CATECHOLAMINE ENZYMES AND NEUROPEPTIDES ARE EXPRESSED IN FIBERS AND SOMATA IN THE INTERMEDIATE GRAY-MATTER IN CHRONIC SPINAL RATS

Spinal cord injury disrupts control of sympathetic preganglionic neuro ns because bulbospinal input has been lost and the remaining regulatio n is accomplished by spinal circuits consisting of dorsal root afferen t and spinal neurons. Moreover, an initial retraction and regrowth of dendrites of preganglionic neurons in response to deafferentation crea tes the potential for remodelling of spinal circuits that control them . Although catecholamines and neuropeptide Y are found in descending i nputs to the preganglionic neurons, their presence in spinal circuits has not been established. Spinal circuits controlling preganglionic ne urons contain substance P but participation of these peptidergic neuro ns in remodelling responses has not been examined. Therefore, we compa red immunoreactivity For the catecholamine-synthesizing enzyme dopamin e beta-hydroxylase, for neuropeptide Y and for substance P in the inte rmediate gray matter of the spinal cord in control rats and in rats se ven or fourteen days after transection at the fourth thoracic cord seg ment. Sympathetic preganglionic neurons were retrogradely labelled by intraperitoneal injection of the tracer FluoroGold. These experiments yielded three original findings. I) At one and two weeks after cord tr ansection, fibres and terminals immunoreactive for dopamine beta-hydro xylase and neuropeptide Y were consistently found in the intermediolat eral cell column in segments caudal to the transection. The area of fi bres and terminals containing these immunoreactivities was markedly re duced compared to control rats or to segments rostral to the transecti on in the spinal rats. 2) Immunoreactivity for substance P was increas ed after cord transection and the distribution of fibres immunoreactiv e for this peptide in segments caudal to the transection extended more widely through the intermediate gray matter. These reactions demonstr ated a plastic reaction to cord transection by spinal neurons expressi ng substance P. 3) Dopamine beta-hydroxylase expression was up-regulat ed in somata within the intermediate gray matter of spinal segments ca udal to the transection. The numbers of somata immunoreactive for this enzyme increased six-fold by 14 days after cord transection, compared to the few somata counted in control rats. In conclusion, the presenc e of a catecholamine synthesizing enzyme and neuropeptides in fibres s urrounding sympathetic preganelionic neurons caudal to a cord transect ion suggests a source of catecholamines and these peptides within spin al circuits in the chronic spinal rat. The presence of dopamine beta-h ydroxylase 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, rev ealing a plastic response to injury within the spinal cord. (C) 1997 I BRO.