Mossy Fiber Pathway Research Articles

Fast Neuronal Calcium Signals in Brain Slices Loaded With Fluo-4 AM Ester.

Staining brain slices with acetoxymethyl ester (AM) Ca2+ dyes is a straightforward procedure to load multiple cells, and Fluo-4 is a commonly used high-affinity indicator due to its very large dynamic range. It has been shown that this dye preferentially stains glial cells, providing slow and large Ca2+ transients, but it is questionable whether and at which temporal resolution it can also report Ca2+ transients from neuronal cells. Here, by electrically stimulating mouse hippocampal slices, we resolved fast neuronal signals corresponding to 1%-3% maximal fluorescence changes. Specifically, by recording Ca2+ fluorescence at 2000 frames/s from multiple sites both in the CA3 and in the CA1 regions, we observed that the signal measured near the stimulating electrode, positioned on the mossy fibre pathway, was not blocked by perfusion with 10 μM NBQX and 50 μM AP5, preventing excitatory synaptic transmission. In contrast, this signal was fully blocked by additional perfusion with 1 μM tetrodotoxin, inhibiting voltage-gated Na+ channels and neuronal action potentials. We also present recordings obtained in the presence of 10 μM of the GABAA receptor antagonist bicuculline, or of 50 μM of the voltage-gated K+ channel inhibitor 4-aminopyridine, exhibiting a wide propagation of the signal from CA3 to CA1 regions under conditions that mimic epileptic seizures. Altogether, while Fluo-4 AM remains a preferable indicator for reporting Ca2+ signals from astrocytes at slow temporal resolution, we demonstrated that it can be also utilised for analysing fast neuronal network activity elicited by electrical stimulation in brain slices.

Understanding Cerebellar Input Stage through Computational and Plasticity Rules.

A central hypothesis concerning brain functioning is that plasticity regulates the signal transfer function by modifying the efficacy of synaptic transmission. In the cerebellum, the granular layer has been shown to control the gain of signals transmitted through the mossy fiber pathway. Until now, the impact of plasticity on incoming activity patterns has been analyzed by combining electrophysiological recordings in acute cerebellar slices and computational modeling, unraveling a broad spectrum of different forms of synaptic plasticity in the granular layer, often accompanied by forms of intrinsic excitability changes. Here, we attempt to provide a brief overview of the most prominent forms of plasticity at the excitatory synapses formed by mossy fibers onto primary neuronal components (granule cells, Golgi cells and unipolar brush cells) in the granular layer. Specifically, we highlight the current understanding of the mechanisms and their functional implications for synaptic and intrinsic plasticity, providing valuable insights into how inputs are processed and reconfigured at the cerebellar input stage.

The Chronic Effects of a Single Low-Intensity Blast Exposure on Phosphoproteome Networks and Cognitive Function Influenced by Mutant Tau Overexpression.

Blast-induced neurotrauma (BINT) is a pressing concern for veterans and civilians exposed to explosive devices. Affected personnel may have increased risk for long-term cognitive decline and developing tauopathies including Alzheimer's disease-related disorders (ADRD) or frontal-temporal dementia (FTD). The goal of this study was to identify the effect of BINT on molecular networks and their modulation by mutant tau in transgenic (Tg) mice overexpressing the human tau P301L mutation (rTg4510) linked to FTD or non-carriers. The primary focus was on the phosphoproteome because of the prominent role of hyperphosphorylation in neurological disorders. Discrimination learning was assessed following injury in the subsequent 6 weeks, using the automated home-cage monitoring CognitionWall platform. At 40 days post injury, label-free phosphoproteomics was used to evaluate molecular networks in the frontal cortex of mice. Utilizing a weighted peptide co-expression network analysis (WpCNA) approach, we identified phosphopeptide networks tied to associative learning and mossy-fiber pathways and those which predicted learning outcomes. Phosphorylation levels in these networks were inversely related to learning and linked to synaptic dysfunction, cognitive decline, and dementia including Atp6v1a and Itsn1. Low-intensity blast (LIB) selectively increased pSer262tau in rTg4510, a site implicated in initiating tauopathy. Additionally, individual and group level analyses identified the Arhgap33 phosphopeptide as an indicator of BINT-induced cognitive impairment predominantly in rTg4510 mice. This study unveils novel interactions between ADRD genetic susceptibility, BINT, and cognitive decline, thus identifying dysregulated pathways as targets in potential precision-medicine focused therapeutics to alleviate the disease burden among those affected by BINT.

Cerebellar Nuclei Receiving Orofacial Proprioceptive Signals through the Mossy Fiber Pathway from the Supratrigeminal Nucleus in Rats.

Proprioception from muscle spindles is necessary for motor function executed by the cerebellum. In particular, cerebellar nuclear neurons that receive proprioceptive signals and send projections to the lower brainstem or spinal cord play key roles in motor control. However, little is known about which cerebellar nuclear regions receive orofacial proprioception. Here, we investigated projections to the cerebellar nuclei from the supratrigeminal nucleus (Su5), which conveys the orofacial proprioception arising from jaw-closing muscle spindles (JCMSs). Injections of an anterograde tracer into the Su5 resulted in a large number of labeled axon terminals bilaterally in the dorsolateral hump (IntDL) of the cerebellar interposed nucleus (Int) and the dorsolateral protuberance (MedDL) of the cerebellar medial nucleus. In addition, a moderate number of axon terminals were ipsilaterally labeled in the vestibular group Y nucleus (group Y). We electrophysiologically detected JCMS proprioceptive signals in the IntDL and MedDL. Retrograde tracing analysis confirmed bilateral projections from the Su5 to the IntDL and MedDL. Furthermore, anterograde tracer injections into the external cuneate nucleus (ECu), which receives other proprioceptive input from forelimb/neck muscles, resulted in only a limited number of ipsilaterally labeled terminals, mainly in the dorsomedial crest of the Int and the group Y. Taken together, the Su5 and ECu axons almost separately terminated in the cerebellar nuclei (except for partial overlap in the group Y). These data suggest that orofacial proprioception is differently processed in the cerebellar circuits in comparison to other body-part proprioception, thus contributing to the executive function of orofacial motor control.

Long-Term Synaptic Plasticity Tunes the Gain of Information Channels through the Cerebellum Granular Layer.

A central hypothesis on brain functioning is that long-term potentiation (LTP) and depression (LTD) regulate the signals transfer function by modifying the efficacy of synaptic transmission. In the cerebellum, granule cells have been shown to control the gain of signals transmitted through the mossy fiber pathway by exploiting synaptic inhibition in the glomeruli. However, the way LTP and LTD control signal transformation at the single-cell level in the space, time and frequency domains remains unclear. Here, the impact of LTP and LTD on incoming activity patterns was analyzed by combining patch-clamp recordings in acute cerebellar slices and mathematical modeling. LTP reduced the delay, increased the gain and broadened the frequency bandwidth of mossy fiber burst transmission, while LTD caused opposite changes. These properties, by exploiting NMDA subthreshold integration, emerged from microscopic changes in spike generation in individual granule cells such that LTP anticipated the emission of spikes and increased their number and precision, while LTD sorted the opposite effects. Thus, akin with the expansion recoding process theoretically attributed to the cerebellum granular layer, LTP and LTD could implement selective filtering lines channeling information toward the molecular and Purkinje cell layers for further processing.

Aβ oligomers from human brain impair mossy fiber LTP in CA3 of hippocampus, but activating cAMP-PKA and cGMP-PKG prevents this

Early cognitive impairment in Alzheimer's disease may result in part from synaptic dysfunction caused by the accumulation oligomeric assemblies of amyloid β-protein (Aβ). Changes in hippocampal function seem critical for cognitive impairment in early Alzheimer's disease (AD). Diffusible oligomers of Aβ (oAβ) have been shown to block canonical long-term potentiation (LTP) in the CA1 area of hippocampus, but whether there is also a direct effect of oAβ on synaptic transmission and plasticity at synapses between mossy fibers (axons) from the dentate gyrus granule cells and CA3 pyramidal neurons (mf-CA3 synapses) is unknown. Studies in APP transgenic mice have suggested an age-dependent impairment of mossy fiber LTP. Here we report that although endogenous AD brain-derived soluble oAβ had no effect on mossy-fiber basal transmission, it strongly impaired paired-pulse facilitation in the mossy fiber pathway and presynaptic mossy fiber LTP (mf-LTP). Selective activation of both β1 and β2 adrenergic receptors and their downstream cAMP/PKA signaling pathway prevented oAβ-mediated inhibition of mf-LTP. Unexpectedly, activation of the cGMP/PKG signaling pathway also prevented oAβ-impaired mf-LTP. Our results reveal certain specific pharmacological targets to ameliorate human oAβ-mediated impairment at the mf-CA3 synapse.

The Cerebellar Cortex Receives Orofacial Proprioceptive Signals from the Supratrigeminal Nucleus via the Mossy Fiber Pathway in Rats.

Proprioceptive sensory information from muscle spindles is essential for the regulation of motor functions. However, little is known about the motor control regions in the cerebellar cortex that receive proprioceptive signals from muscle spindles distributed throughout the body, including the orofacial muscles. Therefore, in this study, we investigated the pattern of projections in the rat cerebellar cortex derived from the supratrigeminal nucleus (Su5), which conveys orofacial proprioceptive information from jaw-closing muscle spindles (JCMSs). Injections of an anterograde tracer into the Su5 revealed that many bilateral axon terminals (rosettes) were distributed in the granular layer of the cerebellar cortex (including the simple lobule B, crus II and flocculus) in a various sized, multiple patchy pattern. We could also detect JCMS proprioceptive signals in these cerebellar cortical regions, revealing for the first time that they receive muscle proprioceptive inputs in rats. Retrograde tracer injections confirmed that the Su5 directly sends outputs to the cerebellar cortical areas. Furthermore, we injected an anterograde tracer into the external cuneate nucleus (ECu), which receives proprioceptive signals from the forelimb and neck muscle spindles, to distinguish between the Su5- and ECu-derived projections in the cerebellar cortex. The labeled terminals from the ECu were distributed predominantly in the vermis of the cerebellar cortex. Almost no overlap was seen in the terminal distributions of the Su5 and ECu projections. Our findings demonstrate that the rat cerebellar cortex receives orofacial proprioceptive input that is processed differently from the proprioceptive signals from the other regions of the body.

Abnormal innate and learned behavior induced by neuron–microglia miscommunication is related to CA3 reconfiguration

Neuron-microglia communication through the Cx3cr1-Cx3cl1 axis is essential for the development and refinement of neural circuits, which determine their function into adulthood. In the present work we set out to extend the behavioral characterization of Cx3cr1-/- mice evaluating innate behaviors and spatial navigation, both dependent on hippocampal function. Our results show that Cx3cr1-deficient mice, which show some changes in microglial and synaptic terminals morphology and density, exhibit alterations in activities of daily living and in the rapid encoding of novel spatial information that, nonetheless, improves with training. A neural substrate for these cognitive deficiencies was found in the form of synaptic dysfunction in the CA3 region of the hippocampus, with a marked impact on the mossy fiber (MF) pathway. A network analysis of the CA3 microcircuit reveals the effect of these synaptic alterations on the functional connectivity among CA3 neurons with diminished strength and topological reorganization in Cx3cr1-deficient mice. Neonatal population activity of the CA3 region in Cx3cr1-deficient mice shows a marked reorganization around the giant depolarizing potentials, the first form of network-driven activity of the hippocampus, suggesting that alterations found in adult subjects arise early on in postnatal development, a critical period of microglia-dependent neural circuit refinement. Our results show that interruption of the Cx3cr1-Cx3cl1/neuron-microglia axis leads to changes in CA3 configuration that affect innate and learned behaviors.

Disrupted mossy fiber connections from defective embryonic neurogenesis contribute to SOX11-associated schizophrenia.

Abnormal mossy fiber connections in the hippocampus have been implicated in schizophrenia. However, it remains unclear whether this abnormality in the patients is genetically determined and whether it contributes to the onset of schizophrenia. Here, we showed that iPSC-derived hippocampal NPCs from schizophrenia patients with the A/A allele at SNP rs16864067 exhibited abnormal NPC polarity, resulting from the downregulation of SOX11 by this high-risk allele. In the SOX11-deficient mouse brain, abnormal NPC polarity was also observed in the hippocampal dentate gyrus, and this abnormal NPC polarity led to defective hippocampal neurogenesis-specifically, irregular neuroblast distribution and disrupted granule cell morphology. As granule cell synapses, the mossy fiber pathway was disrupted, and this disruption was resistant to activity-induced mossy fiber remodeling in SOX11 mutant mice. Moreover, these mutant mice exhibited diminished PPI and schizophrenia-like behaviors. Activation of hippocampal neurogenesis in the embryonic brain, but not in the adult brain, partially alleviated disrupted mossy fiber connections and improved schizophrenia-related behaviors in mutant mice. We conclude that disrupted mossy fiber connections are genetically determined and strongly correlated with schizophrenia-like behaviors in SOX11-deficient mice. This disruption may reflect the pathological substrate of SOX11-associated schizophrenia.

Genetic dissection of glutathione S-transferase omega-1: identification of novel downstream targets and Alzheimer's disease pathways.

Alzheimer's disease (AD) is affected by genetic factors. Polymorphisms in the glutathione S-transferase omega-1 (Gsto1) gene have been shown by genetic correlation analyses performed in different ethnic populations to be genetic risk factors for AD. Gene expression profile data from BXD recombinant inbred mice were used in combination with genetic and bioinformatic analyses to characterize the mechanisms underlying regulation of Gsto1 variation regulation and to identify network members that may contribute to AD risk or progression. Allele-specific assays confirmed that variation in Gsto1 expression is controlled by cis-expression quantitative trait loci. We found that Gsto1 mRNA levels were related to several central nervous system traits, such as glial acidic fibrillary protein levels in the caudate putamen, cortical gray matter volume, and hippocampus mossy fiber pathway volume. We identified 2168 genes whose expression was highly correlated with that of Gsto1. Some genes were enriched for the most common neurodegenerative diseases. Some Gsto1-related genes identified in this study had previously been identified as susceptibility genes for AD, such as APP, Grin2b, Ide, and Psenen. To evaluate the relationships between Gsto1 and candidate network members, we transfected astrocytes with Gsto1 siRNA and assessed the effect on putative downstream effectors. We confirmed that knockdown of Gsto1 had a significant influence on Pa2g4 expression, suggesting that Pa2g4 may be a downstream effector of Gsto1, and that both genes interact with other genes in a network during AD pathogenesis.

Sex differences in the rodent hippocampal opioid system following stress and oxycodone associated learning processes

Over the past two decades, opioid abuse has risen especially among women. In both sexes hippocampal neural circuits involved in associative memory formation and encoding of motivational incentives are critically important in the transition from initial drug use to drug abuse/dependence. Opioid circuits, particularly the mossy fiber pathway, are crucial for associative memory processes important for addiction. Our anatomical studies, especially those utilizing electron microscopic immunocytochemistry, have provided unique insight into sex differences in the distribution of opioid peptides and receptors in specific hippocampal circuits and how these distributions are altered following stress and oxycodone-associative learning processes. Here we review the hippocampal opioid system in rodents with respect to ovarian hormones effects and baseline sex differences then sex differences following acute and chronic stress. Next, we review sex differences in the hippocampal opioid system in unstressed and chronically stressed rats following oxycodone conditioned place preference. We show that opioid peptides and receptors are distributed within hippocampal circuits in females with elevated estrogen states in a manner that would enhance sensitivity to endogenous and exogenous opioids. Moreover, chronic stress primes the opioid system in females in a manner that would promote opioid-associative learning processes. In contrast, chronic stress has limited effects on the opioid system in males and reduces its capacity to support opioid-mediated learning processes. Interestingly, acute stress appears to prime males for opioid associative learning. On a broader scale the findings highlighted in this review have important implications in understanding sex differences in opioid drug use and abuse.

Separable actions of acetylcholine and noradrenaline on neuronal ensemble formation in hippocampal CA3 circuits.

In the hippocampus, episodic memories are thought to be encoded by the formation of ensembles of synaptically coupled CA3 pyramidal cells driven by sparse but powerful mossy fiber inputs from dentate gyrus granule cells. The neuromodulators acetylcholine and noradrenaline are separately proposed as saliency signals that dictate memory encoding but it is not known if they represent distinct signals with separate mechanisms. Here, we show experimentally that acetylcholine, and to a lesser extent noradrenaline, suppress feed-forward inhibition and enhance Excitatory-Inhibitory ratio in the mossy fiber pathway but CA3 recurrent network properties are only altered by acetylcholine. We explore the implications of these findings on CA3 ensemble formation using a hierarchy of models. In reconstructions of CA3 pyramidal cells, mossy fiber pathway disinhibition facilitates postsynaptic dendritic depolarization known to be required for synaptic plasticity at CA3-CA3 recurrent synapses. We further show in a spiking neural network model of CA3 how acetylcholine-specific network alterations can drive rapid overlapping ensemble formation. Thus, through these distinct sets of mechanisms, acetylcholine and noradrenaline facilitate the formation of neuronal ensembles in CA3 that encode salient episodic memories in the hippocampus but acetylcholine selectively enhances the density of memory storage.

Neuronal brain‐derived neurotrophic factor manipulates microglial dynamics

Brain-derived neurotrophic factor (BDNF), a main member of the neurotrophin family that is active in the brain, supports neuronal survival and growth. Microglial BDNF affects both the structural and functional properties of neurons. In contrast, whether and how neuronal BDNF affects microglial dynamics remain largely undetermined. Here, we examined the effects of BDNF on the properties of microglia in the CA3 region of the hippocampus. We chose this site because the axonal boutons of hippocampal mossy fibers, which are mostly formed in the CA3 region, contain the highest levels of BDNF in the rodent brain. We transfected mouse dentate granule cells with an adeno-associated virus that encodes both a BDNF short hairpin RNA (shRNA) and red fluorescent protein to examine the effects of mossy fiber-derived BDNF on microglia. Based on immunohistochemistry, BDNF knockdown with an shRNA resulted in an increase in microglial density in the mossy fiber pathway and increased engulfment of mossy fiber axons by microglia. In addition, we performed time-lapse imaging of microglial processes in hippocampal slice cultures to examine the effects of BDNF on microglial motility. Time-lapse imaging revealed increases in the motility of microglial processes and the engulfment of mossy fiber synapses by microglia when BDNF signaling was pharmacologically blocked. Thus, neuronal BDNF prevents microglia from engulfing mossy fiber synapses in the hippocampus.

Synapse type-specific proteomic dissection identifies IgSF8 as a hippocampal CA3 microcircuit organizer

Excitatory and inhibitory neurons are connected into microcircuits that generate circuit output. Central in the hippocampal CA3 microcircuit is the mossy fiber (MF) synapse, which provides powerful direct excitatory input and indirect feedforward inhibition to CA3 pyramidal neurons. Here, we dissect its cell-surface protein (CSP) composition to discover novel regulators of MF synaptic connectivity. Proteomic profiling of isolated MF synaptosomes uncovers a rich CSP composition, including many CSPs without synaptic function and several that are uncharacterized. Cell-surface interactome screening identifies IgSF8 as a neuronal receptor enriched in the MF pathway. Presynaptic Igsf8 deletion impairs MF synaptic architecture and robustly decreases the density of bouton filopodia that provide feedforward inhibition. Consequently, IgSF8 loss impairs excitation/inhibition balance and increases excitability of CA3 pyramidal neurons. Our results provide insight into the CSP landscape and interactome of a specific excitatory synapse and reveal IgSF8 as a critical regulator of CA3 microcircuit connectivity and function.

Conversion of Graded Presynaptic Climbing Fiber Activity into Graded Postsynaptic Ca2+ Signals by Purkinje Cell Dendrites

The brain must make sense of external stimuli to generate relevant behavior. We used a combination of invivo approaches to investigate how the cerebellum processes sensory-related information. We found that the inferior olive encodes contexts of sensory-associated external cues in a graded manner, apparent in the presynaptic activity of their axonal projections (climbing fibers) in the cerebellar cortex. Individual climbing fibers were broadly responsive to different sensory modalities but relayed sensory-related information to the cortex in a lobule-dependent manner. Purkinje cell dendrites faithfully transformed this climbing fiber activity into dendrite-wide Ca2+ signals without a direct contribution from the mossy fiber pathway. These results demonstrate that the size of climbing-fiber-evoked Ca2+ signals in Purkinje cell dendrites is largely determined by the firing level of climbing fibers. This coding scheme emphasizes the overwhelming role of the inferior olive in generating salient signals useful for instructing plasticity and learning.

Pathogenic tau modifications occur in axons before the somatodendritic compartment in mossy fiber and Schaffer collateral pathways

The deposition of tau pathology in Alzheimer’s disease (AD) may occur first in axons of neurons and then progress back into the cell bodies to form neurofibrillary tangles, however, studies have not directly analyzed this relationship in relatively discrete circuits within the human hippocampus. In the early phases of tau deposition, both AT8 phosphorylation and exposure of the amino terminus of tau occurs in tauopathies, and these modifications are linked to mechanisms of synaptic and axonal dysfunction. Here, we examined the localization of these tau pathologies in well-characterized post-mortem human tissue samples from the hippocampus of 44 cases ranging between non-demented and mild cognitively impaired to capture a time at which intrahippocampal pathways show a range in the extent of tau deposition. The tissue sections were analyzed for AT8 (AT8 antibody), amino terminus exposure (TNT2 antibody), and amyloid-β (MOAB2 antibody) pathology in hippocampal strata containing the axons and neuronal cell bodies of the CA3-Schaffer collateral and dentate granule-mossy fiber pathways. We show that tau pathology first appears in the axonal compartment of affected neurons in the absence of observable tau pathology in the corresponding cell bodies in several cases. Additionally, deposition of tau in these intrahippocampal pathways was independent of the presence of Aβ plaques. We confirmed that the majority of tau pathology positive neuropil threads were axonal in origin and not dendritic using an axonal marker (i.e. SMI312 antibody) and somatodendritic marker (i.e. MAP2 antibody). Taken together, these results support the hypothesis that AT8 phosphorylation and amino terminus exposure are early pathological events and that the deposition of tau pathology, at least in the studied pathways, occurs first in the axonal compartment prior to observable pathology in the somata. These findings highlight the importance on targeting tau deposition, ideally in the initial phases of its deposition in axons.

Axonal organization defects in the hippocampus of adult conditional BACE1 knockout mice.

β-Site APP (amyloid precursor protein) cleaving enzyme 1 (BACE1) is the β-secretase enzyme that initiates production of the toxic amyloid-β peptide that accumulates in the brains of patients with Alzheimer's disease (AD). Hence, BACE1 is a prime therapeutic target, and several BACE1 inhibitor drugs are currently being tested in clinical trials for AD. However, the safety of BACE1 inhibition is unclear. Germline BACE1 knockout mice have multiple neurological phenotypes, although these could arise from BACE1 deficiency during development. To address this question, we report that tamoxifen-inducible conditional BACE1 knockout mice in which the Bace1 gene was ablated in the adult largely lacked the phenotypes observed in germline BACE1 knockout mice. However, one BACE1-null phenotype was induced after Bace1 gene deletion in the adult mouse brain. This phenotype showed reduced length and disorganization of the hippocampal mossy fiber infrapyramidal bundle, the axonal pathway of dentate gyrus granule cells that is maintained by neurogenesis in the mouse brain. This defect in axonal organization correlated with reduced BACE1-mediated cleavage of the neural cell adhesion protein close homolog of L1 (CHL1), which has previously been associated with axon guidance. Although our results indicate that BACE1 inhibition in the adult mouse brain may avoid phenotypes associated with BACE1 deficiency during embryonic and postnatal development, they also suggest that BACE1 inhibitor drugs developed for treating AD may disrupt the organization of an axonal pathway in the hippocampus, an important structure for learning and memory.

Locomotor activity modulates associative learning in mouse cerebellum.

Changes in behavioral state can profoundly influence brain function. Here we show that behavioral state modulates performance in delay eyeblink conditioning, a cerebellum-dependent form of associative learning. Increased locomotor speed in head-fixed mice drove earlier onset of learning and trial-by-trial enhancement of learned responses that were dissociable from changes in arousal and independent of sensory modality. Eyelid responses evoked by optogenetic stimulation of mossy fiber inputs to the cerebellum, but not at sites downstream, were positively modulated by ongoing locomotion. Substituting prolonged, low-intensity optogenetic mossy fiber stimulation for locomotion was sufficient to enhance conditioned responses. Our results suggest that locomotor activity modulates delay eyeblink conditioning through increased activation of the mossy fiber pathway within the cerebellum. Taken together, these results provide evidence for a novel role for behavioral state modulation in associative learning and suggest a potential mechanism through which engaging in movement can improve an individual’s ability to learn.

Shaker-related voltage-gated potassium channels Kv1 in human hippocampus.

In this study, we investigated the tissue expression levels, alpha subunit composition and distribution of Shaker-related voltage-dependent potassium Kv1 channels in human hippocampus by combining western blotting experiments, toxin autoradiography, in vivo radioligand binding studies, immunoprecipitation and immunohistochemistry. Tissue expression of Kv1.1 and Kv1.2 α-subunits in human post-mortem brain tissue was confirmed in immunoblot analysis using a panel of specific monoclonal and polyclonal antibodies. Immunoprecipitation experiments using toxin-prelabeled Kv1 channels revealed that all toxin-sensitive Kv1 channels in human hippocampus contained either a Kv1.1 or Kv1.2 α-subunit with the majority being composed of Kv1.1/Kv1.2 heterotetramers. Receptor autoradiography suggested Kv1.1/Kv1.2 channel expression in the molecular layer of dentate gyrus. In accordance, immunohistochemical experiments also observed Kv1.1 and Kv1.2 α-subunits in the molecular layer of the dentate gyrus, in addition to the CA3 stratum lucidum and the CA1 stratum oriens. These findings indicate expression in axons and terminals of hippocampal pathways, namely the perforant path, the mossy fiber pathway and the Schaffer collaterals. Herein we present the first direct demonstration that Kv1.1 and Kv1.2 channel proteins are targeted to distinct compartments of the human hippocampal formation and that this expression pattern largely reflects their distribution profile in murine brain.

Diminished Dentate Gyrus Filtering of Cortical Input Leads to Enhanced Area Ca3 Excitability after Mild Traumatic Brain Injury.

Mild traumatic brain injury (mTBI) disrupts hippocampal function and can lead to long-lasting episodic memory impairments. The encoding of episodic memories relies on spatial information processing within the hippocampus. As the primary entry point for spatial information into the hippocampus, the dentate gyrus is thought to function as a physiological gate, or filter, of afferent excitation before reaching downstream area Cornu Ammonis (CA3). Although injury has previously been shown to alter dentate gyrus network excitability, it is unknown whether mTBI affects dentate gyrus output to area CA3. In this study, we assessed hippocampal function, specifically the interaction between the dentate gyrus and CA3, using behavioral and electrophysiological techniques in ex vivo brain slices 1 week following mild lateral fluid percussion injury (LFPI). Behaviorally, LFPI mice were found to be impaired in an object-place recognition task, indicating that spatial information processing in the hippocampus is disrupted. Extracellular recordings and voltage-sensitive dye imaging demonstrated that perforant path activation leads to the aberrant spread of excitation from the dentate gyrus into area CA3 along the mossy fiber pathway. These results suggest that after mTBI, the dentate gyrus has a diminished capacity to regulate cortical input into the hippocampus, leading to increased CA3 network excitability. The loss of the dentate filtering efficacy reveals a potential mechanism by which hippocampal-dependent spatial information processing is disrupted, and may contribute to memory dysfunction after mTBI.