Receptor activated C kinase is down-regulated in the male gonad of the marine bivalve mollusc Mya arenaria exposed to tributyltin (TBT)

This study was aimed at investigating the molecular mechanisms by which tributyltin (TBT) impairs the reproductive processes in the marine bivalve Mya arenaria. The suppression polymerase chain reaction subtractive hybridization (SSH) method was used to identify differentially expressed transcripts in the gonads of adult M. arenaria 72 h after a single injection of 160 ng TBT in the adductor muscle. Subtractive cDNA libraries comprising 322 clones were obtained. These clones were sequenced and corresponded to 55 female and 26 single male non-redundant cDNAs. Following similarity searches in genome databases, some of the transcripts could be assigned to cellular functions including mitochondrial respiration, structural proteins, structure of cytoskeleton, nucleic acid regulation, general metabolism and signal transduction. Among the potentially differentially regulated transcripts, Receptor for activated C kinase 1 (RACK1) represented 6% of the total down-regulated clones in males and the corresponding protein exhibited a high degree of similarity (80%) with the human polypeptide. The RACK1 cDNA from M. arenaria consists of 1085 bp, encoding a 318 deduced polypeptide which contains five internal tryptophan-aspartate (WD) repeats, six putative PKC phosphorylation sites, one tyrosine kinase site, four putative N-myristoylation sites as well as a transmembrane segment spanning amino acid 228-251. A significant down-regulation (by approximately 30% (p<0.05)) of RACK1 expression in male gonads exposed to TBT was confirmed by quantitative real-time RT-PCR. Transcript levels of RACK1 were higher in the female gonads than in the mantle, gills and male gonads. Gene expression as detected by in situ hybridization was strong in mature oocytes comparatively to primary germ cells. RACK1 may be a useful biomarker for TBT exposure in the reproductive system of bivalve molluscs.

Functional analysis of chemically-induced mutations at the flounder TP53 locus, the FACIM assay

A functional assay was developed in yeast to identify mutations induced by DNA-damaging agents at the flounder TP53 locus. This assay named FACIM for functional analysis of chemically-induced p53 mutations, is based on the assumption that most genotoxin-induced mutations inactivate transcriptional activity of the TP53 protein. The functional status of the protein expressed in yeast was measured using a p53-responsive reporter gene. The FACIM assay was used to evaluate the mutagenesis of the flounder TP53 exposed in vitro to benzo[a]pyrene diol epoxide (BPDE). A dose-dependent increase of p53 mutation rate was observed with increasing concentrations of BPDE and extension of exposure time. Flounder TP53 gene appeared highly sensitive to point mutations since most of those identified targeted different nucleotides. Mutated base-pairs corresponded predominantly to guanines located on the non-transcribed strand of the DNA. The general distribution of mutations along the flounder TP53 protein was different from that identified in the human homologue suggesting species-differences in mutagenesis of the TP53 gene. Most of flounder TP53 mutants were defective for transactivation and cell growth regulation but some maintained a partial wild-type phenotype. This functional assay in yeast could be used for both evaluation of the genotoxic potency of chemicals or environmental samples and screening of p53 mutations in fish tumours.

Molecular cloning of superoxide dismutase (Cu/Zn-SOD) from aquatic molluscs

The potential of the first line of the active oxygen-scavenging system, partial cDNA encoding Cu/Zn superoxide dismutase (SOD) was isolated in three aquatic mollusc species: Ruditapes decussatus (marine clam), Dreissena polymorpha (continental water mussel) and Bathymodiolus azoricus (hydrothermal vent mussel). These SOD cDNA fragments were amplified by PCR with degenerate oligonucleotide primers derived from the amino acid sequence conserved in the Cu/Zn-SOD from several other organisms. A partial cDNA of CuZn-SOD was obtained for R. decussates (510 bp), D. polymorpha (510 bp) and B. azoricus (195 bp). The deduced amino acid sequence showed high similarity among the three mollusc species (57-63%) and among other species (50-65%). The residues involved in coordinating copper (His-47, 49, 64, 121) and zinc (His-64, 72, 81 and Asp-84) were well conserved among the three Cu/Zn-SOD sequences.

The European flounder (Platichthys flesus) TP53 functions as a temperature-sensitive transcription factor which inhibits cell growth in yeast

Numerous studies focus on biological roles of the TP53 tumor suppressor gene in mammals but little is known about the actual function of TP53 in lower vertebrates. In this study, we used an in vivo functional assay in yeast to address the transactivation capacity of the flounder TP53 protein. We showed that the flounder TP53 acts as a sequence-specific transcription factor which is able to transactivate various human promoters containing a p53-responsive element (RE). This transcriptional activity was completely abrogated in the Val147Glu TP53 mutant previously identified in two flounder hepatic hyperplasia. In addition, we showed that the wild-type (wt) flounder TP53 but not the Val147Glu mutant inhibits cell growth when expressed in yeast. We finally reported that transcription regulation and growth inhibition by the wild-type flounder TP53 is temperature-dependent. The flounder TP53 optimal temperature appeared lower than those reported for the Xenopus and human homologues.

Expression and activity of a multixenobiotic resistance system in the Pacific oyster Crassostrea gigas

The multidrug resistance (MDR) mechanism corresponds to a defence system relying on the expression of high molecular membrane proteins that can actively lower the intracellular concentration of a wide variety of toxins, thus maintaining them below their toxic level. Using RT-PCR, expression levels of a gene belonging to the class I of mammalian mdr genes, has been assessed in different developmental stages of the oyster Crassostrea gigas. While no expression was found in the oocyte or the trocophore stage, a rise of mRNA content was observed from the veliger stage to the juvenile stage, thus indicating the induction of the system as the animal is developing in the environment. The incubation of gill fragments in the dye rhodamine B and subsequent measurements of intracellular fluorescence using a microplate reader indicates that the system can effectively decrease the accumulation of the test compound in a competitive manner with known inhibitors or environmental contaminants as observed in vertebrate cells. The oyster MXR system is thus becoming active in adult oyster and could be of importance in environmentally contaminated areas.

Identification of differentially expressed genes in Dreissena polymorpha exposed to contaminants

Development of transcriptome analysis methods such as differential display PCR and construction of subtractive libraries now makes it possible to profile gene expression in response to xenobiotic exposure. As an example of application of these methods, zebra mussels (Dreissena polymorpha) were treated with various contaminants such as Aroclor 1254, 3-methylcholanthrene, chrysene and atrazine. A total of 242 mRNAs were identified as differentially expressed. Analysis of these mRNAs should provide valuable information regarding detoxification mechanisms in this bivalve species. In addition, the use of cDNA array technology applied to these gene products may constitute a multi-marker approach to monitor the effect of contamination on this aquatic species.

Genetic and immunological characterisation of a multixenobiotic resistance system in the turbot (Scophthalmus maximus)

Pleiotropic resistance driven by transport proteins constitutes a very ubiquitous protection mechanism against natural or synthetic toxic compounds. The multidrug (MDR) or multixenobiotic (MXR) system has been identified in many different species, and may be used as a biomarker for pollution assessment. Here we report the existence of a gene encoding a MXR-related protein in a benthic fish species, the turbot Scophthalmus maximus, and its constitutive expression in several tissues. A 433bp cDNA fragment has been cloned by RT-PCR. The deduced amino-acid sequence shares close to 80% homology with class I or class II mammalian MDR proteins. This cDNA corresponds to a major mRNA of 5.6 kb and encodes a protein having an apparent molecular weight of 83 kDa. Constitutive expression levels assessed by semi-quantitative RT-PCR and Western blot, revealed that the kidney and the brain, and to a lesser extent, the heart, gills and intestine, are the organs which contain the highest amount of both MXR mRNAs or proteins. This tissue specific expression suggests a role for the identified mechanism in protection against endogenous or exogenous toxic compounds.

Validation of an analytical procedure for polychlorinated biphenyls, coplanar polychlorinated biphenyls and polycyclic aromatic hydrocarbons in environmental samples

This work describes an efficient analytical procedure for the analysis of PCBs (polychlorinated biphenyls), coplanar PCBs and PAHs (polycyclic aromatic hydrocarbons) from the same sample. The method includes a solvent extraction followed by a combined purification-separation step on an alumina-silica column. Coplanar PCBs are isolated from the first fraction (PCBs) by a further high-performance liquid chromatography (HPLC) fractionation on a PYE [2-(1-pyrenyl)ethyldimethylsilylated silica gel] column. PCBs are identified and quantified by gas chromatography (GC) with electron-capture detection whereas GC with flame ionization detection or mass spectrometry are used for PAH determinations. This method allows the measurement of these contaminants in biota and sediment at trace levels as low as 1 pg g(-1) for coplanar PCBs with a precision better than 20%.

Flounder health status in the Seine Bay. A multibiomarker study

The Seine Bay is used as a pilot area to assess the usefulness of monitoring programmes using a suite of biological measurements. These biomarkers included ethoxyresorfin-O-deethylase (EROD) and acetylcholinesterase (AChE) activities, multixenobiotic resistance (MXR) protein expression level assessment and gonad histopathology. Samples of European flounder collected in three sites close to the Seine Estuary in late September 1998 showed that 8% of the males were intersex, i.e. had gonads with both male and female tissues. Another 10% of individuals, identified as male by morphological observation during sampling, showed only female tissues on histological sections. These dramatic changes were associated with different patterns of EROD activity, MXR expression or AChE activity inhibition that might reflect shorter time effects of xenobiotics and constitute a starting point to integrate biological responses for the assessment of the health status of flounder in the Seine Bay.

Pharmacological profile of the tachykinin receptor involved in the stimulation of corticosteroid secretion in the frog Rana ridibunda

It has recently been shown that the adrenal gland of the frog Rana ridibunda is densely innervated by a network of fibers containing two novel tachykinins, i.e. ranakinin (the counterpart of substance P) and [Leu3, Ile7]neurokinin A. Both ranakinin and [Leu3, Ile7]neurokinin A stimulate corticosteroid secretion from frog adrenal glands in vitro. In the present study, we have investigated the pharmacological profile of the receptors involved in the stimulatory action of ranakinin on perifused frog adrenal slices. The selective NK-1 receptor antagonists [D-Pro4, D-Trp7,9]substance P 4-11 and CP-96,345, did not affect the stimulatory action of ranakinin. The selective NK-1 agonist substance P 6-11 had no effect on corticosteroid secretion. The non-peptidic NK-1 receptor antagonist RP 67580 significantly reduced the stimulatory effect of ranakinin on corticosterone and aldosterone secretion by 57 and 55%, respectively. In addition, the dual NK-1/NK-2 receptor antagonist FK-224 significantly inhibited the effect of ranakinin on corticosterone (- 80%) and aldosterone secretion (- 95%). Finally, the amphiphilic analogue of substance P, [D-Pro2, D-Phe7, D-Trp9]substance P, had no effect on corticosteroid secretion. These data suggest that in the frog adrenal gland the stimulatory action of ranakinin on steroid secretion is mediated by a novel type of receptor which differs substantially from the mammalian NK-1 receptor subtype.

Effect of ranakinin, a novel tachykinin, on cytosolic free calcium in frog adrenochromaffin cells

It has recently been shown that two novel tachykinins, ranakinin and [Leu3, Ile7]neurokinin A, are present in fibers innervating the frog adrenal gland, and it has been demonstrated that tachykinins stimulate corticosteroid secretion in vitro through activation of chromaffin cells. The purpose of the present study was to investigate the effect of ranakinin on cytosolic free calcium concentrations ([Ca2+]i) and to determine the source of calcium involved. Cultured adrenal cells were loaded with the fluorescent calcium indicator indo-1, and changes in [Ca2+]i were studied using dual emission wavelength microfluorimetry. Administration of a brief pulse of ranakinin (1 microM; 1 sec) in the vicinity of chromaffin cells caused an immediate and transient increase in [Ca2+]i. Repeated pulses of ranakinin resulted in a gradual decline in the [Ca2+]i response, suggesting the occurrence of a desensitization phenomenon. Preincubation of the cells with the calcium channel blockers nifedipine (10 microM) and omega-conotoxin (1 microM) did not alter the response of chromaffin cells to ranakinin. Chelation of extracellular calcium by EGTA (10 mM) caused a marked decrease in the basal [Ca2+]i, but did not suppress the ranakinin-induced [Ca2+]i increase. Conversely, incubation of the cells with thapsigargin (10 microM), an inhibitor of calcium adenosine triphosphatase activity, abolished the stimulatory effect of ranakinin, indicating that the increase in [Ca2+]i can be ascribed to mobilization of calcium from intracellular stores. Preincubation of adrenal cells with the phospholipase C antagonist U-73122 (1 microM; 18 min) or with pertussis toxin (10 microM; 18 h) totally blocked the ranakinin-induced [Ca2+]i rise. Taken together, these data indicate that in frog adrenochromaffin cells, ranakinin causes mobilization of calcium from intracellular stores. The effect of ranakinin is mediated through activation of a phospholipase C via a pertussis toxin-sensitive G protein.

Evidence for the involvement of chromaffin cells in the stimulatory effect of tachykinins on corticosteroid secretion by the frog adrenal gland

The adrenal gland of the frog is innervated by a network of fibers containing two tachykinins (ranakinin and [Leu3,Ile7]neurokinin A), which both stimulate corticosteroid secretion from frog adrenal tissue. The aim of the present study was to determine the mode of action of tachykinins on the frog adrenal gland. Double immunolabeling of tissue sections with a monoclonal antibody to tyrosine hydroxylase and an antiserum to substance P showed that tachykinin-containing fibers are preferentially apposed onto chromaffin cells. Immunocytochemical labeling at the electron microscope level revealed that tachykinin-immunoreactive fibers establish close contacts only with adrenochromaffin cells. Ranakinin stimulated corticosterone and aldosterone secretion from perifused adrenal slices, but had no stimulative effect on dispersed adrenal cells. Cytoautoradiographic labeling of frog adrenal cells in primary culture with [3H]substance P revealed the existence of specific binding sites located exclusively on chromaffin cells. Microfluorimetric measurement of cytosolic calcium concentrations ([Ca2+]i) in cultured adrenal cells showed that ranakinin induced a dose-dependent increase in [Ca2+]i in chromaffin cells (ED50 = 2 x 10(-7) M). In contrast, ranakinin did not affect [Ca2+]i in adrenocortical cells. The present results indicate that in the frog adrenal gland, tachykinin-containing fibers make preferential contacts with chromaffin cells, and tachykinins directly activate chromaffin cells. These data suggest that the stimulative effect of tachykinins on corticosteroid secretion is mediated via presynaptic activation of adrenochromaffin cells.

Immunocytochemical localization and biological activity of 3 beta-hydroxysteroid dehydrogenase in the central nervous system of the frog

The enzyme 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4 isomerase (3 beta-HSD) catalyzes biosynthesis of progesterone (P) and all precursors of glucocorticoids, mineralocorticoids, androgens, and estrogens. Despite the broad interest raised by neurosteroids, the cellular localization of 3 beta-HSD has never been investigated in the brain. We took advantage of the availability of an antiserum raised against human placental 3 beta-HSD to determine the distribution of 3 beta-HSD-immunoreactive structures in the brain of the frog Rana ridibunda by the indirect immunofluorescence technique. Three populations of 3 beta-HSD-immunoreactive cell bodies were observed in the hypothalamus, namely, in the rostral region of the preoptic nucleus, the dorsal infundibular nucleus, and the dorsal part of the ventral infundibular nucleus. A dense network of 3 beta-HSD-immunoreactive nerve fibers was visualized in the dorsal area of the diencephalon, that is, in the lateral neuropil, the corpus geniculatus lateralis, and the nucleus posterolateralis thalami. Reversed-phase HPLC analysis of frog hypothalamic extracts combined with RIA detection showed the presence of substantial amounts of immunoreactive steroids coeluting with P and 17-hydroxyprogesterone (17OH-P). The synthesis of delta 4-3-keto-steroids in the frog hypothalamus was investigated using the pulse-chase technique with 3H-pregnenolone (3H-delta 5P) as a precursor. The formation of five tritiated metabolites of 3H-delta 5P was observed, one of which coeluted with 17OH-P. Conversion of 3H-delta 5P into this radioactive metabolite was significantly reduced by trilostane, a specific inhibitor of 3 beta-HSD. Immunodetection of newly synthesized steroids in HPLC fractions of hypothalamic extracts, using 17OH-P antibodies, revealed the existence of an immunoreactive steroid that exhibited the same retention time as synthetic 17OH-P. The present study provides the first immunocytochemical mapping of 3 beta-HSD, a key enzyme of the steroid biosynthetic pathway, in the CNS of a vertebrate. The data also demonstrate for the first time biosynthesis of neurosteroids in the brain of a nonmammalian vertebrate.

Immunohistochemical distribution, biochemical characterization, and biological action of tachykinins in the frog adrenal gland

The distribution of tachykinin-like immunoreactivity (LI) was studied in the adrenal gland of the frog Rana ridibunda using the immunofluorescence technique. A dense network of varicose fibers immunoreactive to both substance-P (SP) and neurokinin-A (NKA) was found in the adrenal tissue. In contrast, no positive fibers could be detected using antineurokinin-B (NKB) antibodies. At the electron microscope level, the immunogold technique revealed that tachykinin-LI was sequestered in dense core vesicles of 50-70 nm. Bilateral transection of either splanchnic or vagus nerves or total lesion of celiac sympathetic ganglion did not suppress tachykinin-LI. A combination of HPLC analysis and RIA detection was used to characterize tachykinin-LI in frog adrenal extracts. Two major peaks were resolved, which coeluted, respectively, with synthetic ranakinin, a novel tachykinin previously isolated from the frog brain, and [Leu3,Ile7]NKA previously isolated from the frog gut. No NKB could be detected in the extracts. The effects of various synthetic tachykinins on corticosteroid secretion were studied using perifused frog adrenal slices. For concentrations ranging from 10(-8)-10(-4) M, SP induced a dose-dependent stimulation of corticosterone and aldosterone release. A desensitization phenomenon was observed when iterative or prolonged infusions of SP were administered to the tissue. All mammalian or amphibian tachykinin-related peptides tested in our model also enhanced corticosteroid production. The effectiveness of the tachykinins tested was: [Pro7] NKB > NKA > ranakinin > [Pro9]SP > SP > kassinin > physalaemin > NKB > [Leu3,Ile7]NKA. SP also enhanced prostaglandin E2 and prostacyclin release in the effluent perifusate and the response preceded by 10-15 min the increase in corticosteroid output. Indomethacin (5 x 10(-6) M), a specific blocker of cyclooxygenase activity, totally suppressed SP-evoked steroid secretion. These data indicate that tachykinin-induced stimulation of steroidogenesis was mediated through activation of the arachidonic acid cascade. Taken together, our results show that the frog adrenal gland is innervated by a dense network of peptidergic fibers containing both ranakinin and [Leu3,Ile7]NKA, which, in vitro, stimulates corticosteroid secretion by adrenocortical cells through a prostaglandin-dependent mechanism. The present results support the view that tachykinins released by nerve fibers exert a neuroendocrine control on corticosteroid release in amphibians.

In vitro study of catecholamine release from perifused frog adrenal slices

The adrenal gland of amphibians is composed of a mixed population of adrenochromaffin and corticosteroid-secreting cells. It has previously been shown that chromaffin cells synthesize several bioactive substances (including biogenic amines and neurotransmitters) which may act locally to regulate corticosteroid secretion. In the present report, we have studied the secretory activity of adrenochromaffin cells in Rana ridibunda. Frozen sections of adrenal gland were immunolabeled with antisera against tyrosine hydroxylase or phenylethanolamine-N-methyltransferase. Comparison of homologous fields on consecutive sections indicated that 77% of catecholaminergic cells produce adrenaline. The concentrations of catecholamines were measured by means of high performance liquid chromatography analysis coupled to electrochemical detection. The concentrations of dopamine, noradrenaline, and adrenaline in fresh adrenal tissue were 24 +/- 4, 763 +/- 68, and 1032 +/- 118 ng/mg wet weight, respectively. After a 12-hr perifusion period, the concentration of adrenaline in the tissue was reduced by 62%, whereas noradrenaline only decreased by 22%. The secretion rates of adrenaline and noradrenaline from perifused adrenal slices significantly diminished during the first 7 hr of the experiment and then remained relatively stable for about 10 hr. Exposure of adrenal tissue to a depolarizing concentration of potassium (55 mM) induced an immediate and substantial rise of adrenaline and noradrenaline release and a delayed increase in corticosterone output. Acetylcholine, which stimulates corticosterone secretion from frog adrenocortical cells, induced a slight but not significant increase of adrenaline and noradrenaline release. Similarly, the selective cholinergic agonists muscarine and nicotine did not significantly affect catecholamine release, while muscarine mimicked the stimulatory action of acetylcholine on corticosterone secretion. This study validates the use of the perifusion model to investigate the mechanism of control of catecholamine release from frog adrenochromaffin tissue. The results presented herein indicate that, in contrast to mammals, the secretion of catecholamines from the amphibian adrenal gland is not regulated by cholinergic inputs.

Characterization of dopamine receptors associated with steroid secretion in frog adrenocortical cells

We investigated the type of receptors involved in the mechanism of action of dopamine on corticosteroid secretion from the frog interrenal (adrenal) gland, using the in-vitro perifusion technique. Exposure of dispersed interrenal cells to 50 microM dopamine for 20 min had a biphasic effect on corticosterone and aldosterone secretion, i.e. a transient stimulation followed by an inhibitory phase. Repeated administration of equimolar pulses of dopamine, given at 150-min intervals, resulted in an enhancement of corticosteroid secretion followed by a subsequent blockade of the stimulatory phase of the response. In contrast, the dopamine-evoked inhibition of corticosteroid release did not show any sensitization or desensitization phenomena. Infusion of repeated pulses of the D1 receptor agonist SKF38393 (32 microM) stimulated corticosteroid release and mimicked the sensitization-desensitization phenomenon induced by dopamine. Repeated administration of the D2 receptor agonist LY171555 (50 microM) resulted in a reproducible inhibition of corticosterone and aldosterone secretion. These results suggested the presence of two different receptors for dopamine, i.e. D1 and D2, on frog adrenocortical cells, responsible respectively for the stimulatory and inhibitory effects of dopamine on steroid secretion. However, bromocriptine (50 microM) and CV205-502 (50 microM), two other D2 receptor agonists, had no effect on corticosteroid release. In addition, several classical D2 receptor antagonists failed to block the effect of dopamine on steroidogenesis. It was also observed that (-)sulpiride, a specific D2 antagonist, did not alter dopamine-induced inhibition of inositol phosphate formation. On the other hand, dopamine and the selective D1 and D2 antagonists SKF38393 and LY171555 did not affect the formation of cyclic AMP by interrenal tissue. Taken together, these data indicate that dopamine directly regulates corticosteroid secretion from frog adrenocortical cells. The effect of dopamine is not coupled to adenylate cyclase activity but is probably mediated through the phosphoinositide-turnover pathway. The pharmacological characteristics of the receptors involved in the mechanism of action of dopamine clearly differ from those of the D1 and D2 subtypes previously described in mammals.

Self-inhibition of steroid secretion by amphibian adrenocortical cells is not mediated through glucocorticoid receptors

To investigate a possible direct action of glucocorticoids on adrenal steroidogenesis, the effect of corticosterone on the conversion of pregnenolone into various metabolites by frog adrenal tissue was examined. Frog interrenal slices were incubated with [3H]pregnenolone (1 mCi/ml) and the various labelled metabolites analysed by reverse-phase high-performance liquid chromatography. With the methanol gradient used, five identified steroids were resolved: progesterone, 11-deoxycorticosterone, corticosterone, 18-hydroxycorticosterone and aldosterone. Corticosterone (10 micrograms/ml) induced a 45-80% decrease in all steroids synthesized from [3H]pregnenolone. In contrast, the glucocorticoid agonist dexamethasone did not reduce the rate of conversion of pregnenolone into its metabolites. In addition, the inhibitory effect of corticosterone was not reversed by the specific glucocorticoid antagonist RU 43044. These results show that corticosterone exerts a direct inhibitory effect on adrenal steroid secretion. In addition, our data indicate that the ultra-short regulation induced by corticosterone is not mediated through glucocorticoid receptors.

Dopamine inhibits inositol phosphate production, arachidonic acid formation, and corticosteroid release by frog adrenal gland through a pertussis toxin-sensitive G-protein

We have previously shown that dopamine-evoked inhibition of corticosteroid production from adrenocortical cells is mediated through a decrease in prostaglandin biosynthesis. Since the catecholamine did not alter the stimulatory effect of arachidonic acid, it was proposed that dopamine may inhibit the formation of arachidonate from glycerophospholipids. To test this hypothesis, the effect of dopamine on phosphoinositol lipid metabolism was investigated in frog interrenal (adrenal) tissue. In [3H]myo-inositol-prelabeled frog interrenal slices, a short pulse of dopamine (50 microM) induced a biphasic effect on inositol phosphate production: a transient (1-min) increase, followed by a sustained inhibition. Concurrently, dopamine induced a transient reduction followed by a sustained increase in polyphosphoinositides. A 10-min pulse of the D2 dopamine receptor agonist apomorphine (50 microM) elicited a significant inhibition of basal levels of inositol phosphates (tris-, bis-, and mono-), and an increase in plasma membrane phosphoinositol lipid contents. The inhibitory effect of dopamine on inositol phosphate formation and corticosteroid release was abolished by a 24-h incubation of interrenal slices with pertussis toxin. In [3H]arachidonic acid-prelabeled interrenal slices, dopamine also decreased diacylglycerol (DG) and arachidonic acid (AA) concentrations. A delay of 1 min was observed between inhibition of DG and arachidonate, suggesting that AA is probably generated from DG. We conclude that in the adrenal cortex, activation of dopamine D2 receptors is coupled to a phosphoinositide-specific phospholipase-C mediated via a pertussis toxin-sensitive G-protein. Taken together, our data indicate that inhibition of inositol phosphate and AA formation is one of the mechanisms by which dopamine controls corticosteroid production by adrenocortical cells.

Studies on the thymus in Chagas' disease. II. Thymocyte subset fluctuations in Trypanosoma cruzi-infected mice: relationship to stress

Changes in thymic T-cell subsets in mice acutely infected with Trypanosoma cruzi have been studied in both C3H/HeJ and C57BL/6 mice. The significant decrease in thymocyte number, observed in both mouse strains on day 14 post-infection correlated with a drastic decrease in CD4+CD8+ cell number, whereas the number of CD4-CD8-, CD4+CD8- and CD4-CD8+ cells remained essentially unchanged. The important increase in CD3hi cell frequency confirmed that resistant thymocytes during Chagas' disease development were mostly medullary thymocytes, whereas the thymic cortex was largely depleted, as previously observed on thymus sections. This involution of the thymus could have been due to the increase of circulating glucocorticoid levels observed after infection. However, similar cell modifications were found in infected adrenalectomized mice whose serum corticosterone levels were only slightly augmented. Thus, the thymic alterations appear not to be linked to stress responses, at least those dependent on high levels of circulating glucocorticoids.

Reduction of morbidity and cytokine release in anti-CD3 MoAb-treated mice by corticosteroids

In keeping with the in vitro mitogenic properties of anti-CD3 MoAbs, the first injections of anti-CD3 are invariably responsible for an in vivo cellular activation. This activation induces a massive cytokine release in the circulation (TNF, IFN gamma, IL-2, IL-6, and IL-3). Paralleling this release, a severe clinical reaction occurs in OKT3-treated patients and in 145 2C11-treated mice. Corticosteroids both in vitro and in vivo inhibit the production of several cytokines involved in the anti-CD3 reaction. A single 1 mg hydrocortisone dose was administered to 145 2C11-treated mice according to different kinetics schedules. When given 1 hr prior to the anti-CD3 MoAb, hydrocortisone exerted a beneficial effect on the mouse physical reaction. Hypothermia was totally abrogated at the 4-hr time point. Diarrhea decreased by 50%. Hypomotility improved although not significantly. This improvement correlated with a major modification in the anti-CD3 pattern of cytokine release. At the 90-min blood withdrawal time point cytokine serum levels showed a 100% decrease for IFN gamma, an 88% decrease for IL-6, and 85% decrease for IL-2, and a 75% decrease for TNF. At 4 hr IL-2 serum levels were diminished by 65%; IL-6, IL-3, and IFN gamma serum levels were comparable to controls; and, interestingly, TNF was still detected, whereas it has already disappeared when 145 2C11 was administered alone. Importantly, when given more than 1 hr prior to anti-CD3 injection, corticosteroids were ineffective. To conclude, high doses of corticosteroids must be given with a precise kinetics--i.e. 1 hr prior to anti-CD3 MoAb--to achieve their maximal beneficial effect in the prevention of the anti-CD3 reaction.

Adrenal activity in the female lizard Lacerta vivipara Jacquin during artificial hibernation

The variations of interrenal activity were investigated in captive female Lacerta vivipara submitted to artificial hibernation (4 months at 6 degrees) and compared to data obtained in nonhibernating females. Plasma corticosterone levels reached 25 ng/ml during the prehibernal period. During the first day following the transfer to cold conditions, an initial significant peak of plasma corticosterone was observed (up to 63 ng/ml). A second, more gradual, but also significant increase was observed thereafter and levels remained maximum during the two first months of artificial hibernation (75 ng/ml). The circulating levels of corticosterone then decreased gradually. At the time of transfer to warm conditions, a third significant peak of corticosterone was observed (up to 82 ng/ml). The minimal values (15 ng/ml) previously described during vitellogenesis were reached within 1 week. High corticosterone levels appeared to be actually related to the "hibernation state" since they were also observed in hibernating males and not in nonhibernating females. In order to explain the pattern of plasma corticosterone, variations of adrenal sensitivity to synthetic ACTH 1-39 were examined in vitro, using a perifusion system technique. Surprisingly, ACTH-induced stimulation of corticosterone and aldosterone release was significantly reduced during hibernation, whatever the temperature of the perifusion bath (30 or 6 degrees). Nevertheless, a fourfold increase in the half-life of injected tritiated corticosterone was observed during hibernation which likely contributes to maintain high levels of corticosterone despite a low production rate of the hormone.

Dopamine inhibits corticosteroid secretion from frog adrenal gland, in vitro

The effect of dopamine on corticosteroid secretion from frog interrenal (adrenal) tissue was investigated in vitro using a perifusion system technique. Administration of graded concentrations of dopamine (5 X 10(-8) M to 10(-3) M) to interrenal slices induced a dose-dependent inhibition of steroid secretion. The half-maximal effective dose of dopamine was 7 X 10(-6) M for corticosterone and 4 X 10(-6) M for aldosterone. Noradrenaline and adrenaline were also able to elicit a dose-related inhibition of steroid release, but these catecholamines were approximately 100 and 2000 times less potent than dopamine in our model. Administration of repeated pulses of dopamine (5 X 10(-5) M), at 150-min intervals, led to a reproducible inhibition of corticosteroid secretion without any desensitization phenomenon. Similarly, prolonged infusion of dopamine (5 X 10(-6) M) caused a sustained inhibition of steroidogenesis. The inhibitory action of dopamine was also observed using enzymatically dispersed adrenal cells, indicating that dopamine exerts a direct effect on adrenocortical cells. After the second pulse, dopamine also induced a transient stimulation of steroid secretion from acutely dispersed cells. Administration of short pulses of apomorphine (5 X 10(-5) M) induced a transient inhibition of corticosteroid secretion, and the kinetics of the response were very similar to that observed with dopamine. During prolonged administration of dopamine, the steroidogenic actions of ACTH (10(-9) M) and serotonin (5 X 10(-6) M) were not altered. In contrast, dopamine induced a marked inhibition of angiotensin II-evoked corticosteroid secretion. Taken together, these results show that the neurotransmitter dopamine exerts a direct inhibitory effect on steroid secretion from frog adrenocortical cells. Our results also indicate that dopamine and angiotensin II likely act through a common intracellular pathway. These data suggest that dopamine, released by chromaffin cells during neurogenic stress, may modulate the response of adrenocortical cells through a paracrine mode of communication.

Adrenal activity in the female lizard Lacerta vivipara jacquin associated with breeding activities

Variations of adrenal activity were studied in captive viviparous females Lacerta vivipara, in relation to breeding activities. The study was restricted to the period of active life which includes both the phase of annual reproduction and a phase of sexual inactivity. Significant seasonal changes in plasma corticosterone levels were measured with a peak during the second half of gestation followed by an abrupt fall at parturition. No significant variations in plasma aldosterone levels were observed. A limited extraovarian production of progesterone was detected which might be of adrenal origin. The half-life of injected tritiated corticosterone was not longer in pregnant than in nonreproductive females, suggesting that the peak of circulating corticosterone in pregnant females corresponds to an increase in the production rate of the hormone. The functional importance of the pituitary-adrenal axis was demonstrated in vivo: plasma corticosteroid levels dropped to the detection limit after adenohypophysectomy. Seasonal variations of adrenal sensitivity to synthetic ACTH 1-39 were examined in vitro, using a perifusion system. No significant variations were observed throughout the period of active life. These results suggest that the peak of plasma corticosterone during gestation can be ascribed to activation of the pituitary-adrenal axis. Experimental modifications of circulating corticosterone level during late gestation altered the timing of parturition, thus indicating that the fall of corticosterone just before term may be involved in the process of parturition in the female L. vivipara.

Localization and characterization of the N-terminal fragment of atrial natriuretic factor (ANF) precursor in the frog heart

The localization of the N-terminal fragment of the atrial natriuretic factor (ANF) precursor in the heart of the frog Rana ridibunda was examined by the indirect immunofluorescence and the immunogold techniques using an antiserum directed against synthetic rat ANF (Asp11-Ala37). At the optic level, positive material was found in most atrial myocytes. Staining of consecutive sections of frog heart with antibodies against N-terminal and C-terminal regions of the proANF molecule showed that both peptides are contained in the same cardiocytes. In the rat atrium, antibodies against the N-terminal ANF region induced a more intense labeling than in the frog atrium. Electron microscopic studies indicated that all secretory granules present in frog atrial cardiocytes contain N-terminal ANF-like immunoreactive material. The positive material localized in frog atrium was characterized by gel filtration and radioimmunological detection. Serial dilutions of frog atrial extracts exhibited displacement curves which were parallel to that obtained with synthetic human ANF (Asn1-Asp30). Sephadex G-50 gel chromatography of the immunoreactive material showed that the N-terminal ANF-like immunoreactivity eluted in a single peak corresponding to high molecular weight material. These results indicate that the N-terminal fragment of frog proANF is immunologically and biochemically related to the homologous mammalian peptide.

Effect of serotonin on alpha-melanocyte-stimulating hormone secretion from perifused frog neurointermediate lobe: evidence for the presence of serotonin-containing cells in the frog pars intermedia

We have examined the presence of 5-hydroxytryptamine (serotonin; 5-HT) in the intermediate lobe of the frog pituitary and investigated the effect of exogenous 5-HT on alpha-melanocyte-stimulating hormone (alpha-MSH) release from the perifused neurointermediate lobe (NIL). Using a specific antiserum against 5-HT, the indirect immunofluorescence technique revealed the presence of 5-HT-like immunoreactivity (5-HT-LI) in discrete cells, generally gathered in small clusters among parenchymal cells, and in numerous neurites surrounding melanotrophic cells. At the electron microscopic level, using a silver-gold intensification procedure, 5-HT-LI was localized in dense-core secretory vesicles within specific pituitary cells which appear to be different from pituitary melanotrophs. Dense accumulation of gold particles was also observed in nerve fibres running between parenchymal cells. A combination of high-performance liquid chromatography analysis and electrochemical detection showed the presence of both 5-HT and its metabolite 5-hydroxyindol acetic acid (5-HIAA) in frog NIL extracts (534 +/- 40 and 1245 +/- 65 (S.E.M.) pg/mg wet tissue respectively). Administration of graded doses of 5-HT (from 1 to 30 mumol/l) to perifused frog NIL induced a dose-dependent inhibition of alpha-MSH release. Repeated pulses of 5-HT (10 mumol/l each) induced a reproducible inhibition of alpha-MSH without any desensitization phenomena. The inhibitory effect of 5-HT was partially blocked by the serotonergic antagonists methysergide and ICS-205-930 (10 mumol/l each). Concomitant administration of methysergide and ICS-205-930 (10 mumol/l each) totally abolished 5-HT-evoked inhibition of alpha-MSH. Fenfluramine, a releaser of 5-HT, induced a slight but significant reduction of alpha-MSH secretion. While 5-HT caused a marked inhibition of alpha-MSH release from intact NIL, 5-HT was devoid of effect on acutely dispersed pars intermedia cells suggesting that 5-HT does not exert a direct action on pituitary melanotrophs. We have examined the effect of specific dopaminergic, GABAergic and alpha-adrenergic antagonists on 5-HT-induced alpha-MSH inhibition. We observed that sulpiride and SR 95531 (10 mumol/l each) did not affect the response of NIL to 5-HT while yohimbine (10 mumol/l) suppressed the inhibitory action of 5-HT. Taken together, our results indicate that discrete cells of the frog pars intermedia contain the neurotransmitter 5-HT which may act locally to inhibit alpha-MSH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of atrial natriuretic factor (ANF) binding sites in the central nervous system of the frog

The distribution of atrial natriuretic factor (ANF) binding sites was investigated in the central nervous system of the frog Rana ridibunda using the technique of in vitro receptor autoradiography by means of [125I]-labeled ANF-28. The anatomic distribution of ANF recognition sites was determined on Kodak ARX films apposed onto tissue sections, and their distribution was examined in greater detail by analysis of autoradiograms generated by using emulsion-coated sections. The highest levels of ANF binding sites were found in the olfactory bulb, the dorsal pallium, the septum, the habenular nucleus, the dorsal infundibular nucleus, the interpeduncular nucleus, and in the tectum. Moderate levels of ANF binding sites were observed in the thalamus and throughout the mesencephalon, whereas low levels were detected in the lateral and medial pallium, the medial forebrain bundle, and the nucleus rotondus. In the pituitary gland, the neural and distal lobes were densely loaded with ANF binding sites, whereas no autoradiographic labeling was observed in the pars intermedia. In general, there was a good correlation between the location of ANF receptors and the distribution of ANF-containing neurons, as previously determined by immunocytochemistry. Together these results support the view that ANF may act as a neurotransmitter or a neuromodulator in various regions of the frog brain.

Dopamine inhibits corticosteroid secretion in frog adrenocortical cells: evidence for the involvement of prostaglandins in the mechanism of action of dopamine

We have investigated the possible involvement of arachidonic acid metabolites in dopamine-induced inhibition of adrenocortical steroidogenesis. Administration of dopamine (5 x 10(-5) M) for 20 min to perifused frog adrenal slices caused a marked reduction of the release of both prostaglandin E2 (PGE2) and 6-keto-PGF1 alpha, the stable metabolite of prostacyclin (PGI2). Dopamine also induced a significant inhibition of corticosterone and aldosterone secretion. A lag period of 20 min was observed between inhibition of prostanoid and corticosteroid releases. Prolonged dopamine infusion did not prevent the stimulatory effect of PGE1, PGE2 or arachidonic acid on corticosteroid secretion. These observations indicate that activation of dopaminergic receptors in adrenocortical cells is linked to an inhibition of arachidonic acid metabolism. Our data also suggest that the inhibitory effect of dopamine occurs at a step preceding arachidonic acid formation.

Involvement of the cytoskeleton in the steroidogenic response of frog adrenal glands to angiotensin II, acetylcholine and serotonin

In order to determine the role of the cytoskeleton in adrenal steroidogenesis, we have studied the effect of cytochalasin B (a microfilament-disrupting agent) and vinblastine (an antimicrotubular drug) on corticosteroid secretion by frog interrenal tissue in vitro. Perifusion of interrenal fragments with cytochalasin B (50 mumol/l) induced a marked inhibition of basal corticosteroid output. In addition, stimulation of corticosteroidogenesis by all corticotrophic factors was also inhibited by cytochalasin B. Using an immunohistochemical technique and specific anti-tubulin antiserum, we verified that vinblastine (10 mumol/l) was responsible for the disappearance of the microtubular network in adrenocortical cells. Administration of vinblastine (10 mumol/l) did not affect the spontaneous secretion of corticosterone and aldosterone and had no effect on the steroidogenic response of interrenal glands to angiotensin II and acetylcholine. In contrast, vinblastine was responsible for a marked decrease in serotonin-induced stimulation of corticosteroid production. On the other hand, data from high-performance liquid chromatography showed that infusion of cytochalasin B or vinblastine was not associated with the production of any new steroid which could interfere in the radioimmunoassays. Taken together, these data suggest that microfilaments are involved in a late and common step of corticosteroidogenesis while microtubules are only required for the coupling of the secretory response to certain corticotrophic factors such as ACTH and serotonin.

Immunohistochemical and biochemical evidence for the presence of serotonin in amphibian adrenal chromaffin cells

The presence of serotonin (5-HT) in chromaffin cells of the frog adrenal (inter-renal) gland has been demonstrated both by immunocytochemical and biochemical techniques. Using antisera against 5-HT and tyrosine hydroxylase (TH) on consecutive sections, we found by means of the indirect immunofluorescence technique that a majority of chromaffin cells were also immunopositive for 5-HT. When antibodies to 5-HT and phenylethanolamine-N-methyltransferase (PNMT) were applied on consecutive sections, 5-HT-like immunoreactivity was observed in almost all epinephrine-producing cells which represented about 90% of the total chromaffin cells. No 5-HT-containing fibres could be detected. At the ultrastructural level, using a pre-embedding procedure associated with gold-silver intensification of the immunoperoxidase reaction, 5-HT-immunoreactivity was visualized in secretory vesicles essentially located in the periphery of epinephrine cells. Combination of high performance liquid chromatography and electrochemical detection showed the presence of both 5-HT and its metabolite 5-hydroxyindolacetic acid (5-HIAA) in frog adrenal extracts. Transection of the splanchnic nerve enhanced 5-HT immunoreactivity and augmented the amount of 5-HT in adrenal extracts. Taken together, these results indicate that epinephrine-producing cells of the frog adrenal contain significant amounts of serotonin. The observation of the storage of 5-HT in secretory vesicles of epinephrine cells suggests that serotonin may be released with catecholamines under stress conditions.

Effect of atrial natriuretic factor on corticosteroid production by perifused frog interrenal slices

In order to investigate a possible role of atrial natriuretic factor (ANF) in the control of corticosteroid biosynthesis in amphibians, we have examined the effect of synthetic ANF (Arg 101-Tyr 126) on perifused frog interrenal slices. ANF did not affect the spontaneous secretion of corticosterone and aldosterone. In contrast, ANF (10(-6) M) inhibited ACTH-and angiotensin II-stimulated corticosteroid production. ANF was more potent in suppressing aldosterone than corticosterone secretion. Immunocytochemical studies using a specific ANF antiserum revealed the presence of ANF-like immunoreactive fibers in the vicinity of interrenal cells. It is thus proposed that, in amphibians, both "hormonal" ANF secreted by myocytes and "neurohormonal" ANF delivered by peptidergic nerve terminals coursing among interrenal cells may partake in the regulation of corticosteroidogenesis.

Neuronal and paracrine regulation of adrenal steroidogenesis: interactions between acetylcholine, serotonin and vasoactive intestinal peptide (VIP) on corticosteroid production by frog interrenal tissue

The adrenocortical cells of frog interrenal (adrenal) tissue are controlled by multiple factors. Recently, we have shown that corticosteroidogenesis is stimulated by acetylcholine released from splanchnic nerve terminals as well as by serotonin and vasoactive intestinal peptide (VIP) which are both contained in chromaffin cells. Since these 3 putative neuroregulators are known to interact with each other on various target organs, we have investigated possible coordinate actions of acetylcholine, serotonin and VIP on adrenal steroid production, using a perifusion system technique for frog interrenal tissue. Simultaneous infusion of submaximal doses of VIP (10(-5) M) and acetylcholine (5 X 10(-5) M) induced stimulations of corticosteroids (corticosterone and aldosterone) which were strictly additive. When VIP (10(-5) M) and serotonin (5 X 10(-6) M) were infused together, a potentiation of the individual responses was observed. In contrast, concomitant infusion of acetylcholine (5 X 10(-5) M) and serotonin (5 X 10(-6) M) caused a total blockage of the stimulatory effect of serotonin. Muscarine (10(-5) M) caused a similar blockade of the response of adrenocortical cells to serotonin while nicotine (5 X 10(-5) M) did not alter the stimulatory effect of serotonin. The inhibitory effect of acetylcholine on serotonin-induced steroidogenesis was antagonized by atropine (10(-5) M). Thus, acetylcholine appears to block the corticotropic action of serotonin by interacting with typical muscarinic receptors. Taken together our results indicate that 3 of the neuroregulators which participate in the control of adrenal steroidogenesis, namely acetylcholine, serotonin and VIP, may interact on their target cell to modulate the activity of their congeners.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin stimulates corticosteroid secretion by frog adrenocortical tissue in vitro

The mode of action of serotonin (5-HT) in the regulation of frog adrenal steroidogenesis was studied in vitro using the perifusion system technique. Graded doses of 5-HT (from 10(-8) to 10(-6) M) increased both corticosterone and aldosterone production in a dose-dependent manner. Short pulses (20 min) of 10(-6) M 5-HT, administered at 130 min intervals within the same experiment, did not cause any desensitization phenomenon. Indomethacin (IDM; 5 microM), a cyclooxygenase inhibitor which induced a dramatic decrease in the spontaneous secretion of corticosteroids, did not impair the stimulatory effect of 5-HT on corticosterone and aldosterone production. In the absence of calcium, 5-HT (10(-6) M) was still able to stimulate corticosteroid production. Dantrolene (5 x 10(-5) M), a blocker of calcium mobilization from intracellular pools which significantly inhibited the spontaneous production of corticosteroids, did not suppress 5-HT-evoked corticosteroid secretion. These results show that 5-HT, stored in adrenal chromaffin cells, may act as a paracrine factor to stimulate adrenal steroidogenesis in the frog. Our data also indicate that the mechanism of action of 5-HT does not depend on prostaglandin biosynthesis.

Distribution of atrial natriuretic factor-like immunoreactivity in the brain of the frog Rana ridibunda

The localization of atrial natriuretic factor (ANF)-like immunoreactivity in the central nervous system of the frog Rana ridibunda was examined by the indirect immunofluorescence technique, using an antiserum against synthetic ANF (Arg101-Tyr126). Immunoreactive cell bodies were principally found in the dorsal and medial pallium, the medial septal nucleus, the ventrolateral and anteroventral areas of the thalamus, the lateral forebrain bundle, the posterolateral thalamic nuclei, the preoptic nucleus, the dorsal infundibular nucleus, and the anteroventral tegmentum nucleus of the mesencephalon. Numerous cell bodies and a very dense fiber bundle were visualized in the interpeduncular nucleus. All the areas mentioned above contained a high density of immunoreactive fibers. In addition, the amygdala, the infundibular nucleus, the median eminence, and most of the areas of the mesencephalon contained a moderate number of ANF-positive nerve processes. In the frog pituitary, fibers and nerve terminals were found in the peripheral zone of the neural lobe. The intermediate and anterior lobes of the frog pituitary were totally devoid of ANF immunoreactivity. These results indicate that ANF-like material is widely distributed in the frog brain and that ANF may be involved in various brain functions including neuroendocrine regulations.

Interrenal activity in the female lizard Lacerta vivipara J.: in vitro response to ACTH 1-39 and to [Sar1, Val5] angiotensin II (ANG II)

A perifusion system technique was developed in order to determine in vitro the respective roles of ACTH and ANG II in the regulation of adrenal steroidogenesis in the lizard Lacerta vivipara. Synthetic human ACTH 1-39, administered as 20-min pulses, stimulated corticosterone (B) and aldosterone (A) release in a dose-dependent manner. The increase in corticosterone output was higher than that in aldosterone output, leading to an enhancement of the B/A ratio. Iterative stimulations with 1 nM ACTH (20-min pulses every 120 min) led to reproducible increases in corticosterone and aldosterone release. Prolonged stimulation with 1 nM ACTH (up to 240 min) caused a sustained increase in corticosteroid release, suggesting that, in the lizard, ACTH does not induce any desensitization phenomenon. The angiotensin II analogue [Sar1, Val5] ANG II also stimulated corticosterone and aldosterone release in a dose-dependent manner; the stimulatory effects of ANG II on both steroids were very similar. These results indicate that, in lizards, ACTH plays a major role in the regulation of adrenal steroidogenesis. Since ANG II stimulates the production of gluco- and mineralocorticoids, our data raise the question of the existence of two cell types synthesizing corticosterone and aldosterone, respectively, in reptiles.

In vitro study of frog adrenal function--IX. Evidence against the involvement of lipoxygenase metabolites in the control of steroid production

The possible role of arachidonic acid metabolites of the lipoxygenase pathway in the regulation of steroidogenesis was studied in vitro using perifused frog interrenal (adrenal) glands. Graded doses of arachidonic acid (10(-6)-10(-4)M) increased the production of corticosterone and aldosterone in a dose-dependent manner. In the presence of indomethacin (5 X 10(-6)M), the effect of arachidonic acid on steroid secretion was totally abolished. Nordihydroguaiaretic acid (NDGA: 10(-6)M), a lipoxygenase inhibitor, did not alter the spontaneous secretion of corticosteroids and did not impair the stimulatory effect of arachidonic acid. In the presence of NDGA, both ACTH and angiotensin II were still able to stimulate corticosteroid production. Our data support the view that arachidonic acid metabolites play an important role in the regulation of amphibian steroidogenesis. Moreover, the results show that the lipoxygenase pathway is not involved in the spontaneous secretion of corticosteroids and in angiotensin II- or ACTH-induced steroidogenesis.

Localization and identification of immunoreactive atrial natriuretic factor (ANF) in the frog ventricle

The localization of ANF-like immunoreactivity in the ventricle of the frog Rana ridibunda was examined by the indirect immunofluorescence and the immunogold techniques, using an antiserum against synthetic ANF (Arg 101-Tyr 126). At the optic level, an appreciable number of positive cardiocytes was observed in the frog ventricle. Electron microscopic studies showed that all secretory granules present in ventricular cardiocytes contain immunoreactive ANF. The immunoreactive material has been characterized by Sephadex G-50 gel chromatography and reverse phase high performance liquid chromatography (RP-HPLC). After gel filtration, ANF-like immunoreactivity eluted in 3 peaks. The major immunoreactive peak corresponded to high molecular weight material, while one peak co-eluted with synthetic ANF (Arg 101-Tyr 126). Further analysis of frog ventricular extracts by RP-HPLC revealed that the low molecular weight material has the same retention time as synthetic ANF, suggesting a high degree of sequence homology between amphibian and mammalian ANF. These results indicate that in amphibians, ventricular cardiocytes synthesize a peptide immunologically and chemically related to mammalian ANF.

Effect of the intermediate filament inhibitor IDPN on steroid secretion by frog adrenal glands

In order to determine the role of intermediate filaments in adrenal steroidogenesis, we have studied the effect of IDPN (beta-beta'iminodipropionitrile), an intermediate filaments perturbing agent, on corticosteroid secretion by frog interrenal glands in vitro. A 6-h administration of IDPN (10(-3) M) did not affect the spontaneous release of corticosterone and aldosterone. While IDPN did not alter the response of adrenal fragments to ACTH, the drug caused a marked decrease in angiotensin II-induced stimulation of corticosterone and aldosterone production. These results indicate that, in contrast to microfilaments, which play an important role in spontaneous steroidogenesis, intermediate filaments are not required for basal corticosteroid secretion but are involved in the mechanism of action of angiotensin in frog adrenocortical cells.

[Role of microtubules in adrenal gland steroidogenesis]

In order to determine the role of the microtubular network in adrenal steroidogenesis, we have studied the effect of vinblastine, a potent antimicrotubular agent on corticosteroid secretion by frog adrenocortical fragments in perifusion. Administration of vinblastine (10(-5) M) did not alter the spontaneous secretion of corticosteroids and had no effect on the steroidogenic response to angiotensin II and prostaglandin E1. In contrast, vinblastine induced a marked decrease in ACTH-induced stimulation of corticosteroidogenesis. To investigate further the site of action of the microtubular system in ACTH-induced steroidogenesis we have studied the effect of vinblastine on the different steps of the stimulation of the adenylate cyclase system. Our results show that vinblastine does not alter the stimulatory action of cAMP, the second messenger of ACTH. In addition, the corticosteroidogenic effect of forskolin and NaF (which respectively stimulate the adenylate-cyclase subunit and the guanyl nucleotide regularly protein) was not affected by vinblastine. These results indicate that the microtubular network interferes in the coupling of the secretory response of the adrenal glands to ACTH, either at the level of the binding of ACTH to its receptor or in the coupling of the receptor to the guanyl nucleotide regulatory protein.

Atrial natriuretic factor-like immunoreactivity in the central nervous system of the frog

The distribution of atrial natriuretic factor-like immunoreactivity in the central nervous system of the frog Rana ridibunda was investigated by indirect immunofluorescence and the immunogold technique, using an antiserum generated in rabbits against synthetic atrial natriuretic factor (Arg 101-Tyr 126). A stereotaxic atlas of neurons containing atrial natriuretic factor-like material was prepared to show the widespread distribution of atrial natriuretic factor-positive cell bodies and fibres in the brain. Appreciable numbers of immunoreactive perikarya were observed in the dorsal and medial pallium, the medial septal nucleus, the anteroventral and ventrolateral areas of the thalamus, the lateral forebrain bundle, the posterocentral and posterolateral thalamic nuclei, the preoptic nucleus, the dorsal infundibular nucleus and the anteroventral tegmental nucleus of the mesencephalon. A heavy accumulation of atrial natriuretic factor-like immunoreactive cell bodies and very dense fibre bundles were noted in the interpeduncular nucleus of the mesencephalon. Fibres were generally seen where cell bodies were observed, particularly in all regions of the pallium and septum nuclei, in the ventral thalamus, the infundibular nucleus and the tegmental area. Moderate numbers of fibres were also noted in several regions where cell bodies were absent, mainly in the amygdala and the infundibular nucleus, the median eminence and most mesencephalic regions. At the electron microscopic level, the immunoreactivity was restricted to dense core vesicles and absent in clear vesicles. These results provide the first evidence for the presence of atrial natriuretic factor in the brain of a non-mammalian chordate. The localization of atrial natriuretic factor-positive material in the frog central nervous system suggests that this peptide may act as a neuromodulator or a neurotransmitter in amphibians.

Acetylcholine stimulates steroidogenesis in isolated frog adrenal gland through muscarinic receptors: evidence for a desensitization mechanism

The effect of cholinergic agonists on glucocorticoid and mineralocorticoid production by frog interrenal (adrenal) tissue was studied in vitro by means of continuous perifusion. Acetylcholine, at doses ranging from 1 to 100 mumol/l, stimulated both corticosterone and aldosterone output in a dose-dependent manner, with a half-maximal effective dose of 2.5 mumol/l. Corticosteroid production was also stimulated by muscarine (10 mumol/l). In contrast, neither nicotine nor nicotine bitartrate (1-100 mumol/l) enhanced corticosteroid biosynthesis. The kinetics of the response of adrenal cells to acetylcholine and muscarine were similar to those observed during angiotensin II stimulation. In particular, a significant reduction (20-40%) in the spontaneous level of corticosteroid production was recorded after the initial infusion of muscarinic agents, but no further decrease in the basal level occurred after a second cholinergic administration. The effect of acetylcholine was blocked by the muscarinic receptor antagonist atropine (10 mumol/l). These results indicate that acetylcholine can stimulate frog adrenocortical cells through muscarinic receptors. Repeated 20-min pulses of acetylcholine (50 mumol/l) or muscarine (10 mumol/l), given at one pulse per 130 min, resulted in a marked reduction in the secretory response to the second pulse. No reduction in the stimulatory effect of acetylcholine or muscarine was observed when a 6.5-h interval separated two 20-min infusions of the secretagogue. In contrast with these findings, iterative pulses of the muscarinic agonist pilocarpine (in the range 1-100 mumol/l) did not cause any desensitization. These data show that the neurotransmitter acetylcholine can modulate frog adrenocortical function and suggest that, in addition to more conventional regulators, i.e. ACTH and angiotensin II, the cholinergic endings of the splanchnic nerve might participate in the regulation of corticosteroid secretion, at least under some physiological conditions such as neurogenic stress.

Co-distribution of neuropeptide Y and its C-terminal flanking peptide in the brain and pituitary of the frog Rana ridibunda

By means of the peroxidase-anti-peroxidase technique, the distribution of neuropeptide tyrosine (NPY) and its C-terminal flanking peptide (C-PON) has been studied on serial sections of the brain and pituitary of the frog Rana ridibunda. Throughout the brain, NPY and C-PON-immunoreactive perikarya exhibited a remarkable co-distribution. These two peptides were found to be co-located within the same cell bodies in various brain regions including the dorsal and ventral pallium, the dorsal and ventral infundibular nuclei and the preoptic nucleus. The distribution of NPY- and C-PON-containing fibers in the brain and pituitary was similar. Sequential double immunohistochemical staining using the indirect immunofluorescence method, showed that NPY and C-PON were actually located within the same nerve processes throughout the frog brain and in the intermediate lobe of the pituitary. These studies indicate that the deduced C-PON sequence is present within the frog precursor to NPY and is formed in vivo in the frog brain. Like NPY, C-PON is transported distally in nerve terminals and is likely released with NPY in various regions of the brain and in the intermediate lobe of the pituitary.

Localization of atrial natriuretic factor (ANF)-immunoreactive material in the hypothalamo-pituitary complex of the frog

The presence of atrial natriuretic factor (ANF)-like material in the hypothalamo-pituitary complex of the frog (Rana ridibunda) was studied by means of indirect immunofluorescence and immunogold techniques. In the hypothalamus, numerous ANF-positive cell bodies were located in the preoptic nucleus, the lateral forebrain bundle and the dorsal infundibular nucleus. The ventral infundibular area and the median eminence contained only ANF-immunoreactive fibers. A dense network of fibers and nerve terminals was observed in the neural lobe of the pituitary. Scarce nerve endings were also noted in the intermediate lobe whereas ANF-like immunoreactivity was totally absent in the anterior lobe of the pituitary. At the ultrastructural level, positive material appeared restricted to dense core vesicles (about 100 nm in diameter). These results demonstrate that an ANF-like substance is contained in nerve terminals of the frog pars nervosa, and support the hypothesis that ANF may exert a physiological role in the control of release of neurohypophyseal peptides.

Neuropeptide Y in the intermediate lobe of the frog pituitary acts as an alpha-MSH-release inhibiting factor

The presence of neuropeptide tyrosine (NPY) in the intermediate lobe of the frog pituitary was demonstrated using indirect immunofluorescence, the immunogold technique and a specific radioimmunoassay combined with high pressure liquid chromatography (HPLC). A high density of NPY-containing fibers, was found among the parenchymal cells of the intermediate lobe. These fibers originated from the ventral infundibular nucleus, travelled via the median eminence to the pars intermedia. At the electron microscopic level, NPY-like material was found exclusively in nerve fibers where the product of the immunoreaction was associated to dense-core vesicles. High concentrations of NPY-like peptide were found in neurointermediate lobe extracts. After Sephadex G-50 gel filtration the major peak of immunoreactive material appeared to co-elute with synthetic porcine NPY. Conversely, HPLC analysis revealed that the NPY-like peptide of the frog pituitary had a retention time shorter than the porcine NPY. The localization of NPY-like material in the pars intermedia suggested a possible role of NPY in the regulation of melanotropic cell secretion. In fact, graded concentrations of synthetic NPY induced a dose-dependent inhibition of alpha-melanotropin (alpha-MSH) release in vitro. The lack of effect of a dopaminergic antagonist on NPY-induced alpha-MSH release inhibition demonstrated that the local dopaminergic system could not account for the NPY action. These results indicate that NPY located in the hypothalamo-hypophyseal system of the frog may act as a melanotropin-release inhibiting factor.

Involvement of cycloheximide-sensitive mediators in the steroidogenic action of adrenocorticotropin and angiotensin II

The involvement of short-lived proteins in the steroidogenic action of corticotropic peptides has been investigated in vitro by means of a perifusion technique using frog adrenal glands. Graded concentrations of cycloheximide (10(-7) M to 10(-5) M) led to a dose-related inhibition of corticosterone and aldosterone production. The perifusion model gives detailed information on the kinetics of the inhibitory effect of cycloheximide. This effect was rapidly observed (the lag period was about 15 min), maximum inhibition being obtained 25 min after the end of administration of the protein synthesis inhibitor. Whatever the concentration of cycloheximide, corticosteroid output returned to basal values 2 h after the onset of cycloheximide infusion. Stimulation of steroidogenesis by ACTH and angiotensin II was totally inhibited by cycloheximide (10(-6) M) indicating that the synthesis of a labile protein was required for the adrenal response to corticotropic peptides. In addition, the stimulatory effect of cAMP and PGE1, which are considered to be the second messengers of ACTH and angiotensin II in amphibian interrenal gland, was blocked by cycloheximide. Taken together, these data suggest that a labile protein is involved in an early step of corticosteroid biosynthesis in the frog.

Role of prostaglandins in calcium-induced corticosteroid secretion by isolated frog interrenal gland

The role of prostaglandins (PGs) in calcium-induced corticosteroid secretion by frog adrenal (interrenal) gland has been examined in vitro using a perifusion technique. Increasing concentrations of CaCl2 (4-10 mM) stimulated in a dose-dependent manner aldosterone, PGE2 and 6-keto-PGF1 alpha production, whereas TXB2 was not affected. The kinetics of the adrenal response to CaCl2 indicated that the increase in PG output always preceded that of steroid. Administration of cobalt (4 mM), a calcium-channel inhibitor, blocked the calcium-induced stimulation of PGs and corticosteroids. Infusion of indomethacin (5 X 10(-6) M), a specific cyclooxygenase inhibitor, significantly decreased the basal production of PGs and steroids, and prevented the stimulatory effect of CaCl2 (6 mM). Infusion of the calcium ionophore A 23187 (10(-6) M), for 20 min, induced a marked stimulation of PG and steroid production. Taken together, these data support the notion that biosynthesis of prostaglandins is associated with calcium-induced corticosteroid secretion in frog adrenal cells.

Involvement of prostaglandins in the response of frog adrenocortical cells to muscarinic receptor activation

The role of prostaglandins (PGs) in the mechanism of action of acetylcholine (ACh) on frog adrenocortical cells has been examined. Administration of a single dose of ACh (5 X 10(-5) M) to perifused frog interrenal fragments, for 20 min, stimulated the production of corticosterone, aldosterone, PGE2 and 6-keto-PGF1 alpha. In contrast ACh did not significantly alter TXB2 production. The effect of ACh could be mimicked by muscarine (10(-5) M). Conversely, nicotine (10(-6) to 10(-4) M) was totally inactive. The increase in PG biosynthesis preceded the peak of corticosteroid release. Repeated 20-min pulses of ACh (5 X 10(-5) M) or muscarine (10(-5) M) given at 130-min intervals induced a desensitization phenomenon. In presence of indomethacin (5 X 10(-6) M), the effect of ACh on PG and steroid secretion was totally abolished. In calcium-free medium, the effect of ACh on PG and corticosteroid production was completely blocked. These results indicate that, in the frog, ACh stimulates corticosteroid secretion through a PG-dependent mechanism.

Localization and characterization of atrial natriuretic factor (ANF)-like peptide in the frog atrium

The distribution of ANF was studied in the heart of the frog (Rana ridibunda) using indirect immunofluorescence. ANF-like immunoreactivity was localized mainly in the right and left atrium, most of cardiocytes being intensively labelled. At the electron microscopic level, all secretory granules present in atrial cardiocytes contained ANF immunoreactive material. Using a specific radioimmunoassay, we found higher concentrations of ANF in the left atrium (208 +/- 25 ng/mg protein) than in the right atrium (120 +/- 16 ng/mg protein) whilst in the rat, the right atrium contains the highest ANF concentration. The concentration of ANF in the ventricle was 10 times lower than in the whole atrium (32 +/- 4 ng/mg protein). Sephadex G-50 gel filtration of atrial extracts showed that ANF-like immunoreactivity eluted in three peaks. Most of the immunoreactivity corresponded to high molecular weight material eluting at the void volume while 20% of the material co-eluted with synthetic (Arg 101-Tyr 126) ANF. These results indicate that frog cardiocytes synthetize a peptide which is immunologically and biochemically related to mammalian ANF.

Role of calcium in stimulus-secretion coupling on isolated frog interrenal gland

The influence of extracellular calcium concentration on the steroidogenic response to ACTH and to the angiotensin II analogue [Sar1-Val5]AII has been studied in the frog, using a perfusion system technique. The release of corticosterone and aldosterone in the effluent medium was measured by specific radioimmunoassays. In calcium-free medium the stimulatory effect of ACTH (10(-9) M) was completely abolished whereas the response to dbcAMP (5 mM) was unchanged indicating that the role of calcium takes place before the formation of cAMP. Conversely, in the absence of calcium, angiotensin II (10(-7) M) was still able to stimulate corticosterone and aldosterone production. Addition of Co2+ (4 mM), a calcium antagonist, to the perfusion medium, inhibited partially the response of adrenal tissue to ACTH, dbcAMP and angiotensin. The voltage-dependent calcium channel blocker verapamil (10(-6) induced a dose-related inhibition of the corticotropic effect of ACTH. At the higher dose (10(-4) M), verapamil totally inhibited the stimulation of corticosterone and aldosterone production induced by ACTH. By contrast, at the same dose it did not alter the stimulatory effect of forskolin (2.4 X 10(-7)M) on corticosterone output, but significantly diminished forskolin-induced aldosterone response. Similarly, angiotensin-stimulated corticosterone production was slightly inhibited by 10(-4) M verapamil, whereas aldosterone response to angiotensin was totally abolished, indicating that verapamil may act intracellularly to block the conversion of corticosterone to aldosterone. Taken together, these results indicate that, in amphibians extracellular calcium is essential for the action of ACTH, either for the binding of the hormone to its receptor and/or for the transduction of the information from hormone-receptor complex to the adenylate cyclase moiety and that the mechanism of action of angiotensin does not involve calcium uptake by adrenocortical cells.