Hormonally mediated effects on the female reproductive system may manifest as pathologic changes of endocrine-responsive organs and altered reproductive function. Identification of these effects requires proper assessment, which may include investigative studies to profile female reproductive hormones. Here, we briefly describe normal hormonal patterns across the estrous or menstrual cycle and provide general guidance on measuring female reproductive hormones and characterizing hormonal disturbances in nonclinical toxicity studies. Although species used in standard toxicity studies share basic features of reproductive endocrinology, there are important species differences that affect both study design and interpretation of results. Diagnosing female reproductive hormone disturbances can be complicated by many factors, including estrous/menstrual cyclicity, diurnal variation, and age-and stress-related factors. Thus, female reproductive hormonal measurements should not generally be included in first-tier toxicity studies of standard design with groups of unsynchronized intact female animals. Rather, appropriately designed and statistically powered investigative studies are recommended in order to properly identify ovarian and/or pituitary hormone changes and bridge these effects to mechanistic evaluations and safety assessments. This article is intended to provide general considerations and approaches for these types of targeted studies.

Centrally acting dopamine agonists (e.g. bromocriptine) and dopamine transport inhibitors (e.g. GBR12909) are known to inhibit oestradiol-induced prolactin release. The capacity of peripherally restricted compounds to do likewise, however, is unknown. Here, the effects of the peripherally restricted dopamine receptor agonist carmoxirole on oestradiol-induced prolactin release were investigated. Dual-cannulated ovariectomized rats were used, so that a robust, reproducible response to exogenous oestrogen could be induced and sequential blood samples were taken with minimal stress. Carmoxirole (15 mg/kg) inhibited oestradiol-induced prolactin release, similar to bromocriptine and GBR12909. However, carmoxirole also induced a rapid, transient, oestradiol-independent release of prolactin. These data show that peripherally restricted dopamine receptor agonists are sufficient to inhibit oestradiol-induced prolactin release. Like centrally acting compounds, they may therefore be expected to affect the incidence of prolactin-dependent tumours in rat carcinogenesis studies without inducing central-mediated side effects.Understanding the relevance or translation of pre-clinical drug effects to the clinic is critical for accurate human risk assessment for new pharmaceutical agents. In particular, accurate identification of specific mechanisms of serious toxicities observed in non-clinical species, and translation of specific mechanism in human beings can provide a scientific basis with which to evaluate specific risk in human beings. For example, it is known that some endocrine modulators can affect the incidence of particular tumours in life-time rat carcinogenicity bioassays, but not in human beings [1]. More specifically, the centrally active dopamine (D2) receptor agonist, bromocriptine, can induce hypoprolactinaemia in rats and human beings, but increases the incidence of hyperplastic proliferative endometrial lesions and benign and malignant uterine tumours only in rats [2], which has been postulated to be due to a lesser impact of prolactin on ovarian steroidogenesis in human beings [3].Prolactin is a peptide hormone spontaneously produced and secreted by lactotrophs in the anterior lobe of the pituitary gland under control by hypothalamic factors. In the rat, three dopaminergic systems [tuberoinfundibular (TIDA), tuberohypophyseal and periventricular hypophyseal dopaminergic neurons] regulate prolactin release, with the relative contribution varying between reproductive, physiological and pathological stages [4]. Hypothalamic dopaminergic neurons play a central role in the regulation of prolactin release by exerting an effect on the lactotrophs via the D2 receptor [5,6]. However, dopamine can also affect prolactin secretion by acting in the hypothalamus [7,8]. The pituitary lactotroph population is not entirely homogenous; mammosomatotropes, which are particularly common in neonatal rats, are differentiated into lactotrophs under the influence by oestrogens and secrete both prolactin and growth hormone [9]...