The Development of the Hypothalamo-Pituitary Axis in the Neonatal Rat: Hypothalamic Somatostatin and Pituitary and Serum Growth Hormone Concentrations

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Abstract
Using a specific radioimmunoassay tehcnique for somatostatin (GHRIH), the ontogenesis of hypothalamic GHRIH was studied in relation to pituitary and serum GH concentrations in immature rats. Hypothalamic GHRIH concentration rose from minimal levels of 4.5 .+-. 0.2 pg/.mu.g protein (mean .+-. SEM) at 2 days to peak concentrations of 40.6 .+-. 4.1 pg/.mu.g protein at 28 days followed by a progressive decline toward 50 days (7.0 .+-. 0.8 pg/.mu.g protein). Pituitary GH concentration attained peak prepuberal values of 203.5 .+-. 22.8 ng/.mu.g protein at 16 days with a further marked rise after puberty. Serum GH concentration was elevated at 2 days (53.3 .+-. 5.7 ng/ml) and declined progressively to 5.9 .+-. 1.5 ng/ml at 13 days. There was a highly significant inverse correlation between hypothalamic GHRIH and serum GH concentrations (r = -0.743, P < 0.005). The hypothalamic regulatory mechanism for pituitary GH release develops during the neonatal period of rat and GHRIH may play an important physiological role in this process.