Hypothalamic Luteinizing Hormone-Releasing Hormone(LHRH) and LH Secretion in Aging Female Rats.

Abstract

Age-related changes in hypothalamic luteinizing hormone-releasing hormone (LHRH) and luteinizing hormone (LH) secretion were studied in young (6 months), middle-aged (12 months) and old (18 months) female rats. The LHRH levels in the mid-hypothalamic area were higher in intact middle-aged and old females than in young ones.Additionally, there was no age difference in the hypothalamic LHRH levels in male rats. In order to clarify the significance of this age-related increase in female rats, we examined the effects of progesterone treatment in estrogen-primed ovariectomized young and old rats on the LHRH levels in the median eminence (ME) and on plasma LH levels. We found phasic changes in ME-LHRH and plasma LH levels in estrogenprimed rats following progesterone treatment in rats of both ages, but the progesterone-induced change in ME-LHRH levels tended to be delayed in old rats compared with young females.This delay may correspond to the delayed onset, slow and low magnitude of plasma LH increase in old females. The ME-LHRH levels were generally higher in old rats than in young rats. Nevertheless, we found that the increase in plasma LH in response to progesterone treatment in estrogen-primed ovariectomized females was smaller in old rats than young rats.These results suggest that the LHRH secretory mechanism changes with age in female rats.Such alterations may result in the accumulation of LHRH in the mid-hypothalamic area and an increase in ME-LHRH.

Evidence Suggesting that the Potentiating Action of Neuropeptide Y on Luteinizing Hormone (LH)‐Releasing Hormone‐Induced LH Release Remains Unaltered in Aged Female Rats

“…In the present study, a well‐characterized pentobarbital‐blocked steroid‐induced LH surge model was used to assess the pituitary responsiveness to LHRH in the absence of an influence of the endogenous hypothalamic peptide. In agreement with the previous report ( 37), steroid‐induced LH surges in old ovx CE rats were delayed and greatly reduced as compared to those in young rats. Furthermore, pulsatile LHRH administration produced a preovulatory‐like LH surge not only in the pentobarbital‐blocked young rats but also in the pentobarbital‐blocked old rats.…”

“…In this context, an abrupt increase in pituitary responsiveness to LHRH seen in young rats just before, and during the initiation of the LH surge is thought to be mediated by pituitary actions of several factors including progesterone ( 38), LHRH itself ( 39–41), neuropeptide Y ( 23, 24, 42, 43) and galanin ( 44). Thus, it is possible that any alteration in either secretion or action of LHRH ( 37, 39–41, 45, 46), neuropeptide Y ( 22) and/or other neurohormonal signals ( 6, 7) that might occur before pentobarbital administration could result in delayed and attenuated pituitary responsiveness to LHRH. In addition, the stimulatory feedback action of E2 and progesterone may be compromised in old CE rats, and a full recovery in this feedback mechanism may require an increased postovariectomy interval ( 30, 47) and/or >2 days of E2 exposure ( 34).…”