Comparative effects of neonatal hypothyroidism and euthyroidism on TRH and substance P content of lumbar spinal cord in saline and PCPA-treated rats

The effects of neonatal thyroidectomy and thyroid hormone replacement therapy on the content of substance P and TRH in the lumbar segment of the rat spinal cord were studied. The peptide content of discrete spinal cord regions removed by punches of frozen serial slices was measured by RIA. Animals receiving T4 replacement therapy were indistinguishable from normal littermates. In hypothyroid animals without PCPA-treatment, levels of TRH and substance P were significantly increased by 100% in the ventral and the dorsal lumbar spinal cord, respectively. Inhibition of serotonin biosynthesis by PCPA increased by 90% the substance P content in the dorsal horn of euthyroid rats and abolished completely the stimulatory effect of hypothyroidism on the TRH content of the ventral horn. These findings suggest the existence of a physiological relationship between substance P and TRH with the serotoninergic system in the rat spinal cord and that thyroid hormone is implicated in the normal development of the peptide-containing neurons in the rat spinal cord.