Dynamic Pituitary and Ovarian Changes Occurring during the Anestrus to Estrus Transition in the Golden Hamster*

Abstract

Anestrous hamsters exhibit daily afternoon gonadotropin and progesterone surges, but little estrogen secretion. In the first experiment, short day anestrous females were transferred to long days to detect hormonal changes associated with recovery of cyclicity. Morning and afternoon blood samples were taken at increasing durations in long days. Females autopsied at their first vaginal estrus after transfer to long days differed from long day estrous controls only in their lower uterine weights. Some females at all durations exhibited signs of recovery, though they had not yet shown estrus. They did not display afternoon gonadotropin surges and had low circulating progesterone but high estradiol levels, stimulated uteri, and enhanced follicular development. Results of the second and third experiments provided evidence that the daily gonadotropin surges are not the cause per se of anestrus, and that changes in estrogen secretion are essential for the transition to and from anestrus. Phenobarbital blockade of the daily surges in anestrous females did not result in increased follicular growth and estrogen secretion. Furthermore, daily afternoon injections of gonadotropins it appears that the daily surges in anestrous hamsters simply reflect low estradiol levels. It is still not known what signal promotes rapid follicular maturation during the recovery from anestrus. Once initiated, however, this recovery appears to occur within a few days, with a rapid cascade of events. First, follicular development and estrogen secretion resume, and the daily LH surges cease. Then, progesterone levels decline, and an ovulatory surge of gonadotropins is triggered.

Effects of Daily Afternoon Melatonin Administration on Monoamine Accumulation in Median Eminence and Striatum of Ovariectomized Hamsters Receiving Pargyline

“…Concurrent with this anestrous condition, reduced circulating levels of thyroid hormones may also be observed [3,4], This melatonin-induced anestrous condition is similar to that pro duced by short photoperiod [5],…”

Serum Luteinizing Hormone, Prolactin, and Thyrotropin and Their Pituitary Subunit mRNA Levels during Proestrus in the Syrian Hamster

“…Although the pattern is re miniscent of that seen in anovulatory hamsters induced by short photoperiod, the magnitude of the daily increase in serum LH levels in anovulatory animals would fall between those observed normally for proestrus and non-proestrus [25]. In addition, the animals used in this study exhibited normal signs of cyclicity including a copious vaginal discharge at estrous and a proes trous hormone surge every 4 days.…”

“…LHRH and steroid hormones have been shown to regulate pep tide synthesis and glycosylation of hormone peptide subunits [8,26]. Studies of gonadotropin subunit RNAs have revealed that alterations in steady-state levels occur in response to LHRH and the gonadal steroids [25]. Recent reports on LH, FSH, and Prl mRNAs in rats have shown that these transcripts change significantly during the estrous cycle [7,10,11], This is the first report of «, LH|1, Prl, and TSH|5 RNA levels in the female hamster over the 4-day estrous cycle.…”

Food restriction during development delays puberty but does not affect adult seasonal reproductive responses to food availability in Siberian hamsters (Phodopus sungorus)

“…In females, adult FR decreased the proportions of individuals exhibiting normal estrous cycles and uterine horn mass regardless of juvenile food availability. The difference in estrous cyclicity was observed in the absence of changes in basal serum LH concentrations, which is not surprising because morning, or basal, LH levels are very low (<1 ng/ml), near the level of assay detectability, and are not different between noncycling and cycling female Syrian hamsters (Jorgenson & Schwartz, 1987). It is important to note that in view of the pulsatile nature of LH secretion, our sampling regimen, three single blood samples in a 6–7 week period, was too infrequent to detect any changes in LH pulse frequency or amplitude that may have occurred during development and adult FR, and that may be physiologically relevant to HPG axis function.…”