Ovarian influences on primate food intake: Assessment of progesterone actions

Physiology &Behavior,Vol. 21, pp. 923--928.PergamonPress and Brain Research Publ., 1978. Printed in the U.S.A.

Ovarian Influences on Primate Food Intake: Assessment of Progesterone Actions I J O H N A. C Z A J A 2

University of Wisconsin, Regional Primate Research Center, 1223 Capitol Court, Madison, WI 53706 ( R e c e i v e d 5 July 1978) CZAJA, J. A. Ovarian influences on primatefood intake: Assessment of progesterone actions. PHYSIOL. BEHAV. 21(6) 923-928, 1978.--Observations during the rhesus menstrual cycle confirmed that adjustments in food intake accompany changes in ovarian condition and indicated progesterone antagonism of estradior s ability to suppress primate feeding. Food intake was significantly higher during the luteal phase, when progesterone is high and estradiol low, than during the early follicular phase, when levels of both these hormones are low. To test antagonistic effects of progesterone, ovariectomized females given 4 consecutive daily injections of either 15 mg progesterone or vehicle received 20 v-gestradiol benzoate on the second and third treatment days. Progesterone did not modify the decline in feeding and body weight which followed this acute estradiol treatment. In an additional study, females chronically stimulated by subdermal estradiol capsules showed no systematic changes in food intake or body weight when injected for 9 days with 15 rag/day progesterone. This treatment did reduce sexual activity during heterosexual mating tests. These females also responded to 15 rag/day dihydrotestosterone propionate with significant weight gains. A fourth study showed that progesterone treatment of 15 rag/day is adequate to raise circulating levels in ovariectomized rhesus above those of intact females. Overall, these studies failed to confirm a role of progesterone in the adjustments of food intake which accompany changes in ovarian condition of primates. Primates Feeding behavior Mating behavior

Body weight

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F E M A L E primates show systematic variation in food consumption which correlates with changes in reproductive condition. In particular, females show a cyclic pattern of food intake across the menstrual cycle [7,11] and a marked decline in feeding during early pregnancy [5]. Correlative evidence supports the notion that ovarian estrogens are involved in these changes. During the menstrual cycle, a midcycle depression in feeding coincides with the interval of the preovulatory estrogen surge. Such a depression in food intake can be duplicated in ovariectomized primates by inducing a similar surge with estradiol injections [7]. Evidence of progesterone influences on primate feeding is less convincing. The fact that food consumption is relatively elevated during the luteal phase of the menstrual cycle when ovarian progesterone secretion is high led Gillman and Gilbert [11] to propose that progesterone acted as an appetite stimulant. In support of this hypothesis, they found that injecting intact baboons with progesterone during the midportion of their menstrual cycle led to increased food intake. In ovariectomized female monkeys, however, progesterone administration does not influence food intake [7]. It has been shown that artificially raising progesterone levels during the follicular and periovular portions of the menstrual cycle both lowers levels of circulating estradiol and prevents the midcycle estradioi surge that normally precedes ovulation [13].

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Thus, the increased feeding Gilbert and Gillman noted coincident with progesterone injections into the intact baboon might have been secondary to the suppression of circulating estrogen levels. Alternatively, the observed results may have been due to antagonism by progesterone of estrogen effects. Such an antagonism might explain why the feeding depression during early primate pregnancy is only temporary [5] despite the fact that estradiol remains elevated. During early pregnancy, increases in circulating progesterone follow soon after the increases in circulating estradiol [3], suggesting the possibility that the antagonism of progesterone limits the duration of the estradiol effects on feeding in the pregnant animal. Progesterone antagonism of behaviorally significant estrogen effects certainly occurs in female primates, as evidenced by the reports that successful outcome of mating tests involving estrogen stimulated female primates is reduced when these females are concurrently treated with progesterone [1,14]. Whether such a progesterone-estradiol interaction is involved in the control of primate feeding has not been tested. The following studies were therefore undertaken to determine whether progesterone does antagonize the estrogen effects on food intake of the female rhesus monkey. No evidence of such an antagonism was found.

~Publication number 18-017of the Wisconsin Regional Primate Research Center. Support was provided by Grant RR00167 from the National Institutes of Health to the Wisconsin Regional Primate Research Center and by Grant MH21312 from the National Institute of Mental Health. Special thanks are given to Guenther Scheffler for his aid and supervision of the radioimmunoassays. Additional expert technical assistance on the RIAs was provided by Cathy Bolme, Fritz Wegner and Louise Horge. zPresent address: Purdue University, Department of Psychological Sciences, West Lafayette, Indiana 47907.

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FIG. 1. Patterns of food intake and circulating ovarian steroids across the rhesus monkey menstrual cycle. Graphs show 3-day running means for the first 6 (food intake baseline) and last 22 days of the cycle (N=6). STUDY 1: MENSTRUALCYCLE CHANGES Data were first collected from intact cycling females to examine the patterns of food intake and circulating steroids under normal physiological conditions.

Method.s Animals were 12 adult rhesus females which had an established history of menstrual cyclicity. The females were housed in separate cages (60 cm ×60 cm x 80 cm high) within temperature controlled quarters and were checked daily for signs of menses. Food intake was measured in 6 of the animals. To reduce spillage and loss of biscuits, a 3 cm wire mesh was attached to the cage bottoms and extended 10 cm up the sides. Females were given a~known number of biscuits (Purina Monkey Chow) as a food ration at 8:00 a.m. every morning. Six hours later any biscuits remaining were removed and counted, the difference from the allotted ration being recorded as consumption for that day. Water was available continuously. Six other animals had previously been acclimated to the blood drawing procedures, These were caught and bled every morning between 8:00 a.m. and 9:00 a.m. Blood samples (2.5 ml each) drawn from the femoral vein were allowed to clot, spun down and the serum portion removed and frozen for subsequent analysis. All serum samples were analyzed for estradiol by radioimmunoassay as previously described [3]. Samples from every other day were additionally assayed for progesterone [3]. Each animal was monitored until data were collected across a complete menstrual

cycle. Rhesus menstrual cycles were normalized to align endocrine events. The fin'st 6 and last 22 cycle days were aligned by the first day of menses beginning and ending the cycle respectively. As in previous studies of rhesus mating and feeding behavior [6,7], three endocrinologically distinct 6-day cycle segments were then selected for analysis: Days i through 6 of the cycle (early follicular stage); the period 15 through 20 days prior to the first day of menses ending the cycle (per)ovulatory stage); and the period 5 through 10 days prior to the menses ending the cycle (midluteal stage).

Menstrual cycles C X t l l f l } l l l ¢ ~ ] r;.tnged I r o n ] 2 - b , ~ ,.)it},>, il] length with a mean of 29 days. Cycles analyzed for food int',&¢ were comparable in length [o those anal.~zed for chcui~ting steroids, t( 101=0.88, N.S. Patterns of food inlakc and :iclc.id changes are depicted in Fig. I. Analysis of variance of lo~n.t intake during the 3 selected stages of the cycle yiekled a significant F ratio, 1::(2,101~ 20.75, p. 0.01. Additional analysis indicated that tbod intake during the periovuiar stage of the cycle was significantly lower lhan in either J,hc early follicular stage, tl5) 2.94, p. 0.05 or midluteal ~tagc, t/5):-7.37, p :0.01. Furthermore, food intake during the midluteal stage was significanlly greater than during the early follicular stage, 1(5)- 3.1-7 p !).05. Significant effects during the 3 selected stages of the cycle were also found for estradiol, F(2.101 16.89, p. 0.01 and progesterone, F(2.1(1)=22.47, p. 0.01. Estradiol values were highest during the periovular stage ~versus early follicular stage,/(5):::4.23, p~ 0.01; versus midluteal stage, t(Si 4. ]7. p
Methods Four ovariectomized rhesus, weighing between 5.0 and

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8.4 kg, were maintained in single cages as previously described. Females were weighed and received any injections between 7:30 and 8:00 a.m. A ration of biscuits (Purina Monkey Chow) was distributed at 8:00 a.m. and supplemented at noon. At 4:00 p.m. biscuits remaining were removed and counted, the difference from the distributed ration being recorded as food intake for that day. Observations on these females extended over a 6 week period during which females received 2 different injection series in counterbalanced order. Each injection series involved 4 consecutive daily injections, consisting of either oil alone or oil with 15 mg progesterone. On the second and third days of these injections, females were given additional injections of 25/xg estradiol benzoate in oil. For statistical analysis, the 4 day period immediately preceding the first injection of a series was used as the baseline and compared with data from the 3 days following the first estradiol benzoate injection. Results

Results are shown graphically in Fig. 2. Estradiol injections successfully lowered food intake in females pretreated with either oil, t(3)=3.43, p<0.05 or with progesterone, t(3)=5.48, p<0.05. Compared to the oil treatment, progesterone did not noticeably interfere with the actions of estradiol in suppressing food intake, t(3)=0.06, N.S. Likewise, the manner and degree of body weight changes following estradiol benzoate injections were similar whether females were treated with progesterone or oil alone. STUDY3: CHRONICESTROGENSTIMULATION Behavioral evidence of progesterone antagonism in the rhesus has involved changes in the outcome of mating tests following progesterone administration to ovariectomized females chronically stimulated with estrogens [ 1,14]. An additional experiment therefore investigated the effects of progesterone on food intake and mating in such females. Methods

Four adult ovariectomized rhesus, between 4.1 and 6.9 kg

in size, each received a subdermal implant of an estradiolfilled silastic capsule. The capsules, made from silastic tubing (Medical-Grade, Dow Coming, 3.35 mm IDx4.65 mm OD) were 40 mm long. The interior was filled with estradiol (estra-l,3,5(10)-triene-3,17b-diol, Sigma Corporation) and both ends plugged with 5 mm of elastomer (No. 382 Medical-Grade, Dow Coming). Radioimmunoassay of serum samples showed that the subdermal capsules raised circulating estradiol levels within the range encountered during the follicular phase of the menstrual cycle. Prior to implantation of the capsules, levels in the ovariectomized females averaged 13 pg estradiol per ml serum (range 9-19 pg/mi). One week before the start of this experiment, after capsules had been in place for 3 months, circulating estradiol was measured at 99 pg/ml (range 65-145 pg/ml). Circulating estradiol remained relatively constant over time, showing an average decrease in the 4 females of less than 5% per month. Eight adult rhesus males were used as heterosexual partners in standardized mating tests [6,8]. Males and females were maintained in single cages as in the previous studies. Females were weighed and injected every morning between 7:30 and 8:00 a.m. A known number of biscuits (Purina Monkey Chow) were distributed at 8:00 a.m. and supplemented at 11:00 a.m. and 1:00 p.m. At 4:00 p.m. the remaining biscuits were removed and counted, the difference from the total daily ration being recorded as food intake for that day. Water was available in the home cage continuously. Between 9:00 and 11:00 a.m. each female was moved to a test room for a 12-min mating test with one of her 2 assigned male partners. Females were tested with each of their partners on alternative days. No food or water was available in this testing situation. Behavior in the pair test cage (75 cm×150 cmxl20 cm) was recorded on a time lapse video tape deck (7:1 ratio) and subsequently reviewed for occurrence of ejaculation and number of female yawns. Treatment consisted of 18 consecutive daily injections. For Days 1 through 9 of treatment, the intramuscular injections consisted of only corn oil (0.5 mi). During Days 10 through 18, each injection contained in addition 15 mg progesterone. A similar procedure was repeated 2 weeks after the first treatment. Again females received injections of oil alone for 9 days. During Days 10 through 18 of treatment, injections contained 15 mg dihydrotestosterone propionate (DHTP). Dihydrotestosterone is a potent anabolic metabolite of testosterone. It has been shown that a behavioral consequence of DHTP treatment in rhesus females is a marked increase in yawning [20]. Results from this latter study also suggested that dihydrotestosterone given to ovariectomized rhesus may reduce sexual activity during pair mating tests. Data from the last 4 days of oil injections (Days 6-9) were used as a baseline for comparison with data collected during the last 4 days of progesterone injection (Days 15-18). Except as stated, probabilities noted are based on two-tailed statistical tests. Results

Figure 3 summarizes data from the last 4 treatment days under each condition. Although food intake increased by 12% during progesterone treatment, this effect was not consistent, t(3)=0.76, N.S. Food intake was down 11% in one animal and up 2%, 3% and 54% in the other animals. Females showed only minimal changes in body weight during this period, t(3)= 1.19, N.S. In contrast, ejaculation frequency of

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FIG. 5. Clearance of progesterone measured in peripheral circulation of ovariectomized female monkeys following multiple intramuscular injections. Dashed line indicates the average pretreatment levels in these subjects (N=7). terone treatments to interfere with estrogen actions in the second and third studies may be related to the fact that the treatments did not induce circulating progesterone levels comparable to the maximum titers which occur normally during the menstrual cycle. A final study was therefore undertaken to determine the circulating levels induced by intramuscular injections of progesterone into ovariectomized rhesus monkeys.

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FIG. 4. Average levels of circulating progesterone induced in ovariectomized rhesus females (N=7) by 5 days of progesterone injected intramuscularly in oil. Dashed line represents an average of the maximum progesterone levels detected during the menstrual cycles of 6 intact rhesus females. the 8 males showed a significant decline when their partners were treated with progesterone, t(7)=2,96, p<0.05. Dihydrotestosterone also had no systematic effect on food intake, t(3)=0.77, N.S. Nevertheless, body weight increased dramatically during DHTP treatment, t(3)-18.09, p<0.01. No marked change in ejaculation frequency accompanied the androgen treatment of the females, t(7)=0,35, N.S. As expected, DHTP increased the frequency of yawns in all 4 females, t(3)=2.62, p<0.05. I-tailed test. S T U D Y 4: I N D U C E D P R O G E S T E R O N E C H A N G E S

The magnitude of changes in circulating progesterone across the menstrual cycle is relatively large compared to the variations observed in ovarian estrogens and androgens. During the luteal phase, circulating progesterone normally reaches levels over 100 times those which occur during the early follicular phase of the cycle. The failure of proges-

A total of 7 ovariectomized rhesus females, weighing 5.2 to 9.2 kg, were utilized in this study. Injections of O. 10, 1,00, 10.00 and 15.00 mg progesterone per day were g i v e n intramuscularly in a constant volume o f 0.5 ml corn oil. At each dosage, females received 5 consecutive daily injections administered between 8:00 and 8:30 a.m. Blood samples of 3 ml were drawn from the femoral vein 1, 3, 6 and 24 hr following the last injection in a series. Three to 10 days intervened between treatments. Additional samples were collected 72, 120, 240, 408 and 576 hr following the last 15 mg progesterone injection. Samples were handled and assayed for progesterone as in the first study, One month after the last progesterone treatment, the females were additionally injected with 1.00 mg per day dihydrotestosterone propionate in oil for 5 days. Serum from blood samples collected at 1,3, 6 and 24 hr after the last injection was assayed without prior chromatography for levels of circulating dihydrotestosterone using an antibody specific for testosterone and dihydroteslosterone. Result,~ Intramuscular progesterone injections at all dosages examined consistently induced elevated circulating levels of progesterone. Sampled within 6 hr of injection, serum was tbund to have 5.0 to 7.2 times more progesterone than found in serum taken 24 hr after the injection. Across all treatments there was no consistent peak of circulating progesterone, although maximum levels within a female occurred most often at 1 and 3 hr following injection. Figure 4 shows the progesterone levels induced in ovariectomized females compared to the maximum luteal levels measured in the first study. Injections of either 10 or 15 mg per day maintained circulating levels higher than occur during the menstrual cycle. The circulating levels measured within 6 hr after the last

PROGESTERONE AND FEEDING IN MONKEYS 15 mg progesterone injection averaged 103.3 ng/ml serum, approximately 10 times the physiological levels of intact females. Steroid clearance after treatment with 15 mg/day progesterone is depicted in Fig. 5. Five days after the last injection, circulating progesterone in the 7 females was still significantly above preinjection levels, Log,,, ng/ml: t =2.68, df=6, p<0.05. Levels were no longer consistently elevated 10 days after the last injection, Log,,, ng/ml: t= 1.04, dr=6, N.S. The 1 mg injections of dihydrotestosterone induced a much larger absolute increase in that steroid than was noted for progesterone administered at the same dosage. Within 6 hr of the last injection, dihydrotestosterone was elevated by an average 21.4 ng/ml serum versus an elevation of only 6.1 ng/ml progesterone following a similar quantity of progesterone. Clearance after the dihydrotestosterone injections was also slower. Compared to samples taken 1 to 6 hr after the injection, dihydrotestosterone levels at 24 hr were reduced by only 34.6% versus an 86.2% reduction found in the levels of circulating progesterone over the same period. DISCUSSION

Although there clearly is a relationship between ovarian condition and food intake in primates, the exact nature of this interaction is not completely understood. The present studies were designed to elucidate the role of progesterone in this process. Previous attempts to modify food intake by exogenous manipulations of ovarian steroid levels concluded that progesterone was an appetite stimulant. Given to intact cycling rodents [ 12] or primates [ 11], progesterone results in increased food consumption. However, it is now known that progesterone administration to intact females can block ovulation and disrupt the patterns of other ovarian hormones, such as estradiol, which also influence food intake. In the rhesus monkey, for example, exogenous progesterone during the menstrual cycle both lowers overall estradiol levels and prevents the midcycle estradiol surge which normally precedes ovulation [13]. Since estradiol suppresses feeding in monkeys [7], a reduction in endogenous estrogen would also be expected to increase food intake. Thus, the effects of progesterone on feeding in intact primates might be secondary to its actions in lowering circulating estrogens. In support of this latter notion is the fact that progesterone has no effect on feeding when given to ovariectomized monkeys [71. An alternative explanation for the observed progesterone effects in intact females is that progesterone may act to more directly antagonize the effects of estradiol. If this were so, one would still not expect to see any effect of progesterone when given alone to ovariectomized females. Precedence for progesterone antagonism of estrogen influences on behavior can be found in literature on sexual behavior of female rodents and primates [1, 9, 14, 16]. A similar antagonism has also been shown relative to the regulation of feeding in rodents. Given in sufficient quantities to ovariectomized rats, progesterone is found to ameliorate the decline in food intake which accompanies stimulation by estradiol 14, 17, 19]. The present attempts to detect similar interactions in primates were unsuccessful. In the second study, the acute effects of estradiol treatment were not influenced by pretreatment with progesterone. The failure to detect such effects cannot be attributed to use of physiologically inappropriate dosages of the hormones administered. Unpublished estradiol measurements by Goy and Czaja show that although 20/zg estradiol benzoate injections temporarily raise circulating

927 levels higher than normally encountered during the menstrual cycle, the levels attained are within the physiological realm of intact females and by 24 hr after injection are no higher than levels of estradiol which occur during the late follicular phase of the cycle. The third study utilized a test paradigm proven successful in showing progesterone antagonism of estradiol effects on rodent feeding [4]. However, repeated stimulation by progesterone did not have a significant effect on feeding or body weight in the chronically estrogenized rhesus. The apparent differences in progesterone antagonism of estradiol relative to food intake in monkeys and rats may be related to differences in ovarian and steroid patterns which occur in these species. Primates, in contrast to rats, have a spontaneous luteal phase with a prolonged period of elevated progesterone and may have developed differential adaptations relating to progesterone. It should, however, be remembered that even in rodents, the magnitude of the progesterone effect in question is rather limited. For example, after 9 days of injections with somewhat equivalent steroid dosages by body weight as used in Study 2, rats given progesterone with estradiol had food intake only 12% and body weight only 3% above other females injected with estradiol alone for 9 days [191. In another study, female rats chronically stimulated with estradiol benzoate and given 5 mg/day progesterone were only 5% heavier and eating 12% more after 9 days than females receiving estradiol benzoate alone [4]. Of course, one would expect such limited effects to be more obvious in inbred laboratory rats than in the wild caught, genetically heterogenous rhesus utilized in the present studies. Manipulations in rodents show that the possibility of detecting progesterone antagonism is maximized by increasing dosages of both estradiol and progesterone and by prolonging treatment. It might be possible to show the progesterone antagonism in primates by utilizing such procedures. However, researchers have not reported the steroid levels obtained with the replacement therapies used in feeding studies of the rat or related the progesterone treatments to endogenous steroid levels or to the pattern of feeding changes which occur during the rat estrous cycle. The questions explored in the present studies relate to the physiological rather than the pharmacological effects of estradiol and progesterone. Increasing steroid dosages does not seem warrented since measurements in the monkey demonstrated that the treatments utilized were adequate to reinstate levels of the intact animal. Different ratios of progesterone to estradiol or more prolonged treatment may increase the probability of detecting a progesterone antagonism. The present results in the rhesus suggest that any antagonism by progesterone or estradiol effects on primate feeding is relatively weak and appears to contribute little to the pattern of food intake observed across the menstrual cycle. Rather, most of the variation in food intake across the rhesus menstrual cycle appears to be related to levels of circulating estrogens. Major limitations of the present studies should be noted. Treatments were not very long in duration. Also, ovarian changes influence dietary selection [18], a factor not considered here since females were not given a choice of alternative foods. Human and nonhuman primates both show a period of reduced feeding during early pregnancy when circulating estrogens are increasing rapidly [2, 3, 5, 10, 15]. Subsequent improvement in food intake and amelioration of morning sickness symptoms coincide with the time during pregnancy when circulating progesterone increases. The

928

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possibility that these feeding changes during pregnancy might be related to progesterone antagonism of the continued high levels of circulating estradiol is consistent with the behavioral observation that certain primates show a decline in mating during later pregnancy [3]. Actions of gonadal hormones are not limited to the modulation o f strictly reproductive functions. Instead, ovarian steroids appear to be rather ubiquitous biochemical messengers which act to influence many physiological and behav-

ioral processes. The present studies reconfirm the existence of ovarian-feeding relationships in primates. Although such interactions have not been adequately explored in humans, similar phenomena have been documented in various laboratory and domestic animals [5,18]. The generality of certain steroid-feeding interactions and their continuance in higher mammalian forms suggest that these relationships have a significant role in the maintenance or reproduction of these species.

REFERENCES 1. Baum, M. J., E. B. Keverue, B. J. Everitt, J. Herbert and W. J. DeGreef. Effects of progesterone and estradiol on sexual attractivity of female rhesus monkeys. Physiol. Behav. 18: 659670, 1977. 2. Berustein, I. C. An investigation into the etiology of nausea and vomiting of pregnancy. Minn. Med. 35: 34--38, 1952. 3. Bielert, C., J. A. Czaja, S. Eisele, G. Schemer, J. A. Robinson and R. W. Goy. Mating in the rhesus monkey (Macaca mulatta) after conception and its relationship to estradiol and progesterone levels throughout pregnancy. J. Reprod. Fert. 46: 179187, 1976. 4. Blaustein, J. D. and G. N. Wade. Ovarian hormones and meal patterns in rats: Effects of progesterone and role of gastrointestinal transit. Physiol. Behav. 19: 23-27, 1977. 5. Cz~a, J. A. Food rejection by female rhesus monkeys during the menstrual cycle and early pregnancy. Physiol. Behav. 14: 579-587, 1975. 6. Czaja, J. A. and C. Bielert. Female rhesus sexual behavior and distance to a male partner: Relation to stage of the menstrual cycle. Arch. Sex. Behav. 4: 583-597, 1975. 7. Czaja, J. A. and R. W. Goy. Ovarian hormones and food intake in female guinea pigs and rhesus monkeys. Hormones Behav. 6: 329-349, 1975. 8. Czaja, J. A., S. G. Eisele and R. W. Goy. Cyclical changes in the sexual skin of female rhesus: Relationships to mating behavior and successful artificial insemination. Fedn. Proc. 34: 1680-1684, 1975. 9. Czaja, J. A., D. A. Goldfoot and H. J. Karavolas. Comparative facilitation and inhibition of lordosis in the guinea pig with progesterone, 5a-pregnane-3,20-dione, or 3a-hydroxy-5a-pregnan20-one. Hormones Behav. 5: 261-274, 1974. 10. Fairweather, D. V. 1. Nausea and vomiting in pregnancy. Am. J. Obstet. Gynec. 102: 135-175, 1968.

11. Gilbert, C. and J. Gillman. The changing pattern of food intake and appetite during the menstrual cycle of the baboon (Papio ursinus) with a consideration of some of the controlling endocrine factors. S. Afr. J. reed. Sci. 221: 75-88, 1956. 12. Hervey, E. and G. R. Hervey. The effects of progesterone on body weight and composition in the rat. J. Endocr. 37: 361-384, 1967. 13. Hess, D. L. and J. A. Resko. The effects of progesterone on the patterns of testosterone and estradiol concentrations in systemic plasma of the female rhesus monkey during the intermenstrual period. Endocrinology 91: 896-900, 1973. 14. Michael, R. P., G. Saayman and D. Zumpe. The suppression of mounting behavior and ejaculation in male rhesus monkeys (Macaca mulatta) by administering progesterone to their female partners. J. Endocr. 41: 421-431, 1968. 15. Mishell, D. R. Jr., I. H. Thorneycroft, Y. Nagata, T. Murata and R. M. Nakamura. Serum gonadotropin and steroid patterns in early human gestation. Am. J. Obstet. Gynec. 117: 63t-639. 16. Morin, L. P. Progesterone: Inhibition of rodent sexual behavior. Physiol. Behav. 18: 701-715, 1977. 17. Ross, G. E. and I. Zucker. Progesterone and the ovarianadrenal modulation of energy balance in rats. Hormones Behav. 5: 43-62, 1974. 18. Wade, G. N. Gonadal hormones and behavioral regulation of body weight in female rats. Physiol. Behav. 8: 523-534, 1972. 19. Wade, G. N. Some effects of ovarian hormones on food intake and body weight in female rats. J. comp. physiol. PsyehoL 88: 183-193, 1975. 20. Wallen, K. and R. W. Goy. Effects of estradiol benzoate, estrone, and propionates of testosterone or dihydrotestosterone on sexual and related behaviors of ovariectomized rhesus monkeys. Hormones Behav. 9: 228-248, 1977.