LH-releasing activity of hypothalamic extracts not attributable to GnRH

LH secretion is under hypothalamic control, mostly through GnRH. Hypothalamic extracts stimulate LH release in vitro, an action usually attributed to the presence of GnRH. However, the presence of other LH-releasing factors cannot be ruled out since there are other neuromediators endowed with the capacity to influence LH release. We analyzed the presence of an LH-releasing activity in bovine median eminence extracts (ME extracts) different from GnRH using cultured rat pituitary cells. Both, GnRH and ME extracts caused a dose-dependent stimulation of LH release. The maximum response obtained with ME extracts (up to 10-fold the respective baseline) was significantly greater than the maximum response obtained with GnRH (up to 5-fold the respective baseline). Monoclonal or polyclonal GnRH antibodies in the solid phase completely eliminated the LH-releasing activity of 10(-7) M GnRH; in contrast, they only partially removed the activity present in ME extracts. The GnRH receptor blocker, Nal-Glu, 10(-7) M, which completely blocked the effect of GnRH could not totally suppress the LH-releasing activity of ME extracts. These findings indicate that ME extracts contain, in addition to GnRH, an LH-releasing activity not attributable to GnRH because it differs in its immunoreactivity and in its ability to stimulate LH release when GnRH receptors are blocked.