Reproductive Failure due to Experimentally Induced Constant Estrus Does Not Alter the LH-RH Fiber Density in the Median Eminence of the Rat

Abstract

The effects of anterior hypothalamic deafferentation on luteinizing hormone releasing hormone (LH-RH) fiber density in the mediobasal hypothalamus (MBH) were compared to those of a number of nonsurgical treatments which give rise to anovulatory sterility (injections of estradiol valerate, exposure to constant light, or neonatal androgen administration) in the female rat. All of the treatments used (surgical and nonsurgical) disrupted the normal 4-day pattern of estrous cyclicity. Bilateral anterior hypothalamic deafferentation markedly reduced the packing density of LH-RH fibers in the MBH. Unilateral deafferentation reduced the number of fibers on the ipsilateral side of the MBH by 31–64% and on the contralateral side by 15–40%. In contrast, none of the three nonsurgical treatments had any significant effect on the LH-RH fiber density. Electron microscopic examination revealed no morphologic, or distribution differences in MBH LH-RH fibers between control females and animals rendered anovulatory by the nonsurgical experimental procedures. These results demonstrate that estrogen, androgen, and constant light induced anovulatory sterility are not associated with any overt change in the number or morphology of LH-RH immunoreactive fibers in the MBH, suggesting that the primary lesion responsible for the failure of normal estrous cyclicity in such animals resides in the systems responsible for regulating the activity of the LH-RH neurons rather than in the LH-RH neurons themselves.

Novel role of the anorexigenic peptide neuromedin U in the control of LH secretion and its regulation by gonadal hormones and photoperiod

“…Second, from a functional standpoint, our hormone tests demonstrate that LH responses to NMU in females under constant illumination are not similar to (but rather opposite) those observed in OVX rats, as augmented stimulatory responses were observed in the former, whereas in gonadectomized females NMU inhibited LH secretion after its central administration. Overall, on the basis of our current data, it is highly plausible that changes in photoperiodic cues have a primary impact on hypothalamic NMU signaling, which might contribute to the central dysregulation of GnRH secretion caused by constant illumination in the female rat (25). Nonetheless, the relative importance of additional peripheral events, such as changes in ovarian sex steroid secretion, in this phenomenon merits further investigation.…”

Fos Expression in Luteinizing Hormone-Releasing Hormone Neurons of Guinea Pigs, with Knife Cuts Separating the Preoptic Area and the Hypothalamus, Demonstrating Luteinizing Hormone Surges1

“…The large number of LHRH fibers present in the caudal median eminence probably represents remaining deafferented processes from rostral LHRH neurons [10,23], as well as processes emanating from caudal LHRH neurons that were not disturbed by the knife cuts. It is clear that in the present study, some of the processes in the caudal median eminence of knife-cut guinea pigs were capable of releasing LHRH, given that a surge-like pattern of LH secretion was induced by steroid treatment.…”

Effects of Hypothalamic-Pituitary Aging on Reproduction

“…It has been shown by quantitative immunohistochemical staining that the concentration of GnRH in median eminence tissues obtained from older persistent estrous rats are indistinguishable from those obtained from young normal cyclicing rats, indicating that control of the GnRH release mechanism is impaired rather than decreased synthesis and/or storage during aging. 39 This notion is supported by the fact that the surge secretion of LH is abnormal in both middleaged male and female rats, implying abnormal GnRH secretion. [40][41][42] Gonadal and pituitary function in aged, persistently estrous rats can be restored temporarily by pharmacologic neuroendocrine treatment, which lowers estrogen and gonadotrophin levels; under these regimens the animal can sustain levels of endocrine secretion comparable to those of younger cycling females.…”