Direct Pituitary Effects of Kisspeptin: Activation of Gonadotrophs and Somatotrophs and Stimulation of Luteinising Hormone and Growth Hormone Secretion

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Abstract
Recent, compelling evidence indicates that kisspeptins, the products ofKiSS‐1gene, and their receptor GPR54, represent key elements in the neuroendocrine control of reproduction, and that they act primarily by regulating gonadotrophin‐releasing hormone (GnRH) secretion at the hypothalamus. Conversely, and despite earlier reports showing GPR54 expression in the pituitary, the potential physiological roles of kisspeptins at this gland have remained elusive. To clarify this issue, cultures of rat pituitary cells were used to evaluate expression of KiSS‐1 and GPR54, and to monitor the ability of kisspeptin‐10 to stimulate Ca2+responses in gonadotrophs and to elicit luteinising hormone (LH) secretionin vitro. The results obtained show that both GPR54 and KiSS‐1 are expressed in the pituitary of peripubertal male and female rats. Moreover, kisspeptin‐10 induced a rise in free cytosolic Ca2+concentration ([Ca2+]i) in approximately 10% of male rat pituitary cells. Intriguingly, kisspeptin‐responsive cells included not only gonadotrophs, in which a 62.8 ± 16.0%[Ca2+]irise was observed, but also somatotrophs, wherein kisspeptin induced a 60.3 ± 5.5%[Ca2+]iincrease. Accordingly, challenge of dispersed pituitary cells with increasing kisspeptin‐10 concentrations induced dose‐related LH and growth hormone (GH) secretory responses, which were nevertheless of lower magnitude than those evoked by the primary regulators GnRH and GH‐releasing hormone, respectively. In particular, 10−8 M kisspeptin caused maximal increases in LH release (218.7 ± 23.6% and 180.4 ± 7.2% in male and female rat pituitary cells, respectively), and also stimulated maximally GH secretion (181.9 ± 14.9% and 260.2 ± 15.9% in male and female rat pituitary cells, respectively). Additionally, moderate summation of kisspeptin‐ and GnRH‐induced LH responses was observed after short‐term incubation of male rat pituitary cells. In conclusion, our results provide unequivocal evidence that kisspeptins exert direct pituitary effects in peripubertal male and female rats and suggest a possible autocrine/paracrine mode of action. The precise relevance and underlying mechanisms of this potential new actions of kisspeptins (i.e. the direct modulation of gonadotrophic and somatotrophic axis at the pituitary) deserve further analysis.

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