Hippocampal cellular functional organization for fear memory: Effects of sleep.

Memory is vital to our daily existence. Although a large number of studies have suggested that the hippocampus is dedicated to long-term memory, understanding how memory is anatomically encoded within the hippocampal neuronal network is still lacking. Previously our laboratory showed that hippocampal pyramidal cells are organized in cell clusters to encode both spatial and episodic memory. Based on these findings, we hypothesized that "cluster-type" is a functional organization principal in the hippocampus to encode all types of memory. Here, we tested whether contextual fear, another hippocampus-dependent memory, is also organized in cell clusters. We further investigated the possibility that post-learning sleep may affect functional organization. Cluster formation was examined by assessing the topographic localization of active cells using immediate early gene (IEG, Zif268) imaging methods. The first experiment provides evidence of a cluster-type organization in the hippocampus for fear memory by showing a spatial distribution of adjacent Zif268 positive cells. Exposure to the context itself, without electric shocks, induced a similar cellular formation; however, the degree of clustering was significantly lower. The second experiment provides evidence that sleep plays a role in the refinement and long-term stability of the clusters. The present results confirm the existence of a cluster-type topographic functional neuronal organization in the hippocampus for memory, and further suggest that post-learning sleep enhances the cluster-type organization.

Hippocampal functional organization: A microstructure of the place cell network encoding space.

A clue to hippocampal function has been the discovery of place cells, leading to the 'spatial map' theory. Although the firing attributes of place cells are well documented, little is known about the organization of the spatial map. Unit recording studies, thus far, have reported a low coherence between neighboring cells and geometric space, leading to the prevalent view that the spatial map is not topographically organized. However, the number of simultaneously recorded units is severely limited, rendering construction of the spatial map nearly impossible. To visualize the functional organization of place cells, we used the activity-dependent immediate-early gene Zif268 in combination with behavioral, pharmacological and electrophysiological methods, in mice and rats exploring an environment. Here, we show that in animals confined to a small part of a maze, principal cells in the CA1/CA3 subfields of the dorsal hippocampus immunoreactive (IR) for Zif268 adhere to a 'cluster-type' organization. Unit recordings confirmed that the Zif268 IR clusters correspond to active place cells, while blockade of NMDAR (which alters place fields) disrupted the Zif268 IR clusters. Contrary to the prevalent view that the spatial map consists of a non-topographic neural network, our results provide evidence for a 'cluster-type' functional organization of hippocampal neurons encoding for space.

Fear memory consolidation in sleep requires protein kinase A.

It is well established that protein kinase A (PKA) is involved in hippocampal dependent memory consolidation. Sleep is also known to play an important role in this process. However, whether sleep-dependent memory consolidation involves PKA activation has not been clearly determined. Using behavioral observation, animals were categorized into sleep and awake groups. We show that intrahippocampal injections of the PKA inhibitor Rp-cAMPs in post-contextual fear conditioning sleep produced a suppression of long-term fear memory, while injections of Rp-cAMPs during an awake state, at a similar time point, had no effect. In contrast, injections of the PKA activator Sp-cAMPs in awake state, rescued sleep deprivation-induced memory impairments. These results suggest that following learning, PKA activation specifically in sleep is required for the consolidation of long-term memory.

Sleep deprivation impairs spontaneous object-place but not novel-object recognition in rats.

Effects of sleep deprivation (SD) on one-trial recognition memory were investigated in rats using either a spontaneous novel-object or object-place recognition test. Rats were allowed to explore a field in which two identical objects were presented. After a delay period, they were placed again in the same field in which either: (1) one of the two objects was replaced by another object (novel-object recognition); or (2) one of the sample objects was moved to a different place (object-place recognition), and their exploration behavior to these objects was analyzed. Four hours SD immediately after the sample phase (early SD group) disrupted object-place recognition but not novel-object recognition, while SD 4-8h after the sample phase (delayed SD group) did not affect either paradigm. The results suggest that sleep selectively promotes the consolidation of hippocampal dependent memory, and that this effect is limited to within 4h after learning.

Brain-targeted proanthocyanidin metabolites for Alzheimer's disease treatment.

While polyphenolic compounds have many health benefits, the potential development of polyphenols for the prevention/treatment of neurological disorders is largely hindered by their complexity as well as by limited knowledge regarding their bioavailability, metabolism, and bioactivity, especially in the brain. We recently demonstrated that dietary supplementation with a specific grape-derived polyphenolic preparation (GP) significantly improves cognitive function in a mouse model of Alzheimer's disease (AD). GP is comprised of the proanthocyanidin (PAC) catechin and epicatechin in monomeric (Mo), oligomeric, and polymeric forms. In this study, we report that following oral administration of the independent GP forms, only Mo is able to improve cognitive function and only Mo metabolites can selectively reach and accumulate in the brain at a concentration of ∼400 nM. Most importantly, we report for the first time that a biosynthetic epicatechin metabolite, 3'-O-methyl-epicatechin-5-O-ß-glucuronide (3'-O-Me-EC-Gluc), one of the PAC metabolites identified in the brain following Mo treatment, promotes basal synaptic transmission and long-term potentiation at physiologically relevant concentrations in hippocampus slices through mechanisms associated with cAMP response element binding protein (CREB) signaling. Our studies suggest that select brain-targeted PAC metabolites benefit cognition by improving synaptic plasticity in the brain, and provide impetus to develop 3'-O-Me-EC-Gluc and other brain-targeted PAC metabolites to promote learning and memory in AD and other forms of dementia.

Carvedilol as a potential novel agent for the treatment of Alzheimer's disease.

Oligomeric ß-amyloid (Aß) has recently been linked to synaptic plasticity deficits, which play a major role in progressive cognitive decline in Alzheimer's disease (AD). Here we present evidence that chronic oral administration of carvedilol, a nonselective ß-adrenergic receptor blocker, significantly attenuates brain oligomeric ß-amyloid content and cognitive deterioration in 2 independent AD mouse models. We found that carvedilol treatment significantly improved neuronal transmission, and that this improvement was associated with the maintenance of number of the less stable "learning" thin spines in the brains of AD mice. Our novel observation that carvedilol interferes with the neuropathologic, biochemical, and electrophysiological mechanisms underlying cognitive deterioration in AD supports the potential development of carvedilol as a treatment for AD.

Mesenchymal stem cells facilitate axon sorting, myelination, and functional recovery in paralyzed mice deficient in Schwann cell-derived laminin.

Peripheral nerve function depends on a regulated process of axon and Schwann cell development. Schwann cells interact with peripheral neurons to sort and ensheath individual axons. Ablation of laminin γ1 in the peripheral nervous system (PNS) arrests Schwann cell development prior to radial sorting of axons. Peripheral nerves of laminin-deficient animals are disorganized and hypomyelinated. In this study, sciatic nerves of laminin-deficient mice were treated with syngenic murine adipose-derived stem cells (ADSCs). ADSCs expressed laminin in vitro and in vivo following transplant into mutant sciatic nerves. ADSC-treatment of mutant nerves caused endogenous Schwann cells to differentiate past the point of developmental arrest to sort and myelinate axons. This was shown by (1) functional, (2) ultrastructural, and (3) immunohistochemical analysis. Treatment of laminin-deficient nerves with either soluble laminin or the immortalized laminin-expressing cell line 3T3/L1 did not overcome endogenous Schwann cell developmental arrest. In summary, these results indicate that (1) laminin-deficient Schwann cells can be rescued, (2) a cell-based approach is beneficial in comparison with soluble protein treatment, and (3) mesenchymal stem cells modify sciatic nerve function via trophic effects rather than transdifferentiation in this system.

Carvedilol reestablishes long-term potentiation in a mouse model of Alzheimer's disease.

In this study, we examined acute effects of carvedilol, a nonselective alpha/beta-adrenergic receptor blocker, on neuronal transmission and long-term potentiation (LTP) in six month-old TgCRND8 mice and their wild-type age-matched controls. Field excitatory postsynaptic potentials were recorded in the CA1 region of the hippocampus from carvedilol- or vehicle dimethyl sulfoxide-treated slices, and differences in basal synaptic transmission and LTP were assessed. Carvedilol treatment produced a significant increase in basal synaptic transmission and LTP in TgCRND8 mice, as compared to their vehicle-treated slices, in which basal neuronal transmission and LTP decreased. Interestingly, carvedilol significantly suppressed spontaneous seizure activity in TgCRND8 mice as measured by the number of slices showing epileptic discharges as well as the number of spikes within these and the amplitude of the second spike, measured at baseline and end of recording. In contrast, vehicle-treated slices in TgCRND8 mice did not show a significant decrease in epileptic discharges. These results suggest that carvedilol reestablishes basal synaptic transmission, enhances neuronal plasticity and suppresses neuronal hyperexcitability in TgCRND8 mice.

Multimodal sensory responses of nucleus reticularis gigantocellularis and the responses' relation to cortical and motor activation.

The connectivity of large neurons of the nucleus reticularis gigantocellularis (NRGc) in the medullary reticular formation potentially allows both for the integration of stimuli, in several modalities, that would demand immediate action, and for coordinated activation of cortical and motoric activity. We have simultaneously recorded cortical local field potentials, neck muscle electromyograph (EMG), and the neural activity of medullary NRGc neurons in unrestrained, unanesthetized rats to determine whether the activity of the NRGc is consistent with the modulation of general arousal. We observed excitatory responses of individual NRGc neurons to all modalities tested: tactile, visual, auditory, vestibular, and olfactory. Excitation was directly linked to increases in neck muscle EMG amplitude and corresponded with increases in the power of fast oscillations (30 to 80 Hz) of cortical activity and decreases in the power of slow oscillations (2 to 8 Hz). Because these reticular formation neurons can respond to broad ranges of stimuli with increased firing rates associated with the initiation of behavioral responses, we infer that they are part of an elementary "first responder" CNS arousal mechanism.

DEEP BRAIN STIMULATION IN MIDLINE THALAMIC REGION FACILITATES SYNAPTIC TRANSMISSION AND SHORTTERM MEMORY IN A MOUSE MODEL OF ALZHEIMER'S DISEASE.

Based on evidence suggesting that deep brain stimulation (DBS) may promote certain cognitive processes, we have been interested in developing DBS as a means of mitigating memory and learning impairments in Alzheimer's disease (AD). In this study we used an animal model of AD (TgCRND8 mice) to determine the effects of high-frequency stimulation (HFS) on non-amyloidogenic α-secretase activity and DBS in short-term memory. We tested our hypothesis using hippocampal slices (in vitro studies) from TgCRND8 mice to evaluate whether HFS increases α-secretase activity (non-amyloidogenic pathway) in the CA1 region. In a second set of experiments, we performed in vivo studies to evaluate whether DBS in midline thalamic region re-establishes hippocampal dependent short-term memory in TgCRND8 mice. The results showed that application of HFS to isolated hippocampal slices significantly increased synaptic plasticity in the CA1 region and promoted a 2-fold increase of non-amyloidogenic α-secretase activity, in comparison to low frequency stimulated controls from TgCRND8 mice. In the in vivo studies, DBS treatment facilitated acquisition of object recognition memory in TgCRND8 mice, in comparison to their own baseline before treatment. These results provide evidence that DBS could enhance short-term memory in the CA1 region of hippocampus in a mouse model of AD.

Sleep-dependent gene expression in the hippocampus and prefrontal cortex following long-term potentiation.

The activity-dependent transcription factor zif268 is re-activated in sleep following hippocampal long-term potentiation (LTP). However, the activation of secondary genes, possibly involved in modifying local synaptic strengths and ultimately stabilizing memory traces during sleep, has not yet been studied. Here, we investigated changes in hippocampal and cortical gene expression at a time point subsequent to the previously reported initial zif268 re-activation during sleep. Rats underwent unilateral hippocampal LTP and were assigned to SLEEP or AWAKE groups. Eighty minutes after a long rapid-eye-movement sleep (REMS) episode (or an equivalent amount of time for awake group) animals had their hippocampi dissected and processed for gene microarray hybridization. Prefrontal and parietal cortices were also collected for qRT-PCR analysis. The microarray analysis identified 28 up-regulated genes in the hippocampus: 11 genes were enhanced in the LTPed hemisphere of sleep animals; 13 genes were enhanced after sleep, regardless of hemisphere; and 4 genes were enhanced in LTPed hemisphere, regardless of behavioral state. qRT-PCR analysis confirmed the up-regulation of aif-1 and sc-65 during sleep. Moreover, we observed a down-regulation of the purinergic receptor, P2Y4R in the LTP hemisphere of awake animals and a trend for the protein kinase, CaMKI to be up-regulated in the LTP hemisphere of sleep animals. In the prefrontal cortex, we showed a significant LTP-dependent down-regulation of gluR1 and spinophilin specifically during sleep. Zif268 was down-regulated in sleep regardless of the hemisphere. No changes in gene expression were observed in the parietal cortex. Our findings indicate that a set of synaptic plasticity-related genes have their expression modulated during sleep following LTP, which can reflect biochemical events associated with reshaping of synaptic connections in sleep following learning.

The prototoxin lynx1 acts on nicotinic acetylcholine receptors to balance neuronal activity and survival in vivo.

Nicotinic acetylcholine receptors (nAChRs) affect a wide array of biological processes, including learning and memory, attention, and addiction. lynx1, the founding member of a family of mammalian prototoxins, modulates nAChR function in vitro by altering agonist sensitivity and desensitization kinetics. Here we demonstrate, through the generation of lynx1 null mutant mice, that lynx1 modulates nAChR signaling in vivo. Its loss decreases the EC(50) for nicotine by approximately 10-fold, decreases receptor desensitization, elevates intracellular calcium levels in response to nicotine, and enhances synaptic efficacy. lynx1 null mutant mice exhibit enhanced performance in specific tests of learning and memory. Consistent with reports that mutations resulting in hyperactivation of nAChRs can lead to neurodegeneration, aging lynx1 null mutant mice exhibit a vacuolating degeneration that is exacerbated by nicotine and ameliorated by null mutations in nAChRs. We conclude that lynx1 functions as an allosteric modulator of nAChR function in vivo, balancing neuronal activity and survival in the CNS.

Distinct modulatory effects of sleep on the maintenance of hippocampal and medial prefrontal cortex LTP.

Both human and animal studies support the idea that memory consolidation of waking experiences occurs during sleep. In experimental models, rapid-eye-movement (REM) sleep has been shown to be necessary for cortical synaptic plasticity and for the acquisition of spatial and nonspatial memory. Because the hippocampus and medial prefrontal cortex (mPFC) play distinct and important roles in memory processing, we sought to determine the role of sleep in the maintenance of long-term potentiation (LTP) in the dentate gyrus (DG) and mPFC of freely behaving rats. Animals were implanted with stimulating and recording electrodes, either in the medial perforant path and DG or CA1 and mPFC, for the recording of field potentials. Following baseline recordings, LTP was induced and the animals were assigned to three different groups: REM sleep-deprived (REMD), total sleep-deprived (TSD) and control which were allowed to sleep (SLEEP). The deprivation protocol lasted for 4 h and the recordings were made during the first hour and at 5, 24 and 48 h following LTP induction. Our results show that REMD impaired the maintenance of late-phase (48-h) LTP in the DG, whereas it enhanced it in the mPFC. Sleep, therefore, could have distinct effects on the consolidation of different forms of memory.

Tissue plasminogen activator promotes the effects of corticotropin-releasing factor on the amygdala and anxiety-like behavior.

Stress-induced plasticity in the brain requires a precisely orchestrated sequence of cellular events involving novel as well as well known mediators. We have previously demonstrated that tissue plasminogen activator (tPA) in the amygdala promotes stress-induced synaptic plasticity and anxiety-like behavior. Here, we show that tPA activity in the amygdala is up-regulated by a major stress neuromodulator, corticotropin-releasing factor (CRF), acting on CRF type-1 receptors. Compared with WT, tPA-deficient mice responded to CRF treatment with attenuated expression of c-fos (an indicator of neuronal activation) in the central and medial amygdala but had normal c-fos responses in paraventricular nuclei. They exhibited reduced anxiety-like behavior to CRF but had a sustained corticosterone response after CRF administration. This effect of tPA deficiency was not mediated by plasminogen, because plasminogen-deficient mice demonstrated normal behavioral and hormonal changes to CRF. These studies establish tPA as an important mediator of cellular, behavioral, and hormonal responses to CRF.

Site and time dependent effects of acute stress on hippocampal long-term potentiation in freely behaving rats.

The phasic effects of stress-induced elevations of corticosterone on long-term potentiation (LTP) were investigated in the hippocampus of awake, freely behaving rats. Field potential recordings were performed in the dentate gyrus with stimulation of the medial perforant pathway or the CA1 with stimulation of the commissural/associational pathway, on the contralateral hemisphere. LTP was induced either shortly (1 h) after acute stress or 4 h later. Animals were either adrenalectomized or adrenally intact. A subgroup of animals were injected with a low dose of dexamethasone 4 h prior to the stressor, in order to suppress the corticosterone response to restraint stress, and they were tested for LTP in the dentate gyrus 4 h after the stressor. In the dentate gyrus, stress had no effect on LTP induction at 1 h post-stress; however, it produced a significant suppression at the 4 h interval. As expected, adrenalectomized rats did not show stress-suppression of LTP, but showed a lower level of LTP with or without stress. Supporting a role of stress-induced glucocorticoid secretion in LTP suppression, dexamethasone treatment of adrenally intact animals blocked the acute stress suppression of LTP in the dentate gyrus. In the CA1 field, restraint stress did not significantly affect LTP induction at either the 1- or 4-h post-stress intervals. Similarly, stress by itself, did not significantly affect neuronal excitability in either the dentate gyrus or CA1 hippocampal field at either the 1- or 4-h post-stress interval. The present results suggest that stress affects synaptic plasticity differently at the two hippocampal subfields and that the effects are time-dependent and involve the stress-induced surge of glucocorticoids.

Induction of hippocampal long-term potentiation during waking leads to increased extrahippocampal zif-268 expression during ensuing rapid-eye-movement sleep.

Rapid-eye-movement (REM) sleep plays a key role in the consolidation of memories acquired during waking (WK). The search for mechanisms underlying that role has revealed significant correlations in the patterns of neuronal firing, regional blood flow, and expression of the activity-dependent gene zif-268 between WK and subsequent REM sleep. Zif-268 integrates a major calcium signal transduction pathway and is implicated by several lines of evidence in activity-dependent synaptic plasticity. Here we report that the induction of hippocampal long-term potentiation (LTP) during WK in rats leads to an upregulation of zif-268 gene expression in extrahippocampal regions during subsequent REM sleep episodes. This upregulation occurs predominantly in the amygdala, entorhinal, and auditory cerebral cortices during the first REM sleep episodes after LTP induction and reaches somatosensory and motor cerebral cortices as REM sleep recurs. We also show that hippocampal inactivation during REM sleep blocks extrahippocampal zif-268 upregulation, indicating that cortical and amygdalar zif-268 expression during REM sleep is under hippocampal control. Thus, expression of an activity-dependent gene involved in synaptic plasticity propagates gradually from the hippocampus to extrahippocampal regions as REM sleep recurs. These findings suggest that a progressive disengagement of the hippocampus and engagement of the cerebral cortex and amygdala occurs during REM sleep. They are also consistent with the view that REM sleep constitutes a privileged window for hippocampus-driven cortical activation, which may play an instructive role in the communication of memory traces from the hippocampus to the cerebral cortex.

Effects of chronic stress on hippocampal long-term potentiation.

Chronic stress causes atrophy of the apical dendrites of CA3 pyramidal neurons and deficits in spatial memory. We investigated the effects of chronic stress on hippocampal physiology and long-term potentiation (LTP) in the CA3 and dentate gyrus (DG). Rats were subjected to chronic (21 days, 6 h/day) restraint stress and tested for LTP 48 h following the last stress episode. Control animals were briefly handled each day, similar to the experimental group but without restraint. To eliminate acute stress effects, a second control group of rats was subjected to a single acute (6 h) restraint stress and tested for LTP 48 h later. Field potential recordings were made, under chloropent anesthesia, from the stratum lucidum of CA3, with stimulation of either the mossy fiber or commissural/associational pathways, or in the DG granule-cell layer, with stimulation of the medial perforant pathway. Chronic stress produced a suppression of LTP at 48 h compared to controls in a site-specific manner, namely, significantly lower LTP in the medial perforant input to the DG and also in the commissural/associational input to the CA3, but not in the mossy fiber input to CA3. The animals subjected to acute stress and tested 48 h later did not show a suppression in LTP. High-frequency stimulation (HFS) of the commissural/associational and mossy fiber inputs to CA3 produced epileptic afterdischarges in 56% of acutely stressed animals and in 29% of chronically stressed animals, whereas HFS caused afterdischarges in only 9% of nonstressed controls. No afterdischarges were seen in the medial perforant path input to DG. In order to explore the basis for these changes, we performed paired-pulse inhibition/facilitation (PPI/F) and current-source-density (CSD) analysis in stressed and control animals. For PPI/F, acute stress caused an overall significant enhancement of excitation in the commissural/associational input to CA3 and medial perforant path input to DG. In contrast, chronic stress did not produce significant changes in PPI/F. The CSD analysis revealed significant chronic stress-induced shifts in the current sources and sinks in the apical dendrites and pyramidal cell layers of the CA3 field but not in the DG. These results are consistent with the morphological findings for stress effects upon dendrites of CA3 neurons. Furthermore, they suggest that chronic stress produces changes in the input-output relationship in the hippocampal trisynaptic circuit which could affect information flow through this structure.

Single-unit activity in the lateral nucleus of the amygdala and overlying areas of the striatum in freely behaving rats: rates, discharge patterns, and responses to acoustic stimuli.

Acoustic responses of single units were examined in awake, freely behaving rats in the lateral nucleus of the amygdala (AL). Recordings were made from a movable bundle of 9 microwires. Most cells had very low rates of spontaneous activity (about 3 spikes/s average). Firing rates increased during sleep states. Short-latency auditory responses (12-25 ms) were found in the dorsal subnucleus (ALd) of the AL. Cells in the ALd most typically responded in a sustained fashion. Some of the cells in the ALd showed preferences for high frequencies, tone bursts, or frequency-modulated stimuli with center frequencies above 12 kHz. Response latencies were considerably longer in other areas of the amygdala. Our results corroborate the main findings of a previous study (F. Bordi & J. LeDoux, 1992) that examined the acoustic response properties of single cells in the AL in anesthetized rats. Together the findings from awake and anesthetized rats provide the most precise information about sensory processing in amygdala neurons available to date.