Direct inhibitory effect of long term estradiol treatment on dopamine synthesis in tuberoinfundibular dopaminergic neurons: in vitro studies using hypothalamic slices

The mechanism of the inhibitory effect of long term treatment with estradiol on dopamine synthesis in tuberoinfundibular dopaminergic (TIDA) neurons was studied by using hypothalamic slices from ovariectomized rats. Treatment with 2 mg estradiol valerate (EV) at a 3-week interval increased the weight of the anterior pituitary gland and the concentration of serum PRL. In vivo and in vitro dopamine synthesis in TIDA neurons were estimated in EV-treated animals by 3,4-dihydroxyphenylalanine (DOPA) accumulation in the median eminence after injections of 3-hydroxybenzylhydrazine (NSD 1015), a DOPA decarboxylase inhibitor, and after incubation of hypothalamic slices with NSD 1015, respectively. In vivo DOPA accumulation in the median eminence was less in EV-treated rats than in control rats. The basal rate of in vitro DOPA accumulation in the median eminence of hypothalamic slices from EV-treated rats was lower than that in control rats. Ca2+-dependent DOPA accumulation in the median eminence, determined by incubation in medium containing depolarization agents such as 50 mM K+ and veratridine, was decreased in EV-treated rats. Furthermore, cAMP-dependent DOPA accumulation, determined by incubation with Bu2cAMP or forskolin, was also suppressed in EV-treated rats. The decreased depolarization-induced DOPA accumulation in the median eminence recovered after cessation of EV treatment. Hyperprolactinemia lasting for 6 weeks, achieved by transplantation of anterior pituitaries under the kidney capsule, increased the rate of depolarization-induced DOPA accumulation in the median eminence. On the other hand, EV treatment was effective in inhibiting depolarization-induced DOPA accumulation in hypophysectomized rats regardless of the presence of anterior pituitary transplants. These results suggest that chronically administered estradiol inhibits dopamine synthesis in TIDA neurons via a direct action on the hypothalamus and overcomes the facilitatory action of PRL on dopamine synthesis; and estradiol inhibits all three distinct systems that regulate basal, Ca2+-dependent, and cAMP-dependent dopamine synthesis in TIDA neurons.     

Effect of the duration of streptozotocin-induced diabetes on turnover of central biogenic amines in mice.

The relationship between central catecholaminergic and serotonergic neurons and the development of streptozotocin-induced diabetic mice was examined over periods of 3, 14, 50 and 100 days. The accumulation of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) were measured after central decarboxylase activity was inhibited by m-hydroxybenzylhydrazine (NSD-1015). Results indicated that DOPA accumulation in the hypothalamus was not altered during the entire time course of the experiment. On the other hand, DOPA accumulation in the striatum was decreased in 50- and 100-day diabetic mice. The DOPA levels in the pons medulla were not increased until the mice had been diabetic for 100 days. The accumulation of 5-HTP was decreased in the hypothalamus of 14-day diabetic mice and was also present at 50 and 100 days, that in the striatum and pons medulla was not decreased until the mice had been diabetic for 50 days and persisted to 100 days in the striatum. These data showed that both DOPA and 5-HTP accumulation in the striatum and pons medulla were changed only in long-term diabetic mice and suggested that changes in catecholamine and serotonin turn-over rates are not generalized, but restricted only to some particular brain regions and time courses.     

Formation of a direct arterial blood supply to the anterior pituitary gland following complete or partial interruption of the hypophyseal portal vessels

If regions of the anterior pituitary gland received systemic blood via a direct arterial blood supply these regions would escape hypothalamic regulation and thus be a sequela in endocrine disorders. Since, in the untreated rat, all of the blood supply to the anterior pituitary gland is via the hypophyseal portal vessels, we hypothesized that partial interruption of the portal vessels could provoke the establishment of a direct arterial blood supply (arteriogenesis). We utilized the injection of polystyrene microspheres (15 or 9 micron diameter) into the left ventricle of the heart to test this hypothesis. Microspheres are trapped in the first capillary plexus they reach since they are too large to traverse the capillaries. No microspheres reached the anterior pituitary gland of control rats, a finding consistent with the fact that the anterior pituitary gland receives all of its blood supply via the hypophyseal portal blood vessels. Microspheres were observed in the primary portal capillary plexus in the infundibulum (median eminence), infundibular stalk (pituitary stalk), and infundibular process (pars nervosa), the first capillary plexus which they reached. A lesion of the medial basal hypothalamus (MBH) which destroyed the long portal vessels did not result in arteriogenesis since few, if any, microspheres were observed in the anterior pituitary gland. We confirmed, using vascular casts, that these lesions resulted in the long-term destruction of the primary portal capillaries in the infundibulum and of the long portal vessels. In MBH-lesioned animals it appears that all of the blood supply of the anterior pituitary gland is via short portal vessels arising from the infundibular stem and process.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic hypercortisolemia inhibits dopamine synthesis and turnover in the nucleus accumbens: an in vivo microdialysis study

Over 60% of patients with Cushing's syndrome suffer from major depression, which frequently abates after correction of the hypercortisolism. The mesolimbic and mesocortical dopaminergic (DAergic) systems are thought to participate in psychiatric disorders. In this study, we investigated whether hypercortisolemia affects indices of DAergic activity in the nucleus accumbens (NAc) and the prefrontal cortex (PFC) of freely moving rats. Cortisol (CORT, 25 mg/kg/day) was infused subcutaneously for 7 days via an osmotic minipump. Microdialysate collection (30-min periods, 2 microl/min) began 24 h after probe placement. Concentrations of dihydroxyphenylalanine (DOPA) in interstitial fluid in the nucleus accumbens (NAc) and perifrontal cortex (PFC) were measured before and after local perfusion with NSD-1015, an irreversible inhibitor of L-aromatic amino acid decarboxylase, to assess local dopamine (DA) biosynthesis. The sum of microdialysate DA, dihydroxyphenylacetic acid, and homovanillic acid was used as an index of local DA turnover. DOPA accumulation after NSD-1015 was markedly attenuated in CORT-treated compared with saline-treated animals (5,703 +/- 1,849 vs. 10,902 +/- 2,454 pg/ml; p < 0.01). In contrast, the two groups did not differ in DOPA accumulation in the PFC. Values for the turnover index of DA were also significantly lower in CORT-treated animals in the NAc but not in the PFC. The results indicate that CORT inhibits DA synthesis and turnover in the NAc but not in the PFC. Region-specific CORT-induced inhibition of DAergic activity may help to explain depressive symptoms in patients with chronic hypercortisolemia and normalization after medical or surgical correction of hypercortisolism.     

Direct measurement of blood pressure within the long hypophysial portal blood vessels

The microvascular pressures that perfuse the anterior pituitary gland with blood were not known. We now report the direct measurement of these pressures in the urethane-anesthetized rat. The infundibular stalk and ventral surface of the anterior pituitary gland were surgically exposed via a parapharyngeal approach and a micropressure transducer inserted into the lumen of hypophysial portal vessels under direct microscopic observation. A Weiderhielm-type servo-controlled pressure system was used to record the pressures. Continuous pressure recordings up to 30 min in duration were made in long hypophysial portal vessels ranging in diameter from 10 to 50 micron in adult, female Sprague-Dawley rats. The mean pressure recorded from these vessels was 4.0 cm H2O (2.7 mm Hg). A small increase in systemic pressure produced by a rapid saline infusion into a cannulated femoral vein resulted in a mirrored but much greater magnitude increase in pressure to the hypophysial portal vessels. This finding suggests that pressure within the portal vessels is in some instances closely coupled to systemic blood pressure. The low pressures recorded in hypophysial portal vessels correlate well with pressures measured in the hepatic portal vasculature. The porosity of fenestrated capillaries surrounding anterior pituitary cells is hemodynamically essential, since the low hydrostatic pressures alone would be inappropriate for rapid and thorough exchange.     

Immunoneutralization of oxytocin attenuates preovulatory prolactin secretion during proestrus in the rat

Recent studies have demonstrated that oxytocin (OT) stimulates prolactin (PRL) release from the anterior pituitary gland and that the secretion of OT into pituitary portal blood changes during the rat estrous cycle. To better define the role of OT on PRL release during the reproductive cycle, the effect of administration of antiserum specific for OT on preovulatory PRL secretion in female rats was studied. Intravenous injection of OT antiserum into cyclic female rats between 13.30 and 14.00 h of proestrus neutralized the elevated levels of OT in pituitary portal blood and significantly reduced the subsequent PRL surge. The characteristic proestrous surge of luteinizing hormone (LH) was not affected by the OT antiserum treatment. These data support a physiological role for OT in the regulation of PRL release during the reproductive cycle.     

Monoamine biosynthesis in hypothalamic regions of dwarf mice: effect of replacement of deficient anterior pituitary hormones

Female Ames dwarf and phenotypically normal female mice were killed 30 min after treatment with NSD-1015, an aromatic L-amino acid decarboxylase inhibitor. The accumulation of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan were measured by high-performance liquid chromatography with electrochemical detection and provided estimates of the endogenous biosynthesis of dopamine (DA) in the median eminence (ME) and serotonin biosynthesis (5-HT) in all brain regions which were examined. Dopamine synthesis was markedly suppressed in the ME while 5-HT synthesis was enhanced in both the ME and mediobasal hypothalamus (MBH) of dwarfs as compared to phenotypically normal mice. Overall, catecholamine biosynthesis (DOPA accumulation) was suppressed in the MBH of the dwarf mice but was not different from that observed in normal mice in the preoptic area anterior hypothalamus (POA-AH). The biosynthesis of 5-HT was not different in the POA-AH of dwarf mice as compared to normal mice. In the second experiment dwarf mice received saline vehicle, ovine prolactin (PRL), growth hormone (GH) or thyroxin (T4) daily for 14 days. Normal mice received saline only. Replacement with PRL significantly enhanced DA synthesis in the ME and was the only hormone to suppress significantly the elevation of 5-HT synthesis normally observed in the ME and the MBH of the dwarfs. Both GH and T4 only partially reduced 5-HT synthesis in the ME and MBH so that this parameter was no longer statistically different from either saline-treated dwarfs or normal mice.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hormonal control of tyrosine hydroxylase in the median eminence: demonstration of a central role for the pituitary gland

In intact male rats the concentration of dopamine in hypophysial portal plasma of animals treated simultaneously with estradiol and progesterone was twice that of animals treated with the solvent vehicle. Treatment with estradiol or progesterone alone had no effect on dopamine in portal plasma. The rate of synthesis of dihydroxyphenylalanine (DOPA), the precursor of dopamine, in tuberoinfundibular dopaminergic (TID) neurites in the median eminence (ME) was 15 +/- 1.0 (mean +/- SE) pmol DOPA/ME.h in estradiol-progesterone-treated animals compared to 3.2 +/- 0.02 in vehicle-treated controls. Treatment with estradiol or progesterone alone gave a result similar to that seen in controls. In hypophysectomized animals treated with estradiol and progesterone, DOPA synthesis in the ME was greatly attenuated compared to that in intact rats. The in situ activity of tyrosine hydroxylase (TH; expressed as moles of DOPA per mol TH/h) in the ME was 178 +/- 16.5 in estradiol-progesterone-treated intact rats, but was 27 +/- 2.4, 52 +/- 4.2, and 35 +/- 2.5 in animals treated with the solvent vehicle, estradiol, and progesterone, respectively. In hypophysectomized rats the in situ activity of TH in the ME of animals treated with estradiol and progesterone was 53 +/- 8.4, which was significantly (P less than 0.01) less than that in similarly treated intact animals. The circulating PRL level in vehicle-treated animals was 35 +/- 4.6 ng/ml compared to 121 +/- 16 in estradiol-treated animals and 133 +/- 12.2 in estradiol- and progesterone-treated rats, indicating that the difference in the effects of estradiol and estradiol-progesterone on dopamine release, DOPA synthesis, and in situ TH activity was not solely due to a difference in circulating PRL levels. Maintenance for 7 days of anterior pituitary tissue as a graft in a lateral ventricle of intact rats resulted in a 2-fold increase in the synthesis of DOPA and TH activity in the ME compared to that in animals with liver implants. Results obtained in hypophysectomized animals with implants were similar to those in intact animals. The concentrations of PRL in cerebrospinal fluid of intact rats and hypophysectomized rats with anterior pituitary implants in the lateral ventricles were 96 +/- 32 and 127 +/- 35 ng/ml, respectively, which was significantly (P less than 0.001) greater than those in animals with liver implants. We suggest that a factor of pituitary origin stimulates TH activity in TID neurons. This stimulation may be due to PRL, but the existence of another stimulatory substance secreted by pituitary cells cannot be excluded.     

Evidence for prolactin feedback actions on hypothalamic oxytocin, vasoactive intestinal peptide and dopamine secretion

Ovariectomized rats, when transplanted with 4 anterior pituitaries (APs) to the kidney capsule for 2-3 weeks, had elevated plasma prolactin (PRL) levels (3.8-fold) and showed decreased in situ AP weights (0.62-fold) and PRL concentrations (0.63-fold). The concentrations of dopamine (DA) and oxytocin (OT) in pituitary portal plasma of hyperprolactinemic rats were increased 1.7- and 1.9-fold, respectively. However, the levels of vasoactive intestinal peptide (VIP) in pituitary portal plasma of these rats were decreased 0.31-fold. The secretion of DA, dihydroxyphenylalanine (DOPA) and OT from fetal hypothalamic cells in primary culture was increased, whereas VIP secretion from these cells was reduced in a dose-dependent fashion following PRL treatment. These data are the first in vivo and in vitro demonstration of a stimulatory action of PRL on OT release and an inhibitory action of PRL on VIP release. Furthermore, these data suggest that a subtle imbalance between the secretion of the PRL-inhibiting factor (DA) and the PRL-releasing factors (VIP and OT) during elevated systemic levels of PRL is responsible for decreased lactotrophic function.     

Mass and in situ activity of tyrosine hydroxylase in the median eminence: effect of hyperprolactinemia

The role of PRL in the control of catecholaminergic hypothalamic neurons of female rats was investigated. The in situ activity of tyrosine hydroxylase (TH) in neurites of these neurons was assayed by measuring the rate of accumulation of L-3,4-dihydroxyphenylalanine (DOPA) in the median eminence (ME) after the administration of a DOPA decarboxylase inhibitor. The mass of TH was measured by an immunoblot assay using rat TH as the standard. Hyperprolactinemia was induced by pituitaries implanted beneath a renal capsule. Hyperprolactinemia resulted in a significant increase in the in situ activity of TH without an increase in TH mass. The release of dopamine from hypothalamic neurons was assessed by measuring the concentration of dopamine in hypophysial portal plasma. The mean concentration of dopamine in portal plasma of rats bearing pituitary implants was 3 times that in controls. When circulating PRL was neutralized by administration of antiserum against rat PRL, the activity of TH was reduced significantly compared to that in animals treated with preimmune serum. In animals bearing pituitary implants, phosphorylation of TH in the ME was not different from that in control animals. We conclude that the biosynthetic and secretory activities of dopaminergic neurons of the hypothalamus are potentiated by PRL, but the potentiation is not due to an increase in the mass of TH or to the capacity of the neurites of the ME to phosphorylate TH.     

Effect of prostaglandin E1 on vasoactive intestinal polypeptide release from the hypothalamus and on prolactin secretion from the pituitary in rats

In order to elucidate the mechanisms by which prostaglandin (PG) affects PRL secretion, the effect of PGE1 on vasoactive intestinal polypeptide (VIP) release from the rat hypothalamus was examined by determining plasma VIP levels in rat hypophysial portal blood in vivo and VIP release from the perifused hypothalamus in vitro. Intraventricular injection of PGE1 (1 and 5 micrograms/rat) caused a 2- to 3-fold increase in the concentration of plasma VIP in hypophysial portal blood in anesthetized rats. The flow rate of portal blood was slightly increased after the injection of PGE1. VIP release from the perifused rat hypothalamus was stimulated by high potassium levels (56 mM). The infusion of PGE1 (10 microM) resulted in a significant increase in VIP release from the hypothalamus in vitro. Both these responses were calcium dependent. The intraventricular injection of PGE1 (1 and 5 micrograms/rat) resulted in a dose-related increase in peripheral plasma PRL levels in the rat. These findings suggest that PGE1 plays a stimulatory role in regulating VIP release from the hypothalamus into hypophysial portal blood and causes PRL secretion from the pituitary in rats.     

Effects of photoperiod on lactotrophs and on dopaminergic and 5-hydroxytryptaminergic neurones in bull calves

This study was conducted to determine whether photoperiod-induced changes in serum concentrations of prolactin in cattle were associated with changes in activity of dopamine or 5-hydroxytryptamine (5-HT) neurones in the infundibulum/pituitary stalk and the secretion rate and number of lactotrophs in the anterior pituitary gland. Sixteen prepubertal bull calves (approximately 8 weeks of age) were divided into two groups. One group of eight was maintained on a photoperiod of 8 h light: 16 h darkness (8L:16D) and the other group was exposed to 16L:8D for 4 weeks. At this time calves were injected with a decarboxylase inhibitor (m-hydroxybenzylhydrazine dihydrochloride, NSD 1015) which blocks the conversion of dihydroxyphenylalanine (DOPA) to dopamine and of 5-hydroxytryptophan (5-HTP) to 5-HT. Calves were killed with pentobarbital 15 min later. Accumulations of DOPA and 5-HTP in selected brain regions were used as indices of activity of dopamine and 5-HT neurones respectively. Secretory rate and number of prolactin-secreting lactotrophs were determined by reverse haemolytic plaque assay. Relative to calves exposed to 8L:16D, exposure to 16L:8D increased serum concentrations of prolactin by eightfold, anterior pituitary gland weight by 23%, release of prolactin from pituitary explants by 57% and the area of the plaque for prolactin-secreting lactotrophs by 70%. There was no difference in the rates of accumulation of DOPA and 5-HTP in the infundibulum/pituitary stalk of animals exposed to 4 weeks of 16L:8D or 8L:16D.(ABSTRACT TRUNCATED AT 250 WORDS)     

24-Hour changes in catecholamine synthesis in rat and hamster pineal glands

The purpose of this study was to examine the 24-hour variation in endogenous tyrosine hydroxylase (TH) activity and thus catecholamine (CA) synthesis in the hamster and rat pineal gland. To determine CA synthesis a time course of the accumulation of dihydroxyphenylalanine (DOPA) after DOPA decarboxylase inhibition with m-hydroxybenzylhydrazine (NSD-1015) was measured at 0, 15 and 30 min by liquid chromatography with electrochemical detection. Animals were under long photoperiods (LD 14:10; lights on at 06.00 h). In the hamster pineal gland, CA synthesis was greater in the dark (24.00 h) (0.017 ng/pineal/min) than in the light (12.00 h) (0.008 ng/pineal/min). Similarly, CA synthesis in the rat pineal was 0.037 ng/pineal/min (dark) and 0.005 ng/pineal/min (light). In a 24-hour study, animals were injected with NSD-1015,30 min prior to killing to determine if 24-hour changes were present in CA synthesis. In the hamster, DOPA in the dark (p less than 0.001) was significantly greater than in the light (1.33 +/- 0.42 ng/pineal at 06.00 h; 0.33 +/- 0.07 at 13.00 h) in this study. No significant difference was measured in norepinephrine (NE) concentration during this 24-hour period. In the rat, DOPA accumulation was significantly different (p less than 0.001) in the dark as compared to the light (0.86 +/- 0.09 ng/pineal at 03.00 h; 0.21 +/- 0.08 at 12.30 h). Within this 24-hour period, NE concentration fluctuated significantly between 2.28 +/- 0.33 ng/pineal (15.30 h) to 4.65 +/- 0.59 (05.30 h). These results indicate for the first time a definite 24-hour rhythm in endogenous TH activity and NE synthesis in the hamster pineal gland even though NE content does not change. In addition, a 24-hour change in CA synthesis and content is present in the rat pineal gland.     

Stress-induced secretion of alpha-melanocyte-stimulating hormone is accompanied by a decrease in the activity of tuberohypophysial dopaminergic neurons.

The purpose of the present study was to examine the acute effects of stress on the secretion of alpha-melanocyte-stimulating hormone (alpha MSH) and the activity of tuberohypophysial dopamine (DA) neurons in female and male rats. The activity of tuberohypophysial DA neurons was estimated by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following administration of the decarboxylase inhibitor NSD 1015, and the concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the intermediate and neural lobes of the posterior pituitary. The combination of brief (2 min) ether exposure followed by 30 min of supine restraint (immobilization in the supine position) decreased the rate of DOPA accumulation in the intermediate, but not in the neural lobe of both female and male rats. Similarly, brief ether exposure followed by 10, 20 or 30 min of supine restraint increased plasma alpha MSH concentrations and decreased DOPAC concentrations in the intermediate lobe of female and male rats. In the absence of ether, tube restraint (confinement in a cylindrical acrylic tube) increased alpha MSH secretion and decreased intermediate lobe DOPAC concentrations, whereas ether in the absence of physical restraint had no effect. These results suggest that the stress-induced activation of alpha MSH secretion in both female and male rats may be due, in part, to a decrease in the activity of tuberohypophysial DA neurons in the intermediate lobe of the posterior pituitary.     

Effect of electrical stimulation of the ventromedial nucleus and the dorsomedial nucleus on the activity of tuberoinfundibular dopaminergic neurons

Perikarya and terminals of tuberoinfundibular dopaminergic (TIDA) neurons are located in the arcuate nucleus (ARN) and in the median eminence (ME), respectively. Dopamine (DA) released from TIDA terminals in the ME inhibits prolactin secretion from the anterior pituitary. Anatomical studies have described the sources of afferents to ARN and ME, but not to TIDA neurons per se. The ventromedial nucleus (VMN) and the dorsomedial nucleus (DMN) of the hypothalamus project to ARN and ME and have a role in prolactin regulation. In the present study, VMN and DMN were investigated as possible sources of TIDA afferents. Alterations in the activity of TIDA neurons were estimated by measuring plasma concentrations of prolactin and the rates of DA synthesis (3,4-dihydroxyphenylalanine - DOPA - accumulation after administration of the decarboxylase inhibitor NSD 1015) and metabolism (concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid - DOPAC) in the ME following electrical stimulation of ARN, VMN, and DMN in ovariectomized female rats. Thirty minutes of bilateral stimulation of ARN or DMN increased DOPA accumulation in the ME; stimulation of the VMN had no effect. 5-Hydroxytryptamine synthesis in the ME was unaffected by stimulation of any region. Plasma prolactin levels declined during DMN stimulation, varying with the frequency and duration of the electrical stimulus. DA metabolism within TIDA neurons increased with DMN stimulation, as evidenced by increased DOPAC concentrations in the ME. In females whose basal TIDA activity has been increased by haloperidol treatment or decreased by bromocriptine treatment, DMN stimulation was still able to increase DOPA accumulation in the ME. The present data suggest the presence of stimulatory TIDA afferents originating from or passing through the DMN.     

Changes in the function of tuberoinfundibular dopaminergic neurons after long-term estradiol treatment in Fischer 344 rats

The functions of tuberoinfundibular dopaminergic (TIDA) neurons after long-term estradiol treatment were investigated in Fischer 344 (F344) rats which have high susceptibility to estradiol-induced prolactin (PRL)-secreting pituitary tumors. Dopamine synthesis in and release from TIDA neurons were determined in vitro by 3,4-dihydroxyphenylalanine (DOPA) accumulation in the median eminence following incubation with a DOPA decarboxylase inhibitor and endogenous dopamine release from the median eminence, respectively. The concentration of serum PRL and the weight of the anterior pituitary in ovariectomized F344 rats were markedly increased 3 weeks after a single injection of 2 mg estradiol valerate (EV) and decreased thereafter, but still showed higher levels at 15 and 24 weeks than control ovariectomized rats. Dopamine contents in the median eminence were decreased 3 weeks and unchanged 24 weeks after EV treatment. DOPA accumulation and basal dopamine release in the median eminence of F344 rats were decreased 3 weeks and increased 15 and 24 weeks after EV treatment, similarly to those of Wistar rats as reported previously. However, EV treatment, which caused similar increases in the concentrations of serum PRL and estradiol in F344 and Wistar rats, decreased KCl-induced dopamine release in Wistar rats at 3 weeks, but failed to do so in F344 rats. KCl-induced dopamine release 24 weeks after a single EV injection in F344 rats was greater than that in control rats, whereas the dopamine release 24 weeks after the last treatment of 4 injections at 3-week intervals was not different from that in control rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rat placental lactogen-I abolishes nocturnal prolactin surges in the pregnant rat

The twice-daily surges of prolactin (PRL) present during the first half of pregnancy abruptly terminate at midpregnancy concurrent with the appearance of high levels of placental lactogen-I (PL-I) in the blood. This study addressed the role PL-I and other pituitary or placental hormones have in terminating PRL surges in pregnant rats. Implantation of rat PL-I (rPL-I) or ovine PRL into the arcuate-median eminence area of the hypothalamus of day 7 pregnant rats totally eliminated nocturnal PRL surges on days 8 and 9. To assess the specificity of the inhibitory effects of hormones from the PRL-growth hormone (GH) family, rat growth hormone (rGH), human growth hormone (hGH), and rat prolactin-like protein-A (PLP-A) were tested. Only the lactogenic hormone, hGH, had any effect. Since lactogenic hormones may inhibit PRL by stimulation of dopamine synthesis and release into the hypophysial portal blood vessels leading to the anterior pituitary, the effect of these hormones on tyrosine hydroxylase (TH), the rate-limiting enzyme for the synthesis of dopamine activity, was determined. In pregnant rats, both ovine prolactin (oPRL) and hGH significantly increased (64%) TH activity, whereas rPL-I was less effective. In ovariectomized, bromocriptine-treated rats, both rPL-I and oPRL increased TH activity 207 and 151%, respectively. This supports the concept that termination of PRL surges at midpregnancy are owing to secretion of placental lactogens (PLs) from the placenta. However, the mechanism for the inhibition cannot be entirely attributed to an increase in tuberoinfundibular dopaminergic neuronal activity.     

Paradoxical responses of hypothalamic corticotropin-releasing factor (CRF) messenger ribonucleic acid (mRNA) and CRF-41 peptide and adenohypophysial proopiomelanocortin mRNA during chronic inflammatory stress.

We have determined the time course of the neuroendocrine response of Piebald-Viral-Glaxo (PVG) rats during the development of mycobacterially induced adjuvant arthritis. Anterior pituitary POMC mRNA increased at the time of onset of mycobacterially induced arthritis, but, paradoxically, coincident with the first signs of arthritis there was a consistent fall in CRF mRNA in the hypothalamic paraventricular nucleus. Coincident with this fall in CRF message, there was a corresponding decrease in CRF-41 peptide release into the hypophysial portal blood (HPB). In contrast, however, vasopressin release into the HPB was increased. There was an increase in adrenal weight associated with the development of arthritis, reflecting chronic activation of the HPA axis, which was reflected by increased circulating corticosterone concentrations. The synthetic adjuvant CP20961, which has different antigenic determinants, also caused an increase in POMC mRNA in the anterior pituitary, a decrease in CRF mRNA in the hypothalamic paraventricular nucleus, and a decrease in CRF-41 peptide release into the HPB in PVG rats 28 days after the induction of the arthritis. The arginine vasopressin level was not significantly different from the control value. In Sprague-Dawley rats, mycobacterial adjuvant resulted in a similar increase in POMC mRNA in the anterior pituitary 28 days after injection of the adjuvant. In this strain of rat there was no corresponding change in CRF mRNA. While there are some strain differences in the degree of change in CRF mRNA, both strains showed a common paradox of a marked increase in adenohypophyseal POMC mRNA not associated with increased CRF mRNA or peptide release. In the PVG strain of rat, CRF actually appears to be inhibited. The mechanisms involved in this disparity are unclear.     

Dopamine in hypophysial portal plasma of the rat during the estrous cycle and throughout pregnancy

Catecholamine levels in hypophysial portal plasma were determined in pregnant and non-pregnant female rats as well as in intact and castrated male rats, using a radioenzymatic assay for the simultaneous determination of dopamine, norepinephrine, and epinephrine in 50 mul of plasma. Portal and arterial blood were collected from anesthetized rats at 7 mul/min for 60 min. During the collection, blood was kept at 0 C, a temperature at which endogenous catecholamines were relatively stable. Dopamine was present in high concentrations in hypophysial portal plasma thorughout pregnancy, attaining a level near 20 ng/ml on the 20th day of gestation. Dopamine levels in arterial plasma from the same rats were low or undetectable (0.4--0.8 ng/ml1. Norepinephrine and epinephrine was undetectable (less than 0.6 ng/ml) in portal as well as arterial plasma from these rats. The major catecholamine in extracts of the hypothalamus from pregnant rats was norepinephrine, whereas that in the posterior pituitary was dopamine. Dopamine levels in portal plasma collected during proestrus, estrus, diestrus 1, and diestrus 2, were 1.32 +/- 0.21 (mean +/- SE), 3.87 +/- 0.96, 3.11 +/- 0.73, and 2.3 +/- 0.45, respectively. Dopamine in portal plasma from intact and from castrated male rats was approximately 0.6 ng/ml. Norepinephrine and epinephrine were not detectable in either portal or arterial plasma from these animals. It is concluded 1) that dopamine is secreted into hypophysial portal blood in significant quantities during pregnancy, 2) that hypothalamic secretion of dopamine in cyclic rats is greatest during the day of estrus and early diestrus and at least on the day of proestrus, and 3) that these findings support the view that dopamine of hypothalamic origin may have an important role in the regulation of anterior pituitary function.     

Hyperprolactinemia decreases the luteinizing hormone-releasing hormone concentration in pituitary portal plasma: a possible role for beta-endorphin as a mediator

Hyperprolactinemia can reduce the LH secretion in rats, but the mechanism of the effect of PRL is not clear. We have investigated the actions of PRL on the secretion of LHRH and LH and the interaction among PRL, beta-endorphin (beta-EP), and LHRH. The effects of PRL on LHRH and LH secretion were studied in ovariectomized female rats after transplanting four anterior pituitaries to the right kidney capsule of each ovariectomized rat for 2-3 weeks. The level of PRL in rats with pituitary transplants was approximately 5 times higher than that in control rats. The concentration of LHRH in pituitary portal plasma of hyperprolactinemic rats was approximately 4 times lower than that in control rats. Hyperprolactinemic animals also showed lower plasma LH levels than the controls. Since beta-EP inhibits the secretion of LHRH, we have tested whether the reduced secretion of LHRH in hyperprolactinemic ovariectomized rats is associated with an increase in beta-EP activity. This was studied by measuring the concentration of beta-EP in pituitary portal plasma and the response of LHRH and LH to the opiate antagonist naloxone. The level of beta-EP-like immunoreactivity in pituitary portal plasma was significantly higher in hyperprolactinemic rats than in control animals. Naloxone (10 mg/kg, sc) increased both LHRH and LH concentrations in hyperprolactinemic rats, but not in control rats. The present results demonstrate that hyperprolactinemia can reduce LHRH release and suggest a possible involvement of beta-EP in the PRL inhibitory action on LHRH.