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The roles GABAergic and glutamatergic inputs in regulating the activity of the gonadotrophin-releasing hormone (GnRH) neurons at the time of the preovulatory surge remain unclear. We used expansion microscopy to compare the density of GABAergic and glutamatergic synapses on the GnRH neuron cell body and proximal dendrite in dioestrous and pro-oestrous female mice. An evaluation of all synapses immunoreactive for synaptophysin revealed that the highest density of inputs to rostral preoptic area GnRH neurons occurred within the first 45 µm of the primary dendrite (approximately 0.19 synapses µm-1 ) with relatively few synapses on the GnRH neuron soma or beyond 45 µm of the dendrite (0.05-0.08 synapses µm-1 ). Triple immunofluorescence labelling demonstrated a predominance of glutamatergic signalling with twice as many vesicular glutamate transporter 2 synapses detected compared to vesicular GABA transporter. Co-labelling with the GABAA receptor scaffold protein gephyrin and the glutamate receptor postsynaptic density marker Homer1 confirmed these observations, as well as the different spatial distribution of GABA and glutamate inputs along the dendrite. Quantitative assessments revealed no differences in synaptophysin, GABA or glutamate synapses at the proximal dendrite and soma of GnRH neurons between dioestrous and pro-oestrous mice. Taken together, these studies demonstrate that the GnRH neuron receives twice as many glutamatergic synapses compared to GABAergic synapses and that these inputs preferentially target the first 45 µm of the GnRH neuron proximal dendrite. These inputs appear to be structurally stable before the onset of pro-oestrous GnRH surge.

Among all voltage-gated potassium (Kv) channels, Kv2 channels are the most widely expressed in the mammalian brain. However, studying Kv2 in neurons has been challenging due to a lack of high-selective blockers. Recently, a peptide toxin, guangxitoxin-1E, has been identified as a specific inhibitor of Kv2, thus facilitating the study of Kv2 in neurons. The mammalian dorsal cochlear nucleus (DCN) integrates auditory and somatosensory information. In the DCN, cartwheel inhibitory interneurons receive excitatory synaptic inputs from parallel fibers conveying somatosensory information. The activation of parallel fibers drives action potentials in the cartwheel cells up to 130 Hz in vivo, and the excitation of cartwheel cells leads to the strong inhibition of principal cells. Therefore, cartwheel cells play crucial roles in monaural sound localization and cancelling detection of self-generated sounds. However, how Kv2 controls the high-frequency firing in cartwheel cells is unknown. In this study, we performed immunofluorescence labeling with anti-Kv2.1 and anti-Kv2.2 antibodies using fixed mouse brainstem slice preparations. The results revealed that Kv2.1 and Kv2.2 were largely present on the cartwheel cell body membrane but not on the axon initial segment nor the proximal dendrite. Whole-cell patch clamp recordings using mouse brainstem slice preparation and guangxitoxin-1E demonstrated that blockade of Kv2 induced failure of parallel fiber-induced action potentials when parallel fibers were stimulated at high frequencies (30-100 Hz). Thus, somatic Kv2 in cartwheel cells regulates the action potentials in a frequency-dependent manner and may play important roles in the DCN function. Significance statement The mammalian dorsal cochlear nucleus (DCN) plays a role in the monaural sound localization and cancelling detection of self-generated sounds. In the DCN, cartwheel cells receive excitatory synaptic inputs from parallel fibers. Parallel fiber activation can drive action potentials in cartwheel cells at high frequency, but the ionic mechanism of such firing remains unknown. In this study, we found that Kv2.1 and Kv2.2 ion channels were present on the cell bodies of cartwheel cells. Application of a specific blocker of Kv2 induced failure of parallel fiber-induced action potentials only when presynaptic parallel fibers were stimulated at high frequencies. Thus, somatic Kv2 in cartwheel cells regulates action potentials in a frequency-dependent manner and may play important roles in sound processing.

Key points Neurons of the lateral superior olive (LSO) in the brainstem receive powerful glycinergic inhibition that originates from the contralateral ear, and that plays an important role in sound localization. We investigated the ultrastructural basis for strong inhibition of LSO neurons using serial block face scanning electron microscopy. The soma and the proximal dendrite of an LSO neuron are surrounded by a high density of inhibitory axons, whereas excitatory axons are much sparser. A given inhibitory axon establishes contacts via several large axonal thickenings, called varicosities, which typically elaborate several active zones (range 1-11). The number of active zones across inhibitory axon segments is variable. These data thus provide an ultrastructural correlate for the strong and multiquantal, but overall variable, unitary IPSC amplitude observed for inhibitory inputs to LSO neuron. Abstract Binaural neurons in the lateral superior olive (LSO) integrate sound information arriving from each ear, and powerful glycinergic inhibition of these neurons plays an important role in this process. In the present study, we investigated the ultrastructural basis for strong inhibitory inputs onto LSO neurons using serial block face scanning electron microscopy. We reconstructed axon segments that make contact with the partially reconstructed soma and proximal dendrite of a mouse LSO neuron at postnatal day 18. Using functional measurements and the Sr2+ method, we find a constant quantal size but a variable quantal content between 'weak' and 'strong' unitary IPSCs. A 3-D reconstruction of a LSO neuron and its somatic synaptic afferents reveals how a large number of inhibitory axons intermingle in a complex fashion on the soma and proximal dendrite of an LSO neuron; a smaller number of excitatory axons was also observed. A given inhibitory axon typically contacts an LSO neuron via several large varicosities (average diameter 3.7 μm), which contain several active zones (range 1-11). The number of active zones across individual axon segments was highly variable. These data suggest that the variable unitary IPSC amplitude is caused by a variable number of active zones between inhibitory axons that innervate a given LSO neuron. The results of the present study show that relatively large multi-active zone varicosities, which can be repeated many times in a given presynaptic axon, provide the ultrastructural basis for the strong multiquantal inhibition received by LSO neurons.

Proper communication among neurons depends on an appropriately formed dendritic arbor, and thus, aberrant changes to the arbor are implicated in many pathologies, ranging from cognitive disorders to neurodegenerative diseases. Due to the importance of dendritic shape to neuronal network function, the morphology of dendrites is tightly controlled and is influenced by both intrinsic and extrinsic factors. In this work, we examine how brain-derived neurotrophic factor (BDNF), one of the most well-studied extrinsic regulators of dendritic branching, affects the arbor when it is applied locally via microbeads to cultures of hippocampal neurons. We found that local application of BDNF increases both proximal and distal branching in a time-dependent manner and that local BDNF application attenuated pruning of dendrites that occurs with neuronal maturation. Additionally, we examined whether cytosolic PSD-95 interactor (cypin), an intrinsic regulator of dendritic branching, plays a role in these changes and found strong evidence for the involvement of cypin in BDNF-promoted increases in dendrites after 24 but not 48 hours of application. This current study extends our previous work in which we found that bath application of BDNF for 72 hours, but not shorter times, increases proximal dendrite branching and that this increase occurs through transcriptional regulation of cypin. Moreover, this current work illustrates how dendritic branching is regulated differently by the same growth factor depending on its spatial localization, suggesting a novel pathway for modulation of dendritic branching locally.

In this paper, we proposed an extended version of binary code selection algorithm (BCSA) for the variable-order prediction by introducing multiple distal dendrites into BCSA. The proposed model of artificial neurons has a single proximal dendrite to receive the feed-forward inputs (sequences) from the world and multiple distal dendrites to receive the horizontal inputs from nearby neurons. During training, each distal dendrite is able to remember the states of neurons activated at different time and store the temporal correlations. After training, each distal dendrite independently recalls the temporal correlations contained in sequences and makes a local prediction. The variable-order prediction can be obtained by combining these local predictions made by multiple distal dendrites. Experiments show that the proposed method outperforms BCSA and other methods, such as back-propagation networks and radial basis function networks, especially while processing complex sequences.

Neuroscience Hippocampal neurons receive and integrate synaptic input along their dendritic tree. Inputs located near the cell soma or in the distal dendrite contribute differently to neuronal integration by a variety of mechanisms, including NMDA receptor (NMDAR)–dependent plasticity processes. Using superresolution microscopy, single-nanoparticle imaging, and glutamate uncaging, Ferreira et al. investigated the nanoscale organization of NMDARs containing the subunits GluN2A and GluN2B along the dendritic tree. The organization and surface dynamics of GluN2B-NMDARs, but not of GluN2A-NMDARs, changed between proximal and distal clusters, with a gradual increase in receptor local density from proximal to distal dendritic segments. At the proximal dendrite, the nanoscale organization and membrane dynamics of GluN2B-NMDARs were influenced by physical interplay with the protein kinase CaMKII. Proc. Natl. Acad. Sci. U.S.A. 117 , 24526 (2020).

Corticosteroid stress hormones drive a multitude of adaptations in the brain. Hypothalamic corticotropin-releasing hormone (CRH) neurons control the circulating levels of corticosteroid stress hormones in the body and are themselves highly sensitive to corticosteroids. CRH neurons have been shown to undergo various adaptions in response to acute stress hormone elevations. However, their structural and physiological changes under chronically elevated corticosterone are less clear. To address this, we determined the structural and functional changes in CRH neurons in the paraventricular nucleus of the hypothalamus following 14 days of corticosterone treatment. We find that prolonged corticosterone elevation reduces CRH neuron intrinsic excitability as measured by summation of subthreshold postsynaptic depolarisations and spiking output. We find that under normal conditions, CRH neurons have a relatively compact and simple dendritic arbor, with a low density of somatic and dendritic spines. Interestingly, the axon originated from a proximal dendrite close to the soma in approximately half of the CRH neurons reconstructed. While prolonged elevation in corticosterone levels did not result in any changes to gross dendritic morphology, it induced a significant reduction in both somatic and dendritic spine density. Together these data reveal the morphological features of hypothalamic CRH neurons and highlight their capacity to undergo functional and morphological plasticity in response to chronic corticosterone elevations. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Gonadotrophin-releasing hormone (GnRH) neurone function is dependent upon gonadal steroid hormone feedback, which is communicated in large part through an afferent neuronal network. The classical neurotransmitters GABA and glutamate are important regulators of GnRH neurone activity and are implicated in mediating feedback signals. In the present study, we aimed to determine whether GABAergic or glutamatergic input to GnRH neurones differs between males and females and/or exhibits morphological plasticity in response to steroid hormone feedback in females. Tissue collected from GnRH-green fluorescent protein (GFP) male and female mice in dioestrus underwent immunofluorescence labelling of GFP and either the vesicular GABA transporter (VGAT) or the vesicular glutamate transporter 2 (VGLUT2). No differences in the densities or absolute numbers of VGAT-immunoreactive (-IR) or VGLUT2-IR puncta apposed to GnRH neurones were identified between males and females. The most significant input from either neurotransmitter was to the proximal dendritic region and 80% of VGAT-IR puncta apposed to GnRH neurones co-localised with synaptophysin. Putative inputs were also assessed in ovariectomised (OVX) female mice treated with negative (OVX+E) or positive (OVX+E+E) feedback levels of oestrogen, and OVX+E+E mice were killed during the expected GnRH/luteinising hormone surge. No differences in VGLUT2-IR contacts to GnRH neurones were identified between animals under the negative-feedback influence of oestrogen (OVX+E) or the positive influence of oestrogen (OVX+E+E), regardless of cFos activation status. By contrast, a significant elevation in putative GABAergic inputs to GnRH neurones at the time of the preovulatory surge was found in the cFos-negative subset of GnRH neurones, both at the level of the soma and at the proximal dendrite. Taken together, these data suggest that, although GABAergic and glutamatergic innervation of GnRH neurones is not sexually differentiated, cyclic fluctuations in steroid hormone feedback over the female oestrous cycle result in plastic changes in GABAergic inputs to a subpopulation of GnRH neurones.

GnRH neurons are the final output neurons of the hypothalamic network controlling fertility in mammals. In the present study, we used ankyrin G immunohistochemistry and neurobiotin filling of live GnRH neurons in brain slices from GnRH-green fluorescent protein transgenic male mice to examine in detail the location of action potential initiation in GnRH neurons with somata residing at different locations in the basal forebrain. We found that the vast majority of GnRH neurons are bipolar in morphology, elaborating a thick (primary) and thinner (secondary) dendrite from opposite poles of the soma. In addition, an axon-like process arising predominantly from a proximal dendrite was observed in a subpopulation of GnRH neurons. Ankyrin G immunohistochemistry revealed the presence of a single action potential initiation zone ∼27 μm in length primarily in the secondary dendrite of GnRH neurons and located 30 to 140 μm distant from the cell soma, depending on the type of process and location of the cell body. In addition to dendrites, the GnRH neurons with cell bodies located close to hypothalamic circumventricular organs often elaborated ankyrin G–positive axon-like structures. Almost all GnRH neurons (>90%) had their action potential initiation site in a process that initially, or ultimately after a hairpin loop, was coursing in the direction of the median eminence. These studies indicate that action potentials are initiated in different dendritic and axonal compartments of the GnRH neuron in a manner that is dependent partly on the neuroanatomical location of the cell body.

Neurones in the arcuate nucleus that express neurokinin B (NKB), kisspeptin and dynorphin (KNDy) play an important role in the reproductive axis. Oestradiol modulates the gene expression and somatic size of these neurones, although there is limited information available about whether their dendritic structure, a correlate of cellular plasticity, is altered by oestrogens. In the present study, we investigated the morphology of KNDy neurones by filling fluorescent neurones in the arcuate nucleus of Tac2-enhanced green fluorescent protein (EGFP) transgenic mice with biocytin. Filled neurones from ovariectomised (OVX) or OVX plus 17β-oestradiol (E2)-treated mice were visualised with anti-biotin immunohistochemistry and reconstructed in three dimensions with computer-assisted microscopy. KNDy neurones exhibited two primary dendrites, each with a few branches confined to the arcuate nucleus. Quantitative analysis revealed that E2 treatment of OVX mice decreased the cell size and dendritic spine density of KNDy neurones. The axons of KNDy neurones originated from the cell body or proximal dendrite and gave rise to local branches that appeared to terminate within the arcuate nucleus. Numerous terminal boutons were also visualised within the ependymal layer of the third ventricle adjacent to the arcuate nucleus. Axonal branches also projected to the adjacent median eminence and exited the arcuate nucleus. Confocal microscopy revealed close apposition of EGFP and gonadotrophin-releasing hormone-immunoreactive fibres within the median eminence and confirmed the presence of KNDy axon terminals in the ependymal layer of the third ventricle. The axonal branching pattern of KNDy neurones suggests that a single KNDy neurone could influence multiple arcuate neurones, tanycytes in the wall of the third ventricle, axon terminals in the median eminence and numerous areas outside of the arcuate nucleus. In parallel with its inhibitory effects on electrical excitability, E2 treatment of OVX Tac2-EGFP mice induces structural changes in the somata and dendrites of KNDy neurones.

Temporal lobe epilepsy is characterized by recurrent seizures in one or both temporal lobes of the brain; some in vitro models show that epileptiform discharges initiate in entorhinal layer V neurons and then spread into other areas of the temporal lobe. We previously found that, in the presence of GABAA receptor antagonists, stimulation of afferent fibers, terminating both at proximal and distal dendritic locations, initiated hyperexcitable bursts in layer V medial entorhinal neurons. We investigated the differential contribution of Ca2+-dependent mechanisms to the plateaus underlying these bursts at proximal and distal synapses. We found that the NMDA glutamatergic antagonist D,L-2-amino-5-phosphonovaleric acid (APV; 50 μM) reduced both the area and duration of the bursts at both proximal and distal synapses by about half. The L-type Ca2+ channel blocker nimodipine (10 μM) and the R- and T-type Ca2+ channel blocker NiCl2 (200 μM) decreased the area of the bursts to a lesser extent; none of these effects appeared to be location-dependent. Remarkably, the perfusion of flufenamic acid (FFA; 100 μM), to block Ca2+-activated non-selective cation currents (ICAN) mediated by transient receptor potential (TRP) channels, had a location-dependent effect, by abolishing burst firing and switching the suprathreshold response to a single action potential (AP) for proximal stimulation, but only minimally affecting the bursts evoked by distal stimulation. A similar outcome was found when FFA was pressure-applied locally around the proximal dendrite of the recorded neurons and in the presence of a selective blocker of melastatin TRP (TRPM) channels, 9-phenanthrol (100 μM), whereas a selective blocker of canonical TRP (TRPC) channels, SKF 96365, did not affect the bursts. These results indicate that different mechanisms might contribute to the initiation of hyperexcitability in layer V neurons at proximal and distal synapses and could shed light on the initiation of epileptiform activity in the entorhinal cortex.

The parafascicular nucleus (Pf) is an ascending target of the pedunculopontine nucleus (PPN) and is part of the “non-specific” intralaminar thalamus. The PPN, part of the reticular activating system, is mainly involved in waking and rapid eye movement sleep. Gamma oscillations are evident in all Pf neurons and mediated by high threshold voltage-dependent N- and P/Q-type calcium channels. We tested the hypothesis that high-speed calcium imaging would reveal calcium-mediated oscillations in synchrony with patch clamp recorded oscillations during depolarizing current ramps. Patch-clamped 9 to 19-day-old rat Pf neurons (n = 148, dye filled n = 61, control n = 87) were filled with Fura 2, Bis Fura, or Oregon Green BAPTA-1. Calcium transients were generated during depolarizing current ramps and visualized with a high-speed, wide-field fluorescence imaging system. Cells manifested calcium transients with oscillations in both somatic and proximal dendrite fluorescence recordings. Fluorescent calcium transients were blocked with the nonspecific calcium channel blocker, cadmium, or the combination of ω-Agatoxin-IVA (AgA), a specific P/Q-type calcium channel blocker and ω-conotoxin-GVIA (CgTx), a specific N-type calcium channel blocker. We developed a viable methodology for studying high-speed oscillations without the use of multi-photon imaging systems. Fil: Hyde, James. University Of Arkansas For Medical Sciences; Estados Unidos Fil: Kezunovic, Nebojsa. University Of Arkansas For Medical Sciences; Estados Unidos Fil: Urbano Suarez, Francisco Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Fisiología, Biología Molecular y Neurociencias. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Fisiología, Biología Molecular y Neurociencias; Argentina Fil: Garcia Rill, Edgar. University Of Arkansas For Medical Sciences; Estados Unidos

In Purkinje cells (PCs) of the cerebellum, a single “winner” climbing fiber (CF) monopolizes proximal dendrites, whereas hundreds of thousands of parallel fibers (PFs) innervate distal dendrites, and both CF and PF inputs innervate a narrow intermediate domain. It is unclear how this segregated CF and PF innervation is established on PC dendrites. Through reconstruction of dendritic innervation by serial electron microscopy, we show that from postnatal day 9–15 in mice, both CF and PF innervation territories vigorously expand because of an enlargement of the region of overlapping innervation. From postnatal day 15 onwards, segregation of these territories occurs with robust shortening of the overlapping proximal region. Thus, innervation territories by the heterologous inputs are refined during the early postnatal period. Intriguingly, this transition is arrested in mutant mice lacking the type 1 metabotropic glutamate receptor (mGluR1) or protein kinase Cγ (PKCγ), resulting in the persistence of an abnormally expanded overlapping region. This arrested territory refinement is rescued by lentivirus-mediated expression of mGluR1α into mGluR1-deficient PCs. At the proximal dendrite of rescued PCs, PF synapses are eliminated and free spines emerge instead, whereas the number and density of CF synapses are unchanged. Because the mGluR1-PKCγ signaling pathway is also essential for the late-phase of CF synapse elimination, this signaling pathway promotes the two key features of excitatory synaptic wiring in PCs, namely CF monoinnervation by eliminating redundant CF synapses from the soma, and segregated territories of CF and PF innervation by eliminating competing PF synapses from proximal dendrites.

Hierarchical temporal memory is an online machine learning model that simulates some of the structural and algorithmic properties of neocortex. The new implementation of hierarchical temporal memory is proposed in the paper. The main distinction of the implementation is chain extraction module that complements the spatial and temporal polling modules of HTM. The new module simplifies cross-level regions connection implementation (e.g. feedback). An experiment is also described to illustrate how hierarchical temporal memory with explicit states extraction works.

T-type calcium channels play a pivotal role in regulating neural membrane excitability in the nervous system. However, the precise subcellular distributions of T-type channel subunits and their implication for membrane excitability are not well understood. Here we investigated the subcellular distribution of the α1G subunit of the calcium channel which is expressed highly in the mouse dorsal lateral geniculate nucleus (dLGN). Light microscopic analysis demonstrated that dLGN exhibits intense immunoperoxidase reactivity for the α1G subunit. Electron microscopic observation showed that the labeling was present in both the relay cells and interneurons and was found in the somatodendritic, but not axonal, domains of these cells. Most of the immunogold particles for the α1G subunit were either associated with the plasma membrane or the intracellular membranes. Reconstruction analysis of serial electron microscopic images revealed that the intensity of the intracellular labeling exhibited a gradient such that the labeling density was higher in the proximal dendrite and progressively decreased towards the distal dendrite. In contrast, the plasma membrane-associated particles were distributed with a uniform density over the somatodendritic surface of dLGN cells. The labeling density in the relay cell plasma membrane was about 3-fold higher than that of the interneurons. These results provide ultrastructural evidence for cell-type-specific expression levels and for uniform expression density of the α1G subunit over the plasma membrane of dLGN cells. J. Comp. Neurol. 518:4362–4374, 2010. © 2010 Wiley-Liss, Inc.

Perisomatic GABAergic input appears spared or even increased in intractable temporal lobe epilepsy, and has been suggested to contribute to the generation of pathological discharges. Nevertheless, its degree of functional activity during epileptiform synchronization has not been thoroughly investigated. Thus, it remains unclear how structural preservation or loss of domain-specific GABAergic input may affect the network. Here, we have taken advantage of a model of epileptiform activity in vitro to quantify the charge transfer provided by perisomatic GABAA receptor-mediated input to CA1 pyramidal neurons during interictal-like bursts. By recording both firing in GABAergic interneurons and the charge transfer generated by unitary postsynaptic currents to target pyramidal cells, we have estimated the charge transfer that would be dynamically generated by the recruitment of the entire pool of perisomatic-targeting interneurons and the number of perisomatic-targeting interneurons that would be required to generate the experimentally observed GABAergic input. In addition, we have recorded and compared the dynamics and charge density of GABAergic input recorded at different membrane compartments such as the soma vs. the proximal dendrite. Our results suggest that GABAA receptor-mediated perisomatic input is massively activated during burst synchronization and that its kinetic properties and charge density are similar at the soma and proximal dendrite. These functional results match structural data published by other laboratories very well and strengthen the hypothesis that the potential preservation of perisomatic GABAergic input in intractable epilepsies may be a key factor in the generation of pathological network activity.

In the mammalian retina, AII amacrine cells play a crucial role in scotopic vision. They transfer rod signals from rod bipolar cells to the cone circuit, and divide these signals into the ON and OFF pathways at the discrete synaptic layers. AII amacrine cells have been reported to generate tetrodotoxin (TTX)-sensitive repetitive spikes of small amplitude. To investigate the properties of the spikes, we performed whole-cell patch-clamping of AII amacrine cells in mouse retinal slices. The spike frequency increased in proportion to the concentration of glutamate puffer-applied to the arboreal dendrite and to the intensity of the depolarizing current injection. The spike activity was suppressed by L-2-amino-4-phosphonobutyric acid, a glutamate analogue that hyperpolarizes rod bipolar cells, puffer-applied to the outer plexiform layer. Therefore, it is most likely that the spike frequency generated by AII amacrine cells is dependent on the excitatory glutamatergic input from rod bipolar cells. Gap junction blockers reduced the range of intensity of input with which spike frequency varies. Application of TTX to the soma and the proximal dendrite of AII amacrine cells blocked the voltage-gated Na(+) current significantly more than application to the arboreal dendrite, indicating that the Na(+) channels are mainly localized in these regions. Our results suggest that the intensity of the glutamatergic input from rod bipolar cells is coded by the spike frequency at the soma and the proximal dendrite of AII amacrine cells, raising the possibility that the spikes could contribute to the OFF pathway to enhance release of neurotransmitter.

Even though many extracellular factors have been identified as promoters of general dendritic growth and branching, little is known about the cell-intrinsic modulators that allow neurons to sculpt distinctive patterns of dendrite arborization. Here, we identify Lrig1, a nervous system-enriched LRR protein, as a key physiological regulator of dendrite complexity of hippocampal pyramidal neurons. Lrig1-deficient mice display morphological changes in proximal dendrite arborization and defects in social interaction. Specifically, knockdown of Lrig1 enhances both primary dendrite formation and proximal dendritic branching of hippocampal neurons, two phenotypes that resemble the effect of BDNF on these neurons. In addition, we show that Lrig1 physically interacts with TrkB and attenuates BDNF signaling. Gain and loss of function assays indicate that Lrig1 restricts BDNF-induced dendrite morphology. Together, our findings reveal a novel and essential role of Lrig1 in regulating morphogenic events that shape the hippocampal circuits and establish that the assembly of TrkB with Lrig1 represents a key mechanism for understanding how specific neuronal populations expand the repertoire of responses to BDNF during brain development. Synopsis Lrig1 is a novel regulator of dendritogenesis and apical dendrite branching of CA1-CA3 pyramidal hippocampal neurons in vivo, acting as an endogenous inhibitor of neurotrophin-induced proximal dendrite arborization of pyramidal hippocampal neurons. Lrig1 is a physiological regulator of hippocampal dendrite development. Lrig1 is required for proper apical dendrite arborization of CA1-CA3 pyramidal neurons and social behavior. Lrig1 controls TrkB signaling and dendrite development induced by BDNF. Lrig1 regulates dendritogenesis and apical dendrite branching of CA1-CA3 pyramidal hippocampal neurons in vivo, acting as an endogenous inhibitor of neurotrophin-induced proximal dendrite arborization of pyramidal hippocampal neurons. Fil: Alsina, Fernando Cruz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina Fil: Hita, Francisco Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina Fil: Fontanet, Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina Fil: Irala, Dolores. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina Fil: Hedman, Häkan. Universidad de Umea; Suecia Fil: Ledda, Maria Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina Fil: Paratcha, Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Biología Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de Biología Celular y Neurociencia; Argentina

The onset of puberty is initiated by an increase in the release of the gonadotropin-releasing hormone (GnRH) from GnRH neurons in the hypothalamus. However, the precise mechanism that leads to the activation of GnRH neurons at puberty remains controversial. Spines are small protrusions on the surface of dendrites that normally receive excitatory inputs. In this study, we analyzed the number and morphology of spines on GnRH neurons to investigate changes in synaptic inputs across puberty in rats. For morphological estimation, we measured the diameter of the head (DH) of each spine and classified them into small-type (DH 0.65 μm), large-type (DH 0.65 μm) and giant-type (DH 0.9 μm). The greatest number of spines was observed at the proximal dendrite within 50 μm of the soma. At the soma and proximal dendrite, the number of spines was greater in adults than in juveniles in both male and female individuals. Classification of spines revealed that the increase in spine number was due to increases in large- and giant-type spines. To further explore the relationship between spines on GnRH neurons and pubertal development, we next analyzed adult rats neonatally exposed to estradiol benzoate, in which puberty onset and reproductive functions are disrupted. We found a decrease in the number of all types of spines. These results suggest that GnRH neurons become to receive more and greater excitatory inputs on the soma and proximal dendrites as a result of the changes that occur at puberty and that alteration to spines plays a pivotal role in normal pubertal development.

Recent theories have proposed that the unifying principle of brain function is the minimisation of variational free energy and that this is best achieved using a hierarchical predictive coding (HPC) framework. Hierarchical Temporal Memory (HTM) is a model of neocortical function that fits within the free energy framework but does not implement predictive coding. Recent work has attempted to integrate predictive coding and hierarchical message passing into the existing suite of HTM Cortical Learning Algorithms (CLA) producing a PC-CLA hybrid. In this paper we examine for the first time how such hierarchical message passing can be implemented in a pure HTM framework using distal dendrite structures that are already implemented in the CLA temporal pooler. We show this approach outperforms the more simplistic proximal dendrite structures used in the PC-CLA hybrid and also that the new CLA hierarchy is effective for anomaly detection and image reconstruction problems that are beyond the reach of the existing single-level CLA framework.

The mechanisms in the retina that generate light responses selective for the direction of image motion remain unresolved. Recent evidence indicates that directionally selective light responses occur first in the retina in the dendrites of an interneuron, i.e., the starburst amacrine cell, and that these responses are highly sensitive to the activity of Na-K-2Cl (NKCC) and K-Cl (KCC), two types of chloride cotransporter that determine whether the neurotransmitter GABA depolarizes or hyperpolarizes neurons, respectively. We show here that selective blockade of the NKCC2 and KCC2 cotransporters located on starburst dendrites consistently hyperpolarized and depolarized the starburst cells, respectively, and greatly reduced or eliminated their directionally selective light responses. By mapping NKCC2 and KCC2 antibody staining on these dendrites, we further show that NKCC2 and KCC2 are preferentially located in the proximal and distal dendritic compartments, respectively. Finally, measurements of the GABA reversal potential in different starburst dendritic compartments indicate that the GABA reversal potential at the distal dendrite is more hyperpolarized than at the proximal dendrite due to KCC2 activity. These results thus demonstrate that the differential distribution of NKCC2 on the proximal dendrites and KCC2 on the distal dendrites of starburst cells results in a GABA-evoked depolarization and hyperpolarization at the NKCC2 and KCC2 compartments, respectively, and underlies the directionally selective light responses of the dendrites. The functional compartmentalization of interneuron dendrites may be an important means by which the nervous system encodes complex information at the subcellular level.

The firing evoked by injection of simulated barrages of EPSCs into the proximal dendrite of layer 5 pyramidal neurons is greater than when comparable inputs are injected into the soma. This boosting is mediated by dendritic Na+conductances. However, the presence of other active conductances in the dendrites, some of which are nonuniformly distributed, suggests that the degree of boosting may differ along the somatodendritic axis. Here, we injected EPSC barrages at the soma and at the proximal, middle, and distal segments of the apical dendrite and measured boosting of subthreshold and suprathreshold responses. We found that although boosting was maintained throughout the apical dendrite, the degree of boosting changed nonmonotonically with distance from the soma. Boosting dipped in the middle dendritic segments as a result of the deactivation of the hyperpolarization-activated cation current,Ih, but increased in the distal dendrites as a result of the activation of Ca2+conductances. In the distal dendrites, EPSC barrages evoked repetitive bursts of action potentials, and the bursting pattern changed systematically with the magnitude of the input barrages. The quantitative changes in boosting along the somatodendritic axis suggest that inputs from different classes of presynaptic cells are weighted differently, depending on the location of the synaptic contacts. Moreover, the tight coupling between burst characteristics and stimulus parameters indicate that the distal dendrites can support a coding scheme that is different from that at sites closer to the soma, consistent with the notion of a separate dendritic integration site.

Maturation of specific neuronal connections in the mature nervous system includes elimination of redundant synapses formed earlier during development. In the cerebellum of adult animals, each Purkinje cell (PC) is innervated by a single climbing fiber (CF). In early postnatal development each PC is innervated by multiple CFs and elimination of synapses formed by supernumerary CFs occurs until monoinnervation is established at around postnatal day 20 (P20) in mice. It is not clear whether multiple CFs, or only a single CF, translocate from the cell body of immature PCs to the developing dendrite and, in case several CFs translocate, whether they share or segregate their innervation fields. To localize CF innervation fields, we imaged changes in postsynaptic sodium concentration resulting from CF-mediated postsynaptic currents. We found that more than one CF translocates from an innervation field on the cell body of the PC to the developing dendrite and that these CFs share rather than segregate their innervation fields. We concluded that both the soma and the proximal dendrite of the PC are territories of competition for the developing CFs and that the overlapping of their termination fields may be the prerequisite for a local process of elimination of all but one CF, as previously demonstrated in the developing neuromuscular junction.

Nogo-A is known to be a myelin-associated protein with strong inhibitory effect on neurite outgrowth and has been considered one of the major factors that hinder fiber regeneration in the central nervous system. Recent studies have demonstrated widespread occurrence of nogo-A mRNA and Nogo-A protein in neurons. Our concurrent immunohistochemical study substantiated the widespread distribution of neuronal Nogo-A. The present study was thus focused on its intraneuronal distribution in the central nervous system, using Western blotting, immunohistochemical, and immunogold electron microscopic techniques. Western blotting of the nucleus, cytoplasm, and membrane subcellular fractions of the cerebellum and spinal cord tissues demonstrated that all three fractions contained Nogo-A. Nogo-A immunoreactivity could be identified under confocal microscope in the nucleus, perikayon, and proximal dendrite and along the cell membrane. Under the electron microscope, the perikaryonal Nogo-A immunogold particles were mainly distributed at polyribosomes and rough endoplasmic reticulum, suggesting its relationship with translation process. The immunogold particles could also be found beneath or on the plasma membrane. In the nucleus, the Nogo-A immunogold particles were found to be localized at the chromatins of the nucleus, indicating its possible involvement in gene transcription. The presence of Nogo-A in the nucleus was further supported by transfection of COS-7L cells with nogo-A. This study provides the first immunocytochemical evidence for intraneuronal distribution of Nogo-A. Apparently, the significance of Nogo-A in the central nervous system is far more complex than what has been envisioned.

Cisternal stacks are induced during hypoxia, which may be associated with intracellular Ca2+ regulation. Although neurons are divided internally in different compartments, little is known about regional differences in cisternal stack formation. We investigated the effects of hypoxic hypoxia and later reoxygenation on cisternal stack formation and other ultrastructual changes in the proximal dendrite, dendritic spine, and cell body of cerebellar Purkinje cells in rats. After brief hypoxic events, cisternal stacks appeared predominantly in the proximal dendrites and after longer hypoxic events in dendritic spines and cell body. Following reoxygenation, cisternal stacks disappeared first in the cell body, followed by the dendritic spines, then the proximal dendrites. These results showed that stack formation occurred at different degrees and time courses among the three regions, and the effect was reversible, which suggests that these compartments are differentially sensitive to hypoxia.

Cortical columns contain specific neuronal populations with characteristic sets of connections. This wiring forms the structural basis of dynamic information processing. However, at the single-cell level little is known about specific connectivity patterns. We performed experiments in infragranular layers (V and VI) of rat somatosensory cortex, to clarify further the input patterns of inhibitory interneurons immunoreactive (ir) for vasoactive intestinal polypeptide (VIP). Neurons in acute slices were electrophysiologically characterized using whole-cell recordings and filled with biocytin. This allowed us to determine their firing pattern as regular-spiking, intrinsically bursting and fast-spiking, respectively. Biocytin was revealed histochemically and VIP immunohistochemically. Sections were examined for contacts between the axons of the filled neurons and the VIP-ir targets. Twenty pyramidal cells and five nonpyramidal (inter)neurons were recovered and sufficiently stained for further analysis. Regular-spiking pyramidal cells displayed no axonal boutons in contact with VIP-ir targets. In contrast, intrinsically bursting layer V pyramidal cells showed four putative single contacts with a proximal dendrite of VIP neurons. Fast-spiking interneurons formed contacts with two to six VIP neurons, preferentially at their somata. Single as well as multiple contacts on individual target cells were found. Electron microscopic examinations showed that light-microscopically determined contacts represent sites of synaptic interactions. Our results suggest that, within infragranular local cortical circuits, (i) fast-spiking interneurons are more likely to influence VIP cells than are pyramidal cells and (ii) pyramidal cell input probably needs to be highly convergent to fire VIP target cells.

The persistent Na(+) current (I(Nap)) is believed to be an important target of dopamine modulation in prefrontal cortex (PFC) neurons. While past studies have tested the effects of dopamine on I(Nap), the results have been contradictory largely because of difficulties in measuring I(Nap) using somatic whole-cell recordings. To circumvent these confounds we used the cell-attached patch-clamp technique to record single Na(+) channels from the soma, proximal dendrite (PD) or proximal axon (PA) of intact prefrontal layer V pyramidal neurons. Under baseline conditions, numerous well resolved Na(+) channel openings were recorded that exhibited an extrapolated reversal potential of 73 mV, a slope conductance of 14-19 pS and were blocked by tetrodotoxin (TTX). While similar in most respects, the propensity to exhibit prolonged bursts lasting40 ms was many fold greater in the axon than the soma or dendrite. Bath application of the D1/D5 receptor agonist SKF81297 shifted the ensemble current activation curve leftward and increased the number of late events recorded from the PD but not the soma or PA. However, the greatest effect was on prolonged bursting where the D1/D5 receptor agonist increased their occurrence 3 fold in the PD and nearly 7 fold in the soma, but not at all in the PA. As a result, D1/D5 receptor activation equalized the probability of prolonged burst occurrence across the proximal axosomatodendritic region. Therefore, D1/D5 receptor modulation appears to be targeted mainly to Na(+) channels in the PD/soma and not the PA. By circumventing the pitfalls of previous attempts to study the D1/D5 receptor modulation of I(Nap), we demonstrate conclusively that D1/D5 receptor activation can increase the I(Nap) generated proximally, however questions still remain as to how D1/D5 receptor modulates Na(+) currents in the more distal initial segment where most of the I Nap is normally generated.

The amplitude of backpropagating action potentials (BAPs) is attenuated, either activity- or neurotransmitter-dependently in the apical dendrite of hippocampal pyramidal neurons. To test the possibility that this BAP attenuation may contribute to regulating the inducibility of long-term potentiation (LTP), BAPs evoked by theta-burst stimulation (TBS), a standard protocol for LTP induction, to apical dendrite synapses were subjected to perturbation by conditioning stimuli to basal dendrite synapses. During this conditioned TBS (cTBS), the amplitude of BAPs was noticeably attenuated, but that of somatic action potentials was not. In the distal dendrite area, cTBS-induced LTP was much smaller than that induced by TBS. By contrast, no difference was observed between TBS- and cTBS-induced LTP in the proximal dendrite area. These findings suggest that the activity-dependent attenuation of BAPs, propagating along the apical dendrite, may serve to regulate hippocampal synaptic plasticity.

We investigated activity-dependent calcium increases in proximal dendrites of dentate granule cells in the rat hippocampus, and its relationship with induction of LTP at perforant path synapses (PP-synapses). LTP was induced at PP-synapses by high-frequency stimulation (HFS; 100 Hz for 0.4 s), and the same HFS evoked a dendritic calcium increase in the proximal dendrite. However, bath-application of the L-type voltage-dependent calcium channel (VDCC) blocker nimodipine noticeably reduced this calcium increase without abolishing induction of LTP. This calcium increase mediated by high-threshold VDCCs is likely to be evoked by action potentials. LTP induction at PP-synapses is hence suggested to be independent of action potential-induced calcium increases in the proximal dendrite.

From 1939 to 1967, J.L. Conel quantitatively studied the microscopic features of the developing human cerebral cortex and published the findings in eight volumes. We have constructed a database using his neuroanatomical measurements (neuronal packing density, myelinated large fiber density, large proximal dendrite density, somal breadth and height, and total cortical and cortical layer thickness) at the eight age periods (0 [term birth], 1, 3, 6, 15, 24, 48, and 72 postnatal months) he studied. In this report, we examine changes in neuron numbers over the eight age-points for 35 von Economo areas for which Conel gave appropriate data. From birth to 3 months postnatal age, total cortical neuron number increases 23–30%, then falls to within 3.5% of the birth value at 24 months, supporting our previous work showing that the observed decrease in the number of neurons per column of cortex under a 1-mm2 cortical surface from birth to 15 months is almost entirely due to cortical surface expansion. The present study also shows a 60–78% increase in total cortical neuron number above the birth value from postnatal ages 24 to 72 months. The generalization, to humans at least, of the finding of no postnatal neurogenesis in rhesus macaques, a species belonging to a super-family that diverged from that of Homo sapiens more than 25 million years ago, is not warranted until explicitly proven for humans. The data of the present study support the existence of substantial postnatal neurogenesis in humans for the 35 cortical areas studied.

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic activation and signal processing in the neocortex. As a novel approach we have used infra-red videomicroscopy in combination with photostimulation or microiontophoresis in brain slices of rat neocortex to map the distribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on pyramidal neurons of layer V. Both modes of application revealed a spatially distinct distribution of glutamate receptor subtypes: the soma and the proximal dendrite of neurons are highly sensitive to NMDA, whereas the more distal parts of the dendrite are more sensitive to AMPA. An implication is that NMDA receptors near the soma might regulate the amplification of synaptic signals resulting from AMPA receptor activation on remote dendritic sites.

Dendritic morphology was studied in human hippocampal dentate granule cells (DGCs) by intracellularly-injecting biocytin in slice preparations that were obtained from temporal lobe epilepsy patients who underwent a surgical treatment for medically-intractable seizures. These DGCs had a fan-shaped dendritic domain of 54.1 degrees +/- 4.1 S.E.M. with 13.8 +/- 1.1 branch points and an estimated total dendritic length of 11535.6 microns +/- 3045.4. Dendritic spines were counted, and spine density was calculated to be 0.25 spines/microns +/- 0.16 S.E.M.. However, when the cells were categorized into two groups based on the presence or absence of the aberrant mossy fiber collaterals, the number of dendritic branches was significantly lower and spine density was significantly higher in DGCs that had aberrant collaterals. In particular, in the proximal dendrite, the spine density was 5 times higher in DGCs whose own mossy fibers were reorganized sending aberrant collaterals to this dendritic region (0.750 spines/microns +/- 0.203 S.E.M.: P0.01) than the DGCs without such collaterals (0.082 spines/microns +/- p.021 S.E.M.). These results suggest that the axonal reorganization may have an effect on the morphology of DGC dendrites directly or indirectly in such a way that dendritic structure and spines could be protected from seizure-induced excitotoxic cell damage.

A multi-compartmental molecular model has been developed for rodent basal forebrain cholinergic neurons with established gene expression levels. Reconstruction of neurons and network function were acquired using the Transcriptome-To-Physiome™ (TTP™) NeurobioSimulation. Gene expression values [NCBI GEO GSE 13379] were used to derive protein level and kinetic parameters for ligand and voltage gated ion channels in the TTP™ NeurobioSimulator Model using COPASI® software. Global parameters for membrane potential used permeability and ion concentrations inside and outside of the membrane in the Goldman-Hodgkin-Katz equation. Four compartments of the model neuron are included: glutamate synapse, distal dendrite, proximal dendrite, and axon hillock. The simulation of a voltage-gated sodium channel activation, and inactivated states of distal dendrites of cholinergic modeled neurons depends on the excitatory postsynaptic potential (EPSP) event. This distally activated event yielded temporally relevant proximal dendritic activation and inactivation of voltage-gated sodium and potassium channels in the reconstructed neuron. Graded potentials showed temporal summation and a classic action potential occurs at the axon hillock with sodium and potassium fluxes as expected. In future studies, we will reconstruct the electrophysiology of vulnerable neuronal populations in the diseased brain and compare them to controls thus lending substantial insight into molecular and network function corollary to neuropathogenesis.

The coding of odor intensity by an olfactory receptor neuron model was studied under steady-state stimulation. Our model neuron is an elongated cylinder consisting of the following three components: a sensory dendritic region bearing odorant receptors, a passive region consisting of proximal dendrite and cell body, and an axon. First, analytical solutions are given for the three main physiological responses: (1) odorant-dependent conductance change at the sensory dendrite based on the Michaelis-Menten model, (2) generation and spreading of the receptor potential based on a new solution of the cable equation, and (3) firing frequency based on a Lapicque model. Second, the magnitudes of these responses are analyzed as a function of odorant concentration. Their dependence on chemical, electrical, and geometrical parameters is examined. The only evident gain in magnitude results from the activation-to-conductance conversion. An optimal encoder neuron is presented that suggests that increasing the length of the sensory dendrite beyond about 0.3 space constant does not increase the magnitude of the receptor potential. Third, the sensitivities of the responses are examined as functions of (1) the concentration at half-maximum response, (2) the lower and upper concentrations actually discriminated, and (3) the width of the dynamic range. The overall gain in sensitivity results entirely from the conductance-to-voltage conversion. The maximum conductance at the sensory dendrite appears to be the main tuning constant of the neuron because it determines the shift toward low concentrations and the increase in dynamic range. The dynamic range of the model cannot exceed 5.7 log units, for a sensitivity increase at low odor concentration is compensated by a sensitivity decrease at high odor concentration.

Although the majority of mRNAs expressed in neurons are confined to the perikaryon, a growing number appear to be transported into dendrites. It is likely that this allows for the local regulation of protein synthesis within discrete subcellular compartments. Here, three different subcellular distribution patterns are demonstrated for four mRNAs that encode proteins highly expressed in Purkinje cells and their dendrites; mRNAs are found in the perikaryon only, perikaryon and proximal dendrite, or perikaryon and proximal plus distal dendrites. Further, it is shown that transport of an mRNA into the dendrites increases its sensitivity to translational inhibition by diphtheria toxin. These data suggest a simple model whereby the transport machinery can regulate the translation of selected mRNAs. Thus, environmental signals that generally affect translational efficiency in concert with the selectivity provided by the transport machinery could provide a means to locally regulate the synthesis of a restricted pool of proteins.

To examine synaptic input and association of terminals containing dopamine and other transmitters to rat striatal nitric oxide synthase-expressing neurons, an electron microscopic study using tyrosine hydroxylase (TH) immunohistochemistry combined with histochemistry for NADPH-diaphorase (NADPHd) was performed. NADPHd-positive neurons had medium-sized cell bodies containing a highly invaginated nucleus and received relatively sparse synaptic input; 3.6% of boutons apposed to the NADPHd-positive neurons were TH-immunoreactive. Of these TH-immunoreactive boutons, two synaptic contacts showing symmetrical synaptic specializations were found on a cell body and a proximal dendrite of a NADPHd-positive neuron. Other nonsynaptic TH-immunoreactive boutons were occasionally associated with unlabeled terminals adjacent to the NADPHd-positive dendrites and also forming asymmetric synaptic contacts with unlabeled spinous or dendritic profiles. These results suggest that activity of the striatal neurons that release nitric oxide may be regulated by direct synaptic input from dopaminergic neurons and also suggest that the TH-immunoreactive terminals associated with the dendrites of nitric oxide synthase-expressing neurons provide the sites where nitric oxide influences dopamine release from neighboring terminals.

The calcium-binding protein calretinin is present in an intrinsic GABAergic and an extrinsic non-GABAergic system in the rat and monkey hippocampal formation. Important species differences have been noted in hippocampal cell types immunostained for calretinin and the termination pattern of calretinin containing hypothalamic afferents in the hippocampus. In the present study, calretinin-containing neurons were visualized using immunocytochemistry in the human hippocampal formation of individuals which showed no significant neuropathological alterations. Calretinin-immunoreactivity was present exclusively in non-granule cells of the dentate gyrus and in non-pyramidal cells of Ammon's horn. Calretinin-positive neurons were found most frequently in the hilus of the fascia dentata and in strata radiatum and lacunosum-moleculare of CA1, whereas neurons in CA2 and CA3 were rarely immunostained. The majority of calretinin-immunoreactive neurons were small, bipolar or fusiform neurons. The dendritic trees of the calretinin-positive neurons were, for the most part, parallel to the dendrites of the principal cells. In the hilus, however, we observed cells with dendrites restricted to the hilar area. These dendrites were parallel to the granule cell layer. In the stratum lacunosum-moleculare, neurons with dendrites oriented parallel to the hippocampal fissure were frequently detected. In general, dendrites were smooth or sparsely spiny, displaying small conventional spines. The axons usually emerged from the proximal dendrite and could be followed over long distances. Axons were thin, had small varicosities and displayed only few collaterals which branched relatively far away from the cell body. Distinct bands of darkly stained calretinin-positive fibers occupied the innermost portion of the dentate molecular layer and the pyramidal cell layer of CA2. This distribution of calretinin-immunoreactive structures in the human hippocampus is similar to that observed in other primates but differs from that described in lower mammals, i.e., the rat. Our findings suggest that primates may share a common hippocampal calretinin-containing system, presumably both the intrinsic GABAergic and the extrinsic hypothalamic non-GABAergic components.

In this quantitative electron microscopic study we investigated the distribution of GABA axon terminals on rat abducens motoneurons by combining retrograde labelling of montoneurons with post-embedding immunodetection of GABA. We analysed the synapses on 13 cell bodies and 60 proximal dendritic profiles distributed along the entire rostro-caudal extent of the nucleus. For each of these two compartments, we analysed 1754 and 1176 axon terminals in contact with 6042 and 3299 microns of postsynaptic membrane. The axon terminals were classified as Sv-type (containing spherical vesicles) or Pv-type (containing pleomorphic vesicles). The GABAergic terminals contained pleomorphic vesicles and established mainly symmetrical synaptic contacts. Their apposition lengths were greater than those of unlabelled terminals. On cell bodies, the percentage of GABAergic synaptic covering varied from 2.5% to 14.1% and the synaptic frequency of GABAergic axon terminals varied from 0.6% to 8.9%. These two parameters were significantly correlated with the diameter of the motoneurons. The percentage of synaptic covering and synaptic frequency were smaller on dendrites of small motoneurons than on those of large ones. The proximal dendrites of small motoneurons had a lesser GABAergic innervation than large ones. The total synaptic covering and frequency were smaller on somata than on dendrites. However, the percentage of synaptic covering by GABA terminals was higher on cell bodies than on proximal dendrites.

Previous studies revealed mechanisms of dendritic inputs leading to action potential initiation at the axon initial segment and backpropagation into the dendritic tree. This interest has recently expanded toward the communication between different parts of the dendritic tree which could preprocess information before reaching the soma. This study tested for effects of asymmetric voltage attenuation between different sites in the dendritic tree on summation of synaptic inputs and action potential initiation using the NEURON simulation environment. Passive responses due to the electrical equivalent circuit of the three-dimensional neuron architecture with leak channels were examined first, followed by the responses after adding voltage-gated channels and finally synaptic noise. Asymmetric attenuation of voltage, which is a function of asymmetric input resistance, was seen between all pairs of dendritic sites but the transfer voltages (voltage recorded at the opposite site from stimulation among a pair of dendritic sites) were equal and also summed linearly with local voltage responses during simultaneous stimulation of both sites. In neurons with voltage-gated channels, we reproduced the observations where a brief stimulus to the proximal ascending dendritic branch of a pyramidal cell triggers a local action potential but a long stimulus triggers a somal action potential. Combined stimulation of a pair of sites in this proximal dendrite did not alter this pattern. The attraction of the action potential onset toward the soma with a long stimulus in the absence of noise was due to the higher density of voltage-gated sodium channels at the axon initial segment. This attraction was, however, negligible at the most remote distal dendritic sites and was replaced by an effect due to high input resistance. Action potential onset occurred at the dendritic site of higher input resistance among a pair of remote dendritic sites, irrespective of which of these two sites received the synaptic input. Exploration of the parameter space showed how the gradient of voltage-gated channel densities and input resistances along a dendrite could draw the action potential onset away from the stimulation site. The attraction of action potential onset toward the higher density of voltage-gated channels in the soma during stimulation of the proximal dendrite was, however, reduced after the addition of synaptic noise.

Previous patch-clamp studies by our laboratory showed that acute exposure to the pesticide rotenone augments inward currents evoked by N-methyl-d-aspartate (NMDA) in substantia nigra zona compacta (SNC) dopamine neurons in slices of rat brain. The present experiments were done to search for histological evidence of increased neurotoxicity produced by combined rotenone and NMDA treatments. In horizontal slices of rat midbrain, we found that a 30 min superfusion with 100 nM rotenone caused significant injury to tyrosine hydroxylase (TH)-positive proximal dendrites in dorsal and ventral regions of the SNC and ventral tegmental area (VTA). Moreover, treatment with 100 μM NMDA potentiated rotenone toxicity. In contrast, treatment with 30 μM NMDA protected against rotenone-induced injury to dendrites in the ventral SNC and ventral VTA. Interestingly, treatment with 30 μM NMDA-alone produced an apparent increase in proximal dendrite scores in ventral SNC and dorsal VTA. We conclude that NMDA has concentration-dependent actions on rotenone toxicity that differ according to regional subtype of dopamine neuron.

The synaptic input of six spiny stellate neurons in sublamina 4A of cat area 17 was assessed by electron microscopy. The neurons were physiologically characterized and filled with horseradish peroxidase in vivo. After processing the neurons were reconstructed at the light microscopic level using computer-assisted methods and analyzed quantitatively. The extensive branching of the dendritic tree about 50 microns from the soma meant that the distal branches constituted five times the length of proximal dendrite. Proximal and distal portions of a single dendrite from each neuron were examined in series of ultrathin sections (1,456 sections) in the electron microscope. The majority (79%) of the 263 synapses examined were asymmetric; the remainder (21%) were symmetric. Symmetric synapses formed 35% of synapses sampled on proximal dendrites and were usually located on the shaft. They formed only 4% of synapses sampled on distal dendrites. Spines accounted for less than half of the total asymmetric synapses (45%); the remainder were on shafts. Symmetric synapses formed with four of 92 spines. Nine spines formed no synapses. Spiny stellate neurons in cat visual cortex appear to differ considerably from pyramidal neurons in having a significant asymmetric (excitatory) synaptic input to the dendritic shaft.

Alterations in dendrite branching and morphology are present in many neurodegenerative diseases. These variations disrupt postsynaptic transmission and affect neuronal communication. Thus, it is important to understand the molecular mechanisms that regulate dendritogenesis and how they go awry during disease states. Previously, our laboratory showed that cypin, a mammalian guanine deaminase, increases dendrite number when overexpressed and decreases dendrite number when knocked down in cultured hippocampal neurons. Here, we report that exposure to brain-derived neurotrophic factor (BDNF), an important mediator of dendrite arborization, for 72 hours but not for 24 hours or less, increases cypin mRNA and protein levels in rat hippocampal neurons. BDNF signals through cypin to regulate dendrite number since knocking down cypin blocks the effects of BDNF. Furthermore, BDNF increases cypin levels via mitogen-activated protein kinase (MAPK) and transcription-dependent signaling pathways. Moreover, the cypin promoter region contains putative conserved cyclic adenosine 3’,5’-monophosphate (cAMP) response element (CRE) regions, which we found can be recognized and activated by cAMP response element-binding protein (CREB). In addition, exposure of the neurons to BDNF increased CREB binding to the cypin promoter and, in line with these data, expression of a dominant negative form of CREB blocked BDNF-promoted increases in cypin protein levels and proximal dendrite branches. Taken together, these studies suggest that BDNF increases neuronal cypin expression by the activation of CREB, increasing cypin transcription leading to increased protein expression, thus identifying a novel pathway by which BDNF shapes the dendrite network.

SummaryCortical pyramidal neurons receive thousands of synaptic inputs arriving at different dendritic locations with varying degrees of temporal synchrony. It is not known if different locations along single cortical dendrites integrate excitatory inputs in different ways. Here we have used two-photon glutamate uncaging and compartmental modeling to reveal a gradient of nonlinear synaptic integration in basal and apical oblique dendrites of cortical pyramidal neurons. Excitatory inputs to the proximal dendrite sum linearly and require precise temporal coincidence for effective summation, whereas distal inputs are amplified with high gain and integrated over broader time windows. This allows distal inputs to overcome their electrotonic disadvantage, and become surprisingly more effective than proximal inputs at influencing action potential output. Thus, single dendritic branches can already exhibit nonuniform synaptic integration, with the computational strategy shifting from temporal coding to rate coding along the dendrite.

It is a widely held view in contemporary computational neuroscience that the brain responds to sensory input by producing sparse distributed representations. In this paper we investigate a brain-inspired spatial pooling algorithm that produces such sparse distributed representations by modelling the formation of proximal dendrites associated with neocortical minicolumns. In this approach, distributed representations are formed out of a competitive process of inter-column inhibition and subsequent learning. Specifically, we evaluate the performance of a recently proposed binary spatial pooling algorithm on a well-known benchmark of greyscale natural images. Our main contribution is to augment the algorithm to handle greyscale images, and to produce better quality encodings of binary images. We also show that the augmented algorithm produces superior population and lifetime kurtosis measures in comparison to a number of other well-known coding schemes.