Role of the hippocampal formation in learning and memory

Spatially resolved transcriptome of the aging mouse brain

Brain aging is associated with cognitive decline, memory loss and many neurodegenerative disorders. The mammalian brain has distinct structural regions that perform specific functions. However, our understanding in gene expression and cell types within the context of the spatial organization of the mammalian aging brain is limited. Here we generated spatial transcriptomic maps of young and old mouse brains. We identified 27 distinguished brain spatial domains, including layer-specific subregions that are difficult to dissect individually. We comprehensively characterized spatial-specific changes in gene expression in the aging brain, particularly for isocortex, the hippocampal formation, brainstem and fiber tracts, and validated some gene expression differences by qPCR and immunohistochemistry. We identified aging-related genes and pathways that vary in a coordinated manner across spatial regions and parsed the spatial features of aging-related signals, providing important clues to understand genes with specific functions in different brain regions during aging. Combined with single-cell transcriptomics data, we characterized the spatial distribution of brain cell types. The proportion of immature neurons decreased in the DG region with aging, indicating that the formation of new neurons is blocked. Finally, we detected changes in information interactions between regions and found specific pathways were deregulated with aging, including classic signaling WNT and layer-specific signaling COLLAGEN. In summary, we established a spatial molecular atlas of the aging mouse brain (http://sysbio.gzzoc.com/Mouse-Brain-Aging/), which provides important resources and novel insights into the molecular mechanism of brain aging.

Sex-Specific Alterations in Dopamine Metabolism in the Brain after Methamphetamine Self-Administration

Methamphetamine (METH) use disorder affects both sexes, with sex differences occurring in behavioral, structural, and biochemical consequences. The molecular mechanisms underlying these differences are unclear. Herein, we used a rat model to identify potential sex differences in the effects of METH on brain dopaminergic systems. Rats were trained to self-administer METH for 20 days, and a cue-induced drug-seeking test was performed on withdrawal days 3 and 30. Dopamine and its metabolites were measured in the prefrontal cortex (PFC), nucleus accumbens (NAc), dorsal striatum (dSTR), and hippocampus (HIP). Irrespective of conditions, in comparison to females, male rats showed increased 3,4-dihydroxyphenylalanine (DOPA) in the PFC, dSTR, and HIP; increased cys-dopamine in NAc; and increased 3,4-dihydroxyphenylethanol (DOPET) and 3,4-dihydroxyphenylacetic acid (DOPAC) in dSTR. Males also showed METH-associated decreases in DA levels in the HIP but increases in the NAc. Female rats showed METH-associated decreases in DA, DOPAL, and DOPAC levels in the PFC but increases in DOPET and DOPAC levels in the HIP. Both sexes showed METH-associated decreases in NAc DA metabolites. Together, these data document sex differences in METH SA-induced changes in DA metabolism. These observations provide further support for using sex as an essential variable when discussing therapeutic approaches against METH use disorder in humans.

Involvement of protein kinase ζ in the maintenance of hippocampal long-term potentiation in rats with chronic visceral hypersensitivity

The hippocampal long-term potentiation (LTP) was implicated in the formation of visceral hypersensitivity in rats with irritable bowel syndrome in our previous study. Recent studies have shown that protein kinase M ζ (PKMζ) may be responsible for the maintenance of LTP in memory formation. However, it remains unclear whether PKMζ is involved in the visceral hypersensitivity. In this study, a rat model of visceral hypersensitivity was generated by neonatal maternal separation (NMS). The visceral hypersensitivity was assessed by recording responses of the external oblique abdominal muscle to colorectal distension. Our results demonstrated that hippocampal LTP and visceral hypersensitivity were enhanced significantly in rats of NMS. ζ-Pseudosubstrate inhibitory peptide (ZIP) could dose dependently inhibit the maintenance of Cornu Ammonis area 1 LTP in rats of NMS. Furthermore, Western blot data showed that the expression of hippocampal phosphorylated PKMζ (p-PKMζ) significantly increased in rats of NMS. In addition, bilateral intrahippocampal injections of ZIP attenuated the visceral hypersensitivity dose dependently in rats of NMS. The maximal inhibition was observed at 30 min, and significant inhibition lasted for 1.5–2 h after ZIP application. Besides, data from the open-field test and Morris water maze showed that ZIP did not influence the movement and spatial procedural memory in rats of NMS. In conclusion, p-PKMζ might be a critical protein in the maintenance of hippocampal LTP, which could result in visceral hypersensitivity.

Aging, but not tau pathology, impacts olfactory performances and somatostatin systems in THY-Tau22 mice

Somatostatin (SOM) cortical levels decline in Alzheimer's disease (AD) in correlation with cognitive impairment severity, the latter being closely related to the presence of neurofibrillary tangles. Impaired olfaction is another hallmark of AD tightly related to tau pathology in the olfactory pathways. Recent studies showed that SOM modulates olfactory processing, suggesting that alterations in SOM levels participate to olfactory deficits in AD. Herein, we first observed that human olfactory peduncle and cortex are enriched in SOM cells and fibers, in aged postmortem brains. Then, the possible link between SOM alterations and olfactory deficits was evaluated by exploring the impact of age and tau hyperphosphorylation on olfactory SOM networks and behavioral performances in THY-Tau22 mice, a tauopathy transgenic model. Distinct molecular repertoires of SOM peptide and receptors were associated to sensory or cortical olfactory processing structures. Aging mainly affected SOM neurotransmission in piriform and entorhinal cortex in wild-type mice, although olfactory performances decreased. However, no further olfactory impairment was evidenced in THY-Tau22 mice until 12 months although tau pathology early affected olfactory cortical structures. Thus, tau hyperphosphorylation per se has a limited impact on olfactory performances in THY-Tau22 mice.

Inter-modality relationship constrained multi-modality multi-task feature selection for Alzheimer's Disease and mild cognitive impairment identification

Previous studies have demonstrated that the use of integrated information from multi-modalities could significantly improve diagnosis of Alzheimer's Disease (AD). However, feature selection, which is one of the most important steps in classification, is typically performed separately for each modality, which ignores the potentially strong inter-modality relationship within each subject. Recent emergence of multi-task learning approach makes the joint feature selection from different modalities possible. However, joint feature selection may unfortunately overlook different yet complementary information conveyed by different modalities. We propose a novel multi-task feature selection method to preserve the complementary inter-modality information. Specifically, we treat feature selection from each modality as a separate task and further impose a constraint for preserving the inter-modality relationship, besides separately enforcing the sparseness of the selected features from each modality. After feature selection, a multi-kernel support vector machine (SVM) is further used to integrate the selected features from each modality for classification. Our method is evaluated using the baseline PET and MRI images of subjects obtained from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database. Our method achieves a good performance, with an accuracy of 94.37% and an area under the ROC curve (AUC) of 0.9724 for AD identification, and also an accuracy of 78.80% and an AUC of 0.8284 for mild cognitive impairment (MCI) identification. Moreover, the proposed method achieves an accuracy of 67.83% and an AUC of 0.6957 for separating between MCI converters and MCI non-converters (to AD). These performances demonstrate the superiority of the proposed method over the state-of-the-art classification methods.

Cooperation between hippocampal somatostatin receptor subtypes 4 and 2: functional relevance in interactive memory systems

The hippocampal somatostatin (sst) receptor subtype 4 (sst(4)) modulates memory formation by diminishing hippocampus-based spatial function while enhancing striatum-dependent behaviors. sst(4)-mediated regulations on neuronal activity in the hippocampus appear to depend on both competitive and cooperative interactions with sst receptor subtype 2 (sst(2)). Here, we investigated whether interactions with sst(2) receptors are required for sst(4)-mediated effects on hippocampus-dependent spatial memory and striatum-dependent cued memory in a water maze paradigm. Competition was assessed in mice by intrahippocampal injections of the sst(4) agonist L-803,087 alone or combined with sst(2) agonists (L-779,976 or octreotide). Effects of L-803,087 were also tested in sst(2) knockout mice to assess for receptor cooperation. Finally, sst(2a) and sst(4) localizations within hippocampal subregions were analyzed by immunohistochemistry and expression levels of sst(2a) and sst(2b) were quantified by real-time qPCR. Hippocampal injections of L-803,087 impaired spatial memory but enhanced cued memory. The latter effect was lost not only in sst(2) knockout mice but also in the presence of sst(2) agonists, whereas the former effect remained unaffected by sst(2) agonists or gene deletion. Octreotide and L-779,976 did not yield memory effects but reduced swim velocity throughout the acquisition trials suggesting that stimulation of sst(2) affected motivation and/or anxiety. sst(2a) and sst(4) were respectively detected in the dentate gyrus (DG) and the CA1 subfield suggesting that their functional interactions are not mediated by direct receptor coupling. Hippocampus sst(2a) expression was 36-fold higher than sst(2b). Possible neural mechanisms and functional significances for interaction between memory systems in relationship with stress-induced changes in hippocampal functions are discussed.

Roles of the hippocampal formation in pain information processing

Pain is a complex experience consisting of sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Now it has been gradually known that noxious information is processed by a widely-distributed, hierarchically- interconnected neural network, referred to as neuromatrix, in the brain. Thus, identifying the multiple neural networks subserving these functional aspects and harnessing this knowledge to manipulate the pain response in new and beneficial ways are challenging tasks. Albeit with elaborate research efforts on the cortical responses to painful stimuli or clinical pain, involvement of the hippocampal formation (HF) in pain is still a matter of controversy. Here, we integrate previous animal and human studies from the viewpoint of HF and pain, sequentially representing anatomical, behavioral, electrophysiological, molecular/biochemical and functional imaging evidence supporting the role of HF in pain processing. At last, we further expound on the relationship between pain and memory and present some unresolved issues.

Changes in neurogenesis in dementia and Alzheimer mouse models: are they functionally relevant?

Alzheimer's disease and related dementias are devastating disorders that lead to the progressive decline of cognitive functions. Characteristic features are severe brain atrophy, paralleled by accumulation of beta amyloid and neurofibrillary tangles. With the discovery of neurogenesis in the adult brain, the hopes have risen that these neurodegenerative conditions could be overcome, or at least ameliorated, by the generation of new neurons. The location of the adult neurogenic zones in the hippocampus and the lateral ventricle wall, close to corpus callosum and neocortex, indicates strategic positions for potential repair processes. However, we also need to consider that the generation of new neurons is possibly involved in cognitive functions and could, therefore, be influenced by disease pathology. Moreover, aberrant neurogenic mechanisms could even be a part of the pathological events of neurodegenerative diseases. It is the scope of this review to summarize and analyze the recent data from neurogenesis research with respect to Alzheimer's disease and its animal models.

Modulation of behavior and NMDA-R1 gene mRNA expression in adult female mice after sub-acute administration of benzo(a)pyrene

The behavioral performances of adult mice exposed to sub-acute doses of benzo(a)pyrene (B(a)P) were monitored in tests related to learning and memory (Y maze and Morris water maze), locomotor activity (open-field test) and motor coordination (Locotronic apparatus). At low doses (0.02 and 0.2mg/kg), B(a)P impaired short-term learning and spatial memory performance in the Y maze and in the Morris water maze tests. Surprisingly, in the Y maze, the performances of animals exposed to the highest dose of B(a)P (200mg/kg) were quite similar to those of control animals. Hyperactivity/hyperarousal observed in both tests at this dose and attributed to an anxiolytic-like effect of B(a)P may have blurred the learning deficit in these mice faced with a new situation. These deficits seem to be unrelated to motor impairments because B(a)P had no effect on locomotor activity and motor coordination. We demonstrated that sub-acute exposure to B(a)P in adult mice also modulates gene expression of NMDA-R1 subunit in brain areas highly involved in cognitive function like the hippocampus, suggesting a relationship between the expression of functional NMDA-R1 mRNA, impairment of short-term and spatial memory and the B(a)P exposure levels.

Stimulation of hippocampal adenylyl cyclase activity dissociates memory consolidation processes for response and place learning

Procedural and declarative memory systems are postulated to interact in either a synergistic or a competitive manner, and memory consolidation appears to be a highly critical stage for this process. However, the precise cellular mechanisms subserving these interactions remain unknown. To investigate this issue, 24-h retention performances were examined in mice given post-training intrahippocampal injections of forskolin (FK) aiming at stimulating hippocampal adenylyl cyclases (ACs). The injection was given at different time points over a period of 9 h following acquisition in either an appetitive bar-pressing task or water-maze tasks challenging respectively "response memory" and "place memory." Retention testing (24 h) showed that FK injection altered memory formation only when given within a 3- to 6-h time window after acquisition but yielded opposite memory effects as a function of task demands. Retention of the spatial task was impaired, whereas retention of both the cued-response in the water maze and the rewarded bar-press response were improved. Intrahippocampal injections of FK produced an increase in pCREB immunoreactivity, which was strictly limited to the hippocampus and lasted less than 2 h, suggesting that early effects (0-2 h) of FK-induced cAMP/CREB activation can be distinguished from late effects (3-6 h). These results delineate a consolidation period during which specific cAMP levels in the hippocampus play a crucial role in enhancing memory processes mediated by other brain regions (e.g., dorsal or ventral striatum) while eliminating interference by the formation of hippocampus-dependent memory.

A different recruitment of the lateral and basolateral amygdala promotes contextual or elemental conditioned association in Pavlovian fear conditioning

Convergent data suggest dissociated roles for the lateral (LA) and basolateral (BLA) amygdaloid nuclei in fear conditioning, depending on whether a discrete conditioned stimulus (CS)-unconditional stimulus (US) or context-US association is considered. Here, we show that pretraining inactivation of the BLA selectively impaired conditioning to context. In contrast, inactivation of the LA disrupted conditioning to the discrete tone CS, but also either impaired or enhanced contextual conditioning, depending on whether the context was in the foreground or in the background. Hence, these findings refine the current model of the amygdala function in emotional learning by showing that the BLA and the LA not only differentially contribute to elemental and context-US association, but also promote, through their interaction, the most relevant of these two associations.

Hippocampal lesions facilitate instrumental learning with delayed reinforcement but induce impulsive choice in rats

BACKGROUND

Animals must frequently act to influence the world even when the reinforcing outcomes of their actions are delayed. Learning with action-outcome delays is a complex problem, and little is known of the neural mechanisms that bridge such delays. When outcomes are delayed, they may be attributed to (or associated with) the action that caused them, or mistakenly attributed to other stimuli, such as the environmental context. Consequently, animals that are poor at forming context-outcome associations might learn action-outcome associations better with delayed reinforcement than normal animals. The hippocampus contributes to the representation of environmental context, being required for aspects of contextual conditioning. We therefore hypothesized that animals with hippocampal lesions would be better than normal animals at learning to act on the basis of delayed reinforcement. We tested the ability of hippocampal-lesioned rats to learn a free-operant instrumental response using delayed reinforcement, and what is potentially a related ability -- the ability to exhibit self-controlled choice, or to sacrifice an immediate, small reward in order to obtain a delayed but larger reward.

RESULTS

Rats with sham or excitotoxic hippocampal lesions acquired an instrumental response with different delays (0, 10, or 20 s) between the response and reinforcer delivery. These delays retarded learning in normal rats. Hippocampal-lesioned rats responded slightly less than sham-operated controls in the absence of delays, but they became better at learning (relative to shams) as the delays increased; delays impaired learning less in hippocampal-lesioned rats than in shams. In contrast, lesioned rats exhibited impulsive choice, preferring an immediate, small reward to a delayed, larger reward, even though they preferred the large reward when it was not delayed.

CONCLUSION

These results support the view that the hippocampus hinders action-outcome learning with delayed outcomes, perhaps because it promotes the formation of context-outcome associations instead. However, although lesioned rats were better at learning with delayed reinforcement, they were worse at choosing it, suggesting that self-controlled choice and learning with delayed reinforcement tax different psychological processes.

Sexual dimorphism in the induction of LTP: Critical role of tetanizing stimulation

Numerous studies have suggested that sexual dimorphism may exist in learning and memory, particularly in types involving the hippocampus. In the present study, we examined the effects of two different tetani on the induction of long-term potentiation in the CA1 region of hippocampal slices from adult female and male rats to determine the sexual differences in their responses to tetanizing stimulation. We found that the induction of LTP is sex-dependent, and that there were clear sexual differences in the responses to different tetanus patterns, but not impulse number or stimulation frequency. Multiple trains of tetani were more effective in the indution of LTP in male rats than in female ones. These findings suggest that male rats can react to a broader range of tetanizing stimulation compared with female rats. Based on our results and the findings of other studies, we propose that the interaction of gonadal hormones with Ca2+/NMDAR and the subsequent regulation of the ERK/MAP kinase pathway are critical mechanisms for sexual dimorphism in the induction of LTP.

Spatial learning induces differential changes in calcium/calmodulin-stimulated (ACI) and calcium-insensitive (ACII) adenylyl cyclases in the mouse hippocampus

Several lines of evidence indicate that Ca2+/calmodulin-stimulated isoforms of adenylyl cyclase (AC) are involved in long-term potentiation and in certain forms of learning. Recently, we found that training in different types of learning task differentially activates Ca2+-sensitive versus Ca2+-insensitive AC activities in certain brain regions, indicating that AC species other than those stimulated by Ca2+/calmodulin may play an important role in learning processes (Guillou, Rose, & Cooper, 1999). Here, we report the effects of spatial reference memory training in a radial arm maze on the levels of AC1 and AC2 mRNA in the dorsal hippocampus of C57BL/6 mice. Acquisition of the task was associated with a learning-specific and time-dependent increase of AC1 mRNA expression selectively in subfields CA1-CA2. In contrast, AC2 mRNA levels were either reduced or not reliably affected depending on the stage of acquisition. Moreover, no significant changes in AC expression were observed either in the dorsal hippocampus of mice trained in a non-spatial (procedural) version of the task or in cortical regions of mice learning the spatial or procedural task. The regional specificity of these effects indicates that the formation of spatial and non-spatial memory requires distinct contributions from Ca2+-sensitive and Ca2+-insensitive AC in the hippocampus. It is suggested that downregulation of AC2 throughout all hippocampal subfields may play a permissive role during the acquisition of spatial learning whereas an upregulation of AC1 specifically in subfield CA1, may be critical to accurately encode, store or use spatial information.

The effects of ibotenic hippocampal lesions on discriminative fear conditioning to context in mice: impairment or facilitation depending on the associative value of a phasic explicit cue

To what extent the hippocampus is required for contextual conditioning remains a matter of debate. The present experiments examined the effects of ibotenate hippocampal lesions on discriminative fear conditioning to context in mice using measures of freezing in two conditioning paradigms. In both paradigms animals received foot shock as the unconditional stimulus (US) when placed in the (conditioning) context and no foot-shock when placed in the other (neutral) context. In both contexts, animals were presented with a tone as the conditioned stimulus (CS). In the conditioning context there was either no interval (delay condition) or a 30-s interval (trace condition) between tone CS end and shock US onset. These two paradigms were used because theory predicts that in the trace condition animals would learn more about contextual cues as predictors, or not, of shock US occurrence than in the delay condition. In agreement with this, we observed that sham-operated mice learned the context discrimination faster in the trace than in the delay condition. Lesions of the hippocampus significantly retarded, but did not prevent, the acquisition of the context discrimination in the trace condition. In contrast, lesions produced an opposite (facilitatory) effect in the delay condition, which was mainly observed during tone CS presentation. The data suggest that mice used two distinct competing strategies in solving this discrimination task: (i) a strategy relying on the processing of background contextual stimuli allowing direct establishment of context-US associations of different strengths, and (ii) a conditional cue (tone)-based strategy. Hence, hippocampal lesions may impair the use of the former strategy while exacerbating (unmasking) the use of the latter.

Entorhinal cortex of the mouse: cytoarchitectonical organization

The present study describes the cytoarchitectonical and chemoarchitectonical organization of the entorhinal cortex of the mouse (C57BL/6J strain). The entorhinal cortex is medially bordered by the parasubiculum, and laterally by the perirhinal cortex; rostrally and medially it is bordered by the piriform cortex, whereas caudally and dorsally it is bordered by the postrhinal cortex. The entorhinal cortex is divided into two main areas, i.e., the lateral entorhinal area (LEA) and the medial entorhinal area (MEA). Both entorhinal areas are further divided into subfields, i.e., LEA is divided into DLE (dorsolateral entorhinal field), DIE (dorsal intermediate entorhinal field), and VIE (ventral intermediate entorhinal field), whereas MEA is divided into CE (caudal entorhinal field) and ME (medial entorhinal field). Cytoarchitectonically, the main difference between LEA and MEA is displayed by layer II neurons: while these are in a dense layer in LEA, they are more dispersed in MEA. Further, in LEA there is a relatively cell-free zone between layers II and III; this zone is not present in MEA. Histochemically, in acetylcholinesterase (AChE)-stained material, MEA is characterized by darker-stained bands in the superficial layer (i.e., layer I) and in the lamina dissecans, in contrast to LEA, which is more evenly stained for AChE. Further, both the border with the perirhinal cortex and the border with the parasubiculum are characterized by dark-stained bands of AChE. The border between the entorhinal cortex and perirhinal cortex is also easily distinguished in parvalbumin-stained material; while the entorhinal cortex is darkly stained, the perirhinal cortex is lightly stained. In contrast, in sections stained for calretinin, the entorhinal cortex is more lightly stained than the parasubiculum, which has a darkly stained superficial layer, and a densely stained group of neurons in layer III.

Re-evaluation of the spatial memory deficits induced by hippocampal short lasting inactivation reveals the need for cortical co-operation

Evidence has accumulated that the rat hippocampus plays a central role in spatial memory. In complement to lesion studies, reversible lidocaïne-induced inactivations have been used to investigate the time-course of the memory processes mediated by the hippocampus. A number of studies suggest that, in some conditions, the hippocampus is not necessary for online acquisition of spatial information. To test this hypothesis, we examined the effects of bilateral lidocaïne-induced inactivations of the dorsal hippocampus in the acquisition of new spatial information. After initial learning of a place navigation task in the water maze, rats were tested for acquisition of a new platform location and received injections of lidocaïne in the hippocampus prior to each daily four-trial block. The training blocks were separated by a 24-h period allowing the hippocampus to recover from inactivation. The results show that lidocaïne-injected rats were able to learn the new platform location like controls. Inactivations, however, was found to induce a within-block learning impairment. This suggests that the hippocampus can perform off-line processing and that another structure is able to handle spatial information during hippocampal inactivations. Parietal-lesioned rats that received an injection of lidocaïne were still able to learn the new platform location suggesting that the parietal cortex does not sustain this role. Overall, our results suggest that the hippocampus is not necessary for all stages of memory formation and co-operates with other brain, possibly cortical, structures which remain to be determined.

Activation of the dentate gyrus by stimulation of the contralateral perforant pathway: evoked potentials and long-term potentiation after ipsi- and contralateral induction

Rats were chronically implanted with stimulation electrodes in the perforant pathway (pp) bilaterally and a recording electrode in the dentate gyrus (DG) unilaterally. Evoked field potentials (EPs) were recorded upon alternating stimulation of the pp on both sides, and long-term potentiation (LTP) was induced. Besides the EP after ipsilateral stimulation, an EP with a latency of approximately 5.5-6.5 ms was also seen upon stimulation of the contralateral pp. This potential was reversibly abolished during pentobarbital anesthesia and irreversibly after lesioning of the ipsilateral angular bundle. Paired-pulse facilitation and paired-pulse depression, depending on interstimulus interval and intensity, were also observed. Therefore, this long-latency potential could be characterized as polysynaptic and induced perhaps by transsynaptic activation via the ipsilateral entorhinal cortex. Ipsilateral tetanization induced strong E/S potentiation of both the ipsilaterally and contralaterally evoked EP, but with different time courses. Tetanization of the contralateral pp did not induce LTP of the ipsilaterally induced EP in the first 4 h. But afterwards a late and slowly developing potentiation occurred. The contralaterally induced EP also showed potentiation of the population spike, which was not immediately detectable but developed slowly over time. The results can be interpreted such that, after stimulation of the pp, the DG on the opposite side cannot only be activated via the weak crossed entorhinal projection but also transsynaptically via an entorhino/entorhinal connection.

N-methyl-D-aspartate receptor subunit changes are associated with lead-induced deficits of long-term potentiation and spatial learning

The present study demonstrates that impairments of spatial learning and hippocampal long-term potentiation in rats chronically exposed to lead are associated with changes in gene and protein expression of N-methyl-D-aspartate receptor subunits. Rats exposed to 750 and 1500 ppm lead acetate were found to exhibit deficits in acquisition of a water maze spatial learning task. Furthermore, lead-exposed rats show dose-dependent reductions in the maintenance of in vivo hippocampal long-term potentiation induced in entorhinal cortex-dentate gyrus synapses. We found an unexpected, but significant (P<0.05), correlation between spatial learning and long-term potentiation when control and lead-exposed rats were analysed as a single, combined population. Dentate gyrus NR1 subunit messenger RNA was reduced 18% and 28% by exposure to 750 and 1500 ppm lead acetate, respectively. NR2A subunit messenger RNA was reduced 18% but only in the dentate gyrus of rats exposed to 1500 ppm lead acetate. No significant changes in dentate NR2B messenger RNA expression were measured in either of the lead-exposed groups. NR1 subunit protein was reduced 24% and 58% in hippocampal homogenates from rats exposed to 750 and 1500 ppm lead acetate. In contrast, no changes in NR2A or NR2B subunit protein were observed in the same hippocampal homogenates. These data show that reductions of specific N-methyl-D-aspartate receptor subunits are associated with deficits of both hippocampal long-term potentiation and spatial learning, induced in rats by chronic exposure to environmentally relevant levels of lead. These findings strongly suggest that the effects of lead on N-methyl-D-aspartate receptors may be the mechanistic basis for lead-induced deficits in cognitive function.

Neuromodulation of memory in the hippocampus by vasopressin

The involvement of [Arg(8)]vasopressin in memory processes was analyzed in the hippocampal structure, since we have reported that this is one of the main central target structures of the vasopressin-enhancing effect on memory. This structure is functionally differentiated along its dorsoventral axis, and the expression of the vasopressinergic system is dependent upon whether the dorsal or ventral part of the hippocampus is involved. For this reason, the effect of vasopressin injected into hippocampus was evaluated on the basis of the site of injection. We have shown, using a Go-No Go visual discrimination task with mice that both parts of the hippocampus are involved in the effect of endogenous or exogenous vasopressin, but with higher sensitivity for the ventral part. Based on the expression of Fos protein following intracerebroventricular injection of vasopressin in unconditioned or conditioned mice, we confirmed the greater involvement of the ventral hippocampus in the enhancing effect of vasopressin on memory processes. The effect of the peptide seems specific, since only a few of the hippocampal cells that expressed Fos protein in the unconditioned mice did so in the conditioned mice (cells in the dentate gyrus and the CA3 hippocampal field). Moreover, we have shown that in the ventral hippocampus, vasopressin generates different behavioral effects whether treatment is performed at the beginning or in the middle of the learning process, suggesting that the mnemonic context is an important factor for understanding the effect of vasopressin on memory in the ventral hippocampus.

Intrahippocampal injections of cysteamine improve the retention of a bar-pressing task in mice

Cysteamine was used as a tool aimed at investigating the role of central somatostatin (SS-14) and was shown to modulate learning in a task-dependent manner. However, direct arguments have not yet been provided to support the hypothesis that impairments and facilitation of learning produced by cysteamine are both mediated by the hippocampus. Mice were given daily intrahippocampal injections of artificial cerebrospinal fluid (CSF) or cysteamine at doses of either 2.5 microg/0.2 microl or 25 microg/0.2 microl 1 h prior to each learning session of a bar-pressing task, for which the acquisition was previously shown to be improved by systemic injections. The results showed that, with respect to CSF, the mice injected with cysteamine learned the bar pressing task faster whereas no evidence of changes in locomotor activity was provided. Moreover, the results showed that retention was specifically increased in the two groups injected with cysteamine. It is argued that the action of cysteamine on the hippocampus is sufficient to modulate specifically learning-memory processes in a task-dependent manner. In conclusion, the blockade of some hippocampal information processing function by cysteamine is discussed to understand the bidirectional effects of drugs on learning and memory.

Efferent connections of the anteromedial nucleus of the thalamus of the rat

The projections from the anteromedial nucleus of the thalamus (AM) were investigated using anterograde and retrograde tracing techniques. AM projects to nearly the entire rostrocaudal extent of limbic cortex and to visual cortex. Anteriorly, AM projects to medial orbital, frontal polar, precentral agranular, and infraradiata cortices. Posteriorly, AM projects to retrosplenial granular, entorhinal, perirhinal and presubicular cortices, and to the subiculum. Further, AM projects to visual cortical area 18b, and to the lateral and basolateral nuclei of the amygdala. AM projections are topographically organized, i.e., projections to different cortical areas arise from distinct parts of AM. The neurons projecting to rostral infraradiata cortex (IRalpha) are more caudally located in AM than the neurons projecting to caudal infraradiata cortex (IRbeta). The neuronal cell bodies that project to the terminal field in area 18b are located primarily in ventral and lateral parts of AM, whereas neurons projecting to perirhinal cortex and amygdala are more medially located in AM. Injections into the most caudal, medial part of AM (i.e., the interanteromedial [IAM] nucleus) label terminals in the rostral precentral agranular, caudal IRbeta, and caudal perirhinal cortices. Whereas most AM axons terminate in layers I and V-VI, exceptions to this pattern include area 18b (axons and terminals in layers I and IV-V), the retrosplenial granular cortex (axons and terminals in layers I and V), and the presubicular, perirhinal, and entorhinal cortices (axons and terminals predominantly in layer V). Together, these findings suggest that AM influences a widespread area of limbic cortex.

Role of vasopressin in learning and memory in the hippocampus

The involvement of arginine8-vasopressin (VP) in learning and memory in the hippocampus is examined in mice using a discriminative learning task. Bilateral dorsal hippocampal lesion blocks the enhancing effect of intracerebroventricular (i.c.v.) injection of VP on retrieval and relearning processes. An additional study showed that immunoneutralization of dorsal hippocampal endogenous VP inhibited the facilitating effect of i.c.v. injection of VP, suggesting that hippocampus is essential for the expression of VP's behavioral effects. Using in situ microinjection, a greater sensitivity of the ventral part of the hippocampus to the memory enhancing effects of VP has been reported. This effect is mediated by vasopressin V1 type receptors and oxytocin receptors. Then, we examined the effects on behavior of VP applied to the ventral hippocampus, in relation to the time of treatment during learning. When the animals have no previous information about the task to learn, a deleterious effect of VP appears (pre-first session treatment). Regarding memory consolidation, the effects of VP may depend upon the previous level of performance acquired by the animals since, when injected after the first learning session, the peptide slightly delayed performance, whereas when the injection took place after the second learning session, it enhanced learning. Concerning memory retrieval, the effects of VP depend on the quality of the previously stored information. The fact that VP did not generate the same behavioral effects when the treatment was performed at the beginning or in the middle of the learning processes, suggests that mnemonic context is an important factor in understanding the effect of VP on memory in the ventral hippocampus. Finally, the role of hippocampal adrenergic receptors in the enhancing VP effects on memory retrieval has been examined. The facilitatory effects of VP seem to depend upon the functional state of both alpha- and beta-adrenergic receptors, but further studies will be necessary to clarify the role played by each receptor type in retrieval processes, and to determine the relationships that might exist between them.

Estradiol facilitates performance as working memory load increases

A water-escape version of the radial-arm maze was used to assess rat spatial working memory performance. Intact females and ovariectomized females receiving a physiologically low dose, physiologically moderate dose or no estradiol replacement were studied. Subjects were given seven trials a day for 12 days. Females receiving moderate dose estradiol made fewer errors than the other three groups during the latter portion of testing. As trials progressed within a session, the elements of information to be remembered increased. Assessment of individual trials revealed that when the demand on an animal's working memory system was limited to one to four elements of information, the three groups with estrogen (including intact females) maintained successful performance, whereas the ovariectomized females made more errors. However, when the demand on an animal's working memory system was increased to six elements of information, only the moderate dose estradiol females maintained successful performance. These data suggest that, although moderate levels of estradiol replacement are the most beneficial for working memory function, even low-dose estradiol replacement can act to protect working memory systems from the decline seen with the removal of ovarian hormones.

Immunolesion of the cholinergic basal forebrain: effects on functional properties of hippocampal and septal neurons

Deficits in cholinergic function have been documented in a variety of brain disorders including Alzheimer's Disease and, to a lesser extent, in normal ageing. In the present article, we have reviewed our recent findings on the effects of the loss of basal forebrain cholinergic neurons on the functional properties of the septohippocampal pathway. In vivo and ex vivo investigations were performed in rats following basal forebrain cholinergic lesion with the specific immunotoxin 192 IgG-saporin. Our results suggest a significant contribution of cholinergic neurons in the rhythmically bursting activity recorded within the medial septum. In addition, they give evidence that acetylcholine may tonically decrease the glutamatergic synaptic responses in the hippocampus whereas the GABAergic mediated inhibitory potentials are not affected. The possible contribution of these cholinergic mechanisms in the age-related functional alterations of the septohippocampal activity is discussed.

Total number of neurons in the layers of the human entorhinal cortex

The total number of neurons in the major laminae of the human entorhinal cortex were estimated with a design-based stereological technique, the optical fractionator. Detailed descriptions of the laminar organization and the cortical limits of the region required for the analysis are provided, along with detailed descriptions of the sampling scheme employed. The individual, mean values, and variances for estimates made in layers II, III, V, and VI are presented and discussed in terms of the precision of the estimation procedure and the results of other studies. Neuron numbers were estimated to be about 1 million layer II cells, 5 million layer III cells, 2 million layer V cells, and 4 million layer VI cells, for a total of slightly more that 13 million neurons in the entorhinal cortex. Combined with data from a similar study carried out in the human hippocampus, the data presented represent the first rigorous stereological evidence of the divergence of entorhinal projections to the hippocampus. The data presented also indicate that projections from layer II of the entorhinal cortex to the dentate gyrus and CA2/3 and projections from layer III of the entorhinal cortex to CA1 differ in the degrees of their divergence.

Amnesia, memory and brain systems

Bilateral damage to either the medial temporal lobe or the diencephalic midline causes an amnesic syndrome, i.e. a global impairment in the ability to acquire new memories regardless of sensory modality, and a loss of some memories, especially recent ones, from the period before amnesia began. The memory deficit can occur against a background of intact intellectual and perceptual functions. Two themes have been prominent in recent work. First, the amnesic syndrome is narrower than once believed in the sense that a number of learning and memory abilities are preserved (e.g. skill and habit learning, simple forms of conditioning and the phenomenon of priming). Second, the brain system damaged in amnesia has only a temporary role in memory. As time passes after learning, memory is reorganized and consolidated within neocortex, such that eventually medial temporal lobe and diencephalic structures are not needed for storage or retrieval.

Injury-induced physiological events that may modulate gene expression in neurons and glia

Damage to the brain triggers a host of reactive responses in neurons and glia which are seen at sites of focal injury as well as at sites that are at a distance from the injury. Although many of these responses have been studied extensively, the signals that initiate the different responses have not been fully characterized, and it is still not understood how focal injury affects neurons and glia in distant sites. The present review summarizes recent findings that suggest that physiological events that occur at the time of the injury or during the early postlesion period can play an important and variable role in modulating neuronal and glial responses to injury. We focus on the events that occur in the hippocampal formation following unilateral lesions of the entorhinal cortex - a model system that has been used extensively for studies of cellular responses following focal brain injury. This lesion destroys the cells of origin of a massive excitatory projection to the dentate gyrus and hippocampus proper. Over time, the denervated neurons in the hippocampal formation are almost completely reinnervated as a result of local sprouting of systems that survive the lesion. Thus, this model system has been useful for studying cellular responses to both denervation and reinnervation. We summarize the information that this injury triggers physiological events that can strongly modulate gene expression in neurons and glia, including episodes of spreading depression that occur at the time of the injury, seizures that occur during the early postlesion period, the loss of afferent drive which leads to decreases in postsynaptic activity, and the restoration of activity that occurs in conjunction with reinnervation. We describe recent studies which suggest that some of these physiological events occur to a variable extent in different animals, especially the episodes of spreading depression and the recurrent seizures. Thus, the spatial pattern and temporal dynamics of altered gene expression following this "model" experimental injury may vary from animal to animal. The fact that physiological events strongly modulate the reactive changes in gene expression that occur following injury has important implications for understanding the sequelae of injury, and offers new opportunities for experimental and therapeutic interventions that may improve cellular repair, regeneration, and recovery of function.

Memory beyond the hippocampus

Improved neuroanatomical knowledge, technical and methodological innovations (such as PET), and more refined conceptualizations of memory have inspired a reappraisal of theoretical beliefs regarding the role of the hippocampus in memory. In the past few years, it has become apparent that the influence of the medial temporal lobe regions extends beyond memory and that memory processes (such as encoding, consolidation and retrieval) involve not only the hippocampus and the medial temporal and diencephalic regions, but also widely distributed neocortical and perhaps even cerebellar regions.

Differential temporal evolution of post-training changes in regional brain glucose metabolism induced by repeated spatial discrimination training in mice: visualization of the memory consolidation process?

The present study analyses the effects of the stage of learning on the spatial patterns and time-course of [14C]glucose uptake in BALB/c mice brain regions produced by spatial discrimination training in an eight-arm radial maze. Our particular approach was designed to follow, during the post-training period, the level of functional activity in individual brain areas which may underlie the memory consolidation process. Regional mapping of relative [14C]glucose uptake was assessed at three post-training time intervals (5 min, 1 and 3 h) after either the first (Day 1), the fourth (Day 4) or the last (Day 9) daily training session of the discrimination task and compared with sham-conditioned animals placed in the same experimental environment. The results indicated that numerous subcortical and cortical brain regions exhibit metabolic alterations following the acquisition of the spatial discrimination task. These alterations, which were specifically related to learning since they did not appear in sham-conditioned animals, were functions both of the post-training interval studied and of the degree of mastery of the task. On Day 1, a progressive, time-dependent and sequential increase in labelling was found from subcortical (5 min post-training) to cortical regions (3 h post-training). On Day 4, a peak of cortical metabolic activation was identified at 1 h post-training. In contrast, on Day 9, maximum labelling was found 5 min post-training in all subcortical and cortical regions followed by a general monotonic decline at 1 and 3 h post-training. These findings, which show widely distributed changes of metabolic activity in the brain, are consistent with the hypothesis that learning involves distributed neural networks. The sequential activation from subcortical to cortical regions seems to indicate a general mechanism whose function would ultimately be to store cortical memory representations. The acquisition-dependent shifts in the patterns of post-training metabolic labelling observed as a function of task mastery may be taken to represent a visualization of the spatio-temporal evolution of the networks of brain structures actively engaged in the memory consolidation process. In particular, the present data suggest that the duration of post-acquisition memory processing is a function of the quantity of new information which has to be dealt with by the central nervous system.

Isolation of a monoclonal antibody reactive to brain microsome-associated antigen, lap-1, and its preferential localization in limbic system of rat brain

The distribution and subcellular localization of AH9 antigen, recognized by a monoclonal antibody AH9, were examined in rat brain. Highest expression was observed in the lamina lucidum of the dentate gyrus of the rat hippocampus. Synaptic subfields of other limbic areas also expressed AH9 antigen at a substantial level. The molecular size of the AH9 antigen is 15 kDa and it was found in the microsomal fraction of brain but not of heart or kidney. These results indicate that AH9 antigen is a novel synaptosomal protein that is relatively specific to the limbic system, at least in the rat brain. We designated AH9 antigen as a limbic system associated protein-1, lap-1.

Regional zinc staining in postmortem hippocampus from schizophrenic patients

Schizophrenia-associated deficits in learning and memory have been associated with a decrease in the volume of the hippocampus, but the specific nature of the neuronal deficit remains unknown. Many critical afferent pathways in the hippocampus contain ionic zinc. Alterations of these pathways could be manifest as a decrease in ionic zinc levels within hippocampal afferent pathways. This possibility was examined in postmortem hippocampal tissue taken from schizophrenic patients, patients with other psychiatric disorders and matched, non-mentally ill subjects using a modified Timm's silver staining method. The three groups exhibited similar patterns of zinc staining within the hippocampal formation as well as similar levels of zinc within the mossy fiber projection system. A greater prevalence of zinc staining within the inner molecular layer of the dentate gyrus was observed in female as compared to male donors and in older as compared to younger donors. The results of the present study demonstrate that loss of ionic zinc within the hippocampus does not appear to be part of the pathology of schizophrenia.

Spatial memory deficits following stimulation of hippocampal 5-HT1B receptors in the rat

In this study we examined a possible contribution of serotonin (5-hydroxytryptamine, 5-HT) to spatial memory performance in the rat. Rats were trained to run in a radial maze in a manner that involved two kinds of memory function, i.e. working memory and reference memory. They received intrahippocampal microinjections of a 5-HT1A [8-hydroxy-2-(di-n-propylamino)tetralin or 8-OH-DPAT], or a 5-HT1B [3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one or CP-93,129] receptor agonist, and a muscarinic receptor antagonist (scopolamine). 8-OH-DPAT (5 micrograms/microliters), like injections of saline, induced no change in performance levels. In contrast, rats suffered an impairment in both reference and working memory following injection of scopolamine (10 micrograms/microliters). CP-93,129 induced a higher frequency of reference memory errors than of working memory errors at the intermediate (10 micrograms/microliters) and higher doses (16 micrograms/microliters). Thus, the stimulation of 5-HT1B receptors in the CA1 field of the dorsal hippocampus impairs the performance of rats in a spatial learning task.

Changes in exploratory activity following stimulation of hippocampal 5-HT1A and 5-HT1B receptors in the rat

The object exploration task allows the measure of changes in locomotor and exploratory activities, habituation, and reaction to a spatial change and to novelty. The effects of intrahippocampal (dorsal CA1 field) microinjections of serotonin 1 receptor (5-HT1) agonists on these behavioral components were evaluated in the rat. 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, 5 micrograms/microliters) was used as a 5-HT1A agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93,129,16 micrograms/microliters) as a 5-HT1B agonist, and scopolamine (10 micrograms/microliters) as a muscarinic cholinergic antagonist. Scopolamine induced a long-lasting increase in locomotor activity and a lack of reaction to spatial change; both these results are in agreement with the known crucial influence of the septo-hippocampal cholinergic system in hippocampal functioning. Stimulation of 5-HT1A and 5-HT1B receptors induced a decrease in object exploration and habituation without affecting the retrieval of spatial information. But stimulation of hippocampal 5-HT1B receptors induced a selective change in the animal's emotional state, i.e., an initial decrease in locomotor activity and a neophobic reaction in response to a new object; such effects did not occur following stimulation of 5HT1A receptors. These results have to be considered in the light of the anxiogenic property of 5-HT1B agonists. On the whole, they support the hypothesis of the involvement of the serotonergic system, via 5HT1A and 5-HT1B receptors, in the modulation of hippocampal functions.

Serotonin 1B receptor regulation after dorsal subiculum deafferentation

The subiculum may be the key structure in the transfer of relevant processed information from the hippocampal formation to cortical areas. We investigated the location of the serotonin 1B receptor (5-HT1B) in the hippocampus with the specific ligand serotonin-O-carboxymethyl-glycyl[125I]tyrosinamide in rat brain sections using in vitro autoradiography. A high density of 5-HT1B binding sites was found in the dorsal subiculum (DS), in the lacunosum moleculare, and in the most dorsal layer of the stratum oriens of the CA1 field. CA1 pyramidal neurons that contain 5-HT1B mRNA project primarily to the DS. We interrupted the pyramidal CA1 axons unilaterally by a stereotaxic knife cut. Histological analysis showed that the lesion was restricted to a trial of cells lost between CA1 and DS. Specific 5-HT1B binding site density was decreased in the DS on the ipsilateral side of the lesion compared to the contralateral side. We conclude that 5-HT1B receptors are located on CA1 pyramidal axon terminals in the DS. Serotonin, acting on these receptors, should inhibit CA1 neurotransmitter release and, in this way, modulate subicular functions.

Differential involvement of hippocampal G-protein subtypes in the memory process of rats

Inactivation of Gi and G(o) proteins in the dentate gyrus of the hippocampus by pertussis toxin did not affect memory retention of a one-way passive avoidance learning task in rats. Interference of normal Gs activity in the dentate gyrus by cholera toxin impaired retention performance dose-dependently. Cholera toxin also antagonized the memory-enhancing effect of corticotropin-releasing factor in the hippocampus. However, although Gi and G(o) proteins are probably not involved in the memory consolidation process per se, in animals showing a full retention score there was a significant and long-lasting increase of G(o) concentration in the dentate gyrus. Results of ADP-ribosylation experiments have shown that there was a dose-dependent decrease of ADP-ribosylation in vitro as the concentration of in vivo pertussis toxin and cholera toxin increased. These results together suggest that Gs protein is probably involved in the initiation of the memory consolidation process, while enhanced G(o) expression is the ultimate result upon memory formation. These results provide the first in vivo evidence relating the functions of hippocampal G proteins to the memory process of mammals.

The organotypic entorhinal-hippocampal complex slice culture of adolescent rats. A model to study transcellular changes in a circuit particularly vulnerable in neurodegenerative disorders

The entorhinal-hippocampal system is severely altered in many neurodegenerative disorders with mnemonic malfunction, e.g. Alzheimer's, Parkinson's and Huntington's disease. The present approach characterizes an organotypic complex slice culture comprising both the entorhinal cortex and the hippocampal formation in order to establish a tool for experimental studies of the entorhinal-hippocampal interaction and its presumed neurodegenerative alterations in vitro. Slices were obtained from rats at about postnatal day 15 and maintained in culture using the interface technique. Thus, also structures known to be developed gradually during the first weeks postnatally are in accord to structures seen in adult rats. After two-three weeks in vitro, slices in the culture dish still revealed the typical morphological features of the entorhinal-hippocampal formation as visible with the dissecting microscope. Biocytin, which is taken up by and transported within living cells, labeled typical cell bodies, dendrites and axons of stellate neurons in layer II and pyramidal cells in layer III when applied to the outer layers of the entorhinal cortex. Small injections of biocytin within the dentate gyrus displayed living granule cells and the maintenance of their projection to the pyramidal cells in CA3, i.e., a typical suprapyramidal plexus of mossy fibers. The presence of axons of entorhinal neurons traveling towards the hippocampus and growth cones traversing the deep layers of the entorhinal cortex indicate that both brain regions are still interacting. Immunocytochemistry for calbindin D-28K revealed labeled neurons in layer II of the entorhinal cortex and dentate granule cells which are known to contain this calcium-binding protein.