Reciprocal cross talk between gonadotropin-releasing hormone (GnRH) and prostaglandin receptors regulates GnRH receptor expression and differential gonadotropin secretion.

Because COX-2 is the key enzyme in the eicosanoid synthesis pathway, we looked for activation of COX-2 by GnRH. GnRH produced a marked time-dependent induction of COX-2 enzyme expression in the gonadotrope cell line L β T2 ( 13 ), as revealed by RT-PCR analysis ( Fig. 1A ). Preincubation of the cells with the selective MEK inhibitor PD98059 abolished the stimulatory effect of GnRH on COX-2 induction, indicating that activation of COX-2 by GnRH is mediated by ERK. Further quantitative PCR (Q-PCR) revealed a marked induction of COX-2 by GnRH (40-fold; basal and GnRH-stimulated levels were 0.12 and 4.8, respectively, n = 3) ( Fig. 1B ). This stimulatory effect was reduced by the selective inhibitors for MEK (PD98059), JNK (JNKI), p38 (SB203580), phosphatidylinositol 3′-kinase (PI3K) (wortmannin), protein kinase C (PKC) (GF109203X), and c-Src (PP2), with no significant effect on basal levels. The data implicate the known GnRH-activated PKC/c-Src/MAPK pathway ( 14 - 17 ) in GnRH induction of COX-2 ( Fig. 1B ). The substantial inhibition (50–75%) by all the inhibitors implies that the various MAPK cascades do not converge on the same signaling molecules, but rather act in parallel pathways to activate transcription factors, which act as a composite response element during activation of COX-2 gene expression. The epidermal growth factor (EGF) receptor kinase inhibitor, AG1478, had no significant effect on GnRH induced COX-2 expression. This accords with our observation that activation of MAPK by GnRH in gonadotropes is not mediated by transactivation of the EGF receptor ( 18 , 19 ). The selective inhibitors for iPLA 2 [bromoenol lactone (BEL)] and for sPLA 2 [thioetheramide-PC (TE-PC)], but not that for cPLA 2 (AACOCF3), reduced COX-2 induction by GnRH, suggesting that the two PLA 2 isoforms may also be involved in GnRH induction of COX-2. To examine the effect of GnRH on COX-2 promoter activity, L β T2 cells were transiently transfected with the COX-2 promoter and treated with GnRH for various time periods ( Fig. 1C ). Activation was rapid and reached maximal activation after 8 h of stimulation with GnRH (3-fold, P < 0.01), with a decline to basal levels at 24 h. An increase in COX-2 mRNA stability by GnRH may explain the discrepancy between the degree of mRNA induction (40-fold) and the degree and kinetics of promoter activity (3-fold). It is also possible that the transfected promoter construct lacks the full complement of regulatory elements. COX-2 protein expression was similarly regulated by GnRH; maximal protein expression occurring at 8 h (2.5-fold; P < 0.05) ( Fig. 1D ). It is interesting to note that, whereas COX-2 protein levels have declined by 24 h of GnRH stimulation, mRNA levels remained high. This suggests that there is an increased rate of protein degradation and/or decreased translation. To determine whether gonadotropes could elaborate prostanoids, we incubated the L β T2 cells with GnRH for up to 24 h, with and without AA as substrate for the last 30 min of the incubation period. Incubation with GnRH alone for up to 24 h had no effect on PG production but addition of AA for the last 30 min of the incubation period resulted in a 2.5- to 3-fold increase ( P < 0.05) in PGE 2 , PGI 2 , and PGF 2 α production ( Fig. 2 ). Addition of AA alone had no effect on PG production (data not shown). Addition of AA had previously been shown to be required to demonstrate PG production (Ref. 20 and references therein). After biosynthesis, prostanoids are transported out of the cell and act in an autocrine/paracrine manner via cognate GPCRs ( 11 ). We therefore examined prostanoid receptor expression in the immortalized gonadotrope L β T2 cell line ( 13 ), which serve as a gonadotrope cell model to dissect the signaling of the GnRHR. Specific staining for EP1, EP2, EP3, EP4, FP, and IP was observed in L β T2 cells ( Fig. 3A ). Specificity was demonstrated by preabsorption of the antibodies with the specific peptides used for the immunization (C). The expression of EP1, EP2, EP3, EP4, FP, and IP in L β T2 cells was further demonstrated by RT-PCR ( Fig. 3B ). The known marked homologous induction of the GnRHR promoter construct by GnRH itself ( 21 ) was significantly reduced by PGF 2 α and the stable analog of PGI 2 , iloprost, but not by PGE 2 ( Fig. 4A ). Furthermore, cotreatment of L β T2 cells with GnRH and PGF 2 α or iloprost for 6 h reduced GnRH binding [2180 ± 437 counts per minute (cpm) and 2587 ± 454 cpm respectively; sem , n = 3, P < 0.01] when compared with binding in cells incubated with GnRH alone (6134 ± 737 cpm; sem , n = 3), or with GnRH and PGE 2 (6254 ± 1022 cpm; sem , n = 3). To elucidate the mechanisms involved in the inhibitory action of the PGs we first investigated whether they compete with the binding of GnRH for its cognate receptor. As shown in Fig. 4B , GnRH binding to L β T2 cells was not affected by the presence of PGE 2 , PGF 2 α or iloprost (each at 500 n m ). We therefore proceeded to examine initial signaling events after receptor binding. The mechanism of GnRH stimulation of the GnRHR promoter is known to be via activation of its predominant signaling pathway (G α q and PLC- β ) ( 21 ). We confirmed this by demonstrating that GnRH stimulation of the GnRHR promoter was progressively inhibited by increasing concentrations of the PLC- β inhibitor (U73122) ( Fig. 4C ) and that this correlated well with the concomitant decrease in InsP production ( Fig. 4D ). Thus, it was feasible that the inhibitory effects of PGF 2 α and iloprost on the GnRHR promoter were mediated via an inhibition of InsP production. This notion is supported by our demonstration that PGF 2 α and iloprost inhibited GnRH stimulation of InsP formation but had no significant effect on basal levels, whereas PGE 2 was ineffective ( Fig. 4E ). To determine whether prostanoids affect the GnRH transcriptional regulation of gonadotropin subunit genes ( α GSU, LH β , and FSH β ). L β T2 cells were transfected with α GSU, LH β , or FSH β promoters driving a luciferase reporter ( Fig. 5 ). The exogenous addition of the PGs had no significant effect on GnRH induction of α GSU ( Fig. 5A ) and FSH β ( Fig. 5C ) genes. However, PGF 2 α but not PGE 2 or iloprost, reduced GnRH activation of the LH β promoter ( Fig. 5B; P < 0.05). We examined the expression of the various prostanoid receptors in rat pituitaries by immunohistochemistry and immunofluorescent microscopy using confocal microscopy ( Fig. 6 ). Specific expression of a given receptor was followed by colocalization with pituitary hormones for potential cell-specific expression. Specific staining for receptors for PGE 2 (EP1, EP2, EP3, and EP4), for PGF 2 α (FP) and PGI 2 (IP) were demonstrated by immunohistochemistry. Negative controls included omission of the first antibody, pretreatment with preimmune serum and peptide immunogen competition. EP1 and EP2 staining colocalized to LH-containing gonadotropes, which comprise about 10–15% of the total cell population in the adult rat pituitary ( 22 ) ( Fig. 6 ). EP3 and EP4 colocalized to prolactin-containing cells (mammotropes) and the GH-containing cells (somatotropes), respectively, which comprise about 20–30% and 40% of the total cell population in the adult rat pituitary ( 22 ) (data not shown). The PGI 2 receptor IP and the PGF 2 α receptor FP were found to colocalize to the gonadotropes ( Fig. 6 ). Because prostanoids affect gonadal function ( 23 ) and GnRH secretion ( 24 ), it is not feasible to study their direct effects on pituitary function in vivo . We therefore sought to obtain direct evidence for a role for PGs on GnRH stimulation of gonadotropin secretion using an ex vivo approach. Rat pituitaries were stimulated with GnRH alone or in combination with indomethacin (COX enzyme inhibitor) or PGs. Indomethacin and the PGs had no significant effect on basal gonadotropin secretion ( Fig. 7 ). COX enzyme inhibition by indomethacin treatment enhanced the GnRH-stimulated LH but not FSH secretion ( Fig. 7, A and B; P < 0.05). Co-stimulation of pituitaries with GnRH and PGE 2 or iloprost had no effect on the secretion of either gonadotropin in response to GnRH. However, PGF 2 α inhibited the GnRH-stimulated LH release, but had no effect on FSH release ( Fig. 7, C and D; P < 0.01).

GnRH is the central regulator of the reproductive system through the differential and asynchronous stimulation of LH and FSH ( 3 , 4 , 21 , 25 ). The precise cyclical regulation of LH and FSH is crucial to normal reproduction and dysregulation results in conditions such as polycystic ovarian syndrome and amenorrhea anorexic patients ( 26 ). Feedback by ovarian steroid and peptide hormone at the pituitary gonadotrope and changes in GnRH pulse frequency and GnRHR expression have been proposed to contribute to this cyclical secretion ( 27 , 28 ). GnRH auto-sensitizes the gonadotropes by up-regulating its own receptor preparatory to the onset of puberty and for the LH surge that triggers ovulation ( 1 , 2 , 29 ). GnRHR number increases from the evening of diestrus in rodents until late afternoon of proestrus, which culminates in the surge in LH secretion in late proestrus ( 2 , 29 ). Conversely, potential signals to down-regulate GnRHR in mouse gonadotropes include the gonadal steroids and changes in pulse frequencies of GnRH ( 21 , 30 ). In the sheep, the gonadotrope becomes refractory to GnRH after the LH surge ( 31 ). Despite these advances, the mechanisms involved have not been fully elucidated. We have now demonstrated that GnRH stimulates PG biosynthesis, which inhibits GnRHR and LH, but not FSH gene expression and hormone secretion. These findings thus provide a potential autocrine mechanism of refractoriness to GnRH after the LH surge and asynchronous gonadotropin secretion during the ovarian cycle ( Fig. 8 ). Our results further provide a molecular mechanism to modulate GnRHR number and to mediate gonadotrope desensitization during prolonged GnRH stimulation. This is of particular importance because it provides a mechanism for the down-regulation of the GnRHR that is the only mammalian GPCR that lacks a cytoplasmic carboxyl-terminal tail and consequently does not undergo rapid desensitization and internalization as in other GPCRs ( 32 - 35 ). The loss of the carboxyl-terminal tail and lack of rapid desensitization is thought to have evolved to allow the prolonged LH surge, which is required for ovulation in women ( 32 ). Multiple ligands interact with pituitary gonadotropes. In addition to GnRH, gonadal steroids and peptide hormones, growth factors, neurotransmitters, melatonin, and pituitary hormones have all been demonstrated to act as regulators ( 9 ). Hence, addition of a single ligand to cultured cells in vitro does not mimic the in vivo situation. Recognizing these caveats, attempts have been made to study the effect of multiple ligands simultaneously. Here we took another approach, which is based on the assumption that because many ligands stimulate PGs production, one can use these signaling molecules to elucidate the integrative signaling for a given ligand and better mimic the in vivo situation. Moreover, prostanoids may play a role in mediating the effects of ligands other than GnRH. Eicosanoids are hydrolyzed from AA by PLA 2 , a super family of enzymes consisting of at least 14 groups (I–XIV) and at least 19 members ( 36 , 37 ). Once released by PLA 2 , AA is converted to prostanoids (PG and thromboxanes) via COX-1, COX-2, and prostanoid synthases or undergoes dioxygenation to hydroperoxide derivatives by lipoxygenases or to epoxygenase products via cytochrome p450 epoxygenase (cyp2C11) (Ibid). We have previously shown that GnRH stimulates AA release from rat pituitaries ( 38 ) and have implicated 5-lipoxygenase and 12-lipoxygenase products of AA in GnRH actions ( 3 , 12 ). Here, we have investigated the cross talk of GnRH and prostanoid receptors. Because the inducible COX isoform (COX-2) is the key enzyme in the prostanoid synthesis pathway, we looked for activation of COX-2 by GnRH and showed a 40-fold increase in expression. A recent report documented that induction of COX-2 by angiotensin II reflects an immediate-early gene response ( 39 ). Our results support the findings because we show a rapid activation of the COX-2 promoter by GnRH (2 h), mRNA and protein production (4–8 h). The use of pharmacological inhibitors revealed a role for the PKC/c-Src/MAPK pathway, but not for EGF receptor kinase in the GnRH induction of COX-2. The results accord with previous studies suggesting that activation of MAPK by GnRH in gonadotropes is mediated mainly by PKC and c-Src but not via transactivation of the EGF receptor ( 14 , 15 , 18 , 19 , 40 - 46 ). MAPKs have previously been implicated in the induction of COX-2 ( 47 , 48 ) and a role for PI3K and c-Src in COX-2 expression has been demonstrated ( 49 ). Indeed, GnRH activates c-Src and PI3K in L β T2 cells ( 19 , 50 ). We also examined the role of PLA 2 that has been implicated in COX-2 induction ( 51 ). Using specific inhibitors ( 36 , 37 ), we found that iPLA 2 and to a lesser degree sPLA 2 , but not cPLA 2 as in Ref. 51 , participate in GnRH induction of COX-2. Hence, iPLA 2 and sPLA 2 may also be involved in the induction of COX-2 possibly by a feed-forward mechanism via the newly formed PGs as described below. Thus, the signaling pathways and the various PLA 2 and MAPK members are likely to be implicated in COX-2 induction in an agonist and cell context-dependent manner ( 39 , 47 , 48 , 51 ) (present results). Activated ERK phosphorylates various transcription factors ( e.g. c-Jun, c-Fos, ELK-1, and Sap-1) resulting in the activation of activator protein (AP)-1 ( 52 ), which binds and activates the COX-2 promoter via the CRE/ATF response element ( 53 ). ERK phosphorylation is also required for persistent activation of nuclear factor- κ B, which is also involved in COX-2 transcription ( 54 ). AA is converted to prostanoids via activated COX-2 and specific PG synthases ( 11 ). We therefore investigated whether GnRH activation of COX-2 results in PG synthesis. We first failed to detect production of PG after prolonged incubation of the cells with GnRH. We reasoned that although PLA 2 activation by GnRH may be rapid (min), COX-2 induction is a slow process (4–8 h) and might be a limiting step in PG production. Furthermore, due to rapid uptake, the availability of free AA is a rate-limiting step in PG synthesis. Therefore, the two events, namely induction of COX-2 and availability of free AA, may not be appropriately and temporally coordinated during prolonged incubation periods in vitro . Indeed, exogenous AA was needed to demonstrate PGE 2 production by IL-1 ( 20 ). Similarly, we could only detect stimulation of PG production by GnRH when exogenous AA was added for the last 30–60 min of the incubation period. The dose of AA used here (1 μ m ), is below the Michaelis-Menten constant (Km) values of 5-lipoxygenase and COX ( 55 ). In vivo , pituitary gonadotropes are exposed to GnRH pulses every 30–90 min (according to the species) thus, COX-2 induced in previous pulses may act on AA generated by PLA 2 in the current pulse resulting in rapid formation of PGs. Once formed, PGF 2 α and PGI 2 are now capable of inhibiting GnRH-stimulated phosphoinositide turnover (30 min), GnRHR promoter activity (4 h), GnRH binding sites and LH β promoter activity (6 h) and LH release (8 h). Provided COX-2 was activated by GnRH for 2–4 h, once AA was added for 30 min (at any preincubation with GnRH for up to 24 h), we could detect the formation of the PGs at the end of the 30 min exposure to AA ( Fig. 2 ). These temporal differences in activation of enzymes and substrate generation for PG production, which in turn regulate GnRHR and gonadotropin provides an elegant mechanism for translating changes in GnRH pulse frequency into physiological outputs. Prostanoids are known to act in an autocrine/paracrine manner via cognate GPCRs ( 11 ). We therefore examined the expression of the various prostanoid receptors in rat pituitaries. Specific expression of receptors for PGE 2 (EP1, EP2, EP3, and EP4), for PGF 2 α (FP) and PGI 2 (IP) were found. EP1, EP2, IP, and FP colocalize to rat gonadotropes ( Fig. 6 ), which comprise about 10–15% of the total cell population in the adult rat pituitary ( 22 ). EP3 and EP4 are present in the prolactin and GH producing cells, the mammotropes (20–30% of the total cell population) and somatotropes (∼40% of the cells), respectively (data not shown). The prostanoid receptors EP1, EP2, EP3, EP4, IP, and FP were detected in the embryonic-derived mouse L β T2 gonadotrope cells. The staining of the various prostanoid receptors was observed in the cytoplasmic and perinuclear membrane regions, and in some cases in the nucleus ( Fig. 3 ). Because prostanoid receptor signaling is rapidly terminated, it is likely that only a small fraction of the receptor pool is at the plasma membrane under basal conditions and prostanoid receptors may rapidly cycle between the cell surface and the intracellular pool, with a relatively short half-life at the plasma membrane, as for many GPCRs. The predominantly intracellular staining observed here was also reported by others for different cell models. EP1 has been described to reside in the cytoplasm ( 56 ). The thromboxane A 2 /prostaglandin H 2 receptor was found in the endoplasmic reticulum ( 57 ) and EP3 and EP4 were found in the nuclear envelope ( 58 , 59 ). It is interesting to note that the dogma that PLA 2 liberates AA from the sn-2 position of plasma membrane phospholipids has been challenged by the observations that both cPLA 2 α and COX-1/2 reside in the endoplasmic reticulum and the nuclear envelope membrane, where PGs are formed ( 60 ). It is therefore tempting to suggest that, aside from PG secretion and binding to receptors on the plasma membrane, PGs may also act intracellularly in the vicinity of their site of origin. The finding that EP3 and EP4 are present in L β T2 cells but not in rat pituitary gonadotropes may reflect incomplete differentiation of this gonadotrope-derived cell line and that maturation of murine gonadotropes may involve the loss of expression of both EP3 and EP4. These studies begged the question as to whether the newly formed PGs affect the kernal physiological outcome of GnRH actions, namely the transcriptional regulation of gonadotropin subunits ( α GSU, LH β , and FSH β ) and gonadotropin secretion ( 21 ). Treatment with PGs had no significant effect on the induction of α GSU and FSH β genes by GnRH. However, PGF 2 α , but not PGE 2 or PGI 2, significantly reduced the activation of the LH β promoter by GnRH. We also investigated whether the exogenous addition of PGs affected the homologous induction of the GnRHR ( 21 ). The induction of GnRHR by GnRH was markedly reduced by PGF 2 α and PGI 2 , but not by PGE 2 . Homologous activation of the mouse GnRHR identified a role for PKC/MAPK/AP-1 signaling and CRE elements ( 21 ). Phosphoinositide turnover is tightly coupled to the PKC/MAPK/AP-1/CREB pathways ( 14 , 52 , 61 ). We therefore targeted phosphoinositide turnover as the first potential upstream signaling module for PG action. We found inhibition of GnRH-stimulated InsP formation by PGF 2 α and PGI 2, but not PGE 2, in line with their inhibition of the GnRHR. The link of phosphoinositide turnover to GnRHR induction was revealed by the use of the PLC- β inhibitor U73122, which produced a dose-related inhibition of GnRH-induced InsP formation and GnRHR promoter activity. Thus, inhibition of GnRHR by PGF 2 α and PGI 2 appears to be mediated by an inhibition of phosphoinositide turnover, perhaps leading to an alteration of protein complexes at the AP-1 sites in the promoter ( 21 ). Our findings present an enigma in that these ligands bind the FP and IP receptors, which stimulate InsP ( 11 ). Thus, in gonadotropes expressing GnRH, FP, and IP receptors (which are all believed to stimulate lnsP), EP1 and EP2 (which are believed to stimulate lnsP and cAMP, respectively), a more complex picture emerges and activation of the FP and IP receptors actually inhibits GnRHR stimulation of InsP. Hence, the presence of multiple receptors in the same cell may result in a different signaling profile from that obtained by over expressing a single prostanoid receptor. The dogma that PG receptors have discrete signaling pathways ( 11 ) should therefore be revisited because our results suggest that cell context and expression of multiple prostanoid receptors in the same cell can dictate the flavor of the signaling. The mechanism involved in PG inhibition of GnRH-stimulated InsP formation may include potentiating GnRH-stimulated PKC isoforms activation ( 62 ), which can feed back and inhibit PLC- β , or activation of specific InsP phosphatases. Our demonstration of EP1, EP2, FP, and IP receptor expression in rat gonadotropes by double fluorescent confocal microscopy suggested that PGE 2 , PGF 2 α , and PGI 2 might have direct effects on gonadotrope function. In vivo support for our in vitro cell line findings on inhibitory effects of endogenous PG on GnRHRs and LH β expression was obtained by using an ex vivo approach of rat pituitaries incubated with GnRH alone or in the presence of the COX inhibitor, indomethacin or various PGs. Inhibition of COX enzyme activity by indomethacin treatment enhanced GnRH-stimulated LH but not FSH secretion. Furthermore, PGF 2 α , but not PGE 2 or PGI 2, inhibited GnRH-stimulated LH, but had no effect on FSH release. Thus, only the FP receptor in gonadotropes appears to regulate LH in this nondynamic system. Whereas the FP receptor therefore appears to play a significant role in differential gonadotropin secretion in response to GnRH, the findings do not rule out more subtle roles for the IP and EP receptors expressed in gonadotropes. The data suggest that the reduction of GnRHR expression is insufficient to reduce LH secretion as evident from the lack of effect of PGI 2 , which like PGF 2 α reduced GnRHR expression but had no effect on LH secretion. Hence, the reduction of both GnRHR and LH β expression, as observed with PGF 2 α are required to affect the LH exocytotic apparatus. The data also suggest that FSH secretion is less sensitive to the reduction of GnRHRs because it was not affected by PGI 2 and PGF 2 α . The results are in line with a report that found that at high GnRHR concentrations, GnRH activates the α GSU and LH β genes and exerts a selective inhibition of the FSH β gene, whereas the FSH β gene is optimally activated at lower number of GnRHRs ( 27 ). In summary, our results provide a novel inside-out molecular mechanism to regulate GnRHR number and to mediate the differential LH and FSH secretion during GnRH stimulation ( Fig. 8 ). These findings provide a basis for the use of combination therapies of GnRH and prostanoid analogs to treat infertility and diseases with unbalanced LH and FSH secretion such as polycystic ovarian syndrome. Combination therapy may also be contemplated in hormone-dependent disease such as prostatic cancer and endometriosis. These diseases are currently treated with GnRH analogs for inhibition of gonadotropin and hence sex steroids. Because PGs affect GnRHR and LH β , modulation of PG input on gonadotropes has the potential to increase the efficacy of the GnRH analogs.

Culture medium was from Invitrogen Inc. (Paisley, Scotland, UK). Penicillin-streptomycin and fetal calf serum (FCS) were from PAA Laboratories Ltd. (Middlesex, UK). GnRH was from Peninsula (St. Helens, UK). Prostaglandin E 2 (PGE 2 ), the stable PGI 2 analog Iloprost, prostaglandin F 2 α (PGF 2 α ), rabbit polyclonal antibodies to the various PG receptors and respective peptides, arachidonyl trifluoromethyl ketone (AACOCF 3 ), TEPC, BEL, and ELISA kits for the PG were from Cayman Chemical Co. (Alexis Corp., Nottingham, UK). Rabbit antirat antibodies for LH β and prolactin and guinea pig antirat GH were from National Hormone and Peptide Program [National Institutes of Health (NIH)]. Secondary horseradish peroxidase (HRP)-conjugated goat antimouse antibodies or goat antirabbit antibodies were from Jackson ImmunoResearch Laboratories (West Grove, PA). PD980595, a peptide JNK inhibitor (JNKI; Calbiochem 420116), SB203580, GF109203X, wortmannin, PP2, AG1478, and U73122 were from Calbiochem (Nottingham, UK). L β T2 cells were kindly obtained from P. Mellon (University of California, San Diego, San Diego, CA). The cells were cultured in matrigel (Becton Dickinson, Oxford, UK)-coated plastic flasks in DMEM/10% FCS/glutamine medium (Sigma, Poole, UK), with 1% antibiotics (stock 500 IU/ml penicillin and 500 μ g/ml streptomycin) at 37 C and 5% CO 2 (vol/vol) ( 19 ). Adult rat pituitaries were fixed for either 6 or 24 h in Bouins, 4% neutral buffered formaldehyde (NBF) or modified Davidson's fixative. Pituitaries were processed to paraffin using standard procedures and 5 μ m sections were mounted on superfrost plus slides (BDH, Poole, Dorset, UK). Cells were cultured on chamber slides (Lab Tek, Naperville, IL) rinsed for 5 min in PBS followed by 10 min fixation. Each antiserum was tested using a range of dilutions on paraffin sections of rat pituitary fixed as above. They were evaluated with and without heat induced antigen retrieval using a pressure cooker (Tefal, Cambridge, UK) and 0.01 m citrate. For all antisera tested, antigen retrieval was not required. Negative controls were either omission of primary antisera or peptide absorption of antisera using a 10-fold excess of peptide incubated at 4 C overnight. Antisera were evaluated on L β T2 cells after 10 min fixation in 4% NBF. Sections were dewaxed in xylene before being rehydrated in graded ethanol. Sections were blocked in 3% hydrogen peroxide in Methanol for 30 min (HRP and Tyramide detections only), rinsed in running tap water then washed for 5 min in Tris-buffered saline (TBS, 0.05 m Tris; and 0.85% NaCl, pH 7.4). Sections were blocked in 20% normal swine serum in TBS (TBS/NSS) for 30 min at room temperature. Slide were drained and incubated with Primary antiserum diluted in TBS/NSS overnight at 4 C. After two 5-min washes in TBS, sections were incubated for 30 min with Swine anti Rabbit Biotinylated at 1:500 dilution in TBS/NSS for 30 min at room temperature. After two 5-min washes in TBS, sections were incubated for 30 min with HRP-conjugated avidin-biotin complex (ABC-HRP) (Dako, Cambridgeshire, UK). Sections were washed for a further two 5-min washes in TBS before visualizing with diaminobenzidine (Dako). After dewaxing and methanol/hydrogen peroxide blocking (if necessary) as described above. Sections were incubated in 20% normal goat serum in PBS (NGS/PBS). Slides were drained and incubated with Primary antiserum diluted in NGS/PBS overnight at 4 C. After two 5-min washes in PBS, sections were incubated for 30 min with either Goat antiguinea pig biotinylated at 1:500 dilution in PBS/NGS for 30 min at room temperature (prolactin, GH), or goat antimouse Alexa 488 at 1:200 dilution in PBS/NGS (LH). For GH or prolactin staining, after two 5-min washes in PBS, sections were incubated for 30 min with Streptavidin Alexa 488. All sections were washed for a further two 5-min washes in PBS before being counterstained in TOPRO-3 at 1:2000 dilution and coverslipped using permaflour (Coulter, Buckinghamshire, UK). After dewaxing and methanol/hydrogen peroxide blocking as described above. Sections were incubated in 20% NGS/PBS. Slides were drained and incubated with primary antiserum diluted in NGS/PBS overnight at 4 C. After two 5-min washes in PBS, sections were incubated for 30 min with goat antirabbit peroxidase at a 1:200 dilution in PBS/NGS for 30 min at room temperature. After two 5-min washes in PBS, sections were incubated for 10 min with Tyramide Cy3 Sections were washed for a further two 5-min washes in PBS before being counterstained in TOPRO-3 at 1:2000, and cover slipped using permafluour (Coulter). After standard immunofluorescent detection as described above (prolactin, GH, and LH) sections were blocked in peroxide block for 10 min (Dako) before detection of EP1–4, IP, or FP using Fluorescent Tyramide Detection as described above. After two 5-min washes in PBS sections were counterstained in TOPRO-3 at 1:2000 and cover slipped using permaflour. Bright-field images were captured using an Olympus (Center Valley, PA) Provis microscope fitted with a Kodak (Rochester, NY) DCS 330 digital camera Fluorescent images were captured using a Zeiss (Oberkochen, Germany) LSM 510 confocal microscope. Total RNA was isolated from cells using Tri-reagent (Sigma) according to the manufacturer's instructions. The RNA yield was calculated using ultraviolet spectrophotometry and samples were stored as an ethanol precipitate at −80 C before RT-PCR analysis. RNA samples (5 μ g) were reverse-transcribed using deoxynucleotide triphosphate (dNTPs) (0.2 m m each), random primers (200 ng), ribonuclease inhibitor (2 U/ μ l) and SuperScript reverse transcriptase (10 U/ μ l; Invitrogen). The reverse transcription product (200 ng cDNA) was then amplified by PCR using homologous primers designed from the mRNA sequences of the various EP receptors and the COX enzymes. The sequence of the primers (forward and reverse primers respectively) was as follows: EP1, 5′-CGCTCCTTGCGGCATTAGTGTGC, 5′-CCAACACCACCAACACCAGCAGG; EP2, 5′-TAGGGCAGGTGAGGCACAGAAGC, 5′-GAAAGGAGCCACTGACGACTCTTGC; EP3, 5′-ATGTGTGTGCTGTCCGTCTGTTGG, 5′-CAACCAGACTCTCAGATTATCC; EP4, 5′-AGACACCACCTCGCTGAGAACTTTGC, 5′-CTTCAAGCCTGGGCACTCAAGGACC; FP, 5′-TAGGGAGGAAAGAGAGGTGGAACC, 5′-TGACTTCTGTCTAAATCTCTGG; IP, 5′-GATGCCGAAGGTTCTATGGC, 5′-TGTGTCCAGCAATGTCACCTGC; COX-2, 5′-TGCCACCTCTGCGATGCTCTTCC, 5′-CAGACTCCCTTGAAGTGGGTCAGG. Primers were selected to match sequences located in separate exons, enabling detection of spliced transcripts. The PCR mix consisted of 1× reaction buffer containing 1.5 m m MgCl 2 , 0.2 m m dNTPs, 0.2 μ m of each primer and 1.25 U Taq DNA polymerase (Abgene, Epsom, Surrey, UK). Samples were denatured at 94 C for 5min, and then amplified by 35–40 cycles of 94 C for 1 min, 60 C for 30 sec and 72 C for 40 sec with a final extension of 72 C for 10 min. After amplification, 10 μ l of each sample was visualized on a 1% agarose gel. Normal mouse uterus was used as a positive control ( 63 ). RNA samples were quantified and reverse-transcribed using 5.5 m m MgCl 2 , 0.5 m m of each dNTP, 2.5 μ m random hexamers, ribonuclease inhibitor (0.4 U/ μ l) and 1.25 U/ μ l Multiscribe reverse transcriptase (all from Applied Biosystems, Warrington, Cheshire, UK). RNA (200 ng) was added to each 10 μ l reverse transcription reaction and samples were incubated for 60 min at 25 C, 45min at 48 C and 5 min at 95 C. The reaction mix for the PCR consisted of 1× mastermix, ribosomal 18S forward and reverse primers, ribosomal 18S probe (50 n m ; all from Applied Biosystems), forward and reverse primers for COX-2 (300 n m ) and COX-2 probe (200 n m ) (all from Biosource UK, Nivelles, Belgium). The reaction mix (48 μ l) was aliquoted into tubes and 2 μ l cDNA was added. Duplicate 24- μ l samples plus positive and negative controls were placed in a PCR plate and wells were sealed with optical caps. The PCRs were carried out using an ABI Prism 7700 (Applied Biosystems). All primers and probes were designed using the PRIMER express program (Applied Biosystems). The sequences of the COX-2 primers (forward, reverse, and probe, 6-carboxy fluoroscein labeled) were: COX-2, 5′-GCTTCGGGAGCACAACAGA-3′; 5′-TGGTTTGGAATAGTTGCTCATCAC-3′; 5′-TGTGCGACATACTCAAGCAGGAGCATC-3′. Data were analyzed and processed using Sequence Detector version 1.6.3 (Applied Biosystems) according to the manufacturer's instructions. Results were expressed relative to an internal positive standard cDNA obtained from a single sample of mouse uterus ( 63 ). L β T2 cells were grown in 4 × 12-well plates (5 × 10 5 cells/well). Cells were incubated in serum-free DMEM, 0.2% FCS overnight at 37 C. The cells were washed with DMEM and incubated with or without GnRH for various time periods as detailed in the legends. Some of the wells received exogenous AA (1 μ m ) for the last 30 or 60 min of each incubation time. At the end of the incubation time, the media were removed and stored at −20 C for determination of PGE 2 , PGI 2 , and PGF 2 α by ELISA as described in Ref. 64 . The reporter constructs are described in ( 65 ), except 2307/+49 human COX-2 promoter [pGL3.C2.2 (P6P5)], which was obtained from R. Newton (University of Warwick, Coventry, UK). Reporter gene assays were carried out as recently described ( 19 , 65 , 66 ). Whole cell receptor binding assays used the 125 I-[His ( 5 ), d -Tyr ( 6 )]GnRH analog as described ( 67 ). L β T2 cells were incubated with GnRH (100 n m ) in the presence and absence of PGE 2 , iloprost or PGF 2 α (500 n m ) for 30 min and total inositol phosphate production was determined as previously described ( 68 ). Pituitaries from 23-d-old Wistar-derived female rats were preincubated with indomethacin (8 μ m ), PGE 2 , iloprost or PGF 2 α (500 n m ) in Krebs-Ringer-bicarbonate for 4 h, washed twice and further incubated with the drugs with or without GnRH (100 n m ) for 3 h (Indomethacin + GnRH) and 4 h (PG + GnRH). The medium was collected and LH and FSH were determined by RIA using the kit provided by the National Hormone and Peptide Program (NIH). Results from two or three experiments were expressed as mean ± sem . Where appropriate, data were subjected to statistical analysis withy one-way ANOVA and Fisher's protected least significant difference tests (Statview 5.0; Abacus Concepts Inc., Carpinteria, CA), and statistical significance accepted when P < 0.05.