Age-dependent regression analysis of male gonadal axis

Aging and androgens: Physiology and clinical implications

In men >  ~35 years, aging is associated with perturbations in the hypothalamus-pituitary-testicular axis and declining serum testosterone concentrations. The major changes are decreased gonadotropin-releasing hormone (GnRH) outflow and decreased Leydig cell responsivity to stimulation by luteinizing hormone (LH). These physiologic changes increase the prevalence of biochemical secondary hypogonadism-a low serum testosterone concentration without an elevated serum LH concentration. Obesity, medications such as opioids or corticosteroids, and systemic disease further reduce GnRH and LH secretion and might result in biochemical or clinical secondary hypogonadism. Biochemical secondary hypogonadism related to aging often remits with weight reduction and avoidance or treatment of other factors that suppress GnRH and LH secretion. Starting at age ~65-70, progressive Leydig cell dysfunction increases the prevalence of biochemical primary hypogonadism-a low serum testosterone concentration with an elevated serum LH concentration. Unlike biochemical secondary hypogonadism in older men, biochemical primary hypogonadism is generally irreversible. The evaluation of low serum testosterone concentrations in older men requires a careful assessment for symptoms, signs and causes of male hypogonadism. In older men with a body mass index (BMI) ≥ 30, biochemical secondary hypogonadism and without an identifiable cause of hypothalamus or pituitary pathology, weight reduction and improvement of overall health might reverse biochemical hypogonadism. For older men with biochemical primary hypogonadism, testosterone replacement therapy might be beneficial. Because aging is associated with decreased metabolism of testosterone and increased tissue-specific androgen sensitivity, lower dosages of testosterone replacement therapy are often effective and safer in older men.

Pulsatility of Hypothalamo-Pituitary Hormones: A Challenge in Quantification

Neuroendocrine systems control many of the most fundamental physiological processes, e.g., reproduction, growth, adaptations to stress, and metabolism. Each such system involves the hypothalamus, the pituitary, and a specific target gland or organ. In the quantification of the interactions among these components, biostatistical modeling has played an important role. In the present article, five key challenges to an understanding of the interactions of these systems are illustrated and discussed critically.

Effects of aging on the male reproductive system

The study aims to discuss the effects of aging on the male reproductive system. A systematic review was performed using PubMed from 1980 to 2014. Aging is a natural process comprising of irreversible changes due to a myriad of endogenous and environmental factors at the level of all organs and systems. In modern life, as more couples choose to postpone having a child due to various socioeconomic reasons, research for understanding the effects of aging on the reproductive system has gained an increased importance. Paternal aging also causes genetic and epigenetic changes in spermatozoa, which impair male reproductive functions through their adverse effects on sperm quality and count as, well as, on sexual organs and the hypothalamic-pituitary-gonadal axis. Hormone production, spermatogenesis, and testes undergo changes as a man ages. These small changes lead to decrease in both the quality and quantity of spermatozoa. The offspring of older fathers show high prevalence of genetic abnormalities, childhood cancers, and several neuropsychiatric disorders. In addition, the latest advances in assisted reproductive techniques give older men a chance to have a child even with poor semen parameters. Further studies should investigate the onset of gonadal senesce and its effects on aging men.

Interrelationship between NO and androgenic activity in mice, Mus musculus, following temporal phase relation of serotonergic and dopaminergic neural oscillations

The present study was designed to monitor the correlation of gonadal steroidogenic activity and nitric oxide (NO) of plasma and testis of male laboratory mouse, Mus musculus, under (1) control/basal and (2) experimental conditions. In the first study, male mice (n = 6) of three age groups (3-week-old sexually immature, 9-week-old sexually mature and 1.5-year-old) were assayed for plasma testosterone and level of NO in the plasma and testis. The immunoreactivity of 3-β-hydroxysteroid dehydrogenase (3-β-HSD) and androgen receptors (ARs) was also detected in testis and epididymis, respectively. In the second study, the reproductive state of mice was altered by injecting precursors of serotonin 5-hydroxytryptophan (5-HTP) and dopamine L-dihydroxyphenylalanine (L-DOPA) daily for 13 days. In one subgroup, mice received their daily injections 8 h apart and in another subgroup 12 h apart. A third subgroup of control mice received two daily injections of normal saline. After the completion of the experiment, the mentioned parameters were measured. The results showed a significant increase in levels of NO and a lowering of the immune reactivity of 3-β-HSD and AR, in both leydig cells and the epididymis in the sexually immature, old-age mice as well as in the mice that received 5-HTP and L-DOPA in the 8-h temporal relation, whereas opposite effects were observed in the sexually mature mice as well as in the mice that received 5-HTP and L-DOPA in the 12-h temporal relation. These findings led us to conclude that an inverse correlation exists between testicular steroidogenic activity and NO activity of laboratory mice under control and experimentally modulated gonadal conditions.

Thyrotropin secretion in healthy subjects is robust and independent of age and gender, and only weakly dependent on body mass index

CONTEXT

Studies of the influence of sex, age, and body weight on TSH secretion are not unanimous. Most reports are based on a single TSH measurement; studies using frequent blood sampling are scarce and include a limited number of selected subjects.

OBJECTIVE

The goal was to investigate TSH dynamics in 117 healthy adults.

METHODS

TSH was measured by a sensitive immunofluorometric assay. Secretion parameters were quantified by automated deconvolution, approximate entropy [ApEn], spikiness, and diurnal properties.

RESULTS

Mean age was 43 years (range, 22-77 y). Mean body mass index (BMI) was 26.8 kg/m(2) (range, 18.3-39.4 kg/m(2)). Daily TSH secretion was 45.4 mU/L (range, 8.0-207 mU/L). There were no sex differences in secretion parameters, including pulse frequency; basal, pulsatile, and total secretion; pulse mode; half life; pulse regularity; ApEn; spikiness; and nycthemeral properties. BMI was positively related to basal secretion. Total secretion correlated negatively with free T₄ (R = 0.225; P = .018). The onset of the nocturnal surge was delayed by increasing BMI and advanced by increasing age. ApEn and spikiness correlated positively with age, especially in men. The 9 am sample correlated strongly with the total 24-hour secretion, explaining two-thirds of the variability.

CONCLUSION

This study shows that the 24-hour TSH secretion in healthy volunteers is stable and robust and not influenced by sex, BMI, and age. ApEn in the elderly, especially men, is increased, pointing to a less tight feedback control. Furthermore, aging is associated with advance shifting of the TSH rhythm, which is a phenomenon also observed in other biological rhythms.

Testosterone Restoration by Enclomiphene Citrate in Men with Secondary Hypogonadism: Pharmacodynamics and Pharmacokinetics

OBJECTIVES

To determine the pharmacodynamic (PD) profile of serum total testosterone levels (TT) and luteinizing hormone (LH) in men with secondary hypogonadism following initial and chronic daily oral doses of enclomiphene citrate in comparison to transdermal testosterone. To determine the effects of daily oral doses of enclomiphene citrate (Androxal®) in comparison to transdermal testosterone on other hormones and markers in men with secondary hypogonadism.

PATIENTS AND METHODS

This was a randomized, single blind, two-center phase II study to evaluate three different doses of enclomiphene citrate (6.25mg, 12.5mg and 25 mg Androxal®), versus AndroGel®, a transdermal testosterone, on 24-hour LH and TT in otherwise normal healthy men with secondary hypogonadism. Forty-eight men were enrolled in the trial (ITT Population), but 4 men had T levels >350 ng/dL at baseline. Forty-four men completed the study per protocol (PP population). All subjects enrolled in this trial had serum TT in the low range (<350 ng/dL) and had low to normal LH (<12 IU/L) on at least two occasions. TT and LH levels were assessed each hour for 24 hours to examine the effects at each of three treatment doses of enclomiphene versus a standard dose (5 grams) of transdermal testosterone (AndroGel). In the initial profile TT and LH were determined in a naïve population following a single initial oral or transdermal treatment (Day 1). This was contrasted to that seen after six weeks of continuous daily oral or transdermal treatment (Day 42). The pharmacokinetics of enclomiphene was performed in a select subpopulation. Serum samples were obtained over the course of the study to determine levels of various hormones and lipids.

RESULTS

After six weeks of continuous use, the mean ± SD concentration of TT at Day 42 C0hrTT, was 604 ± 160 ng/dL for men taking the highest of dose of enclomiphene citrate (enclomiphene, 25 mg daily) and 500 ± 278 ng in those men treated with transdermal testosterone. These values were higher than Day 1 values but not different from each other (p = 0.23, T-test). All three doses of enclomiphene increased C0hrTT, CavgTT, CmaxTT, CminTT and CrangeTT. Transdermal testosterone also raised TT, albeit with more variability, and with suppressed LH levels. The patterns of TT over 24 hour period following six weeks of dosing could be fit to a non-linear function with morning elevations, mid-day troughs, and rising night-time levels. Enclomiphene and transdermal testosterone increased levels of TT within two weeks, but they had opposite effects on FSH and LH Treatment with enclomiphene did not significantly affect levels of TSH, ACTH, cortisol, lipids, or bone markers. Both transdermal testosterone and enclomiphene citrate decreased IGF-1 levels (p<0.05) but suppression was greater in the enclomiphene citrate groups.

CONCLUSIONS

Enclomiphene citrate increased serum LH and TT; however, there was not a temporal association between the peak drug levels and the Cmax levels LH or TT. Enclomiphene citrate consistently increased serum TT into the normal range and increased LH and FSH above the normal range. The effects on LH and TT persisted for at least one week after stopping treatment.

Changes in pituitary function with ageing and implications for patient care

The pituitary gland has a role in puberty, reproduction, stress-adaptive responses, sodium and water balance, uterine contractions, lactation, thyroid function, growth, body composition and skin pigmentation. Ageing is marked by initially subtle erosion of physiological signalling mechanisms, resulting in lower incremental secretory-burst amplitude, more disorderly patterns of pituitary hormone release and blunted 24 h rhythmic secretion. Almost all pituitary hormones are altered by ageing in humans, often in a manner dependent on sex, body composition, stress, comorbidity, intercurrent illness, medication use, physical frailty, caloric intake, immune status, level of exercise, and neurocognitive decline. The aim of this article is to critically discuss the mechanisms mediating clinical facets of changes in the hypothalamic-pituitary axis during ageing, and the extent to which confounding factors operate to obscure ageing-related effects.

Analysis of the impact of intravenous LH pulses versus continuous LH infusion on testosterone secretion during GnRH-receptor blockade

Gonadotrophin-releasing hormone (GnRH) pulsatility is required for optimal luteinizing hormone (LH) secretion, but whether LH pulsatility is required for physiological testosterone (T) secretion is not known. To test the postulate that pulses of recombinant human (rh) LH stimulate greater T secretion than continuous infusion of the same dose, a potent selective GnRH antagonist was administered overnight to 19 healthy men ages 18-49 yr. Subjects then received saline or rhLH intravenously continuously or as 6-min pulses intravenously every 1 or 2 h at the same total dose. Blood was sampled every 10 min for 10 h to quantify T responses. For the four interventions, the descending rank order of mean LH and mean T concentrations was 1-h = 2-h rhLH pulses > continuous rhLH > saline (P < 10(-3)). Plateau LH and T concentrations correlated positively (R(2) = 0.943, P = 0.029) as did LH concentrations and LH half-lives (R(2) = 0.962, P = 0.019). Percentage pulsatile T secretion assessed by deconvolution analysis (Keenan DM, Takahashi PY, Liu PY, Roebuck PD, Nehra AX, Iranmanesh A, Veldhuis JD. Endocrinology 147: 2817-2828, 2006) was the highest (P = 0.019), and half-time to attain peak T concentrations was the shortest (P < 10(-6)), for 1-h rhLH pulses. Approximate entropy (a pattern-regularity measure) revealed more orderly T secretion for 1- than 2-h rhLH pulses (P = 0.0076). Accordingly, a pulsatile LH signal, while not obligatory to maintain mean T concentrations, controls the mean plasma LH concentration and determines quantifiable patterns of T secretion. These data introduce the question whether blood T patterns in turn supervise distinctive target-tissue responses.

Older men exhibit reduced efficacy of and heightened potency downregulation by intravenous pulses of recombinant human LH: a study in 92 healthy men

Direct sampling of the human spermatic veins has disclosed concomitant LH and testosterone (T) pulses, suggesting pulsatile LH concentration-dependent stimulation of T secretion. However, studies to date have examined this hypothesis using only pharmacological stimulation with hCG. The present study tests the hypothesis that age is marked by decreased T secretory responses to repeated near-physiological iv pulses of recombinant human LH administered in a Clinical Translational Science Center. Participants included 92 healthy men aged 18-75 yr with BMI 18-34 kg/m(2). The contribution of endogenous LH pulses was minimized by combined injection of a selective GnRH receptor antagonist sc and successive pulses of biosynthetic LH iv. A new analytical dose response model was applied to estimate the properties of exogenous LH's drive of T secretion. Regression of LH-T dose response potency estimates on age showed that the efficacy of pulses of biosynthetic LH progressively decreased with age (P = 0.014, r = 0.26). Testis sensitivity to exogenous LH pulses also declined with age (P = 0.011, r = 0.27). Moreover, estimated Leydig cell downregulation by LH pulses rose significantly with age (P = 0.039, r = 0.22). These outcomes were selective, since the recovery potency of infused LH was not affected by age but was reduced by increasing BMI (P = 0.011, r = 0.27). Assuming stable bioactivity of infused recombinant human LH, these novel data indicate that factors associated with age and BMI attenuate LH efficacy and testis sensitivity and augment Leydig cell downregulation in healthy men.

Noninvasive analytical estimation of endogenous GnRH drive: analysis using graded competitive GnRH-receptor antagonism and a calibrating pulse of exogenous GnRH

Homeostatic control of endocrine systems proceeds via feedforward (agonistic, stimulatory) and feedback (antagonistic, inhibitory) interactions mediated via implicit dose-response functions. However, neither the feedback/feedforward pathways nor the dose-response interfaces are directly observed in vivo. Thus, the goal was to formulate and estimate an ensemble construct of time-varying feedback/feedforward interactions among GnRH, LH, and testosterone (T) in the male gonadal axis. The new analytical model revises and extends an earlier construct by: 1) allowing systemic T concentrations to inhibit hypothalamic GnRH output; 2) estimating GnRH outflow after injection of a calibrating pulse of biosynthetic GnRH; 3) framing the pituitary response to GnRH as a secretory burst, rather than continuous LH release; and 4) regressing feedback and feedforward ensemble parameters on age, rather than evaluating age dichotomously. Application of this methodology in 21 men aged 23-72 yr unveiled age-related 1) diminution of GnRH efficacy normalized for the decline in free T with age (P = 0.016), 2) potentiation of maximal T feedback onto (inhibition of) GnRH secretion (P = 0.006), and 3) accentuation of hypothalamic GnRH's sensitivity to T repression (P = 0.003). Outcomes were specific, because injected GnRH agonist and antagonist concentrations were invariant of age. We conclude that combining experimental and analytical strategies may provide a noninvasive means to investigate and decipher feedback determinants of unobserved endocrine signal(s).

Introduction to the interaction between gonadal steroids and the central nervous system

The sex steroids are frequently referred to as the gonadal steroids and are erroneously assumed to be exclusively linked to the ovaries in women or the testes in men and the functions of the reproductive tract. This chapter will provide an overview of some of the extragonadal effects of these hormones, focusing on the central nervous system, and the mechanisms of hormone action. Hormone synthesis and metabolism within the CNS will be discussed with particular focus on the role of aromatase. Sex steroids exert many of their effects via intracellular receptors and these genomic responses tend to be slow in onset, however, some responses to steroids occur more quickly and are mediated via membrane receptors and involve interactions with many different transduction pathways to produce a diverse array of responses. These complexities do pose challenges but also offer opportunity for novel approaches for therapeutic exploitation as the pharmacological tools with which to modulate systems become increasingly available.

Analytical construct of reversible desensitization of pituitary-testicular signaling: illustrative application in aging

Luteinizing hormone (LH) administered in pharmacological amounts downregulates Leydig cell steroidogenesis. Whether reversible downregulation of physiological gonadotropin drive operates in vivo is unknown. Most of the analytical models of dose-response functions that have been constructed are biased by the assumption that no downregulation exists. The present study employs a new analytical platform to quantify potential (but not required) pulsatile cycles of LH-testosterone (T) dose-response stimulation, desensitization, and recovery (pulse-by-pulse hysteresis) in 26 healthy men sampled every 10 min for 24 h. A sensitivity-downregulation hysteresis construct predicted marked hysteresis with a median time delay to LH dose-response inflection within individual T pulses of 23 min and with median T pulse onset and recovery LH sensitivities of 1.1 and 0.10 slope unit, respectively (P < 0.001). A potency-downregulation model yielded median estimates of one-half maximally stimulatory LH concentrations (EC(50) values) of 0.66 and 7.5 IU/l for onset and recovery, respectively (P < 0.001). An efficacy-downregulation formulation of hysteresis forecasts median LH efficacies of 20 and 8.3 ng·dl(-1)·min(-1) for onset and offset of T secretory burst, respectively (P = 0.002). Segmentation of the LH-T data by age suggested greater sensitivity, higher EC(50) (increased LH potency), and markedly (2.7-fold) attenuated LH efficacy in older individuals. Each of the three hysteresis models yielded a marked (P < 0.005) reduction in estimated model residual error compared with no hysteresis. In summary, model-based analyses allowing for (but not requiring) reversible pituitary-gonadal effector-response downregulation are consistent with a hypothesis of recurrent, brief cycles of LH-dependent stimulation, desensitization, and recovery of pulsatile T secretion in vivo and an age-associated reduction of LH efficacy. Prospective studies would be required to prove this aging effect.

Age in men does not determine gonadotropin-releasing hormone's dose-dependent stimulation of luteinizing hormone secretion under an exogenous testosterone clamp

BACKGROUND

Aging is associated with a decline in incremental LH pulse amplitude, which could be due to decreased GnRH secretion or impaired GnRH action.

HYPOTHESIS

Inconsistent published studies of GnRH action in older men may be due to disparate sex-steroid milieus.

FACILITY

This study was conducted at a clinical translational-research unit.

SUBJECTS

We studied 16 healthy men (8 young men and 8 older men).

METHODS

An overnight transdermal testosterone (T) clamp was implemented before randomly ordered injections of 0, 2.5, 10, 25, 250, and 750 ng GnRH on separate days (96 study sessions).

OUTCOMES

LH responses were quantified by variable-waveform deconvolution analysis.

RESULTS

The T clamp maintained age-invariant mean concentrations of total, bioavailable, and free T, SHBG, LH, FSH, and prolactin. By two-way analysis of covariance, GnRH dose (P < 0.001) but not age (0.15 < or = P < or = 0.83) determined mean, peak, incremental, and pulsatile LH responses. Statistical power (median) was 95, 98, 90, and 99% to detect a 30% or greater age contrast at P < or = 0.05 in mean, peak, incremental, and pulsatile LH responses, and greater than 99% to detect a 30% or greater age contrast in bioavailable or total T concentrations. Higher GnRH doses (P < 0.001) abbreviated LH secretory bursts in both age groups.

CONCLUSION

In the face of eugonadal concentrations of total, bioavailable, and free T, young and older men exhibit remarkably similar LH responses to a 300-fold dose range of exogenous GnRH. Accordingly, previously reported disparate effects of age on GnRH action may reflect in part age-discrepant sex-steroid milieus.