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A dual modulatory effect of progesterone on the LHRH-induced LH release
~22-4731/84 $3.00+ 0.00 Pergamon Press Ltd
J. sreroid &&em. Vol. 21, No. 1, pp. 107-110, 1984 Primed in Great Britain
A DUAL MODULATORY EFFECT OF PROGESTERONE ON THE LHRH-INDUCED LH RELEASE JAIME PRILUSKY, NELIA T. VERMOUTH* and RICARDO P. DErst Laboratorio de Reproduction y Lactancia (LARLAC), Casilla de Correos 855, S500 Mendoza, Argentina (Received 27 Jtdy 1983) Summary-The role of progesterone on the release of LH induced by 25 or 50 ng of LHRH was studied in proestrus rats in which spontaneous preovulatory release of LH was prevented by sodium pentobarbitone. After the SC. administration of progesterone (5 mg) at 18.00 h of diestrus day 2 or at 12.00 h of proestrus, serum LH was not detectable at 17.15 h of proestrus. Injections of 25 or 50 ng of LHRH at 17.00 h of proestrus induced a dose response release of LH 15 mm after. However the LH response to LHRH administration increased ~~i~~tly when progesterone was injected at 12.00 h of proestrus. The potentiating effect of progesterone seems to be exerted at pituitary level. Tire effect of LHRH and the enhanced response of the pituitary after progesterone treatment was prevented by the administration of the antiestrogen Tamoxifen in diestrus day 2. The release of LH induced by 50ng of LHRH on proestrus day was blocked by the previous injection of progesterone on diestrus day 2. The inhibition was maintained even though a second dose of progesterone was given at 12.00 h of proestrus. The simultaneous administration of estrogen and progesterone on diestrus day 2 did not prevent the inhibitory effect of progesterone. It is concluded that the facilitatory or inhibitory effect of progesterone on the release of LH induced by LHRH is dependent upon the previous sensitization of the pituitary to estrogen.
INTRODUCEION Previous works demonstrated changes in the response of the pituitary gland to a single injection of synthetic LHRH in different physiological and experimental conditions. The amount of gonadotrophin in plasma
was found to fluctuate accordingly to the stage of the cycle when the LHRH was given. The greatest sensitivity was obtained in the proestrus day [I, 2,3]. The changes in the pituitary sensitivity could be due to estrogen, progesterone, or a combination of both plus a priming effect of the LHRH itself. Many studies have indicated that estrogen has both inhibitory and facilitatory effects on pituitary responsiveness to LHRH [4,5]. The present work was undertaken to study the role of progesterone on the LHRH-induced LH release. A preliminary report of this research was presented in an abstract form [6]. MATERIALS AND METHODS The experiments were performed in virgin female Wistar rats, weighing between 200 and 230 g. The rats were kept in a constant temperature room (24°C k 2) with a lighting schedule of 14 hr light (06.00-20.00 h)/lO hr darkness. The animals were fed Purina lab chow and water ad libitum. Vaginal smears
were evaluated daily, and only those animals showing *Present address: Catedra de Quimica Biologica, Facultad de Ciencias MMicas, Universidad National de Cbrdoba, Cordoba, Argentina. ?To whom correspondence should be addressed.
at least two consecutive 4-day estrous cycles were used. Rats were treated with progesterone, estrogen, antiestrogen, and oil. Progesterone (Proluton, Schering Argentina) was injected S.C.in a dose of 5 mg/rat, at 18.00 h of diestrus day 2 and/or at noon of proestrus day. In a group of 5 rats, a single dose of 1 pg of estrogen (estradiol benzoate, Schering Argentina) was given S.C. on diestrus day 2, few minutes after progesterone administration. The antiestrogen ICI 46.474 (Tamoxifen, ICI, England) was suspended in a solution of 0.5% aqueous Tween 80 and administered by gavage in a dose of 0.5 mg/kg on diestrus day 2 at 12.00 h. Control rats were treated with oil or Tween-80. The study was carried out in proestrus rats treated with 3.5mg/lOOg bw of sodium pentobarbitone (Nembutal, Abbot Argentina) at 13.00 h in order to prevent the spontaneous pre-ovulatory release of LH, The synthetic LHRH used (kindly supplied by Hoechst, Argentina) was stored at -30°C at a concentration of 2.5 ng/pl saline. At 17.00 h of the proestrus day, 25 or 50ng of LHRH were injected into the external jugular vein under light ether anaesthesia. Control rats were injected with the same volume of normal saline. Fifteen minutes after the LHRH or saline injection, blood samples were obtained by heart puncture. This simple method takes only a few seconds without disturbing the animal [7]. Serum was separated by centrifugation at low speed at 4°C and stored frozen (-30°C) until the day of assay. The serum concentration of LH was determined by radioimmunoassay using the method of Niswender et aZ.[8]
107
JAIME PRILUSKY et al.
108
and the results expressed in terms of the NIAMDDrat-LH RP-I standard. The sensitivity of the assay ranged from 7.9 to 125 ng/sample. All serum samples were assayed in a single radioimmunoassay to eliminate inter-assay variations. For the statistical evaluation, the variances within 2 groups were tested for homogeneity using a Fisher test. A posterior analysis was performed using student’s t-test. RESULTS
Effect of progesterone on serum LH concentration
Serum LH was not detectable in control rats injected with oil and in the group of rats treated with progesterone on diestrus day 2 or on proestrus day respectively (Fig. 1). Pituitary response to LHRH gesterone treatment
before and after pro-
A detectable serum LH concentration was obtained 15 min after the administration of 25 or 50 ng of LHRH on proestrus day. Pretreatment with progesterone on proestrus noon potentiated significantly the effect of LHRH on LH release (P < 0.001). A LHRH dose-response for LH secretion was obtained in the rats with or without progesterone treatment. The dose-response elicited by the 25 or 50 ng of LHRH injected, was maintained in those animals treated with progesterone. It is interesting to note that progesterone potentiates the effect of 25 ng of LHRH, allowing a release of LH equivalent to that obtained after 50ng of the releasing hormone.
The release of LH induced by the administration of 50ng of LHRH on proestrus day was significantly blocked by the administration of 5mg of progesterone on diestrus day 2 (P < 0.001). The inhibitory effect of the progesterone administered on diestrus day 2 was maintained in spite of of a second dose of progesterone given at 12.00 h of proestrus day. In a group of rats treated simultaneously with 5 mg of progesterone and 1 pg of estrogen on diestrus day 2, the release of LH induced by LHRH was significantly prevented (Fig. 1). Effect of an antiestrogen on LH release
The administration of Tamoxifen (ICI 46.474) at noon of the diestrus day 2 reduced significantly (P < 0.001) the proestrus release of LH in response to 50 ng of LHRH (Fig. 2). Likewise, the facilitatory effect on the release of LH by progesterone administered on proestrus day, was prevented by the previous administration of Tamoxifen (Fig. 2). DISCUSSION
The present results suggest that the ability of synthetic LHRH to stimulate LH secretion at proestrus is dependent on the sequence of the ovarian hormones to which the hypothalamic-hypophyseal axis is exposed. It is well established an increase in estrogen secretion during the interval between late diestrus and the morning proestrus day (1,9]. The potentiating effect of progesterone administered in proestrus day ap-
160
6
120
-
2 \ F
3
6 _
80
E
2
cz 40 6
6
_
[I_, 16, h 0
LH-AH 5mm l/rg
E
_
P -
02 _
50 P
so
50
50 ng
0,
D2P
0,
dcY
Fig. 1. Serum LH concentration 15 min after the i.v. injection of 25 or 50 ng of LHRH or saline solution into rats in the afternoon of proestrus. The animals where treated with 5 mg of progesterone either at 17.00 h of diestrus day 2 (D2) at noon of proestrus (P) of on both days (D2P). Controls where injected with oil. One group received simultaneously 1 pg of estrogen and 5 mg of progesterone in diestrus day 2. The height of each bar and the line above the bar represent the mean concentration of the hormone + SEM. Number of animals in each group is shown at the top of each bar. Dotted line indicates the sensitivity limit of the assay (approx 10 ng/ml).
Progesterone on pituitary ~nsitivity to LHRH 80
7 -
60
-
6 -
7
OL LH-RH ICI 46474 5w
Pg
u
- .-. -
hours before. On the contrary, small doses of progesterone administered intravenously immediately before LHRH infusion failed to alter the pituitary responsiveness to the releasing hormone [5]. Even large doses of progesterone given S.C. have failed to alter the release of LH induced by LHRH in ovariectomized rats in the absence of large doses of estrogen [ 131. Everett [14], studying the dual action of progesterone in normal rats’ gonadotrophin secretion, suggested that the effect of progesterone might be determined by the levei of estrogen to which the hypothalamic-hypophyseal axis was exposed, before the injection of progesterone. It has been shown in the rabbit that in the absence of estrogen, only the inhibitory effect of progesterone is observed [ 151. On the other hand, the injection of progesterone on the diestrus day 2 suppress the rise of estrogen normally observed at proestrus[2]. It is possible then that the inhibitory effect of progesterone on the LHRHinduced LH release observed in this study, can be due to a lack of endogenous estrogen surge at the appropriate time. This should also explain the absence of a facilitatory effect of progesterone on proestrus day when a previous dose of progesterone on diestrus day 2 was already given. However, since the administration of 1 pg of estrogen at 18.00 h of diestrus day 2 was not able to prevent the inhibitory effect of progesterone given at the same time, this explanation is not fully satisfactory. Progesterone administered on diestrus day 2 may have prevented the modulatory effect of estrogen on the hypothalamic-hypophyseal axis level. Concluding, we can postulate that progesterone have a dual modulatory action, When injected after a priming effect of estrogen, potentiates the stimulatory action of LHRH on LH secretion. If progesterone is administered before the endogenous estrogen surge or together with estrogen, the effect of LHRH is significantly impaired.
1 111 7 -
--
109
--
-
50
50
@-Q
02 -
02 P
day
day
Fig. 2. Serum LH concentration 15 min after the i.v. injection of 50 ng of LHRH or saline solution into rats in the afternoon of proestrus. Two groups received antiestrogen (ICI 46474) by gavage at noon of dies&us day 2. In addition, one group was simultaneously injected with 5mg of progesterone. Control groups received oil or Tween-SO. The
height of each bar and the line above the bar represent the mean concentration of the hormone It SEM. Number of animals in each group is shown at the top of each bar. Dotted line indicates the sensitivity limit of the assay (approx 10 ng/ml).
pears to be dependent on the prior exposure of the animal to estrogen, since the LH response to LWRH injected on the afternoon of proestrus day was prevented by the administration of an antiestrogen at noon on dies&us day 2. In the absence of LHRH, progesterone was not able to release LH nor to increase the basal circulating levels of LH to a detectable amount. The fact that treatment with doses of sodium pentobarbitone sufficient to prevent ovulation, did not affect the potentiating effect of progesterone indicates that the steroid may exerts it facilitating action directly on the pituitary, without participation of a hypothalamic neuronal activity. It is interesting to mention that in ovariectomized rats primed with estrogen, the administration of progesterone induced LH and FSH release[lO]. This effect can be prevented by sodium pentobarbitone treatment [l I] indicating in these cases the participation of the CNS in the facilitatory response to progesterone. Our results show that the pituitary after being sensitized by estrogen is capable of secreting a significantly larger amount of LH in response to LHRH when progesterone is administered 6 h before. Aiyer and Fink[lZ] found that in rats ovariectomized and injected with 2.5 pg of estradiol on the diestrus preceeding the expected day of proestrus, the LH response to LHRH injected on proestrus was restablished when progesterone was administered a few
Acknowledgements-This work was supported by a grant from the Bid-Conicet Program and the Consejo National de investigaciones Cientificas y Technicas of Argentina (CONICET), from which the authors are Career scientists, We are grateful to the NiAMDD for their generous gift of radioimmunoassay reagents. REFERENCES
Brown-Grant K., Exley D. and Naftolin F.: Peripheral plasma oestradiol and luteinizing hormone concentrations during the oestrous cycle of the rat. J. Endow. 48 (1970) 295-296.
Kalra S. P. and Kalra P. S. Temporal interrelationships among circulating levels of estradiol, progesterone and LH during the rat estrous cycle: Effectsof exogenous progesterone. Endocrinolonv 95 (1974) Ill I-1718. kiyer M. S., Fink G. and Greig F.; Changes in the sensitivity of the pituitary gland to luteini~ng hormone releasing factor during the estrous cycle of the rat. J. Endow. 60 (1974) 47-64. 4. Negro-Vilar A., Orias R. and McCann S. M.: Evidence for a pituitary site of action of the acute inhibition of LH release by estrogen in the rat. Endocrinology 92 (1973) 1680-1684.
JAIMEPRILIJSKY et al.
10 5. Libertun C., Orias R. and McCann
S. M.: Biphasic effect of estrogen on the sensitivity of the pituitary to luteinizing hormone-releasing factor (LFR). Endocrinology 94 (1974) 1094-l 100. 6. Vermouth N. T., Deis R. P. and Prilusky J. Efecto de las hormonas ovaricas sobre la secretion de hormona luteinizante (LH) inducida nor LH-RH. Proceedinas of the VII Con&es;
Argentind Biologia, Argentina (1578).
Abstract 110. I. Vermouth N. T. and Deis R. P.: Prolactin release and lactogenesis after ovariectomy in pregnant rats: effect of ovarian hormones. J. Endocr. 63 (1974) 13-20. 8. Niswender G. D., Midgley A. R., Monroe S. E. and Reitchert L. E.: Radioimmunoassay for rat luteinizing hormone with antiovine LH serum and ovine LH-1311. Proc. Sot. exp. Biol. Med. 128 (1968) 807-8 11. 9. Yoshinaga K., Hawkins R. A. and Stocker J. F.: Estrogen secretion by the rat ovary in viuo during the estrous cycle and pregnancy. Endocrinology 85 (1969) 103-I 12.
10. Caligaris L., Astrada J. J. and Taleisnik S.: Release of luteinizing hormone induced by estrogen injection into ovariectomized rats. Endocrinology 89 (1971) 810-815. 11. Brown-Grant K.: Steroid hormone administration and gonadotrophin secretion in the gonadectomized rat. J. Endocr. 62 (1974) 319-332. 12. Aiyer M. S. and Fink G.: The role of sex steroid hormones in modulating the responsiveness of the anterior pituitary gland to luteinizing hormone releasing factor in the female rat. J. Endocr. 62 (1974) 553-572. 13. Libertun C., Cooper K. J., Fawcett C. P. and McCann S. M.: Effects of ovariectomy and steroid treatment on hypophyseal sensitivity to purified LH-Releasing Factor (LRF). Endocrinology 94 (1974) 5 18-525. 14. Everett J. W.: Progesterone and estrogen in the experimental control of ovulation time and other features of the estrous cycle in the rat. Endocrinology 43 (1948) 389-405.
15. Kawakami M. and Sawyer C. H.: Effects of sex hormones and antifertility steroids on brain thresholds in the rabbit. Endocrinology 80 (1967) 857-871.
J. sreroid &&em. Vol. 21, No. 1, pp. 107-110, 1984 Primed in Great Britain
A DUAL MODULATORY EFFECT OF PROGESTERONE ON THE LHRH-INDUCED LH RELEASE JAIME PRILUSKY, NELIA T. VERMOUTH* and RICARDO P. DErst Laboratorio de Reproduction y Lactancia (LARLAC), Casilla de Correos 855, S500 Mendoza, Argentina (Received 27 Jtdy 1983) Summary-The role of progesterone on the release of LH induced by 25 or 50 ng of LHRH was studied in proestrus rats in which spontaneous preovulatory release of LH was prevented by sodium pentobarbitone. After the SC. administration of progesterone (5 mg) at 18.00 h of diestrus day 2 or at 12.00 h of proestrus, serum LH was not detectable at 17.15 h of proestrus. Injections of 25 or 50 ng of LHRH at 17.00 h of proestrus induced a dose response release of LH 15 mm after. However the LH response to LHRH administration increased ~~i~~tly when progesterone was injected at 12.00 h of proestrus. The potentiating effect of progesterone seems to be exerted at pituitary level. Tire effect of LHRH and the enhanced response of the pituitary after progesterone treatment was prevented by the administration of the antiestrogen Tamoxifen in diestrus day 2. The release of LH induced by 50ng of LHRH on proestrus day was blocked by the previous injection of progesterone on diestrus day 2. The inhibition was maintained even though a second dose of progesterone was given at 12.00 h of proestrus. The simultaneous administration of estrogen and progesterone on diestrus day 2 did not prevent the inhibitory effect of progesterone. It is concluded that the facilitatory or inhibitory effect of progesterone on the release of LH induced by LHRH is dependent upon the previous sensitization of the pituitary to estrogen.
INTRODUCEION Previous works demonstrated changes in the response of the pituitary gland to a single injection of synthetic LHRH in different physiological and experimental conditions. The amount of gonadotrophin in plasma
was found to fluctuate accordingly to the stage of the cycle when the LHRH was given. The greatest sensitivity was obtained in the proestrus day [I, 2,3]. The changes in the pituitary sensitivity could be due to estrogen, progesterone, or a combination of both plus a priming effect of the LHRH itself. Many studies have indicated that estrogen has both inhibitory and facilitatory effects on pituitary responsiveness to LHRH [4,5]. The present work was undertaken to study the role of progesterone on the LHRH-induced LH release. A preliminary report of this research was presented in an abstract form [6]. MATERIALS AND METHODS The experiments were performed in virgin female Wistar rats, weighing between 200 and 230 g. The rats were kept in a constant temperature room (24°C k 2) with a lighting schedule of 14 hr light (06.00-20.00 h)/lO hr darkness. The animals were fed Purina lab chow and water ad libitum. Vaginal smears
were evaluated daily, and only those animals showing *Present address: Catedra de Quimica Biologica, Facultad de Ciencias MMicas, Universidad National de Cbrdoba, Cordoba, Argentina. ?To whom correspondence should be addressed.
at least two consecutive 4-day estrous cycles were used. Rats were treated with progesterone, estrogen, antiestrogen, and oil. Progesterone (Proluton, Schering Argentina) was injected S.C.in a dose of 5 mg/rat, at 18.00 h of diestrus day 2 and/or at noon of proestrus day. In a group of 5 rats, a single dose of 1 pg of estrogen (estradiol benzoate, Schering Argentina) was given S.C. on diestrus day 2, few minutes after progesterone administration. The antiestrogen ICI 46.474 (Tamoxifen, ICI, England) was suspended in a solution of 0.5% aqueous Tween 80 and administered by gavage in a dose of 0.5 mg/kg on diestrus day 2 at 12.00 h. Control rats were treated with oil or Tween-80. The study was carried out in proestrus rats treated with 3.5mg/lOOg bw of sodium pentobarbitone (Nembutal, Abbot Argentina) at 13.00 h in order to prevent the spontaneous pre-ovulatory release of LH, The synthetic LHRH used (kindly supplied by Hoechst, Argentina) was stored at -30°C at a concentration of 2.5 ng/pl saline. At 17.00 h of the proestrus day, 25 or 50ng of LHRH were injected into the external jugular vein under light ether anaesthesia. Control rats were injected with the same volume of normal saline. Fifteen minutes after the LHRH or saline injection, blood samples were obtained by heart puncture. This simple method takes only a few seconds without disturbing the animal [7]. Serum was separated by centrifugation at low speed at 4°C and stored frozen (-30°C) until the day of assay. The serum concentration of LH was determined by radioimmunoassay using the method of Niswender et aZ.[8]
107
JAIME PRILUSKY et al.
108
and the results expressed in terms of the NIAMDDrat-LH RP-I standard. The sensitivity of the assay ranged from 7.9 to 125 ng/sample. All serum samples were assayed in a single radioimmunoassay to eliminate inter-assay variations. For the statistical evaluation, the variances within 2 groups were tested for homogeneity using a Fisher test. A posterior analysis was performed using student’s t-test. RESULTS
Effect of progesterone on serum LH concentration
Serum LH was not detectable in control rats injected with oil and in the group of rats treated with progesterone on diestrus day 2 or on proestrus day respectively (Fig. 1). Pituitary response to LHRH gesterone treatment
before and after pro-
A detectable serum LH concentration was obtained 15 min after the administration of 25 or 50 ng of LHRH on proestrus day. Pretreatment with progesterone on proestrus noon potentiated significantly the effect of LHRH on LH release (P < 0.001). A LHRH dose-response for LH secretion was obtained in the rats with or without progesterone treatment. The dose-response elicited by the 25 or 50 ng of LHRH injected, was maintained in those animals treated with progesterone. It is interesting to note that progesterone potentiates the effect of 25 ng of LHRH, allowing a release of LH equivalent to that obtained after 50ng of the releasing hormone.
The release of LH induced by the administration of 50ng of LHRH on proestrus day was significantly blocked by the administration of 5mg of progesterone on diestrus day 2 (P < 0.001). The inhibitory effect of the progesterone administered on diestrus day 2 was maintained in spite of of a second dose of progesterone given at 12.00 h of proestrus day. In a group of rats treated simultaneously with 5 mg of progesterone and 1 pg of estrogen on diestrus day 2, the release of LH induced by LHRH was significantly prevented (Fig. 1). Effect of an antiestrogen on LH release
The administration of Tamoxifen (ICI 46.474) at noon of the diestrus day 2 reduced significantly (P < 0.001) the proestrus release of LH in response to 50 ng of LHRH (Fig. 2). Likewise, the facilitatory effect on the release of LH by progesterone administered on proestrus day, was prevented by the previous administration of Tamoxifen (Fig. 2). DISCUSSION
The present results suggest that the ability of synthetic LHRH to stimulate LH secretion at proestrus is dependent on the sequence of the ovarian hormones to which the hypothalamic-hypophyseal axis is exposed. It is well established an increase in estrogen secretion during the interval between late diestrus and the morning proestrus day (1,9]. The potentiating effect of progesterone administered in proestrus day ap-
160
6
120
-
2 \ F
3
6 _
80
E
2
cz 40 6
6
_
[I_, 16, h 0
LH-AH 5mm l/rg
E
_
P -
02 _
50 P
so
50
50 ng
0,
D2P
0,
dcY
Fig. 1. Serum LH concentration 15 min after the i.v. injection of 25 or 50 ng of LHRH or saline solution into rats in the afternoon of proestrus. The animals where treated with 5 mg of progesterone either at 17.00 h of diestrus day 2 (D2) at noon of proestrus (P) of on both days (D2P). Controls where injected with oil. One group received simultaneously 1 pg of estrogen and 5 mg of progesterone in diestrus day 2. The height of each bar and the line above the bar represent the mean concentration of the hormone + SEM. Number of animals in each group is shown at the top of each bar. Dotted line indicates the sensitivity limit of the assay (approx 10 ng/ml).
Progesterone on pituitary ~nsitivity to LHRH 80
7 -
60
-
6 -
7
OL LH-RH ICI 46474 5w
Pg
u
- .-. -
hours before. On the contrary, small doses of progesterone administered intravenously immediately before LHRH infusion failed to alter the pituitary responsiveness to the releasing hormone [5]. Even large doses of progesterone given S.C. have failed to alter the release of LH induced by LHRH in ovariectomized rats in the absence of large doses of estrogen [ 131. Everett [14], studying the dual action of progesterone in normal rats’ gonadotrophin secretion, suggested that the effect of progesterone might be determined by the levei of estrogen to which the hypothalamic-hypophyseal axis was exposed, before the injection of progesterone. It has been shown in the rabbit that in the absence of estrogen, only the inhibitory effect of progesterone is observed [ 151. On the other hand, the injection of progesterone on the diestrus day 2 suppress the rise of estrogen normally observed at proestrus[2]. It is possible then that the inhibitory effect of progesterone on the LHRHinduced LH release observed in this study, can be due to a lack of endogenous estrogen surge at the appropriate time. This should also explain the absence of a facilitatory effect of progesterone on proestrus day when a previous dose of progesterone on diestrus day 2 was already given. However, since the administration of 1 pg of estrogen at 18.00 h of diestrus day 2 was not able to prevent the inhibitory effect of progesterone given at the same time, this explanation is not fully satisfactory. Progesterone administered on diestrus day 2 may have prevented the modulatory effect of estrogen on the hypothalamic-hypophyseal axis level. Concluding, we can postulate that progesterone have a dual modulatory action, When injected after a priming effect of estrogen, potentiates the stimulatory action of LHRH on LH secretion. If progesterone is administered before the endogenous estrogen surge or together with estrogen, the effect of LHRH is significantly impaired.
1 111 7 -
--
109
--
-
50
50
@-Q
02 -
02 P
day
day
Fig. 2. Serum LH concentration 15 min after the i.v. injection of 50 ng of LHRH or saline solution into rats in the afternoon of proestrus. Two groups received antiestrogen (ICI 46474) by gavage at noon of dies&us day 2. In addition, one group was simultaneously injected with 5mg of progesterone. Control groups received oil or Tween-SO. The
height of each bar and the line above the bar represent the mean concentration of the hormone It SEM. Number of animals in each group is shown at the top of each bar. Dotted line indicates the sensitivity limit of the assay (approx 10 ng/ml).
pears to be dependent on the prior exposure of the animal to estrogen, since the LH response to LWRH injected on the afternoon of proestrus day was prevented by the administration of an antiestrogen at noon on dies&us day 2. In the absence of LHRH, progesterone was not able to release LH nor to increase the basal circulating levels of LH to a detectable amount. The fact that treatment with doses of sodium pentobarbitone sufficient to prevent ovulation, did not affect the potentiating effect of progesterone indicates that the steroid may exerts it facilitating action directly on the pituitary, without participation of a hypothalamic neuronal activity. It is interesting to mention that in ovariectomized rats primed with estrogen, the administration of progesterone induced LH and FSH release[lO]. This effect can be prevented by sodium pentobarbitone treatment [l I] indicating in these cases the participation of the CNS in the facilitatory response to progesterone. Our results show that the pituitary after being sensitized by estrogen is capable of secreting a significantly larger amount of LH in response to LHRH when progesterone is administered 6 h before. Aiyer and Fink[lZ] found that in rats ovariectomized and injected with 2.5 pg of estradiol on the diestrus preceeding the expected day of proestrus, the LH response to LHRH injected on proestrus was restablished when progesterone was administered a few
Acknowledgements-This work was supported by a grant from the Bid-Conicet Program and the Consejo National de investigaciones Cientificas y Technicas of Argentina (CONICET), from which the authors are Career scientists, We are grateful to the NiAMDD for their generous gift of radioimmunoassay reagents. REFERENCES
Brown-Grant K., Exley D. and Naftolin F.: Peripheral plasma oestradiol and luteinizing hormone concentrations during the oestrous cycle of the rat. J. Endow. 48 (1970) 295-296.
Kalra S. P. and Kalra P. S. Temporal interrelationships among circulating levels of estradiol, progesterone and LH during the rat estrous cycle: Effectsof exogenous progesterone. Endocrinolonv 95 (1974) Ill I-1718. kiyer M. S., Fink G. and Greig F.; Changes in the sensitivity of the pituitary gland to luteini~ng hormone releasing factor during the estrous cycle of the rat. J. Endow. 60 (1974) 47-64. 4. Negro-Vilar A., Orias R. and McCann S. M.: Evidence for a pituitary site of action of the acute inhibition of LH release by estrogen in the rat. Endocrinology 92 (1973) 1680-1684.
JAIMEPRILIJSKY et al.
10 5. Libertun C., Orias R. and McCann
S. M.: Biphasic effect of estrogen on the sensitivity of the pituitary to luteinizing hormone-releasing factor (LFR). Endocrinology 94 (1974) 1094-l 100. 6. Vermouth N. T., Deis R. P. and Prilusky J. Efecto de las hormonas ovaricas sobre la secretion de hormona luteinizante (LH) inducida nor LH-RH. Proceedinas of the VII Con&es;
Argentind Biologia, Argentina (1578).
Abstract 110. I. Vermouth N. T. and Deis R. P.: Prolactin release and lactogenesis after ovariectomy in pregnant rats: effect of ovarian hormones. J. Endocr. 63 (1974) 13-20. 8. Niswender G. D., Midgley A. R., Monroe S. E. and Reitchert L. E.: Radioimmunoassay for rat luteinizing hormone with antiovine LH serum and ovine LH-1311. Proc. Sot. exp. Biol. Med. 128 (1968) 807-8 11. 9. Yoshinaga K., Hawkins R. A. and Stocker J. F.: Estrogen secretion by the rat ovary in viuo during the estrous cycle and pregnancy. Endocrinology 85 (1969) 103-I 12.
10. Caligaris L., Astrada J. J. and Taleisnik S.: Release of luteinizing hormone induced by estrogen injection into ovariectomized rats. Endocrinology 89 (1971) 810-815. 11. Brown-Grant K.: Steroid hormone administration and gonadotrophin secretion in the gonadectomized rat. J. Endocr. 62 (1974) 319-332. 12. Aiyer M. S. and Fink G.: The role of sex steroid hormones in modulating the responsiveness of the anterior pituitary gland to luteinizing hormone releasing factor in the female rat. J. Endocr. 62 (1974) 553-572. 13. Libertun C., Cooper K. J., Fawcett C. P. and McCann S. M.: Effects of ovariectomy and steroid treatment on hypophyseal sensitivity to purified LH-Releasing Factor (LRF). Endocrinology 94 (1974) 5 18-525. 14. Everett J. W.: Progesterone and estrogen in the experimental control of ovulation time and other features of the estrous cycle in the rat. Endocrinology 43 (1948) 389-405.
15. Kawakami M. and Sawyer C. H.: Effects of sex hormones and antifertility steroids on brain thresholds in the rabbit. Endocrinology 80 (1967) 857-871.