CHANGES IN IMMUNOREACTIVITY FOR GROWTH-ASSOCIATED PROTEIN-43 SUGGEST REORGANIZATION OF SYNAPSES ON SPINAL SYMPATHETIC NEURONS AFTER CORD TRANSECTION

Cervical or high thoracic spinal cord injury often results in autonomi c dysreflexia, a condition characterized by exaggerated spinal reflexe s and episodic hypertension, that may be caused by reorganization of s ynapses on sympathetic preganglionic neurons after loss of supraspinal input. To assess remodelling of synaptic input to identified pregangl ionic neurons, immunoreactivity for growth associated protein-43 was e xamined by fluorescent and electron microscopy in control rats with in tact spinal cords and in rats seven to 30 days after midthoracic cord transection. This protein is found in mature bulbospinal axons that su pply spinal sympathetic nuclei and it is also known to be up-regulated in growing or sprouting axons. In the thoracic cord of control rats, fibres containing growth associated protein-43 surrounded histochemica lly-or retrogradely-labelled preganglionic neurons and formed a ladder -like pattern in the gray matter. Fibres travelled rostrocaudally alon g the lateral horn and, at approximately regular intervals, they cours ed mediolaterally to form the ''rungs'' of a ladder. Electron microsco py revealed concentrated growth associated protein-43 in many intervar icose axon segments in the intermediolateral cell column. Less frequen tly, faint immunoreactivity for this protein was found in varicosities , some of which synapsed on retrogradely-labelled sympathoadrenal preg anglionic neurons. Electron microscopy of conventionally processed tis sue was used to determine the lime-course of degeneration of severed a xon terminals in the intermediolateral cell column. In spinal rats, te rminals with ultrastructural signs of degeneration were numerous in th e intermediolateral cell column three days after transection, but were rare at seven days and absent at 14 days. Degenerating terminals were never found in this region in control rats. Thus virtually all supras pinal inputs to preganglionic neurons had been eliminated by seven day s after transection. At longer times after injury, terminals containin g immunoreactivity for growth associated protein-43 must therefore ari se from intraspinal neurons. The distribution of fibres immunoreactive for growth associated protein-43 changed markedly in the first 30 day s after cord transection. By 14 days, the ladder-like pattern was dist orted rostral to the transection by enlarged masses of immunoreactive fibres surrounding preganglionic neurons, suggesting sprouting of bulb ospinal or intraspinal axons or accumulation of this protein in their terminals after the parent axon had been severed. Caudal to the transe ction, the ladder-like arrangement of fibres was completely replaced b y a reticular network of immunoreactive fibres that extended throughou t the intermediate gray matter and increased in density between 14 and 30 days. In the intermediolateral cell column, at fourteen days after transection, axons with the ultrastructural features of growth cones contained intense growth associated protein-43 immunoreactivity. Altho ugh varicosities of bulbospinal axons containing this protein had dege nerated by 14 days, weak immunoreactivity was still found in Varicosit ies that synapsed on labelled sympathoadrenal neurons. Furthermore, im munoreactivity appeared in numerous somata of presumed interneurons th roughout the intermediate gray matter by 14 days and the number of som ata increased by 30 days. These interneurons may be the source of this protein in the reticular network, and in growth cones and synapses. T he loss of supraspinal inputs by seven days after cord transection, an d the new intraspinal network of immunoreactive fibres, synapses and c ells are consistent with new synapse formation on preganglionic neuron s. New synapses on preganglionic neurons may be crucial for the develo pment of autonomic dysreflexia. (C) 1997 IBRO. Published by Elsevier S cience Ltd.