Neurotensin modulates the amplitude and frequency of voltage‐activated Ca²⁺ currents in frog pituitary melanotrophs: implication of the inositol triphosphate/protein kinase C pathway

Abstract

Many excitatory neurotransmitters and neuropeptides regulate the activity of neuronal and endocrine cells by modulating voltage-operated Ca2+ channels. Paradoxically, however, excitatory neuromediators that provoke mobilization of intracellular calcium from inositol trisphosphate (IP3)-sensitive stores usually inhibit voltage-gated Ca2+ currents. We have recently demonstrated that neurotensin (NT) stimulates the electrical and secretory activities of frog pituitary melanotrophs, and increases intracellular calcium concentration in these cells. In the present study, we have investigated the effects of NT on Ca2+ currents in cultured frog melanotrophs by using the perforated patch-clamp technique. Frog neurotensin (f NT) reduced the amplitude and facilitated the inactivation of both L- and N-type Ca2+ currents. Application of the membrane-permeant Ca2+ chelator BAPTA-AM, the sarcoendoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, or the IP3 receptor antagonist 2-APB suppressed the reduction of Ca2+ currents induced by f NT. Incubation of melanotrophs with the diacylglycerol analogue PMA, which causes desensitization of protein kinase C (PKC), or with the PKC inhibitors chelerythrine and calphostin C, reduced the inhibitory effect of f NT. The NT-induced action potential waveforms, applied as voltage-clamp commands, decreased the amplitude of Ca2+ currents, and enhanced Ca2+ influx by increasing the Ca2+ spike frequency. Altogether, these data indicate that the inhibitory effect of f NT on Ca2+ currents results from activation of the IP3/PKC pathway. The observation that NT controls Ca2+ signalling through both amplitude and frequency modulations of Ca2+ currents suggests that NT might induce spacial and temporal changes of intracellular Ca2+ concentration leading to stimulation of exocytosis.

Neurotensin Enhances GABAergic Activity in Rat Hippocampus CA1 Region by Modulating L-Type Calcium Channels

“…The activated PKC potentially belonging to the Ca 2ϩ -dependent, conventional PKC family phosphorylates Ltype Ca 2ϩ channels or their associated proteins to reduce AHP and increase GABA release. Consistent with this scenario, the activity of L-type calcium channel is subject to modulation by PKC (Belmeguenai et al 2002).…”

“…This suggests that the actions of NT are not mediated through apamin-or charybdotoxin-sensitive K Ca . Second, calcium channels could also regulate AHP amplitude mainly because different types of calcium channels (L, N, P/Q, T type) are tightly coupled to K Ca channels (Berkefeld et al 2006;Bowden et al 2001;Empson and Jefferys 2001;Hallworth et al 2003;Wolfart and Roeper 2002;Womack et al 2004). Therefore NT-mediated decreases in interneuron AHP amplitude may be explained by NT-mediated modulation of certain subtypes of calcium channels.…”

Neurotensin Stimulates Both Calcium Mobilization from Inositol Trisphosphate-Sensitive Intracellular Stores and Calcium Influx through Membrane Channels in Frog Pituitary Melanotrophs

“…These data indicate that Ca 2ϩ influx from the extracellular medium is required for the responses of melanotrophs to NT. As NT stimulates electrical activity, we hypothesized that NT induced Ca 2ϩ influx through L-and N-type Ca 2ϩ channels, two types of voltage-activated Ca 2ϩ channels expressed in frog melanotrophs (52). Indeed, the L-type channel blocker nifedipine and the N-type channel blocker -CgTx GVIA inhibited the NT-induced [Ca 2ϩ ] c increase and action potential discharge without affecting depolarization.…”

“…However, the residual secretory response to NT observed in the presence of nifedipine and -CgTx GVIA was larger than that obtained either in Ca 2ϩ -free medium or in the presence of the nonselective Ca 2ϩ channel blocker Ni 2ϩ , indicating that NT-evoked ␣MSH secretion also requires Ca 2ϩ influx through nifedipine-/-CgTx GVIA-insensitive Ni 2ϩ -sensitive channels. The fact that coadministration of nifedipine and -CgTx GVIA almost totally suppress voltage-activated calcium currents in frog melanotrophs (52) suggests that the nifedipine-/-CgTx GVIAinsensitive Ni 2ϩ -sensitive channels involved in the secretory effect of NT might be voltage-insensitive channels. Collectively, these observations support the idea that the Ca 2ϩ influx implicated in the secretory response of melanotrophs to NT can be accounted for by activation of store-operated calcium current, which is sensitive to Ni 2ϩ , Gd 3ϩ (53), and 2-APB (45).…”

Polyphenolic Antioxidants Mimic the Effects of 1,4-Dihydropyridines on Neurotensin Receptor Function in PC3 Cells

“…In guinea pig atria, nifedipine alters the ionotropic response to NT but investigators question whether the effect depends on Ca 2ϩ influx (Golba et al, 1995). Also inconsistent is the fact that NT inhibits, rather than stimulates, VGCC currents in frog melanotrophs (Belmeguenai et al, 2002). These contradictory findings have led us to hypothesize that CCBs can alter NT signaling by exerting effects that do not involve Ca 2ϩ channels.…”