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Evidence for an estrogen-like action of raloxifene upon the hypothalamic-pituitary unit: raloxifene inhibits luteinizing hormone secretion and stimulates prolactin secretion in ovariectomized female rats
Neuroscience Letters 311 (2001) 149–152 www.elsevier.com/locate/neulet
Evidence for an estrogen-like action of raloxifene upon the hypothalamic-pituitary unit: raloxifene inhibits luteinizing hormone secretion and stimulates prolactin secretion in ovariectomized female rats L. Pinilla, L.C. Gonzalez, M. Tena-Sempere, E. Aguilar* Department of Cell Biology, Physiology and Immunology. University of Co´rdoba School of Medicine. Co´rdoba, Spain Received 15 June 2001; received in revised form 12 July 2001; accepted 12 July 2001
Abstract Selective estrogen receptor modulators (SERMs) constitute a new family of drugs with growing interest in the management of estrogen-associated pathology. Raloxifene is a SERM that is used to treat and prevent osteoporosis in postmenopausal women. The actions of raloxifene on bone, breast, uterus and serum cholesterol have been widely analyzed, but very few studies have been carried out to evaluate whether raloxifene has an estrogenic activity upon the hypothalamic-pituitary axis in the rat. For this purpose, adult female rats were ovariectomized or sham ovariectomized. One week later, the rats were implanted with intracardiac canullae and 24 h after injected daily with raloxifene (500 mg/ rat/day) or vehicle for 5 days. One hour after the last injection, blood samples were obtained at 5 min intervals for a 3 h. period (10:00–13:00 h). Our results indicate that raloxifene inhibits the pulsatile nature of the post-ovariectomy hypersecretion of luteinizing hormone (LH) and increases prolactin (PRL) secretion in ovariectomized animals. These effects are suggestive of an estrogenic activity of raloxifene on LH and PRL secretion in ovariectomized females. q 2001 Published by Elsevier Science Ireland Ltd. Keywords: Raloxifene; Selective estrogen receptor modulators; luteinizing hormone; Rats
Raloxifene is a non-steroidal selective estrogen receptor modulator developed as a therapeutic agent for menopausal osteoporosis [9]. Interestingly, raloxifene blocks the effects of estrogen in some tissues, such as the breast, while it mimics estrogen action on bone and blood lipid levels [2,6,10,29]. The endocrine (extraneural) regulation of luteinizing hormone (LH) and PRL secretion in females appears to be mainly carried out by estrogen action [8], but at the present the effects of raloxifene on the hypothalamic-pituitary axis have not been extensively evaluated. Raloxifene injected to female rats prevented the effects of estradiol on serum LH levels [19], but stimulated PRL secretion and inhibited follicle stimulating hormone (FSH) release [3]. In human volunteers, raloxifene blunted the effects of estrogen on different anterior pituitary hormones such PRL, and FSH [7]. These data reveal a compound estrogenic/antiestrogenic action of raloxifene in the control of anterior pituitary secretion depending on the experimental approach used. Recently, * Corresponding author. Fax: 134-57-218288. E-mail address: [email protected] (E. Aguilar).
we have demonstrated that raloxifene injected to neonatal female and male rats induced permanent changes in reproductive physiology similar to those induced by estradiol [23]. To further explore the action of raloxifene upon reproductive axis, the present study was undertaken to examine in adult female rats the effects of raloxifene on the pulsatile nature of the post ovariectomy hypersecretion of LH. In addition, the actions of raloxifene upon pulsatile nature of the PRL secretion was also analyzed in this experimental setting. Adult cyclic Wistar rats weighing 250 ^ 20 g were kept under controlled conditions of light (12:12 h light/darkness, light on at 07:00 h) and temperature (228C), with free access to pelleted food (Pacsa Sanders, Seville, Spain) and tap water. Ovariectomy or sham-ovariectomy were performed after light ether anesthesia. One week after, all rats were implanted with intracardiac canullae under sodium pentobarbital (50 mg/Kg) anesthesia. After surgery, the animals were placed directly in isolation test chambers for 6 days. Twenty-four hours after cannulation, the animals were intraperitoneally injected for 5 days with vehicle or 500 mg/day
0304-3940/01/$ - see front matter q 2001 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 1) 02 10 4- 8
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of raloxifene [2-(4-hidroxyphenyl) -6- hidroxybenzo-bthien-3-yl-[4-[2-(1-piperidinyl) ethoxy-phenyl-metanone hydrochloride (Evista w, a selective estrogen receptor modulator, Lilly) dissolved in saline. Each group consisted of 8– 10 animals. One hour after the last injection, blood samples (200 ml) were obtained at 5 min intervals for a 3 h. period (10:00–13:00 h). During the sampling period, the volume of blood withdrawn was replaced by a warmed suspension of blood cells in sterile saline. The experimental procedure was approved by the Co´rdoba University Ethical Committee for animal experimentation and was conducted in accordance with the European Union normative for care and use of experimental animals. After centrifugation (1600 £ g at 48C for 20 min), plasma samples were collected, frozen and stored at 2208C until use. The concentrations of LH and PRL were measured in 10–50 ml using a double-antibody method using radioimmunoassay Kits supplied by NIH (Bethesda, MD). Rat LHI-9 and Rat-PRL-I-6 were labeled with I 125 by the cloramine T method and hormone concentrations were expressed using reference preparation (RP) LH-2 and PRL-3 as standards. Intra-and interassay coefficients of variations were below 7%. The sensitivities of assay were 3.25 and 10 pg/tube for LH and PRL, respectively. Assessment of pulsatile secretion was carried out using the ULTRA program (Van Cauter, E. Department of Medicine, University of Chicago, IL) [28]. Data are expressed as means ^ SEM. Intergroups differences were determined by one or two-way ANOVA followed by Tukey’s Test. Serum LH concentrations at the end of treatments (first blood samples, obtained at 10:00 h) were higher in ovariectomized females than in intact group (6.19 ^ 0.71 ng/ml vs 1.72 ^ 0.27; P # 0:01 in vehicle-injected groups and 6.27 ^ 0.98 ng/ml vs 0.79 ^ 0.17 in raloxifene-injected groups). Analysis of pulsatile secretion during 3 h indicated that ovariectomized females showed significant increases in LH pulse amplitude, mean and through LH levels as well as net LH secretion (estimated as area under curve) as compared to intact group, (Table 1). Raloxifene administered to ovariectomized females decreased significantly all parameters of LH secretion except the pulse frequency and duration (Table 1), but was unable to change the patterns of LH pulsatile secretion in intact females (data not shown). Serum PRL concentrations in ovariectomized females injected with vehicle appeared lower than in the corresponding intact group, although differences were not statistically significant (8.41 ^ 2.08 ng/ml in ovariectomized rats vs. 15.98 ^ 8.87 ng/ml in intact animals). The serum PRL concentrations in ovariectomized females increased at the end of treatment with raloxifene (28.69 ^ 10.59 ng/ml vs. 8.41 ^ 2.08; P # 0:01), while raloxifene was ineffective in intact rats (data not shown). Raloxifene also increased in ovariectomized females some characteristics of pulsatile PRL secretion such as mean PRL levels and the net PRL secretion estimated as AUC (Table 1). In contrast, raloxi-
fene did not change pulsatile PRL secretion in intact females (data not shown). The effects of raloxifene on in vivo LH secretion are conflicting. It has been described that LH secretion was unaffected by raloxifene administration in castrated male rats [19] and in human volunteers [7]. In contrast, administration of raloxifene during 2–4 weeks to mice increased LH secretion [4]. The data now presented indicate that in ovariectomized females raloxifene exerts an estrogenic action in the neuroendocrine mechanisms governing LH and PRL secretion, since treatment with raloxifene during 5 days decreased LH secretion and increased serum PRL levels. The increase in serum LH concentrations after ovariectomy results from increases in both LH pulse amplitude and pulse frequency [12,14,15]. Administration of estradiol inhibited LH secretion in ovariectomized rats by significantly suppressing LH pulse amplitude and mean LH concentration, with a tendency to decrease pulse frequency also [15,18,25–27]. These effects are induced by inhibiting hypothalamic LHRH release and pituitary responsiveness to LHRH [1,12,16,17]. Our results clearly indicate that raloxifene reduced the hypersecretion of LH mimicking the negative feedback effects of estradiol. The inhibitory effect of raloxifene on pulsatile LH release may be exerted either at hypothalamic or pituitary level. The reduction of effectiveness of LHRH on LH release in vitro in presence of raloxifene either in females [11,20] or males (Pinilla L. et al. In preparation) supports a pituitary action of this compound. This hypothesis is reinforced by the reduction in the amplitude of LH pulses and the absence of changes in LH pulse frequency (Table 1). It has been described previously that, in presence of estrogens, raloxifene acts as an antiestrogen in pituitary gonadotrophs, blocking the effects of estradiol on LH release [25]. Thus, it is possible that, at the pituitary level, raloxifene may have either estrogenic (in absence of estradiol) or antiestrogenic activity (in presence of estradiol) depending on the prevailing estrogenic background. The pituitary action of raloxifene does not exclude an additional hypothalamic action; however, it should be considered that implantation of raloxifene in the hypothalamus of estrogen-treated ovariectomized rats attenuated the positive feedback of estradiol on LH secretion but did not affect the negative feedback [21,22]. Another well-known action of estrogens is the stimulatory effect on PRL release due to its action at hypothalamic and pituitary levels [5,8,13,24]. In ovariectomized females, raloxifene increased PRL secretion, an effect not observed in the intact group, which suggests again that the estrogenic effects of raloxifene become apparent in situations with low estrogenic activity. In conclusion our results indicate that in absence of estrogens, raloxifene exerts an estrogenic activity upon neuroendocrine network controlling both LH and PRL secretion.
LH
Pulse frequency (n pulses/3 h) Mean levels (ng/ml) Pulse amplitude (ng/ml) Trough levels (ng/ml) Duration (min/pulse) AUC a
PRL
Intact
Ovx
Ovx 1 Rx (500 mg/kg/day)
Intact
Ovx
Ovx 1 Rx (500 mg/kg/day)
3.40 ^ 0.68 0.44 ^ 0.06 0.34 ^ 0.05 0.18 ^ 0.03 39.40 ^ 6.69 79.16 ^ 11.17
5.33 ^ 0.56 4.98 ^ 0.58** 5.93 ^ 0.66** 2.24 ^ 0.35** 28.87 ^ 2.996 886.77 ^ 103.79**
6.60 ^ 0.60 2.61 ^ 0.41**,# 3.21 ^ 0.63**,# 1.14 ^ 0.30**,# 21.40 ^ 2.91* 460.63 ^ 72.32**,#
8.80 ^ 1.24 7.91 ^ 5.30 6.50 ^ 5.12 0.98 ^ 0.16 18.40 ^ 2.16 1411.63 ^ 955.60
7.83 ^ 0.31 5.01 ^ 1.19 7.35 ^ 4.90 1.68 ^ 0.29 19.00 ^ 0.68 894.87 ^ 211.666
7.00 ^ 0.82 30.16 ^ 10.80*,a 37.34 ^ 15.77* 3.45 ^ 1.69 22.00 ^ 2.92 53796.33 ^ 1936.09*,#
Values are expressed as means ^ SE (n ¼ 8–10 animals per group). *P # 0:05 vs intact group; **P # 0:01 vs ovx group; #P # 0:01 vs ovx group (ANOVA followed by Tukey’s test).
L. Pinilla et al. / Neuroscience Letters 311 (2001) 149–152
Table 1 Characteristics of pulsatile LH and PRL secretion in intact, ovariectomized (ovx) and raloxifen-treated ovariectomized rats a
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The authors are very grateful to RocI´o Campo´ n and Inmaculada Aguilar for their technical assistance. This work was supported by grants PM98-0163 (CICYT. Spain) and IFD97-0696-02 (FEDER). The NIH supplied the materials for hormone determinations. [1] Aiyer, M.S., Sood, M.C. and Brown-Grant, K., The pituitary response to exogenous luteinizing hormone releasing factor in steroid-treated gonadectomized rats, J. Endocrinol., 69 (1976) 255–262. [2] Black, L.J., Sato, M., Rowley, E.R., Magee, D.E., Bekele, A., William, D.C., Cullinan, G.J., Bendele, R., Kaufman, F.R., Bensch, W.R., Frolik, C.A., Termine, J.D. and Bryant, H.U., Raloxifene (LY139481) prevents bone loss and reduces serum cholesterol without causing uterine hypertrophy in ovariectomized rats, J. Clin. Invest., 93 (1994) 63–93. [3] Clemens, J.A., Bennett, D.R., Black, L.J. and Jones, C.D., Effects of a new antiestrogen, Keoxifene (LY 156758), on growth of carcinogen induced mammary tumors and on LH and prolactin levels, Life Sci., 32 (1983) 2869–2875. [4] Cohen, I.R., Sims, M.L., Robbins, M.R., Lakshmann, M.C., Francis, P.C. and Long, G.G., The reversible effects of raloxifene on luteinizing hormone levels and ovarian morphology in mice, Reprod. Toxicol., 14 (2000) 37–44. [5] Chen, C.L. and Meites, J., Effects of estrogen and progesterone on serum and pituitary prolactin levels in ovariectomized rats, Endocrinology, 86 (1970) 503–505. [6] Delmas, P.D., Barnason, N.H., Mitlak, B.H., Ravoux, A.C., Shah, A.S., Huster, W.J., Draper, M.W. and Christiansen, C., Effects of Raloxifene on bone mineral density serum cholesterol concentration and uterine endometrium in postmenopausal woman, New Engl. J. Med., 337 (1997) 1641–1647. [7] Draper, M.W., Flowers, D.E., Neild, J.A., Huster, W.I. and Zerbe, R.L., Antiestrogenic properties of raloxifene, Pharmacology, 50 (1995) 209–217. [8] Freeman, M.E., The neuroendocrine control of the ovarian cycle in the rat, In E. Knobil and J.D. Neill (Eds.), The Physiology of Reproduction, The Physiology of Reproduction, Vol. II, Raven Press, New York, 1994, pp. 613–658. [9] Fuchs-Young, D., Glasebrook, A.L., Short, L.L., Draper, N.W., Rippy, M.K., Cole, H.W., Magee, D.E., Termine, J.D. and Bryant, H.U., Raloxifene is a tissue-selective agonist/ antiagonist that functions through the estrogen receptor, Ann. N. Y. Acad. Sci., 761 (1995) 355–360. [10] Glasebrook, A.L., Phillis, D.L. and Sluka, J.P., Multiple binding sites for the antiestrogen Raloxifene, (156758), J. Bone Miner. Res., , 8(Suppl 1) (1993) A607. [11] Gonzalez, D., Bellido, C., Aguilar, R., Garrido-Garcia, J-C., Hernandez, G., Alonso, R. and Sanchez-Criado, J.E., Luteinizing hormone secretion elicited in a ligand-independent activation of progesterone receptor manner at pituitary level in the rat: different effect of two selective estrogen receptor modulators, Neurosci. Lett., 289 (2000) 111–114. [12] Higuchi, T. and Kawakami, M., Changes in the characteristics of pulsatile luteinizing hormone secretion during the oestrous cycle and after ovariectomy and oestrogen treatment in female rats, J. Endocrinol., 94 (1982) 177–182. [13] Jacques, S. and Gala, R.R., The influence of oestrogen administration in vivo on in vitro prolactin release, Acta Endocrinol., 92 (1979) 437–447. [14] Leipheimer, R.E. and Gallo, R.V., Acute and long-term changes in control and pituitary mechanisms regulating pulsatile luteinizing hormone secretion after ovariectomy in the rat, Neuroendocrinology, 37 (1983) 421–426. [15] Leipheimer, R.E., Bona-Gallo, A. and Gallo, R.V., The influ-
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Evidence for an estrogen-like action of raloxifene upon the hypothalamic-pituitary unit: raloxifene inhibits luteinizing hormone secretion and stimulates prolactin secretion in ovariectomized female rats L. Pinilla, L.C. Gonzalez, M. Tena-Sempere, E. Aguilar* Department of Cell Biology, Physiology and Immunology. University of Co´rdoba School of Medicine. Co´rdoba, Spain Received 15 June 2001; received in revised form 12 July 2001; accepted 12 July 2001
Abstract Selective estrogen receptor modulators (SERMs) constitute a new family of drugs with growing interest in the management of estrogen-associated pathology. Raloxifene is a SERM that is used to treat and prevent osteoporosis in postmenopausal women. The actions of raloxifene on bone, breast, uterus and serum cholesterol have been widely analyzed, but very few studies have been carried out to evaluate whether raloxifene has an estrogenic activity upon the hypothalamic-pituitary axis in the rat. For this purpose, adult female rats were ovariectomized or sham ovariectomized. One week later, the rats were implanted with intracardiac canullae and 24 h after injected daily with raloxifene (500 mg/ rat/day) or vehicle for 5 days. One hour after the last injection, blood samples were obtained at 5 min intervals for a 3 h. period (10:00–13:00 h). Our results indicate that raloxifene inhibits the pulsatile nature of the post-ovariectomy hypersecretion of luteinizing hormone (LH) and increases prolactin (PRL) secretion in ovariectomized animals. These effects are suggestive of an estrogenic activity of raloxifene on LH and PRL secretion in ovariectomized females. q 2001 Published by Elsevier Science Ireland Ltd. Keywords: Raloxifene; Selective estrogen receptor modulators; luteinizing hormone; Rats
Raloxifene is a non-steroidal selective estrogen receptor modulator developed as a therapeutic agent for menopausal osteoporosis [9]. Interestingly, raloxifene blocks the effects of estrogen in some tissues, such as the breast, while it mimics estrogen action on bone and blood lipid levels [2,6,10,29]. The endocrine (extraneural) regulation of luteinizing hormone (LH) and PRL secretion in females appears to be mainly carried out by estrogen action [8], but at the present the effects of raloxifene on the hypothalamic-pituitary axis have not been extensively evaluated. Raloxifene injected to female rats prevented the effects of estradiol on serum LH levels [19], but stimulated PRL secretion and inhibited follicle stimulating hormone (FSH) release [3]. In human volunteers, raloxifene blunted the effects of estrogen on different anterior pituitary hormones such PRL, and FSH [7]. These data reveal a compound estrogenic/antiestrogenic action of raloxifene in the control of anterior pituitary secretion depending on the experimental approach used. Recently, * Corresponding author. Fax: 134-57-218288. E-mail address: [email protected] (E. Aguilar).
we have demonstrated that raloxifene injected to neonatal female and male rats induced permanent changes in reproductive physiology similar to those induced by estradiol [23]. To further explore the action of raloxifene upon reproductive axis, the present study was undertaken to examine in adult female rats the effects of raloxifene on the pulsatile nature of the post ovariectomy hypersecretion of LH. In addition, the actions of raloxifene upon pulsatile nature of the PRL secretion was also analyzed in this experimental setting. Adult cyclic Wistar rats weighing 250 ^ 20 g were kept under controlled conditions of light (12:12 h light/darkness, light on at 07:00 h) and temperature (228C), with free access to pelleted food (Pacsa Sanders, Seville, Spain) and tap water. Ovariectomy or sham-ovariectomy were performed after light ether anesthesia. One week after, all rats were implanted with intracardiac canullae under sodium pentobarbital (50 mg/Kg) anesthesia. After surgery, the animals were placed directly in isolation test chambers for 6 days. Twenty-four hours after cannulation, the animals were intraperitoneally injected for 5 days with vehicle or 500 mg/day
0304-3940/01/$ - see front matter q 2001 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 1) 02 10 4- 8
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of raloxifene [2-(4-hidroxyphenyl) -6- hidroxybenzo-bthien-3-yl-[4-[2-(1-piperidinyl) ethoxy-phenyl-metanone hydrochloride (Evista w, a selective estrogen receptor modulator, Lilly) dissolved in saline. Each group consisted of 8– 10 animals. One hour after the last injection, blood samples (200 ml) were obtained at 5 min intervals for a 3 h. period (10:00–13:00 h). During the sampling period, the volume of blood withdrawn was replaced by a warmed suspension of blood cells in sterile saline. The experimental procedure was approved by the Co´rdoba University Ethical Committee for animal experimentation and was conducted in accordance with the European Union normative for care and use of experimental animals. After centrifugation (1600 £ g at 48C for 20 min), plasma samples were collected, frozen and stored at 2208C until use. The concentrations of LH and PRL were measured in 10–50 ml using a double-antibody method using radioimmunoassay Kits supplied by NIH (Bethesda, MD). Rat LHI-9 and Rat-PRL-I-6 were labeled with I 125 by the cloramine T method and hormone concentrations were expressed using reference preparation (RP) LH-2 and PRL-3 as standards. Intra-and interassay coefficients of variations were below 7%. The sensitivities of assay were 3.25 and 10 pg/tube for LH and PRL, respectively. Assessment of pulsatile secretion was carried out using the ULTRA program (Van Cauter, E. Department of Medicine, University of Chicago, IL) [28]. Data are expressed as means ^ SEM. Intergroups differences were determined by one or two-way ANOVA followed by Tukey’s Test. Serum LH concentrations at the end of treatments (first blood samples, obtained at 10:00 h) were higher in ovariectomized females than in intact group (6.19 ^ 0.71 ng/ml vs 1.72 ^ 0.27; P # 0:01 in vehicle-injected groups and 6.27 ^ 0.98 ng/ml vs 0.79 ^ 0.17 in raloxifene-injected groups). Analysis of pulsatile secretion during 3 h indicated that ovariectomized females showed significant increases in LH pulse amplitude, mean and through LH levels as well as net LH secretion (estimated as area under curve) as compared to intact group, (Table 1). Raloxifene administered to ovariectomized females decreased significantly all parameters of LH secretion except the pulse frequency and duration (Table 1), but was unable to change the patterns of LH pulsatile secretion in intact females (data not shown). Serum PRL concentrations in ovariectomized females injected with vehicle appeared lower than in the corresponding intact group, although differences were not statistically significant (8.41 ^ 2.08 ng/ml in ovariectomized rats vs. 15.98 ^ 8.87 ng/ml in intact animals). The serum PRL concentrations in ovariectomized females increased at the end of treatment with raloxifene (28.69 ^ 10.59 ng/ml vs. 8.41 ^ 2.08; P # 0:01), while raloxifene was ineffective in intact rats (data not shown). Raloxifene also increased in ovariectomized females some characteristics of pulsatile PRL secretion such as mean PRL levels and the net PRL secretion estimated as AUC (Table 1). In contrast, raloxi-
fene did not change pulsatile PRL secretion in intact females (data not shown). The effects of raloxifene on in vivo LH secretion are conflicting. It has been described that LH secretion was unaffected by raloxifene administration in castrated male rats [19] and in human volunteers [7]. In contrast, administration of raloxifene during 2–4 weeks to mice increased LH secretion [4]. The data now presented indicate that in ovariectomized females raloxifene exerts an estrogenic action in the neuroendocrine mechanisms governing LH and PRL secretion, since treatment with raloxifene during 5 days decreased LH secretion and increased serum PRL levels. The increase in serum LH concentrations after ovariectomy results from increases in both LH pulse amplitude and pulse frequency [12,14,15]. Administration of estradiol inhibited LH secretion in ovariectomized rats by significantly suppressing LH pulse amplitude and mean LH concentration, with a tendency to decrease pulse frequency also [15,18,25–27]. These effects are induced by inhibiting hypothalamic LHRH release and pituitary responsiveness to LHRH [1,12,16,17]. Our results clearly indicate that raloxifene reduced the hypersecretion of LH mimicking the negative feedback effects of estradiol. The inhibitory effect of raloxifene on pulsatile LH release may be exerted either at hypothalamic or pituitary level. The reduction of effectiveness of LHRH on LH release in vitro in presence of raloxifene either in females [11,20] or males (Pinilla L. et al. In preparation) supports a pituitary action of this compound. This hypothesis is reinforced by the reduction in the amplitude of LH pulses and the absence of changes in LH pulse frequency (Table 1). It has been described previously that, in presence of estrogens, raloxifene acts as an antiestrogen in pituitary gonadotrophs, blocking the effects of estradiol on LH release [25]. Thus, it is possible that, at the pituitary level, raloxifene may have either estrogenic (in absence of estradiol) or antiestrogenic activity (in presence of estradiol) depending on the prevailing estrogenic background. The pituitary action of raloxifene does not exclude an additional hypothalamic action; however, it should be considered that implantation of raloxifene in the hypothalamus of estrogen-treated ovariectomized rats attenuated the positive feedback of estradiol on LH secretion but did not affect the negative feedback [21,22]. Another well-known action of estrogens is the stimulatory effect on PRL release due to its action at hypothalamic and pituitary levels [5,8,13,24]. In ovariectomized females, raloxifene increased PRL secretion, an effect not observed in the intact group, which suggests again that the estrogenic effects of raloxifene become apparent in situations with low estrogenic activity. In conclusion our results indicate that in absence of estrogens, raloxifene exerts an estrogenic activity upon neuroendocrine network controlling both LH and PRL secretion.
LH
Pulse frequency (n pulses/3 h) Mean levels (ng/ml) Pulse amplitude (ng/ml) Trough levels (ng/ml) Duration (min/pulse) AUC a
PRL
Intact
Ovx
Ovx 1 Rx (500 mg/kg/day)
Intact
Ovx
Ovx 1 Rx (500 mg/kg/day)
3.40 ^ 0.68 0.44 ^ 0.06 0.34 ^ 0.05 0.18 ^ 0.03 39.40 ^ 6.69 79.16 ^ 11.17
5.33 ^ 0.56 4.98 ^ 0.58** 5.93 ^ 0.66** 2.24 ^ 0.35** 28.87 ^ 2.996 886.77 ^ 103.79**
6.60 ^ 0.60 2.61 ^ 0.41**,# 3.21 ^ 0.63**,# 1.14 ^ 0.30**,# 21.40 ^ 2.91* 460.63 ^ 72.32**,#
8.80 ^ 1.24 7.91 ^ 5.30 6.50 ^ 5.12 0.98 ^ 0.16 18.40 ^ 2.16 1411.63 ^ 955.60
7.83 ^ 0.31 5.01 ^ 1.19 7.35 ^ 4.90 1.68 ^ 0.29 19.00 ^ 0.68 894.87 ^ 211.666
7.00 ^ 0.82 30.16 ^ 10.80*,a 37.34 ^ 15.77* 3.45 ^ 1.69 22.00 ^ 2.92 53796.33 ^ 1936.09*,#
Values are expressed as means ^ SE (n ¼ 8–10 animals per group). *P # 0:05 vs intact group; **P # 0:01 vs ovx group; #P # 0:01 vs ovx group (ANOVA followed by Tukey’s test).
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Table 1 Characteristics of pulsatile LH and PRL secretion in intact, ovariectomized (ovx) and raloxifen-treated ovariectomized rats a
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