Estrogen Feedback in the Pig: Sexual Differentiation and the Effect of Prenatal Testosterone Treatment

Transcription

1 BOLOGY OF REPRODTO 2, (1979) Estrogen Feedback in the Pig: Sexual Differentiation and the Effect of Prenatal Testosterone Treatment F ELSAESSER and PAR VZ institut f#{252}r Tierzucbt und Tierverhalten, FAL, Mariensee, 357 eustadt 1, Federal Republic of Germany ABSTRAT We studied the stimulatory estrogen feedback on LH release in newborn (6-day-old) and prepubertal (16-day-old) male and female Landrace pigs and the effect of exposure to testosterone in fetal life on sexual differentiation of the stimulatory estrogen feedback n newborn gonadectomized male pigs neither 6 nor 6 pg estradiol benzoate (EB)/kg BW im had an effect on the level of LH in the plasma for up to 12 h following treatment n female neonates the plasma concentrations of LHwere significantly (P<5) suppressed only at 24 h after 6 pg EB/kg BW Gonadectomized prepubertal male pigs responded to 6 pg EB/kg BW with a significant (P<1) decrease of plasma LH at 24 h and a return to pretreatment levels at 72 h in the face of still elevated estradiol-17 levels Prepubertal guts responded to 6 pg EB/kg BW with a surge of LH 48-6 h following treatment The stimulatory estrogen feedback was significantly (P<5) impaired in females of the same age that had been exposed to testosterone-propionate (TP) via their mother (im injections on 3 occasions separated by 2 day intervals, 5 mg/kg BW) starting on Day 3, 5 or 7 of fetal life The number of gilts that had ovulated by 25 days was reduced and the weight of the ovaries and the uterus was signifcantly (P<1 and P<1, respectively) depressed TP treatment of sows starting on Day 9 or 16 of pregnancy had no effect on the functioning of the stimulatory estrogen feedback mechanism or the ovaries of their female offspring We conclude that the stimulatory estrogen feedback mechanism of the female pig gradually matures and suggest that with respect to the effect of prenatal or neonatal testosterone treatment on the estrogen feedback the pig is similar to the rat, guinea pig and sheep, but different from the rhesus monkey The observed sexual dimorphism in the LH response to estradiol benzoate might be quantitative rather than qualitative TRODTO The concept of sexual differentiation of the control of luteinizing hormone (LH) release by androgens during a critical period of fetal or neonatal life seems to be valid for the rat (Barraclough, 1966; Gorski, 1973), guinea pig (Brown-Grant and Sherwood, 1971), hamster (Buhl et al, 1978) and sheep (Short, 1974; Karsch and Foster, 1975), whereas in monkeys a sexual dimorphism of the stimulatory estrogen feedback mechanism does not seem to exist (Karsch et al, 1973; Hodges and earn, 1978) The stimulatory estrogen feedback mechanism is sexually dimorphic in the miniature pig at 14 days of age (Elsaesser et al, 1978) and cyclic activity of sows was not affected during adulthood when androgens had been given neonatally (Zimbelman and Lauderdale, 1973) t has been suggested that the concept of sexual Accepted January 8, 1979 Received July 31, 1978 hypothalamic differentiation developed for rodents does not apply to the pig (Ford and Schanbacher, 1977) The purpose of this study was to compare the L response to estradiol-17a in male and female Landrace pigs at both 6 and 16 days of age At 16 days, the L surge mechanism is capable of operating in the Landrace gilt (Elsaesser and Foxcroft, 1978) A further experiment was designed to evaluate the effect of testosterone propionate treatment given to the sow at different stages of pregnancy on the stimulatory estrogen feedback and ovarian activity in the female offspring General Details MATERALS AD METHODS Domestic German Landrace pigs were kept under natural lighting conditions and received standard pig chow and water For hormonal analyses, 2-5 ml blood samples were collected at 1 h or 22 h either by jugular venipuncture in 6-day-old piglets or from an ear vein in older animals as described by Hultsch (1979) Jugular venipuncture has no effect on Downloaded from by guest on 3 January

2 1188 ELSAESSER AD PARVZ the plasma levels of LH (Elsaesser et al, 1976) Heparinized plasma was stored at -2#{176}Steroids were always administered im Hormonal Analysis Plasma LH was determined as described previously by Pomerantz et a! (1974) Sensitive and specific porcine LH antiserum (a gift from Dr G D iswender) was diluted to 12, and 1 p1 bound \,4% of LH in the absence of unlabelled LH The LH standard preparation (LER-786-3, a gift from Dr L E Reichert, Jr) was equal to 65 H-LH-S1/mg Plasma testosterone was measured by specific radioimmunoassay without prior chromatography as described previously by Ellendorff et al (1975) The antiserum used had been raised in a miniature pig by intradermal immunization against testosterone 3- carboxyoxime-bovine serum albumin This antiserum was diluted 1:2, The antibody cross reacted 23% with 5-DHT, 28% with 5n-androsten-3, 17-diol, less than 1% with estradiol-17a and less than 1% with androst-4-ene-3, 7-dione, progesterone and hydrocortisone Plasma estradiol-17j3 was measured by radioimmunoassay without prior chromatography (Elsaesser et al, 1978) The estradiol antiserum (provided by Drs Kuss and G#{246}bel, Munich) was directed against estradiol carboxy--methyloxime-bovine serum albumin and cross reacted less than 1% with estrone, estriol and estradiol-benzoate and less than 1% with testosterone, androst-4-ene- 3,17-dione and progesterone The lower limit of detection for this assay was 1 pg The accuracy and precision of each assay were estimated from blank values (for samples of water) and values for plasma samples containing high and low concentrations of testosterone and estradiol-17j3, respectively The inter and intraassay coefficients of variation were 144% and 12%, respectively, for the determination of testosterone and 133% and 9%, respectively, for the determination of estradiol-171 Statistical Analysis Student s t test (Snedecor, 1956) was used to detect significant differences between groups Results are expressed as means ± SEM Estrogen Feedback in 6-Day-Old Male and Female Pigs A total of 35 male piglets was gonadectomized at 3 days of age At 1 h, 6 days after birth, these animals and 31 intact female litter mates received im injections of 6 or 6 pg/kg BW estradiol benzoate (5 mg and 5 mg/m oil, respectively; Progynon B Oleosum, Schering, Berlin) in a total of p1 of sesame oil, or sesame oil alone Blood samples were collected at -12 h, immediately before and 24, 48, 6, 72, 84, 96 and 12 h after the injection Effect of Prenatal Testosterone Pro pionate Treatment on the Functioning of the Estrogen Feedback and the Ovaries A total of 14 pregnant sows, mean body weight 25 kg, was divided into 5 groups of 2 or 3 sows Starting either on Day 3, 5, 7, 9 or 16 of pregnancy, im injections of testosterone propionate (Schering, Berlin; each injection 5 mg/kg BW in a concentration of 5 mg/m sesame oil) were given on 3 occasions separated by 2 day intervals These treatments are referred to as D 3, D 5, etc, for the purpose of convenience A further group of 4 sows remained untreated Blood samples for measurement of plasma testosterone concentrations were taken on Day 2 and Day 3 after the first injection At birth (all animals were born within 2 months of each other), at 16 and at 25 days of age the body weight as well as the effect of the testosterone propionate injections on the size and appearance of the external genitalia of the female offspring was assessed At the age of 16 days, the response of LH to estradiol was tested in the female offspring (for n see Fig 3) as well as in untreated females (n = 8) and males (n = 7) that had been gonadectomized at weaning (6 weeks of age) All animals were treated at 1 h with a single im injection of 6 pg/kg BW of estradiol benzoate, a dose that elicits an LH surge in guts of this age (Elsaesser and Foxcroft, 1978) astrated male (n = 6) and intact female controls (n = 8) received sesame oil Blood samples for measurement of plasma LH concentrations were taken at -12 h, immediately before injection and 24, 48, 6, 72 and 84 h later n males, an additional blood sample was taken at 96 h because Ford and Schanbacher (1977) suggested positive feedback action in the castrated boar 96 h after treatment with estradiol benzoate oncentrations of estradiol-1 73 were measured at 24 and 72 h To check on differences in the degree of sexual maturation, in addition to daily control of estrous behavior starting on Day 16 of age, all females were slaughtered at 25 days of age The reproductive tract was removed, the uterus and the ovaries were weighed and the ovaries were inspected for occurrence of ovulation Estrogen Feedback in 6-Day-Old Male and Female Pigs RESLTS The plasma L concentrations in neonatal gonadectomized male pigs (Fig 1) were not influenced by treatment with either 6 pg or 6 pg estradiol benzoate/kg BW The concentrations of estradiol-17j3 were low ( 2 pg/ml) in castrated males not treated with estradiol benzoate and increased to 26 ± 5 pg/m after receiving 6 pg estradiol benzoate/kg BW After the injection of 6 pg estradiol benzoate/kg BW, the mean estradiol-17a concentrations were 234 ± 36 pg at 24 h and at 96 h the value (2 pg/ml plasma) was not significantly different from that observed in the control animals The plasma concentrations of L in the neonatal female pig were significantly (P 5) suppressed at 24 h only after the administration of 6 pg estradiol benzoate/kg BW and then Downloaded from by guest on 3 January 219

3 SEXAL DFFERETATO OF ESTROGE FEE)BAK THE PG (11) (1) #{176} distance was increased at birth as well as at 16 and 25 days of age ecrotic tissue on the vulva was observed in 3 of 4 D 5 and 4 of 11 D 7 animals at birth, but disappeared later The appearance of the external genitalia was normal in the D 9 and D 16 female offspring (11) #{176}#{176} i-i- iii1 (12) (1),,me (horns) FG 1 oncentrations of LH (open bars) and estradiol-1713 ( logarithmic scale!) in the plasma of orchidectomized male and intact female pigs after treatment with sesame oil (OL) or estradiol benzoate (EB) at 6 days or age V, plasma concentration of estradiol-17p 2 pg/m Results are means ± SEM; number of pigs are shown in parentheses increased again Treatment with 6 pg estradiol benzoate/kg BW was without effect on levels of L The plasma levels of estradiol-17f3 in females after the injection of either 6 or 6 pg estradiol benzoate/kg BW were not different from those observed in the neonatal male pigs Prenatal Testosterone Pro pio nate Treatment n the pregnant sow, plasma levels of testosterone on Day 2 and Day 3 after the first testosterone propionate injection were very high: ng/ml and ng/ml, respectively The stage of pregnancy at which testosterone propionate was administered had no apparent effect on endogenous levels of testosterone Two sows that had been treated with testosterone propionate starting on Day 5 of pregnancy did not deliver All other sows farrowed at the expected time ( days) Parturition was normal except in one sow that had been treated with testosterone propionate starting on Day 16 and in which parturition was prolonged and 5 piglets died n 4 of 1 D 3 females the ano-genital 6 Estrogen L Feedback at 16 Days of Age n the 16-day-old gonadectomized male pig (Fig 2) the concentrations of L were significantly (P1) suppressed 24 h after the injection of 6 pg estradiol benzoate/kg BW and then rose steadily to pretreatment levels in the face of still elevated plasma levels of estradiol-1713 (29 ± 37 pg/m at 72 h) o such depression of L levels after the administration of estradiol benzoate to 6-day-old male pigs was observed (Fig 1) The plasma levels of estradiol-17j3 (Fig 2) at 24 h (172 ± 39 pg/ml) were much higher than in the 6-day-old male pig (Fig 1; 26 ± 5 pg/ml) following administration of 6 pg of estradiol benzoate/ kg BW Female control animals whose mothers were not treated with testosterone propionate (Fig 2) responded to the injection of estradiol benzoate with a significant increase (P5, compared with oil treated controls) in the plasma concentrations of L 48 h and 6 h later The injection of estradiol benzoate (6) (7),,,,e (boo s) rr}1 FG 2 oncentrations of LH (open bars) and estradiol-17i3 (s, logarithmic scale!) in the plasma of orchidectomized male and intact female pigs after treatment with sesame oil (OL) or estradiol benzoate (EB) at 16 days of age V, plasma concentration of estradiol pg/mi Results are means ± SEM; number of pigs are shown in parentheses (8) (8) Downloaded from by guest on 3 January 219

4 119 ELSAESSER AD PARVZ 17j3 in the plasma The concentrations at 24 h and produced 72 h supraphysiological were 725 ± 183 levels and 314 of estradiolestradiol/m plasma, respectively Figure ± 64 pg 3 shows the L response to estradiol benzoate treatment at 16 days of age in gilts that had been exposed to testosterone in fetal life A small increase in plasma L levels was observed = < s 2 9 in D 3 and D 5 females but this was not : : significant compared with oil treated control R animals n both these groups as well as in the D 7 gilts the response to estradiol at 48 or 6 h or both was significantly reduced (P5, compared with estradiol benzoate treated controls; Fig 2) The D 9 and D 16 gilts responded to the injection of estradiol benzoate with a surge of L not significantly different from that observed in estradiol benzoate treated control animals Functioning of the Ovary With the exception of D 3 females, body weight of the gilts pretreated with testosterone 5 propionate was not significantly different from & both control groups atlday,16 or 25Odays of age (Table 1) Between Days 16 and 25 of age occurrence of estrous behavior was erratic in all j groups and not related to the weights of the, 3 (1) 5 (4) 7 ( ,2o2a6,2,s,,rfl4$,o7?, E a 9 (9) 16 (7) #{176} Lst s 6 i a tc 9 :i++j ,m (hours) E us 9 u 8 g F5 FG 3 oncentrations of LH (open bars) and estradiol-17a (, logarithmic scale!) in the plasma of intact female pigs after treatment with estradiol 6 benzoate (6 pg/kg BW) at 16 days of age The mothers of these animals had been treated with testosterone propionate on Day 3, 5, 7, 9 or 16 of pregnancy V, plasma concentration of estradiol- 17) 2 pg/mi Results are means ± SEM; number of pigs are shown in parentheses - E#{176} -,SL), #{149}, n i, >, o S < > -8 a - #{176} - = VSOVS 261 = 2 OS _ 1 Q Q - VS VSOOVSOVSO VS- * n OVS : 1 OSVS OVS V16S*VS VS ,O VS*VS =H=44=44= OOVSVSVSVS o VS 1 *64VVSVS ====,=, ** OVSO- O OVS O O O, VS - VS * VS - VS VS ooooooo t VS VS VS VS VS u : 1, a a a 55 S c a S 5 a a J 2 i >1 8 V a a a Downloaded from by guest on 3 January 219

5 SEXAL DFFERETATO OF ESTROGE FEEDBAK THE PG 1191 ovaries and uteri at 25 days of age owever, despite the fact that the animals were not slaughtered on the same day of the estrous cycle (if they had started to cycle), the weight of the ovaries of D 3 and D 7 animals was significantly reduced (P1 compared with estradiol benzoate or oil treated controls) Also, in these groups the weights of the uteri as well as the percentage of animals with corpora lutea and/or corpora albicantia had declined (Table 1) The weights of the ovaries and of the uterus were also depressed in D 5 animals, but this effect was not significant DSSSO One interesting feature of the present study is the finding that in the Landrace pig, as has been observed previously in the 14-day-old intact and castrated miniature pig (Elsaesser et al, 1978), the stimulatory estrogen feedback on LH release is sexually differentiated: at 16 days of age only the female animal responded to the injection of estradiol benzoate with a clear surge of L Because our treatment produced supraphysiological estradiol levels and the levels following treatment did not differ between sexes, the sexual dimorphism cannot be explained on the basis of differences in sensitivity to estradiol n the 16-day-old male castrate only negative estrogen feedback occurred; LH levels rose to pretreatment levels, although the plasma levels of estradiol were still elevated A similar observation has been made by Ford and Schanbacher (1977) following daily treatment of castrated boars with estradiol benzoate n this context it should be mentioned that, in the castrated male rhesus monkey also, the depressed L concentrations following a single im injection of estradiol benzoate returned to control levels and did not exceed them (Yamaji et al, 1971) f plasma L concentrations are suppressed by the sc implantation of silastic capsules containing crystalline estradiol-17(3, then surges of L secretion resembling those observed in the female can be induced in orchidectomized adult rhesus monkeys by subsequent injections of estradiol benzoate (Karsch et al, 1973) t remains to be determined whether the L surge mechanism can be activated in the castrated male pig by a similar treatment At 6 days of age a sexual dimorphism of the negative estrogen feedback exists, suggesting that the negative estrogen feedback is maturing earlier in the female than in the male With regard to the stimulatory estrogen feedback, we have recently shown that a more pronounced L release in response to various doses of estradiol benzoate occurred in 16-day-old than in 6-day-old gilts (Elsaesser and Foxcroft, 1978) and this study expands our work to the newborn female Landrace pig in which the stimulatory estrogen feedback mechanism has not developed at all These findings point to a gradual maturation of the stimulatory estrogen feedback in the pig similar to that observed in sheep (Land et al, 197; Foster and Karsch, 1975) The possibility that the pituitary of the newborn is unable to release L in a short pulse could be ruled out, because our previous studies showed that the pituitary of the newborn male or female pig responds to LHR in the same way as the adult (Elsaesser et al, 1974) owever, it is possible that the pituitary in these animals is not able to release L in a surgelike manner similar to the preovulatory surge, a possibility which has not been studied so far in the pig A further possibility is that steroid receptors in the hypothalamus or pituitary are lacking Kato et al (1971) and Plapinger and McEwen (1973) have shown an immaturity of steroid receptors in the hypothalamus of the newborn rat, a species in which the stimulatory estrogen feedback mechanism is unable to operate (aligaris et al, 1972; Andrews and Ojeda, 1977) before 22 days of age Another important finding of this study is that the functioning of the stimulatory feedback mechanism and the ovary could be affected by testosterone treatment, when given between Days 3 and 7 of fetal life n this context it is of interest that elevated serum testosterone concentrations have been observed in male pig fetuses between 4-6 days pc by olenbrander et al (1978) and it is tempting to speculate that the critical period of sexual differentiation of the control of L release takes place in the pig at this time The impaired functioning of the ovary might be explained by the impaired functioning of the stimulatory estrogen feedback in these animals Recently, larke et al (1976a,b, 1977) reported strikingly similar effects of testosterone administration in pregnant ewes on the reproductive function of the female offspring, although their technique (Silastic testosterone implants) was very different Androgenization of female rhesus monkeys by exposure to exogenous testosterone in utero did not affect the functioning of the stimulatory feedback mechanism (Stein- Downloaded from by guest on 3 January 219

6 1192 ELSAESSER AD PARVZ er et al, 1976) This is in agreement with other findings, which indicate that the control system governing cyclic gonadotropin secretion seems not to be differentiated in this species (Karsch et al 1973) The pig fetus seems to be well protected against the masculinizing effect of testosterone administered to the mother omplete masculinization of the external genitalia or suppression of LH release following estradiol benzoate as in the genetic male was not achieved by any treatment, althouth testosterone levels in the peripheral plasma of the testosterone propionate treated mothers were at least 5 times as high as in the untreated mothers When we treated 1 sow with im injections of 25 mg testosterone propionate/kg BW on 3 occasions separated by 2 day intervals, starting on Day 3 of pregnancy (Elsaesser, unpublished data), 3 genetic females were born that each had a penis and an empty scrotal sac Testosterone treatments starting on Day 9 or 16 of pregnancy were unable to affect the stimulatory estradiol feedback mechanism at 16 days of age This might be due to a number of factors: unresponsiveness of the target tissue in the brain late in fetal life, the decrease of permeability of the uterus to testosterone, the diminished transfer of testosterone to the fetus as a result of increased aromatization of testosterone in the placenta The last possibility is supported by the findings of Fevre (197) who showed that 19 steroids, including testosterone, can be converted to estrone by the pig placenta Moreover, we know that an increase occurs in plasma concentrations of estradiol and estrone during late gestation (Molokwu and Wagner, 1973; Baldwin and Stabenfeldt, 1975; Ash and eap, 1975) There is also evidence that progesterone can act as a testosterone antagonist to protect against the masculinizing effect of testosterone during neonatal life in the rat (Diamond et al, 1973) and during adulthood in the guinea pig (Diamond and Young, 1963) This protective effect of progesterone could also be functioning in the pig fetus in which the plasma progesterone levels are very high late in pregnancy (Elsaesser et al, 1976; Macdonald et al, 1979) We conclude that with respect to the effect of prenatal or neonatal testosterone treatment on the LH surge mechanism, the pig is similar to the rat (Barraclough, 1966; Gorski, 1973), the guinea pig (Brown-Grant and Sherwood, 1971) and the sheep (larke et al, 1976a,b, 1977) and different from the rhesus monkey (Steiner et al, 1976) owever, because of the similarities between the orchidectomized male pig and the rhesus monkey in the L response to estradiol benzoate, it is possible that the sexual dimorphism observed in the L response to estradiol benzoate is a quantitative rather than a qualitative difference AKOWLEDGMETS This work was supported by the Deutsche Forschungsgemeinschaft We would like to thank Schering AG/Berlin for the generous gift of testosterone propionate REFEREES Andrews, W W and Ojeda, S R (1977) On the feedback actions of estrogen on gonadotropin and prolactin release in infantile female rats Endocrinology 11, Ash, R W and Heap, R B (1975) Oestrogen, progesterone and corticosteroid concentrations in peripheral plasma of sows during pregnancy, parturition, lactation and after weaning J Endocrinol 64, Baldwin, D M and Stabenfeldt, G H (1975) Endocrine changes in the pig during late pregnancy, parturition and lactation Biol Reprod 12, Barraclough, A (1966) Modifications in the S regulation of reproduction after exposure of prepubertal rats to steroid hormones Rec Prog Horm Res 22, Brown-Grant, K and Sherwood, M R (1971) The early androgen syndrome in the guinea-pig J Endocrinol 49, Buh, A E, orman, R L and Resko, J A (1978) Sex differences in estrogen-induced gonadotropin release in hamsters Biol Reprod 18, aligaris, L, Astrada, J J and Taleisnik, 5 (1972) nfluence of age on the release of luteinizing hormone induced by oestrogen and progesterone in immature rats J Endocrinol 55, larke, J, Scaramuzzi, R J and Short, R V (1976) Sexual differentiation of the brain: endocrine and behavioural responses of androgenized ewes to oestrogen J Endocrinol 71, larke, J, Scaramuzzi, R J and Short, R V (1976) Effects of testosterone implants in pregnant ewes on their female offspring J Embryol Exp Morph 36, larke, J, Scaramuzzi, R J and Short, R V (1977) Ovulation in prenatally androgenized ewes J Endocrinol 73, olenbrander, B, de Jong, F H and Wensing, JG (1978) hanges in serum testosterone concentrations in the male pig during development J Reprod Fert 53, Diamond, M and Young, W (1963) Differential responsiveness of pregnant and nonpregnant guinea pigs to the masculinizing action of testosterone propionate Endocrinology 72, Downloaded from by guest on 3 January 219

7 SEXAL DFFERETATO OF ESTROGE FEEDBAK THE PG 1193 Diamond, M, Llacuna, A and Wong L (1973) Sex behaviour after neonatal progesterone, testosterone, estrogen, or antiandrogens Hormones and Behaviour 4, Ellendorff, F, Parvizi,, Pomerantz, D K, Hartjen, A, Konig, A, Smidt, D and Elsaesser, F (1975) Plasma luteinizing hormone and testosterone in the adult male pig: 24 hour fluctuations and the effect of copulation J Endocrinol 67, Elsaesser, F, Ellendorff, F, Parvizi, and Konig A (1974) Response of the pituitary and testes to LHRH in the neonatal miniature pig Acta Endocrinol (Kbh) (Suppl 184), 29 Absrr Elsaesser, F, Ellendorff, F, Pomerantz, D K, Parvizi, and Smidt, D (1976) Plasma levels of luteinizing hormone, progesterone, testosterone and 5cs-dihydrotestosterone in male and female pigs during sexual maturation J Endocrinol 68, Elsaesser, F and Foxcroft, G R (1978) Maturational changes in the characteristics of oestrogeninduced surges of uteinizing hormone in immature domestic guts J Endocrinol 78, Elsaesser, F, Parvizi, and Ellendorff, F (1978) Steroid feedback on luteinizing hormone secretion during sexual maturation in the pig J Endocrinol 78, F#{234}vre,J (197) onversion en oestrone de quelques st&oides -19 chez a truie gestante Ann Biol anim Biochem Biophys 1, Ford, J J and Schanbacher, B D (1977) Luteinizing hormone secretion and female lordosis behavior in male pigs Endocrinology 1, Foster, D L and Karsch, F J (1975) Development of the mechanism regulating the preovulatory surge of luteinizing hormone in sheep Endocrinology Gorski, R A (1973) Perinatal effects of sex steroids on brain development and function Prog in Brain Res 39, Hodges, J K and Hearn, J P (1978) A positive feedback effect of oestradiol on LH release in the male marmoset monkey, allithrix jacchus J Reprod Fert 52, ultsch, K H (1979) Pregnancy and fertility diagnosis in the pig with the aid of plasma progesterone measurements PhD Dissertation, niversity Gottingen Karsch, F J, Dierschke, D J and Knobil, E (1973) Sexual differentiation of pituitary function: apparent differences between primates and rodents Science 179, Karsch, F J and Foster, D L (1975) Sexual differentiation of the mechanism controlling the preovulatory discharge of luteinizing hormone in sheep Endocrinology Kato, J, Atsumi, V and nata, M (1971) Development of estrogen receptors in the rat hypothalamus J Biochem 7, Land, R B, Thimonier, J and Pelletier, J (197) Possibilit d induction d une d#{233}charge de LH par une injection d oestrog#{233}ne chez agneau femelle en fonction de i ge Seances de l Acadmie des Sciences, Serie D Macdonald, A A, Elsaesser, F, Parvizi,, Heilhecker, A, Smidt, D, Ellendorff, F (1979) Progesterone, oestradiol-17a and luteinizing hormone concentrations in the blood and amniotic fluid of chronically catheterized pig foetuses J Endocrinol 8, 14P Abstr Molokwu, E and Wagner, W (1973) Endocrine physiology of the puerperal sow J An Sci 36, Plapinger, L and McEwen, B (1973) Ontogeny of estradiol-binding sites in rat brain Appearance of presumptive adult receptors in cytosol and nuclei Endocrinology 93, Pomerantz, D K, Ellendorff, F, Elsaesser, F, K#{246}nig, A and Smidt, D (1974) Plasma LH changes in intact adult, castrated adult and pubertal male pigs following various doses of synthetic luteinizing hormone-releasing hormone (LH-RH) Endocrinology 94, Short, R V (1974) Sexual differentiation of the brain of the sheep n: Sexual endocrinology of the perinatal period, olloque nternational SERM, Lyon (M G Forest and J Bertrand, eds) SERM Paris pp Snedecor, G W (1956) Statistical Methods owa State niversity Press, Ames, owa p 45 Steiner, R A, lifton, D K, Spies, H G and Resko, J A (1976) Sexual differentiation and feedback control of luteinizing hormone secretion in the Rhesus monkey Biol Reprod 15, van Look, PFA (1976) Failure of positive feedback linics in Obstet Gyn 3, Yamaji, T, Dierschke, D J, Hotchkiss, J, Bhattacharya, A Surve, A H and Knobil, E (1971) Estrogen induction of LH release in the Rhesus monkey Endocrinology Zimbelman, R G and Lauderdale, J W (1973) Failure of prepartum or neonatal steroid injections to cause infertility in heifers, guts and bitches Biol Reprod 8, Downloaded from by guest on 3 January 219