induced suppression was abolished by blocking NMDA receptors, CB(1) receptors and diacylglycerol lipase, but not by inhibiting voltage-gated Ca(2+) channels. When the postsynaptic neuron was dialysed with 30 mm BAPTA, the NMDA-induced suppression was reduced significantly. A lower dose of NMDA (20 microm) exerted little effect when applied alone, but markedly enhanced the cannabinoid-dependent suppression driven by muscarinic receptors or I-mGluRs. These data clearly indicate that the activation of NMDA receptors facilitates the endocannabinoid release either alone or in concert with the G(q)-coupled receptors.
The Journal of Physiology 11/2007; 584(Pt 2):407-18. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid receptors and cause various forms of short-term and long-term synaptic plasticity throughout the brain. Using hippocampal and cerebellar neurons, we have revealed that endocannabinoid release can be induced through two different pathways. One is independent of phospholipase Cbeta (PLCbeta) and driven by Ca(2+) elevation alone (Ca(2+)-driven endocannabinoid release, CaER), and the other is PLCbeta-dependent and driven by activation of G(q/11)-coupled receptors (receptor-driven endocannabinoid release, RER). CaER is induced by activation of either voltage-gated Ca(2+) channels or NMDA receptors. RER is functional even at resting Ca(2+) levels (basal RER), but markedly enhanced by a small Ca(2+) elevation (Ca(2+)-assisted RER). In Ca(2+)-assisted RER, PLCbeta serves as a coincidence detector of receptor activation and Ca(2+) elevation. We have also demonstrated that Ca(2+)-assisted RER is essential for the endocannabinoid release triggered by synaptic activity. Our anatomical data show that a set of receptors and enzymes required for RER are well organized so that the excitatory input can trigger RER effectively. Certain forms of spike-timing-dependent plasticity (STDP) are reported to depend on endocannabinoid signalling. The NMDA receptor and PLCbeta might play key roles in the endocannabinoid-dependent forms of STDP as coincidence detectors with different timing dependences.
The Journal of Physiology 11/2007; 584(Pt 2):373-80. · 4.72 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Current Opinion in Neurobiology 07/2007; 17(3):360-5. · 7.44 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Marijuana affects neural functions through the binding of its active component (Delta(9)-THC) to cannabinoid receptors in the CNS. Recent studies have elucidated that endogenous ligands for cannabinoid receptors, endocannabinoids, serve as retrograde messengers at central synapses. Endocannabinoids are produced on demand in activity-dependent manners and released from postsynaptic neurons. The released endocannabinoids travel backward across the synapse, activate presynaptic CB1 cannabinoid receptors, and modulate presynaptic functions. Retrograde endocannabinoid signaling is crucial for certain forms of short-term and long-term synaptic plasticity at excitatory or inhibitory synapses in many brain regions, and thereby contributes to various aspects of brain function including learning and memory. Molecular identities of the CB1 receptor and enzymes involved in production and degradation of endocannabinoids have been elucidated. Anatomical studies have demonstrated unique distributions of these molecules around synapses, which provide morphological bases for the roles of endocannabinoids as retrograde messengers. CB1-knockout mice exhibit various behavioral abnormalities and multiple defects in synaptic plasticity, supporting the notion that endocannabinoid signaling is involved in various aspects of neural function. In this review article, the authors describe molecular mechanisms of the endocannabinoid-mediated synaptic modulation and its possible physiological significance.
The Neuroscientist 04/2007; 13(2):127-37. · 4.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Endocannabinoids function as retrograde messengers and modulate synaptic transmission through presynaptic cannabinoid CB1 receptors. The magnitude and time course of endocannabinoid signaling are thought to depend on the balance between the production and degradation of endocannabinoids. The major endocannabinoid 2-arachidonoylglycerol (2-AG) is hydrolyzed by monoacylglycerol lipase (MGL), which is shown to be localized at axon terminals. In the present study, we investigated how MGL regulates endocannabinoid signaling and influences synaptic transmission in the hippocampus. We found that MGL inhibitors, methyl arachidonoyl fluorophosphonate and arachidonoyl trifluoromethylketone, caused a gradual suppression of cannabinoid-sensitive IPSCs in cultured hippocampal neurons. This suppression was reversed by blocking CB1 receptors and was attenuated by inhibiting 2-AG synthesis, indicating that MGL scavenges constitutively released 2-AG. We also found that the MGL inhibitors significantly prolonged the suppression of both IPSCs and EPSCs induced by exogenous 2-AG and depolarization-induced suppression of inhibition/excitation, a phenomenon known to be mediated by retrograde endocannabinoid signaling. In contrast, inhibitors of other endocannabinoid hydrolyzing enzymes, fatty acid amide hydrolase and cyclooxygenase-2, had no effect on the 2-AG-induced IPSC suppression. These results strongly suggest that presynaptic MGL not only hydrolyzes 2-AG released from activated postsynaptic neurons but also contributes to degradation of constitutively produced 2-AG and prevention of its accumulation around presynaptic terminals. Thus, the MGL activity determines basal endocannabinoid tone and terminates retrograde endocannabinoid signaling in the hippocampus.
Journal of Neuroscience 02/2007; 27(5):1211-9. · 7.11 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Endocannabinoids work as retrograde messengers and contribute to short-term and long-term modulation of synaptic transmission via presynaptic cannabinoid receptors. It is generally accepted that the CB1 cannabinoid receptor (CB1) mediates the effects of endocannabinoid in inhibitory synapses. For excitatory synapses, however, contributions of CB1, "CB3," and some other unidentified receptors have been suggested. In the present study we used electrophysiological and immunohistochemical techniques and examined the type(s) of cannabinoid receptor functioning at hippocampal and cerebellar excitatory synapses. Our electrophysiological data clearly demonstrate the predominant contribution of CB1. At hippocampal excitatory synapses on pyramidal neurons the cannabinoid-induced synaptic suppression was reversed by a CB1-specific antagonist, N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), and was absent in CB1 knock-out mice. At climbing fiber (CF) and parallel fiber (PF) synapses on cerebellar Purkinje cells the cannabinoid-dependent suppression was absent in CB1 knock-out mice. The presence of CB1 at presynaptic terminals was confirmed by immunohistochemical experiments with specific antibodies against CB1. In immunoelectron microscopy the densities of CB1-positive signals in hippocampal excitatory terminals and cerebellar PF terminals were much lower than in inhibitory terminals but were clearly higher than the background. Along the long axis of PFs, the CB1 was localized at a much higher density on the perisynaptic membrane than on the extrasynaptic and synaptic regions. In contrast, CB1 density was low in CF terminals and was not significantly higher than the background. Despite the discrepancy between the electrophysiological and morphological data for CB1 expression on CFs, these results collectively indicate that CB1 is responsible for cannabinoid-dependent suppression of excitatory transmission in the hippocampus and cerebellum.
Journal of Neuroscience 04/2006; 26(11):2991-3001. · 7.11 Impact Factor
-
Seikagaku. The Journal of Japanese Biochemical Society 03/2006; 78(2):126-30. · 0.04 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Endocannabinoids mediate retrograde signaling and modulate synaptic transmission in various regions of the CNS. Depolarization-induced elevation of intracellular Ca2+ concentration causes endocannabinoid-mediated suppression of excitatory/inhibitory synaptic transmission. Activation of G(q/11)-coupled receptors including group I metabotropic glutamate receptors (mGluRs) also causes endocannabinoid-mediated suppression of synaptic transmission. However, precise mechanisms of endocannabinoid production initiated by physiologically relevant synaptic activity remain to be determined. To address this problem, we made whole-cell recordings from Purkinje cells (PCs) in mouse cerebellar slices and examined their excitatory synapses arising from climbing fibers (CFs) and parallel fibers (PFs). We first characterized three distinct modes to induce endocannabinoid release by analyzing CF to PC synapses. The first mode is strong activation of mGluR subtype 1 (mGluR1)-phospholipase C (PLC) beta4 cascade without detectable Ca2+ elevation. The second mode is Ca2+ elevation to a micromolar range without activation of the mGluR1-PLCbeta4 cascade. The third mode is the Ca2+-assisted mGluR1-PLCbeta4 cascade that requires weak mGluR1 activation and Ca2+ elevation to a submicromolar range. By analyzing PF to PC synapses, we show that the third mode is essential for effective endocannabinoid release from PCs by excitatory synaptic activity. Furthermore, our biochemical analysis demonstrates that combined weak mGluR1 activation and mild depolarization in PCs effectively produces 2-arachidonoylglycerol (2-AG), a candidate of endocannabinoid, whereas either stimulus alone did not produce detectable 2-AG. Our results strongly suggest that under physiological conditions, excitatory synaptic inputs to PCs activate the Ca2+-assisted mGluR1-PLCbeta4 cascade, and thereby produce 2-AG, which retrogradely modulates synaptic transmission to PCs.
Journal of Neuroscience 08/2005; 25(29):6826-35. · 7.11 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Endocannabinoids mediate retrograde signal and modulate transmission efficacy at various central synapses. Although endocannabinoid release is induced by either depolarization or activation of G(q/11)-coupled receptors, it is markedly enhanced by the coincidence of depolarization and receptor activation. Here we report that this coincidence is detected by phospholipase Cbeta1 (PLCbeta1) in hippocampal neurons. By measuring cannabinoid-sensitive synaptic currents, we found that the receptor-driven endocannabinoid release was dependent on physiological levels of intracellular Ca(2+) concentration ([Ca(2+)](i)), and markedly enhanced by depolarization-induced [Ca(2+)](i) elevation. Furthermore, we measured PLC activity in intact neurons by using exogenous TRPC6 channel as a biosensor for the PLC product diacylglycerol and found that the receptor-driven PLC activation exhibited similar [Ca(2+)](i) dependence to that of endocannabinoid release. Neither endocannabinoid release nor PLC activation was induced by receptor activation in PLCbeta1 knockout mice. We therefore conclude that PLCbeta1 serves as a coincidence detector through its Ca(2+) dependency for endocannabinoid release in hippocampal neurons.
Neuron 02/2005; 45(2):257-68. · 14.74 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Stargazin (gamma-2) is disrupted in the ataxic and epileptic mutant mouse, stargazer (stg). The striking defect in the stg cerebellum is the lack of functional AMPA receptors on granule cells. Recently, it has been reported that gamma-2 and its related molecules are crucial for the surface expression, synaptic targeting and recycling of AMPA receptors, being termed collectively as the transmembrane AMPA receptor regulatory proteins (TARPs). However, it is still unclear whether TARPs directly modulate AMPA receptor activity. Here we report that coexpression of GluRalpha1 (GluR1) with gamma-2 using HEK293 cells and Xenopus oocytes markedly enhanced glutamate-induced currents. This effect was far beyond the increase of AMPA receptor surface expression and accompanied by increased glutamate affinity and subunit cooperativity. Other member of TARPs (gamma-3, gamma-4, and gamma-8) also enhanced the current response through the AMPA receptors. The enhancing effect by gamma-2 coexpression was further observed for homomeric GluRalpha2 (GluR2) channels, which, when expressed alone, are known to produce only a small or negligible current response. These results suggest that gamma-2 not only promotes AMPA receptor surface expression but also directly modulates AMPA receptor activity.
Neuroscience Research 01/2005; 50(4):369-74. · 2.25 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The cholinergic system in the CNS plays important roles in higher brain functions, primarily through muscarinic acetylcholine receptors. At cellular levels, muscarinic activation produces various effects including modulation of synaptic transmission. Here we report that muscarinic activation suppresses hippocampal inhibitory transmission through two distinct mechanisms, namely a cannabinoid-dependent and cannabinoid-independent mechanism. We made paired whole-cell recordings from cultured hippocampal neurons of rats and mice, and monitored inhibitory postsynaptic currents (IPSCs). When cannabinoid receptor type 1 (CB1) was blocked, oxotremorine M (oxo-M), a muscarinic agonist, suppressed IPSCs in a subset of neuron pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the M(2)-preferring antagonist gallamine, and was totally absent in neuron pairs from M(2)-knockout mice. When CB1 receptors were not blocked, oxo-M suppressed IPSCs in a gallamine-resistant manner in cannabinoid-sensitive pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the CB1 antagonist AM281, and was completely eliminated in neuron pairs from M(1)/M(3)-compound-knockout mice. Our immunohistochemical examination showed that M(2) and CB1 receptors were present at inhibitory presynaptic terminals of mostly different origins. These results indicate that two distinct mechanisms mediate the muscarinic suppression. In a subset of synapses, activation of M(2) receptors at presynaptic terminals suppresses GABA release directly. In contrast, in a different subset of synapses, activation of M(1)/M(3) receptors causes endocannabinoid production and subsequent suppression of GABA release by activating presynaptic CB1 receptors. Thus, the muscarinic system can influence hippocampal functions by controlling different subsets of inhibitory synapses through the two distinct mechanisms.
European Journal of Neuroscience 06/2004; 19(10):2682-92. · 3.63 Impact Factor
