Antidopaminergic-induced hypothalamic LHRH release and pituitary gonadotrophin secretion in 12 day-old female and male rats.

In previous studies we have shown that the developing rat provides an interesting physiologic model in which the dopaminergic control of both LH and FSH is well defined in contrast to the controversial results obtained in adult rats. We wished to establish the role of testosterone in antidopaminergic induced gonadotrophins release in 12 day-old male and female rats, and evaluate the effect of antidopaminergic drugs at the hypothalamic level during this developmental stage. Haloperidol, an antidopaminergic drug, increased both LH and FSH in female 12 day-old rats but not in male littermates. The effect was blocked by bromocriptine and not by phentolamine indicating that haloperidol acted on the dopaminergic receptor, and that unspecific stimulation of the noradrenergic system was not involved. Haloperidol was ineffective when female rats were previously ovariectomized and injected with testosterone propionate at 9 days of age. If females were treated on the day of birth with testosterone propionate, haloperidol-induced FSH and LH release was also abolished. In control males haloperidol had no effect on the release of LH or FSH. But if males were orchidectomized at birth or at 9 days of age, haloperidol released both LH and FSH during the infantile period. In an attempt to establish the site of action of antidopaminergic drugs on gonadotrophin release, hypothalami (mediobasal and preoptic-suprachiasmatic area) from 12 day-old infant female rats were perifused with either haloperidol or domperidone (2*10(-6) M). Both drugs increased LHRH release into the perifusate. Besides haloperidol did not modify the release of LH or FSH from adenohypophyseal cells incubated in vitro. We therefore conclude that antidopaminergic-induced gonadotrophins release is modulated by serum testosterone concentrations, and that the site of action is probably the LHRH-secreting neuron of the hypothalamus.

Effect of a gonadotropin releasing hormone analog on an experimental ovarian tumor: direct and indirect actions.

An ovary autotransplanted into the spleen of a bilaterally ovariectomized rat develops into a luteoma, which grows under constant gonadotropin hyperstimulation. The effect of a long-acting GnRH agonist (GnRH-a), on tumor growth and hormone secretion was investigated. Two experimental models were used: Model 1: GnRH-a (0.33 mg/rat sc) or estradiol valerianate (50 micrograms/rat sc injected once a week for four weeks) was administered simultaneously with ovary implantation; Model 2: the drugs were administered after 1 month of tumor development. The treatment with estradiol was used as a control of tumor regression. Saline injected ovarian grafted rats and Sham operated animals were used as controls. In Model 1: The GnRH-a significantly inhibited tumor development (Positive tumors: Saline: 100% vs GnRH-a: 43%, p < 0.01). In Model 2: the GnRH-a and estradiol significantly reduced the volume of one month old tumors (52% and 39% of initial volumes respectively, p < 0.01). Gonadotropin secretion was significantly inhibited or its increase blunted by the GnRH-a and by estradiol treatments in both models. Estradiol and progesterone in portal blood, which collects the steroids secreted by the luteoma, were significantly reduced by GnRH-a treatment in both models. On the other hand, in tumor cells cultured "in vitro", the GnRH-a was able to inhibit the LH induced progesterone secretion in a concentration dependent way. These results clearly show that the GnRH-a is effective in inhibiting tumor growth or reducing its volume, when already developed; furthermore, it suppresses tumor steroid hormone production. These actions were exerted at both the hypophyseal and tumor levels.

Alpha-difluoromethylornithine modifies FSH secretion and puberty onset in the female rat.

FSH secretion is high in immature female rats from Postnatal Day 5 to 18 and decreases thereafter. This is a relatively steroid-independent event of cerebral origin and of importance for puberty onset. Polyamines, a group of ubiquitous amines, play an essential role in tissue growth and differentiation, body weight increment, brain organization, and molecular mechanisms of hormonal action. Polyamine levels as well as the activity of ornithine decarboxylase, the limiting enzyme in polyamines biosynthesis, are highest during development. Inhibition of their synthesis during this period by alpha- difluoromethylornithine (DFMO), a specific and irreversible inhibitor of ornithine decarboxylase, impairs normal brain development. The present study tested the hypothesis that polyamines play a role during brain organization of reproduction. DFMO was administered following different schedules in female newborn rats, and the effect on pituitary secretion, puberty onset, and fertility was evaluated. In three groups (daily injections from Day 1 to 9, or from Day 1 to 6, or injections on alternative days from Day 1 to 9), a delay in vaginal opening and first estrous was observed. When vaginal opening was plotted against body weight, it was evident that in groups daily injected with DFMO vaginal opening occurred at a lower body weight. In the group treated on alternate days, a delay occurred but at a higher body weight than in controls. In this group, serum FSH levels on Day 10 and 20, but not on Day 30, were higher in DFMO rats. In the group treated from Day 1 to 6 daily, DFMO increased serum FSH on Postnatal Day 20. After vaginal opening, estrous cyclicity in control and DFMO injected rats was similar. There was no significant effect of treatment on fertility and litter weight or number of offspring at birth. It is concluded that DFMO, an inhibitor of ornithine decarboxylase, administered during the first week of life in female rats is followed by prolonged high FSH serum levels and delayed puberty, but once puberty occurs, fertility is normal.

Octopamine and phenylethylamine inhibit prolactin secretion both in vivo and in vitro.

Trace amines are a group of biogenic amines that are present in neural tissue in concentrations ranging from 0.1 to 100 ng/g. In the present work, we examined the action of two trace amines, octopamine and phenylethylamine, which are found in the hypothalamus, on pituitary hormone secretion in different experimental situations in vivo and in dispersed anterior pituitary cells. Both octopamine and phenylethylamine decreased high prolactin levels due to swimming or immobilization stress without affecting other adenohypophysial hormones. With regard to the hypoprolactinemic potencies in the immobilization stress model, it was observed that p-tyramine, another trace amine, was as potent as octopamine. Phenylethylamine was the least effective. To evaluate the site of action of the effect described, the three trace amines were tested in dispersed anterior pituitary cell cultures in vitro. Tyramine and octopamine reduced prolactin secretion in a concentration-dependent manner, at concentrations of 10(-8) to 10(-5) M, whereas the hypoprolactinemic effect observed for phenylethylamine was very weak. In pharmacologic experiments, neither octopamine nor phenylethylamine reduced prolactin release when dopaminergic receptors were blocked. This could mean that their hypoprolactinemic action was mediated through the release of dopamine, or it could be a direct action at a dopaminergic receptor. This is the first description of a specific endocrine action both in vivo and in vitro for octopamine and phenylethylamine. Further studies are needed to ascertain the physiologic or pathologic implication of these findings.

alpha-difluoromethylornithine modifies gonadotropin-releasing hormone release and follicle-stimulating hormone secretion in the immature female rat.

Polyamines play an essential role in tissue growth and differentiation, in body weight increment, in brain organization, and in the molecular mechanisms of hormonal action, intracellular signaling, and cell-to-cell communication. In a previous study, inhibition of their synthesis by alpha-difluoromethylornithine (DFMO), a specific and irreversible inhibitor of ornithine decarboxylase, during development in female rats, was followed by prolonged high follicle-stimulating hormone (FSH) serum level and a delayed puberty onset. Those changes were relatively independent of body mass and did not impair posterior fertility. The present work studies the mechanisms and site of action of polyamine participation in FSH secretion during development. DFMO was injected in female rats between Days 1 and 9 on alternate days. At 10 days of age, hypothalami from control and DFMO rats were perifused in vitro, and basal and potassium-induced gonadotropin-releasing hormone (GnRH) release were measured. The response to membrane depolarization was altered in DFMO hypothalami. Increased GnRH release in response to a low K+ concentration was evidenced. Adenohypophyses of the same treated prepubertal rats were perifused in vitro and the response to GnRH pulses was checked. In DFMO-treated rats, higher FSH release was observed, with no changes in LH or PRL secretion. Finally, pituitary GnRH receptor number in adenohypophyseal membranes from treated and control groups was quantified. A significant reduction in specific binding was evident in hypophyses from DFMO-treated rats when compared with binding in the control group. In summary, DFMO treatment in a critical developmental period in the female rat impacts the immature GnRH neuronal network and immature gonadotropes. A delay in maturation is evidenced by a higher sensitivity to secretagogs in both pituitary glands and hypothalamic explants. These events could explain the prolonged high FSH serum levels and delayed puberty onset seen in this experimental model.

Effects of LHRH and ANG II on prolactin stimulation are mediated by hypophysial AT1 receptor subtype.

We have used the nonpeptide angiotensin II (ANG II) receptor antagonists losartan (receptor subtype AT1) and PD-123319 (AT2) to determine the participation of ANG II receptor subtypes in luteinizing hormone-releasing hormone (LHRH)-induced prolactin release in a perifusion study using intact pituitaries in vitro. LHRH (1.85 x 10(-7) M) released prolactin consistently, whereas losartan (10(-5) M) abolished prolactin response without modifying basal prolactin or luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release. PD-123319 (10(-5) M) had no effect on basal or LHRH-induced prolactin, LH, or FSH release. We also determined that the effect of ANG II on prolactin release was mediated by the same receptor subtype. In adenohypophysial cells dispersed in vitro ANG II (10(-8) M) released prolactin. Losartan (10(-7) and 10(-6) M), but not PD-123319, inhibited this effect. We conclude that in intact hypophyses of 15-day-old female rats the effect of LHRH on prolactin release is readily demonstrated. LHRH-induced prolactin release appears to be mediated by ANG II acting in a paracrine manner on AT1 receptors located on lactotrophs.