岡田 隆

Circadian changes of performance have been reported in various kinds of learning task. However, the diurnal variation of performance in hippocampus-dependent learning tasks remains unclear. In the present study, rats were subjected to the novel location recognition (NLR) task as well as the novel object recognition (NOR) task to examine whether the circadian pattern of hippocampus-dependent task performance was similar to that in tasks in which brain regions other than the hippocampus contribute. The performance in the NOR task was relatively constant irrespective of the time of day, while the performance in the NLR task was higher at night than during the daytime. When the pineal hormone melatonin was injected into rats before the training phase in order to examine its effects on the pattern of circadian changes of NLR performance, rats showed improvement of performance in the daytime, but impairment at night. These results suggest that the pattern of circadian variation of memory performance depends on the type of task, and that the effects of exogenous melatonin on learning performance vary with the time of day. © 2013 Elsevier B.V.

Bilobalide, a unique constituent of Ginkgo biloba, has been reported to potentiate population spikes in hippocampal CA1 pyramidal cells and to protect the brain against cell death. In this study, the effects of bilobalide on synaptic transmission and its plasticity in rat hippocampal subfields were electrophysiologically investigated. Bilobalide (50 mu M) significantly potentiated the input-output relationship at Schaffer collateral (SC)-CA1 synapses but not at medial perforant path (MPP)-dentate gyrus (DG), lateral perforant path (LPP)-DG, or mossy fiber (MF)-CA3 synapses. Facilitative effects of bilobalide on synaptic plasticity were only observed at MPP-DG synapses, in which the induction of long-term depression was blocked in the presence of bilobalide. However, no effect on synaptic plasticity was observed at SC-CA1 synapses. These results suggest that bilobalide has differential effects on synaptic efficacy in each hippocampal subfield.

Hippocampal long-term potentiation (LIP) is reportedly reduced in the presence of melatonin, but the cellular mechanisms of LIP inhibition by melatonin remain unclear. Since melatonin has the ability to scavenge free radicals such as nitric oxide (NO) and since NO has been suggested as an important contributor to LIP induction, in the present study we electrophysiologically examined whether melatonin inhibits hippocampal LIP by way of the NO signaling pathway. Field EPSP at Schaffer collateral - CA1 pyramidal cell synapses were recorded, and LIP was induced by tetanic stimulation (100 Hz, 1 s). Melatonin (100 nM) reduced the degree of LIP, and L-NAME (100 mu M), an inhibitor of NO synthase, also reduced LTP, but simultaneous application of melatonin and L-NAME did not evoke any additional reduction of LIP in comparison with when only melatonin or only L-NAME were applied. Furthermore, the inhibition of LIP by the application of melatonin and L-NAME was disrupted by the application of an NO donor, DEA/NO (3 mu M). The paired-pulse facilitation ratios before and after LIP induction by tetanic stimulation were nearly identical in the absence and presence of L-NAME. These results demonstrate that the inhibition of LIP in the presence of melatonin is due to the action of melatonin on the postsynaptic NO signaling pathway. (C) 2010 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.

In various neuroscientific studies the hippocampus is reported to play crucial roles in declarative memory. The hippocampus is composed of several synaptic field subdivisions, such as the dentate gyrus, CA3 and CA1. Synaptic plasticity in these areas is thought to be a candidate for the biological basis of memory function. In the last decade there have been efforts to elucidate the memory functions of each synaptic subfield by using gene-manipulated animals and recombinant viral vector-infected animals. The underlying circuit mechanisms have been studied by electrophysiological recordings from the hippocampus of the animals. High spatial resolution functional MRI must be used to clearly define the activated areas within the hippocampus of the living human brain, and studies using a high resolution imaging system for that purpose have recently begun to appear. The prospects and challenges for future integration of animal and human studies are discussed.

Potassium ion channel blockade by tetraethyl ammonium (TEA) reportedly induces long-term potentiation (LTP) at hippocampal mossy fiber (MF)-CA3 synapses, but the characteristics of induction, expression, and modulation of the LTP remain unclear. In the present study, these features of TEA-induced LTP at MF-CA3 synapses were electrophysiologically examined using rat hippocampal slices. Synaptic responses recorded from MF-CA3 synapses were enhanced long-term by TEA application even under the blockade of NMDA receptors with D-AP5, whereas selective pharmacological blockade of T-type voltage-dependent calcium channels (VDCCs) strongly inhibited TEA-induced LTP. Decrease of the paired-pulse facilitation ratio after LTP induction by TEA suggests the involvement of increased neurotransmitter release probability from MF terminals as LTP expression. The facilitative modulation of MF-CA3 LTP by GABA(A) receptor activation reported previously was reversed when bumetanide, a blocker of Na(+)-K(+)-Cl(-) co-transporters (NKCCs), was applied, suggesting that the region-specific modulation of TEA-induced LTP by GABAergic inputs at MF-CA3 synapses is due to the dominance of NKCC action at MF terminals. (C) 2008 Elsevier B.V. All rights reserved.

Tetraethylarnmonium (TEA), a K+-channel blocker, reportedly induces long-term potentiation (LTP) of hippocampal CA1 synaptic responses, but at CA3 and the dentate gyrus (DG), the characteristics of TEA-induced plasticity and modulation by inhibitory interneurons remain unclear. This study recorded field EPSPs from CA1, CA3 and DG to examine the involvement of GABAergic modulation in TEA-induced synaptic plasticity foreach region. In Schaffer collateral-CA1 synapses and associational fiber (AF)-CA3 synapses, bath application of TEA-induced UP in the presence and absence of picrotoxin (PTX), a GABA(A) receptor blocker, whereas TEA-induced UP at mossy fiber (MF)-CA3 synapses was detected only in the absence of GABAA receptor blockers. MF-CA3 UP showed sensitivity to Ni2+, but not to nifedipine. In DG, synaptic plasticity was modulated by GABAergic inputs, but characteristics differed between the afferent lateral perforant path (LPP) and medial perforant path (MPP). LPP-DG synapses showed TEA-induced UP during PTX application, whereas at MPP-DG synapses, TEA-induced long-term depression (LTD) was seen in the absence of PTX. This series of results demonstrates that TEA-induced DG and CA3 plasticity displays afferent specificity and is exposed to GABAergie modulation in an opposite manner. (C) 2007 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.

NMDA receptor-dependent long-term potentiation (LTP) at hippocampal synapses has been considered a crucial component of the cellular basis for learning and memory. This form of LTP occurs in excitatory synapses in both the CA1 area and the dentate gyrus in the hippocampus. However, differential roles of LTP in these areas have not yet been identified. To address this issue, we enhanced the degree of LTP by expressing Ca2+ -permeable AMPA receptors at either hippocampal CA1 or dentate gyrus synapses using Sindbis viral vectors (SINs) encoding both green fluorescent proteins and unedited GluR2 (GluR2Q) subunits, and examined their effects on rat spatial learning. The viral vectors were locally injected into the 8-week-old-rat brain in vivo bilaterally. The postsynaptic expression of Ca2+ -permeable AMPA receptors enhanced the degree of LTP, and induced NMDA receptor-independent LTP in the presence of the NMDA receptor antagonist in SIN-infected regions in both CA1 and dentate gyrus in hippocampal slice preparations. However, the regional expression of Ca2+ -permeable AMPA receptors caused opposite behavioural consequences on the Morris water maze task: rats with SIN-infected CA1 pyramidal cells showed shorter escape latency and better probe test performance, whereas those with SIN-infected dentate gyrus granule cells showed impaired performance. Thus, it was demonstrated that CA1 and dentate gyrus synapses play different functional roles in spatial learning despite their similar mechanism for LTP induction.

Gene manipulation in order to artificially express a particular gene in neurons in the central nervous system is a powerful tool for the analysis of brain function. Sindbis viral vectors have been developed to express high levels of foreign genes in postmitotic brain neurons with little transfection of glial cells. In this study, we expressed the gene encoding the unedited GluR2 (GluR-B) subunit of the AMPA-type glutamate receptor that forms inwardly rectifying and Ca2+-permeable channels, in rat CA1 hippocampal neurons in slice cultures using Sindbis viral vectors. The pyramidal cell layer of the CA1 region was injected with recombinant Sindbis viruses encoding both enhanced green fluorescent protein (GFP) and unedited GluR2. The GFP fluorescence from CA1 neurons could be detected as early as 6 h and reached a maximal level about 48 h postinfection. The inwardly rectifying and Ca2+-permeable AMPA receptors were expressed in most CA1 pyramidal cells expressing GFP. These AMPA receptors expressed by gene transfer were involved in fast excitatory neurotransmission elicited by electrical stimulation of the Schaffer collaterals in the stratum radiatum. Tetanic stimulation of Schaffer collaterals induced NMDA receptor-independent, long-term potentiation due to Ca2+ influx through the newly expressed AMPA receptors in the area densely stained with GFP. Thus, the combined use of Sindbis viral vectors with the GFP reporter allowed physiological examination of the roles of a specific gene product in synaptic function in well-characterized brain neurons.

We simultaneously monitored changes of intracellular free Ca2+ concentration ([Ca2+](i)) following different light stimuli from different inner retinal neurons of the turtle retina slice preparation, [Ca2+](i) increased with an increase of the light stimulus intensity. Some of the cells also showed color opponent Ca2+ signals. 2-Amino-4-phosphonobutyric acid (APB) blocked in particular [Ca2+](i) increases and picrotoxin enhanced the observed [Ca2+](i) changes. These data support the idea that the observed [Ca2+](i) changes result from light stimulation and subsequent retinal processing. Similar Ca2+ signals were observed when the release of Ca2+ from internal stores was blocked with caffeine and thapsigargin. These results indicate that retinal Ca2+ signals evoked by light stimulation depend to a large extent on voltage-dependent Ca2+ influx and might therefore reflect signal processing. (C) 2000 Elsevier Science Ltd. All rights reserved.

The synaptic complex formed by the cone photoreceptor pedicles and the dendrites of horizontal cells in the teleost retina undergoes structural changes during light adaptation. Numerous spinules are formed by the terminal dendrites, and they are subsequently retracted during dark adaptation. In a retina kept under continuous illumination, the retraction process can be initiated by analogues of the neurotransmitter glutamate acting at AMPA/kainate receptors. On the other hand, the retraction process depends on calcium influx and the subsequent activation of CaMkII. We show here that the retraction of spinules induced by AMPA or kainate is not impaired in the presence of cobalt, making an involvement of voltage-gated calcium channels unlikely. Using calcium imaging techniques with isolated horizontal cells, we demonstrate that AMPA and kainate, but not NMDA, increase [Ca2+](i) in the presence of nicardipine, caffeine and thapsigargin. The increase of [Ca2+](i) under these conditions depends on [Ca2+](i) and on the agonist in a dose-dependent manner, suggesting that the increase of [Ca2+](i) is largely due to calcium influx through the agonist-gated channel. Pharmacological studies were performed to determine whether AMPA- and/or kainate-preferring receptors mediate the calcium influx. The AMPA-preferring receptor antagonist LY303070 blocked glutamate- and kainate-evoked increases of [Ca2+](i) in a concentration-dependent manner, indicating that kainate-preferring receptors contributed little or nothing to the observed [Ca2+](i) increase. This was supported by experiments where cyclothiazide (which blocks the desensitization of AMPA receptors) and concanavalin A (which potentiates responses mediated by kainate receptors) were applied. In all cases, LY303070 blocked the agonist-evoked increase of [Ca2+](i). The presence of AMPA-preferring receptors with high Ca2+ permeability on horizontal cells was also supported by measuring agonist-induced currents using whole-cell recording techniques. Furthermore, LY303070 was able to impair the retraction of spinules during dark adaption in the in vivo situation.

In ON-type bipolar cells dissociated from the goldfish retina, a slowly declining inward current (I-tail) was observed after the termination of depolarizing voltage step commands, during which a Ca2+ current was elicited. The properties of I-tail were investigated under the whole-cell voltage clamp. Introduction of the membrane permeant Ca2+ chelator, BAPTA/AM, into the cell suppressed. I-tail, indicating that I-tail was activated by the increase of intracellular free Ca2+ concentration ([Ca2+](i)). The major component of I-tail was identified as the Ca2+-dependent Cl- current (I-Cl(Ca), since the reversal potential of I-tail was almost identical to the Cl- equilibrium potential at various extracellular Cl- concentrations ([Cl-](o)). The contribution of the Na+/Ca2+ exchanger current to I-tail was very small. I-Cl(Ca) was partially suppressed by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) when it was locally applied to the axon terminal but not to the cell body region, suggesting that Ca2+-dependent Cl- channels were localized to the axon terminal. The relationship between the peak amplitude of I-Cl(Ca) and the amount of charge carried by the Ca2+ current was almost linear at levels less than ca. 50 pC, but became saturated at a higher Ca2+ charge.

The aim of this experiment is to study the effect of the length of the inter-food interval (IFI) on immediately following adjunctive drinking in rats, under variable time 112.5 s schedule. The U-shaped relation was found between the length of the IFI and the latency of immediately following licking. The animals began to drink earlier and stopped drinking earlier following intermediate IFI than following shorter or longer IFIs. No systematic relation was observed between the length of the IFI and the number of immediately following licking responses. It was suggested that under FT schedules in previous studies the expected length of the succeeding IFI affected the amount of drinking, whereas under the VT schedule of the present experiment the length of the preceding IFI affected the latency of drinking.

The release of neurotransmitter is evoked by activation of the Ca current (I(Ca)) at presynaptic terminals. Though multiple types of I(Ca) have been reported in various cells, little is known about the properties of presynaptic I(Ca) in the vertebrate CNS. The aim of this article is to identify the type of I(Ca) involved in the release of neurotransmitter from retinal bipolar cells. Bipolar cells with a large axon terminal were isolated enzymatically from the goldfish retina, and studied by the following techniques: (1) recordings of I(Ca) in the whole-cell recording configuration, (2) visualization of intracellular free Ca2 + concentration ([Ca2+]i) with the Fura-2 imaging system, and (3) real-time electrophysiological bioassay of released excitatory amino acid transmitter by a voltage-clamped horizontal cell isolated from the catfish retina. The only I(Ca) found in bipolar cells was the high-voltage-activated, dihydropyridine-sensitive type. This result supports the recent study by Heidelberger and Matthews (1992). When I(Ca) was activated by a short depolarizing pulse, a rapid increase of [Ca2+]i was restricted to the axon terminal. A much slower and smaller increase of [Ca2+]i Was sometimes observed at the cell body, probably due to the diffusion of intracellular free Ca2+ from the axon terminal. The increase of [Ca2+]i was completely suppressed by nicardipine, suggesting that Ca2+ entered through dihydropyridine-sensitive Ca channels located mainly at the axon terminal. Activating I(Ca) of the bipolar cell evoked a transmitter-induced current in the excitatory amino acid probe (i.e., the catfish horizontal cell). Both currents were suppressed concomitantly by nifedipine but not by omega-conotoxin. We conclude that the activation of dihydropyridine-sensitive I(Ca) causes a localized increase of [Ca2+]i at the axon terminal of bipolar cells, and results in the release of neurotransmitter.

Excitatory amino acids are presumed to be the transmitter of retinal bipolar cells. However, one of the essential criteria for the identification of the transmitter, its release from the cells upon depolarization, has not been demonstrated. This article examines the release of endogenous excitatory amino acids from bipolar cells and correlates this release with the influx of Ca2+. Bipolar cells with a large, bulblike axon terminal (ON-type cells with mixed inputs from rods and cones) were enzymatically isolated from the goldfish retina. Horizontal cells dissociated from the catfish retina were used as a probe of excitatory amino acids, because these cells can detect submicromolar concentrations of L-glutamate with high selectivity. An isolated bipolar cell was closely apposed to a dissociated horizontal cell, and each cell was voltage clamped by a patch pipette in the whole-cell clamp configuration. When the bipolar cell was depolarized from -60 mV to a potential more positive than -40 mV using a 500-msec voltage pulse, an outward current (> 20 pA) was recorded from the apposed horizontal cell, which was maintained at +40 mV. The reversal potential of the current induced by the substance released from bipolar cells (I(rs)) was close to 0 mV and was almost identical to the responses evoked with ionophoretically applied L-glutamate. Both reversal potentials were shifted to the same, more negative value when the external Na+ was replaced with choline. Furthermore, the I(rs) was suppressed reversibly by the application of kynurenic acid, a glutamate antagonist. When the Ca current (I(Ca)) of the bipolar cell was blocked by Cd2+, the I(rs) also disappeared. The peak amplitude of I(rs) was closely related to that of the I(Ca). We conclude that mixed rod/cone ON-type bipolar cells of the goldfish retina release an endogenous excitatory amino acid or a closely related compound in a Ca2+-dependent manner.