Fate of the soma and dendrites of cord-projection central neurons after proximal and distal spinal axotomy: an intracellular dye injection study

Neurons survive simultaneous injury to axons and dendrites and regrow both types of processes in vivo

Neurons extend dendrites and axons to receive and send signals. If either type of process is removed, the cell cannot function. Rather than undergoing cell death, some neurons can regrow axons and dendrites. Axon and dendrite regeneration have been examined separately and require sensing the injury and reinitiating the correct growth program. Whether neurons in vivo can sense and respond to simultaneous axon and dendrite injury with polarized regeneration has not been explored. To investigate the outcome of simultaneous axon and dendrite damage, we used a Drosophila model system in which neuronal polarity, axon regeneration, and dendrite regeneration have been characterized. After removal of the axon and all but one dendrite, the remaining dendrite was converted to a process that had a long unbranched region that extended over long distances and a region where shorter branched processes were added. These observations suggested axons and dendrites could regrow at the same time. To further test the capacity of neurons to implement polarized regeneration after axon and dendrite damage, we removed all neurites from mature neurons. In this case a long unbranched neurite and short branched neurites were regrown from the stripped cell body. Moreover, the long neurite had axonal plus-end-out microtubule polarity and the shorter neurites had mixed polarity consistent with dendrite identity. The long process also accumulated endoplasmic reticulum at its tip like regenerating axons. We conclude that neurons in vivo can respond to simultaneous axon and dendrite injury by initiating growth of a new axon and new dendrites.

Is more always better? How different 'doses' of exercise after incomplete spinal cord injury affects the membrane properties of deep dorsal horn interneurons

Interneurons in the deep dorsal horn (DDH) of the spinal cord process somatosensory input, and form an important link between upper and lower motoneurons to subsequently shape motor output. Exercise training after SCI is known to improve functional motor recovery, but little is known about the mechanisms within spinal cord neurons that underlie these improvements. Here we investigate how the properties of DDH interneurons are affected by spinal cord injury (SCI) alone, and SCI in combination with different 'doses' of treadmill exercise training (3, 6, and 9wks). In an adult mouse hemisection model of SCI we used whole-cell patch-clamp electrophysiology to record intrinsic, AP firing and gain modulation properties from DDH interneurons in a horizontal spinal cord slice preparation. We find that neurons within two segments of the injury, both ipsi- and contralateral to the hemisection, are similarly affected by SCI and SCI plus exercise. The passive intrinsic membrane properties input resistance (R_in) and rheobase are sensitive to the effects of recovery time and exercise training after SCI thus altering DDH interneuron excitability. Conversely, select active membrane properties are largely unaffected by either SCI or exercise training. SCI itself causes a mismatch in the expression of voltage-gated subthreshold currents and AP discharge firing type. Over time after SCI, and especially with exercise training (9wks), this mismatched expression is exacerbated. Lastly, amplification properties (i.e. gain of frequency-current relationship) of DDH interneurons are altered by SCI alone and recover spontaneously with no clear effect of exercise training. These results suggest a larger 'dose' of exercise training (9wks) has a strong and selective effect on specific membrane properties, and on the output of interneurons in the vicinity of a SCI. These electrophysiological data provide new insights into the plasticity of DDH interneurons and the mechanisms by which exercise therapy after SCI can improve recovery.

Remote neurodegeneration: multiple actors for one play

Remote neurodegeneration significantly influences the clinical outcome in many central nervous system (CNS) pathologies, such as stroke, multiple sclerosis, and traumatic brain and spinal cord injuries. Because these processes develop days or months after injury, they are accompanied by a therapeutic window of opportunity. The complexity and clinical significance of remote damage is prompting many groups to examine the factors of remote degeneration. This research is providing insights into key unanswered questions, opening new avenues for innovative neuroprotective therapies. In this review, we evaluate data from various remote degeneration models to describe the complexity of the systems that are involved and the importance of their interactions in reducing damage and promoting recovery after brain lesions. Specifically, we recapitulate the current data on remote neuronal degeneration, focusing on molecular and cellular events, as studied in stroke and brain and spinal cord injury models. Remote damage is a multifactorial phenomenon in which many components become active in specific time frames. Days, weeks, or months after injury onset, the interplay between key effectors differentially affects neuronal survival and functional outcomes. In particular, we discuss apoptosis, inflammation, oxidative damage, and autophagy-all of which mediate remote degeneration at specific times. We also review current findings on the pharmacological manipulation of remote degeneration mechanisms in reducing damage and sustaining outcomes. These novel treatments differ from those that have been proposed to limit primary lesion site damage, representing new perspectives on neuroprotection.

Testosterone modulation of dendritic spines of somatosensory cortical pyramidal neurons

Brain structures and functions are increasingly recognized to be directly affected by gonadal hormones, which classically determine reproductive functions and sexual phenotypes. In this regard, we found recently that ovariectomy trimmed the dendritic spines of female rat primary somatosensory cortical neurons and estradiol supplement reversed it. Here, we investigated whether in the male androgen also has a cortical modulatory effect. The dendritic arbors and spines of rat somatosensory cortical pyramidal neurons were studied following intracellular dye injection and three-dimensional reconstruction. Dendritic spines, but not length, of the layers III and V pyramidal neurons were found reduced at 2 weeks and rebounded slightly at 4 weeks and further at 8 and 24 weeks following castration, which, however, remained significantly fewer than those of the intact animals. Two weeks of osmotic pump-delivered testosterone treatment to animals castrated for 4 weeks replenished serum testosterone and reversed the densities of dendritic spines on these neurons to control animal levels. Androgen receptor appears to mediate this effect as its antagonist flutamide reduced the dendritic spines of normal adult rats while causing a mild feedback surge of serum testosterone. On the other hand, blocking the conversion of testosterone to estrogen with the aromatase inhibitor anastrozole failed to alter the dendritic spine densities in male adult rats. In conclusion, these results support our hypothesis that testosterone acts directly on the androgen receptor in males to modulate the dendritic spines of somatosensory cortical output neurons.

Active endocytosis and microtubule remodeling restore compressed pyramidal neuron morphology in rat cerebral cortex

Previous studies have shown that compression alone reduced the thickness of rat cerebral cortex and apical dendritic lengths of pyramidal neurons without apparent cell death. Besides, decompression restored dendritic lengths at different degrees depending on duration of compression. To understand the mechanisms regulating dendritic shortening and lengthening upon compression and decompression, we applied transmission electron microscopy to examine microtubule and membrane structure of pyramidal neurons in rat sensorimotor cortex subjected to compression and decompression. Microtubule densities within apical dendritic trunks decreased significantly and arranged irregularly following compression for a period from 30 min to 24 h. In addition, apical dendritic trunks showed twisted contour. Two reasons are accounted for the decrease of microtubule density within this period. First, microtubule depolymerized and resulted in lower number of microtubules. Second, the twisted membrane widened the diameters of apical dendritic trunks, which also caused a decrease in microtubule density. Interestingly, these compression-induced changes were quickly reversed to control level following decompression, suggesting that these changes were accomplished passively. Furthermore, microtubule densities were restored to control level and the number of endocytotic vesicles significantly increased along the apical dendritic membrane in neurons subjected to 36 h or longer period of compression. However, decompression did not make significant changes on dendrites compressed for 36 h, for they had already shown straight appearance before decompression. These results suggest that active membrane endocytosis and microtubule remodeling occur in this adaptive stage to make the apical dendritic trunks regain their smooth contour and regular microtubule arrangement, similar to that of the normal control neurons.

The immediate large-scale dendritic plasticity of cortical pyramidal neurons subjected to acute epidural compression

Head trauma and acute disorders often instantly compress the cerebral cortex and lead to functional abnormalities. Here we used rat epidural bead implantation model and investigated the immediate changes following acute compression. The dendritic arbors of affected cortical pyramidal neurons were filled with intracellular dye and reconstructed 3-dimensionally for analysis. Compression was found to shorten the apical, but not basal, dendrites of underlying layer III and V cortical pyramidal neurons and reduced dendritic spines on the entire dendritic arbor immediately. Dendrogram analysis showed that in addition to distal, proximal apical dendrites also quickly reconfigured. We then focused on apical dendritic trunks and explored how proximal dendrites were rapidly altered. Compression instantly twisted the microtubules and deformed the membrane contour of dendritic trunks likely a result of the elastic nature of dendrites as immediate decompression restored it and stabilization of microtubules failed to block it. Subsequent adaptive remodeling restored plasmalemma and microtubules to normal appearance in 3 days likely via active mechanisms as taxol blocked the restoration of microtubules and in addition partly affected plasmalemmal reorganization which presumably engaged recycling of excess membrane. In short, the structural dynamics and the associated mechanisms that we revealed demonstrate how compression quickly altered the morphology of cortical output neurons and hence cortical functions consequently.

The cytoarchitecture and soma-dendritic arbors of the pyramidal neurons of aged rat sensorimotor cortex: an intracellular dye injection study

We studied the cytoarchitecture and dendritic arbors of the output neurons of the sensorimotor cortex of aged rats and found that although individual cortical layer became thinner, the overall cytoarchitecture and neuron densities remained comparable to those of young adults. To find out whether aging affects cortical outputs we studied the soma-dendritic arbors of layers III and V pyramidal neurons, main output neurons of the cerebral cortex, using brain slice intracellular dye injection technique. With a fluorescence microscope, selected neurons were filled with fluorescence dye under visual guidance. Injected slices were resectioned into thinner sections for converting the injected dye into non-fading material immunohistochemically. The long apical dendritic trunk and branches could be routinely revealed. This allowed us to reconstruct and study the dendritic arbors of these neurons in isolation in 300-microm-thick dimension. Analysis shows that their cell bodies did not shrink, but the densities of spines on dendrites and the total dendritic length significantly reduced. Among spines, those with long thin stalks thought to be involved in memory acquisition appeared to be reduced. These could underlie the compromise of sensorimotor functions following aging.

The effects of decompression and exogenous NGF on compressed cerebral cortex

Using a rat epidural bead implantation model, we found that compression alone could reduce the overall and individual layer thicknesses of cerebral cortex with no apparent cell death. The dendritic lengths and spine densities of layer II/III and V pyramidal neurons started to decrease within 3 days of compression. Decompression for 14 days resulted in near complete to partial recovery of the cortical thickness and of the dendritic lengths of layer II/III and V pyramidal neurons, depending on the duration of the preceding compression. The recoverability was better following short (3-day) than long (1- or 3-month) periods of compression. The loss of dendritic spines nevertheless persisted. An intraventricular infusion of NGF was performed after decompressing the lesions following 3 days of cortical compression, and this increased the recovery of the spines but not the dendritic length of the cortical pyramidal neurons, nor did it alter the recovery of the cortical thickness. NGF also promoted the increase of the dendritic spines, but not the dendritic length of the cortical pyramidal neurons of normal animals. In short, the data show that a few days of compression alone can cause permanent cortical damage. Exogenous NGF, if applied topically, may restore the dendritic spine density of cortical neurons subjected to compression.

The effect of epidural compression on cerebral cortex: a rat model

We developed a rat model of epidural plastic bead implantation to study the effect of physical compression on the cerebral cortex. Epidural implantation of a bead of appropriate size compressed the underlying sensorimotor cortex without apparent ischemia, since the capillary density of the cortex was increased. Although the thickness of all layers of the compressed cortex was significantly decreased, no apparent changes in the number of NADPH-diaphorase reactive neurons, reactive astrocytes, or microglial cells were observed, nor were apoptotic neurons observed. In fact, the densities of the neurons in most cortical layers apparently increased. To determine how epidural compression affects neuronal morphology, the dendritic arbors of layer III and V pyramidal neurons were evaluated using a fixed tissue intracellular dye injection technique. Neurons in both layers remained pyramidal in shape and their somatic sizes remained unaltered for at least a month after compression. On the other hand, their total dendritic length was significantly reduced beginning at 3 days post implantation. These analyses showed that apical dendrites were affected sooner than basal ones. The reduction of dendritic length was associated with a drop in the number of dendritic branches rather than dendritic trunks, suggesting the trimming of the peripheral part of the dendritic arbor. Detailed analysis showed that dendritic spines on all dendrites were reduced as early as 3 days following implantation. These results suggest that cortical neurons remodel their structures substantially within 3 days after being subjected to epidural compression.