DIFFERENT TYPES OF GLUTAMATE RECEPTORS IN ISOLATED AND IDENTIFIED NEURONS OF THE MOLLUSK PLANORBARIUS-CORNEUS

1. The membrane currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. The fast chloride current (E(r) (reversal potential) = -41 mV) evoked in a Ped-9 neurone by application of glutamate, quisqualate and ibotenic acid could be blocked by furosemide (0.1 mM). The slow potassium current (E(r) = -85 mV) evoked in Ped-8 and Ped-9 neurones by glutamate, quisqualate and kainate could be blocked by tetraethylammonium (50-mu-M). 3. N-Methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA) failed to induce a response in neurones studies. 4. The spider venoms argiopine and argiopinine III (50-500 nM) selectively inhibited quisqualate-induced potassium current, but had no influence on glutamate-, ibotenate- or quisqualate-induced chloride and kainate-induced potassium currents. Glutamate-induced potassium current was partially inhibited by argiopine and argiopinine III. 5. The existence of several types of distinct glutamate receptors was confirmed in cross-desensitization experiments, and a lack of interaction was observed between quisqualate and kainate. 6. Potassium currents induced both by quisqualate and kainate strongly depended on temperature and could be blocked by pertussis toxin. Intracellular injection of the calcium chelator, EGTA, did not affect quisqualate and kainate responses. 7. In neurones loaded with non-hydrolysable GTP analogues, GTP-gamma-S (guanosine-5'-O-(3-thio)triphosphate) or GppNHp (5'-guanylylimidodiphosphate), the potassium current was gradually induced in the absence of agonists. As this current progressed, the magnitude of the glutamate- or kainate-evoked current transients became smaller and finally negligible. The GTP-gamma-S-induced current was not inhibited by argiopine. 8. These data indicate that in the molluscan neurones studied there are at least three pharmacologically distinct glutamate receptors: (1) a receptor of quisqualate-ibotenate type which directly controls chloride channel; (2) quisqualate and (3) kainate receptors which control in calcium-independent manner the common potassium channel by activation of GTP-binding protein.