Functional role of parallel circuits in the hippocampus

The hippocampal formation, one of the most complex and vulnerable brain structures, is recognized as a crucial brain area subserving human long-term memory. Yet, its specific functions in memory are controversial. Recent experimental results suggest that the hippocampal contribution to human memory is limited to episodic memory, novelty detection, semantic (deep) processing of information, and spatial memory. We measured the regional cerebral blood flow by positron-emission tomography while healthy volunteers learned pairs of words with different learning strategies. These led to different forms of learning, allowing us to test the degree to which they challenge hippocampal function. Neither novelty detection nor depth of processing activated the hippocampal formation as much as semantically associating the primarily unrelated words in memory. This is compelling evidence for another function of the human hippocampal formation in memory: establishing semantic associations.

To study the effects of prenatal and postnatal perfluorooctane sulfonate (PFOS) exposure on spatial learning and memory, N-methyl-D-aspartate receptor 2B (NR2B) mRNA and protein level in frontal cortex and hippocampus of rat pups and to explore the mechanism of developmental neurotoxicity induced by PFOS. Twenty-eight pregnant rats were randomly divided into three groups in proportion of 3:2:2, including control group (C), low dose group (L) and high dose group (H) by means of randomized number table, which respectively received 0, 7.2, 14.4 mg/kg PFOS feed from pregnancy day 0 to postnatal day (PND) 30 by free feedings. The animal models of prenatal and postnatal non-exposure (CC), prenatal exposure (LC and HC), postnatal exposure (CL and CH), and prenatal and postnatal exposure (LL and HH) to PFOS were established by cross-fostering method. The spatial learning and memory were measured by water maze experiment,the NR2B mRNA levels in frontal cortex of rat pups was determined with semi-quantitative RT-PCR, NR2B protein express in cerebral cortex (frontal and temporal cortex) and hippocampus (CA1, CA3, CA4 and DG regions) of rat pups was detected by immunohistochemistry. The escape latency of CL, CH, LL and HH groups pups in water maze experiment were (99.83 +/- 25.77) s, (111.30 +/- 17.82) s, (106.40 +/- 18.71) s, (107.70 +/- 16.85) s, and longer as compared with CC group [(54.90 +/- 26.69) s] (q value were 4.349, 4.773, 6.026 and 5.641, respectively, P <0.01). The number of errors of HH group rat pups entering dead end was (22.30 +/- 7.56) at the training day 4, and it was significantly higher than that of CC group (9.80 +/- 4.64) (q = 5.173, P < 0.01). The NR2B mRNA levels of frontal cortex of pups in HC group at PND1, and LC group, HC group and HH group at PND14 were (0.167 +/- 0.008), (0.364 +/- 0.035), (0.341 +/- 0.030) and (0.328 +/- 0.045) respectively,which were significantly lower than CC group (0.271 +/- 0.060) and (0.465 +/- 0.067) (q values were 3.547, 3.739, 4.597 and 5.006, respectively, P< 0.05 ). The results of immunohistochemistry indicated that NR2B protein express of the hippocampus CA1 region of pups in LC group was (0.091 +/- 0.005), and showed significant lower than CC group which was (0.123 +/- 0.009) at PND1 (q = 5.209, P <0.05). At PND14, the effect of PFOS extended to cerebral cortex and hippocampus regions. At PND28, the effects of PFOS were showed in hippocampus CA1, CA3 and temporal cortex regions. Prenatal and postnatal exposure to PFOS should result in the spatial learning and memory damage,and the mechanism might be possibly involved in the decrease of NR2B level in cerebral cortex and hippocampal formation regions.

The research field of functional magnetic resonance imaging (fMRI) has made possible a remarkable progress in the understanding of the human brain enabling neuroscientists to study spatio-temporal alterations in the healthy and the diseased brain. While current theories of schizophrenia stress the critical role that plays aberrant connectivity among brain regions, other theories point towards the crucial role that plays functional excitation-inhibition (E-I) balance. Indeed, recent neuroscientific research has revealed increasing evidence that taking functional brain connectivity into account is essential to understand how the human brain works, and many studies have reviewed that serious behavioural impairments in mental disorders such as schizophrenia result from increases in the functional (E-I) balance within the neural microcircuitry. Particularly, the connection between the dorsolateral prefrontal cortex (DLPFC) and the hippocampal formation (HF) during working memory (WM) was found to be increased in carriers of schizophrenia risk genes and patients. However, less is known about causality, i.e. which region drives the altered connection. Stochastic Dynamic Causal Modelling (sDCM) is a novel mathematical algorithm for studying the causal connectivity among higher cognitive brain regions from fMRI data. The main purpose of this study is to identify alterations on genetic risk carriers and patients from the DLPFC-HF network estimated with sDCM and describe how these alterations have an impact on the behavior. Over the study, we strive to give to the sDCM parameter estimates a neurobiological explanation by linking the concepts of causal connectivity with functional (E-I) balance. In this work, we applied this methodology in two samples by constructing a systematic set of sDCMs describing interactions between right DLPFC and left HF. In a first sample, 180 healthy subjects were measured by fMRI during a standard working memory N-Back task at three different sites (Mannheim, Bonn, Berlin; each with 60 participants). Bayes Model Selection (BMS) revealed the same causal pattern or winning model across the three sites, with the 2-Back working memory condition as driving input to both DLPFC and HF and with a connection from DLPFC to HF. Furthermore, a genome-wide risk variant for schizophrenia: ZNF804A (rs1344706), showed a strong impact on the DLPFC-HF network. On the one hand, risk homozygotes showed higher effective connectivity or higher functional (E-I) balance between DLPFC and HF. On the other hand, risk allele carriers showed higher functional (E-I) balance on the self-connection in the DLPFC. In a second sample, 33 schizophrenia patients were measured by fMRI during the same working memory N-Back task. We pair-wise matched healthy volunteers of the first sample and patients and applied the same methodology. BMS revealed the same winning model in patients but sDCM parameter estimates differed significantly between groups. Patients showed higher functional (E-I) balance on both self-connections in comparison to healthy volunteers. In summary, we observed that risk allele carriers and patients have a higher functional (E-I) balance within the DLPFC-HF network. In view of these research findings, we hypothesized a possible biological functioning of ZNF804A (rs1344706) on the DLPFC-HF network and suggested a mechanistic model for explaining the underlying neurobiology of schizophrenia within this network. Then, we reported causal relations between sDCM parameter estimates and behavior in terms of functional (E-I) balance in both samples. On the one hand, we observed that risk allele carriers and patients require lower functional (E-I) balance on the DLPFC-HF network in order to achieve the best performance during the task. On the other hand, we found that healthy volunteers require higher functional (E-I) balance on the network in order to achieve the optimal behavior. This study investigated the applicability of computational models like sDCM to establish the functional significance of specific genetic polymorphisms for schizophrenia and identify causal mechanisms associated with the disease in relation to the underlying neurobiology and behavior. In forthcoming studies, we plan to investigate whether subject-specific directed connections strengths between DLPFC and HF, and genotype, contain sufficiently rich information to enable accurate predictions of behavior. In order to study how temporal patterns in the neuronal ensembles and genotype convey robust information about behavior, multivariate regressors or statistical decoding algorithms will be used in both samples.

The extent to which individual neurons are interconnected in a selective manner within brain circuits is an unsolved problem in neuroscience. Two opposite views posit dedicated labeled lines of specifically interconnected neurons, versus tabula rasa models of randomly interconnected networks. Even in dense mammalian cortical circuits, apparently equivalent neurons can be organized into preferentially interconnected microcircuits. However, it has remained unclear whether microcircuits might reflect genetically defined subpopulations of selectively interconnected neurons, as opposed to self-organizing random networks. Here we show that the principal neurons of the major hippocampal subdivisions consist of genetically distinct subpopulations that interconnect selectively across subdivisions. In two Thy1 mouse lines, transgene expression in each subdivision visualizes matched principal neuron subpopulations that exhibit unique patterns of gene expression, and share neurogenesis windows, and temporal schedules of synaptogenesis. Marker genes shared among the matched subpopulations map near olfactory receptor gene clusters, a property which we find preferentially associated with neuronal subtype markers. Matched subpopulations exhibit selective connectivity at mossy fiber-to-pyramidal neuron synapses in CA3. Our results provide genetic, developmental and anatomical evidence for the existence of selectively interconnected principal neuron subpopulations in a cortical structure. The results further suggest that unique and co-ordinate schedules of neurogenesis and circuit assembly may underlie the establishment of specific microcircuits in the brain. &#13;\n

Structural synaptic changes have been suggested to underlie long-term memory formation. In this work, we investigate if hippocampal mossy fiber synaptogenesis induced by water maze overtraining can be related with long-term spatial memory performance. Rats were trained in a Morris water maze for one to five identical daily sessions and tested for memory retrieval 1 week and 1 month after training. After the last test session, the rat brains were obtained and processed for Timm's staining to analyze mossy fiber projection. The behavioral results showed that with more training, animals showed a better performance in the memory tests, and this performance positively correlates with Timm's staining in the stratum oriens. Furthermore, with the use of the NMDA antagonist MK801 before, but not after acquisition, water maze spatial memory was impaired. Increased Timm's staining in the stratum oriens was observed in the animals treated with MK801 after acquisition but not in those treated before. Finally, we observed that mossy fiber synaptogenesis occurs mainly in the septal region of the dorsal hippocampus, supporting the idea that this anterior region is important for spatial memory. Altogether, these results suggest that mossy fiber synaptogenesis can be related with spatial long-term memory formation.

Progress on the hippocampal circuits and functions based on sharp wave ripples

Objective To investigate the effects of hippocampal neurogenesis in spatial learning and memory，especially in spatial long-term memory. Methods Totally 24 wistar rats were randomly divided into two groups: normal control group (NC group, n=12) and irradiation group (IRR group, n=12). We use low-dose irradiation at subgranular zone to inhibit adult hippocampal neurogenesis and new neurons were investigated by 5-Bromo-2-deoxyUridine/neuron specific nuclear protein double-labeling. Four weeks after irradiation, Morris water maze, including navigation test and space exploration assay, was used to detect spatial learning and memory. Then, in the day 8 and day 15 after the navigation test, space exploration assay was repeated to detect spatial long-term memory. The expression levels of postsynaptic density protein 95 and synaptophysin were evaluated using western blot and real-time quantitative PCR. Results Hippocampal neurogenesis was inhibited by low dose irradiation（2.80±0.44 vs 23.50±1.12, t=20.21, P<0.01）. After the inhibition, the escape latency did not change, while the time spent in target quadrant was significantly declined in the day 2(（14.76±2.04) s vs (20.47±1.29） s), day 8(（11.95±1.34) s vs (19.52±1.43） s)and day 15(（11.79±1.35) s vs (21.58±1.07） s) after the navigation test （t=2.45, P<0.05; t=3.76, P<0.01; t=5.39,P<0.01）. The postsynaptic density protein 95 and synaptophysin expressions were significantly decreased in IRR group. As to the RNA levels, there was also a significant difference between the two groups. Conclusion Hippocampal neurogenesis in the dentate gyrus plays an important role for the formation of spatial long-term memory. Key words: Neurogenesis; Dentate gyrus; Learning; Memory

Long-range synchrony between medial prefrontal cortex, thalamus and hippocampus underlies working memory behavior in mice. Pia-Kelsey O’Neill Presently, there are no antipsychotic drugs capable of treating the cognitive dysfunctions of schizophrenia. In order to inform the development of better therapies, it is essential to understand the mechanism behind dysfunctional cognition, which requires an understanding of functional cognition. Spatial working memory, a measure of cognitive function, can be assessed in the mouse using a task of delayed alternation: the T-maze. In this thesis, I focus on spatial working memory behavior in the mouse and three brain regions that are implicated in this behavior: the medial prefrontal cortex (mPFC), the hippocampus (HPC) and the medial dorsal thalamus (MD). Lesion and electrophysiological studies in each structure have demonstrated their importance during working memory behavior. Disconnection studies also show that the coordination between the mPFC and either the HPC or MD is important for the behavior, but little is known about the mechanism by which they coordinate. The MD and the ventral region of the hippocampus (vHPC) have robust projections into the mPFC. They are therefore in a good position to influence mPFC activity. Previous reports show that the mPFC and the dorsal region of the hippocampus (dHPC) synchronize activity in the theta range (4-12 Hz) with working memory demand. However, the dHPC does not directly connect with the mPFC so it is unclear how this coordination occurs. We hypothesized that the vHPC may also be involved in spatial working memory behavior and that it may mediate the dHPC-mPFC theta synchrony observed. To test these hypotheses, we recorded neural activity simultaneously from the mPFC, dHPC and vHPC in mice performing the T-maze task. Local field potential oscillations (LFPs), thought to be a measure of synchronized synaptic activity, were obtained from each area. We observed an increase in theta synchrony between the mPFC and both the dHPC and vHPC. Removing the influence of vHPC both analytically and experimentally, we found a decrease in synchrony of the dHPC-mPFC. Aside from the disconnection studies, little is known about the MD-mPFC pathway in rodents. However, due to evidence from schizophrenia patients of altered correlation specifically between the MD and PFC, we hypothesized that an electrophysiological correlate of working memory exists in the MD-mPFC pathway as well and that a decrease in MD activity may lead to prefrontal dysfunction. To test these hypotheses, we recorded LFPs from the mPFC and both single unit activity and LFPs from the MD in mice performing the T-maze task. We observed an increase in phase locking of MD cells to mPFC LFPs in beta (13-30Hz) range during the choice phase of the task. We then utilized a pharmacogenetic technique to decrease firing rate in a small portion of MD cells, which resulted in a deficit in both task acquisition and performance. The increase in MD-mPFC beta phase locking we had observed was not present in MDinactivated animals. Interestingly, beta coherence between the two structures across learning was highly correlated with choice accuracy on the task. This suggests that MD-PFC coordination is predictive of working memory performance. These findings illustrate how long-range synchrony of the mPFC with HPC in the theta frequency range and with the MD in the beta frequency range may be important markers for normal working memory behavior and if disrupted in humans, could contribute to the cognitive symptoms of schizophrenia.

Granulocyte/macrophage colony stimulating factor (GM-CSF) is a growth factor originally discovered in the hematopoietic system. Recently presence of GM-CSF and its receptor (GM-CSFR) was reported on CNS neurons. Here we investigated whether this system is involved in cognition by evaluating learning and memory formation in GM-CSF deficient (GMko) mice. In a complementary approach, GM-CSF signalling was bidirectionally manipulated specifically in the adult hippocampus by adeno-associated virus (AAV) mediated GM-CSF receptor alpha (GM-CSFRalpha) overexpression or knock-down. In GMko mice, broad cognitive deficits were observed in all behavioural paradigms tested. Animals showed impairments in fear memory to conditioned cue, in spatial learning in an active place avoidance paradigm, and in spatial memory and re-learning flexibility in the Morris Water Maze. Additionally, knock-outs also showed diminished exploratory drive. The inferior cognitive performance was not due to secondary factors such as motor function, inherent anxiety, or pain threshold levels. Corroborating these data, spatial memory of AAV-injected mice in the MWM was enhanced or impaired dependent on GM-CSFR alpha overexpression or knock-down, respectively. GMko mice showed diminished overall dendritic length and branching complexity in the dentate gyrus (DG) and reductions in both dendritic spine density and the percentage of mature spines in the hippocampus. Despite these morphological alterations and the strong memory deficits, induction of long-term potentiation (LTP) was not impaired in the hippocampus of knock-out mice, providing an example for the complicated relationship of LTP and memory performance. Collectively, these results suggest that GM-CSF signalling plays a major and hitherto unrecognized role in neuronal structural plasticity relevant to learning and memory. This finding is of particular therapeutic relevance, as GM-CSF readily passes the intact blood-brain barrier.

The acquisition of special skills can induce plastic changes in the human hippocampus, a finding demonstrated in expert navigators (Maguire et al. (2000) Proc Natl Acad Sci USA 97:4,398-403). Conversely, patients with acquired chronic bilateral vestibular loss develop atrophy of the hippocampus, which is associated with impaired spatial memory (Brandt et al. (2005) Brain 128:2,732-741). This suggests that spatial memory relies on vestibular input. In this study 21 professional dancers and slackliners were examined to assess whether balance training with extensive vestibulo-visual stimulation is associated with altered hippocampal formation volumes or spatial memory. Gray matter voxel-based morphometry showed smaller volumes in the anterior hippocampal formation and in parts of the parieto-insular vestibular cortex of the trained subjects but larger volumes in the posterior hippocampal formation and the lingual and fusiform gyri bilaterally. The local volumes in the right anterior hippocampal formation correlated negatively and those in the right posterior hippocampal formation positively with the amount of time spent training ballet/ice dancing or slacklining at the time of the study. There were no differences in general memory or in spatial memory as assessed by the virtual Morris water task. Trained subjects performed significantly better on a hippocampal formation-dependent task of nonspatial memory (transverse patterning). The smaller anterior hippocampal formation volumes of the trained subjects may be the result of a long-term suppression of destabilizing vestibular input. This is supported by the associated volume loss in the parieto-insular vestibular cortex. The larger volumes in the posterior hippocampal formation of the trained subjects might result from their increased utilization of visual cues for balance. This is supported by the concomitant larger volumes in visual areas like the lingual and fusiform gyri. Our findings indicate that there is a spatial separation of vestibular and visual processes in the human hippocampus.

In the last decades a standard model regarding the function of the hippocampus in memory formation has been established and tested computationally. It has been argued that the CA3 region works as an auto-associative memory and that its recurrent fibers are the actual storing place of the memories. Furthermore, to work properly CA3 requires memory patterns that are mutually uncorrelated. It has been suggested that the dentate gyrus orthogonalizes the patterns before storage, a process known as pattern separation. In this study we review the model when random input patterns are presented for storage and investigate whether it is capable of storing patterns of more realistic entorhinal grid cell input. Surprisingly, we find that an auto-associative CA3 net is redundant for random inputs up to moderate noise levels and is only beneficial at high noise levels. When grid cell input is presented, auto-association is even harmful for memory performance at all levels. Furthermore, we find that Hebbian learning in the dentate gyrus does not support its function as a pattern separator. These findings challenge the standard framework and support an alternative view where the simpler EC-CA1-EC network is sufficient for memory storage.

Objective To investigate the effect of schisandrin (SCH) treatment on learning and memory abilities and their mechanism in APP/PS1 dual transgenic dementia mice, and explore the effect of Chinese medicine on Alzheimer's disease (AD). Methods Thirty-five APP /PS1 dementia mouse models were randomly assigned into APP/PS1 model group (n=17) and APP/PS1+SCH group (n=18); another 10 male C57BL/6J mice were chosen as blank control group. The mice in the APP/PS1+SCH group were given intragastric administration of SCH at 2.6 mg/(kg·d) for 30 d; the mice in the APP/PS1 model group and blank control group were treated with distilled water for 30 d. The learning and memory abilities of these APP/PS1 mice (n=7) were detected by Morris water maze. Mice from the three groups were sacrificed; Nissl staining was used to observe Nissl bodies of neurons in brain tissues; real-time fluorescence quantitative PCR (qPCR) was used to detect the mRNA content of terminal glycosylation end products receptor (RAGE) in brain tissues; Western blotting was used to detect the expressions of RAGE and phosphorylated P38 mitogen-activated protein kinase (p-p38) in brain tissues. Results (1) The results of water maze space exploration experiment showed that the times of crossing the platform area in the three groups were statistically significant (P<0.05); as compared with the APP/PS1 model group, the times of crossing the platform area in the APP/PS1+SCH group were significantly increased (P<0.05). (2) Nissl staining results showed that the contents of Nissl bodies in the hippocampal CA1 area and cortical neurons of the APP/PS1 model group were significantly reduced, with light staining and cell body atrophy; the lesions in mice of the APP/PS1+SCH group were less severe than those of APP/PS1 model group, some neurons were atrophic, and the content of the neuronal nileite bodies in the hippocampal CA1 region was relatively abundant. (3) The qPCR results showed that there were statistically significant differences in RAGE mRNA expression levels in the cortex and hippocampus of the three groups (P<0.05); as compared with the APP/PS1 model group, the APP/PS1+SCH group had significantly reduced RAGE mRNA expression in the hippocampal area (P<0.05). (4) Western blotting results showed that RAGE and p-p38 protein expression levels in two parts of mice of APP/PS1+SCH group were significantly reduced as compared with those in the APP/PS1 model group (P<0.05). Conclusion SCH may improve the functional status of hippocampal and cortical neurons and improve the spatial exploratory memory ability of APP/PS1 mice by down regulating the RAGE and P38 expressions. Key words: Schisandrin; Alzheimer's disease; Receptor for advanced glycation end product; p38

Allocentric spatial memory in humans with hippocampal lesions

We carried out a study of the expression patterns of seven developmental regulatory genes (Lef1, Lhx2, Lhx9, Lhx5, Lmo3, Lmo4, and Prox1), in combination with topological position, to identify the medial pallial derivatives, define its major subdivisions, and compare them between mouse and chicken. In both species, the medial pallium is defined as a pallial sector adjacent to the cortical hem and roof plate/choroid tela, showing moderate to strong ventricular zone expression of Lef1, Lhx2, and Lhx9, but not Lhx5. Based on this, the hippocampal formation (indusium griseum, dentate gyrus, Ammon's horn fields, and subiculum), the medial entorhinal cortex, and part of the amygdalo-hippocampal transition area of mouse appeared to derive from the medial pallium. In the chicken, based on the same position and gene expression profile, we propose that the hippocampus (including the V-shaped area), the parahippocampal area (including its caudolateral part), the entorhinal cortex, and the amygdalo-hippocampal transition area are medial pallial derivatives. Moreover, the combinatorial expression of Lef1, Prox1, Lmo4, and Lmo3 allowed the identification of dentate gyrus/CA3-like, CA1/subicular-like, and medial entorhinal-like comparable sectors in mouse and chicken, and point to the existence of mostly conserved molecular networks involved in hippocampal complex development. Notably, while the mouse medial entorhinal cortex derives from the medial pallium (similarly to the hippocampal formation, both being involved in spatial navigation and spatial memory), the lateral entorhinal cortex (involved in processing non-spatial, contextual information) appears to derive from a distinct dorsolateral caudal pallial sector.

The distribution of the major cholinergic regulatory enzyme acetylcholinesterase (AChE, EC 3.1.1.7) has been extensively studied in the human brain, but the distribution of the closely related enzyme butyrylcholinesterase (BuChE, EC 3.1.1.8) is largely unknown. Because of the importance of BuChE and AChE in Alzheimer's disease, we have studied the distribution of BuChE in the normal human amygdala and hippocampal formation and compared it with that of AChE by using histochemical techniques. In the amygdala, the distribution of BuChE differed significantly from that of AChE in that BuChE was found primarily in neurons and their dendritic processes, whereas AChE was found predominantly in the neuropil. BuChE-positive neurons were present in up to 10% of the neuronal profiles in lateral, basolateral (basal), basomedial (accessory basal), central, cortical, and medial amygdaloid nuclei. AChE was found primarily in the neuropil in these nuclei with only a few AChE-positive neurons. In the hippocampal formation, BuChE was also found in neurons and not in the neuropil, whereas AChE was found in both neurons and in the neuropil. BuChE and AChE neurons were present in the polymorphic layer of the dentate gyrus, as well as the stratum oriens and stratum pyramidale of the hippocampus proper. There was considerable overlap in shapes, sizes, and numbers of BuChE- and AChE-positive neurons, suggesting that the enzymes were colocalized in neurons of the hippocampal formation. The distinct distribution of BuChE suggests that it may have specific functions including coregulation of cholinergic and noncholinergic neurotransmission in human amygdala and hippocampal formation.