Differential inhibition of dopamine and bromocriptine on induced prolactin release: multiple sites for the inhibition of dopamine.

Effects of dopamine and bromocriptine on TRH- or dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP)-induced prolactin release from primary cultured rat pituitary cells were studied using a perifusion system. TRH (100 nmol/l) stimulated prolactin release from basal concentrations of 33.8 +/- 0.5 to 151.2 +/- 28.0 ng/ml (net increase) or 447% increase. Dopamine inhibited the basal release of prolactin throughout the experiment, but TRH (100 nmol/l) was still able to stimulate prolactin release under the influence of dopamine. The increment in prolactin release was inversely proportional to the dopamine concentration. When TRH (100 nmol/l) was introduced during a perifusion period with bromocriptine 1 nmol/l, the prolactin concentration was increased to 110.9% of basal levels. The stimulatory effect of TRH under the influence of bromocriptine (1 nmol/l) was significantly lower than that without bromocriptine (control), although the higher concentrations of bromocriptine (10 and 100 nmol/l) did not further reduce the peak concentration of TRH-induced prolactin release. During a perifusion period with a low concentration of dopamine (1 nmol/l plus 0.1 mmol/l ascorbic acid), introduction of dbcAMP (3 mmol/l) stimulated prolactin release to 48% of basal concentration. A higher concentration of dopamine further reduced the stimulatory effect of prolactin release. Bromocriptine impeded the stimulatory effect of dbcAMP (3 mmol/l) on prolactin release in a similar manner as dopamine. Since a higher concentration of bromocriptine (10 and 100 nmol/l) did not further inhibit the TRH-induced prolactin release whereas a higher concentration of dopamine did, it is concluded that dopamine acts through additional mechanism(s) other than the D2 receptor transduction system.     

A study on the prolactin releasing mechanism using an in vitro perfusion system with a cell column of cultured rat anterior pituitary cells

It is well-known that the hypothalamus predominantly exerts an inhibitory control on prolactin secretion and that dopamine (DA) is the main prolactin inhibiting factor (PIF). In addition, the hypothalamus contains prolactin-releasing factors (PRF). Thyrotropin-releasing hormone (TRH), vasoactive intestinal polypeptide (VIP) and peptide-histidine-isoleucine (PHI) are the components of PRF. However, the detailed mechanism by which the peptides release prolactin (PRL) at the pituitary level is still unknown. Therefore, in this paper, an in vitro perifusion system using the cell column of cultured rat pituitary cells attached on Cytodex beads was employed to investigate the mechanism of PRL release. The rat anterior pituitary cells were isolated using collagenase, and the dispersed pituitary cells were cultured with swollen Cytodex beads in Dulbecco's modified Eagle medium (DMEM) containing fetal calf serum at 37 degrees C in 5% CO2 and 95% air for 2--3 days. The cultured anterior pituitary cells attached on Cytodex beads were packed in a column and perifused with DMEM at a constant flow rate of 0.4 ml/min using a peristaltic pump. The following results were obtained. A five minute perifusion with 100 pg/ml to 100 ng/ml TRH caused a significant increase of PRL in a dose-related manner. A continuous perifusion with 2 ng/ml or 10 ng/ml DA inhibited PRL release in a dose-related manner. When TRH at a dose of 1 ng/ml, 10 ng/ml or 100 ng/ml was perifused for 120 min at a rate of 0.4 ml/min, a large amount of PRL was released during the early period of the TRH infusion, and then the PRL release gradually decreased to the basal levels in spite of the continuous TRH infusion. An additional TRH, of which the concentration was ten-fold higher than the TRH level in the continuous infusion, when added at the end of the continuous TRH infusion, had no effect on PRL release. On the other hand, a 5 minute TRH infusion given at 30 min after the end of the continuous TRH infusion caused a significant increase in PRL release. A continuous perifusion with 1 mM 8-bromo-cyclic AMP caused a small but continuous PRL release. An additional continuous 8-bromo-cyclic AMP infusion during the late period of a continuous TRH infusion caused a continuous PRL release similar to that induced by the continuous infusion of cyclic AMP only. A short period perifusion with 1 X 10(-9)M to 1 X 10(-7)M of vasoactive intestinal polypeptide (VIP) enhanced a significant increase of PRL release in a dose-related manner, but the amounts of PRL release induced by VIP were smaller than those induced by TRH.(ABSTRACT TRUNCATED AT 400 WORDS)     

Modulation of thyrotrophin release from an intracellular pool by pre-exposure to thyrotrophin-releasing hormone and dibutyryl cyclic AMP

Amplification of desensitization of TSH response to thyrotrophin-releasing hormone (TRH) may be important mechanisms in the regulation of its secretion. We have investigated this possibility in vitro, using monolayer culture of rat anterior pituitary cells. Cells (1-1.5 X 10(5)/250 microliters per well) were cultured for 72 h, exposed to TRH or dibutyryl cyclic AMP (dbcAMP) for 6 or 8 h, washed, and then treated for 4 h with various doses of TRH, or with K+ (55 mmol/l) as a non-specific secretagogue. Pretreatment with TRH (20 nmol/l) for 8 h reduced subsequent TSH release: basal release fell to 64% of the control value (1.01 +/- 0.10 micrograms/l pretreated, 1.58 +/- 0.16 control) and release in response to TRH (100 nmol/l) to 69% of the control (2.7 +/- 0.19 micrograms/l vs 3.98 +/- 0.22); K+ response was reduced to 86% of the control (3.77 +/- 0.21 micrograms/l vs 4.39 +/- 0.20), significantly less than the other reductions. The extent of the parallel downward shift of the TRH dose-response curve was proportional to dose and duration of prior TRH exposure. There was no significant change in the dose of TRH required to cause half-maximal TSH release (ED50: pretreated 4.8, control 2.8 nmol TRH/l) suggesting depletion of an intracellular pool of TSH rather than 'desensitization'. After 6-h pretreatment with dbcAMP, subsequent TSH responses were augmented: basal release was 130% of the control, response to TRH (100 nmol/l) was 137% and to K+ it was 132% of the control, with a parallel upward shift of the TRH dose-response curve but no change in cellular TSH content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies of TRH-induced prolactin secretion and its inhibition by dopamine, using ovine pituitary cells

Prolactin secretion from ovine pituitary cell cultures was stimulated by thyrotropin-releasing hormone (TRH) (10(-10)-10(-7) M) with a half-maximal effect at approximately 2.5 X 10(-9) M. A maximally effective concentration of TRH produced a peak secretory response, 5-10-fold stimulation over basal release, within 15 min. Dopamine (10(-10)-10(-7) M) but not somatostatin caused a dose-related inhibition of TRH (10(-8) M) stimulated prolactin release. Both dopamine (10(-7) M) and somatostatin (10(-7) M) inhibited basal secretion from the cells. TRH did not significantly increase pituitary cell cyclic AMP levels under any of the conditions tested. Stimulation of prolactin secretion by TRH was not prevented when Ca2+ was omitted from the incubation medium. Dopamine inhibited secretion induced by TRH under low Ca2+ conditions. Our results are consistent with a hypothesis that TRH may stimulate prolactin secretion via release of intracellular Ca2+ rather than increased cellular Ca2+ uptake, and imply that dopamine inhibition involves a lowering of intracellular Ca2+ levels.     

Mechanisms of release of prolactin from fowl anterior pituitary glands incubated in vitro: effects of calcium and cyclic adenosine monophosphate

Fowl anterior pituitary glands were bisected and each half was pretreated in either Medium 199 or medium containing EGTA to deplete endogenous calcium (Ca2+) stores, after which they were incubated in Medium 199, or Ca2+-free medium, containing prolactin release-stimulating agents and verapamil, a Ca2+ channel blocker. High K+ concentrations, hypothalamic extract, synthetic thyrotrophin-releasing hormone (TRH) and dibutyryl cyclic AMP (dbcAMP) all stimulated release of prolactin from control (non EGTA-treated) hemianterior pituitary glands. The effects of TRH and dbcAMP were not additive, but the response to submaximal concentrations of TRH was augmented by theophylline, a phosphodiesterase inhibitor. Reduction of Ca2+ availability with EGTA or verapamil reduced basal release of prolactin, prevented the prolactin-stimulating effects of high K+ concentrations and TRH, and markedly attenuated responses to hypothalamic extract and dbcAMP, EGTA being more effective than verapamil. Increasing the Ca2+ concentration of the medium did not augment basal or stimulated release of prolactin. These results suggest that both Ca2+ and cyclic AMP may act as intracellular mediators in the release of prolactin. Both basal and stimulated release of prolactin depend upon the presence of Ca2+. Although influx from the medium may be the major source of Ca2+, endogenous stores of Ca2+, perhaps mobilized by dbcAMP, may be able to maintain some release of prolactin. The prolactin-stimulating effects of TRH may be mediated by cyclic AMP.     

Effect of serotonin on basal and TRH-induced release of prolactin from rat pituitary glands in vitro

The effect of serotonin on the release of prolactin (PRL) was studied in vitro. Anterior hemipituitary glands from ovariectomized rats were incubated for 1 h in the presence of different doses of serotonin. Serotonin added into the culture medium caused a significant increase in basal PRL release. The effect was dose-related between 10 and 30 nmol/l serotonin, but responsiveness declined towards basal levels with higher concentrations. When studied as a function of incubation time, basal release of PRL was significantly increased up to 1 h but decreased thereafter. Serotonin also enhanced the release of prolactin induced by 30 nmol/l thyrotropin-releasing hormone (TRH), at all doses tested. A serotonin concentration of as little as 30 nmol/l was already effective. A significant response was seen at 15 min and further increases occurred during the following incubation periods. Serotonin (approximately EC50 4.6 X 10(-8) mol/l) was less potent than TRH (EC50 about 1.2 X 10(-8) mol/l) to increase basal PRL release. On the other hand, the indole amine appeared to act with similar potency in stimulating PRL release both basal and TRH-induced. In addition, the combined effect of the releasing agents was found to be additive. These results suggest that serotonin and TRH could act through separate mechanisms. Methysergide, a serotoninergic blocking agent, had no effect on the in vitro PRL release either basal or TRH-induced, but it completely blocked that evoked by serotonin suggesting that serotonin may interact with specific receptors on the lactotropes. These findings clearly demonstrate that serotonin may stimulate the release of PRL by acting directly at the pituitary gland level.     

Sex differences in circadian control of LH secretion

We have studied the secretion of TSH and prolactin from perifused rat anterior pituitary glands in vitro in response to single pulses of thyrotrophin releasing hormone (TRH) and KCl after prior exposure to TRH. Anterior pituitary fragments were incubated in normal medium or in medium containing 28 nmol TRH/1 for 20 h before perifusion. Thyrotrophin releasing hormone (28 nmol/l), administered as a 3-min pulse, stimulated TSH and prolactin release from control tissue to a peak value four or five times that of basal. After exposure of the pituitary tissue to TRH for 20 h, the subsequent response of TSH to a 3-min pulse of TRH was, however, markedly reduced; in contrast, the prolactin response was not significantly reduced. In a similar series of experiments KCl (60 mmol/l) was administered to both control and TRH-'treated' pituitary tissue as a 3-min pulse; no significant differences in TSH responses or prolactin responses were observed. These data indicate that TRH desensitizes the pituitary thyrotroph to a subsequent TRH stimulus but has very little effect on prolactin secretion.     

Weak estrogenic activity of phenol red in the culture medium: its role in the study of the regulation of prolactin release in vitro

Phenol red, which is commonly used in culture media as a pH indicator, has recently been shown to possess estrogenic properties. In this study we investigated the effects of phenol red on prolactin release and synthesis by cultured female and male rat anterior pituitary cells and on the sensitivity of these cells of dopamine, TRH and somatostatin (SRIF). It was shown that phenol red stimulated rat prolactin release and cell content in a dose-dependent manner. The effects of 30 microM phenol red, which is the medium concentration in our regular culture medium, and a submaximally active concentration of 17 beta-estradiol (E2) were additive. Male rat pituitary cells were far more responsive to phenol red and also to E2 than female pituitary cells. The antiestrogen tamoxifen (100 nM) significantly inhibited the phenol red-stimulated prolactin release by male rat pituitary cells but caused a 2-fold increase of prolactin release in the absence of phenol red. 30 microM phenol red did not modulate the responsiveness of female and male rat lactotrophs to dopamine, TRH or SRIF. We propose from our results that the estrogenic effect of 30 microM phenol red is too weak in order to alter the responsiveness of rat lactotrophs to dopamine, TRH and SRIF but the presence of phenol red in culture media should be considered when the effects of estrogens and antiestrogens on rat prolactin release and synthesis in vitro are studied.     

Effects of two enkephalin analogues, morphine sulphate, dopamine and naloxone on prolactin secretion from rat anterior pituitary glands in vitro

Experiments were carried out on the antagonistic effects of opiates on the inhibition by dopamine of prolactin secretion from rat anterior pituitary glands. Dose-response and time-course experiments were carried out using both static incubation of paired hemipituitary glands and perifusion of whole glands. Dopamine (10-1000 nmol/l) was found to have an inhibitory effect on prolactin secretion, but at a lower concentration (0.1 nmol/l) a small stimulation was observed. Against an inhibition established with 100 nmol dopamine/l in static incubation, the three opiates under study, morphine sulphate, Leu5-enkephalin and D-Ala2,Met5-enkephalin (DAME), had a maximum antagonistic effect at 50-1000 nmol/l in a 90-min incubation. Morphine and DAME were rather more effective than Leu5-enkephalin, possibly because of degradation of the latter. Naloxone reversed the effect of morphine. All three opiates showed little effect on dopamine-inhibited prolactin secretion in a perifusion system. The data accord with previous suggestions that prolactin secretion may be stimulated both by very low concentrations of dopamine and by opiates acting to reverse the inhibition exerted by higher dopamine concentrations. It should be noted that both morphine and the enkephalins have similar effects on prolactin secretion, despite their normal specificity for different opiate receptors; their actions on the pituitary may thus be rather non-specific.     

Suppression of prolactin release and mRNA accumulation by two novel dopamine agonist agents

Two novel dopamine agonist drugs, CV 205-502 and CQP 201-403, have been investigated to compare their effects on prolactin secretion and prolactin mRNA accumulation in cultured rat pituitary cells. Both drugs gave dose-dependent suppression of prolactin release over a 24 h incubation period: when each drug was used at 100 nmol/l CV 205-502 and CQP 201-403 induced suppression to 8.9 +/- 1.7 and 10.2 +/- 1.8% of control release, respectively, compared to 26.7 +/- 4.8% of control with 100 nmol/l bromocriptine. There was no consistent effect on growth hormone release. Cytoplasmic accumulation of prolactin mRNA was also inhibited by both drugs at this concentration, to 50.2 +/- 5.5% of control values by CV 205-502 and to 67.4 +/- 8% of control by CQP 201-403, and to a similar extent by 100 nmol/l bromocriptine (50.6 +/- 9.1% of control). None of the drugs had any significant effect on GH mRNA levels. These data suggest that the agents exert their effect at a pretranslational stage of prolactin synthesis, as well as at the level of hormone release.     

2-Hydroxyoestradiol acutely inhibits prolactin secretion from the superfused pituitary glands of normal female rats: evidence for a cyclical effect

Catecholoestrogens are naturally occurring metabolites of oestrogens which are found in brain tissue and for which a neuroendocrine role has been postulated. However, reports of their effects on prolactin secretion are ambiguous and as yet no defined function has been attributed to them. The effects of 2-hydroxyoestradiol (2-OHE2) and dopamine on the release of prolactin in vitro by perfused pituitary glands from normal adult female rats at different stages of the oestrous cycle have been investigated. The purity and stability of the 2-OHE2 preparation before and after exposure to pituitary tissue was confirmed by radioenzymatic assay and subsequent thin-layer chromatography. Dopamine (500 nmol/l, 100 nmol/l) was found consistently to suppress release by 60%; this effect was immediate and reversible upon removal of the dopamine. In contrast, the effects of 2-OHE2 (10 nmol/l, 100 nmol/l) were found to vary during the cycle. No effect on prolactin release was evident during either dioestrus or pro-oestrus, but during oestrus a similar, though less potent, suppression of prolactin secretion to that of dopamine was observed (35% suppression compared with controls). The cyclical variation in the suppressive effect of 2-OHE2 on prolactin secretion in the female rat is compatible with a postulated neuroendocrine role for this catecholoestrogen.     

A high concentration of dopamine preferentially permitted release of newly synthesized prolactin

We used continuous labelling ([3H]leucine) of cultured adenohypophysial cells to investigate the relationship between the storage and release of newly synthesized and stored prolactin in response to dopamine (1 mumol/l) and thyrotropin-releasing hormone (TRH) (0.1 mumol/l) challenge. Newly synthesized prolactin was identified by the tritium radiation activity incorporated in prolactin. A maximal dose of dopamine (1 mumol/l) could not completely block prolactin release from a primary culture of lactotrophs. During 3 h of continuous labelling under maximal dopaminergic inhibition, newly synthesized prolactin was released which was of a significantly higher specific activity than control groups. In contrast, TRH stimulation produced results consistent with previous observations of the release of predominantly old, stored hormone. However, the absolute amount of the newly synthesized prolactin was increased by the TRH administration, and the increased release of the newly synthesized prolactin could be accounted for by increased levels of synthesis. Our results are consistent with the concept of the existence of a regulated route and a dopamine-insensitive constitutive route of prolactin release which predominantly encompasses newly synthesized hormone. However, the possibility that cellular heterogeneity or that non-dopaminergic prolactin-release inhibiting factor(s) (PIF) is responsible for this observed release cannot be ruled out.     

Dual action of dopamine on growth hormone release in vitro

The effect of dopamine (DA) on growth hormone (GH) release was studied in perifused freshly dispersed rat anterior pituitary cells. Pulses of DA (0.01-100 nmol/l), each applied for 30 min, resulted in a prompt rise in GH release. This effect was reversible, concentration-dependent and partially antagonized by metoclopramide, a DA antagonist. The effect of DA was further tested on GH-stimulated secretion by human GH-releasing factor (hGRF). Perifusion with hGRF (6.25 pmol/l) for 2 min elicited an immediate rapid increase in GH release which lasted 20 min. Pretreatment of cells with DA (100 nmol/l) for 10 min and a subsequent hGRF challenge during continuation of DA perifusion significantly reduced the effect of hGRF pulses on GH release. The present data suggest that DA has direct opposite actions at the somatotroph level, stimulating the basal GH release and inhibiting the hGRF-induced GH secretion, and may thus be an important modulator of GH release.     

In-vitro control of growth hormone secretion by synthetic releasing factors in young and adult ducks (Anas platyrhynchos)

Release of GH from perifused duckling hemipituitaries was stimulated, in a biphasic manner, by synthetic TRH and human pancreatic GH-releasing factor (GRF). At all effective concentrations, the level of GH release was increased within 5 min of TRH or GRF perifusion and was maximal after 10 min of TRH perifusion and after 20 min of GRF perifusion. Although TRH was perifused for 20 min the level of GH release declined during the last 10 min. The most effective dose of TRH (1.0 micrograms/ml; 2.7 mumol/l) and GRF (0.5 micrograms/ml; 110 nmol/l) provoked similar (250-300%) increases in the level of GH release. However, since the effect of TRH was only of short duration, the total release of GH induced by GRF was higher than that elicited by TRH, especially with the low dose. The increase in release of GH induced by TRH or GRF was blunted when pituitaries from adult ducks were used. As in young ducks, the GH response to GRF was higher, whereas the response to TRH was very low. The GH response of perifused adult pituitaries to GRF was, however, potentiated when TRH was perifused simultaneously. The basal release of GH from both young and adult pituitary glands was unaffected by perifusion with somatostatin-14 (SRIF-14) at doses of 1 and 2 micrograms/ml. The perifusion of hemipituitary glands with similar doses of SRIF-14 was also unable to suppress the stimulation of GH release induced by prior perifusion with GRF, although when SRIF-14 and TRH were simultaneously perifused TRH-induced GH release was markedly suppressed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of prolactin secretion by low-dose dopamine infusion in patients with hyperprolactinaemia

Dopamine inhibits the secretion of prolactin from the pituitary. We have examined the relation between plasma dopamine and serum prolactin in 12 patients with hyperprolactinaemia during the infusion of dopamine at low doses (0.01, 0.1 and 1 microgram/kg/min). Plasma dopamine levels were raised from less than 100 pg/ml at the lowest rate of infusion to more than 20 000 pg/ml at the highest. Suppression of prolactin secretion was seen in some patients even at the lowest rate of infusion (0.01 microgram/kg/min); marked suppression of prolactin secretion (60%; 17--83%) was found at the intermediate dose (0.1 microgram/kg/min) in 11 of the 12 subjects with little further decrease in serum prolactin (70%; 50--87%) in those in whom the rate of dopamine infusion was increased ten-fold. One patient with the highest serum prolactin (82 500 mu/l) showed no decrease in prolactin either at the lowest or intermediate rates of dopamine infusion. Serum prolactin levels returned to values similar to or greater than basal on cessation of dopamine infusion. Infusion of dopamine at doses much lower than previously used in man exposes the pituitary to a concentration of dopamine sufficient to suppress prolactin secretion. These observations have important implications in understanding the pathophysiology of prolactin secretion from the pituitary gland and for future investigations of the control of hormone release by dopamine.     

Phenoxybenzamine selectively and irreversibly inactivates dopaminergic D2 receptors on primary cultured rat lactotrophs.

Lactotrophs have several different kinds of receptors, such as dopaminergic D2, somatostatin, angiotensin II and thyrotropin-releasing hormone receptors, which stimulate or inhibit prolactin release. We have studied the specificity of phenoxybenzamine on receptors in lactotrophs. Phenoxybenzamine is a beta-haloalkylamine which alkylates chemically active radicals such as hydroxy, sulfhydryl, and amino groups. This alkylation is an irreversible chemical reaction in contrast to the receptor-secretagogue complex which is present in a state of dynamic equilibrium. Primary cultured rat adenohypophyseal cells were used in this study. A dose-response relationship was examined between concentrations of phenoxybenzamine pretreatment and prolactin release using a monolayer cell culture system. The inhibitory action of dopamine (10 mumol/l) on the control group (13.0 +/- 0.1 ng/ml or 86% inhibition relative to the control) was significantly higher than on the 0.1-mumol/l phenoxybenzamine-pretreated group (39.0 +/- 0.2 ng/ml or 58% inhibition relative to the control), but the stimulatory effect of thyrotropin-releasing hormone on prolactin release was not significantly affected up to a 10-mumol/l phenoxybenzamine pretreatment as compared with the control group. We thus selected a phenoxybenzamine concentration of 0.1 mumol/l for the next series of perifusion experiments in order to examine dynamic changes in prolactin release. The basal prolactin release was decreased to almost half by phenoxybenzamine pretreatment. The inhibitory action of dopamine (0.1 mumol/l containing 0.1 mmol/l ascorbic acid) was significantly less in the phenoxybenzamine-pretreated group (68% of the basal prolactin concentration) than in the control group (31% of the basal concentration).(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary gonadotrophin-releasing hormone receptor up-regulation in vitro: dependence on calcium and microtubule function

Exogenous cyclic adenosine nucleotides increase gonadotrophin-releasing hormone (GnRH) receptors in intact cultured rat pituitary cells in a similar manner to that observed with GnRH itself. In this study the calcium and microtubule dependency of GnRH receptor up-regulation was examined in vitro. Treatment of pituitary cells in Ca2+ and serum-containing media with either GnRH (1 nmol/l), K+ (58 mmol/l) or dibutyryl cyclic AMP (dbcAMP; 1 mmol/l) for 7-10 h routinely resulted in a 50-100% increase in GnRH receptors. Incubation of pituitary cells with the calcium channel blocker verapamil, for 7 h, or the calcium chelator EGTA, for 10 h, had no effect on basal receptor levels but prevented the increase in GnRH receptors stimulated by either GnRH, K+ or dbcAMP. Luteinizing hormone release measured with the same stimulators over a 3-h period was prevented by both verapamil and EGTA. Calcium ionophore (A23187) increased GnRH receptors by 40-60% at low concentrations (10 and 100 nmol/l) while higher concentrations (10 and 100 mumol/l) reduced receptor levels. Luteinizing hormone release was not increased by receptor-stimulating concentrations of A23187, but was by higher concentrations (10 mumol/l). None of these pretreatments, for up to 10 h, impaired the subsequent LH response of the cells to increasing doses of GnRH. Vinblastine (1 mumol/l did not affect basal receptor levels but markedly reduced the increase in GnRH receptors stimulated by GnRH, K+ and dbcAMP. This concentration of vinblastine had no effect on LH release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Normal and growth hormone (GH)-secreting adenomatous human pituitaries release somatostatin and GH-releasing hormone

Neuropeptides such as vasoactive intestinal peptide, LHRH, or TRH have been found in rat pituitary tissue and could act via paracrine or autocrine actions in this tissue. In this study we investigated whether normal human pituitary tissue and GH-secreting human pituitary adenomas could release somatostatin (SRIH) and GHRH. Fragments from three human pituitaries and dispersed cells from six GH-secreting adenomas (four adenomas were studied for GHRH release and five for SRIH release) were perifused using a Krebs-Ringer culture medium, and the perifusion medium was collected every 2 min (1 mL/fraction for 5 h). GH, GHRH, and SRIH were measured by RIA under basal conditions and in the presence of 10(-6) mol/L TRH or SRIH. Both normal pituitaries and GH-secreting pituitary adenomas released SRIH and GHRH. SRIH release commenced 90-180 min after initiation of the perifusion, at which time GH secretion had decreased significantly. TRH stimulated SRIH release from normal pituitary tissue and inhibited SRIH release from adenoma tissue. GHRH was present at the start of the perifusion, but rapidly disappeared. However, SRIH stimulated GHRH release from normal pituitary tissue, but not from adenoma tissue. Significant amounts of GHRH and SRIH were released during the experiments, suggesting their local synthesis. These results indicate that pituitary cells can release hypothalamic peptides. The liberation of these neuropeptides is regulated, and moreover, their regulation differs between normal and adenomatous pituitaries.     

Evidence that serotonin is involved in prolactin release by electrical stimulation of the medial basal hypothalamus in the rhesus monkey

The possibility that serotonin plays a role in prolactin secretion after electrical stimulation of the rhesus medial basal hypothalamus (MBH) was investigated. Prolactin responses to electrical stimulation and intravenous injection of 0.5 and 1.0 micrograms of thyrotropin-releasing hormone (TRH) were evaluated before and after administration of methysergide (MES), a serotonin receptor blocker (2 mg orally every 12 h for 48 h), and bromocriptine (CB-154), a dopamine agonist (2.5 mg orally every 12 h for 48 h). Both electrical stimulation and TRH caused prompt increases in serum prolactin. Prestimulation (basal) prolactin levels in both drug-treated groups were not significantly lower than basal levels in control groups. Pretreatment with MES significantly attenuated the electrically induced release of prolactin but had no effect on the TRH-induced release; CB-154 blocked prolactin release induced by both types of stimulation. The study reported here has provided evidence of a possible role for hypothalamic serotinin in releasing pituitary prolactin.     

Interaction between substance P and TRH in the control of prolactin release

Substance P (SP) may participate as a paracrine and/or autocrine factor in the regulation of anterior pituitary function. This project studied the effect of TRH on SP content and release from anterior pituitary and the role of SP in TRH-induced prolactin release. TRH (10(-7) M), but not vasoactive intestinal polypeptide (VIP), increased immunoreactive-SP (ir-SP) content and release from male rat anterior pituitary in vitro. An anti-prolactin serum also increased ir-SP release and content. In order to determine whether intrapituitary SP participates in TRH-induced prolactin release, anterior pituitaries were incubated with TRH (10(-7) M) and either WIN 62,577, a specific antagonist of the NK1 receptor, or a specific anti-SP serum. Both WIN 62,577 (10(-8) and 10(-7) M) and the anti-SP serum (1:250) blocked TRH-induced prolactin release. In order to study the interaction between TRH and SP on prolactin release, anterior pituitaries were incubated with either TRH (10(-7) M) or SP, or with both peptides. SP (10(-7) and 10(-6) M) by itself stimulated prolactin release. While 10(-7) M SP did not modify the TRH effect, 10(-6) M SP reduced TRH-stimulated prolactin release. SP (10(-5) M) alone failed to stimulate prolactin release and markedly decreased TRH-induced prolactin release. The present study shows that TRH stimulates ir-SP release and increases ir-SP content in the anterior pituitary. Our data also suggest that SP may act as a modulator of TRH effect on prolactin secretion by a paracrine mechanism.