Linear relation between the magnitude of long-term potentiation in the dentate gyrus and associative learning in the rat. A demonstration using commissural inhibition and local infusion of an N-methyl-D-aspartate receptor antagonist

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.

Ramipril mitigates radiation-induced impairment of neurogenesis in the rat dentate gyrus

BACKGROUND

Sublethal doses of whole brain irradiation (WBI) are commonly administered therapeutically and frequently result in late delayed radiation injuries, manifesting as severe and irreversible cognitive impairment. Neural progenitors within the subgranular zone (SGZ) of the dentate gyrus are among the most radiosensitive cell types in the adult brain and are known to participate in hippocampal plasticity and normal cognitive function. These progenitors and the specialized SZG microenvironment required for neuronal differentiation are the source of neurogenic potential in the adult dentate gyrus, and provide a continuous supply of immature neurons which may then migrate into the adjacent granule cell layer to become mature granule cell neurons. The extreme radiosensitivity of these progenitors and the SGZ microenvironment suggests the hippocampus as a prime target for radiation-induced cognitive impairment. The brain renin-angiotensin system (RAS) has previously been implicated as a potent modulator of neurogenesis within the SGZ and selective RAS inhibitors have been implicated as mitigators of radiation brain injury. Here we investigate the angiotensin converting enzyme (ACE) inhibitor, ramipril, as a mitigator of radiation injury in this context.

METHODS

Adult male Fisher 344 rats received WBI at doses of 10 Gy and 15 Gy. Ramipril was administered beginning 24 hours post-WBI and maintained continuously for 12 weeks.

RESULTS

Ramipril produced small but significant reductions in the deleterious effects of radiation on progenitor proliferation and neuronal differentiation in the rat dentate gyrus following 10 Gy-WBI, but was not effective following 15 Gy-WBI. Ramipril also reduced the basal rate of neurogenesis within the SGZ in unirradiated control rats.

CONCLUSIONS

Our results indicate that chronic ACE inhibition with ramipril, initiated 24 hours post-irradiation, may reduce apoptosis among SGZ progenitors and/or inflammatory disruption of neurogenic signaling within SGZ microenvironment, and suggest that angiotensin II may participate in maintaining the basal rate of granule cell neurogenesis.

Cortical activation during Pavlovian fear conditioning depends on heart rate response patterns: An MEG study

In the present study, we examined stimulus-driven neuromagnetic activity in a delayed Pavlovian aversive conditioning paradigm using steady state visual evoked fields (SSVEF). Subjects showing an accelerative heart rate (HR) component to the CS+ during learning trials exhibited an increased activation in sensory and parietal cortex due to CS+ depiction in the extinction block. This was accompanied by a selective orientation response (OR) to the CS+ during extinction as indexed by HR deceleration. However, they did not show any differential cortical activation patterns during acquisition. In contrast, subjects not showing an accelerative HR component but rather unspecific HR changes during learning were characterized by greater activity in left orbito-frontal brain regions in the acquisition block but did not show differential SSVEF patterns during extinction. The results suggest that participants expressing different HR responses also differ in their stimulus-driven neuromagnetic response pattern to an aversively conditioned stimulus.

Differential Amplification of Intron-containing Transcripts Reveals Long Term Potentiation-associated Up-regulation of Specific Pde10A Phosphodiesterase Splice Variants

We employed differential display of expressed mRNAs (Liang, P., and Pardee, A. B. (1992) Science 257, 967-971) to identify genes up-regulated after long term potentiation (LTP) induction in the hippocampus of awake adult rats. In situ hybridization confirmed the differential expression of five independently amplified clones representing two distinct transcripts, cl13/19/90 and cl95/96. Neither cl13/19/90 nor cl95/96 showed significant sequence homology to known transcripts (mRNA or expressed sequence tag) or to the mouse or human genome. However, comparison with the rat genome revealed that they are localized to a predicted intron of the phosphodiesterase Pde10A gene. cl13/19/90 and cl95/96 are likely to be part of the Pde10A primary transcript as, using reverse transcriptase-PCR, we could specifically amplify distinct introns of the Pde10A primary transcript, and in situ hybridization demonstrated that a subset of Pde10A splice variants are also up-regulated after LTP induction. These results indicate that amplification of a primary transcript can faithfully report gene activity and that differential display can be used to identify differential expression of RNA species other than mRNA. In transiently transfected Cos7 cells, Pde10A3 reduces the atrial natriuretic peptide-induced elevation in cGMP levels without affecting basal cGMP levels. This cellular function of LTP-associated Pde10A transcripts argues for a role of the cGMP/cGMP-dependent kinase pathway in long term synaptic plasticity.

Long-term synaptic morphometry changes after induction of long-term potentiation and long-term depression in the dentate gyrus of awake rats are not simply mirror phenomena

Mechanisms of expression of long-term synaptic plasticity are believed to involve morphological changes of the activated synapses and remodelling of connectivity. Here, we investigated changes in synaptic and neuronal parameters in the dentate gyrus 24 h after induction of long-term potentiation (LTP) and long-term depression (LTD) in awake rats. In dentate granule cells, tetanization of the medial or lateral perforant paths induces LTP in specific synaptic bands along the dendrites in the middle and outer molecular layers, respectively, and tetanization of the lateral path induces robust LTD heterosynaptically in the middle molecular layer. This functional segregation allowed us to assess morphological changes associated with LTP and LTD in each pathway in the same population of neurons. Electron microscopy and unbiased counting methods were used to estimate neuronal density, axospinous, axodendritic and perforated synapse density, multiple synapse bouton density and postsynaptic density (PSD) area. Whereas there was no change in neuronal density, PSD area and multiple synapse boutons 24 h after either LTP or LTD, there was a noninput-specific increase in unperforated axospinous synapses after both LTP and LTD. However, we found that LTP of the medial, but not lateral, perforant path is associated with a specific increase in perforated axospinous synapses in the potentiated area. We also show that heterosynaptic LTD is associated with an input-specific increase in axodendritic synapse density. These results suggest that each perforant pathway may differ with respect to the nature of LTP-induced long-term changes and show that morphologically LTD is not simply the converse of LTP.

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.

LTP but not seizure is associated with up-regulation of AKAP-150

We have used differential display to profile and compare the mRNAs expressed in the hippocampus of freely moving animals after the induction of long-term potentiation (LTP) at the perforant path-dentate gyrus synapse with control rats receiving low-frequency stimulation. We have combined this with in situ hybridization and have identified A-kinase anchoring protein of 150 kDa (AKAP-150) as a gene selectively up-regulated during the maintenance phase of LTP. AKAP-150 mRNA has a biphasic modulation in the dentate gyrus following the induction of LTP. The expression of AKAP-150 was 29% lower than stimulated controls 1 h after the induction of LTP. Its expression was enhanced 3 (50%), 6 (239%) and 12 h (210%) after induction, returning to control levels by 24 h postinduction. The NMDA receptor antagonist CPP blocked the tetanus-induced modulation of AKAP-150 expression. Interestingly, strong generalized stimulation produced by electroconvulsive shock did not increase the expression of AKAP-150. This implies that the AKAP-150 harbours a novel property of selective responsiveness to the stimulation patterns that trigger NMDA-dependent LTP in vivo. Its selective up-regulation during LTP and its identified functions as a scaffold for protein kinase A, protein kinase C, calmodulin, calcineurin and ionotropic glutamate receptors suggest that AKAP-150 encodes is an important effector protein in the expression of late LTP.

Long-term Activation of Phosphoinositide Turnover Associated with Increased Release of Amino Acids in the Dentate Gyrus and Hippocampus Following Classical Conditioning in the Rat

The release of amino acids and the hydrolysis of inositol phospholipids were examined in parallel in three hippocampal areas following classical conditioning. Paired or unpaired tone(CS) - shock(US) presentations were given to animals engaged in a previously acquired food-motivated lever-pressing task. Conditioned suppression of lever-pressing was the behavioural measure of conditioning. Twenty-four hours after the last conditioning session, the dentate gyrus and areas CA3 and CA1 of the hippocampus were removed bilaterally from conditioned and pseudoconditioned animals, and slices cut and stored in liquid nitrogen for subsequent analysis. Crude synaptosomal pellets were prepared to investigate: (i) potassium-stimulated release of preloaded [3H]glutamate and [14C]aspartate in the presence and absence of extracellular Ca2+; (ii) [3H]inositol labelling of phosphoinositides and inositol phosphates; and (iii) [14C]arachidonic acid labelling of 1,2-diacylglycerol (1,2-DG). Potassium-stimulated, Ca2+-dependent release of [3H]glutamate in synaptosomes prepared from the dentate gyrus and area CA3 was significantly greater in conditioned animals than in pseudoconditioned animals. In area CA1, K+-stimulated, Ca2+-dependent release of [14C]aspartate was significantly increased in conditioned animals. These results confirm in synaptosomes, and extend to a period of 24 h our previous report of an increased release of transmitter in the dentate gyrus and hippocampus associated with classical conditioning. In parallel with the increased release of amino acids, learning was associated with a significant increase in labelling of phosphoinositides and inositol phosphates by [3H]inositol and a significant increase in labelling of 1,2-DG by [14C]arachidonic acid in the three hippocampal areas examined. It is suggested that a long-lasting presynaptic activation of inositol lipid metabolism may contribute to the learning-dependent increase in the capacity of hippocampal terminals to release transmitter and hence to the maintenance of a neurochemical trace which may, at least in part, underlie lasting changes in synaptic function built up during associative learning.

Activation of AP5-sensitive NMDA Receptors is Not Required to Induce LTP of Synaptic Transmission in the Lateral Perforant Path

The role of the N-methyl-d-aspartate (NMDA) type of glutamate receptor in long-term potentiation (LTP) of the medial (MPP) and lateral (LPP) divisions of the perforant path - granule cell system was investigated in urethane-anaesthetized rats. A stimulating electrode was positioned in the dorsomedial or ventrolateral aspect of the angular bundle for selective activation of either the MPP or LPP, respectively. A push - pull cannula served to focally perfuse artificial cerebrospinal fluid (ACSF) across the perforant path synaptic zone, while evoked potentials were monitored in the dentate hilus. Identification of LPP and MPP responses was based on (1) differences in population excitatory postsynaptic potential (EPSP) waveform obtained during stimulus depth profiles, and (2) differential sensitivity of evoked EPSPs to the glutamate receptor agonist l-aminophosphonobutyrate (AP4), and the antagonist gamma-d-glutamylglycine (DGG). High-frequency stimulation (400 Hz, 8 bursts of 8 pulses) applied to the lateral and medial perforant path elicited LTP of the EPSP and population spike in rats perfused with standard medium. In the MPP, LTP was almost completely blocked when d-aminophosphonopentanoate (AP5; 100 microM), a selective NMDA receptor antagonist, was perfused during the tetanus. Surprisingly, in the LPP experiments, AP5 did not impair induction of the 'synaptic' EPSP component of LTP. This occurred despite the ability of AP5 to block LTP of the LPP evoked population spike. The results suggest the existence of a novel, NMDA receptor-independent form of synaptic LTP in the lateral perforant path.

Spatio-temporal heterogeneity and cell-specificity of long-term potentiation-induced mRNA expression in the dentate gyrus in vivo

Gene expression in neurones can vary in response to neuronal activation. In this study, to analyse the spatio-temporal dynamics of the transcriptional response of three genes following the induction of long-term potentiation within the entire dentate gyrus in vivo, two new complementary approaches based on in situ hybridisation were developed: three-dimensional reconstruction of the pattern of mRNA expression within the entire dentate gyrus; and radioactive co-detection of two mRNA species allowing quantification of two different mRNAs in the same brain section. Zif268, Homer and syntaxin 1B genes were studied, and their regulated expression was examined three times after the induction of long-term potentiation. Constitutive expression of each gene under control conditions was homogeneous, but the spatial distribution of mRNA was heterogeneous along the rostro-caudal axis of the dentate gyrus following the induction of long-term potentiation, and different for each gene. In addition, the intensity of each gene-specific pattern of expression varied over time following the induction of long-term potentiation. Our results reveal that long-term potentiation differentially modulates the expression of mRNA species in cells of the dentate gyrus depending on their position along the rostro-caudal axis, on the gene and on time. We suggest that there are several molecular mechanisms of long-term potentiation, differing from one cluster of cells of the dentate gyrus to another, or that the different signaling pathways involved in long-term potentiation are used with varying efficiencies by different cells.

Ntab, a novel non-coding RNA abundantly expressed in rat brain

We have identified a novel transcript that is abundantly and specifically expressed in both the adult and developing rat CNS. Within the full-length cDNA sequence we were unable to identify a clear open reading frame. Moreover, we were unable to detect any protein product derived from the full-length cDNA sequence using an in vitro translation assay. Therefore, we suggest this gene is one of a growing number of non-coding mRNA-like RNA transcripts that exert their cellular functions directly as an RNA. We have named this novel gene Ntab for non-coding transcript abundantly expressed in brain (accession number AY035551). In addition, in some regions of the brain we find evidence for RNA accumulation in cellular processes at some distance from the soma. These findings suggest that Ntab is actively transported and may function within cellular processes. Since Ntab is a targeted non-coding RNA, such cellular functions could include the targeting and/or regulation of localised translation of other mRNA species.

Regulated expression of the neuronal calcium sensor-1 gene during long-term potentiation in the dentate gyrus in vivo

Neuronal calcium sensor-1 (NCS-1), the mammalian homologue of frequenin, is a member of a highly conserved family of neuron-specific calcium-binding proteins which has been implicated in exocytosis and in multiple calcium-signalling pathways, suggesting a potential involvement in mechanisms of neuronal plasticity. Here, using in situ hybridization, we report an increased induction of the mRNA encoding NCS-1 in dentate granule cells following the induction of long-term potentiation in the awake rat. We show that NCS-1 mRNA levels are increased 1 and 3 h after long-term potentiation in an N-methyl-D-aspartate receptor-dependent manner, returning to baseline expression levels by 6 h. Electroconvulsive stimulation also induced NCS-1 mRNA transcription in the dentate gyrus, but at the different time of 6 h post-seizure, returning to baseline by 12 h. These results show that regulated expression of the NCS-1 gene is part of the transcriptional response associated with activity-dependent neuronal plasticity in vivo and suggest a molecular mechanism capable of mediating a functional change in synapse sensitivity to calcium and calcium-signalling pathways after long-term potentiation.

NMDA receptor antagonists sustain LTP and spatial memory: active processes mediate LTP decay

Although long-term potentiation (LTP) is long-lasting, it is not permanent and decays within weeks after its induction. Little is known about the processes underlying this decay. Here we assessed the contribution of synaptic activity to LTP decay by determining the effect of the competitive NMDA receptor antagonist CPP on the decay of perforant path-dentate LTP. CPP blocked decay over a one-week period when administered daily following the induction of LTP, and blocked decay of the late, protein-synthesis-dependent phase of LTP when administered two days after LTP induction. CPP administered for a five-day period following spatial memory training enhanced subsequent memory retention. These data suggest that LTP is normally a persistent process that is actively reversed by NMDA receptor activation, and that both the early and late phases of LTP are dynamic processes regulated by NMDA receptors. These data also support the view that LTP is involved in maintaining spatial memory.

Modulation of hippocampal long-term potentiation by the amygdala: a synaptic mechanism linking emotion and memory

Why are emotionally arousing experiences well-remembered? Since the amygdala and hippocampus play pivotal roles in emotion and memory, respectively, the interaction between these brain regions may underlie the formation of enhanced memory for emotionally arousing events. Behavioral experiments using animals have demonstrated that lesions of the amygdaloid nuclei or infusions of drugs into the amygdaloid nuclei impair or enhance hippocampal-dependent learning. In addition, we have obtained direct evidence that neural inputs from the amygdala modulate synaptic plasticity in the hippocampus, through electrophysiological experiments using anesthetized rats. Electrical stimulation of the basolateral amygdala evoked synaptic potentials in the dentate gyrus of the hippocampus, indicating that there is a neural connection from the amygdala to the hippocampus. Lesion of the basolateral or basomedial, but not central, amygdala resulted in attenuation of long-term potentiation (LTP) at the perforant path-dentate gyrus granule cell synapses. High-frequency stimulation of the basolateral or basomedial amygdala alone did not induce LTP in the dentate gyrus, but facilitated the induction of LTP when applied at the same time as tetanic stimulation of the perforant path. The activity-dependent facilitation of hippocampal LTP by the basomedial and basolateral amygdala may be a synaptic mechanism underlying memory enhancement associated with emotions.

Subfield-specific immediate early gene expression associated with hippocampal long-term potentiation in vivo

It is not known whether NMDA receptor-dependent long-term potentiation (LTP) is mediated by similar molecular mechanisms in different hippocampal areas. To address this question we have investigated changes in immediate early gene and protein expression in two hippocampal subfields following the induction of LTP in vivo and in vitro. In granule cells of the dentate gyrus, LTP induced in vivo by tetanic stimulation of the perforant path was followed by strong induction of the immediate early genes (IEGs) Zif268, Arc and Homer. The increase in Zif268 mRNA was accompanied by an increase in protein expression. In contrast, we were unable to detect modulation of the IEGs Zif268, Arc, Homer and HB-GAM following induction of LTP by high-frequency stimulation of the commissural projection to CA1 pyramidal cells in vivo. In this pathway, we also failed to detect modulation of Zif268 protein levels. Zif268, Arc and Homer can be modulated in CA1 pyramidal cells approximately twofold after electroshock-induced maximal seizure, which demonstrates potential responsiveness to electrical stimuli. When LTP was induced in vitro neither CA1 pyramidal cells nor granule cells showed an increase in Zif268, Arc or Homer mRNA. However, in the slice preparation, granule cells have a different transcriptional state as basal IEG levels are elevated. These results establish the existence of subfield-specific transcriptional responses to LTP-inducing stimulation in the hippocampus of the intact animal, and demonstrate that in area CA1-enhanced transcription of Zif268, Arc and Homer is not required for the induction of late LTP.

Dysfunctional regulation of alphaCaMKII and syntaxin 1B transcription after induction of LTP in the aged rat

Syntaxin 1B and alphaCaMKII are two genes that are upregulated after the induction of LTP and appear to underlie different mechanisms of synaptic plasticity. alphaCaMKII is directly implicated in strengthening the synapses that have been modified, whereas syntaxin 1B has been implicated in a mechanism for the propagation of synaptic plasticity within neural circuits. In these experiments we have investigated whether the regulation of these genes is altered after the induction of LTP in aged rats. We found, three hours after the induction of LTP in the dentate gyrus, that aged rats could be subgrouped into those in which LTP was maintained and those in which LTP had decayed back to basal levels. Both genes were upregulated in young adult rats, whereas there was a differential pattern of LTP-induced expression in the aged rats. Dendritic alphaCaMKII was upregulated in aged rats only when LTP was maintained. In contrast, regulation of syntaxin 1B and alphaCaMKII was absent in the granule cell bodies of the aged rats regardless of whether LTP was maintained or not. These results suggest that molecular mechanisms implicated in two aspects of hippocampal synaptic plasticity malfunction during normal ageing and therefore may have some contributory role in the decline in memory function routinely observed in ageing.

The MAPK/ERK cascade targets both Elk-1 and cAMP response element-binding protein to control long-term potentiation-dependent gene expression in the dentate gyrus in vivo

The mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling cascade contributes to synaptic plasticity and to long-term memory formation, yet whether MAPK/ERK controls activity-dependent gene expression critical for long-lasting changes at the synapse and what the events underlying transduction of the signal are remain uncertain. Here we show that induction of long-term potentiation (LTP) in the dentate gyrus in vivo leads to rapid phosphorylation and nuclear translocation of MAPK/ERK. Following a similar time course, the two downstream transcriptional targets of MAPK/ERK, cAMP response element-binding protein (CREB) and the ternary complex factor Elk-1, a key transcriptional-regulator of serum response element (SRE)-driven gene expression, were hyperphosphorylated and the immediate early gene zif268 was upregulated. The mRNA encoding MAP kinase phosphatase MKP-1 was upregulated at the time point when MAPK/ERK phosphorylation had returned to basal levels, suggesting a negative feedback loop to regulate deactivation of MAPK/ERK. We also show that inhibition of the MAPK/ERK cascade by the MAPK kinase MEK inhibitor SL327 prevented CREB and Elk-1 phosphorylation, and LTP-dependent gene induction, resulting in rapidly decaying LTP. In conclusion, we suggest that Elk-1 forms an important link in the MAP kinase pathway to transduce signals from the cell surface to the nucleus to activate the genetic machinery necessary for the maintenance of synaptic plasticity in the dentate gyrus. Thus, MAPK/ERK activation is required for LTP-dependent transcriptional regulation and we suggest this is regulated by two parallel signaling pathways, the MAPK/ERK-Elk-1 pathway targeting SRE and the MAPK/ERK-CREB pathway targeting CRE.

Effects of saffron extract and its constituent crocin on learning behaviour and long-term potentiation

Crocus sativus L., commonly known as saffron, is used in folk medicine for various purposes. Modern pharmacological studies have demonstrated that saffron extracts have antitumour effects, radical scavenger properties or hypolipaemic effects. Among the constituents of saffron extract, crocetin is mainly responsible for these pharmacological activities. In addition, recent behavioural and electrophysiological studies have demonstrated that saffron extract affects learning and memory in experimental animals. Saffron extract improved ethanol-induced impairments of learning behaviours in mice, and prevented ethanol-induced inhibition of hippocampal long-term potentiation, a form of activity-dependent synaptic plasticity that may underly learning and memory. This effect of saffron extract is attributed to crocin (crocetin di-gentiobiose ester), but not crocetin. Saffron extract or its active constituents, crocetin and crocin, could be useful as a treatment for neurodegenerative disorders accompanying memory impairment.

Correlations between electrophysiological observations of synaptic plasticity modifications and behavioral performance in mammals

Within the past century it has been well established that most mature neurons lose their ability to divide. Since then, it has been assumed that behavioral performance leads to synaptic changes in the brain. The existence of these potential changes has been demonstrated in numerous experiments, and different mechanisms contributing to synaptic plasticity have been discovered. Many structures involved in different types of learning have now been identified. This article reviews the different methods used with mammals to detect electrophysiological modifications in synaptic plasticity following behavior. Evidence of long-term potentiation and long-term depression has been found in the hippocampus and cerebellum, respectively, and empirical data has been used to correlate these mechanisms with specific learning performance. Similar observations were made recently in the septum and amygdala. These phenomena seem to be involved in maintaining the performance in the cortical areas of the brain. Ongoing attempts to find the relationship between behavioral performance and modifications in synaptic efficacy allow to speculate upon the dynamics of cellular mechanisms that contribute to the ability of mammals to modify wide neuronal networks in the brain during their life.

Enhancement by 1-oleoyl-2-docosahexaenoyl phosphatidylcholine of long-term potentiation in the rat hippocampal CA1 region

We reported previously that administration of 1-oleoyl-2-docosahexaenoyl-sn-glycero-3-phosphorylcholine (ODHPC), a kind of phosphatidylcholine, enhanced discriminatory shock avoidance learning in rats. Since long-term potentiation (LTP) of the hippocampus has been suggested to be a physiological substrate of some forms of memory, we investigated the effects of ODHPC on LTP in the rat hippocampal CA1 region. LTP in the amplitude of population spikes in the CA1 region was induced by tetanic stimulation in anesthetized rats. ODHPC significantly increased magnitudes of LTP in a dose-dependent manner when injected intraperitoneally 20 min before inducing LTP. However, administration of 1,2-dioleoyl-sn-glycero-3-phosphorylcholine, in which only docosahexaenoyl residue of ODHPC was replaced with oleoyl residue, did not affect LTP. These results suggest that ODHPC enhances hippocampal LTP by its specific conformation.

Increase in syntaxin 1B and glutamate release in mossy fibre terminals following induction of LTP in the dentate gyrus: a candidate molecular mechanism underlying transsynaptic plasticity

A growing body of evidence suggests that modulation of certain proteins of the exocytotic machinery is, in part, involved in the biochemical changes that underlie long-term synaptic plasticity. We have previously shown that the induction of long-term potentiation (LTP) at perforant path to dentate granule cell synapses in the rat hippocampus induces changes in the mRNA levels of syntaxin 1B and synapsin I, known to be involved in neurotransmitter release. Immunohistochemical staining suggested that concomitant changes in these proteins occurred at mossy fibre synapses, downstream of those synapses at which LTP was induced, leading us to postulate that such a mechanism might underlie a form of transsynaptic plasticity. Here we have used a specific mossy-fibre synaptosome preparation to quantify levels of proteins and measure, using a chemiluminescent glutamate assay, depolarization-induced glutamate release from these synaptosomes after induction of LTP in the dentate gyrus in vivo. We show that 5 h after the induction of LTP, there is an increase in the protein levels of syntaxin 1B and, although to a lesser extent, the synapsins I and II, associated with an increase in depolarization-induced release of glutamate within these terminals. Increases in both the protein levels and glutamate release were not observed when dentate gyrus LTP was blocked by an NMDA receptor antagonist. From these results we propose a molecular mechanism for the propagation of synaptic plasticity through hippocampal circuits.

Differential effects of amygdala lesions on early and late plastic components of auditory cortex spike trains during fear conditioning

In auditory fear conditioning, pairing of a neutral acoustic conditioned stimulus (CS) with an aversive unconditioned stimulus (US) results in an enhancement of neural responses to the CS in the amygdala and auditory cortex. It is not clear, however, whether cortical plasticity governs neural changes in the amygdala or vice versa, or whether learning in these two structures is determined by independent processes. We examined this issue by recording single-cell activity in the auditory cortex (areas Te1, Te1v, and Te3) of freely behaving, amygdalectomized rats using a movable bundle of microwires. Amygdala damage did not affect short-latency (0-50 msec) tone responses, nor did it interfere with conditioning-induced increases of these onset responses. In contrast, lesions of the amygdala interfered with the development of late (500-1500 msec) conditioned tone responses that were not present before conditioning. Furthermore, whereas onset conditioned responses in the control group remained elevated after 30 extinction trials (presentation of CS alone), onset responses in lesioned animals returned to their preconditioning firing level after approximately 10 extinction trials. These results suggest that the amygdala enables the development of long-latency (US anticipatory) responses and prevents the extinction of short-latency onset responses to threatening stimuli. The findings further suggest that auditory cortex cells may participate differently in explicit and implicit memory networks.

Synapsin I and syntaxin 1B: key elements in the control of neurotransmitter release are regulated by neuronal activation and long-term potentiation in vivo

The messenger RNAs encoding proteins of the exocytotic machinery were measured at different times following the induction of long-term potentiation or increasing neuronal activity in the dentate gyrus of the rat in vivo. In situ hybridization revealed that from the many messenger RNAs that encode proteins involved in regulated exocytosis, only those encoding synapsin I and syntaxin 1B were specifically increased. The levels of messenger RNA encoding both synapsin I and syntaxin 1B were increased on the ipsilateral side of the dorsal dentate gyrus 2 and 5 h following the induction of long-term potentiation. Syntaxin 1B was also increased in the ventral dentate gyrus at the same time-points. On the contralateral side of the dentate gyrus there was an increase in both synapsin I and syntaxin 1B at 5 h only. All of these long-term potentiation-induced changes were prevented when the tetanus was delivered in the presence of the N-methyl-D-aspartate receptor antagonist. (D(-)-2-amino-5-phosphonopentanoic acid. Immunocytochemical staining revealed that protein levels for both synapsin I and syntaxin 1B were elevated in the mossy fibre terminal zone of CA3 5 h after the induction of long-term potentiation. In addition to these plasticity-induced changes, a transient increase in the messenger RNA encoding syntaxin 1B was observed at 2 h in conditions of high intensity stimulation of the perforant path to increase the level of cellular activation, but this change was not maintained even when high intensity stimulation was sustained for 5 h. No changes in either of the messenger RNAs were observed under low frequency stimulation and pseudotetanus at either time-points. These results show that an overall increase in neuronal excitation within a neuronal network can be differentiated from a change in synaptic strength at a specific subset of the synapses, where only synaptic plasticity leads to long-term changes in the expression of selective members of the exocytotic machinery. Altered concentrations of key vesicle proteins may thus provide the means for modulation of neurotransmitter release over long time-periods. The persistent long-term potentiation-induced postsynaptic increase in messenger RNAs encoding these presynaptic proteins has important implications for the propagation of signals downstream from the site of long-term potentiation induction in hippocampal neural networks, and highlights a candidate molecular mechanism for mediating the propagation of synaptic plasticity in such networks.

LTP and spatial learning--where to next?

Hebb suggested, in 1949, that memories could be stored by forming associative connections between neurons if the criterion for increasing the connection strength between them be that they were active simultaneously. Much attention has been devoted towards trying to determine a) if there is a physiological substrate of such a rule, and b) if so, whether the phenomenon participates in real-life memory formation. The discovery of the electrically induced increase in synaptic strength known as long-term potentiation (LTP), in the early 1970s, demonstrated that a neural version of the Hebb rule could be observed under laboratory conditions in the hippocampus, a structure important for some types of learning. However, a quarter of a century later, the evidence linking LTP to learning and memory is still contradictory. The purpose of the present article is to review and assess the types of approach that have been taken in trying to determine whether hippocampal synaptic plasticity participates in memory formation.

Long-term potentiation in the rat hippocampal CA1 region is inhibited selectively at the acquisition stage of discriminatory avoidance learning

Long-term potentiation (LTP) of hippocampal synaptic efficacy has been regarded as a synaptic model of learning and memory. We examined whether the induction of LTP in the hippocampal regions was altered with the advance of discriminatory avoidance learning. Evoked potentials in the CA1 region or dentate gyrus were recorded before and after tetanic stimulation in anesthetized rats which had been given training sessions 24 h before. LTP of the amplitude of population spikes and the slope of excitatory postsynaptic potentials recorded in the CA1 region was smaller in rats at a learning stage 24 h after the avoidance rate had first been significantly increased than in naive rats. The magnitude of LTP was not altered in rats which had been exposed to the first training session or in those which had received overtraining. The inhibition of LTP in the CA1 was neither due to stress accompanied with training nor liberation from the stress by learning avoidance response. In contrast, LTP induced in the dentate gyrus was rather enhanced at the learning stage when LTP in the CA1 was inhibited. The results suggest that the acquisition of discriminatory avoidance learning selectively inhibits LTP in the hippocampal CA1 region.

Activation of metabotropic glutamate receptors induce differential effects on synaptic transmission in the dentate gyrus and CA1 of the hippocampus in the anaesthetized rat

Activation of ACPD-sensitive metabotropic receptors induced differential effects on synaptic transmission and the induction of LTP in CA1 and the dentate gyrus of the hippocampus i.c.v. injections of (1.S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced enduring potentiation of the fEPSP in CA1, which occluded tetanically induced LTP. In contrast, ACPD induced a dose-dependent biphasic effect on the fEPSP in the dentate gyrus, consisting of an initial short lasting potentiation, followed by enduring depression of the response, and blockade of LTP. These two effects are likely to be mediated by two different classes of the receptor as in the dentate gyrus the selective class I agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG) induced sustained potentiation of the fEPSP, whereas the mixed mGluR2 agonist-mGluR1 antagonist, (S)-4-carboxy-3-hydrophenylglycine((S)-4C3H-PG) induced only depression. Increasing the concentration of calcium directly in the dentate gyrus prior to, and in conjunction with, injections of ACPD induced sustained potentiation rather than depression. The differential effects indicate that the second messenger cascades the subtypes of receptors are linked with, mediate different forms of synaptic plasticity within the hippocampus and have important implications for their role in learning.

Long-term potentiation and learning

Long-term potentiation (LTP), a relatively long-lived increase in synaptic strength, remains the mot popular model for the cellular process that may underlie information storage within neural systems. The strongest arguments for a role of LTP in memory are theoretical and involve Hebb's Postulate, Marr's theory of hippocampal function, and neural network theory. Considering LTP research as a whole, few studies have addressed the essential question: Is LTP a process involved in learning and memory? The present manuscript reviews research that attempts to link LTP with learning and memory, focusing on studies utilizing electrophysiological, pharmacological, and molecular biological methodologies. Most evidence firmly supports a role for LTP in learning memory. However, an unequivocal experimental demonstration of a contribution of LTP to memory is hampered by our lack of knowledge of the biological basis of memory and of the ways in which memories are represented in ensembles of neurons, the existence of a variety of cellular forms of LTP, and the likely resistance of distributed memory stores to degradation by treatments that incompletely disrupt LTP.

The ascending neuromodulatory systems in learning by reinforcement: comparing computational conjectures with experimental findings

A central problem in cognitive neuroscience is how animals can manage to rapidly master complex sensorimotor tasks when the only sensory feedback they use to improve their performance is a simple reinforcing stimulus. Neural network theorists have constructed algorithms for reinforcement learning that can be used to solve a variety of biological problems and do not violate basic neurophysiological principles, in contrast to the back-propagation algorithm. A key assumption in these models is the existence of a reinforcement signal, which would be diffusively broadcast throughout one or several brain areas engaged in learning. This signal is further assumed to mediate up- and downward changes in synaptic efficacy by acting as a multiplicative factor in learning rules. The biological plausibility of these algorithms has been defended by the conjecture that the neuromodulators noradrenaline, acetylcholine or dopamine may form the neurochemical substrate of reinforcement signals. In this commentary, the predictions raised by this hypothesis are compared to anatomical, electrophysiological and behavioural findings. The experimental evidence does not support, and often argues against, a general reinforcement-encoding function of these neuromodulatory systems. Nevertheless, the broader concept of evaluative signalling between brain structures implied in learning appears to be reasonable and the available algorithms may open new avenues for constructing more realistic network architectures.

Potentiation or depression of synaptic efficacy in the dentate gyrus is determined by the relationship between the conditioned and unconditioned stimulus in a classical conditioning paradigm in rats

Learning a conditioned stimulus (CS)-unconditioned stimulus (US) association is accompanied by a variety of long-lasting changes in physiology and chemistry of the synapse in the dentate gyrus. To determine the time course of synaptic modification during learning, changes in the perforant path-dentate gyrus-evoked field potentials were measured in rats performing a classical conditioning (paired tone and footshock) or pseudoconditioning (unpaired tone and footshock) task. Over the course of 4 days of training, differential changes in the evoked response were observed in the two groups. In the conditioned group, there was an increase in the slope of the excitatory postsynaptic potential (EPSP) which started after five tone-shock paired trials and lasted for more than 40 min, outlasting the training session by 20 min. In contrast, a decrease in the slope of the EPSP which commenced after training and lasted for at least 1 h was observed in the pseudoconditioned group. In both groups there was a prolonged decrease in the amplitude of the population spike. The increase in the EPSP was reduced and the duration tended to shorten over days of training in the conditioned group, whereas in the pseudoconditioned group the decrease in the EPSP tended to increase. Off-line analysis of suppression of lever-pressing for food reward during the presentation of the tone, indicated that the conditioned rats had learned the tone-footshock association. Temperature was measured in the dentate gyrus of rats undergoing an identical procedure. In both groups slight temperature increases were observed, with no difference in amplitude and time-course between the groups. The differential effect of conditioning and pseudoconditioning on the evoked response and changes in temperature eliminate the possibility that effects of stress, arousal and muscular effort are the primary cause of the changes in the EPSP. The results suggest that behavioural events can exert bidirectional control of synaptic strength of entorhinal cortex inputs to the dentate gyrus and that the sign of synaptic modification is at least in part determined by the temporal relationship between these events. The data are discussed in terms of the type of neural activity that may mediate the processing of information in the dentate gyrus.

A long-lasting increase and decrease in synaptic excitability in the rat lateral septum are associated with high and low shuttle box performance, respectively

In a series of experiments with rats, using evoked field potentials, the influence of massed trial training in 2-way shuttle box avoidance and step-through passive avoidance tasks was studied on the synaptic excitability of the lateral septum (LS) neurons and on the induction of long-term potentiation in the lateral septum in vivo. The majority of rats that attained a high performance level in the shuttle box task exhibited, after the shuttle box training, a long-lasting enhancement of synaptic excitability of lateral septum neurons, whereas most of the rats with low performance in the shuttle box showed a long-lasting depression in the LS synaptic excitability. Both types of excitability changes disappeared within 24 h. Neither the first habituation session in the passive avoidance apparatus nor the subsequent one-trial learning in passive avoidance task had a marked influence on lateral septum synaptic excitability. Both high-performance and low-performance rats exhibited a long-term potentiation (LTP)-like potentiation of synaptic excitability of the lateral septum neurons after high frequency stimulation of the fimbria fibers although the amount of LTP in high performance rats was slightly higher than that in low performance animals.

NMDA receptor-dependent long-term potentiation in the hippocampal afferent fibre system to the prefrontal cortex in the rat

This study investigated the role of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor in the induction of long-term potentiation (LTP) in the hippocampal-prefrontal cortex pathway in vivo. Field potentials evoked by electrical stimulation of the CA1/subicular region were recorded in the prelimbic area of the prefrontal cortex under continuous perfusion of artificial cerebrospinal fluid in anaesthetized rats. High-frequency stimulation of the CA1/subicular region induced LTP of the evoked response in the prelimbic area of the prefrontal cortex. LTP was completely blocked when the selective NMDA receptor antagonist D-(-)2-amino-5-phosphonopentanoic acid (D-AP5; 200 microM), was perfused during the tetanus. Perfusion of D-AP5 did not affect normal transmission or pre-established LTP. These results demonstrate that induction of LTP in the hippocampal-prefrontal cortex pathway is an NMDA receptor-dependent process.

Dynamics of the reactivity of cortical neurons to the repeating isolated action of L-glutamate and acetylcholine

A comparative analysis of the probability, directionality, and intensity of the changes in the average frequency of the baseline (BIA) and L-glutamate-induced (GlIA) and acetylcholine-induced (AChIA) impulse activity of individual neurons of the sensorimotor cortex of unanesthetized rats showed that the reactions to Gl are most variable; this is expressed in a significantly higher percent of neurons with potentiation of responses to Gl as compared with the proportion of cells which manifested an increase in the AChIA in the course of 20 applications of the mediators. As a result of 100 applications, the GlIA decreases most frequently; also, the degree of decrease in GlIA over the entire duration of the testing exceeds the degree of decrease in AChIA. It is more characteristic of ACh than of Gl to increase the level of BIA in the periods between the reactions to ACh, while in the case of repeated application of Gl, dynamics of decrease in BIA are characteristic. The results are discussed from the perspective of a possibly different functional role of the glutamatergic and cholinergic neuromediator systems in the cerebral cortex in the process of formation of the dynamics of neuronal activity during learning.

Angiotensin II and its 3-7 fragment improve recognition but not spatial memory in rats

The effects of angiotensin II (AII), its 3-7 fragment [AII(3-7)] and the substituted 3-7 fragment [Leu-5,AII(3-7)] given intracerebroventricularly (ICV) at the dose of 1 nmole each, on spatial memory and recognition were tested. AII(3-7) increased while Leu-5,AII(3-7) slightly decreased session to session foot shock reinforced runtime to the goal in a complex 6 chamber maze. The animals treated with AII performed in the maze similarly to saline injected controls. Overall number of errors was unchanged in all peptide treated groups in comparison with the control group. Object recognition was significantly improved in all the peptide treated groups except for the Leu-5,AII(3-7) group. The results point to the facilitation of recognition and lack of influence on, or even attenuation of, spatial memory by AII and its 3-7 fragment. Leu-5,AII(3-7) caused similar though less pronounced effects.

Spatial and temporal changes in signal transduction pathways during LTP

Following LTP induction in freely moving rats, in situ hybridization revealed discrete changes in the expression of one isoform in each of four families of serine/threonine kinases constitutively expressed in the dentate gyrus of the hippocampus. Expression of the alpha isoform of CaMKII showed a transient increase over the soma and a more persistent increase over the dendritic field of dentate granule cells. Of the PKC isoforms, only gamma PKC was up-regulated substantially 2 hr after LTP induction, declining to control levels 48 hr later. An increase in the expression of mRNA for ERK2 and raf-B was seen at 24 hr only. These results show that, during the maintenance phase of LTP in the hippocampus, there are selective increases in the expression of serine/threonine kinases and that these increases have specific and characteristic temporal and spatial profiles.

Reactivity to novelty during youth as a predictive factor of cognitive impairment in the elderly: a longitudinal study in rats

A life-span study of certain behavioral traits was conducted in rats. Animals were repeatedly tested in a circular corridor for reactivity to novelty and in a recognition memory task for cognitive abilities. These measures revealed important inter-individual differences in young as well as in old subjects. Some of these differences appear with aging (memory deficits) and others disappear (high reactivity to novelty). Moreover, a relationship between high reactivity to novelty in youth and deficits in memory recognition in elderly was found. Rats that are high-responders to novelty had age-related memory impairments whereas the low-responder rats did not. While the biological mechanism linking these two behavioral traits remains to be demonstrated, this study shows that age-related impairments can be predicted by factors detectable early in life.

Suppression of long-term potentiation induction during alert wakefulness but not during 'enhanced' REM sleep after avoidance learning

Major learning events are typically followed by a period during which the number and/or duration of rapid-eye movement sleep episodes is increased. Processes critical to memory formation are thought to take place during this interval of 'enhanced' rapid-eye movement sleep. We therefore compared the capacity for long-term potentiation during rapid-eye movement sleep and alert wakefulness after learning. Rats were chronically implanted with electrodes for stimulation of the perforant path and recording of evoked potentials and EEG in the dentate gyrus. After obtaining baseline recordings, rats were trained on a 40-trial two-way active avoidance task. Conditioned rats exhibited a two-fold increase in the mean duration of rapid-eye movement sleep episodes, as reflected by a prolongation of the hippocampal theta rhythm. There was no change in the sleep pattern of pseudoconditioned controls, which received explicitly unpaired tones and foot shocks in a yoked design. High-frequency stimulation was applied during the second, third, and fourth major rapid-eye movement sleep episodes after active avoidance training. Another group was tetanized at matching time points during alert wakefulness. After pseudoconditioning, tetanus applied during wakefulness or rapid-eye movement sleep readily induced long-term potentiation, and there was no difference between groups in the magnitude of increase for the population excitatory postsynaptic potential slope or the population spike height as measured 1 h, 24 h, and 5 days post-tetanus.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of the medial septal area attenuates LTP of the lateral perforant path and enhances heterosynaptic LTD of the medial perforant path in aged rats

Age-related memory impairments may be due to dysfunction of the septohippocampal system. The medial septal area (MSA) provides the major cholinergic projection to the hippocampus and is critical for memory. Knowledge of the neurobiological mechanisms by which the cholinergic system can attenuate age-related memory loss can facilitate the development of effective cognitive enhancers. At present, one of the best neurobiological models of memory formation is long-term potentiation/long-term depression (LTP/LTD). In previous studies, intraseptal infusion of the muscarinic agonist oxotremorine, which excites MSA neurons, improved memory in aged rats. The present study examined LTP and LTD in aged Fisher 344 rats following intraseptal infusion of oxotremorine. LTP and LTD were assessed using the slope of the EPSP recorded from the hilar region of the dentate gyrus. Induction of LTP was blocked in the lateral perforant path, but not in the medial perforant path, following intraseptal infusions of oxotremorine. The generation and amplitude of heterosynaptic LTD was enhanced in the medial perforant path, but not in the lateral perforant path. The results provide evidence that pharmacological activation of the MSA can modulate LTP and LTD in the hippocampus of aged rats. The implications of these results with respect to memory and synaptic plasticity in the hippocampus are discussed.

Transsynaptic expression of a presynaptic glutamate receptor during hippocampal long-term potentiation

Repetitive activation of excitatory synapses in the hippocampus produces a persistent enhancement of synaptic efficiency known as long-term potentiation (LTP). In anesthetized and in freely moving rats, the induction of LTP in the perforant path led to a transient increase in the amount of messenger RNA (mRNA) coding for a presynaptic glutamate receptor (GR33) in dentate granule cells. The amount of GR33 mRNA was increased for at least 5 hours after the induction of LTP but was indistinguishable from control values 1 day after induction. The N-methyl-D-aspartate receptor antagonist 2-aminophosphonovalerate prevented the induction of both LTP and the increase in GR33 mRNA. The amount of GR33 protein was increased in the mossy fiber terminal zone of dentate granule cells 5 hours after the induction of LTP. These results suggest that the induction of LTP in synapses at one stage in a neural network may lead to modification in synaptic function at the next stage in the network.

Altered synaptic plasticity and memory formation in nitric oxide synthase inhibitor-treated rats

Nitric oxide (NO) is a messenger molecule that is produced in the brain from the metabolism of L-arginine to L-citrulline. Growing evidence suggests a physiological role for NO in long-term potentiation (LTP). Since LTP is a form of synaptic plasticity thought to be involved in learning and memory, we have tested whether inhibition of endogenous NO production affects memory capacities of rats. We found that the NO synthase [L-arginine, NADPH:oxygen oxidoreductase (nitric oxide-forming), EC 1.14.13.39] inhibitor N omega-nitro-L-arginine, at doses blocking LTP in hippocampal slices, impairs spatial learning in a radial arm maze and olfactory memory in a social recognition test. In contrast, N omega-nitro-L-arginine left shock-avoidance learning unaffected. These results indicate that NO is involved in some but not all forms of memory and further support the existence of a causal link between LTP and spatial learning.

GABAA- and GABAB-mediated inhibition in the rat dentate gyrus in vitro

Many studies suggest that the dentate gyrus (DG) is a control point for hippocampal epileptogenesis. However, the importance of GABAergic inhibition in the DG is not quite clear. Intracellular recordings were obtained from granule cells (GC) of the rat DG. In addition to GABAA-mediated spontaneous postsynaptic potentials (PSPs), some GC exhibited spontaneous slow hyperpolarizations (SH). The SH were more commonly observed in a high concentration of external potassium. 2-Hydroxysaclofen, a GABAB antagonist, reduced the SH. Focal stimulation of the perforant path (PP) in the subiculum with a single pulse evoked a depolarization followed by a SH, which were both abolished by the excitatory amino acid (EAA) blockers, 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX) and 2-amino-5-phosphonovaleric acid (APV). When evoked with a train of pulses, the SH was unaffected by the EAA blockers in 40% of the cells, suggesting either the existence of a GABAergic PP, or an unidentified polysynaptic mechanism. In control, the synaptic response to PP stimulation was superficially similar whether the stimulus was applied in the subiculum or stratum moleculare. However, in presence of bicuculline, the subicular PSP was followed by a train of PSPs occurring at a constant frequency of 25 Hz. This 'reverberating' effect of bicuculline was decreased in presence of APV and was abolished in slices in which the excitatory transmission had been interrupted downstream from CA3 neurons, suggesting that reverberation required the integrity of the hippocampo-entorhinal loop. By contrast, bicuculline decreased the amplitude of the stratum moleculare PSP. It is concluded that GC receive tonic inhibition from GABA acting at GABAA and GABAB receptors. The role of GABAB receptors is unclear; by contrast, GABAA-mediated inhibition prevents GC from reverberated excitation. The probability of occurrence of reverberation is higher during activation of the whole temporo-ammonic pathway and is partly dependent on the activation of N-methyl-D-aspartate (NMDA) receptors. Thus, the in vitro brain slice can be used as a model to study reverberation which has been recently demonstrated to underlie epileptiform discharges in the whole brain preparation.

Long-term potentiation of hippocampal afferents and efferents to prefrontal cortex: implications for associative learning

It has been proposed that the physical substrate of memory resides in alterations of the strengths or weights of modifiable synaptic connections. In recent years, the hypothesis that the mechanisms underlying a particular form of synaptic plasticity, known as long-term potentiation, or LTP, are activated during learning and may actually subserve the formation of associative memories, has gained much empirical support. This paper reviews experimental studies suggesting that changes in synapse physiology and chemistry are involved in the formation of neural associative representation in hippocampal networks during classical conditioning. Recent experiments investigating LTP and learning-induced synaptic changes at hippocampal outputs to the prefrontal cortex are reported. The results provide a working framework within which the dynamics of information storage in hippocampal and prefrontal cortical networks is profiled.

Cumulative long-term potentiation in the rat dentate gyrus correlates with, but does not modify, performance in the water maze

The electrically induced increase in hippocampal synaptic strength known as long-term potentiation (LTP) is thought to involve some of the same mechanisms as those mediating information storage during spatial learning. Physiological saturation of synaptic weights might therefore be expected to occlude spatial learning. In support of this, Castro et al. (Castro CA, Silbert LH, McNaughton BL, Barnes CA, 1989, Nature 342:545-548) reported that repeated induction of LTP to asymptotic levels, over a 14-day period, blocked the ability of rats to learn the position of a hidden platform in a water maze shortly afterwards. The authors have attempted to replicate this finding in two experiments. In Experiment 1, rats were given either 400 Hz stimulation as 50 trains/day over a 5-day period, to induce LTP, or were given the same number of stimulus pulses at 1 Hz. Two hours after the last stimulation session they underwent eight spatial learning trials, occurring at 2-hour intervals. In Experiment 2, the procedure of Castro et al. was followed exactly, with LTP induced by 10 trains of 400 Hz stimulation daily for 14 days. Spatial learning trials began 10 minutes after the last stimulation session and consisted of 12 trials with alternating 30-second and 2-minute intervals. In neither experiment was a significant impairment of spatial learning observed. However, there was a strong positive correlation, in both experiments, between the final level of LTP and subsequent performance in the water maze.

Subsensitivity of lead-exposed rats to the accuracy-impairing and rate-altering effects of MK-801 on a multiple schedule of repeated learning and performance

Exposure to lead (Pb) has been reported to inhibit MK-801 binding and to alter other NMDA receptor complex-associated functions. These reported changes are provocative since both NMDA receptor antagonism and Pb exposure are know to impair learning processes. Whether the Pb-induced changes in NMDA function relate to the learning impairments associated with Pb exposure, however, has not been explored. The contention of this study was that if changes in NMDA function produced by Pb serve as the basis of Pb-associated learning impairments, then such changes should be of sufficient biological magnitude and clinical relevance to induce alterations in sensitivity at the level of the whole animal, i.e., changes in behavioral sensitivity to glutamatergic compounds. Thus, in this study, dose-effect curves of control and Pb-treated rats working on a multiple schedule of repeated learning (repeated acquisition, RA) and performance (P) were compared following acute administration of MK-801, the non-competitive NMDA antagonist. Based on the nature of the reported effects of Pb on NMDA systems, it was expected that the curves of Pb-exposed rats would be right-shifted relative to controls, if differential behavioral sensitivity was evident. Rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water from weaning and trained on the multiple RA and P schedule beginning at 55 days old. The RA component required the rat to learn a new 3-member sequence of responses during each experimental session (center right left, RLC, CLR, RCL, or LRC), while the correct sequence of responses for the P component was constant across sessions (LCR), requiring performance of an already learned response. Acute administration of MK-801 (0.05-0.3 mg/k, i.p.) resulted in decrements in accuracy in both the RA and P components of the schedule, indicative of non-specific effects on behavior rather than selective effects on learning. The declines in accuracy during the RA component of the schedule were primarily the result of increased perseverative responding, i.e., repetitive responding on a single lever. Both the decline in RA accuracy and the increases in perseverative responding produced by MK-801 were attenuated by Pb exposure. Moreover, dose-effect curves relating MK-801 dose to changes in rates of responding were significantly shifted to the right in Pb-exposed rats relative to controls. Taken together, these data demonstrate a subsensitivity of Pb-exposed rats to both the accuracy-impairing and response rate-altering properties of MK-801.(ABSTRACT TRUNCATED AT 400 WORDS)

Hippocampal plasticity induced by primed burst, but not long-term potentiation, stimulation is impaired in area CA1 of aged Fischer 344 rats

The effect of two types of electrical stimulation designed to induce long-lasting plasticity of the Schaffer/commissural inputs to CA1 pyramidal neurons was investigated using in vitro hippocampal slices made from young (3-6 month) and old (24-27 month) Fischer 344 rats. The first stimulation paradigm, primed burst (PB) stimulation, consisted of a total of five physiologically patterned stimuli: a single priming pulse followed 170 ms later by a burst of four pulses at 200 Hz. The second stimulation paradigm, long-term potentiation (LTP) stimulation, consisted of a 200 Hz/1 second train (a total of 200 stimuli). Primed burst and LTP stimulation were equally effective at inducing a lasting increase in the population spike recorded from slices made from young rats. However, the enhancement of population spike amplitude produced by PB, but not LTP, stimulation was significantly less in slices made from old rats. These results suggest that the capacity of the hippocampus to demonstrate long-lasting synaptic plasticity is not altered with age, but that engaging plasticity-inducing mechanisms becomes more difficult. Furthermore, these data suggest that physiologically patterned paradigms for inducing long-lasting synaptic plasticity may more accurately assess the functional status of hippocampal memory encoding mechanisms than does conventional LTP stimulation.

Characterizationin vivo of the NMDA receptor-mediated component of dentate granule cell population synaptic responses to perforant path input

The NMDA receptor-mediated component of the hippocampal granule cell population excitatory postsynaptic potential response to low frequency (< 0.2 Hz) stimulation of the medial perforant path was characterized in vivo. Extracellular recordings were obtained from the dentate molecular layer in anesthetized rabbits, and glutamatergic and GABAergic antagonists were applied locally by pressure ejection. To measure the NMDA-mediated component, the NMDA receptor antagonist D-5-aminophosphonovalerate (APV) was applied during the constant ejection of physiological saline, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and/or bicuculline methiodide. In general agreement with the results of attempts by other investigators to identify NMDA responses in vivo, APV did not significantly reduce the response to a single stimulus impulse in the presence of saline. However, an NMDA-mediated response was revealed when alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate receptor-mediated current flow was eliminated by applying the non-NMDA receptor antagonist CNQX. The NMDA component was negative-going as predicted, but its duration was considerably less than indicated in other studies of the dentate in vitro. The relative magnitudes of the NMDA and non-NMDA components of the EPSP were found to vary as a function of stimulus intensity or frequency. The NMDA receptor-mediated component represented 12% of the control response and increased to over 25% in response to higher stimulus intensities. A brief, high-frequency burst of impulses evoked a larger NMDA component in the presence of CNQX and was able to evoke an NMDA component in the presence of saline. Surprisingly, short trains of stimulation at lower frequencies typically produced suppression of the NMDA component. In a final series of experiments, it was found that many characteristics of the NMDA component were substantially altered by GABAergic inhibition. In the presence of the GABAA antagonist bicuculline, the magnitude of NMDA receptor-mediated responses was increased and their duration was greatly extended. Additionally, in the presence of bicuculline, the NMDA component facilitated markedly in response to frequencies of stimulus input > 20 Hz. These results indicate in vivo that the initiation and duration of NMDA current flow depend strongly upon the intensity and frequency of perforant path stimulation. In addition, the NMDA response to a single impulse appears to be reduced and truncated by input from GABAA receptor-mediated feedback and/or feedforward inhibition, and this inhibition affects temporal summation of NMDA receptor-mediated responses over a wide range of input frequencies. It is suggested that such inhibition results from the activation of GABAA receptors located on granule cell dendritic shafts.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of glycine site associated with the NMDA receptor in hippocampal long-term potentiation and acquisition of spatial memory in rats

The effects of 7-chlorokynurenic acid (7-Cl-Kyn), a selective antagonist at the glycine site associated with the N-methyl-D-aspartate (NMDA) receptor, on hippocampal long-term potentiation (LTP) and behavioral performances in a spatial learning task were investigated. Extracellular recordings of evoked potential (population spike) were made in rat hippocampal slices. Perfusion of 7-Cl-Kyn (10(-5) M) inhibited the induction of LTP following a tetanic stimulation (51 or 101 pulses at 100 Hz) both in the Schaffer/commissural-CA1 pyramidal cell synapses and in the perforant path-dentate granule cell synapses. Acquisition of a spatial memory in the Morris water maze was examined using rats chronically cannulated for application of drugs. The intact and vehicle-injected rats learned easily to escape onto a hidden platform with short latencies, while the rats given an injection of 7-Cl-Kyn (10(-8) mol/brain, i.c.v.) prior to every session took a longer time and a longer path to escape even after all 5 sessions of trials. Injection of 7-Cl-Kyn did not affect the swimming speed, an index of swimming ability. This is the first report providing direct evidence that endogenous glycine supports the processes of learning and memory.

Differential expression of K+ channel mRNAs in the rat brain and down-regulation in the hippocampus following seizures

K+ channels are major determinants of membrane excitability. Differences in neuronal excitability within the nervous system may arise from differential expression of K+ channel genes, regulated spatially in a cell type-specific manner, or temporally in response to neuronal activity. We have compared the distribution of mRNAs of three K+ channel genes, Kv1.1, Kv1.2, and Kv4.2 in rat brain, and examined activity-dependent changes following treatment with the convulsant drug pentylenetetrazole. Both regional and cell type-specific differences of K+ channel gene expression were found. In addition, seizure activity caused a reduction of Kv1.2 and Kv4.2 mRNAs in the dentate granule cells of the hippocampus, raising the possibility that K+ channel gene regulation may play a role in long-term neuronal plasticity.