I et2,la dose de l0 pg a provoqu6 une d6charge plus 61ev6e (P<0,001) de LH que

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1 EFFECTS OF GnRH ON LUTEINIZING HORMONE RELEASE AND ONSET OF CYCLIC OVARIAN ACTIVITY POSTPARTUM IN PELIBT]EY EWES A. GONZALEZ R.t'2'3 and B. D. MURPHY1'4 treprductive Bilgy Research Unit, Cllege f Medicine, Universiry f Saskntchewan, Saskatn, Saskatchewan, Canada S7N 0W0; and 2Asciacin Mexicana de Prduccin Animnl, Tampic, Tam., Mexic. Received 2 Apr, 1987, accepted.26 Nv Gxznrzz, R., A. AND MuRpHy, B. D Effects f GnRH n luteinizing hrmne release and nset f cyclic varian activity pstpartum in Pelibuey ewes. Can. J. Anim. Sci. 68: Synthetic gnadtrpin releasing hrmne (GnRH) was administered t Pelibuey ewes t determine its effect n luteinizing hrmne (LH) release and n cyclic varian activity during the first 3-4 wk pstpartum. Tw GnRH dses were given (10 and 100 pg) n days 4 and 5, 7 and 8, 14 and 15, and 2l and 22, after partlritin, respectively. In three trials, 11, 8 and 7 ewes were used. Fr each trial 5, 3 and 2 ewes were used as cntrls, respectively; in trial 3, nly the 100-pg dse was given nce. Results frm the three trials indicated that LH release always ccurred after GnRH treatment, relative t the saline-treated cntrls. In trials I and2, the 10-pg dse induced a higher (P < 0.001) LH release than the 100-pg dse; the LH release in respnse t the 100-pg dse in the three trials was f similar magnitude. Release f LH in respnse t bth dses was lwer (P<0.05) during days 4-5 pstpartum, than during the fllwing 3 wk. Tw types f luteal activity r steridgenesis were bserved in the GnRH-treated ewes, relative t the saline-treated ewes; (1) within 2-4 d after each GnRH injectin small peaks f prgesterne appeared, and (2) within 5-15 d after the last GnRH injectin there were elevatins f prgesterne in peripheral plasma t cncentratins and f duratins characteristic f the luteal phase f the vine estrus cycle. These results indicate that the pituitary f pstpartum Pelibuey ewes regains the ability fr maximal release f LH in respnse t GnRH challenge within 7 d pstpartum. Further, treatment with GnRH induces earlier return t cyclic varian activity. Key wrds: Hair sheep, pstpartum, GnRH, luteal activity [Effets de la GnRH sur la d6charge de I'hrmne lut inisante et le d6clenchement de I'activit6 cyclique varienne aprds la mise bas chez des brebis Pelibuey.l Titre abr6g6: Effects de la GnRH sur le cycle varien des brebis Nus avns inject6 une hrmne synth6tique de d6charge de la gnadtrpine (GnRH) h des brebis Pelibuey afin d'en d6terminer les effets sur la d6charge de l'hrmne lut6inisante (LH) et sur I'activit6 cyclique varienne pendant les 3 u 4 premidres semaines suivant la mise bas. Deux dses de GnRH nt 6t6 inject6es (10 et 100 pg, IV) aux jurs 4 et 5, 7 et 8, 14 et 15 et 2l et 22 aprbs la mise bas respectivement. Au curs de tris essais, nus avns utilisd 11, 8 et 7 brebis. Pur chaque essai, 5, 3 et 2 brebis servaient de t6mins respectivement. Pur I'essai num6r 3, nus n'avns inject6 que la dse de 100 pg, une seule fis. Les r6sultats des tris essais nt laiss6 vir que la d6charge de LH s'bserve tujurs aprds I'injectin de GnRH, cmparativement ir ce qu'n bserve chez les t6mins recevant une slutin saline. Dans les essais I et2,la dse de l0 pg a prvqu6 une d6charge plus 61ev6e (P<0,001) de LH que 3Present address (A.G.R.): Department f Bilgy, University f Saskatchewan, Saskatpn, Saskatchewan, Canada S7N 0W0. 4Address crrespndence t B.D.M. Can. J. Anim. Sci. 68: (June 1988) 359

2 360 canadian JuRNAL F ANTMAL science la dse de l00pg. La ddcharge de LH prvqule par l'injectin d'une dse de 100 1tg 6tait cmparable pur les tris essais. La d6charge de LH d6culant de I'injectin des deux dses 6tait inf6rieure (P<0,05) pendant les jurs 4 et 5 suivant Ia mise bas qu'au curs des 3 semaines suivantes. Deux types d'activit6 de lut6inisatin u de st6ridg6nbse nt 6t6 bserv6s chez les brebis trait6es )r la GnRH, cmparativement aux brebis t6mins: ( 1) dans les 2 ir 4 jurs qui nt suivi chaque injectin de GnRH, n a bserv6 de faibles pics de prgestdrne; (2) dans les 5 )r 15 jurs qui nt suivi la dernibre injectin de GnRH, la teneur en prgest6rne du plasma p6riph6rique a atteint des cncentratins et une dur6e caract6ristiques de la phase de lut6inisatin du cycle estral des vins. Ces rdsultats prtent a crire que I'hypphyse des brebis Pelibuey pst partum redevient en mesure de prduire une s6cr6tin maximale de LH, en r6actin ir la dse d'6preuve de GnRH, dans les 7 jurs qui suivent la mise bas. En utre, le traitement d la GnRH assure un retur plus prdcce de I'activit6 cyclique varienne. Mts c16s: Mutn )r viande, pst partum, GnRH, activit6 de lut6inisatin Gnadtrpin releasing hrmne (GnRH) is a decapeptide that regulates bth luteinizing hrmne (LH) and fllicle stimulating hrmne (FSH) release in mammals (Schally et al. l97l). Since its structural characterizatin, GnRH has been used t induce the release f LH frm the sheep pituitary under varied physilgical cnditins; such as seasnal anestrus, different phases f the estrus cycle and gestatin (Reeves etal. l9l1; Jenkin and Heap 1974; Mcled et al. 1982b) and during pstpartum anestrus (Fster and Crightn 1973; Jenkin and Heap 1974). Anther use f GnRH has been t bring abut the return t cyclic varian activity and estrus cyclicity in pstpartum ewes (Mcled et al. 1982a). It has been shwn that the pituitary f the pstpartum ewe f wl breeds is able t respnd when challenged with GnRH within a few days after lambing (Fster and Crightn 1973; Jenkin and Heap 1974 McLed et al. 1982a). The Pelibuey sheep is a trpical breed that des nt exhibit a seasnal anestrus as ccurs in temperature breeds (Gnzalez et al. 1987). Pelibuey ewes breed thrugh mst f the year, with resumptin f estrual activity after a shrt perid f anestrus after parturitin, during which n vulatry r estrual activities ccur (Gnzalez et al. 1987). Treatment f pstpartum Pelibuey ewes with GnRH culd shrten the interval t first estrus and therefre btain tw lamb crps per year. Therefre, the specific bjectives fthis study were t determine the effects f GnRH n LH release during the pstpartum perid and n the return t cyclic varian activity in the first 3-4 wk pstpartum in Pelibuey ewes. MATERIALS AND METHODS Animals, Management and Experimental Prcedures. This study was carried ut n a flck fpelibuey sheep lcate<i at the Centr de Adiestramient y Mejramient para la Prduccin Animal (CAMPA) in the ranch El Apur, Aldama, Tamaulipas, Mexic. The ranch is lcated at 15 m abve sea level and at a latitude f 22"29'30" and a lngitude f 9'7"55'12" ; the climate at the ranch is classified as semiarid trpic, with an average annual precipitatin f 1080 mm, falling within a well-defined rainy seasn, frm May t August. The dry seasn n pastures f imprved trpical grasses such as Pangla grass (December- February) is characterized by spradic shwers brught in by strng nrth winds. The duratin f the shrtest and lngest days are f apprximately 10 and 15 h light, respectively. The animals in this study were maintained with the flck and grazedyear-rund n pastures f imprved trpical grasses such as Pangla grass (Digitaria decumbens), Guinea grass (Panicum marimum) and (r) Bermuda Cast-Crss One grass (Cyndn dacty ln x C. p le itstachlas). The entire flck had access t water tw t three times a day and was crralled at night. Experimental animals were separated frm the flck nly fr treatments and intensive bleedings, at which times feed and extra water were prvided in lieu f grazing. Estrus behavir was checked daily, using intact brisket-painted rams, and visually twice

3 GONZALEZ ET AL. - EFFEC'TS OF GNRH ON LUTEINIZING HORMONE RELEASE 36I daily, during I h in the mrning and evenings. Lamb were allwed t suckle their dams. The entire flck and experimental animals were run year rund with fertile rams. A ttal f 26 pstpartum ewes was used in three trials, ne each during May 1981 (trial l, n : l1), Nvember 1981 (trial 2, n : 8) and May 1982 (trial 3, n : '7). In each trial ewes were randmly assigned t treatments. In each trial, five, three and tw ewes were used as cntrls, respectively, and each ewe received an equivalent vlume f acidified saline. Mean bdy weights in kilgrams (t SEM) were 24.5 (2.5), 27.5 (1.1) and 28.3 (1.7) and mean ages at lambing in mnths (* SEM) were 2'7.9 (1.9),25.4 (3.2) and2'7.2 (2.3), respectively, fr the ewes in the three trials. Tw synthetic GnRH (Gnadrelin hydrchlride, Factrel, Ayerst, Mntreal, Que.) dses, 10 and 100 pg in 1 ml f acidified saline were administered intravenusly (IV) n days 4 and 5, 7 and 8, 14 and 15 and 2l and 22 pstpartum, respectively. In trial 2, nly the 100-pg dse was administered nce. T mnitr circulating levels f prgesterne, bld samples were taken by venipuncture frm the jugular vein every secnd day, beginning 3-4 d after parturitin and cntinuing thrugh the injectin perid (trials I and 2) until estrus was bserved (trial 3). Jugular bld samples were als taken t mnitr pituitary LH release in respnse t GnRH r saline at 0, 30, 60, 120,240 and 360 min after each injectin. Bld samples were allwed t clt at ambient temperature (26-28"C) fr t h and then placed n ice, transprted t the labratry and centrifuged. Serum was aspirated, frzen and kept at -20'C until hrmne radiimmunassays (RIA) were perfrmed. Hrmne Assays Prgesterne RIA was perfrmed using an antibdy raised against 4-pregnen-6-l-3, 20 dine hemisuccinate-bvine serum albumin. Crssreactivity and validatin f assays using this antiserum have been previusly described (Rawlings et al. 1984). The sensitivity fthe assay, defined as the mean ( * SEM) smallest dse f prgesterne displacing the tracer frm the antibdy, was 199 * l2pgml '(n : 8). The inter- and intra-assay cefficients f variatin were 12.9 and 9.2%. respectively. Serum LH cncentratin was determined using the duble antibdy RIA described by Niswender et al. (1969) as mdified by Rieger (1981). The assay emplyed purified vine LH preparatins fr idinatin (LER-1056-C2l) and as reference standard (NIH-LH-S-22) and Niswender's LH antiserum (GDN n.l5). The mean (t SEM) sensitivity f the assay was ( pg ml-' (n : l7). Inter- and intraassay cefficients f variatin were 11.7 and 15.'7%, respectively. Statistical Analysis. Analysis f data included determinatin f means and standard errrs f means fr LH levels n each day pstpartum in each trial. Areas under release curves were calculated by pltting the amunt f LH determined at each bleeding time versus time, using a Plar cmpensating planimeter. Ttal LH released under the curve and the maximal LH released determined n each bleeding time were nt significantly different, using ne-way ANOVA. Tw-way analysis f variance n days pstpartum and dsages was emplyed and differences between individual means were determined by Student- Newman-Keuls test. Crrelatin and Chi square analysis were used t determine the effect fdsage and days f treatment n LH release and t determine the difference in the prprtin f ewes shwing varian activity after each treatment. Steridgenic activity was assumed t ccur when prgesterne levels were elevated abve 0.5 ng ml-r; varian activity after the last injectin was assumed t ccur when a prgesterne prfile with a luteal phase similar t that f an estrus cycle (Gnzalez et al. 1987) was present. RESULTS Release n LH in respnse t the administratin f synthetic GnRH during the first 3 r 4 wk pstpartum in Pelibuey ewes is depicted in Fig. 1. Table I presents the mean maximum peaks (+ SEM) f LH fr the three trials; maximum levels f LH released ranged frm 1.6 t 40 ng ml--l fr the tw GnRH dses emplyed. Results frm the three trials demnstrated that LH release always increased fllwing GnRH treatment relative t saline treatment. In trials 1 and 3, the 10-pg GnRh dse induced a significantly (P<0.001) greater LH release that the 100-pg dse, which was administered 24 h later. The release f LH in respnse t bth dses was significantly (P<0.05) reduced at days 4-5 pstpartum (Fig. 1), cmpared t the respnses btained during the fllwing 3 wk. The pituitary

4 362 CANADIAN JOURNAL OF ANIMAL SCIENCE L' E CD c llj z =d, I 30 z tr z ut F l J =f u. uj tn 20 l0 Oug 10Ug r'10opg D"vt // /Fd-\\ l"-s4 t..=i# 0.5r246 Pstprtum l4-15 y' Pst p rl u m 2t-22 Dys Pst HOURS POSTINJECTION Fig. 1. Pituitary LH release in respnse t saline (0-0, n : 10, l0 (s--, n : 10) r 100 ptg ( r -- r, n : 16) f GnRH administered t Pelibuey ewes during the first 3-4 wk pstpartum (means + SEM, fr the three trials). N GnRH dse (pg) respnse t a given dse f GnRH did nt vary frm days 7 and 8, r thrugh days 2l ffid 22. One-way ANOVA carried ut n the ttal amunt f LH released (measured as area under the curve) prduced results nt different frm the analysis f the data n maximum cncentratin f LH n each bleeding perid. Similarly, differences between years and amng the three trials due t seasn were nt statistically significant. Thus the LH released in respnse t the 10-pg dse did nt differ significantly in trials 1 and 3, nr did the respnse t the 100-pg dse differ in the three trials. There were elevatins in prgesterne fllwing GhRH treatment, presumably representing increased steridgenic activity in the vary r f luteal frmatin. These appeared with greater frequency in the GnRhtreated ewes relative t the saline-treated ewes (P<0.05). Representative serum prgesterne prfiles fr the GnRH- and the saline-treated ewes are presented in Fig. 2. Tw types f effects were bserved with respect t increased steridgenic activity. There was a shrt-term effect, in which 14 f 15 GnRH-treated ewes displayed prgesterne peaks indicative f luteal activity within2-4d f eachinjectin. These prgesterne peaks persisted during the week fllwing the GnRH injectin. The secnd, lnger-term, effect was bserved 5-15 d after the last GnRH injectin and cnsisted f a sustained luteal phase elevatin f peripheral prgesterne. The prprtin f ewes with luteal like phases 1 and2 wk after Table 1. Pituitary LH respnse (mean peak serum cncentratin + SE fr the three trialsf) fpstpartum Pelibuey ewes in respnse t intravenus GnRH injectins (ng ml-l; 10 l0 l Davs DstDartum 4-s+ 7-8 t t ' l'7.6 x 1.3 2r ! 2.3 fa tw-way ANOVA with unequal cell numbers yielded significant differences fr dsages (P<0.005), days pstpartum (P<0.005) and the interactin dsage x days (p<0.05). $Saline and l0- pg GnRH and saline and 100 pg GnRH were given n the days indicated, respectively.

5 GONZALEZ ET AL. - EFFECTS OF GNRH ON LUTEINIZING HORMONE RELEASE 363 the last GnRH injectin were 87 and 90%, respectively. The crrespnding prprtins fr the saline-treated ewes were 3O and 5l %, respectively, and were significantly (P<0.001) reduced frm thse fr the GnRH-treated ewes. In trial 3, fur f five GnRH-treated ewes exhibited estrus at an average f 43 d pstpartum, with a range f 30-53; the 5th ewe did nt shw estrus by day 54. Variability in LI ) At l FIGURE 2a Dys Pslprlum the prgesterne prfiles f the saline-treated ewes t day 53 pstpartum indicated that sme f these ewes were underging luteal cycles with n behaviral estrus (silent cycles); since, nne f these ewes had exhibited estrus by day 53 pstpartum. DISCUSSION As fr wl breeds f sheep (Knipe 1981; Wright et al. 1980; Wright et al. l98la,b), ll9 {Cnkl) r Dys Pslprtum FIGURI 2b Fig. 2. Representative serum prgesterne levels fpstpartum Pelibuey ewes treated with 10 r 100 pg f GnRH (2a) r physilgical saline (2b) at weekly intervals fr 3 wk. Arrws indicate days f iniectins.

6 364 CANADIAN JOURNAL OF ANIMAL SCIENCE GnRH induces LH release in Pelibuey ewes during the pstpartum perid. The GnRH dsages used in this study are lwer than dses previusly reprted fr pstpartum wl sheep. Release f LH at magnifudes ranging frm 2 t 40 ng ml--l in respnse t 10 and 100 pg in this study are within the ranges reprted in respnse t 150 pg f GnRH at day 10 in pstpartum nnlactating ewes (Rippel et al. 1974) and 2OO p"g at 2, 4 and 6 wk and at 10, 20, 30, 40 and 50 d pstpartum (Jenkin et al. 1977; Knipe 1981). The results frm this study indicate that the 10-pg dse was a maximal dsage and appeared t deplete pituitary reserves; hwever, it did nt prevent a maximal LH release by day 7. This effect has als been demnstrated in pstpartum wl sheep (Pelletier 1976). On the rher hand, the 100-pg dse was a supramaximal dsage, s that it may have induced dwn-regulatin f GnRH receptrs, cnsistent with what has been reprted in wl sheep (Nett et al. 1981). These results indicate rhat the ability f the Pelibuey ewe pituitary gland t release LH in respnse t GnRH is reduced nly during the first 4-5 d pstpartum, with an evfrent recvery taking place by days 7-8. Sme studies in wl sheep (Restall et al. l9j7; Wright et al. 1980) have indicated that the respnse f the pituitary t GnRH was reduced n days relative t days 25-28; thers indicate that secretin f LH in respnse t GnRH did nt change frm days l0-40 in fall lambing ewes (Knipe 1981). When GnRH studies have been carried ut ver lnger perids f time, results suggest a gradual recvery f the ability f the pituitary t secrete LH, f up t l0 wk pstpartum (Jenkin and Heap 1974; Jenkin et al. 1977). There was n indicatin f seasnal effects f GnRH n LH release as the respnse t the 100-pg dse in the three trials did nt vary significantly during May cmpared t that f Nvember. In wl sheep, seasn may (Dmansky et al. 1977;Rieger 1981) r may nt affect (Lincln and Shrt 1980) the LH release induced by GnRH. The effects f seasn n the GnRH-induced LH release result in a diminished peak f LH during anestrus, cmpared t the prevulatry LH peak during the estrus cycle f the breeding seasn (Crightn et al Shareha et al ; Rieger 198 l), r during the pstpartum perid (Pelletier 1976). Shrt- and lng-term effects f GnRH n prgesterne secretin were bserved in this study. The first cnsisted f small increments in prgesterne after each GnRH injectin, suggesting inductin f steridgenic activity. The surce f prgesterne was nt determined, but it culd have riginated frm vulated fllicles, luteinized fllicles r frm the stimulatin f prgesterne prductin frm the interstitial cells. Prgesterne levels fr pstpartum wl sheep did nt increase after a single injectin f GnRH at 10 d pstpartum (Ainswrth et al. 1982). Results frm the present experiment are nt cnsistent with the latter reprt since, when GnRH was given as early as days 4-5 pstpartum, a prgesterne peak was bserved. Other reprts have indicated sme variatins frm regular luteal phase prgesterne prfiles after GnRH treatment in cycling r anestrus ewes. In pstpartum wl sheep, GnRH induces incnsistent prgesterne levels, that are either lw r when a luteal phase is present, it is shrtened (Haresign et ai ; Mcled et al. 1982a). The varian activity induced during the treatrnent perid (the shrt-term effect), indicated by a prgesterne rise and suggestive f a luteal phase, ccurred in 14 f 15 GnRHtreated ewes. A smaller prprtin (40%) t the saline-treated ewes had similar rises within 30 d pstpartum. The lng-term effect f GnRH was that f inducing varian activity I - 2 wk after the treatment perid; at this time, the prprtin f treated ewes with varian activity was greater (P<0.001). Overall, these results suggest that GnRH induces varian activity in the Pelibuey ewe during the pstpartum perid, a finding cnsistent with ther reprts fr wl sheep (Lewis et al. 1981; Wright et al. 1981a).

7 GONZALEZ ET AL. - In summary, results f this study indicated that GnRH induces LH release, regardless f dsages and days pstpartum. A 10-prg dse always induces a greater LH release than a 100-pg dse administered24 hurs later, an effect that culd be due t lss f pituitary respnsiveness, as suggested fr wl sheep (Rippel et al. 1974) r perhaps due t decreased sensitivity induced by suprastimularin (Nett et al. 1981). The LH respnse was significantly reduced at 4 d pstpartum, relative t the fllwing 3 wk, during which time the respnse did nt change significantly. Treatment f pstpartum Pelibuey ewes with GnRH appeared t induce the events leading t vulatin. ACKNOWLEDGMENTS This wrk was supprted partially by grants frm NSERC and IDRC (Canada). CONACYT (Mexic) and the Cllege f Graduate Studies (University f Saskatchewan) prvided schlarships fr A. Gnzalez R. Thanks are expressed t Drs. G. D. Niswender and N. C. Rawlings fr antisera and t L. E. Reichert Jr. fr highly purified LH fr iddinatin. Thanks are als expressed t D. Dyck fr preparatin f figures and t J. Ryan fr typing. Ainswrth, L., Lachance, R. and Labrie, F Effect f GnRH-inCuced endgenus luteinizing hrmne release and exgenus prgestagen treatment n varian activity in the pstpartum ewe. J. Anim. Sci. 54: Crightn, D. B., Fster, J. P., Haresign, W. and Sctt, S. A Plasma LH and prgesterne levels after single r multiple injectins f synthetic LH-RH in anestrus ewes and cmparisn with levels during the estrus cycle. J. Reprd. Fertil. 4: Dmansky, E., Przekp, F., Skubiszewski, B., Wrblewska, B. and Stupnicka, E Effect f prlnged infusin f small dses f LH-RH n the release f LH and vulatin in ewes during mid-anestrus. J. Reprd. Fertil. 51: Fster, J. P. and Crightn, D. B Cmparisn f LH levels in pst-partum anestrus and cycling ewes and the effects f synthetic gnadtrphin-releasing factr. J. Reprd. Fertil. 35: Gnzalez, A., Murphy, B. D., De Alba, J. M. and Manns, J. G. f987. Endcrinlgy f the EFFECTS OF GNRH ON LUTEINIZING HORMONE RELEASE 365 pstpartum perid in the Pelibuey ewe. J. Anim. Sci. 64: Haresign, W., Fster, J. P., Haynes, N. B., Crightn, D. B. and Lamming, D. B Prgesterne levels fllwing treatment f seasnally anestrus ewes with synthetic LHreleasing hrmne. J. Reprd. Fertil. 43: Jenkin, G. and Heap, R. B The lack f respnse fthe sheep pituitary t luteinizing hrmne releasing hrmne stimulatin in gestatin and early lactatin; the prbable rle f prgesterne. J. Endcrinl. 61: xii-xiii. Jenkin, G., Heap, R. B. and Symns, D. B. A Pitrriry respnsiveness t synthetic LH-RH and pituitary LH cntent at varius reprductive stages in the sheep. J. Reprd. Fertil. 49: Knipe, R. K Ovine pituitary respnsiveness t gnadtrpin releasing hrmne during lactatinal anestrus. Diss. Abst. Int.41: 24t68-247t8. Lewis, G. S., Lishman, A. W., Butcher, R. L. and Inskeep, E. K Factrs affecting functin f induced crpra lutea in pstpartum anestrus ewes. J. Anim. Sci. 52: Lincln, G. A. and Shrt, R. V Seasnal breeding: Nature's cntraceptive. Rec. Prg. Hrm. Res. 36: l-52. Mcled, B. J., Haresign, W. and Lamming' G. E. 1982a. The inductin f lulatin and luteal functin in seasnally anestrus ewes treated with small-dse multiple injectins f GnRH. J. Reprd. Fertil. 65: Mcled, B. J., Haresign, W. and Lamming' G. E. 1982b. Respnse f seasnally anestrus ewes t small-dse multiple injectins f GnRH with and withut prgesterne pretreatment. I. Reprd. Fertil. 65: Nett, T. M., Crwder, M. E., Mss, G. E. and Duel, T. M GnRH-receptr interactin. V. Dwn-regulatin f pituitary receptrs fr GnRH in variectmized ewes by infusin f hmlgus hrmne. Bil. Reprd. 24: I 145-l 155. Niswender, G. D., Reichert,L.E., Jr.' Midgley' A. R., Jr., and Nalbandv, A. V Radiimmunassay fr bvine and vine luteinizing hrmne. Endcrinlgy 84: Pelletier, J Influence flh-rf n LH and FSH releases in dmestic animals. Ann. Bil. Anim. Bich. Biphys. 16: Rawlings, N. C., Jeffcate, I. A. and Rieger' D The influence f estradil-l7b and prgesterne n peripheral serum cncentratins f luteinizine hrmne and fllicle stimulating

8 366 CANADIAN JOURNAL OF ANIMAL SCIENCE hrmne in the variectmized ewe. Therigenlgy 22; Reeves, J. J., Arimura, A. and Schally, A. V Pituitary respnsiveness t purified luteinizing hmne-releasing hrmne (LH-RH) at varius stages f the estrus cycle in sheep. J. Anim. Sci. 32l Restall, B. J., Kearins, R. D. and Starr, B. G Studies f pituitary functin in lactating ewes. J. Reprd. Fertil. 49: Rieger, D. f981. The effects fprgesterne and estradil n the spntaneus and GnRH-induced secretin f LH and FSH in the anestrus and cyclic ewe. Ph. D. Thesis, University f Saskatchewan, Saskatn, Sask. 265 pp. Rippel, R. H., Jhnsn, E. S. and White W. F Effect f cnsecutive injectins f synthetic gnadtrpin releasing hrmne n LH release in the anestrus and variectmized ewe. J. Anim. Sci.39: Schally, A. Y., Arimura, A., Kastin, A. J., Matsu, H., Baba, Y., Redding, T. W., Nair, R. M. G. anddebeljuk,l. l9tl. Gnadtrpinreleasing hrmne: One plypeptide regulates secretin f luteinizing and fllicle-stimulating hrmnes. Science 173: Shareha, A. M., Ward, W. R. and Birchall, K Effect f cntinuus infusin f gnadtrphin-releasing hrmne in ewes at different times f the year. J. Reprd. Fertil. 46: 33r-340. Wright, P. J., Geytenbeek, P. E., Clarke, I. J. and Findlay, J. K Pituitary respnsiveness t LH-RH, the ccurrence f estradil-l7b-induced LH-psitive feedback and the resumptin f estrus cycles in ewes pstpartum. J. Reprd. Fertil. 60: 17l-176. Wright, P. J., Geytenbeek, P. E., Clarke, I. J. and Findlay, J. K. 1981a. Evidence fr a change in estradil negative feedback and LH pulse frequency in pst-partum ewes. J. Reprd. Fertil. 61: Wright, P. J., Jenkin, G. and Heap, R. B. l98lb. Prlactin and LH release in respnse t LH-RH and TRH during diestrus, pregnancy and pstpartum. J. Reprd. Fertil. 62: 44'l-453.

9 This article has been cited by: 1. A Gnzález-Reyna, E Márquez-Garcıá, H Lizárraga-Tracy, J.C Martıńez-Gnzález Dse respnse effects f PMSG n vulatin rate and fllicular develpment in Pelibuey ewes treated with Syncr-mate-B implants. Small Ruminant Research 31:2, [Crssref] 2. M.A. Galina, R. Mrales, E. Silva, B. López Reprductive perfrmance f Pelibuey and Blackbelly sheep under trpical management systems in Mexic. Small Ruminant Research 22:1, [Crssref]