Injections Of Retrograde Tracers Research Articles

Projections from the central amygdaloid nucleus to the precuneiform nucleus in the mouse

The mouse precuneiform nucleus has been proposed as the midbrain locomotion center, a function ascribed to its caudal neighbor, cuneiform nucleus, in the rat, cat and other species. The present study investigated the projections from the central amygdaloid nucleus to the precuneiform nucleus in the mouse using retrograde tracer injections (fluoro-gold) into the precuneiform nucleus and anterograde tracer injections (biotinylated dextran amine) into the central amygdaloid nucleus. The entire central amygdaloid nucleus except the rostral pole had retrogradely labeled neurons, especially in the middle portion where labeled neurons were densely packed. Anterogradely labeled amygdaloid fibers approached the precuneiform nucleus from the area ventrolateral to it and terminated in the entire precuneiform nucleus. Labeled fibers were also found in laminae 5 and 6 in the upper cervical cord on the ipsilateral side. The present study is the first demonstration of projections from the central amygdaloid nucleus to the precuneiform nucleus. This projection may underpin the role of the precuneiform nucleus in the modulation of the cardiovascular activity.

Jaw-opening and -closing premotoneurons in the nucleus of the solitary tract making contacts with laryngeal and pharyngeal afferent terminals in rats

This study clarified the neural mechanisms underlying jaw movements in pharyngolaryngeal reflexes such as swallowing in rats. After retrograde tracer injections into the ventromedial division (Vmovm) of the trigeminal motor nucleus (Vmo) containing jaw-opening (JO) motoneurons or into the dorsolateral division (Vmodl) of Vmo containing jaw-closing (JC) motoneurons, JO and JC premotoneurons were labeled with an ipsilateral predominance in the medial and intermediate subnuclei of the rostrocaudal middle two-thirds of the nucleus of the solitary tract (Sol); JC premotoneurons were also in the lateral subnucleus of Sol. After anterograde tracer injections into the Sol, axons were labeled with an ipsilateral predominance in the Vmovm and Vmodl, prominently in the ipsilateral Vmovm. After transganglionic tracer applications to the superior laryngeal nerve (SLN) or the cervical trunk of the glossopharyngeal nerve (GpN-ct), labeled afferents were seen in the medial, intermediate, lateral and interstitial subnuclei of Sol at the rostral three-fourths of Sol, indicating considerable overlap with the JO and JC premotoneurons in the Sol. Double labeling experiments demonstrated contacts between the afferent terminals and the JO and JC premotoneurons. The present study has for the first time revealed the differential distribution of JO and JC premotoneurons in the Sol and features of their projections from the Sol, as well as their connections with SLN and GpN-ct afferent inputs. The JO and JC premotoneurons in the Sol may play an important role in generation and organization of jaw movements in pharyngolaryngeal reflexes evoked by SLN and GpN-ct inputs, such as swallowing.

Orexin neurons are indispensable for prostaglandin E2‐induced fever and defence against environmental cooling in mice

We recently showed using prepro-orexin knockout (ORX-KO) mice and orexin neuron-ablated (ORX-AB) mice that orexin neurons in the hypothalamus, but not orexin peptides per se, are indispensable for stress-induced thermogenesis. To examine whether orexin neurons are more generally involved in central thermoregulatory mechanisms, we applied other forms of thermogenic perturbations, including brain prostaglandin E2 (PGE2) injections which mimic inflammatory fever and environmental cold exposure, to ORX-KO mice, ORX-AB mice and their wild-type (WT) litter mates. ORX-AB mice, but not ORX-KO mice, exhibited a blunted PGE2-induced fever and intolerance to cold (5°C) exposure, and these findings were similar to the results previously obtained with stress-induced thermogenesis. PGE2-induced shivering was also attenuated in ORX-AB mice. Both mutants responded similarly to environmental heating (39°C). In WT and ORX-KO mice, the administration of PGE2 and cold exposure activated orexin neurons, as revealed by increased levels of expression of c-fos. Injection of retrograde tracer into the medullary raphe nucleus revealed direct and indirect projection from the orexin neurons, of which the latter seemed to be preserved in the ORX-AB mice. In addition, we found that glutamate receptor antagonists (D-(-)-2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitroquinoxaline-2,3-dione) but not orexin receptor antagonists (SB334867 and OX2 29) successfully inhibited PGE2-induced fever in WT mice. These results suggest that orexin neurons are important in general thermogenic processes, and their importance is not restricted to stress-induced thermogenesis. In addition, these results indicate the possible involvement of glutamate in orexin neurons implicated in PGE2-induced fever.

Chronic Stress-Induced Alterations of Dendritic Spine Subtypes Predict Functional Decrements in an Hypothalamo–Pituitary–Adrenal-Inhibitory Prefrontal Circuit

Activation of the hypothalamo-pituitary-adrenal (HPA) axis plays a vital role in promoting adaptation during acute stress, but adverse effects of chronic stress may result from overactivity of this system. Recent evidence highlights a subdivision of GABAergic neurons within anterior bed nuclei of the stria terminalis (aBST) that integrates and relays inhibitory influences to HPA-effector neurons in paraventricular hypothalamus during acute stress, notably from medial prefrontal [prelimbic (PL)] and hippocampal [ventral subiculum (vSUB)] cortical fields. Here we localize the site and candidate mechanism of neuroplasticity within upstream regions of this inhibitory network after chronic variable stress (CVS). Rats bearing retrograde tracer injections in aBST underwent CVS for 14 d. Retrogradely labeled and unlabeled neurons in vSUB and PL were selected for intracellular dye filling, followed by three-dimensional imaging and analysis of dendritic arborization and spine morphometry. Whereas PL neurons displayed decreases in dendritic branching and spine density after CVS, aBST-projecting cells showed a selective loss of mature mushroom-shaped spines. In a follow-up experiment, CVS-treated and control rats were exposed to a novel restraint challenge for assay of HPA activation and engagement of aBST-projecting cortical regions. CVS animals showed enhanced HPA output and decreased Fos activation in aBST-projecting PL neurons compared with acutely stressed controls. In contrast, vSUB failed to show any significant differences in structural plasticity or functional activation patterns after CVS. These findings define a mechanism whereby synaptic destabilization in the PL → aBST pathway may dampen its ability to impart inhibitory control over the HPA axis after chronic stress exposure.

Primary visual cortex projections to extrastriate cortices in enucleated and anophthalmic mice

In the mouse, visual extrastriate areas are located within distinct acallosal zones. It has been proposed that the striate-extrastriate and callosal projections are interdependent. In visually deprived mice, the normal patterns of callosal and striate-extrastriate projections are disrupted. It remains unknown whether visual deprivation affects the topography of V1-extrastriate projections and their relationship with callosal projections. Two anterograde tracers were injected in V1 and multiple retrograde tracer injections were performed in the contralateral hemisphere of intact and enucleated C57BL/6 mice and in ZRDCT/An mice to determine the effects of prenatal and postnatal afferent sensory activity on the topography of V1-extrastriate and callosal projections. Greater topographic anomalies were found in striate-extrastriate projections of anophthalmic than enucleated mice. In enucleated mice, the relationship between striate-extrastriate projections and callosal zones was highly variable. In anophthalmic mice, there was also a greater overlap between these projections. These results suggest that the prenatal afferent sensory activity regulates some aspects of the distribution of V1-extrastriate and callosal projections, in addition to the development of a normal topographic representation in extrastriate areas.

Amygdala projections to the lateral bed nucleus of the stria terminalis in the macaque: Comparison with ventral striatal afferents

The lateral bed nucleus of the stria terminalis (BSTL) is involved in mediating anxiety-related behaviors to sustained aversive stimuli. The BSTL forms part of the central extended amygdala, a continuum composed of the BSTL, the amygdala central nucleus, and cell columns running between the two. The central subdivision (BSTLcn) and the juxtacapsular subdivision (BSTLJ) are two BSTL regions that lie above the anterior commissure, near the ventral striatum. The amygdala, a heterogeneous structure that encodes emotional salience, projects to both the BSTL and ventral striatum. We placed small injections of retrograde tracers into the BSTL, focusing on the BSTLcn and BSTLJ, and analyzed the distribution of labeled cells in amygdala subregions. We compared this to the pattern of labeled cells following injections into the ventral striatum. All retrograde results were confirmed by anterograde studies. We found that the BSTLcn receives stronger amygdala inputs relative to the BSTLJ. Furthermore, the BSTLcn is defined by inputs from the corticoamygdaloid transition area and central nucleus, while the BSTLJ receives inputs mainly from the magnocellular accessory basal and basal nucleus. In the ventral striatum, the dorsomedial shell receives inputs that are similar, but not identical, to inputs to the BSTLcn. In contrast, amygdala projections to the ventral shell/core are similar to projections to the BSTLJ. These findings indicate that the BSTLcn and BSTLJ receive distinct amygdala afferent inputs and that the dorsomedial shell is a transition zone with the BSTLcn, while the ventral shell/core are transition zones with the BSTLJ.

Two Projection Streams from Macaque V1 to the Pale Cytochrome Oxidase Stripes of V2

In the primate visual cortex, areas V1 and V2 distribute information they receive from the retina to virtually all extrastriate cortex, parsing this information into dorsal and ventral streams. Therefore, understanding the connectivity between V1 and V2 is crucial to understand visual cortical processing. Cytochrome oxidase staining in V2 reveals a repeating pattern of pale-thick-pale-thin stripes. V1 sends parallel output pathways to distinct V2 stripes. Previous models proposed either three or two parallel V1-to-V2 pathways in macaque, but both models viewed the two pale stripes within a single stripe cycle as a single compartment. However, recent studies have suggested that the two pale stripes may be functionally distinct, and in marmosets they also differ anatomically in the laminar origin of projections they receive from V1. Here we have asked whether the two pale stripes are also anatomically distinct in macaque. We made small retrograde tracer injections in different pale stripe types. We found that while both pale stripes receive a predominant V1 input from layers 2/3, only one set of pale stripes (pale lateral) receives significant projections from layer 4B, while the other set (pale medial) receives few or no layer 4B projections. Moreover, different tracer injections in nearby pale stripe types revealed that 97-99% of layer 2/3 cells only project to a single pale stripe type. These results demonstrate that in macaque, the two pale stripes are anatomically distinct compartments, and support the notion of two distinct projection streams from V1 to the two pale stripes of V2.

Why data coherence and quality is critical for understanding interareal cortical networks

Numerous studies have investigated inter-areal cortical networks using either diffusion MRI or axonal tract-tracing. While both techniques have been used in non-human primates only diffusion MRI can be used in human. The advantage of axonal tract-tracing is that unlike diffusion MRI it has a high single-cell resolution, and most importantly gives the laminar origins and terminations of inter-areal pathways. It, therefore, can be used to obtain the weighted and directed cortical graph. Axonal tract tracing has traditionally been collated from multiple experiments in order to determine the large-scale inter-areal network. Collated data of this kind present numerous problems due to lack of coherence across studies and incomplete exploitation. We have therefore developed a consistent data base which uses standardized experimental and parcellation procedures across brains. Here we review our recent publications analyzing the consistent database obtained from retrograde tracer injections in 29 cortical areas in a parcellation of 91 areas of the macaque cortex. Compared to collated data, our results show that the cortical graph is dense. Density is a graph theoretic measure, and refers to the number of observed connections in a square matrix expressed as a percentage of the possible connections. In our database 66% of the connections that can exist do exist which is considerably higher than the graph densities reported in studies using collated data (7–32%). The consistent data base reports 37% more pathways than previously reported, many of which are unidirectional. This latter and unexpected property has not been reported in earlier studies. Given the high density, the resulting cortical graph shows other unexpected properties. Firstly, the binary specificity is considerably higher than expected. As we show, this property is a consequence of the inter-areal connection probability declining with distance. Secondly, small groups of areas are found to receive high numbers of inputs. This is termed a high domination and is analyzed by a graph theoretic procedure known as a minimum dominating set analysis. We discuss these findings with respect to the long-distance connections, over half of which were previously not reported. These so called new found projections display high specificities and play an important integration role across large regions. It is to be expected that the future examination of the 62 remaining areas will disclose further levels of complexity and enable construction of a weighted directed graph revealing the hierarchical complexity of the cortex.

The role of long-range connections on the specificity of the macaque interareal cortical network

We investigated the influence of interareal distance on connectivity patterns in a database obtained from the injection of retrograde tracers in 29 areas distributed over six regions (occipital, temporal, parietal, frontal, prefrontal, and limbic). One-third of the 1,615 pathways projecting to the 29 target areas were reported only recently and deemed new-found projections (NFPs). NFPs are predominantly long-range, low-weight connections. A minimum dominating set analysis (a graph theoretic measure) shows that NFPs play a major role in globalizing input to small groups of areas. Randomization tests show that (i) NFPs make important contributions to the specificity of the connectivity profile of individual cortical areas, and (ii) NFPs share key properties with known connections at the same distance. We developed a similarity index, which shows that intraregion similarity is high, whereas the interregion similarity declines with distance. For area pairs, there is a steep decline with distance in the similarity and probability of being connected. Nevertheless, the present findings reveal an unexpected binary specificity despite the high density (66%) of the cortical graph. This specificity is made possible because connections are largely concentrated over short distances. These findings emphasize the importance of long-distance connections in the connectivity profile of an area. We demonstrate that long-distance connections are particularly prevalent for prefrontal areas, where they may play a prominent role in large-scale communication and information integration.

Organisation of koniocellular‐projecting ganglion cells and diffuse bipolar cells in the primate fovea

The roles of the midget and parasol pathways as the anatomical foundation for high-acuity vision at the fovea are well established. There is also evidence for the presence of other (non-midget, non-parasol) ganglion cell types in the foveal retina, but it is not established whether these cells receive input from cone photoreceptors in the central few degrees of the visual field, i.e. the region most important for conscious visual perception. To address this question, we targeted injections of retrograde tracer to the koniocellular layers in the posterior aspect of the lateral geniculate nucleus, where the central visual field is represented, in marmoset monkeys (Callithrix jacchus). Labeled ganglion cells were photofilled to reveal their dendritic morphology. Potential inputs to foveal koniocellular cells from diffuse bipolar cells were investigated in vertical sections through the fovea of marmoset and macaque (Macaca fascicularis) monkey retinas using immunohistochemistry. Forty koniocellular-projecting ganglion cells were analysed. We used an established model of marmoset foveal topography to show that all these koniocellular-projecting cells receive cone inputs from the central-most 6°, with about half the cells receiving input from below 2° eccentricity, in the rod-free central bouquet of cones at the foveola. In addition, all diffuse bipolar types investigated were present in the fovea at stratification depths similar to those of their counterparts in the peripheral retina. We conclude that the diverse visual representations established for koniocellular pathways in the peripheral retina are also a feature of the fovea, suggesting that koniocellular pathways contribute to foveal vision.

Medial cerebellar nuclear projections and activity patterns link cerebellar output to orofacial and respiratory behavior

There is ample evidence that the cerebellum plays an important role in coordinating both respiratory and orofacial movements. However, the pathway by which the cerebellum engages brainstem substrates underlying these movements is not well understood. We used tract-tracing techniques in mice to show that neurons in the medial deep cerebellar nucleus (mDCN) project directly to these putative substrates. Injection of an anterograde tracer into the mDCN produced terminal labeling in the ventromedial medullary reticular formation, which was stronger on the contralateral side. Correspondingly, injection of retrograde tracers into these same areas resulted in robust neuronal cell labeling in the contralateral mDCN. Moreover, injection of two retrograde tracers at different rostral–caudal brainstem levels resulted in a subset of double-labeled cells, indicating that single mDCN neurons collateralize to multiple substrates. Using an awake and behaving recording preparation, we show that spiking activity in mDCN neurons is correlated with respiratory and orofacial behaviors, including whisking and fluid licking. Almost half of the recorded neurons showed activity correlated with more than one behavior, suggesting that these neurons may in fact modulate multiple brainstem substrates. Collectively, these results describe a potential pathway through which the cerebellum could modulate and coordinate respiratory and orofacial behaviors.

The interfascicular trigeminal nucleus: A precerebellar nucleus in the mouse defined by retrograde neuronal tracing and genetic fate mapping

We have found a previously unreported precerebellar nucleus located among the emerging fibers of the motor root of the trigeminal nerve in the mouse, which we have called the interfascicular trigeminal nucleus (IF5). This nucleus had previously been named the tensor tympani part of the motor trigeminal nucleus (5TT) in rodent brain atlases, because it was thought to be a subset of small motor neurons of the motor trigeminal nucleus innervating the tensor tympani muscle. However, following injection of retrograde tracer in the cerebellum, the labeled neurons in IF5 were found to be choline acetyltransferase (ChAT) negative, indicating that they are not motor neurons. The cells of IF5 are strongly labeled in mice from Wnt1Cre and Atoh1 CreER lineage fate mapping, in common with the major precerebellar nuclei that arise from the rhombic lip and that issue mossy fibers. Analysis of sections from mouse Hoxa3, Hoxb1, and Egr2 Cre labeled lineages shows that the neurons of IF5 arise from rhombomeres caudal to rhombomere 4, most likely from rhombomeres 6-8. We conclude that IF5 is a significant precerebellar nucleus in the mouse that shares developmental gene expression characteristics with mossy fiber precerebellar nuclei that arise from the caudal rhombic lip.

Neuroanatomical study on the tecto-suprageniculate-dorsal auditory cortex pathway in the rat

Previous anatomical and physiological studies suggest that the superior colliculus sends integrated sensory information to the multimodal cortical areas via the thalamic suprageniculate nucleus (SG). However, the detailed distribution of rat tecto-SG axon terminals and SG neurons projecting to the multimodal cortex, as well as synaptic connections between these tectal axons and SG neurons, remains unclear.In this study, the organization of the tecto-thalamo-cortical pathway was investigated via combined injections of anterograde and retrograde tracers followed by light and electron microscopic observations. Injections of a retrograde tracer, cholera toxin B subunit (CTB), into the temporal cortex, area 2, dorsal part (Te2D), and injections of an anterograde tracer, biotinylated dextran amine (BDA), into the deep layers of the superior colliculus produced the following results: (1) Retrogradely CTB-labeled neurons were found throughout SG, predominantly in its rostral part. CTB-labeled neurons were also found in other cortical areas such as the visual cortex, the auditory cortex, the parietal association cortex, and the perirhinal cortex. (2) Anterogradely BDA-labeled axons and their terminals were also observed throughout SG. Dual visualization of BDA and CTB showed that retrogradely labeled SG neurons and anterogradely labeled tectal axon terminal boutons overlapped considerably in the rostral part of SG, and their direct synaptic contacts were also confirmed via electron microscopy. These findings suggest that multimodal information from the superior colliculus can be processed directly in SG neurons projecting to Te2D.

A Weighted and Directed Interareal Connectivity Matrix for Macaque Cerebral Cortex

Retrograde tracer injections in 29 of the 91 areas of the macaque cerebral cortex revealed 1,615 interareal pathways, a third of which have not previously been reported. A weight index (extrinsic fraction of labeled neurons [FLNe]) was determined for each area-to-area pathway. Newly found projections were weaker on average compared with the known projections; nevertheless, the 2 sets of pathways had extensively overlapping weight distributions. Repeat injections across individuals revealed modest FLNe variability given the range of FLNe values (standard deviation <1 log unit, range 5 log units). The connectivity profile for each area conformed to a lognormal distribution, where a majority of projections are moderate or weak in strength. In the G29 × 29 interareal subgraph, two-thirds of the connections that can exist do exist. Analysis of the smallest set of areas that collects links from all 91 nodes of the G29 × 91 subgraph (dominating set analysis) confirms the dense (66%) structure of the cortical matrix. The G29 × 29 subgraph suggests an unexpectedly high incidence of unidirectional links. The directed and weighted G29 × 91 connectivity matrix for the macaque will be valuable for comparison with connectivity analyses in other species, including humans. It will also inform future modeling studies that explore the regularities of cortical networks.

Axial Organization of a Brain Region That Sequences a Learned Pattern of Behavior

Neural activity within HVC (proper name), a premotor nucleus of the songbird telencephalon analogous to premotor cortical regions in mammals, controls the temporal structure of learned song in male zebra finches (Taeniopygia guttata). HVC is composed of a superficially isomorphic neuronal mosaic, implying that song is encoded in a distributed network within HVC. Here, we combined HVC microlesions (10% focal ablation) with singing-driven immediate-early gene (IEG) labeling to explore the network architecture of HVC during singing. Microlesions produce a transient disruption of HVC activity that results in a temporary (≈ 1 week) loss of vocal patterning. Results showed an asymmetrical reduction in the density of IEG-labeled cells 3-5 d after microlesions: swaths of unlabeled cells extended rostrally and/or caudally depending on the position of the HVC microlesion. Labeling returned once birds recovered their songs. Axial swaths of unlabeled cells occurred whether microlesions were located at rostral or caudal poles of HVC, indicating that the localized reduction in IEG labeling could not be attributable solely to transection of afferents that enter HVC rostrally. The asymmetrical pattern of reduced IEG labeling could be explained if synaptic connectivity within HVC is organized preferentially within the rostrocaudal axis. In vivo retrograde tracer injections and in vitro stimulation and recording experiments in horizontal slices of HVC confirmed a rostrocaudal organization of HVC neural connectivity. Our findings suggest that HVC contains an axially organized network architecture that may encode the temporal structure of song.

Rat Claustrum Coordinates But Does Not Integrate Somatosensory and Motor Cortical Information

The function of the claustrum is a fundamental issue in neuroscience. Anatomical data indicate that the rat claustrum is part of an interhemispheric circuit that could be involved in the bilateral coordination of whisker movements. Given that whisking is a somesthetic-guided motor behavior, the goal of the current study was to elucidate the connections of the claustrum with respect to the whisker representations in the primary somatosensory (wSI) and motor (wMI) cortical areas. Anterograde tracer injections showed that wMI projects most densely to the claustrum in the contralateral hemisphere, whereas wSI does not project to the claustrum in either hemisphere. Injections of different retrograde tracers into wMI and wSI of the same animal revealed intermingled populations of labeled neurons in the claustrum, as well as many double-labeled neurons. This indicates that the same part of the claustrum projects to the whisker representations in both SI and MI. Finally, injections of different anterograde tracers in the wMI regions of both hemispheres were combined with a retrograde tracer injection in wSI, and this produced dense terminal labeling around retrogradely labeled neurons in the claustrum of both hemispheres. Although the rodent claustrum is probably involved in the interhemispheric coordination of the MI and SI whisker representations, it does not receive inputs from both of these cortical regions. Hence, the claustrum should not be universally regarded as an integrator of somesthetic and motor information.

Topography of descending projections from anterior insular and medial prefrontal regions to the lateral habenula of the epithalamus in the rat

The epithalamic lateral nucleus of the habenula (LHb) plays a key role in regulating firing of dopamine and serotonin neurons in the midbrain and is thereby involved in various cognitive and affective behaviors. It is not yet clear, however, from where the LHb receives cognitive and affective information relevant to its regulation of the midbrain monoaminergic systems. The prefrontal cortex would be among the ideal sources. Here, using anterograde and retrograde tracer injections in the rat brain, we characterized the topography of the corticohabenular projections. Following injections of cholera toxin subunit B into the LHb, retrogradely labeled neurons were produced in the anterior insular, cingulate, prelimbic and infralimbic cortices. Consistent with this retrograde tracing, injections of biotinylated dextran amine (BDA) into these cortical regions labeled robust terminals in the LHb. Our quantification of the BDA-impregnated varicosities revealed that projections from the anterior insula terminated mainly in the intersection regions of the lateral and ventral two-thirds of the LHb, while projections from the cingulate cortex terminated mainly in the lateral two-thirds of the LHb. By comparison, BDA-labeled terminals originating from the medial prefrontal regions were contained mainly in the medial plus ventral one-third of LHb. Based on these data, we hypothesize that LHb provides a link for conveying cognitive and affective information from prefrontal and insular regions to the midbrain monoaminergic centers.

Thalamic connections of auditory cortex in marmoset monkeys: lateral belt and parabelt regions.

The primate auditory cortex is comprised of a core region of three primary areas, surrounded by a belt region of secondary areas and a parabelt region lateral to the belt. The main sources of thalamocortical inputs to the auditory cortex are the medial geniculate complex (MGC), medial pulvinar (PM), and several adjoining nuclei in the posterior thalamus. The distribution of inputs varies topographically by cortical area and thalamic nucleus, but in a manner that has not been fully characterized in primates. In this study, the thalamocortical connections of the lateral belt and parabelt were determined by placing retrograde tracer injections into various areas of these regions in the marmoset monkey. Both regions received projections from the medial (MGm) and posterodorsal (MGpd) divisions of the medial geniculate complex (MGC); however, labeled cells in the anterodorsal (MGad) division were present only from injections into the caudal belt. Thalamic inputs to the lateral belt appeared to come mainly from the MGC, whereas the parabelt also received a strong projection from the PM, consistent with its position as a later stage of auditory cortical processing. The results of this study also indicate that the organization of the marmoset auditory cortex is similar to other primates.

Musculotopic organization of the motor neurons supplying forelimb and shoulder girdle muscles in the mouse

We identified the motor neurons (MNs) supplying the shoulder girdle and forelimb muscles in the C57BL/6J mouse spinal cord using Fluoro-Gold retrograde tracer injections. In spinal cord transverse sections from C2 to T2, we observed two MN columns (medial and lateral) both with ventral and dorsal subdivisions. The dorsolateral column consisted of the biceps brachii, forearm extensors, forearm flexors, and hand MNs, and the ventrolateral column consisted of the latissimus dorsi, trapezius, teres major, deltoid, and triceps MNs. The supraspinatus muscle MNs were located in the dorsomedial column, and pectoralis major and serratus anterior MNs were located in the ventromedial columns. MNs of the dorsolateral column innervated the biceps brachii in mid-C4 to mid-C7, forearm extensors in caudal C4 to mid-T1, forearm flexors in rostral C5 to mid-T1, and hand muscles in mid-C8 to mid-T2 segments. The MNs innervating the trapezius were located in mid-C2 to mid-C4, triceps brachii in mid-C6 to rostral T1, deltoid in rostral C4 to mid-C6, teres major in rostral C5 to mid-C8, and latissimus dorsi in mid-C5 to caudal C8. In addition, MNs innervating the supraspinatus were located from rostral C4 to caudal C8, pectoralis major in mid-C6 to mid-T2, and serratus anterior in rostral C5 to caudal C7/rostral C8 segments. While the musculotopic pattern of MN groups was very similar to that documented for other species, we found differences in the position and cranio-caudal extent of some MN pools compared with previous reports. The identification of mouse forelimb MNs can serve as an anatomical reference for studying degenerative MN diseases, spinal cord injury, and developmental gene expression.

Sustained expression of PGC-1 in the rat nigrostriatal system selectively impairs dopaminergic function

Mitochondrial dysfunction and oxidative stress have been implicated in the etiology of Parkinson's disease. Therefore, pathways controlling mitochondrial activity rapidly emerge as potential therapeutic targets. Here, we explore the neuronal response to prolonged overexpression of peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α), a transcriptional regulator of mitochondrial function, both in vitro and in vivo. In neuronal primary cultures from the ventral midbrain, PGC-1α induces mitochondrial biogenesis and increases basal respiration. Over time, we observe an increasing proportion of the oxygen consumed by neurons which are dedicated to adenosine triphosphate production. In parallel to enhanced oxidative phosphorylation, PGC-1α progressively leads to a decrease in mitochondrial polarization. In the adult rat nigrostriatal system, adeno-associated virus (AAV)-mediated overexpression of PGC-1α induces the selective loss of dopaminergic markers and increases dopamine (DA) catabolism, leading to a reduction in striatal DA content. In addition, PGC-1α prevents the labeling of nigral neurons following striatal injection of the fluorogold retrograde tracer. When PGC-1α is expressed at higher levels following intranigral AAV injection, it leads to overt degeneration of dopaminergic neurons. Finally, PGC-1α overexpression does not prevent nigrostriatal degeneration in pathologic conditions induced by α-synuclein overexpression. Overall, we find that lasting overexpression of PGC-1α leads to major alterations in the metabolic activity of neuronal cells which dramatically impair dopaminergic function in vivo. These results highlight the central role of PGC-1α in the function and survival of dopaminergic neurons and the critical need for maintaining physiological levels of PGC-1α activity.