Ionic mechanism mediating Mytilus inhibitory peptides elicited membrane currents in identified Helix neurons

Effects of seven, pressure applied MIP (Mytilus inhibitory peptides) had be en studied on D-neurons of the CNS of Helix pomatia in voltage-clamp experi ments. In physiological saline, the peptides produced a hyperpolarization u sually coupled with the cessation of any spontaneous spiking activity. Clam ped at the resting potential(similar to -60 mV), peptide applications elici ted an outward current, which increased its amplitude by shifting the holdi ng potential towards depolarisation. The response was concentration-depende nt and accompanied by an increased membrane conductance. Reversal potential s obtained at different [K+], were plotted with a slope of 52 mV per ten-fo ld change in [K+](o) showing that the peptide-elicited current was mainly d ue to the increased K+-conductance(s). The peptide-induced outward current could partially be blocked by Ba2+ (5 mM), CdCl2 (1 mM), TEACl (10 mM) or a pamin (2.5 x 10(-5) M) or furosemide (10 mg/ml) and decreased either in Na-free or Cl--free solutions. 4-Aminopyridine at 5 mM concentration complete ly blocked the peptide-induced current. In the presence of high [K+],, the peptide(s) was still found to induce an outward current at membrane potenti als beyond Kf-reversal potential. This component was not present in Cl--fre e saline, suggesting that the current was due to the inward flow of Cl- ion s. Our results show that the MIPs have at least two (three) independent act ions, each associated with different voltage-, concentration-dependence and ionic mechanisms. It is suggested, that the peptide-induced currents are c arried by K+, and Cl- ions. According to our present finding, the observed effects are mediated by the same receptor, activating different second mess enger systems, inducing multiple conductance changes in the membrane of neu rons of the snail ganglia. (C) 1999 Elsevier Science B.V. All rights reserv ed.