Interaction of DHPG‐LTD and synaptic‐LTD at senescent CA3‐CA1 hippocampal synapses

The susceptibility, but not the magnitude, of long‐term depression (LTD) induced by hippocampal CA3‐CA1 synaptic activity (synaptic‐LTD) increases with advanced age. In contrast, the magnitude of LTD induced by pharmacological activation of CA3‐CA1 group I metabotropic glutamate receptors (mGluRs) increases during aging. This study examined the signaling pathways involved in induction of LTD and the interaction between paired‐pulse low frequency stimulation‐induced synaptic‐LTD and group I mGluR selective agonist, (RS)‐3,5‐dihydroxyphenylglycine (DHPG, 100 µM)‐induced DHPG‐LTD in hippocampal slices obtained from aged (22–24 months) male Fischer 344 rats. Prior induction of synaptic‐LTD did not affect induction of DHPG‐LTD; however, prior induction of the DHPG‐LTD occluded synaptic‐LTD suggesting that expression of DHPG‐LTD may incorporate synaptic‐LTD mechanisms. Application of individual antagonist for the group I mGluR (AIDA), the N‐methyl‐d ‐aspartate receptor (NMDAR) (AP‐5), or L‐type voltage‐dependent Ca²⁺ channel (VDCC) (nifedipine) failed to block synaptic‐LTD and any two antagonists severely impaired synaptic‐LTD induction, indicating that activation of any two mechanisms is sufficient to induce synaptic‐LTD in aged animals. For DHPG‐LTD, AIDA blocked DHPG‐LTD and individually applied NMDAR or VDCC attenuated but did not block DHPG‐LTD, indicating that the magnitude of DHPG‐LTD depends on all three mechanisms. © 2014 Wiley Periodicals, Inc.     

The role of Homer1c in metabotropic glutamate receptor‐dependent long‐term potentiation

Group I metabotropic glutamate receptors (mGluR1/5) play a role in synaptic plasticity and they demonstrate direct interactions with the neuronal Homer1c protein. We have previously shown that Homer1c can restore the plasticity deficits in Homer1 knockout mice (H1‐KO). Here, we investigated the role of Homer1c in mGluR‐dependent synaptic plasticity in wild‐type mice, H1‐KO, and H1‐KO mice overexpressing Homer1c (KO+H1c). We used a form of plasticity induced by activation of mGluR1/5 that transforms short‐term potentiaion (STP) induced by a subthreshold theta burst stimulation into long‐term potentiation (LTP). We have shown that although acute hippocampal slices from wild‐type animals can induce LTP using this stimulation protocol, H1‐KO only show STP. Gene delivery of Homer1c into the hippocampus of H1‐KO mice rescued LTP to wild‐type levels. This form of synaptic plasticity was dependent on mGluR5 but not mGluR1 activation both in wild‐type mice and in KO+H1c. mGluR1/5‐dependent LTP was blocked with inhibitors of the MEK‐ERK and PI3K‐mTOR pathways in KO+H1c mice. Moreover, blocking Homer1c–mGluR5 interactions prevented the maintenance of LTP in acute hippocampal slices from KO+H1c. These data indicate that Homer1c–mGluR5 interactions are necessary for mGluR‐dependent LTP, and that mGluR1/5‐dependent LTP involves PI3K and ERK activation. © 2013 Wiley Periodicals, Inc.     

Enhanced astroglial Ca2+ signaling increases excitatory synaptic strength in the epileptic brain

The fine‐tuning of synaptic transmission by astrocyte signaling is crucial to CNS physiology. However, how exactly astroglial excitability and gliotransmission are affected in several neuropathologies, including epilepsy, remains unclear. Here, using a chronic model of temporal lobe epilepsy (TLE) in rats, we found that astrocytes from astrogliotic hippocampal slices displayed an augmented incidence of TTX‐insensitive spontaneous slow Ca²⁺ transients (STs), suggesting a hyperexcitable pattern of astroglial activity. As a consequence, elevated glutamate‐mediated gliotransmission, observed as increased slow inward current (SICs) frequency, up‐regulates the probability of neurotransmitter release in CA3‐CA1 synapses. Selective blockade of spontaneous astroglial Ca²⁺ elevations as well as the inhibition of purinergic P2Y1 or mGluR5 receptors relieves the abnormal enhancement of synaptic strength. Moreover, mGluR5 blockade eliminates any synaptic effects induced by P2Y1R inhibition alone, suggesting that the Pr modulation via mGluR occurs downstream of P2Y1R‐mediated Ca²⁺‐dependent glutamate release from astrocyte. Our findings show that elevated Ca²⁺‐dependent glutamate gliotransmission from hyperexcitable astrocytes up‐regulates excitatory neurotransmission in epileptic hippocampus, suggesting that gliotransmission should be considered as a novel functional key in a broad spectrum of neuropathological conditions. GLIA 2015;63:1507–1521     

Regulation of extracellular signal-regulated kinase phosphorylation in cultured rat striatal neurons

Recent studies demonstrate that activation of Ca²⁺-permeable N-methyl-D-aspartate (NMDA) receptors upregulates phosphorylation of mitogen-activated protein kinases (MAPKs) in heterologous cells and neurons. In cultured rat striatal neurons, the present work systematically evaluated the role of a number of protein kinases in forming a signaling cascade transducing NMDA receptor signals to MAPKs. It was found that a brief NMDA application consistently induced rapid and transient phosphorylation of the extracellular signal-regulated kinase 1/2 (ERK1/2), a best characterized subclass of MAPKs. This ERK1/2 phosphorylation was resistant to the inhibition of protein kinase C, p38 MAPK, cyclin-dependent kinase 5, receptor tyrosine kinase (epidermal growth factor receptors), or non-receptor tyrosine kinases (including Src) by their selective inhibitors. However, the increase in ERK1/2 phosphorylation was partially blocked by a protein kinase A (PKA) inhibitor. The inhibitors for Ca²⁺/calmodulin-dependent protein kinase (CaMK) or phosphatidylinositol 3-kinase (PI3-kinase) completely blocked the NMDA-stimulated ERK1/2 phosphorylation. In an attempt to characterize the sequential role of CaMK and PI3-kinase, we found that NMDA increased PI3-kinase phosphorylation on Tyr⁵⁰⁸, which kinetically corresponded to the ERK1/2 phosphorylation and was blocked by the CaMK inhibitor. These results indicate that the protein kinases are differentially involved in linking NMDA receptors to ERK1/2 in striatal neurons.     

Long‐term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity‐dependent long‐term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64‐channel recording system, we show for the first time that repetitive low‐frequency stimulation (LFS) can elicit frequency‐dependent LTD of glutamate receptor‐mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N‐methyl‐d ‐aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L‐type voltage‐gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin‐dependent protein kinase II did not affect LFS‐evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)‐3,5‐dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor‐independent and could not be occluded by LFS‐induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co‐exist in the same population of IC synapses.     

N-Methyl- -aspartate receptor activation results in regulation of extracellular signal-regulated kinases by protein kinases and phosphatases in glutamate-induced neuronal apototic-like death

Extracellular signal-regulated kinases (ERK1/ERK2) have been shown transiently activated and involved in excitotoxicity. We searched for upstream molecules responsible for the regulation of glutamate-induced ERK1/ERK2 activation and ERK1/ERK2-mediated apototic-like death in cultured rat cortical neurons. ERK1/ERK2 activation (monitored by anti-active ERK1/ERK2 antibody) was almost completely prevented by blockage of NMDA receptor (NMDA-R) or elimination of extracellular Ca²⁺, but not any other glutamate receptor or L-type voltage-gated Ca²⁺ channel. It was prevented largely by inhibition of protein kinase C (PKC), protein-tyrosine kinases (PTK), respectively, but mildly by that of CaM kinase II. Combined inhibition of CaM kinase II (but not PTK) and PKC had an additive effect. Reversion of ERK1/ERK2 activation was largely prevented by inhibition of protein phosphatase (PP) 1 or protein tyrosine phosphatase (PTP). Combined inhibition of PP 1 and PTP had no additive effect. Glutamate-induced apoptotic-like death (determined by DAPI staining) was largely prevented by inhibition of NMDA-R, PKC, CaM kinase II, PTK and MEK1/MEK2 (ERK1/ERK2 kinase), respectively. Combined inhibition of CaM kinase II (but not PKC or PTK) and MEK1/MEK2 had an additive effect. Glutamate-induced apoptotic-like death was promoted by inhibition of PP1 and PTP, respectively. The above results suggested that in glutamate-induced cortical neurotoxicity ERK1/ERK2 activation be mainly mediated by NMDA-R. Subsequently, a pathway dependent on both PKC and PTK was mainly involved, which was also mainly responsible for ERK1/ERK2-mediated apoptotic-like death, and a CaM kinase II-dependent pathway was relatively mildly involved. Reversion of ERK1/ERK2 activation was mainly mediated by a pathway dependent on both PP1 and PTP, which might be involved in the restrain of glutamate-induced neurotoxicity.     

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.     

Constitutive downregulation protein kinase C epsilon in hSOD1G93A astrocytes influences mGluR5 signaling and the regulation of glutamate uptake

Accumulating evidence indicates that motor neuron degeneration in amyotrophic lateral sclerosis (ALS) is a non‐cell‐autonomous process and that impaired glutamate clearance by astrocytes, leading to excitotoxicity, could participate in progression of the disease. In astrocytes derived from an animal model of ALS (hSOD1^G93A rats), activation of type 5 metabotropic glutamate receptor (mGluR5) fails to increase glutamate uptake, impeding a putative dynamic neuroprotective mechanism involving astrocytes. Using astrocyte cultures from hSOD1^G93A rats, we have demonstrated that the typical Ca²⁺ oscillations associated with mGluR5 activation were reduced, and that the majority of cells responded with a sustained elevation of intracellular Ca²⁺ concentration. Since the expression of protein kinase C epsilon isoform (PKC?) has been found to be considerably reduced in astrocytes from hSOD1^G93A rats, the consequences of manipulating its activity and expression on mGluR5 signaling and on the regulation of glutamate uptake have been examined. Increasing PKC? expression was found to restore Ca²⁺ oscillations induced by mGluR5 activation in hSOD1^G93A‐expressing astrocytes. This was also associated with an increase in glutamate uptake capacity in response to mGluR5 activation. Conversely, reducing PKC? expression in astrocytes from wild‐type animals with specific PKC?‐shRNAs was found to alter the mGluR5 associated oscillatory signaling profile, and consistently reduced the regulation of the glutamate uptake‐mediated by mGluR5 activation. These results suggest that PKC? is required to generate Ca²⁺ oscillations following mGluR5 activation, which support the regulation of astrocytic glutamate uptake. Reduced expression of astrocytic PKC? could impair this neuroprotective process and participate in the progression of ALS.     

Nicotinic acetylcholine receptors regulate type 1 inositol 1,4,5‐trisphosphate receptor expression via calmodulin kinase IV activation

Type 1 inositol 1,4,5‐trisphosphate receptors (IP₃R‐1) are among the important calcium channels regulating intracellular Ca²⁺ concentration in the central nervous system. In a previous study, we showed that drugs of abuse, such as cocaine, methamphetamine, and ethanol, induced IP₃R‐1 upregulation via the calcium signal transduction pathway in psychological dependence. Although nicotine, a major component in tobacco smoke, participates in psychological and/or physical dependence, it has not yet been clarified how nicotine alters IP₃R‐1 expression. The present study, therefore, seeks to clarify the mechanism bgy which nicotine modifies IP₃R‐1 expression by using mouse cerebral cortical neurons in primary culture. Nicotine induced dose‐ and time‐dependent upregulation of IP₃R‐1 protein following its mRNA increase, and the latter was significantly suppressed by a nonselective nicotinic acetylcholine receptors (nAChR) antagonist, mecamylamine. Both cFos and phosphorylated‐cJun (p‐cJun) were immediately increased in the nucleus, together with an increase of calmodulin kinase (CaMK) IV but not CaMKII expression after nicotine exposure. A nonselective inhibitor of CaMKs, KN‐93, and a calcium chelating regent, BAPTA‐AM, completely suppressed the expression of cFos and p‐cJun in the nucleus as well as the nicotine‐induced IP₃R‐1 upregulation. These results indicate that nAChR activation by nicotine upregulates IP₃R‐1 via increase of activator protein‐1, which is a cFos and cJun dimmer, in the nucleus, with activation of Ca²⁺ signaling transduction processes. © 2014 Wiley Periodicals, Inc.     

Autism‐associated Shank3 mutations alter mGluR expression and mGluR‐dependent but not NMDA receptor‐dependent long‐term depression

SHANK3 is a postsynaptic structural protein localized at excitatory glutamatergic synapses in which deletions and mutations have been implicated in patients with autism spectrum disorders (ASD). The expression of Shank3 ASD mutations causes impairments in ionotropic glutamate receptor‐mediated synaptic responses in neurons, which is thought to underlie ASD‐related behaviors, thereby indicating glutamatergic synaptopathy as one of the major pathogenic mechanisms. However, little is known about the functional consequences of ASD‐associated mutations in Shank3 on another important set of glutamate receptors, group I metabotropic glutamate receptors (mGluRs). Here, we further assessed how Shank3 mutations identified in patients with ASD (one de novo InsG mutation and two inherited point mutations, R87C and R375C) disrupt group I mGluR (mGluR1 and mGluR5) expression and function. To identify potential isoform‐specific deficits induced by ASD‐associated Shank3 mutations on group I mGluRs, we surface immunolabeled mGluR1 and mGluR5 independently. We also induced mGluR‐dependent synaptic plasticity (R,S‐3,5‐dihydroxyphenylglycine [DHPG]‐induced long‐term depression [LTD]) as well as N‐methyl‐D‐aspartate receptor (NMDAR)‐dependent LTD. ASD‐associated mutations in Shank3 differentially interfered with the ability of cultured hippocampal neurons to express mGluR5 and mGluR1 at synapses. Intriguingly, all ASD Shank3 mutations impaired mGluR‐dependent LTD without altering NMDAR‐dependent LTD. Our data show that the specific perturbation in mGluR‐dependent synaptic plasticity occurs in neurons expressing ASD‐associated Shank3 mutations, which may underpin synaptic dysfunction and subsequent behavioral deficits in ASD.     

Soybean isoflavone ameliorates β‐amyloid 1‐42‐induced learning and memory deficit in rats by protecting synaptic structure and function

This research aims to investigate whether soybean isoflavone (SIF) could alleviate the learning and memory deficit induced by β‐amyloid peptides 1‐42 (Aβ1‐42) by protecting the synapses of rats. Adult male Wistar rats were randomly allocated to the following groups: (1) control group; (2) Aβ1‐42 group; (3) SIF group; (4) SIF + Aβ1‐42 group (SIF pretreatment group) according to body weight. The 80 mg/kg/day of SIF was administered orally by gavage to the rats in SIF and SIF+Aβ1‐42 groups. Aβ1‐42 was injected into the lateral cerebral ventricle of rats in Aβ1‐42 and SIF+Aβ1‐42 groups. The ability of learning and memory, ultramicrostructure of hippocampal synapses, and expression of synaptic related proteins were investigated. The Morris water maze results showed the escape latency and total distance were decreased in the rats of SIF pretreatment group compared to the rats in Aβ1‐42 group. Furthermore, SIF pretreatment could alleviate the synaptic structural damage and antagonize the down‐regulation expressions of below proteins induced by Aβ1‐42: (1) mRNA and protein of the synaptophysin and postsynaptic density protein 95 (PSD‐95); (2) protein of calmodulin (CaM), Ca²⁺/calmodulin‐dependent protein kinase II (CaMK II), and cAMP response element binding protein (CREB); (3) phosphorylation levels of CaMK II and CREB (pCAMK II, pCREB). These results suggested that SIF pretreatment could ameliorate the impairment of learning and memory ability in rats induced by Aβ1‐42, and its mechanism might be associated with the protection of synaptic plasticity by improving the synaptic structure and regulating the synaptic related proteins. Synapse 67:856–864, 2013 . © 2013 Wiley Periodicals, Inc.     

Expression and localization of the calmodulin‐binding protein neurogranin in the adult mouse olfactory bulb

Neurogranin (Ng) is a brain‐specific postsynaptic protein involved in activity‐dependent synaptic plasticity through modulation of Ca²⁺/calmodulin (CaM)‐dependent signal transduction in neurons. In this study, using biochemical and immunohistochemical approaches, we demonstrate Ng expression in the adult mouse olfactory bulb (OB), the first relay station in odor information processing. We show that Ng is principally associated with the granule cell layer (GCL), which is composed of granule cell inhibitory interneurons. This cell type is continuously renewed during adult life and plays a key role in OB circuits, integrating and modulating the activity of mitral/tufted cells. Our results indicate that Ng localizes in the soma and dendrites of a defined subpopulation of mature GABAergic granule cells, enriched in the deep portion of the GCL. Ng‐immunopositive cells largely coexpress the Ca⁺/CaM‐dependent kinase IV (CaMKIV), a downstream protein of CaM signaling cascade, whereas no colocalization was observed between Ng and the calcium‐binding protein calretinin. Finally, we demonstrate that adult neurogenesis contributes to the Ng‐expressing population, with more newly generated Ng‐positive cells integrated in the deep GCL. Together, these results provide a new specific neurochemical marker to identify a subpopulation of olfactory granule cells and suggest possible functional implications for Ng in OB plasticity mechanisms. J. Comp. Neurol. 517:683–694, 2009. © 2009 Wiley‐Liss, Inc.     

Protein kinase Cɛ activity regulates mGluR5 surface expression in the rat nucleus accumbens

Type 5 metabotropic glutamate receptors (mGluR5) activate protein kinase C (PKC) via coupling to G_αq/11 protein signaling. We have previously demonstrated that the epsilon isoform of PKC (PKCɛ) is a critical downstream target of mGluR5 in regulating behavioral and biochemical responses to alcohol. Recent evidence suggests that PKC‐mediated phosphorylation of mGluR5 can lead to receptor desensitization and internalization. We therefore sought to examine the specific involvement of PKCɛ in the regulation of mGluR5 surface expression in the nucleus accumbens (NAc), a key regulator of alcohol‐associated behaviors. Coronal brain sections from male Wistar rats were analyzed for either colocalization of mGluR5 and PKCɛ via immunohistochemistry or changes in mGluR5 surface expression and PKCɛ phosphorylation following local application of PKCɛ translocation activator or inhibitor peptides and/or an orthosteric mGluR5 agonist. We observed colocalization of mGluR5 and PKCɛ in the NAc. We also showed that intra‐NAc infusion of the PKCɛ translocation inhibitor ɛV1–2 increased mGluR5 surface expression under baseline conditions. Stimulation of mGluR5 with an orthosteric agonist DHPG, dose dependently increased ERK1/2 and PKCɛ phosphorylation as well as mGluR5 internalization in acute NAc slices. Finally, we observed that activation of PKCɛ translocation with Tat‐ΨɛRACK peptide mediates agonist‐independent mGluR5 internalization, whereas PKCɛ translocation inhibitor ɛV1–2 prevents agonist‐dependent internalization of mGluR5 in NAc slice preparations. These findings suggest that the subcellular localization of mGluR5 in the NAc is regulated by PKCɛ under basal and stimulation conditions, which may influence the role of mGluR5–PKCɛ signaling in alcohol‐related behaviors. © 2016 Wiley Periodicals, Inc.     

Heterosynaptic long‐term depression mediated by ATP released from astrocytes

Heterosynaptic long‐term depression (hLTD) at untetanized synapses accompanying the induction of long‐term potentiation (LTP) spatially sharpens the activity‐induced synaptic potentiation; however, the underlying mechanism remains unclear. We found that hLTD in the hippocampal CA1 region is caused by stimulation‐induced ATP release from astrocytes that suppresses transmitter release from untetanized synaptic terminals via activation of P2Y receptors. Selective stimulation of astrocytes expressing channelrhodopsin‐2, a light‐gated cation channel permeable to Ca²⁺, resulted in LTD of synapses on neighboring neurons. This synaptic modification required Ca²⁺ elevation in astrocytes and activation of P2Y receptors, but not N‐methyl‐D ‐aspartate receptors. Furthermore, blocking P2Y receptors or buffering astrocyte intracellular Ca²⁺ at a low level prevented hLTD without affecting LTP induced by SC stimulation. Thus, astrocyte activation is both necessary and sufficient for mediating hLTD accompanying LTP induction, strongly supporting the notion that astrocytes actively participate in activity‐dependent synaptic plasticity of neural circuits. © 2012 Wiley Periodicals, Inc.     

Intracellular calcium and calmodulin link brain‐derived neurotrophic factor to p70S6 kinase phosphorylation and dendritic protein synthesis

The mammalian target of rapamycin (mTOR)/p70S6 kinase (S6K) pathway plays an important role in brain‐derived neurotrophic factor (BDNF)‐mediated protein synthesis and neuroplasticity. Although many aspects of neuronal function are regulated by intracellular calcium ([Ca²⁺]_i) and calmodulin (CaM), their functions in BDNF‐induced phosphorylation of p70S6K and protein synthesis are largely unknown. Here, we report that BDNF, via TrkB‐dependent activation of mTOR, induces sustained phosphorylation of p70S6K at Thr389 and Thr421/Ser424. BDNF‐induced phosphorylation at Thr389 was dependent on PI3 kinase but independent of ERK‐MAPK. The previously identified MAPK phosphorylation site at Thr421/Ser424 required both PI3K and MAPK in BDNF‐stimulated neurons. Furthermore, we found that the reduction in [Ca²⁺]_i, but not extracellular calcium, blocked the BDNF‐induced phosphorylation of p70S6K at both sites. Inhibition of CaM by W13 also blocked p70S6K phosphorylation. In correlation, W13 inhibited BDNF‐induced local dendritic protein synthesis. Interestingly, sustained elevation of [Ca²⁺]_i by membrane depolarization antagonized the BDNF‐induced p70S6K phosphorylation. Finally, the BDNF‐induced p70S6K phosphorylation did not require the increase of calcium level through either extracellular influx or PLC‐mediated intracellular calcium release. Collectively, these results indicate that the basal level of intracellular calcium gates BDNF‐induced activation of p70S6K and protein synthesis through CaM. © 2009 Wiley‐Liss, Inc.     

Long‐term depression of inhibitory synaptic transmission induced by spike‐timing dependent plasticity requires coactivation of endocannabinoid and muscarinic receptors

The precise timing of pre‐postsynaptic activity is vital for the induction of long‐term potentiation (LTP) or depression (LTD) at many central synapses. We show in synapses of rat CA1 pyramidal neurons in vitro that spike timing dependent plasticity (STDP) protocols that induce LTP at glutamatergic synapses can evoke LTD of inhibitory postsynaptic currents or STDP‐iLTD. The STDP‐iLTD requires a postsynaptic Ca²⁺ increase, a release of endocannabinoids (eCBs), the activation of type‐1 endocananabinoid receptors and presynaptic muscarinic receptors that mediate a decreased probability of GABA release. In contrast, the STDP‐iLTD is independent of the activation of nicotinic receptors, GABA_BRs and G protein‐coupled postsynaptic receptors at pyramidal neurons. We determine that the downregulation of presynaptic Cyclic adenosine monophosphate/protein Kinase A pathways is essential for the induction of STDP‐iLTD. These results suggest a novel mechanism by which the activation of cholinergic neurons and retrograde signaling by eCBs can modulate the efficacy of GABAergic synaptic transmission in ways that may contribute to information processing and storage in the hippocampus. © 2013 Wiley Periodicals, Inc.     

Anti-apoptotic actions of vasopressin in H32 neurons involve MAP kinase transactivation and Bad phosphorylation

Vasopressin (VP) secreted within the brain modulates neuronal function acting as a neurotransmitter. Based on the observation that VP prevented serum deprivation-induced cell death in the neuronal cell line, H32, which expresses endogenous V1 receptors, we tested the hypothesis that VP has anti-apoptotic properties. Flow cytometry experiments showed that 10 nM VP prevented serum deprivation-induced cell death and annexin V binding. Serum deprivation increased caspase-3 activity in a time and serum concentration dependent manner, and VP prevented these effects through interaction with receptors of V1 subtype. The signaling pathways mediating the anti-apoptotic effect of VP involve mitogen activated protein (MAP) kinase and extracellular signal-regulated kinases (ERK), Ca²⁺/calmodulin dependent kinase (CaMK) and protein kinase C (PKC). Western blot analyses revealed time-dependent decreases of Bad phosphorylation and increases in cytosolic levels of cytochrome c following serum deprivation, effects which were prevented by 10 nM VP. These data demonstrate that activation of endogenous V1 VP receptors prevents serum deprivation-induced apoptosis, through phosphorylation-inactivation of the pro-apoptotic protein, Bad, and consequent decreases in cytosolic cytochrome c and caspase-3 activation. The data suggest that VP has anti-apoptotic activity in neurons and that VP may act as a neuroprotective agent in the brain.     

Nitric-oxide-guanylyl-cyclase-dependent and -independent components of multiple forms of long-term synaptic depression

Long-term depression (LTD) of synaptic strength is induced by glutamate-triggered increases in postsynaptic [Ca²⁺], through either influx or release from intracellular stores. Induction of LTD has also been reported to require release of Ca²⁺ from presynaptic stores and activation of presynaptic Ca²⁺/calmodulin-dependent protein kinase II. This finding leads to the hypothesis that the intercellular messenger nitric oxide (NO) may be a means by which postsynaptic Ca²⁺ triggers changes expressing LTD in presynaptic terminals. We report that bath application of the oxadiazoloquinoxalone derivative ODQ (5 μM), a selective inhibitor of NO-sensitive guanylyl cyclase (NOGC), markedly attenuated (90%) the magnitude of LTD induced by low-frequency stimulation (LFS; 1 Hz/15 min) of Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Both the NO donor S-nitroso-N-acetylpenicillamine (100 μM) and the membrane-permeant cyclic guanine 3′,5′-monophosphate (cGMP) analogue 8-(4-chlorophenylthio) guanosine (8-pCPT)-cGMP (50 μM) enhanced the magnitude of LTD, which is consistent with the hypothesis that activation of NOGC plays a role in the induction of LTD. Nicotinamide (20 mM), an inhibitor of NO-activated ADP ribosyltransferase, did not impair the induction of LTD. In contrast to de novo LTD, the reversal of long-term potentiation by LFS (depotentiation) was only partially blocked (55%) by ODQ, and heterosynaptic LTD was not impaired at all, suggesting that there are both NOGC-dependent and -independent forms of LTD. Because postsynaptic intracellular infusion of ODQ (500 μM) failed to block the induction of LTD, we conclude that activation of presynaptic NOGC is a necessary step in the induction of an NOGC-dependent component of LTD. Hippocampus 7:286–295, 1997. © 1997 Wiley-Liss, Inc.