Brief bursts of high-frequency stimulation produce two types of structural change in rat hippocampus

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)

Activation of NMDA receptors induces rapid dephosphorylation of the cytoskeletal protein MAP2

Hippocampal slices were preincubated with 32P-orthophosphate and used to study the effect of glutamate analogs on protein phosphorylation. NMDA induced a rapid, 70% decrease in the phosphorylation of the microtubule-associated protein MAP2, with no change in the total amount of MAP2. Both competitive and noncompetitive NMDA antagonists blocked the effect of NMDA, but a glutamate antagonist acting at non-NMDA receptors did not. Kainate and quisqualate were less potent than NMDA in stimulating dephosphorylation of MAP2. Other forebrain regions (necortex, striatum, and olfactory bulb) also showed dephosphorylation of MAP2 in response to NMDA. These and other results suggest that NMDA receptor activation induces the dephosphorylation of MAP2 by stimulating a protein phosphatase, possibly the calcium/calmodulin-dependent protein phosphatase calcineurin. Moreover, they indicate that alteration in the properties of a microtubule-associated protein may account for some of the effects of glutamate on postsynaptic neurons.

Target-dependent structural changes accompanying long-term synaptic facilitation in Aplysia neurons

The mechanisms underlying structural changes that accompany learning and memory have been difficult to investigate in the intact nervous system. In order to make these changes more accessible for experimental analysis, dissociated cell culture and low-light-level video microscopy were used to examine Aplysia sensory neurons in the presence or absence of their target cells. Repeated applications of serotonin, a facilitating transmitter important in behavioral dishabituation and sensitization, produced growth of the sensory neurons that paralleled the long-term enhancement of synaptic strength. This growth required the presence of the postsynaptic motor neuron. Thus, both the structural changes and the synaptic facilitation of Aplysia sensorimotor synapses accompanying long-term behavioral sensitization can be produced in vitro by applying a single facilitating transmitter repeatedly. These structural changes depend on an interaction of the presynaptic neuron with an appropriate postsynaptic target.

Intraventricular infusions of antibodies to amyloid-beta-protein precursor impair the acquisition of a passive avoidance response in the rat

Intraventricular infusions of an antiserum raised against a 14 amino acid residue in the extracellular domain of amyloid-beta-protein precursor significantly decreased stepdown latency, at both 24 h and 48 h recall times, in rats trained to avoid an electroshock by remaining on a platform. The antiserum was effective when infused up to 2.5 h following training and no retention deficit was noted when it was administered at 4h or 6h after training. An antiserum generated against a 17 amino acid residue of the A4 amyloid peptide had no effect on learning. Thus the amyloid precursor protein, which is aberrantly processed in Alzheimer's disease, appears to be directly involved in memory formation.

Effect of septal kindling on glutamate binding and calcium/calmodulin-dependent phosphorylation in a postsynaptic density fraction isolated from rat cerebral cortex

Postsynaptic density (PSD) fractions were isolated from the cerebral cortices of control and kindled rats and assayed for glutamate and gamma-aminobutyric acid-binding capacities and for the Ca2+/calmodulin-dependent protein kinase. Glutamate binding was found to be increased by approximately 50% in the PSDs isolated from kindled rats as compared to controls; this increase was almost completely from an increase in Bmax; Kd decreased only slightly. Studies with inhibitors indicate that the receptors involved were of the N-methyl-D-aspartate and quisqualate types. PSDs isolated from control and kindled rats did not differ in gamma-aminobutyric acid or flunitrazepam binding. The in vitro autophosphorylation of the Ca2+/calmodulin-dependent protein kinase was depressed by 45-76% in PSDs isolated from kindled rats as compared to controls, with little change in amount of the kinase. Therefore, we infer that (i) the kindled state is associated with an increase in glutamate activation of postsynaptic sites, allowing Ca2+ to enter dendritic spines, (ii) a change has occurred in activity of the protein kinase, which is the major cerebral cortex PSD protein, and (iii) perhaps major alterations in the PSD are a concomitant to the long-lasting nature of the kindled state.

Striatal preprotachykinin and preproenkephalin mRNA levels and the levels of nigral substance P and pallidal Met5-enkephalin depend on corticostriatal axons that use the excitatory amino acid neurotransmitters aspartate and glutamate: quantitative radioimmunocytochemical and in situ hybridization evidence

Because excitatory amino acid (EAA) neurotransmission has been implicated in long-term postsynaptic events, we conducted an initial study to determine whether or not the EAA-utilizing corticostriatal projection might influence peptide biosynthesis in neurons of the rat's basal ganglia. The content of EAAs in the caudatoputamen was reduced by frontal cortical ablation or by chronic intracerebroventricular infusion of methionine sulfoximine (MS). At 7 days following cortical ablation striatal Asp and Glu were reduced by 15% and 24%, respectively, while MS infusion (24 micrograms/day) for 7 days reduced synaptosomal levels of Asp by 61% and Glu by 48%. With either treatment, quantitative radioimmunocytochemistry revealed that substance P (SP) in the substantia nigra was increased by approximately 38%, while Met5-enkephalin (ME) in the globus pallidus was not changed. In situ hybridization with oligonucleotide probes revealed changes in the rostral striatum of preprotachykinin (PPT) and preproenkephalin (PPE) mRNA levels: cortical ablation reduced PPT mRNA by 17% and PPE mRNA by 20% dorsally, while it increased PPE mRNA (but not PPT mRNA) by 23% ventrally. Likewise, the infusion of MS decreased PPT (32%) and PPE mRNA (28%) dorsally, and increased PPE mRNA (50%) ventrally. In addition to the 7 day time point, the same measurements of EAAs, peptides and mRNAs were made at 14, 21 and 28 days after cortical excisions. At 14 days, the level of striatal Asp had returned to control value, but Glu remained depressed by 21%; nigral SP remained increased by 24%, and pallidal ME decreased by 15%. PPT and PPE mRNA remained depressed dorsally by 15% and 25%, respectively, while the increase in PPE mRNA noted ventrally at 7 days had returned to control values by 14 days. With the exception of Glu, which remained depressed by 18% at 21 and 28 days, all other values had returned to control levels by 21 days. The results indicate that a large reduction in EAA neurotransmission can influence differentially the steady-state levels of neuropeptides in striatal neurons and this change is brought about, at least in part, by an alteration in gene transcription.

Differential effects of advancing age on neurotransmitter cell loss in the substantia nigra and striatum of C57BL/6N mice

The present study was carried out to examine the extrapyramidal motor system of C57BL/6N mice for age-related cell loss in cholinergic neurons of the striatum (ST) and dopaminergic (DA) neurons of the substantia nigra (SN). Immunocytochemistry using antibodies against tyrosine hydroxylase (TH) or choline acetyltransferase (CAT) were used to identify DA or cholinergic neurons of the SN and ST in 6 age groups of young (3 months), middle (6, 10, 20 months) and old (25, 30 months) aged mice. We found that while there was a small decline (11%) in the total number of DA neurons of the SN with age, this decrease did not reach statistical significance. In contrast, the total number of cholinergic neurons of the ST significantly decreased between 25 and 30 months of age with the largest cell loss (38%) found in the rostral ST. In addition, the loss of cholinergic neurons in 30-month-old mice was paralleled by a decline in the mean cross-sectional area of the cell soma and nucleus of remaining cholinergic neurons. These data suggest that advancing age has a differential effect on neurotransmitter neurons of the SN and ST and supports the notion that cell loss is not an inevitable characteristic of senescence but is brain region- and cell type-specific. In addition, these data are consistent with the hypothesis that the proliferation of striatal dendrites described previously in aged C57BL/6N mice may result, in part, from a compensatory growth of these processes secondary to age-related cell loss of striatal neurons.

Quantifying synaptic number and structure: effects of stain and post-mortem delay

Current research indicates the importance of synaptic number and structure in plastic processes such as development, learning and memory, and aging. As such, the examination of these neural features has become an important factor in research on human conditions such as mental retardation, aging and Alzheimer's disease. Synaptic research in human tissue typically involves delayed post-mortem fixation, therefore the current research was designed to examine the effect of post-mortem delay on synaptic number and structure in tissue stained with either routine osmium lead citrate/uranyl acetate (osmium) or ethanol phosphotungstic acid (EPTA). Results indicate that synaptic density shows either a gradual decline (EPTA) or an initial marked drop followed by a plateau (osmium) up to 10-15 h post-mortem depending on the stain used. The number of synaptic vesicles per synapse also undergoes a gradual decline. Measures of synaptic structure were more stable, with the primary change being an initial increase in the cross-sectional length of the synapse. Maximal height of the pre- and postsynaptic dense elements were not affected by post-mortem delay. The EPTA stain gave the best estimates of synaptic parameters with short post-mortem delays. These results indicate that different synaptic measures (and stains) show different responses to post-mortem fixation delay, and that experimental or statistical methods must be used to control for post-mortem effects.

Unpredictable and uncontrollable stress impairs neuronal plasticity in the rat hippocampus

Almost by definition, learning and the effect of stress on learning represent modifications of existing neuronal circuitry. Under some circumstances, this modification can be measured electrophysiologically. One such measure of plasticity is long-term potentiation (LTP), a long-lasting increase in synaptic efficacy following brief exposure to tetanic stimulation. In 1987, Foy et al. reported that hippocampal LTP was impaired by exposure to inescapable shock. We have recent evidence that the impairment in LTP can be prevented by allowing the animal to learn to escape the shock (Shors et al., 1989), indicating that the stress effect is to some extent mediated by "psychological" variables. Regardless of LTP's putative role in learning and memory processes, such a stress-induced decrease in neuronal plasticity is likely to have profound effects on the behaving organism.

Stable depression of potentiated synaptic responses in the hippocampus with 1-5 Hz stimulation

Adult rats with two chronic stimulating electrodes in the Schaffer collateral/commissural system of the hippocampus and one recording electrode in the stratum radiatum (apical dendrites) of field CA1 were administered high-frequency stimulation (10 brief bursts at theta frequency) to produce long-term potentiation (LTP). 'Low frequency' stimulation (100 pulses at 1 Hz alone or followed by 250 pulses at 5 Hz) delivered 5-15 min later had no effect on LTP in 18% of the rats, caused a transient reversal in 18% of the group, but produced an apparent reversal of LTP for the remainder of a 1 h test session in 64% of the animals. LTP did not recover in animals tested 24 h later, at which point a second episode of high-frequency stimulation but without subsequent low-frequency stimulation was administered. This produced an LTP effect that persisted for a 1 h test session in 94% of the cases and that was still present in 86% of the animals tested 24 h later. Low-frequency stimulation applied prior to induction of LTP had no lasting effects on evoked responses not did it affect responses to a control stimulating electrode in those cases in which it reversed LTP. Possible implications of these results for hypotheses concerning the substrates of LTP and mechanisms of forgetting are discussed.

Gangliosides improve synaptic transmission in dentate gyrus of hippocampal rat slices

The effect of perfusion with gangliosides (1 x 10(-6) M) on the response evoked in the granule cell layer of dentate gyrus by stimulation of perforant path in hippocampal rat slices was studied. Gangliosides induced both a decrease in the frequency threshold of stimulation necessary to generate long-term potentiation (LTP) and greater potentiation than under control conditions. It is proposed that gangliosides improve the mechanisms responsible for synaptic plasticity which generate LTP.

The nature and causes of hippocampal long-term potentiation

One of the most fascinating features of the hippocampus is its capacity for plasticity. Long-term potentiation (LTP), a stable facilitation of synaptic potentials after high-frequency synaptic activity, is very prominent in hippocampus and is a leading candidate memory storage mechanism. Here, we discuss the nature and causes of LTP and relate them to endogenous rhythmic neuronal activity patterns and their potential roles in memory. Anatomical studies indicate that LTP is accompanied by postsynaptic structural modifications while pharmacological studies strongly suggest that LTP is not due to an increase in presynaptic transmitter release. In field CA1, LTP induction appears to be triggered by a postsynaptic influx of calcium through NMDA receptor-linked channels. Possible roles of several calcium-sensitive enzyme systems in LTP are discussed and it is argued that activation of a calcium-dependent protease (calpain) could produce the structural changes linked to LTP. Rhythmic bursting activity is highly effective in inducing LTP and it is argued that the endogenous hippocampal theta rhythm plays a role in LTP induction in vivo. Finally, studies indicate that LTP and certain types of memory share a common pharmacology and the use of electrical brain stimulation as a sensory cue suggests that LTP develops when the significance of that cue is learned.

Seizures, neuropeptide regulation, and mRNA expression in the hippocampus

Recent studies have demonstrated that the regulation of neuropeptide expression in forebrain neurons is responsive to external influences including changes in physiological activity. This has been demonstrated most clearly in studies of hippocampus where the synthesis and resting levels of several neuropeptides, localized within well-characterized components of hippocampal circuitry, have been shown to be selectively influenced by seizure activity. In studies described here, we examined the influence of recurrent limbic seizures on the expression of enkephalin, dynorphin, cholecystokinin, and neuropeptide Y (NPY) in rat and mouse hippocampus using immunohistochemical, in situ hybridization and blot hybridization techniques. The data demonstrate that seizures differentially influence the expression of each peptide as a part of a broader cascade of changes in genomic expression within individual hippocampal neurons. In particular, seizures increase preproenkephalin mRNA and enkephalin peptide but decrease dynorphin peptide in the dentate gyrus granule cell/mossy fiber system. Seizure-induced decreases in the concentration of preprodynorphin mRNA in the granule cells have been reported by others. Immunoreactivity for CCK, which is codistributed with the opioid peptides in the mossy fiber system of mouse, is also dramatically reduced in the granule cell axons by seizure. Recurrent seizures induce two temporally distinct changes in NPY expression in hippocampus. First, there is an increase in hybridization to preproNPY mRNA within scattered, probable local circuit neurons in all subfields. This is followed by the seemingly novel appearance of preproNPY mRNA within the dentate gyrus granule cells and pyramidal cells of field CA1. Clues about mechanisms of neuropeptide regulation have come from observations of other, more rapid, transcriptional events induced by seizure. Most notably, our results and those of others demonstrate that seizures increase the expression of messenger RNAs from immediate-early genes (c-fos, c-jun, and NGFI-A) which encode proteins that may mediate neuropeptide gene regulation. In addition, mRNA for nerve growth factor is dramatically increased in the dentate gyrus granule cells by seizure; increased production of this trophic factor might mediate the more delayed changes in genomic expression and growth responses observed to occur in hippocampus and other forebrain areas following seizure activity.

Effects of aging on the dynamics of information processing and synaptic weight changes in the mammalian hippocampus

It is clear that the properties of LTE make it a plausible mechanism for associative information storage at some synapses in the central nervous system. While many of the factors that regulate LTE's induction and expression have been discovered and a strong case is being developed for its role in learning and memory processes, until we understand more clearly the mechanisms underlying both the expression and maintenance of LTE, an understanding of its change with age will be difficult. Judging by the progress that has been made over the past several years in uncovering some of the molecular events that are critical for LTE's expression, one may be optimistic that answers will be forthcoming reasonably soon. Of particular importance to aging mammals, such answers may provide insights into why older organisms show faster forgetting. This may have a profound impact on therapeutic strategies for memory disorders in both normal and pathological conditions of aging.

Morphological correlates of long-term potentiation imply the modification of existing synapses, not synaptogenesis, in the hippocampal dentate gyrus

This report evaluates two morphological markers of synaptogenesis following the induction of long-term potentiation (LTP) in the dentate gyrus of the anesthetized rat. These two morphological features, polyribosomes and multiple synaptic contacts, are known to increase in number with synaptogenesis in the mature hippocampus. The analysis focused on the middle third of the dentate molecular layer. As shown previously, this is the region of primary synaptic activation in our electrophysiological protocol and the region of localized morphological changes with LTP. Here the incidence of a polyribosome at the base of a dendritic spine declined 57% with LTP. In addition, the number of multiple synaptic contacts decreased 18% there with LTP. Both decreases were more pronounced immediately following conditioning stimulation than at later intervals. Because both morphological features decrease with LTP but increase with synaptogenesis, the data do not support the hypothesis that new synapses form with LTP. Instead, the data add further support to the view that the strengthening of existing excitatory synapses underlies LTP.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

The protease inhibitor leupeptin interferes with the development of LTP in hippocampal slices

The effect of leupeptin, an inhibitor of thiol-proteases, was tested on the induction of long-term potentiation (LTP) in field CA1 of hippocampal slices. Two h of drug application did not produce substantial changes while a greater than 3-h application caused a sizeable reduction in the degree of LTP induced. Leupeptin had no obvious effects on the facilitation of postsynaptic responses occurring within or between the short high frequency bursts used to induce LTP, suggesting that the drug acted on cellular chemistries occurring after the initial physiological events that normally trigger LTP. These results are consistent with the hypothesis that a calcium-activated thiol protease (calpain) is involved in the induction of LTP.

Quisqualate and N-methyl-D-aspartate synergistically excite cerebellar Purkinje cells as a long-term effect

Interactions between the excitatory amino acids (EAAs) quisqualate (QUIS) and N-methyl-D-aspartate (NMDA) were investigated in order to explore mechanisms which may help to explain long-term changes in synaptic efficacy induced by these agents. In 93% of the cerebellar Purkinje (Pnj) cells recorded extracellularly which responded to iontophoretic application of QUIS with increases in discharge of up to 100% above spontaneous levels. NMDA administration sensitized the neuron to further QUIS stimulation by 40-200% over control levels of response. NMDA-enhanced QUIS responses reached maximal levels after termination of NMDA application and persisted for up to 2 h post-NMDA. The NMDA receptor blocker 2-amino-5-phosphonovalerate (APV) prevented the observed effect only when administered before but not after NMDA application, suggesting that a long-lasting post-receptor mechanism may be involved in the observed synergistic interaction of QUIS and NMDA.

The neurobiology of learning and memory

The study of memory is a great challenge, perhaps the greatest in biological sciences. Memory involves changes in a tiny fraction of an extremely large pool of elements, a conclusion that makes the task of finding those changes using current technologies formidable. What can be done about this roadblock to neurological investigations of learning? One response that has become particularly productive in recent years is to study learning or learning-like phenomena in relatively simple "model" systems. The idea is to extract basic principles from these models in which molecular and anatomical details can be studied and then to use these in analyzing learning in higher regions of the brain. In this article we discuss current progress and emerging concepts derived from the simple system approach using animal models.

Preservation of neuronal ultrastructure in hippocampal slices using rapid microwave-enhanced fixation

The goal of this study was to obtain fixation as rapidly as possible and to achieve preservation of neuronal ultrastructure to a depth in hippocampal slices where electrophysiological responses are optimal. This study demonstrates that perfusion quality preservation of in vitro hippocampal slices can be achieved within seconds after removal from the incubation chamber by using microwave (MW)-enhanced immersion in mixed aldehydes. The optimal method was determined to be MW irradiation of the slice for 8-11 s, to a tissue temperature of 35-50 degrees C, during immersion in fixative containing 6% glutaraldehyde and 2% paraformaldehyde. Electron microscopy of these slices revealed ultrastructural preservation that was comparable to hippocampi from animals perfused with mixed aldehyde fixative containing 2.5% glutaraldehyde and 2% paraformaldehyde. Excellent ultrastructural preservation extended to 100-175 microns from the hippocampal slice surface after MW-enhanced fixation and therefore was much deeper than the 8-20 microns that can be obtained by rapid freezing. Hippocampal slices are routinely maintained in vitro for electrophysiological or pharmacological studies. This method of MW-enhanced fixation preserves tissue within seconds after removal from incubation, and should provide good preservation of the hippocampal anatomy that might be associated with in vitro physiology.

Developmental changes in synaptic properties in hippocampus of neonatal rats

The properties of synaptic responses in area CA1 of hippocampus were analyzed in slices prepared from 7-9 and 12-15 day old neonate rats. As expected from earlier work, only slices of two-week-old animals showed a consistent degree of long-term potentiation (LTP) in response to patterned high frequency stimulation. Several other synaptic properties were found to change during this developmental period. Inhibitory responses were absent in 7-9 day old but not in 12-15 day old neonates. Paired-pulse facilitation and the calcium sensitivity of postsynaptic responses were considerably reduced in 7-9 as compared to 12-15 day old rats. However, phorbol esters and 4-aminopyridine treatment still produced a strong facilitation of field potentials. The N-methyl-D-aspartate (NMDA) component of responses to single pulse stimulation in low magnesium medium was found to be larger in slices of 7-9 than 12-15 day old or adult animals. At the two time periods examined, trains of high frequency stimulation applied in the presence of regular magnesium elicited an NMDA dependent response. It is concluded that the differences in synaptic properties observed between 7-9 and 12-15 day old neonates may not account for the absence of LTP in the younger animals.

Synaptic structural plasticity following repetitive activation in the rat hippocampus

The morphological effects of repetitive neuronal activation following systemic kainic acid administration were examined in hippocampal CA1 stratum radiatum synapses. Sporadic activation of CA3 and CA1 neurons began approximately 15-25 min after kainic acid administration, which was followed at 1-2 h by repetitive ictal firing until the completion of the experiments at 4 hr. Synaptic density in the CA1 region increased following stimulation, reaching significance at the earliest time period examined, approximately 5-15 min postactivation. There was an initial increase and then a decline in frown (and then flat)-shaped synaptic subtypes, with an ultimate increase in smile-shaped synapses. This pattern is consistent with either a change in synapses from frown to smile shaped or a selective gain/loss of synaptic subtypes. There was also an increase in the size of smile-shaped synapses, but a decrease in the size of frown synapses. By 4 h there was a decline in most indices of synaptic morphology, suggesting that the stimulation had become cytotoxic. These results indicate that the number and morphology of synapses and synaptic subtypes can be modified with relatively short periods of repeated use and suggest their potential role in activity-dependent phenomenon such as information storage and epilepsy.

Absence of long-term potentiation in the subcortically deafferented dentate gyrus

All subcortical afferents to the dorsal hippocampus, running in the fimbria-fornix and supracallosal path, were removed by aspiration. Three to 5 months later the rats were implanted with chronic recording electrodes in the dentate gyrus and CA1 region, and stimulating electrodes in the angular bundle. In non-lesioned rats, high-frequency trains delivered to the angular bundle gave rise to a sustained increase of the evoked population spike in the dentate gyrus. In lesioned animals, high-frequency stimulation resulted in only short-lasting changes, and by 15 min after the conditioning trains the amplitude of both the population spike and field postsynaptic potentials returned to baseline. In lesioned rats large amplitude interictal spikes (less than 40 ms, 3-8 mV) occurred spontaneously. These findings suggest that either (1) coactivation of entorhinal and subcortical inputs is essential for the induction of long-lasting plastic changes in the dentate gyrus, or (2) the long-term potentiation mechanism is saturated by the chronically occurring interictal discharges in the subcortically denervated dentate gyrus.

GAP-43 gene expression during development: persistence in a distinctive set of neurons in the mature central nervous system

GAP-43 is a rapidly transported axonal protein most prominently expressed in regenerating and developing nerves. However, the low level persistence of GAP-43 in the adult CNS where growth and regenerative capacity are minimal may additionally indicate a role for this molecule in neuronal remodeling. Previous studies have revealed GAP-43 immunoreactivity in neurites throughout many regions of the CNS. To identify the CNS neurons that express GAP-43 at different stages of development, we utilized in situ hybridization and immunocytochemistry; the latter was performed with an antibody that recognizes GAP-43 immunoreactivity in both perikarya and neurites. In the perinatal period GAP-43 is expressed in all neurons. Subsequently its expression becomes progressively restricted such that by maturity most neurons no longer express detectable levels, although GAP-43 expression is still moderately high in the adult entorhinal cortex, and strikingly high in the adult hippocampus and olfactory bulb. In light of current notions about the function of GAP-43, it is tempting to speculate that this anatomy denotes neurons engaged in structural remodeling and functional plasticity.

Evidence that changes in presynaptic calcium currents are not responsible for long-term potentiation in hippocampus

We used two approaches to test the possibility that changes in presynaptic calcium currents might be responsible for the long-term potentiation (LTP) effect induced by high-frequency stimulation in area CA1 of hippocampal slices. In a first series of experiments, we compared the effect of LTP induction on paired-pulse facilitation with that produced by changes in extracellular calcium concentration, a procedure that modifies presynaptic calcium currents during depolarization by changing the ionic gradient for calcium. In hippocampus, as in peripheral synapses, increasing concentrations of extracellular calcium caused a marked reduction in the degree of facilitation obtained with paired-pulse stimulation; LTP, conversely, did not affect the facilitation ratio. The differential effect of changing calcium concentrations versus LTP induction on paired-pulse facilitation was observed with different interpulse intervals as well as in conditions in which the changes in response size produced by the two manipulations were comparable. In the second approach, we measured calcium dependency curves of synaptic responses before and after LTP induction or application of 4-aminopyridine, a blocker of potassium channels that increases presynaptic calcium currents by slowing spike repolarization. Procedures that increase calcium entry into terminals during transmission should shift to the left the sigmoidal function relating extracellular calcium to the slope of the extracellular response. This in turn should result in disproportionate effects of the procedure as a function of the calcium concentration. This prediction was realized with 4-aminopyridine but did not occur following LTP induction: control and potentiated responses were similarly affected by changes in calcium concentration. Although indirectly, these data strongly suggest that LTP is not accompanied by alterations in the presynaptic calcium dynamics associated with transmitter release.

Excitatory and inhibitory neurotransmitters in the generation and degeneration of hippocampal neuroarchitecture

The possibility that excitatory and inhibitory inputs to neurons can affect the generation and degeneration of neuroarchitecture was examined in hippocampal pyramidal neurons in isolated cell culture. Dendritic outgrowth and cell survival were directly monitored in neurons exposed to: the excitatory neurotransmitter glutamate, the inhibitory transmitter GABA, anticonvulsants or combinations of these agents. Glutamate caused a graded series of changes in pyramidal neuron cytoarchitecture: a selective inhibition in dendritic outgrowth and dendritic pruning was observed with subtoxic levels of glutamate while cell death was induced by higher levels. Low levels of GABA alone or in combination with diazepam, carbamazepine, phenobarbital or phenytoin were without effect on dendrite outgrowth while higher levels caused moderate reductions in outgrowth. Neither GABA nor the anticonvulsants affected cell survival. GABA plus diazepam, phenobarbital, carbamazepine and phenytoin each significantly reduced the dendritic regression and cell death normally caused by glutamate. Elevation of extracellular K+ to 50 mM caused dendritic regression and 100 mM K+ caused cell death; these effects were greatly reduced by GABA and anticonvulsants. The calcium channel blocker Co2+ prevented the dendritic regression and cell death caused by both glutamate and K+ indicating that calcium influx was required for the neuroarchitectural responses. Taken together, these results demonstrate that neurotransmitters and neuromodulatory drugs can have direct and interactive effects on both neurite outgrowth and cell survival. Such neurotransmitter actions may play roles in both the formation and degeneration of the neuronal circuits in which they participate in information coding.

The development of long-term potentiation in hippocampus and neocortex

The development of long-term potentiation (LTP), an enduring alteration in synaptic efficacy following afferent activation, was examined in CA1 hippocampus and primary visual cortex of rat. Both regions show little LTP prior to postnatal day 5, demonstrate a maximal potentiated response around postnatal day 15, and a subsequent decline to adult levels. These results are discussed with respect to the underlying mechanism of action and behavioral significance of these critical-period phenomena.

Synaptic structural plasticity: role of synaptic shape

Recent research has indicated that synaptic curvature is an important and potentially critical plastic feature of the synapse. Alterations in synaptic shape are related to synaptic function, being found both during maturation and in adulthood following neuronal activation. In this paper we review the evidence supporting synaptic shape as a plastic feature of synaptic structure. We also propose several mechanisms which might underlie these changes in shape. Finally, we suggest the possible functional role of alterations in synaptic curvature, including its potential in altering synaptic transmission efficacy.

Maintenance of long-term potentiation in rat dentate gyrus requires protein synthesis but not messenger RNA synthesis immediately post-tetanization

The involvement of new protein and messenger ribonucleic acid synthesis in long-term potentiation was studied in the anaesthetized rat dentate gyrus using several inhibitors of protein synthesis (anisomycin, emetine, cycloheximide and puromycin) and an inhibitor of messenger ribonucleic acid synthesis (actinomycin D). When injected for 1 h just prior to tetanization, the four inhibitors of protein synthesis produced a mild reduction of long-term potentiation of the excitatory postsynaptic potential measured 10 min after tetanization. Anisomycin produced a significantly faster decay of long-term potentiation, while the other inhibitors had more moderate effects. Actinomycin D failed to affect long-term potentiation. In a second experiment, the time-dependency of the anisomycin effect was examined. Anisomycin injected immediately after tetanization promoted decay of long-term potentiation, but when injected after a 15-min delay, the drug had no effect. Inhibition of protein synthesis for 4 h prior to tetanization did not have any more effect on long-term potentiation than inhibition for 1 h. In no experiment was long-term potentiation of the population spike affected by drug manipulation. These results suggest that for long-term potentiation of the excitatory postsynaptic potential to be maintained for at least 3 h proteins must be synthesized from already existing messenger ribonucleic acid, and that this synthesis is mostly completed within 15 min after tetanization.

Structural alterations of dendritic spines induced by neural degeneration of their presynaptic afferents

Morphological parameters were compared for dendritic spines of spiny stellate neurons in layer IV of the barrel region of mouse somatosensory cortex, which synapse with degenerated thalamocortical afferents (TC spines) and with intact, unidentified axon terminals (UI spines). Spiny stellate neurons were labeled for light and electron microscopic identification by Golgi impregnation and gold toning. Dendritic spines were examined in series of thin sections, and TC spines were ultrastructurally detectable because of the degeneration-induced characteristic appearance of the TC axon terminals. Results show that the means of the width of the spine head and of the length of the spine stalk were significantly higher in TC spines than in UI spines by about 11 and 25%, respectively. The variability of these two morphological parameters was significantly lower for TC spines. The mean of the spine stalk width at the narrowest cross section of the stalk was about 0.12 microns, with no significant difference observed between the two spine groups. No specific relationship was found in either the TC or the UI groups of spines between the length of the spine stalk and the width of the spine stalk at its narrowest profile. As structural features typifying transneuronal degeneration were not observed along the dendritic spines examined, it is speculated that the morphological differences encountered between the TC and UI spines may result, at least in part, from the degeneration-induced synaptic inactivity of the TC axospinous synapses, rather than exclusively from any direct effects of the degeneration process.

Rate-limiting step for transmission at excitatory synapses in hippocampus

To examine mechanisms that might be responsible for limiting transmission at excitatory synapses in hippocampus, we analyzed the relationship between extracellular calcium concentrations (1-6 mM) and postsynaptic responses in field CA1 of hippocampal slices using low stimulation intensities and a paired-pulse paradigm. Three effects were observed: One, the relationship between calcium levels and the slope (or amplitude) of the postsynaptic response was described by a sigmoidal function with an asymptote at about 4 mM. Double reciprocal pilots relating calcium concentration to the initial slope of EPSPs provided evidence for the cooperativity expected between calcium ions and transmitter release. Two, both the rise time and half-decay time of the postsynaptic responses were reduced with increasing calcium concentrations. These effects of calcium were more pronounced on the first response elicited by paired-pulse stimulation and were considerably attenuated by 2 microM bicuculline, indicating that feed-forward inhibition was positively related to calcium concentration and differentially activated by repetitive stimulation. However, inhibition was not responsible for the asymptotic relationship observed between calcium and response size. Three, while increasing the calcium concentration beyond 4 mM did not further affect the initial slope of excitatory postsynaptic potentials (EPSPs), paired-pulse facilitation and 4-aminopyridine were still effective in increasing response size. These results suggest 1) that neither the number of postsynaptic receptors nor the number of transmitter quanta available for release were limiting transmission as a function of the calcium concentration; and 2) that calcium entry into presynaptic terminals was likely to represent the limiting step under the conditions used.

Contributions of quisqualate and NMDA receptors to the induction and expression of LTP

The contributions of two subclasses of excitatory amino acid transmitter receptors to the induction and expression of long-term potentiation (LTP) were analyzed in hippocampal slices. The quisqualate/kainate receptor antagonist DNQX (6,7-dinitro-quinoxaline-2,3-dione) blocked 85% of the evoked field potential, leaving a small response that was sensitive to D-AP5 (D-2-amino-5-phosphonopentanoate), an N-methyl-D-aspartate (NMDA) receptor blocker. This residual D-AP5-sensitive response was of comparable size in control and previously potentiated inputs. High-frequency stimulation in the presence of DNQX did not result in the development of robust LTP. Washout of the drug, however, revealed the potentiation effect. Thus NMDA-mediated responses can induce, but are not greatly affected by, LTP; non-NMDA receptors, conversely, mediate responses that are not needed to elicit LTP but that are required for its expression.

A persistent postsynaptic modification mediates long-term potentiation in the hippocampus

Long-term potentiation (LTP) is a long-lasting enhancement of synaptic transmission that can be induced by brief repetitive stimulation of excitatory pathways in the hippocampus. One of the most controversial points is whether the process underlying the enhanced synaptic transmission occurs pre- or postsynaptically. To examine this question, we have taken advantage of the novel physiological properties of excitatory synaptic transmission in the CA1 region of the hippocampus. Synaptically released glutamate activates both NMDA and non-NMDA receptors on pyramidal cells, resulting in an excitatory postsynaptic potential (EPSP) with two distinct components. A selective increase in the non-NMDA component of the EPSP was observed with LTP. This result suggests that the enhancement of synaptic transmission during LTP is caused by an increased sensitivity of the postsynaptic neuron to synaptically released glutamate.

Long-term potentiation differentially affects two components of synaptic responses in hippocampus

We have used low magnesium concentrations and the specific antagonist D-2-amino-5-phosphonopentanoate (D-AP5) to estimate the effects of long-term potentiation (LTP) on the N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated components of postsynaptic responses. LTP induction resulted in a considerably larger potentiation of non-NMDA as opposed to NMDA receptor-related currents. Increasing the size of postsynaptic potentials with greater stimulation currents or with paired-pulse facilitation produced opposite effects; i.e., those aspects of the response dependent on NMDA receptors increased to a greater degree than did those components mediated by non-NMDA receptors. These results pose new constraints on hypotheses about the locus and nature of LTP and strongly suggest that postsynaptic modifications are part of the effect.

Protein synthesis and processing in cytoplasmic microdomains beneath postsynaptic sites on CNS neurons. A mechanism for establishing and maintaining a mosaic postsynaptic receptive surface

Recent studies have shown that protein synthetic machinery consisting of polyribosomes and associated membranous cisterns is selectively localized beneath synaptic sites on neurons. In the present paper, the role of this machinery in neuronal function will be considered. We will: 1. Summarize the studies that characterize the polyribosomes and define their associations with membranous cisterns. Taken together, these observations suggest the existence of a system for the synthesis and posttranslational processing of proteins at individual synaptic sites; 2. Review the evidence that the protein synthetic machinery is particularly prominent during the initial formation of synaptic contacts (during early development), and during lesion-induced synaptogenesis in mature animals. These observations have led to the hypothesis that the polyribosomes produce proteins that play a role in the formation of the synaptic junction; 3. Review evidence that supports the hypothesis that there is a local synthesis of protein within dendrites, as well as local glycosylation; 4. Describe the evidence suggesting that at least some of the protein constituents of the synaptic junction itself are synthesized locally; and 5. Describe our studies that reveal a mechanism for selective dendritic transport of RNA; this transport mechanism permits the delivery of RNA to postsynaptic sites throughout the dendritic arbor. We will advance the hypothesis that neurons position protein synthetic machinery together with the mRNA's that are appropriate for particular synapses beneath synaptic contact regions. At the synaptic site, this machinery could then direct the synthesis of particular proteins that are critical for synapse formation or maintenance. The positioning of protein synthetic machinery at postsynaptic sites permits a rapid local regulation of the production of key proteins by events at individual synapses.

Stimulation of NMDA receptors induces proteolysis of spectrin in hippocampus

Stimulation of N-methyl-D-aspartate (NMDA) receptors was found to induce proteolysis of brain spectrin in hippocampal slices. The effect was dependent upon extracellular calcium, blocked by the antagonist 2-amino-5-phosphonovalerate (AP5), and was not reproduced by potassium-induced depolarization. These results are consistent with the hypothesis that the involvement of NMDA receptors in plasticity and excitotoxicity is at least partially mediated by calcium-activated proteolysis of cytoskeletal proteins.

Lesions of entorhinal cortex produce a calpain-mediated degradation of brain spectrin in dentate gyrus. II. Anatomical studies

Lesions of the various afferents to the hippocampus have been widely used to investigate the mechanisms underlying growth and degeneration in adult mammalian CNS. It has been proposed that disturbances in intracellular calcium and activation of calcium-dependent proteases represent key steps in producing come of the consequences of the lesions. In this study, we show that lesions of the entorhinal cortex or of the commissural pathway result in profound changes in the distribution of brain spectrin. At 2 days after lesions of the entorhinal cortex, immunoreactivity to spectrin is markedly increased in the outer molecular layer (OML) of the dentate gyrus; conversely at 2 days after commissural lesions, immunoreactivity to the same antigen is increased in the inner molecular layer. The increase in immunoreactivity to spectrin varies with survival time after lesions of the entorhinal cortex. By 24 h post lesion, the increase is homogeneous across the OML, and becomes more intense by 48 h. Between 1 and 3 weeks the increase is much less than at 48 h and is concentrated at the inner border of the OML. Pretreatment of the animals with the calpain inhibitor leupeptin reduces the increase in spectrin immunoreactivity normally seen 48 h after the lesion of the entorhinal cortex. Changes in the pattern of immunoreactivity to GFAP are very different to that seen with spectrin antibodies and are consistent with the known modifications in astrocytes that follow lesions of hippocampal afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term potentiation: persisting problems and recent results

In this paper we discuss recent experimental results pertinent to three unresolved issues regarding the long-term potentiation (LTP) effect: the nature of its enduring substrates, the biochemical mechanisms that produce it, and its potential role in memory. LTP appears to be triggered by a postsynaptic influx of calcium and is associated with alterations in the shape of dendritic spines and probably the formation of new synapses. We discuss the possibility that morphological reorganization also modifies membrane surface chemistry of synaptic elements. Evidence is presented that LTP is not associated with changes in presynaptic calcium currents. Activation of protein kinase C is shown to be insufficient for the induction of LTP, although it may play a modulatory role. The hypothesis that activation of a calcium-sensitive protease (calpain) is pivotal to the establishment of LTP is supported by experiments showing that a calpain inhibitor, leupeptin, blocks LTP. Furthermore, activation of NMDA receptors, an event implicated in LTP induction, is accompanied by calcium-sensitive proteolysis of spectrin, a major dendritic cytoskeletal protein. The finding that stimulation patterns designed to mimic naturally-occurring cell discharge patterns are highly effective for LTP induction greatly strengthens the hypothesis that LTP actually occurs during the encoding of information in cortical systems. Potential contributions of LTP to learning are explored using computer simulations of a simple cortical network.

Hippocampal neurobiological mechanisms of age-related memory dysfunction

Studies are reviewed which indicate that hippocampal frequency potentiation (the growth of neural responses during repetitive synaptic stimulation) is impaired in aged rats, and that this impairment may be important in learning and memory deficits found in these aged animals. Intracellular recording and ultrastructural studies suggest that both hippocampal frequency potentiation and the age deficit in such potentiation are synaptic processes (probably presynaptic), and that the deficit may be due to an age-related increase in calcium influx during depolarization. The latter could in some way result from alterations in the function of a Ca-mediated inactivation of Ca current mechanism recently found in hippocampal neurons. Since major hippocampal changes occur with aging in both rodents and humans, it seems possible that these data are also relevant to human brain aging. Consequently, it is suggested that Alzheimer's disease results from an acceleration of normal age-related neuronal calcium conductance changes by some unknown process (e.g., viruses, aluminum, genetic factors, etc.), leading to a rapid deterioration of brain structure.

Neurochemical characteristics of a postsynaptic density fraction isolated from adult canine hippocampus

Postsynaptic density and synaptic membrane fractions isolated from hippocampal tissue have been compared to those previously isolated from cerebellum and cerebral cortex. In all respects examined, the isolated hippocampal preparations are similar to the cerebral cortex fractions. The morphology of the postsynaptic density (PSD) preparation is the same and the protein composition is similar, but with higher concentrations of the 51-kDa major protein and of calmodulin, and lower concentrations of actin, in the hippocampal PSD fraction. The binding characteristics for glutamate and GABA are also similar between the two fractions, but with higher Bmax and KD glutamate values and lower Bmax and higher KD GABA values for the hippocampal PSD preparation. Both preparations contain GABAA and GABAB receptors. The PSD fraction contains, as does the cerebral cortex fraction, a calmodulin-dependent binding of the Ca2+ channel antagonist, nitrendipine, as well as a cAMP-dependent and a Ca2+/calmodulin-dependent protein kinase, with the same respective substrates. The value of the hippocampal fractions for studies on long-term potentiation and on kindling in the hippocampus is discussed.

Nerve cells with irregular processes: demonstration of anisotropic core geometry of a pyramidal cell

An analytical, recursive method has been developed to demonstrate the anisotropic electrotonic geometry of nerve cells containing varicose or spiny dendrites. The procedure has been based on the distribution of the core geometry of dendrites into modules which consist of module elements where the physical length is much shorter than the actual space constant. The unambiguous representation of the anisotropic core geometry has been possible by plotting the decomposed geometries separated under the condition of the unidirectional spread of the wave front of dendritic potentials. This decomposition has revealed the bidirectional, "smoothed" core geometries as a function of irregular distribution of varicosities or spines. The shape of decomposed core geometries may change according to the position of the input site. The shaping of core geometry reflects the electrotonic effectiveness of a synaptic site to any arbitrary locations which may lead to considerable savings in computations on synaptic effects. The detailed, computer-reconstructed geometry of the apical dendritic field of the pyramidal cell has been analysed by the proposed method. The frequency-dependence of input impedances has been compared between the original and the transformed core geometries assuming that the current is injected into the soma. The significance of dendritic irregularities in the impedance matching has been studied when the shaping of the core geometry has been induced by laminar inputs. The proposed approach may be useful in comparing the input dependence of the receptive fields of different non-smooth cells. The mismatch of the core geometries induced by the opposite travelling waves from the same anatomical location has also been studied and the possible control of the preferred, direction-sensitive activities will be discussed. The important differences between the compartmental modelings based on the known isotropic treatment of dendrites and the more realistic anisotropic approach will be illustrated.

State-dependent recall can be induced by protein synthesis inhibition: behavioural and morphological observations

The possibility that the amnesia induced by protein synthesis inhibition is state dependent was investigated. Chicks injected with the protein synthesis inhibitor anisomycin and then trained in a single-trial passive avoidance learning task showed no recall for the task when tested 6 h later in the absence of the drug. If, however, the same chicks were subsequently retested 30 min after a second administration of the drug they demonstrated clear recall for the task. Control groups showed that this effect was not the result of the administration of anisomycin per se but was due to state-dependent recall. Quantitative morphological characterisation of synapses in a region of the chick forebrain (the intermediate part of the medial hyperstriatum ventrale (IMHV) previously shown to be involved in passive avoidance learning was performed. The characteristic increase in the length of the postsynaptic density in the left IMHV was only evident in chicks killed after the behavioural test in which they had demonstrated recall. No synaptic changes were observed in chicks in which state-dependent recall had been demonstrated in a previous test but which were killed after a test in which they appeared amnestic. These results suggest that a memory trace may be established even in the absence of protein synthesis but that this trace may not normally be accessible. It is also suggested that the synaptic changes observed following learning may be dependent on some aspect of the recall phenomenon.