Kuei Sen Hsu

A Critical Period for Enhanced Synaptic Plasticity in Newly Generated Neurons of the Adult Brain

Active adult neurogenesis occurs in discrete brain regions of all mammals and is widely regarded as a neuronal replacement mechanism. Whether adult-born neurons make unique contributions to brain functions is largely unknown. Here we systematically characterized synaptic plasticity of retrovirally labeled adult-born dentate granule cells at different stages during their neuronal maturation. We identified a critical period between 1 and 1.5 months of the cell age when adult-born neurons exhibit enhanced long-term potentiation with increased potentiation amplitude and decreased induction threshold. Furthermore, such enhanced plasticity in adult-born neurons depends on developmentally regulated synaptic expression of NR2B-containing NMDA receptors. Our study demonstrates that adult-born neurons exhibit the same classic critical period plasticity as neurons in the developing nervous system. The transient nature of such enhanced plasticity may provide a fundamental mechanism allowing adult-born neurons within the critical period to serve as major mediators of experience-induced plasticity while maintaining stability of the mature circuitry.

Presynaptic mechanisms underlying cannabinoid inhibition of excitatory synaptic transmission in rat striatal neurons

1 The striatum is a crucial site of action for the motor effects of cannabinoids (CBs). However, the electrophysiological consequences of activation of CB receptors on the striatal neurons have not been established. Here we report for the first time that the cannabimimetic aminoalkylindole WIN 55,212‐2 and the endogenous cannabinoid anandamide substantially depress corticostriatal glutamatergic synaptic transmission onto striatal neurons in the brain slice preparation. The selective CB1 receptor antagonist SR 141716 effectively reversed this inhibition. 2 WIN 55,212‐2 significantly increased the paired‐pulse facilitation of synaptically evoked EPSCs, while having no effect on the sensitivity of postsynaptic neurons to α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid. WIN 55,212‐2 also reduced the frequency of spontaneous, action potential‐dependent EPSCs (sEPSCs) without altering their amplitude distribution. 3 Superfusion of WIN 55,212‐2 elicited a membrane hyperpolarization accompanied by a decrease in input resistance. Both effects were blocked by intracellular caesium. In contrast, intracellular caesium failed to affect WIN 55,212‐2‐mediated synaptic inhibition. 4 The WIN 55,212‐2‐mediated synaptic inhibition was blocked by the Gi/o protein inhibitor pertussis toxin (PTX), but not by the GABAA receptor antagonist bicuculline or GABAB receptor antagonist SCH 50911. 5 Pretreatment with the N‐type Ca2+ channel antagonist ω‐conotoxin GVIA selectively abolished the WIN‐55,212‐2‐mediated synaptic inhibition. 6 These results suggest that cannabinoids depress the corticostriatal glutamatergic synaptic transmission through the activation of presynaptic CB1 receptors to inhibit N‐type Ca2+ channel activity, which in turn reduces glutamate release. The presynaptic action of cannabinoids is mediated by a PTX‐sensitive Gi/o protein‐coupled signalling pathway.

NADPH Oxidase–Derived Superoxide Anion Mediates Angiotensin II–Induced Pressor Effect via Activation of p38 Mitogen–Activated Protein Kinase in the Rostral Ventrolateral Medulla

The rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons are located, is a central site via which angiotensin II (Ang II) elicits its pressor effect. We tested the hypothesis that NADPH oxidase-derived superoxide anion (O2·−) in the RVLM mediates Ang II–induced pressor response via activation of mitogen-activated protein kinase (MAPK) signaling pathways. Bilateral microinjection of Ang II into the RVLM resulted in an angiotensin subtype 1 (AT1) receptor-dependent phosphorylation of p38 MAPK and extracellular signal-regulated protein kinase (ERK)1/2, but not stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK), in the ventrolateral medulla. The Ang II–induced p38 MAPK or ERK1/2 phosphorylation was attenuated by application into the RVLM of a NADPH oxidase inhibitor, diphenyleneiodonium chloride (DPI), an antisense oligonucleotide that targets against p22phox or p47phox subunit of NADPH oxidase mRNA, or the superoxide dismutase mimetic tempol. DPI or antisense p22phox or p47phox oligonucleotide treatment also attenuated the AT1 receptor-dependent increase in O2·− production in the ventrolateral medulla elicited by Ang II at the RVLM. Functionally, Ang II–elicited pressor response in the RVLM was attenuated by DPI, tempol, or a p38 MAPK inhibitor, SB203580. The AT1 receptor-mediated enhancement of the frequency of glutamate-sensitive spontaneous excitatory postsynaptic currents induced by Ang II in RVLM neurons was also abolished by SB203580. These results suggest that NADPH oxidase-derived O2·− underlies the activation of p38 MAPK or ERK1/2 by Ang II in the ventrolateral medulla. Furthermore, the p38 MAPK signaling pathway may mediate Ang II–induced pressor response via enhancement of presynaptic release of glutamate to RVLM neurons.

Rap1-induced p38 Mitogen-activated Protein Kinase Activation Facilitates AMPA Receptor Trafficking via the GDI·Rab5 Complex POTENTIAL ROLE IN (S)-3,5-DIHYDROXYPHENYLGLYCINE-INDUCED LONG TERM DEPRESSION

Recent evidence has emphasized the importance of p38 mitogen-activated protein kinase (MAPK) in the induction of metabotropic glutamate receptor (mGluR)-dependent long term depression (LTD) at hippocampal CA3-CA1 synapses. However, the cascade responsible of mGluR to activate p38 MAPK and the signaling pathway immediately downstream from it to induce synaptic depression is poorly understood. Here, we show that transient activation of group I mGluR with the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) activates p38 MAPK through G protein βγ-subunit, small GTPase Rap1, and MAPK kinase 3/6 (MKK3/6), thus resulting in mGluR5-dependent LTD. Furthermore, our data clearly show that an accelerating AMPA receptor endocytosis by stimulating the formation of guanyl nucleotide dissociation inhibitor-Rab5 complex is a potential downstream processing of p38 MAPK activation to mediate DHPG-LTD. These results suggest an important role for Rap1-MKK3/6-p38 MAPK pathway in the induction of mGluR-dependent LTD by directly coupling to receptor trafficking machineries to facilitate the loss of synaptic AMPA receptors.

Behavioral Stress Enhances Hippocampal CA1 Long-Term Depression through the Blockade of the Glutamate Uptake

Behavioral stress has been shown to enhance long-term depression (LTD) in the CA1 region of the hippocampus, but the underlying mechanisms remain unclear. In the present study, we found that selectively blocking NR2B-containing NMDA receptors (NMDARs) abolishes the induction of LTD by prolonged low-frequency stimulation (LFS) in slices from stressed animals. Additionally, there is no need to activate NR2A-containing or synaptic NMDARs to induce this LTD, suggesting that LTD observed in slices from stressed animals is triggered primarily by extrasynaptic NMDAR activation. In contrast, stress has no effect on LTD induced by either a brief bath application of NMDA or a combination of LFS with the glutamate-uptake inhibitor dl-threo-β-benzyloxyaspartate (dl-TBOA). Furthermore, saturation of LFS-induced LTD in slices from stressed animals occludes the subsequent induction of LTD by LFS in the presence of dl-TBOA. We also found that stress induces a profound decrease in the glutamate uptake in the synaptosomal fraction of the hippocampal CA1 region. These effects were prevented when the animals were given a glucocorticoid receptor antagonist, 11β,17β-11[4-(dimethylamino)phenyl]-17-hydroxy-17-(1-(propynyl)-estra-4,9-dien-3-one, before experiencing stress. These results suggest that the blockade of glutamate uptake is a potential mechanism underlying the stress-induced enhancement of LTD and point to a novel role for glutamate-uptake machinery in the regulation of synaptic plasticity induction.

Behavioral Stress Modifies Hippocampal Synaptic Plasticity through Corticosterone-Induced Sustained Extracellular Signal-Regulated Kinase/Mitogen-Activated Protein Kinase Activation

The induction of hippocampal long-term synaptic plasticity is exquisitely sensitive to behavioral stress, but the underlying mechanisms are still unclear. We report here that hippocampal slices prepared from adult rats that had experienced unpredictable and inescapable restraint tail-shock stress showed marked impairments of long-term potentiation (LTP) in the CA1 region. The same stress promoted the induction of long-term depression (LTD). These effects were prevented when the animals were given the glucocorticoid receptor antagonist 11β, 17β-11[4-(dimethylamino)phenyl]-17-hydroxy-17-(1-propynyl)-estra-4-9-dien-3-one before the stress. Immunoblotting analyses revealed that stress induced a profound and prolonged extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK1/2 MAPK) hyperphosphorylation through small GTPase Ras, Raf-1, and MAPK kinase 1/2 (MEK1/2). Furthermore, the stress effects were obviated by the intrahippocampal injection of specific inhibitors of MEK1/2 (U0126), protein kinase C (bisindolylmaleimide I), tyrosine kinase (K252a), and BDNF antisense oligonucleotides. These results suggest that the effects of stress on LTP and LTD originate from the corticosterone-induced sustained activation of ERK1/2-coupled signaling cascades.

Characterization of the Mechanism Underlying the Reversal of Long Term Potentiation by Low Frequency Stimulation at Hippocampal CA1 Synapses

Reversal of long term potentiation (LTP) may function to increase the flexibility and storage capacity of neuronal circuits; however, the underlying mechanisms remain incompletely understood. We show that depotentiation induced by low frequency stimulation (LFS) (2 Hz, 10 min, 1200 pulses) was input-specific and dependent onN-methyl-d-aspartate (NMDA) receptor activation. The ability of LFS to reverse LTP was mimicked by a brief application of NMDA. This NMDA-induced depotentiation was blocked by adenosine A1 receptor antagonist. However, the reversal of LTP by LFS was unaffected by metabotropic glutamate receptor antagonism. This LFS-induced depotentiation was specifically prevented by protein phosphatase (PP)1 inhibitors, okadaic acid, and calyculin A but not by the PP2A or PP2B inhibitors. Furthermore, by using phosphorylation site-specific antibodies, we found that LFS-induced depotentiation is associated with a persistent dephosphorylation of the GluR1 subunit of amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor at serine 831, a protein kinase C and calcium/calmodulin-dependent protein kinase II (CaMKII) substrate, but not at serine 845, a substrate of cAMP-dependent protein kinase. This effect was mimicked by bath-applied adenosine or NMDA and was specifically prevented by okadaic acid. Also, the increased phosphorylation of CaMKII at threonine 286 and the decreased PP activity seen with LTP were overcome by LFS, adenosine, or NMDA application. These results suggest that LFS erases LTP through an NMDA receptor-mediated activation of PP1 to dephosphorylate amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and CaMKII in the CA1 region of the hippocampus.

Presynaptic D2 dopaminergic receptors mediate inhibition of excitatory synaptic transmission in rat neostriatum

The effect of dopamine (DA) on excitatory synaptic transmission was studied in rat neostriatal neurons using intracellular- and whole-cell voltage clamp-recording methods. Depolarizing excitatory postsynaptic potentials (EPSPs) were evoked by cortical stimulation. Superfusion of DA (0.01-10 microM) reversibly decreases EPSP in a concentration-dependent manner and with a estimated IC50 of 0.3 microM. In addition, the inhibitory effect induced by DA at a low concentration (0.1 microM) was antagonized by sulpiride (1-10 nM), a selective D2 dopaminergic receptor antagonist. However, D1 dopaminergic receptor antagonist SKF-83566 (1-5 microM) did not affect the blocking effect by DA 0.1 microM. Based on these findings, we conclude that DA at a low concentration (< or = 0.1 microM) reduced the excitatory response of neostriatal neurons following cortical stimulation via the activation of D2, but not D1 dopaminergic receptors, located on the terminals of corticostriatal neurons.

Estrogen modulates sexually dimorphic contextual fear extinction in rats through estrogen receptor β

Females and males are different in brain and behavior. These sex differences occur early during development due to a combination of genetic and hormonal factors and continue throughout the lifespan. Previous studies revealed that male rats exhibited significantly higher levels of contextual fear memory than female rats. However, it remains unknown whether a sex difference exists in the contextual fear extinction. To address this issue, male, normally cycling female, and ovariectomized (OVX) female Sprague‐Dawley rats were subjected to contextual fear conditioning and extinction trials. Here we report that although male rats exhibited higher levels of freezing than cycling female rats after contextual fear conditioning, female rats subjected to conditioning in the proestrus and estrus stage exhibited an enhancement of fear extinction than male rats. An estrogen receptor (ER) β agonist diarylpropionitrile but not an ERα agonist propyl‐pyrazole‐triol administration also enhanced extinction of contextual fear in OVX female rats, suggesting that estrogen‐mediated facilitation of extinction involves the activation of ERβ. Intrahippocampal injection of estradiol or diarylpropionitrile before extinction training in OVX female rats remarkably reduced the levels of freezing response during extinction trials. In addition, the locomotion or anxiety state of female rats does not vary across the ovarian cycle. These results reveal a crucial role for estrogen in mediating sexually dimorphic contextual fear extinction, and that estrogen‐mediated enhancement of fear extinction involves the activation of ERβ.

Inhibition of N-type calcium currents by lamotrigine in rat amygdalar neurones

LAMOTRIGINE (LAG) is a new anticonvulsant drug for the treatment of partial and secondarily generalized seizures. The present study was aimed at elucidating the possible involvement of Ca2+ channels in the action of LAG using whole-cell patch clamp recordings in acutely dissociated amygdalar neurones. Whole-cell Ca2+ currents (ICa) were elicited by 200 ms step commands from −70 mV to −10 mV. Application of LAG reduced the ICa by an average of 40.3 ± 3.2%. The inhibition of ICa by LAG was markedly reduced or eliminated in the presence of the N-type Ca2+ channel blocker ω-cono-toxin-GVIA (1 μM). These results suggest that LAG may exert its anticonvulsant effect through inhibition of presynaptic N-type Ca2+ channels, thereby reducing glutamate release.