Aging of the Reproductive System in the Male Hamster: Behavioral and Endocrine Patterns1

Transcription

1 BOLOGY OF REPRODUCTON 26, (1982) Aging of the Reproductive ystem in the Male Hamster: Behavioral and Endocrine Patterns1 LNDA J WANON,2 3 CLAUDE DEJARDN4 and FRED W TUREK3 Department of Neurobiology and Physiology3 Northwestern University Evanston, llinois 621 and The University of Texas at Austin4 nstitute of Reproductive Biology Department of Zoology Austin, Texas ABTRACT The present experiments were designed to characterize changes in reproductive function, both behavioral and hormonal, in the aging male golden hamster, and to determine whether or not pinealectomy or exercise would alter any reproductive parameters in aging hamsters n the first experiment males were decapitated at 4, 11, 24 and 31 months of age and the weights of the testes and sex accessory glands recorded Trunk blood was collected for determination of serum levels of testosterone, luteinizing hormone (LH) and follicle-stimulating hormone (FH) Although the weights of the single seminal vesicles and epididymis were significantly lower at 31 than at 24 months, they were not less than those of postpubertal males at 4 months Furthermore, these lower values for the 2 measures at 31 months were not accompanied by similar reductions in body weight, ventral prostate or paired testis weight No age dependent declines in serum LH, FH or testosterone were observed Pinealectomy at 14 months of age did not alter any variable measured at 24 or 31 months of age n the second experiment males were given mm tests for copulatory behavior with receptive females Males were tested monthly between 12 and 24 months of age, except for Months 13 and 23 The frequency of mounts and intromissions, the latency to intromit and ejaculate, and the length of the postejaculatory interval were recorded Body weight and testis width were recorded monthly and serum testosterone, LH and FH levels were determined via cardiac puncture at 12, 18 and 24 months Half of the animals had access to a running wheel, but no effect of the wheel on any reproductive variable was observed The percentage of males ejaculating remained above 8% for males at all ages There were no age dependent changes in the number of intromissions, latency to intromit or the length of the postejaculatory interval The latency to ejaculate increased gradually but significantly after the males were 2 months old There were no significant changes in testis width, serum testosterone, LH or FH levels with age The results clearly show that no serious decrements in reproductive function occur, either in behavior or in endocrine milieu, with age in the male golden hamster Although the latency to ejaculate increased gradually with age, even the oldest males exhibited the entire copulatory sequence of behaviors when paired with receptive females Accepted January 18,1982 Received eptember 14, 1981 work was supported by NCHD research grants HD-9885, HD-12622, by NA research grant AG-958, and by, a Research Career Development Award (HD-249) to F W Turek Portions of these findings were reported at the meetings of the Conference on Reproductive Behavior, Nashville, TN, and of the Animal Behavior ociety, Knoxville, TN, in June, Reprint requests: Dr Linda J wanson, Dept of Neurobiology and Physiology, Northwestern University, Evanston, L 621 NTRODUCTON Reproductive hormone profiles of aging males have only been examined in a few rodent species n the aging male rat, mouse and guinea pig there are gradual declines in fertility and testosterone secretion (Ghanadian et al, 1975; Rigaudiere Ct al, 1976; Bronson and Desjardins, 1977; Chan et a!, 1977; Gray, 1978a; Miller and Riegle, 1978; teger et al, 1979; Lupo-di Prisco and Dessi-Fulgheri, 198) erum levels of LH and FH also decline with 791 on 21 August 218

2 792 WANON ET AL age in the rat and mouse, although not to the extent that serum testosterone drops (Gray, 1978a; Bruni et al, 1977) However, no changes in testicular weight and serum LH or FH have been observed with age in the male white-footed mouse (teger et a!, 198) or in testicular weight and serum plasma testosterone in aged healthy laboratory mice (Nelson et al, 1975) Furthermore, in sexually active aged male laboratory mice, levels of serum testosterone, LH and FH, testicular weight and the number of mature sperm in the testes were all similar to those observed in young males (Bronson and Desjardins, 1977) n addition to possible species differences, changes in hormone secretion with age may depend upon the health and/or sexual experience of the male n general, declines in the pattern of sexual behavior are known to occur in the aging male rat, mouse and guinea pig (Grunt and Young, 1952; Larsson, 1958, Gray, 1978b; Huber et al, 198), although for some individuals these declines do not occur Deficits in sexual performance with age may, like the endocrine state of the male, depend upon the genetic makeup of the animal Bronson and Desjardins (1977) were able to divide their male laboratory mice into 2 groups - 1 sexually active and the other not Huber et al (198) observed 3 classes of male laboratory mice One class continued to copulate in a manner similar to young males while the other classes exhibited various degrees of deficit in sexual performance n those species in which decrements in sexual behavior have been observed, it appears that the ejaculatory reflex disappears first, followed by a decline in intromission frequency, and finally, by the disappearance of mounting behavior (Grunt and Young, 1952; Huber et al, 198) Golden hamsters (Mesocricetus auratus) are a commonly used experimental animal that live for about 2 years under laboratory conditions (Kirkman and Yau, 1972), and a great deal is known about their reproductive endocrinology and behavior n view of this, and the fact that the aging of the reproductive system has only been examined in a few species, we sought to determine the endocrine and behavioral events that are associated with aging in the male golden hamster n addition, because pinealectomy or exposure to a running wheel can prevent or delay short-day induced testicular regression in hamsters (Reiter, 1972; Elliott, 1974, 1976), we sought to determine if either of these factors would modify reproductive function in the aging hamster General MATERAL AND METHOD n the first study, retired breeders (1 months of age) were obtained from Lakeview Hamster Colony, Newfield, NJ [LAK:LVG(VR)l The animals were housed in groups of 5/cage in 14L: OD with food and water available ad libitum At 14 months of age half of the animals were pinealectomized and half were sham pinealectomized Males were weighed and sacrificed by decapitation at 11 (n 1), 24 (n = 1) and 31 (n = 12) months of age For comparative purposes, 4-month-old male hamsters maintained under similar conditions were also sacrificed by decapitation The paired testes, ventral prostate, a single seminal vesicle and epididymis were weighed at autopsy, and in the pinealectomized animals the skull was examined for remnants of pineal tissue Trunk blood was collected for determination of serum testosterone, LH and FH levels by radioimmunoassay Throughout the course of this and the following study, the testes of males that died of natural causes were weighed postmortem n a second study, 1-month-old retired breeders from Lakeview Hamster Colony were housed individually in 14L:1OD with food and water available ad libitum Males were given 4-5 screening tests with receptive females and only those males (n = 22) that showed a normal copulatory pattern, made up of a series of mounts with intromissions followed by ejaculation in several tests, were used in the study Only 4/26 (153%) of the males initially tested failed to show consistent copulatory behavior, and these males were excluded from the study At 12 months of age half the males were given access to a running wheel while the other half were housed with running wheels which did not rotate Blood samples were collected via heart puncture under light ether anesthesia at 12, 18 and 24 months for measurement of serum testosterone, LH and FH Testis width was determined as previously described (Turek and Losee, 1978) every other month between Months From 12 to 24 months of age (except for Months 13 and 23), males were tested for copulatory behavior with a receptive female at monthly intervals n order to induce receptivity, ovariectomized females were implanted with 1 mm ilastic capsules filled with estrogen, and were injected with progesterone 3-5 h prior to the time receptivity was desired (see Morin and Zucker, 1978) During a test the male was placed in a wire bottom arena (a mirror beneath the arena was used to help distinguish intromissions from ejaculations, Bunnell et al, 1976) After a 5 min adaptation period a receptive female was introduced into the arena The male s behavior was recorded for 15 min or until the first intromission following the first ejaculation The number of mounts, number of intromissions, latency to first intromission, latency to ejaculate and the length of the postejaculatory interval were recorded n addition, a sex score (modified from Morin and Zucker, 1978) was determined from each animal s behavior One point was given if the male mounted, 2 points if he intromitted and 3 points if on 21 August 218

3 AGNG N THE MALE HAMTER 793 ejaculation occurred A maximum score, where all behaviors occurred, was 6 points Radioimmunoassays The concentration of immunoreactive LH and FH in blood serum was determined according to Turek and Desjardins (1979) using reagents supplied by the Rat Pituitary Hormone Distribution Program, National nstitute of Arthritis, Metabolism, and Digestive Diseases, National nstitutes of Health Reagents included were rat-lh-1-7, rat-f-1-1-3, rat-lh-rp-1, rat-fh-rp-1, anti-rat-lh-4, and antirat-fh--7 mmunoreactive LH and FH values are expressed in terms of nanogram-equivalents of NHrat-LH-RP-1 or NH-rat-FH-RP-1, per ml of serum Final values represent weighted mean potency estimates given by linear regression analysis of the combined log dose-ogit response curves of the reference preparation and unknown samples of blood serum (Duddleson et al, 1972) The minimum amount of hormone detected was 5 ng of rat-lh-rp-1 and 5 ng of rat-fh-rp-1 per assay tube The concentration of testosterone was determined by a radioimmunoassay procedure validated for use with hamster blood serum ndividual samples of blood serum (1 z) were extracted in 1 vol of benzene:hexane (2:1) after adding 15 dpm of (1,2,6,7,16,1 7-3H(N)J testosterone (17 Ci/mmol) in gel-phosphate buffered saline (PB) and incubating samples at 37#{176}Cfor 1 h The organic extract was washed with 2 ml distilled water, evaporated under N2 at 45#{176}C,and reconstituted in gel-pb Duplicate aliquots of the reconstituted samples were used for testosterone determination (Rao et al, 1978), and a third aliquot was dispensed into a scintillation vial to monitor the amount of internal standard 11,2,6,7,16, 17-3H(N)J testosterone recovered tandard curves were run in triplicate for each assay, with 1 concentrations (range from 1 to 5 pg) of chromatographically pure radioinert testosterone The results were analyzed with a computer program that uses a logit-log transformation to obtain a linear inhibition curve (Duddleson et al, 1972) Repeated assessment of intra- and interassay coefficients of variation made on a pool of serum collected from adult male hamsters was 32% and 49%, respectively The antisera used to measure testosterone exhibited less than a 7% cross-reactivity with dihydrotestosterone (Rao et al, 1978) Moreover, the amount of the testosterone present in 1 l aliquots of a large pool of blood serum was unchanged before and after chromatography on ephadex LH-2 This finding indicates that the present determination of testosterone is not compromised by either precursors of testosterone biosynthesis or testosterone metabolites typically present in blood of adult male hamsters tatistics A one-way analysis of variance was used to determine differences with age for the following measures from the first experiment: body weight, single seminal vesicle weight, ventral prostate weight, epididymis weight, paired testis weight, serum testosterone, LH and FH A Dunnett s test was used to compare the means of specific groups A Kruskal-Wallis one-way analysis of variance was used to determine changes over time in the behavioral and hormonal variables determined in the second study A tudent s t test was then used to examine differences between the means of individual groups REULT n the first study there were no significant differences observed between pinealectomized and sham-pinealectomized animals for any measure and thus the data were combined for analysis There was no evidence of pineal tissue remnants in any of the pinealectomized males Eleven-month-old animals were significantly heavier than 4-month-old males and after 11 months no further weight changes were observed (Fig 1) The fact that the males did not lose weight with age, as well as their general appearance, indicated that they were in good health throughout the experiment Although the males were still growing at 4 months, the testes had reached full adult weight ( 3 g) by that time While mean testis weight was slightly reduced in the oldest males, the difference was not significant (Fig 1) erum testosterone remained between 22 and 29 ng/ml in males of all ages and ventral prostate weights remained stable (14-22 mg) as well (Fig 1) Despite the fact that paired testis weight and serum testosterone did not differ among animals of different ages, both single seminal vesicle and epididymis weights (Fig 1) were significantly elevated in 11-month-old males, compared to 4-month-old animals, and significantly reduced in the 31-month-old animals compared to 24-month-old males erum LH levels were not significantly different at any age, while serum FH increased slightly, but significantly (Fig 1, P<1) between 4 and 11 months, and remained stable thereafter n the second study there was no difference in behavior or hormone levels in those animals housed with freely moving wheels and those housed with nonrotating wheels, and thus the data from the 2 groups were combined for analysis Both body weight and mean testis width did not change between 12 and 24 months of age (Fig 2) Animals in this experiment were lighter in weight at all ages than were those from the first experiment This may be related to the fact that these males were housed individually and were sexually active throughout the experiment Levels of serum testosterone, LH and FH did not change between 12 and 24 months of age (Fig 3) erum levels of testosterone in on 21 August 218

4 794 WANON ET AL these males were higher (4-6 ng/ml) than those the former males may have stimulated their observed in the males in the first study Again, higher testosterone levels the fact that sexual activity was maintained in Normal copulatory behavior was observed BODY WT(g) NGLE EMNAL VECLE WT(mg) #{149} VENTRAL PROTATE WT(mg) EPDDYM WT(mg) OO PARED TET WT(mg) ERUM TETOTERONE (ng/m) 3, T 2, 1- ERUMFHj ERUMLH 4 (ng/m) (nglml) T ii AGE in MONTH FG 1 Mean ± EM body weight and single seminal vesicle, ventral prostate, epididymis and paired testis weights, as well as serum testosterone, LH and FH in groups of males autopsied at 4, 11, 24 and 31 months of age The number of males/group is shown within each bar Q 2 on 21 August 218

5 AGNG N THE MALE HAMTER 795 AGE in MONTH FG 2 Mean ± EM testis width and body weight in males from 12 to 24 months old The number of males! group is shown within each bar in males of all ages Mean sex scores remained above 5 at all time points, and even the oldest males showed the full ejaculatory pattern at 22 and 24 months (Fig 4) The percentage of males ejaculating remained above 8% in males of all ages ome changes were noted in the different components of the copulatory sequence, however The latency to the first intromission (Fig 5) decreased between 12 and 15 months (P<1), but remained the same thereafter No significant change in the number of intromissions until the first ejaculation was noted, although there was a slight increase at 2 and 24 months of age There was a gradual, but significant (P<1) increase in the latency to ejaculate (Fig 5) between 2 and 24 months of age The latency to ejaculate increased from a mean of 2 55 for all males less than 21 months old to 7 17 for the aged males at 24 months The length of the postejaculatory interval did not increase with age, however (Fig 5) Figure 6 plots the weight of the testes vs age for all of the animals in both studies at the time of death The animals have been divided into 2 classes, those that died of natural causes and those that were sacrificed (the animals at the 14-month time point died during surgery) No animal that was sacrificed between 4 and 14 months of age had testes weighing less than 2 mg, and only 4/27 males sacrificed at 24 and 31 months had testes which weighed that little However, 22/3 1 males dying from natural causes between 16 and 3 months of age had testes weighing less than 2 mg The fertility of the males used in these studies is not known DCUON The results of the present study showed that there was little change in the reproductive hormone profile in the aging male golden hamster erum levels of testosterone, LH and FH remained, for the most part, unchanged Although seminal vesicle and epididymis weights were reduced in the 31-month-old males, they never weighed less than those of the youngest, postpubertal males, and paired testis and ventral prostate weights did not differ among the different age groups There were relatively few changes in the reproductive behavior patterns which comprised the copulatory sequence in the aging male hamster The full sequence was observed, even in the oldest males at 24 months of age The number of intromissions remained between 1 and 21 per ejaculatory series throughout the 12 months of testing, a frequency comparable to that previously observed in male hamsters that were 8 months old (Bunnell et al, 1976) Furthermore, the latency to intromit also remained stable (between 3 and 2 OO, after an initial decline) as the males increased in age and this latency was, again, comparable to the mean intromission latency (for several ejaculatory series) observed in 8-month-old hamsters by Bunnell et al (1976) Although there were no changes over time in sex score or in the percentage of males ejaculating, the latency to ejacu- on 21 August 218

6 - 796 WANON ET AL late increased gradually between 2 and 24 val than did younger animals (Larrson, 1958) months of age Between 12 and 21 months the The results for the laboratory mouse are somelatency to ejaculate (2 OO to 4 OO ) was corn- what contradictory, but appear to be, at least parable to the latencies observed in the 8- for some animals, similar to those observed here month-old hamster (2 3 to 4 OO ) (Bunnell for the hamster n some male mice the number et al, 1976) Thus, although the older males of intromissions declined with age However, in required a greater amount of time to show this others the number remained constant (Bronson pattern, ejaculation still occurred We observed and Desjardins, 1977; Huber et al, 198) no age dependent decrement in the length of Furthermore, Huber et al (198) observed no the postejaculatory interval, however, deficits in sexual behavior in 4% of the 25- These results differ from those in the rat month-old male mice they tested There may where older males showed fewer intromissions/ be, then, basic species differences in the degree ejaculation, fewer ejaculations, a greater latency to which decrements in sexual performance to ejaculate and a longer postejaculatory inter- occur, with the rat showing definite declines in reproductive behavior with age, while the mouse and hamster show fewer deficits When the weights of the testes and sex accessory organs, as well as serum levels of E testosterone, LH and FH were examined, very few differences were observed with age in the 528O sexually active males (Figs 2 and 3) or among the different age groups in the sexually inactive i () L E 14 (1) a males (Fig 1) The lack of a difference in gonadal steroids in the aging sexually active animals probably accounted for the general lack of change in sexual behavior in these males Paired testis and ventral prostate weights were not significantly different among the different age groups (Fig 1) and single seminal vesicle and epididymis weights were not different between the 11- and 24-month-old animals (Fig 1) The reduced weights for these organs at 31 months may be related to the slight, but nonsignificant, reduction in testis weight in those males These results are in sharp contrast to those observed in the rat and mouse, where testicular a) c PERCTMALE E EX CORE AGE in MONTH FG 3 Mean ± EM serum levels of FH, LH and testosterone in males at 12, 18 and 24 months of age FG 4 The percentage of males ejaculating and The number of males/group is shown within each bar the mean ± EM sex score of males tested monthly of the bottom panel, between 12 and 24 months of age 24 on 21 August 218

7 AGNG N THE MALE HAMTER 797 and sex accessory gland weights, as well as serum testosterone, LH and FH have been reported to decline with age (Bronson and Desjardins, 1977, Bruni et al, 1977; Chan et al, 1977; Gray, 1978a; Pirke et al, 1979; Meites et al, 198; teger et al, 198; Kaler and Neaves, 1981) The decrements in LH and FH secretion are thought to initiate the drop in serum levels of testosterone, which may lead to the observed declines in sexual performance in the aged male rodent However, it has been reported that in sexually active, healthy male mice, and in the male white-footed mouse, neither testicular weight nor plasma or serum levels of testosterone, LH and FH differed between young and old animals (Nelson et al, 1975; Bronson and Desjardins, 1977; teger et al, 198) n agreement with our results, Reiter et al (198b) have recently reported that 2- and 18-month-old male hamsters showed similar plasma levels of LH, FH, prolactin and 3 NUMBER ci NTROHON testosterone at each of 4 time points throughout a 24-hour day Thus, the aging male hamster appears to be comparable in endocrine profile to the aging sexually active male laboratory mouse or male white-footed mouse t has been reported that there are major changes in the pineal melatonin rhythm with age in the golden hamster n younger males (2 months old) there is a rise in pineal melatonin during the dark phase of the light-dark cycle (Reiter et al, 198a) n older (18-month) animals, however, this rise is attenuated, with the peak occurring earlier and at about 1/5 the values observed in the younger males Reiter et al (198b) have further reported that this apparent suppression of pineal function with age develops sometime between 14 and 18 months of age in the male hamster Melatonin is 1 of the putative pineal hormones thought to be involved in modifying reproductive function in the golden hamster However, the attenuation of the nighttime rise in pineal melatonin in aged male hamsters was not accompanied by any change in plasma gonadotrop ins or testosterone, or in the histology of the gonads (Reiter et al, 198b) Furthermore, in the present study, pinealectomy had no effect on serum levels of LH, FH or testosterone in aging males Thus, whether or not an alteration in PARED TET WT (nv) 3OC 2 1Oc 4, o acrificed #{149} Died of Natural Causes o 8 C &oo 8 Post Ejaculatory Ntera (sect 3,C o #{149} 8 7rx o #{149} &O OO LAT AGE in MONTH 2, #{149} #{149} 1, #{149} AGE, MONTH FG 5 Mean ± EM number of intromissions, latency to the first intromission, postejaculatory interval and the latency to ejaculate for males tested monthly between 12 and 24 months of age At points where no standard error bars are shown, the standard error is within the circle #{149}#{149} #{149} #{149} AGE in MONTH FG 6 Paired testis weight for individual males that were killed at autopsy (open circles) and those that died of natural causes prior to the scheduled time of sacrifice (closed circles) on 21 August 218

8 798 WANON ET AL pineal gland function with age modifies the hypothalamic-pituitary-gonadal axis in any way, remains to be determined From the few studies that have been carried out in rodents, it appears that there is a great deal of variability with respect to whether or not reproductive behavior and/or hormones dedine with age The question can be raised: What accounts for these inconsistencies? At least some of the inconsistencies may be due to species and/or strain differences, as well as the variable experimental conditions (eg, sexually active vs sexually inactive) under which the aging studies were performed n addition, it is possible that the declines in reproductive funclion observed in some studies have little to do with an intrinsic aging of the reproductive system, but instead are the consequences of general debilitation with age This possibility is raised by the observation of the dramatic differences in testis weight between males that died of unknown, natural causes and those that were killed at specific autopsy times n the aged animals that were sacrificed (24 and 31 months old), 85% had testes weighing more than 2 mg n contrast, only 29% of the animals that died of natural causes between the ages of 16 and 3 months, had testes that weighed more than 2 mg; indeed, many of these animals had fully regressed testes These data suggest that the decrease in testis weight (and other physiological, hormonal and behavioral events that are associated with reproduction) which has been observed in some previous studies, might be due to the inclusion of animals near death in which reproductive function has declined due to general debilitation Thus, the present study indicates that there is very little change in reproductive behavior and hormones in the aging male hamster The data on paired testis weights in animals dying of natural causes caution against assuming that changes in reproductive function which might be observed with age are intrinsic to the reproductive system ACKNOWLEDGMENT The authors thank andra J Pearson and Donald W Carroll for their assistance with steroid and peptide hormone radioimmunoassays, and usan Losee-Olsen for technical assistance Antiserum for measuring testosterone was kindly provided by Dr P N Rao, Department of Organic and Biological Chemistry, outhwest Foundation for Research and Education, an Antonio, Texas We also thank Dr A Parlow and the Rat Pituitary Hormone Distribution Program of the National nstitute of Arthritis, Metabolism, and Digestive Diseases for providing the reagents used to measure LH and FH REFERENCE Bronson, F H and Desjardins, C (1977) Reproductive failure in aged CBF1 male mice: nterrelationships between pituitary gonadotropic hormones, testicular function, and mating success Endocrinology 11: Bruni, J F, Huang, H H, Marshall, and Meites, J (1977) Effects of single and multiple injections of synthetic GnRH on serum LH, FH and testosterone in young and old male rats Biol Reprod 17: Bunnell, B N, Boland, B D and Dewsbury, D A (1976) Copulatory behavior of golden hamsters (Mesocricetus auratus) Behaviour 61:18-26 Chan, WC, Leathem, J H and Esashi, T (1977) Testicular metabolism and serum testosterone in aging male rats Endocrinology 11: Duddleson, W G, Midgley, A R Jr and Niswender, G D (1972) Computer program sequence for analysis and summary of radioimmunoassay data Comput Biomed Res 5: Elliott, J A (1974) Photoperiodic regulation of testis function in the golden hamster: Relation to the circadian system PhD Dissertation, U of Texas at Austin, pp 248 Elliott, J A (1976) Circadian rhythms and photoperiodic time measurement in mammals Fed Proc 35: Ghanadian, R, Lewis, J G and Chisholm, G D (1975) erum testosterone and dihydrotestosterone changes with age in the rat teroids 25: Gray, G D (1978a) Changes in the levels of luteinizing hormone and testosterone in the circulation of aging male rats J Endocrinol 76: Gray, G D (1978b) Age related changes in penile erections and circulating testosterone in middle aged male rats Adv Exp Med Biol 113: Grunt, J A and Young, W C (1952) Changes in strength of sex drive in the male guinea pig from age 22 to 439 days Anat Rec 113: Huber, MHR, Bronson, F H and Desjardins, C (198) exual activity of aged male mice: Correlation with level of arousal, physical endurance, pathological status, and ejaculatory capacity Biol Reprod 23: Kaler, L W and Neaves, W B (1981) The androgen status of aging male rats Endocrinology 18: Kirkman, H and Yau, PK (1972) Longevity of male and female, intact and gonadectomized, untreated and hormone-treated, neoplastic and nonneoplastic yrian hamsters Am J Anat 135: Larsson, K (1958) exual activity in senile male rats J Gerontol 13: Lupo-di Prisco, C and Dessi-Fulgheri, F (198) Endocrine and behavioral modifications in aging male rats Horm Res 12: Meites, J, teger, R W and Huang, HHH (198) Relation of neuroendocrine system to the repro- on 21 August 218

9 AGNG N THE MALE HAMTER 799 ductive decline in aging rats and human subjects Fed Proc 39: Miller, A E and Riegel, G D (1978) erum LH levels following multiple LHRH injections in aging rats Proc oc Exp Biol Med 157: Morn, L P and Zuker, (1978) Photoperiodic regulation of copulatory behavior in the male hamster J Endocrinol 77: Nelson, J F, Latham, K R and Finch, C E (1975) Plasma testosterone levels in C57BL/6J male mice: Effects of age and disease Acts Endocrinol 8: Pirke, K M, Bofilias,, intermann, R, Langhammer, H, Wolf, and Pabst, H W (1979) Relative capillary blood flow and Leydig cell function in old rats Endocrinology 15: Rao, P N, Moore, Jr, P H, Peterson, D M and Tcholakian, R K (1978) ynthesis of new steroid haptens for radioimmunoassay - Part V 1 9-O-carboxymethyl ether derivative of testosterone A highly specific antiserum for immunoassay of testosterone from both male and female plasma without chromatography J teroid Biochem 9: Reiter, R J (1972) Evidence for refractoriness of the pituitary-gonadal axis to the pineal gland in golden hamsters and its possible implications in annual reproductive rhythms Anat Rec 173: Reiter, R J, Richardson, B A, Johnson, L V, Ferguson, B N and Dinh, D T (198a) Pineal melatonin rhythm: Reduction in aging yrian hamsters cience 21: Reiter, R J, Johnson, L Y, teger, R W, Richardson, B A and Petterborg, L J (198b) Pineal biosynthetic activity and neuroendocrine physiology in the aging hamster and gerbil Peptides 1:(uppl 1)69-77 Rigaudiere, N, Pelardy, G, Robert, A and Delost, P (1976) Changes in the concentrations of testosterone and androstenedione in the plasma and testis of the guinea pig from birth to death J Reprod Fertil 48:291-3 teger, R W, Huang, H H, Hodson, C A, Leung, F C, Meites, J and acher, G A (198) Effects of advancing age on hypothalamic-hypophysealtesticular functions in the male white-footed mouse (Peromyscus leucopus) Biol Reprod 22: teger, R W, Peluso, J J, Bruni, J F Hafez, EE and Meites, J (1979) Gonadotropin binding and testicular function in old rats Endokrinologie 73:1-5 Turek, F W and Desjardins, C (1979) Development of Leydig cell tumors and onset of changes in the reproductive and endocrine systems of aging F344 rats J Nat Cancer nst 63: Turek, F W and Losee, H (1978) Melatonininduced testicular growth in golden hamsters maintained on short days Biol Reprod 18: on 21 August 218