Opposing roles for dopamine D1 and D2 receptors in the regulation of hypothalamic tuberoinfundibular dopamine neurons

The purpose of the present study was to characterize pharmacologically dopamine D1 receptor-mediated inhibition of tuberoinfundibular dopamine neurons in males rats, and to determine if inhibitory dopamine D1 receptors oppose stimulatory dopamine D2 receptors and account for the inability of mixed dopamine receptor agonists to alter the activity of these neurons. Tuberoinfundibular dopamine neuronal activity was estimated by measuring the concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence, the region of the hypothalamus containing terminals of these neurons. Administration of the dopamine D1 receptor agonist (+/-)-1 phenyl-2,3,4,5-tetrahydro-(1 H)-3-benzazepine-7,8-diol (SKF38393) decreased median eminence DOPAC and increased plasma prolactin concentrations, whereas administration of the dopamine D1 receptor antagonist ((-)-trans,6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H -benzo[d]naphtho-[2,1 b]azepine (SCH39166) increased median eminence DOPAC concentrations but had not effect on plasma prolactin. The inhibitory effect of SKF38393 on median eminence DOPAC concentrations was blocked by SCH39166. These results demonstrate that acute activation of dopamine D1 receptors inhibits the activity of tuberoinfundibular dopamine neurons and thereby increases prolactin secretion, and that under basal conditions dopamine D1 receptor-mediated inhibition of tuberoinfundibular dopamine neurons is tonically active. Administration of the dopamine D2 receptor agonist (5aR-trans)-5,5a,6,7,8,9,9a,10-octahydro-6-propyl-pyridol[2, 3-g]quinazolin-2-amine (quinelorane) increased median eminence DOPAC concentrations, and SKF38393 caused a dose-dependent reversal of this effect. Administration of the mixed dopamine D1/D2 receptor agonist R(-)-10,11-dihydroxy-apomorphine (apomorphine) had no effect per se, but blocked quinelorane-induced increases in DOPAC concentrations in the median eminence. These results reveal that concurrent activation of dopamine D1 and D2 receptors nullifies the actions of each of these receptors on tuberoinfundibular dopamine neurons, which likely accounts for the lack of an acute effect of mixed dopamine D1/D2 receptor agonists on these hypothalamic dopamine neurons.     

Reversal of cyclosporine-inhibited low-density lipoprotein receptor activity in HepG2 cells by 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors

The effects of diethylstilbestrol (DES) and of long-acting somatostatin analog, octreotide (SMS) on the rat anterior pituitary microvasculature have been studied by means of computer-assisted image analysis. Additionally, the effects of DES and SMS on prolactin secretion and anterior pituitary cell proliferation have been studied, as well. The vascularization was visualized using Selye's method modified by Poely et al. (1964). The prolactin serum levels were estimated by radio-immunoassay. The proliferation indices were assessed using bromodeoxyuridine incorporation assay. As expected, it was found that DES sharply increased serum prolactin levels and enhanced cell proliferation in the anterior pituitary gland. DES also induced changes in parameters of vascularization. Simultaneous treatment of rats with SMS inhibited the DES-induced elevation of prolactin levels and pituitary cell proliferation. It also suppressed some but not all DES-induced changes in the anterior pituitary vascularization. These data suggest that the angio-inhibitory activity of SMS might be involved in its anti-tumor action on pituitary adenomas, but not as a sole or principal mechanism.     

Supersensitivity of anterior pituitary dopamine receptors involved in the inhibition of prolactin secretion following destruction of the medial basal hypothalamus

The medial basal hypothalamus of ovariectomized rats was destroyed using a modified Halász knife. Large increases in prolactin secretion were observed 1 and 14 days following the lesions. Long- and short-term lesioned animals were anesthetized with chloral hydrate and treated with various doses of apomorphine (0.05, 0.2, 2, 5 mg/kg). Blood samples were obtained before and 10, 30 and 60 minutes after the injection. Both the 0.05 and 0.2 mg/kg doses caused significantly greater and longer-lasting inhibition of prolactin in long-term than in short-term lesioned animals. Since the MBH was totally destroyed this study suggests that anterior pituitary dopamine receptors involved in the inhibition of prolactin secretion become supersensitive in long-term lesioned rats.     

Pulsatile growth hormone and prolactin: effects of (+) butaclamol, a dopamine receptor blocking agent

Growth hormone (rGH) and prolactin (rPRL) secretory profiles were obtained before and after treatment with a dopamine receptor blocking agent, (+) butaclamol, in 10 male rats chronically implanted with right atrial cannulae. Mean rGH plasma concentrations, determined by planimetry, were reduced (202 +/- 20 ng/ml vs. 135 +/- 20ng/ml, P less than .01), but the basic configuration and periodicity of rGH secretory bursts were unaltered. Mean rPRL plasma concentrations were elevated (11.1 +/- 2.1 ng/ml vs 65.5 +/- 8.1 ng/ml, P less than .0005), but rPRL episodic secretion was still apparent. It is concluded that dopaminergic neurons have a minor role in facilitating episodic rGH secretion. Furthermore, persisting episodic rPRL secretion in rats administered a dopamine antagonist suggests that rPRL feedback inhibition does not inactivitate the neural mechanism generating episodic rPRL secretion.     

Adenosine acts by A1 receptors to stimulate release of prolactin from anterior-pituitaries in vitro

Adenosine has been identified in the anterior pituitary gland and is secreted from cultured folliculostellate (FS) cells. To determine whether adenosine controls the secretion of anterior pituitary hormones in vitro, adenosine was incubated with anterior pituitaries. It stimulated prolactin (PRL) release at the lowest concentration used (10(-10) M); the stimulation peaked at 10(-8) M with a threefold increase in release and declined to minimal stimulation at 10(-4) and 10(-3) M. Follicle-stimulating hormone release was maximally inhibited at 10(-8) M, whereas luteinizing hormone release was not significantly inhibited. Two selective A1 adenosine receptor antagonists (10(-7) or 10(-5) M) had no effect on basal PRL release, but either antagonist completely blocked the response to the most effective concentration of adenosine (10(-8) M). In contrast, a highly specific A2 receptor antagonist (10(-7) or 10(-5) M) had no effect on basal PRL release or the stimulation of PRL release induced by adenosine (10(-8) M). We conclude that adenosine acts to stimulate PRL release in vitro by activating A1 receptors. Since the A1 receptors decrease intracellular-free calcium, this would decrease the activation of nitric oxide synthase in the FS cells, resulting in decreased release of nitric oxide (NO). NO inhibits PRL release by activating guanylate cyclase that synthesizes cGMP from GTP; cGMP concentrations increase in the lactotrophs leading to inhibition of PRL release. In the case of adenosine, NO release from the FS cells decreases, resulting in decreased concentrations of NO in the lactotrophs, consequent decreased cGMP formation, and resultant increased PRL release.     

Dose dependent of sister chromatid exchanges in humans lymphocytes induced by in vitro alpha-particle irradiation

AIM: To study the effect of (-)-stepholidine (SPD) on serum prolactin (PRL) level and elucidate its pharmacological action on dopamine D2 receptors. METHOD: After i.p. administration of dopamine receptor agonist, antagonist, or SPD, the serum PRL levels were determined by radioimmunoassay. RESULTS: SPD (24 mg.kg-1, i.p.) caused a rapid rise in serum PRL level, lasting more than 1 h. SPD 0.2-40 mg.kg-1 raised serum PRL level in a dose-dependent manner with ED50 of 3.7 mg.kg-1 (95% confidence limits, 2.6-4.3 mg.kg-1) and PRL maximal level of 448 +/- 64 micrograms.L-1. Pergolide 2 mg.kg-1 i.p. caused a decrease (P < 0.01 vs saline) of PRL level, which was partially attenuated by SPD of 5 mg.kg-1 and completely abolished by 10 mg.kg-1. CONCLUSION: SPD is a dopamine D2 receptor antagonist.     

Identification of endocrine cell populations expressing the AT1B subtype of angiotensin II receptors in the anterior pituitary

Angiotensin II (Ang II) participates in the regulation of anterior pituitary hormone secretion by acting either directly on the anterior pituitary or indirectly on the hypothalamus. When applied directly on pituitary cells, Ang II increases both ACTH and PRL secretion and has also been reported to affect GH secretion. Three distinct subtypes of Ang II receptors (AT1A, AT1B, and AT2) have been identified; they are unequally distributed and differently regulated in various tissues. We have previously demonstrated that only AT1A receptors are present in the hypothalamus while anterior pituitary cells express predominantly the AT1B subtype. Using in situ hybridization in combination with immunohistochemistry, the aim of the present study was to identify the phenotype of the endocrine cell expressing AT1B receptor messenger RNA (mRNA) in the anterior pituitary of adult male Sprague-Dawley rats. Expression of AT1B receptor mRNA was present in 33.9 +/- 1.0% of anterior pituitary cells. AT1B mRNA is predominantly expressed by lactotropes (78.2 +/- 2.1% of AT1B mRNA-expressing cells) and to a lower degree by corticotropes (18.3 +/- 2.1%) and is not detectable in somatotropes, mammosomatotropes, gonadotropes, or thyrotropes. These results indicate that in adult male rats, Ang II, which has been shown to be synthesized in gonadotropes, can directly stimulate PRL and ACTH release from lactotropes and corticotropes through activation of AT1B receptors. As only 53.8 +/- 2.7% of lactotropes and 23.6 +/- 2.8% of corticotropes expressed AT1B mRNA, our findings suggest a functional heterogeneity of both cell types regarding their sensitivity to Ang II.     

Hexadecylphosphocholine and 2-modified 1,3-diacylglycerols as effectors of phospholipase D

The effects of mesulergine (100 and 200 microg/kg s.c.), SB 206553 (1 and 2.5 mg/kg i.p.), RP 62203 (2.5 and 4 mg/kg i.p.) and ritanserin (630 microg/kg i.p.) were studied on the extracellular concentration of dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens of chloral hydrate-anesthetized rats, using intracerebral microdialysis. Mesulergine, a non selective serotonin2C/2B/2A (5-HT2C/2B/2A) receptor antagonist, significantly increased DA release, which reached a peak level (+ 20%) 60 min after drug injection and slowly returned back to baseline values. Mesulergine also caused a dose-dependent increase in DOPAC outflow. Pretreatment with mesulergine (200 microg/kg) did not change the inhibition of DA release induced by apomorphine (100 microg/kg), whereas it prevented the reduction of DOPAC outflow induced by apomorphine (100 microg/kg). Administration of SB 206553, a selective blocker of 5-HT2C/2B receptors, dose-dependently increased DA outflow. The dose of 2.5 mg/kg SB 206553 caused a linear increase of DA output which reached a peak (+75%) 40 min after injection, while 1 mg/kg induced a more gradual increase of DA release which peaked (+54%) 60 min after administration of the drug. Treatment with RP 62203, a selective 5-HT2A receptor antagonist, did not produce any significant effect on DA outflow. Administration of ritanserin, a mixed 5-HT2A/2C receptor antagonist, did not cause any significant change of DA and DOPAC outflow. Taken together, these data indicate that selective blockade of 5-HT2/2B receptor subtypes increases DA release in the rat nucleus accumbens.     

Relationships between dopamine-induced changes in cytosolic free calcium concentration ([Ca2+]i) and rate of prolactin secretion. Elevated [Ca2+]i does not indicate prolactin release

This study was undertaken to investigate the relationship between dopamine (DA) induced changes in the cytosolic calcium concentration ([Ca2+]i) and the rate of prolactin secretion using GH4ZR7, a rat pituitary cell line, which express only one subtype of D2 receptor. GH4ZR7 cells were loaded with Fluo-3, a fluorescent Ca2+ indicator, and then perifused with two different doses of DA (10(-7) mol/L and 5 x 10(-4) mol/L). We monitored changes in [Ca2+]i and rate of prolactin release simultaneously by attaching a spectrofluorometer to a dynamic perifusion system. DA has stimulatory and inhibitory effect on prolactin secretion in GH4ZR7 cells; 10(-7) mol/LDA slightly increased [Ca2+]i and stimulated prolactin release, whereas 5 x 10(-4) mol/LDA decreased [Ca2+]i and inhibited prolactin secretion. When the cells were pretreated with pertussis toxin (PTX), 10(-7) mol/L DA had no significant change in [Ca2+]i while stimulating prolactin release, and 5 x 10(-4) mol/L DA reduced [Ca2+]i without having any significant effect on the rate of prolactin secretion. The results of this study demonstrate that changes in [Ca2+]i do not always correlate with the rate of prolactin release from lactotrophs. The dissociation between [Ca2+]i and prolactin release is somewhat expected considering the diverse role of [Ca2+]i and post-[Ca2+]i events, which can change the rate of prolactin release.     

Differential sensitivity of prolactin release to dopamine and thyrotrophin-releasing hormone in intact and pituitary stalk-sectioned rhesus monkeys

The effects of dopamine and thyrotrophin-releasing hormone (TRH) on prolactin release was studied in 14 intact and six pituitary stalk-sectioned (SS) female rhesus monkeys (Macaca mulatta). Baseline prolactin values were ninefold higher in SS animals (149+/-16 ng/ml) than in intact animals (16+/-1 ng/ml). Prolactin release after intravenous administration of TRH in doses of 0, 125, 250, 500 and 1000 ng revealed that SS monkeys were more sensitive to the prolactin-releasing activity of this tripeptide than were intact animals. A significant (P less than 0.05) increment in serum prolactin was observed in SS animals after injection of 125 ng TRH whereas 250 ng was required to raise prolactin levels in the circulation of intact animals significantly (P less than 0.05). Furthermore, at each comparable dose level of TRH, the increment in serum prolactin was distinctly greater in SS animals than in intact monkeys. Infusion of dopamine at the rate of 10 microgram/kg body weight per min significantly (P less than 0.05) lowered prolactin levels within 60 min in intact animals and no further decline was observed with 20 or 40 microgram dopamine. Serum prolactin concentrations were not affected by saline infusion or by 5 microgram dopamine. Infusion of dopamine at the rate of 10 microgram/kg body wt per min also resulted in significant (P less than 0.01) suppression of serum prolactin in SS animals. This prolactin decrease was apparent within 40 min. Prolactin release after 500 ng TRH was less in these dopamine-treated SS monkeys than after an infusion of saline. Higher doses of dopamine (20 and 40 microgram) did not cause a further decrease in basal serum prolactin concentrations, but these two dopamine treatments blocked the increase in prolactin elicited by 500 ng TRH. The results suggest that the removal of hypothalamic influence, possibly related to the effects of dopamine, renders the pituitary gland more sensitive to the prolactin-releasing action of TRH.     

Inflatable cavernosal body device: feasibility and acute safety study in the canine model

The phenylisopropylamine hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) produced dose-related increases in plasma concentrations of prolactin, adrenocorticotropic hormone (ACTH) and corticosterone but not growth hormone in rats. Pretreatment with metergoline (serotonin, 5-HT1/5-HT2 antagonist), ritanserin and mianserin (5-HT2A/5-HT2C antagonists) significantly attenuated DOM-induced increases in prolactin, ACTH and corticosterone, whereas mesulergine (5-HT2A/5-HT2C antagonist) pretreatment significantly attenuated DOM-induced increases in plasma prolactin and ACTH but not corticosterone. Pretreatment with propranolol (beta adrenoceptor antagonist that also has high binding affinity for 5-HT1A, 5-HT1B and 5-HT2C sites), MDL-72222 and ondansetron (5-HT3 antagonists) attenuated DOM's effect on plasma prolactin, but did not attenuate DOM-induced increases in either ACTH or corticosterone. On the other hand, spiperone (5-HT1A/5-HT2A/D2 antagonist) pretreatment significantly attenuated DOM-induced increases in ACTH but not corticosterone. These findings demonstrate involvement of 5-HT2A/5-HT2C and 5-HT3 receptors in mediating DOM-induced increases in plasma prolactin, whereas DOM-induced increases in ACTH appear to be mediated by stimulation of 5-HT2A receptors. DOM-induced corticosterone secretion appears to be mediated by stimulation of 5-HT2A and/or 5-HT2C receptors. DOM does not affect growth hormone secretion in rats.     

Antagonism of l-stepholidine against bromocriptine-inhibition on prolactin level in lactational rats

AIM: To study the antagonism of l-stepholidine (SPD) against bromocriptine (Bro)-inhibition on prolactin (PRL) level. METHODS: Bro (0.5 mg.kg-1.d-1, s.c.) reduced the PRL and caused a dysplasia of mammary gland in lactational rats. The weight growing of newborn rats was retarded. The PRL of the lactational rats was assessed by immunoradiometric assay (IRMA); the weight of newborn rats and development of mammary glands in lactational rats were also examined. Antagonism of SPD was evaluated. RESULTS: SPD (30 & 100 mg.kg-1.d-1, i.p.) obviously antagonized the Bro that induced lowering the PRL level in lactational rats, the PRL was 11 +/- 4 & 23 +/- 6 micrograms.L-1 (NS 7 +/- 2) respectively on d 15 of postpartum and the development of mammary gland in lactational rats was normal. The newborn rats grew rapidly in 11-15 d. CONCLUSION: SPD possessed an antagonism with Bro inhibition on D2 receptors located in the pituitary gland, and was an antagonist of dopamine D2 receptors.     

Preferential involvement of D3 versus D2 dopamine receptors in the effects of dopamine receptor ligands on oral ethanol self-administration in rats

There is evidence that dopamine transmission is involved in reinforcement processes and the present study investigated the relative involvement of D3 versus D2 dopamine receptors in the effects of dopamine ligands on the reinforcing action of ethanol. Rats were trained to self-administer ethanol (10% v/v) orally in a free-choice two-lever operant task using a saccharin-fading procedure. When preference in responding for ethanol over water had developed the rats were tested with several dopamine agonists and antagonists. Pretreatment with the non-selective dopamine agonist, apomorphine (0.01-0.1 mg/kg), the preferential D2 agonist, bromocriptine (1-10 mg/kg) and the selective D3 agonists, 7-OH-DPAT (0.003-0.1 mg/kg), PD 128907 (0.1-3 mg/kg), (+)3PPP (0.3-3 mg/kg), quinelorane (0.0001-0.003 mg/kg) and quinpirole (0.003-0.03 mg/kg), resulted in dose-dependent decreases in responding for ethanol. The relative potencies of the dopamine agonists to decrease ethanol self-administration were highly correlated with their published potencies to produce in vitro functional D3 but not D2 responses. Active doses could be considered as those selectively stimulating receptors involved in the control of dopamine release, suggesting that reduction of dopamine transmission was associated with a decrease in ethanol-reinforced responding. This conclusion was further supported by the finding that pretreatment with the D2/D3 dopamine antagonists, haloperidol (0.1-0.4 mg/kg) and tiapride (10-60 mg/kg), decreased responding for ethanol at doses which have been shown previously to block dopamine transmission.     

Exon scrambling of MLL transcripts occur commonly and mimic partial genomic duplication of the gene

The aim of this study was to test the hypothesis that prolactin may up- and down-regulate prolactin receptor gene expression in the anterior pituitary gland and hypothalamus respectively. Experiments were carried out in bantams (Gallus domesticus). Comparisons were made of concentrations of PRLR mRNA in the anterior pituitary gland and basal and preoptic hypothalamus in adult males and females held on long days (low vs high plasma prolactin); in 3-week-old juvenile male and females on short days (high vs low plasma prolactin); in 8-week-old juvenile male and females on short days (both low plasma prolactin); in adult laying, incubating, and out-of-lay (high, very high, and low plasma prolactin, respectively); in adult cockerels exposed to long or short days (high vs low prolactin); and in adult hens exposed to long or short days (high vs low prolactin). There was a sex difference in anterior pituitary and basal hypothalamic PRLR mRNA, with lower values in both tissues in females than in males. Compared with laying and out-of-lay hens, anterior pituitary and basal hypothalamic PRLR mRNA concentrations in incubating hens were increased and decreased, respectively. In adult birds of either sex held on long or short days, there was no difference in pituitary PRLR mRNA, while basal hypothalamic PRLR mRNA was lower on short days. PRLR mRNA in the preoptic hypothalamus was not affected by sex, reproductive state, or photoperiod. It is concluded that there is no consistent relationship between plasma prolactin, in the physiological range, and the concentration of PRLR mRNA in the anterior pituitary gland, basal hypothalamus, and preoptic hypothalamus.     

Androgen receptors in the anterior pituitary and central nervous system of the androgen "insensitive" (Tfm) rat: correlation between receptor binding and effects of androgens on gonadotropin secretion

The cytosol fractions of the anterior pituitary, hypothalamus, preoptic area and brain cortex of androgen "insensitive" (Tfm) rats possess androgen receptors. However, in the Tfm rats the androgen binding per mg protein was only 10-15% of that in the corresponding normal littermates (Nl). The physicochemical properties of the androgen receptors in the anterior pituitary of the Tfm rat were indistinguishable from those of the normal rat. Thus, no distinctive differences were observed with regard to electrophoretic mobility in 3.25% polyacrylamide gels, isoelectric point (pI=5.8), binding affinity (KD=1.5 X 10(-9)M), temperature stability, sulfhydryl dependence and steroid specificity. It is, therefore, likely that the very low androgen binding capacity by the anterior pituitary and the central nervous system is due to an extreme reduction in the receptor number rather than to the presence of abnormal receptors. Since in the Tfm animals the androgen receptor number is reduced by 85-90%, it is to be expected that very high doses of androgens would be required to achieve hormonal effects. In fact, low doses of 5alpha-dihydrotestosterone propionate (50 mug/100 g body weight) given sc daily for 12 days had no effect on serum levels of LH and FSH. However, very high doses (2 mg/100 g body weight) of testosterone propionate and 5alpha-dihydrotestosterone propionate, which maintained circulating androgen levels above 20 ng/ml, significantly reduced serum gonadotropin levels in castrated Tfm rats. In normal littermates both low and high doses of the androgens suppressed gonadotropin secretion to low levels. These findings strongly indicate that androgen receptors are essential to androgen action on the anterior pituitary and central nervous system in the rat. The serum levels of testosterone (7.7+/-0.15 (SE) ng/ml) and 5alpha-dihydrotestosterone (0.37+/-0.06 ng/ml) were significantly higher in intact Tfm rats than in normal littermates (2.6+/-0.03 and less than 0.1 ng/ml, respectively). The failure of the elevated concentrations of serum androgens to reduce the high serum levels of LH and FSH in intact Tfm rats is most likely due to the extreme reduction of the androgen receptor number and the consequent insufficient hypothalamic and/or pituitary response to androgens.     

Soluble vascular cell adhesion molecule-1 and E-selectin levels in relation to vascular risk factors and to E-selectin genotype in the first degree relatives of NIDDM patients and in NIDDM patients

OBJECTIVE: The stimulatory and inhibitory effects of N-methyl-D-aspartic acid (NMDA) and kainic acid on prolactin (PRL) secretion have been correlated with the serum prolactin concentrations before drug administration. In the present experiments, we analysed the role of NMDA and kainic acid in PRL secretion in females with different serum concentrations of PRL. METHODS: Hypoprolactinaemic females were obtained by ovariectomy or after administration of diethyldithiocarbamate (an inhibitor of dopamine-beta-hydroxylase). Chronic hyperprolactinaemia was induced by neonatal administration of testosterone or oestradiol and acute hyperprolactinaemia was induced either by administration of alpha-methyl-p-tyrosine (an inhibitor of tyrosine hydroxylase) or by ether exposure. To analyse the role of dopamine in the effects of NMDA, we measured pituitary concentrations of dopamine after NMDA treatment and the effects of pretreatment with domperidone. RESULTS: (1) NMDA, but not kainic acid, stimulated PRL release in cyclic females. This effect was independent of serum PRL concentrations and was not accompanied by a decrease in pituitary concentrations of dopamine. (2) NMDA did not change PRL secretion in neonatally androgenized females, whereas NMDA and kainic acid inhibited PRL release in neonatally oestrogenized females. The inhibitory effects of NMDA and kainic acid were blocked by domperidone. (3) Kainic acid inhibited PRL secretion in prepubertal hyper- and hypoprolactinaemic rats. (4) Hyperprolactinaemia induced by ether stress was counteracted by administration of NMDA and kainic acid. CONCLUSIONS: (a) NMDA has a dual effect on prolactin secretion that is independent of prior prolactin concentrations and of dopamine activity, but kainic acid is only inhibitory. (b) The stimulatory or inhibitory effects of NMDA and kainic acid on PRL secretion were not strictly related to basal PRL concentrations and necessarily involved a change in the secretion of prolactin releasing factors, as no correlations were observed between changes in pituitary concentrations of dopamine and serum PRL concentrations. (c) Females rendered hyperprolactinaemic by neonatal administration of testosterone or oestradiol responded differently after NMDA administration. (d) NMDA and kainic acid blocked the mechanisms involved in stress-induced PRL secretion.     

Hypothalamic dopamine D1 receptors are involved in the stimulation of prolactin secretion by high environmental temperature in the female sheep

Recent evidence suggests that dopamine, acting via its D1 receptors, may function as a neurotransmitter in intrahypothalamic pathways involved in the stimulation of prolactin secretion. Functional dopamine D1 receptors are present in the ventromedial hypothalamic nucleus (VMH) and we hypothesized that they might be part of a prolactin-stimulatory pathway activated by stress. We tested this hypothesis in a series of experiments on sheep involving two different forms of stressors, audiovisual (barking dog) and high environmental temperature. We attempted to block the stimulation of prolactin secretion by infusion into the VMH of an antagonist specific for the D1 receptor. Ovariectomised, oestradiol-implanted merino ewes were surgically implanted with bilateral guide tubes directed at the VMH. After a 180 min pretreatment period, the ewes either were or were not exposed to a stressor (30 min of barking dog or 120 min at 35 degrees C, 65% relative humidity). D1 receptor antagonist, SCH23390 or vehicle (0.9% saline) was infused into the VMH (1.7 microliters/h, 120 nmol/h) for 60 min prior to and during the stressor period. Blood was sampled every 15 min via jugular cannulae and the plasma was assayed for prolactin, cortisol and growth hormone (GH). Both stressors significantly increased prolactin concentrations over control levels. SCH23390 infusion significantly attenuated the prolactin response to high environmental temperature, but had no effect on the prolactin response to audiovisual stress. Cortisol concentrations were significantly increased by audiovisual stress only and were not affected by SCH23390. GH concentrations were not changed by either stressor or infusion. Drug infusion alone did not affect the concentration of the hormones. The data suggest that the VMH D1 receptors are involved in a prolactin stimulatory pathway in response to high environmental temperature. The inability of the D1 antagonist to affect the response to the barking dog indicates that this pathway is stress-specific, implying that there is more than one mechanism or pathway involved in the prolactin response to different stressors.     

In vitro and in vivo characterization of R(+)-FIDA2: a dopamine D2-like imaging agent

R(+)-FIDA2, (R)-(+)-2,3-dimethoxy-5-iodo-N-[(1-(4'-fluorobenzyl)-2-pyrrolid iny l)- methyl]benzamide, is a new dopamine D2-like receptor imaging agent that can be labeled with either 123I or 18F for SPECT or PET imaging. The purpose of this study was to characterize its in vitro and in vivo binding properties. METHODS: In vitro binding studies using [125I]R(+)-FIDA2 were performed in Sf9 cells expressing dopamine D2 or D3 receptors and in rat basal forebrain homogenates, which contain a high density of dopamine D2-like receptors. A series of in vivo SPECT imaging studies in nonhuman primates (cynomologous monkeys) were performed by intravenously injecting 7.1 +/- 1.0 mCi of [123I]R(+)-FIDA2. At least one control study and one displacement experiment, in which a cold compound was injected intravenously 90 min after tracer injection, was performed in each monkey. Data were acquired in 10-min frames for 180 min, and the activity in regions of interest (basal ganglia and cerebellum) were plotted versus time. RESULTS: Iodine-125-R(+)-FIDA2 displayed Kd values for D2 and D3 receptor subtypes expressed in Sf9 cells of 0.11 and 0.04 nM, respectively. As expected, SPECT images of monkey brain (transaxial sections, 2 mm) showed that the radioactivity was localized in the area of the basal ganglia and reached peak concentrations in 11.5 +/- 5.8 min postinjection. An injection of R(+)7-OH-PIPAT, a new ligand that is selective for dopamine D3 receptors and the high affinity state of dopamine D2 receptors, did not show significant displacement of [123I]R(+)-FIDA2 binding in the basal ganglia. CONCLUSION: These studies indicate that R(+)-FIDA2 may be a useful ligand for in vitro pharmacological characterization and in vivo imaging of CNS dopamine D2-like receptors.     

Nephrotic syndrome in a patient with HIV infection and juvenile onset diabetes: diagnostic, therapeutic, and prognostic value of the renal biopsy

The effects of the native somatostatin-14 (SST-14) and of its analog octreotide (OCT) on the activity of protein tyrosine kinases (PTK) in the normal rat anterior pituitary gland, diethylstilbestrol (DES)-induced rat pituitary tumor and murine colonic cancer Colon 38 were studied in vitro. PTK activity was estimated in tissue homogenates using gamma-[32P]ATP and poly (Glu80, Tyr20) as a substrate. It was found that both SST-14 and OCT suppressed the PTK activity in all examined tissues. The suppressive effect was more pronounced in DES-induced pituitary tumor than in normal anterior pituitary gland, and in the former, OCT was more effective than SST-14. In contrast, SST-14 stronger suppressed PTK activity in colonic cancer than OCT. We hypothesize that SST-14 acts on PTK activity in colonic cancer mainly via SSTR-1 subtype of somatostatin receptors.     

Aversive stimulation during the stress-hyporesponsive period does not affect the number of corticotroph cells in neonatal male rats

Immunohistochemistry was used to evaluate the effects of neonatal handling and aversive stimulation during the first 10 days of life on the number of corticotrophs in the anterior lobe of the pituitary of 11-day-old male Wistar rats. Since adult rats handled during infancy respond with reduced corticosterone secretion in response to stressors and with less behavior inhibition in novel environments, we assumed that neonatal stimulation could affect pituitary morphology during this critical period of cell differentiation. Three groups of animals were studied: intact (no manipulation, N = 5), handled (N = 5) and stimulated (submitted to 3 different aversive stimuli, N = 5). The percentage of ACTH-immunoreactive cells in the anterior lobe of the pituitary (number of ACTH-stained cells divided by total number of cells) was determined by examining three slices per pituitary in which a minimum of 200 cells were counted by two independent researchers. Although animals during the neonatal period are less reactive to stress-like stimulation in terms of ACTH and corticosterone secretion, results showed that the relative number of ACTH-stained cells of neonatal handled (0.25 +/- 0.01) and aversive stimulated (0.29 +/- 0.03) rats was not significantly different from intact (0.30 +/- 0.03) animals. Neonatal stimulation may have a differential effect on the various subpopulations of corticotroph cells in the anterior pituitary.