Different effects of melatonin pretreatment on cAMP and LH responses of the neonatal rat pituitary cells.
The rhythm of the pineal hormone melatonin transduces the effect of photoperiod on seasonal functions. Duration of the melatonin pulse provides information about season and the long melatonin pulse induces reproductive involution in the long day breeders such as photoperiodic rodents. The length of melatonin pulse thus carries photoperiodic information, which regulates the function of target cells. Therefore, we have studied the effects of melatonin pretreatment of various lengths on responsiveness of the neonatal rat pituitary cells cultured in vitro to GnRH or forskolin. In these cells, melatonin treatment inhibits the GnRH-induced LH release as well as the forskolin-induced cAMP accumulation. However, long preincubation with melatonin has a paradoxical stimulatory effect on the cellular responsiveness. When the cells are pretreated with melatonin for 16 hr or more, then rinsed thoroughly and treated with forskolin for 30 min, the increase of cAMP is potentiated. Moreover, in the melatonin-pretreated cells. the subsequent melatonin treatment inhibits the forskolin-induced cAMP accumulation relatively more than in the non-pretreated cells. Although melatonin pretreatment does not potentiate the GnRH-induced LH release, it protects the gonadotrophs against the GnRH-induced desensitization: pretreatment with GnRH for 12 hr or more renders the cells insensitive to subsequent GnRH stimulation, while after pretreatment with GnRH and melatonin, the subsequent GnRH treatment induces significant increase of LH release. These observations indicate that long pretreatment with melatonin improves responsiveness of the pituitary cells to the subsequent stimulation, but its effects on cAMP accumulation and LH release are different.
Pelísek
V
V
Institute of Physiology, Academy of Sciences of the Czech Republic, Prague.
Vanecek
J
J
D000818:Animals; D000831:Animals, Newborn; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D007986:Luteinizing Hormone / Q000378:metabolism*; D008550:Melatonin / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar
D005576:Colforsin; D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D008550:Melatonin