Tsai-Hsien Chiu

Spike‐timing‐dependent plasticity at resting and conditioned lateral perforant path synapses on granule cells in the dentate gyrus: different roles of N‐methyl‐d‐aspartate and group I metabotropic glutamate receptors

We examined the mechanisms underlying spike‐timing‐dependent plasticity induction at resting and conditioned lateral perforant pathway (LPP) synapses in the rat dentate gyrus. Two stimulating electrodes were placed in the outer third of the molecular layer and in the granule cell layer in hippocampal slices to evoke field excitatory postsynaptic potentials (fEPSPs) and antidromic field somatic spikes (afSSs), respectively. Long‐term potentiation (LTP) of LPP synapses was induced by paired stimulation with fEPSP preceding afSS. Reversal of the temporal order of fEPSP and afSS stimulation resulted in long‐term depression (LTD). Induction of LTP or LTD was blocked by d,l‐2‐amino‐5‐phosphonopentanoic acid (AP5), showing that both effects were N‐methyl‐d‐aspartate receptor (NMDAR)‐dependent. Induction of LTP was also blocked by inhibitors of calcium–calmodulin kinase II, protein kinase C or mitogen‐activated/extracellular‐signal regulated kinase, suggesting that these are downstream effectors of NMDAR activation, whereas induction of LTD was blocked by inhibitors of protein kinase C and protein phosphatase 2B. At LPP synapses previously potentiated by high‐frequency stimulation or depressed by low‐frequency stimulation, paired fEPSP–afSS stimulation resulted in ‘de‐depression’ at depressed LPP synapses but had no effect on potentiated synapses, whereas reversal of the temporal order of fEPSP–afSS stimulation resulted in ‘de‐potentiation’ at potentiated synapses but had no effect on depressed synapses. Induction of de‐depression and de‐potentiation was unaffected by ap5 but was blocked by 2‐methyl‐6‐(phenylethynyl) pyridine hydrochloride, a group I metabotropic glutamate receptor blocker, showing that both were NMDAR‐independent but group I metabotropic glutamate receptor‐dependent. In conclusion, our results show that spike‐timing‐dependent plasticity can occur at both resting and conditioned LPP synapses, its induction in the former case being NMDAR‐dependent and, in the latter, group I metabotropic glutamate receptor‐dependent.

Striatal glutamate release is important for development of ischemic damage to striatal neurons during rat heatstroke

This study attempted to ascertain whether heatstroke-induced ischemia is associated with augmented striatal glutamate release and can be attenuated by NMDA receptor antagonists. Mean arterial pressure (MAP), striatal cerebral blood flow (CBF), striatal glutamate release and striatal neuronal damage score were assessed in saline-treated rats and in rats treated with NMDA receptor antagonists. Heatstroke was induced by exposing the animals to a high ambient temperature; the moment at which MAP and CBF began to decrease from their peak levels was taken as the onset of heatstroke. During onset of heatstroke, rats displayed higher values of colonic temperature, striatal glutamate release and striatal neuronal damage score, and lower values of MAP and striatal blood flow compared with normothermic control rats. The decreased MAP, the diminished striatal blood flow, the augmented striatal glutamate release and the increased striatal neuronal damage score during onset of heatstroke were significantly attenuated by pretreatment with an NMDA receptor antagonist such as MK-801 or ketamine. In addition, the survival time (interval between onset of heatstroke and death) of the rats was extended by pretreatment with one of these two NMDA receptor antagonists. These results suggest that marked accumulation of glutamate in the striatum is important for the development of ischemic damage to striatal neurons during the onset of heatstroke.

HYPOXIA-INDUCED DIFFERENTIAL ELECTROPHYSIOLOGICAL CHANGES IN RAT LOCUS COERULEUS NEURONS

The effects of hypoxia on rat locus coeruleus (LC) neurons were investigated by intracellular recording from in vitro brain slices. In response to a brief exposure to hypoxic medium (2-5 min), equilibrated with 95% N2 - 5% CO2, two populations of cells could be distinguished, type 1 neurons (61%), showing hyperpolarization (9.3 +/- 0.4 mV, n = 125) and cessation of spontaneous action potentials, and type 2 neurons (39%), displaying gradual pure depolarization (6.0 +/- 0.3 mV, n = 80), instead of hypoxic hyperpolarization. Both types of response were associated with a reduction in membrane input resistance (34 +/- 1% for type 1 cells, n = 125, and 21 +/- 2% for type 2 cells, n = 68). While both types of neurons share similar electrophysiological properties, their membrane input resistance differ significantly (type 1 cells: 144 +/- 5 M omega, n = 125; type 2 cells: 183 +/- 9 M omega, n = 80, p < 0.001). These responses were compared to cyanide-induced chemical hypoxia. Cyanide (2 mM) induced the identical membrane response as effected by nitrogen hypoxia. All cells which responded to nitrogen-saturated hypoxic medium with a pure depolarizing response gave a similar response to cyanide and all neurons hyperpolarized by cyanide were also hyperpolarized by hypoxic medium. Moreover, the K(ATP) channel opener, diazoxide (1 mM), could mimic the hypoxia-induced hyperpolarization in type 1 neurons (10.6 +/- 0.9 mV, n = 18), but was unable to induce hyperpolarization in type 2 cells (n = 13). In addition, the N2-hypoxia-induced hyperpolarization was completely blocked by tolbutamide (200 microM, n = 8) or glibenclamide (3 microM, n = 9). These results indicate that a brief period of hypoxia evokes two different responses in LC neurons and this may be due to the heterogeneous distribution of K(ATP) channels among different LC neurons.

Heat-shock pretreatment prevents suppression of long-term potentiation induced by scopolamine in rat hippocampal CA1 synapses.

We examined the effect of heat-shock pretreatment on long-term potentiation (LTP) in the CA1 hippocampal slices of the rat using the muscarinic blocker scopolamine as the LTP (memory) suppressor. Time course study using immunohistochemical techniques indicated peak expression of HSP70 16 h after heat-shock treatment. Focusing on that time point we found tetanic stimulation (at 100 Hz) induced LTP of 191.1+/-12.2% in control slices (n=7), which was suppressed by scopolamine to 114.5+/-2.8 %. Heat-shock pretreatment successfully prevented such suppression (216.6+/-38.2% and 190.2+/-10.6% with and without scopolamine, respectively, n=7). Both HSP expression and LTP responses were relatively small taken either 2 or 48 h after heat-shock or sham pretreatment. These results suggest that the induction of HSPs is time-dependent and can prevent scopolamine-mediated LTP suppression.

Bidirectional synaptic plasticity induced by conditioned stimulations with different number of pulse at hippocampal CA1 synapses: Roles ofN‐methyl‐D‐aspartate and metabotropic glutamate receptors

In the mammlian brain, the hippocampus has been established as a principle structure for learning and memory processes, which involve synaptic plasticity. Althought a relationship between synaptic plasticity and stimulation frequency has been reported in numerous studies, little is known about the importance of pulse number on synaptic plasticity. Here we investigated whether the pulse number can modulate bidirectional plasticity in hippocampal CA1 areas. When a CA1 area was induced by a paired‐pulse (PP) with a 10‐ms interval, the strength of the synapse was altered to form a long‐term depression (LTD), with a 68 ± 4% decrease in expression. The PP‐induced LTD (PP‐LTD) was blocked by the metabotropic glutamate receptors subtype 5 (mGluR5) antagonist MPEP, suggesting that the PP‐LTD relied on the activation of GluR5. In addition, this modulation of LTD was protein kinase C (PKC)‐ and Group II mGluR‐independent. However, when increasing the pulse number to 4 and 6, potentiated synaptic strength was observed, which was N‐methyl‐D‐aspartate receptor (NMDAR)‐dependent but mGluR5‐independent. Surprisingly, when blocking mGluR, the synaptic efficacy induced by triple‐pulse stimulation was altered to form a long‐term potentiation (LTP) with a 142 ± 7% enhancement, and was further blocked by NMDA antagonist APV. Following treatment with APV and PKC blocker chelerythrine, the LTP expression induced by 4‐ and 6‐pulse stimulation was switched to LTD. We suggest that CA1 synaptic plasticity is regulated by the result of competition between NMDA and mGluR5 receptors. We suggest that the pulse number can bidirectionally modulate synaptic plasticity through the activation of NMDA and mGluR5 in hippocampal CA1 areas. Synapse , 2011.

Presynaptic adenosine A1 receptors modulate excitatory synaptic transmission in the posterior piriform cortex in rats.

The effect of adenosine on the fEPSP was examined in the lateral olfactory tract (Ia input) and associative tract (Ib input) of the rat piriform cortex. The fEPSP evoked in the Ia input showed paired-pulse facilitation, while that in the Ib input showed paired-pulse depression, suggesting a lower resting release probability in the Ia input. This was supported by results showing that MK801 blocked the NMDA receptor-induced fEPSP more rapidly in the Ib input than the Ia input. Adenosine caused dose-dependent inhibition of the fEPSP in both inputs, the sensitivity being higher in the Ib input. This effect was mimicked by the A(1) receptor agonist, CHA, and antagonized by co-application of the A(1) receptor antagonist, DPCPX, showing that adenosine was acting at A(1) receptors. Application of DPCPX alone caused an increase in the fEPSP, the increase being larger in the Ia input. DPCPX also caused paired-pulse depression in both inputs, and the paired-pulse ratio measured in its presence was very similar in both inputs. These results suggest there is a lower endogenous concentration of adenosine in the Ib sublayer than the Ia sublayer, which might account for the native difference in the resting release probability of the two inputs. The adenosine-induced inhibition of the fEPSP in both inputs was associated with a significant reduction in the rate at which MK801 blocked NMDA receptor-mediated fEPSP activity, suggesting a presynaptic location of the A(1) receptors. Blocking of N-, P/Q-type calcium channels occluded the inhibition by adenosine, indicating that they are downstream effectors of presynaptic A(1) receptor activation.

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus.

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC(50) (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [(3)H]rauwolscine binding assay. When treated with acute N(2)-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

(-)Epigallocatechin-3-gallate inhibits the spontaneous firing of rat locus coeruleus neuron.

(-)Epigallocatechin-3-gallate (EGCG), a tea catechin, has been known to cause many biological actions, such as anxiolytic and hypotensive effects in behavioral studies. However, to date, few reports investigate its neuronal modulation. In this study, intracellular recording was used to test the neuronal modulation of different catechins on locus coeruleus (LC) neuron, which has been demonstrated to be affected by cardiovascular function regulation and stressful events. Several catechins (1 -- 1,000 microM) were tested, including: (-)catechin (C), (-)catechingallate (CG), (-)epicatechin (EC), (-)epicatechin-3-gallate (ECG), (?)epigallocatechin (EGC) and EGCG. The results showed that catechins EC, ECG, EGC and EGCG could inhibit the spontaneous firing of the LC neurons; furthermore, these catechins show potency and efficacy in the order of EGCG>ECG>EC approximately EGC. Among the tested catechins, EGCG was the most potent in inhibiting LCs spontaneous firing with IC(50) of 20.5 microM. This caused us to further examine the EGCGs desensitization and tolerance properties. When continuously administering EGCG at 1 -- 300 microM for 20 min, no acute desensitization appeared. However, repeated applications of 300 microM EGCG at 5 min each time showed different results. The second and third applications induced less responses compared to that of the first application, suggesting a development of tolerance towards EGCG in inhibiting LC neuronal activity. Our data suggest that EGCG can inhibit LC neurons spontaneous firing in a dose-dependent manner, with developed tolerance only when high concentration of EGCG is repeatedly applied.