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Restraint stress depresses prolactin surges in pseudopregnant rats and adrenalectomy does not alter the response
Life Sciences, Vol. 41, pp. I~91-1498 Printed in the U.S.A.
Pergamon Journals
RESTRAINT STRESS DEPRESSES PROLACTIN SURGES IN PSEUDOPREGNANT RATS AND ADRENAVECTOMY DOES NOT ALTER THE RESPONSE~ Marilyn H. Morehead2 and Richard R. Gala3 Department of Physiology Wayne State University School of Medicine, Detroit, Michigan 48201 (Received in final form July 14, 1987)
Summary Experiments were performed to determine whether r e s t r a i n t stress decreases the two prolactin (PRL) surges in pseudopregnant (PSP) rats in a manner similar to the stress-induced decrease of the proestrous PRL surge. Adrenal involvement as well as adaptation of the response was also investigated. Vaginal cycles were followed and animals exhibiting 2-3 normal cycles were c e r v i c a l l y stimulated (CS) electromechanically to induce PSP. In one experiment the effect of adrenalectomy (ADX) on the nocturnal surge (NS) was investigated and was found to have no effect. In another set of experiments the effect of restraint stress was investigated. Immediately following an i n i t i a l sample, the animals to be stressed had their hind legs tied together with plastic coated bell wire. Subsequent samples were taken for 3 hours. Restraint stress decreased the NS to 15% of the i n i t i a l value within 30 minutes. ADX did not a l t e r this response. Furthermore, 6-9 days of 3 hours of restraint stress did not attenuate the stress-induced decrease of the NS. Restraint stress also depressed the diurnal surge in PSP rats. These results indicate that restraint stress applied during the two PRL surges of PSP results in significant decreases in plasma PRL and that this response is not altered by ADX or by habituation to the stimulus.
Stress in various forms ( r e s t r a i n t , bleeding, ether) applied to rats when prolactin (PRL) levels are basal stimulates a significant increase in plasma PRL (1-4). This effect is well known and affects both sexes. Paradoxically, when stress is applied to ovariectomized (OVX), estrogen-treated female rats in the afternoon during the PRL surge, or during the afternoon proestrous PRL surge, i t evokes a precipitous decrease in the hormone (2,4,5,6,7). Since stress activates the hypothalamo-hypophyseal-adrenal axis and corticosterone has been shown to depress the stress-induced increase in PRL (3,8), adrenal involvement in the stress-induced decrease was investigated. Adrenalectomized (ADX) animals responded in the same manner as intact sham-operated animals indicating that the adrenal gland is not involved in the mediation of the stress-induced decrease of the afternoon PRL surge (7).
in part by WSUSM BRSG Grant #NS52-S07 RR05384-24. ~Supported Submitted as partial requirement for Ph.D. dissertation. 3To whom a l l correspondence and reprint requests should be directed. 0024-3205/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd.
1492
Stress Decreases PRL Surges in PSP Rats
Vol. 41, No. 12, 1987
The purpose of t h i s investigation was to see i f r e s t r a i n t stress applied during other periods of PRL elevation resulted in a similar decrease. To t h i s end we have examined the effects of r e s t r a i n t stress on the nocturnal (NS) and diurnal surges (DS) of pseudopregnant (PSP) rats. In addition, we have also investigated the influence of ADX on the stress-induced decrease of the PRL NS. Materials and Methods Adult female Sprague-Dawley rats from Charles River (Portage, MI) were housed 2/cage, placed on a 14-10 reverse l i g h t i n g schedule (L on at 1800, o f f at 0800) and provided with food and water ad libitum. Estrous cycles were monitored by vaginal lavage and only 4-5 day normally cycling animals were used. Two to three weeks after i n i t i a t i o n of vaginal lavage, when animals were 250-275g in body weight, they were c e r v i c a l l y stimulated (CS) on the morning of metestrus by mechano-electrical means using electrodes placed in a glass speculum. The electrical stimulus consisting of 5 volts, 20 cps with a 25 msec duration was applied continuously for 45 sec while the speculum and electrode were thrusted against the cervix. Continuousvaginal diestrus was used as an indication of PSP. Forty-eight to seventy-two hours prior to blood sampling, chronic a t r i a l catheters were implanted via the external jugular vein. Adrenals were removed from some animals and others were sham-operated at the time of catheterization. ADX animals were given a 1% saline drinking solution. A l l animals were bled on days 5-8 of PSP (day of CS is day 0). In the f i r s t experiment the influence of ADX on the PRL NS was examined. Here animals were attached to catheter extensions at 2200h rat time and blood samples (0.4 ml) were obtained at 0000, 0200, 0400, 0600, 0800, and lO00h replacing the f l u i d removed each time with warm (37°C), s t e r i l e , heparinized (50 IU/ml) saline (0.9% NaCl). For the stress experiments the animals were attached to catheter extensions at 0000 h rat time for the NS and 1400 h for the DS. The f i r s t blood sample of 0.4 ml was drawn at 0400 h (NS) and 1800 h (DS) and the f l u i d replaced as described above. Immediately following t h i s sample those animals to be stressed had t h e i r hind legs tied together with plastic coated bell wire. Subsequent samples were drawn at 15, 30, 60, 120 and 180 minutes following the i n i t i a l sample, replacing the f l u i d removed each time. The effects of repeated daily stress were examined by daily stressing of the animals from 0400-0700 h rat time beginning on day 1 of PSP and continuing for 6-9 days. On day 6-9 blood was obtained from the animals using the protocol described above for the stress experiments. Each blood sample was added to an equal volume of phosphate-buffered saline without methiolate immediately after i t was taken. At the end of blood sampling the samples were centrifuged and the diluted plasma harvested and stored frozen at -20°C u n t i l assayed. Plasma prolactin was assayed at two d i l u t i o n s in duplicate using a homologous rat prolactin RIA as previously reported (9). The iodination standards were NIAMDD RPI4 and RPI5 and the assay standard was NIAMDD RP-1 which had a biological potency of 111U/mg. Plasma levels were presented as a percent change from i n i t i a l value in the stress experiments. The data were analyzed s t a t i s t i c a l l y using two-way analysis of variance with computer assistance comparing no r e s t r a i n t and r e s t r a i n t PRL values. Oneway analysis of variance was used to compare i n i t i a l values with subsequent stressed values. Duncan'sMultiple Range test was used to determine the level
Vol. 41, No. 12, 1987
600
Stress Decreases PRL Surges in PSP Rats
=
-
1493
= SHAM (7-8)
• . . . . • ADX(7-8)
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100 0
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200
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i
600 400 TIME OF DAY (h)
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I
800
1000
FIG. 1 Plasma prolactin (PRL) levels of adrenalectomized (ADX) pseudopregnant (PSP) rats during the nocturnal surge (NS). Animals were sampled every 2 hrs over a 10 h-period 6-9 days following cervical stimulation. The heavy l i n e on the X-axis represents l i g h t s o f f . The numbers in parentheses are the number of animals per group. The v e r t i c a l lines at each point represent the standard error of the mean. of significance. A level of p r o b a b i l i t y of P < 0.05 was considered s t a t i s t i cally significant. Results The effect of ADX on the NS of PSP animals is shown in Fig. 1. In both ADX and sham-operated animals the surge peaked between 0400-0600 h and PRL was down to basal levels by 0800 h. There was no s i g n i f i c a n t difference in the two curves (P > 0.05). The results of the effects of r e s t r a i n t stress on the NS are shown in Fig. 2. The i n i t i a l 0400 h values in t h i s experiment for the non-stressed animals and stressed animals was 502.8 ng/ml and 822.3 ng/ml respectively. Within 30 minutes of the application of stress, plasma PRL levels were 15% of the i n i t i a l value compared to 97% of the i n i t i a l value for the no r e s t r a i n t group. Restraint stress s i g n i f i c a n t l y decreased the NS (P < 0.001).
1494
Stress Decreases PRL Surges in PSP Rats
M .---a
Vol. 41, No. 12, 1987
NO RESTRAINT (9) RESTRAINT (7)
FIG. 2 Plasma prolactin (PRL) levels of restraint-stressed, pseudopregnant (PSP) rats during the nocturnal surge (NS). At 0400 h following an initial sample, restraint animals had their hind legs tied together. Subsequent samples were taken at the times indicated. Restraint stress significantly decreased the NS (P < 0.001). The heavy line on the X-axis represents lights off. The numbers in parentheses indicate the number of animals per group. The vertical lines at each point represent the standard error of the mean. As shown in Fig. 3, ADX had no effect on the restraint stress-induced decrease of the NS. Initial 0400 h values for sham ADX animals and ADX animals were 412.3 ng/ml and 552.0 ng/ml respectively. There was no significant different between the effect of restraint stress on ADX compared to sham operated animals (P > 0.05). The effect of restraint stress on the DS is shown in Fig. 4. Although this surge is small in magnitude (peak values of 200-250 ng/ml in individual animals), restraint stress significantly decreased plasma PRL (P < 0.01). Fig. 5 shows that 6-9 days of daily restraint stress does not attenuate the stress-induced decrease in plasma PRL compared to no-restraint rats. The response remains intact and the restraint values were significantly different from no restraint animals (P < 0.001).
Stress Decreases PRL Surges in PSP Rats
vol. 41, No. 12, 1987
z
PSP-RESTRAINT
100
M o---e
1495
SHAM ADX (5) ADX (5)
5f 8 d
80
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I
I
500
600
760
TIME OF DAY (h) FIG. 3 Plasma prolactin (PRL) levels of restraint-stressed, adrenalectomized (ADX) pseudopregnant (PSP) rats during the nocturnal surge (NS). Following the initial (0400 h) sample, restraint animals had their hind legs tied together. Subsequent samples were taken at the indicated times. ADX did not affect the restraint stress-induced decrease of the NS (P > 0.05). The heavy line on the X-axis represents lights off. The numbers in parentheses are the number of animals per group. The vertical lines at each point represent the standard error of the mean. Discussion Restraint stress significantly decreased the PRL NS and DS of PSP rats. The response was similar to the stress-induced decrease of the afternoon surge in proestrous and OVX, estrogen-treated rats (2,5,6,7). Adrenalectomy does not affect the stress-induced decrease of the PRL NS, an observation also recorded for the stress-induced decrease of the afternoon surge in OVX, estrogen-treated rats (7). However, unlike the afternoon surge where ADX retarded the initiation of the surge by 2 hours (5), the absence of the adrenal gland had no effect on the timing of the NS as shown in Fig. 1. The lack of an effect of the adrenal gland on the timing of the NS compared to the afternoon surge may reflect the circadian rhythm of the pituitary-adrenal axis. At the peak of the afternoon PRL surge, corticosterone levels are near crest values and have been shown to increase to 3-fold as a result of handling or novel environment stress (3). However during the NS corticosterone levels are near trough levels (approximately 50% of crest
Vol. 41, No. 12, 1987
Stress Decreases PRL Surges in PSP Rats
250 200 15010050-
0!
I
1700
I
1750
I
1800
I
I
1900
I
1
2000
TIME OF DAY (h)
FIG. 4 Plasma prolactin (PRL) levels of restraint-stressed pseudopregnant (PSP) rats during the diurnal surge (DS). Following the initial (1700 h) sample, stressed animals had their hind legs tied together. Subsequent samples were taken at times indicated. Restraint stress significantly depressed the DS of PRL compared to control animals (P < 0.01). The numbers in parentheses are the number of animals per The vertical lines at each point represent the standard error group. of the mean. levels) and have been shown to increase 4-fold as a result of novel environment Moreover, when corticosterone levels are near the crest of the stress (3). circadian rhythm, stress resulted in a prolonged elevation of corticosterone (values were still elevated one hour following stress) while stress during the followed by a decline to near basal trough of the cycle elicited an increase levels within 1 hour of novel environment stress (3). Since corticosterone levels are near or at trough levels during the NS, the resulting loss of corticosterone by ADX could be expected to have little effect on the timing of the PRL NS. Of interest is the fact that daily stress does not result in the conAfter 6-9 days of 3 hours of restraint stress, ditioning out of the response. PRL values are still depressed to 26% of the initial value 30 minutes after the Initial values were 229.2 ng/ml and 383.7 ng/ml initiation of the restraint. for stressed and non-stressed animals respectively (Fig. 5). In contrast, the stress-induced increase in PRL shows adaptation (5,lO). Smith and Gala (5) demonstrated that following daily injections of corn oil for 4 days, PRL
Stress Decreases PRL Surges in PSP Rats
Vol. 41, No. 12, 1987
o-o NO RESTRAINT (6) l--oRESTRAlNT (6)
PSP
140-
1497
120loo806040ZOoioo
04’30
oioo
’
06bo
’
07iIo
TIME OF DAY (h)
FIG. 5 Plasma prolactin (PRL) levels of pseudopregnant (PSP) rats during the nocturnal PRL surge (NS) following 6-9 days of restraint stress. Following the initial (0400 h) sample, stressed animals had their hind legs tied together. Subsequent samples were taken at times indicated. Restraint stress significantly decreased the PRL NS compared to control animals (P < 0.001). The heavy line on the Xaxis represents lights off. The numbers in parentheses are the number of animals per group. The vertical lines at each point represent the standard error of the mean. values of rats subjected to restraint stress showed no increase over levels. Yelvington et al. found that daily handling obliterated the -7 stress-induced increase in PRL (10). This difference in habituation same stimulus when PRL values are low vs. when they are high provides evidence that the regulation of PRL secretion differs depending upon tial level of plasma PRL in different physiological conditions.
basal handling to the further the ini-
Acknowledgement The authors would like to express their Agency of the NIAMDD for providing as a gift iodination and standards, respectively.
appreciation to the Pituitary the RP-I4 & 5 and RP-1 used for
1498
Stress Decreases PFU Surges in PSP Rats
Vol. 41, No. 12, 1987
References 1. 2. 3. 4. 5. 6. 7. 8.
J.D. NEILL, Endocrinol. 87 1192-1197 (1970). W.K. MORISHIGE and I. ROflCHILD, Neuroendocrinol. 16 95-106 (1974). J.A. SEGGIE and G.M. BROWN, Can. J. Physiol. Pharmacol. 53 629-637 (1975). G.D. RIEGLE and J. MEITES, Proc. Sot. Exp. Biol. Med. 152 441-448 (1976). S.W. SMITH and R.R. GALA, J. Endocrinol. 74 303-314 (1977). R.R. GALA and D.J. HAISENLEDER, Neuroendozinol. 43 115-123 (1986). R.R. GALA and D.J. HAISENLEDER, Life Sci. 31 875-879 (1982). M.G. SUBRAMANIAN and R.R. GALA, Proc. Sot. Exp. Biol. Med. 157 415-417 (1978). E.Y.H. KU0 and R.R. GALA, Biochim. Biophys. Acta 264 462-471 (1972). D.B. YELVINGTON, G.K. WEISS and A. RATNER, Psychoneuroendocrinol. -10 95-102 (1985).
1;:
Pergamon Journals
RESTRAINT STRESS DEPRESSES PROLACTIN SURGES IN PSEUDOPREGNANT RATS AND ADRENAVECTOMY DOES NOT ALTER THE RESPONSE~ Marilyn H. Morehead2 and Richard R. Gala3 Department of Physiology Wayne State University School of Medicine, Detroit, Michigan 48201 (Received in final form July 14, 1987)
Summary Experiments were performed to determine whether r e s t r a i n t stress decreases the two prolactin (PRL) surges in pseudopregnant (PSP) rats in a manner similar to the stress-induced decrease of the proestrous PRL surge. Adrenal involvement as well as adaptation of the response was also investigated. Vaginal cycles were followed and animals exhibiting 2-3 normal cycles were c e r v i c a l l y stimulated (CS) electromechanically to induce PSP. In one experiment the effect of adrenalectomy (ADX) on the nocturnal surge (NS) was investigated and was found to have no effect. In another set of experiments the effect of restraint stress was investigated. Immediately following an i n i t i a l sample, the animals to be stressed had their hind legs tied together with plastic coated bell wire. Subsequent samples were taken for 3 hours. Restraint stress decreased the NS to 15% of the i n i t i a l value within 30 minutes. ADX did not a l t e r this response. Furthermore, 6-9 days of 3 hours of restraint stress did not attenuate the stress-induced decrease of the NS. Restraint stress also depressed the diurnal surge in PSP rats. These results indicate that restraint stress applied during the two PRL surges of PSP results in significant decreases in plasma PRL and that this response is not altered by ADX or by habituation to the stimulus.
Stress in various forms ( r e s t r a i n t , bleeding, ether) applied to rats when prolactin (PRL) levels are basal stimulates a significant increase in plasma PRL (1-4). This effect is well known and affects both sexes. Paradoxically, when stress is applied to ovariectomized (OVX), estrogen-treated female rats in the afternoon during the PRL surge, or during the afternoon proestrous PRL surge, i t evokes a precipitous decrease in the hormone (2,4,5,6,7). Since stress activates the hypothalamo-hypophyseal-adrenal axis and corticosterone has been shown to depress the stress-induced increase in PRL (3,8), adrenal involvement in the stress-induced decrease was investigated. Adrenalectomized (ADX) animals responded in the same manner as intact sham-operated animals indicating that the adrenal gland is not involved in the mediation of the stress-induced decrease of the afternoon PRL surge (7).
in part by WSUSM BRSG Grant #NS52-S07 RR05384-24. ~Supported Submitted as partial requirement for Ph.D. dissertation. 3To whom a l l correspondence and reprint requests should be directed. 0024-3205/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd.
1492
Stress Decreases PRL Surges in PSP Rats
Vol. 41, No. 12, 1987
The purpose of t h i s investigation was to see i f r e s t r a i n t stress applied during other periods of PRL elevation resulted in a similar decrease. To t h i s end we have examined the effects of r e s t r a i n t stress on the nocturnal (NS) and diurnal surges (DS) of pseudopregnant (PSP) rats. In addition, we have also investigated the influence of ADX on the stress-induced decrease of the PRL NS. Materials and Methods Adult female Sprague-Dawley rats from Charles River (Portage, MI) were housed 2/cage, placed on a 14-10 reverse l i g h t i n g schedule (L on at 1800, o f f at 0800) and provided with food and water ad libitum. Estrous cycles were monitored by vaginal lavage and only 4-5 day normally cycling animals were used. Two to three weeks after i n i t i a t i o n of vaginal lavage, when animals were 250-275g in body weight, they were c e r v i c a l l y stimulated (CS) on the morning of metestrus by mechano-electrical means using electrodes placed in a glass speculum. The electrical stimulus consisting of 5 volts, 20 cps with a 25 msec duration was applied continuously for 45 sec while the speculum and electrode were thrusted against the cervix. Continuousvaginal diestrus was used as an indication of PSP. Forty-eight to seventy-two hours prior to blood sampling, chronic a t r i a l catheters were implanted via the external jugular vein. Adrenals were removed from some animals and others were sham-operated at the time of catheterization. ADX animals were given a 1% saline drinking solution. A l l animals were bled on days 5-8 of PSP (day of CS is day 0). In the f i r s t experiment the influence of ADX on the PRL NS was examined. Here animals were attached to catheter extensions at 2200h rat time and blood samples (0.4 ml) were obtained at 0000, 0200, 0400, 0600, 0800, and lO00h replacing the f l u i d removed each time with warm (37°C), s t e r i l e , heparinized (50 IU/ml) saline (0.9% NaCl). For the stress experiments the animals were attached to catheter extensions at 0000 h rat time for the NS and 1400 h for the DS. The f i r s t blood sample of 0.4 ml was drawn at 0400 h (NS) and 1800 h (DS) and the f l u i d replaced as described above. Immediately following t h i s sample those animals to be stressed had t h e i r hind legs tied together with plastic coated bell wire. Subsequent samples were drawn at 15, 30, 60, 120 and 180 minutes following the i n i t i a l sample, replacing the f l u i d removed each time. The effects of repeated daily stress were examined by daily stressing of the animals from 0400-0700 h rat time beginning on day 1 of PSP and continuing for 6-9 days. On day 6-9 blood was obtained from the animals using the protocol described above for the stress experiments. Each blood sample was added to an equal volume of phosphate-buffered saline without methiolate immediately after i t was taken. At the end of blood sampling the samples were centrifuged and the diluted plasma harvested and stored frozen at -20°C u n t i l assayed. Plasma prolactin was assayed at two d i l u t i o n s in duplicate using a homologous rat prolactin RIA as previously reported (9). The iodination standards were NIAMDD RPI4 and RPI5 and the assay standard was NIAMDD RP-1 which had a biological potency of 111U/mg. Plasma levels were presented as a percent change from i n i t i a l value in the stress experiments. The data were analyzed s t a t i s t i c a l l y using two-way analysis of variance with computer assistance comparing no r e s t r a i n t and r e s t r a i n t PRL values. Oneway analysis of variance was used to compare i n i t i a l values with subsequent stressed values. Duncan'sMultiple Range test was used to determine the level
Vol. 41, No. 12, 1987
600
Stress Decreases PRL Surges in PSP Rats
=
-
1493
= SHAM (7-8)
• . . . . • ADX(7-8)
.~50013b .n,J n
<
/, I. ......... 1,,k,
400-
il
III
300rlIIII
03 .<
-~ 13- 2 0 0 -
100 0
I
I
0
200
I
i
600 400 TIME OF DAY (h)
I
I
800
1000
FIG. 1 Plasma prolactin (PRL) levels of adrenalectomized (ADX) pseudopregnant (PSP) rats during the nocturnal surge (NS). Animals were sampled every 2 hrs over a 10 h-period 6-9 days following cervical stimulation. The heavy l i n e on the X-axis represents l i g h t s o f f . The numbers in parentheses are the number of animals per group. The v e r t i c a l lines at each point represent the standard error of the mean. of significance. A level of p r o b a b i l i t y of P < 0.05 was considered s t a t i s t i cally significant. Results The effect of ADX on the NS of PSP animals is shown in Fig. 1. In both ADX and sham-operated animals the surge peaked between 0400-0600 h and PRL was down to basal levels by 0800 h. There was no s i g n i f i c a n t difference in the two curves (P > 0.05). The results of the effects of r e s t r a i n t stress on the NS are shown in Fig. 2. The i n i t i a l 0400 h values in t h i s experiment for the non-stressed animals and stressed animals was 502.8 ng/ml and 822.3 ng/ml respectively. Within 30 minutes of the application of stress, plasma PRL levels were 15% of the i n i t i a l value compared to 97% of the i n i t i a l value for the no r e s t r a i n t group. Restraint stress s i g n i f i c a n t l y decreased the NS (P < 0.001).
1494
Stress Decreases PRL Surges in PSP Rats
M .---a
Vol. 41, No. 12, 1987
NO RESTRAINT (9) RESTRAINT (7)
FIG. 2 Plasma prolactin (PRL) levels of restraint-stressed, pseudopregnant (PSP) rats during the nocturnal surge (NS). At 0400 h following an initial sample, restraint animals had their hind legs tied together. Subsequent samples were taken at the times indicated. Restraint stress significantly decreased the NS (P < 0.001). The heavy line on the X-axis represents lights off. The numbers in parentheses indicate the number of animals per group. The vertical lines at each point represent the standard error of the mean. As shown in Fig. 3, ADX had no effect on the restraint stress-induced decrease of the NS. Initial 0400 h values for sham ADX animals and ADX animals were 412.3 ng/ml and 552.0 ng/ml respectively. There was no significant different between the effect of restraint stress on ADX compared to sham operated animals (P > 0.05). The effect of restraint stress on the DS is shown in Fig. 4. Although this surge is small in magnitude (peak values of 200-250 ng/ml in individual animals), restraint stress significantly decreased plasma PRL (P < 0.01). Fig. 5 shows that 6-9 days of daily restraint stress does not attenuate the stress-induced decrease in plasma PRL compared to no-restraint rats. The response remains intact and the restraint values were significantly different from no restraint animals (P < 0.001).
Stress Decreases PRL Surges in PSP Rats
vol. 41, No. 12, 1987
z
PSP-RESTRAINT
100
M o---e
1495
SHAM ADX (5) ADX (5)
5f 8 d
80
g
60
0 do0
I
I 430
I
I
500
600
760
TIME OF DAY (h) FIG. 3 Plasma prolactin (PRL) levels of restraint-stressed, adrenalectomized (ADX) pseudopregnant (PSP) rats during the nocturnal surge (NS). Following the initial (0400 h) sample, restraint animals had their hind legs tied together. Subsequent samples were taken at the indicated times. ADX did not affect the restraint stress-induced decrease of the NS (P > 0.05). The heavy line on the X-axis represents lights off. The numbers in parentheses are the number of animals per group. The vertical lines at each point represent the standard error of the mean. Discussion Restraint stress significantly decreased the PRL NS and DS of PSP rats. The response was similar to the stress-induced decrease of the afternoon surge in proestrous and OVX, estrogen-treated rats (2,5,6,7). Adrenalectomy does not affect the stress-induced decrease of the PRL NS, an observation also recorded for the stress-induced decrease of the afternoon surge in OVX, estrogen-treated rats (7). However, unlike the afternoon surge where ADX retarded the initiation of the surge by 2 hours (5), the absence of the adrenal gland had no effect on the timing of the NS as shown in Fig. 1. The lack of an effect of the adrenal gland on the timing of the NS compared to the afternoon surge may reflect the circadian rhythm of the pituitary-adrenal axis. At the peak of the afternoon PRL surge, corticosterone levels are near crest values and have been shown to increase to 3-fold as a result of handling or novel environment stress (3). However during the NS corticosterone levels are near trough levels (approximately 50% of crest
Vol. 41, No. 12, 1987
Stress Decreases PRL Surges in PSP Rats
250 200 15010050-
0!
I
1700
I
1750
I
1800
I
I
1900
I
1
2000
TIME OF DAY (h)
FIG. 4 Plasma prolactin (PRL) levels of restraint-stressed pseudopregnant (PSP) rats during the diurnal surge (DS). Following the initial (1700 h) sample, stressed animals had their hind legs tied together. Subsequent samples were taken at times indicated. Restraint stress significantly depressed the DS of PRL compared to control animals (P < 0.01). The numbers in parentheses are the number of animals per The vertical lines at each point represent the standard error group. of the mean. levels) and have been shown to increase 4-fold as a result of novel environment Moreover, when corticosterone levels are near the crest of the stress (3). circadian rhythm, stress resulted in a prolonged elevation of corticosterone (values were still elevated one hour following stress) while stress during the followed by a decline to near basal trough of the cycle elicited an increase levels within 1 hour of novel environment stress (3). Since corticosterone levels are near or at trough levels during the NS, the resulting loss of corticosterone by ADX could be expected to have little effect on the timing of the PRL NS. Of interest is the fact that daily stress does not result in the conAfter 6-9 days of 3 hours of restraint stress, ditioning out of the response. PRL values are still depressed to 26% of the initial value 30 minutes after the Initial values were 229.2 ng/ml and 383.7 ng/ml initiation of the restraint. for stressed and non-stressed animals respectively (Fig. 5). In contrast, the stress-induced increase in PRL shows adaptation (5,lO). Smith and Gala (5) demonstrated that following daily injections of corn oil for 4 days, PRL
Stress Decreases PRL Surges in PSP Rats
Vol. 41, No. 12, 1987
o-o NO RESTRAINT (6) l--oRESTRAlNT (6)
PSP
140-
1497
120loo806040ZOoioo
04’30
oioo
’
06bo
’
07iIo
TIME OF DAY (h)
FIG. 5 Plasma prolactin (PRL) levels of pseudopregnant (PSP) rats during the nocturnal PRL surge (NS) following 6-9 days of restraint stress. Following the initial (0400 h) sample, stressed animals had their hind legs tied together. Subsequent samples were taken at times indicated. Restraint stress significantly decreased the PRL NS compared to control animals (P < 0.001). The heavy line on the Xaxis represents lights off. The numbers in parentheses are the number of animals per group. The vertical lines at each point represent the standard error of the mean. values of rats subjected to restraint stress showed no increase over levels. Yelvington et al. found that daily handling obliterated the -7 stress-induced increase in PRL (10). This difference in habituation same stimulus when PRL values are low vs. when they are high provides evidence that the regulation of PRL secretion differs depending upon tial level of plasma PRL in different physiological conditions.
basal handling to the further the ini-
Acknowledgement The authors would like to express their Agency of the NIAMDD for providing as a gift iodination and standards, respectively.
appreciation to the Pituitary the RP-I4 & 5 and RP-1 used for
1498
Stress Decreases PFU Surges in PSP Rats
Vol. 41, No. 12, 1987
References 1. 2. 3. 4. 5. 6. 7. 8.
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