Effects of metachlopromide-induced hyperprolactinemia on serum testosterone and its response to hCG in adult male common marmosets (Callithrix jacchus)

Intramuscular administration of metachlopromide (2.5, 10, and 25 mg) induced increase in serum prolactin levels. Following 10 and 25 mg dose levels, prolactin levels were elevated at least for 8 hr. Single administration of metachlopromide failed to affect the nocturnal rise in circulating levels of testosterone. Daily treatment of metachlopromide (10 mg) for 30 days suppressed the nocturnal elevation of testosterone on day 28 of treatment. However, the hCG-stimulated testosterone production on day 30 was unaffected. The results of the present study demonstrate that metachlopromide-induced hyperprolactinemia in adult male common marmosets affects the hypothalamo-pituitary axis without any effect on testicular response to hCG.     

Prolactin administration during the luteal and follicular phase of the oestrous cycle of gilts

In Exp. I infusions of prolactin (0.5 mg in 2 ml sterile saline) were repeated every 2 h for 36 h on Days 12-13 of the cycle. In Exp. II infusions of prolactin were administered from Days 17 to 19 (60 h) at 2-h intervals. Control gilts were given 2 ml sterile saline at similar intervals during the same period. Basal prolactin concentrations before initiation of infusions ranged from 1.3 +/- 0.1 to 5.6 +/- 2.2 ng/ml in both experiments. By 5 min after a prolactin infusion, mean plasma prolactin concentration ranged from 74.9 +/- 5.8 to 113.0 +/- 9.5 ng/ml, but then declined to approximately equal to 10 ng/ml just before the next infusion of prolactin. Administration of prolactin during the luteal phase of the oestrous cycle of the gilts had no effect on basal levels of progesterone, oestradiol or LH. During the follicular phase there were no differences (P greater than 0.05) between control and prolactin-treated gilt progesterone and LH concentrations, but oestradiol plasma values were decreased (P less than 0.05) on the 2nd and 3rd day of prolactin treatment. Our results would indicate that prolactin does not play a major role in the regulation of the oestrous cycle of the pig.     

Effect of hypothalamic surgery on prolactin release induced by 5-hydroxytryptophan (5-HTP) in rats.

Intravenous injections of varying doses of 5-HTP (1, 3 and 5 mg/100 g body wt), a precursor of serotonin, caused a significant and dose-related increase in plasma prolactin concentrations in urethane-anesthetized rats. Increases in plasma prolactin concentrations caused by 5-HTP (1 mg/100 g body wt iv) were abolished by the concomitant administration of L-DOPA (2 mg/100 g body wt iv). Plasma prolactin levels were also significantly elevated following the injection of 5-HTP in rats with complete hypothalamic deafferentation, whereas 5-HTP had no significant effect on plasma prolactin levels in rats with extensive hypothalamic ablation. These results suggest that 5-HTP causes prolactin secretion by stimulating the serotoninergic mechanism in the hypothalamus.     

Plasma prolactin concentrations during the ovarian cycle and lactation and their relationship to return of fertility post partum in the common marmoset (Callithrix jacchus)

A heterologous double-antibody radioimmunoassay was used to measure plasma prolactin concentrations in 27 marmosets. The assay was valid for the marmoset because plasma levels of prolactin were increased in response to TRH and metoclopramide and suppressed in response to bromocriptine treatment. During the cycle there were no consistent changes in plasma prolactin concentrations. During lactation mothers suckling single or twin infants had higher prolactin levels than did non-suckling females and levels were highest with twins. No statistically significant delay in the resumption of ovulation post partum was observed for the suckling and non-suckling females; conception occurred in all but one marmoset by 70 days post partum. These results show that neither the suckling stimulus nor high levels of prolactin post partum delay the return of ovulation and fertility in the common marmoset, a result in contrast to that for all other primate species so far investigated.     

No evidence for a peripheral effect of L-DOPA on plasma prolactin in the rat

Male and female rats with two permanently indwelling intravenous catheters were infused for 2 hours with ovine prolactin. During equilibrium conditions the effects of intravenously injected L-DOPA and benzerazide (a blocker of dopa-decarboxylase) on steady state levels of ovine prolactin were measured. A dose of 4.5 mg L-DOPA per 100 gr body weight (b.w.) caused a transient increase of plasma ovine prolactin. A dose of 0.3 mg L-DOPA/100 gr b.w. had no effect, neither in males nor in females, while benzerazide (20 mg/100 gr b.w.) had only a slight effect. The experiments suggest that L-DOPA does not affect the peripheral uptake of prolactin from the plasma.     

Effect of propiomazine on plasma prolactin in the rat: counteraction by L-dopa

A single iv injection of 0.31, 0.62, 1.25, 2.5, 5, 10, or 20 mg/kg body wt of a phenothiazine derivative, propiomazine (PP), into male rats significantly (P less than 0.05) increased plasma prolactin concentrations. The higher doses (5, 10, and 20 mg/kg body wt) produced increases that were greater in both magnitude and duration than those produced by the lower doses. The higher doses of PP, along with the elevations in plasma prolactin, also produced concomitant decreases in plasma luteinizing hormone (LH) levels. Pretreatment with L-dopa (100 mg/kg body wt) completely blocked the PP-induced stimulation of prolactin release, indicating that antidopaminergic action of PP either at the hypothalamic or anterior pituitary level was responsible for its effects on the release of prolactin.     

The effects of a sudden decrease or increase in daylength on prolactin secretion in the ram

Six adult Soay rams were preconditioned to an artificial lighting regimen of alternating 4-month periods of long (16L:8D) and short days (8L:16D) for at least 10 months before blood samples were collected at hourly intervals for 24 h at various times. The abrupt change from long to short days resulted in a progressive decrease in plasma levels of prolactin, while that from short to long days had the reverse effect; the first response to the light changes was rapid, beginning within 6 days. During the periods of high secretion there was a 24-h cycle in plasma prolactin concentrations, with a peak in both the early dark and early light phases of each day. Changes in the relative magnitude of these peaks were observed in relation to the long-term alteration in prolactin secretion. Plasma levels of FSH were also measured and a close inverse relationship between gonadotrophin and prolactin secretion was observed.     

The influence of estrogen on plasma prolactin levels induced by thyrotrophin releasing hormone (IRH), clonidine and serotonin in ovariectomized rats.

Prolactin levels were determined in the plasma of ovariectomized and ovariectomized estrogen treated rats by RIA following intraarterial injection of TRH, (1 and 10 μg/rat), clonidine (5 mg/kg) and serotonin (10 mg/kg). In ovariectomized rats, TRH had no effect on plasma prolactin whereas serotonin and clonidine induced slight and moderate increases respectively. In contrast, TRH induced a significant increase in plasma prolactin in estrogen-treated rats while the effects of the other two agents were enhanced only slightly (clonidine) or very markedly (serotonin). These results indicate that the prolactin-releasing activity of TRH is dependent on estrogen and that estrogen differentially affects noradrenergic and serotonergic components of the neuroendocrine mechanism that controls prolactin. It is also suggested that clonidine and serotonin probably do not increase plasma prolactin by releasing endogenous TRH.     

Effect of anxiolytics--buspirone and diazepam--on the prolactin, thyrotropin and cortisol content of the blood in the green monkey

The influence of two anxiolytics--diazepam and buspirone--on prolactin, thyrotrophin and cortisol levels in green monkey (Cercopithecus aethiops) plasma was studied 30 min following i/m injection. Diazepam at 1 mg/kg decreased prolactin and cortisol levels by 30-50% compared to the control animals. Buspirone at 2.5-10 mg/kg induced a 7-10-fold increase in prolactin level but did not change cortisol and thyrotrophin concentration. Buspirone analog--Mj 138-05 at 10 mg/kg produced a 2-3-fold increase in plasma prolactin content in some animals, while at a dose of 5 mg/kg it exerted no detectable effect. Possible neurochemical mechanisms of the effects observed are discussed.     

Prolactin in the marsupial Macropus eugenii, during the estrous cycle, pregnancy and lactation

An heterologous double antibody radioimmunoassay (RIA) using a guinea-pig antiserum (33-9) raised against human prolactin and 125I-ovine prolactin has been developed to measure prolactin (Prl) in plasma and pituitary preparations of marsupials. In this system, purified tammar and kangaroo Prl preparations showed parallel dose-response curves as did serial dilutions of crude pituitary homogenates of tammar, possum and eastern grey kangaroo. Serial dilutions of plasma from ovariectomized and lactating female and castrate male tammars showed immunoreactivity, and plasma Prl levels increased after injection of TRH. The assay has been used to monitor changes in plasma Prl in female tammars in various reproductive states. Plasma Prl remained at basal concentrations of 20 to 30 ng/ml throughout the estrous cycle, at estrus and during pregnancy. However, just prior to parturition, there was a 2- to 3-fold increase in Prl concentrations which declined to basal levels after birth. During early lactation, Prl levels were low but increased to maximum concentration in the second half of lactation.     

Corpus luteum function in the guinea pig: effect of pgf2 alpha and inhibitors of prostaglandin and progesterone biosynthesis.

Exogenous PGF_2α failed to consistently alter estrous cycle length of the guinea pig. A wide range of dose levels were administered with varying frequency, at different stages of the estrous cycle, in different vehicles and by various routes. Massive doses of PGF_2α (5.0–10.0 mg) produced a significant (p<.05), although transient, lowering of plasma progesterone levels. Smaller doses were ineffectual. An i.p. injection of 25 mg of PGF_2α was toxic in four of five treated animals. It would appear that the intact guinea pig is extremely resistant to the luteolytic effects of parenterally administered PGF_2α.Estradiol-17β, administered s.c. on days 3 to 10 of the estrous cycle, significantly (p<.05) reduced corpus luteum diameter and plasma progesterone levels. Estrous cycle length was unaffected. Clomiphene, in the same experiment, caused premature vaginal opening in some treated animals, but corpus luteum size and plasma progesterone levels were unaffected and no ovulations occurred.The prolactin secretion inhibitor, CB-154, administered early in the estrous cycle, did not have any effect on estrous cycle length of the guinea pig alone or in combination with PGF_2α. The prostaglandin precursor, arachidonic acid, also failed to influence estrous cycle length when administered on days 8 and 9 of the cycle. Plasma progesterone levels remained unaltered.Oral administration of a prostaglandin synthetase inhibitor, (MK-715), caused a small, but non-significant (p>.05) prolongation of the estrous cycle. The progesterone biosynthesis inhibitors, aminoglutethimide and 6β-hydroxy-3α, 5α-cyclo-androstane-17-one did not effect estrous cycle length or plasma progesterone levels of the guinea pig.     

Analysis of androgen action on pituitary gonadotropin and prolactin secretion in ewes

To study the role of androgens in the control of gonadotropin and prolactin secretion in ther ewe, we have characterized androgen receptors in pituitary cytosol, and investigated the effect of androgens on pituitary hormone release in vivo and in vitro. High affinity, low capacity receptors, with an affinity for methyltrienolone (R1881) greater than 5 alpha-dihydrotestosterone (5 alpha-DHT) greater than testosterone (T) much greater than androstenedione (A4), estradiol-17 beta (E2) and progesterone (P), were identified in pituitary cytosol. Addition of 1 nM 5 alpha-DHT, but not A4, inhibited luteinizing hormone (LH) release from pituitary cells in vitro, induced by 10(10) to 10(-7) M luteinizing hormone releasing hormone (LHRH). The release of follicle-stimulating hormone (FSH) with 10(-9) M LHRH was inhibited when cells were incubated with 1 nM 5 alpha-DHT. 5 alpha-DHT had no effect when higher or lower doses of LHRH were used. In ovariectomized ewes, neither an i.v. injection of 1 mg, nor intracarotid injections of up to 1 mg, 5 alpha-DHT affected plasma LH, FSH or prolactin levels, despite dose-related increases in plasma 5 alpha-DHT levels. Daily or twice daily i.m. injections of 5 mg 5 alpha-DHT in oil did not affect LH or FSH levels, but daily injections of 20 mg significantly reduced plasma LH levels within 4 days and plasma FSH levels within 6 days. Thus, despite the presence of androgen receptors in the ewe pituitary, we conclude that androgens per se are of minimal importance in the regulation of pituitary LH, FSH and prolactin secretion in the ewe. The low binding affinity of A4 and the lack of its effect on hormone secretion in vitro suggests that A4 may act as an estrogen precursor rather than an androgenic hormone. The function of the pituitary androgen receptor remains to be established.     

The effects of serotonergic and adrenergic receptor antagonist on prolactin release in the monkey.

The influence of serotonergic and adrenergic antagonists on serum prolactin levels was studied in ketamine anesthetized monkeys. Methysergide, a serotonergic receptor blocker, at 0.035, 0.1 and 1 mg/kg body weight induced a rapid and transient increase in serum prolactin. Cyproheptadine, another serotonergic receptor blocker, at 0.05, 0.5 and 1 mg/kg induced a rapid and sustained increase in serum prolactin. SQ 10631, a third serotonergic receptor blocker, had a minimal effect on increasing basal prolactin levels even at doses as high as 10 mg/kg. Propranolol, a β adrenergic blocker, at a dose of 5 mg/kg induced a small sustained increase in serum prolactin, while a lower dose (1 mg/kg) had a slight but significant effect. Phentolamine, an α adrenergic receptor blocker, at a dose of 5 mg/kg induced a rapid and transient increase in plasma prolactin while a lower dose (1 mg/kg) had no effect. Phenoxybenzamine, a potent α adrenergic receptor blocker, had only a minimal effect on prolactin release even at doses of 3 and 5 mg/kg. It appears that the time course and extent of prolactin release differs among neural antagonists even within the same biogenic amine system.     

Endocrinological and behavioural effects of p-chlorophenylalanine (PCPA) oral administration to broody turkey hens (Meleagris gallopavo)

Changes in plasma levels of prolactin and LH, feed intake, water consumption, behavioural pattern and ovarian activity were recorded after oral administration of PCPA to broody turkey hens. A decrease in prolactin concentration was measured, from day 3, in 3 out of the 5 birds treated with 100 mg PCPA/kg body weight (BW) for 3 consecutive days. In these hens, broodiness was disrupted on day 6 and feeding activity subsequently increased to levels of photorefractory hens. Neither LH concentrations nor ovarian activity were affected after treatment with PCPA. Moreover, PCPA treatment was ineffective at a 50 mg/kg BW dose. These results confirm that a serotoninergic mechanism is probably involved in prolactin release and moreover suggest that prolactin is implicated in maintaining broody behaviour. However, the reductions in the plasma concentration of prolactin induced by PCPA were not sufficient to restore the hypothalamic-hypophyseal-ovarian axis to a physiological status characteristic of the laying hen. Therefore, PCPA does not appear to be a useful method of treating broodiness in commercial turkey hens.     

Prolactin and LH release in response to LH-RH and TRH in ewes during dioestrus, pregnancy and post partum

With advancing pregnancy in the ewe there was a marked decline in plasma LH concentrations and pituitary LH-RH responsiveness (integrated LH release) and a marked increase in plasma prolactin values and pituitary TRH responsiveness (integrated prolactin release). In lactating ewes plasma LH levels and pituitary LH-RH responsiveness had returned to values found in the luteal phase of the normal cycle by 21 days post partum, whereas at 42 days post partum prolactin levels were still high. No interaction between TRH and LH-RH on prolactin and LH release in dioestrous ewes was detected. In non-pregnant ewes plasma prolactin levels were significantly higher in June than in January but TRH responsiveness was similar. It is concluded that, in sheep, pituitary LH secretion recovers more rapidly from the chronic negative feedback effect of oestrogens and progesterone in pregnancy than prolactin secretion recovers from the chronic positive feedback effects of oestrogens. This finding may be a contributory factor in the resistance to resumption of breeding activity.     

Prostaglandin F2alpha (PGF2alpha) and prolactin secretion in rats.

Plasma prolactin and F-prostaglandins (PGF) were measured anesthetized male Sprague-Dawley rats before and at 15, 30, 45 and 60 minutes following i.v. injection of either PGF2alpha (4 mg/kg), chlorpromazine, 1 mg/kg or chlorpormazine (1 mg/kg) after pretreatment with i.p. indomethacin (2 mg/kg). Following PGF2alpha administration, plasma prolactin levels increased significantly only at 15 and 30 minutes in spite of extremely high PGF levels throughout 60 minutes. Besides the expected rise in plasma prolactin, chlorpromazine caused a transient but statistically significant increase in PGF. Indomethacin blocked the chlorpormazine-induced PGF rise but not prolactin increase. Animals stressed with ether anesthesia showed elevation of plasma prolactin, which was not blocked by indomethacin although PGF concentration fell. Theese results indicate that PGF2alpha can stimulate prolactin release. This effect does not appear to be physiologic since very high PGF levels are required. Furthermore, blockade of prostaglandin synthesis by indomethacin does not prevent the release of prolactin in response to chlorpormazine or stress. Our findings do not support a possible role of PGFs as intermediaries in prolactin release. However, it is possible that PGFs may work through other mechanisms not investigated in our study.     

Effects of a dopaminergic stimulant,amfonelic acid,on anterior pituitary hormone release in conscious rats

The response of plasma LH, Prolactin, GH and TSH levels to systematic administration of a specific central dopaminergic stimulant, amfonelic acid (AFA), by intravenous pulse injection in ovariectomized (OVX) and OVX estrogen-progesterone primed conscious rats has been evaluated. Intravenous injection of 0.2 mg/kg of AFA had no influence on plasma LH concentration until 60 min after injection when it was significantly elevated. Increasing the dose to 1 mg/kg reduced LH titers at 15 and 30 min with a return to preinjection levels by 60 min. AFA produced a dose-dependent decrease in plasma prolactin levels; the decrease occurred as early as 5 min after injection. AFA, both at 0.2 and 1 mg/kg doses, was effective in producing a sharp, dose-related rise in plasma GH levels. By contrast, TSH levels were significantly suppressed by both doses of AFA. Injection of the 1 mg/kg dose of AFA did not modify plasma LH levels in OVX-steroid-primed animals, white producing a comparable effect on plasma prolactin, GH and TSH levels to that observed in OVX animals. The present results indicate that endogenously released DA can have profound effects on pituitary hormone release, inhibiting PRL and TSH discharge, stimulating GH release and either inhibiting or stimulating LH release.     

Novel hyperprolactinemia and hyperprolactinemic anovulation model using the cynomolgus monkey (Macaca fascicularis)

We investigated the relationship between the menstrual cycle and hormone levels in cynomolgus monkeys, and developed a sulpiride-induced hyperprolactinemic anovulation model. On this study, we demonstrated the usefulness of the commercial human prolactin immunoradiometric assay kit for the measurement of cynomolgus monkey serum samples. In the normal menstrual cycle of the cynomolgus monkey, serum prolactin concentrations were not significantly different between luteal and follicular phases. However, the serum prolactin concentration tended to elevate at the ovulation stage. And serum progesterone began to increase after an estradiol surge, and then declined before the ensuing preovulatory rise in estradiol. During the luteal phase, the serum concentration of progesterone was elevated. Moreover, we aimed to develop an anovulation model, using sulpiride-induced hyperprolactinemia in the cynomolgus monkey. The serum prolactin level gradually increased during the twice-daily administration of sulpiride, and the drug produced as big a response at 5 mg/kg. In this study, the length of the menstrual cycle was approximately 29 days in normal cynomolgus monkeys. When treatment with sulpiride had been continued for more than one month, serum progesterone and estradiol levels fell to within the range seen in the follicular phase of the normal cycle, and the absence of ovulation was recognized by laparoscopy. Moreover, in this period we found that amenorrhea or anovulatory menstruation in the experimental animals. We could produce an anovulatory model induced by sulpiride repeatedly administered over a long time period. Our findings suggest that the cynomolgus monkey is useful as a endocrinological model that uses prolactin as a parameter and as an anovulatory model; thus, it could be a useful model for the hyperprolactinemic amenorrhea and/or anovulation seen in humans.     

Serum prolactin levels and crop-sac development in ring doves during a breeding cycle

Using a turkey prolactin radioimmunoassay, the serum prolactin levels of male and female ring doves (Streptopelia risoria) during the breeding cycle were measured and their circulating prolactin levels were compared to crop-sac weight on a within-bird basis. During the early phase of the incubation period, crop weight showed a delayed response to prolactin stimulation and there was no correlation; during the midincubation period when prolactin and crop-sac weight were increasing, there was a strong positive correlation; around hatching, when prolactin was at its peak, there was no correlation. There were again strong positive correlations at later samples, when squabs were developing and both prolactin and crop-sac weight were declining. Thus, it appears that the correlation between the circulating prolactin level and crop-sac development depends on the stage of the breeding cycle. While males and females showed similar pattern of circulating prolactin during the period of incubation and parental care, only females consistently showed a postovulatory rise of prolactin. These results were discussed in the context of the role of prolactin in the breeding cycle.     

Twenty-four hour rhythm of plasma prolactin in female rabbit pups. Correlation with hypothalamic and adenohypophysial dopamine, serotonin, gamma-aminobutyric acid and taurine content

Lactation in the rabbit is a nocturnal activity, extremely short and regular, that can be a strong synchronizer for the development of circadian rhythmicity in the pups. In the present study, 24-h rhythmicity of plasma prolactin and median eminence and anterior pituitary content of dopamine (DA), serotonin (5HT), gamma-aminobutyric acid (GABA) and taurine were examined in 11 days old female pups kept under 16 h light:8 h dark photoperiods (lights on at 08:00 h). Groups of six to seven female rabbit pups were killed by decapitation at six different time points throughout a 24-h cycle, starting at 09:00 h. Plasma prolactin levels changed significantly throughout the day, showing two peaks, one at first half of rest span (at 13:00 h) and another one at the beginning of the scotophase (at 01:00 h), just preceding doe visit. Median eminence DA content changed in a bimodal way as a function of time of day, displaying two maxima, at the beginning of the rest span and of the activity phase. Median eminence DA and plasma prolactin correlated significantly in an inverse way. Two maxima in median eminence 5HT levels were found, about 4 h in advance to the prolactin peaks. Circulating prolactin correlated inversely with median eminence 5HT content and directly with adenohypophysial 5HT content. Median eminence GABA content reached its maximum at the beginning of the scotophase and correlated significantly with plasma prolactin concentration. A positive correlation between plasma prolactin and adenohypophysial taurine content was observed. These results show that the circadian rhythmicity in prolactin secretory mechanisms in female rabbit pups develops during the early neonatal life.