Peripheral Nerve Bridge Research Articles

1714 Axonal regeneration and long-time survival of retinal ganglion cells with a peripheral nerve bridge to the cat cerebral cortex

1714 Axonal regeneration and long-time survival of retinal ganglion cells with a peripheral nerve bridge to the cat cerebral cortex

Nerve Growth Factor in CNS Repair

The hypothesis that neurotrophic factors play important roles in the adult central nervous system (CNS) has been successfully investigated in the past decade with regard to experimental and pathologic situations. Trophic roles in adult CNS axonal regeneration, on the other hand, have received much less attention. We review three groups of recent studies that demonstrate the relevance of nerve growth factor (NGF) for the regeneration of selected axons into adult central nervous tissue. The first group concerns a septohippocampal model where transected septal cholinergic axons are allowed to regrow into the hippocampal formation through a peripheral nerve bridge implanted into the transection lesion gap. NGF is required in the bridge, enhances penetration of the hippocampal tissue when infused there, and both attracts and promotes sprouting within the septum when infused in the lateral ventricle or the septal tissue itself. The second group of studies concerns the development of a spinal cord sensory regeneration model, where dorsal root ganglionic axons regrow into a nerve bridge placed within the dorsal spinal cord. Preliminary data indicate that NGF infusion rostral to the bridge once again promotes substantial penetration of the adult cord tissue by the regenerating NGF-sensitive fibers. In the third group of studies, attention has been shifted to the location of endogenous NGF in the adult rat hippocampal formation and the normal or lesion-induced occurrence of extrasomal NGF immunoreactivity. These regions of anchored NGF have the ability to attract NGF-sensitive growing axons and may provide opportunities to investigate local cues for final definition of terminal fields.

The Ability of Developing Spinal Neurons to Reinnervate a Muscle through a Peripheral Nerve Conduit Is Enhanced by Cografted Embryonic Spinal Cord

The ability of neurons in the spinal cord of rats aged 5-12 days to reinnervate a muscle via a peripheral nerve bridge was examined and the possible influence of the cografted ED-12 embryonic spinal cord was tested. The soleus muscle was transferred paravertebrally and connected to the contralateral L4-L5 hemicord by its nerve. In some experiments embryonic spinal cord was grafted at the same level. Six to 12 weeks later fast blue and diamidino yellow were injected into the muscle or applied on the cut nerve bridge. The animals were perfused after 3-4 days and their spinal cords were examined using fluorescent microscopy, but retrogradely labeled neurons were only rarely seen. The embryonic spinal cord grafts survived well but had no influence on the outcome of these experiments. However, when neuromuscular implants from adult immunocompatible rats were used instead of the immature autologous ones, a variety of neurons including motoneurons extended their axons into the imp]ants. The numbers of retrogradely labeled neurons were significantly higher in the spinal cords with embryonic grafts. These retrogradely labeled neurons were in the host's grey matter and only exceptionally in the grafts. Thus, the developing neurons can extend their axons outside the spinal cord into the implants of adult soleus muscle and nerve, but immature nerve-muscle implants fail to attract and/or support axonal outgrowth. The reinnervation potential of the host's spinal neurons was enhanced by cografting of embryonic spinal cord.

Growth of axons through a lesion in the intact CNS of fetal rat maintained in long-term culture.

The ability of neurons in the central nervous system (CNS) to grow through a lesion and restore conduction has been analysed in developing spinal cord in vitro. The preparation consists of the entire CNS of embryonic rat, isolated and maintained in culture. Conduction of electrical activity and normal morphological appearance (light microscopical and electron microscopical) were maintained in the spinal cord of such preparations for up to 7 d in culture. A complete transverse crush of the spinal cord abolished all conduction for 2 d. After 3-5 d, clear recovery had occurred: electrical conduction across the crush was comparable with that in uninjured preparations. Furthermore, the spinal cord had largely regained its gross normal appearance at the crush site. Axons stained in vivo by carbocyanine dyes had, by 5 d, grown in profusion through the lesion and several millimetres beyond it. These experiments, like those made in neonatal opossum (Treherne et al. 1992) demonstrate that central neurons of immature mammals, unlike those in adults, can respond to injury by rapid and extensive outgrowth of nerve fibres in the absence of peripheral nerve bridges or antibodies that neutralize inhibitory factors. However, unlike the opossum, in which outgrowth occurred at 24 degrees C, although there was prolonged survival of rat spinal cords at this temperature, outgrowth of axons across the lesion required a temperature of 29 degrees C. With rapid and reliable regeneration in vitro it becomes practicable to assay the effects of molecules that promote or inhibit restoration of functional connections.

Restoration of conduction and growth of axons through injured spinal cord of neonatal opossum in culture.

The ability of neurons in the central nervous system to grow through a lesion and restore conduction has been analyzed in a developing spinal cord. The preparation consists of the entire central nervous system of the newly born opossum (Monodelphis domestica), isolated and maintained in culture. Cell division, cell migration, and reflexes are maintained in such preparations for up to 8 days in culture. In the present experiments, massive lesions were produced by crushing the spinal cord, which abolished all conduction for a day. By 2-3 days after injury, electrical conduction across the crush could be observed. After 4-5 days, clear recovery had occurred: the amplitude of the conducted volley was comparable to that in acute preparations. In such preparations, the spinal cord had largely regained its normal appearance at the crush site. Axons stained by carbocyanine dyes or horseradish peroxidase had, by 4 days, grown in profusion through the lesion and several millimeters beyond it. These experiments demonstrate that neurons in the central nervous system of newly born mammals, unlike those in adults, can respond to injury by rapid and extensive outgrowth in the absence of peripheral nerve bridges or antibodies that neutralize inhibitory factors of myelin. With rapid and reliable regeneration occurring in vitro, it becomes practicable to assay the effects of molecules that promote or inhibit the restoration of functional connections.

Septohippocampal cholinergic axonal regeneration through peripheral nerve bridges: Quantification and temporal development

Axons of the adult mammalian CNS have been shown to regrow vigorously into peripheral nerve grafts. Using a cholinergic septohippocampal model for adult CNS regeneration, involving complete denervation of the hippocampal formation from its basal forebrain cholinergic afferents, this study has established quantitative parameters and a temporal baseline of cholinergic fiber regeneration into the dorsal hippocampal tissue through a peripheral sciatic nerve graft. In nerve-implanted animals (i) the nerve grafts are maximally invaded by AChE-positive fibers between 2 weeks and 1 month postlesion, (ii) the fibers entering the hippocampal formation from the graft show a peak numerical increase and rate of elongation around the first month and/or in the proximal hippocampal region, (iii) an apparently normal innervation pattern and fiber density in the most rostral 1.5 mm of the dorsal hippocampal formation is reached by 6 months postlesion. The present study provides a basis for future quantitative comparisons of manipulations of different components of the system, e.g., the contributing neurons, the bridging material, and the receiving central nervous tissue. The temporal/spatial pattern of fiber regeneration suggests that the hippocampal CNS tissue can be a good axonal growth-promoting environment, albeit with temporal and/or spatial limitations, and is therefore not an immutably restrictive environment for axonal regeneration.

Formation of functional endplates by spinal axons regenerating through a peripheral nerve graft. A study in the adult rat

Peripheral nerve (PN) autografts were used in the adult rat to join the midcervical spinal cord to a nearby denervated skeletal muscle. Retrograde tracing, morphological and electrophysiological studies indicated the following: 1) a great number of neurons, located bilaterally, between C3 and C7, in most laminae of the grey matter, extended axons into the PN grafts, 2) a lesser number of neurons regenerated up to the reconnected muscle, but most of them were typical motoneurons, 3) neuromuscular junctions were formed in ectopic locations, around the tip of the grafted nerve, and at the sites of original endplates, 4) these junctions were functional and formed by axons that had regenerated into the PN bridges, as muscle contraction was obtained by electrical stimulation of the grafted nerves, 5) they were proved to be cholinergic since endplate potentials, evoked by stimulating the PN graft, were suppressed by curare. These results strongly suggest that spinal neurons, and especially motoneurons, are involved in the formation, through PN bridges, of new functional cholinergic connections with denervated skeletal muscles.

Chapter 28 Functional reinnervation of a denervated skeletal muscle of the adult rat by axons regenerating from the spinal cord through a peripheral nervous system graft

Publisher Summary In adult mammals, different types of neurons whose processes have been damaged in the central nervous system (CNS) may regrow axons along peripheral nerve (PN) grafts that have been inserted near their neuronal somata. This chapter aims to determine whether CNS neurons are capable of establishing synaptic connections through PN grafts, with peripheral targets, or not. The horseradish peroxidase (HRP)-labelling experiments from the grafted nerve discussed in this chapter are in full agreement with the results of other investigations, which demonstrate that, following spinal injury, intrinsic neurons, and among them, motoneurons of the ventral horn, have the capability to extend axons into PNS conduits, such as adjacent ventral roots, grafted dorsal and ventral roots, and blind-ended segments of autologous PN grafts. Both electrophysiological and morphological data indicate that a population of axons b, that have regrown from the spinal cord along the PN bridge have established new functional cholinergic connections with the reconnected muscle. However, further investigations should aim at confirming the actual participation of motoneurons to the functional reinnervation, through the PN bridges, of denervated skeletal muscles.

Regeneration of monoaminergic and cholinergic neurons in the mammalian central nervous system

Regeneration of monoaminergic and cholinergic neurons in the mammalian central nervous system