The biochemistry of memory: a new and specific hypothesis

Therapeutic potential of positive AMPA modulators and their relationship to AMPA receptor subunits. A review of preclinical data

BACKGROUND

Positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) modulators enhance glutamate transmission via the AMPA receptor by altering the rate of desensitization; alone they have no intrinsic activity. They are the only class of compounds known that may pharmacologically separate AMPA subtypes.

OBJECTIVE

This manuscript will review preclinical work on positive AMPA modulators, with clinical examples where relevant.

RESULTS

The activity of these compounds appears to be determined by the AMPA receptor subunit composition. Studies have shown that splice variant and/or subunit combinations change the desensitization rate of this receptor. Also, these subunits are heterogeneously expressed across the central nervous system. Therefore, the functional outcome of different positive AMPA modulators could indeed be different. The origins of this pharmacological class come from hippocampal long-term potentiation studies, so quite naturally they were first studied in models of short- and long-term memory (e.g., delayed match to sample, maze performance). In general, these agents were procognitive. However, more recent work with different chemical classes has suggested additional therapeutic effects in models of schizophrenia (e.g., amphetamine locomotor activity), depression (e.g., forced swim test), neuroprotection (e.g., NMDA agonist lesions) and Parkinson's disease (e.g., 6-hydroxydopamine lesion).

CONCLUSIONS

In conclusion, positive modulation of AMPA may offer numerous therapeutic avenues for central nervous system drug discovery.

Memantine: targeting glutamate excitotoxicity in Alzheimer's disease and other dementias

The management of dementia has changed since the development of new antidementia drugs. The benefits observed in Alzheimer's disease (AD) with selective cholinergic transmission treatments are mainly symptomatic, without clear evidence of neuroprotection. The hypothesis that glutamate-mediated neurotoxicity is involved in the pathogenesis of AD is finding increasingly more acceptance in the scientific community. Glutamate receptors are overactive, and N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential for the treatment of AD and other neurological disorders. Memantine is a noncompetitive NMDA antagonist that is considered a neuroprotective drug. Memantine's capacity has been demonstrated in preclinical studies, and it is considered a useful symptomatic treatment for AD. Memantine has been shown to benefit cognition, function, and global outcome in patients with moderate to severe AD, and it is currently approved by the US Food and Drug Administration (FDA) for the treatment of moderate to severe AD. Recently, memantine has also demonstrated efficacy in the initial stages of AD, although FDA authorization is pending. This review highlights the important pharmacological and clinical aspects of memantine, as well as some basic mechanisms mediating glutamatergic neurodegeneration.

Cystatin C prevents degeneration of rat nigral dopaminergic neurons: in vitro and in vivo studies

Destruction of nigrostriatal dopaminergic (DA) pathway triggers various persistent responses, such as inflammation and increased synthesis of neural growth factors, both in striatum and in substantia nigra. The pathological processes involved in such responses are poorly characterized and could contribute to secondary damage and/or regeneration in the central nervous system (CNS). Cystatin C was previously implicated in the process of neurodegeneration. However, its biological role during neurodegeneration is not understood and remains controversial. The present study identified an increased cystatin C mRNA level in the DA-depleted rat striatum, starting from the second week following a 6-OHDA-induced lesion. Immunohistochemical analysis confirmed the increase in cystatin C protein level in the striatum following DA depletion. Double-labeled fluorescence immunohistochemistry revealed that nigrostriatal neurons, astrocytes, and microglia contributed to the elevated level of cystatin C. Exposure to 6-hydroxydopamine, a DA-specific neurotoxin, resulted in DA neurons loss in the fetal mesencephalic cultures, an effect which could be partially reversed by treatment with cystatin C. Moreover, in vivo DA neurons survival study showed that administration of cystatin C in rats with 6-OHDA-induced lesion partially rescued the nigral DA neurons. The results indicate that the 6-OHDA lesioning induced a relatively slow but sustained up-regulation of cystatin C expression and suggest that the inhibitor may exert a neuroprotective action on DA neurons. The findings raise the possibility that cysteine proteinase inhibitors may be new candidates for neuroprotective treatment of Parkinson's disease. Cystatin C may be useful therapeutically in limiting neuropathy in Parkinson's disease.

The calpain-system of Drosophila melanogaster: coming of age

Drosophila melanogaster is one of the most popular and powerful model organisms that help our understanding of mammalian (human) life processes at the molecular level. Calpains are Ca(2+)-activated cytoplasmic proteases thought to play multiple roles in intracellular signal processing by limited proteolysis of target substrate proteins, thereby changing their function. The calpain superfamily consists of 14 genes in mammals, but only 4 genes in Drosophila. One may assume that the calpain system, i.e. recognizing calpain-dependent life processes and identifying the substrates cleaved while exerting their functions, would prove easier to solve in Drosophila than in mammals. Recently, major progress has been made in characterizing Drosophila Calpain A, Calpain B and Calpain C. The fourth member, Calpain D (or SOL), was analyzed earlier. At this juncture, it seems justifiable to summarize our knowledge about the Drosophila enzymes, in comparison to the ubiquitous mammalian ones, as regards structure-function relations, mode of activation by Ca(2+) and other factors, inhibition, potential targeting, expression pattern in vivo, etc. Equipped with all this information, we may now embark on the genetic modification of family members, interpreting mutant phenotypes in terms of the cell biology of calpains.

Nuclear calpain regulates Ca2+-dependent signaling via proteolysis of nuclear Ca2+/calmodulin-dependent protein kinase type IV in cultured neurons

Accumulating evidence indicates that calpains can reside in or translocate to the cell nucleus, but their functions in this compartment remain poorly understood. Dissociated cultures of cerebellar granule cells (GCs) demonstrate improved long-term survival when their growth medium is supplemented with depolarizing agents that stimulate Ca(2+) influx and activate calmodulin-dependent signaling cascades, notably 20 mm KCl. We previously observed Ca(2+)-dependent down-regulation of Ca(2+)/calmodulin-dependent protein kinase (CaMK) type IV, which was attenuated by calpain inhibitors, in GCs supplemented with 20 mm KCl (Tremper-Wells, B., Mathur, A., Beaman-Hall, C. M., and Vallano, M. L. (2002) J. Neurochem. 81, 314-324). CaMKIV is highly enriched in the nucleus and thought to be critical for improved survival. Here, we demonstrate by immunolocalization/confocal microscopy and subcellular fractionation that the regulatory and catalytic subunits of m-calpain are enriched in GC nuclei, including GCs grown in medium containing 5 mm KCl. Calpain-mediated proteolysis of CaMKIV is selective, as several other nuclear and non-nuclear calpain substrates were not degraded under chronic depolarizing culture conditions. Depolarization and Ca(2+)-dependent down-regulation of CaMKIV were associated with significant alterations in other components of the Ca(2+)-CaMKIV signaling cascade: the ratio of phosphorylated to total cAMP response element-binding protein (a downstream CaMKIV substrate) was reduced by approximately 10-fold, and the amount of CaMK kinase (an upstream activator of CaMKIV) protein and mRNA was significantly reduced. We hypothesize that calpain-mediated CaMKIV proteolysis is an autoregulatory feedback response to sustained activation of a Ca(2+)-CaMKIV signaling pathway, resulting from growth of cultures in medium containing 25 mm KCl. This study establishes nuclear m-calpain as a regulator of CaMKIV and associated signaling molecules under conditions of sustained Ca(2+) influx.

Polychlorinated biphenyl mixtures (Aroclors) induce apoptosis via Bcl-2, Bax and caspase-3 proteins in neuronal cell cultures

Polychlorinated biphenyls (PCBs) are a group of persistent and widely dispersed environmental pollutants, some of which may be neurotoxic. In the present study, we have investigated the effect of PCB commercial mixtures (Aroclors) on neuronal cell cultures by assessing cell viability and apoptotic cell death. We have combined morphological and biochemical techniques to establish the relevance of apoptosis in neuronal cell death induced by Aroclors. Treatment with both Aroclor 1248 and Aroclor 1260 caused the loss of cell viability and accelerated apoptosis both in a concentration- and time-dependent manner. However, the extent of apoptosis resulted greater for Aroclor 1248 than for Aroclor 1260. This is correlated with the loss of cell viability since Aroclor 1248 is more cytotoxic. The apoptosis induced by Aroclors involves the increase of caspase-3 activity. To correlate the caspase-3 activity with respect to changes in protein processing, caspase-3 precursor protein (procaspase-3) was evaluated by Western blot analysis. Also, Bcl-2 and Bax protein were assessed in order to elucidate the cell death machinery induced in cortical neuronal cell cultures by Aroclor 1248. The results indicate that the increase in Aroclor-induced apoptosis correlates with a reduction in the expression of antiapoptotic Bcl-2 and an increase in the expression of proapoptotic Bax. These results suggest that, with our experimental conditions, Aroclors induce apoptosis in primary cultures of cortical neurons via proteins of the Bcl-2 and caspase families.

Quantitative proteomics analysis of specific protein expression and oxidative modification in aged senescence-accelerated-prone 8 mice brain

The senescence-accelerated mouse (SAM) is a murine model of accelerated senescence that was established using phenotypic selection. The SAMP series includes nine substrains, each of which exhibits characteristic disorders. SAMP8 is known to exhibit age-dependent learning and memory deficits. In our previous study, we reported that brains from 12-month-old SAMP8 have greater protein oxidation, as well as lipid peroxidation, compared with brains from 4-month-old SAMP8 mice. In order to investigate the relation between age-associated oxidative stress on specific protein oxidation and age-related learning and memory deficits in SAMP8, we used proteomics to identify proteins that are expressed differently and/or modified oxidatively in aged SAMP8 brains. We report here that in 12 month SAMP8 mice brains the expressions of neurofilament triplet L protein, lactate dehydrogenase 2 (LDH-2), heat shock protein 86, and alpha-spectrin are significantly decreased, while the expression of triosephosphate isomerase (TPI) is increased compared with 4-month-old SAMP8 brains. We also report that the specific protein carbonyl levels of LDH-2, dihydropyrimidinase-like protein 2, alpha-spectrin and creatine kinase, are significantly increased in the brain of 12-month-old SAMP8 mice when compared with the 4-month-old SAMP8 brain. These findings are discussed in reference to the effect of specific protein oxidation and changes of expression on potential mechanisms of abnormal alterations in metabolism and neurochemicals, as well as to the learning and memory deficits in aged SAMP8 mice.

Ischemia-induced increase in long-term potentiation is warded off by specific calpain inhibitor PD150606

In the present study, the effect of specific, membrane-permeable calpain inhibitor, PD150606, was analysed on synaptic efficacy in in vitro brain slices experiments after ischemic insult of rats in vivo, and on cell viability in a glutamate excitotoxicity test in mouse cell culture. Bilateral common carotid artery ligation (BCCL) for 24 h markedly increased calpain activity and enhanced LTP induction in rat hippocampus, although the CA1 layer significantly shrank. The enhancement of LTP could be diminished by short-term application of PD150606 (40 microM) into the perfusion solution. Intracerebroventricular administration of PD150606 (100 microM) parallel with ischemic insult prevented LTP and effectively inhibited hippocampal calpain activity. Intracerebroventricularly applied PD150606 inhibited the CA1 layer shrinkage after common carotid ligation. High level of exogenous glutamate caused marked decrease of cell viability in mouse cerebellar granule cell cultures, which could be partly warded off by 20 microM PD150606. Our data witness that calpain action is intricately involved in the regulation of synaptic efficacy.

The AMPA receptor potentiator LY404187 increases cerebral glucose utilization and c-fos expression in the rat

AMPA receptor potentiators enhance AMPA receptor-mediated glutamatergic neurotransmission and may have therapeutic potential as cognitive enhancers or antidepressants. The anatomical basis for the action of AMPA receptor potentiators is unknown. The aim of this study was to determine the effects of the biarylpropylsulfonamide AMPA receptor potentiator, LY404187 (0.05 to 5 mg/kg subcutaneously), upon cerebral glucose utilization and c-fos expression using 14C-2-deoxglucose autoradiography and c-fos immunocytochemistry. LY404187 (0.5 mg/kg) produced significant elevations in glucose utilization in 28 of the 52 anatomical regions analyzed, which included rostral neocortical areas and the hippocampus, as well the dorsal raphe nucleus, lateral habenula, and locus coeruleus. No significant decreases in glucose utilization were observed in any region after LY404187 administration. The increases in glucose utilization with LY404187 (0.5 mg/kg) were blocked by pretreatment with the AMPA receptor antagonist LY293558 (25 mg/kg), indicating that LY404187 acts through AMPA receptor-mediated mechanisms. LY404187 (0.5 mg/kg) also produced increases in c-fos immunoreactivity in the cortex, locus coeruleus, and the dorsal raphe nucleus. These studies demonstrate neuronal activation in key brain areas that are associated with memory processes and thus provide an anatomical basis for the cognitive enhancing effects of AMPA receptor potentiators.

Ubiquitin-proteasome-mediated local protein degradation and synaptic plasticity

A proteolytic pathway in which attachment of a small protein, ubiquitin, marks the substrates for degradation by a multi-subunit complex called the proteasome has been shown to function in synaptic plasticity and in several other physiological processes of the nervous system. Attachment of ubiquitin to protein substrates occurs through a series of highly specific and regulated steps. Degradation by the proteasome is subject to multiple levels of regulation as well. How does the ubiquitin-proteasome pathway contribute to synaptic plasticity? Long-lasting, protein synthesis-dependent, changes in the synaptic strength occur through activation of molecular cascades in the nucleus in coordination with signaling events in specific synapses. Available evidence indicates that ubiquitin-proteasome-mediated degradation has a role in the molecular mechanisms underlying synaptic plasticity that operate in the nucleus as well as at the synapse. Since the ubiquitin-proteasome pathway has been shown to be versatile in having roles in addition to proteolysis in several other cellular processes relevant to synaptic plasticity, such as endocytosis and transcription, this pathway is highly suited for a localized role in the neuron. Because of its numerous roles, malfunctioning of this pathway leads to several diseases and disorders of the nervous system. In this review, I examine the ubiquitin-proteasome pathway in detail and describe the role of regulated proteolysis in long-term synaptic plasticity. Also, using synaptic tagging theory of synapse-specific plasticity, I provide a model on the possible roles and regulation of local protein degradation by the ubiquitin-proteasome pathway.

Latent inhibition of cued fear conditioning: an NMDA receptor-dependent process that can be established in the presence of anisomycin

Much of the research examining the biological basis for long-term memories has focused on mechanisms that support the formation of conditioned associations. Less information is available on biological mechanisms which underlie processes that modify the strength of conditioned associations. Latent inhibition is a phenomenon by which pre-exposure to a to-be-conditioned stimulus (CS) weakens subsequent conditioning of that CS to an unconditioned stimulus (US). Here we report that latent inhibition of cued fear conditioning is dependent on NMDA receptor activation. MK-801 (1 mg/kg), an NMDA receptor antagonist, abolished latent inhibition of cued fear conditioning. This dose of MK-801 administered before training did not disrupt cued fear conditioning. Conversely, anisomycin (150 mg/kg), a protein synthesis inhibitor, had no effect on latent inhibition of cued fear conditioning when administered 20 min before, immediately after, or 2, 4, 6, or 8 h after CS pre-exposure. Furthermore, continuous anisomycin administration (50 mg/kg, administered every 2 h for 6 h starting 20 min prior to pre-exposure) did not disrupt latent inhibition of cued fear conditioning. In addition, anisomycin had no effect on a long-lasting version of latent inhibition of cued fear conditioning that was maintained over a 7-day interval. Anisomycin administered before training, however, disrupted learning of the CS-US association. These findings suggest that latent inhibition of cued fear conditioning is a long-lasting NMDA receptor-dependent process that can develop during the inhibition of protein synthesis.

Synaptic plasticity and AMPA receptor trafficking

Alterations in neuronal activity can elicit long-lasting changes in the strength of synaptic transmission at excitatory synapses and, as a consequence, may underlie many forms of experience-dependent plasticity, including learning and memory. The best-characterized forms of such synaptic plasticity are the long-term depression (LTD) and long-term potentiation (LTP) observed at excitatory synapses in the CA1 region of the hippocampus. It is now well accepted that the trafficking of AMPA receptors to and away from the synaptic plasma membrane plays an essential role in both LTP and LTD, respectively. Here we review current models of AMPA receptor trafficking and how this trafficking may be regulated at the molecular level in order to produce the observed changes in synaptic strength. We also review recent work from our lab suggesting that synaptic plasticity in the mesolimbic dopamine system may contribute importantly to the neural adaptations elicited by drugs of abuse.

Synapses with a segmented, completely partitioned postsynaptic density express more AMPA receptors than other axospinous synaptic junctions

Axospinous perforated synapses of one morphological subtype exhibit multiple transmission zones, each one being formed by an axon terminal protrusion apposing a postsynaptic density (PSD) segment and separated from others by complete spine partitions. Such segmented, completely partitioned (SCP) synapses have been implicated in synaptic plasticity and postulated to be exceptionally efficacious. The present study explored the validity of this supposition. Postembedding immunogold electron microscopy was used for quantifying the postsynaptic AMPA receptor (AMPAR) expression, which is widely regarded as a major determinant of synaptic efficacy. Various subtypes of axospinous synapses were examined in the rat CA1 stratum radiatum. The results showed that the number of immunogold particles for AMPARs in SCP synapses markedly and significantly exceeded that in other perforated subtypes (by 101% on the average) and in nonperforated immunopositive synapses (by 1086%). Moreover, the particle number per single PSD segment, each of which also contained NMDA receptors, was significantly higher than that per nonperforated PSD (by 485%). SCP synapses also exhibited a higher particle density per unit PSD area, as well as a larger overall PSD area as compared with other synaptic subtypes. Analysis of covariance revealed that the high AMPAR expression in SCP synapses was related to the segmented PSD configuration, not only to the PSD size. Moreover, the subpopulations of SCP and other perforated synapses with either overlapping or equal PSD sizes differed in AMPAR content and concentration, with both measures being significantly higher in SCP synapses. Thus, the elevated AMPAR expression in SCP synapses is associated with the presence of separate PSD segments, not only with their large PSD area. These findings are consistent with the idea that SCP synapses have a relatively greater efficacy and may support maximal levels of synaptic enhancement characteristic of certain forms of synaptic plasticity such as the early LTP phase.

Effects of fimbria-fornix transection on calpain and choline acetyl transferase activities in the septohippocampal pathway

The ability of fimbria-fornix bilateral axotomy to elicit calpain and caspase-3 activation in the rat septohippocampal pathway was determined using antibodies that selectively recognize either calpain- or caspase-cleaved products of the cytoskeletal protein alphaII-spectrin. Radioenzymatically determined choline acetyl transferase (ChAT) activity was elevated in the septum at day 5, but reduced in the dorsal hippocampus at days 3, 5 and 7, after axotomy. Prominent accumulation of calpain-, but not caspase-3-, cleaved spectrin proteolytic fragments was observed in both the septum and dorsal hippocampus 1-7 days after axotomy. ChAT-positive neuronal cell bodies in the septum also displayed calpain-cleaved spectrin indicating that calpain activation occurred in cholinergic septal neurons as a consequence of transection of the septohippocampal pathway. Calpain-cleaved alphaII-spectrin immunoreactivity was observed in cholinergic fibers coursing through the fimbria-fornix, but not in pyramidal neurons of the dorsal hippocampus, suggesting that degenerating cholinergic nerve terminals were the source of calpain activity in the dorsal hippocampus following axotomy. Accumulation of calpain-cleaved spectrin proteolytic fragments in the dorsal hippocampus and septum at day 5 after axotomy was reduced by i.c.v. administration of two calpain inhibitors. Calpain inhibition partially reduced the elevation of ChAT activity in the septum produced by transection but failed to decrease the loss of ChAT activity in the dorsal hippocampus following axotomy. These findings suggest that calpain activation contributes to the cholinergic cell body response and hippocampal axonal cytoskeletal degradation produced by transection of the septohippocampal pathway.

Inhibition of Mitogen-Activated Protein Kinase-Extracellular Signal-Regulated Kinase Disrupts Latent Inhibition of Cued Fear Conditioning in C57BL/6 Mice

The mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) cascade has been implicated in a variety of associative conditioning tasks. However, the role of the MAPK-ERK cascades in modulating conditioning is less clear. The authors examined the effect of the potent and selective MAPK-ERK inhibitor SL327 on latent inhibition of cued fear conditioning. The results demonstrate that 50 mg/kg and 100 mg/kg SL327 disrupt latent inhibition of cued fear conditioning. These data provide evidence for an essential role of the MAPK-ERK cascade in tasks that modulate the strength of associative conditioning. The results are discussed in relation to the molecular mechanisms that support latent inhibition of cued fear conditioning.

Memantine: an antiglutamatergic option for dementia

Alzheimer's disease (AD) is the most common form of dementia in occidental countries. Currently approved treatments for AD provide mainly symptomatic benefits without clear evidence of neuroprotection. N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in several central nervous system disorders, including neuroprotective treatment in chronic neurodegenerative diseases, and symptomatic treatment in other neurologic diseases. Memantine, an NMDA antagonist, has been recently approved for the treatment of advanced AD. Due to its mechanism of action, memantine is considered a neuroprotective drug, whose utility has been demonstrated in preclinical studies. In addition, memantine is a useful symptomatic treatment for AD and vascular dementia. This paper reviews both aspects of memantine as well as some basic mechanisms mediating cognition and glutamatergic neurodegeneration.

Transient and sustained types of long-term potentiation in the CA1 area of the rat hippocampus

Synaptic potentiation induced by high frequency stimulation was investigated by recording field excitatory postsynaptic potentials (f-EPSPs) in rat hippocampal slices. Potentiation consisted of a transient period of decaying f-EPSPs (short-term potentiation, STP) that led to a plateau of continuously potentiated f-EPSPs (long-term potentiation, LTP). Here we show that a previously unknown type of transient, use-dependent, long-lasting potentiation (t-LTP) can account for STP. t-LTP could be stored for more than 6 h and its decay was caused by synaptic activation. Both the expression and the decay of t-LTP were input specific. t-LTP was induced differently from conventional LTP in that the amplitude of t-LTP was dependent upon the stimulation frequency, whereas the magnitude of LTP was dependent on the number of stimuli in the induction train. The decay of t-LTP could not be prevented by the blockage of glutamate receptors, but was prevented by the blockage of stimulus-evoked neurotransmitter release, suggesting that t-LTP is expressed presynaptically. Paired-pulse stimulation experiments showed that the decay of t-LTP was mediated by a decrease in the probability of neurotransmitter release. The decline of t-LTP could be prolonged by the activation of NMDA receptors. Hence, both single and paired-pulse stimuli prolonged the decline of the t-LTP. This decline could be prevented by high frequency burst stimulation (200 Hz). We conclude that t-LTP allows dynamic modulation of synaptic transmission by providing not only spatial association but also temporal convergence between synaptic inputs. Therefore, t-LTP might be a substrate for the encoding of synaptic memory.

Ubiquitination, protein turnover, and long-term synaptic plasticity

For at least half a century, alteration of synaptic strength through growth at specific nerve terminals has been favored as the mechanism underlying long-term changes in behavior and synaptic plasticity. Although new proteins for synapses can either be synthesized locally or transported from the cell body, recent work on the postsynaptic element (dendritic spines) of cortical excitatory synapses indicates that transmission can also be modified by controlling the density of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors (AMPARs) in the postsynaptic density (PSD). This regulation is mediated by mono-ubiquitination, which governs turnover of AMPAR subunits by determining whether the endocytosed subunits are sent to lysosomes to be degraded or recycled back to the membrane by exocytosis. Also important is activity-dependent multi-ubiquitination and degradation of proteins that make up the scaffolding complexes that confine receptors to the PSD.

Adsorption of surfactant to bronchial epithelium: possible role of receptor 'unmasking' in asthma

BACKGROUND

In a recent study in animals it has been shown how surface-active phospholipid (SAPL) in the form of a commercially available micronized (5 micromphi) dry powder (ALECT/PumactantT) was able to reduce afferent neural feedback to the brainstem in response to a methacholine challenge by the same order of magnitude as drugs commonly prescribed for asthma. The underlying theory assumed that adsorption of SAPL to bronchial epithelium masked irritant receptors eliciting the bronchoconstrictor reflex, thus providing a barrier to noxious stimuli entering the lungs.

OBJECTIVE

To test the underlying assumption that SAPL was actually adsorbed (i.e., bound to bronchial epithelium), especially the major and most surface-active component of lung surfactant, namely dipalmitoyl phosphatidylcholine (DPPC). A secondary objective was to investigate any role of phosphatidylglycerol (PG) in promoting the adsorption of DPPC.

METHODS

Radiolabeled DPPC dispersed ultrasonically in saline was used to incubate excised sections of porcine bronchial epithelium. The adsorbed DPPC was then quantified by rigorously rinsing the tissue of adhering fluid and then digesting it for beta-scintillation counting. Each test (n=8 runs) was repeated for ratios of DPPC:PG of 9:1, 7:3 (as per ALECT/PumactantT) and 1:1 for both dipalmitoyl PG (DPPG) and EggPG (as incorporated in ALECT/PumactantT).

RESULTS

Despite rigorous rinsing postincubation, bronchial epithelium was found to adsorb DPPC at a level roughly equivalent to one close-packed monolayer; whereas both DPPG and EggPG promoted the adsorption of DPPC in a dose-dependent manner, reaching an approximate threefold increase for 7:3 DPPC:PG.

CONCLUSION

DPPC adsorbs to bronchial epithelium in amounts necessary for the masking of receptors, and this adsorption (probably chemisorption) is quite strongly promoted by PG either in its indigenous state (DPPG) or in the form (EggPG) used in ALECT to suppress the sensitivity of bronchial irritant receptors in our previous study and in clinical trials just completed.

The ubiquitin proteasome system functions as an inhibitory constraint on synaptic strengthening

BACKGROUND

Long-lasting forms of synaptic plasticity have been shown to depend on changes in gene expression. Although many studies have focused on the regulation of transcription and translation during learning-related synaptic plasticity, regulated protein degradation provides another common means of altering the macromolecular composition of cells.

RESULTS

We have investigated the role of the ubiquitin proteasome system in long-lasting forms of learning-related plasticity in Aplysia sensory-motor synapses. We find that inhibition of the proteasome produces a long-lasting (24 hr) increase in synaptic strength between sensory and motor neurons and that it dramatically enhances serotonin-induced long-term facilitation. The increase in synaptic strength produced by proteasome inhibitors is dependent on translation but not transcription. In addition to the increase in synaptic strength, proteasome inhibition leads to an increase in the number of synaptic contacts formed between the sensory and motor neurons. Blockade of the proteasome in isolated postsynaptic motor neurons produces an increase in the glutamate-evoked postsynaptic potential, and blockade of the proteasome in the isolated presynaptic sensory cells produces increases in neurite length and branching.

CONCLUSIONS

We conclude that both pre- and postsynaptic substrates of the ubiquitin proteasome function constitutively to regulate synaptic strength and growth and that the ubiquitin proteasome pathway functions in mature neurons as an inhibitory constraint on synaptic strengthening.

Fodrin CaM-binding domain cleavage by Pet from enteroaggregative Escherichia coli leads to actin cytoskeletal disruption

We have previously shown that the plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli produces cytotoxic and enterotoxic effects. Pet-intoxicated epithelial cells reveal contraction of the cytoskeleton and loss of actin stress fibres. Pet effects require its internalization into epithelial cells. We have also shown that Pet degrades erythroid spectrin. Pet delivery within the intestine suggests that Pet may degrade epithelial fodrin (non-erythroid spectrin). Here we demonstrate that Pet has affinity for alpha-fodrin (formally named alphaII spectrin) in vitro and in vivo and cleaves epithelial fodrin, causing its redistribution within the cells. When Pet has produced its cytoskeletal effects, fodrin is found in intracellular aggregates as membrane blebs. Pet cleaves recombinant GST-fodrin, generating two breakdown products of 37 and 72 kDa. Sequencing of the 37 kDa fragment demonstrated that the cleavage site occurred within fodrin's 11th repetitive unit between M1198 and V1199, in the calmodulin binding domain. Site-directed mutagenesis of these amino acids prevented fodrin degradation by Pet. Pet also cleaves epithelial fodrin from cultured Pet-treated cells. A mutant in the Pet serine protease motif was unable to cause fodrin redistribution or to cleave GST-fodrin. This is the first report showing cleavage of alpha-fodrin by a bacterial protease. Cleavage occurs in the middle of the calmodulin binding domain, which leads to cytoskeleton disruption.

Organization of working memory processes in monkeys: the effects of a dopamine receptor agonist

The effects of the dopamine receptor agonist agroclavin on cognitive processes associated with mechanisms of visual recognition and long-term and working (short-term) memory were studied in delayed visual differentiation and delayed spatial selection tasks in monkeys (rhesus macaques). Measurements made before and after p.o. pharmacological treatment with this agent were used to identify the p.o. dose (5 mg/kg) inducing a significant effect. The psychotropic effect of agroclavin, which induced cognitive dysfunction, was present in all the monkeys studied to one extent or another. Behavioral criteria were: the probability of correct solutions of the visual differentiation task, the probability of refusals to resolve the task, and the time taken for correct motor responses. Despite individual differences in these behavioral characteristics in monkeys, significant changes due to agroclavin were consistently evident in all animals. There was a reduction in the probability of correct solutions, due to worsening of the characteristics of short-term memory; most monkeys showed increases in the numbers of refusals to solve tasks and increases in the time for correct motor responses during these solutions. In fact, all monkeys showed no increases in the number of erroneous solutions in visual differeniation and spatial selection tasks without delays, though in most cases there were increases in the time taken for correct motor reactions and the number of refusals to solve tasks. Data were obtained indicating that the effect of agroclavin was not uniform with respect to different types of visual information. The possible structural-functional organization of processes underlying working memory is discussed on the basis of the conclusion that the behavioral characteristics studied here reflect different components of cognitive processes realized by structures with different functional properties and different locations.

Long-term potentiation in the Eocene

The first ten years of long-term potentiation (LTP) research are reviewed. Surprisingly, given the intensity of current interest, the discovery paper did not trigger a wave of follow-on experiments. Despite this, the initial work laid out what ultimately became standard questions and paradigms. The application of the then still novel hippocampal slice technique oriented LTP towards basic neuroscience, perhaps somewhat at the cost of lesser attention to its functional significance. The use of slices led to the discovery of the events that trigger the formation of LTP and provided some first clues about its extraordinary persistence. Signs of the intense controversy over the nature of LTP expression (release vs receptors) emerged towards the end of the first decade of work. What appears to be lacking in the literature of that time is a widespread concern about LTP and memory. This may reflect a somewhat different attitude that neurobiologists then had towards memory research and a perceived need to integrate the new potentiation phenomenon into the web of established science before advancing extended arguments about its contributions to behaviour.

AMPA receptor trafficking and long-term potentiation

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. A prominent example is long-term potentiation (LTP), whose mechanisms have been the subject of considerable scrutiny over the past few decades. I review studies from our laboratory that support a critical role for AMPA receptor trafficking in LTP and experience-dependent plasticity.

Expression mechanisms underlying long-term potentiation: a postsynaptic view

This review summarizes the various experiments that have been carried out to determine if the expression of long-term potentiation (LTP), in particular N-methyl-D-aspartate (NMDA) receptor-dependent LTP, is presynaptic or postsynaptic. Evidence for a presynaptic expression mechanism comes primarily from experiments reporting that glutamate overflow is increased during LTP and from experiments showing that the failure rate decreases during LTP. However, other experimental approaches, such as monitoring synaptic glutamate release by recording astrocytic glutamate transporter currents, have failed to detect any change in glutamate release during LTP. In addition, the discovery of silent synapses, in which LTP rapidly switches on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function at NMDA-receptor-only synapses, provides a postsynaptic mechanism for the decrease in failures during LTP. It is argued that the preponderance of evidence favours a postsynaptic expression mechanism, whereby NMDA receptor activation results in the rapid recruitment of AMPA receptors as well as a covalent modification of synaptic AMPA receptors.

Long-term potentiation and memory

The discovery of long-term potentiation (LTP) transformed research on the neurobiology of learning and memory. This did not happen overnight, but the discovery of an experimentally demonstrable phenomenon reflecting activity-driven neuronal and synaptic plasticity changed discussions about what might underlie learning from speculation into something much more concrete. Equally, however, the relationship between the discovery of LTP and research on the neurobiology of learning and memory has been reciprocal; for it is also true that studies of the psychological, anatomical and neurochemical basis of memory provided a developing and critical intellectual context for the physiological discovery. The emerging concept of multiple memory systems, from 1970 onwards, paved the way for the development of new behavioural and cognitive tasks, including the watermaze described in this paper. The use of this task in turn provided key evidence that pharmacological interference with an LTP induction mechanism would also interfere with learning, a finding that was by no means a foregone conclusion. This reciprocal relationship between studies of LTP and the neurobiology of memory helped the physiological phenomenon to be recognized as a major discovery.

Elements of a neurobiological theory of the hippocampus: the role of activity-dependent synaptic plasticity in memory

The hypothesis that synaptic plasticity is a critical component of the neural mechanisms underlying learning and memory is now widely accepted. In this article, we begin by outlining four criteria for evaluating the 'synaptic plasticity and memory (SPM)' hypothesis. We then attempt to lay the foundations for a specific neurobiological theory of hippocampal (HPC) function in which activity-dependent synaptic plasticity, such as long-term potentiation (LTP), plays a key part in the forms of memory mediated by this brain structure. HPC memory can, like other forms of memory, be divided into four processes: encoding, storage, consolidation and retrieval. We argue that synaptic plasticity is critical for the encoding and intermediate storage of memory traces that are automatically recorded in the hippocampus. These traces decay, but are sometimes retained by a process of cellular consolidation. However, we also argue that HPC synaptic plasticity is not involved in memory retrieval, and is unlikely to be involved in systems-level consolidation that depends on HPC-neocortical interactions, although neocortical synaptic plasticity does play a part. The information that has emerged from the worldwide focus on the mechanisms of induction and expression of plasticity at individual synapses has been very valuable in functional studies. Progress towards a comprehensive understanding of memory processing will also depend on the analysis of these synaptic changes within the context of a wider range of systems-level and cellular mechanisms of neuronal transmission and plasticity.

Stress, synaptic plasticity, and psychopathology

It is now generally recognized that stressful events play a critical role in the genesis of psychopathology. The neurobiological mechanisms that mediate the contribution of stressful events to the manifestation of psychiatric disorders may include changes in synaptic efficacy in different brain areas. Numerous studies in animals have begun to identify some of these areas through experiments manipulating stressful components. This review focuses on alterations of synaptic efficacy in the hippocampus, the lateral septum, and the medial prefrontal cortex that mimic the pathophysiology of depression and post-traumatic stress disorder.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

Contribution of NMDA receptor channels to the expression of LTP in the hippocampal dentate gyrus

The role of glutamatergic NMDA receptor channels (NMDARs) in the induction of long-term potentiation (LTP) has been well established. In contrast, whether or not NMDARs contribute to the expression of LTP has been an issue of debate. In this study, we investigated the contribution of NMDARs to LTP expression in the hippocampal dentate gyrus (DG) by stimulating perforant path afferents with short bursts of pulses delivered at a moderate frequency (40 Hz), instead of using the traditional protocol of a single stimulus at a low frequency (<0.1 Hz). The synaptic summation provided by the "burst" protocol enabled us to measure the NMDAR-mediated component of synaptic responses (NMDA component), defined as the NMDAR antagonist D-2-amino-5-phosphonovalerate (APV2+)-sensitive component, in the presence of physiological concentrations of Mg (1 mM). Intracellular recordings were obtained from DG granule cells of rabbit hippocampal slices, and excitatory postsynaptic potentials (EPSPs) were measured in terms of the integrated area of their profiles. At 40 Hz, frequency facilitation of the evoked EPSPs was observed. The NMDA component gradually increased during the five-pulse train and frequency facilitation was significantly reduced after the application of APV. We tested the hypothesis that NMDARs undergo potentiation in LTP by comparing the NMDA/non-NMDA ratio of the synaptic responses in control and LTP groups. An increase in the ratio was observed in the LTP group, strongly suggesting potentiation of NMDARs. To infer changes in conductance at individual synapses based on EPSPs recorded at the soma, we constructed a compartmental model of a morphologically reconstructed DG granule cell. The effect on the NMDA/non-NMDA ratio of changes in AMPA and NMDA component synaptic conductance, and of differences in the distribution of activated synapses, was studied with computer simulations. The results confirmed that NMDARs are potentiated after the induction of LTP and contribute significantly to the expression of potentiation under physiological conditions.

New life in an old idea: the synaptic plasticity and memory hypothesis revisited

The notion that changes in synaptic efficacy underlie learning and memory processes is now widely accepted, although definitive proof of the synaptic plasticity and memory hypothesis is still lacking. This article reviews recent evidence relevant to the hypothesis, with particular emphasis on studies of experience-dependent plasticity in the neocortex and hippocampus. In our view, there is now compelling evidence that changes in synaptic strength occur as a consequence of certain forms of learning. A major challenge will be to determine whether such changes constitute the memory trace itself or play a less specific supporting role in the information processing that accompanies memory formation.

Constitutive cycling: a general mechanism to regulate cell surface proteins

Cells can change their function by rapidly modulating the levels of certain proteins at the plasma membrane. This rapid modulation is achieved by using a specialised trafficking process called constitutive cycling. The constitutive cycling of a variety of transmembrane proteins such as receptors, channels and transporters has recently been directly demonstrated in a wide range of cell types. This regulation is thought to underlie important biological phenomena such as learning and memory, gastric acid secretion and water and blood glucose homeostasis. This review discusses the molecular mechanisms of constitutive cycling, its regulation by extracellular agents such as hormones and its misregulation in disease states.

Proteases involved in long-term potentiation

Much attention has been paid to proteases involved in long-term potentiation (LTP). Calpains, Ca-dependent cysteine proteases, have first been demonstrated to be the mediator of LTP by the proteolytic cleavage of fodrin, which allows glutamate receptors located deep in the postsynaptic membrane to move to the surface. It is now generally considered that calpain activation is necessary for LTP formation in the cleavage of substrates such as protein kinase Czeta, NMDA receptors, and the glutamate receptor-interacting protein. Recent studies have shown that serine proteases such as tissue-type plasminogen activator (tPA), thrombin, and neuropsin are involved in LTP. tPA contributes to LTP by both receptor-mediated activation of cAMP-dependent protein kinase and the cleavage of NMDA receptors. Thrombin induces a proteolytic activation of PAR-1, resulting in activation of protein kinase C, which reduces the voltage-dependent Mg2+ blockade of NMDA receptor-channels. On the other hand, neuropsin may act as a regulatory molecule in LTP via its proteolytic degradation of extracellular matrix protein such as fibronectin. In addition to such neuronal proteases, proteases secreted from microglia such as tPA may also contribute to LTP. The enzymatic activity of each protease is strictly regulated by endogenous inhibitors and other factors in the brain. Once activated, proteases can irreversibly cleave peptide bonds. After cleavage, some substrates are inactivated and others are activated to gain new functions. Therefore, the issue to identify substrates for each protease is very important to understand the molecular basis of LTP.

Kainate-induced excitotoxicity is dependent upon extracellular potassium concentrations that regulate the activity of AMPA/KA type glutamate receptors

In addition to well-known N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity, recent studies suggest that non-NMDA type ionotropic glutamate receptors are also important mediators of excitotoxic neuronal death, and that their functional expression can be regulated by the cellular environment. In this study, we used cerebellar granule cells (CGCs) in culture to investigate kainate (KA)-induced excitotoxicity. Although previous reports indicated that KA induces apoptosis of CGCs in culture, no KA-induced excitotoxic cell death was observed in CGCs treated with KA when cells were maintained in high potassium media (24 mm K+). In contrast, when mature CGCs were shifted into low potassium media (3 mm K+), KA produced significant excitotoxicity. In electrophysiological studies, the KA-induced inward current density was significantly elevated in CGCs shifted into low K+ media compared with those maintained in high K+ media. Non-desensitizing aspects of KA currents observed in this study suggest that these responses were mediated by AMPA rather than KA receptors. In immunofluorescence studies, the surface expression of GluR1 subunits increased when mature CGCs were shifted into a low K+ environment. This study suggests that KA-induced excitotoxicity in mature CGCs is dependent upon the extracellular potassium concentration, which modulates functional expression and excitability of AMPA/KA receptors.

Regulation of AMPA receptors during synaptic plasticity

Ionotropic neurotransmitter receptors mediate rapid synaptic transmission in the CNS and PNS. Owing to this central role in trans-synaptic signal transduction, modulation of these receptors could play a crucial role in the expression of synaptic plasticity in the brain. AMPA receptors mediate the majority of rapid excitatory synaptic transmission in the CNS. Recent studies have indicated that the activity and synaptic distribution of these receptors is dynamically regulated and could be crucial for the short- and long-term modification of synaptic efficacy. Here we review recent data on the molecular mechanisms that underlie the modulation of AMPA receptors and the role of AMPA-receptor regulation in mediating synaptic plasticity.

Memory enhancement: the search for mechanism-based drugs

Rapid progress has been made in understanding the synaptic changes required for memory encoding. Several companies are now attempting to use information about the induction and consolidation phases of this process to build memory-enhancing drugs. These efforts have produced novel compounds that improve retention scores across a broad range of tests and species. Initial clinical results are encouraging. Issues now arise about appropriate applications of candidate drugs and optimal cellular targets for future development.

Glutamate receptor trafficking in synaptic plasticity

Ionotropic glutamate receptors mediate excitatory synaptic transmission at most central mammalian synapses. In addition to converting the chemical signal released from the presynaptic terminal to an electrical response in the postsynaptic neuron, these receptors are critically involved in activity-dependent, long-term changes in synaptic strength and, therefore, are central to processes thought to underlie learning and memory. Several mechanisms have been proposed to play roles in altering synaptic strength, and it is clear that there are several different forms of long-term synaptic plasticity in the mammalian brain. Here, we review recent evidence that some forms of synaptic strengthening rely on the modification of the glutamate receptor complement at synapses in response to activity-dependent processes.

AMPA receptor trafficking and synaptic plasticity

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. Two prominent examples are long-term potentiation (LTP) and long-term depression (LTD), whose mechanisms have been the subject of considerable scrutiny over the past few decades. Here we review the growing literature that supports a critical role for AMPA receptor trafficking in LTP and LTD, focusing on the roles proposed for specific AMPA receptor subunits and their interacting proteins. While much work remains to understand the molecular basis for synaptic plasticity, recent results on AMPA receptor trafficking provide a clear conceptual framework for future studies.

Chronic exposure to trichloroethylene affects neuronal plasticity in rat hippocampal slices

Inhalational exposure to organic solvents is known to exert neurotoxic effects. Using the new multielectrode dish system (Panasonic) the effects of chronic exposure to trichloroethylene (TCE) on neuronal plasticity were assessed in different regions of the adult rat brain. Two groups of Long-Evans rats were exposed to 0 ppm or 500 ppm TCE, respectively, 6 h/day, 5 days/week for 6 months. Long-term potentiation (LTP) as well as paired-pulse potentiation/inhibition were assessed in slices from the visual cortex and the hippocampus. In addition, several behavioral tests were performed. Trichloroethanol concentrations were measured in blood and trichloroacetic acid concentrations were determined in urine. While TCE exposure impaired LTP as well as paired-pulse potentiation in hippocampal slices, no effects were seen in cortical slices. Our data demonstrate brain region specific functional changes following TCE exposure with the hippocampus being more vulnerable than the visual cortex. The behavioral measurements revealed no TCE related effects.

Neural plasticity and the brain renin-angiotensin system

The brain renin-angiotensin system mediates several classic physiologies including body water balance, maintenance of blood pressure, cyclicity of reproductive hormones and sexual behaviors, and regulation of pituitary gland hormones. In addition, angiotensin peptides have been implicated in neural plasticity and memory. The present review initially describes the extracellular matrix (ECM) and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases, and tissue inhibitors of metalloproteinases in the maintenance and degradation of the ECM. It is the ECM that appears to permit synaptic remodeling and thus is critical to the plasticity that is presumed to underlie mechanisms of memory consolidation and retrieval. The interrelationship among long-term potentiation (LTP), CAMs, and synaptic strengthening is described, followed by the influence of angiotensins on LTP. There is strong support for an inhibitory influence by angiotensin II (AngII) and a facilitory role by angiotensin IV (AngIV), on LTP. Next, the influences of AngII and IV on associative and spatial memories are summarized. Finally, the impact of sleep deprivation on matrix metalloproteinases and memory function is described. Recent findings indicate that sleep deprivation-induced memory impairment is accompanied by a lack of appropriate changes in matrix metalloproteinases within the hippocampus and neocortex as compared with non-sleep deprived animals. These findings generally support an important contribution by angiotensin peptides to neural plasticity and memory consolidation.

A Role for Nitric Oxide in Long-term Potentiation

Nitric oxide production in the cerebellum and induction of long-term potentiation (LTP) in the hippocampus have some characteristics in common: both phenomena are induced by activation of N-methyl-d-aspartate receptors and both are highly dependent on calcium-mediated processes. Here we provide evidence that endogenous nitric oxide production is necessary for synaptic plasticity in the CA1 hippocampus of the rat. LTP recorded in slices was blocked in a concentration-dependent manner by the nitric oxide synthase inhibitors l-NG-nitroarginine and l-NG-nitroarginine methyl ester, but l-NG-monomethylarginine was only marginally active. Bathing the slices with haemoglobin, a protein that scavenges nitric oxide, also resulted in a concentration-dependent blockade of LTP. Nitric oxide released locally from hydroxylamine produced a stable potentiation of synaptic transmission that was not additive with LTP induced by high-frequency stimulation. These results are fully consistent with the presumed retrograde messenger role of nitric oxide in LTP.

Long-term potentiation alters the modulator pharmacology of AMPA-type glutamate receptors

Changes in the biophysical properties of AMPA-type glutamate receptors have been proposed to mediate the expression of long-term potentiation (LTP). The present study tested if, as predicted from this hypothesis, AMPA receptor modulators differentially affect potentiated versus control synaptic currents. Whole cell recordings were collected from CA1 pyramidal neurons in hippocampal slices from adult rats. Within-neuron comparisons were made of the excitatory postsynaptic currents (EPSCs) elicited by two separate groups of Schaffer-collateral/commissural synapses. LTP was induced by theta burst stimulation in one set of inputs; cyclothiazide (CTZ), a drug that acts on the desensitization kinetics of AMPA receptors, was infused 30 min later. The decay time constants of the potentiated EPSCs prior to drug infusion were slightly, but significantly, shorter than those of control EPSCs. CTZ slowed the decay of the EPSCs, as reported in prior studies, and did so to a significantly greater degree in the potentiated synapses. Additionally, infusion of CTZ resulted in significantly greater effects on amplitude in potentiated pathways as compared with control pathways. The interaction between LTP and CTZ was also obtained in a separate set of experiments in which GABA receptor antagonists were used to block inhibitory postsynaptic currents. Additionally, there was no significant change in paired-pulse facilitation in the presence of CTZ, indicating that presynaptic effects of the drug were negligible. These findings provide new evidence that LTP modifies AMPA receptor kinetics. Candidates for the changes responsible for the observed effects of LTP were evaluated using a model of AMPA receptor kinetics; a simple increase in the channel opening rate provided the most satisfactory match with the LTP data.

Molecular bases of long-term memories: a question of persistence

The most distinctive attribute of long-term memory is persistence over time. New studies have uncovered many aspects of the molecular and cellular biology of synaptic plasticity, and the acquisition and consolidation of memory, which are thought to depend on synaptic plasticity. Much less, however, is known about the molecular and cellular biology of long-term memory persistence. Recent findings in the field are construed within the conceptual framework that proposes that consolidation and persistence of long-term memories require modulation of gene expression, which can culminate in synaptic remodeling. Whether modulation of gene expression, and particularly the ensuing morphological plasticity of the synapse, is permissive, causal or sufficient for the materialization and persistence of the long-term trace is, as yet, undetermined. How persistent is persistence? Renewed interest is focused on the possibility that some long-term memories consolidate anew with retrieval, and could, under certain conditions, become transiently shaky in this period of reconsolidation.

Gelatinase B and TIMP-1 are regulated in a cell- and time-dependent manner in association with neuronal death and glial reactivity after global forebrain ischemia

Matrix metalloproteinases (MMPs) belong to a large family of endopeptidases that regulate the pericellular environment through the cleavage of protein components of the extracellular matrix, membrane receptors and cytokines. MMP activity is controlled by the multifunctional tissue inhibitors of metalloproteinases (TIMPs). Proteases and their inhibitors are critically involved in developmental and pathological processes in numerous organs, including the brain. Global transient cerebral ischemia induces selective delayed neuronal death and neuroinflammation. We compared, in discrete vulnerable and resistant areas of the ischemic rat hippocampus, the kinetics and cellular distribution of gelatinase B and its principal inhibitor TIMP-1 and we assessed by in situ zymography, the net gelatinolytic activity at the cellular level. We show that gelatinases are expressed and active in neurons, suggesting that MMPs play a role in maintaining neural homeostasis. In the ischemic rat brain, expression and activity of gelatinase B, and expression of TIMP-1 are altered in a time-, region- and cell-dependent manner. Gelatinase B is induced first in reactive microglia and subsequently in reactive astrocytes. In situ, increases in gelatinase activity accompanied the progression of neuronal death and glial reactivity. Our results suggest that MMPs and TIMPs are involved in cell viability and tissue remodelling in the ischemic brain, and reinforces the idea that the MMP/TIMP system contributes both to neuronal demise and tissue repair in the context of glial reactivity.

Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

Considerable evidence now suggests an interrelationship among long-term potentiation (LTP), extracellular matrix (ECM) reconfiguration, synaptogenesis, and memory consolidation within the mammalian central nervous system. Extracellular matrix molecules provide the scaffolding necessary to permit synaptic remodeling and contribute to the regulation of ionic and nutritional homeostasis of surrounding cells. These molecules also facilitate cellular proliferation, movement, differentiation, and apoptosis. The present review initially focuses on characterizing the ECM and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), in the maintenance and degradation of the ECM. The induction and maintenance of LTP is described. Debate continues over whether LTP results in some form of synaptic strengthening and in turn promotes memory consolidation. Next, the contribution of CAMs and TIMPs to the facilitation of LTP and memory consolidation is discussed. Finally, possible roles for angiotensins, MMPs, and tissue plasminogen activators in the facilitation of LTP and memory consolidation are described. These enzymatic pathways appear to be very important to an understanding of dysfunctional memory diseases such as Alzheimer's disease, multiple sclerosis, brain tumors, and infections.

The ubiquitin-proteasome cascade is required for mammalian long-term memory formation

It has been recently demonstrated that ubiquitin-proteasome-mediated proteolysis is required for long-term synaptic facilitation in Aplysia. Here we show that the hippocampal blockade of this proteolytic pathway is also required for the formation of long-term memory in the rat. Bilateral infusion of lactacystin, a specific proteasome inhibitor, to the CA1 region caused full retrograde amnesia for a one-trial inhibitory avoidance learning when given 1, 4 or 7h, but not 10 h, after training. Proteasome inhibitor I produced similar effects. In addition, inhibitory avoidance training resulted in an increased ubiquitination and 26S proteasome proteolytic activity and a decrease in the levels of IkappaB, a substrate of the ubiquitin-proteasome cascade, in hippocampus 4 h after training. Together, these findings indicate that the ubiquitin-proteasome cascade is crucial for the establishment of LTM in the behaving animal.

Restless AMPA receptors: implications for synaptic transmission and plasticity

A central assumption in neurobiology holds that changes in the strength of individual synapses underlie changes in behavior. This concept is widely accepted in the case of learning and memory where LTP and LTD are the most compelling cellular models. It is therefore of great interest to understand, on a molecular level, how the brain regulates the strength of neuronal connections. We review a large body of evidence in support of the very straightforward regulation of synaptic strength by changing the number of postsynaptic receptors, and discuss the molecular machinery required for insertion and removal of AMPA receptors.