Untersuchung zur Wiederherstellung des positiven Feedback Mechanismus von Estradiol-17[beta] und zur Sekretion von Estradiol-17[beta] im Verlauf des Anöstrus der Hündin

Abstract

Investigations on the reestablishment of the positive feedback of estradiol-17[beta] and the secretion of estradiol-17[beta] during anoestrus of the bitch

The mechanisms involved in the neuroendocrine regulation of the reproduction of bitches, especially in respect to anoestrus, are still largely unknown. The objective of the work presented, therefore has been to elaborate information on the relevance of both, the negative and positive feedback of estradiol-17[beta] as factors of neuroendocrine regulation of the anoestrus.
The experiments were carried out using 5 sexually healthy beagle bitches at the age of 1 - 3 years.

With reference to the situation during heat an application scheme for estradiol-17[beta] was used resulting in a concentration curve close to that of proestrous estradiol-17[beta] concentrations. This was achieved by subcutaneous application of 1, 1,5, 5 and 5 mg estradiol-benzoate/kg b.w., respectively at times 0, 12, 24 and 48 hours. The applications within the anoestrus started 7 days following the decrease of plasma progesterone concentrations to levels < 1 ng/ml. They were repeated in 5 weeks intervals unto onset of heat. Additionally treatments were performed during the dioestrus (day 50 following onset of proestrous bleeding) and after castration of the bitches. With reference to the first estradiol-benzoate injection 4 blood samples were taken in 6 hours intervals and then until the 168. hour in 8 hour intervals. These periods are characterised as experimental phases. The bitches served individually as their own controls: in another reproduction cycle, only the vehicle was administered using the identical time regime for dosing and blood sampling. These periods are characterised as control phases. The duration of the experimental and the control phases had been fixed according to the duration of proestrous estradiol-17[beta] release and the following pre-ovulatory LH-peak.

All animals treated with estradiol-17[beta] developed heat immediately after the last experimental phase during the anoestrus. In these experiments blood sampling was continued until plasma progesterone levels were >3ng/ml. Blood sampling with bitches coming to heat during controls was performed from day 1 of proestrous bleeding until plasma progesterone levels were found >3 ng/ml.

LH, estradiol-17[beta], and progesterone were analysed using validated radioimmunoassay methods. LH and estradiol-17[beta] concentrations were determined in all samples taken during the experimental and control phases. Progesterone was analysed for in the daily 8 o'clock samples of the experimental and control phases during the dioestrus and heat, and additionally two times weekly for the whole duration of the experimental work to assess ovarial activity. These analyses were complemented by the measurement of the electric conductivity of the vaginal mucosa. LH release was characterised by its maximal value, the Area Under Curve (AUC), the basal levels, the number of peaks, and the time to first peak following treatment.

Estradiol-17[beta] release during the control phases was quantified upon calculation of the AUC, the basal levels, the maximal levels, and the number of peaks.

To establish the course of the electrical conductivity in the vaginal epithelium, the AUC and the maximal values were calculated, too. The statistical evaluation included mono- and bi-factorial analysis of variance, trend analysis and - when appropriate - the t-test. The assessment of the AUC and the maximal levels resulted in highly significant time and treatment effects for the parameter LH release. As compared to the control, treatment with estradiol-17[beta] resulted at any time of sampling within the cycle in a decrease in LH release. It decreased from dioestrus to anoestrus to slowly increase in the course of the anoestrus, and was paralleled by an increase of estradiol-17[beta] availability detected in the control phases. The application of estradiol-17[beta] did not lead to a detectable shift in the course of the cycle. The heats observed following the last experimental phase in the anoestrus immediately after estradiol-17[beta] treatment may be attributed with certain reservation to induction, especially since the pre-ovulatory LH release is different from that of the "regular" heat. This demonstrates that the negative feedback effect of estradiol-17[beta] is an important and sustaining regulator for the duration of the anoestrus.

Irrespective of the obvious desensibilisation of the hypothalamic-hypophysial system (simultaneous increase of the availability of LH and estradiol-17[beta] in the blood) in the course of the anoestrus the change to the positive feedback mechanism becomes obvious only shortly to the end of the anoestrus.

From the experiments on changing of the electrical conductivity in the vaginal mucosa it may be concluded that the availability of estrogen receptors in the vaginal epithelium plays an important role: during the anoestrus - especially the cycle phase immediately preceding heat - changes in the conductivity of the vaginal secreta could be demonstrated, which was not the case after castration and during the dioestrus.

For all animals used in the experiments the complete functionality of the hypothalamic-hypophysial-gonadal-axis may be granted since, after castration the expected increase of LH release, and following estradiol-17[beta] treatment a remarkable negative feedback occurred both.

Zur "Frontpage" des Volltextes	Zur "Giessener Elektronischen Bibliothek"
Fragen und Kommentare bitte an: koordinierung@ub.uni-giessen.de	Zuletzt geändert am 21.01.2000