Mechanisms of age-related changes in gonadotropin-releasing hormone receptor messenger ribonucleic acid content in the anterior pituitary of male rats

Molecular correlates of hypothalamic development in songbird ontogeny in comparison with the telencephalon

Development of the songbird brain provides an excellent experimental model for understanding the regulation of sex differences in ontogeny. Considering the regulatory role of the hypothalamus in endocrine, in particular reproductive, physiology, we measured the structural (volume) and molecular correlates of hypothalamic development during ontogeny of male and female zebra finches. We quantified by relative quantitative polymerase chain reaction (rqPCR) the expression of 14 genes related to thyroid and steroid hormones actions as well as 12 genes related to brain plasticity at four specific time points during ontogeny and compared these expression patterns with the expression of the same genes as detected by transcriptomics in the telencephalon. These two different methodological approaches detected specific changes with age and demonstrated that in a substantial number of cases changes observed in both brain regions are nearly identical. Other genes however had a tissue-specific developmental pattern. Sex differences or interactions of sex by age were detected in the expression of a subset of genes, more in hypothalamus than telencephalon. These results correlate with multiple known aspects of the developmental and reproductive physiology but also raise a number of new functional questions.

Aging attenuates the pituitary response to gonadotropin-releasing hormone

CONTEXT

Complex changes in GnRH secretion occur with aging in women, but little is known about the effect of aging on the pituitary per se.

OBJECTIVE

The aim of the study was to determine whether pituitary responsiveness to GnRH is attenuated with aging.

DESIGN AND SETTING

A GnRH antagonist and graded doses of GnRH were used to isolate pituitary responsiveness in Clinical Research Center studies at an academic medical center.

SUBJECTS

Subjects were healthy postmenopausal women (PMW) aged 48-57 yr (n = 10) or 70-77 yr (n= 9).

INTERVENTIONS

A suppressive dose of the NAL-GLU GnRH antagonist (150 microg/kg sc) was administered and was followed by GnRH doses of 25, 75, 250, or 750 ng/kg iv every 4 h.

RESULTS

The LH response to GnRH was attenuated with aging (P = 0.05) with an interaction between age and dose (P = 0.01) such that the LH amplitude was less in older PMW at the higher doses (250 ng/kg, 50 +/- 9 vs. 29 +/- 4.9 IU/liter, for young and old PMW, respectively, P = 0.02; and 750 ng/kg, 97.7 +/- 11 vs. 70.2 +/- 9.3 IU/liter, P = 0.002), but not the lower doses of GnRH. The FSH response to GnRH was also attenuated with aging in PMW (P = 0.005).

CONCLUSIONS

In studies that isolated the pituitary from endogenous GnRH stimulation, aging attenuated the LH and FSH responses to exogenous GnRH in PMW. These studies indicate that the pituitary plays a role in the decline in gonadotropin levels with aging, further supporting the potential contribution of age-associated changes in both hypothalamic and pituitary function to reproductive senescence.

Effects of ageing and exogenous melatonin on pituitary responsiveness to GnRH in ewes during anestrus and the reproductive season

The study examined the effect of melatonin implants on in vivo pituitary responsiveness to GnRH in control, fully productive (5.7+/-0.4 years old, n=17) and aged (10.7+/-0.3 years old, n=14) ovariectomized, estradiol-treated Rasa Aragonesa ewes. On 27 February, eight ewes in each age group received a single implant containing 18 mg melatonin. On 10 April, blood samples to be assayed for LH were collected at 10-min intervals over 4h (starting at 09:00 and 22:00 h). After samples 6 and 18 were collected, ewes received a single i.v. injection of GnRH (20 ng/kg liveweight). The pituitary response to GnRH was assessed using the difference between plasma LH concentrations before and after (highest value) each injection (DLH1, DLH2)), and the area under the LH response curve for 1h after each GnRH injection (AUC1, AUC2). On 23 September, the previously implanted ewes received a new melatonin implant and, on 17 November, all of the ewes were subjected to the same diurnal and nocturnal sampling protocols, again. Generally, non-implanted aged ewes exhibited a lower pituitary response to GnRH than did non-implanted control ewes, particularly in November and after the first injection (P<0.05 for DLH1 and AUC1 in both the diurnal and nocturnal tests). The response was significantly affected by the interaction of age and melatonin treatment, particularly in the diurnal tests (P<0.1 for DLH1 and AUC1, and P<0.05 for AUC2 in April; P<0.05 for DLH1, AUC1 and AUC2 in November), which indicated that exogenous melatonin increased LH levels after GnRH injections in aged ewes compared to non-implanted ewes, this effect being the opposite in control females. Thus, melatonin can restore in ewes the functionality of the neuroendocrine system, after it has been reduced by senescence.

Changes in expression of hypothalamic releasing hormone receptors in individual rat anterior pituitary cells during maturation, puberty and senescence

Anterior pituitary (AP) is formed by five different cell types, each one producing a different AP hormone whose secretion is regulated by a specific hypothalamic-releasing hormone (HRH). On the other hand, a significant number of AP cells express multiple HRH receptors (multiresponsive cells). Plastic changes in expression of HRH receptors in individual AP cells are involved in critical endocrine events. Here we have characterized the changes in functional responses to CRH, LHRH, TRH, and GHRH in individual AP cells throughout the whole life span of the rat. To this end, calcium responses to the HRHs were followed by single-cell imaging in freshly dispersed AP cells prepared from rats of different ages (0-540 postnatal days). Three different cell pools were identified: 1) monoresponsive cells, holding a single class of HRH receptor; 2) multiresponsive cells; and 3) nonresponsive cells. The relative abundance of each pool changed with age. Nonresponsive cells were abundant at birth, multiresponsive cells were abundant at puberty, and monoresponsive cells dominated at senescence. The relative abundance of each HRH receptor changed largely with age but not gender. In addition, the contribution of monoresponsive and multiresponsive cells to responses to each HRH changed very much with age. Thus, the anterior pituitary shows large changes in cell populations typed by functional responses to HRHs during maturation, puberty, and senescence.