Effects of Age and Testicular Function on the Pituitary–thyroid System in Male Rats

Chen, H. J.; Walfish, P. G.

doi:10.1677/joe.0.0820053

Cited by 40 publications

(15 citation statements)

References 9 publications

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“…It is well recognized that castration decreases levels of TSH in peripheral blood of male rats [27], The castrationinduced reduction of TSH levels is completely reversed by testosterone substitution [27] (present study). Pekary et al [28] reported that castration increased TRH (as deter mined by RIA and HPLC analysis) in the PP, while reduc ing in vitro TRH release from the hypothalamus.…”

Section: Discussionsupporting

confidence: 59%

Regulation of Thyrotropin-Releasing Hormone in the Posterior Pituitary

Rondeel

Klootwijk²,

Linkels³

et al. 1995

Neuroendocrinology

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Although the presence of thyrotropin-releasing hormone (TRH) in the posterior pituitary (PP) was reported more than one decade ago, knowledge on its origin, regulation and functional significance is lacking. In the present study we investigated the regulation of TRH in the rat PP. Analysis by specific RIA, anion and cation exchange chromatography and reverse-phase HPLC showed that all TRH immunoreactivity in the PP is accounted for by authentic TRH. Induction of hyperthyroidism with thyroxine increased levels of TRH in the PP by 20%, whereas in methimazole-treated, hypothyroid rats the content decreased by 25% versus untreated, euthyroid controls. Food deprivation for 3 days increased levels by 35% and refeeding completely normalized TRH content again. Also 14–17 days after castration, TRH in the PP was increased by 25% while testosterone substitution prevented this increase. Castration did not affect proTRH mRNA levels in the hypothalamus. One week after adrenalectomy or daily subcutaneous dexamethasone injections, TRH content in the PP was not affected. Treatment with disulfiram, an inhibitor of the peptidylglycine α-amidating monooxygenase (PAM), reduced levels of TRH in the PP by 20%. ProTRH and PAM mRNA levels were not affected in the hypothalamus by this treatment. Since TRH in the PP has been suggested to play a role in prolactin (PRL) release, we determined the content of TRH in the PP during a 6-hour suckling stimulus that increased PRL levels in peripheral blood 30-fold. Whereas TRH in the median eminence increased by 35%, 6 h after the initiation of suckling, TRH levels in the PP remained constant. In conclusion, (1) authentic TRH accounts for all TRH immunoreactivity in the PP, (2) TRH content in the PP is increased in conditions in which hypothalamic TRH release is reportedly decreased (hyperthyroidism, starvation, castration) and decreased when TRH release is increased (hypothyroidism) – its levels may thus be used as a parameter of TRH content in hypothalamic hypophysiotropic neurons; (3) the changes in TRH levels in the PP in conditions that alter thyroid-stimulating hormone (TSH) levels in peripheral plasma suggest a role in TSH secretion, and (4) TRH in the PP does not seem to be important for the suckling-induced PRL release.

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Section: Discussionsupporting

confidence: 59%

Regulation of Thyrotropin-Releasing Hormone in the Posterior Pituitary

Rondeel

Klootwijk²,

Linkels³

et al. 1995

Neuroendocrinology

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“…Age-related decreases in hypothalamic tuberoinfundibular dopamingergic neuronal activity contribute to in creased basal and stimulated PRL levels in old rats [41,42], Because testosterone inhibits these neurons during stress [43], and circulating levels of testosterone decrease with age [44], alterations in dopaminergic and androgenic inputs may have contributed to the exaggerated PRL response to Immo in old rats in this study. Failure of the ultrashort loop negative feedback mechanism of PRL secretion in old rats might also be a contributing factor [45].The increased levels of immunoreactive PRL in old rats during Immo do not necessarily correspond to in creased biological effects, as the biological activity of PRL declines with age [46], In the current study, the levels of immunoreactive GH were similar at baseline and were significantly inhibited by Immo in both young and old rats.…”

Section: Discussionmentioning

confidence: 63%

Stress-Induced Inhibition of the Hypothalamic-Pituitary-Thyroid Axis Is Attenuated in the Aged Fischer 344/N Male Rat

et al. 1995

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Aging is associated with a progressive decrement in the basal activity of the hypothalamic-pituitary-thyroid axis in male Fischer 344/N rats. The aim of this study was to determine whether stress influences the activity of this axis in young and old rats. As prolactin and growth hormone share some regulatory mechanisms with thyrotropin-releasing and thyroid-stimulating hormones, which are influenced by stress, the plasma levels of these two hormones were also determined during immobilization (Immo). To accomplish this, young (3-month-old) and old (23-month-old) male 344/N Fischer rats were immobilized for 2 h; blood was collected by cannulation from the tail artery at different intervals during Immo (0, 5, 30, 60, and 120 min), and brains were removed at the end of Immo. The basal plasma levels of thyroid-stimulating hormone were similar in both groups, but were significantly and progressively inhibited by Immo in young, but not in old rats. The baseline plasma levels of total triiodothyronine were slightly lower in old than in young rats; Immo caused a significant decrease of total triiodothyronine levels only in the young animals. The baseline plasma levels of free triiodothyronine were similar and were not altered by Immo in either age group. The paraventricular nucleus thyrotropin-releasing hormone mRNA levels were lower in old than in young rats under basal conditions; under stress they were significantly inhibited in young, but remained unchanged in old rats. The basal thyroid-stimulating hormone beta mRNA levels in the anterior pituitary were significantly lower in old than in young rats, but were not affected by Immo in either age group. The plasma prolactin levels were similar at baseline and were significantly increased by Immo in both age groups, but significantly more in old than in young rats. The plasma growth hormone levels were also similar at baseline; they were significantly decreased by Immo to a similar extent in both age groups. In summary, these data indicate that the stress-induced decrease in plasma thyroid-stimulating hormone is in part mediated at the level of the hypothalamic thyrotropin-releasing hormone neuron and that this phenomenon is attenuated in the aged rat.

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“…Intact male rodents exhibit wide individual differences in androgen production as a function of such factors as age (Chen and Walfish, 1979;Gray, 19781, exposure to a female (Taylor et al, 1983a), and prior sexual experience (Taylor et al, 1983b). The present findings suggest that different suprathreshold levels of TP can provoke different structural changes in the penes of male rats.…”

Section: Discussionmentioning

confidence: 99%

Light and scanning electron microscopic study of testosterone‐restored penile papillae in castrated rats

1983

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Structural changes in various penile tissue from rats with differing circulating androgen levels were analyzed with light and scanning electron microscopy. Adult males were castrated and daily injected for 6 weeks with one of four physiologic amounts of testosterone propionate (TP). Results indicated that epithelium, subepithelial connective tissue, and the papillae protruding from the surface of the glans penis responded, more or less, in a dose-dependent fashion to the steroid. The least androgen-sensitive penile tissue was the epidermal stratum corneum, more sensitive was the maligiphian layer, and more sensitive still was the subepithelium. The penile papillae and their follicles were the most androgen-sensitive tissue. Specifically, the high testosterone, 400 micrograms and 800 micrograms TP, males experienced papillae that were thicker, taller, more densely arranged, and more highly keratinized than the lower testosterone, 100 micrograms and 200 micrograms TP, males. The function of the papillae may be to provide the female with the mechanical stimulation necessary to ensure a proper hormonal milieu for successful impregnation.

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Effects of Age and Testicular Function on the Pituitary–thyroid System in Male Rats

Cited by 40 publications

References 9 publications

Regulation of Thyrotropin-Releasing Hormone in the Posterior Pituitary

Regulation of Thyrotropin-Releasing Hormone in the Posterior Pituitary

Stress-Induced Inhibition of the Hypothalamic-Pituitary-Thyroid Axis Is Attenuated in the Aged Fischer 344/N Male Rat

Light and scanning electron microscopic study of testosterone‐restored penile papillae in castrated rats

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