In search of cellular mechanisms of memory

Distribution of visinin-like protein (VILIP) immunoreactivity in the hippocampus of the Mongolian gerbil (Meriones unguiculatus)

Visinin-like protein (VILIP) is a neuronal EF-hand Ca(2+)-binding protein. In the chick brain, it is widely expressed, e.g. in neurons of the visual pathway and the cerebellum. In the cerebellum, a presynaptic localization of VILIP in glutamatergic parallel- and climbing-fiber terminals has been observed. Here, we describe the distribution of immunoreactivity (IR) detected by antibodies against chick VILIP in the gerbil hippocampus at the light and electron microscopic level. VILIP antibodies stain neurons in the whole hippocampal formation including pyramidal cells in the CA1 and CA3 region of the Ammon's horn and granule cells of the dentate gyrus. In CA3 neurons, VILIP-IR is localized in dendrites and dendritic spines.

Corticosterone facilitates long-term memory formation via enhanced glycoprotein synthesis

Long-term memory formation for a passive avoidance task in one-day-old chicks requires a late phase of synaptic glycoprotein synthesis (including the neural cell adhesion molecule), commencing 5.5 h post-training. This phase occurred in chicks trained with a strong, but not a weak aversant, which only retained this memory for a few hours (< 10). In addition, previous work has shown that a corticosteroid action through central corticosteroid receptors is also required for long-term passive avoidance memory. Here, we tested the hypothesis that the corticosteroid action on memory formation might be exerted via modulation of the late phase of neural glycoprotein synthesis. One-day-old chicks were used as experimental subjects. Incorporation of the radiolabelled glycoprotein precursor [3H]fucose into synaptic membranes of the chick forebrain was used as an index of glycoprotein fucosylation. Bilateral intracerebral injections of a corticosterone dose (0.5 micrograms/hemisphere) that facilitates long-term retention of weak learning were able to induce the late phase of glycoprotein synthesis in undisturbed chicks. A further experiment examined the effect of antibodies against the neural cell adhesion molecule on the facilitatory action of corticosterone on long-term memory for the weak passive avoidance training. Chicks trained on a weak aversant were injected with corticosterone (0.5 micrograms/hemisphere) 30 min post-training and testing occurred 24 h post-training. Administration of the neural cell adhesion molecule antibodies during the late phase (5.5 h post-training) blocked the facilitatory action of corticosterone on long-term memory. These findings further support the view that corticosteroids have a role in memory consolidation. In addition to previously proposed effects on gene transcription, these data suggest a post-translational glycosylation mechanism for the modulatory effect of corticosteroids on long-term memory formation.

Administration of glutamate following a reminder induces transient memory loss in day-old chicks

Monosodium glutamate (4.0 mM) administered immediately after a visual reminder presented to day-old chickens between 7.5 min and 24 h following a single trial passive avoidance learning task produced transient losses of memory on retention test, an effect not observed in the absence of a reminder or when the reminder was given 48 h post-learning. The duration of the transient deficit decreased with increasing interval between the training and the reminder trial. The time of onset of memory loss after the reminder trial appeared to increase with increasing interval between the training and the reminder trials. The results suggest that, for a period of at least up to 24 h after passive avoidance training, retrieval of memory may lead to processes which are sensitive to inhibition by glutamate, with the duration of sensitivity post-retrieval decreasing as the period of memory consolidation increases. The results extend previously reported findings with rodents and suggest the possibility that consolidation of a stable memorial representation of a learning experience may take place over several days and may entail the concurrent laying down of a stable retrieval mechanism.

Decrements in auditory responses to a repeated conspecific song are long-lasting and require two periods of protein synthesis in the songbird forebrain

Earlier work showed that playbacks of conspecific song induce expression of the immediate early gene ZENK in the caudo-medial neostriatum (NCM) of awake male zebra finches and that this response disappears with repeated presentations of the same stimulus. In the present study, we investigated whether repetitions of a song stimulus also elicited a decrement in the electrophysiological responses in the NCM neurons of these birds. Multiunit auditory responses in NCM were initially vigorous, but their amplitude decreased (habituated) rapidly to repeated stimulation, declining to about 40% of the initial response during the first 50 iterations. A similar time course of change was seen at the single unit level. This habituation occurred specifically for each song presented but did not occur when pure tones were used as a stimulus. Habituation to conspecific, but not heterospecific, song was retained for 20 h or longer. Injections of inhibitors of protein or RNA synthesis at the recording site did not affect the initial habituation to a novel stimulus, but these drugs blocked the long-term habituation when injected at 0.5-3 h and at 5.5-7 h after the first exposure to the stimulus. Thus, at least two waves of gene induction appear to be necessary for long-lasting habituation to a particular song.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Glycoproteins and memory formation

One-trial passive avoidance training in day-old chicks results in a biochemical cascade occurring in two forebrain regions, the intermediate medial hyperstriatum ventrale and the lobus parolfactorius. This cascade, initiated by synaptic transients, results in the activation of immediate early genes and culminates in the de novo synthesis of a family of pre- and post-synaptic membrane glycoproteins, that, inserted into the membrane, serve in the remodelling of synaptic connectivity which is a requirement for the brain representations constituting long-term memory. There are two waves of glycoprotein synthesis consequent on training, the first occurring within an hour of the training experience and the second 5.5-8 h post-training. Blocking synthesis during these time windows results in amnesia for the task. Amongst the glycoproteins involved are two cell adhesion molecules, NCAM and L1. Injection of antibodies to L1 result in amnesia if injected during either time window, but not outside these times; antibodies to NCAM result in amnesia only if injected at the 5.5-h timepoint. I interpret these results as indicating that de novo synthesis of NCAM during the second time window is necessary for producing a persistent memory trace.

Structural dynamics of synapses and synaptic components

Learning and memory formation are apparently based on cascades of molecular and cellular processes with increasing time constants (ms to days and weeks), but even the most long-lasting effects are transient. Memory traces may permanently modify the behavior (activity patterns, gene expression) of neurons and neuronal networks. Therefore the question is raised whether our current view on the stability of synapses under normal conditions is tenable. Evidence is reviewed suggesting that as direct or indirect effects of modifications in bioelectrical activity and chemical trophicity, synapses may be remodeled and removed within days and weeks, and possibly within hours. Accordingly, species-specific connectivity patterns are not restricted to the standard architecture of the CNS, but (morpho-)genetics allow for a considerable number of alternative wiring patterns, which appear under unusual conditions during ontogenesis and in adulthood. Our present knowledge suggests that, rather than the formation of synapses, they are a selective process. Until now there is no direct method of measuring either synaptic reorganization or the average life span of synapses. Specific cases, however, allow to estimate synapse turnover during ontogenesis, at its lowest possible level. Such data suggest that each synapse is on average remodeled or replaced several to many times during normal developmental, e.g. in the cerebral cortex of Marmoset monkeys at the very least 5 to 10 times (corresponding to 250 million synapses eliminated per hour in area 17!). It is discussed how the consequences of synapse turnover could be utilized by learning processes. Conclusions are followed by an outlook.

Passive avoidance learning induced change in GAP43 phosphorylation in day-old chicks

Day-old chicks trained on a single trial passive discriminated avoidance task demonstrated a significant increase in in vitro phosphorylation of a 50 kDa protein in P2M fractions of total forebrain. The increase occurred 30 min posttraining, at a time when previous reports suggest that mechanisms for triggering protein synthesis-dependent long-term memory consolidation are activated. These changes in phosphorylation rates were accompanied by a substantial enhancement of total kinase activity. Immunoblotting studies with monoclonal anti-GAP43 antibody indicate that this protein is GAP43. These results contradict previous reports of a decrease in in vitro GAP43 phosphorylation following the same learning paradigm. A number of procedural differences may account for this discrepancy. The results suggest that changes in the phosphorylation state may be associated with mechanisms triggering long-term memory consolidation.

Deficient long-term memory in mice with a targeted mutation of the cAMP-responsive element-binding protein

The cAMP-responsive element-binding protein (CREB) has been implicated in the activation of protein synthesis required for long-term facilitation, a cellular model of memory in Aplysia. Our studies with fear conditioning and with the water maze show that mice with a targeted disruption of the alpha and delta isoforms of CREB are profoundly deficient in long-term memory. In contrast, short-term memory, lasting between 30 and 60 min, is normal. Consistent with models claiming a role for long-term potentiation (LTP) in memory, LTP in hippocampal slices from CREB mutants decayed to baseline 90 min after tetanic stimulation. However, paired-pulse facilitation and posttetanic potentiation are normal. These results implicate CREB-dependent transcription in mammalian long-term memory.

Requirement of a critical period of transcription for induction of a late phase of LTP

Repeated high-frequency trains of stimuli induce long-term potentiation (LTP) in the CA1 region that persists for up to 8 hours in hippocampal slices and for days in intact animals. This long time course has made LTP an attractive model for certain forms of long-term memory in the mammalian brain. A hallmark of long-term memory in the intact animal is a requirement for transcription, and thus whether the late phase of LTP (L-LTP) requires transcription was investigated here. With the use of different inhibitors, it was found in rat hippocampal slices that the induction of L-LTP [produced either by tetanic stimulation or by application of the cyclic adenosine monophosphate (cAMP) analog Sp-cAMPS (Sp-cyclic adenosine 3',5'-monophosphorothioate)] was selectively prevented when transcription was blocked immediately after tetanization or during application of cAMP. As with behavioral memory, this requirement for transcription had a critical time window. Thus, the late phase of LTP in the CA1 region requires transcription during a critical period, perhaps because cAMP-inducible genes must be expressed during this period.

Plastic genes are in!

Even though the synthesis of new proteins is thought to be essential for long-term changes in synaptic plasticity, as well as for long-term memory, little is known about the identity of the required proteins. The hunt for these molecules is under way, however, and in the past year several groups of researchers have entered this fascinating search by introducing new approaches that have lead to the identification of several potential candidates, amongst which are trophic factors, kinases, ion channels, and proteases. The results will have much to say not only about the nature of memory, but also about the mechanisms of learning.

LTP in hippocampal CA1 of urethane-narcotized rats requires stronger tetanization parameters

Rats with chronically implanted electrodes in the hippocampal CA1 region were tested in their capacity to express and maintain long-term potentiation (LTP) of the population spike (PS) or of the field excitatory postsynaptic potential (fEPSP). Two different states were compared: a) freely moving animals; b) urethane-anesthetized animals (1 g/kg, IP). We found that a short, high-frequency tetanus (six bursts of 15 pulses; 200 Hz; double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in double-pulse width; interburst interval 10 s) increased PS amplitudes and fEPSP slopes up to 300% in response to test stimuli in the awake rat. The PS amplitude slowly decreased in time, returning to baseline levels 4 h post-tetanically, whereas the fEPSP slope remained at higher values for 24 h. Urethane injection reduced the fEPSP slope and abolished the PS to normal test pulses. We thus increased the strength of the test stimuli until we again recorded magnitudes of PSs and fEPSPs comparable to those in the awake animal. In conjunction with these stronger stimuli, tetanus-induced LTP was elicited that for the PS was increased in magnitude and prolonged in duration compared to the untreated control group. Although, stronger tetanic stimuli were applied to the narcotized fEPSP group too, no difference was found compared to controls. These results suggest that urethane narcosis influences the sensitivity of CA1 neurons to express LTP. Stronger stimulation was required to induce and maintain a long-lasting potentiation of the fEPSP slope and PS amplitude.

Recruitment of long-lasting and protein kinase A-dependent long-term potentiation in the CA1 region of hippocampus requires repeated tetanization

To study how the late phase of long-term potentiation (LTP) in hippocampus arises, we examined the resulting LTP for its time course and its dependence on protein synthesis and different second-messenger kinases by applying various conditioning tetani. We find that one high-frequency train (100 Hz) produces a form of LTP that lasts longer than 1 hr but less than 3 hr (the early phase of LTP, or E-LTP). It is blocked by inhibitors of calcium/calmodulin kinase II (Cam kinase II) but is not affected by an inhibitor of cAMP-dependent protein kinase [protein kinase A (PKA) and the protein synthesis inhibitor anisomycin] nor is it occluded by the cAMP activator forskolin. In contrast, when three high-frequency trains are used, the resulting potentiation persists for at least 6-10 hr. The L-LTP induced by three trains differs from the E-LTP in that it requires new protein synthesis, is blocked by an inhibitor of cAMP-dependent protein kinase, and is occluded by forskolin. These results indicate that the two mechanistically distinctive forms of LTP, a transient, early component (E-LTP) and a more enduring form (L-LTP), can be recruited selectively by changing the number of conditioning tetanic trains. Repeated tetani induce a PKA and protein synthesis-dependent late component that adds to the amplitude and duration of the potentiation induced by a single tetanus.

Fucose and fucose-containing sugar epitopes enhance hippocampal long-term potentiation in the freely moving rat

Male Wistar rats were intrahippocampally injected with L-fucose and the sugar epitope 2'-fucosyl-lactose prior to induction of long-term potentiation (LTP). Both substances had only a minimal and short-lasting depressive effect on the monosynaptically evoked field potential recorded in the dorsal blade of the dentate gyrus of freely moving rats upon stimulation of the perforant pathway. However, LTP induced by fractionated tetanization of the perforant pathway, which declined within 24 h in control animals injected with Lactose, remained at the initial level even 48 h after tetanization (difference to the control group significant with P < 0.01). The results support earlier findings which have indicated a participation of fucosylated macromolecules in the maintenance of LTP. Different molecular mechanisms concerning the effect of both substances and the significance of the data in elucidation of the relationship between LTP and memory formation are discussed.

Quantal analysis of hippocampal long-term potentiation

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

Physiological concentrations of melatonin inhibit nitric oxide synthase in rat cerebellum

In the present paper we show the inhibitory effect of melatonin on rat cerebellar nitric oxide synthase (NOS) activity. NO production was monitored by the stoichiometric conversion of L-arginine to L-citrulline. The inhibitory effect of melatonin was dose-dependent, with an IC50 value of about 0.1 mM. However, a significant inhibition of enzyme activity (> 22%) was observed at 1 nM melatonin which is in the range of the physiological serum concentration of the hormone at night. The inhibitory effect of melatonin was observed exclusively in the presence of Ca++. Results suggest a new and important role of the pineal hormone melatonin on central nervous system processes, i.e., by modulating NO production.

Differential effects of protein kinase inhibitors and activators on memory formation in the 2-day-old chick

Thirteen protein kinase inhibitors (PKIs) were investigated in chicks for their in vitro effects on PKC activity and for their in vivo effects on memory formation for a peak-avoidance task. Amnesia occurred by 15-30 min post-training when agents that inhibit primarily Ca2+/calmodulin were injected into brain. Amnesia occurred by 60 min post-training when agents that inhibit PKC-, PKA-, and/or PKG-dependent protein kinases, but not Ca2+/calmodulin, were injected. Enhancement of memory formation was accomplished by injecting bradykinin, but not forskolin. Both of these agents, however, attenuated the amnesia produced by H-7. These results are discussed as relevant neural processes involved in memory and synaptic plasticity.

The pharmacology of the nootropics; new insights and new questions

Up to now, the memory-enhancing effect of the nootropics has chiefly been investigated in the context of effects on energy metabolism and on cholinergic and glutamatergic neurotransmission. Recent studies have also shown that the effect on memory is steroid-sensitive. The present review article summarizes the available results and discusses them in the context of a new hypothesis on the mechanism of action and with respect to clinical implications.

Nitric oxide: a new messenger in the brain

The important role played by nitric oxide (NO) in the central nervous system has largely been emphasized in the recent literature. It can originate at least from four different sources: the endothelium of cerebral vessels, the immunostimulated microglia and astrocytes, the nonadrenergic noncholinergic nerve, and the glutamate neuron. NO has been implicated in a large number of pathologies (such as neurotoxicity in Alzheimer's disease and Huntington's disease, cerebral ischemia, stroke, and anxiety) and also in normal physiological functions (such as memory and learning, regulation of the cerebrovascular system, modulation of the wakefulness, mediation of nociception, olfaction, food intake and drinking, regulation of noradrenaline, and dopamine release). The aim of this paper is to review and to integrate the most recent advances in our understanding of the roles of NO in the brain.

The time course of learning a visual skill

Several examples of experience-dependent perceptual improvement (perceptual learning) suggest that plasticity in specific neuronal loci could underlie the learning process. For a basic visual discrimination task (using an optimal stimulus for 'automatic' pre-attentive texture segregation), discrete retinal input-dependent changes within a very early stage in the stream of visual processing were indicated as the locus of a large and consistent learning effect. When do these changes occur? Here we report that except for a fast, rapidly saturating improvement early in the first practice session, performance was very stable within sessions. Indeed, observers showed little or no improvement until up to 8 hours after their last training session (latent phase). But large improvements occurred thereafter. Finally, there was almost no forgetting; what was gained was retained for at least 2-3 years. We conjecture that some types of perceptual experience trigger permanent neural changes in early processing stages of the adult visual system. These may take many hours to become functional.

A role for the neural cell adhesion molecule in a late, consolidating phase of glycoprotein synthesis six hours following passive avoidance training of the young chick

We have investigated the effect of intracranial injections of the amnestic anti-metabolite, 2-deoxygalactose, and antibodies to the neural cell adhesion molecule on retention of a one-trial passive avoidance task in chicks. Groups of chicks received bilateral intracranial injections of 10 mumol/hemisphere 2-deoxygalactose or 10 microliters/hemisphere anti-neural cell adhesion molecule and were tested 24 h following training. 2-Deoxygalactose injections were amnestic when administered at a previously established time (30 min pre-training). Here we show that the agent is also amnestic when injected within a second time window occurring specifically 6-8 h after training. Administration of 2-deoxygalactose between 2 and 6 h or after 8 h post-training was without effect on retention tested 24 h following training. Anti-neural cell adhesion molecule injections were amnestic only when performed at a time which coincided with the second phase of 2-deoxygalactose susceptibility. Further experiments demonstrated that the neural cell adhesion molecule is one of the molecules into which 2-deoxygalactose is incorporated. Additionally, we investigated the extent of diffusion of 2-deoxygalactose and anti-neural cell adhesion molecule following their injection, with respect to their residence in forebrain loci known to be involved in the memory for passive avoidance. We interpret these data as indicating that two waves of glycoprotein synthesis are necessary for the establishment of long-term memory for the experience of passive avoidance training. The evidence is discussed in the context of earlier results indicating that the two waves involve different glycoprotein species and, possibly, different forebrain regions. We speculate that the late phase of glycoprotein synthesis coincides with, and is required for, modulation of cell-cell adhesion processes, reflecting the selection and stabilization of synapses which maintain an enduring representation of long-term memory.

Early postnatal hypoxia induces long-term changes in the dopaminergic system in rats

A rat model of a mild, chronic, early postnatal hypoxia, characterized by long-term consequences in the behavioural outcome, was used to study long-term consequences in the dopaminergic system. Exposure of newborn rats to an early postnatal hypoxia (hypobaric hypoxia, 11 kPa pO2 in the inspiratory air, 2nd-10th day of life, 10 hours daily) brings about the following lasting neurochemical changes: an increased stimulated dopamine release rate from striatum slices by about 30%, an increased low affinity, high capacity dopamine uptake into striatum synaptosomes by about 100%. The critical period to produce an increased release rate of dopamine was estimated as day 2-6 postnatally. There are no long-term changes in the concentration of dopamine and its metabolites and in the tyrosine hydroxylase activity in consequences of this early postnatal hypoxia. Treatment of newborn animals with L-DOPA (10-50 micrograms/g body weight) previous to hypoxia normalizes the DA release rate.

Influence of dopaminergic agonists/antagonists on fucose metabolism in the rat brain

Valid indications of a key role for dopaminergic drugs in glycoprotein fucosylation in neuronal tissue in vitro led us to investigate whether the administration of dopaminergic agonists/antagonists influences fucose metabolism in the rat brain in vivo. Three test groups were set up. Group 1 was given L-DOPA (210 mg/l), Group 2 haloperidol (10.5 mg/l) in drinking water, Group 3 served as control. The rats were sacrificed after 8 weeks and enzyme activities in 5 different brain areas were determined concerning the enzymes of the anabolic fucose metabolism: fucokinase, fucose-1-phosphate pyrophosphorylase and fucosyltransferase 1 and 2. Only the specific activity of fucokinase was affected by haloperidol or L-DOPA administration. In the olfactory bulbus, thalamus and cortex, haloperidol decreased fucokinase activity by 21%, 37%, and 39%, respectively (p < 0.05 in each case). No changes were observed in the cerebellum and striatum. Surprisingly, fucokinase activity in the cortex was decreased by L-DOPA (31%, p < 0.05).

CONCLUSIONS

Cerebral fucokinase activity is influenced in vivo by dopaminergic drugs in well circumscribed brain areas. Other enzymes of the anabolic fucose metabolism showed no change in activity. It is, therefore, conceivable that these drugs, apart from known mechanisms, exert part of their pharmacological action via a modulation of fucose metabolism.

Differential expression of immediate early genes after hippocampal long-term potentiation in awake rats

The pattern of expression of fos and jun family immediate early genes following the induction of long-term potentiation (LTP) was investigated in the dentate gyrus of awake rats. Rapid, transient increases in the levels of c-jun and jun-B mRNA and protein, and in the levels of Fos-related proteins (FRAs), occurred in the dentate gyrus after LTP-inducing tetanization of the perforant path. A delayed, and more prolonged induction occurred for jun-D mRNA and protein. The induction of c-Jun, Jun-B, Jun-D and Fos-related proteins was prevented by administration of an N-methyl-D-aspartate receptor antagonist, which also blocked LTP induction, and by pentobarbital, which reduced but did not block LTP. These findings show that differential expression of fos and jun gene family members occurs in a distinct pattern following LTP in awake rats. The responsive genes may participate in the biochemical cascade leading to the long-term stabilization of synaptic modifications.

Long-term potentiation as synaptic dialogue

We have proposed a testable model of the physiological and biochemical events underlying LTP that offers the following novel features. (1) The focus is not on a single mechanism or synaptic site, but rather on the integration and interaction of mechanisms occurring on both sides of the synapse, (2) beta PKC plays a critical presynaptic role in LTP, while gamma PKC functions postsynaptically. (3) These stages can be ordered in a time-delimited sequence of post- then presynaptic molecular events based on the period of effectiveness of inhibitor compounds. (4) The distinction is made between the time when kinase activation occurs and the time when the potentiated response requiring this kinase activation is observed.

Synaptic reorganization in developing and adult nervous systems

Evidence is accumulating that synapse reorganization already starts during development, soon after first synapses appear. Although remodeling continues throughout ontogenesis, there are apparently (critical) periods which are characterized by enhanced synaptic reorganization. In certain parts of the peripheral and central nervous system, synapses may undergo remodeling which leads to changes in their transmission efficiency or complete elimination of the synaptic junctions, even in adulthood. Synaptic reorganization includes progressive and regressive changes on branches of dendritic and/or axonal processes that accompany the formation and elimination of synapses. Three modes of elimination are presently known: Physiological cell death of synaptically connected neurons is involved, especially during certain developmental periods, during hormonally induced metamorphosis and in the olfactory bulb. Synaptic disconnection ("stripping") and lysosomal degradation predominantly of presynaptic elements occur under different conditions. In order to undergo plastic changes, neurons seem to respond to exogenous or intrinsic factors such as lesions (partial deafferentation and axotomy), long-lasting changes in neuronal activity (e.g. drug application and sensory deprivation), hormonal influences (e.g. sexual hormones) or learning conditions.

Postsynaptic then presynaptic protein kinase C activity may be necessary for long-term potentiation

Protein kinase C inhibitor was injected intracellularly by iontophoresis into CA1 somata either before or after long-term potentiation in the hippocampal slice preparation. Two different protein kinase C inhibitors, polymyxin B (PMXB) or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), injected 10 min before long-term potentiation induction caused potentiated responses to return to baseline 15-35 min after induction without significantly affecting the initial magnitude of potentiation. There was no effect on long-term potentiation persistence when H-7 or PMXB was injected intracellularly 5 min after long-term potentiation induction. In contrast, focal extracellular micro-pressure ejection of protein kinase C inhibitor in the stratum radiatum, 15 or 30 min, but not 60 min after long-term potentiation induction caused decay of long-term potentiation to baseline. This is probably a presynaptic action since intracellular inhibitors injected postsynaptically were ineffective 5 min after long-term potentiation induction. Focal application to stratum pyramidale produced a weaker decay than to stratum radiatum suggesting a Schaffer collateral presynaptic terminal site of action. We propose that activation of postsynaptic protein kinase C activity is necessary for long-term potentiation persistence but this activity persists for less than 5 min after induction. Presynaptic protein kinase C activity is also necessary for persistence and is time-limited to less than 60 min. It is attractive to think that these two events are sequentially activated and employ different protein kinase C subtypes differentially localized to presynaptic or postsynaptic elements.

New insights into activation of the steroid hormone receptor superfamily

For many years the prevailing view of how steroid hormone receptors exert their effects on gene transcription has been that these intracellular receptor proteins, upon association with their specific cognate ligands, undergo a transformation to a state where they are capable of interacting with chromatin and regulating the transcription of specific genes. It has become implicit dogma in the field of biochemical endocrinology that receptor activation is absolutely dependent upon this ligand-binding event. However, recent studies described here by Bert O'Malley and colleagues have shown that certain members of the steroid hormone receptor superfamily can be activated in a totally ligand-independent manner by a cell membrane receptor agonist, the neurotransmitter dopamine.

C-fos protooncogene expression in rat brain after long-term training of two-way active avoidance reaction

C-fos nuclear protooncogene encodes a regulatory protein (Fos), able to directly influence both expression of itself and other genes. It has been repeatedly shown that c-fos expression coincides with different forms of cell activation, probably being functionally involved in the coupling of extracellular ligands to long-term cellular responses. In this study it has been found that c-fos mRNA accumulation in rat brain, as measured by northern blotting coincides with increase of performance level of learned behavior of a two-way active avoidance task. We have previously reported (Nikolaev et al., Brain Res. Bull., in press) that a single training session of two-way active avoidance strongly induces c-fos mRNA accumulation but that after long-term training up to the asymptotic level of performance no c-fos expression was detectable. In this paper we show that c-fos still remains inducible even after long-term, asymptotic training to darkness as conditioned stimulus (CS), provided that a novel stimulus, wide band noise, which elevated performance level, was given together with darkness as compound CS.

Phorbol ester-induced changes in rat hippocampal glycoprotein fucosylation

Studies on glycoprotein fucosylation were carried out using hippocampal slices from rat brain. These slices were incubated in the presence of the protein kinase C (PKC) activating phorbol ester, 4 beta-phorbol-12,13-dibutyrate (PDBu), or an inactive isoform, 4 alpha-phorbol-12,13-didecanoate (PDD), respectively, for 7 min followed by a 60 min pulse of [3H]fucose. PDBu caused an increase in [3H]fucose incorporation into glycoproteins by 29% as well as an activation of the fucokinase enzyme reaction by 21%. The PDBu-induced stimulation of [3H]fucose insertion into hippocampal glycoproteins was abolished by the PKC inhibitors, staurosporine and H7. The importance of a PKC-regulated glycoprotein fucosylation in mechanisms underlying changes in neuronal plasticity is discussed.

Effect of MK-801 on the induction and subsequent decay of long-term potentiation in the unanesthetized rabbit hippocampal dentate gyrus

Several investigations have shown that the N-methyl-D-aspartate (NMDA) antagonist MK-801 interferes both with learning and a candidate neural mechanism for learning, the long-term potentiation (LTP) phenomenon. The low doses of MK-801 reported to block learning, however, may not be sufficient to block LTP. The present experiments examined the effects of 4 doses (0.05, 0.10, 0.50 and 1.00 mg/kg) of MK-801 on LTP of the perforant path-granule cell population excitatory postsynaptic potential (EPSP) and spike in unanesthetized rabbits. MK-801 did not significantly affect the threshold intensity for LTP of the population EPSP but the 3 highest doses did increase the threshold for LTP of the population spike. The 3 highest doses also reduced the peak magnitude and the duration of LTP of the spike to less than 24 h. The 0.05 m/kg dose did not affect the threshold or peak magnitude of spike LTP, but did decrease the decay time constant to 10.4 days, compared to 20.8 days in control rabbits. Only the 1.00 mg/kg dose reduced the magnitude of LTP of the EPSP. It was not possible to determine if MK-801 altered the longer-lasting component of LTP of the EPSP as it never persisted for longer than 24 h. These studies demonstrate that MK-801 disrupts LTP at doses also known to interfere with learning.

Distributed changes in rat brain DNA synthesis with long-term habituation and potentiation of the perforant path-granule cell synapse

The involvement of brain deoxyribonucleic acid (DNA) synthesis in adaptive neural events was studied in the adult rat during long-term habituation (LTH) or potentiation (LTP) of the perforant path-granule cell synapse. Male Long-Evans rats were given 50 muCi [3H]thymidine intraventricularly under urethane anesthesia. Soon thereafter, field excitatory postsynaptic potential (EPSP) slope and population spike were monitored from the right dentate gyrus before and at various times (5, 10, 15, 60 min) following the delivery to the ipsilateral perforant bundle of a low frequency (LFS: 1.0 Hz, 160 s) or a high-frequency train (HFS: 400 Hz, 200 ms), repeated once after 5 min. Unstimulated implanted rats served as controls. DNA synthesis was evaluated by the incorporation of the radioactive precursor into DNA of several brain areas at the end of a 1 h incorporation period. In CA1, LTH and LTP increased DNA synthesis by 30% on the stimulated side. In the entorhinal cortex, LTH but not LTP increased DNA synthesis (by 30%) on the stimulated side. Conversely, in the frontal cortex, LTP but not LTH increased DNA synthesis (by 100%) on both sides. Long-lasting changes in synaptic efficacy covaried non-linearly with DNA synthesis in mono- and polysynaptically stimulated hippocampal regions, and in functionally associated neocortical areas. The co-variations of population spike amplitude were positive for LTH and negative for LTP in the dentate gyrus and frontal cortex of both sides, and in CA3/CA1 of the stimulated side, indicating higher DNA synthesis at lower values of LTH and LTP, and viceversa. Further, regional cross-correlation analyses revealed a high degree of synchronization among brain sites, following low- or high-frequency train pulses, indicating that (i) extra-target sites participate on the stimulated and on the contralateral side, and (ii) small distributed changes take place across the sampled neural networks. A modulatory role of information flow on brain DNA synthesis is inferred to take place in a diffuse, distributed manner.

The influence of long-term potentiation on the spatial relationship between astrocyte processes and potentiated synapses in the dentate gyrus neuropil of rat brain

The influence of long-term potentiation (induced by repeated high-frequency stimulation of the perforant pathway) on the distribution pattern of astrocyte processes in the neuropil of the hippocampal dentate area containing the potentiated synapses was investigated by quantitative electronmicroscopy. It has been found that significant changes occurred in the ramification of astrocyte processes as well as in their topographic relation to synaptic complexes. When comparing the results obtained in LTP animals with active control or sham-operated animals, we found significant higher numerical density, but smaller volume, higher surface density and closer apposition of astrocyte processes to the synaptic clefts, boutons terminaux or spines in the potentiated synapses containing neuropil. The glial reaction to synaptic activation has been seen most pronounced 8 h after the LTP induction. The results are pointing to a participation of the glia cells in the maintenance of the LTP effect as well as to a metabolic coupling between synaptic transmission and glia function for equilibrating the homeostasis by clearing the extracellular space next to the transmission zones.

The role of immediate early genes in the stabilization of long-term potentiation

Immediate early genes (IEGs) are a class of genes that show rapid and transient but protein synthesis-independent increases in expression to extracellular signals such as growth factors and neurotransmitters. Many IEGs code for transcription factors that have been suggested to govern the growth and differentiation of many cell types by regulating the expression of other genes. IEGs are expressed in adult neurons both constitutively and in response to afferent activity, and it has been suggested that during learning, IEGs may play a role in the signal cascade, resulting in the expression of genes critical for the consolidation of long-term memory. Long-term potentiation (LTP) is a persistent, activity-dependent form of synaptic plasticity that stands as a good candidate for the mechanism of associative memory. A number of IEGs coding for transcription factors have been shown to transiently increase transcription in the dentate gyrus of rats following LTP-inducing afferent stimulation. These include zif/268 (also termed NGFI-A, Krox-24, TIS-8, and egr-l), c-fos-related genes, c-jun, junB, and junD. Of these, zif/268 appears to be the most specifically related to LTP since it is evoked under virtually all LTP-inducing situations and shows a remarkably high correlation with the duration of LTP. There are a number of outstanding questions regarding the role of zif/268 and other IEGs in LTP, including which second messenger systems are important for activating them, which "late effector" genes are regulated by them, and the exact role these genes play, if any, in the stabilization and maintenance of LTP.

The amnesic substance 2-deoxy-D-galactose suppresses the maintenance of hippocampal LTP

Male Wistar rats were intraventricularly injected with 2-deoxy-D-galactose (do-gal), a substance interfering with the fucosylation of glycomacromolecules and impairing memory consolidation in various learning tasks. Do-gal was found to have no influence on the monosynaptically evoked field potential (MEFP) recorded in the dentate gyrus upon stimulation of the perforant pathway. However, hippocampal long-term potentiation (LTP) induced in do-gal-pretreated animals by fractionated tetanization of the perforant pathway declined to control levels 2 h after tetanization, whereas it remained constant for 24 h in saline-treated rats. Similar effects were observed in the CA1 region of hippocampal slices. The results indicate a participation of fucosylated macromolecules in the maintenance of LTP. The possible significance of processes involved in LTP for memory formation is discussed.

Accumulation of c-fos mRNA in rat hippocampus during acquisition of a brightness discrimination

Training rats to attain a foot-shock-motivated brightness discrimination in a Y-maze results in an early and transient increase of hippocampal c-fos mRNA levels. Maximal accumulation was observed immediately after training, returning to basal levels during the following 2 h. A similar increase was obtained when rats were subjected to a pseudotraining with an equal number of runs, but with random pairing of the choice of bright and dark alleys with foot shock. It is suggested that induction of hippocampal c-fos mRNA expression is a necessary, but not sufficient, prerequisite for the formation of long-term memory trace. This early gene expression seems rather to correspond to an initial stage induced by complex stimulus presentation of both the training and the pseudotraining procedure. The subsequent late synthesis or processing of target proteins finally contributing to the formation of a permanent trace requires the action of further convergent signals to principal cells, probably mediating reward or emotional influences.

Posttetanic long-term potentiation in rat dentate area increases postsynaptic 411B immunoreactivity

411B is a monoclonal antibody raised to chick forebrain postsynaptic densities (PSDs) which also recognises an antigen in brain tissue from adult Wistar rats but not liver, heart, or lung. This antigen is enriched in the PSD fraction and appears to be a useful biochemical marker for plastic changes of postsynaptic structures in the rat brain. The aim of this study was to investigate whether 411B immunoreactivity is changed in various hippocampal subregions by post-tetanic long-term potentiation (LTP). LTP was elicited in freely moving rats by applying four trains of 300 square-wave pulses (frequency 200 Hz, pulse duration 0.2 ms, and intensity 300 mA) into the right perforant path; this included an increase in transmission efficacy at the ipsilateral perforant path-granular cell synapse of the dentate gyrus lasting several days. Eight hours after tetanisation, antigens recognised by monoclonal 411B and a polyclonal anti-actin antiserum were assayed in lysed homogenates of ipsi- and contralateral CA1. CA3, and CA4/dentate area hippocampal subfields as well as in visual cortex, cerebellum, and olfactory bulb dissected from LTP rats, and compared to passive controls. Under these experimental conditions, tetanisation of the perforant path resulted in a significant increase in the titre of 411B in the ipsilateral CA4/dentate area subfield (+34.0%; p less than 0.001) compared with passive controls, whereas in all other brain regions studied no differences between experimental and control rats were observed. In no region were anti-actin titres significantly different from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopaminergic antagonists prevent long-term maintenance of posttetanic LTP in the CA1 region of rat hippocampal slices

The involvement of dopaminergic mechanisms in the induction and maintenance of posttetanic long-term potentiation (LTP) was investigated on CA1 cells of rat hippocampal slices. The presence of the dopamine receptor blocker domperidone in a concentration of 1 microM during tetanization with 3 trains of 100 impulses (100 Hz) and a train interval of 10 min influences neither the synaptic transmission nor the induction of LTP. However, the potentiation of both the population spike and the population EPSP gradually decreases, thus significantly differing from control LTP about 4 h after initiation and reaching the level of non-tetanized controls about 7-8 h after tetanization. The simultaneous presence of 1 microM apomorphine during tetanization abolishes this effect of domperidone indicating the specific dopaminolytic nature of its action. Also the presence of the dopamine antagonists sulpiride and flupenthixol, respectively, in a concentration of 1 microM during tetanization likewise prevents the occurrence of the late LTP maintenance. The determination of [14C]dopamine in 2 min fractions from the superfused slices after preloading during a preincubation period revealed that a low frequency stimulation of the Schaffer collaterals with 0.33 Hz does not influence the spontaneous efflux of dopamine, whereas the tetanization with an impulse train of 100 Hz produces a significantly enhanced release. The observations suggest that dopaminergic influences during and immediately after tetanization at least additionally contribute to the induction of postsynaptic mechanisms subserving a late, long-lasting maintenance of potentiation. The results also support the assumed existence of different subsequent stages of LTP.

Long-term enhancement of synaptic transmission by synthetic mast cell degranulating peptide and its localization of binding sites in hippocampus

A bee venom, mast-cell-degranulating (MCD) peptide, was synthesized by stepwise formation of the two disulfide bridges. This synthetic MCD peptide induced long-term potentiation (LTP) in the CA1 region of a hippocampus slice at concentrations ranging from 10(-7) to 10(-5) M. The potentiating effect of MCD was not lost by biotinylation of its N-terminus, and thus it became possible to investigate localization of its binding site at a cellular level in the hippocampus slice at a LTP-inducible concentration (1 microM). We found that even after brief application of N-biotinyl-MCD (1 microM) to perfusate, its presence was observed in pyramidal cell bodies in the CA1 and CA3 regions, but not in granule cells in the dentate gyrus of the hippocampus slice. This observation does not coincide with the localization of its high-affinity binding sites revealed by 125I-MCD binding, suggestive that another type(s) of MCD binding site, such as low-affinity binding sites, are present in the hippocampus.

Methylglucamine orotate, a memory-improving drug, prolongs hippocampal long-term potentiation

We showed earlier that the memory-improving substance, methylglucamine orotate, a precursor of pyrimidine nucleotide synthesis, improved memory retention and enhanced the postconditioning potentiation of field potentials in a special learning task. Since long-term potentiation (LTP) is frequently regarded as a mechanism of memory formation, we decided to test whether it can be influenced in a similar manner. The present study shows that the same dose of methylglucamine orotate (225 micrograms i.c.v.) prolongs LTP but has only a slight effect on the field potential recorded in the dentate gyrus. In controls, LTP of the population spike returned to 154% of the control value 24 h after tetanization and to 126% after 48 h. However, in drug treated animals LTP of the population spike remained at 246 and 216% of the control, respectively. The results support the assumption that LTP is at least a component of memory formation.