Abstract

The concentration of growth hormone (GH) in serum of neonatal rats was determined by radioimmunoassay after administration of various drugs that after serum GH in adult rats. Administration of the serotonin precursor 5-hydroxytryptophan or morphine increased serum GH in pups of all ages. The 5-hydroxytryptophan-induced rise was prevented by the serotonergic antagonist cyproheptadine but not by the dopaminergic antagonist haloperidol. Pentobarbital decreased serum GH in 5-day-old pups but increased GH in 15-, 20- and 35-day-old pups. Clonidine decreased serum GH in 5- and 10-day-old pups and did not increase serum GH until pups were 35 days old. Finally, significant ultradian surges of GH secretion were observed in 35-day-old animals but only slight secretory episodes occurred in 24-day-old pups. These results suggest that the serotonergic and endogenous opiate systems involved in regulation of GH secretion in rats are functional within the first postnatal week. In contrast, the extrahypothalamic mechanisms through which pentobarbital acts do not mature until 2 to 3 weeks after birth. Finally, the inability of clonidine to stimulate GH secretion and the absence of significant ultradian surges before day 35 suggest that the noradrenergic mechanism and possibly other neural systems which trigger the surges of GH secretion observed in adult rats do not mature until after puberty.