Serum testosterone and testicular response to HCG in young and aged male rats

Hypogonadism induced by surgical stress and brain trauma is reversed by human chorionic gonadotropin in male rats: A potential therapy for surgical and TBI-induced hypogonadism?

INTRODUCTION

Hypogonadotropic hypogonadism (HH) is an almost universal, yet underappreciated, endocrinological complication of traumatic brain injury (TBI). The goal of this study was to determine whether the developmental hormone human chorionic gonadotropin (hCG) treatment could reverse HH induced by a TBI.

METHODS

Plasma samples were collected at post-surgery/post-injury (PSD/PID) days -10, 1, 11, 19 and 29 from male Sprague-Dawley rats (5- to 6-month-old) that had undergone a Sham surgery (craniectomy alone) or CCI injury (craniectomy + bilateral moderate-to-severe CCI injury) and treatment with saline or hCG (400 IU/kg; i.m.) every other day.

RESULTS

Both Sham and CCI injury significantly decreased circulating testosterone (T), 11-deoxycorticosterone (11-DOC) and corticosterone concentrations to a similar extent (79.1% vs. 80.0%; 46.6% vs. 48.4%; 56.2% vs. 32.5%; respectively) by PSD/PID 1. hCG treatment  returned circulating T to baseline concentrations by PSD/PID 1 (8.9 ± 1.5 ng/ml and 8.3 ± 1.9 ng/ml; respectively) and was maintained through PSD/PID 29. hCG treatment significantly, but transiently, increased circulating progesterone (P4) ~3-fold (30.2 ± 10.5 ng/ml and 24.2 ± 5.8 ng/ml) above that of baseline concentrations on PSD 1 and PID 1, respectively. hCG treatment did not reverse hypoadrenalism following either procedure.

CONCLUSIONS

Together, these data indicate that (1) craniectomy is sufficient to induce persistent hypogonadism and hypoadrenalism, (2) hCG can reverse hypogonadism induced by a craniectomy or craniectomy +CCI injury, suggesting that (3) craniectomy and CCI injury induce a persistent hypogonadism by decreasing hypothalamic and/or pituitary function rather than testicular function in male rats. The potential role of hCG as a cheap, safe and readily available treatment for reversing surgery or TBI-induced hypogonadism is discussed.

Testosterone impairs the acquisition of an operant delayed alternation task in male rats

The current study examined the effects of gonadectomy (GDX) and subsequent testosterone treatment of male Long-Evans rats on an operant variable delay spatial alternation task (DSA). Gonadally-intact rats (intact-B), GDX rats receiving implants that delivered a physiological level of testosterone (GDX-T), and GDX rats receiving blank implants (GDX-B) were tested for 25 sessions on a DSA task with variable inter-trial delays ranging from 0 to 18 s. Acquisition of the DSA task was found to be enhanced following GDX in a time and delay dependent manner. Both the GDX-T and the intact-B rats had lower performance accuracies across delays initially, relative to GDX-B rats, and this deficit persisted into subsequent testing sessions at longer delays. The GDX-T and intact-B rats also had a tendency to commit more perseverative errors during the early testing sessions, with both groups persisting in pressing a lever which had not been associated with reinforcement for at least two consecutive trials. However, both the GDX-T and intact-B groups were able to achieve performance accuracy similar to that of the GDX-B rats by the final sessions of testing. Overall, these results suggest that castration of adult male rats enhances their acquisition of an operant DSA task.

Sexually dimorphic aging of dendritic morphology in CA1 of hippocampus

During aging, rats of both sexes experience a decline in performance on hippocampal-dependent tasks. Investigations into the neuroanatomical correlates of this functional decline have been conducted almost exclusively in male subjects. In the present study, dendritic spine density in stratum radiatum and complexity of the entire apical dendritic tree were quantified using Golgi-Cox-stained tissue in young (3-5 months) and aged (19-22 months) rats of both sexes. Because both cognitive decline and hippocampal morphology may be influenced by ovarian hormonal state, young adult females were examined during either proestrus or estrus, and aged females were examined in one of two reproductively senescent states: persistent estrus or persistent diestrus. A sex difference in dendritic branching of CA1 pyramidal cells was found among young adults. However, this difference disappeared during aging, due to a reduction in branching with age for males but not for females. Spine density was not influenced by age or sex, nor did ovarian hormone status influence either measure. These results are consistent with our previous findings in the rat medial prefrontal cortex and primary motor cortex and with the human literature, which indicate that age-related atrophy of cognitive brain regions is more severe for males than females.

Age-related increase in mitochondrial superoxide generation in the testosterone-producing cells of Brown Norway rat testes: relationship to reduced steroidogenic function?

Aging in Brown Norway rats is accompanied by the reduced production of testosterone by the Leydig cells, the testicular cells responsible for synthesizing and secreting this essential steroid. As yet, the mechanism by which Leydig cell steroidogenesis is reduced is unknown. Herein we assess the production of mitochondrial reactive oxygen species by intact Leydig cells isolated from the testes of young and old rats. To this end, Leydig cells were incubated with lucigenin (bis-N-methylacridinium nitrate), a probe that enters cells, localizes to mitochondria, and yields a significant chemiluminescent response following its reaction with intramitochondrial superoxide. Leydig cells from old rats elicited significantly greater lucigenin-derived chemiluminescence (LDCL) than those from young rats. Electron microscopic stereological analysis revealed that the absolute volume of mitochondria in the old cells was reduced from that in the young. These results, taken together, suggest that there are age-related changes in the production of reactive oxygen species by the mitochondria of Leydig cells, with those of old Leydig cells producing significantly greater levels than those of young Leydig cells. The results are consistent with the proposal that mitochondrial-derived reactive oxygen may play a role in the irreversible decline in the ability of old Leydig cells to produce testosterone.

Assessment of modifications of the rate of aging in the rat

Multivariate models of BA are, at present, the only way of measuring the main subject of gerontological research, namely, aging of the organism as a whole. Aging is obviously not a completely random process without any regularity, but shows a pattern which is determined by the organism's principles of order and its answer to the stochastic events of primary aging. Based on this theoretical concept, which is supported by results of factor analysis, multicellular aging may be described by a single complex variable, BA. However, the non-linearity of age changes and the different extent to which primary and secondary aging are involved, as well as factors other than aging, restrict the sensitivity of multiple regression models. Sensitivity may be increased by selecting parameters according to their discriminating power revealed by discriminant analysis. Wherever influences on aging are to be tested, a combination of the three types of multivariate methods will allow for an objectivation of effects and provide clues to a possible mode of action.

Effects of aging on pituitary and testicular luteinizing hormone-releasing hormone receptors in the rat

Aging exerts profound influences on the function of the hypothalamic-pituitary-testicular-axis. This work has been performed in order to verify whether, in male rats, the decreased secretion of LH and testosterone (T) occurring in old animals is reflected by modifications of luteinizing hormone-releasing hormone (LHRH) receptors at the level of the anterior pituitary and of the testes. To this purpose, the affinity constant (Ka) and the maximal binding capacity (Bmax) for the LHRH analog [D-Ser(tBu)6]des-Gly10-LHRH-N-ethylamide were evaluated, by means of a receptor binding assay, in membrane preparations derived from the anterior pituitary and testicular Leydig cells of male rats of 3 and 19 months of age. Serum levels of LH and T were measured by specific RIAs. The results obtained show that, in aged male rats, the concentration of pituitary LHRH receptors is significantly lower than that found in young animals. On the other hand, the concentration of LHRH binding sites is significantly increased on the membranes of Leydig cells of old rats. In no instance the Ka for the LHRH analog is significantly affected. Serum levels of LH and T are significantly lower in old than in young male rats. In conclusion, these results suggest that the reduced secretion of LH in old male rats may be linked, at least partially, to a decrease of the number of pituitary LHRH receptors. The impaired production of testosterone occurring in aged rats is accompanied by a significant increase of the number of testicular LHRH receptors, indicating that also the intratesticular mechanisms controlling testosterone release undergo significant alterations with aging.

Age-related alterations in the stimulated release in vitro of catecholamines and luteinizing hormone-releasing hormone from the male rat hypothalamus

Using an in vitro perifusion system, the present study investigated the possibility that alterations in catecholamine and luteinizing hormone-releasing hormone (LHRH) secretion from the male rat mediobasal hypothalamus are present during the period of middle-age. The results indicate that, while tissue concentrations and baseline secretion of norepinephrine, dopamine and LHRH were similar between age groups, the patterns of dopamine and LHRH release in response to a series of depolarizing stimuli was different in the older animals. After all challenges, dopamine concentrations in the perifusate declined much more sharply for the middle-aged group, a finding that may be associated with a decrease with age in the pool of transmitter available for ready release. Also, tissue fragments from young adult rats were able to maintain the release of LHRH to a greater extent than tissue from the middle-aged animals, but only for the initial challenge period. The typical episodic pattern of LHRH release appeared to be disrupted in the older group following a second stimulus. It is possible that these age-related changes are early components of a disruption in the hypothalamic mechanisms governing gonadotropin secretion.

Hypothalamic aromatase activity in young and old male rats

We studied the capability of hypothalamic tissue from young (3-4 months) and old (22-24 months) male rats to aromatize androgens to estrogens. Aromatase activity was measured in homogenates of brain tissue by using a radiometric assay that quantifies the stereospecific production of 3H2O from [1 beta-3H]androstenedione as an index of estrogen formation. Despite the 40% drop in circulating testosterone (T) levels associated with aging in males, we found that hypothalamic aromatase activity was unaffected by age. This finding suggested that chronic exposure to low levels of circulating T can maintain brain aromatase activity in aged male rats. In an experiment designed to examine the acute response of hypothalamic aromatase activity to induction by T, we found a significant positive correlation between circulating T levels and hypothalamic aromatase activity in both age groups. These results demonstrate that T remains an effective regulator of hypothalamic aromatase in old male rats.

Androgen and precursor levels in serum and testes of adult rats under basal conditions and after hCG stimulation

In adult male Wistar rats, serum and testicular concentration of testosterone (T), estradiol (E2), dihydrotestosterone (DHT), 5 alpha-androstane 3 alpha, 17 beta-diol (Adiol), and their precursors were measured under basal conditions as well as 4 and 8 h respectively after i.m. injection of 100 I.U. of hCG. Under basal conditions T (203 +/- 30 (SE) ng/dl) was quantitatively by far the most important serum steroid, followed by progesterone (P) (76.5 +/- 12 ng/dl), 17-hydroxyprogesterone (170HP) (37.3 +/- 4.1 ng/dl), androstenedione (A) (24.6 +/- 2.5 ng/dl) and pregnenolone (P5) (22.9 +/- 4 ng/dl). Estradiol (E2) was present in a low concentration (1.06 +/- 0.26 ng/dl). In the testes, T was quantitatively the most important steroid (89 +/- 7.2 ng/g), followed by 5 alpha-androstane-3 alpha-17 beta-diol (Adiol) (26.5 +/- 2.8 ng/g) whereas 170HP (11.8 +/- 1.0 ng/g), P (11.5 +/- 1.0 ng/g) and P5 (16.6 +/- 1.8 ng/g) were present in roughly the same concentration, concentrations of A, DHT, dehydroepiandrosterone (DHEA), 17 hydroxy-pregnenolone (170HP5) and androst-5-ene-3 beta-17 beta-diol (D5diol) being much lower; E2 (0.06 +/- 0.01 ng/g) was hardly detectable. Within 4-8 h after hCG stimulation, serum androgen concentrations increased by a factor of 4-12, except for DHEA and D5diol (X2), and E2 (X 1.5). Intratesticular androgens and delta 4 steroid precursors increased by a factor of 5-10, delta 5 precursors by a factor of 2-4 and E2 by a factor of 2, the data tending to confirm that the delta 4 pathway is preferred over the delta 5 pathway. After hCG the relative increase of T precursors was the most important for P, suggesting that 17 hydroxylation might be the rate limiting step in T biosynthesis.

The aging Leydig cell. VIII. Protein kinase activity

The effects of aging on cyclic AMP (cAMP) dependent protein kinase activity of purified Leydig cells were evaluated. Purified Leydig cells from 20-day-, 40-day-, 60-day- and 30-month-old rats were incubated with or without hCG for 1 hour. At the end of incubation, tubes were centrifuged and supernatants were saved for cAMP and testosterone determinations, and the pellets were prepared for protein kinase assay. Basal cAMP levels of Leydig cells from 20-day- and 40-day-old rats were lower than those of 60-day- and 30-month-old rats. In response to hCG stimulation, Leydig cells of 40-day-old rats produced the highest amounts of cAMP, while 30-month-old had the lowest levels. Testosterone responses of 30-month-old rates were also significantly lower than those of 60-day-old mature rats. When cAMP-dependent protein kinase activities were measured, 30-month-old rats had the lowest total kinase activity. These results suggest that reduced protein kinase activity might contribute to the decreased testosterone synthesis of aged rats.

The pituitary-adrenal response to novel stimulation and ether stress in young adult and aged rats

Pituitary-adrenal responsiveness to stress induced by either psychological (novelty) or physiological (ether) stimuli was examined in young (3 to 6 months) and old (24 to 27 months) male and female rats. In Experiment 1, subjects were placed in a novel environment for 3 min. Blood samples were collected at 0, 15, 30, 45, and 60 min. In Experiment 2, a blood sample was collected from experimental subjects immediately following 15 min in a novel environment; a basal sample was collected from control subjects. In Experiment 3, basal and stress blood samples were collected from subjects exposed to ether vapors. Blood samples were assayed for plasma corticosterone content. The major findings were: (1) no change with age in either sex in basal corticosterone levels or in the time course of the adrenocortical response; (2) no change with age in males in stress-induced increments in corticoids; (3) no change with age in females in corticoid elevations induced by a mild stressor (Experiment 1), but a decrement in elevations induced by more potent stressors (Experiments 2 and 3).

Age-related changes in the hypothalamic-pituitary-testicular function of the rat

The activity of the hypothalamic-pituitary-testicular axis was investigated in 3-4 months (young) and 24 months (old) rats. The results clearly demonstrate that aging reduces (p less than 0.01) the hypothalamic content of gonadotrophin releasing hormone (GnRH), decreases the capacity of the pituitary (p less than 0.01) to synthesize and or release follicle stimulating hormone and luteinizing hormone (LH) following a single stimulation of GnRH (50 ng/100 g body weight), lowers the capacity of the testes to produce testosterone (p less than 0.01) following multiple subcutaneous injections of human chorionic gonadotrophin (hCG 3IU/100 g body weight for 3 consecutive days), decreases the number of Leydig cell LH receptors and decreases the in vitro responsiveness of the hCG-challenged Leydig cell to synthesize testosterone (p less than 0.01). These phenomena are independent of a major alteration in the capacity of the hCG challenged Leydig cell to produce adenosine 3',5'-monophosphate. It is concluded that the decline in testicular activity accompanying senescence is not inherent to the testes only but is also associated with alteration in the function of the hypothalamus and pituitary which eventually lead to the loss of fecundity.

The aging Leydig cell: 2. Two distinct populations of Leydig cells and the possible site of defective steroidogenesis

Using metrizamide gradient centrifugation two populations of Leydig cells were found in both 60-90 day-old and 24 month-old rats. Cells from both Band 2 (B2) and Band 3 (B3) responded to LH stimulation with increased cyclic AMP formation; however, only B3 cells produced significant amounts of testosterone. Cells from both B2 and B3 of the old rats synthesized less cyclic AMP and testosterone than cells from their younger counterparts. In response to LH stimulation, 0.01 - 1.0 mIU/ml, no appreciable difference of cyclic AMP formation could be detected between young and old Leydig cells. Maximal testosterone production occurred when 1 mIU/ml LH was used. Only when LH concentration was increased to 10 and 100 mIU/ml, did young Leydig cells produce significantly more cyclic AMP than old Leydig cells. After addition of 5X10(-7)M of pregnenolone or progesterone to the incubation medium, both young and old Leydig cells produced comparable amounts of testosterone. These results demonstrate no impairment of old rat Leydig cells to synthesize testosterone from pregnenolone and progesterone.