Subcortical auditory structures in the Mongolian gerbil: I. Golgi architecture

Anatomy of superior olivary complex and lateral lemniscus in Etruscan shrew

Based on the auditory periphery and the small head size, Etruscan shrews (Suncus etruscus) approximate ancestral mammalian conditions. The auditory brainstem in this insectivore has not been investigated. Using labelling techniques, we assessed the structures of their superior olivary complex (SOC) and the nuclei of the lateral lemniscus (NLL). There, we identified the position of the major nuclei, their input pattern, transmitter content, expression of calcium binding proteins (CaBPs) and two voltage-gated ion channels. The most prominent SOC structures were the medial nucleus of the trapezoid body (MNTB), the lateral nucleus of the trapezoid body (LNTB), the lateral superior olive (LSO) and the superior paraolivary nucleus (SPN). In the NLL, the ventral (VNLL), a specific ventrolateral VNLL (VNLLvl) cell population, the intermediate (INLL) and dorsal (DNLL) nucleus, as well as the inferior colliculus's central aspect were discerned. INLL and VNLL were clearly separated by the differential distribution of various marker proteins. Most labelled proteins showed expression patterns comparable to rodents. However, SPN neurons were glycinergic and not GABAergic and the overall CaBPs expression was low. Next to the characterisation of the Etruscan shrew's auditory brainstem, our work identifies conserved nuclei and indicates variable structures in a species that approximates ancestral conditions.

Glycine is a transmitter in the human and chimpanzee cochlear nuclei

Introduction

Auditory information is relayed from the cochlea via the eighth cranial nerve to the dorsal and ventral cochlear nuclei (DCN, VCN). The organization, neurochemistry and circuitry of the cochlear nuclei (CN) have been studied in many species. It is well-established that glycine is an inhibitory transmitter in the CN of rodents and cats, with glycinergic cells in the DCN and VCN. There are, however, major differences in the laminar and cellular organization of the DCN between humans (and other primates) and rodents and cats. We therefore asked whether there might also be differences in glycinergic neurotransmission in the CN.

Methods

We studied brainstem sections from humans, chimpanzees, and cats. We used antibodies to glycine receptors (GLYR) to identify neurons receiving glycinergic input, and antibodies to the neuronal glycine transporter (GLYT2) to immunolabel glycinergic axons and terminals. We also examined archival sections immunostained for calretinin (CR) and nonphosphorylated neurofilament protein (NPNFP) to try to locate the octopus cell area (OCA), a region in the VCN that rodents has minimal glycinergic input.

Results

In humans and chimpanzees we found widespread immunolabel for glycine receptors in DCN and in the posterior (PVCN) and anterior (AVCN) divisions of the VCN. We found a parallel distribution of GLYT2-immunolabeled fibers and puncta. The data also suggest that, as in rodents, a region containing octopus cells in cats, humans and chimpanzees has little glycinergic input.

Discussion

Our results show that glycine is a major transmitter in the human and chimpanzee CN, despite the species differences in DCN organization. The sources of the glycinergic input to the CN in humans and chimpanzees are not known.

Neuronal morphology and synaptic input patterns of neurons in the intermediate nucleus of the lateral lemniscus of gerbils

The lateral lemniscus encompasses processing stages for binaural hearing, suppressing spurious frequencies and frequency integration. Within the lemniscal fibres three nuclei can be identified, termed after their location as dorsal, intermediate and ventral nucleus of the lateral lemniscus (DNLL, INLL and VNLL). While the DNLL and VNLL have been functionally and anatomically characterized, less is known about INLL neurons. Here, we quantitatively describe the morphology, the cellular orientation and distribution of synaptic contact sites along dendrites in mature Mongolian gerbils. INLL neurons are largely non-inhibitory and morphologically heterogeneous with an overall perpendicular orientation regarding the lemniscal fibers. Dendritic ranges are heterogeneous and can extend beyond the nucleus border. INLL neurons receive VGluT1/2 containing glutamatergic and a mix of GABA- and glycinergic inputs distributed over the entire dendrite. Input counts suggest that numbers of excitatory exceed the inhibitory contact sites. Axonal projections indicate connectivity to ascending and descending auditory structures. Our data show that INLL neurons form a morphologically heterogeneous continuum and incoming auditory information is processed on thin dendrites of various length and biased to perpendicular orientation. Together with the different axonal projection patterns, this indicates that the INLL is a highly complex structure that might hold many unexplored auditory functions.

Structural arrangement of auditory brainstem nuclei in the bats Phyllostomus discolor and Carollia perspicillata

The structure of the mammalian auditory brainstem is evolutionarily highly plastic, and distinct nuclei arrange in a species-dependent manner. Such anatomical variability is present in the superior olivary complex (SOC) and the nuclei of the lateral lemniscus (LL). Due to the structure-function relationship in the auditory brainstem, the identification of individual nuclei supports the understanding of sound processing. Here, we comparatively describe the nucleus arrangement and the expression of functional markers in the auditory brainstem of the two bat species Phyllostomus discolor and Carollia perspicillata. Using immunofluorescent labeling, we describe the arrangement and identity of the SOC and LL nuclei based on the expression of synaptic markers (vesicular glutamate transporter 1 and glycine transporter 2), calcium-binding proteins, as well as the voltage-gated ion channel subunits Kv1.1 and HCN1. The distribution of excitatory and inhibitory synaptic labeling appears similar between both species and matches with that of other mammals. The detection of calcium-binding proteins indicates species-dependent differences and deviations from other mammals. Kv1.1 and HCN1 show largely the same expression pattern in both species, which diverges from other mammals, indicating functional adaptations in the cellular physiology of bat neurons.

Dendritic Kv4.2 potassium channels selectively mediate spatial pattern separation in the dentate gyrus

The capacity to distinguish comparable experiences is fundamental for the recall of similar memories and has been proposed to require pattern separation in the dentate gyrus (DG). However, the cellular mechanisms by which mature granule cells (GCs) of the DG accomplish this function are poorly characterized. Here, we show that Kv4.2 channels selectively modulate the excitability of medial dendrites of dentate GCs. These dendrites are targeted by the medial entorhinal cortex, the main source of spatial inputs to the DG. Accordingly, we found that the spatial pattern separation capability of animals lacking the Kv4.2 channel is significantly impaired. This points to the role of intrinsic excitability in supporting the mnemonic function of the dentate and to the Kv4.2 channel as a candidate substrate promoting spatial pattern separation.

Early Sensory Loss Alters the Dendritic Branching and Spine Density of Supragranular Pyramidal Neurons in Rodent Primary Sensory Cortices

Multisensory integration in primary auditory (A1), visual (V1), and somatosensory cortex (S1) is substantially mediated by their direct interconnections and by thalamic inputs across the sensory modalities. We have previously shown in rodents (Mongolian gerbils) that during postnatal development, the anatomical and functional strengths of these crossmodal and also of sensory matched connections are determined by early auditory, somatosensory, and visual experience. Because supragranular layer III pyramidal neurons are major targets of corticocortical and thalamocortical connections, we investigated in this follow-up study how the loss of early sensory experience changes their dendritic morphology. Gerbils were sensory deprived early in development by either bilateral sciatic nerve transection at postnatal day (P) 5, ototoxic inner hair cell damage at P10, or eye enucleation at P10. Sholl and branch order analyses of Golgi-stained layer III pyramidal neurons at P28, which demarcates the end of the sensory critical period in this species, revealed that visual and somatosensory deprivation leads to a general increase of apical and basal dendritic branching in A1, V1, and S1. In contrast, dendritic branching, particularly of apical dendrites, decreased in all three areas following auditory deprivation. Generally, the number of spines, and consequently spine density, along the apical and basal dendrites decreased in both sensory deprived and non-deprived cortical areas. Therefore, we conclude that the loss of early sensory experience induces a refinement of corticocortical crossmodal and other cortical and thalamic connections by pruning of dendritic spines at the end of the critical period. Based on present and previous own results and on findings from the literature, we propose a scenario for multisensory development following early sensory loss.

Ultrastructure of giant thalamic terminals in the auditory cortex

The auditory system comprises some very large axonal terminals like the endbulb and calyx of Held and "giant" corticothalamic synapses. Previously, we described a hitherto unknown population of giant thalamocortical boutons arising from the medial division of the medial geniculate body (MGm) in the Mongolian gerbil, which terminate over a wide cortical range but in a columnar manner particularly in the extragranular layers of the auditory cortex. As a first step towards an understanding of their potential functional role, we here describe their ultrastructure combining anterograde tract-tracing with biocytin and electron microscopy. Quantitative ultrastructural analyses revealed that biocytin-labelled MGm boutons reach much larger sizes than other, non-labelled boutons. Also, mitochondria occupy more space within labelled boutons whereas synapses are of similar size. Labelled boutons are very heterogeneous in size but homogeneous with respect to their ultrastructural characteristics, with asymmetric synapses containing clear, round vesicles and targeting dendritic spines. Functionally, the ultrastructure of the MGm terminals indicates that they form excitatory contacts, which may transmit their information in a rapid, powerful and high-fidelity manner onto strategically advantageous compartments of their cortical target cells.

Ablation of the presynaptic organizer Bassoon in excitatory neurons retards dentate gyrus maturation and enhances learning performance

Bassoon is a large scaffolding protein of the presynaptic active zone involved in the development of presynaptic terminals and in the regulation of neurotransmitter release at both excitatory and inhibitory brain synapses. Mice with constitutive ablation of the Bassoon (Bsn) gene display impaired presynaptic function, show sensory deficits and develop severe seizures. To specifically study the role of Bassoon at excitatory forebrain synapses and its relevance for control of behavior, we generated conditional knockout (Bsn cKO) mice by gene ablation through an Emx1 promoter-driven Cre recombinase. In these animals, we confirm selective loss of Bassoon from glutamatergic neurons of the forebrain. Behavioral assessment revealed that, in comparison to wild-type littermates, Bsn cKO mice display selectively enhanced contextual fear memory and increased novelty preference in a spatial discrimination/pattern separation task. These changes are accompanied by an augmentation of baseline synaptic transmission at medial perforant path to dentate gyrus (DG) synapses, as indicated by increased ratios of field excitatory postsynaptic potential slope to fiber volley amplitude. At the structural level, an increased complexity of apical dendrites of DG granule cells can be detected in Bsn cKO mice. In addition, alterations in the expression of cellular maturation markers and a lack of age-dependent decrease in excitability between juvenile and adult Bsn cKO mice are observed. Our data suggest that expression of Bassoon in excitatory forebrain neurons is required for the normal maturation of the DG and important for spatial and contextual memory.

Crossmodal Connections of Primary Sensory Cortices Largely Vanish During Normal Aging

During aging, human response times (RTs) to unisensory and crossmodal stimuli decrease. However, the elderly benefit more from crossmodal stimulus representations than younger people. The underlying short-latency multisensory integration process is mediated by direct crossmodal connections at the level of primary sensory cortices. We investigate the age-related changes of these connections using a rodent model (Mongolian gerbil), retrograde tracer injections into the primary auditory (A1), somatosensory (S1), and visual cortex (V1), and immunohistochemistry for markers of apoptosis (Caspase-3), axonal plasticity (Growth associated protein 43, GAP 43), and a calcium-binding protein (Parvalbumin, PV). In adult animals, primary sensory cortices receive a substantial number of direct thalamic inputs from nuclei of their matched, but also from nuclei of non-matched sensory modalities. There are also direct intracortical connections among primary sensory cortices and connections with secondary sensory cortices of other modalities. In very old animals, the crossmodal connections strongly decrease in number or vanish entirely. This is likely due to a retraction of the projection neuron axonal branches rather than ongoing programmed cell death. The loss of crossmodal connections is also accompanied by changes in anatomical correlates of inhibition and excitation in the sensory thalamus and cortex. Together, the loss and restructuring of crossmodal connections during aging suggest a shift of multisensory processing from primary cortices towards other sensory brain areas in elderly individuals.

Brain atlas of the Mongolian gerbil (Meriones unguiculatus) in CT/MRI-aided stereotaxic coordinates

A new stereotaxic brain atlas of the Mongolian gerbil (Meriones unguiculatus), an important animal model in neurosciences, is presented. It combines high-quality histological material for identification of brain structures with reliable stereotaxic coordinates. The atlas consists of high-resolution images of frontal sections alternately stained for cell bodies (Nissl) and myelinated fibers (Gallyas) of 62 rostro-caudal levels at intervals of 350 μm. Brain structures were named according to the Paxinos nomenclature for rodents. The accuracy of the stereotaxic coordinate system was improved substantially by comparing and matching the series of histological sections to in vivo brain images of the gerbil obtained by magnetic resonance imaging (MRI). The skull outlines corresponding to the MR images were acquired using X-ray computerized tomography (CT) and were used to establish the relationship between coordinates of brain structures and skull. Landmarks such as lambda, bregma, ear canals and occipital crest can be used to line up skull and brain in standard atlas coordinates. An easily reproducible protocol allows sectioning of experimental brains in the standard frontal plane of the atlas.

All the way from the cortex: a review of auditory corticosubcollicular pathways

Enrico Mugnaini has devoted part of his long and fruitful neuroscientific career to investigating the structural similarities between the cerebellar cortex and one of the first relay stations of the mammalian auditory pathway: the dorsal cochlear nucleus. The hypothesis of the cerebellar-like nature of the superficial layers of the dorsal cochlear nucleus received definitive support with the discovery and extensive characterization in his laboratory of unipolar brush cells, a neuron type unique to certain regions of the cerebellar cortex and to the granule cell domains of the cochlear nuclei. Paradoxically, a different line of research carried out in his laboratory revealed that, unlike the mammalian cerebellar cortex, the dorsal cochlear nucleus receives direct projections from the cerebral cortex, a fact that constitutes one of the main differences between the cerebellum and the dorsal cochlear nucleus. In an article published in 1995, Mugnaini's group described in detail the novel direct projections from the rat auditory neocortex to various subcollicular auditory centers, including the nucleus sagulum, the paralemniscal regions, the superior olivary complex, and the cochlear nuclei (Feliciano et al., Auditory Neuroscience 1995; 1:287-308). This review gives Enrico Mugnaini credit for his seminal contribution to the knowledge of auditory corticosubcollicular projections and summarizes how this growing field has evolved in the last 20 years.

Microelectrode mapping of tonotopic, laminar, and field-specific organization of thalamo-cortical pathway in rat

The rat has long been considered an important model system for studying neural mechanisms of auditory perception and learning, and particularly mechanisms involving auditory thalamo-cortical processing. However, the functional topography of the auditory thalamus, or medial geniculate body (MGB) has not yet been fully characterized in the rat, and the anatomically-defined features of field-specific, layer-specific and tonotopic thalamo-cortical projections have never been confirmed electrophysiologically. In the present study, we have established a novel technique for recording simultaneously from a surface microelectrode array on the auditory cortex, and a depth electrode array across auditory cortical layers and within the MGB, and characterized the rat MGB and thalamo-cortical projections under isoflurane anesthesia. We revealed that the ventral division of the MGB (MGv) exhibited a low-high-low CF gradient and long-short-long latency gradient along the dorsolateral-to-ventromedial axis, suggesting that the rat MGv is divided into two subdivisions. We also demonstrated that microstimulation in the MGv elicited cortical activation in layer-specific, region-specific and tonotopically organized manners. To our knowledge, the present study has provided the first and most compelling electrophysiological confirmation of the anatomical organization of the primary thalamo-cortical pathway in the rat, setting the groundwork for further investigation.

Structural Changes and Lack of HCN1 Channels in the Binaural Auditory Brainstem of the Naked Mole-Rat (Heterocephalus glaber)

Naked mole-rats (Heterocephalus glaber) live in large eu-social, underground colonies in narrow burrows and are exposed to a large repertoire of communication signals but negligible binaural sound localization cues, such as interaural time and intensity differences. We therefore asked whether monaural and binaural auditory brainstem nuclei in the naked mole-rat are differentially adjusted to this acoustic environment. Using antibody stainings against excitatory and inhibitory presynaptic structures, namely the vesicular glutamate transporter VGluT1 and the glycine transporter GlyT2 we identified all major auditory brainstem nuclei except the superior paraolivary nucleus in these animals. Naked mole-rats possess a well structured medial superior olive, with a similar synaptic arrangement to interaural-time-difference encoding animals. The neighboring lateral superior olive, which analyzes interaural intensity differences, is large and elongated, whereas the medial nucleus of the trapezoid body, which provides the contralateral inhibitory input to these binaural nuclei, is reduced in size. In contrast, the cochlear nucleus, the nuclei of the lateral lemniscus and the inferior colliculus are not considerably different when compared to other rodent species. Most interestingly, binaural auditory brainstem nuclei lack the membrane-bound hyperpolarization-activated channel HCN1, a voltage-gated ion channel that greatly contributes to the fast integration times in binaural nuclei of the superior olivary complex in other species. This suggests substantially lengthened membrane time constants and thus prolonged temporal integration of inputs in binaural auditory brainstem neurons and might be linked to the severely degenerated sound localization abilities in these animals.

The serine/threonine kinase Ndr2 controls integrin trafficking and integrin-dependent neurite growth

Integrins have been implicated in various processes of nervous system development, including proliferation, migration, and differentiation of neuronal cells. In this study, we show that the serine/threonine kinase Ndr2 controls integrin-dependent dendritic and axonal growth in mouse hippocampal neurons. We further demonstrate that Ndr2 is able to induce phosphorylation at the activity- and trafficking-relevant site Thr(788/789) of β1-integrin to stimulate the PKC- and CaMKII-dependent activation of β1-integrins, as well as their exocytosis. Accordingly, Ndr2 associates with integrin-positive early and recycling endosomes in primary hippocampal neurons and the surface expression of activated β1-integrins is reduced on dendrites of Ndr2-deficient neurons. The role of Ndr2 in dendritic differentiation is also evident in vivo, because Ndr2-null mutant mice show arbor-specific alterations of dendritic complexity in the hippocampus. This indicates a role of Ndr2 in the fine regulation of dendritic growth; in fact, treatment of primary neurons with Semaphorin 3A rescues Ndr2 knock-down-induced dendritic growth deficits but fails to enhance growth beyond control level. Correspondingly, Ndr2-null mutant mice show a Semaphorin 3A(-/-)-like phenotype of premature dendritic branching in the hippocampus. The results of this study show that Ndr2-mediated integrin trafficking and activation are crucial for neurite growth and guidance signals during neuronal development.

Hearing in action; auditory properties of neurons in the red nucleus of alert primates

The response of neurons in the Red Nucleus pars magnocellularis (RNm) to both tone bursts and electrical stimulation were observed in three cynomolgus monkeys (Macaca fascicularis), in a series of studies primarily designed to characterize the influence of the dopaminergic ventral midbrain on auditory processing. Compared to its role in motor behavior, little is known about the sensory response properties of neurons in the red nucleus (RN); particularly those concerning the auditory modality. Sites in the RN were recognized by observing electrically evoked body movements characteristic for this deep brain structure. In this study we applied brief monopolar electrical stimulation to 118 deep brain sites at a maximum intensity of 200 μA, thus evoking minimal body movements. Auditory sensitivity of RN neurons was analyzed more thoroughly at 15 sites, with the majority exhibiting broad tuning curves and phase locking up to 1.03 kHz. Since the RN appears to receive inputs from a very early stage of the ascending auditory system, our results suggest that sounds can modify the motor control exerted by this brain nucleus. At selected locations, we also tested for the presence of functional connections between the RN and the auditory cortex by inserting additional microelectrodes into the auditory cortex and investigating how action potentials and local field potentials (LFPs) were affected by electrical stimulation of the RN.

Wnt7a regulates multiple steps of neurogenesis

Although Wnt7a has been implicated in axon guidance and synapse formation, investigations of its role in the early steps of neurogenesis have just begun. We show here that Wnt7a is essential for neural stem cell self-renewal and neural progenitor cell cycle progression in adult mouse brains. Loss of Wnt7a expression dramatically reduced the neural stem cell population and increased the rate of cell cycle exit in neural progenitors in the hippocampal dentate gyrus of adult mice. Furthermore, Wnt7a is important for neuronal differentiation and maturation. Loss of Wnt7a expression led to a substantial decrease in the number of newborn neurons in the hippocampal dentate gyrus. Wnt7a(-/-) dentate granule neurons exhibited dramatically impaired dendritic development. Moreover, Wnt7a activated β-catenin and its downstream target genes to regulate neural stem cell proliferation and differentiation. Wnt7a stimulated neural stem cell proliferation by activating the β-catenin-cyclin D1 pathway and promoted neuronal differentiation and maturation by inducing the β-catenin-neurogenin 2 pathway. Thus, Wnt7a exercised critical control over multiple steps of neurogenesis by regulating genes involved in both cell cycle control and neuronal differentiation.