INHIBITORY EFFECTS OF AROMATIC OLIGOPEPTIDES ON THE EXCITABILITY OF IDENTIFIABLE GIANT-NEURONS OF AN AFRICAN GIANT SNAIL (ACHATINA-FULICA-FERUSSAC)

Abstract

Effects of an aromatic tripeptide, L-Lys-L-Phe-L-Tyr and its related dipeptides on the excitability of 2 identifiable giant neurons (the TAN, tonically autoactive neuron and the PON, periodically oscillating neuron) in the subesophageal ganglia of the African giant snail A. fulica, were examined. Of 33 substances examined, 3 aromatic oligopeptides showed a marked inhibitory effect on the TAN excitability. The critical concentrations of these substances in bath application were: L-Lys-L-Phe-L-Tyr (6.6 .cntdot. 10-6 .apprx. 2.2 .times. 10-5 M), L-Phe-L-Try (9.1 .times. 10-6 .apprx. 3.0 .times. 10-5 M) and L-Phe-L-Trp (2.4 .times. 10-5 .apprx. 7.3 .times. 10-5 M). The microdrop application (100 .mu.m in diameter) of a solution of the 3 inhibitory oligopeptides on the TAN surface hyperpolarized the TAN membrane potential and eliminated the spike discharges in the physiological and the chloride-free (replaced with acetate) conditions. The current-voltage relationships (I-V curve) of the TAN neuromembrane were measured by the injection of a transmembrane triangular current. The TAN I-V curve measured in the presence of L-Lys-L-Phe-L-Tyr or L-Phe-L-Trp was almost concordant to that of the normal state, if the 2 I-V curves were superimposed using the firing level as the reference point. The same 3 aromatic oligopeptides also showed an inhibitory effect on the PON excitability. The critical concentrations needed to produce the effects were much higher than those in the case of the TAN.