Adaptive and maladaptive motor axonal sprouting in aging and motoneuron disease.

Motor unit (MU) enlargement by sprouting is an important compensatory mechanism for loss of functional MUs during normal aging and neuromuscular disease. Perisynaptic Schwann cells at neuromuscular junctions extend processes that bridge between denervated and reinnervated endplates, and guide axonal sprouts to reinnervate the denervated endplates. In a rat model of partial denervation, high levels of daily neuromuscular activity have been shown to inhibit the outgrowth of sprouts by preventing Schwann cell bridging. In this review, we consider (1) the relative roles of increasing levels of oxidative stress and neuromuscular activity to the destabilization of neuromuscular junctions with age and disease, and (2) how a progressive increase in the neuromuscular activity of declining numbers of functional MUs contributes to the progressive failure of adaptive sprouting and, in turn, to the progressive muscle weakness in the motoneuron diseases of post-polio syndrome and amyotrophic lateral sclerosis. We conclude that there is a time-related progression of MU loss, adaptive sprouting followed by maladaptive sprouting, and continuing recession of terminals during normal aging. The progression is accelerated in motoneuron disease, progressing more rapidly in the post-polio syndrome after prolonged denervation and extremely rapidly in ALS.

NOS inhibition restores renal responses to atrial distension during pregnancy.

Nitric oxide (NO) biosynthesis increases during pregnancy and has been shown to suppress baroreceptor activity. The renal response to a simulated increase in circulating blood volume (atrial distension) is also attenuated at this time. We hypothesized that blocking NO biosynthesis during pregnancy would restore the renal response. Female rats were implanted with indwelling intracardiac balloons and central venous cannulas. After recovery, they were mated, and on day 14 of pregnancy, osmotic minipumps containing the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) or its inactive enantiomer N(G)-nitro-D-arginine methyl ester (D-NAME) (120 mg/2 ml at 10 microg/min) were implanted. In response to atrial distension (1 h), urine output increased in the D- and L-NAME-treated virgin rats. During pregnancy (day 20), this response was attenuated in the D-NAME-treated, but not the L-NAME-treated, animals, i.e., after a simulated increase in circulating blood volume, inhibition of NO biosynthesis restored the renal response of pregnant rats to that seen in virgin animals. We conclude that, during normal pregnancy, increased NO biosynthesis blunts the reflex renal response to atrial distension.

Mechanisms controlling axonal sprouting at the neuromuscular junction.

This review considers the relative roles of sprouting stimuli, perisynaptic Schwann cells and neuromuscular activity in axonal sprouting at the neuromuscular junction in partially denervated muscles. A number of sprouting stimuli, including insulin-like growth factor II, which are generated from inactive muscle fibers in partially denervated and paralyzed skeletal muscles, has been considered. There is also evidence that perisynaptic Schwann cells induce and guide axonal sprouting in adult partially denervated muscles. Excessive neuromuscular activity significantly reduces bridging of perisynaptic Schwann cell processes between innervated and denervated endplates and thereby inhibits axonal sprouting in partially denervated adult muscles. Elimination of neuromuscular activity is also detrimental to sprouting in these muscles, suggesting that calcium influx into the nerve is crucial for axonal sprouting. The role of neuromuscular activity in axonal sprouting will be considered critically in the context of the roles of sprouting stimuli and perisynaptic Schwann cells in the process of axonal sprouting.

Electrical stimulation accelerates and enhances expression of regeneration-associated genes in regenerating rat femoral motoneurons.

1. In this study we investigated whether electrical stimulation accelerates the upregulation of Talpha1-tubulin and GAP-43 (regeneration-associated genes; RAGs) and the downregulation of the medium-molecular-weight neurofilament (NFM), in concert with stimulation-induced acceleration of BDNF and trkB gene expression and axonal regeneration. 2. Two weeks prior to unilateral femoral nerve transection and suture, fluorogold (Fluorochrome Inc., Denver) or fluororuby (Dextran tetramethylrhodamine, Mol. Probes, D-1817, Eugene, OR) was injected into quadriceps muscles of the left and right hindlimbs to label the femoral motoneuron pools as previously described. Over a period of 7 days, fresh spinal cords were processed for semiquantitation of mRNA by using in situ hybridization. 3. There was an increase in Talpha1-tubulin and GAP-43 mRNA and a decline in the NFM mRNA at 7 days after nerve suture and sham stimulation but not in intact nerves. In contrast, 1-h stimulation of sutured but not intact nerves dramatically accelerated the changes in gene expression: mRNA levels of Talpha1-tubulin and GAP-43 were significantly elevated above control levels by 2 days while NFM mRNA was significantly reduced by 2 days in the sutured nerves. Thereby, the neurofilament/tubulin expression ratio was reduced at 2 days after suture and stimulation, possibly allowing more tubulin to be transported faster into the growing axons to accelerate the elongation rate following stimulation. Importantly, the changes in RAGs and NFM gene expression were delayed relative to the accelerated upregulation of BDNF and trkB mRNA by electrical stimulation. 4. The temporal sequence of upregulation of BDNF and trkB, altered gene expression of RAGs and NFM, and accelerated axonal outgrowth from the proximal nerve stump are consistent with a key role of BDNF and trkB in mediating the altered expression of RAGs and, in turn, the promotion of axonal outgrowth after electrical stimulation.

Effect of NOS inhibition on central response to atrial distension during pregnancy.

Atrial distension increases c-fos expression in the paraventricular nucleus of virgin, but not pregnant, rats. We proposed that nitric oxide (NO), biosynthesis of which increases during pregnancy, blunts this reflex and that blocking NO biosynthesis would restore the response. Female rats were implanted with indwelling intracardiac balloons. On day 14 of pregnancy, osmotic minipumps containing either D- or N(G)-nitro-L-arginine methyl ester (L-NAME) (120 mg/2 ml at 10 microg/min) were implanted. On day 20, the rats were infused with saline (3 ml/h) with or without atrial balloon inflation (1 h). The brains were then processed for quantitation of c-fos expression. In the virgin rats, and in the pregnant rats treated with L-NAME, atrial distension significantly increased hypothalamic c-fos expression. In the pregnant animals treated with D-NAME, the response was greatly attenuated. NO had no effect on the increase in atrial receptor afferent discharge (single-fiber recordings) elicited by atrial distension. We conclude that, during pregnancy, NO attenuates central processing of the reflex response to atrial distension but does not alter the transducer properties of the volume receptors.

Effect of exercise on stability of chronically enlarged motor units.

Chronic denervation syndromes such as the post-polio syndrome are associated with progressive muscle weakness and fatigue after motoneuron death. Neither the etiology nor the management of these syndromes is clear. To address this issue, we partially denervated rat hindlimb muscles for 1 or 12 months and examined whether chronically enlarged motor units (MUs) become destabilized with time and further destabilized by daily running on exercise wheels. MU enlargement, measured electrophysiologically and morphologically was significantly reduced at 12 months in extensively denervated muscles, and to a lesser extent in moderately denervated muscles, as compared to the findings at 1 month. A 1-month period of running exercise further reduced the size of the chronically enlarged MUs in the extensively denervated muscles. We have therefore (1) successfully established a rat model of time-related MU size reduction, in which destabilization of chronically enlarged MUs results in loss of axonal terminals, and (2) demonstrated that nonphysiological activity has small but significant effects of further destabilizing the chronically enlarged MUs.

Changes in spinal cord architecture after brachial plexus injury in the newborn.

Obstetric brachial plexus palsy is a devastating birth injury. While many children recover spontaneously, 20-25% are left with a permanent impairment of the affected limb. So far, concepts of pathology and recovery have focused on the injury of the peripheral nerve. Proximal nerve injury at birth, however, leads to massive injury-induced motoneuron loss in corresponding motoneuron pools and therefore limits the extent of functional recovery. In the present study, the role of spinal cord plasticity after injury and recovery from obstetric brachial plexus lesions was investigated. A selective injury to spinal roots C5 and C6 was induced in newborn Sprague-Dawley rats, leading to motoneuron loss in corresponding motoneuron pools. Recovery of extremity function was evaluated with different behavioural paradigms. Permanent changes of adjacent motoneuron pools were quantitatively evaluated by retrograde tracing and functional muscle testing. We report that the adjacent C7 motoneuron contribution to biceps muscle innervation increased four-fold after upper trunk lesions in newborns, thus compensating for the injury-induced motoneuron loss. These results indicate that, in obstetric brachial plexus palsy, changes in spinal cord architecture are an integral part not only of primary pathology but also of the subsequent recovery process. While present treatment is directed towards the restoration of neural continuity, future treatment strategies must recognize and take advantage of CNS participation in the injury and recovery process.