Chronic estradiol exposure induces oxidative stress in the hypothalamus to decrease hypothalamic dopamine and cause hyperprolactinemia

Estrogens are known to cause hyperprolactinemia, most probably by acting on the tuberoinfundibular dopaminergic (TIDA) system of the hypothalamus. Dopamine (DA) produced by TIDA neurons directly inhibits prolactin secretion and, therefore, to stimulate prolactin secretion, estrogens inhibit TIDA neurons to decrease DA production. However, the mechanism by which estrogen produces this effect is not clear. In the present study, we used a paradigm involving chronic exposure to low levels of estradiol-17β (E(2)) to mimic prolonged exposures to environmental and endogenous estrogens. We hypothesized that chronic exposure to low levels of E(2) induces oxidative stress in the arcuate nucleus (AN) of the hypothalamus that contains TIDA neurons and causes nitration of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of DA. This results in a significant decrease in DA and consequently, hyperprolactinemia. To investigate this, adult, intact female cycling rats were implanted with slow-release E(2) pellets (20 ng/day) for 30, 60, or 90 days and were compared with old (16-18 mo old) constant estrous (OCE) rats. Chronic E(2) exposure significantly increased the expression of glial fibrillary acidic protein and the concentrations of interleukin-1β (IL-1β) and nitrate in the AN that contains perikarya of TIDA neurons and increased nitration of TH in the median eminence (ME) that contains the terminals. These levels were comparable to those seen in OCE rats. We observed a significant decrease in DA concentrations in the ME and hyperprolactinemia in an exposure-dependent manner similar to that seen in OCE rats. It was concluded that chronic exposure to low levels of E(2) evokes oxidative stress in the AN to inhibit TIDA neuronal function, most probably leading to hyperprolactinemia.     

Prostaglandin D2 stimulates vasoactive intestinal polypeptide release into rat hypophysial portal blood

The effect of prostaglandin D2 (PGD2) on vasoactive intestinal polypeptide (VIP) release from the hypothalamus was examined by determining plasma VIP levels in rat hypophysial portal blood. Intraventricular injection of PGD2 (5 micrograms/rat) caused a 3-fold increase in the concentration of plasma VIP in hypophysial portal blood in anesthetized rats. A PGD2 metabolite, 13,14-dihydro-15-keto PGD2, did not affect VIP levels in portal blood. The flow rate of hypophysial portal blood was not changed after the injection of PGD2. The intraventricular injection of PGD2, but not PGD2 metabolite, resulted in an increase in peripheral plasma prolactin (PRL) levels in the rat. These findings suggest that PGD2 plays a stimulatory role in regulating VIP release from the hypothalamus into hypophysial portal blood and causes PRL secretion from the pituitary in rats.     

Catecholamines and pituitary function. III. Restoration of the prolactin response to thyrotropin-releasing hormone by low-dose dopamine infusion in women with pathological hyperprolactinemia

Previous studies in Rhesus monkeys have demonstrated that a dopamine (DA) infusion rate of 0.1 microgram/kg X min induces peripheral DA levels similar to those measured in hypophysial stalk blood and normalizes serum prolactin (PRL) levels in stalk-transected animals. We therefore examined the effect of such DA infusion rate on basal and thyrotropin-releasing hormone (TRH)-stimulated PRL secretion in both normal cycling women and women with pathological hyperprolactinemia. 0.1 microgram/kg X min DA infusion fully normalized PRL serum levels in 8 normal cycling women whose endogenous catecholamine synthesis had been inhibited by alpha-methyl-p-tyrosine (AMPT) pretreatment. Furthermore, DA significantly reduced, but did not abolish, the rise in serum PRL concentrations induced by both acute 500 mg AMPT administration and 200 micrograms intravenous TRH injection in normal women. A significant reduction in serum PRL levels in response to 0.1 microgram/kg X min DA, similar to that observed in normal cycling women when expressed as a percentage of baseline PRL, was documented in 13 amenorrheic patients with TRH-unresponsive pathological hyperprolactinemia. However, a marked rise was observed in the serum PRL of the same patients when TRH was administered during the course of a 0.1-microgram/kg X min DA infusion. The PRL response to TRH was significantly higher during DA than in basal conditions in hyperprolactinemic patients, irrespective of whether this was expressed as an absolute increase (delta PRL 94.4 +/- 14.2 vs. 17.8 +/- 14.1 ng/ml, p less than 0.002) or a percent increase (delta% PRL 155.4 +/- 18.9 vs. 17.9 +/- 7.1, p less than 0.0005), and there was a significant linear correlation between the PRL decrements induced by DA and the subsequent PRL responses to TRH. These data would seem to show that the 0.1-microgram/kg X min DA infusion rate reduces basal PRL secretion and blunts, but does not abolish, the PRL response to both TRH and acute AMPT administration. The strong reduction in PRL secretion and the restoration of the PRL response to TRH by 0.1 microgram/kg X min DA infusion in high majority of hyperprolactinemic patients, seem to indicate that both PRL hypersecretion and abnormal PRL response to TRH in women with pathological hyperprolactinemia are due to a relative DA deficiency at the DA receptor site of the pituitary lactotrophs.     

Pure uptake blockers of dopamine can reduce prolactin secretion: studies with diclofensine

The effects of diclofensine, a pure dopamine (DA) uptake inhibitor on 1) 3H-DA uptake in rat arcuate-periventricular nucleus-median eminence synaptosomes, 2) basal and K+-evoked endogenous DA release from tuberoinfundibular dopaminergic (TIDA) neurons and 3) in vivo prolactin (PRL) secretion were studied. Diclofensine, in concentrations of 0.01, 0.1 and 1 microM caused a marked decrease of 3H-DA uptake. In addition, it was unable to stimulate basal endogenous DA release which, on the contrary, was elicited by d-amphetamine in the same concentration (50 microM). On the other hand, diclofensine (50 microM) caused a 3 fold enhancement of K+-evoked DA release. Finally, the compound, when administered in vivo to male rats, significantly reduced basal serum PRL levels. The results of the present study seem to indicate that the pharmacological blockade of DA uptake in TIDA neurons is a condition sufficient to cause a reduction of PRL release.     

Bromocryptine prevents the decline in tuberoinfundibular neuronal release of dopamine after removal of chronic estrogen treatment

Prolonged exposure to estradiol 17-beta (E2) in rats has been shown to decrease dopamine (DA) synthesis in and release from tuberoinfundibular dopaminergic (TIDA) neurons in Fischer 344 rats. The objective of the present study was to determine whether inhibition of the E2-induced increase in anterior pituitary (AP) weight and prolactin (PRL) secretion by concomitant administration of the dopaminergic agonist, bromocryptine, could prevent the decrease in TIDA neuronal function produced by chronic E2 administration. TIDA neuronal function was evaluated by in vitro superfusion and electrical stimulation of median eminence (ME) tissue after allowing for accumulation of [3H]dopamine (DA). The effect of chronic E2 and/or bromocryptine treatment on catecholamine content in tuberohypophyseal neurons in the neurointermediate lobe was also measured to determine whether increased pituitary size possibly damaged the tuberohypophyseal neurons. Treatment with E2 for 30 days significantly increased AP weight, serum PRL concentration, and AP PRL and DNA content over values in non-E2-treated controls. When bromocryptine was injected daily during E2 treatment, bromocryptine completely inhibited the E2-induced increase in serum PRL and AP DNA content, and AP weight was only moderately increased. The evoked release of 3H at the end of the 30-day E2 treatment was reduced during electrical stimulation and there was no augmented release of 3H from the ME tissue after 10 microM nomifensine infusion in E2-treated rats and in rats given both bromocryptine and E2. However, neurointermediate lobe DA content was diminished only in E2-treated rats and not in animals given bromocryptine together with E2. When all treatments were discontinued for 30 days, animals previously given only E2 showed sustained increases in AP weight, serum PRL levels, and AP PRL and DNA content, but reduced stimulation-evoked release of 3H, absence of response to nomifensine, and reduced neurointermediate lobe DA and norepinephrine content when compared with values in non-E2-treated controls. After withdrawal of E2 treatment for 30 days, animals previously given bromocryptine and E2 together were not different from control animals in any of the parameters measured. These results suggest that the decline in TIDA neuronal release of DA induced by chronic E2 treatment was at least partly exerted via the marked hyperprolactinemia and/or by compression of the medial basal hypothalamus by the enlarged AP.     

Chronic hyperprolactinemia depletes hypothalamic dopamine concentrations in male rats

The tuberoinfundibular dopamine (TIDA) system appears to tonically inhibit pituitary prolactin secretion while moderate elevations in serum prolactin levels, in turn, augment the turnover rate of dopamine (DA) without affecting the steady state concentrations of DA in the TIDA neurons (1–5). The present study demonstrates that chronic elevations in serum prolactin, to greater than 2,000 ng/ml, induced by the prolactin secreting MtTW15 tumor, decreased DA concentrations by 47% in the median eminence-arcuate nucleus (ME-ARC) region, by 43% in the medial basal hypothalamus (MBH) and 14% in the preoptic area-anterior hypothalamic region (POA-AH) without influencing the norepinephrine levels in these regions. Thus, chronic stimulation of hypothalamic DA neurons by prolactin may lead to depletion of DA concentrations and this may be an important factor in the reduced DA levels observed in hyperprolactinemia of senescent rats or that produced by chronic estrogen treatment.     

Involvement of dopamine D2 receptor in the diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in female rats

Background

An endogenous dopaminergic (DA) tone acting on D₃ receptors has been shown to inhibit tuberoinfundibular (TI) DA neuron activity and stimulate prolactin (PRL) surge in the afternoon of estrogen-primed ovariectomized (OVX+E₂) rats. Whether D₂ receptor (D₂R) is also involved in the regulation of TIDA and PRL rhythms was determined in this study.

Results

Intracerebroventricular (icv) injection of PHNO, a D₂R agonist, in the morning inhibited TIDA and midbrain DA neurons’ activities, and stimulated PRL secretion. The effects of PHNO were significantly reversed by co-administration of raclopride, a D₂R antagonist. A single injection of raclopride at 1200 h significantly reversed the lowered TIDA neuron activity and the increased serum PRL level at 1500 h. Dopamine D₂R mRNA expression in medial basal hypothalamus (MBH) exhibited a diurnal rhythm, i.e., low in the morning and high in the afternoon, which was opposite to that of TIDA neuron activity. The D₂R rhythm was abolished in OVX+E₂ rats kept under constant lighting but not in OVX rats with regular lighting exposures. Pretreatment with an antisense oligodeoxynucleotides (AODN, 10 μg/3 μl/day, icv) against D₂R mRNA for 2 days significantly reduced D₂R mRNAs in central DA neurons, and reversed both lowered TIDA neuron activity and increased serum PRL level in the afternoon on day 3. A diurnal rhythm of D₂R mRNA expression was also observed in midbrain DA neurons and the rhythm was significantly knocked down by the AODN pretreatment.

Conclusions

We conclude that a diurnal change of D₂R mRNA expression in MBH may underlie the diurnal rhythms of TIDA neuron activity and PRL secretion in OVX+E₂ rats.     

Involvement of hypothalamic vasoactive intestinal polypeptide (VIP) in prolactin secretion induced by serotonin in rats

To study the possible involvement of hypothalamic vasoactive intestinal polypeptide (VIP) in regulating the secretion of prolactin (PRL), the effect of anti-VIP rabbit serum on serotonin (5-HT)-induced PRL release was examined in urethane-anesthetized male rats. Anti-VIP serum (AVS) or normal rabbit serum (NRS) was infused into a single hypophysial portal vessel of the rat for 40 min at a rate of 2 microliters/min with the aid of a fine glass cannula and 5-HT was injected into a lateral ventricle 10 min after the start of the infusion. Intraventricular injection of 5-HT (10 micrograms/rat) caused an increase in plasma PRL levels in control animals infused with NRS and 5-HT-induced PRL release was blunted in animals infused with AVS (mean +/- SE peak plasma PRL: 118.9 +/- 19.8 ng/ml vs 54.7 +/- 16.2 ng/ml, p less than 0.05). These findings suggest that the secretion of PRL induced by 5-HT is mediated, at least in part, by hypothalamic VIP release into the hypophysial portal blood in the rat.     

Effects of orphanin FQ on central dopaminergic neuronal activities and prolactin secretion

Effects of orphanin FQ (OFQ) on central dopaminergic (DA) neurons and serum prolactin (PRL) were examined in ovariectomized, estrogen-primed Sprague-Dawley rats. The activities of central DA neurons, including the tuberoinfundibular (TI), nigrostriatal, mesolimbic, and incertohypothalamic ones, were determined by measuring the levels of 3,4-dihydroxyphenylacetic acid (DOPAC), the major metabolite of dopamine, in their projection regions in the brain by HPLC plus electrochemical detection. Intracerebroventricular administration of OFQ lowered DOPAC levels in the median eminence (ME), striatum, nucleus accumbens, and hypothalamic paraventricular nucleus in a dose (0.01-10 microg)- and time (30-90 min)-dependent manner. In contrast, OFQ increased DOPAC in the suprachiasmatic nucleus and had no effect in the periventricular nucleus. Serum PRL levels exhibited a typical inverse relationship with the activity of TIDA neurons, as determined by DOPAC levels in the ME. In the afternoon, we observed an endogenous decrease of ME DOPAC level accompanied by a PRL surge in estrogen-primed female rats. Although OFQ caused further decrease of ME DOPAC in the afternoon, it failed to augment the PRL surge level. Although pretreatment of an antisense oligodeoxynucleotide against the opioid receptor-like receptor gene had no effect on basal ME DOPAC levels in the morning or afternoon, it attenuated the afternoon PRL surge. Furthermore, it blocked the effects of exogenous OFQ on ME DOPAC and serum PRL levels, whereas the sense or missense oligodeoxynucleotide had no effect. These results indicate that OFQ and its receptors may be involved in the regulation of central DA neuronal activity and PRL secretion.     

Effects of immobilization stress on tuberoinfundibular dopaminergic (TIDA) neuronal activity and prolactin levels in lactating and non-lactating female rats.

The effects of immobilization stress on the prolactin secretory response and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons were determined in intact, virgin female rats on the morning of diestrus or proestrus and in post-partum, lactating female rats. The virgin females exhibited a significant increase in circulating levels of prolactin which was evident by 1 minute and persisted during the immobilization (5 minutes). In contrast, the prolactin secretory response in lactating females was significantly attenuated compared to non-lactating animals. The activity of the TIDA neurons was not altered by the 5 minutes of stress. Even after 30 minutes of immobilization, TIDA neuronal activity was not affected in either the lactating or cycling females. These data suggest that the cycling female rat is capable of a prolactin secretory response to the stressor without inhibition of TIDA neuronal activity. It seems likely that prolactin releasing factors mediate this response. In contrast, stress did not produce a similar prolactin increase during lactation. It seems likely that, during lactation, the pituitary is not sensitive to releasing factors unless the TIDA neurons are inhibited. There appear to be differences in the sensitivity of the pituitary depending on the physiological state of the model employed.     

Neuroendocrine control of gonadotropin secretion

Luteinizing hormone releasing hormone (LHRH), a hypothalmic peptide that is concentrated in granules of neurons, has the capacity to release gonadotropins (luteinizing hormone (LH) and follicle stimulating hormone) from the pituitary gland. LHRH has been found in hypophysial portal blood of rats, monkeys, and rabbits. Antibodies to LHRH depress plasma LH concentrations in castrated animals and evoke testicular atrophy, but passive immunization against LHRH does not block the LH surge induced by estrogen in monkeys. Estrogens, progestin, prolactin, and dopamine have marked effects on LH secretion, yet an association between these effects and altered hypophysial portal blood concentrations of LHRH is not established. In view of the paucity of evidence demonstrating such a cause and effect relationship, two alternative proposals have become tenable. One, hormones and neurotransmitters may not alter the levels of portal blood LHRH, but rather alter the frequency of pulsatile LHRH secretion. Two, hormones, such as estrogens, progesterone, and prolactin, may alter the responsiveness of the gonadotropin-secreting cells to LHRH by affecting the secretion of dopamine.     

Persistence of low hypothalamic dopaminergic activity after removal of chronic estrogen treatment

The purpose of this study was to determine whether inhibition of tuberoinfundibular dopaminergic (TIDA) neuron function which occurs during chronic estrogen administration persists after removal of the estrogen. Ovariectomized (OVX) Fischer 344 (F344) rats were implanted for 4 weeks with a Silastic capsule containing estradiol-17 beta (E2) and controls with an empty capsule for 4 weeks. Other rats which received E2 for 4 weeks had the capsule removed and experiments performed 4 weeks later. At the end of 4 weeks of E2 treatment, anterior pituitary (AP) weight was increased sixfold, serum prolactin (PRL) 65-fold, and AP DNA content fivefold over OVX control rats. Four weeks after removal of E2, AP weight, serum PRL, and AP DNA content declined, but remained significantly above OVX control values. At the end of 4 weeks of E2 treatment and after E2 withdrawal, release of [3H]dopamine (DA) from median eminence (ME) tissue superfused in vitro was lower than from ME of OVX control rats although [3H]DA accumulation was not significantly different among the treatment groups. Administration of apomorphine (APO), a dopamine agonist, significantly reduced plasma prolactin levels in OVX control rats, in rats at the end of 4 weeks E2 treatment, and in rats after 4 weeks of E2 withdrawal. Injection of haloperidol (HALO) produced similar increases in plasma PRL/estimated PRL-cell DNA in OVX controls, at the end of E2 treatment or after E2 withdrawal. However, injection of morphine (MOR), a drug which increases the release of PRL by inhibiting hypothalamic dopaminergic activity, resulted in a rise in plasma PRL/estimated PRL-cell DNA in OVX control rats that was significantly greater compared to rats at the end of E2 treatment or after E2 withdrawal. Since rats treated with E2 released less [3H]DA from ME tissue in vitro, and were less responsive to MOR, it can be that animals treated for 4 weeks with E2 show a decreased ability to release DA from TIDA neurons which persists even after termination of E2 treatment. These results suggest that chronic high circulating E2 levels result in a depression of TIDA neuronal activity which is sustained after E2 is removed.     

Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.     

Alterations in concentrations of dihydroxyphenylacetic acid in the median eminence of rats euthanatized with pentobarbital

Concentrations of dopamine (DA) and one of its major metabolites, dihydroxyphenylacetic acid (DOPAC), were determined in selected brain regions of rats that were euthanatized either by decapitation or by intravenous injections of pentobarbital or Fatal Plus, a commercial preparation that contains pentobarbital. When compared with values in decapitated brains, pentobarbital increased the concentration of DOPAC in the median eminence, which contains terminals of tuberoinfundibular dopaminergic (TIDA) neurons. Fifteen minutes of restraint reduced the concentration of DOPAC in the median eminence of rats killed by decapitation or by injections of pentobarbital, indicating that pentobarbital does not mask restraint-induced decrease in TIDA neuronal activity. In contrast, none of the manipulations altered DA or DOPAC concentrations in the striatum, which contains terminals of nigrostriatal dopaminergic neurons. Thus, changes in the concentrations of DOPAC in the median eminence (an index of TIDA neuronal activity) induced by stress can be detected in rats euthanatized by either decapitation or an injection of pentobarbital.     

Ca2(+)-independent secretory mechanism of thyrotropin-releasing hormone (TRH) involves protein kinase C in rat pituitary cells

Thyrotropin-releasing hormone (TRH) stimulates biphasic prolactin (PRL) secretion from rat pituitary GH3 cells. The pretreatment of cells with EGTA (100 microM) plus arachidonic acid (15 microM), a condition which decreased TRH-responsive intracellular Ca2+ pools, eliminated the activity of TRH on burst PRL secretion (2 min) but did not alter that on sustained PRL secretion (30 min). However, the treatment of cells with EGTA, arachidonic acid and H-7 (300 microM), a potent inhibitor of protein kinase C (PKC), almost completely suppressed the activity of TRH for sustained PRL secretion. In cells down-modulated for PKC, TRH abolished this Ca2(+)-independent sustained PRL secretion. These results suggest that TRH acts through a separate, Ca2(+)-independent secretory mechanism, besides by modulating the Ca2(+)-dependent mechanism and that PKC is involved in this Ca2(+)-independent secretory pathway.     

Prolactin secretory rhythm of mated rats induced by a single injection of oxytocin

Mating or vaginocervical stimulation [copulatory stimulus (CS)] induces two daily surges of the hormone prolactin (PRL) in rats. This unique secretory pattern of PRL surges is characteristic for the first half of pregnancy and is also present in ovariectomized (OVX) rats. Studies have shown that CS additionally provokes an acute release of the hormone oxytocin (OT). In this study, we tested whether a single injection of OT (iv) is sufficient to initiate the PRL secretion pattern of OVX/CS rats. Furthermore, we measured the 24-h profile of dopamine (DA) content in the anterior lobe of the pituitary gland, because DA is the major inhibitory factor of PRL secretion. The results indicated that a single injection of OT induces a PRL secretory rhythm and a DA release pattern similar to that initiated by CS. Immunocytochemical investigation showed that particular OTergic neurons in the hypothalamus express receptors for PRL, as well as for vasoactive intestinal polypeptide, which indicates an involvement in generating the PRL rhythm and entraining it to the ambient photoperiod. On the basis of this study, we suggest that the PRL-DA inhibitory feedback loop between lactotrophs and DAergic neurons plays a crucial role in generating the oscillatory PRL secretion pattern in CS rats. A timing signal, likely provided by the hypothalamic suprachiasmatic nucleus, entrains the autonomous PRL oscillation to a particular time of day. Mathematical modeling was used to illustrate the proposed network function. The experimental results further suggest an additional feedback mechanism in which certain hypothalamic OTergic neurons are influenced by PRL.     

Cooperative synthesis of dopamine in rat mediobasal hypothalamus as a compensatory mechanism in hyperprolactinemia

Dopamine (DA), synthesized in the mediobasal hypothalamus by dopaminergic neurons containing two enzymes of DA synthesis–tyrosine hydroxylase and decarboxylase of aromatic L-amino acids, or by monoenzymatic non-dopaminergic neurons containing one DA synthesis enzyme in cooperation, is known to have an inhibitory effect on prolactin secretion. Deterioration of this inhibitory control leads to an increase in prolactin concentration in the blood and to the development of hyperprolactinemia syndrome. In a rat model of hyperprolactinemia induced by administration of a neurotoxin causing degeneration of dopaminergic and noradrenergic neurons, the level of DA first decreases, leading to an increase in prolactin level (decompensation stage), while later both levels are restored to normal (compensation stage). However, the mechanism of such compensation is still not clear. The aim of the present study was to analyze whether the increase in cooperative synthesis of DA by monoenzymatic neurons during hyperprolactinemia is a manifestation of a compensatory mechanism representing a particular case of neuroplasticity. The level of cooperative synthesis in the hyperprolactinemia model and in the control was estimated as the level of synthesis of DA and L-dihydroxyphenylalanine (L-DOPA)–an intermediate product of DA synthesis, when L-DOPA transfer from neurons containing tyrosine hydroxylase into neurons containing aromatic L-amino acid decarboxylase is inhibited. The level of DA synthesis during the decompensation stage was not changed, while during the compensation stage it was lower than the control. Along with a reduction in DA level, during the compensation stage an increase in the extracellular L-DOPA level in the medium was detected. Thus, the compensation of DA deficiency after degeneration of dopaminergic neurons in the mediobasal hypothalamus is due to the increase in cooperative synthesis of DA by monoenzymatic neurons containing one of the complementary enzymes of the DA synthesis pathway.     

Acute administration of bromocriptine abolishes the hyperprolactinemic response induced by submaximal exercise in man.

The effective control of hypophysial prolactin (PRL) secretion with a pharmacological agent is a prerequisite for the investigation of the role of hyperprolactinemia observed during exercise. Using bromocriptine, a potent inhibitor of PRL secretion, this study established the proper experimental conditions whereby any significant increase in plasma PRL level can be prevented and basal circulating levels maintained during physical exercise. On three occasions at weekly intervals, 15 male adults, separated into two groups, exercised on an ergocycle (40 min at 65% VO2max) either 1 or 3 h after ingesting either placebo or 1.25 or 2.50 mg of bromocriptine mesylate (Parlodel; Sandoz Canada Inc., Dorval, Qué.). Under all conditions, the plasma PRL elevation observed during exercise after placebo was prevented by the administration of bromocriptine. Resting plasma PRL levels were maintained when exercise was performed 1 h after bromocriptine ingestion, but were significantly reduced when exercise was performed 3 h after administration of either bromocriptine dosages. Considering the primary and secondary effects observed, 1.25 mg of bromocriptine administered 1 h before exercise provides suitable experimental conditions to investigate the role of the increase in plasma PRL during physical exercise.     

Studies of anterior pituitary-grafted rats: II. Normal growth hormone secretion

Of the various animal models used to study chronic hyperprolactinemia, the otherwise intact rat implanted with extra anterior pituitary glands (AP) under the kidney capsule is assumed to be normal except for excess circulating prolactin (PRL). Since the ectopic glands contain numerous somatotropes in addition to abundant and active lactotropes, it was important to assess growth hormone (GH) secretion as well in this model of hyperprolactinemia. The structural and functional similarities of PRL and GH are such that it is necessary to demonstrate that metabolic abnormalities noted in AP-implanted rats are due to hyperprolactinemia and not to altered GH secretion. AP-implanted female rats have significantly higher resting serum PRL concentrations when compared to sham-operated control rats, but baseline serum GH levels are similar in normal and pituitary-grafted rats. Suppression of GH by insulin and clonidine is comparable in AP-implanted and control rats. The intrasellar pituitary GH concentration is also similar (ca. 20 μg/mg wet weight) in hyperprolactinemic and normal rats. We conclude that GH secretion is normal in the non-hypophysectomized AP-implanted rat, in contrast to the hypophysectomized AP-implanted rat model which has been reported to have diminished GH secretion. Despite the presence of recognizable somatotropes, the ectopic anterior pituitary does not appear to secrete significant amounts of GH, making the intact rat bearing multiple pituitary grafts an excellent model of chronic hyperprolactinemia.     

Interactions between prolactin and dopaminergic neurons

The secretion of prolactin from the adenohypophysis is tonically inhibited by dopamine that is released into the hypophysial portal blood from terminals of tuberoinfundibular neurons located in the external layer of the median eminence. These tuberoinfundibular neurons are unique among other dopaminergic neurons in the brain (including the well-characterized nigrostriatal neurons) in that they are not directly regulated by dopaminergic receptor-mediated mechanisms, but instead are selectively responsive to changes in prolactin concentrations in blood and cerebrospinal fluid. In the rat, the activity of the tuberoinfundibular dopaminergic neurons is higher in the female than in the male, exhibits a characteristic cyclical pattern during the first half of pregnancy and is constantly high as a result of stimulation by placental lactogen during the last 9 days of pregnancy, and is reduced in lactating animals and acutely inhibited during suckling.