Sesonl differences in endocrine nd ovrin ptterns of Bos turus indicus (Nelore) heifers estrous cycles Diferençs szonis no pdrão endócrino e ovrino do ciclo estrl de novilhs Bos turus indicus (Nelore) Anivldo Olivio CORTE JUNIOR 1 ; Bruno José Ferreir NAVARRETE 1 ; Guilherme de Pul NOGUEIRA 1 1 Curso de Medicin Veterinári d Universidde Estdul Pulist Júlio de Mesquit Filho, Arçtu-SP, Brzil Astrct Estrous cycle of eight Nelore heifers were evluted during different sesons of the yer (utumn n=11; winter n=8; spring n=9 nd summer n=9) with dily count nd mesurement of follicles 3mm, lood ws collected every 1h for LH nd progesterone (P), nd fter estrous every 3h for LH pek. Five ovriectomized heifers were injected with 17β-estrdiol (μg/kg) every seson nd lood smples collected every 3h (for 3h) therefter for LH quntifiction. The monthly percent ody weight difference (Δ%) did not vry mong sesons. P concentrtion ws higher (p<.1) nd follicle numer lower during utumn nd summer compred to winter nd spring. During winter there were more estrous cycles with three nd during summer only cycles with two folliculr wves (p<.1). As LH secretion did not vry despite P concentrtion nd s there ws negtive correltion etween higher P vlues nd dily percentile vrition of photoperiod (Δ%, p<.1; r= -.5) it is possile to suppose tht there is sesonl vrition on lutel cell sensitivity to LH. In the ovriectomized Nelore heifers, the LH sl concentrtion (without estrdiol stimulus, p=.) nd the LH response to estrdiol (p<.1) were lower during summer, leding to the hypothesis tht there is sesonl vrition of hypothlmic sensitivity to estrdiol. According to the present experiment there re suggestions of sesonl reproduction in Nelore heifers. Keywords: Sesonl. Estrdiol. Luteinizing hormone. Ovrin follicle. Progesterone. Resumo Ciclos estris em oito novilhs Nelore form compnhdos durnte diferentes estções (outono n=11; inverno n=8; primver n=9; verão n=9) com contgem e diâmetro de folículos 3 mm dirimente, com colheits de sngue cd 1 h pr LH e P, e cd 3 h pr o pico de LH. Cinco novilhs ovriectomizds receerm 17β estrdiol (μg/kg) em cd estção, com colheits de sngue cd 3 h pr quntificr LH. A diferenç percentul (Δ%) do peso entre os meses não vriou entre s estções. Durnte o ciclo estrl, concentrção de P (médi e máxim) foi mior (p<,1), e o número de folículos menor (p<,1), no outono e verão, se comprdos o inverno e primver. No inverno houve mis ciclos com três onds e no verão somente ciclos com dus onds foliculres (p<,1). Como s crcterístics d secreção de LH não form diferentes pesr d vrição n concentrção de P, e como houve correlção negtiv entre os vlores máximos de P e vrição percentul (Δ%) diári no fotoperíodo (p<,1; r= -,5), supõe-se que hj vrição szonl n sensiilidde ds céluls luteínics o LH. Ns novilhs ovriectomizds, concentrção sl (sem estrdiol) circnul de LH foi menor no verão (p=,), ssim como su secreção em respost o estrdiol (p<,1), o que permite supor que tmém hj um vrição szonl n sensiilidde hipotlâmic o estrdiol. De cordo com o presente experimento, há indícios de szonlidde reprodutiv em novilhs Nelore. Plvrs-chve: Estções do no. Estrdiol. Hormônio luteiniznte. Folículo ovrin. Progesteron. Introduction Considering the Bovide fmily there re severl species tht reproduces sesonlly, such s sheep, gots nd uffloes. Although the Bos turus species is considered yerly polyestrous, some reproductive chrcteristics vries throughout the yer, such s higher mounting frequency of eef cttle in winter compred to summer 1. The literture generlly presents conflicting results regrding the sesonl reproduction in cttle. In Bos indicus oth the sence of Correspondênci pr: Guilherme de Pul Nogueir R: Clóvis Pestn, 793, Arçtu-SP CEP.:15-8 e-mil: gpn@fmv.unesp.r Receido: 1/9/9 Aprovdo: 9//1
7 sesonl effect on serum progesterone concentrtions (P), greter P during decresing photoperiod 3, or even the opposite 5 were descried. Also, the Bos indicus corpus luteum (CL) showed lrger re nd volume during the summer compred to winter 5, ssocited to n incresed cpcity of P production in response to luteinizing hormone (LH),7. There ws no sesonl influence on plsm LH concentrtion in intct Brhmn cows (Bos indicus), ut in ovriectomized cows there ws mrked LH decrese during summer 8. In ovriectomized Bos turus cows, there ws higher LH concentrtion nd greter pulse mplitude in response to estrdiol round the spring equinox, compred to other yer sesons 9,1. Higher LH concentrtion ws found in spring nd summer compred to winter, in intct Bos indicus cows. Winter strts fter the winter solstice, fter which the photoperiod grdully increses until the summer solstice when the photoperiod egins to regress. A reproductive sesonl niml uses photoperiod s cue, iming more fvorle period for offspring irth. Mechnism my include ltering the hypothlmic gondotropin-relesing-hormone (GnRH) secretion, nd, consequently LH nd follicle stimulting hormone (FSH) relese from the pituitry, so tht ovultion in concentrted in more successful seson. Throughout evolution, nimls were selected y environmentl pressure to give irth in sesons when food supply ws gretest. For exmple, the Indin domestic ufflo (Bulus ulis) from the sme geogrphic region of Bos indicus, elongs to the sme Bovide fmily nd hs gesttion lenght not very different from cttle. Tht species, hs defined reeding seson concentrted in the lower photoperiod dys 11. Anlyzing the Indin climtic conditions, the psture supply increses with the onset of summer, when the monsoon winds increses rinfll 1. According to Drwinin evolutionry theory, nimls with gesttion period of 9 (cttle) nd 1 (ufflo) months, shll hve reproductive ctivity etween lte summer nd utumn (diminishing photoperiod), to give irth in lte spring nd erly summer, t fvorle environmentl conditions for the offspring s survivl. In the ovine estrous cycle, follicle growth exhiits wve pttern tht occurs periodiclly in oth the turine (Bos turus) 13 nd in the zeu (Bos indicus) 1,15 reeds. Ech folliculr wve is composed y three distinct phses: recruitment, selection nd dominnce 1,17,18 ; the developing dominnt follicle requires LH 19 nd restrints the growth of other follicles through the suppression of FSH concentrtions. This event depends on inhiin, produced y the growing follicles nd lso on estrdiol produced y the dominnt follicle 1,. The hypothlmic estrdiol sensitivity is stimulted y previous exposure to P, tht is lso needed to stimulte the relese of the LH preovultory pek 1. Tretment with P filed to suppress the surge of FSH in cttle,3, however plys n importnt role in the LH suppression 19,. The motivtion of the present experiment cme from results of preliminry study, conducted t the Lortory of Animl Endocrinology t the Universidde Estdul Pulist (UNESP) Cmpus Arçtu- SP, Brzil, in which sesonl vrition ws oserved in serum LH concentrtions in ovriectomized Nelore heifers. In tht study, there ws negtive correltion with the h photoperiod vrition. In the presentstudy, we used intct Nelore heifers to test the hypothesis tht Nelore femles hve different LH concentrtion during the estrous cycle in different sesons. We lso used ovriectomized heifers to test whether such sesonl vrition in LH concentrtion occurs due to vrition in hypothlmic sensitivity to estrdiol. Mteril nd Method This study ws pproved y the ethics committee on niml experimenttion from the locl institution (7/31). We used 11 Nelore heifers t 3
8 months of ge, orn with in n intervl of 3 dys. Five heifers were ovriectomized for more thn two yers nd six were intct (not-ovriectomized). Animls were kept t the Veterinry Medicine College- UNESP Cmpus Arçtu -SP (South 1 1 3 West nd 5 5 58 ), with free ccess to wter nd minerl supplementtion. Heifers were kept in grzing conditions (Brchiri riznth) during the summer months. During the dry seson, heifers were fed on semi-feedlot system with mixture of sugr cne gsse supplemented with kg/dy/niml of concentrte (soyen, corn nd minerl nd vitmins). Heifers were weighed monthly. Climtic vriles were collected t the CATI (Integrted Technicl Assistnce Coordintion) in Arçtu, nd locl photoperiod vrition ws otined from the Apollo 115 we site. From the six intct heifers, two estrous cycles per seson per niml were nlyzed, heifers were exmined y trnsrectl ultrsonogrphy nd lood smples were collected. Before ech evluted cycle, estrus ws synchronized with two luteolytic gent (cloprostenol sodium - Ciosin or d-cloprostenol - Prolise ; 53 μg dose, I.M.) injections given t 1 dys intervl. After the second injection, heifers were evluted dily y ultrsound, to indentify ovultions induced y the luteolytic gent (D). Ultrsound exmintions were conducted there fter throughout the estrous cycle either until the following ovultion, defined s the disppernce of the dominnt or until D. Estrous cycle dt from heifers tht did not ovulte until the D were not included in the sttisticl nlysis. A GE ultrsound (Generl Electric ) Model Logic-1, with 5 MHz liner rry trnsducer ws used; ultrsound exmintions were performed y single opertor, lwys round pm.both ovries were evluted, ll ntrl follicles 3 mm were counted nd the dimeter mesured (verge of follicle height nd width). The follicles were then sorted into the following size ctegories nd counted etween 3 nd 5 mm pre-folliculr devition, etween 5 < nd 9 mm nd etween 9 < nd 1 mm. During the estrous cycle evlution, two lood smples were collected dily from ll intct heifers, the first smple efore performing the ultrsound exmintion (round m), nd the second t pm. Upon the detection of estrus signs (vginl mucus, het ehvior nd lrge dominnt follicle dimeter) lood smples were collected every three hours until ovultion, to chrcterize the LH surge. For P quntifiction, morning smples from every three dys in ech cycle, were used. Once during ech seson, five ovriectomized heifers were injected with 17β estrdiol (μg/kg) nd hd lood smples collected (once efore injection) every three hours (for 3 h) to evlute the LH concentrtion. Both intct nd ovriectomized heifers hd lood smples collected every four dys to ssess the circnnul plsm LH profile. For this nlysis ll LH peks were excluded from intct heifers nd ll estrdiol-induced LH peks were excluded from ovriectomized heifers. The hormone mesurements in plsm smples were performed y rdioimmunossy (RIA) t the Animl Endocrinology Lortory t Unesp Arçtu. For P the DPC kit ws used, with.88% of intr-ssy CV for.7 ng/ml nd 7.9% for.11 ng/ml plsm stndrds, sensitivity ws.191 ng/ml. The LH quntifiction ws performed y RIA s descried y Bolt et l. 3, intr-ssy CV ws 9.3% for 7. ng/ml nd 13.31% for.3 ng/ml plsm stndrds, the interssy CV ws 1.37% for the high nd 13.7% for low stndrds nd the sensitivity ws.8 ng / ml. Dily photoperiod percentile chnge (Δ%) nd climtic vriles (temperture, rinfll nd photoperiod) etween different sesons were compred using Kruskl-Wllis nd Dunn s post-hoc test, nd the numer of cycles with two nd three folliculr wves in ech seson evluted with the exct Fisher test.
9 Averge of the dominnt follicle dimeter from first nd second folliculr wve in cycles with two wves were compred using the pired t test, nd dominnt follicle dimeter etween three wves with repeted mesures nlysis of vrince (ANOVA) with Tukey s post-hoc test. Among the sesons, oth for intct nd ovriectomized heifers, the men weight nd weight percentile chnge (Δ%) were compred y repeted mesures ANOVA with Tukey post-hoc test. The sme test ws used for estrous cycles length nd the dt otined from folliculr ultrsound exmintions. Some of these vriles were trnsformed into logrithm (LOG1 (X +1)) or squre root (SQRT (X +1)), when the ssumption of normlity of residues ws not fulfilled. To identify LH peks nd to clculte LH secretion vriles (totl re, re under the pek, mplitude etween the first nd lst pek point nd highest concentrtion) from ovriectomized nd intct heifers smples, the progrm GrfPd 3: PRISM ws used. Vriles nlyzed in intct heifers were: LH secretion chrcteristics, the sl LH plsm concentrtion (without peks) during the cycles, circnnul plsm LH seline profile, P plsm concentrtions, the mximum P concentrtion in ech cycle nd LH concentrtion in four smples (D, D9, D1 nd D15) during ech cycle were compred etween the different sesons. Vriles nlyzed in ovriectomized heifers were: the LH secretion vriles, the LH concentrtion (with peks) fter estrdiol tretment nd circnnul seline of plsm LH profile were compred mong the different sesons. Originl dt were lso trnsformed when the ssumption of normlity of residues ws not fulfilled. For clrity, dt on tles nd figures were shown untrnsformed. The mximum P vlues otined from intct heifers, were correlted to the weight percentge chnge y Person correltion, nd dily photoperiod chnge (Δ%) y Spermn correltion. A 5% significnce level ws considered nd the dt were presented s men nd stndrd error of men. Results nd Discussion Locl tropicl climte ws well chrcterized y high tempertures throughout the yer, with higher summer rinfll nd dry seson in winter. There were differences in men percentge delt (Δ%) etween sesons in the dily photoperiod (Tle 1), showing difference of h nd 3 min fter the solstices lthough it is n inter-tropicl zone. Tle 1 - Environmentl vriles (men ± SEM) in different sesons during the experiment (South 1 1 3 West nd 5 5 58 ) - Arçtu-SP - 9 Vriles 7 8 Sesonl Autumn Winter Spring Summer effect Mximum Temperture (oc) 9.7±. c 3.±.5 c 33.3±. 31.81±.3 1 p<.1 Minimun Temperture (oc) 15.3±.5 1.19±.3 19.7±..±.1 p<.1 Averge Temperture (oc).9±..±..53±.3.3±. p<.1 Rinfll mm/dy.5±1.1.98±.5.7±.8 5.9±1.5 p<.1 Photoperiod (h:min) 11: 11: 1:55 1:55 - Dily Photoperiod vrition (%) -.1±..1±..1±. -.1±. p<.1 Vlues followed y different letters on the sme line, differ (p <.1) y Dunn s test 1 - Different letters differ verge numer (p <.5)
5 On totl, 37 estrous cycles were considered, 11 in the fll, 8 in winter, 9 in spring nd 9 in summer. Another 11 estrous cycles were excluded for exceeding dys: one in the fll, four in winter, three in spring nd three in the summer. During the experimentl period 5 cycles with two folliculr wves (7.%) nd 1 cycles with three wves (3.%) were oserved. In cttle, more thn 95% of estrous cycles hve two or three folliculr wves 1 nd there my e vritions in the sme niml 1,15, lthough the mjority of the heifers kept the sme numer of folliculr wves in 1 consecutive cycles. Nelore cows hve predominnce of cycles with two 1, or three folliculr wves 7, while heifers with three folliculr wves 1,. Considering tht the heifers from this experiment were 3 months old, the 7.% frequency of two wves cycles ws similr to previously descried for cows. In the winter there ws greter numer of three folliculr wves estrous cycles, different (p <.1) from the summer when two folliculr wve cycles previled (Tle ). However, the estrous cycles length verge did not differ mong sesons, lsting for 1±. dys throughout the experiment (Tle ), greeing with previous report 1. Normlly the durtion of the lutel phse of the estrous cycle determines the numer of folliculr wves nd the estrous cycle length 13,1,8. However, fctors tht influence the development of the dominnt follicle in the first folliculr wve my lso e responsile for wve pttern regultion 1. There re reports ssociting poor qulity diet nd het stress to the increse in the proportion of cycles Tle Effect of seson (men ± SEM) on the numer of follicles in different ctegories, the preovultory follicle dimeter, the estrous cycles length, numers of estrous cycles with two or three folliculr wves, verge ody weight nd ody weight percent chnges in 8 intct Nelore heifers nd ody weight nd percentile ody weight chnge in the months of estrdiol tretment in ovriectomized Nelore heifers (n=5) - Arçtu-SP, 9 Vrile 7 8 Sesonl Autumn (n=13) Winter (n=8) Spring (n=9) Summer (n=9) effect Totl follicles 7.8±.1 9.±.3 9.7±.3.8±. p<.1 3mm Follicles 5mm.±.1 7.8±.3 7.7±.3.9±. p=. 5mm < Follicles 9mm 1.±. 1.±.1 1.±.1 1.±.1 p=. 9mm < Follicles 1mm.±..±..±..±. p=.93 Pre-ovultory follicle dimeter (mm) 1 1.±.5 11.±. 1.1±.3 13.1±.5 p=. Numer of cycles with nd 3 folliculr wves 7 nd 3 nd 5 nd 3 9 nd p<.1 Cycles length (dys).8±..9±.5 1.1±.5.±.5 p=.85 Intct heifers ody weight (kg) 583.±8.8 9.3±13.7 55.±.9 9.9±1.8 p<.1 Intct heifers ody weight chnge (%).±.3 3.±.8.1±1.9 -.±1.7 p=.19 Ovriectomized heifers ody weight (kg, n=5) Ovriectomized heifers ody weight chnge (%, n=5) 579.±19.5 ±.1 73.±.8 83.±5. p=.1.8±..±.3.9±3.5 -.±.3 p=.8 Mens followed y different letters on the line re different (p <.5) y Tukey test 1 Mens followed y different letters on the line, re different (p =.) y Tukey test Mens followed y different letters on the line re different (p <.1) y Fisher s test
51 Progesterone (ng/ml) 7 5 3 1 (A) Cycle dy Autumn Winter Spring Summer Progesterone (ng/ml) 3 1 (B) Cycle verge 9 85 8 75 7 5 Photoperiod (min) 3 9 1 15 18 1 Cycle dy Progesterone (ng/ml) 7 5 3 1 (C) Higher pek 9 85 8 75 7 5 Photoperiod (min) Numer of follicles 1 8 (D) Follícles 9 85 8 75 7 5 Photoperiod (min) Autumn Winter Spring Summer Autumn Winter Spring Summer Figure 1 (A) Plsm progesterone concentrtion from smples collected every 3 dys; (B) verge progesterone concentrtion, (C) Mximum progesterone concentrtion nd (D) numer of follicles 3 mm from dily oservtions (verge±sem) during estrous cycles evluted t different sesons of the yer (Autumn-n=11; Winter-n=8; Spring-n=9; Summer-n=9) in 8 Nellore heifers, depicted relted to photoperiod vrition. Brs with different superscript letters re different (p<,1) ccording Tukey Arçtu-SP - 9 with three wves 1,. Despite the sesonlity vrition found in the numer of folliculr wves, some reports found no difference in these vriles Nelore 1,7 or in Brhmn. The estrous cycle length with two folliculr wves (.53±. dys) were shorter (p=.) compred to estrous cycles with three folliculr wves (1.1±.35 dys). In estrous cycles with two folliculr wves, there ws no difference (p=.3) etween the mximum dimeter of the dominnt follicle from the first wve (1.1 ±.3 mm) nd pre-ovultory follicle dimeter (1.5 ±,3 mm). But in cycles with three folliculr wves, the dominnt follicle dimeter from the second wve ws smller (9.7 ±. mm, p <.1) thn the dominnt follicle from first wve (11.3 ±. mm) nd the pre-ovultory follicles (1.1 ±. mm). This difference etween the dominnt follicle dimeter in cows presenting three folliculr wves is consistent with the literture for oth Bos turus 17 nd for Bos indicus 1 cttle nd cn e explined y the different P production from CL, tht reches mximum etween 5 nd 11 dys of the estrous cycle 9. The higher P levels decreses LH levels 19 nd decreses LH pulses frequency, leding to smller dominnt follicle dimeter, s consequence of shorter growth period of the dominnt follicle in the second folliculr wve 1. In contrst, low P concentrtions results in lrger dominnt follicle dimeter, nd longer dominnce phse, delying the emergence of the next folliculr wve 19,.
5 1 8 Autumn 1 8 Spring 1 8 Winter 1 8 Summer :h 1:h 18:h :h 3:h :h 1:h 18:h :h 3:h Figure LH plsmtic concentrtion from 5 ovriectomized Nellore heifers every 3 hours fter 17β-estrdiol (µg/kg) injection, in different sesons of the yer. = identified LH peks Arçtu-SP - 9 1. (A).75.5 Ovriectomized..3. (B) Intct.5.1. Autumn Winter Spring Summer. Autumn Winter Spring Summer Figure 3 - (A) Averge LH concentrtion in ech seson of the yer, lood smples were collected every dys, from 5 ovriectomized Nellore heifers (excluding 17β-estrdiol injection dy) (verge±sem). (B) Averge LH concentrtion, lood smples collected every dys (ovultory LH peks were excluded) from intct Nellore heifers during different sesons of the yer. Brs with different superscripts re different (p<,5) ccording Tukey Arçtu-SP - 9 During the utumn nd summer (i.e. decresing photoperiod) there ws higher plsm P concentrtion (verge over the entire cycle) in intct heifers, compred winter nd spring (i.e. incresing photoperiod) (Figure 1B). The mximum P concentrtion in ech cycle ws higher t sesons with decresing pho-
53 toperiod (p <.1, Figure 1C). Higher concentrtions of P in cows under decresing photoperiod were lso found y other uthors 3,. One possile explntion would e higher sensitivity of CL to LH in nimls exposed to decresing photoperiod, with greter P production, s previously oserved in cultured lutel cells from Bos turus indicus,7. Knowing tht the energy lnce influences P concentrtion 3, heifers were weighed monthly. Intct heifers verge weight during the experiment ws.5±8.9 kg, while the weight of ovriectomized heifers ws 7.±.7 kg. The verge weight of intct heifers in the utumn ws lower (p =.1) thn in the spring, ut ovriectomized heifers showed no weight difference (p =.1) etween sesons (Tle ). The verge weight percentile chnge of intct (p=.1) nd ovriectomized (p=.8) heifers showed no sesonl vrition (Tle ). There ws no correltion etween the mximum P vlues found in ech estrous cycle with the weight percentile chnge (Δ%) in the respective month of the cycle (p=., r= -.8). Also the periods in which supplementtion ws more intense (i.e. utumn nd winter) were not ssocited with lower P concentrtions periods (i.e. winter nd spring), suggesting tht P vrition ws not influenced y diet. Since the dily photoperiod percentge chnge (Δ%) were highly negtively correlted with mximum P vlue (p <.1, r =-.5), possile sesonl effect on lutel function of Zeu heifer cn e considered. It should e noted tht the heifers were mintined on psture nd feeding with sugr cne gsse nd supplemented with up to kg of concentrte per niml/dy. Considering the intct heifers verge weight of.5 kg, they received no more thn.3% live weight of high energy food/dy. During utumn nd summer there ws lower dily totl numer of follicles (p <.1) compred with winter nd spring (Tle nd Figure 1B). Among the ctegories, there were fewer follicles 3 mm 5 mm (p=.) in summer compred to winter nd spring (Tle ). Compring the estrous cycles, fewer follicles 3mm were oserved in sesons with higher P concentrtion (Figures 1C nd 1D). Different from the lower numer of mm follicles oserved in Holstein heifers treted with lower P thn physiologicl concentrtion, proly consequence of the extended dominnt folliculr phse, delying the FSH surge nd the next wve growth phse. Thus, in Nelore heifers there ws greter numer of follicles emerging on estrous cycles presenting lower concentrtion of P. Despite the difference etween sesons on P concentrtion, there ws no difference on LH secretion ptterns. Specificlly, oth the totl re (p =.3), the re under the pek (p =.), pek mplitude (intervl etween the first nd the lst point of the pek; p =.5), the LH mximum concentrtion (p =.3), seline plsm LH during the cycle (p =.35) nd circnnul plsm LH concentrtions (seson verge of smples collected every four dys; p =.3; Figure 3C) were similr mong sesons. From the five ovriectomized heifers stimulted with 17β estrdiol, three showed no LH pek in summer (Figure ), s result, the LH secretion fter estrdiol stimultion ws lower (p <.1) in summer (.8±. ng/ml) thn in utumn (1.±. ng/ml), winter (1.3±. ng/ml) or spring (1.3±.5 ng/ml). Also in ovriectomized heifers the circnnul LH plsm concentrtion (smples tken every four dys) ws lower in summer thn in utumn (p =., Figure 3A) similr to descried previously. However, like the intct nimls, in ovriectomized heifers showed no sesonl vrition in LH secretion pttern, considering totl secretion re (p =.), re under the pek (p =.5), pek mplitude (p =.9) nd mximum LH concentrtion (p =.18). Interestingly, in the summer, only three out of five ovriectomized heifers showed pek LH in response to estrdiol (Figure ). A decrese in LH plsm concentrtion in ovriectomized Brhmn cows in the summer ws oserved previously 8. Also in
5 ovriectomized Bos turus turus cows there were n incresed LH secretion in response to estrdiol 9,1 in the spring equinox nd during the winter 31. Throughout the estrous cycle, LH is secreted in pulses every 1- h when there is lower P concentrtion fter ovultion, decresing to one every 3- h, in the middle of estrous cycle when P is mximum. LH pulse frequency increses gin fter luteolysis nd stimultes estrdiol production, culminting in the preovultory pek of GnRH,9. As there ws vrition in P concentrtion etween sesons (Figure 1A), LH concentrtion ws quntified in estrous cycle smples from dys D, D9, D1 nd D15, iming to evlute the LH influence on P concentrtion nd of Pconcentrtion on LH. However, there ws no sesonl vrition in LH plsm concentrtions from these smples (p =.7), which reinforces the hypothesis tht etween sesons, there is some vrition in the lutel cells sensitivity to LH, s descried in Zeu y Rndel nd Rhodes, Rndel nd Long 7. Although it hs suppressive effect on the LH secretion,9, previous P exposure is required for the hypothlmic sensitiztion y estrdiol tht is needed to stimulte the GnRH/LH pek 1. This hppens ecuse P increses the concentrtion of estrdiol receptors in the mediosl hypothlmus, incresing the sensitivity to estrdiol tht will led to the relese of the GnRH (nd, consequently, the LH) pek 3,33. The reson for fewer follicles oserved, in sesons with higher P concentrtions during this experiment (Figure 1C nd 1D) my e consequence of estrdiol sensitivity vrition in the hypothlmus. The greter hypothlmic sensitivity to estrdiol during the lutel phse (with higher concentrtions of P) llowed estrdiol to suppress FSH secretion premturely, nticipting the moment of folliculr devition. It is importnt to evlute differences on estrdiol sensitivity s the hormonl mnipultion of the estrous cycle llows vlule increse in livestock production. Estrus synchroniztion protocols hve een widely pplied in different species, however nimls with sesonl estrous cyclicity do not respond dequtely to progestins nd estrogens tretment during the unfvorle reeding seson. This ws oserved in ovriectomized gots 3, demonstrting possile chnge in estrdiol/ progesterone receptor expression cross the sesons. In uffloes, the folliculr development decrese fter estrdiol vlerte nd norgestomet injection ws less intense outside the reeding seson 35. Bsl 17β-estrdiol nd LH concentrtions in wter-uffloes with regulr estrous cycles, did not vry throughout the yer, however, concentrtions were significntly higher close to estrus during the fvorle reeding seson 3. In sheep there ws incresed estrdiol receptors expression in the preoptic re during long dys compred to short dys 37. In the current experiment, ecuse of the sesonl vrition on LH concentrtion fter 17β-estrdiol injection (Figure 3) it is possile to suppose tht in Nelore heifers there ws lower expression hypothlmic estrdiol receptors (responsile for negtive feedck) during summer, the most fvorle seson for LH pek occurrence. During zeu domestiction, nimls were selected ginst sesonl reproductive chrcteristic, tody these suspecies shows reproductive cpcity throughout the yer 38 confirmed in this experiment, ut some fetures still hve heritge from the evolutionry process. Conclusion Nelore heifers showed sesonl chnges during the estrous cycle. Specific chnges were in the P concentrtion, the numer of follicles nd numer of folliculr wves. However, there ws no chnge in LH etween sesons, despite the vrition in LH relese in response to 17β-estrdiol injection in ovriectomized heifers throughout the sesons.we speculte tht there re sesonl effects on lutel cells sensitivity to LH nd lso on the hypothlmic response to estrdiol. Acknowledgements : FAPESP nd UNESP-Arçtu
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