Raphe dorsalis-spinal cord cografts in oculo: electrophysiological evidence for an excitatory serotonergic innervation of transplanted spinal neurons

Functional interactions between spinal cord grafts suggest asymmetries dictated by graft maturity

Fetal spinal cord tissue grafts have been advocated as a possible repair strategy for spinal cord injury. In the present study, we used intraocular spinal cord grafts to model the interactions which may occur between fetal and adult spinal cord after making such a graft and to study to which extent functional connections can be expected to occur between the host and graft tissue. We first grafted fetal spinal cord to the anterior chamber of the eye where it was allowed to mature. A second piece of fetal spinal cord was then sequentially grafted in contact with the first graft. Electrophysiological recordings made from the older graft while electrically stimulating the younger graft provided evidence for an excitatory innervation from the younger spinal cord graft to the mature spinal cord which appeared to be glutamatergic. However, we only rarely found excitatory inputs from the first, mature spinal cord graft to the younger graft. Fiber connections between the two spinal cord grafts were verified by retrograde tracing and neurofilament immunohistochemistry. In no case was a trophic influence on graft volume observed between spinal cord grafts regardless of whether the transplantations were performed sequentially or at the same time. Even the introduction of a second graft to immature spinal cord tissue was ineffective. In contrast, we found a marked trophic, neuron-rescuing effect of spinal cord grafts upon cografts of fetal dorsal root ganglia. This latter observation is consistent with the hypothesis that spinal cord tissue can exert a trophic effect on developing sensory ganglia and demonstrates that many sensory neurons can survive in the presence of a central target and in the absence of the appropriate peripheral target. These intraocular experiments predict that fetal spinal cord grafted to the injured adult spinal cord may develop effective excitatory inputs with the host, while host-to-graft inputs may develop to a considerably smaller extent. Our results also suggest that the adult spinal cord does not exert marked trophic effects on growth of fetal spinal cord, while it does exert a trophic influence on central projections of dorsal root ganglia.

Interactions of neurotrophic factors GDNF and NT-3, but not BDNF, with the immune system following fetal spinal cord transplantation

Glial cell line-derived neurotrophic factor (GDNF) is known to stimulate survival of dopaminergic and spinal cord motor neurons. However, little is known of the possible immune sequelae of GDNF exposure, or that of other putative trophic factors. To address these questions we utilized in oculo grafts of spinal cord, wherein we could induce different levels of immune responses via allogeneic vs. syngeneic combinations. Adult female Sprague-Dawley and Fisher rats were used as hosts for allogeneic and syngeneic grafts, respectively. Embryonic age 14-15-day-old fetuses were taken from pregnant dams of each strain, and cervical spinal cords were removed and dissected. Pieces of the spinal cord were transplanted into the anterior chamber of the eye within each strain. At 5-day intervals, 0.5 microgram of GDNF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or cytochrome c (CC) was injected into the anterior chamber of the eye and the sizes of the transplants were measured for the Sprague-Dawley rats. The same injections and measurements, but only for GDNF and CC, were carried out using Fisher rats. As expected, GDNF increased transplant survival and growth in both the Sprague-Dawley and Fisher animals. At day 41-42, all rats were sacrificed. Cameral graft appearance was evaluated by cresyl violet and immunohistochemically using antibodies against neurofilament (NF), calcitonin gene-related peptide (CGRP) and glial fibrillary acidic protein (GFAP). To monitor immune responses, the following monoclonal antibodies were used: OX38 against CD4, OX18 against MHC class I (MHCI), OX8 against CD8, OX6 against MHC class II (MHCII), OX42 against CD11b, R73 against alpha and beta T cell receptor (TcR), and ED1. In the Sprague-Dawley grafts, significantly higher amounts of CD8+, T lymphocyte+, MHCI+ and MHCII+ antigen-presenting cells (APC) were observed in GDNF-treated transplants. These markers were also increased in NT-3-treated groups. There were two types of OX-42+ cells, one was the ordinary ramified microglial cell, the other appeared to be a phagocytic cell, looking like the interstitial proliferating variety. Interestingly, the phagocytic OX-42+ cells had the same distribution as ED1+ and MHCII+ cells. In contrast, there were few immunoreactive cells after GDNF treatment in the inbred Fisher animals, similar to the CC control group. These results suggest that GDNF and to some extent NT-3, can activate the immune system in allogeneic graft combinations, but that these trophic factors do not produce overt rejection, and do not per se induce immune responses.

Glial cell line-derived neurotrophic factor enhances survival and growth of prenatal and postnatal spinal cord transplants

Glial cell line-derived neurotrophic factor was first described as a trophic factor for developing dopamine neurons. However, it has been shown that glial cell line-derived neurotrophic factor messenger RNA is also expressed in several areas of the developing brain and spinal cord, suggesting that it may have additional roles in the nervous system. Intraocular transplantation of neural tissue provides a unique method to examine in vivo effects of trophic factors. We have therefore studied the effects of glial cell line-derived neurotrophic factor on spinal cord survival and development following grafting to the anterior chamber of the eye of adult rats. We used spinal cord tissue from fetal stages (embryonic days 14 and 18) and postnatal days 1 and 14 as donors. The spinal cord tissue was allotransplanted to the anterior eye chamber of Sprague-Dawley host rats after incubation in buffered saline containing 100 micrograms glial cell line-derived neurotrophic factor/ml or 100 micrograms cytochrome C/ml. One group of postnatal day 1 spinal cord grafts was also treated with concentrations of 20 and 10 micrograms glial cell line-derived neurotrophic factor/ml. In all cases, 5 microliters of the same solution was injected into the anterior eye chamber on postgrafting days 5, 10, 15 and 20 (total amounts 0.5, 0.1 and 0.05 microgram/eye/injection, respectively). We found that all glial cell line-derived neurotrophic factor-treated spinal cord grafts grew more than controls. The effect of glial cell line-derived neurotrophic factor was most prominent in grafts from newborn rats. In these grafts we found a dose-dependent effect of glial cell line-derived neurotrophic factor on growth. Moreover, grafts treated with the highest dose (0.5 microgram) grew to sizes exceeding the initial size at transplantation. In these transplants we also found greater numbers of large neurons compared to controls. Glial fibrillary acidic protein immunoreactivity, in contrast, showed increased gliosis in controls. Similar results were found with syngeneic spinal cord postnatal day 1 grafts in Fisher hosts. Spinal cord tissue grafts from two-week-old rats treated with the highest glial cell line-derived neurotrophic factor dose every fifth day, through day 35 postgrafting, responded with increased growth and less necrotic tissue compared with controls; however, we could not detect neurofilament immunoreactivity in these transplants. Taken together, these results suggest that glial cell line-derived neurotrophic factor may be a potent trophic factor for neurons in the spinal cord and in spinal cord transplants. Of particular importance is that glial cell line-derived neurotrophic factor treatment can be used to obtain survival of postnatal spinal cord tissue, that would otherwise show minimal or no survival. Thus, glial cell line-derived neurotrophic factor allows successful transplantation of more mature spinal cord tissue, which may have important implications for both basic and clinical neuroscience.

Spinal cord-skeletal muscle cografts: trophic and functional interactions

Skeletal muscle from embryonic day 20 (E20) was combined with E15 rat spinal cord in the anterior chamber of the eye of adult albino rats. The two grafts were either transplanted concomitantly or sequentially, in which case muscle tissue was added 4 months after the spinal cord. Control groups received a single graft of either spinal cord or skeletal muscle. Survival and intraocular growth were observed through the cornea. After maturation in oculo, the double grafts were examined immunohistologically utilizing antisera to neurofilament (NF) and acetylcholinesterase (AChE). The grafts were also evaluated using electrical stimulation to determine functional connectivity. The spinal cord and skeletal muscle grafts were found to exert reciprocal trophic effects on each other, evidenced as a larger muscle mass in skeletal muscle grafts allowed to develop in the presence of spinal cord tissue, and a larger volume of spinal cord grafts allowed to develop together with a skeletal muscle graft, respectively. Immunohistochemistry revealed NF-positive nerve fibers leaving the spinal cord graft and entering the muscle tissue. AChE-positive endplates developed in the muscle grafts. Electrical stimulation of the spinal cord part of double-graft combinations generally elicited contractile responses in specific areas of the muscle cograft. These results demonstrate both structural and functional connections between grafts of spinal cord and skeletal muscle tissue in vivo. The fact that such connections were also established between a mature (adult) spinal cord graft and fetal skeletal muscle tissue suggests that some alpha-motoneurons are able to survive for many months in the intraocular grafts without an appropriate target, and that they are able to subsequently innervate skeletal muscle targets.

Intraocular grafts of fetal rat spinal cord: a Golgi study of neuronal morphology and organization

A modified Golgi staining technique was used to study the neuronal morphology and organization of the fetal rat spinal cord which had developed in the anterior eye chamber of adult rat hosts. Individual neurons in spinal cord grafts displayed profiles which were strikingly similar to previously described spinal cord neurons stained in situ. Further, for the population of 464 graft neurons studied, the distributions of average cell body diameters and numbers of primary dendrites were comparable to those of previously published in situ studies. An internal organization of graft neurons was also observed. Similar-shaped neurons were grouped together, which made it possible to identify different regions within a graft, such that a comparison could be made with normal spinal cord morphology. These observations provide further evidence that the intraocular spinal cord graft is a suitable model for studying spinal cord growth, development, and plasticity and indicate that the Golgi stain may be useful for the quantitative study of concomitant changes in neuronal morphology.

Evidence for functional contact between cografted locus coeruleus and spinal cord in oculo: electrophysiological studies

The functional consequences of the locus coeruleus innervation of the spinal cord are not yet clearly understood. In a recent histological study it was shown that intraocular spinal cord grafts will become innervated by tyrosine hydroxylase-positive nerve fibers from a cografted locus coeruleus. In the present study we use this intraocular model of the descending coeruleo-spinal pathway to investigate functional contact between locus coeruleus and the spinal cord. We have pharmacologically characterized the receptor mediation of norepinephrine-induced, as well as locus coeruleus-mediated depressions of spinal cord neurons grafted in oculo. We found that electrical stimulation of the locus coeruleus part of the double grafts predominantly caused an inhibition of cografted spinal cord neurons. Norepinephrine-induced inhibition of the firing rate of single grafted spinal cord neurons was antagonized by phentolamine, an alpha-adrenergic antagonist, but was unaffected by timolol, a beta-adrenergic antagonist. Similarly, inhibition of the firing rate of grafted spinal cord neurons by stimulation of cografted locus coeruleus was antagonized by phentolamine but not by timolol. Interestingly, single spinal cord grafts were more sensitive to the depressant effects of perfused norepinephrine than was the spinal cord cografted with locus coeruleus. We conclude that spinal cord grafts can be functionally innervated by cografted locus coeruleus and that the noradrenergic inputs to spinal cord from cografted locus coeruleus are alpha-adrenergically mediated. Furthermore, the postsynaptic receptors in single spinal cord grafts appear to be supersensitive to norepinephrine application.

Thyrotropin releasing hormone augments growth of spinal cord transplants in oculo

The effects of thyrotropin releasing hormone (TRH) on spinal cord growth were evaluated using the in oculo transplant model. The growth of fetal spinal cord allografts, placed into the anterior eye chamber of Sprague-Dawley rats, was markedly augmented by acute exposure of the graft and host animal to TRH at the time of transplantation. No significant growth augmentation was seen after equimolar administration of a mixture of the amino acids that comprise the TRH molecule. It is concluded that acutely administered TRH, at the time of grafting, elicits a significant stimulation of the growth of spinal cord tissue. Our data strengthen the rationale for continued clinical trials of this peptide in spinal cord injury.

Expression of eight neuropeptides in intraocular spinal cord grafts: organotypical and disturbed patterns as evidenced by immunohistochemistry

The present study examines the distribution of several neuropeptides, as revealed by immunohistochemistry in the isolated cord. Fetal rat spinal cord was grafted to the anterior chamber of the adult Sprague-Dawley albino rats. After intraocular maturation for 2-3 months, the amount and distribution of somatostatin, neuropeptide Y, substance P, enkephalin, vasoactive intestinal peptide, peptide histidine-isoleucine, calcitonin gene-related peptide and cholecystokinin immunoreactive terminals and cell bodies were analysed using indirect fluorescence immunohistochemistry. The visualization of immunoreactive cell bodies in the grafts was enhanced using a novel intraocular colchicine treatment. In the graft a rich network of somatostatin-positive terminals was found with a high density in well-demarcated areas reminiscent of substantia gelatinosa of the dorsal horn of normal spinal cord. A large number of small- to medium-sized somatostatin neurons was found throughout the grafts without colchicine treatment. This is in contrast to normal spinal cord, where positive neurons were difficult to visualize without colchicine and were mainly confined to the dorsal horn. Neuropeptide Y had a distribution in the grafts similar to that of somatostatin and neuropeptide Y cells were found throughout the grafts without colchicine treatment. In normal spinal cord, neuropeptide Y-positive fibers were found mainly in substantia gelatinosa with a sparse network in the ventral horn. Enkephalin-positive fibers were found throughout the grafts. The distribution of fibers resembled that of somatostatin and neuropeptide Y with distinct zones of high fiber density in well-demarcated areas, whereas the density of nerve fibers in the rest of the graft neuropil was moderate to low. The distribution of substance P was similar to that of enkephalin. After colchicine treatment, both enkephalin- and substance P-positive cell bodies were visualized. In the intact spinal cord both peptides were seen in the entire gray matter with the highest concentrations in the superficial laminae of the dorsal horn. Antisera against calcitonin gene related-peptide, revealed a sparse terminal network and many large cells, which might represent motoneurons. A sparse network of varicose cholecystokinin-immunoreactive fibers was found evenly distributed in the grafts. In normal spinal cord a dense cholecystokinin-positive network of primary sensory afferent origin was found in the dorsal horn. In the grafts cholecystokinin cell bodies were seen after colchicine treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

The innervation of intraocular spinal cord transplants by cografts of locus ceruleus and substantia nigra neurons

The nucleus locus ceruleus and the substantia nigra were sequentially cografted with spinal cord in the anterior chamber of the eye of adult rats. The double grafts were allowed to mature in oculo. After a survival time of 2 months, half of the number of rats were sacrificed, the grafts were dissected out and prepared for tyrosine hydroxylase immunohistochemistry and Falck-Hillarp monoamine histochemistry. The remaining animals were sacrificed after 12-14 months and processed according to the same protocol. No ingrowth from the sympathetic ground plexus in the host iris was seen in single spinal cord grafts at any of the survival times. At the shorter survival time there were no or very few catecholaminergic nerve fibers from the locus ceruleus or substantia nigra that were innervating the spinal cord cograft. In double grafts that were allowed to remain in oculo for 12-14 months, the entire spinal cord graft was innervated with varicose, catecholamine-containing nerve fibers. The locus grafts were able to innervate the spinal cord cografts much more densely than the substantia nigra grafts. In conclusion, the results demonstrate that isolated intraocular spinal cord grafts can be innervated by adjacent central catecholamine neurons. However, this is done in a delayed manner as compared to other models. The isolated replicas of descending adrenergic pathways to the spinal cord thus formed should provide useful information about the function of descending spinal catecholamine pathways.