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      Electrophysiological Properties of Pituitary Cells in Primary Culture from Atlantic Cod (Gadus morhua)

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          The aim of the present study was to explore the electrophysiological properties of pituitary cells from Atlantic cod (Gadus morhua), as a basis for future studies of the signaling pathways involved in the control of pituitary secretion in this species. Primary cultures of pituitary cells from maturing Atlantic cod were prepared by trypsin treatment and mechanical dispersion. Electrophysiological recordings were performed using the perforated patch clamp method. A subpopulation of large cells were selected for recordings. Spontaneous action potentials were observed in about 30% of the cells. The action potentials displayed a fast initial spike followed by a prolonged plateau. Correspondingly, the inward current elicited by depolarizing steps consisted of both a transient, tetrodotoxin-sensitive Na<sup>+</sup> component and a nifedipine-sensitive Ca<sup>2+</sup> component that was sustained when Ba<sup>2+</sup> replaced Ca<sup>2+</sup> as current carrier. The outward current was partially blocked both by 5 m M tetraethylammonium and 10 m M 4-aminopyridine. The voltage-activated ion channels present in these cells largely correspond to the ion channels of pituitary cells in other teleosts (goldfish, Carassius auratus, and tilapia, Oreochromis mossambicus) and mammals, although differences exist regarding the shape and duration of action potentials.

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          Most cited references 18

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          The brain-pituitary-gonad axis in male teleosts, with special emphasis on flatfish (Pleuronectiformes).

          The key component regulating vertebrate puberty and sexual maturation is the endocrine system primarily effectuated along the brain-pituitary-gonad (BPG) axis. By far most investigations on the teleost BPG axis have been performed on salmonids, carps, catfish and eels. Accordingly, earlier reviews on the BPG axis in teleosts have focused on these species, and mainly on females (e.g. 'Fish Physiology, vol. IXA. Reproduction (1983) pp. 97'; 'Proceedings of the Fourth International Symposium on the Reproductive Physiology of Fish. FishSymp91, Sheffield, UK, 1991, pp. 2'; 'Curr. Top. Dev. Biol. 30 (1995) pp. 103'; 'Rev. Fish Biol. Fish. 7 (1997) pp. 173'; 'Proceedings of the Sixth International Symposium on the Reproductive Physiology of Fish. John Grieg A/S, Bergen, Norway, 2000, pp. 211'). However, in recent years new data have emerged on the BPG axis in flatfish, especially at the level of the brain and pituitary. The evolutionarily advanced flatfishes are important model species both from an evolutionary point of view and also because many are candidates for aquaculture. The scope of this paper is to review the present status on the male teleost BPG axis, with an emphasis on flatfish. In doing so, we will first discuss the present understanding of the individual constituents of the axis in the best studied teleost models, and thereafter discuss available data on flatfish. Of the three constituents of the BPG axis, we will focus especially on the pituitary and gonadotropins. In addition to reviewing recent information on flatfish, we present some entirely new information on the phylogeny and molecular structure of teleost gonadotropins.
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            Biophysical basis of pituitary cell type-specific Ca2+ signaling-secretion coupling.

            All secretory pituitary cells exhibit spontaneous and extracellular Ca2+-dependent electrical activity. Somatotrophs and lactotrophs fire plateau-bursting action potentials, which generate Ca2+ signals of sufficient amplitude to trigger hormone release. Gonadotrophs also fire action potentials spontaneously, but as single, high-amplitude spikes with limited ability to promote Ca2+ influx and secretion. However, Ca2+ mobilization in gonadotrophs transforms single spiking into plateau-bursting-type electrical activity and triggers secretion. Patch clamp analysis revealed that somatotrophs and lactotrophs, but not gonadotrophs, express BK (big)-type Ca2+-controlled K+ channels, activation of which is closely associated with voltage-gated Ca2+ influx. Conversely, pituitary gonadotrophs express SK (small)-type Ca2+-activated K+ channels that are colocalized with intracellular Ca2+ release sites. Activation of both channels is crucial for plateau-bursting-type rhythmic electrical activity and secretion.
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              Mechanism of GnRH receptor signaling: combinatorial cross-talk of Ca2+ and protein kinase C.

               Z. Naor,  D. Harris,  S Shacham (1998)
              Gonadotropin-releasing hormone (GnRH), the first key hormone of reproduction, is synthesized in the hypothalamus and is released in a pulsatile manner to stimulate pituitary gonadotrope-luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and release. Gonadotropes represent only about 10% of pituitary cells and are divided into monohormonal cells (18% LH and 22% FSH cells) and 60% multihormonal (LH + FSH) cells. GnRH binds to a specific seven transmembrane domain receptor which is coupled to Gq and activates sequentially different phospholipases to provide Ca2+ and lipid-derived messenger molecules. Initially, phospholipase C is activated, followed by activation of both phospholipase A2 (PLA2) and phospholipase D (PLD). Generation of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG) lead to mobilization of intracellular pools of Ca2+ and activation of protein kinase C (PKC). Early DAG and Ca2+, derived via enhanced phosphoinositide turnover, might be involved in rapid activation of selective Ca(2+)-dependent, conventional PKC isoforms (cPKC). On the other hand, late DAG, derived from phosphatidic acid (PA) via PLD, may activate Ca(2+)-independent novel PKC isoforms (nPKC). In addition, arachidonic acid (AA) which is liberated by activated PLA2, might also support selective activation of PKC isoforms (PKCs) with or without other cofactors. Differential cross-talk of Ca2+, AA, and selective PKCs might generate a compartmentalized signal transduction cascade to downstream elements which are activated during the neurohormone action. Among those elements is the mitogen-activated protein kinase (MAPK) cascade which is activated by GnRH in a PKC-, Ca(2+)-, and protein tyrosine kinase (PTK)-dependent fashion. Transcriptional regulation can be mediated by the activation of transcription factors such as c-fos by MAPK. Indeed, GnRH activates the expression of both c-jun and c-fos which might participate in gene regulation via the formation of AP-1. The signaling cascade leading to gonadotropin (LH and FSH) gene regulation by GnRH is still not known and might involve the above-mentioned cascades. AA and selective lipoxygenase products such as leukotriene C4 also participate in GnRH action, possibly by cross-talk with PKCs, or by an autocrine/paracrine amplification cycle. A complex combinatorial, spatial and temporal cross-talk of the above messenger molecules seems to mediate the diverse effects elicited by GnRH, the first key hormone of the reproductive cycle.

                Author and article information

                S. Karger AG
                August 2007
                11 June 2007
                : 86
                : 1
                : 38-47
                Department of Molecular Biosciences, University of Oslo, Oslo, Norway
                103867 Neuroendocrinology 2007;86:38–47
                © 2007 S. Karger AG, Basel

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                Page count
                Figures: 5, References: 27, Pages: 10
                Cellular Communication, Receptors and Signalling


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