TrkA, the nerve growth factor (NGF) tyrosine kinase receptor, is expressed
not only in the nervous system, but also in nonneural cells, including disc
rete cellular subsets of the endocrine and immune system. In the rat ovary,
trkA receptor abundance increases strikingly in thecal-interstitial cells
during the hours preceding the first ovulation. Blockade of either trkA tra
nsducing capacity or NGF biological activity inhibited ovulation, suggestin
g a role for NGF in the ovulatory process of this species. To identify some
of the processes that may be affected by trkA activation in the thecal com
partment, we used purified thecal cells/thecal fibroblasts from bovine ovar
ies (heretofore referred to as thecal cells). Ribonuclease protection assay
s employing bovine-specific cRNA probes demonstrated the presence of the me
ssenger RNAs (mRNAs) encoding NGF and its receptors, p75 NTR and trkA, in t
he thecal compartment of small, medium, and large antral follicles and show
ed that trkA mRNA is also expressed in granulosa cells. In situ hybridizati
on and immunohistochemical examination of intact ovaries confirmed these ce
llular sites of NGF and trkA synthesis. TrkA mRNA, but not NGF mRNA, was lo
st within 48 h of placing thecal cells in culture. Thus, to study trkA-medi
ated actions of NGF on these cells me transiently expressed the receptor by
transfection with a vector containing a full-length rat trkA complementary
DNA under transcriptional control of the cytamegalovirus promoter. Because
ovulation is preceded by an LH-dependent increase in androgen and progeste
rone production, the ability of NGF to modify the release of these steroids
was determined in freshly plated cells still containing endogenous trkA re
ceptors and in cells undergoing luteinization in culture that were transien
tly transfected with the trkA-encoding plasmid. NGF stimulated bath androge
n and progesterone release in freshly plated thecal cells, but not in lutei
nizing cells provided with trkA receptors. As ovulation in rodents requires
an increased formation of PGE, and has been shown to be antedated by proli
feration of thecal fibroblasts, we determined the ability of NGF to affect
these parameters in trkA-transfected thecal cells. The neurotrophin rapidly
stimulated PGE, release and amplified the early steroidal response to hCG
in trkA-expressing cells, but not in cells lacking the receptor. Likewise,
NGF stimulated [H-3]thymidine incorporation into trkA-containing cells, but
not into cells that had lost the receptor in culture. Induction of ovulati
on in immature rats by gonadotropin treatment verified that an increased ce
ll proliferation in the thecal compartment, determined by the incorporation
of bromodeoxyuridine into cell nuclei, occurs 4-5 h before ovulation in th
is species. These results suggest that the contribution of NGF to the ovula
tory process includes a stimulatory effect of the neurotrophin on steroidog
enesis, PGE(2) formation, and proliferative activity of thecal compartment
cells.