Inhibition of prolactin release by serotonin antagonists in hyperprolactinemic subjects

Hyperprolactinemic African elephant (Loxodonta africana) females exhibit elevated dopamine, oxytocin and serotonin concentrations compared to normal cycling and noncycling, low prolactin elephants†

Many zoo elephants do not cycle normally, and for African elephants, it is often associated with hyperprolactinemia. Dopamine agonists successfully treat hyperprolactinemia-induced ovarian dysfunction in women, but not elephants. The objective of this study was to determine how longitudinal dopamine, serotonin, and oxytocin patterns in African elephants are related to ovarian cycle function. We hypothesized that dopamine concentrations are decreased, while oxytocin and serotonin are increased in non-cycling, hyperprolactinemic African elephants. Weekly urine and serum samples were collected for eight consecutive months from 28 female African elephants. Females were categorized as follows: (1) non-cycling with average prolactin concentrations of 15 ng/ml or greater (HIGH; n = 7); (2) non-cycling with average prolactin concentrations below 15 ng/ml (LOW; n = 13); and (3) cycling with normal progestagen and prolactin patterns (CYCLING; n = 8). Both oxytocin and serotonin were elevated in hyperprolactinemic elephants. Thus, we propose that stimulatory factors may play a role in the observed hyperprolactinemia in this species. Interestingly, rather than being reduced as hypothesized, urinary dopamine was elevated in hyperprolactinemic elephants compared to CYCLING and LOW prolactin groups. Despite its apparent lack of regulatory control over prolactin, this new evidence suggests that dopamine synthesis and secretion are not impaired in these elephants, and perhaps are augmented.

Update on Comparative Biology of Elephants: Factors Affecting Reproduction, Health and Welfare

Asian (Elephas maximus) and African (Loxodonta africana) elephants serve as important keystone, umbrella and flagship species. Despite that, population numbers are declining, due mainly to poaching and habitat destruction. Understanding reproductive mechanisms is vital to effective management, particularly insurance populations in captivity, and to that end, long-term biological databases are key to understanding how intrinsic and extrinsic factors affect reproductive function at individual and population levels. Through decades of hormonal and ultrasonographic monitoring, many unique aspects of zoo elephant reproduction have been identified, including differences in luteal steroidogenic activity, follicular maturation, pituitary gonadotropin secretion, fetal development and reproductive tract anatomy. Reproductive problems also hamper captive propagation efforts, particularly those related to abnormal or lack of ovarian cyclicity. Recent large-scale, multi-institutional studies and use of epidemiological approaches have identified factors important for good welfare and reproduction, which include enrichment, feeding diversity, good elephant-keeper relations, social compatibility, exercise, and not being obese. There are notable differences in reproductive mechanisms between Asian and African elephants, as well as the factors that influence reproduction and welfare, suggesting species-targeted management approaches are needed to maximize fitness. In the first edition, we discussed reproductive function in male and female elephants. Since then, a number of significant advances have been made primarily in female elephants, which will be the focus of this updated review.

Validity of urinary monoamine assay sales under the "spot baseline urinary neurotransmitter testing marketing model"

Spot baseline urinary monoamine assays have been used in medicine for over 50 years as a screening test for monoamine-secreting tumors, such as pheochromocytoma and carcinoid syndrome. In these disease states, when the result of a spot baseline monoamine assay is above the specific value set by the laboratory, it is an indication to obtain a 24-hour urine sample to make a definitive diagnosis. There are no defined applications where spot baseline urinary monoamine assays can be used to diagnose disease or other states directly. No peer-reviewed published original research exists which demonstrates that these assays are valid in the treatment of individual patients in the clinical setting. Since 2001, urinary monoamine assay sales have been promoted for numerous applications under the "spot baseline urinary neurotransmitter testing marketing model". There is no published peer-reviewed original research that defines the scientific foundation upon which the claims for these assays are made. On the contrary, several articles have been published that discredit various aspects of the model. To fill the void, this manuscript is a comprehensive review of the scientific foundation and claims put forth by laboratories selling urinary monoamine assays under the spot baseline urinary neurotransmitter testing marketing model.

Drug-induced changes in prolactin secretion. Clinical implications

Prolactin secretion is affected by various diseases as well as by many drugs in humans and animals. While marked hyperprolactinaemia suggests the presence of a pituitary tumor, moderate changes may also occur in various endocrine or non-endocrine disorders. Drugs can interfere with prolactin regulation via complex mechanisms at the hypothalamus or at the pituitary site, but possible changes in prolactin metabolism are poorly understood as yet. This survey of the literature up to June 1986 covers the influence of various groups of drugs and agents on the plasma prolactin level under various conditions. It contains information that will facilitate evaluation of whether hyper- or hypoprolactinaemia may result from therapeutic intervention or must be related to an underlying disease. It is obvious that more subtle changes can be revealed by the use of dynamic tests either to stimulate or to suppress prolactin secretion.

Effect of dihydroergokryptine administration on serum prolactin and growth hormone levels in normal, hyperprolactinaemic and acromegalic subjects: evidence of potent and long-lasting pituitary dopamine receptor stimulation

The endocrine effects of a relatively potent dopaminergic agent, dihydroergokryptine, have been studied in normal subjects, and in hyperprolactinaemic and acromegalic patients. A single 6 mg oral dose of the drug caused a marked, long lasting fall in prolactin (PRL) plasma levels in healthy subjects, in hyperprolactinaemic patients and in normoprolactinaemic acromegalics. Growth hormone (GH) levels decreased in 1-DOPA - responder, acromegalic patients, but dihydroergokryptine did not affect GH levels in normal volunteers or in 1-DOPA non-responder, acromegalic patients. The PRL- and GH- lowering activity of 6 mg dihydroergokryptine was significantly greater than that of 6 mg dihydroergocristine, and was similar to that of an oral dose of 500 mg 1-DOPA.

Effect of fenfluramine on prolactin secretion in obese patients: evidence for serotoninergic regulation of prolactin in man

The present investigation was carried out to study the effect of serotoninergic stimulation on prolactin (PRL) secretion in man. Fenfluramine (60 mg, orally), an anorexiant drug which under acute circumstances stimulates the serotoninergic system, was administered to eight obese patients. Compared with placebo, drug administration increased PRL significantly (P less than 0.05 at 180 and 300 min, P less than 0.01 at 240 min). No significant changes were observed after fenfluramine in blood pressure, plasma aldosterone (PA), plasma cortisol, plasma renin activity, serum electrolytes or growth hormone. Since it has been reported that dopaminergic blockade raises PA concentration, the lack of change in PA in obese patients treated with fenfluramine suggests that the observed increase in PRL induced by fenfluramine is likely to be mediated by serotoninergic stimulation.

Prolactin secretion in man: a useful tool to evaluate the activity of drugs on central 5-hydroxytryptaminergic neurones. Studies with fenfluramine

Acute oral administration of various doses of fenfluramine, a 5-HT releaser, induced a dose-related increase of PRL secretion in nine healthy volunteers. Fenfluramine reached the maximum effect on PRL secretion at 4 h after its administration. This effect was already significant at 2 h and lasted till 8 h. Metergoline, a 5-HT receptor blocker, when administered alone, decreased serum PRL levels in six healthy subjects. The pretreatment with this drug significantly antagonized the PRL-releasing action of fenfluramine (60 mg) suggesting that the effect of fenfluramine on PRL release may be mediated through a 5-HT mechanism in the brain. These findings suggest the possibility that serum PRL levels in humans may represent a useful tool to evaluate, in vivo, the activity of drugs possessing putative 5-hydroxytryptaminergic properties.

Effect of dopamine infusion on serum prolactin concentration in normal and hyperprolactinaemic subjects

The serum prolactin response to intravenous dopamine infusion (5 micrograms . kg-1 . min-1) was measured in twenty-one healthy subjects, in seven hyperprolactinaemic patients without evidence of a pituitary tumour, and in twenty-one patients with prolactinomas. Mean serum prolactin values were significantly suppressed in all three groups, without any significant difference between the degree of suppression. A decrease of serum prolactin to below 50% of basal values occurred in fifteen healthy subjects, in four patients without evidence of pituitary tumour, and in fourteen patients with prolactinomas. These findings demonstrate that most human prolactin-secreting pituitary adenomas are normally suppressible by exogenously administered dopamine and that dopamine infusion is not able to distinguish between tumorous and non-tumorous hyperprolactinaemia. Since intravenously infused dopamine is believed to inhibit prolactin secretion by acting at pituitary level, it is suggested that a normal functioning of pituitary dopamine receptors is maintained in most human prolactinomas.

Effect of metergoline on serum prolactin stimulated by TRH

The PRL response to TRH was evaluated before and after acute administration of metergoline, an antiserotonin drug, in healthy subjects. The drug was given to two groups of 6 subjects each, at the dose of 4 and 8 mg respectively, either po or by im route. Metergoline significantly inhibited the PRL response to TRH; no significant difference was found between the two doses and the administration routes. Although these findings suggest that the drug may directly inhibit lactotropic cells, an action of metergoline on central nervous system to stimulate PIF or to inhibit PRF release cannot be ruled out, and might as well explain the inhibition of the PRL response to TRH.

Restoration of cyclic ovarian function by metergoline treatment in a patient with a prolactin-secreting pituitary microadenoma

A patient with amenorrhea due to a prolactin-secreting pituitary microadenoma was treated with the antiserotoninergic drug metergoline for 8 months. The first menstruation occurred after 1 month of therapy, and it was followed by regular menses by the 3rd month. Presumptive evidence of ovulation was obtained in at least some instances by serum progesterone and gonadotropin determination. Serum prolactin was only slightly lowered by treatment. The patient had menses and possibly ovulation in the 2 months following drug withdrawal. Metergoline might restore ovarian function in hyperprolactinemic amenorrhea either by prolactin suppression or perhaps by direct stimulation of gonadotropin release.