Rat Hippocampal Synapses Research Articles

3P217 ラット脳海馬におけるニューロステロイドのシナプスへの効果の電気生理学的解析(神経・感覚 : 細胞・膜蛋白・分子)

3P217 ラット脳海馬におけるニューロステロイドのシナプスへの効果の電気生理学的解析(神経・感覚 : 細胞・膜蛋白・分子)

Postsynaptic depolarisation enhances transmitter release and causes the appearance of responses at ?silent? synapses in rat hippocampus

Recent data indicate that most “silent” synapses in the hippocampus are “presynaptically silent” due to low transmitter release rather than “postsynaptically silent” due to “latent” receptors of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid type (AMPARs). That synapses bearing only N -methyl- d -aspartate (NMDAR) receptors do exist is suggested by the decreased number of transmission failures during postsynaptic depolarisation and by the presence of NMDA-mediated excitatory postsynaptic currents (EPSCs) in synapses silent at rest. We tested whether these effects could be due to potentiated transmitter release at depolarised postsynaptic potentials rather than removal of Mg 2+ block from NMDARs. Using whole-cell recordings of minimal EPSCs from CA1 and CA3 neurones of hippocampal slices we confirmed decreased incidence of failures at +40 mV as compared with −60 mV. This effect was associated with a gradual increase of EPSC amplitude after switching to +40 mV and with a decrease of paired-pulse facilitation. In initially silent synapses, potentiation of pharmacologically isolated AMPAR-mediated EPSCs was still observed at +40 mV and this persisted after stepping back to −60 mV. All above effects were blocked when the cell was dialysed with the Ca 2+ chelator BAPTA (20 mM). These observations are difficult to reconcile with the “latent AMPAR” hypothesis and suggest an alternative explanation, namely that the reduction in failure rates at positive potentials is due to potentiation of transmitter release following Ca 2+ influx through NMDARs. Our results suggest that silent synapses can be mainly “presynaptically” rather than “postsynaptically silent” and thus increased transmitter release rather than insertion of AMPARs is a major mechanism of early long-term potentiation maintenance.

Activity-dependent release of adenosine contributes to short-term depression at CA3-CA1 synapses in rat hippocampus.

High-frequency stimulation results in a transient, presynaptically mediated decrease in synaptic efficacy called short-term depression (STD). Stimulation of Schaffer-collateral axons at 10 Hz for 5 s resulted in approximately 75% depression of excitatory postsynaptic current (EPSC) slope recorded from CA1 cells in rat organotypic slice cultures. An adenosine A(1) receptor antagonist decreased the magnitude of STD elicited with 10-Hz stimulation by approximately 30%. The A(1) receptor antagonist had no effect on STD elicited with 3-Hz stimulation. The activation of A(1) receptors during 10-Hz stimulation was not due to the extracellular conversion of released ATP to adenosine, because block of 5'-ectonucleotidases did not significantly affect STD. The adenosine transport inhibitor dipyridamole did not reduce STD, indicating that adenosine was not released by facilitated transport. We conclude that 10-Hz, but not 3-Hz, stimulation causes the vesicular release of adenosine and the rapid (<3 s) activation of presynaptic inhibitory A(1) receptors, which account for approximately 40% of homosynaptic EPSC depression.

Solubilization and immunological identification of presynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the rat hippocampus

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been identified mostly as postsynaptic receptors mediating fast glutamatergic synaptic transmission. However, neurochemical studies based on the modulation of neurotransmitter release have suggested the existence of presynaptic AMPA receptors. We have used a recently described technique that allows a high-purity fractionation of the pre- and postsynaptic proteins of synaptic junctions to evaluate the distribution of the different AMPA receptor subunits in rat hippocampal synapses. Surprisingly, we found very high levels of GluR1- and GluR2/3-like immunoreactivity in the presynaptic fraction, but also in the postsynaptic and extrasynaptic fractions. GluR4-like immunoreactivity was much less abundant but was still detected, predominantly in the postsynaptic fraction. This methodology appears to be far more sensitive than the classical immunogold electron microscopy to determine the localization of synaptic receptors.

Vesicle pool partitioning influences presynaptic diversity and weighting in rat hippocampal synapses

Hippocampal synapses display a range of release probabilities. This is partially the result of scaling of release probability with the total number of releasable vesicles at each synapse. We have compared synaptic release and vesicle pool sizes across a large number of hippocampal synapses using FM 1-43 and confocal fluorescence microscopy. We found that the relationship between the number of recycling vesicles at a synapse and its release probability is dependent on firing frequency. During firing at 10 Hz, the release probability of each synapse is closely related to the number of recycling vesicles that it contains. In contrast, during firing at 1 Hz, different synapses turn over their recycling vesicle pools at different rates leading to an indirect relationship between recycling vesicle pool size and release probability. Hence two synapses may release vesicles at markedly different rates during low frequency firing, even if they contain similar numbers of vesicles. Both further kinetic analyses and manipulation of the number of vesicles in the readily releasable pool using phorbol ester treatment suggested that this imprecise scaling observed during firing at 1 Hz resulted from synapse-to-synapse differences in the proportion of recycling vesicles partitioned into the readily releasable pool. Hence differential partitioning between vesicle pools affects presynaptic weighting in a frequency-dependent manner. Since hippocampal single unit firing rates shift between 1 Hz and 10 Hz regimes with behavioural state, differential partitioning may be a mechanism for encoding information in hippocampal circuits.

G-protein alpha subunit isoforms couple differentially to receptors that mediate presynaptic inhibition at rat hippocampal synapses.

Presynaptic receptors that are coupled to heterotrimeric G-proteins are found throughout the brain and are responsible for modulating synaptic transmission. At least 10 G-protein-coupled receptors (GPCRs) reduce transmission in hippocampal neurons. Additionally, hippocampal neurons express up to 17 different Galpha, Gbeta, and Ggamma subunits, making for a striking array of possible heterotrimer compositions and GPCR-heterotrimer interactions. The identity of the Galpha subunit is likely a critical determinant in coupling specificity between GPCRs and their molecular effectors mediating presynaptic inhibition. We studied the role of four Galpha(i/o) subunits (Galpha(o1), Galpha(i1,) Galpha(i2), and Galpha(i3)) in mediating presynaptic inhibition in hippocampal neurons by expressing pertussis toxin-insensitive (PTx-ins) Galpha(i/o) mutants. PTx treatment of these cells disrupts coupling of endogenous subunits, leaving only the mutant Galpha subunits to couple with native GPCRs and betagamma subunits. Successful rescue of presynaptic inhibition indicates that the expressed mutant Galpha subunit can couple to the GPCR of interest. All four PTx-ins Galpha subunits rescued presynaptic inhibition by adenosine A1 receptors. A PTx-ins Galpha subunit also rescued adenosine A1-mediated inhibition of spontaneous vesicle fusion frequency. Of the remaining GPCRs tested, cannabinoid CB1, somatostatin, and GABA(B) receptors displayed an alpha subunit-dependent selectivity in binding to G-protein heterotrimers, whereas group III metabotropic glutamate receptor-mediated inhibition was not rescued by expression of any of the four PTx-ins Galpha subunits. Differential coupling of G-protein alpha subunits may be a means of achieving specificity between different GPCRs and their molecular targets for mediating presynaptic inhibition.

G-Protein α Subunit Isoforms Couple Differentially to Receptors that Mediate Presynaptic Inhibition at Rat Hippocampal Synapses

Presynaptic receptors that are coupled to heterotrimeric G-proteins are found throughout the brain and are responsible for modulating synaptic transmission. At least 10 G-protein-coupled receptors (GPCRs) reduce transmission in hippocampal neurons. Additionally, hippocampal neurons express up to 17 different Gα, Gβ, and Gγ subunits, making for a striking array of possible heterotrimer compositions and GPCR–heterotrimer interactions. The identity of the Gα subunit is likely a critical determinant in coupling specificity between GPCRs and their molecular effectors mediating presynaptic inhibition. We studied the role of four Gαi/osubunits (Gαo1, Gαi1,Gαi2, and Gαi3) in mediating presynaptic inhibition in hippocampal neurons by expressing pertussis toxin-insensitive (PTx-ins) Gαi/omutants. PTx treatment of these cells disrupts coupling of endogenous subunits, leaving only the mutant Gα subunits to couple with native GPCRs and βγ subunits. Successful rescue of presynaptic inhibition indicates that the expressed mutant Gα subunit can couple to the GPCR of interest. All four PTx-ins Gα subunits rescued presynaptic inhibition by adenosine A1 receptors. A PTx-ins Gα subunit also rescued adenosine A1-mediated inhibition of spontaneous vesicle fusion frequency. Of the remaining GPCRs tested, cannabinoid CB1, somatostatin, and GABABreceptors displayed an α subunit-dependent selectivity in binding to G-protein heterotrimers, whereas group III metabotropic glutamate receptor-mediated inhibition was not rescued by expression of any of the four PTx-ins Gα subunits. Differential coupling of G-protein α subunits may be a means of achieving specificity between different GPCRs and their molecular targets for mediating presynaptic inhibition.

Hippocampal Long-Term Potentiation Is Reduced by Chronic Opiate Treatment and Can Be Restored by Re-Exposure to Opiates

Chronic exposure to opiates eventually leads to drug addiction, which is believed to involve maladaptive changes in brain function, but the underlying neuronal mechanisms remain primarily unknown. Given the known effects of opiates such as morphine and heroin on hippocampal function, we investigated the potential effect of chronic opiate treatment on long-term potentiation (LTP) at CA1 synapses in rat hippocampus, a leading experimental model for studying synaptic plasticity. Our results revealed that chronic exposure of rats to morphine or heroin, which induced severe drug tolerance and dependence, markedly reduced the capacity of hippocampal CA1 LTP during the period of drug withdrawal (from approximately 190% in control to approximately 120%). More interestingly, the capacity of LTP could be restored to the normal level by re-exposure of the animals to opiates, indicating that the synaptic function was already adapted to opiates. Morris water maze test, which measures behavioral consequences of synaptic plasticity, showed parallel learning deficits after chronic exposure to opiates. Moreover, the opiate-reduced LTP could also be restored by inhibitors of cAMP-dependent protein kinase A (PKA), suggesting that upregulation of cAMP pathway was likely one of the underlying mechanisms of the observed phenomena. These findings demonstrated that chronic opiate treatment can significantly modulate synaptic plasticity in the hippocampus, leading to an opiate dependence of the plasticity.

2E1630 神経ステロイドによる海馬歯状回・長期増強の分子機構 : 膜電位イメージングによる解析(16.神経回路・脳の情報処理,一般演題,日本生物物理学会第40回年会)

2E1630 神経ステロイドによる海馬歯状回・長期増強の分子機構 : 膜電位イメージングによる解析(16.神経回路・脳の情報処理,一般演題,日本生物物理学会第40回年会)

Endogenous regulators of G protein signaling proteins regulate presynaptic inhibition at rat hippocampal synapses.

Presynaptic inhibition mediated by G protein-coupled receptors (GPCRs) can develop and decay in a few seconds. This time course is too rapid to be accounted for by the intrinsic GTPase activity of Galpha subunits alone. Here, we test the hypothesis that endogenous regulators of G protein signaling (RGS proteins) are required for rapid, brief presynaptic inhibition. Endogenous G protein alpha subunits were uncoupled from GPCRs by treating cultures with pertussis toxin (PTX). Adenoviral expression of mutant PTX-insensitive (PTX-i) Galpha(i1-3) or Galpha(o) subunits rescued adenosine-induced presynaptic inhibition in cultured hippocampal neurons. Expression of double mutant Galpha(i1) or Galpha(o) subunits that were both PTX-insensitive and unable to bind RGS proteins (PTX/RGS-i) also rescued presynaptic inhibition. Presynaptic inhibition mediated by PTX/RGS-i subunits decayed much more slowly after agonist removal than that mediated by PTX-i subunits or native G proteins. The onset of presynaptic inhibition mediated by PTX/RGS-i Galpha(o) was also slower than that mediated by PTX-i Galpha(o). In contrast, the onset of presynaptic inhibition mediated by PTX/RGS-i Galpha(i1) was similar to that mediated by PTX-i Galpha(i1). These results suggest that endogenous RGS proteins regulate the time course of G protein signaling in mammalian central nervous system presynaptic terminals.

Paired-pulse modulation at individual GABAergic synapses in rat hippocampus.

1. Unitary inhibitory postsynaptic currents (uIPSCs) were recorded in synaptically coupled pairs of CA1 hippocampal interneurons and pyramidal neurons in rat brain slices by using dual patch-clamp techniques. Paired-pulse modulation of uIPSCs at individual GABAergic synapses was tested. 2. GABAergic synapses could be divided into two subgroups, high and low failure, depending on their failure rate. 3. The external Ca2+ levels modulate the failure rate of uIPSCs. In 0.51 mM Ca2+, low-failure pairs had a high-failure characteristic, whereas high-failure pairs had a low-failure characteristic in 8 mM Ca2+. The results suggest that uIPSC failures result from the Ca2+-dependent release mechanism rather than axon propagation failures. 4. Paired-pulse facilitation (PPF) occurred in high-failure pairs when the interspike interval was 20 ms. Paired-pulse depression (PPD) was not predominant in high-failure pairs. 5. Potency of uIPSCs, the average amplitude of non-failure events, was enhanced by PPF, suggesting that multiple synapses connect each pair. Differing numbers of activated synapses contributed to the variable amplitude of uIPSCs from a given pair. 6. PPD occurred in low-failure pairs at the tested range of interspike intervals (20-200 ms). The uIPSC2 after a large uIPSC1 was smaller than the uIPSC2 after a small uIPSC1, suggesting that PPD is use dependent and due to a decrease in the quantal content (m) after the first release. 7. In 8 mM Ca2+, PPD occurred in high-failure pairs and was larger in low-failure pairs, suggesting that the occurrence of PPF or PPD depends on the baseline release probability. 8. The GABAB receptor antagonist CGP 55845A (5 microM) decreased the baseline release probability of inhibitory synapses and attenuated PPD indirectly, rather than by blocking presynaptic GABAB autoreceptors.

Analysis of NMDA-independent long-term potentiation induced at CA3-CA1 synapses in rat hippocampus in vitro.

1. Excitatory postsynaptic currents (EPSCs) were evoked at synapses formed by Schaffer collaterals/commissural (CA3) axons with CA1 pyramidal cells using the rat hippocampal slice preparation. Long-term potentiation (LTP) was induced at these synapses using a pairing protocol, with 50 microM d,l-APV present in the artificial cerebrospinal fluid (ACSF). 2. Quantal analysis of the amplitudes of the control and conditioned EPSCs showed that the enhancement of synaptic strength was due entirely to an increase in quantal content of the EPSC. No change occurred in the quantal current. 3. These results were compared with those obtained from a previous quantal analysis of LTP induced in normal ACSF, where both quantal current and quantal content increased. The results suggest that calcium entering via NMDA receptors initiates the signalling cascade that results in enhanced AMPA currents because it is adding to cytoplasmic calcium from other sources to reach a threshold for this signalling pathway, or because calcium entering via NMDA receptors specifically activates this signalling pathway.

Presynaptic alpha2-adrenoceptors control excitatory, but not inhibitory, transmission at rat hippocampal synapses.

1. The effects of noradrenaline on neurotransmission at rat hippocampal synapses were investigated by recording autaptic currents in single neurons isolated on glial microislands. Noradrenaline reduced excitatory, but not inhibitory, autaptic currents in a pertussis toxin-sensitive manner, but the amine did not affect glutamate-evoked currents. 2. The inhibition of excitatory autaptic currents by noradrenaline was half-maximal at 0. 11 +/- 0.06 microM. The alpha2-adrenoceptor agonists UK 14 304 and clonidine were equipotent to noradrenaline in reducing these currents, whereas the alpha1-adrenoceptor agonist methoxamine and the beta-adrenoceptor agonist isoprenaline (isoproterenol) were ineffective. The reduction of excitatory autaptic currents by noradrenaline was not altered by the alpha1-adrenergic antagonist urapidil or the beta-antagonist propranolol, but reduced by the alpha2-antagonist yohimbine. The subtype-preferring antagonists rauwolscine and phentolamine (both at 0.3 microM) caused 9-fold and 36-fold rightward shifts in the concentration-response curve for the noradrenaline-dependent reduction of excitatory autaptic currents, respectively. Prazosine (1 microM) did not affect this concentration-response curve. 3. Noradrenaline reduced voltage-activated Ca2+ currents in excitatory, but not in inhibitory, microisland neurons. For comparison, the GABAB agonist baclofen reduced both excitatory and inhibitory autaptic currents and diminished voltage-activated Ca2+ currents in both types of neurons. The inhibition of Ca2+ currents by noradrenaline was half-maximal at 0.17 +/- 0.05 microM, and UK 14 304 and clonidine were equipotent to noradrenaline in reducing these currents. The noradrenaline-induced reduction of Ca2+ currents was antagonized by yohimbine, but not by urapidil or propranolol; the subtype-preferring alpha2-adrenergic antagonists displayed the following rank order of activity: phentolamine > rauwolscine > prazosine. 4. Noradrenaline did not affect K+ currents and failed to alter the frequency of miniature excitatory postsynaptic currents measured in mass cultures of hippocampal neurons. 5. These results show that noradrenaline regulates transmission at glutamatergic, but not at GABAergic, hippocampal synapses via presynaptic alpha2-adrenoceptors of the alpha2A/D subtype. This inhibitory action involves an inhibition of voltage-activated Ca2+ currents, but no modulation of spontaneous vesicle exocytosis or of voltage-activated K+ currents.

Rapid report: postsynaptic bursting is essential for 'Hebbian' induction of associative long-term potentiation at excitatory synapses in rat hippocampus.

1. The biologically relevant rules of synaptic potentiation were investigated in hippocampal slices from adult rat by mimicking neuronal activity seen during learning behaviours. Synaptic efficacy was monitored in two separate afferent pathways among the Schaffer collaterals during intracellular recording of CA1 pyramidal neurones. The effects of pairing presynaptic single spikes or bursts with postsynaptic single spikes or bursts, repeated at 5 Hz ('theta' frequency), were compared. 2. The pairing of ten single evoked excitatory synaptic events with ten postsynaptic single action potentials at 5 Hz, repeated twelve times, failed to induce synaptic enhancement (EPSP amplitude 95% of baseline amplitude 20 min after pairing; n = 5). In contrast, pairing the same number of action potentials, but clustered in bursts, induced robust synaptic potentiation (EPSP amplitude 163%; P < 0.01, Student's t test; n = 5). This potentiation was input specific, long lasting ( > 1 h; n = 3) and its induction was blocked by an antagonist at NMDA receptors (20-50 microM D(-)-2-amino-5-phosphonopentanoic acid; EPSP amplitude 109%; n = 6). 3. Presynaptic bursting paired with postsynaptic single action potentials did not induce input specific synaptic change (113 % in the test input vs. 111 % in the control; n = 8). In contrast, postsynaptic bursting when paired with presynaptic single action potentials was sufficient to induce synaptic potentiation when the presynaptic activity preceded the postsynaptic activity by 10 ms (150 vs. 84 % in the control input; P < 0.01; n = 10). 4. These results indicate that, under our conditions, postsynaptic bursting activity is necessary for associative synaptic potentiation at CA1 excitatory synapses in adult hippocampus. The existence of a distinct postsynaptic signal for induction of synaptic change calls for refinement of the common interpretation of Hebb's rule, and is likely to have important implications for our understanding of cortical network operation.

Morphometry of Synapse and Granule Cell Changes in the Hippocampus of Aging Fischer 344 Rats

Morphologic changes in Alzheimer’s disease (AD) include a decrease in synapse number while the surviving synapses are increased in size by a process known as compensatory synaptogenesis. There is disagreement among investigators as to whether the synapse changes observed in AD in humans also occurs in the aging rat. The hippocampus, plays a significant role in learning and memory in both humans and rats and is known to be particularly susceptible to cell loss associated with aging. s In the rat hippocampus, age-related synaptic density declines as much as 27% have been reported by some authors, while other investigators have failed to detect a change in synapse number with age. A secondary factor in the total number of synapses in the dentate gyrus is the number of granule cells. Proliferation or regeneration (neurogenesis) of hippocampal granule cells is known to occur well into the adult life of the rat. The goal of the current study was to further explore age-related changes in rat hippocampal synapses and granule cells using the disector morphometry method.

Paired-pulse facilitation and depression at unitary synapses in rat hippocampus: quantal fluctuation affects subsequent release.

1. Excitatory synaptic transmission between pairs of monosynaptically coupled pyramidal cells was examined in rat hippocampal slice cultures. Action potentials were elicited in single CA3 pyramidal cells impaled with microelectrodes and unitary excitatory postsynaptic currents (EPSCs) were recorded in whole-cell voltage-clamped CA1 or CA3 cells. 2. The amplitude of successive unitary EPSCs in response to single action potentials varied. The amplitude of EPSCs was altered by adenosine or changes in the [Mg2+]/[CA2+] ratio. We conclude that single action potentials triggered the release of multiple quanta of glutamate. 3. When two action potentials were elicited in the presynaptic cell, the amplitude of the second EPSC was inversely related to the amplitude of the first. Paired-pulse facilitation (PPF) was observed when the first EPSC was small, i.e. the second EPSC was larger than the first, whereas paired-pulse depression (PPD) was observed when the first EPSC was large. 4. The number of trials displaying PPD was greater when release probability was increased, and smaller when release probability was decreased. 5. PPD was not postsynaptically mediated because it was unaffected by decreasing ionic flux with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or receptor desensitization with aniracetam. 6. PPF was maximal at an interstimulus interval of 70 ms and recovered within 500 ms. Recovery from PPD occurred within 5 s. 7. We propose that multiple release sites are formed by the axon of a CA3 pyramidal cell and a single postsynaptic CA1 or CA3 cell. PPF is observed if the first action potential fails to release transmitter at most release sites. PPD is observed if the first action potential successfully triggers release at most release sites. 8. Our observations of PPF are consistent with the residual calcium hypothesis. We conclude that PPD results from a decrease in quantal content, perhaps due to short-term depletion of readily releasable vesicles.

Erratum for Harris et al., Three-Dimensional Structure of Dendritic Spines and Synapses in Rat Hippocampus (CAl) at Postnatal Day 15 and Adult Ages: Implications for the Maturation of Synaptic Physiology and Long-term Potentiation

In “Three-Dimensional Structure of Dendritic Spines and Synapses in Rat Hippocampus (CAl) at Postnatal Day 15 and Adult Ages: Implications for the Maturation of Synaptic Physiology and Long-term Potentiation” (K.M. Harris, F.E. Jensen, and B. Tsao;J Neurosci12:2685–2705, July 1992), figures 9C and 9E were inadvertently switched with one another. Figure 9C is the electron micrograph of the reference section of the spine reconstructed in figure 10E, and figure 9E is the electron micrograph of the reference section of the spine reconstructed in figure 10C. The authors regret this error.

Three-dimensional structure of dendritic spines and synapses in rat hippocampus (CA1) at postnatal day 15 and adult ages: implications for the maturation of synaptic physiology and long-term potentiation [published erratum appears in J Neurosci 1992 Aug;12(8):following table of contents

It has long been hypothesized that changes in dendritic spine structure may modify the physiological properties of synapses located on them. Due to their small size, large number, and highly variable shapes, standard light microscopy of Golgi impregnations and electron microscopy (EM) of single thin sections have not proved adequate to identify most spines in a sample or to quantify their structural dimensions and composition. Here we describe a new approach, the series sample, that was developed to classify by shape and subcellular composition all of the spines and synapses in a sample of neuropil by viewing them through serial EM sections. Spines in each class are then randomly selected for serial reconstruction and measurement in three dimensions. This approach was used to assess whether structural changes in hippocampal CA1 spines could contribute to the enhanced synaptic transmission and the greater endurance of long-term potentiation (LTP) that occur with maturation. Our results show a near doubling in the total density of synapses in the neuropil and along reconstructed dendrites between postnatal day 15 (PND 15) and adult ages. However, this doubling does not occur uniformly across all spine and synapse morphologies. Thin spines, mushroom spines containing perforated postsynaptic densities (PSDs) and spine apparatuses, and branched spines increase by about four-fold in density between PND 15 and adult ages. In contrast, stubby spines decrease by more than half and no change occurs in mushroom spines with macular PSDs or in dendritic shaft synapses. The stubby spines that remain are smaller in adults than at PND 15 and the mushroom spines are larger, while no change occurs in the three-dimensional structure of thin spines. Only a few spine necks at either age are constricted or long enough to attenuate charge transfer; therefore, the doubling in synapses should mediate the enhancement of synaptic transmission that occurs with maturation. In addition, LTP is not likely to be mediated by widening of spine necks at either age. However, the constricted spine necks could serve to concentrate specific molecules at activated synapses, thereby enhancing the specificity and endurance of LTP with maturation. These results demonstrate that the new series sample method combined with three-dimensional reconstruction reveals quantitative changes in the frequency and structure of spines and synapses that are not discernable by other methods and are likely to have dramatic effects on synaptic physiology and plasticity.

Kainic acid depresses the ex vivo release of dynorphin B and glutamate from rat hippocampal mossy fiber synaptosomes

This experiment examined the effects of intracerebroventricularly (i.c.v.) administered kainic acid (KA) on the subsequent ex vivo release of l-glutamate (Glu) and dynorphin B-like immunoreactivity (Dyn B-LI) from isolated rat hippocampal mossy fiber (MF) synaptosomes at 4.5 h, 20 h or 48 h after administration of 0.5 μg/μl KA. The Dyn B-LI content in the synaptosomal fraction initially decreased at 4.5 h and then rebounded and remained elevated above control levels at 20 h and 48 h. The K +-evoked release of Dyn B-LI from the synaptosomes was markedly depressed at 4.5 h after KA and remained significantly below control levels at 20 h and 48 h. In contrast, KA caused no change in the K +-evoked release of Glu at 4.5 h as compared to control levels, but did result in a significant decrease in Glu release at 20 h and 48 h. These data indicate a persistent effect of i.c.v. KA on neurotransmission at MF-CA3 synapses in rat hippocampus, resulting in a suppression of the release of Glu as well as the opioid peptide, Dyn B.