Endocrine Status Versus Chronologic Age as Predictors of Altered Luteinizing Hormone Secretion in the “Aging” Rat*

The spontaneous luteinizing hormone (LH) surge on proestrus as well as the steroid-induced LH surge and hypersecretion of LH following ovariectomy are attenuated in middle-aged female rats when compared to their young counterparts. It is generally assumed that the lower titers of serum LH measured in aging animals result, in part, from age-related alterations in luteinizing hormone-releasing hormone (LHRH) neurosecretion, yet no direct measurements of LHRH release from the hypothalamus of aging females are currently available. The present study utilized the push-pull perfusion technique to characterize and compare in vivo LHRH output from the mediobasal hypothalamus of ovariectomized middle-aged and young females during a steroid-induced LH surge. Twelve-minute perfusates were collected for a period of 6 h from middle-aged and young animals outfitted with push-pull cannulae resting in the mediobasal hypothalamus in close proximity to the median eminence. The overall pattern of LHRH release differed in the two age groups. Mean levels of LHRH measured in brain perfusates from middle-aged females were significantly lower than mean levels measured in young animals. Moreover, mean levels of LHRH detected in perfusates during the 1- and 2-hour intervals prior to and the 1-hour interval after the time of maximal LHRH output were lower in middle-aged animals, and a larger percentage of samples from middle-aged females contained levels of LHRH below the detectability of the assay. The mean amplitude of identified LHRH pulses and maximal LHRH output, however, did not differ in the two age groups. The alterations in hypothalamic LHRH output observed in middle-aged females in the present study would be expected to interfere with self-priming of the pituitary gonadotrophs by LHRH and might therefore contribute to diminished LH release.

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Alterations in Luteinizing Hormone-Releasing Hormone Release from the Mediobasal Hypothalamus of Ovariectomized, Steroid-Primed Middle-Aged Rats as Measured by Push-Pull Perfusion

Rubin

Bridges²

1989

“…Middle-aged animals have been shown to present substantial changes in the regulation of LH compared to young females in the afternoon of proestrus [13,17,30,53]. It is interesting to notice that the pituitary LHRH-R in crease observed on the second day of diestrus in young animals is abolished in middle-aged cycling rats; further more, maximal proestrus values were lower in this experi mental group.…”

Section: Discussionmentioning

confidence: 74%

“…Our results are in agreement and extend previous fin dings of Marian et al [40] reporting a significant reduction in LHRH-R capacity in the pituitary from old CVC ani mals. Such decrease of LHRH binding sites appears to be a consequence of the endocrine status, the CVC condition it self [13,40], Neural aging has also been studied in terms of capacity of the pituitary gonadotroph to respond to direct hypotha lamic stimulation [15]. It sems interesting to notice in young control animals a different response of the pituitary LHRH-R to ES, according to the phase of the estrous cycle.…”

Section: Discussionmentioning

confidence: 99%

Opposite Changes of Pituitary and Ovarian Receptors for LHRH in Ageing Rats: Further Evidence for a Direct Neural Control of Ovarian LHRH Receptor Activity

Marchetti

Cioni

1988

Hypophyseal and ovarian receptors for the neurohormone LHRH (LHRH-R) have been measured in young (3–4 months), middle-aged (8–11 months), constant estrous (CE, 10–14 months) and pseudopregnant (PR, 16–18 months) rats in order to study whether changes in hypothalamic and/or peripheral LHRH-like peptide production might precede and/or accompany the onset of reproductive failure observed in aging rats. Furthermore, we have investigated whether the neural efferent system from the brain to the ovary is affected with aging. The pattern of pituitary LHRH-R modifications during the estrous cycle of middle-aged rats shows lower LHRH-R levels on the second day of diestrus, resulting in a shift of the maximal LHRH binding capacity in the morning of proestrus. On the other hand, when comparing pituitary LHRH-R of animals exhibiting constant vaginal cornification (CVC) or repetitive PP with young estrus rats, no significant difference could be observed. Young rats responded to electrical stimulation (ES) of the medial preoptic area with an acute elevation of LHRH-R while ES performed in CVC, or PP animals resulted in a significant increase of hypophyseal LHRH binding capacity similar to the one observed in young estrous controls, indicating an impairment in the neural signals impinging in the pulse generating system, in old rats, and not an intrinsic defect of the LHRH-R per se. Ovarian LHRH-R concentration is higher in middle-aged cycling animals on the day of vaginal proestrus, compared to levels measured in young animals at any phase of the estrous cycle. Similarly, CE rats displaying CVC as well as PP animals show significantly higher numbers of LHRH-R with no change in affinity, than young estrus rats. CVC and PP rats receiving unilaterally an intraovarian injection of the potent LHRH antagonist, Ac-D-Cl-Phe^1,2, D-Trp³, D-Phe⁶, D-Ala¹⁰, LHRH, showed an acute drop of LHRH-R measured within the treated ovary with no significant changes taking place in the vehicle-treated contralateral gland, suggesting that changes of endogenous ovarian LHRH-like peptide might participate in the mechanism(s) responsible for LHRH receptor increase observed in aging rats. In order to investigate the participation of a direct neural efferent pathway in ovarian LHRH-R regulation, young and old rats were subjected to spinal cord transection (above T10–T11). Bilateral transection of the spinal cord in young animals in the morning of proestrous markedly increased ovarian LHRH-R concentration in the afternoon (17.00 h) of the same day. When spinalectomy was performed unilaterally, a marked increase of ovarian LHRH-R capacity in the gonad ipsilateral to the intervention was measured, with no effect taking place in the contralateral gland. On the other hand, the same intervention did not modify the already stimulated LHRH-R levels measured in old rats. It is suggested that an increase of ovarian LHRH-R activity might trigger the neuroendocrine mechanisms leading to reproductive dysfunction in aging female ...

“…Investigations of the underlying mechanisms have indi cated the possibility that significant changes might be tak ing place at the hypothalamic and/or pituitary level. The response of the pituitary gland to gonadotropin-releasing hormone (GnRH) infused into the vascular system of mid dle-aged rats has been found to be lower than that of young rats [5,6]. Early in vitro studies showed that pituitary glands from middle-aged rats released less LH in response to a prolonged infusion of GnRH [7], In more recent stud ies, we have shown that pituitary tissue from the specific sub-population of middle-aged rats with attenuated LH surges shows a decreased response to pulses of GnRH [8].…”

Section: Introductionmentioning

confidence: 99%

Luteinizing Hormone Secretion by Dispersed Anterior Pituitary Gland Cells from Middle-Aged Rats

Krieg

Brito²,

Sayles³

et al. 1995

Several studies have shown that a certain subpopulation of middle-aged rats have an attenuated preovulatory LH surge, and that this is an early indicator of a general decline in reproductive cyclicity. The pituitary glands of middle-aged animals with attenuated LH surges have been shown to be significantly less responsive to GnRH than those from young (Y) animals. The present study was designed to investigate the relationship between the in vivo LH surge and the in vitro secretory capacity of dispersed anterior pituitary gland cells from the same animals. Individual LH-secreting cells were also studied with respect to their LH secretory capacity and proportionate numbers in the pituitary gland using reverse hemolytic plaque assay (RHPA). Two groups of animals, young (4–5 months old) and middle-aged (10–12 months old), were sampled on the afternoon of proestrus to determine the amplitudes of their preovulatory LH surges. The middle-aged animals were then classified as either normal (MAN) or attenuated (MAA) with regard to the peak levels of the surges. On the next proestrus, dispersed anterior pituitary gland cells were prepared for either perifusion or RHPA. Perifused anterior pituitary gland cells were exposed to three 2.5-min pulses of GnRH (30 nM) at 60-min intervals. In the RHPA, red blood cells were treated with protein-A to allow coating with rabbit antiovine LH-beta, and then co-incubated with anterior pituitary gland cells. The cells were then treated with one of several concentrations of GnRH (0, 0.1, 1.0, 10, or 100 nM). GnRH responsiveness of dispersed anterior pituitary gland cells from MAA animals was decreased compared to cells from Y animals. Comparison of MAA to Y animals showed that proportional LH release was greater in vitro than in vivo. This corroborated results from previous studies using perifused pituitary fragments. Responsiveness of individual LH-secreting cells, as indicated by mean plaque area, was not different between Y and middle-aged rats at 12.00 h. However, the percentage of anterior pituitary gland cells from MAA animals that formed LH plaques were significantly lower than those from Y animals at 12.00 h, under both basal and GnRH-stimulated conditions. These results verify that the pituitary glands of MAA animals are less responsive than those of Y animals, and imply potential changes in GnRH output from the hypothalamus in vivo. They also indicate that the decreased responsiveness of pituitary tissue from MAA animals may be due more to a decrease in the percentage of anterior pituitary gland cells that release LH than to a decrease in LH responsiveness of individual cells.