成年Wistar大鼠空间学习与海马苔藓纤维可塑性

目的 :验证空间学习记忆是否能引起海马苔藓纤维抽芽。方法 :用Morris水迷宫及Timm显影染色方法 ,研究正常成年Wistar大鼠进行定位航行试验后海马CA3区苔藓纤维分布的可塑性变化。结果 :大鼠经 1 4天学习训练 ,其逃避潜伏期逐日缩短 ,由 39 8± 1 5 2s下降到 1 0 9± 7 6s。苔藓纤维主要集中在门区和CA3透明层 ,但在学习组 1 0只和对照组 1 0只大鼠 ,均发现在CA3区始层多少不等地存在苔藓纤维终扣分布 ,两组苔藓纤维分布面密度分别为平均 0 0 5 89± 0 0 335 9,0 0 71 7± 0 0 5 2 84。经“t”检验 ,P >0 0 5 ,差异无显著性。结论 :成年Wistar大鼠Morris水迷宫 1 4天空间学习记忆未见引起海马CA3区苔藓纤维抽芽。     

Sexual dimorphism in the mossy fiber synapses of the rat hippocampus

Summary The presence of sexual dimorphism in the hippocampal formation has long been recognized. Differences between male and female rats have been detected with respect to the number of dentate granule cells and branching patterns of dentate granule and hippocampal pyramidal cell dendrites. Groups of 6 male and 6 female Sprague-Dawley rats were studied at 180 days of age. Based on light microscopical Timm-staining and Golgi-impregnation and electron microscopy, and applying morphometric techniques, we now report that the total number of synapses between mossy fibers and the apical dendritic excrescences of CA3 pyramidal cells is the same in male and female rats, despite a higher numerical density in the latter. Moreover, the volume of the mossy fiber system was found to be smaller in females. Because the number of dentate granule cells is smaller in females than in males, the increased numerical density of synapses may be thought of as a compensatory mechanism to equalize the number of synaptic contacts between dentate granule and CA3 pyramidal cells in the two sexes. We demonstrate that an increase in the number of mossy fiber boutons in female rats is a determining factor for the sexual differences found.     

Noradrenergic enhancement of long-term potentiation at mossy fiber synapses in the hippocampus

1. We tested several hypotheses related to the modulation of long-term potentiation (LTP) by norepinephrine (NE) at the mossy fiber synapses in the rat hippocampal slice preparation using extracellular and intracellular recording techniques. 2. NE exerted frequency-dependent effects on mossy fiber synaptic transmission. It had little effect on extracellular population excitatory postsynaptic potentials (pEPSPs) sampled during low-frequency stimulation, whereas it had marked effects on the duration, magnitude, and probability of induction of LTP at these synapses. 3. The beta-adrenoceptor agonist isoproterenol mimicked all of the effects of NE, whereas the beta-adrenoceptor antagonists propranolol and timolol reversibly blocked the induction of LTP, suggesting the effects of NE are mediated by a beta-adrenoceptor and that beta-adrenoceptor activation may be an important constituent for the expression of LTP at these synapses. 4. Frequency-dependent effects of NE and isoproterenol on mossy fiber pEPSPs were also observed in the presence of the gamma-aminobutyric acid (GABA) antagonist, picrotoxin, suggesting that NE can enhance LTP by a mechanism that does not depend on intact inhibition. However, propranolol did not block LTP in these disinhibited slices and did not affect LTP magnitude. 5. The adenylate cyclase activator forskolin augmented pEPSPs sampled during low-frequency stimulation in disinhibited slices and significantly enhanced LTP. Forskolin, however, did not produce LTP in the absence of tetanic stimulation. This supports the hypothesis that NE and isoproterenol augment features of LTP by stimulating adenosine 3',5'-cyclic monophosphate (cAMP) production and that cAMP plays a modulatory role in the induction of LTP. 6. The postsynaptic injection of the cAMP analogue 8-bromoadenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) significantly increased the probability of induction of LTP measured intracellularly under voltage-clamp conditions with intact inhibition. An analysis of the inhibitory synaptic slope conductance during these experiments indicated that changes in this measure could neither account for the increase in mossy fiber synaptic slope conductance in those cells that displayed it nor account for the group differences in this variable. 7. The amplitude and duration of the postsynaptic depolarization during tetanic stimulation in the cells that displayed LTP in the 8-bromo-cAMP-injected group were significantly greater than in the cells that did not display LTP in the adenosine 5'-monophosphate-injected group.(ABSTRACT TRUNCATED AT 400 WORDS)     

Voltage-clamp analysis of posttetanic potentiation of the mossy fiber to CA3 synapse in hippocampus

1. Short-term changes in synaptic efficacy were studied at the mossy fiber (MF) to CA3 (MF-CA3) synapse in the in vitro hippocampus. Monosynaptic excitatory postsynaptic currents (EPSCs) were recorded before and during posttetanic potentiation (PTP) with the use of intracellular recording and single-electrode voltage-clamp (SEVC) techniques. 2. Repetitive stimulation (100 Hz for 1 s) of the MF synaptic inputs to CA3 pyramidal cells resulted in PTP averaging 170 +/- 19% (SE, n = 42) over control and decaying with a time constant (tau p) of 59.7 +/- 5 s(n = 23). Reproducible episodes of PTP could be recorded if low stimulus intensities were used. Also, after MF tetanization, a faster component, termed augmentation, preceded PTP but could not be accurately resolved within the experimental protocol; only estimates of this component are included. 3. Biophysical parameters of the EPSC that were monitored before and during PTP included synaptic conductance (G), synaptic reversal potential (Erev), decay time constant (tau EPSC), and input resistance of the postsynaptic cell. During PTP the EPSC synaptic conductance increased from 9.8 to 32.7 nS (P less than 0.02, n = 6), whereas there was no statistical change in Erev (-6.0 compared with -6.7 mV, n = 6), tau EPSC (4.3 compared with 4.5 ms, n = 9), or postsynaptic input resistance (59 compared with 63 M omega, n = 12). 4. A presynaptic contribution to PTP was studied directly by observing changes in transmitter release during PTP. Presynaptic mechanisms were assessed by determining the ratio of evoked synaptic excitatory postsynaptic potentials (EPSPs) over the total number of stimuli (EPSP-to-stimuli ratio). The ratio of EPSP to stimuli changed from 0.64 to 0.90 (P less than 0.01, n = 7) during PTP. A reduction in the number of synaptic failures can only be explained by a presynaptic mechanism. No assumptions concerning the statistical distribution of transmitter release were necessary because no statistical parameters were determined. 5. Changes in postsynaptic cell properties do not appear to contribute to PTP studied under the present experimental conditions. Direct stimulation of the postsynaptic neuron via the intracellular recording electrode (20-100 Hz/1 s) failed to produce potentiation of the EPSC; in fact, a slight depression was observed at 50 and 100 Hz direct stimulation. Likewise, the postsynaptic input resistance and synaptic Erev did not change during PTP. 6. The specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovaleric acid (APV, 20 microM) had no effect on either the magnitude or duration of PTP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Activation of kainate receptors controls the number of functional glutamatergic synapses in the area CA1 of rat hippocampus

The expression and functions of kainate-type glutamate receptors (KARs) in the hippocampus are developmentally regulated. In particular, presynaptic KARs depressing glutamate release are tonically activated during early postnatal development, and this activity is down-regulated in parallel with maturation of the synaptic circuitry. In order to understand the physiological relevance of the tonic KAR-mediated signalling, we have here studied the effect of long-term pharmacological activation of KARs on glutamatergic synaptic connectivity in hippocampal slice cultures where presynaptic KARs are expressed but not endogenously activated. Prolonged (16–20 h) activation of the GluR5 subunit-containing KARs using the agonist ATPA (1 μ m ) caused a specific and enduring increase in the number of glutamatergic synapses in area CA1, evidenced as an increase in the frequency of action potential-independent spontaneous EPSCs (mEPSCs) and in immunostaining against synaptic marker proteins. The long-term ATPA treatment had no detectable effect on GABAergic transmission or on glutamate release probability. Further, the effect of ATPA on synaptic density was independent of action potential firing and dependent on protein kinase C. A critical role of endogenous KAR activity in synaptic development was revealed by chronic treatment of the cultures with the selective GluR5 antagonist LY382884, which caused a significant impairment of glutamatergic transmission to CA1 pyramidal neurons. Together, these data suggest a role for the GluR5 subunit-containing KARs in the formation and/or stabilization of functional glutamatergic synapses in area CA1.     

Evidence of reorganization in the hippocampal mossy fiber synapses of adult rats rehabilitated after prolonged undernutrition

We have previously demonstrated that prolonged low-protein diet leads to irreversible cell loss in the hippocampal formation of the adult rat. Because the extent of the resulting hippocampal synaptic alterations is not well characterized, we studied the contacts between mossy fibers and the dendritic excrescences of CA3 pyramidal cells (MF-CA3 synapses) using quantitative methods. Moreover, we investigated whether rehabilitation from undernutrition would influence the morphology of hippocampal synapses. To address these issues, three groups of adult rats were compared: (a) rats fed with a normal diet for 12 months (control rats); (b) rats treated during the same period with low-protein diet (undernourished rats); and (c) rats undernourished for 6 months and then switched to normal diet for 6 months (recovery rats). Timm staining and electron microscopy were employed to estimate the volume of the mossy fiber system and the number and related quantitative features of MF-CA3 synapses. The volume of the suprapyramidal bundle of the mossy fiber system and its total number of synapses were smaller in undernourished rats than in control and recovery animals. These parameters did not differ between the latter two groups. The size of mossy fiber terminals and dendritic excrescences and the surface area of synapses were smaller in undernourished than in control and recovery groups. Conversely, in recovery animals, the volume of the suprapyramidal bundle of the mossy fiber system, the size of mossy fiber terminals and dendritic excrescences, and the total number and surface area of synapses were similar to those of controls. These findings indicate that, following rehabilitation, the pre- and postsynaptic compartments of MF-CA3 synapses undergo structural alterations which compensate for the neuronal loss induced by undernutrition.     

Postnatal maturation of mossy fibre excitatory transmission in mouse CA3 pyramidal cells: a potential role for kainate receptors

Kainate receptors (KARs) are abundantly expressed in the central nervous system at a period of intense synaptogenesis and might participate in the maturation of neural networks. We have described the postnatal development of mossy fibre excitatory synaptic transmission in CA3 pyramidal cells and we have explored the potential role of KARs in synaptic maturation. In CA3 pyramidal cells, mossy fibre stimulation evokes EPSCs as early as postnatal day 3 (P3). At this early stage, mossy fibre (MF)-EPSCs are fully blocked by GYKI 53655, an AMPA receptor (AMPAR) antagonist. A postsynaptic KAR component can only be detected from P6. Thus, AMPAR-EPSCs precede KAR-EPSCs during postnatal maturation at this synapse. All MF-EPSCs display a KAR component after P10. A key issue of the present work is that between P6 and P9, the presence of a postsynaptic KAR component tightly coincides with AMPAR-mediated EPSCs of large amplitude, and with the onset of low frequency facilitation (from 0.1 Hz to 1 Hz), a presynaptic form of short-term synaptic plasticity. In addition, mice lacking functional KARs throughout postnatal development display MF-EPSCs of significantly smaller amplitude at stages of maturation where synaptic KARs are normally present, due to both pre- and postsynaptic impairment of synaptic transmission. These data suggest a role for KARs in the maturation of mossy fibre synapses.     

Enhancement of postsynaptic responsiveness during long-term potentiation of mossy fiber synapses in guinea pig hippocampus.

The primary site responsible for the long-lasting enhancement of synaptic transmission during long-term potentiation (LTP) was examined by quantal analysis of excitatory postsynaptic potentials in thin sections of the guinea pig hippocampus. With induction of LTP in mossy fiber synapses, estimated values of quantal amplitude (q) and Pascal parameters p and r were increased significantly. No increases in quantal content (m) were detected. The magnitude of increases in q was almost equal to that of LTP. These results indicate that LTP in mossy fiber synapses results from increases in responsiveness of postsynaptic neurons.     

Contrary roles of kainate receptors in transmitter release at corticothalamic synapses onto thalamic relay and reticular neurons

Corticothalamic fibres, which originate from layer VI pyramidal neurons in the cerebral cortex, provide excitatory synaptic inputs to both thalamic relay neurons and reticular neurons; reticular neurons in turn supply inhibitory inputs to thalamic relay neurons. Pyramidal cells in layer VI in the mouse somatosensory cortex highly express mRNA encoding kainate receptors, which facilitate or depress transmitter release at several synapses in the central nervous system. We report here that contrary modulation of transmitter release from corticothalamic fibres onto thalamic relay and reticular neurons is mediated by activation of kainate receptors in mouse thalamic ventrobasal complex and thalamic reticular nucleus. Exogenous kainate presynaptically depresses the synaptic transmission at corticothalamic synapses onto thalamic relay neurons, but facilitates it at corticothalamic synapses onto reticular neurons. Meanwhile, the lemniscal synaptic transmission, which sends primary somatosensory inputs to relay neurons, is not affected by kainate. In addition, GluR5-containing kainate receptors are involved in the depression of corticothalamic synaptic transmission onto relay neurons, but not onto reticular neurons. Furthermore, synaptically activated kainate receptors mimic these effects; high-frequency stimulation of corticothalamic fibres depresses synaptic transmission onto relay neurons, but facilitates it onto reticular neurons. Our results suggest that the opposite sensitivity of kainate receptors at the two corticothalamic synapses is governed by cortical activity and regulates the balance of excitatory and inhibitory inputs to thalamic relay neurons and therefore their excitability.     

Assessing the roles of presynaptic ryanodine receptors and adenosine receptors in caffeine-induced enhancement of hippocampal mossy fiber transmission

Caffeine robustly enhances transmitter release from the hippocampal mossy fiber terminals, although it remains uncertain whether calcium mobilization through presynaptic ryanodine receptors mediates this enhancement. In this study, we adopted a selective adenosine A1 blocker to assess relative contribution of A1 receptors and ryanodine receptors in caffeine-induced synaptic enhancement. Application of caffeine further enhanced transmission at the hippocampal mossy fiber synapse even after full blockade of adenosine A1 receptors. This result suggests that caffeine enhances mossy fiber synaptic transmission by two distinct presynaptic mechanisms, i.e., removal of A1 receptor-mediated tonic inhibition and ryanodine receptor-mediated calcium release from intracellular stores.     

Presynaptic inclusions in mossy fibre terminals of the cerebellar cortex following long-term undernutrition in adult rats

Summary Past work on the C.N.S. of nutritionally deprived immature rats shows widespread structural and functional alterations that were not found in nutritionally deprived adults. In the present work we studied cerebellar mossy fibre terminals of adult rats, given a 8% casein diet for periods of 6, 12 and 18 months. After 12 months of the diet, mossy fibre terminals presented a large number of spherical or disc-shaped membrane-free inclusions herein referred to as presynaptic inclusions, with diameters up to 1.3 m and formed by fine-textured subunits, compacted into 3 nm strands. This material is an extension of the cytoplasmic network of the terminal. No special relationship was observed with the active zones; synaptic vesicles found in the proximity or embedded in these inclusions were often of the flat type, whereas those close to the active zones always appeared spherical. These results show that even in adult rats synaptic terminals are areas sensitive to protein deprivation.     

Characteristics of structural organization of interneurons in CA 3 region and mossy fibers of hippocampus

The paper reviews the current data on the structural peculiarities of the interneurons of hippocampal area CA 3. Special attention is paid to horizontal spiny interneurons located in the stratum lucidum and forming direct contacts with the system of mossy fibers. Their axons terminate at the proximal portions of dendrites and cell body of pyramidal neurons, occasionally they make contacts with the main cells of the hilus and other hippocampal interneurons. Some portion of axon collaterals is in close vicinity to direct and indirect entorhinal-hippocampal connections, indicating their accommodation to selectively suppress mossy fiber input. The latter form the synapses of various types on the principal cells and on various types of interneurons of area CA 3, indicating different mechanisms of transmitter uptake and presynaptic control of their release.     

Y5 receptors mediate neuropeptide Y actions at excitatory synapses in area CA3 of the mouse hippocampus.

Neuropeptide Y (NPY) is a potent modulator of excitatory synaptic transmission and limbic seizures. NPY is abundantly expressed in the dentate gyrus and is thought to modulate hippocampal excitability via activation of presynaptic Y2 receptors (Y2R). Here we demonstrate that NPY, and commonly used Y2R-preferring (NPY(13-36)) and Y5 receptor (Y5R)-preferring ([D-Trp(32)]NPY and hPP) peptide agonists, evoke similar levels of inhibition at excitatory CA3 synapses in hippocampal slices from wild-type control mice (WT). In contrast, NPYergic inhibition of excitatory CA3 synaptic transmission is absent in mice lacking the Y5R subtype (Y5R KO). In both analyses of evoked population spike activity and spontaneous excitatory postsynaptic synaptic currents (EPSCs), NPY agonists induced powerful inhibitory effects in all hippocampal slices from WT mice, whereas these peptides had no effect in slices from Y5R KO mice. In slices from WT mice, NPY (and NPY receptor-preferring agonists) reduced the frequency of spontaneous EPSCs but had no effect on sEPSC amplitude, rise time, or decay time. Furthermore, NPYergic modulation of spontaneous EPSCs in WT mice was mimicked by bath application of a novel Y5R-selective peptide agonist ([cpp]hPP) but not the selective Y2R agonist ([ahx(5-24)]NPY). In situ hybridization was used to confirm the presence of NPY, Y2, and Y5 mRNA in the hippocampus of WT mice and the absence of Y5R in knockout mice. These results suggest that the Y5 receptor subtype, previously believed to mediate food intake, plays a critical role in modulation of hippocampal excitatory transmission at the hilar-to-CA3 synapse in the mouse.     

Kindling is associated with the formation of novel mossy fibre synapses in the CA3 region

Summary A Golgi and electron microscopy study of the hippocampal CA3 region was performed on control and kindled Wistar rats. The observations provide evidence that, in epileptic rats, mossy fibres sprout and establish novel synapses with the basilar dendrites of CA3 pyramidal neurons. These newly-developed synapses showed the typical features of mossy synapses observed in the stratum lucidum, including the appearance of complex giant spines. The morphological changes reported here may represent a histopathological substrate for the epilepsy in the absence of overt signs of a hippocampal lesion.     

BK potassium channels control transmitter release at CA3–CA3 synapses in the rat hippocampus

Augmentation is a component of short-term synaptic plasticity with a gradual onset and duration in seconds. To investigate this component at the corticogeniculate synapse, whole cell patch-clamp recordings were obtained from principal cells in a slice preparation of the rat dorsal lateral geniculate nucleus. Trains with 10 stimuli at 25 Hz evoked excitatory postsynaptic currents (EPSCs) that grew in amplitude, primarily from facilitation. Such trains also induced augmentation that decayed exponentially with a time constant τ= 4.6 ± 2.6 s (mean ± standard deviation). When the trains were repeated at 1–10 s intervals, augmentation markedly increased the size of the first EPSCs, leaving late EPSCs unaffected. The magnitude of augmentation was dependent on the number of pulses, pulse rate and intervals between trains. Augmented EPSCs changed proportionally to basal EPSC amplitudes following alterations in extracellular calcium ion concentration. The results indicate that augmentation is determined by residual calcium remaining in the presynaptic terminal after repetitive spikes, competing with fast facilitation. We propose that augmentation serves to maintain a high synaptic strength in the corticogeniculate positive feedback system during attentive visual exploration.     

Mossy fiber synapses on glutamate decarboxylase-immunoreactive neurons: evidence for feed-forward inhibition in the CA3 region of the hippocampus

Summary Mossy fibers are known to form excitatory synapses on pyramidal neurons in regio inferior of the hippocampus. This study demonstrates that the mossy fibers also establish synaptic contacts with glutamate decarboxylase-immunoreactive, supposedly GABAergic inhibitory neurons in the CA3 region. The observed connection provides a morphological basis for feed-forward inhibition of the pyramidal cells.     

Presynaptic Inhibition of GABAA receptor-mediated unitary IPSPs by cannabinoid receptors at synapses between CCK-positive interneurons in rat hippocampus

There is growing evidence to link cholecystokinin (CCK)-positive interneurons and anxiety disorders. Despite this, little is known about the physiology and pharmacology of synaptic interactions between CCK-positive interneurons. This study aims to investigate the local circuit connections among CCK-positive Schaffer collateral associated (SCA) interneurons in stratum radiatum (SR) and their modulatory interactions using paired whole cell recordings combined with biocytin and double immunofluorescence labeling in slices of rat hippocampus. The cell bodies of SCA interneurons were located in SR, and their sparsely spiny dendrites projected toward s. pyramidale (SP) and along SR. Their axons innervated SR, SP, and s. oriens (SO) with predominant ramification in SR. These cells were immunopositive for CCK and immunonegative for parvalbumin (PV). SCA interneurons often displayed an accommodating firing pattern with or without a "sag" in response to hyperpolarizing current injection. Pairs of these cells exhibited electrical coupling and reciprocal chemical connections in which inhibitory postsynaptic potentials (IPSPs) displayed powerful frequency-dependent facilitation and augmentation. The synaptic connections were modulated by the endogenous cannabinoid receptor (CB) agonist, anandamide and by depolarization-induced suppression of inhibition (DSI), both of which reduced the amplitude of unitary IPSPs to 50% of control and increased the number of apparent failures of transmission. These effects were blocked by the CB1 receptor antagonist, AM-251. I suggest that synaptic facilitation between CCK-positive SCA interneurons may modify the onset of CB1 receptor-mediated regulation of inhibition, thereby affecting spike timing, and that this process could influence the expression of anxiety.     

Frequency-dependent shift from paired-pulse facilitation to paired-pulse depression at unitary CA3-CA3 synapses in the rat hippocampus

Paired recordings between CA3 interconnected pyramidal neurons were used to study the properties of short-term depression occurring in these synapses under different frequencies of presynaptic firing (   n = 22  ). In stationary conditions (0.05-0.067 Hz) pairs of presynaptic action potentials (50 ms apart) evoked EPSCs whose amplitude fluctuated from trial to trial with occasional response failures. In 15/20 cells, paired-pulse ratio (PPR) was characterized by facilitation (PPF) while in the remaining five by depression (PPD). Increasing stimulation frequency from 0.05-0.067 Hz to 0.1-1 Hz induced low frequency depression (LFD) of EPSC amplitude with a gradual increase in the failure rate. Overall, 9/12 cells at 1 Hz became almost 'silent'. In six cells in which the firing rate was sequentially shifted from 0.05 to 0.1 and 1 Hz, changes in synaptic efficacy were so strong that PPR shifted from PPF to PPD. The time course of depression of EPSC1 could be fitted with single exponentials with time constants of 98 and 36 s at 0.1 and 1 Hz, respectively. In line with the inversion of PPR at 1 Hz, the time course of depression of EPSC2 was faster than EPSC1 (7 s). Recovery from depression could be obtained by lowering the frequency of stimulation to 0.025 Hz. These results could be explained by a model that takes into account two distinct release processes, one dependent on the residual calcium and the other on the size of the readily releasable pool of vesicles.     

Functional roles of protein interactions with AMPA and kainate receptors

The glutamate receptor subtypes AMPA and kainate are involved in synaptic transmission and synaptic plasticity in the CNS. Recently there has been considerable interest in understanding the molecular regulation of these receptors by proteins that directly bind to AMPA and kainate receptor subunits. Amongst the first interaction partners to be discovered were NSF, ABP, GRIP and PICK1, which bind the AMPA receptor subunit GLUA2. We have studied the functional roles of the interactions of these proteins in regulating AMPA receptor-mediated synaptic transmission and synaptic plasticity in the hippocampus. We have also started to investigate the functions of PICK1 and GRIP on kainate receptor-mediated synaptic transmission in this region. In this article we reflect upon this work, which has led to some new ideas about how AMPA and kainate receptors are regulated at synapses.