Augmenting Effect of Serotonin on the Voltage-Dependent Ca2+ Current and Subsequently Activated K+ Current in Aplysia Neurons

Abstract

Receptor-induced activation of protein kinase C (PKC) plays an important role in modulation of various types of ionic channels in neurons. For example, PKC causes facilitation or long-lasting activation of certain ionic channels involved in spike firing after the receptor stimulation. We investigated the effect of serotonin (5-HT) on the voltage-dependent Ca 2+ channels in RB and RC neurons of Aplysia ganglia under voltage clamp. An outward current response was induced by voltage change of the cell membrane from −60 mV to +10 mV. Application of 5-HT significantly augmented the outward current response to the voltage change. Both the outward current and the augmenting effect of 5-HT markedly decreased when examined in either Ca 2+ -free, 10 mM tetraethylammonium, or 0.3 mM Cd 2+ -solution, indicating the current to be Ca 2+ -activated K + current produced by Ca 2+ entry. Intracellular application of either guanosine 5′-O-(2-thiodiphosphate) or cholera toxin (CTX), reagents for G-proteins, irreversibly blocked the augmenting effect of 5-HT. Application of phorbol dibutylate (PDBu), an activator of PKC, augmented the outward current and the effect of 5-HT was occluded after PDBu application. Staurosporine, a specific inhibitor of PKC, markedly suppressed the augmenting effects of both 5-HT and PDBu on the outward current. However, either 5-HT or PDBu did not augment the Ca 2+ -activated K + current induced by intracellular injection of Ca 2+ but rather depressed it. These results suggest that stimulation of 5-HT receptor may activate a novel type of CTXsensitive G-protein and subsequent PKC, and that phosphorylation of voltage-dependent Ca 2+ channels may result in the increase in Ca 2+ entry and subsequent Ca 2+ -activated K + current. The mechanism may contribute to retain the long-lasting activation without broadening of the spike width during the excitatory response to 5-HT in these neurons. serotonin receptor; cholera toxin; G-protein; protein kinase C; calcium channel

A Gonadotropin-Releasing Hormone-Like Molecule Modulates the Activity of Diverse Central Neurons in a Gastropod Mollusk, Aplysia Californica

“…In addition to direct modulation, ap-GnRH could target the afferents of the recorded neurons to elicit trans-synaptic changes. As the neurocircuits governing movement are complex, it is at present difficult to identify these afferent targets, but the presence of ample sensory–motor connections and interneurons within the central ganglia certainly provides neural substrates for such regulation (Matsumoto et al, 1988; Bley and Tsien, 1990; Kawaski et al, 2007). Overall, ap-GnRH, through both direct and trans-synaptic mechanisms, can generate the diverse responses observed in our intracellular recordings.…”

Effects of Divalent Cations on Outward Potassium Currents in Leech Retzius Nerve Cells

“…Cd 2+ is a well-known specific blocker of Ca 2+ channels and inhibits Ca 2+ cellular uptake. Electrophysiological investigations in various experimental models have shown that Cd 2+ is commonly used to block voltage-dependent Ca 2+ currents (25) and Ca 2+ -activated K + currents (26)(27)(28)(29). In rat pyramidal neurons (26), 200 µM cadmium was found to markedly reduce the early and late components of Ca 2+ -activated K + current (I K(Ca) ).…”