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

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.

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.

Ultrastructure of spinal cord grafts with and without cografts of locus coeruleus in oculo

The ultrastructure of spinal cord and spinal cord-locus coeruleus double grafts transplanted to the anterior chamber of the eye of adult rats was studied. The present results show that single spinal cord grafts express several morphologically organotypical characteristics of normal spinal cord at the ultrastructural level. Thus, the parenchyma was divided into a cell-rich layer corresponding to the normal gray matter and an axonal layer with a large amount of myelinated fibers and immature oligodendrocytes. In the cellular layer, a variety of cell types of different sizes were observed. The neurons had different patterns of cytoplasmic organelles, including Nissl bodies, Golgi apparatus, mitochondria, and polysomes. The Nissl substance was variable and some neurons appeared to be immature. Although the spinal cord grafts are in a state of relative gliosis, surrounded by a glial barrier, cografted fetal locus coeruleus catecholamine neurons are able to innervate the spinal cord grafts and form anatomically relevant synapses with the spinal cord neuronal elements as revealed by TH-immunoelectron microscopy. In conclusion, several organotypical features of normal spinal cord are found. Examples also were found, however, of a disturbed and delayed development that have to be considered when evaluating the functional potential of grafted cells.