Effect of chronic prolactin infusion on pituitary prolactin and hypothalamic proopiomelanocortin

Pharmacokinetics and pharmacodynamics of ropinirole in patients with prolactinomas

AIMS

Treatment of prolactinomas with ergoline dopamine agonists can be complicated by intolerance and resistance. This study investigated the pharmacokinetics and pharmacodynamics of the nonergot dopamine agonist ropinirole, to assess its therapeutic potential as a novel therapy for prolactinomas.

METHODS

Five female subjects with prolactinomas participated in this dose-response study. Subjects received up to three doses of ropinirole (0.5, 1.0 and 2.0 mg), each on separate occasions. Frequent blood samples for prolactin and ropinirole were collected for 24 h following drug administration. Data were analysed using noncompartmental and compartmental pharmacokinetic-pharmacodynamic (PKPD) techniques.

RESULTS

Seven 24-h curves revealed increased systemic drug exposure with increasing ropinirole doses. Ropinirole concentrations peaked at 4.4 ± 2.7 h and exhibited a half-life of 5.8 ± 1.7 h. A dose-dependent prolactin nadir occurred 4.4 ± 1.2 h after drug intake and prolactin concentrations transiently normalized in two of five subjects. PKPD modelling revealed that single-dose PK of ropinirole is dose-independent and can be described with a one-compartment model with linear absorption and elimination. An indirect response model successfully captures the inhibitory effect of ropinirole on prolactin secretion and incorporates time-dependent receptor desensitization for three of five subjects whose prolactin concentrations nadired before ropinirole reached C_max .

CONCLUSIONS

This data-rich study has informed our understanding of the clinical pharmacokinetics and pharmacodynamics of ropinirole, which are successfully captured by the proposed semi-mechanistic PKPD model. This model can be used to further investigate the PKPD of ropinirole and may facilitate the identification of optimal dose regimens for the treatment of prolactinomas and the establishment of a new therapeutic option for patients impacted by this rare disease.

Expression of prolactin receptors in rat reproductive tissues during periovulatory prolactin imbalance

Indirect immunoperoxidase technique in combination with cytophotometry showed that prolactin imbalance in the periovulatory period is characterized by considerable changes in the expression of prolactin receptors in uterine glands and endometrial stroma (but not in the myometrium) and in the pituitary gland and hypothalamus, while receptor compartmentalization pattern remains unchanged.

Rhythmicity of beta-endorphinergic neuronal activity in the mediobasal hypothalamus during pregnancy in the rat

During the first half of gestation in the rat, prolactin (PRL) from the anterior pituitary gland exerts its luteotropic function on the ovary to stimulate progesterone secretion. During this period, beta-endorphin stimulates PRL secretion by regulation of dopaminergic neurons in the hypothalamus. During the second half, placental lactogens (PLs) take the place of PRL in maintenance of pregnancy, and initiate a negative feedback to suppress PRL secretion. However, the effect of PLs on beta-endorphinergic neurons is not known. The aim of this study was to examine the possibility that PLs suppress PRL secretion by inhibiting beta-endorphinergic neuronal activity. To accomplish this aim, we examined the changes in the neuronal activity of beta-endorphinergic neurons in the mediobasal hypothalamus, as measured by Fos immunoreactivity, after manipulating the levels of PRL and PLs during pregnancy. On day 4 of pregnancy, animals received either Rcho-1 cells in the lateral ventricle that secrete PLs or HRP-1 cells as controls. In a separate experiment on day 12, hysterectomy was performed to remove the intrinsic source of PLs. These rats received Rcho-1 cells, HRP-1 cells, or nothing. Intracerebroventricular (i.c.v.) injection of Rcho-1 into hysterectomized rats was done to examine the effect of PL replacement. Sham-hysterectomy was also performed as a control. Animals were sacrificed 2 days after each treatment at 0200 h, 1400 h, and 1800 h. Brains were used for dual immunocytochemistry of Fos/beta-endorphin. The neuronal activity of beta-endorphinergic neurons of HRP-1 i.c.v. injected animals showed a daily rhythm, with high levels at 0200 h and 1800 h, and a low level at 1400 h. These animals also exhibited two surges of PRL secretion on day 6 of pregnancy. This rhythmicity of beta-endorphinergic neurons was also observed in Rcho-1 i.c.v. injected animals, which showed very low and unchanging PRL levels. However, the magnitude of neuronal activity was reduced. On day 14 of pregnancy, all four experimental groups showed diurnal rhythms of beta-endorphinergic neurons. This rhythmicity occurred even though PRL was elevated at all three time points in the hysterectomized rats and very low in the Rcho-1 i.c.v. injected hysterectomized and sham-hysterectomized rats. Our results demonstrate that there is a diurnal rhythm of beta-endorphinergic neuronal activity in the mediobasal hypothalamus during pregnancy in the rat. PLs might reduce the neuronal activity of beta-endorphinergic neurons, but only during the first half of pregnancy, partially explaining the suppression of PRL surges.

Effects of photoperiod and androgen on proopiomelanocortin gene expression in the arcuate nucleus of golden hamsters

In golden hamsters, seasonal changes in day length act via a pineal-dependent mechanism to regulate feedback and behavioral effects of androgen. Endogenous opiates participate in photoperiodically regulated neuroendocrine functions, but the effects of androgen on expression of the gene encoding POMC, the precursor of beta-endorphin, have been controversial. We used quantitative in situ hybridization to examine regulation of POMC messenger RNA (mRNA) by testosterone and to test the hypothesis that short day lengths act through the pineal gland to amplify POMC mRNA expression. We studied intact hamsters and castrates with or without androgen treatment held in long (14 h of light, 10 h of darkness) or short (5 h of light, 19 h of darkness) days for 10 weeks. POMC gene expression differed with rostral-caudal plane, photoperiod, and surgical treatment (castration and testosterone administration). Testosterone increased the number of silver grains in labeled cells throughout the arcuate nucleus, and short day castrates given androgen consistently had more silver grains per labeled cell than did their long day counterparts. Testosterone exerted an inhibitory effect, however, on the number of POMC mRNA-positive cells, and more POMC mRNA-labeled cells were found in the arcuate nucleus of long than short day castrates treated with testosterone. Photoperiod had no significant influence in castrates not receiving androgen. Testosterone treatment had generally similar effects whether it was begun at the time of castration or 5 weeks later. Pinealectomy blocked the influence of photoperiod on both the mean number of silver grains per labeled cell and the number of labeled cells. The results indicate that day length regulates POMC gene expression when androgen levels are held constant, but that androgen is necessary for photoperiod effects to be expressed.