Effect of substance P sequences on isolated neurons of Lymnaea stagnalis

Effects of peptides and of acetylcholine (Ach) on electrical properties of nerve cells were compared using isolated neurons from brain of the freshwater mollusc, Lymnaea stagnalis. The peptides included certain partial sequences of Substance P (SP) and eledoisin (E). The essential C-terminal pentapeptide of eledoisin (E7-11) did not significantly influence the resting membrane potential (MP). SP hexapeptide (SP6-11) caused a dramatic depolarization. SP nonapeptide (SP3-11) had a qualitatively similar effect, but of less magnitude. Both SP6-11 and SP3-11 had a maximal effect on MP when used at a concentration of 10(-6) M. Ach also caused depolarization of the membrane, but recovery subsequent to wash-out was shorter after Ach than after peptide. Ach depolarization was blocked by d-tubocurarine chloride whereas SP6-11-induced depolarization was not antagonized. Unlike Ach, an eledoisin related peptide (EH) was found to decrease discharge rate of spontaneously discharging neurons, and whereas Ach increased membrane conductance, EH did not. These results would further suggest that Ach and these neurotropic peptides act via different mechanisms.