Regulation of pro-opiomelanocortin synthesis by dopamine and cAMP in the amphibian pituitary intermediate lobe

Continuous illumination through larval development suppresses dopamine synthesis in the suprachiasmatic nucleus, causing activation of α-MSH synthesis in the pituitary and abnormal metamorphic skin pigmentation in flounder

In order to better understand the endocrine aberrations related to abnormal metamorphic pigmentation that appear in flounder larvae reared in tanks, this study examined the effects of continuous 24-h illumination (LL) through larval development on the expression of tyrosine hydroxylase-1 (th1), proopiomelanocortin (pomc), α-melanophore-stimulating hormone (α-MSH) and melanin concentrating hormone (MCH), which are known to participate in the control of background adaptation of body color. We observed two conspicuous deviations in the endocrine system under LL when compared with natural light conditions (LD). First, LL severely suppressed th1 expression in the dopaminergic neurons in the anterior diencephalon, including the suprachiasmatic nucleus (SCN). Second, pomc and α-MSH expression in the pars intermedia melanotrophs was enhanced by LL. Skin color was paler under LL than LD before metamorphic pigmentation, and abnormal metamorphic pigmentation occurred at a higher ratio in LL. We therefore hypothesize that continuous LL inhibited dopamine synthesis in the SCN, which resulted in up-regulation of pomc mRNA expression in the melanotrophs. In spite of the up-regulation of pomc in the melanotrophs, larval skin was adjusted to be pale by MCH which was not affected by LL. Accumulation of α-MSH in the melanotrophs is caused by uncoupling of α-MSH synthesis and secretion due to inhibitory role of MCH on α-MSH secretion, which results in abnormal metamorphic pigmentation by affecting differentiation of adult-type melanophores. Our data demonstrate that continuous illumination at the post-embryonic stage has negative effects on the neuroendocrine system and pituitary in flounder.

Extracellular-signal regulated kinase regulates production of pro-opiomelanocortin in pituitary melanotroph cells

The extracellular signal-regulated kinase (ERK) pathway is important in the regulation of neuronal plasticity, although a role for the kinase in regulating plasticity of neuroendocrine systems has not been examined. The melanotroph cells in the pars intermedia of pituitary gland of the amphibian Xenopus laevis are highly plastic, undergoing very strong growth to support the high biosynthetic and secretory activity involving α-melanophore-stimulating hormone (α-MSH), a peptide that causes pigment dispersion in dermal melanophores during the adaptation of the animal to a dark background. In the present study, we tested our hypothesis that ERK-signalling is involved in the regulation of melanotroph cell function during black-background adaptation, namely in the production of pro-opiomelanocortin (POMC), the precursor of α-MSH. Using western blot analyses, we found elevated levels of the activated (phosphorylated) form of ERK in melanotrophs of black- versus white-adapted animals. Treatment of melanotrophs in vitro with the mitogen-activated protein kinase kinase inhibitor U0126 markedly reduced ERK phosphorylation and lowered the transcription as well as the translation of POMC. This same treatment also reduced the expression of BDNF transcript IV and of the immediate early genes c-Fos and Nur77. We conclude that ERK-mediated signalling is important for the maintenance of the melanotroph cells in an active state.

Brain-derived neurotrophic factor stimulates growth of pituitary melanotrope cells in an autocrine way

Brain-derived neurotrophic factor (BDNF) is expressed in the mammalian pituitary gland, in both the anterior and intermediate lobes, where its functional significance is unknown. Melanotrope cells in the intermediate pituitary lobe of the amphibian Xenopus laevis also produce BDNF, which co-exists in secretory granules with α-melanophore-stimulating hormone (α-MSH), a peptide that causes pigment dispersion in dermal melanophores during adaptation of the toad to a dark background. Xenopus melanotropes are highly plastic, undergoing very strong growth to support the high biosynthesis and release of α-MSH in black-adapted animals. In this study we have tested our hypothesis that this enhanced growth of the melanotrope is maintained by autocrine release of BDNF. Furthermore, since the extracellular-regulated kinase (ERK) pathway is a major component of BDNF signaling in neuronal plasticity, we investigated its involvement in melanotrope cell growth. For these purposes melanotropes were treated for 3 days in vitro, with either an anti-BDNF serum or a recombinant tropomyosin-receptor kinase B (TrkB) receptor fragment to eliminate released BDNF, or with the ERK inhibitor U0126. We also applied a novel inhibitor of the TrkB receptor, cyclotraxin-B, to test this receptor's involvement in melanotrope cell growth regulation. All treatments markedly reduced melanotrope cell growth. Therefore, we conclude that autocrine release of BDNF and subsequent TrkB-dependent ERK-mediated signaling is important for melanotrope cell growth during its physiologically induced activation.

Differential neuroendocrine expression of multiple brain-derived neurotrophic factor transcripts

Brain-derived neurotrophic factor (BDNF) is a neurotrophin with important growth-promoting properties. We report here the first characterization of a BDNF gene in an amphibian, Xenopus laevis, and demonstrate that environmental factors can activate this gene in a promoter-specific fashion. The Xenopus BDNF gene contains six promoter-specific 5'-exons and one 3'-protein-encoding exon. We examined the expression of promoter-specific transcripts in Xenopus neuroendocrine melanotrope cells. These cells make a good model to study how environmental factors control gene expression. In animals placed on a black background melanotrope cells more actively produce and release alphaMSH than in animals on a white background. BDNF is cosequestered and coreleased with alphaMSH and stimulates biosynthesis of proopiomelanocortin (POMC), the precursor protein for alphaMSH. Our analysis of the expression of the BDNF transcripts revealed that there is differential use of some BDNF promoters in melanotrope cells, depending on the adaptation state of the frog. During black-background adaptation, stimulation of expression of BDNF transcript IV preceded that of the POMC transcript, suggesting the BDNF gene is an effector gene for POMC expression. The possible mechanisms regulating expression of the various transcripts are discussed on the basis of the potential calcium- and cAMP-responsive elements in the promoter region of exon IV. Finally, we show that the upstream open reading frames of BDNF transcripts I and IV markedly decrease BDNF translation efficiency, giving the first indication for a functional role of untranslated BDNF exons.

Distribution and corticosteroid regulation of glucocorticoid receptor in the brain of Xenopus laevis

Glucocorticoids (GCs) play essential roles in physiology, development, and behavior that are mediated largely by the glucocorticoid receptor (GR). Although the GR has been intensively studied in mammals, very little is known about the GR in nonmammalian tetrapods. We analyzed the distribution and GC regulation of GR in the brain of the frog Xenopus laevis by immunohistochemistry. GR-immunoreactive (GR-ir) cells were widely distributed, with the highest densities in the medial pallium (mp; homolog of the mammalian hippocampus), accumbens, anterior preoptic area (POA; homolog of the mammalian paraventricular nucleus), Purkinje cell layer of the cerebellum, and rostral anterior pituitary gland (location of corticotropes). Lower but distinct GR-ir was observed in the internal granule cell layer of the olfactory bulbs, dorsal and lateral pallium, striatum, various subfields of the amygdala, bed nucleus of the stria terminalis (BNST), optic tectum, various tegmental nuclei, locus coeruleus, raphe nuclei, reticular nuclei, and the nuclei of the trigeminal motor nerves. Treatment with corticosterone (CORT) for 4 days significantly decreased GR-ir in the POA, mp, medial amygdala (MeA), BNST, and rostral pars distalis. Treatment with the corticosteroid synthesis inhibitor metyrapone (MTP) also significantly reduced GR-ir in the POA, mp, MeA and BNST, but not in the rostral pars distalis. Replacement with a low dose of CORT in MTP-treated animals reversed these effects in brain. Thus, chronic increase or decrease in circulating corticosteroids reduces GR-ir in regions of the frog brain. Our results show that the central distribution of GR-ir and regulation by corticosteroids are highly conserved among vertebrates.

Neuronal, Neurohormonal, and Autocrine Control ofXenopusMelanotrope Cell Activity

Amphibian pituitary melanotropes are used to investigate principles of neuroendocrine translation of neural input into hormonal output. Here, the steps in this translation process are outlined for the melanotrope cell of Xenopus laevis, with attention to external stimuli, neurochemical messengers, receptor dynamics, second-messenger pathways, and control of the melanotrope secretory process. Emphasis is on the pathways that neurochemical messengers follow to reach the melanotrope. The inhibitory messengers, dopamine, gamma-aminobutyric acid, and neuropeptide Y, act on the cells by synaptic input from the suprachiasmatic nucleus, whereas the locus coeruleus and raphe nucleus synaptically stimulate the cells via noradrenaline and serotonin, respectively. Autoexcitatory actions are exerted by acetylcholine, brain-derived neurotrophic factor (BDNF), and the calcium-sensing receptor. At least six messengers released from the pituitary neural lobe stimulate melanotropes in a neurohormonal way: corticotropin-releasing hormone, thyrotropin-releasing hormone, BDNF, urocortin, mesotocin, and vasotocin. They all are produced by the magnocellular nucleus and coexist in various combinations in two types of neurohemal axon terminal. Most of the relevant receptors of the melanotropes have been elucidated. Apparently, the neural lobe has a dominant role in activating melanotrope secretory activity. The intracellular mechanisms translating the various inputs into cellular activities like biosynthesis and secretion constitute the adenylyl cyclase-cAMP pathway and Ca(2+) in the form of periodic changes of the intracellular Ca(2+) concentration, known as Ca(2+) oscillations. It is proposed that the pattern of these oscillations encodes specific regulatory information and that it is set by first messengers that control, for example, via G proteins and cAMP-related events, specific ion channel-mediated events in the membrane of the melanotrope cell.

Multiple control and dynamic response of the Xenopus melanotrope cell

Some amphibian brain-melanotrope cell systems are used to study how neuronal and (neuro)endocrine mechanisms convert environmental signals into physiological responses. Pituitary melanotropes release alpha-melanophore-stimulating hormone (alpha-MSH), which controls skin color in response to background light stimuli. Xenopus laevis suprachiasmatic neurons receive optic input and inhibit melanotrope activity by releasing neuropeptide Y (NPY), dopamine (DA) and gamma-aminobutyric acid (GABA) when animals are placed on a light background. Under this condition, they strengthen their synaptic contacts with the melanotropes and enhance their secretory machinery by upregulating exocytosis-related proteins (e.g. SNAP-25). The inhibitory transmitters converge on the adenylyl cyclase system, regulating Ca(2+) channel activity. Other messengers like thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH, from the magnocellular nucleus), noradrenalin (from the locus coeruleus), serotonin (from the raphe nucleus) and acetylcholine (from the melanotropes themselves) stimulate melanotrope activity. Ca(2+) enters the cell and the resulting Ca(2+) oscillations trigger alpha-MSH secretion. These intracellular Ca(2+) dynamics can be described by a mathematical model. The oscillations travel as a wave through the cytoplasm and enter the nucleus where they may induce the expression of genes involved in biosynthesis and processing (7B2, PC2) of pro-opiomelanocortin (POMC) and release (SNAP-25, munc18) of its end-products. We propose that various environmental factors (e.g. light and temperature) act via distinct brain centers in order to release various neuronal messengers that act on the melanotrope to control distinct subcellular events (e.g. hormone biosynthesis, processing and release) by specifically shaping the pattern of melanotrope Ca(2+) oscillations.

Dynamics and plasticity of peptidergic control centres in the retino-brain-pituitary system of Xenopus laevis

This review deals particularly with the recent literature on the structural and functional aspects of the retino-brain-pituitary system that controls the physiological process of background adaptation in the aquatic toad Xenopus laevis. Taking together the large amount of multidisciplinary data, a consistent picture emerges of a highly plastic system that efficiently responds to changes in the environmental light condition by releasing POMC-derived peptides, such as the peptide alpha-melanophore-stimulating hormone (alpha-MSH), into the circulation. This plasticity is exhibited by both the central nervous system and the pituitary pars intermedia, at the level of molecules, subcellular structures, synapses, and cells. Signal transduction in the pars intermedia of the pituitary gland of Xenopus laevis appears to be a complex event, involving various environmental factors (e.g., light and temperature) that act via distinct brain centres and neuronal messengers converging on the melanotrope cells. In the melanotropes, these messages are translated by specific receptors and second messenger systems, in particular via Ca(2+) oscillations, controlling main secretory events such as gene transcription, POMC-precursor translation and processing, posttranslational peptide modifications, and release of a bouquet of POMC-derived peptides. In conclusion, the Xenopus hypothalamo-hypophyseal system involved in background adaptation reveals how neuronal plasticity at the molecular, cellular and organismal levels, enable an organism to respond adequately to the continuously changing environmental factors demanding physiological adaptation.

Altered biosynthesis and secretion of pro-opiomelanocortin in the intermediate and anterior pituitary of carboxypeptidase E-deficient, Cpe(fat)/ Cpe(fat)mice

The biosynthesis and secretion of pro-opiomelanocortin (POMC) was examined in the pituitary of Cpe(fat)/ Cpe(fat)mice, which are deficient in carboxypeptidase E, a sorting receptor for the regulated secretory pathway (Cool D R, Normant E, Shen F S, et al. Cell 1997; 83: 73-83). Dopamine inhibited forskolin-stimulated accumulation of cAMP in the intermediate lobe of Cpe(fat)/ Cpe(fat)mice, showing that their dopamine receptors were fully functional. This result indicates that the elevated, dopamine-insensitive POMC secretion previously observed in the intermediate pituitary of Cpe(fat)/ Cpe(fat)mice was constitutive, rather than due to defective dopamine receptors. Concomitant with the increase in POMC secretion was a twofold increase in POMC mRNA levels and [(35)S]-methionine incorporation into POMC. In the anterior pituitary of Cpe(fat)/ Cpe(fat)mice, a 1.6-fold increase in basal release of POMC was accompanied by a similar increase in [(35)S]-methionine incorporation into POMC, although POMC mRNA levels were unchanged. Thus, the intermediate and anterior pituitary of Cpe(fat)/ Cpe(fat)mice compensate for the constitutive secretion of POMC by upregulating biosynthesis.

Differential effects of dopamine on two frog melanotrope cell subpopulations

The frog intermediate lobe consists of a single endocrine cell type, the melanotrope cells, which are under the tonic inhibitory control of dopamine. Separation of dispersed pars intermedia cells in a Percoll density gradient has revealed the existence of two melanotrope cell subpopulations, referred to as high-density (HD) and low-density (LD) cells. The aim of the present study was to investigate the effects of dopamine on each of these melanotrope cell subsets. Increasing doses of dopamine, ranging from 10(-9)-10(-6) M, inhibited the release of alpha-melanocyte-stimulating hormone (alpha-MSH) in LD (but not in HD) melanotrope cells. In addition, dopamine provoked a significant reduction of the rate of acetylation of alpha-MSH in LD cells but not in HD cells. Similarly, dopamine significantly decreased the accumulation of POMC messenger RNA in LD cells, whereas it did not affect POMC gene expression in the HD melanotrope subset. On the other hand, microfluorimetric studies revealed that dopamine induced a significant reduction of KCl-stimulated cytosolic free calcium concentration in both LD and HD cells. The present study provides additional evidence for functional heterogeneity of melanotrope cells in the frog pars intermedia. Because dopamine plays a pivotal role in the regulation of alpha-MSH secretion, these data suggest the involvement of cell heterogeneity in the physiological process of background color adaptation in amphibians.

Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia

This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.

Expression of the Xenopus D2 dopamine receptor. Tissue-specific regulation and two transcriptionally active genes but no evidence for alternative splicing

In the amphibian Xenopus laevis the D2 dopamine receptor is involved in the regulation of the melanotrope cells of the intermediate pituitary during background adaptation of the animal. The Xenopus D2 receptor has been found to be pharmacologically different from the mammalian D2 receptor. In a number of mammalian species alternative splicing generates two molecular forms of the D2 receptor. These isoforms differ by the presence or absence of 29 amino acids in the third cytoplasmic loop which is thought to be involved in guanine-nucleotide-binding-regulatory-protein (G-protein) binding of the receptor. We previously described a cDNA encoding the large isoform of the Xenopus D2 receptor. Here we report on the isolation of a brain cDNA encoding a second, structurally different Xenopus D2 dopamine receptor. Both Xenopus receptors correspond to the large isoform of the D2 receptor and they display a high degree of sequence identity with their mammalian counterparts. Their occurrence reflects the expression of two Xenopus D2 receptor genes and they are expressed to approximately the same level. In contrast to mammals, PCR analysis gave no evidence for alternative splicing during D2 receptor expression in Xenopus brain and pituitary. Tissue-specific expression of the Xenopus D2 receptor was observed in the pituitary during background adaptation. The low level of receptor mRNA in melanotrope cells of white animals compared to that of black animals may be caused by chronic dopamine stimulation of melanotrope cells in white animals with consequent cellular desensitization and down regulation of the D2 receptor gene.

Structural analysis of the entire proopiomelanocortin gene of Xenopus laevis

In the pars intermedia of the pituitary the prohormone proopiomelanocortin (POMC) is tissue-specifically processed to, among other peptides, alpha-melanotropin (alpha MSH). In the South African clawed toad Xenopus laevis this hormone mediates the process of background adaptation: release of alpha-MSH causes darkening of the animal, while inhibition of alpha-MSH release results in a pale toad. Elevated release of alpha-MSH coincides with a higher rate of POMC gene transcription. The present study aims to find possible transcriptional regulatory elements in the Xenopus POMC gene. For that purpose the complete nucleotide sequence of the POMC gene and its 5'- and 3'- flanking regions were determined and analyzed. The Xenopus POMC gene promoter contains several regions which may be regulatory DNA elements in view of their similarity with corresponding regions of mammalian POMC gene promoters. In the rat POMC gene promoter, many of these regions represent protein-binding sequences. Besides the promoter sequence and the protein-coding sequences, no other segments with significant identity between the Xenopus and human POMC genes were found. Intron A of the Xenopus POMC gene contains a simple sequence, (TATC)76, and a JH12 repetitive element, while the 3'-flanking region contains a repetitive-EcoRI-monomer-2 element. Comparison of the JH12 sequence of the POMC gene with JH12 sequences from other Xenopus genes revealed a 335-bp consensus sequence which is flanked by a 30-bp inverted repeat. This JH12 consensus sequence is significantly larger than the previously reported JH12 core region. Alignment of intron B of the Xenopus POMC gene with database sequences revealed a consensus sequence of a novel Xenopus repetitive element of 330 bp flanked by a nearly perfect inverted repeat, indicating that this element may be a transposon-like element.

Cloning and sequence analysis of brain cDNA encoding a Xenopus D2 dopamine receptor

A D2 dopamine receptor pharmacologically different from the mammalian D2 receptor has previously been characterized in the amphibian Xenopus laevis. Here we report the cloning of a Xenopus D2 receptor which revealed about 75% amino acid sequence identity with its mammalian counterpart and the presence of an additional 33 amino acid sequence in the 3rd cytoplasmic loop instead of the additional 29 residues in the large form of the mammalian D2 receptor. All 7 predicted transmembrane domains are highly conserved between the Xenopus and mammalian D2 receptors, as are the 1st and 2nd intracellular loop, the 1st and 3rd extracellular loop and the carboxy-terminal portion of the receptors. The amino-terminal portion, the 2nd extracellular loop and the middle portion of the 3rd intracellular loop of these receptors, however, differ considerably. Knowledge of the locations of these regions of conservation and divergence within the D2 receptors of Xenopus and mammals will help to delineate portions of the receptor molecule that are functionally important. Interestingly, the 5'-untranslated region of the Xenopus D2 receptor mRNA contains 4 small open reading frames which may affect translational efficiency.

The CRF-related peptide sauvagine stimulates and the GABAB receptor agonist baclofen inhibits cyclic-AMP production in melanotrope cells of Xenopus laevis

Release of alpha-MSH from the pars intermedia melanotrope cells of Xenopus laevis is regulated by various classical neurotransmitters and neuropeptides. We have examined the effect of two of these regulatory substances, the neurotransmitter GABA and the CRF-related peptide sauvagine, on the adenylate cyclase system of the melanotrope cells. Sauvagine treatment, which stimulates alpha-MSH release, lead to an elevation in the level of cyclic-AMP, an effect which was potentiated by cholera toxin. Treatment with baclofen, a GABAB receptor agonist, gave a pertussis toxin-sensitive decrease in the cyclic-AMP level and an inhibition of alpha-MSH release. We conclude that sauvagine stimulates alpha-MSH secretion through activation of adenylate cyclase and that GABAB receptor activation inhibits secretion through inhibition of cyclic-AMP production. Baclofen treatment sensitized melanotrope cells to the stimulatory action of 8-bromo-cyclic-AMP on the secretion of alpha-MSH. This observation supports the conclusion that GABAB receptor activation inhibits cyclic-AMP production.

Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism

Recently we have demonstrated that dopamine inhibits action potentials in cultured frog melanotrophs through D2 receptor-mediated activation of hyperpolarizing potassium current and reduction of calcium and sodium currents. Herein, the respective roles of G proteins, guanosine-5'-triphosphate and adenosine-3':5'-cyclic-monophosphate in dopamine-induced electrical responses were investigated using the whole-cell patch-clamp technique. Pretreatment of melanotrophs with pertussis toxin (1 microgram/ml) abolished the hyperpolarization and arrest of action potentials evoked by dopamine (1 microM) in 77% of the cells studied. Addition of guanosine-5'-O-(2-thiodiphosphate) (500 microM) to the intracellular solution did not alter the effects of a first exposure to dopamine, but completely blocked the response of cultured melanotrophs to subsequent pulses of dopamine. In cells which were dialysed with guanosine-5'-O-(3-thiotriphosphate) (100 microM) dopamine caused a sustained hyperpolarization and an irreversible inhibition of spikes. Voltage-clamp recordings with electrodes containing guanosine-5'-O-(3-thiotriphosphate), showed that the increase of potassium current and decrease of calcium and sodium currents caused by dopamine were irreversible. These effects were not modified when the pipette contained, in addition to guanosine-5'-O-(3-thiotriphosphate), a high concentration of adenosine-3':5'-cyclic-monophosphate (100 microM) together with the inhibitor of phosphodiesterases 3-isobutyl-1-methylxanthine (100 microM). It is concluded that, in cultured frog melanotrophs, a pertussis toxin-sensitive G protein is implicated in the coupling of dopamine D2 receptors to activation of potassium channels and inhibition of calcium and sodium channels. Our results also indicate that the G protein-mediated signal transduction does not involve the adenylate cyclase system.

Molecular and cellular regulation of neuropeptide expression: the bag cell model system

The bag cell neuroendocrine system of Aplysia californica has been under intensive investigation for nearly two decades. The favorable morphology and hardiness in organ culture of this preparation have permitted a wide range of electrophysiological, cellular, and molecular studies. In this review we have focused our attention on the biochemical and physiological processes that serve the principle function of the bag cells: the synthesis and secretion of the neuropeptide egg-laying hormone. Although these cells were at first considered a model system for the most elementary neuroendocrine mechanisms, increasing knowledge has disclosed a surprising degree of complexity in both neuropeptide biosynthesis and the electrophysiological processes responsible for secretion. Not only may various components of the prohormone be sorted into different classes of neurosecretory granules, which may in turn have different probabilities of secretion, but biosynthesis itself appears to be regulated by the same intracellular messengers that mediate the electrophysiological discharge cycle. Hence, the bag cells, and presumably other peptidergic neurons, appear to possess an array of regulatory processes that can modulate the amount and character of their secretory output. The interactions of these processes may confer a degree of plasticity to the functional expression of peptidergic neurons unanticipated in studies of other neuron types.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Regulation of cyclic-AMP synthesis in amphibian melanotrope cells through catecholamine and GABA receptors

Catecholamines and GABA are neurotransmitters involved in the regulation of release of pro-opiomelanocortin (POMC) derived peptides from the neurointermediate lobe of Xenopus laevis. The present study concerns the relation of these neurotransmitters to the adenylate cyclase system of the melanotrope cell. During in vitro incubation of isolated melanotrope cells it was found that dopamine, adrenaline and LY 171555 induced inhibition of forskolin-stimulated cAMP production and concomitantly inhibited MSH release. Activation of the GABAb receptors by baclofen also induced inhibition of cAMP production and alpha MSH secretion. Activation of the GABAa receptors evoked stimulation of cAMP production, while alpha MSH release was slightly inhibited, indicating that the GABAa mechanism may prove to be complex. A dual regulation through two subtypes of this receptor might be involved, one stimulating release through the adenylate cyclase system, while the other would inhibit secretion.

Identification of the D2-dopamine receptor binding subunit in several mammalian tissues and species by photoaffinity labeling

Photoaffinity labeling of the D2-dopamine receptor in plasma membrane preparations of various tissues from several mammalian species was performed using the recently developed D2-dopaminergic antagonist probe [125I]N-(p-azidophenethyl)spiperone ([125I]N3-NAPS). In tissues containing D2-receptors such as the corpus striatum from rat, dog, calf, hamster, guinea pig, and rabbit as well as the anterior pituitary of rat, bovine, and hamster, the probe covalently labels a peptide of Mr = 94,000. Specificity of the labeling is typically D2-dopaminergic in character. The covalent labeling is blocked by (+)-butaclamol but not by the inactive (-)isomer. Agonists block incorporation with the order of potency: N-n-propylnorapomorphine greater than apomorphine greater than dopamine. The D2-selective antagonist spiperone blocks labeling of the Mr = 94,000 peptide whereas the D1-selective antagonist SCH-23390 is ineffective. Thus, these results indicate that the ligand binding subunit of the D2-dopamine receptor resides on a Mr = 94,000 peptide in these various tissues from several species. Under conditions where proteolysis is not stringently controlled, peptides of lower Mr (32-38,000) are labeled at the expense of the Mr = 94,000 peptide. The most efficient protease inhibitor tested in these systems was EDTA, suggesting that the generation of these lower Mr receptor fragments might be the result of a metal-dependent proteolysis in the membrane preparations. In the rat neurointermediate lobe, a tissue containing D2-receptors, [125I]N3-NAPS specifically labels a major peptide of Mr approximately equal to 120,000 in addition to the Mr = 94,000 peptide.(ABSTRACT TRUNCATED AT 250 WORDS)