Characterization of the dopaminergic input to the pituitary gland throughout the estrous cycle of the rat

Changes in the concentrations of dopamine (DA) and its major metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were characterized in the pituitary gland throughout the 4-day estrous cycle of the rat. Female rats were sacrificed at 2- to 3-hour intervals throughout each day of the 4-day estrous cycle. Pituitary glands were removed, and the concentrations of DA and DOPAC were determined by high-performance liquid chromatography coupled to electrochemical detection. The concentration of prolactin (PRL) in serum from these same animals was determined by radioimmunoassay. The concentration of DA in the anterior lobe was constant throughout most of the 4-day estrous cycle. Prior to initiation of the proestrous surge of PRL, there were significant (p < 0.05) decreases in the concentrations of both DA and DOPAC in the anterior lobe which returned to elevated baseline levels just prior to the termination of the proestrous surge of PRL. The concentrations of DA and DOPAC in the intermediate lobe exhibited a daily rhythm. However, in the intermediate as well as in the anterior lobe, there were significant (p < 0.001) decreases in the concentrations of both DA and DOPAC, coincident with the initiation of the proestrous surge of PRL. Similarly, coincident with the peak of the proestrous surge of PRL, there were significant (p < 0.001) increases in the concentrations of DA and DOPAC in the intermediate lobe, followed by a return to basal levels and resumption of the daily rhythm. The pattern of the concentrations of DA and DOPAC in the neural lobe was also daily in nature, with peaks occurring between 13.00 and 15.00 h each day of the 4-day estrous cycle. These data, taken together: (1) confirm that a decrease of the concentrations of DA and DOPAC occurs in the anterior lobe prior to the proestrous surge of PRL; (2) reveal that DA is released in a daily pattern at intermediate and neural lobes, and (3) suggest an apparent role for DA released to the intermediate lobe in the regulation of the proestrous surge of PRL.

Changes in the local metabolism of dopamine in the anterior and neural lobes but not in the intermediate lobe of the pituitary gland during nursing

Dopamine (DA), produced by tubero-infundibular dopaminergic (TIDA) neurons of the arcuate nucleus (ARN) is the established inhibitor of the secretion of prolactin (PRL). Changes in dopaminergic (DAergic) neuronal activity in the median eminence-long portal vessels (ME-LPV) and/or the concentration of DA in the anterior lobe (AL) are inversely related to the secretion of PRL. However, conflicting reports concerning DAergic neuronal activity during the suckling-induced release of PRL persist. In addition to TIDA neurons, PeVN-hypophysial DAergic (PHDA) and tubero-hypophysial DAergic (THDA) neurons which, respectively, innervate the intermediate lobe (IL) and the IL/neural lobe (NL) also have a significant role. We measured the concentrations of DA and its main metabolite, 3, 4-dihydroxyphenylacetic acid (DOPAC), in the median eminence and the three pituitary lobes of lactating mothers. Concentrations of DA and DOPAC from tissues and the concentration of PRL in plasma were measured by HPLC-EC and RIA, respectively. There were no changes in the concentration of DA and DOPAC of the IL due to the suckling stimulus. In the NL, a decrease in the concentration of DOPAC was detected due to the suckling stimulus. In addition, there were no changes of DA or DOPAC in the outer zone of the AL (AL-OZ) due to suckling. However, a decrease in the concentrations of DA and DOPAC was detected in the inner zone of the AL (AL-IZ). These data suggest lactotrophs from the AL-IZ are responsible for the changes in the concentration of plasma PRL in response to the suckling stimulus. In addition TIDA and THDA neurons, but not PHDA neurons, regulate the control of the secretion of PRL in response to suckling.

Endothelin activates large-conductance K+ channels in rat lactotrophs: reversal by long-term exposure to dopamine agonist

Endothelin-1 (ET-1) inhibits PRL secretion from cultured rat lactotrophs. However, ET-1 stimulates PRL secretion after cultured lactotrophs have been exposed for 48 h to dopamine or D2 dopamine agonists. In the present study, we have used cell-attached and inside-out patch recordings to establish an ionic basis for these effects. Bath application of 20 nM ET-1 to untreated lactotrophs evoked a robust and persistent activation of large-conductance K+ channels in cell-attached patches. This effect of ET-1 had a long latency to onset, was maintained for as long as ET-1 was present, and required at least 10 min of washing in control saline before complete recovery was achieved. The stimulatory effect of 20 nM ET-1 on these channels was markedly attenuated in the presence of the selective ET(A) receptor antagonist BQ-610 (200 nM), or after pertussis toxin (200 ng/ml, 16 h) pretreatment. The unitary slope conductance of the ET-1 activated channels in cell attached patches was 165 and 95 pS when the recording electrodes contained 150 and 5.4 mM KCl, respectively. These channels were voltage-sensitive and their activity increased upon patch depolarization. Previously activated channels in cell-attached patches became quiescent immediately upon patch excision into Ca2+-free bath saline. Exposure of the intracellular surface to 0.1 microM Ca2+ restored the activity of these channels similar to the level seen before patch excision. In addition, preincubating the cells with the membrane-permeable Ca2+-chelator BAPTA-AM, or using Ca2+-free solution in the recording pipettes, prevented the activation of these channels by ET-1. The ET-1 activated large-conductance Ca2+-dependent K+ (BK(Ca)) channels were blocked by 20 mM tetraethylammonium but were insensitive to the K+ channel blockers apamin (1 microM), charybdotoxin (200 nM), or iberiotoxin (200 nM). Acute application of 10 microM dopamine and 20 nM ET-1 caused activation of BK(Ca) channels with indistinguishable kinetic properties, although the effect of dopamine occurred with shorter latency. After 48-h exposure to the specific D2 dopamine receptor agonist (+/-)-2-(N-phenyl-N-propyl) amino-5-hydroxytetralin hydrochloride (PPHT, 500 nM), bath application of 20 nM ET-1 resulted in inhibition of spontaneously active BK(Ca) channels. These data suggest that both the stimulatory and inhibitory effects of ET-1 on PRL secretion are mediated, at least in part, by actions on BK(Ca) channels, and that long term exposure to dopamine or D2 agonists alters the signaling pathways from the ET(A) receptor to BK(Ca) channels.

Effects of ovarian steroid hormones on dopamine-controlled prolactin secretory responses in vitro

Dopamine (DA), a well-established inhibitor of prolactin (PRL) secretion, has also been shown to stimulate PRL secretion from the anterior lobe of the pituitary gland of the rat. It has been reported that low doses of DA stimulate PRL whereas high doses inhibit PRL secretion. Our laboratory has previously reported that these PRL secretory responses are dependent upon the stages of the estrous cycle of the rat. The objective of the present study was to determine the steroid requirements for the differing PRL secretory responses to low and high doses of DA in perifusion. Animals were ovariectomized (OVX) and immediately given Silastic implants containing estradiol (E2) which has previously been shown in our laboratory to produce blood levels of 70-100 pg/ml, progesterone which has previously been shown in our laboratory to produce blood levels of 30-40 ng/ml, or the combination. OVX rats served as controls. Ten days later, the anterior lobes of the pituitary glands were harvested and enzymatically dissociated. Cells were mixed with Sephadex G-10 and placed in six 0.5-ml perifusion chambers (1 x 10(6) cells/chamber). Cells were perifused for 24 h with Dulbecco's modified Eagle's medium containing 0.2% bovine serum albumin and 0.1 mM ascorbic acid. The PRL secretory pattern was characterized in response to the following treatment sequence: (1) 30 min media alone (maximally uninhibited); (2) 24 min 100 pM DA; (3) 30 min media alone (DA withdrawal); (4) 24 min 1 microM DA, and (5) 30 min media alone (DA withdrawal). PRL secretion in the presence of 100 pM DA was unchanged in cells obtained from OVX animals, but exposure to 1 microM DA inhibited PRL release in these cells. Subsequent withdrawal stimulated PRL secretion relative to that of the initial exposure to media alone. The responses of cells from OVX rats implanted with progesterone alone was indistinguishable from those of the OVX controls. In cells obtained from animals implanted with E2 alone, 100 pM DA and its subsequent withdrawal neither stimulated nor inhibited PRL secretion. In contrast, 1 microM DA exposure initially stimulated and then inhibited PRL secretion in cells from E2-treated animals. Here, subsequent withdrawal of DA enhanced PRL secretion. In cells obtained from E2+progesterone-treated animals, 100 pM DA exposure robustly enhanced PRL secretory responses. Withdrawal of this dose of DA had no further effect on PRL secretion. However, exposure of E2+progesterone-treated cells to 1 microM DA robustly stimulated and subsequently only slightly inhibited PRL secretion. The results of these studies suggest that inhibition of PRL secretion by DA is independent of ovarian steroids while E2 and progesterone in combination favors stimulation of PRL secretion in response to DA. Taken together, these data suggest that PRL secretory responses in this system are determined by the ovarian steroid milieu.

Dopaminergic neurons in periventricular and arcuate nuclei of proestrous and ovariectomized rats: endogenous diurnal rhythm of Fos-related antigens expression

The expression of Fos-related antigens (FRAs) in the A12 and the A14 hypothalamic dopaminergic neurons was compared throughout the day in proestrous (PRO) and ovariectomized (OVX) rats to establish the relationship between secretion of prolactin (PRL) and dopaminergic neuronal activation. Animals with intact ovaries were sacrificed at 11:00, 13:00, 15:00, 17:00 and 21:00 h on the day of proestrus and 06:00 and 09:00 h in the morning of estrus. OVX animals were sacrificed at the same time points on the 12th day after surgery. Double-label immunocytochemistry was performed by using antibodies against FRAs as markers of tonic neuronal activity and tyrosine hydroxylase to identify dopaminergic neurons. Serum PRL levels were determined by radioimmunoassay. A pattern of FRAs expression was present in the A14 and A12 neurons of PRO rats. The incidence of FRAs expressing neurons was the highest in the first half of the day when the PRL levels were low, and decreased prior to the surge of serum PRL during the afternoon. This pattern was present in the dopaminergic neurons of the periventricular nucleus (A14), and in all portions of the arcuate nucleus (A12) except the ventrolateral portion of the middle arcuate nucleus. A similar pattern of FRAs expression existed in the A14 and A12 neurons of OVX rats in spite of that there were no detectable changes in serum PRL levels. However, the amplitude of decrease in the incidence of FRAs-labeled neurons was lower in OVX than in PRO rats. These data suggested that similar to the control of hypothalamic PRL releasing factors, the activation/deactivation pattern of the A12 and the A14 neurons is governed by a seemingly endogenous rhythmic input. The incidence of this rhythm is independent of the reproductive state, but its amplitude is enhanced by ovarian steroids. The disparity between the FRAs expression and serum PRL levels in OVX rats indicated that the effect of this endogenous dopaminergic rhythm upon PRL secretion is dependent on the ovarian steroid background.

Dopaminergic neurons of periventricular and arcuate nuclei of pseudopregnant rats: semicircadian rhythm in Fos-related antigens immunoreactivities and in dopamine concentration

The activity of the A14 and A12 hypothalamic dopaminergic (DAergic) neurons were characterized throughout a day of pseudopregnancy to establish their relationship to the generation of PRL surges during that time. Double-label immunocytochemistry was performed using fos-related antigens as markers of neuronal activity, and tyrosine hydroxylase antibody to identify DAergic neurons of the periventricular nucleus and the arcuate nucleus (ARN). Dopamine concentrations were measured with HPLC with electrochemical detection from micropunched samples to approximate synthetic activities of A14 and A12 neurons. Serum PRL levels were determined by RIA. On the fifth and sixth day of pseudopregnancy, five rats/time point were killed at 1100 h, 1300 h, 1500 h, 1800 h, 2100 h, 2400 h, 0300 h, and 0600 h. When the serum PRL was low, the incidence of fos-related antigens/tyrosine hydroxylase double-labeled neurons was high, and it decreased twice a day by 1500 and 0300 h in the periventricular nucleus, rostral ARN, dorsomedial subpopulation of the middle ARN, and in the caudal ARN, corresponding to the initiation of PRL surges. Concentrations of DA were high in these areas, and decreased before PRL surges in agreement with the immunocytochemical data. These findings suggest a semicircadian rhythm in the activities of A14 and A12 neurons which, in concert with PRL releasing factors, contribute to the generation of PRL surges in pseudopregnant animals.

Effects of the estrous cycle stage on the prolactin secretory response to dopamine in vitro

Dopamine (DA) will both stimulate and inhibit prolactin (PRL) secretion from the anterior pituitary gland in vitro and in vivo. The present study was designed to determine if there are selected times during the estrous cycle of the rat when one function is favored over the other. Anterior pituitary glands collected on diestrus-1 (D1), diestrus-2 (D2), the morning of proestrus (Pro-AM), the afternoon of proestrus (Pro-PM), and estrus (E) were enzymatically dissociated and placed in monolayer culture. On the fourth day in culture, cells were challenged for 10, 20, 30, 60, 120, 180, or 240 min with media alone or media containing either 100 pM or 1 μM DA. The concentration of PRL in the media was determined by radioimmunoassay. Regression analysis revealed that in the absence of DA, PRL secretion from cultured cells differed significantly depending on the stage of the estrous cycle during which they were obtained. Cells obtained during the morning of diestrus-2 secreted PRL at the greatest rate compared to other stages of the cycle. When all stages were compared, the rates of PRL secretion were: D2>E>D1>Pro-AM>Pro-PM (each significantly different from the others,P<0.01). By 20-30 min of exposure to 100 pM DA, the rate of PRL secretion from cells obtained during each stage of the cycle was significantly enhanced. This enhanced secretion persisted in cells obtained during D2 and Pro-PM but was short-lived in cells obtained during other stages. No inhibition of PRL secretion was induced by this dose of DA. PRL secretion was inhibited when treated with 1 μM DA in cells obtained at all stages of the estrous cycle. Inhibition was more prolonged in cells obtained on D1, D2, and Pro-AM. DA was least effective as an inhibitor of PRL secretion in cells obtained during Pro-PM and E. Prior to inhibiting PRL secretion in cells obtained during Pro-PM, 1 μM DA rapidly stimulated PRL secretion. This effect persisted for 60 min. These data suggest that in the absence of DA, the dynamics of PRL secretion from anterior pituitary cells in vitro differ depending on the stage of the estrous cycle during which the cells were obtained. Moreover, the in vivo environment of the cell determines the direction and magnitude of the PRL-secretory response to DA.

The genetic and molecular organization of the Dopa decarboxylase gene cluster of Drosophila melanogaster

We report the complete molecular organization of the Dopa decarboxylase gene cluster. Mutagenesis screens recovered 77 new Df(2L)TW130 recessive lethal mutations. These new alleles combined with 263 previously isolated mutations in the cluster to define 18 essential genes. In addition, seven new deficiencies were isolated and characterized. Deficiency mapping, restriction fragment length polymorphism (RFLP) analysis and P-element-mediated germline transformation experiments determined the gene order for all 18 loci. Genomic and cDNA restriction endonuclease mapping, Northern blot analysis and DNA sequencing provided information on exact gene location, mRNA size and transcriptional direction for most of these loci. In addition, this analysis identified two transcription units that had not previously been identified by extensive mutagenesis screening. Most of the loci are contained within two dense subclusters. We discuss the effectiveness of mutagens and strategies used in our screens, the variable mutability of loci within the genome of Drosophila melanogaster, the cytological and molecular organization of the Ddc gene cluster, the validity of the one band-one gene hypothesis and a possible purpose for the clustering of genes in the Ddc region.

Long term exposure to dopamine reverses the inhibitory effect of endothelin-1 on prolactin secretion

This study was undertaken to assess the possibility that endothelin-1 (ET-1) and dopamine (DA) can act in concert to modulate PRL secretion. Enzymatically dispersed anterior pituitary cells obtained from random cycling female rats were perifused with Dulbecco's Modified Eagle's Medium supplemented with 0.2% BSA and 100 microM ascorbic acid. In the absence of dopamine, ET-1 (applied at 20 nM for 60 min) rapidly evoked a small transient elevation of PRL release, followed by a sustained inhibitory phase. Overnight perfusion with 500 nM DA-supplemented medium did not change the basic character of ET-1's effects on PRL secretion. Continuation of DA exposure for 48 h dramatically shifted the responsiveness of the lactotrophs to ET-1; the fast stimulatory response was robustly enhanced, whereas the inhibitory phase was replaced by a modest secondary elevation of basal PRL secretion. The stimulatory effect of ET-1 on PRL secretion after DA pretreatment was blocked by an ETA receptor antagonist, BQ-123. The effect of DA can be mimicked completely by a specific D2 receptor agonist (+/-)-2-(N-phenyl-N-propyl)amino-5-hydroxytetraline hydrochloride, whereas pretreatment with a D1 agonist, SKF-39393, failed to change the responsiveness of lactotrophs to ET-1. Our data indicate that persistent activation of D2 receptors, a condition most closely resembling the in vivo environment of the lactotrophs, uncouples the inhibitory signaling pathway from the ETA receptor while synergistically affecting signal transduction, which mediates the ET-induced stimulation of PRL secretion.

A transformed murine Leydig cell line expresses the ETA receptor subtype

We recently demonstrated that transformed murine Leydig cells (MA-10) responded to endothelin-1 (ET-1) via increased steroidogenesis. This study addresses the endothelin receptor subtype present on this cell line and whether or not the cells produce ET-1. The expression of the preproendothelin-1 (PPET-1) gene was investigated by Northern blot analysis, and PPET-1 mRNA was found to be < 0.2% of that present in pulmonary endothelial cells. The medium from MA-10 cells, maintained under serum-free conditions, was analyzed by radio-immunoassay to determine immunoreactive-ET-1 production and ET-1 levels were found to be below the sensitivity of the assay (< 10 pg/ml). The data from competitive binding experiments with [125I]ET-1 and unlabeled ET-1, ET-3 and receptor subtype selective ligands yielded a single class of high affinity binding sites with ETA receptor subtype characteristics. The results of this study demonstrate that MA-10 cells possess the ETA receptor subtype but do not produce significant quantities of ET-1 under basal conditions.

Comparison of the forms of the dopamine D2 receptor expressed in GH4C1 cells

The effects of dopamine (DA) on prolactin (PRL) secretion, phosphoinositide metabolism, cytosolic calcium concentrations ([Ca2+]i), and cAMP production in GH4C1 cells expressed either the short (GH4ZR7 cells) or long (GH4I12 cells) form of the rat DA D2 receptor were compared in this study. The GH4C1 cell line is derived from the rat anterior pituitary gland and lacks functional DA receptors. The GH4ZR7 and GH4I12 cell lines have been transfected with either the short or the long form of the D2 receptor, respectively. In this study, the functional coupling of these receptors to both basal and stimulated PRL secretion and to common second messenger systems was examined. Both cell types expressing DA receptors exhibited similar saturable binding to the D2 antagonist [3H]spiperone (Kd GH4ZR7 = 96 +/- 8 pM, Kd GH4I12 = 107 +/- 49 pM). In GH4ZR7 cells, 1 and 10 microM DA inhibited basal PRL secretion by 37% and 58%, respectively. In GH4I12 cells, 1 and 10 microM DA inhibited basal PRL secretion by 63% and 54%, respectively. In GH4ZR7 cells, 10 microM DA completely reversed the stimulatory effects of 1-100 nM thyrotropin-releasing hormone (TRH), and 1 microM DA attenuated the stimulatory effects of 10 and 100 nM TRH. Interestingly, GH4I12 cells were not responsive to TRH. In both cell lines, the inhibitory effects of DA were blocked by the specific D2 antagonist, eticlopride. The stimulatory effects of TRH on [Ca2+]i were dose dependent and could be blocked (at least in GH4ZR7 cells) by prior treatment of the cells with 1 microM DA. The ability of dopamine to block the TRH-mediated increase in [Ca2+]i was attenuated by eticlopride. DA (1 microM) had no effect on resting [Ca2+]i in either cell line expressing DA receptor. TRH (100 nM) maximally stimulated total inositol phosphate (IP) accumulation to values approximately three times greater than controls in GH4C1 and GH4ZR7 cells only. DA had no effect on basal or TRH-stimulated IP accumulation in any of the cell lines. DA (1 and 10 microM) inhibited cAMP production by 40% and 39%, respectively, in GH4ZR7 cells. Similarly, in GH4I12 cells, DA (1 and 10 microM) inhibited cAMP production by 48% and 60%, respectively. These data indicate that both the long and short forms of the D2 receptors play similar roles when expressed in GH4C1 cells. However, maximal inhibition of PRL in these cell lines is approximately 20% less than that of normal lactotrophs.

Characterization of endothelin receptors in the anterior pituitary gland

To characterize endothelin (ET) receptors modulating pituitary hormone secretion, potencies of ET-like agonists were compared on prolactin (PRL), thyrotropin (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) secretion from primary cultures of female rat pituitary cells. ET-1 was more potent than ET-3 in all cases. Sarafotoxin (SRTX) S6b an ETA agonist, was also more potent than ET-3 in all cases. SRTX-c, an ETB receptor agonist, was inactive. The ET-1-to-ET-3 potency ratio was three orders of magnitude higher on PRL or TSH secretion than on LH and FSH secretion, whereas SRTX-b-to-ET-3 potency ratios were similar on all four hormones. The ETA antagonist BQ-123 caused a parallel dextral displacement of dose-response curves of ET-1 and ET-3 on all four hormones. Schild regressions for BQ-123 on ET-1-induced PRL, TSH, LH, and FSH secretion indicated that BQ-123 has a similar affinity for the receptors mediating ET-1's effects. When BQ-123 was assessed against ET-3, Schild regressions indicated greater affinity for ET-3 on lactotrophs and thyrotrophs than gonadotrophs. Thus changes in pituitary hormone secretion are mediated by ETA-like receptors. ET receptors in lactotrophs and thyrotrophs are clearly distinguishable from gonadotrophs. We suggest the existence of distinct ETA receptor subtypes (ETA1 and ETA2) on these differing pituitary cells.

Low concentrations of dopamine increase cytosolic calcium in lactotrophs

Dopamine (DA) plays a dual role in the neuroendocrine regulation of PRL secretion from the anterior pituitary gland. DA normally exerts a tonic inhibitory effect on PRL secretion. However, DA at concentrations much lower than those required for maximal inhibition of PRL secretion actually stimulates PRL secretion. In this study, the effects of stimulatory concentrations of DA on common second messenger systems were examined in pituitary cells enriched for lactotrophs in an attempt to identify the signal transduction mechanisms used in DAergic stimulation of PRL secretion. Rat pituitary cells were enriched for lactotrophs (88% lactotrophs) by differential sedimentation on a discontinuous Percoll gradient. Enriched cells in monolayer culture were responsive to both inhibitory and stimulatory doses of DA; 10 microM DA inhibited PRL secretion by 80%, and 0.1 nM DA stimulated PRL secretion by 40% after a 4-h incubation. No dose of DA (10 pM to 10 microM) had an effect on phosphatidylinositol turnover. However, 1 and 10 microM DA inhibited cAMP formation by 58% and 72%, respectively. DA at doses in the stimulatory range had no effect on cAMP formation. Single cell cytosolic calcium concentrations ([Ca2+]i) were examined by quantitative fluorescence microscopy using fura-2 as a probe. Inhibitory doses of DA (1 microM) decreased [Ca2+]i in 83% of the cells examined. Two subpopulations of cells were noted that varied in the degree of response to 1 microM DA. One subpopulation responded to DA by decreasing [Ca2+]i 50-100 nM. The other subpopulation responded to DA by decreasing [Ca2+]i 300-400 nM. A stimulatory dose of DA (0.1 nM) increased [Ca2+]i in 44% of the cells examined. This dose of DA increased [Ca2+]i by 100-400 nM. These data indicate that the second messenger mediating DAergic stimulation of PRL secretion is most likely Ca2+.

Infusion of dopamine at low concentrations stimulates the release of prolactin from alpha-methyl-p-tyrosine-treated rats

Prolactin (PRL) secretion from the anterior pituitary gland is inhibited by dopamine (DA) released into the hypophysial portal vasculature from neurons in the hypothalamus. We have shown previously that DA also stimulates PRL secretion in vitro. Here we report that DA has a dual effect on PRL release in vivo. Injection of rats with alpha-methyl-p-tyrosine (200 mg/kg, ip) induced an immediate 35-fold enhancement of PRL secretion which reached a plateau by 90 min after injection on diestrus 1. When DA was infused intravenously at varying doses beginning at 90 min after alpha-methyl-p-tyrosine, differing effects on PRL secretion were observed. These effects were dose dependent: higher doses of DA (1000 ng/kg/min) inhibited and lower doses (10 ng/kg/min) stimulated PRL secretion. These data suggest that DA may be an important stimulator of PRL secretion in vivo.

Activity of vasoactive intestinal peptide and serotonin in the paraventricular nucleus reflects the periodicity of the endogenous stimulatory rhythm regulating prolactin secretion

PRL secretion in the female rat is regulated by an endogenous stimulatory rhythm (ESR) of prolactin-releasing factors of hypothalamic origin which has a bimodal periodicity with distinct nocturnal (N) and diurnal (D) phases. The N phase reaches peak magnitude by 0300 h and the D phase reaches peak magnitude by 1700 h. This rhythm was first unmasked in ovariectomized rats by correctly timed injection of a dopamine antagonist. OT, vasoactive intestinal peptide (VIP), and serotonin (5-HT) are differentially involved in generating the ESR. Pharmacological studies suggest that OT is the neurohormone and VIP and 5-HT are neuromodulators which act to stimulate OT release. Recently, we reported that activity of OTergic neurons in the paraventricular nucleus (PVN) and OT concentrations in the anterior pituitary mirror the periodicity of the ESR. The present experiments were conducted to determine if VIP and 5-HT activity in the hypothalamus also mirrors the periodicity of the ESR. Push-pull cannulae were surgically implanted in the PVN of ovariectomized female rats. Following recovery, push-pull perfusion was conducted from either 0600-1400 h, 1400-2200 h, or 2200-0600 h. VIP was measured in perfusates by RIA. There was no difference in VIP pulse frequency between rats perfused during the three periods studied. However, animals perfused from 2200-0600 h had significantly greater pulse amplitude as compared to rats at either 0600-1400 h or 1400-2200 h. Activity of 5-HTergic neurons in the hypothalamus was studied by estimating the turnover of 5-HT 10 min following the injection of pargyline. Hypothalamic nuclei were dissected using Palkovits' punch technique and 5-HT concentration assayed by HPLC in conjunction with electrochemical detection. Turnover of 5-HT was estimated by calculating the slope of the accumulation of 5-HT over 10 min at differing times of day using least squares regression analysis. There was a distinct diurnal rhythm of 5-HT accumulation in the PVN. Rats killed at 1700 h had significantly greater slopes of 5-HT accumulation in the PVN than rats killed at either 0300 or 1200 h. Similarly, there was a diurnal rhythm of 5-HT turnover in the suprachiasmatic nucleus. Rats sampled at either 1200 or 1700 h had significantly greater slopes of 5-HT accumulation in the suprachiasmatic nucleus than rats sampled at 0300 h. There was no diurnal rhythm of 5-HT turnover evident in either the median eminence or the supraoptic nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

The stimulatory and inhibitory effects of dopamine on prolactin secretion involve different G-proteins

The reverse hemolytic plaque assay (RHPA) was used in this study to further characterize the mechanism whereby low concentrations of dopamine (DA) stimulate PRL secretion in vitro. Female Sprague-Dawley rats were used as a source of anterior pituitary cells for the RHPA. Pituitary cells were infused into Cunningham chambers along with a suspension of protein-A-coated ovine red blood cells. Excess cells were rinsed from the chambers leaving a monolayer of cells attached to the glass. The cells were then incubated with solutions containing PRL antiserum (1:40) and various concentrations of DA. After 4 h, a solution containing guinea pig complement (1:60) was infused into the chambers. Thirty minutes later, the cells were fixed and plaques (zones of hemolysis) surrounding PRL-producing cells (lactotrophs) were measured and used as an index of the amount of PRL secreted. Control cells that received no DA had a mean plaque area of 8,000 microns 2 and two distinct subpopulations of plaque sizes. This biphasic population of cells consisted of a small and a large plaque producing population. The mean plaque area surrounding lactotrophs was significantly (P less than 0.05) decreased if 1 microM or 10 microM DA was present (4,500 microns 2 and 3,500 microns 2, respectively). These cells which received inhibitory concentrations of DA demonstrated a monophasic distribution of plaque-forming cells. On the other hand, mean plaque area was significantly (P less than 0.05) increased if 0.1 nM or 1 nM DA was presented to the cells (15,000 microns 2 and 14,500 microns 2, respectively). These cells receiving stimulatory doses of DA exhibited a multiphasic distribution of plaque-forming cells. The possibility that the two physiological opposing actions of DA on PRL secretion might be mediated by different GTP binding proteins was also examined using cholera toxin (CTX) and pertussis toxin (PTX). Anterior pituitary cells were pretreated with either CTX (50 micrograms/ml) or PTX (5 micrograms/ml) for 1 h before initiation of the RHPA. In the RHPA, cells received no DA, a stimulatory dose of DA (0.1 nM), or a inhibitory dose of DA (10 microM). The effects of toxin pretreatment on mean plaque area of DA-treated cells was determined. PTX pretreatment significantly attenuated the inhibitory effects of DA while having no effect on the stimulatory effects of DA on PRL secretion. CTX significantly (P less than 0.05) potentiated the stimulatory effects of DA on PRL secretion and had no effect on inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity of oxytocinergic neurons in the paraventricular nucleus mirrors the periodicity of the endogenous stimulatory rhythm regulating prolactin secretion

PRL secretion is regulated by an endogenous stimulatory rhythm of PRL-releasing factors within the hypothalamus. The endogenous rhythm has a bimodal periodicity with a nocturnal component which peaks at approximately 0300 h and a diurnal component that peaks at approximately 1700 h. Several PRL-releasing factors are known to be involved in this rhythm. Among these are oxytocin (OT), vasoactive intestinal peptide, and serotonin. We have proposed that OT is the neurohormone that stimulates PRL release from the lactotroph. In this study, we examined the activity of OTergic neurons in the paraventricular nucleus using the expression of the protooncogene c-fos (Fos) as a marker of neuronal activity. Ovariectomized rats were killed at either 0300, 1200, or 1700 h and brains quickly fixed by perfusion with 2.5% acrolein in 4% paraformaldehyde. Brains were blocked and processed for OT/Fos immunohistochemistry. Rats killed at 0300 and 1700 h had significantly greater proportion of Fos expressing OTergic neurons than control rats (1200 h). Percent of Fos-positive OTergic neurons were 2- and 1.5-fold greater at 0300 and 1700 h than 1200 h, respectively. The majority of these neurons were located in the medial parvocellular paraventricular nucleus and periventricular area. In another experiment, groups of OVX rats were killed every 2 h over a 24-h period and OT extracted from their anterior and posterior pituitaries. OT was present in the anterior pituitary in a bimodal rhythm. OT concentrations were greatest at approximately 0400 h and slowly declined to baseline by 1000 h. Another peak of OT was present in the anterior pituitary at approximately 2000 h and quickly declined to baseline by 2400 h. This rhythm of OT was not reflected in either the posterior pituitary or trunk blood. These data suggest that activity of a specific population of OTergic neurons of the paraventricular nucleus is rhythmic. The periodicity of these neurons mirrors that of the endogenous stimulatory rhythm. Furthermore, the anatomical location of these neurons suggests that they may project to the median eminence. Indeed, this heightened activity is reflected in a bimodal rhythm of OT in the anterior pituitary. Taken together, the data presented here provide compelling support for the role of OT as the neurohormone in the mechanism of the endogenous stimulatory rhythm.

The effects of endothelins on the secretion of prolactin, luteinizing hormone, and follicle-stimulating hormone are mediated by different guanine nucleotide-binding proteins

Different bacterial toxins capable of modifying specific alpha-subunits of G-proteins were used to characterize the guanine nucleotide-binding protein (G-protein) dependency of the effects of endothelins (ETs) on PRL, LH, and FSH secretion. Primary cultures of anterior pituitary cells obtained from female rats were preincubated for 24 h with 20 ng/ml pertussis toxin (PTX) or 2 micrograms/ml cholera toxin (CTX) before challenge with ETs. Both ET-1 and ET-3 elicited a concentration-dependent inhibition of PRL secretion and stimulated the release of LH and FSH secretion on pituitary cells not treated with toxins. Based on the calculated ration of the half-maximal effective concentrations (EC50) of ET-1 and ET-3, ET-1 showed 7800, 20, and 14 times greater potency than ET-3 on PRL, LH, and FSH secretion, respectively. PTX, a selective inhibitor of Gi and several other G proteins, increased the basal secretion of PRL and completely eliminated the responsiveness of lactotroph cells to ET-1 and ET-3. Pretreatment with PTX caused a markedly different effect on LH and FSH secretion: while basal LH release was slightly increased, FSH secretion was markedly depressed by PTX. Moreover, while ET-induced LH secretion was enhanced by PTX, the effectiveness of ETs on FSH release was completely abolished. CTX, known as an activator of Gs proteins, decreased the basal secretory activity of lactotrophs but did not influence the ET-induced decrease of PRL release. CTX pretreatment (like PTX before) elicited a strikingly different effect on LH and FSH: while basal LH secretion was enhanced, basal FSH secretion was markedly inhibited by CTX. Moreover, while the effectiveness of ETs on LH secretion was not changed significantly, the stimulatory effect of ETs on FSH secretion was diminished after CTX pretreatment. Thus, the inhibition of PRL secretion by ETs requires a PTX-sensitive G protein while the ET-induced stimulation of FSH secretion involves both PTX- and CTX-sensitive elements. The fact that pretreatments with PTX or CTX influenced basal secretion of PRL, LH, and FSH suggests that PTX- and/or CTX-sensitive G proteins are directly involved in the process of exocytosis. Additionally, these findings might indicate an active paracrine/autocrine regulation of pituitary cells in culture that are impaired or enhanced by the bacterial toxins employed. Though the broad substrate specificity of PTX and CTX and the multiplicity of G protein families did not allow us to identify the specific G protein(s) involved, these data reveal the diversity of ET-induced intracellular signaling mechanisms in lactotrophs and gonadotrophs.

Pharmacologic evidence that a D2 receptor subtype mediates dopaminergic stimulation of prolactin secretion from the anterior pituitary gland

The ability of low concentrations of dopamine (DA) to stimulate the secretion of prolactin (PRL) was examined in perifused or monolayer cultures of anterior pituitary cells. In cultures perifused with media containing 100 nM DA, changing the DA concentration to either 1 or 100 pM caused a significant dose-dependent stimulatory PRL secretory response within 6 min when compared to the PRL secretory response to removal of DA altogether. Picomolar concentrations of DA caused a biphasic PRL secretory response. This response is characterized by an immediate increase in the rate of PRL secretion similar to that seen when the cells were treated with 100 nM thyrotropin-releasing hormone followed by a decrease in the rate of PRL secretion to levels comparable to cells receiving media alone. In a monolayer culture system DA, at concentrations between 10 nM and 1.0 pM, caused significant stimulation of PRL secretion relative to media alone. Maximal stimulation occurred at nanomolar concentrations of DA (approximately 60% greater than control). Although the D2 agonists, bromocriptine and 2-(N-phenethyl-N-propyl)-amino-5-hydroxytetralin hydrochloride (PPHT) caused significant (p less than 0.05) inhibition of PRL secretion at nanomolar concentrations and above, neither had stimulatory activity. The D1 agonists, SKF 38393 and SKF 82958, had no effect on PRL secretion when tested at 0.1 pM to 1 microM. These data suggest that DA not only inhibits PRL secretion in vitro, but also stimulates PRL secretion at relatively low concentrations. Stimulation is mediated by a DA receptor which is neither recognized by D2 nor D1 agonists, suggesting a possible third DA receptor subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of prolactin secretion by endothelin-3 is pertussis toxin-sensitive

The effect of pertussis toxin (PTX) pretreatment on endothelin-3 (ET-3)-mediated inhibition of prolactin secretion from primary cultures of rat anterior pituitary cells was examined. Monolayer cultures of anterior pituitary cells were treated with either 20 ng/ml PTX dissolved in media or with media alone (control) on the third day of culture. Exactly 24 h after PTX pretreatment, cells were challenged with either 100 nM ET-3 dissolved in media or media alone (control) for 4 or 48 h. ET-3 significantly (P less than 0.01) inhibited prolactin secretion in both the 4 and 48 h incubations. However, if the cells had been previously treated with PTX, ET-3 did not significantly affect prolactin secretion. These data suggest that a PTX-sensitive G protein mediates ET-3-induced inhibition of prolactin secretion and that ET-3 may invoke a signal transduction mechanism in the lactotroph which is distinct from those described in other cell types.

Ontogeny of the endogenous stimulatory rhythm regulating prolactin secretion in immature female rats

PRL secretion in the female rat is regulated by an endogenous stimulatory rhythm (ESR). This rhythm consists of two components: a nocturnal (N) component whose activity is greatest by 0300 h and a diurnal (D) component that peaks at approximately 1700 h. This periodicity coincides with the periods of the N and D surges of PRL in responses to the mating stimulus. Furthermore, we have shown that the ESR is involved in the regulation of mating-induced PRL surges. Mating causes a lowering of dopaminergic tone which reveals the ESR for PRL. The ability of immature female rats to express PRL surges induced by copulomimetic stimuli begins at 25 days of age. In this study we investigated the ontogeny of the ESR in immature female rats in order to observe the relationship between the onset of PRL secretion induced by copulomimetic stimuli and the development of the ESR. Immature female rats were raised in our colony and kept with their dams until used in an experiment or weaned at 23 days of age where appropriate. At 15, 20, 23, 24, 25, or 30 days of age female rats received a single ip injection of domperidone (DOM; 5 mg/kg) or saline vehicle at 0300, 1200, or 1700 h. Thirty minutes after the injection the rats were decapitated, and trunk blood was collected. PRL was measured by RIA. DOM had no effect on PRL secretion as compared to that in saline-treated controls at 15 days of age. However, in all other age groups DOM induced a significant increase in PRL levels compared to those in saline-treated animals regardless of the time of injection. In addition, there was no time of day difference in the PRL secretory response to DOM in rats 15-23 days of age. However, rats treated with DOM at 0300 h at 24 days of age secreted approximately 2-fold greater PRL than rats treated similarly at 1200 or 1700 h. Moreover, at 25 and 30 days of age, rats treated with DOM at either 0300 or 1700 h secreted significantly greater PRL than rats treated similarly at 1200 h. These results suggest that the ESR for PRL secretion begins by 24 days of age. In addition, they indicate that the hypothalamic developmental event preceding and required for expression of mating-induced surges of PRL is the establishment of the ESR.

Endothelin-3 inhibits prolactin and stimulates LH, FSH and TSH secretion from pituitary cell culture

The influence of endothelin-3 (ET-3) on anterior pituitary hormone secretion was investigated over a wide range of concentrations (from 10(-14) to 10(-6) M) and incubation times (from 4 to 48 hours). ET-3 elicited a concentration-dependent inhibition of prolactin (PRL) secretion and stimulated the release of luteinizing hormone (LH), follicle stimulating hormone (FSH) and thyroid stimulating hormone (TSH) from primary monolayer cultures of anterior pituitary cells derived from female rats. The responsiveness of different pituitary cells to ET-3 differs markedly in terms of onset and duration: the maximal inhibition of PRL secretion occurred after 12 hours and the stimulation of LH, FSH and TSH reached the maximum after 4, 48 and 48 hours of incubation, respectively. These data corroborate the concept that ET-3 has an important role as a neuroendocrine modulator. Moreover, the data presented suggest different intracellular mechanisms underlying ET-3 actions.

The endogenous stimulatory rhythm regulating prolactin secretion is present in the lactating rat

Prolactin (PRL) secretion in the female rat is regulated by an endogenous stimulatory rhythm (ESR), which is normally under the tonic inhibition of dopamine (DA). The ESR consists of a nocturnal (N) component which peaks at approximately 03.00 h, and a diurnal (D) component which peaks at approximately 17.00 h. This ESR has been shown to be present in ovariectomized and cervically stimulated rats. We have proposed that the ESR is continuously present in the female rat and that any suppression of the tonic inhibitory influence on PRL secretion can reveal its existence. In this study, the effects of the DA-lowering stimulus of suckling was investigated at different times of day in lactating rats. In addition, the pattern of PRL secretion in freely lactating rats throughout a 24-hour period was studied. Female rats were separated from their pups for 6 h prior to reunion at either 03.00 (coincident with the N component), 12.00 (control) or 17.00 h (coincident with the D component) at various stages of lactation. Blood samples were collected from intra-atrial cannulae immediately before separation of pups and dams, immediately before reunion of pups and dams (0 time), and 15, 30, 60 and 120 min following reunion of pups and dams. Four days following parturition, dams suckled at either 03.00 or 17.00 h secreted significantly greater PRL than rats suckled at 12.00 h. Peak levels of PRL were 60-, 90- and 25-fold greater than 0 time levels, at 03.00, 17.00 and 12.00 h, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)