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A variety of neurotrophic factors have been used in attempts to improve morphological and behavioural outcomes after experimental spinal cord injury (SCI). Here we review many of these factors, their cellular targets, and their therapeutic impact on spinal cord repair in different, primarily rodent, models of SCI. A majority of studies report favourable outcomes but results are by no means consistent, thus a major aim of this review is to consider how best to apply neurotrophic factors after SCI to optimize their therapeutic potential. In addition to which factors are chosen, many variables need be considered when delivering trophic support, including where and when to apply a given factor or factors, how such factors are administered, at what dose, and for how long. Overall, the majority of studies have applied neurotrophic support in or close to the spinal cord lesion site, in the acute or sub-acute phase (0-14 days post-injury). Far fewer chronic SCI studies have been undertaken. In addition, comparatively fewer studies have administered neurotrophic factors directly to the cell bodies of injured neurons; yet in other instructive rodent models of CNS injury, for example optic nerve crush or transection, therapies are targeted directly at the injured neurons themselves, the retinal ganglion cells. The mode of delivery of neurotrophic factors is also an important variable, whether delivered by acute injection of recombinant proteins, sub-acute or chronic delivery using osmotic minipumps, cell-mediated delivery, delivery using polymer release vehicles or supporting bridges of some sort, or the use of gene therapy to modify neurons, glial cells or precursor/stem cells. Neurotrophic factors are often used in combination with cell or tissue grafts and/or other pharmacotherapeutic agents. Finally, the dose and time-course of delivery of trophic support should ideally be tailored to suit specific biological requirements, whether they relate to neuronal survival, axonal sparing/sprouting, or the long-distance regeneration of axons ending in a different mode of growth associated with terminal arborization and renewed synaptogenesis. This article is part of a Special Issue entitled SI: Spinal cord injury., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Porous hydrophilic sponges made from 2-hydroxyethyl methacrylate (HEMA) have a number of possible biomedical applications. We have investigated whether these poly(HEMA) hydrogels, when coated with collagen and infiltrated in vitro with cultured Schwann cells, can be implanted into the lesioned optic tract and act as prosthetic bridges to promote axonal regeneration. Nineteen rats (20-21 days old) were given hydrogel/Schwann cell implants. No obvious toxic effects were seen, either to the transplanted glia or in the adjacent host tissue. Schwann cells survived the implantation technique and were immunopositive for the low affinity nerve growth factor receptor, S100 and laminin. Immunohistochemical studies showed that host non-neuronal cells (astrocytes, oligodendroglia and macrophages) migrated into the implanted hydrogels. Astrocytes were the most frequently observed host cell in the polymer bridges. RT97-positive axons were seen in about two thirds of the implants. The axons were closely associated with transplanted Schwann cells and, in some cases, host glia (astrocytes). Individual axons regrowing within the implanted hydrogels could be traced for up to 900 microns, showing that there was continuity in the network of channels within the polymer scaffold. Axons did not appear to be myelinated by either Schwann cells or by migrated host oligodendroglia. In three rats, anterograde tracing with WGA/HRP failed to demonstrate the presence of retinal axons within the hydrogels. The data indicate that poly(HEMA) hydrogels containing Schwann cells have the potential to provide a stable three-dimensional scaffold which is capable of supporting axonal regeneration in the damaged CNS.

Retrograde tracing techniques have been used to study the host visual cortical projection to fetal tectal tissue grafted to the midbrain of newborn host rats. To determine whether there is any topographic order in these cortical afferents, different parts of the grafts were injected with 3 different tracers: Fast blue (FB), Diamidino yellow dihydrochloride (DY), and either horseradish peroxidase (HRP) or rhodamine-labelled microspheres (Rh). The comparative visual cortical distribution of cells retrogradely labelled with the different dyes was then examined. Tectal tissue from 15-day-old pigmented rat embryos was injected via a glass micropipette onto the dorsal midbrain of anaesthetised newborn rats of the same strain. In adulthood, host rats were examined for the presence of grafts; 21 grafts were injected with retrograde tracers and the cortices of 12 of these animals were mapped to show the relative location of FB-, DY-, HRP- or Rh-labelled cells. Qualitative inspection of area 17 did not reveal consistent evidence of point-to-point visuotopic mapping in the cortico-transplant projection. However, within area 17 statistical analysis (chi 2 tests) revealed significant differences in most brains in the relative distribution of FB-, DY-, HRP- or Rh-labelled neurons. Areas 18 and 18a contained greater numbers of retrogradely labelled cells. In these extrastriate regions, statistical analysis also indicated significant differences in the relative distribution of neurons labelled with different tracers. These data thus provide evidence for a non-random pattern of cortical innervation of tectal grafts. Possible reasons for the absence of coherent, topographically organized cortico-transplant maps typical of the normal corticotectal projection are considered.

The development and distribution of alpha-bungarotoxin (alpha-BTX) binding sites in tectal grafts was examined autoradiographically using the radioligand [125I]-alpha-BTX. High alpha-BTX binding was observed in localized areas within grafts; these areas corresponded to regions which contained high acetylcholinesterase activity and received retinal input. Receptor differentiation also occurred in the absence of specific host afferents. The graft data show that, as in normal superior colliculus, development of high alpha-BTX binding is limited to areas containing presumptive superficial layer cells.

Ablation of one auditory cortex at birth in rats results in the formation of aberrant crossed projections from the intact hemisphere. These aberrant projections extend throughout most of the corticorecipient zone of the contralateral inferior colliculus and can be traced as far rostral as the contralateral medial geniculate nucleus. The aberrant crossed corticothalamic axons arise from layer V pyramidal neurons in the intact auditory cortex.

Suction lesions of the left rostral superior colliculus (SC) and pretectal area were made in young rats. One to 50 days later, the right eye of each animal was injected with horseradish peroxidase (HRP). Animals were subsequently perfused and the brains processed with tetramethylbenzidine. In many older animals, lesioned areas were covered with thin membranes composed of glial and connective tissue cells. HRP-labelled optic axons were traced running through these membranes for up to 3 mm. On occasion, regrowing retinal fibres reached the rostral border of the remaining SC but did not grow into the tectal neuropil.

It has been shown previously that tectal tissue obtained from young embryos can be successfully transplanted to the neonatal rat brain. In the present study, tecta from E15 rat embryos were maintained as free-floating explants for 3-14 days in vitro (DIV) before being transplanted to the midbrain of newborn rats. We wished to determine how explant culture affected (i) graft survival, (ii) the subsequent morphological and histochemical development of tectal grafts and (iii) the specificity with which host retinal and cortical axons grew into and innervated the graft neuropil. Grafts were examined 6-40 weeks posttransplantation. Host retinal input was assessed by injecting the host eyes with either [3H]proline, horseradish peroxidase (HRP) or wheat-germ agglutinin conjugated HRP. The host cortical projection was examined using anterograde degeneration techniques. Frozen tissue sections were also stained for Nissl, neurofibrils or reacted for acetylcholinesterase (AChE). All 3 DIV and 7 DIV explants survived transplantation and many grew considerably in size within the host brain. 14 DIV grafts were smaller and were found in only 50% of host brains. The cellular organization, fibre architecture and pattern of AChE staining in cultured grafts was similar to that found in non-cultured tectal transplants. Furthermore, host retina and cortex projected into the grafts in a manner similar to their innervation of non-cultured tectal tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

We have examined the time course of maturation of embryonic tectal tissue transplanted to the midbrain region of newborn rats and studied the development of the host retinal projection to the grafts. Transplants were examined 2-19 days after transplantation. The morphology of developing transplants was studied using Holmes silver and neutral red stained material. Tectal transplants attained their mature morphology about 17 days after transplantation. The time course of tectal transplant maturation appeared to be similar to that of normal superior colliculus in situ. The development of the host retinal projection into the transplants was examined by injecting the host eyes with horseradish peroxidase (HRP) at various times after transplantation. Retinal fibers anterogradely labeled with HRP were first seen growing into the transplants 3-4 days after transplantation. Ingrowing fibers were always located close to the surface of the transplants. The rate of growth of optic axons into the grafts was estimated to be about 250 to 300 micron per day. Patch-like arborizations of retinal afferents were formed soon after innervation and the mature pattern of optic innervation was established by about two weeks. There was no evidence for an initial transitory phase in which the axons invaded the whole transplant. The development of the host retinal projection preceded morphological maturation of the transplants. The mode of ingrowth of retinal axons into tectal grafts was in many respects similar to the way optic fibers grow into the superior colliculus during normal in situ development. The transplant technique thus provides an opportunity to manipulate and analyze the factors which guide optic fiber growth in intact brains.