The neuropeptide FMRFa both inhibits and enhances the Ca2+ current in dissociated Helix neurons via independent mechanisms.
1. The modulation of the voltage-activated Ca2+ current by the neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFa) was investigated in dissociated central neurons from Helix aspersa using whole-cell voltage-clamp recording techniques. External Ba2+ was always used as the charge carrier in this study, and the intracellular Ca2+ concentration was buffered to 20 nM with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). 2. Run-down of the Ca2+ currents was not a problem as long as the neurons were dialyzed with a patch electrode filling solution containing ATP (1 or 2 mM). In ATP-dialyzed neurons, the rate of inactivation of the calcium current increased with time without any significant change in the rate of activation. However, when neurons were dialyzed with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 100 microM; with ATP), the rate of inactivation decreased with time. There was no effect of GTP gamma S on the rate of activation of the Ca2+ current. This suggests that guanosine 5'-triphosphate (GTP)-binding proteins (G proteins) are able to modulate the rate of inactivation of the Ca2+ current in Helix neurons. 3. FMRFa both decreased and enhanced the amplitude of the Ca2+ current in these neurons. This inhibition was observed in most neurons, while the enhancement was observed in 20% of the neurons. Although the enhancement usually was preceded by the inhibitory response, sometimes the enhancement was observed separately. 4. The FMRFa-induced inhibition of the Ca2+ current usually consisted of a decrease in both the amplitude and the rate of inactivation of this current, effects that were reduced as the membrane potential was stepped to more depolarized potentials. A pertussis toxin (PTX)-sensitive G protein mediated this response, whereas no evidence was found to suggest the involvement of any known intracellular messenger. Therefore this inhibition may have resulted from a direct coupling between the FMRFa receptor and the Ca2+ channels via a PTX-sensitive G protein. 5. Arachidonic acid (100 microM) irreversibly reduced the amplitude of the Ca2+ current, but it did not alter the relative inhibition of this current by FMRFa. 6. The FMRFa-induced enhancement of the Ca2+ current was difficult to study because it was observed infrequently, and was rarely observed independently of the FMRFa-induced inhibitory response. In addition, the ability of FMRFa to enhance this current usually disappeared with time.(ABSTRACT TRUNCATED AT 400 WORDS)
Yakel
J L
JL
Laboratoire de Neurobiologie, Ecole Normale Supérieure, Paris, France.
D000818:Animals; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects*; D004566:Electrodes; D004594:Electrophysiology; D019835:FMRFamide; D019204:GTP-Binding Proteins / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006372:Helix, Snails / Q000502:physiology*; D009243:NAD / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009479:Neuropeptides / Q000494:pharmacology*; D037342:Pertussis Toxin; D013569:Synapses / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D001095:Arachidonic Acids; D015220:Calcium Channels; D009479:Neuropeptides; D010566:Virulence Factors, Bordetella; D009243:NAD; D016718:Arachidonic Acid; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019835:FMRFamide; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins