Relationship between Hypothalamic Serotoninergic Activity and Prolactin and Growth Hormone Secretion in the Domestic Cockerel

Cockerels were injected with drugs which affect brain serotoninergic activity. Concentrations of plasma prolactin and growth hormone have been measured and correlated with hypothalamic serotonin and 5-hydroxyindole acetic acid (5HIAA) levels. Tryptophan concentrations in the brain and the activity of monoamine oxidase (MAO types A and B) were also measured in some studies. The administration of the MAO inhibitor, pargyline, produced dose- and time-related reductions in brain MAO type A and B activities, hypothalamic 5HIAA concentrations and plasma growth hormone levels, but increased the hypothalamic serotonin and plasma prolactin concentrations. Clorgyline administration inhibited MAO type A (but not type B) activity and also increased hypothalamic serotonin and plasma prolactin levels, while reducing hypothalamic 5HIAA and circulating growth hormone concentrations. Deprenyl treatment inhibited MAO type B (but not type A) activity but did not significantly affect serotonin, 5HIAA, prolactin or growth hormone levels. The serotonin precursor, tryptophan, elevated brain tryptophan levels when given systemically. It also increased hypothalamic serotonin in a dose-related manner, increased plasma prolactin and reduced plasma growth hormone concentrations. Pargyline and tryptophan treatments affected hormone levels more markedly in 3-week-old than in 18-week-old cockerels. These results demonstrate a strong relationship between hypothalamic serotoninergic activity, MAO type A activity and the secretion of prolactin and growth hormone in the cockerel.