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

Hormonal and pheromonal studies on amphibians with special reference to metamorphosis and reproductive behavior

In this article, we review studies which have been conducted to investigate the hormonal influence on metamorphosis in bullfrog (Rana catesbeiana) and Japanese toad (Bufo japonicus) larvae, in addition to studies conducted on the hormonal and pheromonal control of reproductive behavior in red-bellied newts (Cynops pyrrhogaster). Metamorphosis was studied with an emphasis on the roles of prolactin (PRL) and thyrotropin (TSH). The release of PRL was shown to be regulated by thyrotropin-releasing hormone (TRH) and that of TSH was evidenced to be regulated by corticotropin-releasing factor. The significance of the fact that the neuropeptide that controls the secretion of TSH is different from those encountered in mammals is discussed in consideration of the observation that the release of TRH, which stimulates the release of PRL, is enhanced when the animals are subjected to a cold temperature. Findings that were made by using melanin-rich cells of Bufo embryos and larvae, such as the determination of the origin of the adenohypophyseal primordium, identification of the pancreatic chitinase, and involvement of the rostral preoptic recess organ as the hypothalamic inhibitory center of α-melanocyte-stimulating hormone (α-MSH) secretion, are mentioned in this article. In addition, the involvement of hormones in eliciting courtship behavior in male red-bellied newts and the discovery of the peptide sex pheromones and hormonal control of their secretion are also discussed in the present article.

Some aspects of the hypothalamic and pituitary development, metamorphosis, and reproductive behavior as studied in amphibians

In this review article, information about the development of the hypothalamo-hypophyseal axis, endocrine control of metamorphosis, and hormonal and pheromonal involvements in reproductive behavior in some amphibian species is assembled from the works conducted mainly by our research group. The hypothalamic and pituitary development was studied using Bufo embryos and larvae. The primordium of the epithelial hypophysis originates at the anterior neural ridge and migrates underneath the brain to form a Rathke's pouch-like structure. The hypothalamo-hypophyseal axis develops under the influence of thyroid hormone (TH). For the morphological and functional development of the median eminence, which is a key structure in the transport of regulatory hormones to the pituitary, contact of the adenohypophysis with the undeveloped median eminence is necessary. For the development of proopiomelanocortin-producing cells, contact of the pituitary primordium with the infundibulum is required. The significance of avascularization in terms of the function of the intermediate lobe of the pituitary was evidenced with transgenic Xenopus frogs expressing a vascular endothelial growth factor in melanotropes. Metamorphosis progresses via the interaction of TH, adrenal corticosteroids, and prolactin (PRL). We emphasize that PRL has a dual role: modulation of the speed of metamorphic changes and functional development of organs for adult life. A brief description about a novel type of PRL (1B) that was detected was made. A possible reason why the main hypothalamic factor that stimulates the release of thyrotropin is not thyrotropin-releasing hormone, but corticotropin-releasing factor is considered in light of the fact that amphibians are poikilotherms. As regards the reproductive behavior in amphibians, studies were focused on the courtship behavior of the newt, Cynops pyrrhogaster. Male newts exhibit a unique courtship behavior toward sexually developed conspecific females. Hormonal interactions eliciting this behavior and hormonal control of the courtship pheromone secretion are discussed on the basis of our experimental results.

Arginine vasotocin is the major adrenocorticotropic hormone-releasing factor in the bullfrog Rana catesbeiana

In a previous study, we showed that corticotropin-releasing factor (CRF) is the major thyroid-stimulating hormone (TSH)-releasing factor in the bullfrog (Rana catesbeiana) hypothalamus. Our findings prompted us to ascertain whether CRF or arginine vasotocin (AVT), a known adrenocorticotropic hormone (ACTH) secretagogue in several vertebrates, is the main stimulator of the release of ACTH from the bullfrog pituitary. Both the frog CRF and AVT stimulated the release of immunoassayable ACTH from dispersed anterior pituitary cells in vitro in a concentration-dependent manner. AVT, however, exhibited far more potent ACTH-releasing activity than CRF. Although CRF by itself weakly stimulated ACTH release, it acted synergistically with AVT to enhance the release of ACTH markedly. Mesotocin and AVT-related peptides such as hydrin 1 and hydrin 2 showed relatively weak ACTH-releasing activity. Subsequently, cDNAs encoding the bullfrog AVT V1a-type and V1b-type receptors were molecularly cloned. Reverse transcriptase-PCR using specific primers revealed that the anterior lobe of the pituitary predominantly expressed AVT V1b-type receptor mRNA but scarcely expressed AVT V1a-type receptor mRNA. Abundant signals for V1b-type receptor mRNA in the corticotropes were also detected by in situ hybridization. The results obtained by the experiments with the bullfrog pituitary indicate that AVT acts as the main ACTH-releasing factor through the AVT V1b-type receptor and that CRF acts synergistically with AVT to enhance the release of ACTH.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Regulation of neurosteroid biosynthesis by neurotransmitters and neuropeptides

The enzymatic pathways leading to the synthesis of bioactive steroids in the brain are now almost completely elucidated in various groups of vertebrates and, during the last decade, the neuronal mechanisms involved in the regulation of neurosteroid production have received increasing attention. This report reviews the current knowledge concerning the effects of neurotransmitters, peptide hormones, and neuropeptides on the biosynthesis of neurosteroids. Anatomical studies have been carried out to visualize the neurotransmitter- or neuropeptide-containing fibers contacting steroid-synthesizing neurons as well as the neurotransmitter, peptide hormones, or neuropeptide receptors expressed in these neurons. Biochemical experiments have been conducted to investigate the effects of neurotransmitters, peptide hormones, or neuropeptides on neurosteroid biosynthesis, and to characterize the type of receptors involved. Thus, it has been found that glutamate, acting through kainate and/or AMPA receptors, rapidly inactivates P450arom, and that melatonin produced by the pineal gland and eye inhibits the biosynthesis of 7α-hydroxypregnenolone (7α-OH-Δ(5)P), while prolactin produced by the adenohypophysis enhances the formation of 7α-OH-Δ(5)P. It has also been demonstrated that the biosynthesis of neurosteroids is inhibited by GABA, acting through GABA(A) receptors, and neuropeptide Y, acting through Y1 receptors. In contrast, it has been shown that the octadecaneuropetide ODN, acting through central-type benzodiazepine receptors, the triakontatetraneuropeptide TTN, acting though peripheral-type benzodiazepine receptors, and vasotocin, acting through V1a-like receptors, stimulate the production of neurosteroids. Since neurosteroids are implicated in the control of various neurophysiological and behavioral processes, these data suggest that some of the neurophysiological effects exerted by neurotransmitters and neuropeptides may be mediated via the regulation of neurosteroid production.

Identification and characterization of mesotocin and V1a-like vasotocin receptors in a urodele amphibian, Taricha granulosa

The cDNA sequences encoding the mesotocin receptor (MTR) and vasotocin 1a receptor (VTR-1a) were identified in a urodele amphibian, the rough-skinned newt, Taricha granulosa. Saturation binding of [(3)H]oxytocin (OT) to the Taricha MTR (tMTR) was best fit by a two-state model; a high affinity-low abundance site and a lower affinity-high abundance site. Competition-binding studies found the following rank-order affinities for the tMTR: mesotocin (MT)>OT≈vasotocin (VT)>vasopressin (VP)>isotocin (IT). Inositol phosphate (IP) accumulation studies demonstrated functional activity of both the tMTR and Taricha VTR-1a (tVTR-1a) in a heterologous cell culture system. The rank-order potencies for the tMTR were MT>OT>VT≈VP>IT. The combined binding and IP results indicate that VT may act as a partial agonist of the tMTR. Rank-order potencies for the tVTR-1a were VT>VP>MT≈OT>IT. For both receptors, stimulation of IP accumulation was blocked by d(CH(2))(5)[Tyr(Me)(2)]AVP (Manning compound) and d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2)]OVT (OTA). OTA was a more potent antagonist for the transiently expressed tMTR while Manning compound was relatively more potent at inhibiting IP accumulation in tVTR-1a expressing cells. In contradiction to earlier assumptions, the absolute IC(50) of Manning compound was lower for the tMTR (27nM±13) than the tVTR-1a (586nM±166) indicating its potential higher affinity for the tMTR, a finding with special relevance to interpretation of comparative studies investigating the behavioral and physiological actions of neurohypophysial peptides in non-mammalian species.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Vasotocin and mesotocin stimulate the biosynthesis of neurosteroids in the frog brain

The neurohypophysial nonapeptides vasopressin (VP) and oxytocin (OT) modulate a broad range of cognitive and social activities. Notably, in amphibians, vasotocin (VT), the ortholog of mammalian VP, plays a crucial role in the control of sexual behaviors. Because several neurosteroids also regulate reproduction-related behaviors, we investigated the possible effect of VT and the OT ortholog mesotocin (MT) in the control of neurosteroid production. Double immunohistochemical labeling of frog brain sections revealed the presence of VT/MT-positive fibers in close proximity of neurons expressing the steroidogenic enzymes 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase (3beta-HSD) and cytochrome P450 17alpha-hydroxylase/c17, 20-lyase (P450(C17)). High concentrations of VT and MT receptor mRNAs were observed in diencephalic nuclei containing the 3beta-HSD and P450(C17) neuronal populations. Exposure of frog hypothalamic explants to graded concentrations of VT or MT produced a dose-dependent increase in the formation of progesterone, 17-hydroxypregnenolone, 17-hydroxyprogesterone, and dehydroepiandrosterone. The stimulatory effect of VT and MT on neurosteroid biosynthesis was mimicked by VP and OT, as well as by a selective V1b receptor agonist, whereas V2 and OT receptor agonists had no effect. VT-induced neurosteroid production was completely suppressed by selective V1a receptor antagonists and was not affected by V2 and OT receptor antagonists. Concurrently, the effect of MT on neurosteroidogenesis was markedly attenuated by selective OT and V1a receptor antagonists but not by a V2 antagonist. The present study provides the first evidence for a regulatory effect of VT and MT on neurosteroid biosynthesis. These data suggest that neurosteroids may mediate some of the behavioral actions of VT and MT.

Vasotocin/Isotocin-immunoreactive Neurons in the Medaka Fish Brain Are Sexually Dimorphic and Their Numbers Decrease After Spawning in the Female

In teleosts, the distribution of neurons in the preoptic-hypothalamic region and their associated neurohypophysial hormones, such as vasotocin (VT), appears to be different among species. This differential distribution is thought to reflect the social and/or sexual status of individuals within a species. In the present study, we analyzed the number, size and the distribution of vasotocin/isotocin (VT/IT) neurons in the brains of both male and female medaka (Oryzias latipes) using immunohistochemistry. VT/IT neurons were similarly located in an inverted L-shape in the nucleus preopticus in both gender, as has been already reported in salmonids. However, computer-assisted image analysis revealed sexual dimorphism in the number of VT/IT-immunoreactive (ir) neurons, with greater numbers found in males as compared to females. Further, in the female brain, the number of VT/IT-ir neurons decreased significantly after spawning. In pre-spawning compared to post-spawning females, the small-sized VT/IT-ir neurons dominated. Sexual differentiation of the medaka is fully dependent upon the steroid status during the early developmental stages and steroids are also known to trigger gender-specific behavior in the adult medaka. Our findings strongly suggest that VT and/or IT neurons may be functionally related to ovulation and/or the reproductive axes through connections to their steroidal status.

Identification of Amino Acid Residues That Direct Differential Ligand Selectivity of Mammalian and Nonmammalian V1a Type Receptors for Arginine Vasopressin and Vasotocin

Arginine vasotocin (VT) is the ortholog in all nonmammalian vertebrates of arginine vasopressin (AVP) in mammals. We have previously cloned an amphibian V1atype vasotocin receptor (VT1R) that exhibited higher sensitivity for VT than AVP, while the mammalian V1a type receptor (V1aR) responded better to AVP than VT. In the present study, we identified the amino acid residues that confer differential ligand selectivity for AVP and VT between rat V1aR and bullfrog VT1R (bfVT1R). A chimeric rat V1aR having transmembrane domain (TMD) VI to the carboxyl-terminal tail (C-tail) of bfVT1R showed a reverse ligand preference for AVP and VT, whereas a chimeric VT1R with TMD VI to the C-tail of rat V1aR showed a great increase in sensitivity for AVP. A single mutation (Ile(315(6.53)) to Thr) in TMD VI of V1aR increased the sensitivity for VT, while a single mutation (Phe(313(6.51)) to Tyr or Pro(334(7.33)) to Thr) reduced sensitivity toward AVP. Interestingly the triple mutation (Phe(313(6.51)) to Tyr, Ile(6.53) to Thr, and Pro(7.33) to Thr) of V1aR increased sensitivity to VT but greatly reduced sensitivity to AVP, behaving like bfVT1R. Further, like V1aR, a double mutant (Tyr(306(6.51)) to Phe and Thr(327(7.33)) to Pro) of bfVT1R showed an increased sensitivity to AVP. These results suggest that Phe/Tyr(6.51), Ile/Thr(6.53), and Pro/Thr(7.33) are responsible for the differential ligand selectivity between rat V1aR and bfVT1R. This information regarding the molecular interaction of VT/AVP with their receptors may have important implications for the development of novel AVP analogs.

Pituitary adenylate cyclase-activating polypeptide and its receptors in amphibians

Pituitary adenylate cyclase-activating polypeptide (PACAP), a novel peptide of the secretin/glucagon/vasoactive intestinal polypeptide superfamily, has been initially characterized in mammals in 1989 and, only 2 years later, its counterpart has been isolated in amphibians. A number of studies conducted in the frog Rana ridibunda have demonstrated that PACAP is widely distributed in the central nervous system (particularly in the hypothalamus and the median eminence) and in peripheral organs including the adrenal gland. The cDNAs encoding the PACAP precursor and 3 types of PACAP receptors have been cloned in amphibians and their distribution has been determined by in situ hybridization histochemistry. Ontogenetic studies have revealed that PACAP is expressed early in the brain of tadpoles, soon after hatching. In the frog Rana ridibunda, PACAP exerts a large array of biological effects in the brain, pituitary, adrenal gland, and ovary, suggesting that, in amphibians as in mammals, PACAP may act as neurotrophic factor, a neurotransmitter and a neurohormone.

Evidence for the involvement of nitric oxide in the control of steroid secretion by the frog adrenal gland

Nitric oxide (NO) has been found to modulate the response of rat, bovine and human adrenocortical cells to corticotropic factors. The aim of the present study was to investigate the possible involvement of NO in the control of corticosteroid secretion in the frog Rana ridibunda. Histochemical studies using the NADPH-diaphorase reaction and immunohistochemical labeling with antibodies against NO synthase (NOS) revealed that NOS is exclusively expressed in chromaffin cells. The NO donor sodium nitroprusside (SNP) and the NO synthase inhibitor Nw-nitro-L-arginine (L-NO(2)Arg) did not modify the spontaneous production of corticosterone and aldosterone by perifused adrenal slices. Similarly, L-NO(2)Arg had no effect on the secretory responses induced by ACTH, angiotensin II (AII) and endothelin-1 (ET-1). In contrast, SNP significantly inhibited the stimulatory effects of ACTH, AII and ET-1 on corticosterone and aldosterone secretion. These data provide the first evidence for a modulatory role of NO on adrenocortical cell activity in amphibians.

Immunohistochemical localization, biochemical characterization, and biological activity of neurotensin in the frog adrenal gland

The primary structure of neurotensin has been recently determined for the frog Rana ridibunda (Endocrinology 139: 4140-4146, 1998). In the present study, we have investigated the distribution and biochemical characterization of neurotensin-like immunoreactivity in the frog adrenal gland, using an antiserum directed against the conserved C-terminal region of the peptide. Neurotensin-like immunoreactivity was detected in two populations of nerve fibers: numerous varicose fibers coursing between adrenal cells, and a few processes located in the walls of blood vessels irrigating the gland. Reversed-phase HPLC analysis of frog adrenal gland extracts revealed the existence of a major peak of neurotensin-like immunoreactivity that exhibited the same retention time as synthetic frog neurotensin. The possible involvement of neurotensin in the regulation of steroid secretion was studied in vitro using perifused frog adrenal slices. For concentrations ranging from 10(-10) to 10(-5) M, synthetic frog neurotensin increased corticosterone and aldosterone production in a dose-dependent manner (EC50 = 1.2 x 10(-9) M and 5.8 x 10(-10) M, respectively). Repeated administration of neurotensin induced a reproducible stimulation of steroid output without any tachyphylaxis. Prolonged administration (3 h) of frog neurotensin caused a transient increase in corticosterone and aldosterone secretion followed by a decline of corticosteroid secretion. Neurotensin also produced a significant stimulation of corticosteroid secretion from dispersed frog adrenal cells. This study demonstrates that neurotensin is located in nerve processes innervating the adrenal gland of amphibians. The results also show that synthetic frog neurotensin exerts a direct stimulatory effect on corticosteroid output. Taken together, these data support the view that neurotensin, released by nerve fibers, may act as a local regulator of corticosteroid secretion.

Immunohistochemical demonstration of syntaxin and SNAP-25 in chromaffin cells of the frog adrenal gland

The release of catecholamines from chromaffin cells involves specific proteins such as synaptobrevin present in the secretory vesicles as well as syntaxin and synaptosomal-associated protein of 25 kDa (SNAP-25), both present in the plasma membrane. We have found syntaxin and SNAP-25 in chromaffin cells of the frog adrenal gland by immunohistochemistry. This result suggests that the secretion of catecholamines from chromaffin cells involves these proteins in the frog.

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.

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.

An additional arginine-vasotocin-related peptide, vasotocinyl-Gly-Lys, in Xenopus neurohypophysis

The neurohypophysis of Xenopus and that of Ranidae and Bufonidae contain hydrin 1 (vasotocinyl-Gly-Lys-Arg) and hydrin 2 (vasotocinyl-Gly), respectively. In order to test the aldosterone-releasing activity of arginine vasotocin (AVT) and hydrin 1, purification of these peptides from an acid-extract of the neurointermediate lobe of Xenopus laevis was performed using an ODS-silica cartridge and reverse-phase and ion-exchange HPLC columns. As a result, an additional AVT-related peptide was newly found. Amino-acid analysis revealed that this peptide is vasotocinyl Gly-Lys (AVT-GK). The aldosterone-releasing activity of AVT-GK was equivalent to that of hydrin 1 (AVT-GKR) and lower than that of AVT. Like AVT and AVT-GKR, AVT-GK were effective in stimulating water flux from the isolated urinary bladder of the toad. Since AVT-GK is regarded as an intermediate between hydrin 1 and hydrin 2 in terms of its C-terminal form, it was designated hydrin 1'.

Purification and characterization of joining peptide and N-terminal peptide of proopiomelanocortin from the pars distalis of the bullfrog pituitary

The joining peptide (JP) and the N-terminal peptide of proopiomelanocortin (NPP) were isolated from an acid-acetone extract of the distal lobe of the pituitary of the bullfrog, Rana catesbeiana, and purified by gel filtration and reverse-phase high performance liquid chromatography. The amino acid sequence of the bullfrog JP resembled the sequences of the JPs of Rana ridibunda (86% similarity) and Xenopus laevis (54% similarity), as deduced from the nucleotide sequences of their cDNAs. The amino acid sequence of bullfrog NPP showed 100%, 85%, and 50% similarity with those of Rana ridibunda, Xenopus laevis, and human NPPs, respectively. Administration of bullfrog NPP (0.05-5 micrograms/ml) to perifused Rana ridibunda interrenal slices induced a dose-dependent stimulation of corticosterone and aldosterone release. The present results indicate that the primary structure of NPP has been highly conserved during evolution. These data also reveal that NPP, which has no sequence homology with ACTH, exhibits a substantial corticotropic activity.

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.

Interactions between vasotocin and other corticotropic factors on the frog adrenal gland

The adrenocortical cells of the amphibian interrenal (adrenal) gland are controlled by multiple factors including neuropeptides and classical neurotransmitters. In particular, it has recently been shown that vasotocin (AVT), the amphibian counterpart of vasopressin, is a potent stimulator of frog corticosteroidogenesis. In the present study, we have investigated the possible interactions between AVT and other regulatory factors on frog interrenal tissue. When AVT (10(-9) M) and serotonin (10(-6) M) were infused together, a strict addition of the individual effects was observed. Similar results were obtained with concomitant infusion of AVT and vasoactive intestinal peptide or AVT and ACTH. In contrast, when AVT (10(-9) M) and acetylcholine (5 x 10(-5) M) were added together, the increase in corticosteroid secretion was less than additive. Dopamine induced a significant reduction of AVT-evoked stimulation of corticosterone production. These results indicate that regulatory peptides or classical neurotransmitters which participate in the control of adrenal steroidogenesis may interact on their target cell to modulate the activity of their congeners.

Localization and quantification of nonapeptide binding sites in the kidney of Xenopus laevis: evidence for the existence of two different nonapeptide receptors

The distribution and properties of nonapeptide binding sites in the kidney of the anuran Xenopus laevis were investigated using quantitative in vitro autoradiography. The binding studies were performed with [3H]arginine vasopressin (AVP) as ligand because [125I]arginine vasotocin (AVT) lacks biological activity. Specific binding sites for [3H]AVP are located in the glomeruli of the kidney. [3H]AVP binding results in a steady state of association and dissociation between ligand and binding sites. Scatchard and Hill analyses of saturation experiments showed that [3H]AVP binds to a single class of binding sites with a dissociation constant (Kd) of 430 +/- 109 pM and a maximum binding capacity (Bmax) of 5.306 +/- 1.379 fmol/mm2 (n = 8). Displacement studies demonstrated the same affinity of these [3H]AVP binding sites to [3H]AVP, unlabeled AVP, and AVT, whereas mesotocin possesses only weak affinity. Further nonapeptides like oxytocin and isotocin or the mammalian-specific V1 receptor antagonist [1-beta-mercapto-beta,beta-cyclopentamethylene propionic acid)-2-(O-methyl)-tyrosine)-AVP or the V2 receptor agonist (1-deamino-8-D-arginine)-vasopressin or unrelated peptides did not alter the binding of [3H]AVP. The localization of nonapeptide binding sites in the glomeruli with the same affinity to AVP as to AVT agrees with the finding that AVT causes antidiuresis in Xenopus laevis. An earlier study demonstrated Xenopus laevis interrenal tissue to possess a higher sensitivity for AVT than AVP which points to a nonapeptide receptor with a higher affinity for AVT than AVP.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin synthesis in adrenochromaffin cells

The inter-renal (adrenal) gland of amphibians is composed of chromaffin and steroidogenic cells which can interact through a paracrine mode of communication. We have previously shown that serotonin is present in secretory granules of frog adrenochromaffin cells; concurrently, we have demonstrated that serotonin is a potent stimulator of corticosterone and aldosterone secretion by adrenocortical cells. The aim of the present study was to determine the origin of the amine contained in frog chromaffin cells. Using 3H-labelled tryptophan as a precursor, we observed the formation of substantial amounts of serotonin and its metabolite 5-hydroxyindoleacetic acid by frog inter-renal slices. Newly synthesized serotonin was secreted into the incubation medium and the release process was enhanced by depolarizing concentrations of KCl. Fluoxetine, and inhibitor of serotonin uptake, caused an increase of 3H-labelled serotonin in the incubation medium, suggesting that the indoleamine was taken up again by adrenal chromaffin cells. The capacity of the frog inter-renal gland to synthesize serotonin was also demonstrated by incubating inter-renal slices with non-labelled tryptophan or 5-hydroxytryptophan. In these conditions, we observed that the rate of synthesis was higher when 5-hydroxytryptophan was used as a a precursor, rather than tryptophan. Taken together, these results indicate that chromaffin cells, which have the capacity for synthesizing and releasing serotonin, behave like authentic serotonergic paraneurons. As far as is known, these data provide the first evidence for the occurrence of tryptophan-5-hydroxylase activity within the adrenal gland.

Arginine vasotocin (AVT) and AVT-related peptide are major aldosterone-releasing factors in the bullfrog neurointermediate lobe

Two major components which stimulate aldosterone release from Xenopus adrenocortical tissue were isolated from an acid-acetone extract of the neurointermediate lobes of the bullfrog (Rana catesbeiana) using C18 Sep-Pak cartridges, Sephadex G-50, and reverse-phase HPLC columns. One of the components was identified as arginine vasotocin (AVT) from its HPLC profile and amino acid sequence analysis. The other was an AVT-like decapeptide with an extra glycine residue at the C-terminus of nonamidated AVT, which was recently termed hydrin 2. The yields of these two peptides were almost the same. They also showed equipotent activity in stimulating water flux from the isolated urinary bladder of the toad (Bufo japonicus).

Intracerebroventricular injection of arginine vasotocin induces elevation of blood pressure in anesthetized trout

The present investigation assessed the ability of the neurohypophysial nonapeptide arginine vasotocin (AVT) to centrally regulate the cardiovascular activity in fish. Intracerebroventricular (ICV) injection of AVT (0.4 to 50 ng/kg b.wt.) in anesthetized trout resulted in a dose-related increase in blood pressure (BP) without any consistent changes in heart rate. For doses of AVT ranging from 2 to 50 ng/kg b.wt., BP remained elevated during at least 25 min after ICV injection. Systemic (intraarterial) administration of the same doses of AVT appeared to be less efficient than ICV injection, except for the highest dose (50 ng/kg) which evoked a similar rise in BP as that observed after ICV administration. In contrast to AVT, a high concentration of neuropeptide Y (10 micrograms/kg b.wt., ICV) caused only a slight increase of BP. The results suggest that AVT acts centrally to regulate BP in fish. These data, together with the widespread distribution of AVT-immunoreactive fibers and AVT binding sites in the brain, support the notion that, in fish, AVT may play neuromodulator and/or neurotransmitter functions.

A novel 1745-dalton pyroglutamyl peptide derived from chromogranin B is in the bovine adrenomedullary chromaffin vesicle

1. Following the recent demonstration of a glutaminyl cyclase activity localized in adrenomedullary chromaffin vesicles, an assay was developed to isolate and characterize posttranslationally modified peptides from this tissue which contain pyroglutamate. This assay consisted of spectrometric identification of peptides before and after enzymatic removal of pyroglutamyl residues. 2. Using this procedure, a pyroglutamyl peptide (BAM-1745) was isolated and sequenced and was shown to be a significant component of adrenomedullary secretory vesicles. 3. A computer search through the Swiss-Prot protein sequence database revealed a 93% identity of BAM-1745 and a fragment of human chromogranin B (Gln580-Tyr593).

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.