Endocrine mechanisms of puberty in heifers: estradiol negative feedback regulation of luteinizing hormone secretion

The hypothesis that luteinizing hormone (LH) secretion in prepubertal females is responsive to estradiol negative feedback and that decreased feedback occurs as puberty approaches was tested in heifers. In the first experiment, seven heifers were maintained prepubertal by dietary energy restriction until 508 days of age (Day 0). All heifers were placed on a high-energy diet on Day 0 at which time they received no additional treatment (CONT), were ovariectomized (OVX) or were ovariectomized and subcutaneously implanted with estradiol-17 beta (OVX-E2). This feeding regimen was used to synchronize reproductive state in all heifers. A second experiment was performed with 16 prepubertal heifers using the same treatments at 266 days (Day 0) of age (CONT, OVX and OVX-E2) but no dietary intake manipulation. In both experiments, LH secretion increased rapidly following ovariectomy in OVX heifers. In the initial experiment, LH secretion was maintained at a low level in OVX-E2 heifers until a synchronous rapid increase was noted coincidental with puberty in the CONT heifer. In the second experiment, LH secretion increased gradually in OVX-E2 heifers and attained castrate levels coincidental with puberty in CONT heifers. A gradual increase in LH secretion occurred as puberty approached in CONT heifers. These results indicate that: a) LH secretion in prepubertal heifers is responsive to estradiol negative feedback; and b) estradiol negative feedback decreases during the prepubertal period in beef heifers.     

Reduced growth hormone secretion prolongs puberty but does not delay the developmental increase in luteinizing hormone in the absence of gonadal negative feedback

Previous studies have shown that the growth hormone (GH) axis is important for timing the later stages of puberty in female monkeys. However, it is not clear whether these growth-related signals are important for the initiation of puberty and early pubertal events. The present study, using female rhesus monkeys, used two approaches to answer this question. Experiment 1 tested the hypothesis that reduced GH secretion would blunt the rise in nocturnal LH secretion in young (17 mo; n = 7) but not older adolescent ovariectomized females (29 mo; n = 6). Reduced GH secretion was induced by treating females with the sustained release somatostatin analogue formulation, Sandostatin LAR (625 microg/kg). Morning (0900-0930 h) and evening (2200-2230 h) concentrations of bioactive LH were higher in older adolescent compared to young adolescent females. However, diurnal concentrations were not affected by the inhibition of GH secretion in either age group when compared to the placebo-treated, control condition. Experiment 2 tested the hypothesis that reduced GH secretion induced in young juvenile females would delay the initial increase in nocturnal LH secretion and subsequent early signs of puberty. In order to examine this hypothesis, puberty in control females (n = 7) was compared to those in which puberty had been experimentally arrested until a late adolescent age (29 mo) by the use of a depot GnRH analogue, Lupron (750 microg kg(-1) mo(-1); n = 7). Once the analogue treatment was discontinued, the progression of puberty was compared to a group treated in a similar fashion but made GH deficient by continuous treatment with Sandostatin LAR (n = 6). Puberty occurred as expected in control females with the initial rise in evening LH at 21 mo, menarche at 22 mo, and first ovulation at 30 mo. As expected, Lupron arrested reproductive maturation, but elevations in morning and evening LH and menarche occurred within 2 mo of the cessation of Lupron in both Lupron and Lupron-GH-suppressed females. In contrast, first ovulation was delayed significantly in the Lupron-GH-suppressed females (41 mo) compared to the Lupron-only females (36 mo). These data indicate that within this experimental model, reduced GH secretion does not perturb the early stages of puberty but supports previous observations that the GH axis is important for timing the later stages of puberty and attainment of fertility. Taken together, the data indicate that factors that reduce GH secretion may have a deleterious effect on the completion of puberty.     

Exogenous estradiol reduces inhibition of luteinizing hormone by estradiol in prepubertal heifers

The hypothesis that high levels of exogenous estradiol administered to heifers during the prepubertal period would decrease subsequent negative feedback of estradiol on luteinizing hormone (LH) secretion was tested. Fourteen prepubertal heifers were ovariectomized on Day 0. Ovariectomized heifers received either no further treatment (OVX, n = 4), a single estradiol implant on Day 0 (OVXE, n = 5), or the single implant on Day 0 and two additional implants between Days 16 and 30 (OVXE+ E, n = 5). Ten ovary-intact heifers received either no treatment (INT, n = 5) or were administered the two estradiol implants between Days 16 and 30 (INT+ 5, n = 5). Comparison of LH secretion in OVXE to OVXE+E, and in INT to INT+E resulted in significant time-by-treatment interactions (p less than 0.05 for both). As pubertal age approached, mean concentration of LH (p less than 0.05) and pulse frequency (p less than 0.05) increased more rapidly in OVXE+E and INT+E than in OVXE and INT, respectively. Amplitude of LH pulses was unaffected by treatment. When data were standardized to day of puberty in INT and INT+E heifers, mean LH concentration and LH pulse frequency increased as puberty approached in both groups. These data confirm earlier reports indicating that secretion of LH increases gradually as puberty approaches in heifers. It was concluded that administration of estradiol during the prepubertal period hastened the decline in the subsequent negative feedback of estradiol. Precocious puberty was not induced in ovary-intact females.     

Roles of estradiol and gonadotropin-releasing hormone in controlling negative and positive feedback associated with the luteinizing hormone surge in ovariectomized pigs.

Ovariectomized gilts (n = 63) were given estradiol benzoate (estradiol), antiserum to neutralize endogenous GnRH, and pulses of a GnRH agonist (GnRH-A) to stimulate release of LH. GnRH-A was given as 200-ng pulses hourly from 0 to 54 h and as 100- or 200-ng pulses every 30 or 60 min from 54 to 96 h after estradiol. Estradiol alone suppressed LH from 6 to 54 h and elicited an LH surge that peaked at 72 h. When GnRH-A was given every 30-60 min from 0 to 96 h, estradiol suppressed LH for 6-12 h, but then LH returned to pre-estradiol concentrations. When pulses of GnRH-A were given only between 54 and 96 h after estradiol, the surge of LH was related positively to dose and frequency of GnRH-A. We conclude that 1) estrogen acts at the hypothalamus to inhibit release of GnRH for 54 h and then causes a synchronous release of GnRH; 2) estrogen acts at the pituitary to block its response to GnRH for 6-12 h and enhances the accumulation of releasable LH; and 3) magnitude of the LH surge is dependent on the amount of GnRH stimulation.     

Estradiol regulation of luteinizing hormone secretion in heifers of two breed types that reach puberty at different ages

The working hypothesis was that 17 beta-estradiol (E(2)) negative feedback on the hypothalamic-pituitary axis in regulation of LH secretion decreases during peripuberty in heifers of 2 different genotypes. We investigated whether Bos indicus heifers had a period postpuberty, as compared with prepuberty, of greater E(2) inhibition of LH secretion at a time when heifers of this genotype have been reported to have a period of anestrus. Prepubertal heifers 9 mo of age of 2 genotypes (B. indicus and B. taurus) were assigned to 3 groups (6 animals/group) to either remain intact (control), be ovariectomized, or be ovariectomized and implanted with E(2). Variables evaluated from 10 to 28 mo of age were circulating concentrations of progesterone (P(4)), presence of corpora lutea, and pulsatile pattern of LH release. Results confirmed that B. taurus heifers attained puberty at younger ages (P < 0.001) and at lower live weights (P = 0.015) than did B. indicus heifers (507 +/- 37 days of age vs. 678 +/- 7 days of age; 259 +/- 14 kg vs. 312 +/- 11 kg; respectively). There was cessation of E(2) inhibition of LH pulses coincident with the onset of puberty in heifers of both breed types but at a much younger age in B. taurus heifers. There was no evidence of enhanced negative feedback of E(2) on LH secretion subsequent to puberty in B. indicus heifers nor was there cessation of estrous cycles in control heifers of either breed type after puberty.     

The negative feedback effect of estradiol-17beta on secretion of luteinizing hormone in beef cows

Thirty-two ovariectomized cows were used to determine the time course for the negative feedback effect of estradiol-17beta (E) on secretion of the luteinizing hormone (LH). The cows were injected with gonadotropin releasing hormone (GnRH; 40 mug) 2.5 or 5 h after pretreatment with E (1 mug/kg body weight) or with a vehicle for control (C). Pretreatment with E resulted in lower serum concentrations of LH at 2.5 h (0.27 vs 0.90 ng/ml; P < 0.01) and at 5 h (0.27 vs 0.67 ng/ml; P < 0.01); less LH was released in response to GnRH at 2.5 h after treatment compared to cows treated with C (10 +/- 4.9 vs 27 +/- 3.8 ng/ml; P < 0.001). However, when GnRH was administered 5 h after E or C, there was no difference in the total amount of LH released (34 +/- 1.8 vs 26 +/- 4.4 ng/ml; P > 0.2). Time to half area (estimate of decay for the induced surge of LH) was longer for cows treated with E when compared to those treated with C (1.3 vs 0.9 h, P < 0.001; 1.5 vs 0.8 h, P < 0.001). Time to half area was not affected by the time of administration of GnRH after E (P > 0.4). These results suggest that E acts in the pituitary to cause the initial decrease in concentrations of LH. Pituitaries in animals pretreated with E regained the capacity to release as much LH at 5 h after treatment as those treated with C at a time when LH concentrations were still suppressed by E. Thus, the hypothalamus or an extra-hypothalamic area may be involved in maintaining the suppression of LH secretion after the initial effect on the pituitary has declined.     

Changes in episodic luteinizing hormone secretion leading to puberty in the lamb

In this study, we monitored episodic luteinizing hormone (LH) secretion throughout development in eight April-born ewe lambs to determine if a change in LH pulse patterns preceded first ovulation at puberty. LH pulses were measured in samples collected every 12 min for 6 h once in July, twice a month from 22 August to 2 October, and then weekly until puberty. Progesterone concentrations, measured in samples taken 3/wk, were used as an index of first ovulation, which occurred at 29.3 +/- 0.7 wk of age. LH pulse frequencies throughout most of this period ranged from 0 to 2 pulses/6 h, with no change over time. However, during the week prior to the first progesterone rise, there was a significant increase in pulse frequency to a level seen during the follicular phase in post-pubertal lambs. This increase in pulse frequency was evident in 7 of 8 lambs; pulses were not analyzed in the last lamb because samples were taken during the LH surge. In contrast, LH pulse amplitude did not increase prior to puberty. In fact, pulse amplitude declined linearly during the 3 wk before first ovulation and then increased during the follicular phase in post-pubertal animals. These results support the hypothesis that an increase in the frequency of episodic LH secretion is a key event leading to the onset of ovarian cycles in the lamb. Whether an increase in pulse amplitude is also necessary remains unclear. If so, it must occur just before the LH surge, since it was not detected in any samples taken before puberty in this study.     

Effects of pulsatile infusion of luteinizing hormone-releasing hormone on luteinizing hormone secretion and ovarian function in hypophysial stalk-transected beef heifers

Hypothalamic regulation of luteinizing hormone (LH) secretion and ovarian function were investigated in beef heifers by infusing LH-releasing hormone (LHRH) in a pulsatile manner (1 microgram/ml; 1 ml during 1 min every h) into the external jugular vein of 10 hypophysial stalk-transected (HST) animals. The heifers were HST approximately 30 mo earlier. All heifers had increased ovarian size during the LHRH infusion. The maximum ovarian size (16 +/- 2.7 cm3) was greater (P less than 0.01) than the initial ovarian size (8 +/- 1.4 cm3). Ovarian follicular growth occurred in 4 of 10 HST heifers in response to pulsatile LHRH infusion. In 2 heifers, an ovarian follicle developed to preovulatory size, but ovulation occurred in only 1 animal after the frequency of LHRH was increased (1 microgram every 20 min during 8 h). In blood samples obtained at 20-min intervals every 5th day, LH concentrations in peripheral serum remained consistently low (0.9 ng/ml) and nonepisodic in the 10 HST heifers during infusion of vehicle on the day before beginning LHRH. In 7 of 10 HST animals, episodic LH secretion occurred in response to pulsatile infusion of LHRH. In 3 of these long-term HST heifers, however, serum LH remained at basal levels and the isolated pituitary seemingly was unresponsive to pulsatile infusion of LHRH as indicated by sequential patterns of gonadotropin secretion obtained at 5-day intervals. These results indicate that pulsatile infusion of LHRH induces LH release in HST beef heifers.     

Effects of an opioid antagonist on pulsatile luteinizing hormone secretion in the ewe vary with changes in steroid negative feedback

In ewes during the breeding season, estradiol (E) and progesterone (P) synergistically regulate pulsatile luteinizing hormone (LH) secretion. E primarily inhibits LH pulse amplitude and P inhibits LH pulse frequency. To determine if endogenous opioid peptides (EOP) mediate these negative feedback effects, we administered the long-acting opioid antagonist WIN 44,441-3 (WIN) to intact ewes during the luteal and follicular phases of the estrous cycle and to ovariectomized ewes treated with no steroids, E, P, or E plus P. Steroid levels were maintained at levels seen during the estrous cycle by Silastic implants placed shortly after surgery. WIN increased LH pulse frequency, but not amplitude, in luteal phase ewes. In contrast, during the follicular phase, LH pulse amplitude was increased by WIN and pulse frequency was unchanged. Neither LH pulse frequency nor pulse amplitude was affected by WIN in long-term ovariectomized ewes untreated with steroids. In contrast, WIN slightly increased LH pulse frequency in short-term ovariectomized ewes. WIN also increased LH pulse frequency in ovariectomized ewes treated with P or E plus P. WIN did not affect pulse frequency but did increase LH pulse amplitude in E-treated ewes. These results support the hypothesis that EOP participate in the negative feedback effects of E and P on pulsatile LH secretion during the breeding season and that the inhibitory effects of EOP may persist for some time after ovariectomy.     

Influence of season and estradiol on secretion of luteinizing hormone in ovariectomized cows

The hypotheses that secretion of luteinizing hormone (LH) varies with season and that estradiol may modulate the seasonal fluctuation in secretion of LH in cows were investigated. Seven mature cows were ovariectomized approximately 30 days before initiation of the experiment. Three of the ovariectomized cows (OVX-E2) were administered a subcutaneous estradiol implant that provided low circulating levels of 17 beta-estradiol. The remaining 4 cows (OVX) were not implanted. From December 21, 1982, to September 20, 1984, blood samples were collected sequentially (at 10-min intervals for 6 h) at each summer and winter solstice, and each spring and autumn equinox. In addition, from March 17, 1983, to March 17, 1984, sequential samples were collected midway between each solstice and equinox. Concentration of LH was measured in all samples, and concentration of estradiol was measured in pools of samples. An annual cycle in mean serum concentration of LH and amplitude of LH pulses was detected in both groups of cows. The seasonal pattern did not differ in the two treatment groups. Serum concentration of LH and amplitude of LH pulses were highest around the spring equinox, decreased gradually to the autumn equinox, and then increased and peaked again during the following spring equinox. Frequency of LH pulses and concentration of estradiol in serum did not vary with season. Circulating concentrations of LH and amplitude of pulses tended to be higher in OVX-E2 than OVX cows throughout the experimental period. Frequency of pulses of LH was lower in OVX-E2 than OVX cows throughout the experiment. Concentrations of estradiol were higher in the implanted cows.(ABSTRACT TRUNCATED AT 250 WORDS)     

Negative effect of estradiol on luteinizing hormone throughout the ovulatory luteinizing hormone surge in mares

The negative effect of estradiol-17beta (E2) on LH, based on exogenous E2 treatments, and the reciprocal effect of LH on endogenous E2, based on hCG treatments, were studied throughout the ovulatory follicular wave during a total of 103 equine estrous cycles in seven experiments. An initial study developed E2 treatment protocols that approximated physiologic E2 concentrations during the estrous cycle. On Day 13 (ovulation = Day 0), when basal concentrations of E2 and LH precede the ovulatory surges, exogenous E2 significantly depressed LH concentrations to below basal levels. Ablation of all follicles > or = 10 mm when the largest was > or =20 mm resulted in an increase in percentage change in LH concentration within 8 h that was greater (P < 0.03) than for controls or E2-treated/follicle-ablated mares. Significant decreases in LH occurred when E2 was given when the largest follicle was either > or =25 mm, > or =28 mm, > or =35 mm, or near ovulation. Treatment with 200 or 2000 IU of hCG did not affect E2 concentrations during the initial portion of the LH surge (largest follicle, > or =25 mm), but 2000 IU significantly depressed E2 concentrations before ovulation (largest follicle, > or =35 mm). Results indicated a continuous negative effect of E2 on LH throughout the ovulatory follicular wave and may be related to the long LH surge and the long follicular phase in mares. Results also indicated that a reciprocal negative effect of LH on E2 does not develop until the E2 surge reaches a peak.     

Seasonal changes in pulsatile luteinizing hormone (LH) secretion in the ewe: relationship of frequency of LH pulses to day length and response to estradiol negative feedback

Seasonal changes in pulsatile luteinizing hormone (LH) secretion in ovariectomized ewes were examined over the course of 2 yr in relation to annual changes in environmental photoperiod, shifts in response to estradiol negative feedback control of LH secretion, and timing of the breeding season. Under natural environmental conditions, the frequency of LH pulses in individual ovariectomized ewes changed gradually and in close association with the annual cycle of day length. As days became shorter in late summer and autumn, LH pulse frequency increased; conversely, as day length increased in late winter and spring, frequency declined. Under artificial conditions in which ovariectomized ewes were exposed to different photoperiods, a similar inverse relationship was observed between day length and LH pulse frequency. The seasonal changes in frequency of LH pulses in ovariectomized ewes, although symmetric with the annual photoperiodic cycle, were not temporally coupled to the dramatic shifts in response to estradiol feedback inhibition of LH secretion at the transitions between breeding season and anestrus. The feedback shifts occurred abruptly and at times when LH pulse frequency in ovariectomized ewes was at, or near, the annual maximum or minimum. The tight coupling between LH pulse frequency and photoperiod leads to the conclusion that there is a photoperiodic drive to the LH pulse-generating system of the ewe. The temporal dissociation between changes in this photoperiodic drive and the seasonal shifts in response to estradiol negative feedback support the hypothesis that the neuroendocrine basis for these two phenomena is not one and the same.     

Influences of month of birth and age on patterns of luteinizing hormone secretion in prepubertal heifers

An experiment was done to determine if month of birth and age influenced patterns of luteinizing hormone (LH) secretion in prepubertal heifers. Mean concentrations of LH increased linearly (P < .05) in March-born heifers between one and seven months of age. This was partially due to an increase in number of LH pulses. The prepubertal pattern of LH concentrations was quadratic (P < .05) for heifers born in September because concentrations were slightly higher (P = .15) than those in March-born heifers at one month of age. There were no differences between groups during the remainder of the prepubertal period (3 to 7 months). There was a tendency (P = .18) for September-born animals to reach puberty at younger ages than those born in March. September-born heifers also had greater (P = .06) average daily gains, but body weights at puberty were similar for the two groups. These results show that season of birth influenced LH concentrations at one month of age, but did not significantly affect the increase between three and seven months of age.     

Role of estradiol in inducing an ovulatory-like surge of luteinizing hormone in sheep

Two experiments were performed to examine the effect of estradiol on secretion of luteinizing hormone (LH) and on the number of receptors for gonadotropin-releasing hormone (GnRH) after down regulation of GnRH receptors in ovariectomized ewes. In the first experiment, ovariectomized ewes were administered one of four treatments: Group 1) infusion of GnRH i.v. for 40 h; Group 2) injection of 100 micrograms estradiol i.m.; Group 3) infusion of GnRH i.v. for 16 h followed immediately by an injection of 100 micrograms estradiol i.m.; and Group 4) infusion of GnRH i.v. for 40 h plus injection of 100 micrograms estradiol i.m. after the 16th h of infusion. Ewes in Groups 1, 3 and 4 responded to the infusion of GnRH with an immediate increase in serum concentrations of LH, with maximum values occurring between 2 and 4 h after the start of infusion; serum concentrations of LH then began to decline and were approaching the pretreatment baseline within 16 h. Administration of estradiol resulted in a surge of LH regardless of whether the pituitary had been desensitized by infusion of GnRH or not. In all cases the magnitude of the surge was similar to that induced by the initial infusion of GnRH. In Groups 2 and 3 the surge of LH began at 12.3 +/- 0.1 and 11.9 +/- 0.1 h after administration of estradiol. In contrast, the ewes in Group 4 had a surge of LH beginning 3.7 +/- 0.1 h after administration of estradiol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible negative ultra-short loop feedback of luteinizing hormone releasing hormone (LHRH) in the ovariectomized rat

To determine if LHRH might act within the brain to modify its own release, repeated blood samples were removed from conscious ovariectomized rats and minute doses of LHRH were injected into the third ventricle (3V). The effect of these injections on plasma LH and FSH was measured by radioimmunoassay (RIA). The higher dose of intraventricular LHRH (10 ng in 2 microliter) induced an increase in plasma LH within 10 min after its injection. Plasma LH decreased for the next 60 min. This was followed by restoration of LH pulses characteristic of the ovariectomized rat. This dose of LHRH slightly elevated plasma FSH concentrations. In stark contrast, a 10 fold lower dose of 1 ng of LHRH injected into the ventricle resulted in a highly significant decrease of plasma LH at 10 min following injection, followed by return of LH pulsations. There was no effect on the pulsatile release of FSH. The results are interpreted to mean that at the higher dose, sufficient LHRH reached the site of origin of the hypophyseal portal vessels in the median eminence so that it diffused into portal vessels and was delivered to the gonadotrophs to induce LH release. In contrast, the lower dose provided sufficient hypothalamic concentrations of the peptide to suppress the discharge of the LHRH neurons, thereby leading to a decline in plasma LH, indicative of an ultrashort-loop negative feedback of LHRH to suppress its own release.     

Effect of estradiol on secretion of luteinizing hormone during the follicular phase of the bovine estrous cycle

Mean concentrations of luteinizing hormone (LH) increase during the follicular phase of the estrous cycle in cows. The working hypotheses in the present study were (1) that increasing concentrations of 17 beta-estradiol (E2) during the follicular phase of the estrous cycle cause an increase in mean concentration of LH by increasing amplitude of pulses of LH, and (2) that increasing E2 concentrations during this stage of the estrous cycle decrease frequency of pulses of LH in bovine females. Day of estrus was synchronized in seventeen mature cows. Treatments were initiated on Day 16 of the experimental estrous cycle (Day 0 = estrus). At Hour 0 (on Day 16), 4 cows were lutectomized. Lutectomy of these cows (EE; n = 4) allowed for endogenous secretion of E2. The remaining cows were ovariectomized at Hour 0 and were assigned to one of three E2 treatments: luteal phase E2 (LE, n = 5), increasing then decreasing E2 (DE, n = 5), and no E2 (NE, n = 3). Cows in the group that received LE were administered one E2 implant at Hour 0, which provided low circulating concentrations of E2 similar to those observed during the luteal phase of the estrous cycle. Cows in the group that received DE were administered one E2 implant at Hour 0, and additional implants were administered at 8-h intervals through Hour 40; then, two implants were removed at Hours 48 and 56, and one implant was removed at Hour 64.(ABSTRACT TRUNCATED AT 250 WORDS)     

Absence of an increase in gonad-independent drive to pulsatile luteinizing hormone secretion during photoperiod-induced puberty

This study was conducted to determine if photoperiod can influence the pattern of luteinizing hormone (LH) secretion in the absence of the ovaries in the developing female sheep. Lambs were raised in a photoperiod sequence (short, long, short days) known to induce puberty between 30 and 35 wk of age, or in a photoperiod (only short days) that prevents puberty during the first year. Their ovaries were removed at 10 wk of age, and the detailed pattern of LH was assessed (samples at 12-min intervals for 4 h) each 3- to 5-wk period between 9 and 45 wk of age. Rapid LH pulses (40- to 50-min interpulse interval) were evident within a few weeks after ovariectomy in both groups of females. Those exposed to the artificial photoperiod sequence that induces normal sexual maturity did not increase their pulse frequency further during the pubertal period. Moreover, their LH pulse frequencies were not greater than those in agonadal females exposed to the photoperiod that delays puberty. These findings indicate that photoperiodic induction of puberty in the sheep does not require steroid-independent modulation of pulsatile LH secretion.     

Abnormalities of the thyroid hormone negative feedback regulation of TSH secretion in spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) are characterized by several neuroendocrine abnormalities including a chronic hypersecretion of thyrotropin (TSH) of unknown etiology. We hypothesized that the inappropriately high TSH secretion in SHR may be the result of an impaired thyroid hormone negative feedback regulation of hypothalamic thyrotropin-releasing hormone (TRH) and/or pituitary TSH production. To test this hypothesis, SHR or their normotensive Wistar-Kyoto (WKY) controls were treated with either methimazole (0.02% in drinking water) to induce hypothyroidism or administered L-thyroxine (T4) at a dose of 0.8 or 2.0 micrograms/100 g body weight/day to induce hyperthyroidism. All treatments were continued for 14 days after which animals were killed under low stress conditions. TSH concentrations in plasma and anterior pituitary tissue were 2-fold higher (P less than 0.01) in euthyroid SHR compared to WKY control rats while thyroid hormone (T3 and T4) levels were in the normal range. Hypothyroidism induced by either methimazole or thyroidectomy caused a significant (P less than 0.01) rise of plasma TSH levels in both WKY and SHR rats. However, relative to the TSH concentrations in control animals, the increase of plasma TSH in SHR was significantly blunted (P less than 0.01) in comparison to the WKY group. Hypothyroidism caused a significant depletion of TRH in stalk-median eminence (SME) tissue in both groups of rats. However, no differences between SHR and WKY rats were observed. The administration of thyroid hormone caused a dose dependent suppression of plasma TSH levels in both strains of rats. However, at both doses tested plasma TSH concentrations in SHR rats were significantly less suppressed (P less than 0.05) than those in WKY animals. Under in vitro conditions basal and potassium induced TRH release from SMEs derived from SHR was significantly (P less than 0.05) higher than that from WKY rats, whether expressed in absolute terms or as percent of content. These findings suggest that the thyroid hormone negative feedback regulation of TSH secretion may be impaired in SHR rats. Our data do not allow conclusions as to whether defects in the regulation of TSH production are located exclusively at the hypothalamic level. Since the overproduction of hypothalamic TRH and hypophysial TSH should lead to an increased thyroid hormone biosynthesis other defects in the hypothalamus-pituitary-thyroid-axis may contribute to the abnormal regulation of TSH secretion in SHR rats.     

Role of the central ascending noradrenergic system in the regulation of luteinizing hormone responsiveness to testosterone negative feedback in the adult male rat

It is well established that testosterone (T) feeds back on the brain and the anterior pituitary to inhibit gonadotropin secretion. However, the precise mechanism by which T exerts its central inhibitory action is poorly understood. We hypothesized that central noradrenergic activity decreases hypothalamic sensitivity to T negative feedback. To test this hypothesis, we compared the dose-response relationships between T and luteinizing hormone (LH) and between T and follicle-stimulating hormone (FSH) in adult male rats chronically depleted of hypothalamic norepinephrine (NE) to the dose-response curves exhibited by control animals. Depletion of hypothalamic NE was achieved by two independent methods: 1) by bilateral transection of the ascending noradrenergic system at the level of the mesencephalon, and 2) by intracerebroventricular infusion of the neurotoxin, 6-hydroxydopamine. After allowing 2-3 weeks for recovery from this initial surgery, all animals were castrated, and 3 weeks later were outfitted with subcutaneous T-containing or empty (sham) implants for a period of 48 hours. We observed that despite a profound chronic reduction in hypothalamic noradrenergic activity, the dose-response relationship between plasma T and the gonadotropins remained unaltered. These data demonstrate that normal amounts of hypothalamic noradrenergic activity are not essential for T to exert its negative feedback effect on gonadotropin secretion. Furthermore, they suggest that chronic removal of hypothalamic NE does not alter gonadotropin sensitivity to T negative feedback.