Influences of both thyroid and bovine growth hormones on substance P, thyrotropin-releasing hormone, serotonin and 5-hydroxyindoleacetic acid contents in the lumbar spinal cord of developing rats.

Neonatal hypothyroidism was induced by injection of 131I on the first living day whilst neonatal hyperthyroidism was induced by daily administration of high doses of thyroxine (T4). Following decapitation, segments of the lumbar spinal cord were microdissected by a punch technique. We measured serotonin and 5-hydroxyindoleacetic acid (5-HIAA) contents by high performance liquid chromatography and both substance P and thyrotropin-releasing hormone (TRH) levels by radioimmunoassay. We demonstrated that: (1) neonatal hyperthyroidism decreased substance P and TRH levels in the dorsal and ventral horns respectively, without modifying serotonin and 5-HIAA contents; (2) neonatal hypothyroidism increased the concentration of substance P in dorsal horn, of TRH in ventral horn (confirming our previous work), of serotonin in ventral horn, and of 5-HIAA in both ventral and dorsal horns; (3) T4-replacement therapy abolishes hypothyroid effects on substance P, TRH, and 5-HIAA, but not on 5-HT accumulation; and (4) bovine growth hormone-replacement therapy has no therapeutic action on the hypothyroid-induced accumulation of substance P, TRH, serotonin and 5-HIAA.