EFFECT OF ADMINISTRATION OF DOPAMINERGIC AGONISTSON THE UTERINE RESPONSIVENESS TO OESTROGEN IN

MATURING RATS

D. E. HERNANDEZ AND E. O. ALVAREZInstituto de Fisiología, Facultad de Medicina, Universidad Austral de Chile, Casilla 567,

Valdivia, Chile

(Received 2 July 1979)SUMMARY

The purpose of the present work was to investigate whether the administration of a

dopamine agonist (2-bromo-\g=a\-ergocryptine)was able to interfere with the normal change inuterine sensitivity to oestrogen that occurs around the onset of puberty in the rat.

To determine the change in uterine sensitivity groups of rats at 25, 30 or 35 days of agewere killed (to find the initial uterine weight) or ovariectomized with or without oestrogenreplacement therapy (0\m=.\05\g=m\g/100g body wt, daily for 5 days) and then killed to determinethe final uterine weight. Comparisons were made between initial and final uterine weights.Five days before the animals reached 25, 30 or 35 days of age, one group was injected with2-bromo-\g=a\-ergocryptine(0\m=.\5 mg/100gbody wt daily for 5 days) and another one withsolvent (control rats). At the ages specified (25, 30 and 35 days of age) both groups were sub-jected to the ovariectomy schedule. Results showed that the bromocriptine treatment waseffective in blocking the normal uterine change in sensitivity that occurs at 30 days of age.At 35 days of age the dopamine agonist was able to counteract the action of oestrogen inmaintaining the uterine weight 5 days after ovariectomy. The results are interpreted as

suggesting that the mechanism of change in uterine responsiveness to oestrogen in maturingrats is mediated by prolactin.

introduction

The onset of puberty in the rat is associated with a change in the functional activity of thehypothalamic-pituitary-gonadal system (McCann, Ojeda & Negro-Vilar, 1974). Among thechanges related to puberty in the female rat are an increased responsiveness of the ovaries(Zarrow & Wilson, 1961) and uterus (Liu, 1960) to stimulation with gonadotrophins andsteroid hormones respectively. Previous work in our laboratory indicated that the increasedresponsiveness of the uterus to oestradiol is ultimately dependent on a mechanism ofcentralorigin since lesions of the anterior hypothalamus which lead to a precocious onset ofpubertyare associated with an enhanced response of the uterus to oestradiol (Advis & Alvarez, 1977).Furthermore, anterior hypothalamic lesions have been shown to modify the pattern ofprolactin secretion (Alvarez, Hancke & Advis, 1977; Alvarez, 1979) and the precocious onsetofpuberty is completely abolished by the simultaneous administration to the lesioned rats of2-bromo-a-ergocryptine (Alvarez et al. 1977). These results suggest a probable role forprolactin in the induction of puberty.

The purpose of the present investigation, therefore, was to determine whether treatmentwith bromocriptine, a well-known dopaminergic agonist (Dray & Oakley, 1976), couldantagonize the natural change in uterine sensitivity to oestrogen which is associated with theonset of puberty.

MATERIALS AND METHODS

Female rats of the Wistar-Holtzman strain maintained under controlled conditions oftemperature (22 ± 2 °C) and light (lights on 05.00-19.00 h) were used. Daily i.p. injections of2-bromo-a-ergocryptine methanesulphonate (bromocriptine; Sandoz A. G., Basel,Switzerland; 0-5mg/100g body weight) dissolved in 1ml 0-5% tartaric acid in 1%ethanol-saline were given for 5 days starting at 20, 25 or 30 days of age. Control rats weretreated similarly with vehicle alone. At 25,30 and 35 days of age half the rats were killed andthe uterus was dissected and weighed (initial weight) to the nearest 0-5 mg. The remaininganimals were ovariectomized by the lateral approach under light ether anaesthesia anddivided into two groups which were either given or not given a replacement dose ofoestradiol (oestradiol valerate, Primofol Depot; Schering, Berlin; 005 pg/100 g body weightfor 5 days). Five days after ovariectomy all the animals were killed and their uterine weightsdetermined (final weight).

The significance of differences between the uterine weights in the respective groups weredetermined using a one-way analysis of variance followed by Duncan's New Multiple RangeTest (Duncan, 1955). A value of <005 was considered to be significant.

RESULTSTable 1 shows the effect of bromocriptine treatment on the body weight of maturing femalerats at different ages. The administration of bromocriptine for 5 days at a dosé of0-5 mg/100 g body wt induced a slight but statistically significant decrease in body weight.This decrease in body weight reached in some cases up to 19-3% (ovariectomized ratswithout oestrogen replacement therapy at 35 days) but in general, it was around 10%.

Table 1. Body weight (g) ofmaturing rats ovariectomized and treated with bromocriptinefor 5days before ovariectomy (0-5 mg/100 g body wt) at three different ages (values aremeans ± s.e.m.). The figures in parentheses indicate the number of animals used per group

Final wt 5 days after ovariectomy

Treatment25 days of age at ovariectomy

ControlBromocriptine

30 days of age at ovariectomyControlBromocriptine

35 days of age at ovariectomyControlBromocriptine

Initial wt

471 ±1-7 (20)49-8 + 1-6(18)

63-8 ±1-3 (9)58-7± 1-5 (18)*

101 0±5-2(5)91-4±2 6 (10)*

Without oestrogenreplacement

76-9 + 0-7(9)70-3 + 2-2(8)*

87-6+1-7(8)95-5 + 2-3(9)**

112-2 + 4-5(6)90-5 + 2-4(10)****

With oestrogenreplacement

76-5 + 1-6(10)70-5 + 1-5(10)*

101-9+1-8(7)84-9 + 2-6(10)****

112-0 + 3-2(8)96-1 + 1-5***

* <0·05; ** <0·02; *** <0 1; ****P<0001: compared with the appropriate control wt (2-way analysisof variance and /-test of multiple comparisons).

The mean uterine weights in the various treatment groups are summarized in Fig. 1.Treatment with bromocriptine did not significantly affect the initial uterine weight at any ofthe ages studied. Ovariectomy at 25 days of age was followed by a significant (P < 001 ) fall inuterine weight in both the bromocriptine-treated and control rats. The replacement dose ofoestrogen, however, was sufficient to prevent this fall. After ovariectomy at 30 days of agetreatment with oestradiol resulted in a significantly (P<00\) higher uterine weightcompared with the initial weight in the controls but not in the bromocriptine-treated group.

Fig. 1. Effect of administration of bromocriptine on the uterine weight in prepubertal rats at 25,30 and 35days of age. Results are expressed as means ± s.e.m. The figures in parentheses indicate the number ofanimals used per group. Control rats, open columns; bromocriptine-treated rats, shaded columns; I, initialuterine weight; F, final uterine weight 5 days after ovariectomy without treatment with oestrogen; F + OEfinal uterine weight 5 days after ovariectomy followed by treatment with oestrogen. *P<0-0\: differentfrom the respective initial uterine weight (Duncan's test).

In both groups, however, oestradiol blocked the decrease in uterine weight induced byovariectomy. At 35 days of age, however, previous treatment with bromocriptine wasassociated with a failure of oestrogen to enhance the uterine weight (P<0-01).

DISCUSSION

In accordance with previous findings (Advis & Alvarez, 1977), the present results confirm thechange in uterine sensitivity to oestrogen in control rats between 30 and 35 days of age.Although in the former study the initial and final uterine weights were somewhat lower thanthe corresponding ones observed here, the percentage increase in the final v. initial uterineweight was practically identical, e.g. 45-3 and 42-8% in the former and present studyrespectively. Treatment with bromocriptine effectively blocked the increase in uterine weightwhich occurs between 30 and 35 days of age in ovariectomized rats. Furthermore, thedopamine agonist counteracted the action of oestrogen in the maintenance of uterine weight5 days after ovariectomy at 35-40 days of age. Considering that the induction of the changein uterine responsiveness to oestrogen is dependent on central mechanisms, as indicated byhypothalamic-lesion studies (Advis & Alvarez, 1977), the present results can be interpretedto imply that the increase in uterine sensitivity to oestrogen is mediated by a diminisheddopaminergic activity. An analysis of our data (2-way analysis of variance and /-test ofmultiple comparisons) revealed that bromocriptine induced a slight but significant decreasein the body weight of the three groups of rats used in this study (25, 30 and 35 days old)compared with the control rats. As Table 1 shows, the decrease in body weight was, ingeneral, around 10% and in some cases reached 19-3%. If the assumption is made that theaction of bromocriptine is on body weight rather than on uterine weight of rats, then the

general decrease observed in our study in animals treated with the drug should increase therelative uterine weight. Figure 1 shows that this expectation was not fulfilled. The onlyincrease in body weight due to the drug treatment was seen in the ovariectomized ratswithout oestrogen replacement therapy at 30 days (Table 1 ) but the decreased relative uterineweight observed can be easily explained by the absence of oestrogen. It has been reportedthat bromocriptine decreases the concentration ofprolactin in plasma without affecting thatof luteinizing hormone (Döhler & Wuttke, 1974; Flückinger, Doepfner, Marko & Niederer,1976). In the present study levels ofprolactin in plasma were not measured but Alvarez et al.(1977) reported that the dose ofbromocriptine used prevents the precocious puberty inducedby hypothalamic lesions but does not affect ovulation.

The present results suggest that prolactin may be involved in the mechanism which bringsabout the change in uterine responsiveness to oestrogen in rats during sexual maturation.

This work was supported by a grant (23.53.2.75-R) from PLAMIRH (ProgramaLatinoamericano de Investigaciones en Reproducción Humana, Colombia). The technicalassistance of Mrs I. Hoppe and S. Troncoso is gratefully acknowledged. Dr H. Weidmannand H. Friedly (Pharmazeutisches Department, Sandoz, Basel, Switzerland) kindly supplied2-bromo-a-ergocryptine methanesulphonate (CB-154).

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