Advances in understanding hilar mossy cells of the dentate gyrus.

Hilar mossy cells (MCs) of the dentate gyrus (DG) distinguish the DG from other hippocampal subfields (CA1-3) because there are two glutamatergic cell types in the DG rather than one. Thus, in the DG, the main cell types include glutamatergic granule cells (GCs) and MCs, whereas in CA1-3, the only glutamatergic cell type is the pyramidal cell. In contrast to GCs, MCs are different in morphology, intrinsic electrophysiological properties, afferent input and axonal projections, so their function is likely to be very different from GCs. Why are MCs necessary to the DG? In past studies, the answer has been unclear because MCs not only excite GCs directly but also inhibit them disynaptically, by exciting GABAergic neurons that project to GCs. Results of new studies are discussed that shed light on this issue. These studies take advantage of recently available transgenic mice with Cre recombinase expression mostly in MCs and techniques such as optogenetics and DREADDs (designer receptors exclusively activated by designer drugs). The recent studies also address in vivo behavioral functions of MCs. Some of the results support past hypotheses whereas others suggest new conceptualizations of how the MCs contribute to DG circuitry and function. While substantial progess has been made, additional research is still needed to clarify the characteristics and functions of these unique cells.

Local and Long-Range Circuit Connections to Hilar Mossy Cells in the Dentate Gyrus.

Hilar mossy cells are the prominent glutamatergic cell type in the dentate hilus of the dentate gyrus (DG); they have been proposed to have critical roles in the DG network. To better understand how mossy cells contribute to DG function, we have applied new viral genetic and functional circuit mapping approaches to quantitatively map and compare local and long-range circuit connections of mossy cells and dentate granule cells in the mouse. The great majority of inputs to mossy cells consist of two parallel inputs from within the DG: an excitatory input pathway from dentate granule cells and an inhibitory input pathway from local DG inhibitory neurons. Mossy cells also receive a moderate degree of excitatory and inhibitory CA3 input from proximal CA3 subfields. Long range inputs to mossy cells are numerically sparse, and they are only identified readily from the medial septum and the septofimbrial nucleus. In comparison, dentate granule cells receive most of their inputs from the entorhinal cortex. The granule cells receive significant synaptic inputs from the hilus and the medial septum, and they also receive direct inputs from both distal and proximal CA3 subfields, which has been underdescribed in the existing literature. Our slice-based physiological mapping studies further supported the identified circuit connections of mossy cells and granule cells. Together, our data suggest that hilar mossy cells are major local circuit integrators and they exert modulation of the activity of dentate granule cells as well as the CA3 region through "back-projection" pathways.

Excitability tuning of axons in the central nervous system.

The axon is a long neuronal process that originates from the soma and extends towards the presynaptic terminals. The pioneering studies on the squid giant axon or the spinal cord motoneuron established that the axon conducts action potentials faithfully to the presynaptic terminals with self-regenerative processes of membrane excitation. Recent studies challenged the notion that the fundamental understandings obtained from the study of squid giant axons are readily applicable to the axons in the mammalian central nervous system (CNS). These studies revealed that the functional and structural properties of the CNS axons are much more variable than previously thought. In this review article, we summarize the recent understandings of axon physiology in the mammalian CNS due to progress in the subcellular recording techniques which allow direct recordings from the axonal membranes, with emphasis on the hippocampal mossy fibers as a representative en passant axons typical for cortical axons.

Differential regulation of BDNF, synaptic plasticity and sprouting in the hippocampal mossy fiber pathway of male and female rats.

Many studies have described potent effects of BDNF, 17ß-estradiol or androgen on hippocampal synapses and their plasticity. Far less information is available about the interactions between 17ß-estradiol and BDNF in hippocampus, or interactions between androgen and BDNF in hippocampus. Here we review the regulation of BDNF in the mossy fiber pathway, a critical part of hippocampal circuitry. We discuss the emerging view that 17ß-estradiol upregulates mossy fiber BDNF synthesis in the adult female rat, while testosterone exerts a tonic suppression of mossy fiber BDNF levels in the adult male rat. The consequences are interesting to consider: in females, increased excitability associated with high levels of BDNF in mossy fibers could improve normal functions of area CA3, such as the ability to perform pattern completion. However, memory retrieval may lead to anxiety if stressful events are recalled. Therefore, the actions of 17ß-estradiol on the mossy fiber pathway in females may provide a potential explanation for the greater incidence of anxiety-related disorders and post-traumatic stress syndrome (PTSD) in women relative to men. In males, suppression of BDNF-dependent plasticity in the mossy fibers may be protective, but at the 'price' of reduced synaptic plasticity in CA3. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Bone morphogenetic protein signaling in the developing telencephalon controls formation of the hippocampal dentate gyrus and modifies fear-related behavior.

The cortical hem is an embryonic signaling center that generates bone morphogenetic proteins (BMPs) and acts as an organizer for the hippocampus. The role of BMP signaling in hippocampal neurogenesis, however, has not been established. We therefore generated mice that were deficient in Bmpr1b constitutively, and deficient in Bmpr1a conditionally in the dorsal telencephalon. In double mutant male and female mice, the dentate gyrus (DG) was dramatically smaller than in control mice, reflecting decreased production of granule neurons at the peak period of DG neurogenesis. Additionally, the pool of cells that generates new DG neurons throughout life was reduced, commensurate with the smaller size of the DG. Effects of diminished BMP signaling on the cortical hem were at least partly responsible for these defects in DG development. Reduction of the DG and its major extrinsic output to CA3 raised the possibility that the DG was functionally compromised. We therefore looked for behavioral deficits in double mutants and found that the mice were less responsive to fear- or anxiety-provoking stimuli, whether the association of the stimulus with fear or anxiety was learned or innate. Given that no anatomical defects appeared in the double mutant telencephalon outside the DG, our observations support a growing literature that implicates the hippocampus in circuitry mediating fear and anxiety. Our results additionally indicate a requirement for BMP signaling in generating the dorsalmost neuronal lineage of the telencephalon, DG granule neurons, and in the development of the stem cell niche that makes neurons in the adult hippocampus.

The dentate mossy fibers: structural organization, development and plasticity.

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

Learning-induced axonal remodeling: evolutionary divergence and conservation of two components of the mossy fiber system within Rodentia.

Damage to the hippocampal formation results in profound impairments in spatial navigation in rats and mice leading to the widely accepted assumption that the hippocampal cellular and molecular memory mechanisms of both genera are conserved. Recently our group has shown in two rat strains that hippocampal-dependent training in the water maze specifically induces robust 'sprouting' of granule cell suprapyramidal mossy fiber axon terminal fields. Here we sought to investigate whether the pronounced remodeling of adult hippocampal circuitry observed in the rat is also present in the mouse motivated by the thought that subsequent studies using genetically-engineered mice could then be implemented to explore the molecular mechanisms underlying training-dependent axonal growth in adult rodents. However, in contrast to Wistar rats, no changes in the Timm's-stained area of mossy fiber terminal fields (MFTFs) were observed in C57BL/6J or 129Sv/EmsJ inbred wild-type mice after water maze training. Neither extending the duration of training nor scaling down the size of the apparatus was able to induce sprouting in mouse mossy fiber pathways. Though there may be similarities in the ultimate output of the hippocampus of rats and mice as inferred from lesion studies, the current results, as well as differences in learning and memory characteristics between the two genera, suggest that the way in which the component circuitry functions is likely to be different; a not too surprising conclusion given the substantial evolutionary distance between them (>20 million years). The present findings afford an opportunity for uncovering linkages between evolutionarily significant alterations in hippocampal circuitry in relation to genera-specific information storage requirements.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures.

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

Hippocampal mossy fibre terminal field size is differentially affected in a rat model of risk-taking behaviour.

Individual differences in novelty-induced exploratory activity identify rats which can serve as a model of human sensation-seeking, risk-taking behaviour. Experimentally naïve rats, when exposed to mild stress of a novel environment, exhibit variability in their exploratory activity. Some rats display high rates of locomotor reactivity to novelty (high responders (HR)), and others display low rates (low responders (LR)). The LRHR phenotype is a reliable predictor of drug-taking behaviour and is linked to differences in hippocampal glucocorticoid receptor mRNA expression. In this study, we investigated whether the LRHR phenotype is associated with differences in the quantitative morphology of the hippocampal field CA3, dentate gyrus molecular layer, granule cell layer and mossy fibres. LRs and HRs showed no significant differences in the volumes of CA3 and dentate molecular layer volume or the number of dentate granule cells. However, LRs had a significantly larger suprapyramidal mossy fibre terminal field volume when compared to HRs. The infrapyramidal mossy fibres did not differ between phenotypes. Also, we found a LRHR phenotype-independent significant negative correlation between molecular layer volume per granule cell and the total number of granule cells. These findings implicate the SP-MF in vulnerability for risk-taking behaviour, and we propose that LR and HR hippocampi may differ in the way novelty information is processed.

Associational sprouting in the mouse fascia dentata after entorhinal lesion in vitro.

Collateral sprouting is a form of neuronal plasticity observed in brain following injury. In order to establish an in vitro model of collateral sprouting, entorhino-hippocampal slice cultures were prepared from brain of C57BL/6 mouse pups (P1-4) and incubated for 14-16 days in vitro. Thereafter, entorhino-hippocampal fibers were cut and the outer molecular layer of the fascia dentata was denervated. At this age, entorhino-hippocampal fibers do not regenerate, as could be shown using anterograde tracing with Miniruby. Sprouting of associational mossy cell axons was monitored using calretinin-immunocytochemistry. Control and lesioned entorhino-hippocampal slices were studied at 1, 5, and 10 days postlesion. Whereas only the inner portion of the molecular layer was occupied by calretinin-positive mossy cell axons in controls and after 1 and 5 days postlesion, the entire width of the molecular layer was occupied by associational fibers by 10 days postlesion. Thus, robust sprouting of associational mossy cell axons occurs in response to entorhinal denervation in vitro. Using organotypic entorhino-hippocampal slices of genetically engineered mice, this sprouting model can be used to identify molecules involved in the regulation of sprouting following brain injury.

Structural and functional asymmetry in the normal and epileptic rat dentate gyrus.

The rat dentate gyrus is usually described as relatively homogeneous. Here, we present anatomic and physiological data which demonstrate that there are striking differences between the supra- and infrapyramidal blades after status epilepticus and recurrent seizures. These differences appear to be an accentuation of a subtle asymmetry present in normal rats. In both pilocarpine and kainic acid models, there was greater mossy fiber sprouting in the infrapyramidal blade. This occurred primarily in the middle third of the hippocampus. Asymmetric sprouting was evident both with Timm stain as well as antisera to brain-derived neurotrophic factor (BDNF) or neuropeptide Y (NPY). In addition, surviving NPY-immunoreactive hilar neurons were distributed preferentially in the suprapyramidal region of the hilus. Extracellular recordings from infrapyramidal sites in hippocampal slices of pilocarpine-treated rats showed larger population spikes and weaker paired-pulse inhibition in response to perforant path stimulation relative to suprapyramidal recordings. A single stimulus could evoke burst discharges in infrapyramidal granule cells but not suprapyramidal blade neurons. BDNF exposure led to spontaneous epileptiform discharges that were larger in amplitude and longer lasting in the infrapyramidal blade. Stimulation of the infrapyramidal molecular layer evoked larger responses in area CA3 than suprapyramidal stimulation. In slices from the temporal pole, in which anatomic evidence of asymmetry waned, there was little evidence of physiological asymmetry either. Of interest, some normal rats also showed signs of greater evoked responses in the infrapyramidal blade, and this could be detected with both microelectrode recording and optical imaging techniques. Although there were no signs of hyperexcitability in normal rats, the data suggest that there is some asymmetry in the normal dentate gyrus and this asymmetry is enhanced by seizures. Taken together, the results suggest that supra- and infrapyramidal blades of the dentate gyrus could have different circuit functions and that the infrapyramidal blade may play a greater role in activating the hippocampus.

Variation in hippocampal dynorphin b-immunoreactive mossy fiber terminal fields of apomorphine-(un)susceptible rats.

The size of distinct hippocampal sub-fields were measured in the apomorphine-susceptible and apomorphine-unsusceptible rat lines. Mossy fiber terminal fields were delineated using dynorphin B immunoreactivity and area measurements were taken from (1) the supra-pyramidal mossy fiber terminal field; (2) the intra- and infra-pyramidal mossy fiber terminal field; (3) the hilus of the fascia dentata (4) the non dynorphin B immunoreactive area of the regio inferior and fascia dentata and (5) the total area of regio inferior and fascia dentata. The data indicate that statistically significant differences in the morphometry of the hippocampal subfields of the apomorphine susceptible and unsusceptible rats are confined to the intra- and infra terminal field: the relative size of the left and right intra- and infra terminal field of apomorphine unsusceptible rats are significantly larger than those of the apomorphine susceptible rats. These data explain at least in part the differential response of these rats to novelty.

Frequencia da mutacao delta-mtDNA 4977 e acumulo de placas neuriticas e emaranhados neurofibrilares no giro para-hipocampal humano, no envelhecimento normal e na doenca de Alzheimer / Delta mtDNA 4977 mutational frequence and neuritic plaques and neurofibrillary tangles in human parahippocampal gyrus, in normal aging and Alzheimer's disease

A presenca da mutacao-delecao mtDNA no giro para-hipocampal humano foi investigada em 95 pacientes autopsiados de tres series de origens geograficas distintas, Alemanha, Brasil e Japao, incluindo 70 pacientes sem doencas neuropsiquiatricas e 25 pacientes portadores da doenca de Alzheimer. Somente a serie alema, caracterizada por maiores proporcoes de neuronios medios e grandes, e alta incidencia de placas neuriticas e emaranhados neurofibrilares no giro para-hipocampal, apresentou a delta-mtDNA em niveis detectaveis pela reacao de cadeia da polimerase (PCR). As series brasileira e japonesa, caracterizadas por menores proporcoes de neuronios medios e grandes e baixa incidencia de placas e emaranhados, nao apresentaram niveis detectaveis da alfa-mtDNA. A frequencia f da alfa-mtDNA foi tres vezes menor no grupo de pacientes portadores da doenca de Alzheimer (f=0,12) que no grupo controle (f=0,37) (p=0,03)...