Dopamine decreases release of luteinizing hormone releasing hormone from superfused rat mediobasal hypothalamus

The effects of dopamine hydrochloride (DA) on the releases of LHRH and LH were examined in a serial sequential double chamber perifusion system by perifusing the mediobasal hypothalamus including the preoptic area and/or pituitaries excised from diestrus female rats. DA, perifused at a dose of 4.2 X 10(-4) M, significantly (p less than 0.05) lowered LH secretion from the pituitary in series with the hypothalamus 40-80% below the preinfusion level, but it had no effect on LH efflux from the pituitary perifused alone. DA also significantly (p less than 0.05) reduced the LHRH level 20-40% below the initial level. Perifusion with the DA receptor blocker haloperidol at a concentration of 10(-6) M abolished the suppressive effect of DA on LH secretion. These findings indicate that DA suppressed hypothalamic LHRH release, resulting in decrease in LH secretion from the pituitary in diestrus rats.     

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.     

Effect of insulin-like growth factor I on gonadotropin release from the hypothalamus-pituitary axis in vitro

The effects of insulin-like growth factor-I on gonadotropin release were studied using primary culture of rat anterior pituitary cells incubated with IGF-I (20-5000 micrograms/l) and a hypothalamus-pituitary perifusion system, in which either the mediobasal hypothalamus-pituitary unit or the pituitary were perifused with IGF-I (20-2000 micrograms/l). In primary cultures of rat anterior pituitary cells, IGF-I (2000 micrograms/l) caused a significant increase in the release of both LH (46% increase) and FSH (27% increase). It also caused a significant decrease in the cellular content of LH (9%) and FSH (19%). Its effects in stimulating gonadotropin release were suppressed by administration of anti IGF-I receptor antibody (1 mg/l). In the perifusion system, IGF-I (2000 micrograms/l) did not affect the LH release from the hypothalamus-pituitary or pituitary alone. However, it caused a significant increase in the GnRH (10(-9) mol/l) stimulated LH release from perifused pituitary. These data suggest that IGF-I enhances pituitary gonadotropin release via the IGF-I receptor, but its effect on the hypothalamus was not confirmed.     

Prolonged sensitisation of pituitary glands in vitro to repeated administration of luteinising hormone-releasing hormone: effects of pulse frequency, ovariectomy, estradiol and progesterone

Anterior pituitary gland fragments were obtained from female Wistar-derived rats on dioestrus or pro-oestrus and perifused in Biogel columns in vitro. They were subjected at the beginning of each of 5 h of perifusion to a volley of 6 1-min pulses of luteinising hormone-releasing hormone (LHRH, 10 nM), given 4 min apart. Luteinising hormone (LH) was measured by radioimmunoassay in sequential 2-min fractions of the perifusate. Pituitary glands removed at 14.00 h on dioestrus showed a characteristic pattern of sensitisation followed by desensitisation to the repeated volleys of LHRH, whereas tissues removed at 10.00 or 14.00 h on pro-oestrus showed no evidence of desensitisation over the 5-hour period, the response to each LHRH volley being greater than the preceding one. Estradiol (E, 3-100 pg/ml) added to the medium from the start of perifusion had no significant effect on the pattern of response from tissues removed on pro-oestrus, but the highest concentration significantly enhanced the response of dioestrus pituitaries to all but the last of the LHRH volleys. Progesterone (P, 1-50 ng/ml) added to the medium produced a dose-related inhibition of the response of pro-oestrous tissues to the LHRH volleys. Groups of animals were ovariectomised (OVX) on dioestrus and used for experiment the next morning. OVX at 10.00 h on dioestrus produced the pattern of response characteristic of dioestrus the next morning, but with much higher levels of LH release, which were unaltered by the addition of E to the medium. OVX at 17.00 h on dioestrus produced an entirely different pattern of response the next day, with high basal and moderate LHRH-induced LH release, and no evidence of changes in sensitivity of the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of time in culture on the stimulatory effect of 17-beta estradiol on adenohypophyseal hormone release (author's transl)]

Secretion of LH, FSH, TSH and prolactin was studied in rat anterior pituitary cells in primary culture. Basal LH release, LH cell content and LH responsiveness to LHRH decreased with a half-life of 4 days. Basal FSH decreased only 20% up to 20 days thus indicating the relative independence of FSH secretion compared to that of LH. The stimulatory effect of 17-beta estradiol on LH and FSH secretion was higher during the first days in culture. The stimulatory effect of 17-beta estradiol on prolactin secretion did however remain maximal up to 20 days.     

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)     

Autoregulation of follicle-stimulating hormone secretion in the hamster: evidence for control at the level of the anterior pituitary gland

Pituitary tissue obtained from proestrous, phenobarbital-treated hamsters was placed in organ culture, and the release rates of FSH and LH were monitored. Addition of LHRH to the culture medium increased gonadotropin release rates. Preincubation of pituitaries in medium that contained 1 microgram purified FSH ( NIADDK rat FSH-I-5) resulted in enhanced basal and LHRH-induced FSH release rates. Full expression of FSH hypersecretion by pituitary tissue occurred after 3-h exposure to purified FSH. This phenomenon appears to require adequate tissue calcium concentrations. Further, exposure of pituitaries to purified FSH slightly but significantly enhanced basal but not LHRH-stimulated LH release compared with that in untreated controls. Experiments that employed labeled FSH, reduced exposure time to purified FSH, or decreased calcium concentration in the medium proved that the increased FSH secretory rate was not due to contamination of the medium with the purified FSH used to stimulate the pituitary. These studies suggest that FSH has the ability to influence its own secretion by an action at the level of the anterior pituitary gland.     

Effects of ovarian steroids on in vitro release of LHRH from mediobasal hypothalamus

The phasic luteinizing hormone (LH) release observed in ovariectomized (OVX), estrogen-implanted rats was further amplified and advanced when progesterone (P) was given 4 h prior to the gonadotropin surge. In contrast, an inhibitory effect of P on the daily LH surge was observed when P was administered 16-36 h prior to LH peak. In order to determine whether this biphasic action of P is primarily exerted on the release of luteinizing hormone releasing hormone (LHRH), on the pituitary response to LHRH, or on both, mediobasal hypothalamic slices or pituitary fragments of adult OVX rats or of OVX rats pretreated with estrogen alone or in combination with P were tested in a perifusion system. Mediobasal hypothalamic slices were perifused in buffered (pH 7.2) oxygenated Locke's medium containing bacitracin (2 X 10(-5) M). In the absence of estrogen pretreatment, high (56 mM) concentrations of K+ were barely effective in releasing LHRH. Subcutaneous implantation of 17 beta-estradiol for 5 days markedly increased the amplitude of the LHRH secretory response to K+ depolarization. Additional administration of P (25 mg/rat s.c.) 4 h before sacrifice further amplified the K+-induced LHRH release. In contrast, the K+-evoked LHRH secretion was significantly inhibited when P was given 16 or 36 h before. Estradiol thus appears to facilitate the LHRH secretory response to depolarizing stimuli, whereas P either enhances or blocks the induced LHRH release depending upon its time of administration. At the pituitary level, the sensitivity of LHRH-induced LH release was also increased after estrogen pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adenosine and its analogue (-)-N6-R-phenyl-isopropyladenosine modulate anterior pituitary adenylate cyclase activity and prolactin secretion in the rat

The effect of adenosine and its analogue (-)-N6-R-phenylisopropyladenosine (PIA) on both anterior pituitary adenylate cyclase activity and prolactin secretion was examined in the rat. Adenosine inhibited basal adenylate cyclase activity in a dose-dependent manner and also reduced the stimulation of the enzyme by vasoactive intestinal peptide (VIP). Likewise, in primary cultures of anterior pituitary cells, adenosine decreased prolactin secretion in both basal and VIP-stimulated conditions. In perifusion experiments, adenosine also inhibited prolactin release in both basal and TRH-stimulated conditions. PIA produced a biphasic pattern of response of basal adenylate cyclase activity, being inhibitory at low and stimulatory at high concentrations. In VIP-stimulated conditions, low concentrations of PIA inhibited both adenylate cyclase activity and prolactin release from primary cultures of pituitary cells, while no additive stimulatory effect was seen at high concentrations. Similarly, low concentrations of PIA reduced both basal and TRH-stimulated prolactin release from perifused pituitaries, while increasing PIA concentrations restored prolactin release. These data show that adenosine affects basal and stimulated prolactin secretion from anterior pituitary cells. Adenosine receptors seem to be coupled to the adenylate cyclase system in the anterior pituitary gland, suggesting a possible relationship between the effect of adenosine on adenylate cyclase activity and prolactin secretion.     

Median eminence serotonin involved in the proestrus gonadotropin release

Numerous studies have suggested that serotonin (5-HT) is involved in the regulation of anterior pituitary hormone release. In the present study, the 5-HT concentrations of the median eminence and anterior pituitary lobe were measured during the estrous cycle and lactation in order to correlate changes in 5-HT levels with changes in serum luteinizing hormone, follicle-stimulating hormone, and prolactin. On the day of proestrus, median eminence 5-HT concentrations declined significantly between 14.00 and 16.30 h at the beginning of the gonadotropin and prolactin surges. No changes in 5-HT concentrations were found between the morning and afternoon on other days of the cycle. In the anterior pituitary, the levels of 5-HT did not change during the estrous cycle. 5-HT turnover rates were also estimated in the median eminence on proestrus and diestrus 1. The median eminence 5-HT synthesis rate increased in the afternoon of proestrus at 16.30 h. 5-HT was also measured in the anterior pituitary and the median eminence of lactating rats in four experimental situations: mothers with their litter until decapitation, mothers separated from their pups 4 h earlier, and mothers separated from their pups 4 h earlier, after which the pups were allowed to suckle for 5 or 30 min. In spite of the acute changes in circulating prolactin, 5-HT levels in the median eminence were not affected in any situation studied. These results suggest that 5-HT in the median eminence is involved in the control of gonadotropin release. The data further suggest that 5-HT does not act directly on the anterior pituitary to modulate gonadotropin or prolactin release.     

Permissive role for ornithine decarboxylase and putrescine in the luteinising hormone surge

Anterior pituitary gland fragments removed from Wistar-derived rats at 10.00 h on pro-oestrus were perifused with Krebs-bicarbonate medium in a column and exposed to hourly volleys of 6 1-min pulses of 10 nM luteinising hormone (LH)-releasing hormone (LHRH). LH release showed a characteristic pattern of prolonged (over 5 h) sensitisation to the releasing hormone, with the response to each volley becoming progressively greater. The addition of 2 mM difluoromethylornithine (DFMO; an inhibitor of ornithine decarboxylase) to the perifusion medium completely inhibited the response to all volleys of LHRH. This effect of DFMO was reversed by the concurrent inclusion of 2 mM putrescine in the medium. Putrescine alone had a small but non-significant enhancing effect on LHRH-induced release, and no significant effect on basal LH release in this system. In a second series of experiments, tissues were loaded with 1-14C-ornithine and the radioactive carbon dioxide released into the medium during the perifusion monitored. Unstimulated pituitary tissues showed constant low levels of carbon dioxide release during 5 h of incubation, but those given hourly volleys of LHRH showed progressively increasing release of radioactivity, which was blocked by the addition of DFMO. No LHRH-stimulated increase in 14CO2 production from 1-14C-ornithine was seen from pituitary tissue removed at 10.00 h on dioestrus. Administration of DFMO (10-100 mg/rat) on the afternoon of pro-oestrus 4 h before the expected peak of the LH surge reduced the magnitude of the subsequent surge and the concentration of the hormone found in the anterior pituitary gland in a dose-related manner. In addition, the concentration of putrescine, but not of spermidine or spermine, was significantly reduced in treated animals (50 mg/rat) at the time of this attenuated surge.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prolactin inhibits oestradiol-induced luteinizing hormone release at the pituitary level

The direct action of prolactin (Prl) on pituitary LH secretion was studied by examining its effect on oestradiol (E2)-induced luteinizing hormone (LH) release from the rat pituitary in a perifusion system and determining the oestrogen receptor (ER) content of the pituitary in hyperprolactinaemic female rats. When the pituitary from rats in pro-oestrus was perifused with medium alone, 10(-7) M E2 significantly (P less than 0.05) increased the LH concentration in the effluent by 60-130% of the basal level, but in medium containing 1 microgram/ml Prl it did not cause LH release. In hyperprolactinaemic rats produced by implanting 2 anterior pituitary glands under the kidney capsule, the ER content of the cytosol of the pituitary (53.2 +/- 2.9 fmol/pituitary) was significantly increased (P less than 0.05), but that of the nuclei (6.8 +/- 0.2 fmol/pituitary) was significantly decreased (P less than 0.05) compared with the contents in rats in pro-oestrus. These data suggest that Prl has a direct suppressive effect on LH secretion from the pituitary in the rat, and that decreased translocation of ER into the nucleus might relate to impaired LH release by E2 from the pituitary of hyperprolactinaemic rats.     

The priming effect of LH-releasing hormone: effects of cold and involvement of new protein synthesis

The possible involvement of protein synthesis in the priming effect of LH-releasing hormone (LHRH) has been investigated in vitro using hemipituitary glands from pro-oestrous rats. Cycloheximide (7.1 mumol/l) blocked the priming effect of LHRH (elicited by 8.5 nmol LHRH/l) and protein synthesis (assessed by gel electrophoresis of 35S-labelled pituitary proteins). Pituitary glands were also incubated at 0-1 degrees C followed by incubation at 37 degrees C. While incubation at 0 degrees C for either 1 or 2 h reduced LH release and blocked protein synthesis, the LH response to LHRH in a subsequent 1-h incubation at 37 degrees C was similar to that during the corresponding period in pituitary glands incubated throughout at 37 degrees C. Incubation with medium alone at 0 degrees C during the first hour followed by incubation with LHRH at 37 degrees C during the second hour resulted in an LH response to LHRH which was similar to that in glands incubated with LHRH for 2 successive hours at 37 degrees C. Two-dimensional gel electrophoresis showed that LHRH priming was associated with the synthesis of a new protein of approximately 69 000 molecular weight and with changes in the isoelectric point of two other high molecular weight proteins. These results suggest that the priming effect of LHRH involves the synthesis of a new protein as well as post-translational changes (possibly phosphorylation) in two other proteins and that exposure to cold may prime the pituitary gland to LHRH possibly by stimulating intracellular Ca2+ release and/or protein phosphorylation.     

Interactions of delta 9-tetrahydrocannabinol (THC) with hypothalamic neurotransmitters controlling luteinizing hormone and prolactin release

The effects of delta 9-tetrahydrocannabinol (THC) on hypothalamic norepinephrine (NE) and dopamine (DA) turnover and hypothalamic serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and LHRH content preceding and during a progesterone- (P) induced LH and prolactin (PRL) surge were investigated in ovariectomized estrogen-primed rats. THC had no effect on basal LH levels, but it inhibited basal PRL levels and blocked the surges of both LH and PRL. The turnover of NE, as estimated by measuring NE depletion after inhibition of tyrosine hydroxylase with alpha-methyl tyrosine (250 mg/kg), in both the anterior (AH) and medial basal hypothalamus (MBH) was significantly inhibited by THC. THC did not significantly affect AH or MBH DA or 5-HT content nor MBH-DA-turnover. Hypothalamic LHRH levels were significantly elevated 4 h after THC administration as compared to the vehicle-injected controls, but pituitary response to exogenous LHRH was not affected. These data suggest that THC inhibits the steroid-induced positive feedback release of LH by reducing NE metabolism and the release of hypothalamic LHRH. Although the mechanism for the inhibition of PRL release by THC is not clear from these experiments, it does not appear that alterations in DA turnover are a contributing factor.     

Prolactin suppresses luteinizing hormone secretion and pituitary responsiveness to luteinizing hormone-releasing hormone by a direct action at the anterior pituitary

In pathological or experimental hyperprolactinemia, the elevated circulating levels of PRL are the usual cause of the impairment in gonadotropic function. The present study was undertaken to determine whether PRL could suppress basal LH secretion and LHRH-stimulated LH release by a direct action at the anterior pituitary. Anterior pituitaries from ovariectomized rats were incubated in medium 199 alone or in medium 199 containing ovine PRL, and basal and the LHRH-stimulated LH release were followed for 2 or 3 h in vitro. Ovine PRL at 40 and 80 micrograms/ml suppressed basal LH release by 41% and 72%, respectively, at 2 h of incubation. This suppressive effect of both concentrations of PRL continued to the third hour of incubation. LHRH at 5 ng/ml increased the release of LH from pituitaries incubated in medium alone by 57%, 61%, and 107% at 1, 2, and 3 h of incubation, respectively. However, in the pituitaries treated with 40 micrograms/ml ovine PRL, the stimulatory effects of LHRH were diminished at all time points measured. Pretreatment of anterior pituitaries with ovine PRL for 6 h significantly inhibited by 81% the LHRH (5 ng/ml) stimulation of LH release at 2 h of incubation. On the other hand, inhibition of endogenous PRL release by 10(-6) M bromocriptine enhanced the stimulatory effects of 5 ng/ml LHRH by 2.5-fold at 2 h of incubation. The inhibitory effects of PRL on basal and stimulated LH secretion appeared unique, since neither BSA nor vasopressin could elicit similar suppressive effects on LH. These results suggest that in anterior pituitaries exposed to elevated levels of PRL, LH secretion and pituitary responsiveness to LHRH could be impaired. This phenomenon may contribute in part to the antigonadotropic effects of PRL.     

Autonomous progesterone secretion from the bovine corpus luteum in vitro

While the regulation of progesterone secretion from the corpus luteum by LH has been convincingly demonstrated, the secretory patterns in the absence of any pituitary LH inputs are yet unclear. Consequently, we investigated the progesterone secretion by an in vitro perifusion system to characterize spontaneous progesterone release from the isolated bovine corpus luteum. Slices (120 mg) of midluteal corpora lutea were placed in perifusion chambers and continuously perifused by Medium-199 for 160-320 min. Progesterone was determined by radioimmunoassay in the effluent fractions collected at 2-min intervals. The spontaneous progesterone release from all bovine corpora lutea was pulsatile. Pulses were observed at mean (+/- SEM) intervals of 17.7 +/- 1.5 min with amplitudes of 6.7 +/- 0.5 ng and release rates of 29.5 +/- 2.4 ng.ml-1.(2 min)-1 (N = 5). Addition of 6.7 nmol/l hCG to the perifusion medium appeared to increase the pulse amplitudes and release rates (195 +/- 25% over unstimulated conditions), but did not change the pulse frequencies (N = 3). Perifusions with calcium-free medium containing 50 mumol/l verapamil and 20 mmol/l EGTA tended to suppress the pulse frequencies and amplitudes of this spontaneous progesterone release, whereas addition of hCG reversed this decrease again (N = 3). When prostaglandin activity was inhibited by 100 mumol/l indomethacin added to the perifusion medium, both pulse frequencies and amplitudes of this progesterone release were enhanced (N = 4). During perifusions with 100 mumol/l of the anti-progesterone ZK 96.734, both the pulse frequencies and amplitudes increased (N = 4). These studies demonstrate an episodic progesterone release from the bovine corpus luteum perifused in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Feedback of follicle-stimulating hormone to inhibit luteinizing hormone and stimulate follicle-stimulating hormone release in ovariectomized rats

Injection of ovine follicle-stimulating hormone (o-FSH) into the third cerebral ventricle significantly suppressed plasma luteinizing hormone (LH) release in conscious, unrestrained ovariectomized rats. Intraventricular injection of o-FSH (0.5 microgram) significantly lowered the plasma levels of LH by 70 min and decreased pulse frequency, whereas the pulsatile release of FSH was unchanged during the 180-min duration of the experiment. In a 12-hour experiment, the suppression of LH release reached the maximum within 3 h (p less than 0.001), followed by a return of plasma LH level to initial values 6-8 h after intraventricular injection of o-FSH at doses of 0.5, 4 and 8 micrograms. Plasma levels of FSH were not significantly changed by the low dose of o-FSH (0.5 microgram), but there was a delayed elevation of plasma FSH during the 8-12 h after intraventricular injection of o-FSH at higher doses (4 and 8 micrograms). On the other hand, plasma prolactin was unchanged after intraventricular injection of o-FSH. Since the pituitary responsiveness to synthetic LH-releasing hormone (LHRH) was unimpaired at the end of the sampling and incubation with o-FSH suppressed the release of LHRH from median eminence terminals, these data indicate that FSH acts intrahypothalamically to suppress LHRH release. The late elevation of plasma FSH may be the result of enhanced release of FSH-releasing factor or of a delayed, direct effect of the hormone on the gonadotropes to increase release of FSH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pulsatile gonadotropin-releasing hormone on the release of luteinizing hormone and follicle-stimulating hormone in vitro by anterior pituitaries from lactating and cycling rats

The ability of pituitaries from lactating animals to secrete LH and FSH in response to gonadotropin-releasing hormone (GnRH) was studied in vitro using a pituitary incubation system. Hemipituitaries were exposed to GnRH for 6 min during each hour of incubation. LH release by anterior pituitaries (APs) from day 5 postpartum rats nursing eight pups, in response to pulsatile exposure to GnRH, was significantly less than that released by APs from diestrous cycling females. Even though the amount of LH released by APs increased as lactation progressed, LH release by APs from day 15 postpartum rats nursing eight pups was still less than LH release by APs from diestrous females. In contrast pituitaries from lactating females nursing two pups released amounts of LH similar to that released by pituitaries from diestrous females, whereas females deprived of their litters for 48 h showed a greater response than diestrous females. Generally, there was a good quantitative relationship between the amount of LH released in vitro and plasma LH concentrations for all the intact groups studied. The ability of lactation to suppress the postcastration rise in serum LH also was demonstrated in vitro as pituitaries from ovariectomized or intact females nursing eight pups released similar amounts of LH on days 5 and 10 postpartum. However, by day 15 postpartum, even though serum LH concentrations were still very low, pituitaries from ovariectomized lactating females released LH in vitro at a rate similar to pituitaries from nonlactating rats. Serum FSH concentrations were not suppressed but similar in intact and cycling females. Also, the total amount of FSH released in vitro in response to GnRH by pituitaries from lactating and cycling females did not differ significantly, even though LH release differed greatly among these groups of animals. However, the patterns of GnRH-stimulating FSH secretion differed among intact lactating, ovariectomized lactating, and nonlactating females. Pituitary LH concentrations were similar on day 5 postpartum and diestrus and on day 15 postpartum and proestrus. Pituitary FSH concentrations on day 5 postpartum were similar to those during diestrus and proestrus and had increased 2-3 times by day 15 postpartum. Generally, there was no correlation between the amount of LH or FSH released by pituitaries in response to GnRH and pituitary gonadotropin content. In summary, the inability of pituitaries from lactating rats to respond adequately to large doses of GnRH in vitro suggests that the suckling stimulus indirectly suppresses pituitary responsiveness to GnRH. This suppression differentially affects basal LH secretion, but not basal FSH secretion, and may be the direct result of inadequate GnRH stimulation in vivo.     

Selectivity of melatonin pituitary inhibition for luteinizing hormone-releasing hormone

The pineal indole melatonin suppresses the neonatal rat luteinizing hormone (LH) and follicle-stimulating hormone (FSH) responses to LH-releasing hormone (LHRH), as shown in previous studies from this laboratory. We show in this study that the melatonin inhibition is a selective effect and is not due to general inhibition of pituitary function. The effects of the indole on the responses to thyrotropin-releasing hormone (TRH) and somatostatin (SRIF) and on basal pituitary hormone secretion were examined with cells in culture. Neonatal rat anterior pituitary cells dissociated with collagenase and hyaluronidase were cultured overnight and distributed to 35-mm dishes at the time of use. For examination of melatonin effects on the response to releasing hormones, the cells were incubated for 3 h in control medium or medium containing LHRH (10-9-10-6 M), TRH (10-10-10-6 M), or SRIF (10-9-10-6 M), either alone or in the presence of melatonin (10-8 or 10-6 M). For examination of basal hormone secretion, the cells were incubated for 1.5, 3, 6, 15, or 24 h in either medium alone or medium containing melatonin (10-6 M). Medium and cell lysate concentrations of LH, FSH, thyroid-stimulating hormone (TSh), prolactin (PRL) and growth hormone (GH) were determined by double antibody RIA. As previously, melatonin (10-8 M) significantly suppressed LH and FSH release by all concentrations of LHRH. This concentration of the indole produced maximal suppression of both LH and FSH responses to LHRH. By contrast, melatonin at a 100-fold greater concentration (10-6 M) had no effect on TRH stimulation of TSH or PRL release or on SRIF inhibition of GH release. Similarly, melatonin had no effect on basal release of TSH, PRL, or GH at the times examined. These findings show that melatonin inhibition of the gonadotroph response to LHRH is a selective effect.     

Hypothalamic catecholamine biosynthesis and pituitary gonadotropin secretion in vitro: Effect of hyperprolactinemia

The effect of hyperprolactinemia on central catecholamine biosynthesis and anterior pituitary hormone release was studied using an in vitro methodology. The incorporation of [³H]tyrosine into hypothalamic and neurohypophyseal catecholamines was determined using a new method which combines high performance liquid chromatography (HPLC) with amperometric detection (LCEC). Elevated plasma prolactin levels, induced by pituitary transplants, resulted in increased in vitro biosynthesis of medial basal hypothalamic (MBH) dopamine (DA), but not norepinephrine (NE). Neurohypophyseal DA biosynthesis (including the intermediate lobe) was not affected. Plasma LH levels were depressed by hyperprolactinemia although the content of hypothalamic luteinizing hormone-releasing hormone (LHRH) was not changed. In parallel studies, the anterior pituitaries from these animals were incubated in vitro using a paired-half technique and LH and PRL release measured. While the basal release of prolactin was not altered by hyperprolactinemia, LH release was significantly decreased. Gonadotroph responsiveness to LHRH was significantly increased, while the inhibition of prolactin by dopamine was not altered. There was a decrease in pituitary prolactin content with normal LH levels. These experiments confirm several in vivo reports which show that hypothalamic dopaminergic but not noradrenergic activity is increased by prolactin. This action is specifically localized in the tuberoinfundibular dopaminergic neurons. Furthermore, these experiments suggest that these central changes result in alterations in both gonadotroph and mammotroph function.