Moderate traumatic brain injury causes acute dendritic and synaptic degeneration in the hippocampal dentate gyrus

Hippocampal injury-associated learning and memory deficits are frequent hallmarks of brain trauma and are the most enduring and devastating consequences following traumatic brain injury (TBI). Several reports, including our recent paper, showed that TBI brought on by a moderate level of controlled cortical impact (CCI) induces immature newborn neuron death in the hippocampal dentate gyrus. In contrast, the majority of mature neurons are spared. Less research has been focused on these spared neurons, which may also be injured or compromised by TBI. Here we examined the dendrite morphologies, dendritic spines, and synaptic structures using a genetic approach in combination with immunohistochemistry and Golgi staining. We found that although most of the mature granular neurons were spared following TBI at a moderate level of impact, they exhibited dramatic dendritic beading and fragmentation, decreased number of dendritic branches, and a lower density of dendritic spines, particularly the mushroom-shaped mature spines. Further studies showed that the density of synapses in the molecular layer of the hippocampal dentate gyrus was significantly reduced. The electrophysiological activity of neurons was impaired as well. These results indicate that TBI not only induces cell death in immature granular neurons, it also causes significant dendritic and synaptic degeneration in pathohistology. TBI also impairs the function of the spared mature granular neurons in the hippocampal dentate gyrus. These observations point to a potential anatomic substrate to explain, in part, the development of posttraumatic memory deficits. They also indicate that dendritic damage in the hippocampal dentate gyrus may serve as a therapeutic target following TBI.     

Short-term synaptic plasticity in the dentate gyrus of monkeys

The hippocampus plays an important role in learning and memory. Synaptic plasticity in the hippocampus, short-term and long-term, is postulated to be a neural substrate of memory trace. Paired-pulse stimulation is a standard technique for evaluating a form of short-term synaptic plasticity in rodents. However, evidence is lacking for paired-pulse responses in the primate hippocampus. In the present study, we recorded paired-pulse responses in the dentate gyrus of monkeys while stimulating to the medial part of the perforant path at several inter-pulse intervals (IPIs) using low and high stimulus intensities. When the stimulus intensity was low, the first pulse produced early strong depression (at IPIs of 10-30 ms) and late slight depression (at IPIs of 100-1000 ms) of field excitatory postsynaptic potentials (fEPSPs) generated by the second pulse, interposing no depression IPIs (50-70 ms). When the stimulus intensity was high, fEPSPs generated by the second pulse were depressed by the first pulse at all IPIs except for the longest one (2000 ms). Population spikes (PSs) generated by the second pulse were completely blocked or strongly depressed at shorter IPIs (10-100 or 200 ms, respectively), while no depression or slight facilitation occurred at longer IPIs (500-2000 ms). Administration of diazepam slightly increased fEPSPs, while it decreased PSs produced by the first pulse. It also enhanced the facilitation of PSs produced by the second stimulation at longer IPIs. The present results, in comparison with previous studies using rodents, indicate that paired-pulse responses of fEPSPs in the monkey are basically similar to those of rodents, although paired-pulse responses of PSs in the monkey are more delayed than those in rodents and have a different sensitivity to diazepam.     

Electron microscopic 3D-reconstruction of dendritic spines in cultured hippocampal neurons undergoing synaptic plasticity

Dendritic spines are assumed to constitute the locus of neuronal plasticity, and considerable effort has been focused on attempts to demonstrate that new memories are associated with the formation of new spines. However, few studies that have documented the appearance of spines after exposure to plasticity-producing paradigms could demonstrate that a new spine is touched by a bona fida presynaptic terminal. Thus, the functional significance of plastic dendritic spine changes is not clearly understood. We have used quantitative time lapse confocal imaging of cultured hippocampal neurons before and after their exposure to a conditioning medium which activates synaptic NMDA receptors. Following the experiment the cultures were prepared for 3D electron microscopic reconstruction of visually identified dendritic spines. We found that a majority of new, 1- to 2-h-old spines was touched by presynaptic terminals. Furthermore, when spines disappeared, the parent dendrites were sometime touched by a presynaptic bouton at the site where the previously identified spine had been located. We conclude that new spines are most likely to be functional and that pruned spines can be transformed into shaft synapses and thus maintain their functionality within the neuronal network.     

Time discrete model of recurrent inhibition in hippocampal dentate gyrus

A time discrete model of recurrent inhibition in the hippocampal dentate gyrus is made and analyzed. This model assumes that (1) each granule cell can generate only one action potential in response to a single stimulation of the perforant path, (2) each interneuron receives synaptic inputs from many granule cells, and (3) an output of the interneuron is inhibitory for granule cells. Although each granule cell generates an action potential in the all-or-none fashion, the population spike is shown to be approximated by a piecewise linear function of the population excitatory post-synaptic potential (EPSP). From this model six patterns in the population spike responses to the paired-pulse stimulation are deduced. Each pattern is composed of some broken lines whose slopes and intercepts are explicitly expressed by the average and variance of the microscopic parameters and the population size of the cells. This model clarifies the relation between the measured quantity in the field potential experiment and the microscopic quantities peculiar to the granule cell and to the interneuron.     

Interneurons: an electron microscopic study of the cat's hippocampal formation, II.

The ultrastructure of interneurons was studied in the cat and it was found to differ from that of the pyramidal and granule cells. The size, shape and topographic situation of the cell body of interneurons were the criteria for approaching their identification with the cells found in Golgi material. Some interneuron dendrites are also described. The varicose and spindle-shaped dendritic profiles belonging to interneurons are different in size. In Golgi material the interneurons have beaded (varicose) or spindle-shaped dendrites. The arrangement of synaptic terminals on the interneuron dendritic surface seems to be their characteristic feature.     

Priming of associative long-term depression in the dentate gyrus by theta frequency synaptic activity.

Associative long-term synaptic depression (LTD) was investigated utilizing negatively correlated activity patterns in the medial and lateral perforant path inputs to the dentate gyrus in anesthetized rats. Normally only nonassociative, or heterosynaptic, LTD is elicited in naive pathways. We report here, however, that associative LTD in the lateral path is readily induced after being "primed" by a brief period of lateral path synaptic activity at a theta rhythm frequency (5 Hz). Priming of associative LTD lasts at least 2 hr and is not seen following priming activity at non-theta frequencies (1 and 15 Hz). N-methyl-D-aspartate receptor activation is critical for establishing the priming effect, but not for the subsequent induction of the associative LTD. These data suggest that theta rhythm activity in the dentate gyrus may predispose the system to a specific form of synaptic plasticity, associative LTD.     

Temporal proteomic profile of memory consolidation in the rat hippocampal dentate gyrus

Information storage in the brain depends on the ability of neurons to alter synaptic connectivity within key circuitries such as the hippocampus. Memory-associated synaptic plasticity is mediated by a temporal cascade of de novo protein synthesis and altered protein processing. Here, we have used two-dimensional difference in gel electrophoresis (2-D DIGE) to investigate memory-specific protein changes in the hippocampal dentate gyrus at increasing times following spatial learning. We identified 42 proteins that were significantly regulated in the first 24 h of spatial memory consolidation. Two distinct waves of protein expression regulation were evident, at 3 and 12 h post-learning and this is in agreement with studies employing inhibitors of global translation. Functional classification of the memory-associated proteins revealed that the majority of regulated proteins contributed either to cellular structure or cellular metabolism. For example, actins, tubulins and intermediate filament proteins, core proteins of the three major cytoskeletal components, were dynamically regulated at times that suggest a role in memory-associated synaptic reorganization. Increased proteasome-mediated protein degradation was evident in the early post-training period including the down-regulation of phosphoprotein enriched in astrocytes 15 kDa, a key inhibitor of extracellular signal-regulated kinase signaling. Some of the most substantial protein expression changes were observed for secreted carrier proteins including transthyretin and serum albumin at 6-12 h post-learning, regulations that could serve an important role in increasing the supply of retinoic acid and thyroid hormone, key synaptic plasticity-promoting signals in the adult brain. Together these observations provide further insight into protein level regulations occurring in the hippocampus during spatial memory consolidation.     

The neuronal primary cilium: Driver of neurogenesis and memory formation in the hippocampal dentate gyrus?

Considerable attention has recently been focused on the postnatal persistence of neurogenesis in the dentate gyrus of the hippocampus and the roles of signals from the primary cilium in the different functions of an increasing number of tissues and their malfunctions. Here we summarize the evidence that ties sonic hedgehog-triggered proliferogenic signaling from the primary cilia on granule cell progenitors in the adult dentate subgranular zone to maintain a pool of new “blank slate” dentate granule cells. These can be recruited to bundle and encode novel inputs flowing from various regions of the brain into the dentate gyrus via the entorhinal cortex without altering and erasing the synaptic patterns from previous inputs inscribed on older granule cells.     

(-)-Clausenamide potentiates synaptic transmission in the dentate gyrus of rats

The effect of clausenamide on synaptic transmission in the dentate gyrus of rats in vivo and its possible mechanism of action were investigated in this study. Four of 16 enantiomers showed potentiating effects on basal synaptic transmission in anesthetized animals. By comparing one pair of enantiomers, (-)-clausenamide and (+)-clausenamide, we can report three primary findings: (1) (-)-clausenamide potentiated synaptic transmission in both anesthetized and freely moving animals while (+)-clausenamide showed no or little effect; (2) (-)-clausenamide increased the magnitude of long-term potentiation (LTP) induced by high-frequency stimulation (HFS) in anesthetized animals whereas (+)-clausenamide had no effect; (3) voltage-dependent calcium channels (VDCCs) calcineurin and calpain are involved in (-)-clausenamide-induced potentiation of synaptic transmission. Because hippocampal LTP is thought to reflect a cellular mechanism involved in learning and memory, our findings may provide the pharmacological basis for understanding the nootropic mechanisms of (-)-clausenamide, which is the first chiral nootropic agent developed in China.     

The complement inhibitor CD59 is required for GABAergic synaptic transmission in the dentate gyrus

1. Download : Download high-res image (249KB)
2. Download : Download full-size image

     

Lithium enhances long-term potentiation independently of hippocampal neurogenesis in the rat dentate gyrus

We measured the temporal and spatial profiles of neural precursor cells, hippocampal long-term potentiation (LTP), and signaling molecules in neurogenesis-induced adult rats. Chronic lithium treatment produced a significant 54% and 40% increase in the numbers of bromodeoxyuridine [BrdU(+)] cells after 12 h and 28 days, respectively, after treatment completion in the dentate gyrus (DG). Both LTP obtained from slices perfused with artificial cerebrospinal fluid (ACSF-LTP) and LTP recorded in the presence of bicuculline (bicuculline-LTP) were significantly greater in the lithium group than in the saline controls. Although the number of BrdU(+) cells, approximately 90% of which were double-labeled with a neural marker neuronal nuclear protein, were markedly increased in the granule cell layer (GCL) 28 days after the completion of the 28-day lithium treatment, the magnitude of LTP observed at this time was similar to that observed 12 h after completing the 28-day lithium treatment. However, protein levels of calcium and calmodulin-dependent protein kinase II, p-Elk and TrkB were highly elevated until 28 days after the 28-day lithium treatment. Acute lithium treatment for 2 days also enhanced LTP, which was accompanied by the elevated expression of p-CREB, but not by neurogenesis. Our results suggest that the enhancement of LTP is independent of the increased number of neurons per se and it is more closely associated with key molecules, which are probably involved in neurogenesis.     

Postnatal development of the dentate gyrus: a karyometric and topographic study.

We have analysed the postnatal development of the nuclear sizes of the granular cells of the dentate gyrus in 5- to 190-day-old male mice. The study was performed in three topographic levels: rostral, intermediate and caudal. Three subdivisions were analysed in each level: suprapyramidal blade, infrapyramidal blade and the transition between them, the angular zone. Additionally, each of these subdivisions was measured in its external and internal layer, separately. Three gradients of postnatal karyometric development can be described: external-to-internal, suprapyramidal-to-infrapyramidal, and caudal-to-rostral, indicating that the external, suprapyramidal and caudal cells show higher karyometric sizes than the other subdivisions. These gradients are related to the ontogenetic gradients of these neurons.     

Input integration around the dendritic branches in hippocampal dentate granule cells

Recent studies have shown that the dendrites of several neurons are not simple translators but are crucial facilitators of excitatory postsynaptic potential (EPSP) propagation and summation of synaptic inputs to compensate for inherent voltage attenuation. Granule cells (GCs)are located at the gateway for valuable information arriving at the hippocampus from the entorhinal cortex. However, the underlying mechanisms of information integration along the dendrites of GCs in the hippocampus are still unclear. In this study, we investigated the input integration around dendritic branches of GCs in the rat hippocampus. We applied differential spatiotemporal stimulations to the dendrites using a high-speed glutamate-uncaging laser. Our results showed that when two sites close to and equidistant from a branching point were simultaneously stimulated, a nonlinear summation of EPSPs was observed at the soma. In addition, nonlinear summation (facilitation) depended on the stimulus location and was significantly blocked by the application of a voltage-dependent Ca²⁺ channel antagonist. These findings suggest that the nonlinear summation of EPSPs around the dendritic branches of hippocampal GCs is a result of voltage-dependent Ca²⁺ channel activation and may play a crucial role in the integration of input information.     

维生素E对应激大鼠海马齿状回长时程增强的保护作用

目的：探讨维生素E对应激大鼠海马齿状回长时程增强的保护作用。方法：束缚应激条件下,通过补充维生素E（VE）,观察大鼠在应激过程中的行为效应。结果：接受应激大鼠在旷场实验中的穿行格数明显增加;长时程增强（LTP）诱发率降低,突触传递功能减弱;血浆糖皮质激素水平明显升高。而应激同时适量补充VE的大鼠未出现上述异常变化。结论：适当补充维生素E可减轻应激性海马突触传递功能障碍,提高机体应激适应能力。     

Borna disease virus infection alters synaptic input of neurons in rat dentate gyrus

Granule cells are major targets of entorhinal afferents terminating in a laminar fashion in the outer molecular layer of the dentate gyrus. Since Borna disease virus (BDV) infection of newborn rats causes a progressive loss of granule cells in the dentate gyrus, entorhinal fibres become disjoined from their main targets. We have investigated the extent to which entorhinal axons react to this loss of granule cells. Unexpectedly, anterograde DiI tracing has shown a prominent layered termination of the entorhinal projection, despite an almost complete loss of granule cells at 9 weeks after infection. Combined light- and electron-microscopic analysis of dendrites at the outer molecular layer of the dentate gyrus at 6 and 9 weeks post-infection has revealed a transient increase in the synaptic density of calbindin-positive granule cells and parvalbuminergic neurons after 6 weeks. In contrast, synaptic density reaches values similar to those of uninfected controls 9 weeks post-infection. These findings indicate that, after BDV infection, synaptic reorganization processes occur at peripheral dendrites of the remaining granule cells and parvalbuminergic neurons, including the unexpected persistence of entorhinal axons in the absence of their main targets.