More nerve growth factors?

Changes in eye blink responses following hypoglossal-facial anastomosis in the cat: evidence of adult mammal motoneuron unadaptability to new motor tasks

Hypoglossal-facial anastomosis is used in humans to restore the activity of the mimic musculature following irrecoverable facial nerve lesions. As eyelid movement kinetics is very well known, we have used this experimental model in cats to follow the evolution of blink responses and the adaptability of hypoglossal motor pools to new motor tasks. Although the electromyographic activity of the orbicularis oculi muscle in response to corneal air puffs, flashes of light or electrical stimulation of the supraorbital nerve was not recovered in the seven months following this crossed anastomosis, reflex blinks were got back by the increased activity of the retractor bulbi and extraocular recti muscles. The lid of the anastomosed side oscillated in perfect synchronization with tongue movements during licking, while it was severely affected in its motor function during optokinetic stimulation because of the spontaneous appearance of tongue-related hypoglossal activity. Present results suggest that adult mammal motoneurons are unable to readapt their motor programs to the kinetic needs of new motor targets and that most of the functional recovery observed in the cat was achieved by the compensatory hyperactivity of motor systems not directly affected by the surgery.

Denervated skeletal muscle stimulates migration of Schwann cells from the distal stump of transected peripheral nerve: an in vivo study

We have tested the stimulation of Schwann cell migration from the distal stump of a 1 week transected sciatic nerve of adult rats by denervated skeletal muscle. Migrating Schwann cells were distinguished by the presence of non-specific cholinesterase (nChE) activity and glial fibrillary acidic protein (GFAP) at a distance of about 6 mm among denervated muscle fibres 4 weeks after insertion of the distal stump. In addition, the distal stump was introduced into the open end of a silicone chamber packed with artificial fibrin sponge (Gelaspon) soaked in homogenate from intact or denervated muscles. A larger amount of migrated Schwann cells was observed in the chambers filled with homogenate from denervated muscles. An alteration in the amounts of Schwann cells migrating into the silicone chambers observed after histochemical staining (nChE or GFAP) was supported by biochemical measurements of the nChE activity. The biochemical assessment of the nChE activity revealed the increased amounts of migrated Schwann cells in proportion to the protein contents of homogenates from the denervated muscles. In addition, heating of homogenate from the denervated muscles resulted in a diminution of Schwann cell migration. Bromodeoxyuridine incorporation did not show an increased proliferation of Schwann cells inside the chambers following application of homogenate from the denervated muscles in comparison with the homogenate from the innervated muscles. Our results suggest a stimulation of Schwann cell migration from the distal stump of the transected sciatic nerve by soluble factor(s) produced by denervated skeletal muscles.

Recovery of original nerve supply after hypoglossal-facial anastomosis causes permanent motor hyperinnervation of the whisker-pad muscles in the rat

Hypoglossal-facial anastomosis (HFA), used in humans for the treatment of facial palsy, was experimentally performed in adult female Wistar rats. The time course of facial reinnervation and the extent of the new motor nerve supply of the vibrissal muscles that develops after HFA were estimated by counting all motoneurons in the brainstem labeled by injection of horseradish peroxidase (HRP) into the whisker pad; muscle innervation by motor endplates was not studied. In untreated animals, HRP injection labels 1,254 +/- 54 (mean +/- S.D.; n = 6) motoneurons, localized exclusively in the lateral subdivision of the facial nucleus. Immediately following HFA, this number drops to zero. The first HRP-labeled motoneurons appear in the hypoglossal nucleus at 28 days postoperation (dpo) and at 56 dpo their number reaches 1,096 +/- 48. Unexpectedly, the facial nerve, whose proximal stump has been left as blind end during surgery, additionally sends axons to the facial periphery. This resprouting is first detected at 42 dpo with HRP-marked neurons throughout the facial nucleus lacking somatotopic organization. The number of these labeled neurons also rises with time, and at 56 dpo, a total of 1,797 +/- 142 facial and hypoglossal motoneurons, that is, 43% more motoneurons than in normal animals, supplies the whisker pad. This hyperinnervation, that is, the projection of more motoneurons into the target muscle than under normal conditions--further increases to 1,978 +/- 92 motoneurons at 224 dpo and may provide a new animal model for studying the competitive relationships between motoneurons in their search for peripheral targets.

Rapid neural regulation of muscle urokinase-like plasminogen activator as defined by nerve crush

Muscle plasminogen activators (PAs), such as urokinase-like PA and, to a lesser extent, tissue PA, increase dramatically after denervation induced by axotomy. The PA/plasmin system has also been implicated in degradation of specific components of the muscle fiber basement membrane after local activation of plasminogen. These results suggest that neural regulation of muscle extracellular matrix metabolism accompanies or precedes regeneration after injury and is mediated by activation of PAs. In the present study, we have used nerve crush to explore the neural regulation of muscle PA activities directly after subtotal axon interruption and during the process of reinnervation. Muscle contraction after nerve stimulation and estimation of choline acetyltransferase activity were used to monitor reinnervation. Within 24 hr of nerve crush, muscle urokinase (but not tissue PA) activity rose in soluble and membrane-bound muscle fractions, as shown by an amidolytic assay and a fibrin zymography. Membrane-bound activity was 5-fold higher than cytosol activity, but there was no shift between cellular compartments during the time course of denervation. Coincident with the return of choline acetyltransferase activity and muscle contractility, muscle urokinase returned almost to baseline levels. These results show tight regulation of muscle urokinase levels by some neural influence.

Neurotrophic action of autopsied ventral spinal cord extracts in patients with amyotrophic lateral sclerosis on the ventral spinal cord of rat embryo

The effects of ventral spinal cord extracts from normal controls and patients with amyotrophic lateral sclerosis (ALS) on neurite appearance in ventral spinal cord explant of 13-day-old Sprague-Dawley rats were investigated. Ventral spinal cord extracts from normal controls and ALS were significantly more effective in stimulating growth than control medium only. There was no statistically significant difference between normal and ALS ventral spinal cord extracts in growth of neurites. Our results may be an important for the consideration of the hypothesis that ALS may be a disorder of motor neuron growth factors.

Extracts of muscle from patients with amyotrophic lateral sclerosis enhance neurite outgrowth from ventral spinal cord explants of rat embryo

The effects of muscle extracts from normal controls and patients with amyotrophic lateral sclerosis (ALS) on neurite appearance in ventral spinal cord explants of 13-15-day-old Sprague-Dawley rats were investigated. Muscle extracts from normal controls and ALS muscle were significantly more effective in stimulating growth than control medium alone. There was no statistically significant difference between normal muscle extracts and ALS muscle extracts in growth of neurties. Our results may contribute to the hypothesis that ALS may be a disorder of motor neuron growth factors.

Suppression of terminal axonal sprouting at the neuromuscular junction by monoclonal antibodies against a muscle-derived antigen of 56,000 daltons

After the partial denervation or paralysis of a muscle, the remaining motor axon terminals may sprout fine, neuritic processes (terminal sprouts) which escape the endplate region of the neuromuscular junction. We previously identified a muscle-derived, protein antigen of 56,000 daltons (56 kD) which plays a necessary role in terminal sprouting. A panel of monoclonal antibodies have been produced against the 56-kD antigen, some of which also partially suppress motor axon terminal sprouting. These monoclonal antibodies define at least two different epitopes upon the surface of the antigen, one of which is necessary for it to effect its biological role in vivo.

Nerve dependent regulation of neural cell adhesion molecule expression in skeletal muscle

The expression of neural cell adhesion molecule was analysed by indirect immunofluorescence on adult mouse skeletal muscle subjected to a variety of experimental lesions. Adult mouse muscle does not express neural cell adhesion molecule at the sarcolemma. However, following denervation there is a rapid rise in neural cell adhesion molecule levels; this is initially in the cytoplasm of the myofibres but by 18 days there is intense reactivity at the sarcolemma. A nerve crush lesion was used to show that the increase in neural cell adhesion molecule levels following denervation is accompanied by a switch-off of neural cell adhesion molecule expression following reinnervation. Paralysis of skeletal muscle by botulinum toxin injection is sufficient to activate neural cell adhesion molecule expression. As paralysis of skeletal muscle by botulinum toxin is not accompanied by activation of muscle satellite cells or degeneration products of nerve or myelin, it suggests that the observed levels of neural cell adhesion molecule are synthesised by myofibres. As the expression of neural cell adhesion molecule in these lesions parallels the ability of skeletal muscle to accept innervation it is possible that neural cell adhesion molecule acts as a molecular cue at the sarcolemma in regulating synaptogenesis.

Expression of several adhesive macromolecules (N-CAM, L1, J1, NILE, uvomorulin, laminin, fibronectin, and a heparan sulfate proteoglycan) in embryonic, adult, and denervated adult skeletal muscle

Levels of the neural cell adhesion molecule N-CAM in muscle are regulated in parallel with the susceptibility of muscle to innervation: N-CAM is abundant on the surface of early embryonic myotubes, declines in level as development proceeds, reappears when adult muscles are denervated or paralyzed, and is lost after reinnervation (Covault, J., and J. R. Sanes, 1985, Proc. Natl. Acad. Sci. USA, 82:4544-4548). Here we used immunocytochemical methods to compare this pattern of expression with those of several other molecules known to be involved in cellular adhesion. Laminin, fibronectin, and a basal lamina-associated heparan sulfate proteoglycan accumulate on embryonic myotubes after synapse formation, and their levels change little after denervation. L1, J1, nerve growth factor-inducible large external protein, uvomorulin, and a carbohydrate epitope (L2/HNK-1) shared by several adhesion molecules are undetectable on the surface of embryonic, perinatal, adult, or denervated adult muscle fibers. Thus, of the molecules tested, only N-CAM appears on the surface of muscle cells in parallel with the ability of the muscle cell surface to accept synapses. However, four antigens--N-CAM, J1, fibronectin, and a heparan sulfate proteoglycan--accumulate in interstitial spaces near denervated synaptic sites; regenerating axons traverse these spaces as they preferentially reinnervate original synaptic sites. Of particular interest is J1, antibodies to which block adhesion of central neurons to astrocytes (Kruse, J., G. Keihauer, A. Faissner, R. Timpl, and M. Schachner, 1985, Nature (Lond.), 316:146-148). J1 is associated with collagen and other fibrils in muscle and thus may be an extracellular matrix molecule employed in both the central and peripheral nervous systems.

Trophic effects of skeletal muscle extracts on ventral spinal cord neurons in vitro: separation of a protein with morphologic activity from proteins with cholinergic activity

Protein factors derived from skeletal muscle separately promote neurite elongation and acetylcholine synthesis in cultured rat ventral spinal neurons. Morphologic factor activity (neurite-inducing activity) is specifically found in rat skeletal muscle and cord neuron extracts, decreases with the postnatal age of the rats from which muscle extract is prepared, and increases in rat hindlimb muscle after 5 d of denervation. Cholinergic factor activity (acetylcholine synthesis-stimulating activity) is found in extracts of rat cerebral cortex and cardiac muscle in addition to spinal cord and skeletal muscle, increases with animal age, and decreases following 5 d of denervation. Biochemically, the factors responsible for these activities differ in their lability to denaturing conditions, apparent molecular weights, isoelectric points, and lectin-binding specificities. Under reducing conditions, morphologic activity is isolated in a single acidic glycoprotein with an Mr of 35,000, while acetylcholine synthesis-stimulating activity is found in multiple species of different molecular weights. Thus, acetylcholine synthesis-promoting activities and neurite growth-promoting activity appear to reside in different molecules. Significant purification of several of these factors has been achieved.

Neural cell adhesion molecule (N-CAM) accumulates in denervated and paralyzed skeletal muscles

We have used immunofluorescence and immunoblotting methods to study the amount and distribution of the neural cell adhesion molecule (N-CAM) in rat skeletal muscle; this molecule is thought to mediate adhesion of neurons to cultured myotubes. N-CAM is present on the surface of embryonic myotubes, but it is lost as development proceeds and is nearly absent from adult muscle. However, denervation of adult muscle results in the reappearance of N-CAM. In denervated muscle, N-CAM is associated both with muscle fibers and with cells in interstitial spaces between fibers. The N-CAM in interstitial spaces is concentrated near denervated endplates, which are known to be preferential sites for reinnervation. Paralysis of innervated muscle, known to mimic denervation in many respects, also induces the accumulation of N-CAM. Axons that regenerate to reinnervate muscle bear N-CAM on their terminals, and reinnervation results in the disappearance of N-CAM from muscle. Denervation induces accumulation of N-CAM in mouse and chicken, as well as in rat muscles. Thus, the expression of N-CAM in muscle is regulated by the muscle's state of innervation. In that N-CAM-rich muscles (embryonic, denervated, and paralyzed) are known to be competent to accept synapses, while N-CAM-poor muscles (normal adult and reinnervated) are refractory to hyperinnervation, N-CAM might, in turn, participate in regulating muscle's susceptibility to innervation.

Molecular regulators of brain function: a new view

A working hypothesis is proposed based on two mutually dependent concepts: neurons may be functionally regulated not only by presently known neurotransmitters but by many kinds of informational substances. Known informational substances are considered in categories corresponding to major regulators of the central nervous system, including transmitters, peptides, hormones, "factors" and various proteins. Many new informational substances are being discovered by the application of DNA technology. Alongside neuronal circuitry that forms the basis for conventional neuroanatomy and neurophysiology, and that operates through conventional synaptic junctions, is a system here called parasynaptic, i.e. in "parallel with" synapse-linked circuitry. In the parasynaptic system, informational substances reach specific target cell receptors by diffusion from release points through extracellular fluids. The parasynaptic system has the same degree of selectivity as synaptic circuitry but possesses, in addition, a domain of versatility and plasticity lacking in "hardwired" circuitry; the latter is, however, also influenced by highly potent informational substances in the ambient extracellular fluid.