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Hormonal Interrelationships Between Hypothalamus, Pituitary and Testis of Rams and Bulls University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln U.S. Department of Agriculture: Agricultural Publications from USDA-ARS / UNL Faculty Research Service, Lincoln, Nebraska 1982 Hormonal Interrelationships Between Hypothalamus, Pituitary and Testis of Rams and Bulls Bruce D. Schanbacher USDA-ARS Follow this and additional works at: https://digitalcommons.unl.edu/usdaarsfacpub Part of the Agricultural Science Commons Schanbacher, Bruce D., "Hormonal Interrelationships Between Hypothalamus, Pituitary and Testis of Rams and Bulls" (1982). Publications from USDA-ARS / UNL Faculty. 763. https://digitalcommons.unl.edu/usdaarsfacpub/763 This Article is brought to you for free and open access by the U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Publications from USDA-ARS / UNL Faculty by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Journal of Animal Science 55 (1982), pp. 56-67. Hormonal Interrelationships Between Hypothalamus, Pituitary and Testis of Rams and Bulls Bruce D. Schanbacher1 US Department of Agriculture2, Clay Center, NE 68933 Summary Introduction This mini-review aims to summarize some of Despite the need for efficient reproduction our recent findings relating to testicular func• by males and the important contribution males tion and feedback control of the hypothalamic- make to propagation of the species, limited pituitary axis by testicular steroids in rams and information has been accumulated on the basic bulls. Testosterone secretion in intact males is physiological mechanisms that regulate repro• not tonic, but is characterized by episodic ductive processes in males. This is true for pulses. This pattern of secretion is dictated by domestic species, particularly rams and bulls. inputs of the central nervous system via secre• Livestock production contributes substantially tions of the hypothalamus (luteinizing hor• to the economic stability of agriculture in this mone-releasing hormone; LHRH) and anterior country; therefore, a better understanding of pituitary (luteinizing hormone; LH). A tem• those factors contributing to the reproductive poral relationship exists between concentra• success or failure in these species is warranted. tions of LH and testosterone in serum and A number of reproductive characteristics are evidence is presented that strongly suggests that quantifiable in males, however, it seems that their episodic secretion is dependent on discrete the testes should be of paramount importance. episodes of LHRH discharge from the hypo• Two major functions of the testis include thalamus. Based on data from experiments with androgen biosynthesis by the interstitial cells rams and bulls, I suggest that acutely castrated and sperm production within the seminiferous males (but not chronic castrates) remain sus• tubules. Although these two functions are ceptible to the negative feedback effects of segregated anatomically, intercellular communi• testosterone, i.e., LH concentrations remain cation exists between them. Recent reviews suppressed in serum of animals given testos• have been prepared on quantitiative aspects of terone replacement therapy immediately fol• spermatogenesis (Amann, 1981) and regulation lowing castration. Estradiol-17j3, on the other of the hypothalamic-pituitary seminiferous hand, abolishes pulsatile LH release and sup• tubule axis by inhibin (Setchell et al, 1977). presses mean LH concentrations in both acute Therefore, this paper will review our current and chronic castrates. Therefore, testosterone understanding of testosterone secretion and the feedback on LH secretion may, in part, involve hypothalamic-pituitary-Leydig cell axis in rams extragonadal conversion to estradiol-17j3 to and bulls. block pulsatile LHRH release. The potent inhibitory effects of estradiol on LH secretion provide an experimental probe for future in• Hypothalamic-Pituitary-Gonadal vestigations relating to mechanisms controlling Axis male reproduction. The anterior pituitary through its secretion of the gonadotropins, luteinizing hormone (LH) (Key Words; Hypothalamus, Pituitary, Testicu• and follicle stimulating hormone (FSH) partic• lar Steroids, Negative Feedback.) ipate in regulation of testicular function. The anterior pituitary, itself, is regulated by inputs from the central nervous system, which are 'The author acknowledges the cooperation of the coordinated via the hypothalamic secretion of Nebraska Agr. Exp. Sta., Univ. of Nebraska, Lincoln, luteinizing hormone releasing hormone (LHRH; and the assistance of Michael J. D'Occhio, James E. Kinder and Cindy Rainbolt in obtaining some of the also referred to as GnRH). The interrelation• original data cited. Appreciation is extended to Kathy ships of hypothalamus, pituitary and testis are Leising for typing the manuscript. schematically presented in figure 1. 2 Roman L. Hruska U.S. Meat Animal Research Center, Agr. Res. Ser. Testicular steroidogenesis depends on con- 56 ENDOCRINOLOGY AND REGULATION OF TESTICULAR FUNCTION 57 tinuous support from the pituitary in the form of LH. This gonadotropin binds to specific receptors on the plasma membrane of Leydig cells to activate adenyl cyclase. This enzyme catalyzes the conversion of ATP to cAMP, which serves as a second messenger to activate a cytoplasmic protein kinease that is responsi• ble for the conversion of cholesterol to sex steroids, primarily testosterone (Neaves, 1975). The remainder of this report will deal with the interrelationships between inputs of the hypo• thalamus (i.e., LHRH), LH secretion and feed• back control of the hypothalamic-pituitary axis by testicular steroids. Hypothalamic Control of Testosterone Secretion Testosterone secretion in intact males is not tonic, but is characterized by episodic pulses. This pattern of secretion varies from one ani• mal to the next and is probably a function of age, reproductive status, health, external en• vironment and other unidentified factors (Schanbacher and Ford, 1977; Lincoln, 1978; Lacroix and Pelletier, 1979a,b; Schanbacher, 1980a). Diurnal rhythms in serum testosterone are not evident in rams or bulls (Falvo et al., Figure 1. Hypothalamic-pituitary control of tes• ticular function. Gonadotropin releasing hormone 1975). (GnRH) from the hypothalamus mediates pituitary Several investigators have reported a tem• release of LH and FSH, which in turn binds to specific poral relationship between secretory episodes membrane receptors on Leydig cells and Sertoli cells, of LH and testosterone with a lag period of respectively. Testicular secretions (testosterone and estradiol from Leydig cells and inhibin from Sertoli approximately 60 min between peak concen• cells) act on the brain and anterior pituitary, where trations (Katongole et al., 1971, 1974;Sanford they exert control of gonadotropin release via negative et al., 1974; Schanbacher and Ford, 1976; feedback mechanisms. Testosterone also facilitates Lincoln et al., 1977; Schams et al., 1978; spermatogenesis by binding to cytoplasmic receptors D'Occhio et al., 1982b). The ability of iv injec• in the Sertoli cells of the seminiferous tubules. tions of either LH or LHRH to induce normal surges of testosterone in serum (Thibier, 1976; Schanbacher and Echternkamp, 1978; Schan• (Fink and Jamieson, 1976) varies in a pulsatile bacher, 1979a, 1980b) has provided supplemen• manner. tary evidence that pulsatile testosterone secre• Lincoln and Frazer (1979) studied the im• tion is the result of a pulsatile pattern of LHRH portance of neural inputs from the hypothala• secretion from the hypothalamus. Further evi• mus indirectly by neutralizing LHRH secretion dence for this conclusion is illustrated in figure with LHRH antibodies. Passive immunization 2, wherein striking similarity exists for the of intact Soay rams with LHRH antiserum re• naturally occurring LH-testosterone episode ob• sulted in an immediate blockage of the episo• served in an intact bull (figure 2A) and the LH- dic fluctuations of both LH and testosterone. testosterone episodes that have been induced Similar observations have been made for ram by an injection of synthetic LHRH (figure 2B). lambs (Schanbacher, 1982) and for bull calves Based on these data I suggest that an intermit• (B. D. Schanbacher, unpublished data) follow• tent pattern of LH secretion is dictated by a ing active immunization against LHRH. Failure similar pattern of LHRH secretion by the hypo• of these lambs and calves to respond to exogen• thalamus. Indeed, LHRH concentrations in ous LHRH showed that LHRH immunoneutral- blood taken from pituitary portal vessels of ization provides a physiological block to the rhesus monkeys (Carmel et al., 1976) and rats hypothalamic-pituitary-Leydig cell axis. Data 58 SCHANBACHER m c E 2 s m c CO —I * 10 o ' B 10 ^ m 33 • _ m A <* W 6 • • ' * * ' i 0600 1000 1400 1800 TIME OF DAY (h) Figure 2. Luteinizing hormone (o) and testosterone (•) concentrations in serum of a mature bull during a 14-h intensive bleeding period (A) and another mature bull injected iv every 4 h with 1,000 ng of LHRH (B). (From Schanbacher and D'Occhio, 1982). that illustrate the biological consequences of concentrations of LH and testosterone and pro• LHRH immunization in lambs are presented in vide evidence that their episodic secretion is table 1. Note the marked reduction in testicu• dependent on secretory episodes of LHRH lar weight, the low to nondetectable concentra• from
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