Inhibitory Action Of Dopamine Research Articles

Hyperprolactinemia and Galactorrhea Associated with Risperidone-Fluoxetine Combination Therapy: A Case Report

Prolactin is a polypeptide hormone secreted and released by lactotroph cells in the anterior pituitary gland in response to diverse physiological stimuli, principally via the inhibitory action of dopamine and serotonin. This paper describes a 44-year-old woman with depression and obsessive-compulsive disorder (OCD) who called the 13-Aban drug and poison information center (DPIC). She was being treated with fluoxetine (80 mg/day) for 10 months until risperidone was added to her regimen for augmentation therapy (0.5 mg/day). Her symptoms improved within less than two months without significant side effects until she experienced painful bilateral breast discharge along with spotting and menstrual irregularity, besides amenorrhea for the previous 2 cycles and serum prolactin level of 33.7 ng/mL, presenting hyperprolactinemia. After discontinuing risperidone, within two weeks, galactorrhea and breast pain disappeared and amenorrhea resolved. Further prolactin level measurement showed the significant reduction. This neuroendocrine effect observed with very low-dose risperidone plus fluoxetine is apparently exerted through both pharmacokinetic and pharmacodynamic augmentation of this combination therapy.

Time- and dose-related effects of a gonadotropin-releasing hormone agonist and dopamine antagonist on reproduction in the Northern leopard frog (Lithobates pipiens)

Gonadotropin-releasing hormone (GnRH) stimulates luteinizing hormone release to control ovulation and spermiation in vertebrates. Dopamine (DA) has a clear inhibitory role in the control of reproduction in numerous teleosts, and emerging evidence suggests that similar mechanisms may exist in amphibians. The interactions between GnRH and DA on spawning success and pituitary gene expression in the Northern leopard frog (Lithobates pipiens) were therefore investigated. Frogs were injected during the natural breeding season with a GnRH agonist [GnRH-A; (Des-Gly10, D-Ala6, Pro-NHEt9)-LHRH; 0.1μg/g and 0.4μg/g] alone and in combination with the dopamine receptor D2 antagonist metoclopramide (MET; 5μg/g and 10μg/g). Injected animals were allowed to breed in outdoor mesocosms. Time to amplexus and oviposition were assessed, and egg mass release, incidences of amplexus, egg mass weight, total egg numbers and fertilization rates were measured. To examine gene expression, female pituitaries were sampled at 12, 24 and 36h following injection of GnRH-A (0.4μg/g) alone and in combination with MET (10μg/g). The mRNA levels of the genes lhb, fshb, gpha, drd2 and gnrhr1 were measured using quantitative real-time PCR. Data were analyzed by a two-way ANOVA. Both GnRH-A doses increased amplexus, oviposition and fertilization alone. Co-injection of MET with GnRH-A did not further enhance spawning success. Injection of GnRH-A alone time-dependently increased expression of lhb, fshb, gpha and gnrhr1. The major effect of MET alone was to decrease expression of drd2. Importantly, the stimulatory effects of GnRH-A on lhb, gpha and gnrhr1 were potentiated by the co-injection of MET at 36h. At this time, expression of fshb was increased only in animals injected with both GnRH-A and MET. Spawning success was primarily driven by the actions of GnRH-A. The hypothesized inhibitory action of DA was supported by pituitary gene expression analysis. The results from this study provide a fundamental framework for future time- and dose–response investigations to improve current spawning methods in amphibians.

Stonin 2 Overexpression is Correlated with Unfavorable Prognosis and Tumor Invasion in Epithelial Ovarian Cancer.

Stonin 2 (STON2), which functions in adjusting endocytotic complexes, is probably involved in the monitoring of the internalization of dopamine D2 receptors which have an inhibitory action of dopamine on tumor progression. However, its clinical significance in tumor progression and prognosis remains unclear. We explored the association between STON2 and the clinicopathological characteristics of epithelial ovarian cancer (EOC). The STON2 levels in ovarian cancer and normal cell lines and tissues were detected by real-time PCR and Western blot analyses. STON2 protein expression was also detected by an immunohistochemical analysis. The clinical significance of STON2 expression in ovarian cancer was statistically analyzed. STON2 significantly increased in the ovarian cancer cell lines and tissues compared to the normal ones. In the 89 EOC samples tested, STON2 expression was significantly correlated with intraperitoneal metastasis, intestinal metastasis, intraperitoneal recurrence, ascites containing tumor cells, and CA153 level. Moreover, patients with STON2 protein overexpression were more likely to exhibit platinum resistance and to have undergone neoadjuvant chemotherapy. Patients with high STON2 protein expression had a tendency to have a shorter overall survival and a poor prognosis. A multivariate analysis showed that STON2 was an independent prognostic predictor for EOC patients. In conclusion, STON2 plays an important role in the progression and prognosis of ovarian carcinoma, especially in platinum resistance, intraperitoneal metastasis, and recurrence. STON2 can be a novel antitumor drug target and biomarker which predicts an unfavorable prognosis for EOC patients.

EVALUATION OF ANTI-OBESITY EFFECT OF AQUEOUS EXTRACT OF MUCUNA PRURIENS SEEDS ON RATS

Objective: Evaluation of the anti-obesity effect of aqueous extract of Mucuna pruriens seeds on rats.Methods: Male Sprague-Dawley (SD) rats were subjected to high-fat diet (HFD) for 12 wk. L-DOPA (12.5 mg/kg, p. o.) as standard drug and aqueous extract of Mucuna pruriens (AEMP) seeds (200 mg/kg, p. o. and 400 mg/kg, p. o.) as test drugs were administered in last 4 wk along with HFD. Body weight, food intake, body mass index (BMI), serum total cholesterol (TC), triglyceride (TG) and high-density lipoprotein (HDL) levels were measured at the end of fourth, eighth and twelfth wk, while white adipose tissue (WAT) mass and brain dopamine levels were measured at the end of the twelfth wk.Results: AEMP (200 mg/kg, p. o.) and (400 mg/kg, p. o.) treated groups showed a significant decrease in food intake and weight gain without altering BMI. Moreover, TG levels were lower in treated groups as compared to the HFD group, but no significant changes were observed in TC and HDL levels. L-DOPA-treated group showed a significant decrease in body weight, food intake, BMI and WAT. Both AEMP and L-DOPA-treated groups showed an increase in brain dopamine levels as compared to disease control group (p<0.05).Conclusion: L-DOPA and AEMP showed anti-obesity activity by reducing body weight gains, food intake and WAT weights; modulating TG with increased brain dopamine level which correlates to the inhibitory action of dopamine on reward mechanism.

Dopaminergic modulation of the voltage-gated sodium current in the cochlear afferent neurons of the rat.

The cochlear inner hair cells synapse onto type I afferent terminal dendrites, constituting the main afferent pathway for auditory information flow. This pathway receives central control input from the lateral olivocochlear efferent neurons that release various neurotransmitters, among which dopamine (DA) plays a salient role. DA receptors activation exert a protective role in the over activation of the afferent glutamatergic synapses, which occurs when an animal is exposed to intense sound stimuli or during hypoxic events. However, the mechanism of action of DA at the cellular level is still not completely understood. In this work, we studied the actions of DA and its receptor agonists and antagonists on the voltage-gated sodium current (INa) in isolated cochlear afferent neurons of the rat to define the mechanisms of dopaminergic control of the afferent input in the cochlear pathway. Experiments were performed using the voltage and current clamp techniques in the whole-cell configuration in primary cultures of cochlear spiral ganglion neurons (SGNs). Recordings of the INa showed that DA receptor activation induced a significant inhibition of the peak current amplitude, leading to a significant decrease in cell excitability. Inhibition of the INa was produced by a phosphorylation of the sodium channels as shown by the use of phosphatase inhibitor that produced an inhibition analogous to that caused by DA receptor activation. Use of specific agonists and antagonists showed that inhibitory action of DA was mediated both by activation of D1- and D2-like DA receptors. The action of the D1- and D2-like receptors was shown to be mediated by a Gαs/AC/cAMP/PKA and Gαq/PLC/PKC pathways respectively. These results showed that DA receptor activation constitutes a significant modulatory input to SGNs, effectively modulating their excitability and information flow in the auditory pathway.

Increased excitability of spinal pain reflexes and altered frequency-dependent modulation in the dopamine D3-receptor knockout mouse

Frequency-dependent modulation and dopamine (DA) receptors strongly modulate neural circuits in the spinal cord. Of the five known DA receptor subtypes, the D3 receptor has the highest affinity to DA, and D3-mediated actions are mainly inhibitory. Using an animal model of spinal sensorimotor dysfunction, the D3 receptor knockout mouse (D3KO), we investigated the physiological consequences of D3 receptor dysfunction on pain-associated signaling pathways in the spinal cord, the initial integration site for the processing of pain signaling. In the D3KO spinal cord, inhibitory actions of DA on the proprioceptive monosynaptic stretch reflex are converted from depression to facilitation, but its effects on longer-latency and pain-associated reflex responses and the effects of FM have not been studied. Using behavioral approaches in vivo, we found that D3KO animals exhibit reduced paw withdrawal latencies to thermal pain stimulation (Hargreaves' test) over wild type (WT) controls. Electrophysiological and pharmacological approaches in the isolated spinal cord in vitro showed that constant current stimulation of dorsal roots at a pain-associated frequency was associated with a significant reduction in the frequency-dependent modulation of longer-latency reflex (LLRs) responses but not monosynaptic stretch reflexes (MSRs) in D3KO. Application of the D1 and D2 receptor agonists and the voltage-gated calcium-channel ligand, pregabalin, but not DA, was able to restore the frequency-dependent modulation of the LLR in D3KO to WT levels. Thus we demonstrate that nociception-associated LLRs and proprioceptive MSRs are differentially modulated by frequency, dopaminergics and the Ca2+ channel ligand, pregabalin. Our data suggest a role for the DA D3 receptor in pain modulation and identify the D3KO as a possible model for increased nociception.

A review of the control of reproduction and hormonal manipulations in finfish species

Control of reproductive is essential for the sustainability of commercial aquaculture production. The reasons for breakage of ovulation and spawning is failure of the pituitary to release gonadotropin hormones. Gonadotropin hormones secretion is under a dual control, by gonadotropin releasing hormone (GnRH), and inhibition by dopamine on the actions of GnRH. The inhibitory actions of dopamine on GTH secretion can vary in potency between species. In some fish it is necessary to use dopamine blocker, such as domperidone, results in potentiation of the actions of GnRH analoge, leading to a large release of GTH-II and ovulation. In some other injection of a high dose of GnRH analoge alone is effective. In several marine species GTH-II secretion is not under dopaminergic inhibitory control but sensitive to exogenous analoge hormones (GnRHa). In many fishes, synthetic analoges of lutenizing hormone-releasing hormone (LHRHa) are widely used for inducing spawning. In several tropical freshwater and marine finfishes the human chorionic gonadotropin (HCG), pituitary homogenate and semi-purified fish gonadotropin are used. Rrecently the replacement of natural GnRH are used and had resulted in a centuple increase in the effectiveness of luteinizing hormones secretion induction. Key word: Reproduction induced spawning, dopamine antagonists, GTH-II- HCG

Recent Clinical Aspects of Hyperprolactinemia Induced by Antipsychotics

This review will address the current understanding of the relationship between hyperprolactinemia and antipsychotic drugs. Hyperprolactinemia is a frequent but often neglected side effect of typical, but also of many atypical antipsychotics. Release of PRL from lactotrope cells is influenced by several factors, such as stress, physical and sexual activity and food assumption. PRL secretion is regulated by hypothalamic-pituitary portal system and its homeostasis is the result of a complex balance between stimulating and inhibitory factors, both endogeneous and esogeneous. The main physiological control mechanism of secretion is played by the inhibitory action of dopamine. Conversely, among stimulation factors, serotonin is probably the main modulator of PRL release. An high number of drugs may cause PRL increase too, such as drugs that reduce dopaminergic functions at SNC level, or drugs with an antagonistic action towards dopaminergic receptors and those increasing serotonergic neurotransmission. Hyperprolactinemia is one of the most frequent endocrine pathologies of the hypothalamic-pituitary axis. Antipsychotics (AP) are the most common cause of druginduced hyperprolactinemia. Not all AP have the same impact on inducing hyperprolactinemia. In this review we will focus on the subdivision of AP in 'PRL-raising' (stimulators) and 'PRL-sparing' (sparers) and on their differences in inducing hyperprolactinemia. Finally we evaluated different complications in patients with antipsychotics induced hyperprolactinemia that may cause not only short-term side effects but also important systemic long-term effects. At the end of the review we finally report the possible options of treatment considering however that at present there are no ideal therapies or evaluations, and decisions have to be made on a case by case basis.

Dopamine inhibition of glycine release in the rat trigeminal nucleus pars caudalis: possible involvement of trace amine receptors

Dopamine (DA)-induced pre-synaptic inhibition of glycinergic transmission was studied from substantia gelatinosa (SG) neurons of the trigeminal nucleus pars caudalis using a conventional whole-cell patch clamp technique. The action potential-dependent glycinergic inhibitory post-synaptic currents (IPSCs) were recorded from SG neurons in the presence of 3 mM kynurenic acid and 10 μM 6-imino-3-(4-methoxyphenyl)-1(6H)-pyridazinebutanoic acid HBr (SR95531). In these conditions, bath applied DA (100 μM) reduced the amplitude of glycinergic IPSCs and increased the paired-pulse ratio, suggesting that DA acts pre-synaptically to reduce the probability of glycine release. However, the inhibitory action of DA on glycinergic IPSCs was not blocked by SCH23390 (10 μM) and spiperone (1 μM), selective D(1) - and D(2) -like receptor antagonists, respectively. In addition, either SKF38393 (100 μM), a selective D(1) -like receptor agonist, or quinpirole (100 μM), a selective D(2) -like receptor agonist, had no pre-synaptic effect on glycinergic IPSCs. The results suggest that both D(1) - and D(2) -like receptors are not involved in the DA-induced decrease in glycinergic IPSCs. On the other hand, tyramine (100 μM), one of representative trace amines, reduced the amplitude of glycinergic IPSCs and increased the paired-pulse ratio, suggesting that tyramine acts pre-synaptically to reduce the probability of glycine release. Considering that DA can activate trace amine (TA) receptors and that TA receptors are exclusively expressed on the trigeminal nucleus pars caudalis, DA might act on putative pre-synaptic TA receptors, rather than classical DA receptors, to inhibit glycinergic transmission onto SG neurons of the trigeminal nucleus pars caudalis.

Actions of angiotensin II and dopamine in the medial preoptic area on prolactin secretion

Dopamine (DA) is known as a primary regulator of prolactin secretion (PRL) and angiotensin II (Ang II) has been recognized as one brain inhibitory factor of this secretion. In this work, estrogen-primed or unprimed ovariectomized rats were submitted to the microinjection of saline or Ang II after previous microinjection of saline or of DA antagonist (haloperidol, sulpiride or SCH) both in the medial preoptic area (MPOA). Our study of these interactions has shown that 1) estrogen-induced PRL secretion is mediated by Ang II and DA actions in the MPOA, i.e. very high plasma PRL would be prevented by inhibitory action of Ang II, while very low levels would be prevented in part by stimulatory action of DA through D(2) receptors, 2) the inhibitory action of Ang II depends on estrogen and is mediated in part by inhibitory action of DA through D(1) receptors and in other part by inhibition of stimulatory action of DA through D(2) receptors.

A 68930 and dihydrexidine inhibit locomotor activity and d-amphetamine-induced hyperactivity in rats: a role of inhibitory dopamine d 1/5 receptors in the prefrontal cortex?

The behavioral and biochemical effects of the full dopamine D 1/5 receptor agonists, dihydrexidine and (1 R,3 S)-1-aminomethyl-5,6-dihydroxy-3-phenylisochroman HCl (A 68930), were examined in rats. Both A 68930 (0–4.6 mg kg −1, s.c.) and dihydrexidine (0–8.0 mg kg −1, s.c.) caused a dose-dependent suppression of locomotor activity, as assessed in an open-field. This locomotor suppression was dose-dependently antagonized by the selective dopamine D 1/5 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine HCl (SCH 23390; 0–5.0 μg kg −1, s.c.), but not by the selective dopamine D 2/3 receptor antagonist raclopride (0–25.0 μg kg −1, s.c.). Furthermore, A 68930 and dihydrexidine did not cause any locomotor activity in habituated rats that displayed a very low base-line activity. Neither did A 68930 nor dihydrexidine produce any excessive stereotypies that could possibly interfere with and mask ambulatory activity. In fact, both A 68930 and dihydrexidine potently blocked hyperactivity produced by d-amphetamine (0–4.0 mg kg −1, s.c.). Such findings traditionally would be interpreted as a sign of potential antipsychotic properties of A 68930 and dihydrexidine. Examination of neuronal activation, as indexed by the immediate early gene c-fos, showed that A 68930 and dihydrexidine caused a highly significant expression of c-fos in the medial prefrontal cortex. This c-fos expression was sensitive to treatment with SCH 23390, but not with raclopride. The effects of A 68930 and dihydrexidine on c-fos expression in caudate putamen or nucleus accumbens were less marked, or undetectable. The results indicate that stimulation of dopamine D 1/5 receptors, possibly in the medial prefrontal cortex, is associated with inhibitory actions on locomotor activity and d-amphetamine-induced hyperactivity. Assuming an important role of prefrontal dopamine D 1/5 receptors in schizophrenia, such inhibitory actions of dopamine D 1/5 receptor stimulation on psychomotor activation mya have interesting clinical implications in the treatment of schizophrenia.

Physiological involvement of DA in ovarian development of the freshwater giant prawn, Macrobrachium rosenbergii

Biogenic amines such as dopamine (DA) and serotonin (5-hydroxytryptamine, 5-HT) are able to affect numerous physiological processes in crustaceans through their actions as neuroregulators. Both DA and serotonin have been shown to be involved in the synthesis and release of neurohormones such as crustacean hyperglycemic hormone (CHH), vitellogenesis-inhibiting hormone (VIH), molt-inhibiting hormone (MIH) and those related to pigmentation which in turn subsequently influence homeostasis and homeokinesis. The effect of DA and 5-HT on the ovarian development of the freshwater giant prawn, Macrobrachium rosenbergii, as well as the pathway of these actions is presented in this paper. During a 20-day experimental period, hemolymph was collected from both intact and bilaterally eyestalk-ablated prawns 2 days after each administration of DA and 5-HT which was injected into the prawns on every fourth day. The vitellogenin concentration in the hemolymph was then quantified using ELISA technique. The results show that DA depressed vitellogenin synthesis while 5-HT enhanced the process. When DA was injected with selective receptor antagonists, SCH23390, a DA D 1 receptor antagonist, was able to suppress DA's effect on vitellogenin synthesis while domperidone, a DA D 2 receptor antagonist, had no apparent effect, suggesting that the actions of DA are primarily channeled through D 1 receptor. Given that DA is able to inhibit vitellogenin synthesis in eyestalk-ablated prawns in a similar manner as in intact prawns, it is clear that the inhibitory action of DA is at the thoracic ganglia through inhibition of vitellogenesis-stimulating hormone (VSH) release, but not at the eyestalk level through stimulation of VIH release from the X organ–sinus gland complex.

Influence of VIP on prolactinemia in turkey anterior pituitary cells: role of cAMP second messenger in VIP-induced prolactin gene expression.

Vasoactive intestinal peptide (VIP) is the avian prolactin (PRL)-releasing factor. In the turkey, hypothalamic VIP immunoreactivity and mRNA content, as well as VIP levels in hypophyseal portal blood, are closely related to the state of prolactinemia and the reproductive stage. The present study investigated the role of VIP on prolactinemia in turkey anterior pituitary (AP) cells through PRL gene expression and the role of a cAMP second messenger system on VIP-induced PRL expression. In primary AP cells harvested from hens in different prolactinemic states, steady state promoter activities were positively correlated with secreted PRL levels. VIP increased PRL promoter activities in AP cells from hens with intermediate PRL levels (laying), but not in AP cells from hypoprolactinemic hens (nonphotostimulated reproductively quiescent). However, in AP cells from hyperprolactinemic hens (incubating), PRL promoter activity was down-regulated by VIP. PRL mRNA steady state levels were significantly decreased by the cAMP analogue, 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), and PRL secretion was down-regulated by the phosphodiesterase blocker, 3-isobutyl-1-methylxanthine (IBMX) in a dose-dependent manner, suggesting that the cAMP second messenger system might be involved in the inhibitory action of dopamine upon VIP-stimulated PRL secretion and gene expression at the pituitary level. In a study of VIP immediate and long-term effects on c-fos expression in relation to PRL expression, VIP dramatically induced c-fos mRNA expression within 5 min, suggesting that VIP-induced c-fos expression might be involved in VIP-stimulated PRL secretion and gene expression. These results provide additional evidence of the functional significance of VIP in PRL gene expression and suggest that changes in PRL promoter activity by VIP may be one of the important inductive mechanisms leading to prolactinemia.

Role of cerebral dopamine but not plasma insulin, leptin and glucocorticoid in the development of tolerance to the anorectic effect of amphetamine.

Repeated treatment with amphetamine (AMPH), an anorectic agent, induced a marked anorexia on day 1 followed by a gradual reversion of this anorexia to the normal level of feeding (tolerant anorexia). The mechanism for this tolerant anorexia remained unknown because it might be related with multiple parameters, such as the change of cerebral dopamine (DA) or the plasma levels of insulin, leptin and glucocorticoid. Results revealed that plasma insulin and leptin concentrations remained unchanged during repeated AMPH administration, revealing that these two factors are not involved. Also, glucocorticoids were not required for the development of tolerant anorexia, as this effect could not be prevented by adrenalectomy. However, AMPH-induced anorexia was decreased by the pretreatment of alpha-methyl-p-tyrosine, an inhibitor of central catecholamine synthesis, and was increased by the pretreatment of nomifensine, a blocker of DA transporter that increased extracellular DA content in brain, revealing that the change of DA content could modify the action of tolerant anorexia. It is suggested that the decrease of inhibitory action of DA in brain during repeated AMPH plays a functional role in the development of tolerance to the anorectic response of AMPH.

Pretreatment with antisense oligodeoxynucleotide against D 2 or D 3 receptor mRNA diminished dopamine’s inhibitory effect on dorsomedial arcuate neurons in brain slices of estrogen-treated ovariectomized rats

A high percentage of dopamine (DA)-responsive neurons has been repeatedly shown in hypothalamic dorsomedial arcuate nucleus (dmARN) using single-unit recording in brain slices. Both D 2 and D 3 receptors may be involved in the inhibitory action of DA as indicated by results obtained from using specific DA agonists and antagonists. To further delineate the DA receptor types involved, ovariectomized, estrogen-primed Sprague–Dawley rats pretreated with antisense oligodeoxynucleotide (ODN, 10 μg/3 μl, i.c.v.) against either D 2 or D 3 receptor mRNA for 2 days were used in this study for brain slice preparation. Rats pretreated with aCSF, random antisense or sense ODNs were used as controls. DA (5-50 nmol) inhibited a majority of dmARN neurons in brain slices prepared from rats pretreated with aCSF (71.4% of 35 U), random antisense/sense ODN for D 2 (67.6%, n=34), D 3 (59.5%, n=42), or D 2 plus D 3 (60.5%, n=38) mRNAs. In contrast, DA inhibited 43.6% ( n=39) of dmARN neurons in slices prepared from D 2, and 38.5% ( n=39) from D 3 antisense ODN-pretreated rats. Furthermore, in brain slices prepared from rats pretreated with combined D 2 and D 3 antisense ODNs, DA only inhibited 18.4% ( n=38) of dmARN neurons. We conclude that both D 2 and D 3 receptors are involved in the action of DA on dmARN neurons in ovariectomized, estrogen-treated rats.

Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe.

In ewes, photoperiod modulates LH release and dopaminergic terminals in the median eminence (ME) have a critical role in the establishment of long-day inhibition of LH secretion. This study was undertaken to determine the type of dopaminergic receptors, D1-like or D2-like, that mediate the action of dopamine on LH secretion at the ME level in this situation. This was assessed, in ovariectomized and estradiol-treated ewes, with the use of reverse microdialysis in the ME in three experiments: first, when LH secretion was stimulated by short days, by determining the response to three doses (0.01, 0.1 or 1 mg/ml) of a D1-like (SKF38393) and a D2-like (quinpirole) agonist; secondly, during early long-day inhibition of LH secretion, by determining the ability of SKF38393 and quinpirole (1 mg/ml) to mimic the inhibitory effects of dopamine, after a blockade of its synthesis with alpha-methyl-para-tyrosine (alphaMPT; 2 mg/ml); and thirdly, during early long-day inhibition of LH secretion, by determining the response to three doses (0.009, 0.09 or 0.9 mg/ml) of a D1-like (SCH23390) and a D2-like (sulpiride) antagonist. In none of the conditions was effect of the D1-like analogs on LH secretion found, compared with the control treatments. In contrast, the D2-like analogs caused changes in LH secretion. First, with short days, quinpirole in the highest dose significantly reduced mean LH concentration (P<0.05) and LH pulse frequency (P<0.01). Secondly, with long days, addition of quinpirole to alphaMPT caused a significant decrease in LH secretion relative to alphaMPT alone (P<0.05). Thirdly, with long days, sulpiride at the highest dose significantly increased mean LH concentration (during the first 3 h of treatment, P<0.05) and LH pulse frequency (P<0.05). Prolactin secretion was also determined in these experiments, and D2-like agonist and antagonist caused an inhibition and a stimulation of prolactin secretion, respectively. These results demonstrate that, in the ME, inhibitory action of dopamine on LH secretion, critical for the initiation of long-day-induced inhibition, is mediated by D2-like, not D1-like, dopaminergic receptors.

Regulation of prolactin secretion: Dopamine is the prolactin‐release inhibiting factor (PIF), but also plays a role as a releasing factor (PRF)

Many in‐depth reviews related to regulations of prolactin secretion are available. We will, therefore, focus on controversial aspects using personal opinion in this review. The neuroendocrine control of prolactin secretion from the anterior pituitary gland involves multiple factors including prolactin‐release inhibiting factor (PIF) and prolactin releasing factor (PRF). The PIF exerts a tonic inhibitory control in the physiological conditions. The PIF should be able to effectively inhibit prolactin release for a lifetime, but the inhibitory action of dopamine cannot be sustained for a long period of time. Perifusion of a high concentration of dopamine (1,000 nM) could not sustain inhibitory action on prolactin release but when a small amount of ascorbic acid (0.1 mM) is added in a low concentration of dopamine (3 nM) solution, prolactin release was inhibited for a long period. Ascorbate is essential for dopamine action to inhibit prolactin release. We have, therefore, concluded that the PIF is dopamine pl...

Protein Phosphatase 2A Plays a Role in the Suckling-Induced Changes in the Responsiveness of Pituitary Mammotropes

It is well established that PRL secretion is under a tonic inhibition exercised by the hypothalamic dopamine (DA). One feature of this regulation is an immediate withdrawal reaction (elevation of PRL release) of mammotropes after disruption of hypothalamic influence. Although plasma PRL rises rapidly, the suckling stimulus does not cause an acute diminution of hypothalamic DA, but, as we have previously demonstrated, it results in an almost immediate (within 10 min) desensitization of mammotropes as indicated by the change in dose response of DA to inhibit PRL release. Our present investigations relate to the phenomenon of this change in responsiveness of PRL cells. This was accomplished by using the reverse hemolytic plaque assay to evaluate the secretory characteristics of individual PRL secretors derived from lactating rats either before or after a 10-min suckling stimulus. To investigate the mechanism of these changes, the binding characteristics of [3H]spiperone on pituitary membranes from nonsuckled and suckled rats have been compared, and the possible involvement of dephosphorylating enzymes was tested by using okadaic acid (OA) in a dose of 2 nm that preferentially and selectively inhibits protein phosphatase-2A (PP2A) activity. We have also determined the activities of PP1 and PP2A in pituitary tissue samples as well as in enzymatically dispersed cells. Mammotropes from nonsuckled rats exhibited a depression of PRL release after both DA and OA treatment and an elevation after withdrawal of DA. This suggests that the secretory response of mammotropes obtained from nonsuckled rats still shows those two responses that are characteristic of the tonic inhibitory regulation. In contrast, superimposition of suckling in vivo or application of OA together with DA pretreatment in cells from nonsuckled rats in vitro resulted in a disappearance of the dissociation-induced elevation of PRL release, indicating an abolishment of the tonic inhibitory action of DA. Evidence is also presented that the PP2A, but not the PP1, activity of the anterior lobe is significantly lower after a 10-min suckling stimulus. Moreover, DA is able to decrease PP2A activity in dispersed pituitary cells obtained from nonsuckled, but not from suckled, animals. In contrast, there were no differences in either the affinity or the number of binding sites between nonsuckled and suckled rats. Taken together, our results suggest that the suckling-induced decrease in PP2A activity plays a role in the uncoupling of D2 receptors on mammotropes from the tonic inhibitory signaling pathway.

Protein phosphatase 2A plays a role in the suckling-induced changes in the responsiveness of pituitary mammotropes.

It is well established that PRL secretion is under a tonic inhibition exercised by the hypothalamic dopamine (DA). One feature of this regulation is an immediate withdrawal reaction (elevation of PRL release) of mammotropes after disruption of hypothalamic influence. Although plasma PRL rises rapidly, the suckling stimulus does not cause an acute diminution of hypothalamic DA, but, as we have previously demonstrated, it results in an almost immediate (within 10 min) desensitization of mammotropes as indicated by the change in dose response of DA to inhibit PRL release. Our present investigations relate to the phenomenon of this change in responsiveness of PRL cells. This was accomplished by using the reverse hemolytic plaque assay to evaluate the secretory characteristics of individual PRL secretors derived from lactating rats either before or after a 10-min suckling stimulus. To investigate the mechanism of these changes, the binding characteristics of [3H]spiperone on pituitary membranes from nonsuckled and suckled rats have been compared, and the possible involvement of dephosphorylating enzymes was tested by using okadaic acid (OA) in a dose of 2 nM that preferentially and selectively inhibits protein phosphatase-2A (PP2A) activity. We have also determined the activities of PP1 and PP2A in pituitary tissue samples as well as in enzymatically dispersed cells. Mammotropes from nonsuckled rats exhibited a depression of PRL release after both DA and OA treatment and an elevation after withdrawal of DA. This suggests that the secretory response of mammotropes obtained from nonsuckled rats still shows those two responses that are characteristic of the tonic inhibitory regulation. In contrast, superimposition of suckling in vivo or application of OA together with DA pretreatment in cells from nonsuckled rats in vitro resulted in a disappearance of the dissociation-induced elevation of PRL release, indicating an abolishment of the tonic inhibitory action of DA. Evidence is also presented that the PP2A, but not the PP1, activity of the anterior lobe is significantly lower after a 10-min suckling stimulus. Moreover, DA is able to decrease PP2A activity in dispersed pituitary cells obtained from nonsuckled, but not from suckled, animals. In contrast, there were no differences in either the affinity or the number of binding sites between nonsuckled and suckled rats. Taken together, our results suggest that the suckling-induced decrease in PP2A activity plays a role in the uncoupling of D2 receptors on mammotropes from the tonic inhibitory signaling pathway.

Dopamine-D2 actions on voltage-dependent calcium current and gonadotropin-II secretion in cultured goldfish gonadotrophs.

Dopamine D2-receptor activation directly inhibits GnRH-induced gonadotropin-II (maturational gonadotropin, GTH-II) secretion from goldfish pituitary cells. In this study, we show that dopamine and its D2 agonist, quinpirole, reduced GTH-II secretion induced by either high extracellular K+ concentration or the voltage-gated Ca2+ channel agonist, Bay K 8644. These actions of dopamine were blocked by addition of the dopamine D2-receptor antagonist, spiperone. The actions of dopamine on Ca2+ current in single identified goldfish gonadotrophs were assessed in voltage-clamp experiments using Ba2+ as the charge carrier through voltage-gated Ca2+ channels. Dopamine caused a concentration-dependent reduction in Ba2+ current amplitude with an EC50 of 1.0+/-0.3 nM, but did not shift the current-voltage relationship. The D2 agonist quinpirole also caused a dose-dependent reduction in the Ba2+ current amplitude with an EC50 of 2.7+/-1.4 nM. Quinpirole slowed the activation and inactivation kinetics, as well as removing the steady-state inactivation properties of the Ba2+ current. In contrast to the actions of quinpirole, the dopamine D1-receptor agonist, SKF 38393, did not affect the Ba2+ current. The inhibitory action of dopamine on voltage-dependent Ca2+ currents was reversed by spiperone, but not by the D1 antagonist SKF 83566. Voltage-dependent Na+ and K+ currents were not affected by dopamine or dopamine agonists. These data indicate that dopamine D2-receptor activation reduces Ca2+ influx through voltage-dependent Ca2+ channels to inhibit GTH-II secretion.