Long-term depression in cerebral cortex: a possible substrate of "forgetting" that should not be forgotten

The Intriguing Contribution of Hippocampal Long-Term Depression to Spatial Learning and Long-Term Memory

Long-term potentiation (LTP) and long-term depression (LTD) comprise the principal cellular mechanisms that fulfill established criteria for the physiological correlates of learning and memory. Traditionally LTP, that increases synaptic weights, has been ascribed a prominent role in learning and memory whereas LTD, that decreases them, has often been relegated to the category of “counterpart to LTP” that serves to prevent saturation of synapses. In contradiction of these assumptions, studies over the last several years have provided functional evidence for distinct roles of LTD in specific aspects of hippocampus-dependent associative learning and information encoding. Furthermore, evidence of the experience-dependent “pruning” of excitatory synapses, the majority of which are located on dendritic spines, by means of LTD has been provided. In addition, reports exist of the temporal and physical restriction of LTP in dendritic compartments by means of LTD. Here, we discuss the role of LTD and LTP in experience-dependent information encoding based on empirical evidence derived from conjoint behavioral and electrophysiological studies conducted in behaving rodents. We pinpoint the close interrelation between structural modifications of dendritic spines and the occurrence of LTP and LTD. We report on findings that support that whereas LTP serves to acquire the general scheme of a spatial representation, LTD enables retention of content details. We argue that LTD contributes to learning by engaging in a functional interplay with LTP, rather than serving as its simple counterpart, or negator. We propose that similar spatial experiences that share elements of neuronal representations can be modified by means of LTD to enable pattern separation. Therewith, LTD plays a crucial role in the disambiguation of similar spatial representations and the prevention of generalization.

G-protein coupled receptors and synaptic plasticity in sleep deprivation

Insufficient sleep has been correlated to many physiological and psychoneurological disorders. Over the years, our understanding of the state of sleep has transcended from an inactive period of rest to a more active state involving important cellular and molecular processes. In addition, during sleep, electrophysiological changes also occur in pathways in specific regions of the mammalian central nervous system (CNS). Activity mediated synaptic plasticity in the CNS can lead to long-term and sometimes permanent strengthening and/or weakening synaptic strength affecting neuronal network behaviour. Memory consolidation and learning that take place during sleep cycles, can be affected by changes in synaptic plasticity during sleep disturbances. G-protein coupled receptors (GPCRs), with their versatile structural and functional attributes, can regulate synaptic plasticity in CNS and hence, may be potentially affected in sleep deprived conditions. In this review, we aim to discuss important functional changes that can take place in the CNS during sleep and sleep deprivation and how changes in GPCRs can lead to potential problems with therapeutics with pharmacological interventions.

Brain is modulated by neuronal plasticity during postnatal development

Neuroplasticity is referred to the ability of the nervous system to change its structure or functions as a result of former stimuli. It is a plausible mechanism underlying a dynamic brain through adaptation processes of neural structure and activity patterns. Nevertheless, it is still unclear how the plastic neural systems achieve and maintain their equilibrium. Additionally, the alterations of balanced brain dynamics under different plasticity rules have not been explored either. Therefore, the present article primarily aims to review recent research studies regarding homosynaptic and heterosynaptic neuroplasticity characterized by the manipulation of excitatory and inhibitory synaptic inputs. Moreover, it attempts to understand different mechanisms related to the main forms of synaptic plasticity at the excitatory and inhibitory synapses during the brain development processes. Hence, this study comprised surveying those articles published since 1988 and available through PubMed, Google Scholar and science direct databases on a keyword-based search paradigm. All in all, the study results presented extensive and corroborative pieces of evidence for the main types of plasticity, including the long-term potentiation (LTP) and long-term depression (LTD) of the excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs).

Toward a Symptom-Guided Neurostimulation for Gilles de la Tourette Syndrome

Therapy resistance of approximately one-third of patients with Gilles de la Tourette syndrome (GTS) requires consideration of alternative therapeutic interventions. This article provides a condensed review of the invasive and non-invasive stimulation techniques that have been applied, to date, for treatment and investigation of GTS. Through this perspective and short review, the article discusses potential novel applications for neurostimulation techniques based on a symptom-guided approach. The concept of considering the physiological basis of specific symptoms when using stimulation techniques will provide a platform for more effective non-pharmacological neuromodulation of symptoms in GTS.

Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo

The expression of homosynaptic long-term depression (LTD) is thought to mediate a crucial role in sustaining memory function. Our in vivo investigations of LTD expression at lateral (LPP) and medial perforant path (MPP) synapses in the dentate gyrus (DG) corroborate prior demonstrations that PP-DG LTD is difficult to induce in intact animals. In freely moving animals, LTD expression occurred inconsistently among LPP-DG and MPP-DG responses. Interestingly, following acute electrode implantation in anesthetized rats, low-frequency stimulation (LFS; 900 pulses, 1 Hz) promotes slow-onset LTP at both MPP-DG and LPP-DG synapses that utilize distinct induction mechanisms. Systemic administration of the N-methyl-d-aspartate (NMDA) receptor antagonist (+/-)-cyclopiperidine-6-piperiperenzine (CPP; 10 mg/kg) 90 min before LFS selectively blocked MPP-DG but not LPP-DG slow onset LTP, suggesting MPP-DG synapses express a NMDA receptor-dependent slow onset LTP whereas LPP-DG slow onset LTP induction is NMDA receptor independent. In experiments where paired-pulse LFS (900 paired pulses, 200-ms paired-pulse interval) was used to induce LTD, paired-pulse LFS of the LPP resulted in rapid onset LTP of DG responses, whereas paired-pulse LFS of the MPP induced slow onset LTP of DG responses. Although LTD observations were very rare following acute electrode implantation in anesthetized rats, LPP-DG LTD was demonstrated in some anesthetized rats with previously implanted electrodes. Together, our data indicate in vivo PP-DG LTD expression is an inconsistent phenomenon that is primarily observed in recovered animals, suggesting perturbation of the dentate through surgery-related tissue trauma influences both LTD incidence and LTP induction at PP-DG synapses in vivo.

Increased NR2A:NR2B ratio compresses long-term depression range and constrains long-term memory

The NR2A:NR2B subunit ratio of the NMDA receptors is widely known to increase in the brain from postnatal development to sexual maturity and to aging, yet its impact on memory function remains speculative. We have generated forebrain-specific NR2A overexpression transgenic mice and show that these mice had normal basic behaviors and short-term memory, but exhibited broad long-term memory deficits as revealed by several behavioral paradigms. Surprisingly, increased NR2A expression did not affect 1-Hz-induced long-term depression (LTD) or 100 Hz-induced long-term potentiation (LTP) in the CA1 region of the hippocampus, but selectively abolished LTD responses in the 3–5 Hz frequency range. Our results demonstrate that the increased NR2A:NR2B ratio is a critical genetic factor in constraining long-term memory in the adult brain. We postulate that LTD-like process underlies post-learning information sculpting, a novel and essential consolidation step in transforming new information into long-term memory.

Hippocampal long-term depression mediates spatial reversal learning in the Morris water maze

Synaptic plasticity at hippocampal excitatory synapses has been proposed as the cellular mechanism underlying spatial learning and memory. However, most previous studies have focused on the role of long-term potentiation (LTP) in learning and memory, and much less is known about the role of long-term depression (LTD). Here, we report that hippocampal-dependent spatial learning in the Morris water maze facilitated hippocampal CA1 LTD induction in vivo. The LTD can be blocked by systemic application of the selective GluN2B antagonist Ro25-6981 (6 mg/kg, i.p.) or a synthetic peptide Tat-GluA2(3Y) (3 μmol/kg, i.p.) that interferes with the endocytosis of AMPA receptors. In addition, systemic or intrahippocampal administration of these two mechanistically and structurally distinct inhibitors impaired spatial reversal learning of a novel target location, when the hidden platform was moved to the quadrant opposite the initial target location. Notably, acute elevated-platform stress, which facilitates hippocampal LTD induction, enhanced both acquisition and retrieval of spatial reversal memory. The present study demonstrates that reversal learning is impaired by blocking hippocampal LTD, and enhanced by facilitating hippocampal LTD, suggesting that hippocampal LTD may be necessary and sufficient to mediate new information processing. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The role of the entorhinal cortex in extinction: influences of aging

The entorhinal cortex is perhaps the area of the brain in which neurofibrillary tangles and amyloid plaques are first detectable in old age with or without mild cognitive impairment, and very particularly in Alzheimer's disease. It plays a key role in memory formation, retrieval, and extinction, as part of circuits that include the hippocampus, the amygdaloid nucleus, and several regions of the neocortex, in particular of the prefrontal cortex. Lesions or biochemical impairments of the entorhinal cortex hinder extinction. Microinfusion experiments have shown that glutamate NMDA receptors, calcium and calmodulin-dependent protein kinase II, and protein synthesis in the entorhinal cortex are involved in and required for extinction. Aging also hinders extinction; it is possible that its effect may be in part mediated by the entorhinal cortex.

Hippocampal long-term depression: master or minion in declarative memory processes?

The neural mechanisms for the formation of declarative memory (memory for facts and events) are believed to be integrated from processes mediated by hippocampal long-term potentiation (LTP) and long-term depression (LTD). Traditionally, LTP has been designated as the main mediator of spatial memory storage in the hippocampus, whereas LTD has been assigned an auxiliary role in signal-to-noise regulation or in forgetting. It has recently become apparent, however, that LTD contributes directly to hippocampal information storage. In fact, LTD could dominate in the processing of precise spatial characteristics. Accumulating evidence supports the idea that LTP and LTD enable distinct and separate forms of information storage, which together facilitate the generation of a spatial cognitive map.

Developmental regulation of cognitive abilities: modified composition of a molecular switch turns on associative learning

N-methyl-D-aspartate receptors (NMDARs) act as molecular coincidence detectors and allow for association or dissociation between pre- and postsynaptic neurons. NMDA receptors are central to remodeling of synaptic connections during postnatal development and associative learning abilities in adults. The ability to remodel neural networks is altered during postnatal development, possibly due to a change in the composition of NMDARs. That is, as forebrain systems (and cerebellum) develop, synaptic NR2B-containing NMDARs (NR2B-NMDARs) are replaced by NR2A-containing NMDARs (NR2A-NMDARs) and NR2B-NMDARs move to extrasynaptic sites. During the initial phase of the switch, synapses contain both NR2A- and NR2B-NMDARs and both long-term potentiation and long-term depression are enhanced. As NMDAR subunit expression decreases and NR2A-NMDARs come to predominate in the synapse, channel function and synaptic plasticity are reduced, and remodeling ability dissipates. The end result is a balance of plasticity and stability that is optimal for information processing and storage. Associative learning abilities involving different sensory modalities emerge sequentially, in accordance with synaptic maturation in related cortical and underlying brain structures. Thus, developmental alterations in NMDAR composition that occur at different ages in various brain structures may explain the protracted nature of the maturation of various associative learning abilities.

Competitive interactions between endogenous LTD and LTP in the hippocampus underlie the storage of emotional memories and stress-induced amnesia

This speculative review serves two purposes. First, it as an extension of the ideas we developed in a previous review (Diamond et al., Hippocampus, 2004;14:281-291), and second, it is a rebuttal to Abraham's (Hippocampus, 2004;14:675-676) critique of that review. We had speculated on the functional significance of the finding that post-training LTP induction produces retrograde amnesia. We noted the similarities between the findings that strong tetanizing stimulation can produce LTP and retrograde amnesia, and that a strong emotional experience can produce a long-lasting memory and retrograde amnesia, as well. The commonalities between LTP induction and emotional learning provided the basis of our hypothesis that an emotional experience generates endogenous LTD/depotentiation, which reverses synaptic plasticity formed during previous learning experiences, and endogenous LTP, which underlies the storage of new information. Abraham raised several concerns with our review, including the criticism that our speculation "falters because there is no evidence that stress causes LTD or depotentiation," and that research on stress and hippocampus has "failed to report any LTP-like changes." Abraham's points are well-taken because stress, in isolation, does not appear to generate long-lasting changes in baseline measures of hippocampal excitability. Here, within the context of a reply to Abraham's critique, we have provided a review of the literature on the influence of stress, novelty, fear conditioning, and the retrieval of emotional memories on cognitive and physiological measures of hippocampal functioning. An emphasis of this review is our hypothesis that endogenous forms of depotentiation, LTD and LTP are generated only when arousing experiences occur in conjunction with memory-related activation of the hippocampus and amygdala. We conclude with speculation that interactions among the different forms of endogenous plasticity underlie a form of competition by synapses and memories for access to retrieval resources.

Roles of oxytocin in spatial learning and memory in the nucleus basalis of Meynert in rats

The present study was performed to explore the role of oxytocin (OT) in spatial learning and memory in the nucleus basalis of Meynert (NBM) of rats. The latency, distance and swimming path to find the platform were tested by Morris water maze and recorded by a video camera connected to a computer. Intra-NBM injections of 2 or 10 nmol of OT, but not 0.2 nmol of OT, induced significant increase on the latency of spatial learning. Rats receiving intra-NBM administrations of 2 or 10 nmol of OT showed a more random search pattern. There were no significant changes in the swimming speed in Morris water maze test after the injection of OT. Furthermore, the impaired effect of OT on the latency of spatial learning was blocked by intra-NBM injection of the selective OT antagonist Atosiban, indicating that the effect of OT was mediated by OT receptor in the NBM of rats. Moreover, there were no influences of OT or Atosiban on the retention performance in rats. The results suggest that OT plays an inhibitory role in spatial learning in the NBM; the effect is mediated by OT receptor.

Hippocampal long-term depression and long-term potentiation encode different aspects of novelty acquisition

The hippocampus is required for encoding spatial information. Little is known however, about how different attributes of learning are related to different types of synaptic plasticity. Here, we investigated the association between long-term depression (LTD) and long-term potentiation, both cellular models for learning, and novelty exploration. We found that exploration of a new environment containing unfamiliar objects and/or familiar objects in new locations facilitated LTD, whereas exploration of the new environment itself, in the absence of objects, impaired LTD. Furthermore, we found this phenomenon to be modulated by 5-hydroxytryptamine 4 receptor activation. In contrast, long-term potentiation was facilitated by exploration of an empty novel environment, but simultaneous object exploration caused depotentiation. We also found that no further LTD could be induced. These findings support a decisive role for LTD in the acquisition of object-place configuration and consolidate its candidacy as a learning mechanism.

Pharmacological modulation of cortical excitability shifts induced by transcranial direct current stimulation in humans

Transcranial direct current stimulation (tDCS) of the human motor cortex results in polarity-specific shifts of cortical excitability during and after stimulation. Anodal tDCS enhances and cathodal stimulation reduces excitability. Animal experiments have demonstrated that the effect of anodal tDCS is caused by neuronal depolarisation, while cathodal tDCS hyperpolarises cortical neurones. However, not much is known about the ion channels and receptors involved in these effects. Thus, the impact of the sodium channel blocker carbamazepine, the calcium channel blocker flunarizine and the NMDA receptor antagonist dextromethorphane on tDCS-elicited motor cortical excitability changes of healthy human subjects were tested. tDCS-protocols inducing excitability alterations (1) only during tDCS and (2) eliciting long-lasting after-effects were applied after drug administration. Carbamazepine selectively eliminated the excitability enhancement induced by anodal stimulation during and after tDCS. Flunarizine resulted in similar changes. Antagonising NMDA receptors did not alter current-generated excitability changes during a short stimulation, which elicits no after-effects, but prevented the induction of long-lasting after-effects independent of their direction. These results suggest that, like in other animals, cortical excitability shifts induced during tDCS in humans also depend on membrane polarisation, thus modulating the conductance of sodium and calcium channels. Moreover, they suggest that the after-effects may be NMDA receptor dependent. Since NMDA receptors are involved in neuroplastic changes, the results suggest a possible application of tDCS in the modulation or induction of these processes in a clinical setting. The selective elimination of tDCS-driven excitability enhancements by carbamazepine proposes a role for this drug in focussing the effects of cathodal tDCS, which may have important future clinical applications.

Effects of arginine-vasopressin (AVP) on long-term potentiation in intact anesthetized rats

We studied the effects of the neuropeptide arginine-vasopressin (AVP) on the long-term potentiation (LTP) paradigm in the dentate gyrus (DG) of urethane intact anesthetized rats. Intracerebroventricular injection of 1 microg of the hormone in 1 microl of physiological solution 3 min before tetanization, produced a significant increase in both components of the perforant path-evoked potentials (EP) in the DG. The effects were already evident 1 min after tetanization. Amplitude of the EPs increased continuously for the 2h of recording time, reaching values 100% above baseline, reference levels. In contrast, in previous in vitro studies, enhancement of LTP with AVP appeared only after 15 min of exposure of the hippocampal slice to the hormone, increased EPSPs were no higher than 50% from baseline, reached a plateau at 40 min decreasing slowly thereafter. Not only quantitative but also qualitative differences can be observed between in vitro and in vivo intact preparations in response to identical hormones. This study emphasizes the importance of hormone neurotransmitter interactions in determining electrophysiological characteristics of response to AVP.

Imaging of calcineurin activated by long-term depression-inducing synaptic inputs in living neurons of rat visual cortex

Long-term depression (LTD) of synaptic transmission is induced by low-frequency stimulation (LFS) of afferents lasting for a long time, typically for 10-15 min, in neocortical and hippocampal slices. It is suggested that calcineurin, Ca2+/calmodulin-dependent protein phosphatase, plays a role in the induction of LTD, based on the results that pharmacological or genetic manipulation of calcineurin activity interfered in its induction. However, questions as to why it takes so long to induce LTD and in which compartment of neurons calcineurin is activated remain unanswered. With a fluorescent indicator for calcineurin activity, we visualized the spatiotemporal pattern of its activation in living neurons in layer II/III of visual cortical slices of rats during the LFS of layer IV that induced LTD of synaptic responses. During LFS, the fluorescence intensity gradually increased with a latency of a few minutes in dendrites and soma of neurons, and remained increased during the whole observation period (10-25 min) after LFS. The onset latency of the increase in the soma was slower than that in the distal dendritic region. The LFS-induced rise in fluorescence was not observed in neurons which were loaded with inhibitors of calcineurin, indicating that the intensity of fluorescence reflects calcineurin activity. Control stimulation at 0.05 Hz and theta-burst stimulation did not significantly change the intensity of fluorescence. Only LFS-type inputs effectively activate calcineurin in postsynaptic neurons in an augmenting manner, and such a time-consuming activation of calcineurin may be a reason why long-lasting LFS is necessary for the induction of LTD.

Hippocampal long-term depression as an index of spatial working memory

Long-term potentiation (LTP), a form of synaptic plasticity in the hippocampus, is a cellular model for the neural basis of learning and memory, but few studies have investigated the contribution of long-term depression (LTD), a counterpart of LTP. To address the possible relationship between hippocampal LTD and spatial performance, the spatial cognitive ability of a rat was assessed in a spontaneous alternation test and, thereafter, LTD in response to low-frequency burst stimulation (LFBS) was monitored in the dentate gyrus of the same rat under anaesthesia. To enhance a divergence in the ability for spatial performance, some of the animals received fimbria-fornix (FF) transection 14 days before the experiments. LTD was reliably induced by application of LFBS to the medial perforant path of intact rats, while no apparent LTD was elicited in rats with FF lesions. The behavioural parameters of spatial memory showed a significant correlation with the magnitude of LTD. We found no evidence that the cognitive ability correlated with other electrophysiological parameters, e.g. basal synaptic responses, stimulus intensity to produce half-maximal responses, paired-pulse facilitation or paired-pulse depression. These results suggest that the magnitude of LTD in the dentate gyrus serves as a reliable index of spatial cognitive ability, providing insights into the functional significance of hippocampal LTD.

Oxytocin induces long-term depression on the rat dentate gyrus: possible ATPase and ectoprotein kinase mediation

We studied the effects of the neuropeptide oxytocin (OT) on the long-term potentiation (LTP) paradigm in the dentate gyrus (DG) of urethane anesthetized rats. Intracerebroventricular injection of 1 microg of the hormone in 1 microl of physiological solution 2min before tetanization produced a significant decrease in both components of the perforant path evoked potentials (EP) in the DG. The effects appeared right after the tetanization stimuli and were more pronounced in the excitatory postsynaptic components of the EPs. The decrements lasted for the 2h of recording time. We concluded that OT induced and maintained long-term depression on the DG. In contrast, injection of OT in the absence of tetanic stimulation did not significantly affect perforant path EP in the DG. The results are discussed taking particular consideration of the inhibitory effects the OT has on (Ca(2+)+Mg(2+)) ATPase at membrane levels and the potential interference that this action may have with phosphorylation processes via an ectoprotein kinase isolated from membranes of hippocampal pyramidal neurons. Blocking of this ectoprotein kinase in vitro significantly impairs establishment and maintenance of LTP.

17beta-estradiol: effect on CA1 hippocampal synaptic plasticity

An understanding of synaptic plasticity in the mammalian brain has been one of R. F. Thompson's major pursuits throughout his illustrious career. A current series of experiments of significant interest to R. F. Thompson is an examination of the interactions between sex hormones, synaptic plasticity, aging, and stress. This research is contained within a broader project whose aim is to investigate animal models that evaluate estrogen interactions with Alzheimer's disease. This paper reviews the recent results that have led to a better understanding of how the sex hormone estrogen influences synaptic plasticity in an important structure within the mammalian brain responsible for learning and memory: the hippocampus. In this review, a number of experiments have been highlighted that investigate the molecular mechanisms that underlie estrogen's effect on two specific forms of synaptic plasticity commonly studied in neurophysiology and the behavioral neurosciences: long-term potentiation and long-term depression.

LTD induction in adult visual cortex: role of stimulus timing and inhibition

One Hertz stimulation of afferents for 15 min with constant interstimulus intervals (regular stimulation) can induce long-term depression (LTD) of synaptic strength in the neocortex. However, it is unknown whether natural patterns of low-frequency afferent spike activity induce LTD. Although neurons in the neocortex can fire at overall rates as low as 1 Hz, the intervals between spikes are irregular. This irregular spike activity (and thus, presumably, irregular activation of the synapses of that neuron onto postsynaptic targets) can be approximated by stimulation with Poisson-distributed interstimulus intervals (Poisson stimulation). Therefore, if low-frequency presynaptic spike activity in the intact neocortex is sufficient to induce a generalized LTD of synaptic transmission, then Poisson stimulation, which mimics this spike activity, should induce LTD in slices. We tested this hypothesis by comparing changes in the strength of synapses onto layer 2/3 pyramidal cells induced by regular and Poisson stimulation in slices from adult visual cortex. We find that regular stimulation induces LTD of excitatory synaptic transmission as assessed by field potentials and intracellular postsynaptic potentials (PSPs) with inhibition absent. However, Poisson stimulation does not induce a net LTD of excitatory synaptic transmission. When the PSP contained an inhibitory component, neither Poisson nor regular stimulation induced LTD. We propose that the short bursts of synaptic activity that occur during a Poisson train have potentiating effects that offset the induction of LTD that is favored with regular stimulation. Thus, natural (i.e., irregular) low-frequency activity in the adult neocortex in vivo should not consistently induce LTD.

17beta-estradiol suppresses expression of long-term depression in aged rats

It has been recently reported that the female steroid hormone 17beta-estradiol enhances synaptic transmission and the magnitude of long-term potentiation (LTP) in adult rodent hippocampus. Moreover, 17beta-estradiol ameliorates cognitive and memory function in postmenopausal women. Since aging is associated with an alteration of synaptic plasticity (e.g., higher susceptibility to long-term depression [LTD]), we examined whether 17beta-estradiol alters the expression of LTD in aged rats. We now report that the induction of LTD recorded from CA1 hippocampal neurons of aged rats is suppressed by 17beta-estradiol treatment, which produced only a minimal effect in suppressing LTD in adult rats. These results suggest that estrogen may act to improve memory by suppressing forgetfulness via a synaptic mechanism, such as LTD.

Behavioural pharmacology and its contribution to the molecular basis of memory consolidation

Recent findings have significantly advanced our understanding the mechanisms of memory formation. Most of these advances stemmed from behavioural pharmacology research involving, particularly, the localized infusion of drugs with specific molecular actions into specific brain regions. This approach has revealed brain structures involved in different memory types and the main neurotransmitter systems and sequence of metabolic cascades that participate in memory consolidation. Biochemical studies and, in several cases, studies of genetically manipulated animals, in which receptors or enzymes affected by the various drugs were absent or overexpressed, have complemented the pharmacological research. Although most studies have concentrated on the involvement of the hippocampus, many have also investigated the entorhinal cortex, other regions of the cortex, and the amygdala. Behavioural pharmacology has been of crucial importance in establishing the major neurohumoral and hormonal systems involved in the modulation of memory formation. These systems act on specific steps of memory formation in the hippocampus and in the entorhinal, parietal, and cingulate cortex. A specialized system mediated by the basolateral amygdaloid nucleus, and involving several neuromodulatory systems, is activated by emotional arousal and serves to regulate memory formation in other brain regions. The core mechanisms involved in the formation of explicit (declarative) memory are in many respects similar to those of long-term potentiation (LTP), particularly in the hippocampus. However, there are also important differences between memory formation and LTP. Memory formation involves numerous modulatory influences, the co-participation of various brain regions other than the hippocampus, and some properties that are specific to memory and absent in LTP (i.e. flexibility of response). We discuss the implications of these similarities and differences for understanding the neural bases of memory.

Primed-bursts induced long-term potentiation in rat visual cortex: effects of dark-rearing

Theta burst stimulation (TBS) and primed bursts (PBs) stimulation are among the effective tetanic stimulations for induction of long-term potentiation (LTP) in the hippocampus. Recent studies have indicated that TBS is effective in LTP induction of layer III synapses of neocortex, only if applied to layer IV. However, the possibility of neocortical LTP induction using PBs has not been investigated yet. Sensory deprivation greatly influences the development of neocortex. According to the effect of sensory deprivation on synaptic plasticity of developing neocortex, we studied the induction of LTP by PBs in visual cortical slices of control and dark-reared rats. The results showed that application of PBs to layer IV could effectively induce LTP of layer II/III field potentials. These potentials are consisted of two components: pEPSP1, (population excitatory postsynaptic potential 1) and pEPSP2. In control slices PBs led to selective potentiation of pEPSP2. Visual deprivation increased the incidence of LTP of pEPSP1 and decreased the amount of LTP of pEPSP2. These findings showed that PBs could be used as an effective tetanic stimulation to study the synaptic plasticity in neocortex. The effects of visual deprivation on PBs-induced LTP are consistent with its role in the development of excitatory system in neocortex.

Induction of stable long-term depression in vivo in the hippocampal-prefrontal cortex pathway

We studied excitatory field potentials in the medial prefrontal cortex (mPFC, prelimbic area) to electrostimulation of the ventral hippocampus (CA1/subicular region) in the anaesthetized rat. Nine hundred stimulus trains (5 pulses at 250 Hz) applied at 1 Hz to the ventral hippocampus significantly and persistently depressed the amplitude and maximal slope ( approximately 55% for each index) of the prelimbic field potentials, but did not change the latency of the maximal slope or peak negativity. Twelve stimulus trains (50 pulses at 250 Hz) applied subsequently at 0.1 Hz restored the depression back to control level, and this reversible depression was maintained for at least 13 h. Cumulative depressive effects on the prelimbic field potential amplitude and maximal slope were observed upon addition of stimulus trains in the hippocampus. An important implication of the results is that the direct pathway from the hippocampus to the mPFC in the rat retains long-term depression (LTD) as a neuroplastic form in vivo. This form could cooperate with long-term potentiation (LTP) and such a bi-directional synaptic plasticity in the prefrontal cortex contributes to how cortical neural networks store information.

Novelty acquisition is associated with induction of hippocampal long-term depression

Homosynaptic long-term depression (LTD) consists of a persistent nonpathological decrease in synaptic transmission, which is induced by low-frequency stimulation. In vivo, low-frequency stimulation (1 Hz, 900 pulses) induces LTD in Wistar but not Hooded Lister rats. In this study, we investigated the influence of behavioral learning and behavioral state on the expression of LTD in both rat strains. Recordings were taken from freely moving animals that had undergone chronic implantation of a recording electrode in the hippocampal CA1 region and a bipolar stimulating electrode in the ipsilateral Schaffer collateral-commissural pathway. Exposure of the rat strains to stress induced a significant elevation in serum corticosterone levels but did not facilitate LTD expression. However, LFS given during exploration of a novel environment resulted in LTD expression in Hooded Lister, and LTD enhancement in Wistar, rats. Reexposure to the same environment did not result in new expression of LTD. Behavioral comparison between the first and second environmental exposure confirmed that the animals had habituated to the novel environment. These observations strongly implicate an association between novelty acquisition and LTD.

Hebbian synapses in cortical and hippocampal pathways

Use-dependent alterations in synaptic efficacy are believed to form the basis for such complex brain functions as learning and memory and significantly contribute to the development of neuronal networks. The algorithm of synapse modification proposed by Hebb as early as 1949 is the coincident activation of pre- and postsynaptic neurons. The present review considers the evolution of experimental protocols in which postsynaptic cell depolarization through the recording microelectrode was used to reveal the manifestation of Hebb-type plasticity in the synaptic inputs of the neocortex and hippocampus. Special attention is focused on the inhibitory control of the Hebb-type plasticity. Disinhibition within the local neuronal circuits is considered to be an important factor in Hebbian plasticity, contributing to such phenomena as priming, primed burst potentiation, hippocampal theta-rhythm and cortical arousal. The role of various transmitters (acetylcholine, norepinephrine, gamma-amino-butyric acid) in disinhibition is discussed with a special emphasis on the brain noradrenergic system. Possible mechanisms of Hebbian synapse modification and their modulation by memory enhancing substances are considered. It is suggested that along with their involvement in disinhibition processes these substances may control Hebb-type plasticity through intracellular second messenger systems.

Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent A delta-fibres in the adult rat

Long-term potentiation (LTP) of spinal C-fibre-evoked field potentials can be induced by brief electrical stimulation of afferent C-fibres, by natural noxious stimulation of skin or by acute nerve injury. Here, we report that in urethane anaesthetized, adult rats prolonged high frequency burst stimulation of the sciatic nerve at Adelta-fibre strength produced long-term depression (LTD) of C-fibre-evoked field potentials, and also depressed the increased amplitudes of C-fibre-evoked field potentials recorded after LTP had been established (depotentiation). Electrical stimulation of Abeta-fibres failed to induce LTD or depotentiation. In spinalized rats, prolonged Adelta-fibre conditioning stimulation induced LTP rather than LTD of C-fibre-evoked field potentials. Thus, tonic descending inhibition may determine the direction of plastic changes in C-fibre-mediated synaptic transmission. Spinal application of the N-methyl-D-aspartic acid receptor antagonist D-APV blocked induction of LTD in intact rats and LTP in spinalized rats. The presently described LTD and the depotentiation of established LTP of C-fibre-evoked field potentials in spinal dorsal horn may underlie some forms of prolonged analgesia induced by peripheral nerve stimulation procedures.

Memory formation: the sequence of biochemical events in the hippocampus and its connection to activity in other brain structures

Recent data have demonstrated a biochemical sequence of events in the rat hippocampus that is necessary for memory formation of inhibitory avoidance behavior. The sequence initially involves the activation of three different types of glutamate receptors followed by changes in second messengers and biochemical cascades led by enhanced activity of protein kinases A, C, and G and calcium-calmodulin protein kinase II, followed by changes in glutamate receptor subunits and binding properties and increased expression of constitutive and inducible transcription factors. The biochemical events are regulated early after training by hormonal and neurohumoral mechanisms related to alertness, anxiety, and stress, and 3-6 h after training by pathways related to mood and affect. The early modulation is mediated locally by GABAergic, cholinergic, and noradrenergic synapses and by putative retrograde synaptic messengers, and extrinsically by the amygdala and possibly the medial septum, which handle emotional components of memories and are direct or indirect sites of action for several hormones and neurotransmitters. The late modulation relies on dopamine D1, beta-noradrenergic, and 5HT1A receptors in the hippocampus and dopaminergic, noradrenergic, and serotoninergic pathways. Evidence indicates that hippocampal activity mediated by glutamate AMPA receptors must persist during at least 3 h after training in order for memories to be consolidated. Probably, this activity is transmitted to other areas, including the source of the dopaminergic, noradrenergic, and serotoninergic pathways, and the entorhinal and posterior parietal cortex. The entorhinal and posterior parietal cortex participate in memory consolidation minutes after the hippocampal chain of events starts, in both cases through glutamate NMDA receptor-mediated processes, and their intervention is necessary in order to complete memory consolidation. The hippocampus, amygdala, entorhinal cortex, and parietal cortex are involved in retrieval in the first few days after training; at 30 days from training only the entorhinal and parietal cortex are involved, and at 60 days only the parietal cortex is necessary for retrieval. Based on observations on other forms of hippocampal plasticity and on memory formation in the chick brain, it is suggested that the hippocampal chain of events that underlies memory formation is linked to long-term storage elsewhere through activity-dependent changes in cell connectivity.

Suppression of pair-pulse depression of the population spike in the dentate gyrus during carbachol-induced theta-like activity in guinea pig hippocampal slices

In pair-pulse stimulation experiments, pair-pulse depression (PPD) of the population spike (PS) occurred at intervals shorter than 20 ms in the dentate gyrus in guinea pig hippocampal slices. Application of 50 microM carbachol resulted in an increase in the test PS amplitude and caused suppression of PPD. This suppression was antagonized by atropine sulfate, a muscarinic receptor antagonist. Carbachol at 50 microM induced intermittent bursts of theta-like activity (TLA). We compared the pair-pulse index (PPI) during TLA with that in a rest period between bursts of TLA. The PPI was defined as the ratio of the amplitude of the test PS to that of the conditioning PS. The PPI during TLA were significantly larger than that during the rest period, although there were no significant differences in the conditioning PS amplitude and the test pEPSP slope. When TLA was induced, the PPI during the rest period was increased by bicuculline. The PPI during TLA did not change significantly with the drug. The increase by bicuculline in the PPI during the rest period was caused by increase in the test PS amplitude. PPD can occur due to inhibition of granule cell activity by inhibitory neurons. Our findings suggest that the action of inhibitory neurons on granule cell activity is suppressed by activation of muscarinic receptors, with stronger suppression during TLA than during the rest period between bursts of TLA.

Theta-activity-dependent and -independent muscarinic facilitation of long-term potentiation in guinea pig hippocampal slices

Cholinergic input to the hippocampus originates in the septum and diagonal band. Guinea pig hippocampal slices in a bath of carbachol, a cholinergic agonist, displayed different patterns of rhythmical activities depending on the carbachol concentration. Exposure to 50 or 100 microM led to intermittent induction of theta-like activities (TLAs). Long-term potentiation (LTP), induced by tetanus. was facilitated at concentrations within the optimum range for generating TLAs. This facilitation was blocked by the muscarinic receptor antagonist atropine. Augmentation of LTP during TLAs was greater than that during the rest period of TLAs which, in turn, was greater than that induced without activation of the muscarinic receptors. These results suggest that there are two muscarinic facilitation processes of LTP, one dependent on and the other independent of TLAs, with the former being more easily facilitated than the latter.

Long-term potentiation and depression in layer III and V pyramidal neurons of the cat sensorimotor cortex in vitro

Synaptic plasticity of the cat sensorimotor cortex was examined intracellularly in vitro. After tetanic stimulation of the white matter, layer III and V pyramidal neurons showed long-term potentiation (LTP) of EPSPs in high incidence without GABA(A) antagonist. The incidence and magnitude of LTP were very conspicuous in layer V cells. After an NMDA receptor antagonist application, the synaptic potentiation was blocked completely in layer III but not in layer V cells. Long-term depression (LTD) of the evoked EPSPs was also induced by the same stimulation in some layer III cells, where a transient hyperpolarization of the membrane potential was observed during tetanus.

A facilitatory effect on the induction of long-term potentiation in vivo by chronic administration of antisense oligodeoxynucleotides against catalytic subunits of calcineurin

A rise in Ca2+ concentration at postsynaptic sites provides an initial step in inducing both the long-term potentiation (LTP) and long-term depression (LTD) in the CA1 region of the hippocampus. LTP induction requires the activation of Ca(2+)-sensitive protein kinases following the rise in Ca2+. By contrast, the activity of protein phosphatase(s) appears to be critical to induce LTD. Here we demonstrate that inhibition of the synthesis of calcineurin A alpha and A beta, catalytic subunits of Ca2+/calmodulin- (CaM) dependent protein phosphatase, reduces the threshold of induction for commissural-CA1 LTP in anesthetized rats. In rats administered antisense oligodeoxynucleotides (ODNs) against calcineurin A alpha and A beta intraventricularly for 7 days, a brief tetanic stimulation to the CA3 region, which in the control case was below threshold for the induction of LTP, now produced a long-lasting increase in both the EPSP slope and the amplitude of population spike recorded from the commissural-CA1 pathway. Western blot analysis of calcineurin showed that the threshold reduction was accompanied by a selective decrease in the protein levels in the hippocampus. Thus our study provides direct evidence that calcineurin per se has an antagonizing role in LTP induction. Complementary experiments with the selective calcineurin inhibitor, FK506, also showed the reduction of LTP threshold in a dose-dependent manner. These results, together with previous studies, support the hypothesis that the quantitative phosphorylation level of critical intracellular proteins determines whether the synaptic efficacy will increase or decrease after the activity-dependent rise in postsynaptic Ca2+.

Ionotropic glutamate receptors. Their possible role in the expression of hippocampal synaptic plasticity

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds--the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and the N-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/ LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/ decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.

Long-term depression in rat visual cortex is associated with a lower rise of postsynaptic calcium than long-term potentiation

To test the hypothesis that an input-associated rise of Ca2+ at postsynaptic sites beyond a certain threshold leads to the induction of long-term potentiation (LTP) while a lower rise below the threshold leads to long-term depression (LTD), the method of microscopic Ca2+ fluorometry was employed simultaneously with recordings of synaptic activity from layer II/III of visual cortical slices prepared from young rats. The conventional Ca2+ indicators, such as fura-2 or fluo-3, may interfere with intracellular processes for the induction of LTP/LTD because of their strong Ca(2+)-chelating action. To minimize such a problem, another Ca2+ indicator, rhod-2, was used since it has a much weaker Ca(2+)-chelating action than those indicators. In 16 slices loaded with rhod-2 through the perfusion medium, tetanic stimulation of theta-burst type was applied to layer IV of the cortex and changes in Ca2+ concentration were analyzed in layer II/III from which field potentials to test stimulation of layer IV were recorded simultaneously. In 7 slices in which weak tetanic stimulation consisting of 0.1 ms duration pulses was applied to layer IV, LTD of field responses was induced, while LTP was induced in 6 of the 9 slices in which strong tetanus consisting of 0.2 ms pulses was applied. In the 6 slices in which LTP was induced, the peak rise of fluorescence intensity during tetanus was 13.9 +/- 0.2 (S.E.M.) %, which was significantly (t-test, P < 0.01) higher than that (10.4 +/- 0.3%) in the 9 slices in which LTD was induced. In another series of experiments, rhod-2 was injected directly into 12 pyramidal cell-like neurons in layer II/III through patch pipettes, and changes in Ca2+ concentration in apical dendritic areas during tetanus were measured simultaneously with recordings of excitatory postsynaptic potentials (EPSPs) evoked by test stimulation of layer IV. It was found that LTP of EPSPs was induced in 4 cells which exhibited a strong rise of dendritic Ca2+ signal (197.1 +/- 18.5%) while LTD was induced in other 5 cells which showed a weak rise of the signal (31.0 +/- 4.1%). These results seem consistent with the above-mentioned, Ca(2+)-switching hypothesis for the induction of LTP and LTD in visual cortex.

Postsynaptic calcium and calcium-dependent processes in synaptic plasticity in the developing visual cortex

In this paper we describe some of the results obtained from recent experiments on mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD) in the visual cortex of young rats. In particular, we focus on experiments which tested the hypotheses that the induction of LTP in the visual cortex is of Hebbian type and that an input-associated Ca2+ rise at postsynaptic sites and subsequent activation of protein kinases or protein phosphatases may play roles in the induction of LTP or LTD in the developing visual cortex.

Development and plasticity of cortical processing architectures

One of the basic functions of the cerebral cortex is the analysis and representation of relations among the components of sensory and motor patterns. It is proposed that the cortex applies two complementary strategies to cope with the combinatorial problem posed by the astronomical number of possible relations: (i) the analysis and representation of frequently occurring, behaviorally relevant relations by groups of cells with fixed but broadly tuned response properties; and (ii) the dynamic association of these cells into functionally coherent assemblies. Feedforward connections and reciprocal associative connections, respectively, are thought to underlie these two operations. The architectures of both types of connections are susceptible to experience-dependent modifications during development, but they become fixed in the adult. As development proceeds, feedforward connections also appear to lose much of their functional plasticity, whereas the synapses of the associative connections retain a high susceptibility to use-dependent modifications. The reduced plasticity of feedforward connections is probably responsible for the invariance of cognitive categories acquired early in development. The persistent adaptivity of reciprocal connections is a likely substrate for the ability to generate representations for new perceptual objects and motor patterns throughout life.

Metabotropic glutamate receptors and visual cortical synaptic plasticity

Two forms of use-dependent synaptic plasticity, called long-term potentiation (LTP) and long-term depression (LTD), can be elicited in the visual cortex following different paradigms of electrophysiological stimulation. These neurobiological phenomena often are considered as necessary components of models for the alteration in function of the nervous system that must occur at some level for the establishment and (or) maintenance of memory engrams, for learning processes, or for the consolidation of active neural connections and regression of inactive contacts in the developing brain. It has been postulated that for LTP and LTD to be produced in the hippocampus, activation of a particular subtype of excitatory amino acid receptor, the metabotropic receptor, is a critical requirement. Only recently has it become possible to test this hypothesis directly, as a new compound, (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG), has been introduced and the suggestion made that it selectively antagonizes the metabotropic receptor. This substance has been tested in the present study on responses recorded from slices of rat visual cortex and has been found both to block the activation of the metabotropic receptor and to interfere selectively with the form of synaptic plasticity called LTD. It thus appears from the experiments reported in this paper as though the metabotropic receptor subtype that is blocked by MCPG is required for the expression of LTD but not for the expression of LTP, in the visual cortex of adult rats.

Simultaneous expression of excitatory postsynaptic potential/spike potentiation and excitatory postsynaptic potential/spike depression in the hippocampus

Tetanic stimulation of afferents in the stratum radiatum of the CA1 area of the rat hippocampus results in long-term potentiation of excitatory synaptic responses in pyramidal cells. Previous studies have reported a greater increase in the population spike amplitude following the induction of long-term potentiation than could be accounted for by the increase of the slope of the population excitatory postsynaptic potential. Two hypotheses have been proposed to explain this phenomenon (called excitatory postsynaptic potential/spike potentiation): a modification of the firing threshold and/or a modification of the inhibitory drive. Previous studies have not, however, addressed the question of possible changes in spike threshold in association with long-term depression. This paper examines whether the concomitant long-term potentiation of pharmacologically isolated N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials, reported previously, is also associated with a change in spike threshold. When the amplitude of the population spike is plotted as a function of the slope of the population excitatory postsynaptic potential (excitatory postsynaptic potential/spike curve), excitatory postsynaptic potential/spike potentiation (depression) is seen as a shift of the excitatory postsynaptic potential/spike curve to the left (right) following a conditioning stimulus. In this study, using kainic acid lesioned hippocampus, we have shown that tetanic stimulation produced excitatory postsynaptic potential/spike potentiation of the control synaptic response and excitatory postsynaptic potential/spike depression of the isolated N-methyl-D-aspartate receptor-mediated responses.

Plasticity of excitatory synaptic transmission in kitten visual cortex depends on voltage-dependent Ca2+ channels but not on NMDA receptors

Long-term potentiation (LTP) of excitatory synaptic transmission occurs in in vitro slices of cat and rat visual cortex. Earlier studies suggested that activation of N-methyl-D-aspartate (NMDA)-selective glutamate receptors is essential for the induction of LTP. However, our studies on kitten visual cortex demonstrate that LTP induction requires the activation of low-threshold Ca2+ channels in postsynaptic cells but not of NMDA receptors.

Synaptic plasticity: LTP and LTD

Long-term potentiation (LTP) is a synaptic enhancement that follows brief, high-frequency electrical stimulation in the hippocampus and neocortex. Recent evidence suggests that induction of LTP may require, in addition to postsynaptic Ca2+ entry, activation of metabotropic glutamate receptors and the generation of diffusible intercellular messengers. A new form of synaptic plasticity, homosynaptic long-term depression (LTD) has also recently been documented, which, like LTP, requires Ca2+ entry through the NMDA receptor. Current work suggests that this LTD is a reversal of LTP, and vice versa, and that the mechanisms of LTP and LTD may converge at the level of specific phosphoproteins.