An electrophysiological investigation of the actions of FMRF amide and related peptides on neurones of 'Helix aspersa'

Abstract

1. The actions of FMRFamide and related peptides on identified and some unidentified Helix aspersa neurones have been investigated using electrophysiological techniques. 2. Neurones were voltage clamped using a single microelectrode. The electrode was rapidly switched between recording the membrane potential and passing current using a sample and hold amplifier. 3. Peptides were applied to the neurones by ionophoresis, pressure ejection, or bath application. No qualitative differences were observed between responses induced by different application methods. 4. The ionic mechanisms of the responses were investigated by ion substitution experiments and application of channel blocking compounds. 5. Three responses were induced by FMRFamide: a hyperpolarizing response resulting from a slow increase in gK, a rapidly desensitizing depolarizing response resulting mainly from an increase in gNa, and a voltage dependent response resulting from a suppression of an outward K current (a decrease in gK). 6. Combinations of the above responses occurred in individual neurones. The increase in and the increase in gNa occurred simultaneously in the F2 neurone, while the increase in gK and the decrease in gK occurred simultaneously in the Cl neurone. 7. The responses induced by FLRFamide and FIRFamide had similar properties as FMRFamide induced responses. It is concluded that these three tetrapeptides activate the same receptors. 8. Responses to pQDPFLRFamide and YGGFMRFamide were different to those induced by the tetrapeptides. Both of these heptapeptides induced a fast hyperpolarization which was the result of a fast increase in however, they failed to activate the increase in gNa. 9. Four responses were observed to peptides with the common sequence -Met (or Leu) -Arg-Phe-NH2: a slow increase in gK and an increase in gNa which were activated mainly by the tetrapeptides, and a fast increase in gK and a decrease in gK which were activated mainly by the heptapeptides. 10. The results are consistent with the presence of four types of FMRFamide receptors on Helix neurones. 11. Both FMRFamide and pQDPFLRFamide are present in Helix. The different actions of these two peptides reported here imply that they have different physiological roles in Helix.