Ghrelin Inhibits Prolactin Secretion in Prepubertal Rats

Ghrelin, a novel 28-amino-acid peptide primarily expressed in stomach and hypothalamus, has recently emerged as the endogenous ligand for the GH-secretagogue receptor with ability to stimulate GH secretion in humans and rats. In addition, ghrelin also stimulates prolactin (PRL) secretion in humans. However, its role in the regulation of PRL secretion in rats remains largely unknown. In this context, the present experiments were carried out to analyze the effects of ghrelin on PRL secretion in male and female rats. In detail, the ontogeny and potential sexual dimorphism in the PRL response to ghrelin was evaluated. In addition, the hypothalamic and/or pituitary site of primary action of ghrelin, as well as the possible interactions between ghrelin and other neurotransmitters, as nitric oxide, dopamine, serotonin or excitatory amino acids, in the precise control of PRL secretion were assessed. Experiments were conducted in prepubertal male and female animals. Systemic (i.p.) and central (i.c.v.) administration of ghrelin significantly inhibited PRL secretion. Such an inhibitory effect became evident after day 10 of age, was similar in males and females, and was also observed in hyperprolactinemic aged female rats. In contrast, however, challenge of pituitary samples in vitro with increasing doses of ghrelin (10^–9–10^–7<i>M</i>) failed to inhibit PRL secretion. Analysis of interactions between ghrelin and other systems involved in the control of PRL secretion revealed that neither blockade of dopaminergic receptors with domperidone, nor enhancement of serotoninergic tone with fluoxetine + 5-hydroxytryptophan altered the inhibitory response to ghrelin in terms of PRL secretion. Similarly, blockade of nitric oxide synthases with <i>L</i>-nitro-arginine-methyl ester failed to modify the magnitude of ghrelin-induced inhibition of PRL secretion, whereas ghrelin was unable to further decrease serum PRL levels after activation of ionotropic excitatory amino acid receptors by administration of NMDA or AMPA. In conclusion, our data indicate that ghrelin is able to inhibit PRL secretion in male and female rats, likely through an extrapituitary primary site of action that is independent of nitric oxide, dopamine, and serotonin systems.