Effects of L-glutamic acid and its agonists on snail neurones

The sensitivities of 22 giant neurone types of an African giant snail (Achatina fulica Férussac) to threo-beta-hydroxy-L-glutamic acid (threo-L-BHGA), a derivative of L-glutamic acid (L-Glu), applied by brief pneumatic pressure ejection, were examined under current clamp. The 5 neurone types were depolarized by this compound, whereas 2 were hyperpolarized. The 4 neurone types, PON (periodically oscillating neurone), RAPN (right anterior pallial nerve neurone), d-RPLN (dorsal-right parietal large neurone) and RPeNLN (right pedal nerve large neurone) that are excited by threo-L-BHGA and one type, v-LCDN (ventral-left cerebral distinct neurone), inhibited by this compound, were selected to study their pharmacological features in detail. Effects of the stereoisomers of L-Glu and threo-L-BHGA, and mammalian L-Glu receptor agonists, ejected by brief pressure, on the 5 Achatina neurone types were examined under voltage clamp. d-RPLN produced an inward current (Iin) by L-Glu and threo-L-BHGA, whereas this neurone type was insensitive to D-Glu and erythro-L-, threo-D- and erythro-D-BHGA. This was also excited by AMPA, indicating that the pharmacological features of the L-Glu receptors in this neurone type were similar to those of the mammalian ionotropic AMPA type L-Glu receptors. RAPN produced Iin by L-Glu and threo-L-BHGA. This neurone type was also excited by quisqualic acid and ibotenic acid, indicating that the features of the L-Glu receptors were similar to those of the mammalian metabotropic L-Glu receptors. PON and RPeNLN produced Iin by L-Glu and threo-L-BHGA. These neurone types were also excited by quisqualic acid, AMPA and ibotenic acid, indicating that their L-Glu receptors seemed to be in the mixed type, of the two types mentioned. On the other hand, v-LCDN produced an outward current (Iout) by threo-L- and erythro-L-BHGA, but was insensitive to L-Glu, indicating that the receptors activated by L-BHGA were not L-Glu receptors. This neurone type was also inhibited by quisqualic acid.