Redistribution of synaptic vesicles during long-term potentiation in the hippocampus

Mitochondrial support of persistent presynaptic vesicle mobilization with age-dependent synaptic growth after LTP

Mitochondria support synaptic transmission through production of ATP, sequestration of calcium, synthesis of glutamate, and other vital functions. Surprisingly, less than 50% of hippocampal CA1 presynaptic boutons contain mitochondria, raising the question of whether synapses without mitochondria can sustain changes in efficacy. To address this question, we analyzed synapses from postnatal day 15 (P15) and adult rat hippocampus that had undergone theta-burst stimulation to produce long-term potentiation (TBS-LTP) and compared them to control or no stimulation. At 30 and 120 min after TBS-LTP, vesicles were decreased only in presynaptic boutons that contained mitochondria at P15, and vesicle decrement was greatest in adult boutons containing mitochondria. Presynaptic mitochondrial cristae were widened, suggesting a sustained energy demand. Thus, mitochondrial proximity reflected enhanced vesicle mobilization well after potentiation reached asymptote, in parallel with the apparently silent addition of new dendritic spines at P15 or the silent enlargement of synapses in adults.

DOI:http://dx.doi.org/10.7554/eLife.15275.001

Presynaptic ultrastructural plasticity along CA3→CA1 axons during long-term potentiation in mature hippocampus

In area CA1 of the mature hippocampus, synaptogenesis occurs within 30 minutes after the induction of long-term potentiation (LTP); however, by 2 hours many small dendritic spines are lost, and those remaining have larger synapses. Little is known, however, about associated changes in presynaptic vesicles and axonal boutons. Axons in CA1 stratum radiatum were evaluated with 3D reconstructions from serial section electron microscopy at 30 minutes and 2 hours after induction of LTP by theta-burst stimulation (TBS). The frequency of axonal boutons with a single postsynaptic partner was decreased by 33% at 2 hours, corresponding perfectly to the 33% loss specifically of small dendritic spines (head diameters <0.45 μm). Docked vesicles were reduced at 30 minutes and then returned to control levels by 2 hours following induction of LTP. By 2 hours there were fewer small synaptic vesicles overall in the presynaptic vesicle pool. Clathrin-mediated endocytosis was used as a marker of local activity, and axonal boutons containing clathrin-coated pits showed a more pronounced decrease in presynaptic vesicles at both 30 minutes and 2 hours after induction of LTP relative to control values. Putative transport packets, identified as a cluster of less than 10 axonal vesicles occurring between synaptic boutons, were stable at 30 minutes but markedly reduced by 2 hours after the induction of LTP. APV blocked these effects, suggesting that the loss of axonal boutons and presynaptic vesicles was dependent on N-methyl-D-aspartic acid (NMDA) receptor activation during LTP. These findings show that specific presynaptic ultrastructural changes complement postsynaptic ultrastructural plasticity during LTP.

Nanoscale analysis of structural synaptic plasticity

Structural plasticity of dendritic spines and synapses is an essential mechanism to sustain long lasting changes in the brain with learning and experience. The use of electron microscopy over the last several decades has advanced our understanding of the magnitude and extent of structural plasticity at a nanoscale resolution. In particular, serial section electron microscopy (ssEM) provides accurate measurements of plasticity-related changes in synaptic size and density and distribution of key cellular resources such as polyribosomes, smooth endoplasmic reticulum, and synaptic vesicles. Careful attention to experimental and analytical approaches ensures correct interpretation of ultrastructural data and has begun to reveal the degree to which synapses undergo structural remodeling in response to physiological plasticity.

Involvement of the nitric oxide pathway in synaptic dysfunction following amyloid elevation in Alzheimer's disease

Amyloid-beta (Abeta), a peptide thought to play a crucial role in Alzheimer's disease (AD), has attracted scientific interest with the aim of characterizing the mechanisms by which it is involved in AD pathogenesis. Abeta has been found to markedly impair hippocampal long-term potentiation (LTP), a widely studied cellular model of synaptic plasticity that is thought to underlie learning and memory. The overall purpose of this review is to define the role of the nitric oxide (NO)/cGMP/cAMP-regulatory element binding (CREB) pathway in beta-amyloid-induced changes of basal neurotransmission and synaptic plasticity in the hippocampus, a structure within the temporal lobe of the brain critical for memory storage.

BDNF mobilizes synaptic vesicles and enhances synapse formation by disrupting cadherin-beta-catenin interactions

Neurons of the vertebrate central nervous system have the capacity to modify synapse number, morphology, and efficacy in response to activity. Some of these functions can be attributed to activity-induced synthesis and secretion of the neurotrophin brain-derived neurotrophic factor (BDNF); however, the molecular mechanisms by which BDNF mediates these events are still not well understood. Using time-lapse confocal analysis, we show that BDNF mobilizes synaptic vesicles at existing synapses, resulting in small clusters of synaptic vesicles “splitting” away from synaptic sites. We demonstrate that BDNF's ability to mobilize synaptic vesicle clusters depends on the dissociation of cadherin–β-catenin adhesion complexes that occurs after tyrosine phosphorylation of β-catenin. Artificially maintaining cadherin–β-catenin complexes in the presence of BDNF abolishes the BDNF-mediated enhancement of synaptic vesicle mobility, as well as the longer-term BDNF-mediated increase in synapse number. Together, this data demonstrates that the disruption of cadherin–β-catenin complexes is an important molecular event through which BDNF increases synapse density in cultured hippocampal neurons.

Myristoylated alanine rich C kinase substrate (MARCKS) heterozygous mutant mice exhibit deficits in hippocampal mossy fiber-CA3 long-term potentiation

The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary protein kinase C (PKC) substrate in brain thought to transduce PKC signaling into alterations in the filamentous (F) actin cytoskeleton. Within the adult hippocampus, MARCKS is highly expressed in the dentate gyrus (DG)-CA3 mossy fiber pathway, but is expressed at low levels in the CA3-CA1 Schaffer collateral-CA1 pathway. We have previously demonstrated that 50% reductions in MARCKS expression in heterozygous Marcks mutant mice produce robust deficits in spatial reversal learning, but not contextual fear conditioning, suggesting that only specific aspects of hippocampal function are impaired by reduction in MARCKS expression. To further elucidate the role of MARCKS in hippocampal synaptic plasticity, in the present study we examined basal synaptic transmission, paired-pulse facilitation, post-tetanic potentiation, and long-term potentiation (LTP) in the hippocampal mossy fiber-CA3 and Schaffer collateral-CA1 pathways of heterozygous Marcks mutant and wild-type mice. We found that LTP is significantly impaired in the mossy fiber-CA3 pathway, but not in the Schaffer collateral-CA1 pathway, in heterozygous Marcks mutant mice, whereas basal synaptic transmission, paired-pulse facilitation, and post-tetanic potentiation are unaffected in both pathways. These findings indicate that a 50% reduction in MARCKS expression impairs processes required for long-term, but not short-term, synaptic plasticity in the mossy fiber-CA3 pathway. The implications of these findings for the role of the mossy fiber-CA3 pathway in hippocampus-dependent learning processes are discussed.

5-HT offsets homeostasis of synaptic transmission during short-term facilitation

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Tenascin-R-deficient mice show structural alterations of symmetric perisomatic synapses in the CA1 region of the hippocampus

Accumulating evidence suggests that extracellular matrix (ECM) molecules play important roles in formation of synapses. Our previous electrophysiologic study of mice deficient in the extracellular matrix glycoprotein tenascin-R (TN-R) showed an impaired gamma-aminobutyric acid release at perisomatic inhibitory synapses in the CA1 pyramidal cell layer of the hippocampus. The present study investigated possible ultrastructural correlates of abnormal perisomatic inhibition. Topographic, morphometric, and stereologic methods were applied at the light and electron microscopic levels to quantify the density and spatial arrangement of cell bodies of CA1 pyramidal neurons and density and architecture of symmetric synapses formed on them in TN-R(-/-) and wild-type mice of different ages. The spatial arrangement of neuronal cell bodies in the CA1 pyramidal cell layer was found more diffuse and disordered in TN-R(-/-) mice than in wild-type animals. The coverage of the plasma membrane of pyramidal cell bodies by active zones of symmetric synapses was reduced by at least 40% in TN-R(-/-) animals compared with control animals. Further, the length of active zone profiles of perisomatic inhibitory synapses in the CA1 pyramidal cell layer was 8-14% smaller, whereas the number of active zones calculated per length unit of cell body profile was 30-40% smaller in TN-R mutants than in wild-type animals. The density and spatial arrangement of synaptic vesicles in the synaptic terminals provided ultrastructural evidence for reduced synaptic activity in TN-R mutants. Thus, TN-R appears to play an important role in the regulation of the number and architecture of perisomatic inhibitory synapses, which play crucial roles in the synchronization of neuronal activity and modulation of synaptic plasticity in the hippocampus.

Model systems for assessing cognitive function: implications for HIV-1 infection and drugs of abuse

Memory deficits are common among drug abusers and in those with chronic neurodegenerative disorders. Currently, the mechanisms through which diverse neurophysiologic processes alter memory are not known. This review describes the current systems and rationale for studying memory formation, consolidation, and recall. Special attention is given to physiologic (hippocampal long-term potentiation) and behavioral animal models. The principles and methods described can be applied to studies of diverse clinical disorders.

Protein synthesis in entorhinal cortex and long-term potentiation in dentate gyrus

Despite the concentration of effort in recent years, the mechanisms underlying the expression of long-term potentiation (LTP) in the hippocampus remain elusive, but amidst the uncertainty and sometimes controversy, one consistent finding is emerging; this is that late-phase LTP requires synthesis of proteins. This hypothesis was first proposed by a number of groups who reported that the more persistent components of LTP were blocked by protein synthesis inhibitors, and was supported by a significant literature which indicated that morphological changes accompanied LTP. Recent evidence indicated that the increase in protein synthesis may be cAMP-dependent and that subsequent activation of the transcription factor, CREB, represented one step in the cascade of events leading to protein synthesis. Whether protein synthesis occurs in presynaptic or postsynaptic neurons, or both, is still a subject of debate. Here we present evidence which suggests that LTP in perforant path-granule cell synapses is accompanied by protein synthesis, specifically synthesis of synaptic vesicle proteins, in the entorhinal cortex. We also show that protein synthesis is decreased in the entorhinal cortex of aged rats and a strain of rat which is genetically hypertensive, both of which exhibited impaired LTP. We propose that that the observed increase in protein synthesis in the entorhinal cortex, which accompanied LTP in the dentate gyrus, contributes to the reported changes in morphology in the presynaptic terminal.

Spontaneous synaptogenesis in ex vivo sympathetic ganglion and the blockade by serum treatment

Central denervation for more than 1 month has been shown to cause an increase in the number of adrenergic synapses in sympathetic ganglia in vivo. Here, we report several lines of evidence that adrenergic synapses may be generated de novo in ex vivo superior cervical ganglion (SCG) of adult rats only several hours after the isolation. Structures immunoreactive for synaptophysin, a marker of presynaptic elements, were drastically decreased 6 days after the preganglionic denervation. A significant increase in number of synaptophysin positive boutons was observed over 3-8 hours in the denervated SCGs maintained ex vivo at 36 degrees C in oxygenated physiologic saline, and this increase was blocked by adding normal serum in the saline. Electron microscopic analysis confirmed that the number of adrenergic synapses specifically labeled with 5-hydroxydopamine was increased by several-fold under the same condition. Intracellular labeling of SCG neurons revealed an increase in the incidence (from 8 to 50%) of neurons having dendritic plexus after the in vitro incubation. No evidence of axonal sprouting within the ganglion was observed. Intracellular recordings from single neurons of denervated SCGs revealed that maximum amplitudes of inhibitory postsynaptic potentials, which were completely blocked by yohimbine, an alpha2-adrenoceptor antagonist, in response to focal stimulation were increased over the several hours. These results suggest that dendrites of SCG neurons rapidly develop and exhibit local efferent characteristics that underlie the inhibitory synaptic transmission once they are subjected to serum deprivation.

Analysis of the mechanisms underlying the age-related impairment in long-term potentiation in the rat

The process of ageing is common to all organisms but despite its universality, understanding of the cellular and molecular changes which accompany ageing is poor. With the increase in the ageing population, the need for systematic study of the process becomes a priority but the development of a systematic approach to the problem requires the development of a testable theory of ageing. While several theories of ageing have been proposed /336/, between which there is at least some overlap, no generally-accepted theory has been identified. In relation to the brain, four interrelated theories of ageing have received significant attention; these are (1) the membrane hypothesis, (2) the free radical hypothesis, (3) the calcium hypothesis and (4) the glucocorticoid theory. The overlap between these hypotheses is such that a causal relationship between them is very likely with age-related changes in membrane composition triggered by changes in free radical production, while changes in membrane composition are likely to account for changes in membrane function including age-related changes in calcium homeostasis. The glucocorticoid theory could equally well be termed the stress hypothesis of ageing, which in the hippocampus at least, might trigger age-related changes in free radical production. This review is not intended to be a balanced appraisal of each theory but emphasizes aspects of the membrane hypothesis of ageing and discusses some relevant aspects of the other hypotheses. Attention is focussed on analysis of the biochemical changes which might underlie age-related changes in hippocampal function, particularly maintenance of long-term potentiation (LTP), and these changes are discussed in the context of the four hypotheses of ageing.

Stability in synapse number and size at 2 hr after long-term potentiation in hippocampal area CA1

Long-term potentiation (LTP) is an important model for examining synaptic mechanisms of learning and memory. A key question is whether the enhanced synaptic transmission occurring with LTP involves the addition of new synapses, the enlargement of existing synapses, or a redistribution in synaptic weight among synapses. Two experimental designs were used to address this question. In the first experimental design three conditions were evaluated across hippocampal slices maintained in vitro, including slices with LTP analyzed at 2 hr post-tetanus, slices tetanized in the presence of APV, and control slices receiving test stimulation only. In the second experimental design independent LTP and control (low-frequency stimulation) sites were examined. Synapse density was estimated by an unbiased volume sampling procedure. Synapse size was computed by three-dimensional reconstruction from serial electron microscopy (EM). Serial EM also was used to compute synapse number per unit length of dendrite. In both experimental designs there were no significant effects of LTP on total synapse number, on the distribution of different types of synapses (thin, mushroom, stubby, or branched dendritic spines and macular, perforated, or segmented postsynaptic densities), on the frequency of shaft synapses, nor on the relative proportion of single or multiple synapse axonal boutons. There was also no increase in synapse size. These results suggest that LTP does not cause an overall formation of new synapses nor an enlargement of synapses at 2 hr post-tetanus in hippocampal area CA1, and these results support the hypothesis that LTP could involve a redistribution of synaptic weights among existing synapses.

Time-dependent changes in rat hippocampal synapsin I mRNA expression during long-term potentiation

We studied the time-dependent changes in synapsin I mRNA levels after hippocampal long-term potentiation (LTP) in rats in vivo. Following LTP induction by stimulating the perforant path, synapsin I mRNA expression in the granule cell layer of the dentate gyrus ipsilateral to stimulation increased significantly in a time-dependent manner. From 2 to 8 h after stimulation, the synapsin I mRNA levels in the ipsilateral dentate gyrus were significantly higher than those of controls subjected to a sham procedure. The synapsin I mRNA level (157.4+/-7.1% of the control level, mean+/-SEM) was at a maximum 8 h after stimulation. The synapsin I mRNA level of animals that received only test pulses did not increase significantly, compared with the control level. These results suggest that the increased level of synapsin I mRNA is related to persistent enhancement of synaptic activity within the neural networks in which dentate granule cells participate in LTP.

Changes in protein synthesis and synthesis of the synaptic vesicle protein, synaptophysin, in entorhinal cortex following induction of long-term potentiation in dentate gyrus: an age-related study in the rat

We have examined protein synthesis in entorhinal cortex following induction of long-term potentiation (LTP) in perforant path-granule cell synapses. The data presented here indicate that there was an increase in [35S]methionine labelling of TCA-precipitated proteins and [35S]methionine labelling of synaptophysin in the ipsilateral entorhinal cortex 40 min after induction of LTP in dentate gyrus. Intraventricular injection of both the NMDA antagonist, D-amino-phosphonovalerate, and the protein synthesis inhibitor, anisomycin reduced protein synthesis though the decrease caused by anisomycin was much more profound. Both agents blocked induction of LTP and the increase in protein synthesis and synaptophysin synthesis which accompanied LTP. These data indicate a close coupling of increased protein synthesis in the entorhinal cortex and expression of LTP in the dentate gyrus. This coupling was further suggested by the absence of an LTP-associated increase in protein synthesis in aged animals, in which LTP was markedly attenuated. The possibility that these changes impact on morphological changes which accompany LTP is discussed.

Injury-induced remodelling and regeneration of the ribbon presynaptic terminal in vitro

The neuronal response to axonal injury may relate to the type of insult incurred. Recently, neuritic and presynaptic varicosity regeneration by isolated adult salamander photoreceptors was demonstrated. We have used this system to compare the rod photoreceptor response to two types of injury: denervation/detargeting, the removal of pre- and postsynaptic partners from the axon terminal, and axotomy, the removal of the axon terminal itself. Cells were followed with time-lapse video microscopy for 24-48 h in culture and immunolabelled for SV2 or synaptophysin to identify synaptic vesicle-containing varicosities. Although all injured cells responded with regenerative growth, denervated/detargeted photoreceptors (i.e. neurons which retain their axon terminal) grew 80% more processes and fourfold more presynaptic varicosities than axotomized neurons. In cells which retained their original axon and terminal, varicosity formation generally began with axon retraction. Retraction was followed by elaboration of a lamellipodium and, by 48 h, development of varicosity-bearing neurites from the lamellipodium. Synaptic vesicle protein localization in denervated/detargeted cells paralleled axon terminal reorganization. Axotomized cells, in contrast, lacked synaptic vesicle protein immunoreactivity during this period. To detect synaptic protein synthesis, photoreceptors were examined for colocalization of synaptic vesicle protein with rab6, a Golgi marker, by confocal microscopy. As expected, synaptic vesicle protein staining was present in the Golgi complex during regeneration; however, in cells with an axon, new synaptic vesicle protein-labelled varicosities were found at early stages, prior to the appearance of immunolabel in the Golgi complex. The data demonstrate remarkable plasticity in the ribbon synapse, and suggest that in adult rod cells with an intact axon terminal, synaptic vesicle protein synthesis is not a prerequisite for the formation of new presynaptic-like terminals. We propose that preexisting axonal components are reutilized to expedite presynaptic renewal as an early response to denervation/detargeting.

Direct spinal projections to limbic and striatal areas: anterograde transport studies from the upper cervical spinal cord and the cervical enlargement in squirrel monkey and rat

With the anterograde tracers Phaseolus vulgaris-leucoagglutinin (PHA-L) and biotinylated dextranamine (BD), direct spinal connections from the upper cervical spinal cord (UC; C1 and C2) and the cervical enlargement (CE; C5-T1) were demonstrated in various striatal and limbic nuclei in both squirrel monkey and rat. Within each species and from each spinal level, the total number of terminals seen in the limbic and striatal areas was approximately 50-80% of the number seen within the thalamus. Labeled terminal structures were seen in the hypothalamic nuclei, ventral striatum, globus pallidus, amygdala, preoptic area, and septal nuclei. In both species, the number of labeled terminals in limbic and striatal regions was larger from UC than from CE, although the distributions to each nucleus varied with the specific lamina injected. In both species and from both UC and CE, approximately one-half of the projections to striatal and limbic areas terminated in the hypothalamus. The only region that demonstrated a topographical organization was the globus pallidus, where terminals from the CE were located dorsomedially to those from the UC. In the rat, UC and CE injections into the lateral dorsal horn and pericentral laminae resulted in the largest number of limbic and striatal terminations. The proportion of ipsilateral terminations was greatest when the medial laminae in the UC or the lateral dorsal horn in the CE received injections. Analysis of the morphology of these spinohypothalamic and spinotelencephalic terminals showed that, in the squirrel monkey, terminals from CE injections were larger than terminals from UC injections; no such size difference was evident in the rat. However, limbic and striatal terminals in the rat were generally larger than those in the squirrel monkey following injections into the UC or CE. The exact function of these direct spinal projections to various striatal and limbic areas in primates and in rodents remains to be determined. These findings, however, support recent imaging studies that suggest that the limbic system plays an important role in the mediation of chest pain, perhaps directly through these spinolimbic and spinostriatal pathways.

The human synapsin II gene promoter. Possible role for the transcription factor zif268/egr-1, polyoma enhancer activator 3, and AP2

Synapsin II is a neuron-specific phosphoprotein that selectively binds to small synaptic vesicles in the presynaptic nerve terminal. Here we report the cloning and sequencing of the 5'-flanking region of the human synapsin II gene. This sequence is very GC-rich and lacks a TATA or CAAT box. Two major transcriptional start sites were mapped. A hybrid gene consisting of the Escherichia coli chloramphenicol acetyltransferase gene under the control of 837 base pairs of the synapsin II 5'-upstream region was transfected into neuronal and nonneuronal cells. While reporter gene expression was low in neuroblastoma and non-neuronal cells, high chloramphenicol acetyltransferase activities were monitored in PC12 pheochromocytoma cells. However, there was no correlation between reporter gene expression in the transfected cells and endogenous synapsin II immunoreactivity. Using DNA-protein binding assays we showed that the transcription factors zif268/egr-1, polyoma enhancer activator 3 (PEA3), and AP2 specifically contact the synapsin II promoter DNA in vitro. Moreover, the zif268/egr-1 protein as well as PEA3 were shown to stimulate transcription of a reporter gene containing synapsin II promoter sequences. In the nervous system, zif268/egr-1 functions as a "third messenger" with a potential role in synaptic plasticity. PEA3 is expressed in the brain and its activity is regulated by proteins encoded from non-nuclear oncogenes. We postulate that zif268/egr-1 and PEA3 couple extracellular signals to long-term responses by regulating synapsin II gene expression.

Morphogenesis in memory formation: synaptic and cellular mechanisms

We review some of the evidence for structural changes in synapses in response to environmental stimulation. These include changes in synapse number, in distribution of presynaptic vesicles, in synaptic bouton size, and complex changes in the shape and size of synaptic contact zones. Increased numbers of postsynaptic polyribosomal aggregates (PRA) are correlated histologically with developmental plasticity. We discuss the role that dendritically targeted mRNAs and polyribosomes might play in providing rapid, localized synthesis of proteins necessary for structural change. Using synaptoneurosomes, we have demonstrated that depolarization leads to a rapid (1-2 min) increase in PRA and in [35S]methionine incorporation into polypeptides. We have shown that this process is initiated by metabotropic glutamate receptors, which trigger phosphatidyl inositol hydrolysis, leading to release of internal Ca2+ stores and activation of protein kinase C. Entry of external Ca2+, however, seems to downregulate polyribosomal aggregation, via a calmodulin-dependent mechanism, suggesting that translation may be controlled by interaction of ionotropic receptors, voltage-dependent calcium channels, and metabotropic receptors.

Membrane arachidonic acid concentration correlates with age and induction of long-term potentiation in the dentate gyrus in the rat

We examined the induction and maintenance of long-term potentiation (LTP) in vivo in the dentate gyrus of 4-month-old and 22-month-old urethane-anaesthetized rats. High-frequency stimulation of the perforant path induced an immediate increase in the slope of the population excitatory postsynaptic potential (EPSP), which was sustained in the 4-month-old animals for the duration of the experiment (45 min post-tetanus). In the 22-month-old group, the mean slope of the population EPSP decreased almost to baseline by the end of the experiment. Examination of the individual records indicated that LTP was sustained for the duration of the experiment in half of the 22-month-old animals, while in the others only post-tetanic potentiation was observed. Membrane arachidonic acid concentration was reduced in aged compared with young animals and was lowest in the subgroup of aged animals which failed to sustain LTP. Potassium-stimulated, calcium-dependent release of glutamate was also decreased in aged compared with young animals, but LTP was associated with an increase in glutamate release in the 4-month-old group and 22-month-old subgroup in which LTP was successfully sustained; no change was observed in the 22-month-old group in which LTP was not sustained. The results indicate a correlation between membrane arachidonic acid concentration, glutamate release and ability to sustain LTP in aged animals.

Increase in synaptic vesicle proteins accompanies long-term potentiation in the dentate gyrus

Maintenance of long-term potentiation in synapses formed by the perforant path on to granule cells of the dentate gyrus is accompanied by a sustained increase in the extracellular concentration of glutamate, the presumed transmitter at this excitatory hippocampal pathway. Quantal analysis indicates that, at least in the first hour of induction, this reflects an increase in transmitter release rather than a decrease in glutamate uptake, while biochemical studies have suggested that the increase in release persists for several hours. Morphological studies have described early but persistent increases in the spine number and area. Increases in the number of segmented/perforated synapses persisting for at least 1 h after induction of long-term potentiation, have also been reported. These morphological changes suggest both presynaptic and postsynaptic modifications. Increases in synaptic vesicle number and distribution lasting for at least 1 h specifically indicate presynaptic changes. To explore further the role of the presynaptic terminal in long-term potentiation, we have investigated changes in three synaptic vesicle proteins, synapsin, synaptotagmin and synaptophysin, in control tissue and in tissue prepared from potentiated dentate gyrus 45 min and 3 h after induction of long-term potentiation. We found that there was an increase in the concentration of the three proteins 3 h after induction of long-term potentiation. No such increase was observed 45 min after induction or in tissue prepared from animals in which an intraventricular injection of the N-methyl-D-aspartate receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid, blocked induction of long-term potentiation.

Quantal analysis of hippocampal long-term potentiation

Long-term potentiation (LTP) is a lasting (hours, days) increase in electrical responses after brief (seconds) high-frequency activation of monosynaptic pathways. It represents a popular model to study mechanisms of learning and memory. There is a general agreement on mechanisms of LTP induction, at least for LTP in hippocampal area CA1. However, a controversy exists about mechanisms of LTP maintenance: there is evidence for both pre- and postsynaptic locations of LTP mechanisms. Publications on statistical (quantal) analysis of fluctuations of excitatory postsynaptic potentials in hippocampal and some other structures are reviewed. The analysis suggests two independent mechanisms for LTP maintenance during the first hour. They are termed LTPm and LTPv and are expressed as changes in the mean number of transmitter quanta or quantal content (m) and changes in the effect of one quantum or quantal size (v), respectively. The increased number of transmitter quanta per presynaptic impulse (LTPm) can account for the many-fold increase in synaptic efficacy during LTP, especially when initially "silent" connections increase their release probabilities (p). The increase in the number of effective release sites is considered to be secondary to the increase in p. Appearance of new subsynaptic receptors, which can produce an apparent increase in m, is not excluded. The additional mechanism (LTPv) can account for an essential part of potentiation when the LTP magnitude is relatively small (< 60% increase over pretetanic amplitude). Experiments with paired-pulse facilitation support postsynaptic mechanisms for quantization and for LTPv. Intriguing problems for future statistical analysis of quantal synaptic mechanisms for behavioral memory and conditioning are understanding the different mechanisms for induction of LTPm and LTPv, and their contribution to the maintenance of LTP during post-tetanic periods of > 1 hour.

An ultrastructural size principle

Recent ultrastructural descriptions of synaptic contacts suggest that potential synaptic efficacy may be directly correlated with bouton size. The characteristics of a synaptic bouton which presumably underlie its potential physiological strength (such as vesicle number, active zone number and area, and mitochondrial volume) are all linearly related to the volume of the bouton. Furthermore, at synapses which contact dendritic spines in both the hippocampus and cerebellum, the volume of the spine is linearly related to bouton volume. The existence of these scaling relationships has widespread implications for interpreting synaptic anatomy and variability, and for examining synaptic plasticity. We review evidence in support of the "ultrastructural size principle" outlined above and its potential generality.

Potentiation of nerve growth factor-induced alterations in cholinergic fibre length and presynaptic terminal size in cortex of lesioned rats by the monosialoganglioside GM1

The effect of monosialoganglioside GM1 and/or nerve growth factor treatment on the cholinergic innervation of the rat cortex was studied using both light- and electron-microscopic techniques assisted by image analysis. Adult male Wistar rats were unilaterally decorticated and received continuous infusions, via minipump, of vehicle, GM1 (1.5 mg/day) and/or nerve growth factor (12 micrograms/day) into the cerebroventricular space. Treatments were initiated immediately post-lesion and ended after seven days. Thirty days post-lesion (i.e. 23 days after the end of drug administration) brains were processed for choline acetyltransferase immunocytochemistry for either light- or electron-microscopic analysis. At this time-point choline acetyltransferase-immunoreactive neurons in the ipsilateral nucleus basalis magnocellularis were significantly reduced in size especially in the mid portion of this nucleus, in lesion vehicle-treated rats. Moreover, decreases in choline acetyltransferase immunoreactive fibre length (ranging from 31 to 50%) and varicosity number (ranging from 26 to 39%) occurred in all cortical layers within a portion of the remaining cortex of these animals. Monosialoganglioside GM1 or nerve growth factor treatment equally attenuated deficits in nucleus basalis magnocellularis cell size and cortical choline acetyltransferase immunoreactive fibre length. However, nerve growth factor, but not monosialoganglioside GM1 treatment also increased choline acetyltransferase-immunoreactive varicosity number above control levels. In lesioned rats which received both nerve growth factor and the monosialoganglioside GM1, the mean cross-sectional area of nucleus basalis magnocellularis cholinergic neurons did not differ significantly from control values. By contrast, cortical choline acetyltransferase-immunoreactive fibre length and varicosity number were significantly increased above control values and that induced by nerve growth factor treatment alone. Quantitative electron-microscopic analysis showed that cholinergic boutons in cortical layer V were considerably shrunken in lesioned vehicle-treated rats and that GM1 treatment failed to significantly attenuate this deficit. However, exogenous nerve growth factor provoked a significant increase (35% above control values) in cortical cholinergic presynaptic terminal size which was even further augmented by concurrent GM1 treatment (69% above control values). This trophic factor-induced increase in bouton size was confirmed using serial electron microscopy and computer-assisted three-dimensional reconstruction of the cholinergic varicosities. The number of synaptic contacts in cortical layer V was also found to be significantly reduced (45% of control values) in lesioned vehicle-treated rats but was maintained at control levels by exogenous GM1 treatment. In addition, a significant increase (95% above control levels) in the number of choline acetyltransferase-immunoreactive boutons with synaptic differentiations was noted in lesioned nerve growth factor-treated rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantal analysis and synaptic anatomy--integrating two views of hippocampal plasticity

The excitatory synapses onto CA1 pyramidal cells have become a model system for understanding the activity-dependent changes in synapses that underlie learning and memory. Here we examine physiological and anatomical results that are relevant to understanding the mechanisms of synaptic transmission and plasticity at these synapses. Three main points are discussed. First, quantal analysis indicates a large heterogeneity of postsynaptic efficacies for different synapses on the same cell. Reconstructions from electron microscopy show that synapse size is also highly heterogeneous. Reasons for suspecting a relationship between synaptic size and efficacy are discussed. Second, physiological evidence indicates that the changes during long-term potentiation are both pre- and postsynaptic. Similarly, several lines of anatomical evidence suggest that plasticity affects the structure of both the pre- and postsynaptic elements. The detailed registration of structures across the synapse and the physical linkage between pre- and postsynaptic elements suggest a 'structural unit hypothesis' for coordinating pre- and postsynaptic modifications. Third, quantal analysis indicates that stimulation of a single axon can release multiple quanta. Anatomical evidence shows that cell pairs can be connected by multiple synapses, suggesting that multiple quanta may be released at independent sites. These results raise the possibility that one component of synaptic plasticity is mediated by changes in the number of functional synaptic sites.

Principal neurons as local circuit neurons in the rat superior cervical ganglion: the synaptology of the neuronal processes revealed by intracellular injection of biocytin

To analyze the local circuitry of the sympathetic ganglion, the synaptic relations of the neuronal processes of the principal neurons in the rat superior cervical ganglion were investigated by correlated light and electron microscopy combined with intracellular injection of biocytin. Intracellular iontophoresis of biocytin followed by avidin-biotinylated horseradish peroxidase cytochemistry allowed complete visualization of the neuronal processes of the principal neurons. The stained principal neurons have a single process (axon), which leaves the ganglion, and several intraganglionic processes (dendrites), some of which show specific terminal arborizations. Some terminals of the dendritic collaterals formed pericellular plexuses or intercellular glomerular plexuses. Electron microscopically, the dendrites and their collaterals contain numerous small vesicles. Synaptic membrane specializations were observed between the stained dendritic collaterals and unlabeled neurites. These may be both preganglionic axon terminals and processes of principal neurons. The likely direction of neurotransmission often could not be determined because of the bidirectional synaptic structures. Our findings show that the dendritic collaterals of principal neurons appear to make both post- and presynaptic contacts with both the principal neurons and the preganglionic axons. It is suggested that the principal neurons might participate in local circuits involving not only preganglionic axons but also neighboring principal neurons.

Statistical analysis of long-term potentiation of large excitatory postsynaptic potentials recorded in guinea pig hippocampal slices: binomial model

Excitatory postsynaptic potentials (EPSPs) were recorded in guinea pig hippocampal slices (area CA1) from 15 neurons after stimulation of stratum radiatum (str. rad.) and stratum oriens. EPSP amplitudes increased in 8 neurones (10 post-tetanic regions) recorded 15 to 45 min after tetanic stimulation of str. rad. The increase was considered to represent long-term potentiation (LTP). Quantal analysis was performed by two methods assuming binomial statistics: the histogram method using deconvolution of noise and the variance method. According to both methods, LTP was associated with an increase in mean quantal content (m) which correlated with LTP magnitude. A statistically significant increase in quantal size (v) was found only by the histogram method and the increase was not correlated with LTP magnitude. A separate analysis of EPSPs with small LTP magnitude demonstrated that with the histogram method only v was increased but not m. A smaller increase in m for the pooled data of both methods did not correlate with LTP magnitude for this EPSP subset. The increase in m for the whole EPSP set corresponds to previous results on the quantal analysis of LTP in in vivo preparations and favours a presynaptic location of major mechanisms underlying LTP maintenance. The increase in v indicates the existence of another mechanism responsible for the maintenance of a small part of LTP. This mechanism might involve either pre- or postsynaptic changes or both.

Nerve growth factor-induced synaptogenesis and hypertrophy of cortical cholinergic terminals

In this study light and EM quantitative analysis were used to examine whether exogenous nerve growth factor (NGF) could affect terminal fields and synaptic connections in the adult rat brain in vivo. Adult rats received, immediately after unilateral decortication, 2.5S NGF (12 micrograms/day) or vehicle intracerebroventricularly for 7 days. Thirty days after the lesion cholinergic fiber length was quantified, using image analysis, in the remaining cortical area adjacent to the lesion site in each animal. Rats that had received vehicle showed a significantly reduced cortical choline acetyl-transferase-immunoreactive fiber network in the remaining cortex when compared with control animals. By contrast, the network in lesioned rats that had received 2.5S NGF was not different from control animals. Furthermore, the number of cortical choline acetyltransferase-immunoreactive varicosities, which decreased in vehicle-treated lesioned rats, significantly increased above control in lesioned rats that had received 2.5S NGF. At the ultrastructural level, 30 days after the lesion, animals that had received vehicle showed shrunken cholinergic boutons in cortical layer V and fewer synapses compared with control animals. Exogenous NGF, administered to lesioned rats, increased to supernormal levels both size of cholinergic boutons and number of synaptic contacts. These parameters were unaltered in unlesioned rats treated with NGF. This study demonstrates that exogenous NGF can cause significant compensatory changes in terminal fields and synaptic connections in the adult fully differentiated central nervous system.

Increased synthesis of two polypeptides in area CA1 of the hippocampus in response to repetitive electrical stimulation

Changes in the pattern of newly synthesized polypeptides were investigated in the in vitro hippocampal slice following exposure to repetitive stimulation with and without the induction of long-term potentiation. Using [35S]methionine labeling of polypeptides and two-dimensional gel electrophoresis, we detected an increase in the rate of synthesis of two polypeptides (48 kDa and 89 kDa) in CA1 in response to repetitive stimulation of the Schaffer collaterals. The synthesis of the 48 kDa polypeptide (pI approximately 6.6) increased 240% in response to high-frequency stimulation (100 Hz) relative to the same protein from unstimulated slices (n = 14), and increased 220% in response to low-frequency stimulation (1 Hz) (n = 5). Blockade of the N-methyl-D-aspartate (NMDA) receptor induced the protein 180%, with no further increase following tetanic stimulation. An 89 kDa doublet (pI approximately 6.8) increased 150% following high-frequency and 140% following low-frequency stimulation. Blockade of the NMDA receptor increased this protein as well (180% of the unstimulated control) and no further increase was observed following high-frequency stimulation. Based on physicochemical and electrophysiological properties, these proteins are not identifiable as any of those previously associated with long-term potentiation or repetitive electrical stimulation.

Triggers and substrates of hippocampal synaptic plasticity

It is widely assumed that behavioral learning reflects adaptive properties of the neuronal networks underlying behavior. Adaptive properties of networks in turn arise from the existence of biochemical mechanisms that regulate the efficacy of synaptic transmission. Considerable progress has been made in the elucidation of the mechanisms involved in synaptic plasticity at central synapses and especially those responsible for the phenomenon of long-term potentiation (LTP) of synaptic transmission in hippocampus. While the nature and the timing requirements of the triggering steps are reasonably well known, there is still a lot of uncertainty concerning the mechanisms responsible for the long-term changes. Several biochemical processes have been proposed to play critical roles in promoting long-lasting modifications of synaptic efficacy. This review examines first the triggers that are necessary to produce LTP in the hippocampus and then the different biochemical processes that have been considered to participate in the maintenance of LTP. Finally, we examine the relationships between LTP and behavioral learning.

Morphological changes in the geniculocortical pathway associated with monocular deprivation

To summarize (Fig. 10), the structural consequences of monocular deprivation include the following changes: the relay cells in the binocular segments of the deprived geniculate layers shrink and contain less of the possible neurotransmitter NAAG. These changes appear to be secondary to a loss of terminal synaptic arbor. Certainly, deprived geniculocortical cells project to smaller ocular dominance patches in layer IV of visual cortex, where they make fewer and abnormal synapses. As a result, they activate ocular activation columns that, in addition to being small, are faint and usually fail to extend into extragranular layers. This failure to extend to other layers probably results from a failure of the poorly activated deprived-eye cells in layer IV to compete successfully with neighboring experienced-eye cells in layer IV, resulting in a loss of connections from layer IV to other layers (Fig. 11). Thus, the primary effect of monocular deprivation is probably the disruption of the geniculocortical synapse, with the other changes, such as cell size, and possibly the change in neurotransmitter content, being secondary. The disrupted synapse would result in poorly driven cortical cells and faint ocular activation columns, which in turn would bias a secondary competition for access to cells in extragranular layers. There are certain general principles that unite the findings presented in this chapter with the others in this session. First, there are similarities in the types of morphological changes observed, for example, changes in the number and size of synaptic terminals, as well as mitochondrial changes. This implies that there are similar changes during development and adult plasticity and also similar changes in vertebrates and invertebrates. Second, it is not so much the amount of activity that determines these changes, but the pattern of activity. In my results, the relative imbalance in activity is important, but not the absolute amount (for example, the columns activated by the 8-hr eye of an AME 8/1 are different from those activated by the 8-hr eye of an AME 8/8). Similarly, the binocular segment, where there was an imbalance and competition could occur, was affected, whereas the monocular segment, where there was no imbalance and competition could not occur, was not. Finally, the recent results of Reiter and Stryker suggest that monocular deprivation produces changes only when the activity of the presynaptic cell and the postsynaptic cell are correlated.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term potentiation-induced synaptic changes in hippocampal dentate gyrus of rats with an inborn low or high learning capacity

Structural changes induced by high frequency (HF) stimulation on granule cells of the hippocampal dentate gyrus of rats with inborn high (HP) and low (LP) learning capacity was examined in this study. Two types of axodendritic synapses were processed morphometrically; the axonal bouton on dendritic shafts and those making contact through dendritic spines. The occurrence of granule cell spine synapses on LP rats before tetanus is less than the one in control animals. However, after long-term potentiation (LTP) produced by HF stimulation the number of spine synapses decreased, as happened in the control rats. Moreover, the number of granule cells shaft synapses counted on LP rats, before tetanus is less than the one on control rats, and after LTP this number increased greatly. There was no change in the number of granule cell shaft synapses in HP rats before and after LTP. LP rats which did not show LTP in spite of HF stimulation did not show changes in the number of spine and shaft synapses. From these results we can conclude that: (1) the great hippocampal synaptic plasticity observed in HP rats compared to LP rats can be attributed, at least in part, to the increased number of spine and shaft synapses observed in HP rats; (2) the minor number of spine and shaft synapses in LP rats may account for the impossibility to obtain LTP after HF stimulation in these animals.

Contributions of dendritic spines and perforated synapses to synaptic plasticity

The dynamic nature of synaptic connections has presented morphologists with considerable problems which, from a structural perspective, have frustrated the development of ideas on synaptic plasticity. Gradually, however, progress has been made on concepts such as the structural remodelling and turnover of synapses. This has been considerably helped by the recent elaboration of unbiased stereological procedures. The major emphasis of this review is on naturally occurring synaptic plasticity, which is regarded as an ongoing process in the postdevelopmental CNS. The focus of attention are PSs, with their characteristically discontinuous synaptic active zone, since there is mounting evidence that this synaptic type is indicative of synaptic remodelling and turnover in the mature CNS. Since the majority of CNS synapses can only be considered in terms of their relationship to dendritic spines, the contribution of these spines to synaptic plasticity is discussed initially. Changes in the configuration of these spines appears to be crucial for the plasticity, and these can be viewed in terms of the significance of the cytoskeleton, of various dendritic organelles, and also of the biophysical properties of spines. Of the synaptic characteristics that may play a role in synaptic plasticity, the PSD, synaptic curvature, the spinule, coated vesicles, polyribosomes, and the spine apparatus have all been implicated. Each of these is assessed. Special emphasis is placed on PSs because of their ever-increasing significance in discussions of synaptic plasticity. The possibility of their being artefacts is dismissed on a number of grounds, including consideration of the results of serial section studies. Various roles, other than one in synaptic plasticity have been put forward in discussing PSs. Although relevant to synaptic plasticity, these include a role in increasing synaptic efficacy, as a more permanent type of synaptic connection, or as a route for the intercellular exchange of metabolites or membrane components. The consideration of many estimates of synaptic density, and of PS frequency, have proved misleading, since studies have reported diverse and sometimes low figures. A recent reassessment of PS frequency, using unbiased stereological procedures, has provided evidence that in some brain regions PSs may account for up to 40% of all synapses. All ideas that have been put forward to date regarding the role of PSs are examined, with particular attention being devoted to the major models of Nieto-Sampedro and co-workers.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic enhancement of synaptic transmission in hippocampal cell cultures by phorbol esters

The calcium-diacyglycerol activated protein kinase (C kinase) plays an important role in synaptic plasticity, and possibly in long-term potentiation (LTP). We have found that phorbol esters, which activate this protein kinase, increase the frequency of miniature excitatory postsynaptic currents (mEPCs) in hippocampal cell cultures. An analysis of distributions of these miniature synaptic currents shows that amplitudes, and thus postsynaptic function, are not influenced by phorbol esters. Synaptic transmission is therefore enhanced by phorbol esters at a presynaptic locus. The method of analysis employed here is applicable to a broad range of situations involving modification of synaptic transmission in the mammalian central nervous system.

Presynaptic ultrastructural correlates of long-term potentiation in the CA1 subfield of the hippocampus

To determine if long-term potentiation (LTP) is accompanied by changes in the ultrastructural distribution of calcium within presynaptic terminals, calcium was localized at the electron microscopic level using an oxalate/pyroantimonate histochemical technique. Following the induction of LTP at the Schaffer collateral/commissural synapses in the CA1 subfield of the rat hippocampal slice, there was a significant decrease (30%) in the percentage of synaptic vesicles containing calcium deposits. This effect could be accounted for by both a significant reduction in the average number of calcium deposit-bearing vesicles and a significant increase in the average number of synaptic vesicles per terminal profile in slices that displayed LTP. These changes persisted for at least one hour following the induction of LTP and were not observed in slices that received high-frequency stimulation in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV, 50 microM), which blocked LTP. These data suggest that LTP may be accompanied by long-term changes in intraterminal calcium homeostasis and the number of synaptic vesicles. These effects may be related to the reported increase in transmitter release following the induction of LTP.

Long-term potentiation at nicotinic synapses in the rat superior cervical ganglion

1. Nicotinic fast excitatory postsynaptic potentials (fast EPSPs) were recorded intracellularly from postganglionic neurones in the isolated rat superior cervical ganglion. 2. An hours-long potentiation of the fast EPSP could be induced by brief tetanic stimulation of the preganglionic nerve (5 Hz for 5 s to 20 Hz for 20 s). While long-term potentiation (LTP) can be detected in every ganglion by extracellular techniques, LTP was induced in only two-thirds of the nicotinic synaptic responses. 3. Muscarinic blockade with atropine did not prevent LTP of the fast EPSP. 4. LTP of the fast EPSP did not correlate with changes in input resistance nor cell potential, as recorded in the soma. 5. The formation of nicotinic LTP appeared to depend upon stimulation of the nerve terminals. Non-synaptic tetanic depolarization of the postganglionic neurone, effected by injecting depolarizing current pulses through the intracellular microelectrode, was not sufficient. LTP could be induced by synaptic tetani in two-thirds of the same neurones. 6. The response to exogenous 1,1-dimethyl-4-phenylpiperazinium (DMPP), a selective nicotinic agonist, was not increased during nicotinic synaptic LTP. This was true whether DMPP was applied by pressure-ejection from an extracellular micropipette during intracellular recording, or by brief superfusion during sucrose-gap recording of postganglionic responses. 7. Responses to exogenous acetylcholine and carbachol were increased during nicotinic LTP when these non-selective cholinergic agonists were applied by pressure-ejection during intracellular recording. However, the potentiation of the fast EPSP was always at least twofold greater than the potentiation of the response to these exogenous agonists. 8. Potentiation of the responses to acetylcholine and carbachol may have been due to long-term enhancement of muscarinic responses. Thus, no postsynaptic basis for nicotinic LTP was uncovered in these studies.

Synaptic interface surface area increases with long-term potentiation in the hippocampal dentate gyrus

The present study continues our attempt to understand the ultrastructural changes that accompany and may underlie long-term potentiation (LTP). This report describes changes with LTP in the surface area of the pre- and postsynaptic membrane apposition at the synapses formed by entorhinal cortical (EC) axons with granule cell dendritic spines of the dentate gyrus (DG). The electrophysiology and electron microscopy of the DGs from each animal followed conventional procedures. The trace length of the pre- and postsynaptic apposition was measured for identified asymmetric synapses in the dentate molecular layer. The total apposed membrane surface area per unit volume (Sv) was then computed for 4 categories of synaptic profiles for each third of the molecular layer. Statistical analysis of the Sv data used multivariate analyses of variance. Across the entire molecular layer, total apposed Sv does not change significantly with LTP. However, in the activated portion of the molecular layer, total apposed Sv increases significantly, reflecting a significant increase in the apposed Sv for the concave spine profiles there. For these spine profiles, the increased apposed Sv is due to the increased membrane area both at the postsynaptic density and beyond. The average apposed surface area per individual synapse also increases markedly with LTP. The present data support the hypothesis of coordinated pre- and postsynaptic anatomical changes with LTP in the EC-DG system.

Long-term regulation of synaptic acetylcholine release and nicotinic transmission: the role of cyclic AMP

1. Using the rat superior cervical ganglion in vitro, the relative efficacy of nicotinic synaptic transmission was estimated by recording the postganglionic compound action potential and the amount of endogenous acetylcholine (ACh) released. These two parameters were correlated in individual ganglia by sampling the bathing medium for the assay of ACh while simultaneously recording the postganglionic response. 2. The beta-adrenoceptor agonist isoprenaline potentiated both the evoked release of ACh and the postganglionic response by about 20% during preganglionic stimulation at 0.2 Hz. 3. The adenosine receptor agonist 2-chloroadenosine inhibited ACh release and the postganglionic response by about 35%. 4. Tetanic preganglionic stimulation for a few seconds induced a long-term potentiation of nicotinic responses and of ACh release. Both of these potentiations were dependent upon extracellular Ca2+ during the tetani. 5. Forskolin and analogues of cyclic AMP also caused a long-lasting potentiation of both the evoked release of ACh and the postganglionic response, indicating that cyclic AMP may regulate transmission by a presynaptic mechanism. The specificity of the cyclic AMP analogues was tested using various butyryl- and bromo-purine nucleotides. 6. The effects of forskolin and 8-bromo-cyclic AMP did not appear to be dependent upon extracellular Ca2+. 7. The potentiation caused by forskolin was consistently augmented by three phosphodiesterase inhibitors--AH 21-132, papaverine and SQ 20-006. However, the effect of forskolin was not consistently enhanced by theophylline, nor was it reduced by the adenylate cyclase inhibitor SQ 22-536. 8. The neurogenic long-term potentiation was augmented by two of the phosphodiesterase inhibitors that also augmented the forskolin-induced potentiation--papaverine and SQ 20-006. 9. It was concluded that cyclic AMP can enhance nicotinic transmission, and can do so by increasing the evoked release of ACh. However, it was not possible to prove that cyclic AMP mediates the long-term potentiation induced by tetanic preganglionic stimulation.