Supraspinal fiber outgrowth and apparent synaptic remodelling across transected-reconstructed feline spinal cord

Following complete transection of the spinal cord at T9, 12 cats were separated into two groups: Group 1 received a collagen matrix (CM) treated with a neuroactive agent or with saline to bridge the spinal cord stumps and an omental transposition which was placed on the dorsal surface of the matrix; Group 2 received spinal cord transection only. Two cats received no spinal cord transection. After 90 days, all animals were killed and their brains and spinal cords were removed for immunohistochemical examination. Two weeks prior to sacrifice, spinal cord blood flows (SCBF) were measured and the retrograde axonal tracer Fluoro-Gold was injected below the transection site. Results show that omental transposition to the CM bridge in Group 1 animals increased SCBF an average 59% (assessed by clamping the omental blood supply to the cord). Examination of the brain 90 days after cord transection revealed Fluoro-Gold accumulation in the cytoplasm and processes of neurons located in the brainstem, midbrain, and diencephalic region which are known to contribute pathways to the spinal cord. Immunohistochemical staining with antibodies against the catecholamine synthesizing enzymes tyrosine hydroxylase and dopamine-B-hydroxylase, indicated that only Group I treated cats developed dense bundles of dopaminergic and noradrenergic fibers within the CM bridge and distal spinal cord tissue. These fibers were seen to extend 90 mm below the transection site. In addition, the synaptogenic marker synaptophysin (SYN) was observed in association with dopaminergic and noradrenergic fibers distal to the collagen matrix bridge, an indication that synaptic remodelling (regeneration) by previously denervated supraspinal axons may have occurred. Immunostaining for glial fibrillary acidic protein (GFAP) showed little to none reactive astrocytosis near the transection site of cats treated with the CM and omentum transposition (Group 1). No catecholaminergic fibers or SYN expression below the transection site were observed in Group 2 treated cats. Group 2 treated cats also showed dense immunostaining of GFAP near the transection site indicating significant astrocytic proliferation. These findings indicate that following complete spinal cord transection in cats and reconstruction with a treated collagen matrix and omental transposition, disconnected supraspinal fibers have the ability to regenerate for long anatomic distances and seemingly engage in synaptic remodelling with distal target tissue.