Estradiol-induced prolactinomas: differential effects on dopamine in posterior pituitary and median eminence

Developmental exposure to a very low dose of bisphenol A induces persistent islet insulin hypersecretion in Fischer 344 rat offspring

BACKGROUND

In children with obesity, accentuated insulin secretion has been coupled with development of type 2 diabetes mellitus (T2DM). Bisphenol A (BPA) is a chemical with endocrine- and metabolism-disrupting properties which can be measured in a majority of the population. Exposure to BPA has been associated with the development of metabolic diseases including T2DM.

OBJECTIVE

The aim of this study was to investigate if exposure early in life to an environmentally relevant low dose of BPA causes insulin hypersecretion in rat offspring.

METHODS

Pregnant Fischer 344 rats were exposed to 0.5 (BPA0.5) or 50 (BPA50) µg BPA/kg BW/day via drinking water from gestational day 3.5 until postnatal day 22. Pancreata from dams and 5- and 52-week-old offspring were procured and islets were isolated by collagenase digestion. Glucose-stimulated insulin secretion and insulin content in the islets were determined by ELISA.

RESULTS

Basal (5.5 mM glucose) islet insulin secretion was not affected by BPA exposure. However, stimulated (11 mM glucose) insulin secretion was enhanced by about 50% in islets isolated from BPA0.5-exposed 5- and 52-week-old female and male offspring and by 80% in islets from dams, compared with control. In contrast, the higher dose, BPA50, reduced stimulated insulin secretion by 40% in both 5- and 52-week-old female and male offspring and dams, compared with control.

CONCLUSION

A BPA intake 8 times lower than the European Food Safety Authority's (EFSA's) current tolerable daily intake (TDI) of 4 µg/kg BW/day of BPA delivered via drinking water during gestation and early development causes islet insulin hypersecretion in rat offspring up to one year after exposure. The effects of BPA exposure on the endocrine pancreas may promote the development of metabolic disease including T2DM.

Chronic exposure to low levels of oestradiol-17beta affects oestrous cyclicity, hypothalamic norepinephrine and serum luteinising hormone in young intact rats

Chronic exposure to oestrogens is known to inhibit the secretion of luteinising hormone (LH) in rats, leading to anovulation. Hypothalamic catecholamines, norepinephrine and dopamine play an important role in LH regulation. However, the effects of chronic exposure to low levels of oestradiol on hypothalamic catecholamines have not been investigated thoroughly. In the present study, adult female Sprague-Dawley rats were either sham implanted or implanted with 17beta-oestradiol (E(2)) pellets (20 ng/day) for 30 (E-30), 60 (E-60) or 90 (E-90) days. E(2) exposure affected oestrous cyclicity and ovarian morphology in a duration-dependent manner. There was no change in oestrous cyclicity in E-30 rats; however, 75% of E-60 and 95% of E-90 rats were acyclic (P < 0.05). Cycling rats from E-30 or the control group were killed at different time points on the afternoon of pro-oestrous. E-30 rats in oestrous, constant oestrous rats in the E-60 and E-90 groups and a group of old constant oestrous (OCE) rats were killed at 12.00 h. LH was measured in the serum by radioimmunoassay. Individual hypothalamic nuclei that are involved in LH regulation were microdissected and analysed for norepinephrine and dopamine levels using high-performance liquid chromatography/electrochemical detection. Norepinephrine levels in the hypothalamic nuclei increased significantly in control and E-30 groups during the afternoon of pro-oestrous, which was accompanied by a rise in LH levels (P < 0.05). On the day of oestrous, norepinephrine concentrations in hypothalamic nuclei and serum LH were significantly lower in E-60, E-90 and OCE rats compared to E-30 and control rats. On the other hand, dopamine levels declined significantly in one hypothalamic nucleus. These results indicate that chronic E(2) exposure affects hypothalamic catecholamine and serum LH levels in a duration-dependent manner. This coincides well with the loss of cyclicity observed in these animals. These results suggest that repeated exposure to endogenous oestrogens could play a role in reproductive senescence.

Inhibitory effects of melatonin on the growth of pituitary prolactin-secreting tumor in rats

The in vivo effects of melatonin on proliferation and apoptosis of 17-beta-estradiol (E2)-induced pituitary prolactin-secreting tumor (prolactinoma) were investigated in rats kept in 12 L/12 D (lights on: 06:00-18:00 hr). As melatonin was shown to induce apoptosis of breast and liver tumor cells, we examined whether melatonin would induce apoptosis of rat pituitary prolactinoma cells. 0.125, 0.25, 0.50 or 1.0 mg melatonin/day/rat was administrated subcutaneously at 17:30-18:00 hr. The weight of prolactinomas was measured. Apoptosis was evaluated using the TdT-mediated dUTP nick-end labeling method. It was found that treatment with 0.25 and 0.50 mg melatonin for 97 days inhibited prolactinoma cell proliferation and increased prolactinoma cell apoptosis. Furthermore, melatonin induced mRNA expression of Bax and cytochrome c protein expression. Conversely, mRNA expression of Bcl-2, and mitochondrial membrane potential were inhibited by melatonin treatment. These results suggest that melatonin inhibits the proliferation and induces apoptosis of rat pituitary prolactin-secreting tumor via perturbation of mitochondria physiology.

Fibroblast growth factor-2 and epidermal growth factor modulate prolactin responses to TRH and dopamine in primary cultures

Fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) are expressed in most tissues of the organism including pituitary. FGF-2 increases PRL levels and PRL mRNA in GH3 cells and primary cultures, and it has been involved in the lactotroph proliferation and hyperplasia. EGF also increases PRL levels in vitro. However, the effects of these two factors in the responses of lactotroph cells to TRH and dopamine (DA) remain to be clarified. In the present work we have studied the modulator activity of FGF-2 and EGF on in vitro PRL in responses to TRH and DA in primary cultures from in vivo vehicle- or estrogen (E2)-treated rats. We have found that FGF-2 (2 x 10(-11) M) prevents the EGF-induced dose-dependent increase in PRL levels in control cells, and reversed the EGF-stimulating effects in cells from E2-treated rats. Both FGF-2 (2 x 10(-11) M) and EGF (6.6 x 10(-9) M) significantly increase (>30% and >120%, respectively) the PRL levels in response to TRH (10(-6), 10(-5) M). FGF-2 blocked the inhibitory effects of low doses of DA (10(-9) M). EGF was unable to do so, although markedly increased (>200%) the post-DA PRL rebound. In cells from in vivo E2-treated rats, FGF-2 increased (>50%) the PRL secretion in response to TRH, while EGF reduced responses to high doses of TRH (10(-6), 10(-5) M). In addition, FGF-2 reversed and EGF increased the inhibitory effects of DA. Both FGF-2 and EGF completely blocked the post-DA PRL rebound, in these cells. Taken together our data suggest that FGF-2 and EGF are important regulators of lactotroph responsiveness to TRH and DA in vitro, although their actions are highly dependent on estrogenic milieu.

Diethylstilbestrol-induced prolactinoma: dose-related tumor growth and effect of catecholaminergic cells on prolactin tumor cells

BACKGROUND

Prolactinoma is a pituitary adenoma originating from prolactin-secreting epithelial cells of the adenohypophysis. Unfortunately, there appears to be a relatively high recurrence rate despite all pharmacological, radiological, and surgical therapeutic interventions. The aim of the present study was to evaluate the extent of involvement of the dopaminergic dysregulation hypothesis of prolactinomas. We transplanted, in rats, DES-induced prolactinoma cells into the adrenal medulla or under the renal capsule, two tissues rich and poor in catecholaminergic innervation, respectively.

METHODS

Prolactinoma was dose-dependently induced in ovariectomized female rats implanted with 10 and 20 mg DES, and tumor cells taken from prolactinoma induced by 20 mg DES were either transplanted under the renal capsule or into the adrenal medulla.

RESULTS

Although the adrenal medulla, with its high dopamine content to inhibit prolactin secretion, was devoid of any tumoral development, a significant tumoral development was evident under the renal capsule, seemingly because of no inhibitory control over prolactin secretion coexisting with the dopamine deficiency of the tissue. Results are discussed for an alternatively possible regression and prevention of any relapse of prolactinoma, most possibly occurring because of tuberoinfundibular dopamine deficiency, by the implantation of another dopamine-rich tissue beside the tumoral mass.

CONCLUSION

Regression and prevention of any relapse of a tumoral outgrowth, most possibly occurring because of tuberoinfundibular dopamine deficiency, can well be alternatively achieved by the implantation of another dopamine-rich tissue beside the tumoral mass prolactinoma.

Heparin increases prolactin and modifies the effects of fgf-2 upon prolactin accumulation in pituitary primary cultures

We have studied the effects of heparin on prolactin accumulation in the medium from primary pituitary cultures, and whether heparin interferes with the effects of fibroblast growth factor-2 (FGF-2) on PRL regulation in vitro. In the absence of exogenous FGF-2, hepa-rin increased prolactin accumulation in the culture medium in a dose-dependent manner. FGF-2 also increased the prolactin levels of primary cells in a time- and dose-dependent manner. However, low doses of heparin reduced the effects of FGF-2, but higher doses of heparin increased the maximal FGF-2-induced prolactin secretion and ED50. In vivo estrogenization of rats resulted in the abolition of FGF-2 capability to promote prolactin release in vitro. However, heparin restored cell responsiveness to FGF-2. Our results suggest that heparin, when present in the medium, binds FGF-2, therefore reducing its ability to interact with FGF receptors in a dose-dependent manner up to a critical molar concentration, at which heparin itself starts to activate the FGF receptor, and strengthens the activation induced by its proper ligand, FGF-2. Prolactin responses to FGF-2 are blocked by estrogen pretreatment, and it is probable that this introduces lactotroph cells in the proliferative stage. In conclusion, heparin modulates PRL secretion and PRL responses to FGF-2 in vitro.

Dominant negative ER induces apoptosis in GH(4) pituitary lactotrope cells and inhibits tumor growth in nude mice

The ER plays an important role in the proliferation and differentiation of lactotrope tumor cells. GH(4) cells were infected with adenoviral vectors (AdL540Q and Ad1-536) to investigate the ability of dominant negative ER mutants to affect the regulation of gene expression and cell growth by endogenous ER. The dominant negative mutants suppressed estradiol stimulation of an estrogen-responsive reporter gene and the PRL promoter in these cells. AdL540Q or Ad1--536 infection also inhibited GH(4) cell growth and induced apoptosis, increasing the expression of the proapoptotic Bax protein and decreasing the expression of antiapoptotic Bcl-2. AdwtER-infected cells also showed decreased Bcl-2 protein. E2-induced activation of p38 MAPK, an enzyme that may participate in apoptosis, was observed in cells infected with AdwtER, AdL540Q, and Ad1--536. Consistent with the apoptotic effects in vitro, infection of GH(4) cells with AdL540Q or Ad1--536 inhibited the ability of the cells to form tumors in nude mice. These results indicate that dominant negative ER mutants induce apoptosis of GH(4) cells and suppress tumor formation and development. The delivery of dominant negative ERs by adenoviral vectors may provide an alternative modality for the targeted therapy of pituitary lactotrope adenomas and other estrogen-responsive tumors.

Influence of 2-bromopropane on reproductive system--short-term administration of 2-bromopropane inhibits ovulation in F344 rats

The present study was performed to investigate the toxic effects of 2-bromopropane (2BP) on the female reproductive system. Female F344 rats were administered 2BP (500 or 1000 mg/kg, i.p.) at intervals of 2 or 3 days for 15-17 days. The body weights were measured and estrous stages were observed throughout the experimental period. Ovulation, organ weights, ovarian histology, and blood biochemistry were investigated on the terminal day of the experiment. Uterine weights in rats treated with 2BP were significantly lower than those in control animals. Body, liver, kidney, and adrenal weights in 2BP-treated rats showed no significant differences from control values. 2BP treatment prolonged estrous cycles and decreased the number of ovulated ova in spontaneous ovulation. In addition, histological examinations showed that the preovulatory follicles in the ovary were altered markedly in 2BP groups. These results show that even in short-term treatment, 2BP injured the ovary, particularly the preovulatory follicles. It appears that these damages of the preovulatory follicles induced by 2BP reduced the numbers of spontaneously ovulated ova in female F344 rats.

The xenoestrogen bisphenol A induces growth, differentiation, and c-fos gene expression in the female reproductive tract

The xenoestrogen bisphenol A (BPA) has been shown to mimic estrogen both in vivo and in vitro. BPA stimulates PRL secretion and the expression of a PRL regulating factor from the posterior pituitary in the estrogen-sensitive Fischer 344 rat (F344), but not in Sprague-Dawley (SD) rats. The goal of the present studies was to examine the in vivo actions of BPA on the reproductive tract. The specific objectives were 1) to characterize the short term effects of BPA on cell proliferation and c-fos expression in the uterus and vagina, and 2) to compare the effects of prolonged exposure to low doses of BPA on the reproductive tract of F344 and SD rats. Treatment with single high doses of BPA induced cell proliferation in the uterus and vagina of ovariectomized F344 rats, as determined by bromodeoxyuridine immunostaining. This proliferation was dose dependent (from 37.5-150 mg/kg) and followed a time course similar to that of estradiol (E2). Quantitative RT-PCR revealed that both BPA and E2 increased c-fos messenger RNA levels in the uterus 14- to 16-fold within 2 h, which returned to basal levels after 6 h. In the vagina, BPA-induced c-fos expression remained elevated for up to 6 h, compared with the transient increase caused by E2. Treatment of F344 rats for 3 days with continuous release capsules that supplied a much lower dose of BPA (approximately 0.3 mg/kg x day) resulted in hypertrophy, hyperplasia, and mucus secretion in the uterus and hyperplasia and cornification of the vaginal epithelium. The reproductive tract of SD rats did not respond to this treatment paradigm with BPA. These studies demonstrate that 1) the molecular and morphological alterations induced by BPA in the uterus and vagina are nearly identical to those induced by estradiol; 2) the vagina appears to be especially sensitive to the estrogenic actions of BPA; 3) the reproductive tract of the inbred F344 rat appears more sensitive to BPA than that of the outbred SD rat; and 4) continuous exposure to microgram levels of BPA is sufficient for exerting estrogenic actions.

Xenoestrogens: the emerging story of bisphenol a

Xenoestrogens are chemicals with diverse structure that mimic estrogen. Bisphenol A (BPA), a monomer of polycarbonate and epoxy resins, has been detected in canned food and human saliva. BPA stimulates cell proliferation and induces expression of estrogen-responsive genes in vitro, albeit with a relatively low potency. In vivo, BPA increases prolactin release and stimulates uterine, vaginal and mammary growth and differentiation. BPA shares similarities in structure, metabolism and action with diethylstilbestrol (DES), a known human teratogen and carcinogen. This review considers the hypothesis that BPA is converted in vivo to hydroxylated metabolite(s) with enhanced estrogenicity and genotoxicity.

The environmental estrogen bisphenol A stimulates prolactin release in vitro and in vivo

Environmental estrogens (xenoestrogens) are a diverse group of chemicals that mimic estrogenic actions. Bisphenol A (BPA), a monomer of plastics used in many consumer products, has estrogenic activity in vitro. The pituitary lactotroph is a well established estrogen-responsive cell. The overall objective was to examine the effects of BPA on PRL release and explore its mechanism of action. The specific aims were to: 1) compare the potency of estradiol and BPA in stimulating PRL gene expression and release in vitro; 2) determine whether BPA increases PRL release in vivo; 3) examine if the in vivo estrogenic effects are mediated by PRL regulating factor from the posterior pituitary; and 4) examine if BPA regulates transcription through the estrogen response element (ERE). BPA increased PRL gene expression, release, and cell proliferation in anterior pituitary cells albeit at a 1000- to 5000-fold lower potency than estradiol. On the other hand, BPA had similar efficacy to estradiol in inducing hyperprolactinemia in estrogen-sensitive Fischer 344 (F344) rats; Sprague Dawley (SD) rats did not respond to BPA. Posterior pituitary cells from estradiol- or BPA-treated F344 rats strongly increased PRL gene expression upon coculture with GH3 cells stably transfected with a reporter gene. Similar to estradiol, BPA induced ERE activation in transiently transfected anterior and posterior pituitary cells. We conclude that: a) BPA mimics estradiol in inducing hyperprolactinemia in genetically predisposed rats; b) the in vivo action of estradiol and BPA in F344 rats is mediated, at least in part, by increasing PRL regulating factor activity in the posterior pituitary; c) BPA appears to regulate transcription through an ERE, suggesting that it binds to estrogen receptors in both the anterior and posterior pituitaries. The possibility that BPA and other xenoestrogens have adverse effects on the neuroendocrine axis in susceptible human subpopulations is discussed.

Cell-specific induction of c-fos expression in the pituitary gland by estrogen

Estrogens regulate many functions of pituitary lactotrophs, including PRL gene expression, release, storage, and cellular proliferation. The mechanism by which estrogens exert such a variety of functions is poorly understood. In the uterus, estrogens rapidly and transiently induce the expression of the immediate early genes c-fos and c-jun in specific cell types. The Fos/Jun proteins form the activating protein-1 (AP1) transcription factor that mediates ligand-activated cell proliferation, differentiation, and secretion. Here we used Fischer 344 (F344) rats that develop hyperprolactinemia and prolactinomas in response to estrogens. The objectives were to: 1) determine whether estrogen induces c-fos expression in the pituitary gland and identify the responsive cells; 2) compare the dynamics of c-fos induction in the pituitary and uterus; and 3) examine the temporal relationship between c-fos expression and PRL release. Ovariectomized F344 rats were injected with 1 microg estradiol and killed at different times thereafter. Pituitaries were subjected to in situ hybridization for c-fos and immunostaining for selected pituitary cells. Estradiol stimulated c-fos expression in lactotrophs and folliculo-stellate cells within the anterior lobe without affecting either the intermediate or neural lobes. In a second experiment, c-fos messenger RNA levels were measured by solution hybridization in anterior pituitaries and uteri from estradiol-treated rats. Trunk blood was analyzed for PRL by RIA. The estrogen-induced c-fos rise in the uterus was rapid, robust, and transient, whereas that in the anterior pituitary was delayed, lower, and sustained. The profile of serum PRL levels resembles that of c-fos induction in the anterior pituitary. We conclude that: 1) both lactotrophs and folliculo-stellate cells increase c-fos expression in response to estrogens; 2) induction of c-fos expression may mediate some estrogenic effects on PRL synthesis and release and lactotroph proliferation in F344 rats; and 3) the atypical dynamics of c-fos induction in the pituitary could be due to indirect effects of estrogens on PRL-regulating factors within the hypothalamo-pituitary complex as well as to pituitary-specific estrogen receptor isoforms, coactivators, or repressors.

Multiple tyrosine hydroxylase transcripts and immunoreactive forms in the rat: differential expression in the anterior pituitary and adrenal gland

Several regulatory neurofactors, classically associated with the hypothalamus, may be synthesized in the anterior pituitary (AP). Dopamine (DA) is the main prolactin-inhibiting factor. Its de novo synthesis in the normal AP has not been proved, although the TH transcript has been previously demonstrated by RT/PCR in the AP. We investigated tyrosine hydroxylase (TH) gene expression at both the protein and mRNA levels in the AP of normal random cycling female rats and in a catecholaminergic tissue, the adrenal gland (AG). The Western blot analysis of AP homogenates revealed two immunoreactive forms of TH in the AP, both differing from the TH present in the AG. RT/PCR products from AP and AG mRNA were subcloned and sequenced. In addition to the full-length form, we identified two TH transcripts generated by alternative splicing either involving the use of a new alternate splice-donor site within exon 2 or skipping exon 11. The form lacking exon 11 was not isolated from the AG. In the AP, all three forms were present. Although the AP contained the full-length TH mRNA, the expected size protein was not detected. Thus, there is alternative splicing of the TH primary transcript, and putative additional post-translational regulation may yield TH proteins with no enzymatic activity, at least in non-catecholaminergic tissues.

Growth of a microprolactinoma to a macroprolactinoma during estrogen therapy

Following presentation and diagnosis, microprolactinomas usually follow a benign course and rarely progress to macroprolactinomas. However, clinically significant enlargement of prolactinomas during pregnancy, presumably related to estrogen stimulation, has been reported. This report describes a patient with amenorrhea and hyperprolactinemia and a microadenoma by computed tomography scan who developed a macroprolactinoma within 10 months after being placed on estrogen therapy. We propose that exogenous estrogen administration in this patient most likely promoted growth from a microprolactinoma to a macroprolactinoma. This case emphasizes the primary role of dopaminergic agonist therapy in the management of pathological hyperprolactinemia and suggests that estrogen therapy should not be casually given to patients with known prolactinomas to avoid the possibility of promoting tumor growth. A correlate of this approach is that caution regarding estrogen therapy should also be exercised in patients with idiopathic hyperprolactinemia who might have an occult microprolactinoma which could grow following estrogen stimulation. If estrogen treatment is deemed necessary, dopaminergic agonist therapy should also be used prophylactically to prevent potential tumor growth due to estrogen. The patient should then be carefully monitored with periodic serum PRLs and for the development of clinical manifestations suggesting pituitary growth. An imaging study should be performed when there is a significant increase in serum PRL or the development of new clinical manifestations.

A compartmentalized chamber for studying dopamine neurons in an hypothalamo-pituitary explant

An in vitro technique was developed which could be used to study whole neurons of the tuberohypophyseal dopaminergic (THDA) tract. Explants containing the medial basal hypothalamus (MBH; THDA cell bodies), pituitary stalk (THDA axons) and posterior pituitary (PP; THDA nerve terminals) were carefully dissected and placed in specially designed chambers. The chambers consist of separate compartments for the incubation of the MBH and PP while maintaining an intact pituitary stalk which traverses through a notched barrier. Dopamine (DA) synthesis in the explants was constant for 3 h and was significantly elevated by electrical stimulation. Electrical stimulation of both the PP and MBH increased endogenous DA release from the PP. DA release in response to potassium depolarization and dopaminergic drugs was significantly lower in explants than in the isolated PP even though spontaneous release was similar.

CONCLUSIONS

(1) this method is suitable for studying intact THDA neurons in vitro, and (2) intact THDA neurons respond differently to various stimuli than their excised terminals.

Evidence of tyrosine hydroxylase mRNA in the anterior and neurointermediate lobes of female rat pituitary

The anterior pituitary is thought to be unable to synthesize dopamine (DA) except under experimental conditions where a tyrosine hydroxylase (TH) activity, the rate-limiting step of its synthesis, has been demonstrated. In this work, we tested whether the enzyme described as active under particular conditions comes from de novo TH gene transcription or from a pre-existing TH mRNA poorly translated or untranslated under physiological conditions. Therefore, we searched for the presence of TH mRNA in normal female rat pituitary using the polymerase chain reaction following reverse transcription (RT/PCR) and in situ hybridization (ISH). The neurointermediate lobe (NIL) of the hypophysis was used as negative tissue, since it is thought to be unable to synthesize TH. As expected, no ISH labelling could be seen in the neural lobe (NL). However, scarce labelled cells were found in the intermediate lobe (IL) confirming the positive results observed in the NIL by RT/PCR. The anterior lobe (AL) also presented TH mRNA by PCR and ISH. The TH gene expression in sparse cells of the AL is discussed in regard to the ability of the AL to synthesize DA under particular conditions from a pre-existing mRNA.

Sexual differences in the activity of periventricular-hypophysial dopaminergic neurons in rats

The activities of periventricular-hypophysial dopaminergic (DA) neurons were compared in male and female rats by measuring dopamine synthesis (accumulation of 3,4-dihydroxyphenylalanine [DOPA] after inhibition of L-aromatic amino acid decarboxylase) and metabolism (concentrations of 3,4-dihydroxyphenylacetic acid [DOPAC]) in terminals of these neurons in the intermediate lobe of the pituitary. For comparison, the synthesis and metabolism of dopamine in the neural lobe of the pituitary and median eminence were also determined. The concentrations of DOPAC and accumulation of DOPA were higher in females than in males in both the intermediate lobe and median eminence, revealing a sexual difference in the basal activity of periventricular-hypophysial and tuberoinfundibular DA neurons. In contrast, there were no differences between male and female rats in activity of DA neurons terminating in the neural lobe. One week following gonadectomy, DOPA accumulation in the median eminence was decreased in females and increased in males, but remained unchanged in the intermediate lobe. These results indicate that sexual differences in the activity of periventricular-hypophysial DA neurons terminating in the intermediate lobe are not dependent upon the presence of circulating gonadal steroids, and in this respect, these neurons differ from tuberoinfundibular DA neurons.

Effects of reuptake inhibitors on dopamine release from the stalk-median eminence and posterior pituitary in vitro

Similar to other dopaminergic systems, the dopaminergic neurons innervating the stalk-median eminence (SME) and posterior pituitary (PP) possess an uptake mechanism for dopamine (DA). However, the extent of DA reuptake in these tissues and its physiological significance are debated since much of the released DA is removed by the hypophysial portal vasculature before recapture. The objectives of this study were: (1) to establish in vitro conditions for examining the effects of reuptake inhibitors on DA release from the PP and SME; (2) to compare the effects of nomifensine, diclofensine and amphetamine on DA release from the SME and PP; and (3) to distinguish between reuptake and releasing properties of these drugs. Individual SME and PP were dissected from ovariectomized rats and incubated in either a static or perifusion system. Media DA was extracted with alumina and quantitated by high performance liquid chromatography with electrochemical detection. The reuptake inhibitors, nomifensine, dichlofensine and amphetamine, in the presence of pargyline, a monoamine oxidase inhibitor, stimulated both basal and K(+)-evoked release of DA from the SME and PP under static incubation conditions. The drugs elicited a 2-3-fold higher increase in basal DA release from the SME as compared to the PP. Only amphetamine stimulated DA release in the perifusion system whereas nomifensine and diclofensine were without effects. We concluded that: (1) a mechanism for the reuptake of DA is operable in both the SME and PP; (2) the reuptake of DA appears to be more active in the SME than the PP; and (3) unlike amphetamine, nomifensine and diclofensine are pure reuptake inhibitors devoid of direct DA releasing activities.