Mechanisms underlying the prolactin-lowering effect of meterogoline in the rat

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.

Does metergoline selectively attenuate 5-HT mediated prolactin release?

Administration of the non-selective 5-HT receptor antagonist metergoline (0.5 mg/kg) to male rats attenuated the prolactin response to the 5-HT releasing agent d-fenfluramine (7.5 mg/kg) but not to the dopamine receptor antagonist haloperidol (1.5 mg/kg). In contrast, in healthy male volunteers, pretreatment with metergoline (4 mg orally) abolished the prolactin response to intravenous haloperidol (5 micrograms/kg). The findings suggest that in humans blockade of a prolactin response by a conventional oral dose of metergoline cannot be taken as evidence of involvement of 5-HT-mediated mechanisms.

Methysergide decreases prolactin release after FK 33-824 [Tyr-D-Ala-Gly-MePhe-Met(o)-ol], a potent analogue of methionine enkephalin. A study in man

This study investigated the mechanisms which may underly the prolactin (PRL) stimulating effects of FK 33-824, a potent enkephalin analogue. FK 33-824 (1 mg) was infused in healthy volunteers before and after pretreatment with 3 mg methysergide, a serotonin receptor blocker. All subjects showed a release of PRL after FK 33-824, which was significantly diminished after pretreatment with methysergide.

Correlation between inhibitory effect on prolactin secretion and antitumor activity of new ergoline compounds on DMBA-induced tumors in rats

Five recently synthesized (355/1057, 355/1000, 355/1101, 355/1138 and FCE 21336) and 4 well-known (bromocriptine, metergoline, 1-demethylmetergoline and pergolide) prolactin-lowering ergoline derivatives and 1 ergoline (nicergoline) without antiprolactin activity were tested against 7-12-dimethyl-benzanthracene (DMBA)-induced mammary carcinomas in rats. Nicergoline did not show any activity, while the other compounds, tested at doses inhibiting prolactin secretion, proved active against established tumors and on the onset of new tumors. The activity of 3 of the new ergolines (355/1000, 355/1057 and FCE 21336) and of bromocriptine and pergolide was also tested at different oral doses and was correlated with serum prolactin levels 24 hr after the last dose. All the compounds proved highly effective, inducing 50-60% regression of the initial tumors. The inhibition of serum prolactin levels was dose-related and, for all the compounds tested except bromocriptine, a good correlation was found between doses administered and complete tumor remissions.

Serotonergic mechanism in the control of beta-endorphin and ACTH release in male rats

The role of the serotonergic mechanism in the regulation of beta-endorphin (beta-EP and adrenocorticotropin (ACTH)-like immunoreactivity in plasma was investigated. Increases in beta-EP and ACTH-LI produced by quipazine maleate (QPZ), a serotonergic agonist, 1 hr after injection could be completely prevented by the serotonin (5-HT) antagonist, cinanserin (CIN), which when injected alone, decreased basal plasma concentrations of both beta-EP-LI and ACTH-LI. Concurrent injections of L-5-HTP with the 5-HT reuptake inhibitor, fluoxetine, produced an additive increase in plasma beta-EP-LI 1 hr after injection. Injection of the 5-HT antagonist, cyproheptadine, significantly decreased plasma beta-EP-LI. Stress by immobilization for 30 min or exposing the rats to 40 degree +/- 1 degree C for 30 min produced an approximate 4-fold increase in plasma beta-EP-LI and ACTH-LI, which was potentiated by I.P. injections of fluoxetine. Furthermore, the stress induced increases in plasma concentrations of beta-EP-LI and ACTH-LI were significantly reduced by the serotonin antagonists metergoline and cinanserin. These results suggest that 5-HT is a potent stimulator of both beta-EP and ACTH release and the increase in plasma concentrations of ACTH and beta-EP induced by stress are probably mediated, at least in part, by central serotonergic mechanisms.

Role of central nervous system neurotransmitters in mediating the effects of morphine on growth hormone- and prolactin-secretion in the rat

Unanesthetized adult male rats with indwelling right atrial cannulae were used in the majority of experiments. Morphine (MOR, 3.0 mg/kg) caused a large but transient increase in both GH and PRL levels, which could be prevented with naloxone. Disruption of central noradrenergic function with diethyldithiocarbamate (400 mg/kg) or phenoxybenzamine (15 mg/kg) abolished the GH-releasing effect of MOR, without interfering with the PRL secretory response. Depletion of brain serotonin stores with p-chlorophenylalanine (300 mg/kg) or 5,7-dihydroxytryptamine or administration of serotonin receptor blocker, cyproheptadine (2.5 mg/kg), did not diminish the GH respnse to MOR but it inhibited, or in the case of 5,7-DHT treatment abolished the activation of PRL secretion. Additionally, metergoline (0.1 and 1.0 mg/kg), another serotonin receptor blocker, caused an inhibition of the GH-releasing action of MOR; however, this inhibition was reversed by pretreatment with spiroperidol (0.1 mg/kg). Metergoline also markedly diminished the MOR-induced elevation of PRL. Inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine (alpha-MT, 250 mg/kg) blunted the effect of MOR on GH; however, dopamine receptor blockers, spiroperidol (0.01 and 0.1 mg/kg) or (+)butaclamol (0.3 and 1.3 mg/kg), were without any influence. alpha-MT or spiroperidol did not alter the effect of MOR on PRL secretion, but the higher dose of (+)butaclamol suppressed it. It is concluded that the GH-releasing action of MOR requires unimpaired functioning of the central noradrenergic system, while the serotonergic and dopaminergic systems appear to play no significant role in it. In contrast, serotonergic systems seem to be essential for the activation of PRL secretion, whereas the noradrenergic system is not involved. It remains uncertain whether morphine activtes PRL secretion also through inhibition of dopaminergic activity. We favor the view that the dopaminergic component participates in the PRL activation by MOR, but that its contribution to the overall effect is rather small.

Metergoline and cyproheptadine suppress prolactin release by a non-5-hydroxytryptaminergic, non-dopaminergic mechanism

A dispersed rat anterior pituitary cell system has been used to investigate the effects of cyproheptadine and metergoline on prolactin secretion. Both drugs were potent inhibitors of prolactin secretion. However, the inhibition was not antagonized by either 5-hydroxytryptamine or a variety of dopamine antagonists. We conclude that both drugs act through mechanisms that are neither 5-hydroxytryptaminergic nor dopaminergic.

Further evidence for an inhibitory role of central 5-HT in male rat sexual behavior

The 5-hydroxytryptamine (5-HT) reuptake inhibitors zimelidine (10 mg/kg IP 1.5h pretest) or alaproclate (20 mg/kg IP 1.5h pretest) produced a prolongation of the ejaculation latency and of the post-ejaculatory interval in male rats treated with a subthreshold dose of 5-hydroxytryptophan (5-htp) (12.5 mg/kg IP 1h pretest). The 5-HTP-induced (50 mg/kg IP 1h pretest) prolongation of ejaculatory latency was effectively counteracted by administration of the 5-HT receptor blocking agent metergoline (1 mg/kg IP 1.5h pretest). All animals were treated with an inhibitor of peripheral aromatic amino acid decarboxylase, indicating that the effects are of central origin. The results support the contention that central 5-HT plays an inhibitory role in male rat sexual behavior.

Interaction of sulfpride and ergot derivatives on rat brain DOPAC concentration and prolactin secretion in vivo

Sulpiride, which differs from classical neuroleptics by not producing major extrapyramidal side effects, is a potent antiemetic agent and stimulates prolactin secretion in both laboratory animals and man. In parallel it increases dopamine synthesis in both striatum and nucleus accumbens. Bromocriptine and metergoline are two effective agents in suppressing prolactin release and postulated to stimulate dopamine receptors. The interactions of these two ergot derivatives with sulpiride have been investigated on prolactin release and on striatal and limbic DOPAC accumulation. Bromocriptine at all doses tested was able to suppress the increased in vivo prolactin secretion observed after sulpiride administration. Metergoline antagonized the sulpiride-induced prolactin increase only at low doses; on the contrary higher doses potentiated it. High concentrations of bromocriptine suppressed the sulpiride-induced increased of DOPAC levels in striatum and n. accumbens, while metergoline potentiated the sulpiride-induced accumulation of brain DOPAC.