H.J. Leenders

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.

Differential effects of coexisting dopamine, GABA and NPY on α-MSH secretion from melanotrope cells of Xenopus laevis

The secretion of alpha-MSH from the intermediate lobe of the pituitary gland of the amphibian Xenopus laevis is under complex neural control. Three neurotransmitters, dopamine, GABA and NPY, coexist in nerve terminals that contact the melanotrope cells. All three neurotransmitters inhibit alpha-MSH release. We have investigated the significance of this neurotransmitter coexistence for the regulation of alpha-MSH release, using an in vitro superfusion system. From experiments where lobes were treated with various combinations of receptor agonists we conclude that the transmitters act in an additive way but have clear, differential actions. Inhibition of secretion by either dopamine, isoguvacine (GABAA receptor agonist) or baclofen (GABAB receptor agonist) occurs rapidly and alpha-MSH secretion rapidly returns when treatment is terminated (recovery from baclofen being relatively fast, that from dopamine relatively slow); in contrast, inhibition by NPY and recovery from NPY-induced inhibition occurs only very slowly. Differential effects of the transmitters were also seen in experiments with 8-bromo-cyclic AMP, which strongly stimulates alpha-MSH secretion from isoguvacine- or baclofen-treated lobes, but is relatively ineffective in stimulating secretion from lobes treated with dopamine or NPY. NPY, furthermore, enables a short phasic stimulation of secretion by isoguvacine and attenuates the inhibitory action of dopamine and baclofen. Altogether it is concluded that the coexisting factors differentially affect the secretory process of the melanotrope cells of Xenopus laevis. NPY has a slow, sustained action whereas dopamine and GABA act fast.

New aspects of signal transduction in the Xenopus laevis melanotrope cell.

Light and temperature stimuli act via various brain centers and neurochemical messengers on the pituitary melanotrope cells of Xenopus laevis to control distinct subcellular activities such as the biosynthesis, processing, and release of alpha-melanophore-stimulating hormone (alphaMSH). The melanotrope signal transduction involves the action of a large repertoire of neurotransmitter and neuropeptide receptors and the second messengers cAMP and Ca(2+). Here we briefly review this signaling mechanism and then present new data on two aspects of this process, viz. the presence of a stimulatory beta-adrenergic receptor acting via cAMP and the egress of cAMP from the melanotrope upon a change of alphaMSH release activity.

Inhibition of alpha-MSH secretion is associated with increased cyclic-AMP egress from the neurointermediate lobe of Xenopus laevis.

Cyclic-AMP is known to be released from cells and tissues and the amounts released have been reported to reflect intracellular cAMP levels. To measure cAMP release the phosphodiesterase inhibitor IBMX is often used to increase the amount of cAMP to a detectable level. Using this method to follow cAMP dynamics of melanotrope cells in the neurointermediate pituitary lobe of the amphibian Xenopus laevis we show that the alpha-MSH secreto-inhibitors baclofen (GABAB receptor agonist) and dopamine inhibit cAMP release, confirming the idea that these factors inhibit alpha-MSH secretion by reducing adenylyl cyclase activity. Using a sensitive cAMP radioimmunoassay we were able to measure cAMP release from Xenopus neurointermediate lobes in the absence of IBMX. Both baclofen and dopamine appeared to inhibit alpha-MSH secretion but strongly stimulated the release of cAMP. This indicates that the extracellular cAMP level is not a reliable parameter to measure the intracellular cAMP level in the absence of IBMX. The data furthermore suggest that cAMP release is a physiologically regulated process, which might be involved in lowering intracellular cAMP levels associated with a cellular secretory compartment. No apparent differences could be found in the lobe content of cAMP at the termination of secreto-inhibitor treatment, leading to the idea that the cAMP compartment associated with secretion in small relative to the total amount of cAMP present in the lobe.

Respiration of larval salivary glands of Drosophila in relation to the activity of specific genome loci.

Abstract Oxygen consumption of intact larval salivary glands of Drosophila hydei was measured after the addition of intermediates of the citric acid cycle or amino acids to the incubation medium. The effect of these substances on respiration of glands previously submitted to anaerobiosis in vivo was compared with that of glands of control larvae. Only isocitrate and tyrosine stimulated respiration of anaerobically treated glands to a much higher extent than glands of control larvae. This stimulatory effect was abolished when RNA or protein synthesis was inhibited. It is suggested that some of the specific puffs occurring as a consequence of anaerobiosis reflect gene activity required for an increase in utilization of isocitrate and tyrosine for respiration under conditions of stress.

Studies on the mitochondrial NAD-dependent isocitrate dehydrogenase of Drosophila larvae after induction of gene activity by anaerobiosis

Abstract Electrophoretic profiles of the mitochondrial NAD-dependent isocitrate dehydrogenase (EC 1.1.1.41) (IDH) obtained from salivary glands of Drosophila larvae which had been subjected to CO 2 anaerobiosis followed by a period of recovery in air compared with the enzyme from the untreated larvae revealed significant changes in the relative enzyme activity of different protein bands. No qualitative changes in the electrophoretic pattern were detected. The enzyme affinity for NAD + and isocitrate was significantly higher in treated larvae than in the controls. However, the enzyme response to ADP stimulation was lower in treated larvae than that in the control larvae. The difference in properties of IDH from anaerobically treated and untreated larvae suggests that the enzyme from treated larvae was activated to a higher extent than in control larvae. The possible relationship between newly induced gene activities and the changes in mitochondrial IDH occurring in a definite sequence during recovery from anaerobiosis, are discussed.

A change in the affinity of larval α-glycerophosphate dehydrogenase in relation to the activity of the heat shock genes of Drosophila hydei

Abstract After activation of the heat shock genes in Drosophila hydei salivary glands, the K m of α-glycerophosphate dehydrogenase (E.C.1.1.1.8., α-GPDH) for α-glycerophosphate decreases, whereas the app. V m remains the same. Although RNA and protein synthesis are required for this change to occur in vivo, this decrease is not due to de novo synthesis of the enzyme: a similar change in K m can be produced in vitro when extracts from control glands are mixed with extracts from heat-shocked cells. The in vitro reaction is probably due to a deacetylation of the enzyme, since it can be prevented by eserine sulphate and acetyl CoA but not by ATP or a trypsin inhibitor. The activation of the heat shock genes is thus accompanied by the appearance of a factor capable of modifying α-GPDH activity. Alpha-glycerophosphate levels in induced and control glands are determined. The larval α-GPDH has been purified 1150-fold by affinity chromatography. The purified enzyme has a subunit mol. wt of 32,000 and thus differs from other known heat shock proteins. The K m of the enzyme purified from extracts of control or induced glands is the same as that measured in crude extracts, the decrease in app. K m found is therefore a property of the enzyme itself and not due to some contaminant present in the crude extract.

A slow and a fast secretory compartment of POMC-derived peptides in the neurointermediate lobe of the amphibian Xenopus laevis

1. Peptide release from the neurointermediate lobe of Xenopus laevis has been studied using dual pulse-chase incubation, superfusion and HPLC techniques. 2. Lobes release pulse-labelled material in two phases, the first phase lasting about 6 hr, the second persisting up to 14 hr. 3. In both phases similar, POMC-derived peptides are released. Their release can be inhibited by dopamine. 4. When release during the first phase is inhibited, newly synthesized peptides are shunted into the second release pathway. 5. It is concluded that the neurointermediate lobe contains two release compartments. The possible locations of these compartments within melanotrope cells have been discussed.

Evidence for Independently Regulated Secretory Pathways of Proopiomelano‐cortin‐Related Peptides in the Mouse Pars Intermedia

The existence of multiple secretory pathways within endocrine cells has been receiving increasing attention (1–5). In some cases there is evidence for independently regulated pathways, for example in prolactin‐producing cells (6) and parathyroid hormone‐producing cells (7, 8). These pathways concern newly synthesized hormone versus hormone sequestered in mature secretory compartments. In the present study, we have analysed secretion of newly synthesized and mature peptides from proopiomelanocortin (POMC)‐producing cells of the mouse pars intermedia. Evidence has been found for independent regulation and we show that the peptide composition of the two secretory pathways differs with respect to some of the POMC‐derived peptides.