Effects of glutamic acid relatives on the electrical activity of an identified molluscan giant neurone (Achatina fulica Férussac)

Abstract

Effects of glutamic acid and its relatives were examined on the electrical activity of two kinds of neurones (the PON, periodically oscillating neurone, and the TAN, tonically autoactive neurone) identified in the suboesophageal ganglia of an African giant snail, Achatina fulica Férussac. L- and D-Glu, L-Asp, Gly and beta-Ala did not show any effect on the two neurones. However, beta-hydroxyglutamic acid (BHGA) showed a remarkable inhibitory effect on the PON. Erythro-L-BHGA had the strongest effect of these stereoisomers, and the critical concentration of this substance to produce the effect was 10(-6)-3 X 10(-5) g/ml (6-18 muM) when administered in the bath application. We confirmed by the microdrop application that erythro-L-BHGA directly hyperpolarized the PON neuromembrane. When the curve of current-voltage relationships (I-V curve) of the PON neuromembrane measured under erythro-L-BHGA at 10(-5) g/ml (61 muM) was superimposed on the curve of the normal state using the firing level as the common standard, these two curves showed concordance in a wide range of membrane polarization level. This concordance implies that the membrane resistance was maintained normally under erythro-L-BHGA at this concentration. A higher concentration of 10(-4) g/ml (0.61 mM) of this substance caused a decrease of PON membrane resistance and a remarkable elevation of its firing level. The TAN was not sensitive to BHGA, but sensitive to GABA and its derivatives.