Catecholamine enzymes and neuropeptides are expressed in fibres and somata in the intermediate gray matter in chronic spinal rats

Abstract

Spinal cord injury disrupts control of sympathetic preganglionic neurons because bulbospinal input has been lost and the remaining regulation is accomplished by spinal circuits consisting of dorsal root afferent and spinal neurons. Moreover, an initial retraction and regrowth of dendrites of preganglionic neurons in response to deafferentation creates the potential for remodelling of spinal circuits that control them. Although catecholamines and neuropeptide Y are found in descending inputs to the preganglionic neurons, their presence in spinal circuits has not been established. Spinal circuits controlling preganglionic neurons contain substance P but participation of these peptidergic neurons in remodelling responses has not been examined. Therefore, we compared immunoreactivity for the catecholamine-synthesizing enzyme dopamine β-hydroxylase, for neuropeptide Y and for substance P in the intermediate gray matter of the spinal cord in control rats and in rats seven or fourteen days after transection at the fourth thoracic cord segment. Sympathetic preganglionic neurons were retrogradely labelled by intraperitoneal injection of the tracer FluoroGold. These experiments yielded three original findings. 1) At one and two weeks after cord transection, fibres and terminals immunoreactive for dopamine β-hydroxylase and neuropeptide Y were consistently found in the intermediolateral cell column in segments caudal to the transection. The area of fibres and terminals containing these immunoreactivities was markedly reduced compared to control rats or to segments rostral to the transection in the spinal rats. 2) Immunoreactivity for substance P was increased after cord transection and the distribution of fibres immunoreactive for this peptide in segments caudal to the transection extended more widely through the intermediate gray matter. These reactions demonstrated a plastic reaction to cord transection by spinal neurons expressing substance P. 3) Dopamine β-hydroxylase expression was up-regulated in somata within the intermediate gray matter of spinal segments caudal to the transection. The numbers of somata immunoreactive for this enzyme increased six-fold by fourteen days after cord transection, compared to the few somata counted in control rats.In conclusion, the presence of a catecholamine synthesizing enzyme and neuropeptides in fibres surrounding sympathetic preganglionic neurons caudal to a cord transection suggests a source of catecholamines and these peptides within spinal circuits in the chronic spinal rat. The presence of dopamine β-hydroxylase in a markedly greater number of neuronal somata after cord transection reflects significant up-regulation of gene expression and may indicate a switch by these neurons to an adrenergic phenotype, revealing a plastic response to injury within the spinal cord.