Calcium oscillations in melanotrope cells of Xenopus laevis are differentially regulated by cAMP-dependent and cAMP-independent mechanisms

The role of cAMP dependent protein kinase in modulating spontaneous intracellular Ca²⁺ waves in interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) serve as electrical pacemakers in the rabbit urethra. Pacemaking activity in ICC results from spontaneous intracellular Ca(2+) waves that rely on Ca(2+) release from endoplasmic reticulum (ER) stores. The purpose of this study was to investigate if the action of protein kinase A (PKA) affected the generation of Ca(2+) waves in ICC. Intracellular [Ca(2+)] was measured in fluo-4 loaded ICC, freshly isolated from the rabbit urethra using a Nipkow spinning disc confocal microscope. Application of the PKA inhibitor H-89 (10 μM) significantly inhibited the generation of spontaneous Ca(2+) waves in ICC and this was associated with a significant decrease in the ER Ca(2+) load, measured with 10mM caffeine responses. Ca(2+) waves could be rescued in the presence of H-89 by stimulating ryanodine receptors (RyRs) with 1mM caffeine but not by activation of inositol 1,4,5 tri-phosphate receptors (IP3Rs) with 10 μM phenylephrine. Increasing intracellular PKA with the cAMP agonists forskolin and 8-bromo-cAMP failed to yield an increase in Ca(2+) wave activity. We conclude that PKA may be maximally active under basal conditions in ICC and that inhibition of PKA with H-89 leads to a decreased ER Ca(2+) load sufficient to inactivate IP3Rs but not RyRs.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Plasticity in the melanotrope neuroendocrine interface of Xenopus laevis

Melanotrope cells of the amphibian pituitary pars intermedia produce alpha-melanophore-stimulating hormone (alpha-MSH), a peptide which causes skin darkening during adaptation to a dark background. The secretory activity of the melanotrope of the South African clawed toad Xenopus laevis is regulated by multiple factors, both classical neurotransmitters and neuropeptides from the brain. This review concerns the plasticity displayed in this intermediate lobe neuroendocrine interface during physiological adaptation to the environment. The plasticity includes dramatic morphological plasticity in both pre- and post-synaptic elements of the interface. Inhibitory neurons in the suprachiasmatic nucleus, designated suprachiasmatic melanotrope-inhibiting neurons (SMINs), possess more and larger synapses on the melanotrope cells in white than in black-background adapted animals; in the latter animals the melanotropes are larger and produce more proopiomelanocortin (POMC), the precursor of alpha-MSH. On a white background, pre-synaptic SMIN plasticity is reflected by a higher expression of inhibitory neuropeptide Y (NPY) and is closely associated with postsynaptic melanotrope plasticity, namely a higher expression of the NPY Y1 receptor. Interestingly, melanotrope cells in such animals also display higher expression of the receptors for thyrotropin-releasing hormone (TRH) and urocortin 1, two neuropeptides that stimulate alpha-MSH secretion. Possibly, in white-adapted animals melanotropes are sensitized to neuropeptide stimulation so that, when the toad moves to a black background, they can immediately initiate alpha-MSH secretion to achieve rapid adaptation to the new background condition. The melanotrope cell also produces brain-derived neurotrophic factor (BDNF), which is co-sequestered with alpha-MSH in secretory granules within the cells. The neurotrophin seems to control melanotrope cell plasticity in an autocrine way and we speculate that it may also control presynaptic SMIN plasticity.

The effects of disruption of A kinase anchoring protein-protein kinase A association on protein kinase A signalling in neuroendocrine melanotroph cells of Xenopus laevis

The secretory activity of melanotroph cells from Xenopus laevis is regulated by multiple neurotransmitters that act through adenylyl cyclase. Cyclic adenosine monophosphate (cAMP), acting on protein kinase A (PKA), stimulates the frequency of intracellular Ca(2+) oscillations and the secretory activity of the melanotroph cell. Anchoring of PKA near target proteins is essential for many PKA-regulated processes, and the family of A kinase anchoring proteins (AKAPs) is involved in the compartmentalisation of PKA type II (PKA II) regulatory subunits. In the present study, we determined to what degree cAMP signalling in Xenopus melanotrophs depends on compartmentalised PKA II. For this purpose, a membrane-permeable stearated form of Ht31 (St-Ht31), which dislodges PKA II from AKAP (thus disrupting PKA II signalling), was used. The effect of St-Ht31 on both secretion of radiolabelled peptides and intracellular Ca(2+) signalling by superfused Xenopus melanotrophs was assessed. St-Ht31 stimulated secretion but had no effect on Ca(2+) signalling. We conclude Xenopus melanotrophs possess a St-Ht31-sensitive PKA II that is associated with the exocytosis machinery and, furthermore, that Ca(2+) signalling is regulated by an AKAP-independent signalling system. Moreover, our results support a recent proposal that AKAP participates in regulating PKA activity independently from cAMP.

Receptors for neuropeptide Y, gamma-aminobutyric acid and dopamine differentially regulate Ca2+ currents in Xenopus melanotrope cells via the G(i) protein beta/gamma-subunit

Secretion of alpha-melanophore-stimulating hormone (alphaMSH) from pituitary melanotrope cells of the amphibian Xenopus laevis is under inhibitory synaptic control by three neurotransmitters produced by the suprachiasmatic nucleus: gamma-aminobutyric acid (GABA), neuropeptide Y (NPY) and dopamine (DA). These inhibitory effects occur through G(i)-protein-coupled receptors (G(i)PCR), and differ in strength: GABA(B)-receptor-induced inhibition is the weakest, whereas DA (via a D2-receptor) and NPY (via a Y1-receptor) strongly inhibit, with NPY having a long-lasting effect. Previously it was shown that DA inhibits two (R- and N-type channel) of the four voltage-operated Ca2+ channels in the melanotrope, and that only part of this inhibition is mediated by beta/gamma-subunits of the G(i) protein. We here demonstrate that also the Y1- and GABA(B)-receptor inhibit only part of the total Ca2+ current (I(Ca)), with fast activation and inactivation kinetics. However, GABA(B)-mediated inhibition is weaker than the inhibitions induced via Y1- and D2-receptors (-21 versus -27% and -30%, respectively). Using a depolarizing pre-pulse protocol it was demonstrated that GABA(B)-induced inhibition of I(Ca) most likely depends on Gbeta/gamma-subunit activation whereas Y1- and D2- induced inhibitions are only partially mediated by Gbeta/gamma-subunits. No differences were found between the Y1- and D2-induced inhibitions. These results imply that activation of different G(i)PCR inhibits the I(Ca) through different mechanisms, a phenomenon that may underlie the different potencies of the suprachiasmatic neurotransmitters to inhibit alphaMSH secretion.

Dopamine D2-receptor activation differentially inhibits N- and R-type Ca2+ channels in Xenopus melanotrope cells

Dopamine inhibits pituitary melanotrope cells of the amphibian Xenopus laevis through activation of a dopamine (D2) receptor that couples to a Gi protein. Activated Gi protein subunits are known to affect voltage-operated Ca2+ currents (ICa). In the present study we investigated which Ca2+ currents are regulated by D2-receptor activation and which Gi protein subunits are involved. Whole-cell voltage-clamp patch-clamp experiments from holding potentials (HPs) of -80 and -30 mV show that 28.6 and 36.9%, respectively, of the total ICa was inhibited by apomorphin, a D2-receptor agonist. The inhibited current had fast activation and inactivation kinetics. From an HP of -80 mV, inhibition of N-type Ca2+ currents with omega-conotoxin GVIA and R-type current by SNX-482 reduced the efficacy of the apomorphin-induced inhibition. From an HP of -30 mV this reduction for omega-conotoxin GVIA was still observed. Blocking L-type current by nifedipine or P/Q-type current by omega-agatoxin IVA did not affect apomorphin-induced inhibition at either HP. Our results imply that D2-receptor activation inhibits both N- and R-type Ca2+ currents. Using a strong depolarizing pre-pulse partially reversed the inhibition of the total current by apomorphin. About 50% of this inhibition was achieved through interaction of Gbeta/gamma proteins, and this part of the inhibited ICa had fast activating and inactivating kinetics. However, the other part of the current inhibited by D2-receptor activation may proceed through Galpha-PKA phosphorylation.

Ca2+ oscillations in melanotropes of Xenopus laevis: their generation, propagation, and function

The melanotrope cell of the amphibian Xenopus laevis is a neuroendocrine transducer that converts neuronal input concerning the color of background into an endocrine output, the release of alpha-melanophore-stimulating hormone (alpha-MSH). The cell displays intracellular Ca(2+) oscillations that are thought to be the driving force for secretion as well as for the expression of genes important to the process of background adaptation. Here we review the functioning of the Xenopus melanotrope cell, with emphasis on the role of Ca(2+) oscillations in signal transduction in this cell. We start by giving a general overview of the evolution of Ca(2+) as an intracellular messenger molecule. This is followed by an examination of the melanotrope as a neuroendocrine integrator cell. Then, the evidence that Ca(2+) oscillations drive the secretion of alpha-MSH is reviewed, followed by a similar analysis of the evidence that the same oscillations regulate the expression of proopiomelanocortin (POMC), the precursor protein for alpha-MSH. Finally, the possible importance of the pattern of Ca(2+) signaling to melanotrope cell function is considered.

Sauvagine regulates Ca2+ oscillations and electrical membrane activity of melanotrope cells of Xenopus laevis

Ca2+ oscillations regulate secretion of the hormone alpha-melanphore-stimulating hormone (alpha-MSH) by the neuroendocrine pituitary melanotrope cells of the amphibian Xenopus laevis. These Ca2+ oscillations are built up by discrete increments in the intracellular Ca2+ concentration, the Ca2+ steps, which are generated by electrical membrane bursting firing activity. It has been demonstrated that the patterns of Ca2+ oscillations and kinetics of the Ca2+ steps can be modulated by changing the degree of intracellular Ca2+ buffering. We hypothesized that neurotransmitters known to regulate alpha-MSH secretion also modulate the pattern of Ca2+ oscillations and related electrical membrane activity. In this study, we tested this hypothesis for the secretagogue sauvagine. Using high temporal-resolution Ca2+ imaging, we show that sauvagine modulated the pattern of Ca2+ signalling by increasing the frequency of Ca2+ oscillations and inducing a broadening of the oscillations through its effect on various Ca2+ step parameters. Second, we demonstrate that sauvagine caused a small but significant decrease in K+ currents measured in the whole-cell voltage-clamp, whereas Ca2+ currents remained unchanged. Third, in the cell-attached patch-clamp mode, a stimulatory effect of sauvagine on action current firing was observed. Moreover, sauvagine changed the shape of individual action currents. These results support the hypothesis that the secretagogue sauvagine stimulates the frequency of Ca2+ oscillations in Xenopus melanotropes by altering Ca2+ step parameters, an action that likely is evoked by an inhibition of K+ currents.

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.

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.

Kinetics of calcium steps underlying calcium oscillations in melanotrope cells of Xenopus laevis

Melanotrope cells of Xenopus laevis display intracellular calcium oscillations which are generated at the plasma membrane and travel as a wave through the cytoplasm into the nucleus. An oscillation involves discrete increases in intracellular Ca2+ ('steps'), followed by a relatively smooth return to the basal Ca2+ level. The aim of our investigation was to determine what role these steps play in shaping the Ca2+ signal in melanotrope cells, by conducting a high resolution spatio-temporal analysis of the kinetics of the Ca2+ steps. To this end Fura-red loaded cells were analysed by confocal laser scanning microscopy using the line scanning method to achieve 6 ms time resolution. Furthermore, the kinetics of the steps were analysed in 3 different intracellular areas, to see if there are spatial differences in Ca2+ signalling kinetics. The results showed that each calcium oscillation is built up by 3-4 steps that were generated very quickly and had approximately the same size. Following each Ca2+ step, there was a slow removal of calcium before the next step boosted the overall level of Ca2+. Since the Ca2+ steps were most pronounced directly beneath the plasma membrane, they appear to be generated in this region. The speed of the Ca2+ wave near the membrane exceeded 40 microns/s, indicating an active mechanism for wave propagation. In deeper regions of the cell, the wave speed was much slower (about 8 microns/s) and the size of each step was smaller, indicating that regulation occurs within a narrower range of [Ca2+]i. Inside the nucleus, however, the calcium wave accelerated again (23 microns/s). Treatment with TRH evoked a high amplitude Ca2+ transient and increased the number of Ca2+ steps to 5 or 6. Each step had approximately the same size as the steps of the pretreatment Ca2+ oscillations. Caffeine treatment, which increased the frequency of the oscillations, had no effect on the number or the size of the Ca2+ steps, but it reduced the time needed for each step to reach its maximum height. We suggest a possible 'building block' function for the Ca2+ steps, whereby a cell generates more steps to achieve a high oscillation amplitude or accelerates the speed of the steps to increase the frequency of oscillations. Both phenomena may play a crucial role in the encoding of information transduced from an extracellular input to the intracellular target.