Effects of compounds related to β-hydroxyglutamic acid (BHGA) on identifiable giant neurones of an African giant snail (Achatina fulica Férussac)

Abstract

The present study aimed to further elucidate the pharmacological features, wiht respect to sensitivity to L-BHGA agonists,of the receptors sensitive to β-hydroxy-L-glutamic acid (L-BHGA) in five Achatina giant neurones: PON (periodically oscillating neurone), d-RPLN (dorsal-right parietal large neurone), VIN (visceral intermittently firing neurone), RAPN (right anteior pallial neurone) and v-RCDN (ventrral-right cerebral distinct neurone). Of these neurones, d-RPLN and RAPN were depolarized by L-BHGA, while PON, VIN and v-RCDN were inhibited. Threo-β-hydroxy-DL-aspartic acid markedly depolarized d-RPLN and RAPN (effective potency quotient (EPQ) in relation to the more effective L-BHGA isomer: 1 for d-RPLN and 0.3 for RAPN). This compound produced only slight inhibitory effects on PON, VIN and v-RCDN with EPQs calculated to be<0.03, <0.03 and 0.03, respectively. On the other hand, erythro-β-hydroxy-DL-aspartic acid at 10 −3 M was almost ineffective, except on v-RCDN where it elicited some slight inhibitory effects (EPQ: 0.01). L-Aspartic and D-aspartic acid at 10 −3 M, also had almost no effect except for slight effects of D-aspartic acid on d-RPLN (EPQ: 0.1). N-Methyl-L- and N-methyl-D-aspartic acid were slightly effective only on v-RCDN (EPQ: <0.021 and 0.01 respectively). The other compounds, including β-hydroxypyrroglutamic acid (cyclic BHGA) and proline derivatives, were almost ineffective at 10 −3 M; very weak effects were occasionally observed on some neurones. These results have a two-fold meaning: the relative sensitivities of these neurones to the erythro- and threo-L-BHGA stereoisomers do not correspond to the sensitivities of the stereoisomers of β-hydroxy-DL-aspartic acid and the zwitterionic character of the α-amino group, the presence of a β-hydroxyl group and a C4 or C5 carboxyl group are necessary to produced the effects on these neurones.