Neuroglial response to sciatic neurectomy. I. Light microscopy and autoradiography

Histological Assessment of Wallerian Degeneration of the Rat Tibial Nerve Following Crush and Transection Injuries

BACKGROUND

 Wallerian degeneration (WD) following peripheral nerve injury (PNI) is an area of growing focus for pharmacological developments. Clinically, WD presents challenges in achieving full functional recovery following PNI, as prolonged denervation of distal tissues for an extended period of time can irreversibly destabilize sensory and motor targets with secondary tissue atrophy. Our objective is to improve upon histological assessments of WD.

METHODS

 Conventional methods utilize a qualitative system simply describing the presence or absence of WD in nerve fibers. We propose a three-category assessment that allows more quantification: A fibers appear normal, B fibers have moderate WD (altered axoplasm), and C fibers have extensive WD (myelin figures). Analysis was by light microscopy (LM) on semithin sections stained with toluidine blue in three rat tibial nerve lesion models (crush, partial transection, and complete transection) at 5 days postop and 5 mm distal to the injury site. The LM criteria were verified at the ultrastructural level. This early outcome measure was compared with the loss of extensor postural thrust and the absence of muscle atrophy.

RESULTS

 The results showed good to excellent internal consistency among counters, demonstrating a significant difference between the crush and transection lesion models. A significant decrease in fiber density in the injured nerves due to inflammation/edema was observed. The growth cones of regenerating axons were evident in the crush lesion group.

CONCLUSION

 The ABC method of histological assessment is a consistent and reliable method that will be useful to quantify the effects of different interventions on the WD process.

Spinal Motor Circuit Synaptic Plasticity after Peripheral Nerve Injury Depends on Microglia Activation and a CCR2 Mechanism

Peripheral nerve injury results in persistent motor deficits, even after the nerve regenerates and muscles are reinnervated. This lack of functional recovery is partly explained by brain and spinal cord circuit alterations triggered by the injury, but the mechanisms are generally unknown. One example of this plasticity is the die-back in the spinal cord ventral horn of the projections of proprioceptive axons mediating the stretch reflex (Ia afferents). Consequently, Ia information about muscle length and dynamics is lost from ventral spinal circuits, degrading motor performance after nerve regeneration. Simultaneously, there is activation of microglia around the central projections of peripherally injured Ia afferents, suggesting a possible causal relationship between neuroinflammation and Ia axon removal. Therefore, we used mice (both sexes) that allow visualization of microglia (CX3CR1-GFP) and infiltrating peripheral myeloid cells (CCR2-RFP) and related changes in these cells to Ia synaptic losses (identified by VGLUT1 content) on retrogradely labeled motoneurons. Microgliosis around axotomized motoneurons starts and peaks within 2 weeks after nerve transection. Thereafter, this region becomes infiltrated by CCR2 cells, and VGLUT1 synapses are lost in parallel. Immunohistochemistry, flow cytometry, and genetic lineage tracing showed that infiltrating CCR2 cells include T cells, dendritic cells, and monocytes, the latter differentiating into tissue macrophages. VGLUT1 synapses were rescued after attenuating the ventral microglial reaction by removal of colony stimulating factor 1 from motoneurons or in CCR2 global KOs. Thus, both activation of ventral microglia and a CCR2-dependent mechanism are necessary for removal of VGLUT1 synapses and alterations in Ia-circuit function following nerve injuries.SIGNIFICANCE STATEMENT Synaptic plasticity and reorganization of essential motor circuits after a peripheral nerve injury can result in permanent motor deficits due to the removal of sensory Ia afferent synapses from the spinal cord ventral horn. Our data link this major circuit change with the neuroinflammatory reaction that occurs inside the spinal cord following injury to peripheral nerves. We describe that both activation of microglia and recruitment into the spinal cord of blood-derived myeloid cells are necessary for motor circuit synaptic plasticity. This study sheds new light into mechanisms that trigger major network plasticity in CNS regions removed from injury sites and that might prevent full recovery of function, even after successful regeneration.

Botulinum Neurotoxin Application to the Severed Femoral Nerve Modulates Spinal Synaptic Responses to Axotomy and Enhances Motor Recovery in Rats

Botulinum neurotoxin A (BoNT) and brain-derived neurotrophic factor (BDNF) are known for their ability to influence synaptic inputs to neurons. Here, we tested if these drugs can modulate the deafferentation of motoneurons following nerve section/suture and, as a consequence, modify the outcome of peripheral nerve regeneration. We applied drug solutions to the proximal stump of the freshly cut femoral nerve of adult rats to achieve drug uptake and transport to the neuronal perikarya. The most marked effect of this application was a significant reduction of the axotomy-induced loss of perisomatic cholinergic terminals by BoNT at one week and two months post injury. The attenuation of the synaptic deficit was associated with enhanced motor recovery of the rats 2–20 weeks after injury. Although BDNF also reduced cholinergic terminal loss at 1 week, it had no effect on this parameter at two months and no effect on functional recovery. These findings strengthen the idea that persistent partial deafferentation of axotomized motoneurons may have a significant negative impact on functional outcome after nerve injury. Intraneural application of drugs may be a promising way to modify deafferentation and, thus, elucidate relationships between synaptic plasticity and restoration of function.

Synaptic defects in the spinal and neuromuscular circuitry in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a major genetic cause of death in childhood characterized by marked muscle weakness. To investigate mechanisms underlying motor impairment in SMA, we examined the spinal and neuromuscular circuitry governing hindlimb ambulatory behavior in SMA model mice (SMNΔ7). In the neuromuscular circuitry, we found that nearly all neuromuscular junctions (NMJs) in hindlimb muscles of SMNΔ7 mice remained fully innervated at the disease end stage and were capable of eliciting muscle contraction, despite a modest reduction in quantal content. In the spinal circuitry, we observed a ∼28% loss of synapses onto spinal motoneurons in the lateral column of lumbar segments 3–5, and a significant reduction in proprioceptive sensory neurons, which may contribute to the 50% reduction in vesicular glutamate transporter 1(VGLUT1)-positive synapses onto SMNΔ7 motoneurons. In addition, there was an increase in the association of activated microglia with SMNΔ7 motoneurons. Together, our results present a novel concept that synaptic defects occur at multiple levels of the spinal and neuromuscular circuitry in SMNΔ7 mice, and that proprioceptive spinal synapses could be a potential target for SMA therapy.

Adult ependymal cells are postmitotic and are derived from radial glial cells during embryogenesis

Ependymal cells on the walls of brain ventricles play essential roles in the transport of CSF and in brain homeostasis. It has been suggested that ependymal cells also function as stem cells. However, the proliferative capacity of mature ependymal cells remains controversial, and the developmental origin of these cells is not known. Using confocal or electron microscopy (EM) of adult mice that received bromodeoxyuridine (BrdU) or [3H]thymidine for several weeks, we found no evidence that ependymal cells proliferate. In contrast, ependymal cells were labeled by BrdU administration during embryonic development. The majority of them are born between embryonic day 14 (E14) and E16. Interestingly, we found that the maturation of ependymal cells and the formation of cilia occur significantly later, during the first postnatal week. We analyzed the early postnatal ventricular zone at the EM and found a subpopulation of radial glia in various stages of transformation into ependymal cells. These cells often had deuterosomes. To directly test whether radial glia give rise to ependymal cells, we used a Cre-lox recombination strategy to genetically tag radial glia in the neonatal brain and follow their progeny. We found that some radial glia in the lateral ventricular wall transform to give rise to mature ependymal cells. This work identifies the time of birth and early stages in the maturation of ependymal cells and demonstrates that these cells are derived from radial glia. Our results indicate that ependymal cells are born in the embryonic and early postnatal brain and that they do not divide after differentiation. The postmitotic nature of ependymal cells strongly suggests that these cells do not function as neural stem cells in the adult.

Quantitative real-time RT-PCR assessment of spinal microglial and astrocytic activation markers in a rat model of neuropathic pain

Activated spinal glial cells have been strongly implicated in the development and maintenance of persistent pain states following a variety of stimuli including traumatic nerve injury. The present study was conducted to characterize the time course of surface markers indicative of microglial and astrocytic activation at the transcriptional level following an L5 nerve transection that results in behavioral hypersensitivity. Male Sprague-Dawley rats were divided into a normal group, a sham surgery group with an L5 spinal nerve exposure and an L5 spinal nerve transected group. Mechanical allodynia (heightened response to a non-noxious stimulus) of the ipsilateral hind paw was assessed throughout the study. Spinal lumbar mRNA levels of glial fibrillary acidic protein (GFAP), integrin alpha M (ITGAM), toll-like receptor 4 (TLR4) and cluster determinant 14 (CD14) were assayed using real-time reverse transcription polymerase chain reaction (RT-PCR) at 4 h, 1, 4, 7, 14 and 28 days post surgery. The spinal lumbar mRNA expression of ITGAM, TLR4, and CD14 was upregulated at 4 h post surgery, CD14 peaked 4 days after spinal nerve transection while ITGAM and TLR4 continued to increase until day 14 and returned to almost normal levels by postoperative day 28. In contrast, spinal GFAP mRNA did not significantly increase until postoperative day 4 and then continued to increase over the duration of the study. Our optimized real-time RT-PCR method was highly sensitive, specific and reproducible at a wide dynamic range. This study demonstrates that peripheral nerve injury induces an early spinal microglial activation that precedes astrocytic activation using mRNA for surface marker expression; the delayed but sustained expression of mRNA coding for GFAP implicates astrocytes in the maintenance phase of persistent pain states. In summary, these data demonstrate a distinct spinal glial response following nerve injury using real-time RT-PCR.

Microglia, but not astrocytes, react to sciatic nerve injury in aging rats

Peripheral nerve axotomy activates microglia and astrocytes within regions of brainstem or spinal cord from which the nerve arises. The present study demonstrates that unilateral sciatic axotomy in rats 2 to 18 months of age results in differing responses with age between these two glial populations. By 4 days postaxotomy, both astrocytes and microglia become activated in 2-month-old rats, whereas only the microglial population shows evidence of activation in rats 8 to 18 months of age.

Ependymal development, proliferation, and functions: a review

A survey of the literature shows that proliferation of ependyma occurs largely during the embryonic and early postnatal periods of development in most species. Differentiation of these cells proceeds along particular regional and temporal gradients as does the expression of various cytoskeletal (vimentin, cytokeratins, glial fibrillary acidic protein) and secretory proteins (S-100). Turnover declines significantly postnatally, and only low levels of residual activity persist into adulthood under normal conditions. Although the reported response of ependyma to injury is somewhat equivocal, only limited regenerative capacity appears to exist and to varying degrees in different regions of the neuraxis. Proliferation has been most often observed in response to spinal cord injury. Indeed, the ependyma plays a significant role in the initiation and maintenance of the regenerative processes in the spinal cord of inframammalian vertebrates. In the human, however, ependyma appears never to regenerate at any age nor re-express cytoskeletal proteins characteristic of immature cells. The functions of ependyma including tanycytes, a specialized form of ependymal cell that persists into adulthood within circumscribed regions of the nervous system, are still largely speculative. Fetal unlike mature ependyma is believed to be secretory and is believed to play a role in neurogenesis, neuronal differentiation/axonal guidance, transport, and support. In the adult brain, mature ependyma is not merely an inert lining but may regulate the transport of ions, small molecules, and water between the cerebrospinal fluid and neuropil and serve an important barrier function that protects neural tissue from potentially harmful substances by mechanisms that are still incompletely understood.

Gliogenesis in postnatal rat optic nerve: LC1 + microglia and S100-beta + astrocytes

Lipocortin 1 (LC1) and S100-beta, two Ca(2+)-binding proteins that serve as specific markers for microglia and astrocytes, respectively, have been used to study postnatal gliogenesis in the rat optic nerve. Computerized image analysis was used to quantify and map the stained and unstained glia in transverse sections (10 microns thick) taken 1-2 mm from the chiasm in optic nerves from rat pups at postnatal day 0 (P0), P7, P14, P21, P28, P38 and adults. The number of astrocytes was remarkably constant (100 per section) at all ages. Because the area of the nerve increases 10-fold from P0 to adult, the population density of astrocytes begins al > 5000 mm-2 and drops to 400 mm-2 in the mature nerve; however, because the nerve length increases two-fold, the number of astrocytes doubles over the same period. In contrast, the number of LC1 + cells per section initially is sparse (4 at P0), increases rapidly up to 36 at P21 and levels off at 49 in adults. The microglia population density is relatively stable throughout development (200-300 mm-2) except during the peak of oligodendroblast apoptosis (P21) when it rises to 450 mm-2. Neonatally, LC1 immunoreactivity predominantly labels spherical-ameboid cells; but by P28 they are replaced by mature ramified microglia. The number of unstained cells (putative oligodendrocytes) per section increases from 11 at P0 to a peak of 308 at P21, and declines slightly to 269 in adults. While generally confirming concepts of astrocyte and oligodendrocyte ontogeny from the literature, the present report adds considerable detail regarding microglia, which often have been ignored. Microglia identified by LC1 immunoreactivity comprise 12% of the glia in adult optic nerve near the chiasm.

Infusion of cytosine-arabinoside into the cerebrospinal fluid of the rat brain inhibits the microglial cell proliferation after hypoglossal nerve injury

The present study describes a method to inhibit selectively the microglial cell proliferation following peripheral nerve injury. Continuous infusion of cytosine-arabinoside (ARA-C) from an osmotic minipump to the fourth ventricle or cisterna magna completely blocks the proliferation of microglial cells that normally occurs following hypoglossal nerve transection. This treatment had no significant effect on other glial cells or on the expected morphological changes in the axotomized hypoglossal motorneurons. The method opens up new possibilities for analyzing the functional role of the axotomy-induced microglial cell reaction.

Regeneration of hypoglossal nerve axons following blockade of the axotomy-induced microglial cell reaction in the rat

The aim of the present study was to examine whether inhibition of the microglial cell reaction around axotomized motoneurons affects the subsequent regeneration process of the injured axons. The microglial cell reaction in the hypoglossal nucleus of the rat was blocked by infusion of cytosine-arabinoside (ARA-C) into the ventricular system. Axon regeneration was evaluated by determining the number and size distribution of myelinated axons at a defined level distal to the crush site, the number of neurons which could be retrogradely labelled from the distal stump as well as the number of motor endplates in the tongue at various times following injury. No significant difference was observed for any of these parameters between ARA-C-treated and untreated animals. Therefore, it is concluded that the microglial cell reaction is not necessary for peripheral nerves to regenerate and restore target contact at a normal rate and to a normal extent.

Transsynaptic degeneration in the superficial dorsal horn after sciatic nerve injury: effects of a chronic constriction injury, transection, and strychnine

The lumbar and cervical spinal dorsal horns of adult rats with a chronic (8 days) constriction injury of the sciatic nerve on one side (and a sham operation on the other) were examined for signs of transsynaptic degeneration. The incidence of neurons with signs of degeneration (pyknosis and hyperchromatosis; 'dark neurons') was significantly increased in the lumbar dorsal horn on both sides. The ipsilateral lumbar increase was significantly greater than the contralateral increase. There was no increase in the incidence of dark neurons in the cervical dorsal horns of the same rats. The distribution of lumbar dark neurons was similar bilaterally. The majority of the dark neurons were found in the sciatic nerve's territory in laminae I-II. A second group of rats received the same surgery but in addition received a series of 7 daily subconvulsive doses of strychnine. Dark neurons were again found bilaterally (with ipsilateral predominance) in the sciatic nerve's territory in lumbar laminae I-II, but the incidence was significantly greater than that found in the group that did not receive strychnine. The same result was obtained in a third group of strychnine-treated rats when the sham operation was omitted. Thus the appearance of contralateral dark neurons is not dependent on unintentional nerve damage created by the sham procedure. An additional group of rats was sacrificed 8 days after receiving a unilateral sciatic nerve transection, a contralateral sham operation, and the 7 daily strychnine injections. There was no increase in the incidence of dark neurons in any of these rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Perineuronal glial responses after axotomy of central and peripheral axons. A comparison

In each of 6 mature rats, unilateral rubrospinal tractotomy and hypoglossal neurectomy were done at one operative sitting. Paired operated animals were killed by formaldehyde and ethanol-acetic acid perfusion 3, 14 and 28 days later. One pair of unoperated control rats was perfused also. All rats were injected i.p. with [3H]thymidine 24 h before death. Immunohistochemical methods were applied to paraffin sections to visualize glial fibrillary acidic protein (GFAP) and transferrin in astrocytes and oligodendroglia, respectively. Microglia were demonstrated by both lectin-binding and histoautoradiographic methods. Neuroglia and nerve cells were counted in hematoxylin-eosin and azure B stains. Cell areas and the RNA concentration of hypoglossal neurons were determined by the Zeiss Image Scan System. Three days after hypoglossal neurectomy, increased astroglial staining (GFAP) and microglial hyperplasia (radiolabeled nuclei) were evident in the ipsilateral hypoglossal nucleus (HN). Microglial hyperplasia waned rapidly after 3 days and microglial numbers decreased. However, astroglial hypertrophy, demonstrable by GFAP staining, persisted 4 weeks postoperatively when astroglial processes were concentrated in a perineuronal position. Oligodendroglia were unaltered. In contrast to the HN, the axotomized red nucleus (RN) contained few radiolabeled microglia while a slight increase in GFAP-positive astroglial processes was seen only in animals killed 28 days postoperatively. Again, oligodendroglia were unchanged. In neither HN nor RN did axotomy cause nerve cell death. Although axotomized rubral neurons atrophy and become depleted of RNA, no statistically significant changes in somal size and RNA content of axotomized hypoglossal neurons occurred. The apparent absence of a neuroglial response of putatively supportive nature in the environs of axotomized rubral neurons may relate to their failure to regenerate. The neuroglial response likely is originated by the axotomized neuron and its absence may be an innate defect in the reaction of intrinsic neurons to axonic severance. Somas of axotomized peripherally projecting nerve cells appear to have the capacity to summon a neuroglial response.

Astrocytic reactions in spinal gray matter following sciatic axotomy

Astrocytic responses following unilateral sciatic nerve axotomy were examined in the spinal gray matter. Using an antiserum to glial fibrillary acidic protein (GFAP), immunoreactive astrocytes were studied in both dorsal and ventral gray matter at intervals from 2 days through 34 days post-axotomy. In all axotomized animals, increased numbers of strongly immunoreactive astrocytes were present in the gray matter ipsilateral to the surgery. Such astrocytes were absent from the contralateral intact side and from gray matter bilaterally in adjacent spinal segments not involved in formation of the sciatic nerve. These GFAP-positive astrocytes occurred not only in association with large motor neurons in the ventral gray matter but also in association with central processes of dorsal root ganglion neurons in the dorsal gray matter. The response was quite rapid, being discernible both dorsally and ventrally as early as the second post-operative day. This increased GFAP immunoreactivity persisted throughout the entire observation period, with the perikarya of large ventral motor neurons appearing to become surrounded or encapsulated by the immunoreactive processes. A further alteration noted at the longest post-operative intervals was the presence in the ventral gray matter of astrocytes appearing to be binucleate. The data obtained indicate that the astrocytic response is not related solely to reactions in motor neurons and, furthermore, the rapidity with which it develops in the dorsal gray matter suggests that its induction is not dependent upon transganglionic degeneration, which others have reported to occur weeks after peripheral nerve injury.

Remote astrocytic response as demonstrated by glial fibrillary acidic protein immunohistochemistry in the visual cortex of dorsal lateral geniculate nucleus lesioned rats

The reaction of astroglia was investigated after unilateral destruction of the dorsal lateral geniculate nucleus in the primary visual cortex of adult albino rats. The destruction of the dorsal lateral geniculate nucleus was performed by stereotaxic injections of ibotenic acid, and the location was verified in Nissl stained sections in each animal. Electron microscopic observations demonstrated the presence of degenerating axon terminals surrounded by hypertrophic astroglial processes mainly in layers III and IV of the ipsilateral primary visual cortex. The ipsilateral (impaired) and contralateral (control) sides of the primary visual cortex showed light microscopically a clearly differing appearance and distribution of glial fibrillary acidic protein (GFAP) immunoreactivity 7 to 11 days after the unilateral injection of ibotenic acid into the dorsal lateral geniculate nucleus. Whereas the control side of the primary visual cortex showed GFAP staining only in the subpial zone of layer I and close to the white matter, all layers of the impaired cortex showed an intense GFAP immunoreactivity. The increase in immunoreactivity was confined to the primary visual cortex. The extent of and increase in immunoreactivity was corroborated by image analysis. These findings were interpreted as a localized hypertrophy of astroglia caused by the anterograde degeneration of geniculocortical terminals. This hypertrophy is accompanied by an increase in GFAP, which may represent the stabilization of the cytoskeleton of newly formed glial processes involved in the rearrangement of the impaired neuropil.

Intracellular transport and neuronal activation of phospholipid and glycoprotein synthesis during axonal regeneration of cranio-spinal nerves

In the present work, the hypothesis that the increased rapid intracellular transport of newly-synthesized material along the axons of a regenerating system is sustained by an alteration of the transport of proteolipid complexes through subcellular compartments of a neuronal cell body was tested by a biochemical methodology. The motoneurons of spinal cord ventral horn, 4 wk after unilateral lesion (crush) of cervico-thoracic nerves of the rabbit at the level of brachial plexus, were chosen as the model system of regeneration. A time-staggered procedure of in vivo and in vitro double labeling with metabolic precursors, such as [3H]-choline, [14C]-choline, [3H]-fucose, and [14C]-fucose, was used. Subcellular fractions (RER, SER, Golgi apparatus, and plasma membranes) of ventral horn tissue, taken from spinal cord hemisections (regenerating and contralateral side), were further isolated. Twenty-eight days after axotomy, we did not observe any change of intracellular transport kinetics (14C/3H ratio) of newly-synthesized choline-phospholipids and glycoproteins in regenerating motoneurons compared to controls. However, associated with regenerating phenomenon in Golgi apparatus, we observed an increase of labeled choline-phospholipid and glycoprotein material that could contribute to the increased fast axonal transport and delivery of membrane proteolipid complexes to plasma membrane and axonal compartments. The increase of glycoprotein labeling was more pronounced in the SER portion (vesicles and elements of smooth membranes). This result is in favor of the hypothesis that membrane-bound proteins are transported from the Golgi to the axon through the perikaryal SER.

Changes of phospholipid-metabolizing and lysosomal enzymes in hypoglossal nucleus and ventral horn motoneurons during regeneration of craniospinal nerves

In order to study the biochemical changes associated with the cell body response to axonal crush injury, two systems, hypoglossal nucleus and spinal cord ventral horn, were used. The time intervals chosen were 7, 14, and 28 days after unilateral crushing of the right hypoglossal nerve and cervicothoracic nerves of the rabbit. Non-crushed, contralateral nerves were used as controls. Three groups of enzyme activities were tested: (a) phospholipase A2, acyl CoA:2-acyl-sn-glycero-3-phosphocholine acyltransferase, and choline phosphotransferase, as indicators of phospholipid degradation and biosynthesis; (b) seven hydrolases, namely, beta-D-glucuronidase, beta-N-acetyl-D-hexosaminidase, arylsulfatase A, galactosylceramidase, GM1-ganglioside beta-galactosidase, and acid RNase, as indicators of lysosomal activity; and (c) free and inhibitor-bound alkaline RNase, as an index of RNA metabolism. Changes could be grouped into three distinct patterns. Compared to contralateral control, choline phosphotransferase showed a slight increase, whereas phospholipase A2 and most lysosomal hydrolases showed a significant increase of activity, especially evident in the ventral spinal cord neurons 14-28 days after crushing. These changes correlate with known increases of membrane and organelle numbers, including lysosomes, in motor and sensory neurons during peripheral regeneration. In contrast, free and acid alkaline RNase activity significantly decreased in the injured sides compared to the controls. This change can probably be correlated with a stabilization of RNAs needed for increased protein synthesis. No changes in total alkaline RNase and acyltransferase activities in either regeneration model were observed.

Imaging of primary and remote ischaemic and excitotoxic brain lesions. An autoradiographic study of peripheral type benzodiazepine binding sites in the rat and cat

Seven days after unilateral middle cerebral artery occlusion in rats, peripheral type benzodiazepine binding sites (PTBBS), using [3H]PK 11195 as a specific radioligand, were greatly increased in the cortical and striatal regions surrounding the focus of infarction with smaller increases in the ventrolateral and posterior thalamic complexes and in the substantia nigra, all ipsilateral to the occlusion. Similarly, PTBBS increases were observed in the caudate nucleus and entorhinal cortex of cats likewise subjected to prior unilateral occlusion of the middle cerebral artery. Intrastriatal administration of N-methyl-D-aspartate (250 nmol) in the rat resulted in a dramatic ipsilateral increase in PTBBS levels in the striatum and in the deeper laminae of the ipsilateral frontoparietal cortex. Intrastriatal kainic acid administration (12 nmol) also elicited PTBBS increases ipsilaterally in rat striatum and cortex; a bilateral elevation of PTBBS levels was observed in the hippocampus. With all these interventions there existed a good spatial correlation between the PTBBS increase and neuronal loss as assessed either histologically or by the autoradiographic detection of the putative neuronal marker [3H]SCH 23390 (a D1 dopamine receptor ligand). Moreover, a glial proliferation of non-neuronal cells (macrophage and glial cells) was observed in brain regions noted to have increased PTBBS levels. PTBBS autoradiography thus constitutes a suitable technique for the localization of damaged areas in several experimental models of brain injury. PTBBS label not only the primary lesions but also functionally related areas and could further our understanding of phenomena such as partial neuronal loss and diaschisis. The study of PTBBS could be envisaged for the detection, localization and quantification of all neuropathological situations which engender a glial reaction or macrophage invasion and is potentially applicable to both experimental and human subjects, in which both autoradiographic and tomographic approaches could be undertaken.

Response of endogenous glial cells to motor neuron degeneration induced by toxic ricin

The injection of toxic lectin from Ricinus communis into the rat facial nerve resulted in suicide transport and rapid degeneration of facial motor neurons. The reaction of glial cells to neuronal death in comparison with nerve crush lesions was studied by using lectin-HRP conjugates derived from Griffonia simplicifolia for the selective staining of microglial cells at both light and electron microscopic levels. In addition, the proliferative activity of microglia was assessed by quantification of 3H-thymidine incorporation. The astrocytic response was evaluated by light microscopic immunocytochemistry for glial fibrillary acidic protein. In the degenerating facial nucleus local microglial cells responded by rapid proliferation and phagocytosis of neuronal debris. After nerve crush, no phagocytes were observed, but microglial proliferation and perineuronal satellitosis were prominent. The astrocytic expression of glial fibrillary acidic protein in response to nerve crush proceeded gradually over a period of several weeks after which it declined, contrasting with accelerated astrocytic hypertrophy and permanent glial scarring after neuronal degeneration. These results show that the expression of glial fibrillary acidic protein by fibrous astrocytes is intensified after lethal neuronal injury compared to sublethal insults. In the absence of any observations indicating participation of hematogenous elements, it is proposed that local microglial cells transform into brain macrophages.

Effect on the rat hypoglossal nucleus of vinblastine and colchicine applied to the intact or transected hypoglossal nerve

The possibility that interruption of axonal transport in otherwise intact axons induces retrograde neuronal and nonneuronal reactions was examined. In addition, the proposal that blockade of axonal transport proximal to nerve injury might inhibit or delay the axon reaction was examined. Cuffs containing various doses of vinblastine were applied to the intact hypoglossal nerve. Colchicine was applied in a similar way to the intact hypoglossal nerve, injected directly into the intact nerve, or administered proximal to the site of hypoglossal nerve transection. The effect on retrograde axonal transport in the nerve was evaluated in the vinblastine experiments by the retrograde horseradish peroxidase (HRP) technique following injection of HRP or wheat germ agglutinin-conjugated HRP into the tongue. A dose of 0.01% caused an almost complete, but transient, blockade of the retrograde transport of the tracer, and induced a clearcut chromatolytic reaction in hypoglossal neurons. The chromatolytic changes were accompanied by a significant increase in the number of glial cells, many of which were identified as microglia. Similar results were obtained with colchicine alone or in combination with nerve transection. Signs of Wallerian degeneration after vinblastine treatment (0.01%) were observed only in a small number of myelinated fibers. The findings are compatible with the view that depletion of retrogradely transported factors from the peripheral innervation territory (including the distal nerve stump) to the perikaryon and/or a premature return of anterogradely transported substances at the site of drug exposure are factors inducing retrograde neuronal and nonneuronal changes.

Changes in microglial cell numbers in the spinal cord dorsal horn following brachial plexus transection in the adult rat

The effect of peripheral nerve transection on the size of the microglial cell population in cytoarchitecturally distinct regions of the spinal cord dorsal horn of rats was evaluated at selected intervals 2 through 35 days after unilateral brachial plexotomy. The identification of cells was verified by electron microscopic examination of a representative random sample of cells included in the counts. Microglial cell numbers were increased in laminae I, II as well as the arbitrarily defined deeper laminae 3.5 days after surgery. Although microglial cell numbers in laminae I were within normal range 35 days after axotomy, those of the more ventrally located laminae remained significantly greater than control values for the duration of the experimental period. These findings demonstrate that: 1) microglial cell proliferation in the dorsal horn is an early event in the central changes that are attendant to peripheral nerve injury 2) the time course of the response varies in cytoarchitecturally different regions.

Effect of axotomy on perineuronal glial cells in the hypoglossal and dorsal motor vagal nuclei of the cat

An approximate twofold increase in microglial cell densities occurred in the hypoglossal and dorsal motor vagal nuclei of the cat after transection of their respective nerves. The densities of astrocytes and oligodendrocytes were unaffected. A significant loss of neurons was demonstrated in both nuclei. These findings indicate that a perineuronal microglial cell reaction occurs in craniomotor nuclei of the cat following nerve transection, but to a considerably lesser extent than in rats and rabbits. We suggest that the different glial response after nerve transection compared with nerve crush in the cat may be related to differences in the degree of axotomy-induced neuronal degeneration.

Ultrastructural evidence of mitotic ependymal cells in 6-aminonicotinamide-treated suckling mice

Mitotic ependymal cells were encountered in 10-day-old mice treated with 6-aminonicotinamide, an antagonist of niacin. These occurred along the medial surface of the lateral ventricle and the ventral portion of the aqueduct. Electron microscopy revealed that both mitotic ependymal cells had eccentrically placed chromosomes without a nuclear membrane and well-formed gap junctions in contact with adjacent ependymal cells. Microtubules from a centriole radiated to the chromosomes. These data show that cell division occurs in morphologically matured ependymal cells in the postnatal brain under pathological conditions. We believe this to be the first ultrastructural demonstration of this phenomenon.

Failure to demonstrate pluripotential hemopoietic stem cells in mouse brains

Hemopoietic stem cells as defined by the capacity to produce spleen colonies in lethally irradiated recipients were reported by P. F. Bartlett [(1982) Proc. Natl. Acad. Sci. USA 79, 2722-2725] to be present in high frequencies in mouse brain. He also reported similar numbers of colony-forming units, spleen (CFU-s), in the brains of Wf/Wf mice, the bone marrow of which lacks detectable spleen colony-forming cells. To verify these observations, single cell suspensions were produced from murine brains by incubation with trypsin and DNase, followed by removal of myelin by Percoll gradient centrifugation. Two to 13 CFU-s were detected per brain. This low number suggested contamination of the brains by either blood or bone marrow leaking from the skull bones during dissection. When the isolated, intact brains were washed carefully in balanced salt solution, the recovered number of CFU-s decreased to 0.1-0.4 per brain. No CFU-s could be detected in the brains of W/Wv mice. It is concluded that the CFU-s observed by Bartlett in preparations of mouse brain did not originate from the brain tissue.

A morphological study of glial cells in the hypoglossal nucleus of the cat during nerve regeneration

The cat hypoglossal nerve and nucleus have been used as a model for the study of the occurrence and time course of modifications in the size and composition of the perineuronal glial cell population as they relate to cytological changes in the nerve cell body and the initiation and progress of axon regeneration. Animals were killed at 2, 5, 10, 20, 35, 65, and 115 days after crush injury to the hypoglossal nerve. At 5 days after surgery, growth cones and regenerating unmyelinated axons were present at the lesion site, but no conspicuous changes were apparent in the nerve cell bodies. At 10 days after surgery, the granular endoplasmic reticulum was disaggregated and depleted. The elongation phase appeared to be completed at 20 days, as judged by the bilateral retrograde labeling of the hypoglossal nuclei with horseradish peroxidase. By 35 days, the cytoarchitecture of the nerve cell bodies and maturation of axons, as determined by a comparison of the relative frequency distribution of cross sectional areas proximal and distal to the lesion, were completely restored. Comparative quantitative light microscopic examination of the hypoglossal nuclei of intact and experimental animals failed to reveal any statistically significant differences in the total number of glial cells, number of glial cells/unit area of neuropil, or relative proportions of glial cell types at any of the postoperative time intervals. Moreover, electron microscopic quantitation of the microglial cell population did not reveal any significant alterations in the number, density, location, or morphology of these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nerve section on perineuronal glial cells in the CNS of rat and cat

The effect of axotomy on the numbers and density of perineuronal cell populations was evaluated in rats, cats and kittens. Cats were sacrificed at different postoperative time intervals two through 90 days after unilateral plexotomy. Kittens (6-10 weeks of age) were subjected to the same surgical procedure and sacrificed one through 28 days after surgery. Rats were sacrificed 10 and 15 days after unilateral section of the brachial plexus or at 7 or 10 days after section of the left hypoglossal nerve. A marked increase in the total number and density of perineuronal cells occurred in the rat ventral horn 10 and 15 days after axotomy. A similar response was noted in the rat hypoglossal nucleus 7 and 10 days after neurotomy. In contrast, no significant change in these parameters was observed in the ventral horns of cats and kittens at any of the postoperative time intervals. Although quantitatively demonstrable increases in the perineuronal cell populations occur in the ventral horns and hypoglossal nuclei of rats, similar modifications do not occur in the cat following axon injury. These findings suggest that evolutionary modifications may have occurred in how perineuronal glia respond to peripheral axon injury.

A fine structural study of the cells that proliferate in the partially denervated dentate gyrus of the rat

Tritiated thymidine autoradiography has established that after interrupting the commissural afferents to the dentate gyrus a number of non-neuronal cells proliferate in the molecular layer. In the present study the fine structure of the proliferating cells was analyzed by reembedding the 2-microns thick plastic sections of the dentate gyrus which had been previously coated with a nuclear emulsion and processed for light microscopic autoradiography. The location of the labeled cells was plotted with a camera lucida and a few ultrathin sections were taken from the re-embedded sections. In these the labeled cells were re-identified and photographed in an electron microscope. Most of the identified proliferating cells exhibited the following morphological features: The nuclei were irregularly oval, sometimes with deep indentations and contained dense clumps of chromatin; their diameters ranged between 4.5 and 6.5 microns. The cytoplasm was generally disposed to one side of the nucleus and often extended into a few broad processes. The Golgi apparatus was well developed. Many rosettes of free ribosomes were scattered throughout the cytoplasm, and the rough endoplasmic reticulum usually consisted of a few short cisternae. Small multilamellated bodies were common, but dense inclusion bodies were infrequent. The observations reported in this paper suggest: 1. that the nonneuronal cells which proliferate in a neuropil undergoing a mild denervation are morphologically closely related to microglia; 2. that in young adult animals these cells do not seem to have been previously involved in intense phagocytic activity; and 3. that the proliferating cells are present in the neuropil at the time of the denervation.

Glial cell responses in the adult rabbit dorsal motor vagal nucleus during axon reaction

Following transection of the vagal nerve on the neck, a dramatic loss of neurons occurs in the dorsal motor vagal nucleus (DMV) of the adult rabbit. The present study describes the light and electron microscopical characteristics of the accompanying glial cell response. The number of microglial cells was considerably increased a few days after nerve injury and remained at a higher than normal level throughout almost the whole period examined. Astrocytes increased to some extent at a later post-operative stage. Ultrastructurally, microglial cells hypertrophied, surrounded degenerating neurons, and contained a variety of inclusion bodies, some of which appeared to be derived from degenerating neuroplasm. Astrocytes also hypertrophied, displayed numerous filaments and glycogen granules in their cytoplasm as well as a substantial number of heterogenous dense bodies. The findings indicate that the principal features of the glial cell response in the rabbit DMV is similar to previously described glial cell changes in other systems. However, the intensity in the response of astroglial cells in the DMV appears to be greater than usually observed, possibly reflecting the degenerative nature of the nerve cell body response.

Non-neuronal cells in the spinal cord of nude and heterozygous mice. I. Ventral horn neuroglia

A quantitative light microscopic analysis of the ventral grey matter in the lumbar spinal cord of homozygous nude (nu/nu) and heterozygous (nu/+) mice was performed to determine the possible contribution of lymphocytes to normal C.N.S. tissue. If lymphocytes were present in the neuropil, they could be mistaken for neuroglial cells. Athymic nude mice offer a good model, since they lack T-lymphocytes and symptoms of neurological involvement. Mean cell counts from 1 micrometer sections were tested by analysis of variance. There were not strain differences for the area and number of neurons. The total neuroglial cell count was also similar, but the number of oligodendrocytes decreased 28%, astrocytes increased 51% and microglia were unchanged in the nude compared with the heterozygous mouse. There were no qualitative differences at the ultrastructural level among the neuroglia of either strain. Either the genetic defect retards and alters neuroglial cell development, or some of the small, round dark nuclei belong to lymphocytes, which have earlier migrated into the C.N.S. parenchyma. Lymphocytes could then participate in a cell-mediated immune response with brain macrophages, which are thought to be primarily derived from mononuclear leukocytes.

Non-neuronal cells in the spinal cord of nude and heterozygous mice. II. Agranular leukocytes in the subarachnoid and perivascular space

The lumbar spinal cord of the athymic nude mouse and its heterozygous control were examined at the light and electron microscopic levels for differences in the cellular constituents of the subarachnoid and perivascular spaces. Both macrophages and lymphocytes were found in these spaces associated with the adventitial and leptomeningeal cells. Occasionally, fixed-cells were associated with extensions of the basal lamina and subjacent astrocytic processes. The appearance of the basal lamina of the external glia limitans and perivascular space was enhanced by tannic acid treatment. There were twice as many macrophages as lymphocytes, but no significant strain differences. Comparison was made between cells in the lumina of incompletely perfused vessels and neuroglial cells in the spinal cord. The cellular morphology is distinct in each of these compartments and no cell migrations were observed between the blood, cerebrospinal fluid and C.N.S. interstitial space. The normal presence of both macrophages and lymphocytes in the subarachnoid and perivascular space suggests that these cells could penetrate the basal lamina and gain access to the C.N.S.

Variations in the perineuronal glial changes after different types of nerve lesion: light and electron microscopic investigations on the facial nucleus of the rat

The perineuronal glial changes were studied by light and electron microscopy after avulsion or a crush lesion of the facial nerve in rats. The changes consisted of proliferation of microglia, ensheathment of neurons by thin astrocytic processes, and separation of the synaptic boutons from the neuronal surface. Quantitative estimates of the glial proliferation were made with the light microscope. In spite of marked differences in the acute nerve cell reaction, 4 days after the two types of lesion the glial and synaptic changes did not differ significantly. Ultimately, all changes were reversible after crush lesions, while neuronophagia occurred after nerve avulsion. It is concluded that the acute synaptic and glial reactions were not influenced by the type of nerve lesion or the severity of the nerve cell reaction, but the latter stages differed depending upon whether the neurons recovered or disintegrated.

Morphological studies on neuroglia. II. Response of glial cells to kainic acid-induced lesions

The cellular response of non-neuronal elements of the pyramidal cell layer of the rat hippocampus, especially the area CA3, was observed electron microscopically following destruction of this formation by means of intraventricular administration of kainic acid (KA). The neuroglial cell types responding to the KA-induced lesion included astrocytes and the "microglia-like reactive cells". In addition, numerous brain macrophages appeared in the damaged area CA3. Oligodendrocytes and pericytes revealed no morphological changes. Swollen astrocytes were seen in the KA-induced lesion during the early stage. Glial filaments gradually developed in the soma and cell processes of these cells. Brain macrophages were seen in the KA-induced lesion during the early stage; they gradually decreased in number with time. Numerous small cells displaying a dark nucleus appeared in the damaged area CA3 during the first two days after the KA-administration, and gradually increased in number. During the later stage this cell type could hardly be distinguished from the intrinsic microglial cells. It is open to discussion whether this cell type originates from the intrinsic microglial cells or from the hematogenic monocytes; therefore it is designated as "microglia-like reactive cell" in the present study.

Non-specific esterase activity in reactive cells in injured nervous tissue labeled with 3H-thymidine or 125iododeoxyuridine injected before injury

Tritiated thymidine (3H-TdR) injected before a stab wound of the spinal cord or transection of the hypoglossal nerve has resulted in many labeled reactive cells in the CNS after injury, most of which have the ultrastructural features of microglia. To test for the possible origin of these labeled cells from monocytes, we examined them for the presence of sodium fluoride- (NaF) sensitive non-specific esterase (NSE), an enzyme characteristic of monocytes. Some of the labeled cells in stab wounds had NaF-sensitive NSE, but no such cells were found in the nucleus of the injured hypoglossal nerve. To test for the possibility that the NSE-negative labeled cells had been labeled by reutilization of 3H-TdR, we used 125I-5-iodo-2'-deoxyuridine (125I-UdR), a thymidine analogue with a much lower rate of reutilization, to label blood mononuclear cells prior to either a spinal cord stab wound or hypoglossal axotomy. The number of labeled cells was decreased in the spinal cord wound, but more than half were NSE-negative. No labeled blood mononuclear cells were found in the hypoglossal nucleus, although there was no decrease in the hyperplasia of unlabeled non-neuronal cells. When 125I-UdR was injected on the fourth day after hypoglossal axotomy, or when both 3H-TdR and 125I-UdR were injected simultaneously before hypoglossal axotomy, many labeled cells were found in the hypoglossal nucleus, indicating that 125I-UdR can be used by the reactive cells and that it did not inhibit their proliferation. Therefore, the microglial cells that proliferate in response to peripheral nerve injury are not recently derived from any type of circulating large blood mononuclear cell. The most likely explanation for the presence of the 3H-TdR-labeled cells in the nucleus of the injured hypoglossal nerve is that they were proliferating intrinsic cells labeled by reutilization of 3H-TdR.

Initiation and time course of mitosis of non-neuronal cells after spinal motoneuron axotomy

The mitotic response of non-neuronal cells following motor axon transection was measured after in vitro incorporation of [3H]thymidine in frog spinal cord. This predominantly ipsilateral response occurs more rapidly and is of greater magnitude when motor axons are unilaterally transected at the ventral root than after sciatic nerve transection. No increase in incorporation occurred when regenerating fibers were transected a second time before reinnervation, but an increase was observed when the second operation was performed after the formation of functional neuromuscular connections had taken place. Autoradiographic studies after dorsal or ventral root transection showed that the distribution of labeled cells approximated the anatomical extent of the injured cellular elements within the spinal cord. These data are discussed in relation to the characteristics of the dividing cells and the nature of the events eliciting mitosis.

Endocytic activity of subependymal microglial cells in the toad brain: a cytochemical study of peroxidase uptake

A population of microglial cells that rapidly incorporate extracellular material introduced into the ventricular system has been identified just beneath the ependyma of all four cerebral ventricles in the toad (Bufo marinus). In untreated tissue these cells appear to be scattered, possess few processes and have an elongate shape with their long axes lying parallel to the ventricular surface. Their most distinctive ultrastructural features are nuclei containing clumps of chromatin, cytoplasmic dense bodies and single strands of granular endoplasmic reticulum. When horseradish peroxidase (HRP) is perfused through the ventricular system and the tissue processed using the DAB cytochemical method, the cells change shape and incorporate HRP into cytoplasmic structures. Even after very short perfusion periods (2-5 minutes) cells become rounded, the surface is ruffled and pseudopodia develop that contain characteristic flocculent material. Reaction product for HRP is contained in plain and coated vesicles, tubules, vacuoles and long structures composed of two closely apposed membranes. At these early times, relatively few multivesicular bodies and dense bodies contain reaction product, but when the cells are viewed at longer time periods after the ventricular perfusion of HRP an increasing proportion of the multivesicular bodies and dense bodies contain reaction product. By 320 minutes reaction product is found almost exclusively in these two organelles. In addition, many pseudopodia containing dense bodies with peroxidase activity are found in the neurophile; some, but not all, can be traced from the subependymal microglial cells. The cell bodies have resumed their flattened shape. When compared to the subependymal microglial cells, other brain cells--oligodendrocytes, astrocytes, ependymal cells and neurons--contain relatively little reaction product at short time intervals; only by 320 minutes are moderate amounts of HRP present. Because of the position of the microglial cells and their ingestive capacity, it is suggested that they function to protect the brain from foreign substances entering from the CSF.

Intraspinal non-neuronal cellular responses to peripheral nerve injury

The accumulation of non-vascular, non-neuronal cells (designated herein as reactive cells) in association with perikarya of axotomized motor neurons has been described by many investigators. Recently Gilmore ('75) found that sciatic axotomy in immature rats resulted in the occurrence of reactive cells not only in the spinal ventral gray matter but also in the dorsal gray matter. To determine if the presence of these cells in the dorsal gray matter, a finding not reported by others, was related to the immaturity of the animal, sciatic axotomy was performed in rats ranging in age from 17 days to 16 months in the present study. Light microscopic evaluation of the spinal cords three or seven days post-operatively showed that the reactive cells occurred consistently in both dorsal and ventral gray matter irrespective of age. Transection of tibial nerve or the nerve to the medial head of the gastrocnemius muscle elicited a cellular response in both dorsal and ventral gray matter, although transection of the latter nerve resulted in a much less obvious response. Crushing of the sciatic nerve was followed by a response of reactive cells not qualitatively different from that noted following transection. Transection of the sural nerve, primarily a sensory nerve, resulted in the presence of reactive cells in dorsal gray matter but not in the environs of motor neurons in the ventral gray matter. These findings suggest that the reactive cells in the dorsal gray matter of the spinal cord are associated with altered central processes of dorsal root ganglion cells.

Proliferative and migratory activity of glial cells in the partially deafferented hippocampus

The proliferative response of the glial cell population of the adult rat hippocampus deafferented by unilateral lesion of the entorhinal cortex was studied using 3H-thymidine autoradiography. Two experimental paradigms were used, involving: (1) intraventricular 3H-thymidine injection at a number of post-lesion intervals with sacrifice six hours later and (2) intraventricular injection at 30 hours post-lesion with sacrifice at 6, 96, or 192 hours later. The first increase in the number of labeled glial cells was obtained at 20 hours post-lesion and was confined to areas of degenerating axons. By 30 hours a large and uniformly dense proliferative response was observed throughout the ipsilateral, and medial aspects of the contralateral, hippocampus encompassing both deafferented and intact regions. Cell division continued through 50 and 65 hours post-lesion particularly in directly deafferented regions, but diminished to control levels by 80 hours. Although oligodendroglia and astrocyte-like cells were sometimes found to have incorporated the label the most common proliferative element within the hippocampus corresponded to previous light microscopic descriptions of "microglial" cells. The experiments using thymidine injection given at the peak proliferative period followed by survival periods of varying lengths indicated that a progressive redistribution of labeled nuclei occurred resulting in an accumulation of labeled cells in the zones of deafferentation. Multiple division of cells within these areas as well as the migration of nuclei from non-deafferented regions was found to contribute to this effect. The possible involvement of glial proliferation with other morphological effects of deafferentation, including the sprouting response of intact afferents, is discussed.