Axodendritic contacts onto calcium/calmodulin-dependent protein kinase type II-expressing neurons in the barn owl auditory space map

Distinct Cellular Profiles of Hif1a and Vegf mRNA Localization in Microglia, Astrocytes and Neurons during a Period of Vascular Maturation in the Auditory Brainstem of Neonate Rats

Defining the relationship between vascular development and the expression of hypoxia-inducible factors (Hifs) and vascular endothelial growth factor (Vegf) in the auditory brainstem is important to understand how tissue hypoxia caused by oxygen shortage contributes to sensory deficits in neonates. In this study, we used histology, molecular labeling, confocal microscopy and 3D image processing methods to test the hypothesis that significant maturation of the vascular bed in the medial nucleus of the trapezoid body (MNTB) occurs during the postnatal period that precedes hearing onset. Isolectin-B4 histochemistry experiments suggested that the MNTB vasculature becomes more elaborate between P5 and P10. When combined with a cell proliferation marker and immunohistochemistry, we found that vascular growth coincides with a switch in the localization of proliferating cells to perivascular locations, and an increase in the density of microglia within the MNTB. Furthermore, microglia were identified as perivascular cells with proliferative activity during the period of vascular maturation. Lastly, combined in situ hybridization and immunohistochemistry experiments showed distinct profiles of Hif1a and Vegf mRNA localization in microglia, astrocytes and MNTB principal neurons. These results suggest that different cells of the neuro-glio-vascular unit are likely targets of hypoxic insult in the auditory brainstem of neonate rats.

Toric Spines at a Site of Learning

We discovered a new type of dendritic spine. It is found on space-specific neurons in the barn owl inferior colliculus, a site of experience-dependent plasticity. Connectomic analysis revealed dendritic protrusions of unusual morphology including topological holes, hence termed "toric" spines (n = 76). More significantly, presynaptic terminals converging onto individual toric spines displayed numerous active zones (up to 49) derived from multiple axons (up to 11) with incoming trajectories distributed widely throughout 3D space. This arrangement is suited to integrate input sources. Dense reconstruction of two toric spines revealed that they were unconnected with the majority (∼84%) of intertwined axons, implying a high capacity for information storage. We developed an ex vivo slice preparation and provide the first published data on space-specific neuron intrinsic properties, including cellular subtypes with and without toric-like spines. We propose that toric spines are a cellular locus of sensory integration and behavioral learning.

A novel relay nucleus between the inferior colliculus and the optic tectum in the chicken (Gallus gallus)

Processing multimodal sensory information is vital for behaving animals in many contexts. The barn owl, an auditory specialist, is a classic model for studying multisensory integration. In the barn owl, spatial auditory information is conveyed to the optic tectum (TeO) by a direct projection from the external nucleus of the inferior colliculus (ICX). In contrast, evidence of an integration of visual and auditory information in auditory generalist avian species is completely lacking. In particular, it is not known whether in auditory generalist species the ICX projects to the TeO at all. Here we use various retrograde and anterograde tracing techniques both in vivo and in vitro, intracellular fillings of neurons in vitro, and whole-cell patch recordings to characterize the connectivity between ICX and TeO in the chicken. We found that there is a direct projection from ICX to the TeO in the chicken, although this is small and only to the deeper layers (layers 13-15) of the TeO. However, we found a relay area interposed among the IC, the TeO, and the isthmic complex that receives strong synaptic input from the ICX and projects broadly upon the intermediate and deep layers of the TeO. This area is an external portion of the formatio reticularis lateralis (FRLx). In addition to the projection to the TeO, cells in FRLx send, via collaterals, descending projections through tectopontine-tectoreticular pathways. This newly described connection from the inferior colliculus to the TeO provides a solid basis for visual-auditory integration in an auditory generalist bird. J. Comp. Neurol. 525:513-534, 2017. © 2016 Wiley Periodicals, Inc.

Hunting increases phosphorylation of calcium/calmodulin-dependent protein kinase type II in adult barn owls

Juvenile barn owls readily adapt to prismatic spectacles, whereas adult owls living under standard aviary conditions do not. We previously demonstrated that phosphorylation of the cyclic-AMP response element-binding protein (CREB) provides a readout of the instructive signals that guide plasticity in juveniles. Here we investigated phosphorylation of calcium/calmodulin-dependent protein kinase II (pCaMKII) in both juveniles and adults. In contrast to CREB, we found no differences in pCaMKII expression between prism-wearing and control juveniles within the external nucleus of the inferior colliculus (ICX), the major site of plasticity. For prism-wearing adults that hunted live mice and are capable of adaptation, expression of pCaMKII was increased relative to prism-wearing adults that fed passively on dead mice and are not capable of adaptation. This effect did not bear the hallmarks of instructive information: it was not localized to rostral ICX and did not exhibit a patchy distribution reflecting discrete bimodal stimuli. These data are consistent with a role for CaMKII as a permissive rather than an instructive factor. In addition, the paucity of pCaMKII expression in passively fed adults suggests that the permissive default setting is "off" in adults.

Input clustering in the normal and learned circuits of adult barn owls

Experience-dependent formation of synaptic input clusters can occur in juvenile brains. Whether this also occurs in adults is largely unknown. We previously reconstructed the normal and learned circuits of prism-adapted barn owls and found that changes in clustering of axo-dendritic contacts (putative synapses) predicted functional circuit strength. Here we asked whether comparable changes occurred in normal and prism-removed adults. Across all anatomical zones, no systematic differences in the primary metrics for within-branch or between-branch clustering were observed: 95-99% of contacts resided within clusters (<10-20 μm from nearest neighbor) regardless of circuit strength. Bouton volumes, a proxy measure of synaptic strength, were on average larger in the functionally strong zones, indicating that changes in synaptic efficacy contributed to the differences in circuit strength. Bootstrap analysis showed that the distribution of inter-contact distances strongly deviated from random not in the functionally strong zones but in those that had been strong during the sensitive period (60-250 d), indicating that clusters formed early in life were preserved regardless of current value. While cluster formation in juveniles appeared to require the production of new synapses, cluster formation in adults did not. In total, these results support a model in which high cluster dynamics in juveniles sculpt a potential connectivity map that is refined in adulthood. We propose that preservation of clusters in functionally weak adult circuits provides a storage mechanism for disused but potentially useful pathways.

Natural and lesion-induced decrease in cell proliferation in the medial nucleus of the trapezoid body during hearing development

The functional interactions between neurons and glial cells that are important for nervous system function are presumably established during development from the activity of progenitor cells. In this study we examined proliferation of progenitor cells in the medial nucleus of the trapezoid body (MNTB) located in the rat auditory brainstem. We performed DNA synthesis labeling experiments to demonstrate changes in cell proliferation activity during postnatal stages of development. An increase in cell proliferation correlated with MNTB growth and the presence of S100β-positive astrocytes among MNTB neurons. In additional experiments we analyzed the fate of newly born cells. At perinatal ages, newly born cells colabeled with the astrocyte marker S100β in higher numbers than when cells were generated at postnatal day 6. Furthermore, we identified newly born cells that were colabeled with caspase-3 immunohistochemistry and performed comparative experiments to demonstrate that there is a natural decrease in cell proliferation activity during postnatal development in rats, mice, gerbils, and ferrets. Lastly, we found that there is a stronger decrease in MNTB cell proliferation after performing bilateral lesions of the auditory periphery in rats. Altogether, these results identify important stages in the development of astrocytes in the MNTB and provide evidence that the proliferative activity of the progenitor cells is developmentally regulated. We propose that the developmental reduction in cell proliferation may reflect coordinated signaling between the auditory brainstem and the auditory periphery. J. Comp. Neurol. 522:971–985, 2014.

Input clustering and the microscale structure of local circuits

The recent development of powerful tools for high-throughput mapping of synaptic networks promises major advances in understanding brain function. One open question is how circuits integrate and store information. Competing models based on random vs. structured connectivity make distinct predictions regarding the dendritic addressing of synaptic inputs. In this article we review recent experimental tests of one of these models, the input clustering hypothesis. Across circuits, brain regions and species, there is growing evidence of a link between synaptic co-activation and dendritic location, although this finding is not universal. The functional implications of input clustering and future challenges are discussed.

Early-life stress affects the structural and functional plasticity of the medial prefrontal cortex in adolescent rats

Early life experiences are crucial factors that shape brain development and function due to their ability to induce structural and functional plasticity. Among these experiences, early-life stress (ELS) is known to interfere with brain development and maturation, increasing the risk of future psychopathologies, including depression, anxiety, and personality disorders. Moreover, ELS may contribute to the emergence of these psychopathologies during adolescence. In this present study, we investigated the effects of ELS, in the form of maternal separation (MS), on the structural and functional plasticity of the medial prefrontal cortex (mPFC) and anxiety-like behavior in adolescent male rats. We found that the MS procedure resulted in disturbances in mother-pup interactions that lasted until weaning and were most strongly demonstrated by increases in nursing behavior. Moreover, MS caused atrophy of the basal dendritic tree and reduced spine density on both the apical and basal dendrites in layer II/III pyramidal neurons of the mPFC. The structural changes were accompanied by an impairment of long-term potentiation processes and increased expression of key proteins, specifically glutamate receptor 1, glutamate receptor 2, postsynaptic density protein 95, αCa(2+) /calmodulin-dependent protein kinase II and αCa(2+)/calmodulin-dependent protein kinase II phosphorylated at residue Thr305, that are engaged in long-term potentiation induction and maintenance in the mPFC. We also found that the MS animals were more anxious in the light/dark exploration test. The results of this study indicate that ELS has a significant impact on the structural and functional plasticity of the mPFC in adolescents. ELS-induced adaptive plasticity may underlie the pathomechanisms of some early-onset psychopathologies observed in adolescents.

The representation of sound localization cues in the barn owl's inferior colliculus

The barn owl is a well-known model system for studying auditory processing and sound localization. This article reviews the morphological and functional organization, as well as the role of the underlying microcircuits, of the barn owl's inferior colliculus (IC). We focus on the processing of frequency and interaural time (ITD) and level differences (ILD). We first summarize the morphology of the sub-nuclei belonging to the IC and their differentiation by antero- and retrograde labeling and by staining with various antibodies. We then focus on the response properties of neurons in the three major sub-nuclei of IC [core of the central nucleus of the IC (ICCc), lateral shell of the central nucleus of the IC (ICCls), and the external nucleus of the IC (ICX)]. ICCc projects to ICCls, which in turn sends its information to ICX. The responses of neurons in ICCc are sensitive to changes in ITD but not to changes in ILD. The distribution of ITD sensitivity with frequency in ICCc can only partly be explained by optimal coding. We continue with the tuning properties of ICCls neurons, the first station in the midbrain where the ITD and ILD pathways merge after they have split at the level of the cochlear nucleus. The ICCc and ICCls share similar ITD and frequency tuning. By contrast, ICCls shows sigmoidal ILD tuning which is absent in ICCc. Both ICCc and ICCls project to the forebrain, and ICCls also projects to ICX, where space-specific neurons are found. Space-specific neurons exhibit side peak suppression in ITD tuning, bell-shaped ILD tuning, and are broadly tuned to frequency. These neurons respond only to restricted positions of auditory space and form a map of two-dimensional auditory space. Finally, we briefly review major IC features, including multiplication-like computations, correlates of echo suppression, plasticity, and adaptation.

Neuronal morphology in subdivisions of the inferior colliculus of chicken (Gallus gallus)

The avian inferior colliculus (IC), also referred to as the nucleus mesencephalicus lateralis pars dorsalis (MLd), is an auditory midbrain nucleus that converges auditory cues from tonotopically organized brainstem nuclei. This information is relayed onto the optic tectum on the one hand and to nucleus ovoidalis on the other hand. Morphologically, there has been considerable debate about the number and nomenclature of the subnuclei within the IC. Here, we provide morphological characteristics of single cells in five IC subnuclei in chicken. The cellular structure within the IC was studied by whole-cell patch technique and biocytin iontophoresis. In addition, histological staining was performed, to delineate the borders between subnuclei of the IC. We were able to discriminate between 5 subnuclei: the core of the central nucleus (ICCc), the medial and lateral shell of the central nucleus (ICCms and ICCls), the external nucleus (ICX) and the superficial nucleus (ICS) of the IC. Our findings suggest the existence of at least two different morphologies of neurons with two subtypes each. The IC in chicken is a largely homogenous nucleus in terms of neuronal anatomy on a cellular level. However, its compartmentation into diversified subnuclei with different neurophysiological characteristics suggests a complex system to process auditory information. The auditory system in chicken is not as hypertrophied as in specialists such as the barn owl, but appears to have comparable connectivity and cellular morphology.

Auditory processing, plasticity, and learning in the barn owl

The human brain has accumulated many useful building blocks over its evolutionary history, and the best knowledge of these has often derived from experiments performed in animal species that display finely honed abilities. In this article we review a model system at the forefront of investigation into the neural bases of information processing, plasticity, and learning: the barn owl auditory localization pathway. In addition to the broadly applicable principles gleaned from three decades of work in this system, there are good reasons to believe that continued exploration of the owl brain will be invaluable for further advances in understanding of how neuronal networks give rise to behavior.

Perceptron learning rule derived from spike-frequency adaptation and spike-time-dependent plasticity

It is widely believed that sensory and motor processing in the brain is based on simple computational primitives rooted in cellular and synaptic physiology. However, many gaps remain in our understanding of the connections between neural computations and biophysical properties of neurons. Here, we show that synaptic spike-time-dependent plasticity (STDP) combined with spike-frequency adaptation (SFA) in a single neuron together approximate the well-known perceptron learning rule. Our calculations and integrate-and-fire simulations reveal that delayed inputs to a neuron endowed with STDP and SFA precisely instruct neural responses to earlier arriving inputs. We demonstrate this mechanism on a developmental example of auditory map formation guided by visual inputs, as observed in the external nucleus of the inferior colliculus (ICX) of barn owls. The interplay of SFA and STDP in model ICX neurons precisely transfers the tuning curve from the visual modality onto the auditory modality, demonstrating a useful computation for multimodal and sensory-guided processing.

Endocannabinoid-mediated long-term depression in the avian midbrain expressed presynaptically and postsynaptically

Here, we examined long-term synaptic plasticity in the avian auditory midbrain, a region involved in experience-dependent learning. We found that coactivation of N-methyl-D-aspartate receptors (NMDAR) and type 1 cannabinoid receptors (CB1R) induces long-term depression (LTD) at the synapse between the central shell and the external portion of the inferior colliculus of the chicken. Although endocannabinoids are commonly thought of as presynaptic modulators, recent reports have suggested that they can also modulate the postsynaptic site. In the avian midbrain, we found that LTD is mediated by both presynaptic and postsynaptic changes. The presynaptic mechanism consists of a decrease in neurotransmitter release, whereas a depression of NMDAR-mediated current takes place on the postsynaptic side. Both the presynaptic and the postsynaptic effects depend on CB1R activation. The reduction of postsynaptic NMDAR currents represents a novel role of endocannabinoids in synaptic modulation.

Experience-dependent refinement of the inhibitory axons projecting to the medial superior olive

Neurons in the medial superior olive (MSO) analyze interaural time differences (ITDs) by comparing the arrival times of the two excitatory inputs from each ear using a coincidence detection mechanism. They also receive a prominent inhibitory, glycinergic projection from the ipsilateral medial nucleus of the trapezoid body (MNTB), which contributes to the fine-tuning of ITD analysis. Here, we investigated developmental changes of the axonal arborisation pattern of single Microruby-labeled MNTB neurons projecting to the MSO region. During the first 2 weeks after hearing onset, the axonal arborisation of MNTB neurons was significantly refined resulting in a narrowed projection area across the tonotopic axis of the MSO and a redistribution of the axonal endsegments to a mostly somatic location. Rearing the animals in omnidirectional noise prevented the structural changes of single MNTB projections. These results indicate that the functional elimination of inhibitory inputs on MSO neurons after hearing onset, as described previously, is paralleled by a structural, site-specific refinement of the inputs and is dependent on the normal acoustic experience of the animal.

Visual modulation of auditory responses in the owl inferior colliculus

The barn owl's central auditory system creates a map of auditory space in the external nucleus of the inferior colliculus (ICX). Although the crucial role visual experience plays in the formation and maintenance of this auditory space map is well established, the mechanism by which vision influences ICX responses remains unclear. Surprisingly, previous experiments have found that in the absence of extensive pharmacological manipulation, visual stimuli do not drive neural responses in the ICX. Here we investigated the influence of dynamic visual stimuli on auditory responses in the ICX. We show that a salient visual stimulus, when coincident with an auditory stimulus, can modulate auditory responses in the ICX even though the same visual stimulus may elicit no neural responses when presented alone. For each ICX neuron, the most effective auditory and visual stimuli were located in the same region of space. In addition, the magnitude of the visual modulation of auditory responses was dependent on the context of the stimulus presentation with novel visual stimuli eliciting consistently larger response modulations than frequently presented visual stimuli. Thus the visual modulation of ICX responses is dependent on the characteristics of the visual stimulus as well as on the spatial and temporal correspondence of the auditory and visual stimuli. These results demonstrate moment-to-moment visual enhancements of auditory responsiveness that, in the short-term, increase auditory responses to salient bimodal stimuli and in the long-term could serve to instruct the adaptive auditory plasticity necessary to maintain accurate auditory orienting behavior.

Bidirectional regulation of the cAMP response element binding protein encodes spatial map alignment in prism-adapting barn owls

The barn owl midbrain contains mutually aligned maps of auditory and visual space. Throughout life, map alignment is maintained through the actions of an instructive signal that encodes the magnitude of auditory-visual mismatch. The intracellular signaling pathways activated by this signal are unknown. Here we tested the hypothesis that CREB (cAMP response element-binding protein) provides a cell-specific readout of instructive information. Owls were fitted with prismatic or control spectacles and provided rich auditory-visual experience: hunting live mice. CREB activation was analyzed within 30 min of hunting using phosphorylation state-specific CREB (pCREB) and CREB antibodies, confocal imaging, and immunofluorescence measurements at individual cell nuclei. In control owls or prism-adapted owls, which experience small instructive signals, the frequency distributions of pCREB/CREB values obtained for cell nuclei within the external nucleus of the inferior colliculus (ICX) were unimodal. In contrast, in owls adapting to prisms or readapting to normal conditions, the distributions were bimodal: certain cells had received a signal that positively regulated CREB and, by extension, transcription of CREB-dependent genes, whereas others received a signal that negatively regulated it. These changes were restricted to the subregion of the inferior colliculus that received optically displaced input, the rostral ICX, and were not evident in the caudal ICX or central nucleus. Finally, the topographic pattern of CREB regulation was patchy, not continuous, as expected from the actions of a topographically precise signal encoding discrete events. These results support a model in which the magnitude of CREB activation within individual cells provides a readout of the instructive signal that guides plasticity and learning.

Micro-rewiring as a substrate for learning

How does the brain encode life experiences? Recent results derived from vital imaging, computational modeling, cellular physiology and systems neuroscience have pointed to local changes in synaptic connectivity as a powerful substrate, here termed micro-rewiring. To examine this hypothesis, I first review findings on micro-structural dynamics with focus on the extension and retraction of dendritic spines. Although these observations demonstrate a biological mechanism, they do not inform us of the specific changes in circuit configuration that might occur during learning. Here, computational models have made testable predictions for both the neuronal and circuit levels. Integrative approaches in the mammalian neocortex and the barn owl auditory localization pathway provide some of the first direct evidence in support of these 'synaptic-clustering' mechanisms. The implications of these data and the challenges for future research are discussed.

Learning drives differential clustering of axodendritic contacts in the barn owl auditory system

Computational models predict that experience-driven clustering of coactive synapses is a mechanism for information storage. This prediction has remained untested, because it is difficult to approach through time-lapse analysis. Here, we exploit a unique feature of the barn owl auditory localization pathway that permits retrospective analysis of prelearned and postlearned circuitry: owls reared wearing prismatic spectacles develop an adaptive microcircuit that coexists with the native one but can be analyzed independently based on topographic location. To visualize the clustering of axodendritic contacts (potential synapses) within these zones, coactive axons were labeled by focal injection of fluorescent tracer and their target dendrites labeled with an antibody directed against CaMKII (calcium/calmodulin-dependent protein kinase type II, alpha subunit). Using high-resolution confocal imaging, we measured the distance from each contact to its nearest neighbor on the same branch of dendrite. We found that the distribution of intercontact distances for the adaptive zone was shifted dramatically toward smaller values compared with distributions for either the maladaptive zone of the same animals or the adaptive zone of normal juveniles, which indicates that a dynamic clustering of contacts had occurred. Moreover, clustering in the normal zone was greater in normal juveniles than in prism-adapted owls, indicative of declustering. These data demonstrate that clustering is bidirectionally adjustable and tuned by behaviorally relevant experience. The microanatomical configurations in all zones of both experimental groups matched the functional circuit strengths that were assessed by in vivo electrophysiological mapping. Thus, the observed changes in clustering are appropriately positioned to contribute to the adaptive strengthening and weakening of auditory-driven responses.

Transcriptome changes associated with instructed learning in the barn owl auditory localization pathway

Owls reared wearing prismatic spectacles learn to make adaptive orienting movements. This instructed learning depends on re-calibration of the midbrain auditory space map, which in turn involves the formation of new synapses. Here we investigated whether these processes are associated with differential gene expression, using longSAGE. Newly fledged owls were reared for 8-36 days with prism or control lenses at which time the extent of learning was quantified by electrophysiological mapping. Transciptome profiles were obtained from the inferior colliculus (IC), the major site of synaptic plasticity, and the optic tectum (OT), which provides an instructive signal that controls the direction and extent of plasticity. Twenty-two differentially expressed sequence tags were identified in IC and 36 in OT, out of more than 35,000 unique tags. Of these, only four were regulated in both structures. These results indicate that regulation of two largely independent gene clusters is associated with synaptic remodeling (in IC) and generation of the instructive signal (in OT). Real-time PCR data confirmed the changes for two transcripts, ubiquitin/polyubiquitin and tyrosine 3-monooxgenase/tryotophan 5-monooxygenase activation protein, theta subunit (YWHAQ; also referred to as 14-3-3 protein). Ubiquitin was downregulated in IC, consistent with a model in which protein degradation pathways act as an inhibitory constraint on synaptogenesis. YWHAQ was up-regulated in OT, indicating a role in the synthesis or delivery of instructive information. In total, our results provide a path towards unraveling molecular cascades that link naturalistic experience with synaptic remodeling and, ultimately, with the expression of learned behavior.

Proteomic Analyses of Zebra Finch Optic Tectum and Comparative Histochemistry

Proteomic analyses of zebra finch (Taeniopygia guttata) optic tectum resulted in identification of 176 proteins. In the Swissprot database, only 52 proteins were identified as bird homologs and only 71 proteins were identified in songbird transcriptome databases, reflecting a lack of completeness in the T. guttata genomic sequence. Analysis in Kyoto encyclopedia of genes and genome (KEGG) pathway database found that identified proteins most frequently belong to glucose, pyruvate, glyoxylate, dicarboxylate, alanine, and aspartate metabolism pathways. A number of identified proteins have been previously reported to exist in the avian optic tectum. The immunohistochemical localization of selected proteins showed their distribution in similar laminae of the owl (Tyto alba) and chicken (Gallus gallus) tectum. Immunohistochemical analysis of identified proteins can provide clues about cell types and circuit layout of the avian optic tectum in general. As the optic tectum of nonmammals is homologous to the superior colliculus of mammals, the analysis of the tectal and collicular proteome may provide clues about conserved cell and circuit layout, circuit function, and evolution.

Adaptation in the Auditory Space Map of the Barn Owl

Auditory neurons in the owl’s external nucleus of the inferior colliculus (ICX) integrate information across frequency channels to create a map of auditory space. This study describes a powerful, sound-driven adaptation of unit responsiveness in the ICX and explores the implications of this adaptation for sensory processing. Adaptation in the ICX was analyzed by presenting lightly anesthetized owls with sequential pairs of dichotic noise bursts. Adaptation occurred in response even to weak, threshold-level sounds and remained strong for more than 100 ms after stimulus offset. Stimulation by one range of sound frequencies caused adaptation that generalized across the entire broad range of frequencies to which these units responded. Identical stimuli were used to test adaptation in the lateral shell of the central nucleus of the inferior colliculus (ICCls), which provides input directly to the ICX. Compared with ICX adaptation, adaptation in the ICCls was substantially weaker, shorter lasting, and far more frequency specific, suggesting that part of the adaptation observed in the ICX was attributable to processes resident to the ICX. The sharp tuning of ICX neurons to space, along with their broad tuning to frequency, allows ICX adaptation to preserve a representation of stimulus location, regardless of the frequency content of the sound. The ICX is known to be a site of visually guided auditory map plasticity. ICX adaptation could play a role in this cross-modal plasticity by providing a short-term memory of the representation of auditory localization cues that could be compared with later-arriving, visual–spatial information from bimodal stimuli.

Branching of calyceal afferents during postnatal development in the rat auditory brainstem

Cells in the anteroventral cochlear nucleus (aVCN) send out calyceal axons that form large excitatory somatic terminals, the calyces of Held, onto principal cells of the contralateral medial nucleus of the trapezoid body (MNTB). It is unclear which fraction of these axons might form more than one calyx and whether this fraction changes during development. We combined in vitro anterograde tracing, stereological cell counts, analysis of apoptosis, and immunohistochemistry to study the development of calyceal afferents in rats of different postnatal ages. We found that some principal cells were contacted by multiple large axosomatic inputs, but these invariably originated from the same axon. Conversely, at least 18% of traced afferents branched to form multiple calyces, independently of age. Calyces from the same axon generally innervated nearby principal cells, and most of these branch points were <50 microm away from the synaptic terminals. Our results show that the projection from the aVCN to the MNTB is divergent, both when calyces have just been formed and in the adult. Cell counts did not provide evidence for principal cell loss during development, although analysis of apoptosis showed a large increase in nonneuronal cell death around the onset of hearing. Our data suggest that, once a calyceal synapse forms in the MNTB, it stays.

Projections of the nucleus lentiformis mesencephali in pigeons (Columba livia): A comparison of the morphology and distribution of neurons with different efferent projections

The avian nucleus lentiformis mesencephali (LM) is a visual structure involved in the optokinetic response. The LM consists of several morphologically distinct cell types. In the present study we sought to determine if different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of LM neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, nucleus of the basal optic root (nBOR), and midline mesencephalon. From injections into the latter two structures, small LM cells were labeled. More were localized to the lateral LM as opposed to medial LM. From injections into the dorsal thalamus, small neurons were found throughout LM. From injections into the VbC, large multipolar cells were found throughout LM. From injections into IO, a strip of medium-sized fusiform neurons along the border of the medial and lateral subnuclei was labeled. To investigate if neurons project to multiple targets we used fluorescent retrograde tracers. After injections into IO and VbC, double-labeled neurons were not observed in LM. Likewise, after injections into nBOR and IO, double-labeled neurons were not observed. Finally, we processed sections through LM for glutamic acid decarboxylase (GAD). Small neurons, mostly in the lateral LM, were labeled, suggesting that projections from LM to nBOR and midline mesencephalon are GABAergic. We conclude that two efferents of LM, VbC and IO, receive input from morphologically distinct neurons: large multipolar and medium-sized fusiform neurons, respectively. The dorsal thalamus, nBOR, and midline mesencephalon receive input from small neurons, some of which are likely GABAergic.