Toxicity induced by cumene hydroperoxide in PC12 cells: protective role of thiol donors

Oxidative shock and production of reactive oxygen species are known to play a major role in situations leading to neuron degeneration, but the precise mechanisms responsible for cell degeneration remain uncertain. In the present article, we have studied in PC 12 cells the effect of cumene hydroxyperoxide on both cell metabolism and morphology. We observed that relatively low concentrations of the drug (100 μM) led to a significant decrease in the cellular content of ATP and reduced glutathione as well as to a decreased mitochondrial potential. These metabolic alterations were followed by an important increase in intracellular free calcium and membrane disruption and death. In parallel, we observed profound changes in cell morphology with a shortening of cell extensions, the formation of ruffles and blebs at the cell surface, and a progressive detachment of the cells from the surface of the culture flasks. We also showed that addition of thiol donors such as N-acetylcysteine or β-mercaptoethanol, which were able to enhance cell glutathione content, almost completely protected PC 12 cells from the toxic action of cumene hydroperoxide whereas pretreatment by buthionine sulfoximine, a selective inhibitor of GSH synthesis, enhanced its action.

The endozepine triakontatetraneuropeptide diazepam-binding inhibitor [17-50] stimulates neurosteroid biosynthesis in the frog hypothalamus

Neurons and glial cells are capable of synthesizing various bioactive steroids, but the neuronal mechanisms controlling neurosteroid-secreting cells are poorly understood. In the present study, we have investigated the possible effect of an endogenous ligand of benzodiazepine receptors, the triakontatetraneuropeptide [17-50] (TTN), on steroid biosynthesis in the frog hypothalamus. Immunohistochemical studies revealed that most hypothalamic neurons expressing 3 beta-hydroxysteroid dehydrogenase/delta 5-delta 4-isomerase also contained peripheral-type benzodiazepine receptor-like immunoreactivity. Confocal laser scanning microscopic analysis revealed that the peripheral-type benzodiazepine receptor-immunoreactive material was located both in the cytoplasm and at the periphery of the cell bodies. By using the pulse-chase technique, TTN was found to stimulate the conversion of [3H]pregnenolone into various steroids, including 17-hydroxypregnenolone, 5 alpha-dihydrotestosterone and 17-hydroxyprogesterone, in a dose-dependent manner. The peripheral-type benzodiazepine receptor agonist Ro5-4864 mimicked the stimulatory effect of TTN on the formation of neurosteroids. The peripheral-type benzodiazepine receptor antagonist PK11195 significantly reduced the effect of TTN on neurosteroid synthesis, while the central-type benzodiazepine receptor antagonist flumazenil did not affect the formation of neurosteroids evoked by TTN. These data indicate that TTN stimulates the biosynthesis of 3-keto-17 alpha-hydroxysteroids in frog hypothalamic neurons through activation of peripheral-type benzodiazepine receptors likely located at the plasma membrane level.

Effect of the triakontatetraneuropeptide (TTN) on corticosteroid secretion by the frog adrenal gland

Diazepam-binding inhibitor (DBI) was initially isolated from the rat brain as a result of its ability to compete with benzodiazepines for their receptors. Immunohistochemical studies have recently shown the presence of peripheral-type benzodiazepine receptor (PBR)- and DBI-like immunoreactivity in the frog adrenal gland. The aim of the present study was to investigate the effect of two biologically active DBI-derived peptides, the triakontatetraneuropeptide [TTN; DBI(17-50)] and the octadecaneuropeptide [ODN; DBI(33-50)], on corticosteroid secretion by frog adrenocortical cells. Exposure of frog adrenal explants to graded concentrations of TTN (3.16 x 10(-8) to 3.16 x 10(-6) M) induced a dose-related increase in corticosterone and aldosterone secretion. In contrast, ODN did not modify corticosteroid output. When repeated pulses of TTN (10(-6) M) were administered at 2-h intervals, the response of the adrenal explants to the second dose of TTN was markedly reduced, suggesting the existence of a desensitization phenomenon. Exposure of dispersed adrenal cells to TTN also induced a marked stimulation of corticosteroid secretion, indicating that TTN acts directly on adrenocortical cells. The central-type benzodiazepine receptor (CBR) agonist, clonazepam, did not stimulate corticosteroid secretion and the CBR antagonist, flumazenil, did not block the stimulatory action of TTN. Similarly, the PBR agonist, Ro5-4864, did not mimic the stimulatory effect of TTN and the PBR antagonist, flunitrazepam, did not affect the stimulatory action of TTN. The present study provides the first evidence for a stimulatory effect of TTN on intact adrenocortical cells. The receptor mediating the corticotropic action of TTN is not related to central- or peripheral-type benzodiazepine receptors. Our data suggest that TTN, released by chromaffin cells, may act as a paracrine factor regulating the activity of adrenocortical cells.

Characterization of pars intermedia connections in amphibians by biocytin tract tracing and immunofluorescence aided by confocal microscopy

Biocytin, recently introduced in neuroanatomical studies, was used as a retrograde tract tracer in combination with immunofluorescence in order to analyse the neurochemical characters of some central neuronal projections to the pars intermedia in two amphibian species, the anuran Rana esculenta and the urodele Triturus carnifex. After biocytin insertions in the pars intermedia, neurons became retrogradely labelled in the suprachiasmatic hypothalamus and the locus coeruleus of the brainstem in both species. Some scattered biocytin-labelled neurons were observed in the preoptic area. Moreover, working on the same sections, immunofluorescence revealed a number of codistributions and, in some cases, colocalization in the same neurons of biocytin labellings and immunopositivity for (1) tyrosine hydroxylase in the suprachiasmatic hypothalamus and the locus coeruleus of Rana and Triturus, (2) gamma-aminobutyric acid in the suprachiasmatic hypothalamus of Rana and Triturus and (3) neuropeptide Y in the suprachiasmatic hypothalamus of Rana. The specificity of such colocalizations was fully confirmed using dual-channel confocal laser scanning microscopy analysis.

In vivo and in vitro evidence for the biosynthesis of testosterone in the telencephalon of the female frog

Neurons and glial cells are capable of synthesizing various steroid hormones, but biosynthesis of testosterone in the CNS has never been reported. The aim of the present study was to demonstrate the synthesis of testosterone in the frog brain. The presence of 17beta-hydroxysteroid dehydrogenase (17beta-HSD)-like immunoreactivity was detected in a population of glial cells located in the telencephalon. Reversed-phase HPLC analysis of brain tissue extracts combined with radioimmunoassay detection revealed the presence of substantial amounts of testosterone and 5alpha-dihydrotestosterone (5alpha-DHT) in the telencephalon where 17beta-HSD-positive cells were visualized. In male frogs, castration totally suppressed testosterone and 5alpha-DHT in the blood and in the rhombencephalon but did not affect the concentration of these two steroids in the telencephalon. Chemical characterization of testosterone in female frog telencephalon extracts was performed by coupling HPLC analysis with gas chromatography-mass spectrometry. Using the pulse-chase technique with [3H]pregnenolone as a precursor, the formation of a series of metabolites was observed, including dehydroepiandrosterone, androstenedione, testosterone, 5alpha-DHT, and estradiol. These data demonstrate the existence of an active form of 17beta-HSD in the frog telencephalon, which is likely involved in testosterone biosynthesis within the brain.

Localization of diazepam-binding inhibitor-related peptides and peripheral type benzodiazepine receptors in the frog adrenal gland

The adrenal gland of mammals contains high concentrations of peripheral-type benzodiazepine receptors (PBR) and diazepam-binding inhibitor (DBI), a polypeptide which acts as an endogenous ligand for PBR. The aim of the present study was to investigate the localization of DBI and PBR in the adrenal gland of the frog Rana ridibunda. Reverse transcription followed by polymerase chain reaction with specific primers for the frog DBI cDNA showed the presence of DBI mRNA in frog adrenal gland extracts. The cellular distribution of DBI and PBR was investigated using an antiserum against the octadecaneuropeptide DBI [33-50] (ODN) and antibodies against the 18-kDa isoquinoline binding protein subunit of PBR (IBP), respectively. ODN-like immunoreactivity was found in chromaffin cells and in Stilling s cells, but not in adrenocortical cells. IBP-like immunoreactivity was observed in chromaffin cells, in Stilling s cells and in a small proportion (11%) of steroid-secreting cells. The ODN- and IBP-immunoreactive materials were homogeneously distributed in the cytoplasm of chromaffin cells and concentrated at the periphery of large cytoplasmic vesicles in Stilling s cells. The proportion of ODN-positive Stilling s cells showed marked circannual variations with a maximum in July. Similarly, the proportion of IBP-positive Stilling s cells was 17 times higher in July than in December. These results indicate that, in the frog adrenal gland, DBI-related peptides and PBR are simultaneously expressed in chromaffin cells and Stilling s cells, suggesting that endogenous ligands for PBR may play a physiological role in the control of adrenal cell activity.

Localization of 17beta-hydroxysteroid dehydrogenase and characterization of testosterone in the brain of the male frog

Several enzymes involved in the formation of steroids of the pregnene and pregnane series have been identified in the brain, but the biosynthesis of testosterone has never been reported in the central nervous system. In the present study, we have investigated the distribution and bioactivity of 17beta-hydroxysteroid dehydrogenase (17beta-HSD) (EC 1.1.1.62; a key enzyme that is required for the formation of testosterone and estradiol) in the brain of the male frog Rana ridibunda. By using an antiserum against human type I placental 17beta-HSD, immunoreactivity was localized in a discrete group of ependymal glial cells bordering the telencephalic ventricles. HPLC analysis of telencephalon and hypothalamus extracts combined with testosterone radioimmunoassay revealed the existence of two peaks coeluting with testosterone and 5alpha-dihydrotestosterone. After HPLC purification, testosterone was identified by gas chromatography/mass spectrometry. Incubation of telencephalon slices with [3H]pregnenolone resulted in the formation of metabolites which coeluted with progesterone, 17alpha-hydroxyprogesterone, dehydroepiandrosterone, androstenedione, testosterone, and 5alpha-dihydrotestosterone. The newly synthesized steroid comigrating with testosterone was selectively immunodetected by using testosterone antibodies. These data indicate that 17beta-HSD is expressed in a subpopulation of gliocytes in the frog telencephalon and that telencephalic cells are capable of synthesizing various androgens, including dehydroepiandrosterone, androstenedione, testosterone, and 5alpha-dihydrotestosterone.

Confocal microscopy analysis of NPY and TH immunoreactivities in the hypothalamo-hypophysial system of the frog

Co-expression of tyrosine hydroxylase (TH) and neuropeptide Y (NPY) in local circuits innervating the hypothalamo-pituitary complex of the green frog, Rana ridibunda, was investigated using simultaneous double immunohistochemical technique, aided by dual-channel confocal laser scanning microscopy. NPY and TH immunoreactivities were observed co-occurring within a discrete neuronal population located in the suprachiasmatic region. In other hypothalamic areas, NPY-immunoreactive (IR) perikarya were generally codistributed, but distinct from TH-IR cells. In the adenohypophysial pars intermedia, the overlap between the two markers was partial, demonstrating the existence of multiple neuronal sources for the inputs to the gland.

Distribution of FMRFamide-like immunoreactivity in the brain of the lungfish Protopterus annectens

The distribution of FMRFamide-like immunoreactive peptides was studied in the brain of the African lungfish, Protopterus annectens, using the indirect immunofluorescence technique. The main populations of FMRFamide-positive cell bodies were detected in the forebrain and in the mesencephalic tegmentum. In the telencephalon, only a small number of FMRFamide-immunoreactive neurons was localized at the level of the subpallium, in the nucleus septi medialis. The diencephalon contained two prominent groups of FMRFamide-positive cell bodies located in the preoptic and periventricular preoptic nuclei. The thalamus exhibited only scattered FMRFamide-immunoreactive perikarya in its ventral part. In the mesencephalon, a group of positive cell bodies was identified in the caudal region of the tegmentum. A strong immunoreaction was also detected in the nervus terminalis. In the pituitary, most of the cells of the intermediate lobe were brightly stained. FMRFamide-like immunoreactive fibers and nerve terminals were widely distributed in the brain. In the telecephalon, numerous fibers were observed in several regions of the pallium and subpallium. A dense plexus of fibers was found in the hypothalamus and the thalamus. Immunoreactive fibers were seen coursing along the ventral wall of the infundibular cavity and terminating in the pars nervosa of the pituitary. The tectum and the ventral mesencephalon were also densely innervated. In contrast, the caudal brainstem only showed scarce immunoreactive processes. Coexistence of FMRFamide- and neuropeptide Y-like immunoreactivity was observed in the preoptic nucleus and in the nervus terminalis. The widespread distribution of FMRFamide-immunoreactive neurons in the brain and pituitary of P. annectens suggests that the peptide may exert both neuromodulator and neuroendocrine functions. The similarity between the distribution patterns of FMRFamide and neuropeptide Y in the brain of lungfish and amphibians supports the concept of a close phylogenetic link between these two groups.

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.

Pituitary adenylate cyclase-activating polypeptide stimulates both adrenocortical cells and chromaffin cells in the frog adrenal gland

In a previous report, we have shown that frog pituitary adenylate cyclase-activating polypeptide (fPACAP38) is a potent stimulator of corticosteroid secretion by frog adrenal slices in vitro. The aim of the present study was to determine the mode of action of PACAP on the frog adrenal gland. Immunoelectron microscopic labeling revealed that PACAP-like immunoreactivity is present in electron-dense vesicles within nerve endings located in the vicinity of both adrenocortical and chromaffin cells. Exposure of dispersed adrenal cells to fPACAP38 caused stimulation of corticosteroid secretion. Labeling of cultured adrenal cells with [125I]PACAP27 revealed the existence of PACAP-binding sites on both adrenocortical and chromaffin cells. Saturation and competition experiments showed the occurrence of high affinity and selective receptors for fPACAP38 on cultured adrenal cells. fPACAP38 (10(-8)-10(-5) M) provoked a dose-dependent stimulation of cAMP production by frog adrenal slices. Microflurimetric studies demonstrated that fPACAP38 induced a substantial elevation of the intracellular calcium concentration in both adrenocortical and chromaffin cells. The present results indicate that in the frog adrenal gland, PACAP fibers innervate both adrenocortical and chromaffin cells. The data show the presence of PACAP receptors on the two cell types. PACAP exerts a direct stimulatory effect on corticosteroid-producing cells. This effect is probably mediated through stimulation of adenylyl cyclase activity and/or augmentation of intracellular Ca2+. PACAP also increases intracellular Ca2+ in chromaffin cells. These data suggest that PACAP, released locally in the adrenal gland, acts as a neuroendocrine factor, regulating the activity of adrenocortical and chromaffin cells.

Visualization of gamma-aminobutyric acid A receptors on proopiomelanocortin-producing neurons in the rat hypothalamus

It has recently been shown that gamma-aminobutyric acid (GABA) and central-type benzodiazepine receptor agonists inhibit the expression of the POMC gene and the release of POMC-derived peptides from hypothalamic neurons. To determine whether the inhibitory effect of GABA could be accounted for by a direct action on POMC neurons, we investigated the localization of the beta 1-subunit of the GABAA-benzodiazepine-receptor complex in the arcuate nucleus. Using a monoclonal antibody raised against a synthetic fragment of the beta 1-subunit, we demonstrate the presence of GABAA receptor on POMC neurons. The proportion of POMC neurons that exhibit immunoreactivity for the beta 1-subunit of the GABAA receptor was not significantly different in the posterior portion (73.0-76.0%) and anterior portion (61.3-62.7%) of the arcuate nucleus. The data also revealed that in the arcuate nucleus, a majority of neurons that were immunostained by the antibody to the beta 1-subunit were not POMC positive. The present results support the concept that GABAA and central-type benzodiazepine receptor agonists exert a direct inhibitory action on POMC neurons. The data also indicate the existence of subsets of POMC neurons within the arcuate nucleus.

Involvement of the cytoskeleton in the mechanism of action of endothelin on frog adrenocortical cells

In a previous report, we have shown that endothelin-1 (ET-1) is a potent stimulator of corticosterone and aldosterone secretion by frog adrenocortical cells. In the present study, we examine the possible involvement of cytoskeletal elements in the mechanism of action of ET-1 on corticosteroid secretion from frog adrenal gland. The microfilament disrupting agent cytochalasin B (5 x 10(-5) M) induced a reversible inhibition of the spontaneous secretion of corticosteroid and blocked the response of adrenocortical cells to ET-1 (5 x 10(-9) M). In contrast, the antimicrotubular agent vinblastine (10(-5) M) and the intermediate filament inhibitor beta-beta' iminodipropionitrile (10(-3) M) had virtually no effect on both spontaneous and endothelin-induced steroidogenesis. Taken together, these results indicate that, in the frog adrenal gland, the integrity of the microfilament network is required for the corticotropic activity of ET-1 whereas microtubules and intermediate filaments are apparently not involved in the mechanism of action of ET-1.

In vitro study of the effect of urotensin II on corticosteroid secretion in the frog Rana ridibunda

Urotensin II is a cyclic dodecapeptide that was originally isolated from the fish urophysis, the terminus of a neurosecretory system located in the caudal area of the spinal cord. We have recently isolated and characterized urotensin II in the brain of a tetrapod, the frog Rana ridibunda. Recent reports, suggesting that urotensin II may stimulate cortisol secretion in fish, prompted us to investigate the possible effects of fish and frog urotensin II on corticosteroid secretion in amphibians. Exposure of perifused frog adrenal slices to goby (Gillichthys mirabilis) urophysis extracts induced a marked stimulation of corticosterone and aldosterone secretion. In contrast, at concentrations ranging from 10(-10) to 10(-6) M, synthetic goby urotensin II had no effect on corticosteroid production. Similarly, infusion of synthetic frog urotensin II (10(-10) to 10(-6) M) did not modify the spontaneous release of corticosterone and aldosterone. In addition, frog urotensin II had no effect on ACTH- and angiotensin II-induced secretion of corticosteroids. These results show that in frog, urotensin II does not modulate spontaneous and ACTH- or angiotensin II-evoked adrenal steroidogenesis.

Effects of selective disruption of cytoskeletal elements on steroid secretion by human adrenocortical slices

The role of the cytoskeleton in corticosteroid secretion in normal human adrenal gland was investigated in vitro, using the perifusion technique and confocal laser scanning microscopy. Vinblastine, which selectively disrupted microtubules in adrenocortical cells, did not modify the basal release of cortisol but induced a 58% inhibition of the response to adrenocorticotropic hormone (ACTH). In contrast, vinblastine did not alter dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP)-induced steroid secretion. Cytochalasin B treatment caused disappearance of microfilaments and blocked the stimulatory action of ACTH and DBcAMP on cortisol secretion. beta,beta'-Iminodipropionitrile disorganized the cytokeratin intermediate filaments but did not alter spontaneous and ACTH-evoked cortisol release. These results, which provide the first evidence for an action of cytoskeleton inhibitors on cortisol release from normal human adrenocortical cells, show that microtubules are involved in the mechanism of action of ACTH at a step preceding adenosine 3',5'-cyclic monophosphate formation, whereas microfilaments are involved in a late and common step of adrenal steroidogenesis.

Distribution of FMRFamide-like immunoreactivity in the brain of the lizard Podarcis sicula

The distribution of FMRFamide-like immunoreactive peptides was investigated in the brain of the lizard, Podarcis sicula, using the indirect immunofluorescence technique. The main populations of FMRFamide-immunoreactive cell bodies were located in the forebrain. In the telencephalon, FMRFamide-containing neurons were found both in the pallium and subpallium, namely in the medial cortex, the anterior olfactory nucleus, the nucleus accumbens, the septal nuclei, the nucleus of the medial forebrain bundle, and the nucleus of the diagonal band of Broca. In the diencephalon, a dense accumulation of FMRFamide-immunoreactive neurons was observed in the area preoptica lateralis, the nucleus suprachiasmaticus, the nucleus periventricularis hypothalami, the area lateralis hypothalami, and the dorsal region of the nucleus geniculatus lateralis. In the midbrain, sparse immunoreactive perikarya were found in the tegmentum of the mesencephalon. FMRFamide-immunoreactive fibers were visualized in all regions containing positive cell bodies. In particular, dense bundles of immunoreactive processes were seen in the area preoptica lateralis, in the hypothalamus, and in the median eminence. The tectum and the basal mesencephalon were also densely innervated. Conversely, the caudal brain stem only exhibited scarce immunoreactive processes. The distribution pattern of FMRFamide-immunoreactive neurons in the brain of Podarcis sicula exhibits a number of similarities with that reported in mammals, but significantly differs from that reported in amphibians and fish, suggesting that the neuromodulatory functions of FMRFamide may have diverged during the emergence of terrestrial life.

Effect of cytochalasin-B on the metabolism of polyphosphoinositides in andrenocortical cells

We have previously shown that microfilament-disrupting agents inhibit steroid secretion by frog adrenocortical cells. To determine the role of microfilaments in the process of corticosteroid production, we studied the effects of cytochalasin-B and chaetoglobosin-C on polyphosphoinositide metabolism in myo-[3H]inositol-prelabeled frog interrenal (adrenal) slices. Immunocytochemical labeling of adrenocortical cells in primary culture with actin antiserum showed that cytochalasin-B (5 x 10(-5) M) induced a complete and reversible disruption of microfilaments, whereas chaetoglobosin-C, a cytochalasin analog that cannot interact with actin, did not modify the organization of the microfilament network. Cytochalasin-B caused a dramatic inhibition of corticosteroid release from perifused frog interrenal slices, whereas chaetoglobosin-C did not affect steroid secretion. Analysis of labeled inositol phosphates and phosphoinositides revealed that cytochalasin-B, but not chaetoglobosin-C, caused a significant increase in tritiated inositol content (+38%) and concurrently inhibited the formation of polyphosphoinositides (-48%). Cytochalasin-B reduced the production of phosphatidylinositol (-63%), phosphatidylinositol monophosphate (-46%), phosphatidylinositol bisphosphate (-46%), and lyso-phosphatidylinositol (-66%). Cytochalasin-B also blocked the stimulatory effect of angiotensin-II on the breakdown of phosphatidylinositol, phosphatidylinositol monophosphate, and phosphatidylinositol bisphosphate and the formation of lyso-phosphatidylinositol and inositol phosphates. The present results provide evidence of a role for microfilaments in polyphosphoinositide metabolism in adrenocortical cells. These data indicate that microfilaments are required for the incorporation of inositol into membrane phospholipids and are necessary for angiotensin-II-induced phospholipase activation.

Localization, characterization and activity of pituitary adenylate cyclase-activating polypeptide in the frog adrenal gland

Pituitary adenylate cyclase-activating polypeptide (PACAP) has recently been isolated from the frog brain and the sequence of the peptide appears to be strikingly similar to that of mammalian PACAP. In the present study, we have investigated the localization of PACAP in the frog interrenal (adrenal) gland by immunocytochemistry using antisera directed against PACAP 38 or PACAP 27. Two types of PACAP-immunoreactive fibres were observed: thick varicose fibres coursing between adrenal cells and thin processes located in the walls of blood vessels irrigating the gland. Bilateral transection of the splanchnic nerves did not affect the intensity and distribution of PACAP immunoreactivity. The mean +/- S.E.M. concentration of PACAP, measured by radioimmunoassay in crude adrenal extracts, was 0.65 +/- 0.16 nmol/g wet tissue. Two molecular forms of PACAP in the adrenal gland were characterized by reversed phase high-performance liquid chromatography combined with radioimmunoassay quantification. The elution profiles revealed the existence of two peaks exhibiting the same retention times as synthetic frog PACAP 38 (fPACAP 38) and PACAP 27, the predominant form being PACAP 38. The possible involvement of PACAP in the regulation of adrenal steroidogenesis was investigated in vitro using a perifusion system for frog adrenal slices. Graded doses of fPACAP 38 (0.1-10 mumol/l) increased the secretion of both corticosterone and aldosterone in a dose-dependent manner. Administration of repeated pulses of fPACAP 38 (1 mumol/l), at 120-min intervals, led to a reproducible stimulation of corticosteroid secretion without any tachyphylaxis. Prolonged infusion (2 h) of the peptide induced a rapid increase in corticosterone and aldosterone output, followed by a gradual decline in the secretion rate, suggesting the occurrence of a desensitization phenomenon. Synthetic porcine vasoactive intestinal peptide, which is structurally related to PACAP, was about ten times less potent than fPACAP 38 in stimulating steroidogenesis while the [Des-His1]-fPACAP 38 analogue was 100 times less effective. These results demonstrate that a peptide closely related to fPACAP 38 is present in fibres innervating the frog adrenal gland and could participate in the regulation of corticosteroid secretion, particularly during neurogenic stress.

Neuroanatomical and physiological evidence for the involvement of pituitary adenylate cyclase-activating polypeptide in the regulation of the distal lobe of the frog pituitary

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a 38 amino-acid peptide which belongs to the glucagon/secretin/vasoactive intestinal peptide superfamily. The sequence of PACAP is identical in all mammalian species studied so far but frog PACAP differs by one amino-acid from mammalian PACAP. The aim of the present study was to investigate the presence of PACAP in the hypothalamo-pituitary complex of the frog Rana ribibunda and to determine the biological activity of frog PACAP on homologous pituitary cells. The distribution of PACAP-containing neurons and fibers was examined by the indirect immunofluorescence method using an antiserum raised against the N-terminal region of the peptide. In the hypothalamus, PACAP-immunoreactive perikarya were localized in the preoptic nucleus and the dorsal and ventral infundibular nuclei. Beaded nerve fibers were observed coursing from the ventral infundibular nucleus to the external vascular layer of the median eminence. A dense network of immunoreactive axons terminated in the vicinity of the capillaries of the hypophysial portal system. The neurointermediate lobe and the distal lobe of the pituitary were devoid of immunoreactive elements. The amount of PACAP-like immunoreactive material in hypothalamus extracts was measured by radioimmunoassay; the apparent concentration of PACAP was 4.5 ng/mg protein. Synthetic frog PACAP38 and PACAP27 induced a similar dose-dependent stimulation of cAMP production in isolated frog distal lobe pituitary fragments (ED50 = 2 x 10(-8) M). At the maximum dose tested (5 x 10(-6) M), both frog PACAP38 and PACAP27 produced a 4-fold increase in cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical localization of atrial natriuretic factor (ANF)-like peptides in the brain and heart of the treefrog Hyla japonica: effect of weightlessness on the distribution of immunoreactive neurons and cardiocytes

The localization of atrial-natriuretic factor (ANF)-like immunoreactivity was investigated in the brain and heart of the treefrog Hyla japonica by the indirect immunofluorescence technique. Concurrently, the effect of weightlessness on the distribution of ANF-containing neurons and cardiocytes was studied in frogs that were sent into space for 9 days on the space station "MIR." In control animals, the amygdala contained the most prominent group of ANF-immunoreactive cells and fibers. ANF-positive neurons and nerve processes were also detected in other areas of the telencephalon such as the nucleus olfactorius, the pallium mediale, and the striatum. In "space frogs," the intensity of labeling of the amygdala and nucleus olfactorius was similar to that seen in control animals. In contrast, the pallium and the striatum of "space frogs" were totally devoid of positive cell bodies. In the diencephalon, of all animals, numerous ANF-immunoreactive perikarya and fibers were seen in the hypothalamus, the anterior thalamus, the infundibulum, and the median eminence. ANF-positive cell bodies were also noted in the lateral forebrain bundle of control frogs but were absent in "space frogs." The major difference between control and "space frogs" was observed in the posterior nuclei of the thalamus. In "space frogs," the nucleus posterocentralis thalami and the nucleus posterolateralis thalami exhibited large ANF-immunoreactive perikarya, while, in control frogs, these nuclei only contained scarce positive nerve fibers. In the mesencephalon, ANF-positive cell bodies and nerve processes were seen in the nucleus tegmenti mesencephali, the interpeduncular nucleus, and the nucleus cerebelli of all animals. However, stained perikarya were only observed in the nucleus reticularis isthmi of control frogs. In the heart, atrial cardiocytes exhibited intense ANF-like immunoreactivity. ANF-positive myocytes were also detected in the subpericardial region of the ventricle. The density and distribution of the staining were identical in the heart of control and "space frogs." These data support the concept that prolonged exposure to microgravity affects biosynthesis and/or release of ANF-related peptides in discrete regions of the amphibian brain.

Immunohistochemical distribution and biological activity of pituitary adenylate cyclase-activating polypeptide (PACAP) in the central nervous system of the frog Rana ridibunda

The primary structure of frog pituitary adenylate cyclase-activating polypeptide (PACAP) has recently been determined and the results show that the sequence of PACAP has been highly conserved during evolution. In particular, the structure of the 1-27 fragment of PACAP is identical in frog and mammals. Using an antiserum raised against PACAP27, we have investigated the distribution of PACAP-containing neurons in the central nervous system of the frog Rana ridibunda by the immunofluorescence technique. The main populations of immunoreactive perikarya were located in the medial and ventral diencephalon, i.e., the preoptic nucleus, the ventral and dorsal infundibular nuclei, the nucleus posterocentralis thalami, and the ventral and ventrolateral areas of the thalamus. In the telencephalon, sparse cell bodies were found in the nucleus accumbens septi, the amygdala, the pallial commissure, and the bed nucleus of the pallial commissure. In the hindbrain, the torus semicircularis, the nucleus profundus and the nucleus anteroventralis tegmenti of the mesencephalon also contained populations of PACAP-immunoreactive perikarya. Beaded nerve fibers were observed throughout the brain. Occasionally they formed bundles, e.g., from the ventral infundibulum to the external vascular layer of the median eminence, from the central thalamus to the optic tectum, and rostrocaudally, from the nucleus accumbens septi to the nucleus entopeduncularis. Other areas, such as the interpeduncular nucleus, the nucleus isthmi and the roots of cranial nerves V and VIII in the medulla oblongata, were also densely innervated. The adenylate cyclase-stimulating activity of PACAP was tested by using a static incubation technique for hypothalamic slices.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical localization of delta sleep-inducing peptide (DSIP) in the brain and pituitary of the cartilaginous fish Scyliorhinus canicula

The distribution of delta sleep-inducing peptide (DSIP) in the brain and pituitary of the cartilaginous fish Scyliorhinus canicula was investigated using the indirect immunofluorescence technique. Delta sleep-inducing peptide-like immunoreactive cell bodies were mainly observed in the nucleus lateralis tuberis of the hypothalamus. Immunolabeled perikarya were also distributed in the nucleus lobi lateralis hypothalami and in the dorso-lateral wall of the recessus posterioris. Most of these cells, located in the subependymal layers of the infundibulum and lateral lobes, had the typical aspect of cerebrospinal fluid-contacting elements. The DSIP-like immunoreactive fibers were localized in the basal telencephalon, within the regions of the nucleus interstitialis commissurae anterioris and the nucleus entopeduncularis. A dense network of DSIP-positive fibers was seen throughout the midcaudal hypothalamus, the lateral lobes, and the posterior lobe. In the pituitary, numerous DSIP-like immunoreactive cells were detected in the median lobe of the pars distalis. In particular, a high concentration of cells was seen in the dorsal wall of the median lobe, an area which is known to contain melanin-concentrating hormone (MCH)-producing cells. Comparison of the distribution of DSIP- and MCH-like immunoreactive cells revealed that the two neuropeptides are stored in the same cells of the median lobe of the pituitary. These findings provide the first evidence for the presence of a DSIP-related peptide in fish. The distribution of the immunoreactive material supports the view that DSIP may act as a neuromodulator and/or a hypophysiotropic factor. Moreover, the presence of DSIP-like immunoreactive cells in the pars distalis suggests that this peptide may exert autocrine or paracrine effect in the pituitary.

In vitro study of the effect of adenosine on frog adrenocortical cells

Previous reports have shown that adenosine in rat inhibits both spontaneous and ACTH-induced release of corticosteroids through activation of adenosine A1 receptors. In the present study, we have investigated the possible effect of adenosine in the secretion of corticosteroids in amphibians using a perfusion technique for frog adrenocortical slices. Infusion of adenosine, at concentrations ranging from 10(-7) to 10(-4) M, had no effect on the basal output of corticosterone and aldosterone by frog interrenal cells. Similarly, adenosine did not affect the response of frog adrenocortical slices to ACTH, vasoactive intestinal peptide, or angiotensin II. The stable adenosine A1 receptor agonist N6-phenylisopropyl adenosine (PIA) was also totally devoid of effect on the spontaneous or ACTH-induced release of corticosteroids. These results show that in amphibians, adenosine does not modulate adrenal steroidogenesis.

Serotonin-induced stimulation of cortisol secretion from human adrenocortical tissue is mediated through activation of a serotonin4 receptor subtype

The occurrence of serotonin in the human adrenal gland was demonstrated both by immuno-histochemical and biochemical approaches. Using specific polyclonal antibodies to serotonin, the presence of numerous immunoreactive cells was revealed by means of the peroxidase-antiperoxidase technique. These cells exhibited the morphological characteristics of mast cells. Combination of high performance liquid chromatography and electrochemical detection showed the presence of substantial amounts of both serotonin and its metabolite 5-hydroxyindolacetic acid in adrenocortical extracts. The role of serotonin in the regulation of steroidogenesis from human adrenocortical slices was studied in vitro using a perifusion system technique coupled to a specific radioimmunoassay for cortisol. Graded doses of serotonin (from 10(-8) M to 3 x 10(-7) M) increased cortisol production in a dose-dependent manner. Prolonged exposure of adrenal fragments to serotonin (10(-7) M) induced a biphasic response, i.e. a rapid and transient increase in cortisol secretion followed by a plateau phase, suggesting the existence of a desensitization phenomenon. The stimulatory effect of serotonin (10(-7) M) was not altered during infusion of the serotonin1 and/or serotonin2 receptor antagonists methysergide (10(-6) M) and ketanserin (10(-6) M), respectively. In contrast, ICS 205 930 (10(-6) M), a non-selective serotonin3/serotonin4 antagonist, totally abolished the response of adrenal slices to serotonin (10(-7) M). The benzamide derivative zacopride, considered as a serotonin4 agonist, induced a robust stimulation of cortisol secretion. In addition, the corticotropic effects of serotonin (10(-7) M) and zacopride (10(-6) M) were not additive. Incubation of adrenocortical fragments with zacopride (10(-6) M) or serotonin (10(-6) M) caused a significant increase in cAMP formation. Taken together, these data suggest that serotonin, locally released by intra-adrenal mast-like cells, may act as a paracrine factor to stimulate cortisol secretion in man. Our results also indicate that serotonin-induced corticosteroid production is mediated through activation of a serotonin4 receptor subtype positively coupled to adenylate cyclase.

Immunohistochemical localization of delta sleep-inducing peptide-like immunoreactivity in the central nervous system and pituitary of the frog Rana ridibunda

The purpose of the present study was to investigate the distribution of delta sleep-inducing peptide in the brain and pituitary of the frog Rana ridibunda and to determine the possible effect of this nonapeptide on adrenocorticotropic hormone and corticosteroid secretion. Delta sleep-inducing peptide-like immunoreactive fibres were observed throughout the brain of the frog. These fibres generally exhibited the characteristics of glial cell processes. Scarce delta sleep-inducing peptide-positive fibres were seen in the olfactory bulb and in the periventricular areas of the telencephalon. In the diencephalon, numerous delta sleep-inducing peptide-containing processes were noted in the preoptic nucleus, the infundibular nuclei and the median eminence. A few cerebrospinal fluid-contacting cells were visualized in the ventral nucleus of the infundibulum. Delta sleep-inducing peptide-positive fibres were also observed in the mesencephalon, radiating through the different layers of the tectum. In the cerebellum, all Purkinje cells exhibited delta sleep-inducing peptide-like immunoreactivity. More caudally, numerous delta sleep-inducing peptide-positive fibres were noted in the vestibular nucleus of the rhombencephalon. A dense network of delta sleep-inducing peptide-containing fibres was seen in the pars nervosa of the pituitary. In the distal lobe, a population of endocrine cells located in the anteroventral region contained delta sleep-inducing peptide-immunoreactive material. Labelling of consecutive sections of the pituitary by delta sleep-inducing peptide and adrenocorticotropic hormone antiserum revealed that a delta sleep-inducing peptide-related peptide is expressed in corticotroph cells. The possible role of delta sleep-inducing peptide in the control of adrenocorticotropic hormone and corticosteroid release was studied in vitro, using the perifusion system technique. Administration of graded doses of delta sleep-inducing peptide (from 10(-8) to 10(-6) M) to perifused frog anterior pituitary cells did not affect the spontaneous release of adrenocorticotropic hormone. In addition, prolonged infusion of delta sleep-inducing peptide (10(-6) M) did not alter the stimulatory effect of corticotropin-releasing factor (10(-7) M) on adrenocorticotropic hormone secretion. Similarly, exposure of frog interrenal slices to delta sleep-inducing peptide did not induce any modification of spontaneous or adrenocorticotropic hormone-evoked secretion of corticosterone and aldosterone. Our results provide the first evidence for the presence of a delta sleep-inducing peptide-related peptide in lower vertebrates. The occurrence of delta sleep-inducing peptide-like immunoreactivity in specific areas of the brain suggests that the peptide may act as a neuromodulator.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of FMRFamide-like immunoreactivity in the brain of the elasmobranch fish Scyliorhinus canicula

The distribution of FMRFamide-like-immunoreactive peptides was investigated in the brain and pituitary of the elasmobranch fish Scyliorhinus canicula using the indirect immunofluorescence technique. FMRFamide-immunoreactive cells and fibers were mainly observed in the telencephalon and the diencephalon, while other brain structures were almost unstained. In the telencephalon, FMRFamide-like-containing neurons were seen in the caudal part of the area periventricularis pallialis, in the posterior area of the nucleus septi medialis and in the nucleus septi caudoventralis. In the diencephalon, numerous FMRFamide-positive cell bodies were observed in the hypothalamus, ventral thalamus and posterior tuberculum. The highest density of immunofluorescent perikarya was found in the nucleus lobi lateralis hypothalami and in the nucleus periventricularis hypothalami. More caudally, the mesencephalon and the caudal brainstem only contained scattered varicose FMRFamide-immunoreactive fibers. Stained fibers were also identified in the median eminence and several FMRFamide-like-positive cells were detected in the dorsal and rostral parts of the neurointermediate lobe of the pituitary. These data indicate that substances related to the molluscan cardioexcitatory peptide FMRFamide are widely distributed in the brain of S. canicula, suggesting their implication in neuroendocrine and/or neuromodulatory functions.

Distribution and characterization of immunoreactive growth hormone (GH) in the pituitary of the frog Rana ridibunda using an antiserum against purified bullfrog GH

The presence of growth hormone (GH) in the pituitary of the frog Rana ridibunda was investigated using an antiserum raised against purified bullfrog GH. The immunofluorescence technique revealed that GH-containing cells are exclusively located in the dorsal area of the distal lobe of the pituitary. The relative abundance of these GH-positive cells, which correspond to acidophilic type 2 cells, was 18 +/- 1% of the total population of endocrine cells of the pars distalis. Frontal sections of the distal lobe indicated that GH-producing cells are distributed in an arc of a circle occupying all of the dorsal part of the lobe. At the electron microscopic level, GH-immunoreactive material was sequestered in large polymorphic granules (200-700 nm). GH was quantified in R. ridibunda pituitary extracts using a radioimmunoassay for bullfrog GH. The displacement curves obtained with serial dilutions of pars distalis extracts were not strictly parallel to the standard curve made with purified bullfrog GH. In contrast, Western blot analysis revealed that GH from R. ridibunda had a molecular weight (22 kDa) similar to that of bullfrog GH. In the pars distalis, the apparent amount of GH was 0.61 +/- 0.14 microgram per lobe, corresponding to 0.92 +/- 0.17% of total proteins in the extracts. In contrast, frog neurointermediate lobe or hypothalamus did not contain significant concentrations of immunoreactive GH (less than 0.006% of total proteins in the extracts). Taken together, these results validate the use of an antiserum to bullfrog GH to investigate the regulation of GH secretion in R. ridibunda.

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.

Immunohistochemical mapping of galanin-like immunoreactivity in the brain of the dogfish Scyliorhinus canicula

The distribution of galanin-like immunoreactivity in the brain of the dogfish Scyliorhinus canicula was investigated using the indirect immunofluorescence technique. In the telencephalon, positive cells and fibers were located in the mid-caudal part of the area superficialis basalis, the n. septi caudoventralis and in the n. interstitialis commissurae anterioris. Most of the galanin-containing neurons observed in the hypothalamus were located in the magnocellular preoptic nucleus. Positive perikarya were also found in the n. lobi lateralis hypothalami and in the n. lateralis tuberis. A dense network of positive nerve processes was noted in the caudal part of the median eminence. In the dorso-caudal part of the diencephalon numerous immunoreactive neurons were seen in the recessus posterioris. A large bundle of galanin-containing fibers, which divided in two branches, was observed in the basal midbrain tegmentum. The widespread distribution of galanin-like material suggests that, in the dogfish, galanin may be involved in various brain functions including neuroendocrine regulations.

Localization of atrial natriuretic factor (ANF)-related peptides in the central nervous system of the elasmobranch fish Scyliorhinus canicula

We have investigated the localization of atrial natriuretic factor (ANF)-like immunoreactivity in the central nervous system of the cartilaginous fish, Scyliorhinus canicula, using the indirect immunofluorescence technique. Immunoreactive perikarya and fibers were observed in two regions of the telencephalon, the area superficialis basalis and the area periventricularis ventrolateralis. In the diencephalon, the hypothalamus exhibited a moderate number of ANF-containing neurons and fibers located in the preoptic and periventricular nuclei and in the nucleus lateralis tuberis. The most important group of ANF-immunoreactive cells was observed in the nucleus tuberculi posterioris of the diencephalon. In contrast, the mesencephalon showed only a few ANF-positive nerve processes located in the tegmentum mesencephali. Numerous fine fibers and nerve terminals were found in the dorsal area of the neurointermediate lobe of the pituitary. These results provide the first evidence for the presence of ANF-related peptides in the brain of a cartilaginous fish. The widespread distribution of ANF-positive cells and fibers in the brain and pituitary suggests that this peptide may act both as a neurotransmitter and (or) a neurohormone in fish.

Structure-activity relationships of monomeric and dimeric synthetic ACTH fragments in perifused frog adrenal slices

The effect of synthetic monomeric and dimeric ACTH fragments on spontaneous and ACTH(1-39)-evoked steroidogenesis in frog interrenal tissue was studied in vitro. Infusion of ACTH fragment 11-24 (10(-6) M) or its dimeric conjugates, attached either by their N-terminal, Glu(11-24)2, or their C-terminal amino acid, (11-24)2Lys, had no effect on the spontaneous release of corticosteroids. The monomer ACTH(11-24) and the dimer Glu(11-24)2 were also totally devoid of effect on the steroidogenic response to ACTH(1-39) (10(-9)M). In contrast, the (11-24)2Lys conjugate (10(-6)M) significantly decreased ACTH-induced stimulation of corticosterone and aldosterone (-63 and -62%, respectively). The dimeric conjugate of the fragment ACTH(7-24), linked through the C-terminal ends, (7-24)2Lys (10(-6)M), was also completely devoid of effect on basal steroidogenesis but caused a marked decrease of ACTH-evoked corticosterone and aldosterone release (-72 and -80%, respectively). Conversely, infusion of the dimer (1-24)2Lys gave rise to a dose-related stimulation of corticosterone and aldosterone release. The time-course of the steroidogenic response to the dimer was similar to that of ACTH(1-24). The 1-24 conjugate was 70 times less potent than the monomers ACTH(1-24) and ACTH(1-39). These results suggest that amphibian adrenocortical cells contain only one class of ACTH receptor which recognizes the 11-24 domain of ACTH with an affinity which depends on the presence of a strong potentiator segment, located at the N-terminus end of ACTH(1-39). Since the ACTH-dimers are thought to induce cross-linking of the receptors, our results suggest that aggregation of ACTH receptors causes a down-regulation of the receptors.

Identification of vasotocin-like immunoreactivity in chromaffin cells of the frog adrenal gland: effect of vasotocin on corticosteroid secretion

The presence of neurohypophyseal nonapeptides in the adrenal gland of nonmammalian vertebrates and the possible action of these regulatory peptides on corticosteroid secretion have never been investigated. We have applied the indirect immunofluorescence technique to examine whether vasotocin (AVT) and/or mesotocin (MT) are located in frog adrenal (interrenal) tissue. Using antisera against AVT and tyrosine hydroxylase, we found that all chromaffin cells contain an AVT-like peptide. Labeling of consecutive sections with phenylethanolamine-N-methyltransferase or AVT antibodies showed that both noradrenaline- and adrenaline-storing cells contain AVT-like immunoreactivity. In contrast no labeling of frog adrenal slices was observed using a MT antiserum. At the ultrastructural level, the immunogold technique revealed that the AVT-immunoreactive peptide is sequestered in chromaffin granules with varying electron densities. Filtration of frog adrenal tissue extracts on Sep-Pak C-18 cartridges showed that the elution profile of the AVT-like peptide was similar to that of synthetic AVT. The apparent concentration of AVT in the adrenal was 2.7 ng/g tissue. Since chromaffin cells represent approximately one third of all interrenal cells, the actual concentration of AVT in chromaffin tissue was about 8 ng/g tissue. The role of AVT in the regulation of frog adrenal steroidogenesis was studied in vitro using perifused frog interrenal slices. Graded doses of AVT (10(-10)-10(-7) M) induced a dose-dependent stimulation of both corticosterone and aldosterone secretion. The other neurohypophyseal peptides (vasopressin, oxytocin, and MT) were also able to enhance corticosteroid secretion, but AVT was by far the most potent stimulator of steroidogenesis. Prolonged administration (4 h) of AVT induced a rapid increase in corticosterone and aldosterone output, followed by a gradual decline of corticosteroid secretion. These results show that an AVT-like peptide is stored in chromaffin granules of frog adrenal gland. Our data also indicate that synthetic AVT is a potent stimulator of corticosteroid secretion by frog interrenal cells. Since in amphibians adrenocortical and chromaffin cells are intimately intermingled, these results suggest that AVT produced by chromaffin cells may regulate corticosteroid release locally, through a cell to cell mode of communication.

Mechanism of action of serotonin on frog adrenal cortex

The mechanism of action of serotonin (5-HT) on frog adrenal cortex has been investigated in vitro using the perifusion system technique. The direct effect of 5-HT on corticosteroid secreting cells was demonstrated, using enzymatically dispersed adrenocortical cells. Melatonin and 5-HTP appeared to be less potent than 5-HT to enhance corticosteroid secretion. In contrast Trp and 5-HIAA were totally devoid of effect on steroid secretion. To investigate the type of receptor involved in the stimulatory effect of 5-HT on adrenocortical cells, adrenal slices were stimulated with 5-HT in absence or presence of various antagonists. We observed that classical antagonists of 5-HT1, 5-HT2 and 5-HT3 type receptors failed to block 5-HT-induced corticosteroid secretion in our model. These results show that 5-HT exerts a direct effect on corticosteroid-secreting cells. Our data also indicates that the type of receptor involved in the action of 5-HT in frog adrenal cortex differs from mammalian 5-HT receptors.

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.

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.

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.

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.

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.

Formation of 11 beta-hydroxysteroids requires the integrity of the microfilament network in adrenocortical cells

In order to determine the role of microfilaments in adrenal steroidogenesis, we have studied the effect of cytochalasin B, a microfilament-disrupting agent, on the kinetics of [3H] pregnenolone conversion to labelled metabolites by frog interrenal tissue in vitro. Cytochalasin B (5 x 10(-5)M) induced a 50 to 70% decrease in corticosterone, 18-hydroxycorticosterone and aldosterone biosynthesis while the formation of progesterone and 11-desoxycorticosterone was not affected. These results suggest that microfilaments interfere in the conversion of 11-desoxycorticosterone to corticosterone probably by controlling the movement of 11-desoxycorticosterone from the reticulum to the mitochondria.