Changes of Pituitary Thyrotropin Releasing Hormone (TRH) Receptor Level and Prolactin Response to TRH During the Rat Estrous Cycle A. DE LF.AN,1 M. GARON, P. A. KELLY, AND F. LABRIE2 Medical Research Council Group in Molecular Endocrinology, he Centre Hospitalier de I'Universite Laval, Quebec, G1V 4G2, Canada of [3H]TRH to anterior pituitary gland, a 3-fold difference being observed between the minimal binding measured on the morning of diestrus II and the maximal value found on the evening of proestrus. Contrary to findings with LHRH and LH, repeated injections of a small dose (10 ng) of TRH in the afternoon of proestrus abolished PRL and TSH responses to subsequent injection of the neurohormone. (Endocrinology 100: 1505, 1977)
ABSTRACT. The plasma PRL and TSH responses to TRH injected iv at different stages of the estrous cycle in normal rats under Surital anesthesia were maximal during the afternoon of proestrus and morning of estrus and lowest on diestrus I. As calculated from the areas under the plasma response curves, a 10-fold difference was found between the maximal and minimal PRL responses while a 2-fold difference was measured for TSH. The plasma PRL and TSH responses to TRH showed a correlation with the binding
M
UCH evidence indicates that the preovulatory surge of LH occurring in the afternoon of proestrus in the rat and at midcycle in women can, at least partly, be explained by increased pituitary sensitivity to LHRH (1-4). It has also been clearly shown that estrogens play an essential role in this increased pituitary response to the neurohormone (5-7). The rate of prolactin secretion is also increased in the afternoon of proestrus (8,9) and the stimulatory effect of estrogens on PRL secretion is well documented (10,11). In order to investigate possible changes of pituitary sensitivity to TRH during the estrous cycle, the plasma PRL and TSH response curves to the neurohormone were investigated in 4-day cycling rats under Surital anesthesia. Since we had recently found a close correlation between PRL responses to TRH and the concentration of pituitary TRH receptors (12), [3H]TRH binding was also measured at the same stages of the cycle. Received June 21, 1976. 1 Fellow of the Medical Research Council of Canada (MRC). 2 MRC Associate.
Materials and Methods Animals Female Sprague-Dawley rats (obtained from Canadian Breeding Farms, St. Constant, Quebec) weighing 200-225 g on arrival, were housed in individual cages, exposed to 14 h light-10 h dark cycles (lights on at 0500 and off at 1900 h) in a sound-attenuated and temperaturecontrolled (24 ± 2 C) room. Purina rat chow and water were available ad libitum. Animals were kept for one week before daily examination of vaginal smears which was performed between 0800 and 1000 h. Only animals showing 2 or 3 consecutive 4-day cycles were used in the present studies. Materials TRH (L-[2,3-3H]proline-labelled, 40 Ci/mmol) was obtained from New England Nuclear while the unlabelled hormone was kindly provided by Dr. Roger Guillemin, La Jolla, Ca. Cellulose nitrate filters (pore size, 0.45 /am) and purified goat anti-rabbit y-globulins were obtained from Sartorius and Endocrinolab Ltd, respectively. Thiamylal (Surital) was a product of Parke-Davis. [3H]TRH binding assay [3H]TRH binding was measured according to slight modifications (12) of the procedure
1505
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Endo i 1977 Vol 100 . No 6
DE LEAN ET AL.
1506
described (13), at a final concentration of 5 x 10~8M [3H]TRH in the presence or absence of 5 x 10" 6 M unlabelled TRH.
Diseases, Rat Pituitary Hormone Program. All data are presented as mean ± SEM of triplicate determinations.
In vivo TRH assay
Calculations
The assay was performed essentially as described (12) in rats under Surital anesthesia. Hormone assays Plasma TSH and PRL levels were measured by double-antibody radioimmunoassays (14,15) using purified rat hormones (TSH-RP-1, PRLRP-1, TSH-I-1 and PRL-I-1) and rabbit antisera (anti-rat TSH-S-1 and anti-rat PRL-S-2) kindly provided by Dr. A. F. Parlow for the National Institute of Arthritis, Metabolism, and Digestive
200 r
Radioimmunoassay data were analyzed with a Hewlett-Packard desk-top calculator using a program based on model II of Rodbard and Lewald (16). Statistical significance was measured using Student's t test or the multiple-range test of Duncan-Kramer (17). Binding data were analyzed according to Cleland (18) or Scatchard (19). The TSH and PRL responses to TRH were measured by calculating the net areas under the responses curves after subtraction of the plasma hormone concentration at time zero (20).
OIESTRUS
ESTRUS
FIG. 1. Plasma PRL response to TRH at different stages of the estrous cycle (D D, 0900 h; O O, 1200h; O---O, 1700 h; A - — - A , 2100 h). Rats were anesthetized with Surital (50 mg/kg, ip) 1 h before iv injection of 1 fig of TRH through a catheter inserted in the right superior vena cava. Plasma samples were withdrawn at the indicated time intervals for measurement of PRL. Data are presented as mean ± SEM of individual values obtained with 8 animals per group.
PROESTRUS
DIESTRUS 200
175 150 125 100
75 50 25
o!'0
15
30
45
60
0
15
45
60
MINUTES AFTER TRH INJECTION
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PRL RESPONSE TO TRH DURING ESTROUS CYCLE 3500
1507
ESTRUS
OIESTRUS I
3000
2500 •
FIG. 2. Plasma TSH response to TRH at different stages of the estrous cycle in the rat. Same experiment as described in Fig. 1.
15
30
45
60
0
15
30
45
60
MINUTES AFTER TRH INJECTION
Results In order to minimize possible interference by changes of endogenous TRH secretion during the course of experiments, the animals were anesthetized with Surital 1 h before the iv injection of TRH and were kept deeply anesthetized up to the last blood collection. In agreement with previous data (8,9), the basal plasma PRL levels were elevated in the afternoon of proestrus (Fig. 1). It can be seen that the amplitude of the secretory PRL response is markedly changed during the estrous cycle. As judged from the areas under the plasma response curves, the lowest response was observed on the morning of diestrus I (242 ± 31 ng PRL-RP-
1/min/ml). The response increased during the morning of proestrus to reach maximal values (approximately 2200 ng PRL-RP-1/min/ml) on the evening of proestrus and morning of estrus. At noon on estrus, a 50% decrease was already observed. As is illustrated in Fig. 2, the TSH response to TRH also changed during the rat estrous cycle, a 2-fold difference being observed between the lowest response at 1700 h on diestrus I (56.2 ± 2.6 ng TSH-RP-1/ min/ml) and the maximal value measured on the morning of estrus (105 ± 6.2 ng TSH-RP-1/min/ml) (F < 0.01). Contrary to PRL, the basal levels of plasma TSH did not change significantly during the estrous cycle, the values being only slightly higher
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 16 November 2015. at 04:42 For personal use only. No other uses without permission. . All rights reserved.
DE LEAN ETAL.
1508
Endo • 1977 Vol 100 • No 6
As observed for PRL, previous injection of TRH prevents the response to subsequent injections of the neurohormone. Discussion The present data show clearly that the PRL response to TRH varies markedly during the rat estrous cycle, a 10-fold higher response being found late on proestrous and early in estrus as compared 500 ESTRUS
DIESTRUS I
OIESTRUS H
PROESTRUS
FIG. 3. Changes in [3H]TRH binding to anterior pituitary
tissue
during
the
estrous
cycle.
400
4-Day
cycling rats were sacrificed at the indicated time intervals and [3H]TRH binding measured as described in Materials and Methods.
s> 300
200
in the morning than afternoon on each day. 100 As is illustrated in Fig. 3, modulation of the binding of [3H]TRH to anterior pituitary gland is also observed during the estrous cycle. Minimal binding is observed on 600 the morning of diestrus II (27.4 ± 3.0 fmol/ pituitary) while a maximum is reached in 500 the evening of proestrus (94.8 ± 9.8 fmol/ pituitary). A 2- to 3-fold difference is £ 400 thus observed between the minimal (diestrus II) and maximal (proestrus) values of 300 pituitary TRH receptor levels during the estrous cycle. Since LHRH can potentiate the LH 200 response to subsequent injection of the neurohormone (6,21,22), the PRL and TSH J2 100 responses to TRH were measured in the afternoon of proestrus after repeated in17 14 15 16 jection of a small dose (10 ng) of the TIME OF INJECTION (hoars) tripeptide. As is illustrated in Fig. 4, a FIG. 4. Effect of successive iv injections of TRH significant PRL response to this small on plasma PRL and TSH release. Animals were dose of TRH is observed only at 1530 h anesthetized with Surital at 1230 h of proestnis, and this response is completely abolished before iv injection of 10 ng of TRH at the indicated by previous injection of the neurohormone. times (arrows). The abscissa indicates the time of the day ( D - D , TRH injections at 1330, A significant TSH response to the same 1430 and 1530 h; O O, TRH injections at dose of TRH is seen in all groups although 1430 and 1530 h; O O, TRH injection at 1530 h a greater response is measured at 1530 h. only).
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 16 November 2015. at 04:42 For personal use only. No other uses without permission. . All rights reserved.
PRL RESPONSE TO TRH DURING ESTROUS CYCLE to diestrus I. Although a significant increase of the PRL response can be detected at 0900 h and 1200 h on proestrus, maximal sensitivity occurs at 1700 h, a time which coincides with the peak levels of plasma PRL. The high PRL response to TRH measured at 0900 h on the morning of estrus shows that the maximal secretory ability of the pituitary is maintained for at least 16 h in the rat. While some groups have found that the PRL surge lasts only a few hours in the afternoon and evening of proestrus (8,23,24), it should be mentioned that increased PRL secretion extending up to the morning of estrus has also been reported (25) and has occasionally been seen in our laboratory. The markedly increased PRL response to TRH found in the afternoon of proestrus and morning of estrus may offer an explanation for the high PRL secretion rate measured at the same stage of the estrous cycle. This increased pituitary sensitivity to TRH does not exclude, but certainly could facilitate an added hypothalamic influence which could allow finer adjustment of the PRL secretion rate. Estrogens are well known to stimulate PRL secretion in both man (10) and rat (11), this effect being at least partly exerted at the pituitary level (26). We have also recently reported that estrogen treatment leads to parallel increases in PRL secretion and pituitary TRH receptor levels (12). It is thus likely that both the increased level of pituitary TRH receptors and PRL response to the neurohormone observed on proestrus and estrus are secondary to increased estrogen secretion on the evening of diestrus II and morning of proestrus (27). There are many similarities between the LH and PRL surges in the afternoon of proestrus. The proestrus increased release of both hormones can be inhibited by pentobarbital (28) and nicotine (23). Moreover, both LH and PRL surges depend
1509
upon prior estrogen secretion (5,29). There are, however, instances of a dissociation between the secretion of these two hormones. For example, prevention of the LH surge following administration of a LHRH antiserum does not affect PRL secretion (30) while ergot derivatives, at low dose, inhibit completely the PRL surge without affecting that of LH (31). While the increased LH response to LHRH measured in the afternoon of proestrus in the rat (1,2) or at midcycle in women (3,4) could be explained by potentiation of the LH and FSH responses by previous secretion of the neurohormone (6,21,22), the present data indicate that a similar mechanism does not apply to the increased PRL response measured at about the same stage of the estrous cycle. Previous injection of TRH fails to potentiate the PRL response to subsequent injection of the neurohormone, and instead completely blocks the response. In agreement with our previous findings (12), the present data show that increased level of [3H]TRH binding is associated with increased TSH response to TRH. The TSH changes are, however, much smaller than those observed with PRL and their physiological significance remains to be assessed. References 1. Ferland, L., P. Borgeat, F. Labrie, J. Bernard, A. De Lean, and J. P. Raynaud, Mol Cell Endocrinol 2: 107, 1975. 2. Gordon, J. H., and S. Reichlin, Endocrinology 94: 974, 1974. 3. Nillius, S. J., and L. J. Wide, Obstet Gynaecol Br Commonw 78: 822, 1971. 4. Yen, S. S. C , G. Vandenberg, R. Rebar, and Y. Ehara, / Clin Endocrinol Metab 35: 931, 1972. 5. Ferin, M., A. Tempone, P. A. Zimmering, and R. L. Vande Wiele, Endocrinology 85: 1070, 1969. 6. Ferland, L., J. Drouin, and F. Labrie, In Labrie, F., J. Meites, and G. Pelletier (eds.), Hypothalamus and Endocrine Functions, Plenum Press, New York, 1976, p. 191. 7. Drouin, J., L. Lagace, and F. Labrie, Endocrinology 99: 1477, 1976.
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1510
DE LEAN ET AL.
8. Wuttke, W., and J. Meites, Proc Soc Exp Biol Med 135: 648, 1970. 9. Neill, J. D., M. E. Freeman, and S. A. Tillson, Endocrinology 89: 1448, 1971. 10. Yen, S. S. C , Y. Ehara, and T. M. Siler, J Clin Invest 53: 652, 1974. 11. Chen, L., and J. Meites, Endocrinology 86: 503, 1970. 12. De Lean, A., L. Ferland, J. Drouin, P. A. Kelly, and F. Labrie, Endocrinology 100: 1496, 1977. 13. Labrie, F., N. Barden, G. Poirier, and A. De Lean, Proc Natl Acad Sci USA 69: 283, 1972. 14. Birge, C. A., G. T. Peake, I. K. Maris, and W. H. Daughaday, Endocrinology 81: 195, 1967. 15. Odell, W. D., P. L. Rayford, and G. T. Ross, J Lab Clin Med 70: 973, 1967. 16. Rodbard, D., and J. E. Lewald, In Diczfalusy, E. (ed.), Karolinska Symposium on Research Methods in Reproductive Endocrinology, vol. 2, Stockholm, 1970, p. 79. 17. Kramer, C. Y., Biometrics 12: 307, 1956. 18. Cleland, W. W., Adv Enzymol 29: 1, 1967. 19. Scatchard, G., Ann NY Acad Sci 53: 660, 1949.
Endo • 1977 Vol 100 • No 6
20. Normand, M., and C. Fortier, Can J Physiol Pharmacol 48: 274, 1970. 21. Aiyer, M. S., S. A. Chiappa, G. Fink, and F. GriegJ Physiol (Lond) 234: 81P, 1973. 22. Castro-Vasquez, A., and S. M. McCann, Endocrinology 97: 13, 1975. 23. Blake, C. A., R. L. Norman, R. J. Scaramuzzi, and C. H. Sawyer, Endocrinology 92: 1334, 1973. 24. Kalra, S. P., K. Ajika, L. Krulich, C. P. Fawcett, M. Quijada, and S. M. McCann, Endocrinology 88: 1150, 1971. 25. Amenomori, Y., C. L. Chen, and J. Meites, J Endocrinol 86: 506, 1970. 26. Nicoll, C. S., and J. Meites, Proc Soc Exp Biol Med 11: 579, 1964. 27. Brown-Grant, K., D. Exley, and F. Naftolin, 7 Endocrinol 48: 295, 1970. 28. Wuttke, W., M. Gelato, and J. Meites, J Endocrinol 89: 1191, 1971. 29. Neill, J. D., Endocrinology 90: 1154, 1972. 30. Koch, Y., P. Chobsieng, U. Zor, M. Fridkin, and H. R. Lindner, Biochem Biophys Res Commun 55: 623, 1973. 31. Wuttke, W., E. Cassel, and J. Meites, Endocrinology 88: 737, 1971.
VI International Congress of Endocrinology The VI International Congress of Endocrinology will be held in Melbourne, Australia, from February 10-16, 1980. The Chairman of the Local Organizing Committee is: Dr. Bryan Hudson, Howard Florey Institute of Experimental Physiology & Medicine, University of Melbourne, Parkville, Victoria 3052, Australia. The Secretary (who will receive any enquiries re satellite meetings) is: Dr. D. M. de Kretser, Medical Research Centre, St. Kilda Road, Prince Henry's Hospital, Melbourne, Victoria 3004, Australia. The Treasurer is: Dr. J. K. Findlay, S. S. Cameron Laboratory, State Research Farm, Werribee, Victoria 3030, Australia. Subsequent bulletins will be issued from the Local Organizing Committee in Melbourne.
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ABSTRACT. The plasma PRL and TSH responses to TRH injected iv at different stages of the estrous cycle in normal rats under Surital anesthesia were maximal during the afternoon of proestrus and morning of estrus and lowest on diestrus I. As calculated from the areas under the plasma response curves, a 10-fold difference was found between the maximal and minimal PRL responses while a 2-fold difference was measured for TSH. The plasma PRL and TSH responses to TRH showed a correlation with the binding
M
UCH evidence indicates that the preovulatory surge of LH occurring in the afternoon of proestrus in the rat and at midcycle in women can, at least partly, be explained by increased pituitary sensitivity to LHRH (1-4). It has also been clearly shown that estrogens play an essential role in this increased pituitary response to the neurohormone (5-7). The rate of prolactin secretion is also increased in the afternoon of proestrus (8,9) and the stimulatory effect of estrogens on PRL secretion is well documented (10,11). In order to investigate possible changes of pituitary sensitivity to TRH during the estrous cycle, the plasma PRL and TSH response curves to the neurohormone were investigated in 4-day cycling rats under Surital anesthesia. Since we had recently found a close correlation between PRL responses to TRH and the concentration of pituitary TRH receptors (12), [3H]TRH binding was also measured at the same stages of the cycle. Received June 21, 1976. 1 Fellow of the Medical Research Council of Canada (MRC). 2 MRC Associate.
Materials and Methods Animals Female Sprague-Dawley rats (obtained from Canadian Breeding Farms, St. Constant, Quebec) weighing 200-225 g on arrival, were housed in individual cages, exposed to 14 h light-10 h dark cycles (lights on at 0500 and off at 1900 h) in a sound-attenuated and temperaturecontrolled (24 ± 2 C) room. Purina rat chow and water were available ad libitum. Animals were kept for one week before daily examination of vaginal smears which was performed between 0800 and 1000 h. Only animals showing 2 or 3 consecutive 4-day cycles were used in the present studies. Materials TRH (L-[2,3-3H]proline-labelled, 40 Ci/mmol) was obtained from New England Nuclear while the unlabelled hormone was kindly provided by Dr. Roger Guillemin, La Jolla, Ca. Cellulose nitrate filters (pore size, 0.45 /am) and purified goat anti-rabbit y-globulins were obtained from Sartorius and Endocrinolab Ltd, respectively. Thiamylal (Surital) was a product of Parke-Davis. [3H]TRH binding assay [3H]TRH binding was measured according to slight modifications (12) of the procedure
1505
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 16 November 2015. at 04:42 For personal use only. No other uses without permission. . All rights reserved.
Endo i 1977 Vol 100 . No 6
DE LEAN ET AL.
1506
described (13), at a final concentration of 5 x 10~8M [3H]TRH in the presence or absence of 5 x 10" 6 M unlabelled TRH.
Diseases, Rat Pituitary Hormone Program. All data are presented as mean ± SEM of triplicate determinations.
In vivo TRH assay
Calculations
The assay was performed essentially as described (12) in rats under Surital anesthesia. Hormone assays Plasma TSH and PRL levels were measured by double-antibody radioimmunoassays (14,15) using purified rat hormones (TSH-RP-1, PRLRP-1, TSH-I-1 and PRL-I-1) and rabbit antisera (anti-rat TSH-S-1 and anti-rat PRL-S-2) kindly provided by Dr. A. F. Parlow for the National Institute of Arthritis, Metabolism, and Digestive
200 r
Radioimmunoassay data were analyzed with a Hewlett-Packard desk-top calculator using a program based on model II of Rodbard and Lewald (16). Statistical significance was measured using Student's t test or the multiple-range test of Duncan-Kramer (17). Binding data were analyzed according to Cleland (18) or Scatchard (19). The TSH and PRL responses to TRH were measured by calculating the net areas under the responses curves after subtraction of the plasma hormone concentration at time zero (20).
OIESTRUS
ESTRUS
FIG. 1. Plasma PRL response to TRH at different stages of the estrous cycle (D D, 0900 h; O O, 1200h; O---O, 1700 h; A - — - A , 2100 h). Rats were anesthetized with Surital (50 mg/kg, ip) 1 h before iv injection of 1 fig of TRH through a catheter inserted in the right superior vena cava. Plasma samples were withdrawn at the indicated time intervals for measurement of PRL. Data are presented as mean ± SEM of individual values obtained with 8 animals per group.
PROESTRUS
DIESTRUS 200
175 150 125 100
75 50 25
o!'0
15
30
45
60
0
15
45
60
MINUTES AFTER TRH INJECTION
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PRL RESPONSE TO TRH DURING ESTROUS CYCLE 3500
1507
ESTRUS
OIESTRUS I
3000
2500 •
FIG. 2. Plasma TSH response to TRH at different stages of the estrous cycle in the rat. Same experiment as described in Fig. 1.
15
30
45
60
0
15
30
45
60
MINUTES AFTER TRH INJECTION
Results In order to minimize possible interference by changes of endogenous TRH secretion during the course of experiments, the animals were anesthetized with Surital 1 h before the iv injection of TRH and were kept deeply anesthetized up to the last blood collection. In agreement with previous data (8,9), the basal plasma PRL levels were elevated in the afternoon of proestrus (Fig. 1). It can be seen that the amplitude of the secretory PRL response is markedly changed during the estrous cycle. As judged from the areas under the plasma response curves, the lowest response was observed on the morning of diestrus I (242 ± 31 ng PRL-RP-
1/min/ml). The response increased during the morning of proestrus to reach maximal values (approximately 2200 ng PRL-RP-1/min/ml) on the evening of proestrus and morning of estrus. At noon on estrus, a 50% decrease was already observed. As is illustrated in Fig. 2, the TSH response to TRH also changed during the rat estrous cycle, a 2-fold difference being observed between the lowest response at 1700 h on diestrus I (56.2 ± 2.6 ng TSH-RP-1/ min/ml) and the maximal value measured on the morning of estrus (105 ± 6.2 ng TSH-RP-1/min/ml) (F < 0.01). Contrary to PRL, the basal levels of plasma TSH did not change significantly during the estrous cycle, the values being only slightly higher
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 16 November 2015. at 04:42 For personal use only. No other uses without permission. . All rights reserved.
DE LEAN ETAL.
1508
Endo • 1977 Vol 100 • No 6
As observed for PRL, previous injection of TRH prevents the response to subsequent injections of the neurohormone. Discussion The present data show clearly that the PRL response to TRH varies markedly during the rat estrous cycle, a 10-fold higher response being found late on proestrous and early in estrus as compared 500 ESTRUS
DIESTRUS I
OIESTRUS H
PROESTRUS
FIG. 3. Changes in [3H]TRH binding to anterior pituitary
tissue
during
the
estrous
cycle.
400
4-Day
cycling rats were sacrificed at the indicated time intervals and [3H]TRH binding measured as described in Materials and Methods.
s> 300
200
in the morning than afternoon on each day. 100 As is illustrated in Fig. 3, modulation of the binding of [3H]TRH to anterior pituitary gland is also observed during the estrous cycle. Minimal binding is observed on 600 the morning of diestrus II (27.4 ± 3.0 fmol/ pituitary) while a maximum is reached in 500 the evening of proestrus (94.8 ± 9.8 fmol/ pituitary). A 2- to 3-fold difference is £ 400 thus observed between the minimal (diestrus II) and maximal (proestrus) values of 300 pituitary TRH receptor levels during the estrous cycle. Since LHRH can potentiate the LH 200 response to subsequent injection of the neurohormone (6,21,22), the PRL and TSH J2 100 responses to TRH were measured in the afternoon of proestrus after repeated in17 14 15 16 jection of a small dose (10 ng) of the TIME OF INJECTION (hoars) tripeptide. As is illustrated in Fig. 4, a FIG. 4. Effect of successive iv injections of TRH significant PRL response to this small on plasma PRL and TSH release. Animals were dose of TRH is observed only at 1530 h anesthetized with Surital at 1230 h of proestnis, and this response is completely abolished before iv injection of 10 ng of TRH at the indicated by previous injection of the neurohormone. times (arrows). The abscissa indicates the time of the day ( D - D , TRH injections at 1330, A significant TSH response to the same 1430 and 1530 h; O O, TRH injections at dose of TRH is seen in all groups although 1430 and 1530 h; O O, TRH injection at 1530 h a greater response is measured at 1530 h. only).
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 16 November 2015. at 04:42 For personal use only. No other uses without permission. . All rights reserved.
PRL RESPONSE TO TRH DURING ESTROUS CYCLE to diestrus I. Although a significant increase of the PRL response can be detected at 0900 h and 1200 h on proestrus, maximal sensitivity occurs at 1700 h, a time which coincides with the peak levels of plasma PRL. The high PRL response to TRH measured at 0900 h on the morning of estrus shows that the maximal secretory ability of the pituitary is maintained for at least 16 h in the rat. While some groups have found that the PRL surge lasts only a few hours in the afternoon and evening of proestrus (8,23,24), it should be mentioned that increased PRL secretion extending up to the morning of estrus has also been reported (25) and has occasionally been seen in our laboratory. The markedly increased PRL response to TRH found in the afternoon of proestrus and morning of estrus may offer an explanation for the high PRL secretion rate measured at the same stage of the estrous cycle. This increased pituitary sensitivity to TRH does not exclude, but certainly could facilitate an added hypothalamic influence which could allow finer adjustment of the PRL secretion rate. Estrogens are well known to stimulate PRL secretion in both man (10) and rat (11), this effect being at least partly exerted at the pituitary level (26). We have also recently reported that estrogen treatment leads to parallel increases in PRL secretion and pituitary TRH receptor levels (12). It is thus likely that both the increased level of pituitary TRH receptors and PRL response to the neurohormone observed on proestrus and estrus are secondary to increased estrogen secretion on the evening of diestrus II and morning of proestrus (27). There are many similarities between the LH and PRL surges in the afternoon of proestrus. The proestrus increased release of both hormones can be inhibited by pentobarbital (28) and nicotine (23). Moreover, both LH and PRL surges depend
1509
upon prior estrogen secretion (5,29). There are, however, instances of a dissociation between the secretion of these two hormones. For example, prevention of the LH surge following administration of a LHRH antiserum does not affect PRL secretion (30) while ergot derivatives, at low dose, inhibit completely the PRL surge without affecting that of LH (31). While the increased LH response to LHRH measured in the afternoon of proestrus in the rat (1,2) or at midcycle in women (3,4) could be explained by potentiation of the LH and FSH responses by previous secretion of the neurohormone (6,21,22), the present data indicate that a similar mechanism does not apply to the increased PRL response measured at about the same stage of the estrous cycle. Previous injection of TRH fails to potentiate the PRL response to subsequent injection of the neurohormone, and instead completely blocks the response. In agreement with our previous findings (12), the present data show that increased level of [3H]TRH binding is associated with increased TSH response to TRH. The TSH changes are, however, much smaller than those observed with PRL and their physiological significance remains to be assessed. References 1. Ferland, L., P. Borgeat, F. Labrie, J. Bernard, A. De Lean, and J. P. Raynaud, Mol Cell Endocrinol 2: 107, 1975. 2. Gordon, J. H., and S. Reichlin, Endocrinology 94: 974, 1974. 3. Nillius, S. J., and L. J. Wide, Obstet Gynaecol Br Commonw 78: 822, 1971. 4. Yen, S. S. C , G. Vandenberg, R. Rebar, and Y. Ehara, / Clin Endocrinol Metab 35: 931, 1972. 5. Ferin, M., A. Tempone, P. A. Zimmering, and R. L. Vande Wiele, Endocrinology 85: 1070, 1969. 6. Ferland, L., J. Drouin, and F. Labrie, In Labrie, F., J. Meites, and G. Pelletier (eds.), Hypothalamus and Endocrine Functions, Plenum Press, New York, 1976, p. 191. 7. Drouin, J., L. Lagace, and F. Labrie, Endocrinology 99: 1477, 1976.
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 16 November 2015. at 04:42 For personal use only. No other uses without permission. . All rights reserved.
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VI International Congress of Endocrinology The VI International Congress of Endocrinology will be held in Melbourne, Australia, from February 10-16, 1980. The Chairman of the Local Organizing Committee is: Dr. Bryan Hudson, Howard Florey Institute of Experimental Physiology & Medicine, University of Melbourne, Parkville, Victoria 3052, Australia. The Secretary (who will receive any enquiries re satellite meetings) is: Dr. D. M. de Kretser, Medical Research Centre, St. Kilda Road, Prince Henry's Hospital, Melbourne, Victoria 3004, Australia. The Treasurer is: Dr. J. K. Findlay, S. S. Cameron Laboratory, State Research Farm, Werribee, Victoria 3030, Australia. Subsequent bulletins will be issued from the Local Organizing Committee in Melbourne.
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