Bone marrow adipose tissue (BMAT) is increased in both obesity and anorexia. This is unique relative to white adipose tissue (WAT), which is generally more attuned to metabolic demand. It suggests ...that there may be regulatory pathways that are common to both BMAT and WAT and also those that are specific to BMAT alone. The central nervous system (CNS) is a key mediator of adipose tissue function through sympathetic adrenergic neurons. Thus, we hypothesized that central autonomic pathways may be involved in BMAT regulation. To test this, we first quantified the innervation of BMAT by tyrosine hydroxylase (TH) positive nerves within the metaphysis and diaphysis of the tibia of B6 and C3H mice. We found that many of the TH+ axons were concentrated around central blood vessels in the bone marrow. However, there were also areas of free nerve endings which terminated in regions of BMAT adipocytes. Overall, the proportion of nerve-associated BMAT adipocytes increased from proximal to distal along the length of the tibia (from ~3-5 to ~14-24%), regardless of mouse strain. To identify the central pathways involved in BMAT innervation and compare to peripheral WAT, we then performed retrograde viral tract tracing with an attenuated pseudorabies virus (PRV) to infect efferent nerves from the tibial metaphysis (inclusive of BMAT) and inguinal WAT (iWAT) of C3H mice. PRV positive neurons were identified consistently from both injection sites in the intermediolateral horn of the spinal cord, reticular formation, rostroventral medulla, solitary tract, periaqueductal gray, locus coeruleus, subcoeruleus, Barrington's nucleus, and hypothalamus. We also observed dual-PRV infected neurons within the majority of these regions. Similar tracings were observed in pons, midbrain, and hypothalamic regions from B6 femur and tibia, demonstrating that these results persist across mouse strains and between skeletal sites. Altogether, this is the first quantitative report of BMAT autonomic innervation and reveals common central neuroanatomic pathways, including putative "command" neurons, involved in coordinating multiple aspects of sympathetic output and facilitation of parallel processing between bone marrow/BMAT and peripheral adipose tissue.
Cell death was examined by studying the spinal cords of rats subjected to traumatic insults of mild to moderate severity. Within minutes after mild weight drop impact (a 10 gm weight falling 6.25 ...mm), neurons in the immediate impact area showed a loss of cytoplasmic Nissl substances. Over the next 7 d, this lesion area expanded and cavitated. Terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL)-positive neurons were noted primarily restricted to the gross lesion area 4-24 hr after injury, with a maximum presence at 8 hr after injury. TUNEL-positive glia were present at all stages studied between 4 hr and 14 d, with a maximum presence within the lesion area 24 hr after injury. However 7 d after injury, a second wave of TUNEL-positive glial cells was noted in the white matter peripheral to the lesion and extending at least several millimeters away from the lesion center. The suggestion of apoptosis was supported by electron microscopy, as well as by nuclear staining with Hoechst 33342 dye, and by examination of DNA prepared from the lesion site. Furthermore, repeated intraperitoneal injections of cycloheximide, beginning immediately after a 12.5 mm weight drop insult, produced a substantial reduction in histological evidence of cord damage and in motor dysfunction assessed 4 weeks later. Present data support the hypothesis that apoptosis dependent on active protein synthesis contributes to the neuronal and glial cell death, as well as to the neurological dysfunction, induced by mild-to-moderate severity traumatic insults to the rat spinal cord.
Mechanisms of oligodendrocyte death after spinal cord injury (SCI) were evaluated by T9 cord level hemisection in wild-type mice (C57BL/6J and Bax+/+ mice), Wlds mice in which severed axons remain ...viable for 2 weeks, and mice deficient in the proapoptotic protein Bax (Bax-/-). In the lateral white-matter tracts, substantial oligodendrocyte death was evident in the ipsilateral white matter 3-7 mm rostral and caudal to the hemisection site 8 d after injury. Ultrastructural analysis and expression of anti-activated caspase-3 characterized the ongoing oligodendrocyte death at 8 d as primarily apoptotic. Oligodendrocytes were selectively preserved in Wlds mice compared with C57BL/6J mice at 8 d after injury, when severed axons remained viable as verified by antereograde labeling of the lateral vestibular spinal tract. However, 30 d after injury when the severed axons in Wlds animals were already degenerated, the oligodendrocytes preserved at 8 d were lost, and numbers were then equivalent to control C57BL/6J mice. In contrast, oligodendrocyte death was prevented at both time points in Bax-/- mice. When cultured oligodendrocytes were exposed to staurosporine or cyclosporin A, drugs known to stimulate apoptosis in oligodendrocytes, those from Bax-/- mice but not from Bax+/+ or Bax+/- mice were resistant to the apoptotic death. In contrast, the three groups were equally vulnerable to excitotoxic necrosis death induced by kainate. On the basis of these data, we hypothesize that the Wallerian degeneration of white matter axons that follows SCI removes axonal support and induces apoptotic death in oligodendrocytes by triggering Bax expression.
The mouse trigeminal (V) system undergoes significant postnatal structural and functional developmental changes. Histological modules (barrelettes, barreloids and barrels) in the brainstem, thalamus ...and cortex related to actively moved (whisking) tactile hairs (vibrissae) on the face allow detailed studies of development. High‐resolution 3H2‐deoxyglucose (2DG) emulsion autoradiography with cytochrome oxidase histochemistry was used to analyze neuronal activity changes related to specific whisker modules in the developing and mature mouse V system provoked by passive (experimenter‐induced) and active (animal‐induced) displacements of a single whisker (D4). We tested the hypothesis that neuronal activity patterns change in relation to the onset of active touch (whisking) on postnatal day (P)14. Quantitative image analyses revealed: (i) on P7, when whisker‐like patterns of modules are clear, heightened 2DG activity in all appropriate modules in the brainstem, thalamus and cortex; (ii) on P14, a transitory activity pattern coincident with the emergence of whisking behavior that presages (iii) strong labeling of the spinal V subnucleus interpolaris and barrel cortex produced by single‐whisker‐mediated active touch in adults and (iv) at all above‐listed ages and structures, significant suppression of baseline activity in some modules surrounding those representing the stimulated whisker. Differences in activity patterns before and after the onset of whisking behavior may be caused by neuronal activity induced by whisking, and by strengthening of modulatory projections that alter the activity of subcortical inputs produced by whisking behavior during active touch.
Little is known of transcriptional mechanisms underlying the development of the trigeminal (V) principal sensory nucleus (PrV), the brainstem nucleus responsible for the development of the ...whisker-to-barrel cortex pathway. Lmx1b, a LIM homeodomain transcription factor, is expressed in embryonic PrV. In Lmx1b knockout (
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−) mice, V primary afferent projections to PrV are normal, albeit reduced in number, whereas the PrV-thalamic lemniscal pathway is sparse and develops late. Excess cell death occurs in the embryonic Lmx1b
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− PrV, but not in Lmx1b/Bax double null mutants. Expression of Drg11, a downstream transcription factor essential for PrV development and pattern formation, is abolished in PrV, but not in the V ganglion. Consequently, whisker patterns fail to develop in PrV by birth. Rescued PrV cells in Lmx1b/Bax double
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−s failed to rescue whisker-related PrV pattern formation. Thus, Lmx1b and Drg11 may act in the same genetic signaling pathway that is essential for PrV pattern formation.
Abstract
Trigeminal (V) nucleus principalis (PrV) is the requisite brainstem nucleus in the whisker-to-barrel cortex model system that is widely used to reveal mechanisms of map formation and ...information processing. Yet, little is known of the actual PrV circuitry. In the ventral "barrelette" portion of the adult mouse PrV, relationships between V primary afferent terminals, thalamic-projecting PrV neurons, and gamma-aminobutyric acid (GABA)-ergic terminals were analyzed in the electron microscope. Primary afferents, thalamic-projecting cells, and GABAergic terminals were labeled, respectively, by Neurobiotin injections in the V ganglion, horseradish peroxidase injections in the thalamus, and postembedding immunogold histochemistry. Primary afferent terminals (Neurobiotin- and glutamate-immunoreactive) display asymmetric and multiple synapses predominantly upon the distal dendrites and spines of PrV cells that project to the thalamus. Primary afferents also synapse upon GABAergic terminals. GABAergic terminals display symmetric synapses onto primary afferent terminals, the somata and dendrites (distal, mostly) of thalamic-projecting neurons, and GABAergic dendrites. Thus, primary afferent inputs through the PrV are subject to pre- and postsynaptic GABAergic influences. As such, circuitry exists in PrV "barrelettes" for primary afferents to directly activate thalamic-projecting and inhibitory local circuit cells. The latter are synaptically associated with themselves, the primary afferents, and with the thalamic-projecting neurons. Thus, whisker-related primary afferent inputs through PrV projection neurons are pre- and postsynaptically modulated by local circuits.
Little is known about the molecular mechanisms underlying the formation of the principal sensory nucleus (PrV) of the trigeminal nerve, a major relay station for somatotopic pattern formation in the ...trigeminal system. Here, we show that mice lacking Drg11, a homeodomain transcription factor, exhibit defects within the PrV, which include an aberrant distribution of Drg11-/- cells, altered expression of a molecular marker, unusual projections of primary afferents from trigeminal ganglion cells, and, subsequently, increased cell death. In addition, surviving PrV cells exhibit delayed and more spatially restricted ascending projections to the ventral posterior medial nucleus of the thalamus (VPm). These early embryonic abnormalities in the PrV lead to the failure to develop whisker-related patterns in the PrV, VPm, and somatosensory cortex. By contrast, somatotopic patterns exist in the spinal trigeminal subnuclei interpolaris (SpVi) and subnuclei caudalis (SpVc) and the dorsal column nucleus-based lemniscal and cortical pathway. Thus, the deficits in the trigeminal system of Drg11-/- mice are specific to the PrV. Our results demonstrate that Drg11 is essential for proper cellular differentiation and, subsequently, for the formation of the whisker-related lemniscal and cortical structures.
Abstract
Single whiskers are topographically represented in the trigeminal (V) nucleus principalis (PrV) by a set of cylindrical aggregates of primary afferent terminals and somata (barrelettes). ...This isomorphic pattern is transmitted to the thalamus and barrel cortex. However, it is not known if terminals in PrV from neighboring whiskers interdigitate so as to violate rules of spatial parcellation predicted by barrelette borders; nor is it known the extent to which higher order inputs are topographic. The existence of inter-whisker arbor overlap or diffuse higher order inputs would demand additional theoretical principles to account for single whisker dominance in PrV cell responses. In adult rats, first, primary afferent pairs responding to the same or neighboring whiskers and injected with Neurobiotin or horseradish peroxidase were rendered brown or black to color-code their terminal boutons. When collaterals from both fibers appeared in the same topographic plane through PrV, the percentage of the summed area of the two arbor envelopes that overlapped was computed. For same-whisker pairs, overlap was 5 ± 6% (mean ± SD). For within-row neighbors, overlap was 2 ± 5%. For between-row neighbors, overlap was 1 ± 4%. Second, the areas of whisker primary afferent arbors and their corresponding barrelettes in the PrV were compared. In the transverse plane, arbor envelopes significantly exceeded the areas of cytochrome oxidase-stained barrelettes; arbors often extended into neighboring barrelettes. Third, bulk tracing of the projections from the spinal V subnucleus interpolaris (SpVi) to the PrV revealed strict topography such that they connect same-whisker barrelettes in the SpVi and PrV. Thus, whisker primary afferents do not exclusively project to their corresponding PrV barrelette, whereas higher order SpVi inputs to the PrV are precisely topographic.
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