|Weber, Gregory - Greg|
Submitted to: General and Comparative Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2004
Publication Date: 1/7/2005
Citation: Tipsmark, C., Weber, G.M., Strom, C., Grau, E.G., Hirano, T., Borski, R. 2005. Involvement of phospholipase c and intracellular calcium signaling in the gonadotropin-releasing hormone regulation of prolactin release from lactotrophs of tilapia (oreochromis mossambicus). General and Comparative Endocrinology. 142:227-233. Interpretive Summary: Gonadotropin-releasing hormone (GnRH) is a hormone released from the brain that regulates the release of hormones called gonadotropins that regulate the development and function of the gonads. GnRH treatment is used in aquaculture to induce gonad growth and spawning in fish. It has recently been shown that GnRH is also a potent stimulator of secretion of another pituitary hormone, prolactin (PRL), which is known to regulate many important physiological functions in fish including growth, the immune system, and the ability to live in different salinity waters. Therefore, it is important to understand how GnRH affects PRL release and its impact on these other physiological functions, as side effects to the intended use of GnRH to control reproduction. In addition, there is potential to develop methods to control PRL release to aid aquaculture. The first step in understanding the effects of GnRH on PRL actions is to understand how GnRH regulates PRL cell function. The mechanism by which GnRH regulates PRL cell function is poorly understood. Generally, such control is accomplished through signal transduction pathways. The objective of the current studies was to identify signal transduction pathways involved in GnRH-induced release of PRL. We determined GnRH stimulates PRL release through activation of signal transduction pathways which involve increases in phospholipase C (PLC), inositol triphosphate (IP3), and intracellular calcium (Ca2+i). This information can be used to predict complications associated with the use of GnRH in aquaculture and to develop strategies to overcome these complications. It also provides information to possibly develop strategies to use GnRH-induced PRL release in aquaculture, such as for stimulation of the immune system.
Technical Abstract: Gonadotropin-releasing hormone (GnRH) is a potent stimulator of prolactin (PRL) secretion in various vertebrates including the tilapia, Oreochromis mossambicus. The mechanism by which GnRH regulates lactotroph cell function is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated, we examined whether GnRH might stimulate PRL release through an increase in phospholipase C (PLC), inositol triphosphate (IP3), and intracellular calcium (Ca2+i) signaling. Using Ca2+i imaging and the calcium-sensitive dye fura-2, we found that chicken GnRH-II (cGnRH-II) induced a rapid dose-dependent increase in Ca2+i in dispersed tilapia lactotrophs. The Ca2+i signal was abolished by U-73122, an inhibitor of PLC-dependent phosphoinositide hydrolysis. Correspondingly, cGnRH-II-induced tPRL188 secretion was inhibited by U-73122, suggesting that activation of PLC mediates cGnRH-II's stimulatory effect on PRL secretion. Pretreatment with 8-(N,N-diethylamino) octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), an inhibitor of Ca2+ release from intracellular stores, impeded the effect of cGnRH-II on Ca2+i. To further address the possible involvement of intracellular stores, IP3 concentrations in the tilapia rostral pars distalis (RPD containing 95-99% PRL cells) was determined by a radioreceptor assay. We found that GnRH-II induces a rapid (< 5 min) and sustained increase in the IP3 concentration in the RPD. Secretion of tPRL188 in response to cGnRH-II was suppressed by Ca2+ antagonists (TMB-8 and nifedipine). These data, along with our previous findings that show PRL release increases with a rise in Ca2+i, suggest that GnRH may elicit its PRL releasing effect by increasing Ca2+i. Furthermore, the rise in Ca2+i may be derived from PLC/IP3-induced mobilization of Ca2+ from intracellular stores along with influx through L-type voltage-gated Ca2+ channels.