Increasing evidence exist that multiple G proteins mediate the effects of gonadotropin-releasing
hormone (GnRH) on the synthesis and release of pituitary gonadotropins. In the present
study, we have expressed the rat GnRH receptor (GnRH-R) in insect cells, by infection
with a recombinant baculovirus. Under the conditions used, insect cells expressed,
48 h post-infection, a maximum of 7800 +/- 650 receptors/cell which bound GnRH agonist
[D-Trp6]GnRH with a Kd = 0.52 +/- 0.06 nM indicating characteristics similar to those
of the natural receptor. No binding was observed in non-infected cells or cells infected
with wild-type baculovirus. In presence of GnRH, GnRH-R expressing cells elicited
a time- and dose-dependent production of inositol trisphosphate, with a maximum level
reached within 30 min and an EC50 = 5 nM. These recombinant insect cells also produced
cAMP in response to GnRH. However, in contrast to other heterologous systems, or rat
pituitary gonadotropes wherein GnRH induced a weak and delayed elevation of cAMP,
in insect cells the rise of cAMP was comparatively rapid, attaining a maximum level
after 2 h, and the EC50 was 5 nM. Finally, a clear activation of adenylyl cyclase
(AC) in response to GnRH was shown for the first time by measuring the conversion
of [alpha-32P]ATP into labeled cAMP, using membrane preparations from GnRH-R expressing
insect cells. These data demonstrate that rat GnRH-R has the potential for dual coupling
to both phosphoinositidase C and AC and suggest a major influence of the host cell
for this coupling and/or its expression, probably in relation with the G protein repertoire
and preference. This notion could be extended to several target cells other than pituitary
gonadotropes that normally express the GnRH-R in mammals, including hippocampal, Leydig,
granulosa, placental and GnRH-secreting hypothalamic cells.