The Effects of Serotonin on Prolactin and Growth Hormone Concentrations in Normal and Pituitary Stalk- Sectioned Monkeys*

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Abstract
The effects of serotonin on PRL and GH secretion were studied in normal and pituitary stalk-sectioned female rhesus monkeys. Serotonin was administered iv at doses of 50, 500, and 5000 μg. Pretreatment concentrations of serum PRL were elevated in stalk-sectioned monkeys compared to normal monkeys [34 ± 5 vs. 3 ± 1 ng/ml (mean ± SEM)], while serum GH concentrations were lower in the stalk-sectioned animals (vs. 1.3 ± 0.2 ng /ml). In both normal and stalk-sectioned monkeys, the 50-μg dose of serotonin failed to alter PRL concentrations. However, with the 500-μg dose, PRL rose from 3 ± 1 to 22 ± 6 ng /ml in normal monkeys and from 27 ± 6 to 57 ± 10 ng / ml in stalk-sectioned monkeys. Likewise, with the 5000- μg dose, PRL rose from 4 ± 2 to 82 ± 27 and from 30 ± 6 to 75 ± 26 ng / ml in the two respective groups. No dose of serotonin stimulated GH secretion in stalk-sectioned monkeys, although GH did increase from approximately 2 to 4 –11 ng /ml in normal monkeys. Since the pituitary is devoid of direct hypothalamic influences in the pituitary stalk-sectioned animals, these results suggest that serotonin can modulate PRL secretion either directly at the pituitary level or via some yet to be determined peripheral mechanism. In contrast, this neurotransmitter appears to incorporate hypothalamic factors in its modulation of GH secretion.