GnRH Signaling in Goldfish Pituitary Cells

Abstract

In goldfish, maturational gonadotropin (GTH) and growth hormone (GH) release are stimulated by two native GTH-releasing hormones (sGnRH and cGnRH-II). Both GnRHs stimulate GTH and GH release via activation of phospholipase C, protein kinase C, Ca²⁺ entry through voltage-sensitive channels and calmodulin. However, sGnRH-induced GTH release also involves arachidonic acid and intracellular Ca²⁺ components absent from its action on GH, as well as from cGnRH-II action on GTH and GH secretion. The relative roles and interactions of these signaling pathways in mediating sGnRH and cGnRH-II action on acute and prolonged GTH and GH release are compared. How two GnRHs bind to similar receptors but induce similar and dissimilar transduction mechanisms in two cell types and within one cell type is unknown.

Signal transduction mechanisms mediating secretion in goldfish gonadotropes and somatotropes

“…Results from studies on the brain distribution of GnRH protein and mRNA also support the view that both peptides act as hypophysiotropic neurohormones in goldfish (reviewed in Yu et al 1998). The presence of GnRH receptors on both GTH-II and GH cells, the ability of sGnRH and cGnRH-II to stimulate GTH-II and GH release from pituitary cell preparations, as well as correlative changes in brain GnRH content during physiological modulations of serum GTH-II and GH levels, are consistent with a role of both sGnRH and cGnRH-II as neuroendocrine regulators of gonadotrope and somatotrope functions (reviewed in Chang et al 1996b;Peng and Peter 1997). It has been suggested that GTH-II and GH cells possess different GnRH receptors, or suites of receptors (reviewed in Chang et al 1996b;Illing et al 1999).…”

“…The presence of GnRH receptors on both GTH-II and GH cells, the ability of sGnRH and cGnRH-II to stimulate GTH-II and GH release from pituitary cell preparations, as well as correlative changes in brain GnRH content during physiological modulations of serum GTH-II and GH levels, are consistent with a role of both sGnRH and cGnRH-II as neuroendocrine regulators of gonadotrope and somatotrope functions (reviewed in Chang et al 1996b;Peng and Peter 1997). It has been suggested that GTH-II and GH cells possess different GnRH receptors, or suites of receptors (reviewed in Chang et al 1996b;Illing et al 1999). Recently, cDNAs encoding two goldfish GnRH receptors have been cloned, and their presence in the pituitary confirmed (Yu et al 1998;Illing et al 1999).…”

“…The long-term GTH-II response to cGnRH-II is more sensitive to (and often abolished by) the reduction of [Ca 2+ ] in the incubation medium and treatments with VSCC inhibitors, than that elicited by sGnRH (reviewed in Chang et al 1996b;1997). These results suggest that prolonged cGnRH-II-induced GTH-II release is more dependent on the availability of extracellular Ca 2+ .…”

“…Pharmacological studies indicate that prolonged sGnRH and cGnRH-II stimulation of GH secretion also involve extracellular Ca 2+ entry through VSCC present on identified goldfish somatotropes (reviewed in Chang et al , 1996bFig. 2).…”

“…Emphasis is placed on more current developments in these areas since these topics were last reviewed Chang et al 1996bChang et al , 1997. Recent results on the signal transduction mechanisms mediating the effects of SRIF, NE, and PACAP on GH secretion are also presented and reviewed.…”