Augmenting effect of serotonin on the voltage-dependent Ca2+ current and subsequently activated K+ current in Aplysia neurons.
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 CTX-sensitive 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.
Kawasaki
Satoshi
S
Department of Physiology, School of Medicine, Iwate Medical University, Morioka 020-8505, Japan. skawasak@iwate-med.ac.jp
Kimura
Shingo
S
Watanabe
Shuji
S
Fujita
Reiko
R
Matsumoto
Mitsuhiko
M
Sasaki
Kazuhiko
K
D000818:Animals; D001048:Aplysia / Q000166:cytology / Q000502:physiology*; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D002772:Cholera Toxin / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000037:antagonists & inhibitors / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D019319:Okadaic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D037342:Pertussis Toxin / Q000494:pharmacology; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D024681:Potassium Channels, Calcium-Activated / Q000502:physiology*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D011985:Receptors, Serotonin / Q000502:physiology; D012701:Serotonin / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D019311:Staurosporine / Q000494:pharmacology; D019789:Tetraethylammonium / Q000494:pharmacology; D013873:Thionucleotides / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004791:Enzyme Inhibitors; D051036:Large-Conductance Calcium-Activated Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D011985:Receptors, Serotonin; D013873:Thionucleotides; D002104:Cadmium; D006153:Guanosine Diphosphate; D019319:Okadaic Acid; D012701:Serotonin; D015240:Phorbol 12,13-Dibutyrate; D019789:Tetraethylammonium; C023427:guanosine 5'-O-(2-thiodiphosphate); D002772:Cholera Toxin; D037342:Pertussis Toxin; D011493:Protein Kinase C; D019311:Staurosporine; D002118:Calcium