Integration of cytoplasmic calcium and membrane potential oscillations maintains calcium signaling in pituitary gonadotrophs.

Inositol 1,4,5-trisphosphate (InsP 3 ) and cAMP are the two second messengers that play an important role in neuronal signaling. Here, we investigated the interactions of InsP 3 -and cAMP-mediated signaling pathways activated by dopamine in striatal medium spiny neurons (MSN). We found that in ϳ40% of the MSN, application of dopamine elicited robust repetitive Ca 2؉ transients (oscillations). In pharmacological experiments with specific agonists and antagonists, we found that the observed Ca 2؉ oscillations were triggered by activation of D1 class dopamine receptors (DARs). We further demonstrated that activation of phospholipase C was required for induction of dopamine- Dopamine is an important transmitter and neuromodulator in the brain. The cellular mechanisms by which dopamine affects neuronal function are only beginning to be elucidated (1, 2). Striatal medium spiny neurons (MSN) 1 express multiple subtypes of dopamine receptors (DARs) (3-5). On the basis of their molecular structure and pharmacological properties, DARs are divided into the D1 class (D1R and D5R) and D2 class (D2R, D3R, and D4R) (6). D1 class DARs are coupled to G s/olf , activation of adenylyl cyclase, and cAMP production (3). Activation of D2 class DARs has dual effects of inhibiting cAMP production (3) and activating phospholipase C␤ (PLC␤) (7). A putative D1 class DAR subtype coupled to PLC activation and phosphatidylinositol 4,5-diphosphate hydrolysis, but not to cAMP production, has been postulated (8 -10), but has not yet been isolated or cloned. Recently, a specific agonist for this receptor (SKF83959) was identified (11). The D1R-binding protein calcyon has been isolated by yeast two-hybrid methods (12). Association of D1/D5 receptors with calcyon enables coupling of D1/D5 receptors with G q/11 , resulting in PLC activation and inositol 1,4,5-trisphosphate (InsP 3 ) generation (12, 13). Cross-talk between cAMP and Ca 2ϩ signaling pathways plays an important role in dopaminergic signaling in the neostriatum (1). Activation of D1 class DARs enhances L-type Ca 2ϩ channel activity (14 -16) and currents via the ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (17) and the N-methyl-D-aspartic acid (NMDA) receptor (18,19). In contrast, activation of D2 class DARs reduces L-type Ca 2ϩ currents (7) and NMDA receptor activity (20). D1/D5 receptormediated facilitation of L-type Ca 2ϩ channels and AMPA and NMDA receptors results from increased phosphorylation of these channels by protein kinase A (PKA) (16, 21) and decreased dephosphorylation of these channels by protein phosphatase-1 (PP1) (17,22,23). DARPP-32 (dopamine-and cAMPregulated phosphoprotein of M r 32,000) (24, 25) is partly responsible for inhibition of PP1 activity following activation of D1/D5 receptors (26). DARPP-32 phosphorylated by PKA at a single threonine residue (Thr-34) is transformed into a potent inhibitor of PP1, which in turn regulates the phosphorylation state of many neurotransmitter receptors and voltage-gated ion channels (1).The type 1 inos...

Section: Dopamine and Ca 2ϩ Signaling In Msnsupporting

confidence: 67%

Dopamine Receptor-mediated Ca2+ Signaling in Striatal Medium Spiny Neurons

Tang

Bezprozvanny

2004

“…Consistent with this, cultured somatotrophs (8,9), lactotrophs (10,11), and immortalized GH cells (12)(13)(14)(15)(16), as well as in situ somatotrophs (17), spontaneously fire APs, and the majority of unstimulated male rat gonadotrophs are quiescent (18). In ovariectomized rats, however, gonadotropin secretion remained low despite the observation that about 50% of the cells examined exhibited spontaneous AP firing (1,19). These observations raise the possibility that the nature of spontaneous AP firing, such as Ca 2ϩ -dependent versus Na ϩ -dependent spiking, or variations in the proportion of excitable cells, and/or frequency of spontaneous firing account for the cell type-specific patterns of basal hormone secretion.…”

mentioning

confidence: 64%

Dependence of Pituitary Hormone Secretion on the Pattern of Spontaneus Voltage-gated Calcium Influx

Goor¹,

Živadinović

Martínez-Fuentes

et al. 2001

110

126

In excitable cells, voltage-gated calcium influx provides an effective mechanism for the activation of exocytosis. In this study, we demonstrate that although rat anterior pituitary lactotrophs, somatotrophs, and gonadotrophs exhibited spontaneous and extracellular calcium-dependent electrical activity, voltage-gated calcium influx triggered secretion only in lactotrophs and somatotrophs. The lack of action potential-driven secretion in gonadotrophs was not due to the proportion of spontaneously firing cells or spike frequency. Gonadotrophs exhibited calcium signals during prolonged depolarization comparable with signals observed in somatotrophs and lactotrophs. The secretory vesicles in all three cell types also had a similar sensitivity to voltagegated calcium influx. However, the pattern of action potential calcium influx differed among three cell types. Spontaneous activity in gonadotrophs was characterized by high amplitude, sharp spikes that had a limited capacity to promote calcium influx, whereas lactotrophs and somatotrophs fired plateau-bursting action potentials that generated high amplitude calcium signals. Furthermore, a shift in the pattern of firing from sharp spikes to plateau-like spikes in gonadotrophs triggered luteinizing hormone secretion. These results indicate that the cell type-specific action potential secretion coupling in pituitary cells is determined by the capacity of their plasma membrane oscillator to generate threshold calcium signals.Although anterior pituitary secretory cells are derived from the same progenitor cells, they differ with respect to their secretory patterns in vitro and in vivo. In vitro, basal prolactin (PRL) 1 and growth hormone (GH) secretion from pituitary fragments, dispersed pituitary cells, and immortalized lacto-somatotrophs is high and is dependent on the extracellular calcium concentration (1-4). In contrast, basal luteinizing hormone (LH) secretion is low and not dependent on the extracellular calcium concentration (1). In vivo, animals bearing ectopic pituitary grafts release high levels of PRL and low levels of LH for a prolonged period, leading to pseudo-pregnancy (5). Because of the high levels of basal GH and PRL secretion, it is not surprising that lactotrophs and somatotrophs are under negative hypothalamic control by G i/o -coupled dopamine and somatostatin receptors, in addition to positive control by Ca 2ϩ -mobilizing and G s -coupled receptors, such as GH-releasing hormone and thyrotropin-releasing hormone receptors. On the other hand, LH secretion from gonadotrophs is under positive hypothalamic control by Ca 2ϩ -mobilizing receptors, including gonadotropin-releasing hormone (GnRH) and endothelin-A, but no inhibitory hypothalamic factor has been identified (6, 7).It is not known what endows lactotrophs and somatotrophs, but not gonadotrophs, with the ability to secrete high levels of hormone in the absence of any stimuli. One possibility is that lactotrophs and somatotrophs fire spontaneous action potentials (APs) that are capable of driving ...

“…Cell Cultures and Hormone Secretion-Anterior pituitary glands from adult female Harlan Sprague-Dawley rats were obtained from Charles River Inc. (Wilmington, MA) and were dispersed into single cells by a trypsin/DNase (Sigma) treatment procedure as described previously (23). All experiments were performed in either identified cells from mixed pituitary cell populations or purified somatotrophs.…”

Section: Methodsmentioning

confidence: 99%

“…In addition to cAMP, GHRH also stimulated cGMP production in a dose-dependent manner but with a shift in the EC 50 of about 1 log unit (Fig. 14B) (23,(37)(38)(39)(40). Calcium-mobilizing agonist-induced [Ca 2ϩ ] i oscillations do not occur synchronously within the cell but originate from a specific sub-plasma membrane locus and propagate throughout the cell (41), including the nucleoplasm.…”

Section: Characterization Of a Ca 2ϩ Oscillator In Somatotrophs 35697mentioning

confidence: 99%

“…This is in contrast with the results by Sims et al (20) and Holl et al (21), who observed spontaneous firing of APs and extracellular Ca 2ϩ -dependent fluctuation in [Ca 2ϩ ] i in a fraction of somatotrophs. Several laboratories have also reported that gonadotrophs, lactotrophs, and corticotrophs exhibit periods of spontaneous firing of APs (22)(23)(24)(25). In gonadotrophs, voltage-gated Ca 2ϩ influx forms membrane-localized [Ca 2ϩ ] i signals, which are not sufficient to trigger gonadotropin secretion (13).…”

Section: In Excitable Cells Oscillations In Intracellular Free Calcimentioning

confidence: 99%

See 1 more Smart Citation

Characterization of a Plasma Membrane Calcium Oscillator in Rat Pituitary Somatotrophs

Tomić

Koshimizu

Yuan

et al. 1999