Actions of Kainic Acid and Related Analogues on Hirudo, Helix and Limulus Central Neurones

ABSTRACT Intracellular recordings have been made from central neurones from Hirudo medicinali s, Helix aspersa and Limulus polyphemus. All three preparations have neurones which respond to L-glutamate either by excitation, inhibition or a biphasic response, inhibition followed by excitation. However on all three preparations, kainate and its analogues only activate the excitatory receptors. In the case of Hirudo Retzius neurones and Limulus neurones, kainate is more potent than L-glutamate, with potency ratios of 0.006 and 0.063 respectively while on Helix neurones, kainate is over 100 times less potent than L-glutamate. On Hirudo Retzius neurones, the potency ratios for dihydrokainate, α-ketokainate and allo-α-ketokainate are 0.02, 0.05 and 0.14 respectively while on Limulus neurones the potency ratios are 0.7, 0.22 and 1.7 respectively. Quisqualate has a biphasic action on Hirudo Retzius cells, mimics both the inhibitory and excitatory action of L-glutamate on Helix neurones but is only excitatory on Limulus neurones, even those which are only inhibited by L-glutamate. Ibotenate mimics the action of L-glutamate at all sites where glutamate is active, being approximately equipotent in most cases. It is concluded that either kainate is unable to activate glutamate receptors linked to chloride ionophores or else if there are specific kainate receptors, then these are not linked to chloride ionophores.