Orthogonal topography in the parallel input architecture of songbird HVC

Neural activity within the cortical premotor nucleus HVC (acronym is name) encodes the learned songs of adult male zebra finches (Taeniopygia guttata). HVC activity is driven and/or modulated by a group of five afferent nuclei (the Medial Magnocellular nucleus of the Anterior Nidopallium, MMAN; Nucleus Interface, NIf; nucleus Avalanche, Av; the Robust nucleus of the Arcopallium, RA; the Uvaeform nucleus, Uva). While earlier evidence suggested that HVC receives a uniformly distributed and nontopographic pattern of afferent input, recent evidence suggests this view is incorrect (Basista et al., 2014 ). Here, we used a double‐labeling strategy (varying both the distance between and the axial orientation of dual tracer injections into HVC) to reveal a massively parallel and in some cases topographic pattern of afferent input. Afferent neurons target only one rostral or caudal location within medial or lateral HVC, and each HVC location receives convergent input from each afferent nucleus in parallel. Quantifying the distributions of single‐labeled cells revealed an orthogonal topography in the organization of afferent input from MMAN and NIf, two cortical nuclei necessary for song learning. MMAN input is organized across the lateral‐medial axis whereas NIf input is organized across the rostral‐caudal axis. To the extent that HVC activity is influenced by afferent input during the learning, perception, or production of song, functional models of HVC activity may need revision to account for the parallel input architecture of HVC, along with the orthogonal input topography of MMAN and NIf.     

Laminar and cellular targets of individual thalamic reticular nucleus axons in the lateral geniculate nucleus in the prosimian primate Galago

The visual sector of the thalamic reticular nucleus is the source of the primary inhibitory projection to the visual thalamic relay nucleus, the dorsal lateral geniculate nucleus. The purpose of this study was to investigate laminar and cellular targets of individual thalamic reticular nucleus axons in the highly laminated lateral geniculate nucleus of the prosimian primate Galago to better understand the nature and function of this projection. Thalamic reticular axons labeled anterogradely by means of biotinylated dextran amine were examined by using light microscopic serial reconstruction and electron microscopic analysis in combination with postembedding immunohistochemical labeling for the neurotransmitter gamma-aminobutyric acid (GABA). The synaptic targets of labeled reticular terminal profiles were primarily GABA-negative dendrites (79-84%) of thalamocortical cells, whereas up to 16% were GABA-positive dendritic shafts or F2 terminals of interneurons. Reconstructed thalamic reticular nucleus axons were narrowly aligned along a single axis perpendicular to the geniculate laminar plane, exhibiting a high degree of visuotopic precision. Individual reticular axons targeted multiple or all geniculate laminae, with little laminar selectivity in the distribution of swellings with regard to the eye of origin or to the parvocellular, koniocellular, or magnocellular type neurons contained in the separate layers of the Galago lateral geniculate nucleus. These results suggest that cells in the visual thalamic reticular nucleus influence the lateral geniculate nucleus retinotopically, with little regard to visual functional streams.     

A comparative analysis of the physiological properties of neurons in the anterolateral bed nucleus of the stria terminalis in the Mus musculus,Rattus norvegicus,and Macaca mulatta

The anterolateral group of the bed nucleus of the stria terminalis (BNST_ALG) is a critical modulator of a variety of rodent and primate behaviors spanning anxiety behavior and drug addiction. Three distinct neuronal cell types have been previously defined in the rat BNST_ALG based on differences in the voltage‐response to hyperpolarizing and depolarizing current injection. Differences in genetic expression profile between these three cell types suggest electrophysiological cell type may be an indicator for functional differences in the circuit of the rat BNST_ALG. Although the behavioral role of the BNST is conserved across species, it is unknown if the same electrophysiological cell types exist in the BNST_ALG of the mouse and nonhuman primate. Here, we used whole‐cell patch clamp electrophysiology and neuronal reconstructions of biocytin‐filled neurons to compare and contrast the electrophysiological and morphological properties of neurons in the BNST_ALG from the mouse, rat, and rhesus macaque. We provide evidence that the BNST_ALG of all three species contains neurons that match the three defined cell types found in the rat; however, there are intriguing differences in the relative frequency of these cell types as well as electrophysiological and morphological properties of the BNST_ALG neurons across species. This study suggests that the overall landscape of the BNST_ALG in the primate and mouse may be similar to that of the rat in some aspects but perhaps significantly different in others.     

A profile of auditory‐responsive neurons in the larval zebrafish brain

Many features of auditory processing are conserved among vertebrates, but the degree to which these pathways are established at early stages is not well explored. In this study, we have observed single cell activity throughout the brains of larval zebrafish with the goal of identifying the cellular responses, brain regions, and brain‐wide pathways through which these larvae perceive and process auditory stimuli. Using GCaMP and selective plane illumination microscopy, we find strong responses to auditory tones ranging from 100 Hz to 400 Hz. We also identify different categories of auditory neuron with distinct frequency response profiles. Auditory responses occur in the medial octavolateral nucleus, the torus semicircularis, the medial hindbrain, and the thalamus, and the flow of information among these regions resembles the pathways described in adult fish and mammals. The details of these patterns, however, indicate that auditory processing is still rudimentary in larvae. The range of frequencies detected is small, and while different neurons have distinct response profiles, most are sensitive to multiple frequencies, and distinct categories show substantial overlap in their responses. Likewise, while there are signs of nascent spatial representations of frequency in the larval brain, this only faintly resembles the clear tonotopy seen in adult fish and mammals. Overall, our results show that many fundamental properties of the auditory system are established early in development, and suggest that zebrafish will provide a good model in which to study the development and refinement of these pathways.     

The representation of the visual field in parvicellular and magnocellular layers of the lateral geniculate nucleus in the macaque monkey

Two-dimensional maps of individual layers of the dorsal lateral geniculate nucleus (LGN) in the macaque monkey were constructed and used as a basis for comparing laminar size, shape, and topographic organization. Topographical data from the electrophysiological investigation of the LGN by Malpeli and Baker ('75) were displayed on maps of all six layers. As known from previous studies, there is a significant over-representation of central vision in the LGN. Unexpectedly, though, the visual representation is anisotropic over portions of most LGN layers. That is, the linear magnification factor (millimeters along the laminar surface per degree of visual field) is not equal for all directions from a given point in the visual field. Moreover, the visual representations in the parvicellular and magnocellular divisions of the LGN differ both in their emphasis on central vision and in their anisotropies. To determine the degree of individual variability, laminar maps were prepared from the LGN of seven other hemispheres. The shapes of laminar maps varied considerably between LGNs, from nearly circular to highly elliptical, but the surface area was relatively constant for each layer. Topographical organization, determined by mapping the optic disc representation on the LGN laminae and by labeling from anterograde and retrograde tracer injections in striate cortex, showed significant individual variability. Interestingly, the visual representations in the LGN and striate cortex are topologically inverted with respect to one another. This indicates that the establishment of geniculocortical connections involves a systematic crossing-over of fibers. Information on cell densities and magnification factors in striate cortex obtained from other studies was compared to the results of the present study in order to estimate ratios of cortical neurons to LGN neurons at different eccentricities. The total number of cortical neurons per LGN neuron is about 130 on average, but it extends over approximately a tenfold range, from less than 100 in the far periphery to nearly 1,000 in the fovea. The estimated number of cells in layers 4A and 4Cβ per parvicellular layer neuron is smaller and extends over a slightly narrower range, from 30 to 240, whereas the number of layer 4Cα neurons per magnocellular neuron varies more widely, from about 45 to 7,000.     

Visuomotor signals for reaching movements in the rostro‐dorsal sector of the monkey thalamic reticular nucleus

The thalamic reticular nucleus (TRN) collects inputs from the cerebral cortex and thalamus and, in turn, sends inhibitory outputs to the thalamic relay nuclei. This unique connectivity suggests that the TRN plays a pivotal role in regulating information flow through the thalamus. Here, we analyzed the roles of TRN neurons in visually guided reaching movements. We first used retrograde transneuronal labeling with rabies virus, and showed that the rostro‐dorsal sector of the TRN (TRNrd) projected disynaptically to the ventral premotor cortex (PMv). In other experiments, we recorded neurons from the TRNrd or PMv while monkeys performed a visuomotor task. We found that neurons in the TRNrd and PMv showed visual‐, set‐, and movement‐related activity modulation. These results indicate that the TRNrd, as well as the PMv, is involved in the reception of visual signals and in the preparation and execution of reaching movements. The fraction of neurons that were non‐selective for the location of visual signals or the direction of reaching movements was greater in the TRNrd than in the PMv. Furthermore, the fraction of neurons whose activity increased from the baseline was greater in the TRNrd than in the PMv. The timing of activity modulation of visual‐related and movement‐related neurons was similar in TRNrd and PMv neurons. Overall, our data suggest that TRNrd neurons provide motor thalamic nuclei with inhibitory inputs that are predominantly devoid of spatial selectivity, and that these signals modulate how these nuclei engage in both sensory processing and motor output during visually guided reaching behavior.     

Morphology of neurons in the thalamic reticular nucleus (TRN) of mammals as revealed by intracellular injections into fixed brain slices

I have investigated the morphology of neurons in the thalamic reticular nucleus (TRN) by means of intracellular injections in fixed tissue in order to study whether neurons in visual (dorsocaudal part), somatosensory (intermediate part), or limbic/motor (rostral part) sectors in the rat, rabbit, and cat differ morphologically in relation to their different sensory cortical or thalamic inputs. In addition, I have compared the different mammalian species to ask whether there is a morphological difference of TRN neurons according to reported differences in the intrinsic thalamic organisation, for example, due to the presence of GABAergic local circuit neurons in the majority of thalamic nuclei in the cat and the lack of those neurons in most of the rat thalamic nuclei, and presynaptic dendrites in the cat but not in the rat. In all animals investigated so far, neurons in the caudal (visual) and intermediate (somatosensory) part of the TRN have an elongated dendritic morphology in all three species, but some neurons in the rostral part, in particular in dorsal sections, have a distinctive multipolar morphology. Neurons have round, ovoid, or elongated somata ranging in area between 150 and 860 μm². In general 4–8 first order dendrites emerge directly from the two poles of the soma or from a thick stem segment. Most of the dendrites then run parallel to the borders of the nucleus extending for relatively long distances, up to 450 μm, but remain inside the border of the nucleus. Only a few (1–3) dendrites could be observed to run perpendicular to the border of the nucleus and generally only for a short distance (20–70 μm). Some of the smooth first order dendrites give rise to second order dendrites (up to 200 μm in length), which then branch into short (15–70 μm) third order dendrites. Dendritic spines and varicosities, spine-like protusions and/or hair-like processes are mainly found on second and third order dendrites. Surprisingly, the shape, arrangement, and the size of the dendritic field are not strictly related to the shape and size of the nucleus. In mammalian species with a comparatively narrow TRN (rat and cat) the dendritic field size was similar to that in the rabbit with a broad TRN. There was considerable variability in dendritic morphology in the caudal and intermediate parts of TRN. However, in contrast to two recent studies in the rat TRN I have found no obvious basis for classification of neurons in the mammalian TRN according to dendritic morphology. In addition, there seems to be no difference in neuronal morphology of TRN neurons in relation to different intrinsic thalamic organisation within or between species. © 1993 Wiley-Liss, Inc.     

Demonstration of lack of dorsal lateral geniculate nucleus input to extrastriate areas MT and Visual 2 in the macaque monkey

Injections of wheat germ conjugated and normal horseradish peroxidase in the identifiable extrastriate cortical areas MT and Visual 2 of the macaque monkey, indicate that these areas do not receive an input from the dorsal lateral geniculate nucleus as do other regions of extrastriate visual cortex.     

Recovery from the anatomical effects of long‐term monocular deprivation in cat lateral geniculate nucleus

Monocular deprivation (MD) imposed early in postnatal life elicits profound structural and functional abnormalities throughout the primary visual pathway. The ability of MD to modify neurons within the visual system is restricted to a so‐called critical period that, for cats, peaks at about one postnatal month and declines thereafter so that by about 3 months of age MD has little effect. Recovery from the consequences of MD likewise adheres to a critical period that ends by about 3 months of age, after which the effects of deprivation are thought to be permanent and without capacity for reversal. The attenuation of plasticity beyond early development is a formidable obstacle for conventional therapies to stimulate recovery from protracted visual deprivation. In the current study we examined the efficacy of dark exposure and retinal inactivation with tetrodotoxin to promote anatomical recovery in the dorsal lateral geniculate nuclues (dLGN) from long‐term MD started at the peak of the critical period. Whereas 10 days of dark exposure or binocular retinal inactivation were not better at promoting recovery than conventional treatment with reverse occlusion, inactivation of only the non‐deprived (fellow) eye for 10 days produced a complete restoration of neuron soma size, and also reversed the significant loss of neurofilament protein within originally deprived dLGN layers. These results reveal a capacity for neural plasticity and recovery that is larger than anything previously observed following protracted MD in cat, and they highlight a possibility for alternative therapies applied at ages thought to be recalcitrant to recovery.     

Retrograde analyses of spinothalamic projections in the macaque monkey: input to the ventral lateral nucleus

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in monkeys following variously sized injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input from different sets of STT cells. This report focuses on STT input to the ventral lateral nucleus (VL), where prior anterograde tracing studies identified dense or moderately dense STT terminations. Large and very large injections in VL produced large numbers of labeled cells predominantly in laminae V and VII (more than half as many as from massive injections in the entire thalamus). Medium-sized and small injections in VL labeled STT cells almost exclusively in laminae V and VII, in segments consistent with the coarse mediolateral VL topography; few or no cells were labeled in lamina I. All injections labeled the deep cerebellar nuclei (see accompanying article: Evrard and Craig, 2008). Notably, even the most anterior injection in VL that produced dense pallidal labeling still labeled both STT and deep cerebellar cells. These observations indicate that VL receives STT input originating from laminae V and VII neurons that may be coextensive with its cerebellothalamic input. These findings support the role of laminae V and VII STT cells in sensorimotor integration and suggest a significant ongoing influence on both motor and premotor thalamocortical function. Together with the preceding observations of selective STT projections to other thalamic regions, these results provide compelling evidence that the primate STT consists of anatomically and functionally differentiable components.     

A comparison of the organization of the projections of the dorsal lateral geniculate nucleus, the inferior pulvinar and adjacent lateral pulvinar to primary visual cortex (area 17) in the macaque monkey

Both anterograde and retrograde transport tracing methods were used to study the organization of the projections of the dorsal lateral geniculate (DLG), the inferior pulvinar and subdivisions of the lateral pulvinar to primary visual cortex (striate cortex or area 17). The DLG projects only to striate cortex. These projections are retinotopically organized, and do not extend to any cortical layers above layer IVA. In contrast the inferior pulvinar (PI) and the immediately adjacent portion of the lateral pulvinar (PL alpha 48) project to both striate and prestriate cortex. The projections from these two thalamic areas to the striate cortex are also retinotopically organized and exist in parallel with those from the DLG. In contrast to the DLG, the projections from PI and PL alpha terminate above layer IVA in striate cortex, i.e. layers I, II and III. In prestriate cortex the layers of termination include layers IV, III and I. The pulvinar terminations in layers II and III of area 17 occur in segregated patches as do the geniculate terminations in layers IVC and IVA. On the other hand the pulvinar terminations in layer I which overlie those in layers II and III of area 17 appeared to be continuous. Control studies show that the remainder of the lateral pulvinar overlying PL alpha does not project to striate cortex. It is concluded that there are 3 visuotopically organized inputs from the lateral thalamus to primary visual cortex and that each of these inputs have different layers of termination. The inputs from PI and DLG can convey direct retinal inputs while those from PI and PL alpha can also be involved in intrinsic cortico-thalamocortical connection with prestriate cortex. It remains, then that it cannot be tacitly assumed that the ascending inputs which influence the response properties of the primary cortical neurons arise solely from the dorsal lateral geniculate nucleus. It is also argued that these inputs to the supragranular layers may be excitatory as those from the DLG to the IVth layer.     

Post‐crossing segment of dI1 commissural axons forms collateral branches to motor neurons in the developing spinal cord

The dI1 commissural axons in the developing spinal cord, upon crossing the midline through the floor plate, make a sharp turn to grow rostrally. These post‐crossing axons initially just extend adjacent to the floor plate without entering nearby motor columns. However, it remains poorly characterized how these post‐crossing dI1 axons behave subsequently to this process. In the present study, to address this issue, we examined in detail the behavior of post‐crossing dI1 axons in mice, using the Atoh1 enhancer‐based conditional expression system that enables selective and sparse labeling of individual dI1 axons, together with Hb9 and ChAT immunohistochemistry for precise identification of spinal motor neurons (MNs). We found unexpectedly that the post‐crossing segment of dI1 axons later gave off collateral branches that extended laterally to invade motor columns. Interestingly, these collateral branches emerged at around the time when their primary growth cones initiated invasion into motor columns. In addition, although the length of the laterally growing collateral branches increased with age, the majority of them remained within motor columns. Strikingly, these collateral branches further gave rise to multiple secondary branches in the region of MNs that innervate muscles close to the body axis. Moreover, these axonal branches formed presynaptic terminals on MNs. These observations demonstrate that dI1 commissural neurons develop axonal projection to spinal MNs via collateral branches arising later from the post‐crossing segment of these axons. Our findings thus reveal a previously unrecognized projection of dI1 commissural axons that may contribute directly to generating proper motor output.     

Sexual dimorphism in visual and olfactory brain centers in the perfume‐collecting orchid bee Euglossa dilemma (Hymenoptera,Apidae)

Insect mating behavior is controlled by a diverse array of sex‐specific traits and strategies that evolved to maximize mating success. Orchid bees exhibit a unique suite of perfume‐mediated mating behaviors. Male bees collect volatile compounds from their environment to concoct species‐specific perfume mixtures that are presumably used to attract conspecific females. Despite a growing understanding of the ecology and evolution of chemical signaling in orchid bees, many aspects of the functional adaptations involved, in particular regarding sensory systems, remain unknown. Here we investigated male and female brain morphology in the common orchid bee Euglossa dilemma Bembé & Eltz. Males exhibited increased relative volumes of the Medulla, a visual brain region, which correlated with larger compound eye size (area). While the overall volume of olfactory brain regions was similar between sexes, the antennal lobes exhibited several sex‐specific structures including one male‐specific macroglomerulus. These findings reveal sexual dimorphism in both the visual and the olfactory system of orchid bees. It highlights the tendency of an increased investment in the male visual system similar to that observed in other bee lineages, and suggests that visual input may play a more important role in orchid bee male mating behavior than previously thought. Furthermore, our results suggest that the evolution of perfume communication in orchid bees did not involve drastic changes in olfactory brain morphology compared to other bee lineages.     

Individual mediodorsal thalamic neurons project to multiple areas of the rat prefrontal cortex: A single neuron‐tracing study using virus vectors

The prefrontal cortex has an important role in a variety of cognitive and executive processes, and is generally defined by its reciprocal connections with the mediodorsal thalamic nucleus (MD). The rat MD is mainly subdivided into three segments, the medial (MDm), central (MDc), and lateral (MDl) divisions, on the basis of the cytoarchitecture and chemoarchitecture. The MD segments are known to topographically project to multiple prefrontal areas at the population level: the MDm mainly to the prelimbic, infralimbic, and agranular insular areas; the MDc to the orbital and agranular insular areas; and the MDl to the prelimbic and anterior cingulate areas. However, it is unknown whether individual MD neurons project to single or multiple prefrontal cortical areas. In the present study, we visualized individual MD neurons with Sindbis virus vectors, and reconstructed whole structures of MD neurons. While the main cortical projection targets of MDm, MDc, and MDl neurons were generally consistent with those of previous results, it was found that individual MD neurons sent their axon fibers to multiple prefrontal areas, and displayed various projection patterns in the target areas. Furthermore, the axons of single MD neurons were not homogeneously spread, but were rather distributed to form patchy axon arbors approximately 1 mm in diameter. The multiple‐area projections and patchy axon arbors of single MD neurons might be able to coactivate cortical neuron groups in distant prefrontal areas simultaneously. Furthermore, considerable heterogeneity of the projection patterns is likely, to recruit the different sets of cortical neurons, and thus contributes to a variety of prefrontal functions. J. Comp. Neurol. 525:166–185, 2017. © 2016 Wiley Periodicals, Inc.     

Differential contributions of intra‐cellular and inter‐cellular mechanisms to the spatial and temporal architecture of the suprachiasmatic nucleus circadian circuitry in wild‐type,cryptochrome‐null and vasoactive intestinal peptide receptor 2‐null mutant mice

To serve as a robust internal circadian clock, the cell‐autonomous molecular and electrophysiological activities of the individual neurons of the mammalian suprachiasmatic nucleus (SCN) are coordinated in time and neuroanatomical space. Although the contributions of the chemical and electrical interconnections between neurons are essential to this circuit‐level orchestration, the features upon which they operate to confer robustness to the ensemble signal are not known. To address this, we applied several methods to deconstruct the interactions between the spatial and temporal organisation of circadian oscillations in organotypic slices from mice with circadian abnormalities. We studied the SCN of mice lacking Cryptochrome genes (Cry1 and Cry2), which are essential for cell‐autonomous oscillation, and the SCN of mice lacking the vasoactive intestinal peptide receptor 2 (VPAC2‐null), which is necessary for circuit‐level integration, in order to map biological mechanisms to the revealed oscillatory features. The SCN of wild‐type mice showed a strong link between the temporal rhythm of the bioluminescence profiles of PER2::LUC and regularly repeated spatially organised oscillation. The Cry‐null SCN had stable spatial organisation but lacked temporal organisation, whereas in VPAC2‐null SCN some specimens exhibited temporal organisation in the absence of spatial organisation. The results indicated that spatial and temporal organisation were separable, that they may have different mechanistic origins (cell‐autonomous vs. interneuronal signaling) and that both were necessary to maintain robust and organised circadian rhythms throughout the SCN. This study therefore provided evidence that the coherent emergent properties of the neuronal circuitry, revealed in the spatially organised clusters, were essential to the pacemaking function of the SCN.     

Expression of m1‐type muscarinic acetylcholine receptors by parvalbumin‐immunoreactive neurons in the primary visual cortex: A comparative study of rat,guinea pig,ferret, macaque,and human

Cholinergic neuromodulation is a candidate mechanism for aspects of arousal and attention in mammals. We have reported previously that cholinergic modulation in the primary visual cortex (V1) of the macaque monkey is strongly targeted toward GABAergic interneurons, and in particular that the vast majority of parvalbumin‐immunoreactive (PV) neurons in macaque V1 express the m1‐type (pirenzepine‐sensitive, Gq‐coupled) muscarinic ACh receptor (m1AChR). In contrast, previous physiological data indicates that PV neurons in rats rarely express pirenzepine‐sensitive muscarinic AChRs. To examine further this apparent species difference in the cholinergic effectors for the primary visual cortex, we have conducted a comparative study of the expression of m1AChRs by PV neurons in V1 of rats, guinea pigs, ferrets, macaques, and humans. We visualize PV‐ and mAChR‐immunoreactive somata by dual‐immunofluorescence confocal microscopy and find that the species differences are profound; the vast majority (>75%) of PV‐ir neurons in macaques, humans, and guinea pigs express m1AChRs. In contrast, in rats only ～25% of the PV population is immunoreactive for m1AChRs. Our data reveal that while they do so much less frequently than in primates, PV neurons in rats do express Gq‐coupled muscarinic AChRs, which appear to have gone undetected in the previous in vitro studies. Data such as these are critical in determining the species that represent adequate models for the capacity of the cholinergic system to modulate inhibition in the primate cortex. J. Comp. Neurol. 522:986–1003, 2014. © 2013 Wiley Periodicals, Inc.