Neuronal activity related to memory in the intermediate and medial part of the hyperstriatum ventrale of the chick brain

The intermediate and medial part of the hyperstriatum ventrale (IMHV) in the forebrain of the domestic chick Gallus gallus domesticus has been shown in previous studies to be critically involved in the learning process of imprinting. In the present study, 1-day-old chicks were imprinted by exposing them to one of two artificial stimuli. 24 h later each chick was given a preference test in which the two stimuli were presented in sequence. A preference score, an index of the strength of imprinting (i.e., of learning), was calculated from approach activity during the preference test. The chicks were divided into groups with low, medium and high preference scores (corresponding to weak, medium and strong learning respectively) and coded so that all subsequent procedures were performed blind. Each chick was then aneesthetized and spontaneous action potentials recorded extracellularly from groups of neurones in the left IMHV. The mean neuronal firing rate in chicks with medium and high preference scores was significantly greater than that in chicks with low preference scores. This relation between neuronal activity and preference score was not attributable to the chicks' locomotor activity per se. The results demonstrate an association between spontaneous electrical activity in the left IMHV and a measure of learning 1 day after the learning occurred.     

Hypothesis regarding the cellular mechanisms responsible for long-term synaptic potentiation in the hippocampus

A possible cellular mechanism accounting for the long-term potentiation (LTP) of synaptic transmission in the hippocampus is presented. Our multiple-stage hypothesis postulates that repetitive stimulaion causes an increase of intracellular calcium in the target dendritic regions which results in the activation of a membrane-associated protease which in turn exposes additional glutamate receptors.     

Subfornical organ disconnection and Fos-like immunoreactivity in hypothalamic nuclei after intragastric hypertonic saline

The subfornical organ (SFO) may act as a sodium- or osmoreceptor that drives hypothalamic and other nuclei to secrete vasopressin and to elicit drinking. However, in response to mild doses of hypertonic saline, Fos-like immunoreactivity (Fos-ir) is absent in the SFO whereas it is well expressed in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. This suggests that the hypothalamus may be activated in advance of the SFO. In this study, the fibers connecting the SFO and hypothalamus were disconnected by a wire knife cut so that Fos-ir could be examined in both the SFO and hypothalamus after an intragastric (ig) load of 0.5% of body weight of 0.6 M NaCl. Compared with Fos-ir in isotonic-loaded rats, Fos-ir after the hypertonic load was not significantly elevated in the SFO or median preoptic nucleus in sham-cut or knife-cut rats and was only slightly elevated in the OVLT in sham-cut rats. However, the hypertonic load in sham-cut rats greatly elevated Fos-ir in the SON and in the entire PVN, but this expression was reduced significantly by 30-50% in knife-cut rats. Thus, the connectivity between SFO and the hypothalamus is critical for the full expression of Fos-ir in the hypothalamus during moderate ig hypertonic saline loading even when the SFO itself does not yet express Fos-ir.     

LTP研究进展(Ⅲ)——LTP和神经趋向因子

长时程增强（LTP）是学习和记忆过程的分子水平现象。参与LTP机制的因素很多，最近研究发现神经趋向因子，特别是其中的脑衍生的神经趋向因子（BDNF）对LTP起着重要的调节作用，而且对短时程及长时程突触可塑性均有影响。已经明确的神经趋向因子的功能包括调节神经分化，神经元轴突和树突的生长和修复，以及突触形成。本文综述了BDNF与LTP相关性的实验性根据。总结了BDNF通过突触前以及突触后机制影响LTP的引发和后期维持。BD-NF的直接作用机制是作用于突触前后膜上的受体，导致突触前递质小泡增多从而增加递质释放。在突触后引起突触后膜去极化，从而打开电压依赖性钙通道、征离子浓度增高，最终导致AMPA受体数目增多，功能强化，产生LTP。     

Both phasic sensory stimulation and tonic pharmacological activation increase Fos-like immunoreactivity in the rat locus coeruleus.

Neuronal activation increases levels of Fos protein, the product of the early immediate gene c-fos. Since most studies used stimuli that evoke sustained elevations in activity; the present study examined Fos-like immunoreactivity (Fos-LI) in the nucleus locus coeruleus (LC). Halothane-anesthetized rats were given either footshock to elicit phasic activation of LC neurons or yohimbine (10 mg/kg, i.p.) to induce a tonic increase in firing. Both treatments markedly increased Fos-LI in a subpopulation of LC neurons. These results demonstrate that c-fos induction does not require high tonic levels of neuronal activity and that Fos-LI may underestimate the proportion of LC neurons neurophysiologically activated by a given stimulus and suggest that factors beyond neuronal activity per se contributes to c-fos expression.     

Fos-like immunoreactivity in the circadian timing system of calorie-restricted rats fed at dawn: daily rhythms and light pulse-induced changes

Daily rhythms of pineal melatonin, body temperature, and locomotor activity are synchronized to the light–dark cycle (LD) via a circadian clock located in the suprachiasmatic nuclei (SCN). A timed caloric restriction in rats fed at dawn induces phase-advances and further phase-stabilization of these rhythms, suggesting that the circadian clock can integrate conflicting daily photic and non-photic cues. The present study investigated the daily expression of Fos-like immunoreactivity (Fos-ir) and light pulse-induced Fos-ir in the SCN, the intergeniculate leaflet (IGL) and the paraventricular thalamic nucleus (PVT) in calorie-restricted rats fed 2 h after the onset of light and in controls fed ad libitum. A daily rhythm of Fos-ir in the SCN was confirmed in control rats, with a peak approximately 2 h after lights on. At this time point (i.e. just prior to the feeding time), the level of SCN Fos-ir was lowered in calorie-restricted rats. Concomitantly, IGL Fos-ir was higher in calorie-restricted vs. control rats. In response to a light pulse during darkness, Fos-ir induction was found to be specifically (i.e. phase-dependently) lowered in the SCN and IGL of calorie-restricted rats. Observed changes of Fos-ir in the PVT were possibly related to the wake state of the animals. This study shows that repetitive non-photic cues presented in addition to a LD cycle affect the Fos expression in the circadian timing system.     

Correlation between the induction of an immediate early gene, zif/268, and long-term potentiation in the dentate gyrus.

Expression of the immediate early gene zif/268 (also termed NGFI-A, Krox 24, TIS8 and Egr-1) was investigated in awake rats following various long-term potentiation (LTP) induction protocols.zif/268 mRNA (Northern blots) and protein (immunohistochemistry) levels sharply increased following LTP, and followed a time course characteristic of other immediate early genes. When measured across 3 tetanization protocols known to produce differing degrees of LTP persistence,zif/268 induction was found to be more highly correlated with LTP duration than with the magnitude of initial LTP. These data support the hypothesis that the immediate early gene zif/268 plays a role as a third messenger in the cascade of cellular and nuclear events that govern the persistence of LTP.     

Involvement of nitric oxide in long-term potentiation in the dentate gyrus in vivo

Effects of NO synthase inhibitors on long-term potentiation (LTP) were investigated in the dentate gyrus of anesthetized rats. Administration ofN^G-nitro-L-arginine methyl ester (L-NAME; 50 nmol i.c.v.) greatly blocked the generation of LTP without affecting basal responses.N^G-nitro-L-arginine (50 nmol) orN^G-methyl-L-arginine (50 nmol) also showed similar effects. The LTP-blocking effect of L-NAME was reversed by coadministration ofL-arginine (500 nmol). These results suggest that NO participates in the generation of LTP at the dentate gyrus in vivo.     

Correlation between the induction of an immediate early gene,zif/268, and long-term potentiation in the dentate gyrus

Expression of the immediate early gene zif/268 (also termed NGFI-A, Krox 24, TIS8 and Egr-1) was investigated in awake rats following various long-term potentiation (LTP) induction protocols.zif/268 mRNA (Northern blots) and protein (immunohistochemistry) levels sharply increased following LTP, and followed a time course characteristic of other immediate early genes. When measured across 3 tetanization protocols known to produce differing degrees of LTP persistence,zif/268 induction was found to be more highly correlated with LTP duration than with the magnitude of initial LTP. These data support the hypothesis that the immediate early gene zif/268 plays a role as a third messenger in the cascade of cellular and nuclear events that govern the persistence of LTP.     

Placement in a novel environment induces Fos-like immunoreactivity in supramammillary cells projecting to the hippocampus and midbrain

Injections of fluorescent retrograde tracers into either the hippocampal formation or the midbrain raphe nuclei resulted in retrograde labeling of many cells in the supramammillary region of the hypothalamus. Double labeling studies indicated that these two projections originate from different populations of supramammillary cells. Expression of the proto-oncoprotein Fos could be induced in some retrogradely labeled cells by placing rats in a novel open field before sacrifice. Although seen in both cell types, Fos-like immunoreactivity was significantly more common in supramammillary cells projecting to the hippocampus than in those projecting to the midbrain. These findings suggest that the supramammillary region may contain several populations of neurons which are differentially responsive to certain behavioral manipulations.     

Alterations in the balance of protein kinase and phosphatase activities and age-related impairments of synaptic transmission and long-term potentiation

Aging is associated with an impaired ability to maintain long-term potentiation (LTP), but the underlying cause of the impairment remains unclear. To gain a better understanding of the cellular and molecular mechanisms responsible for this impairment, the synaptic transmission and plasticity were studied in the CA1 region of hippocampal slices from adult (6-8 months) and poor-memory (PM)-aged (23-24 months) rats. The one-way inhibitory avoidance learning task was used as the behavioral paradigm to screen PM-aged rats. With intracellular recordings, CA1 neurons of PM-aged rats exhibited a more hyperpolarized resting membrane potential, reduced input resistance, and increased amplitude of afterhyperpolarization and spike threshold, compared with those in adult rats. Although a reduction in the size of excitatory synaptic response was observed in PM-aged rats, no obvious differences were found between adult and PM-aged rats in the pharmacological properties of excitatory synaptic response, paired-pulse facilitation, or frequency-dependent facilitation, which was tested with trains of 10 pulses at 1, 5, and 10 Hz. Slices from the PM-aged rats displayed significantly reduced early-phase long-term potentiation (E-LTP) and late-phase LTP (L-LTP), and the entire frequency-response curve of LTP and LTD is modified to favor LTD induction. The susceptibility of time-dependent reversal of LTP by low-frequency afferent stimulation was also facilitated in PM-aged rats. Bath application of the protein phosphatase inhibitor, calyculin A, enhanced synaptic response in slices from PM-aged, but not adult, rats. In contrast, application of the cAMP-dependent protein kinase inhibitors, Rp-8-CPT-cAMPS and KT5720, induced a decrease in synaptic transmission only in slices from the adult rats. Furthermore, the selective beta-adrenergic receptor agonist, isoproterenol, and pertussis toxin-sensitive G-protein inhibitor, N-ethylmaleimide, effectively restored the deficit in E-LTP and L-LTP of PM-aged rats. These results demonstrate that age-related impairments of synaptic transmission and LTP may result from alterations in the balance of protein kinase/phosphatase activities.     

Zif268 and Fos-like immunoreactivity in tetanus toxin-induced epilepsy: reciprocal changes in the epileptic focus and the surround

Altered gene expression for a number of molecules has been suggested as one of the underlying mechanisms of epileptogenesis. Changes in expression of the immediate early genes, zif268 and c-fos, were investigated in chronic focal epilepsy induced by tetanus toxin (TT, 20–35 ng) injected in the rat motor cortex. Most rats injected with TT and perfused on postoperative day 5, 7 or 14 had recurrent focal seizures after a latent period of 4–13 days, and showed enhanced Zif268 immunoreactivity in a cluster of neurons at the injection site, as well as reduced Zif268 immunoreactivity in a distinct cortical zone around this cluster. C-fos or Fos-related immunoreactivity was decreased over widespread areas of frontoparietal and piriform cortex in epileptic rats, except for a focus at the injection site which, in most cases, showed increases in Fos-like immunoreactivity. Some epileptic rats showed increased Zif268 immunoreactivity in neurons of the ipsilateral ventral lateral and central lateral thalamic nuclei and increased Zif268 and Fos-like immunoreactivity in the pontine nuclei. Rats perfused before onset of seizures, showed no overt changes other than a slight decrease in Zif268 and Fos-like immunoreactivity at the injection site. The reciprocal changes in Zif268 immunoreactive neurons in the epileptic focus and the immediate surround parallel changes in gene expression for a number of molecules important in epileptogenesis and suggest a state of functional disconnection of the epileptic focus from other cortical areas that may contribute to the development and maintenance of focal epilepsy.     

Dissociation between apoptosis, neurogenesis, and synaptic potentiation in the dentate gyrus of adrenalectomized rats

Removal of adrenal hormone corticosterone in rats aged 3-4 months results within 3 days in acceleration of apoptosis and proliferation of newborn cells in the dentate gyrus (DG). A critical question is whether such a shift in the maturity of dentate cells after adrenalectomy (ADX) affects synaptic plasticity. To address this question, male rats were adrenalectomized and synaptic potentiation was recorded in vitro in hippocampal slices, as well as in vivo, in response to high frequency stimulation of the perforant path, 3 days after ADX. At this time-point, cell loss was assessed and proliferation was examined. Based on two independent parameters, bromodeoxyuridine and Ki-67, we found that removal of the adrenal glands increases proliferation rate. This increase in proliferation was, in particular, evident in those animals that displayed substantial cell loss. The accelerated cell-turnover after ADX was accompanied by reduced synaptic potentiation, both when recorded in vitro and in vivo. Corticosterone replacement in vivo (in adrenalectomized animals), at levels that activate the mineralocorticoid receptor, prevented ADX-induced proliferation, apoptosis, and restored synaptic potentiation to control levels. Importantly, corticosterone applied to slices from adrenalectomized rats also normalized synaptic potentiation, despite increased proliferation. This suggests that changes in cell proliferation and apoptotic cell death in the DG are not necessarily key factors determining the efficacy of synaptic potentiation.     

Adult hippocampal glucocorticoid receptor expression and dentate synaptic plasticity correlate with maternal care received by individuals early in life

Maternal care in mammals is the prevailing environmental influence during perinatal development. The adult rat offspring of mothers exhibiting increased levels of pup licking/grooming (LG; High LG mothers), compared to those reared by Low LG dams, show increased hippocampal glucocorticoid receptor expression, complex dendritic tree structure, and an enhanced capacity for synaptic potentiation. However, these data were derived from studies using the total amount of maternal care directed toward the entire litter, thus ignoring possible within-litter variation. We show that the amount of LG received by individual pups within a litter varies considerably. Therefore, we questioned if the amount of LG received by individual pups correlates with and thus putatively predicts later hippocampal structure and function. To this end, LG-scores were determined during the first postnatal week for all pups in 32 litters and correlated with neuroendocrine and hippocampal parameters in young-adulthood. Pup LG-score positively correlated with the glucocorticoid receptor mRNA expression in the adult hippocampus. Moreover, the ability to induce synaptic potentiation in the dentate gyrus in vitro was enhanced in animals with high LG-scores. Structural plasticity correlated less reliably with LG-scores early in life and differed between sexes. Male offspring with high LG-scores displayed fewer newborn neurons, higher brain derived neurotrophic factor expression and tended to have more complex granule cell dendritic trees. We conclude that even moderate variations in early life environment have a major impact on adult hippocampal function. This principle could provide a mechanistic basis for individual differences in susceptibility to psychopathology.     

Two selected models of missense mutations in mice for the study of learning behaviour

A large number of genome-wide association studies have linked missense mutations, mutations altering the amino acid sequence of proteins, with cognitive impairment in humans. However, these studies are correlative. As there may be multiple mutations for one particular patient, it is essential to address the functional impact of a missense mutation in a model system. The most suitable model system is the generation of knock-in mice with the homologous missense mutation followed by behavioural phenotyping. Here, we review selected mutants demonstrating an impact of single mutations on learning and memory in mice and discuss the relevance of such studies for understanding the role of these polymorphisms in human behaviour. We conclude that using these animal models has been instrumental in decoding the mechanisms underlying behaviour, and assists the design of therapeutic strategies for humans.     

Distinct mechanisms of bidirectional activity-dependent synaptic plasticity in superficial and deep layers of rat entorhinal cortex

The entorhinal cortex plays a key role in processing memory information in the brain; superficial layers relay information to, and deep layers receive information from, the hippocampus. The cellular mechanisms of memory are thought to include a number that produce long-term potentiation (LTP) and depression (LTD) of synaptic strength. Our work presents evidence that LTP and LTD occur simultaneously at memory-relevant synapses. We report here that low frequency stimulation generates NMDA receptor-dependent LTD in Wistar rat superficial (layers II and III), and LTP in the deep entorhinal cortex layers (layers V and VI). LTP in deep layers is masked by simultaneously occurring voltage-gated calcium channel-dependent LTD. Our data support a novel mechanism for the sliding-threshold (BCM) model of synaptic plasticity: The sliding thresholds for induction of LTP and LTD in entorhinal cortex deep layers will be driven by the relative activation state of NMDA receptors and voltage-gated calcium channels. The co-expression of LTD and LTP at presynaptic sites in the entorhinal cortex deep layers reveals an intriguing mechanism for differential processing of synaptic information, which may underlie the vast dynamic capacity for information storage by this cortical structure.     

Long-term potentiation in the amygdala: a mechanism for emotional learning and memory

In the mammalian brain, LTP is an enduring form of synaptic plasticity that is posited to have a role in learning and memory. Compelling new evidence for this view derives from studies of LTP in the amygdala, a brain structure that is essential for simple forms of emotional learning and memory, such as Pavlovian fear conditioning in rats. More specifically, antagonists of the NMDA receptor block both amygdaloid LTP induction and fear conditioning, fear conditioning induces increases in amygdaloid synaptic transmission that resemble LTP, and genetic modifications that disrupt amygdaloid LTP eliminate fear conditioning. Collectively, these results provide the most-convincing evidence to date that LTP mediates learning and memory in mammals.     

Substance P receptor regulates the photic induction of Fos-like protein in the suprachiasmatic nucleus of Syrian hamsters

Substance P (SP) is a candidate neurotransmitter or neuromodulator for conveying light information from the retina to the hypothalamic suprachiasmatic nucleus (SCN) where a circadian oscillator(s) is located in mammals. Immediate early gene c-fos has been demonstrated to be induced in the SCN with a brief light exposure at the subjective night, and suggested to play an important role in the photic entrainment of the oscillator. To clarify the possibility of an involvement of the SP receptor in the photic-induction of c-fos in the SCN, we examined effects of a SP receptor antagonist, spantide, on the light-induced Fos-like protein immunoreactivity (Fos-lir) in the SCN of Syrian hamster. The light-induced Fos-lir was inhibited with the pretreatment of spantide in a dose-related manner and in an anatomically distinctive way. The higher dose of spantide (8 nmol) blocked light-induced Fos-lir substantially in the rostral and central areas of the SCN, and in the dorsal portion of the caudal SCN. However, it blocked Fos-lir only slightly in the ventral portion of the caudal SCN. These results suggest that the SP is involved in conveying light information to induce Fos protein in the hamster SCN, and that different neurotransmitter systems are involved in the light-induced Fos-lir in the different portions of hamster SCN.     

Plasticity of synaptic glun receptors is required for the Src‐dependent induction of long‐term potentiation at CA3‐CA1 synapses

The induction of long‐term potentiation (LTP) of CA3‐CA1 synapses requires activation of postsynaptic N‐methyl‐D ‐aspartate receptors (GluNRs). At resting potential, the contribution of GluNRs is limited by their voltage‐dependent block by extracellular Mg²⁺. High‐frequency afferent stimulation is required to cause sufficient summation of excitatory synaptic potentials (EPSPs) to relieve this block and to permit an influx of Ca²⁺. It has been assumed that this relief of Mg²⁺ block is sufficient for induction. We postulated that the induction of LTP also requires a Src‐dependent plasticity of GluNRs. Using whole‐cell recordings, LTP (GluARs) of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptors‐EPSCS was induced by pairing postsynaptic depolarization with presynaptic stimulation. This LTP was both GluNR and Src‐dependent, being sensitive to AP‐5, a GluNR selective antagonist, or to SU6656, a Src‐selective inhibitor. When CNQX was used to block all GluARs, we observed a long‐lasting potentiation of GluNR‐mediated EPSCs. This plasticity was prevented by transiently blocking GluNRs during the induction protocol or by chelating intracellular Ca²⁺. GluNRs plasticity was also prevented by bath applications of SU6656 or intracellular applications of the Src‐selective inhibitory peptide, Src(40–58). It was also blocked by preventing activation of protein kinase C, a kinase that is upstream of Src‐kinase‐dependent regulation of GluNRs. Both GluN2A and GluN2B receptors were found to contribute to the plasticity of GluNRs. The contribution of GluNRs and, in particular, their plasticity to the maintenance of LTP was explored using AP5 and SU6656, respectively. When applied >20 min after induction neither drug influenced the magnitude of LTP. However, when applied immediately after induction, treatment with either drug caused the initial magnitude of LTP to progressively decrease to a sustained phase of reduced amplitude. Collectively, our findings suggest that GluNR plasticity, although not strictly required for induction, is necessary for the maintenance of a nondecrementing component of LTP. © 2010 Wiley‐Liss, Inc.