Kainate receptors, cellular signaling and excitotoxic neuronal death: New pharmacological evidence for the functional involvement of low-affinity and metabotropic kainate receptors

The characteristics of the kainic acid (KA) subtype of ionotropic receptor for L-glutamate (GluR) are less well understood than those of other ion-gated GluRs. Described here are studies that focus on KA receptor neurobiology,, especially novel aspects of neurochemical anatomy, pathology and cellular signaling. Low-affinity KA receptors (GluR5-7) as mapped by receptor autoradiography using [H-3](2S,4R)-4-methylglutamic acid (4MG), were widely and topographically distributed through the neuroaxes of old and new world monkeys. Binding of [H-3]4MG (KD 20nM) was fully inhibited by KA, domoate and Glu, whilst putative GluR5-7 agonists inhibited 40-50% of specific binding. These low affinity KA agonists produced concentration-dependent neurotoxicity of mouse neocortical neurons in culture across an apoptotic-necrotic continuum of injury. Rank order of neurotoxic potency differed from that found in [H-3]4MG binding suggestive of mixed actions at ionotropic GluRs, a conclusion supported by the neuroprotection found with various ionotropic GluR antagonists. Using cerebellar granule cells (CGCs), a metabotropic KA receptor linked to the phosphoinositide cascade was revealed. KA (EC50 150 mum) produced a concentration- and calcium-dependent stimulation (200% Li+ control) of phosphoinositide turnover that was insensitive to concanavalin A. KA receptor-mediated neurotoxicity of CGCs was purely apoptotic, likely to involve low affinity receptors, and was accompanied by the expression of BAX and cyclin D1 protein. Olomoucine, a cyclin-dependent kinase inhibitor, attenuated KA-induced neurotoxicity emphasizing that post-receptor events are neuroprotective targets in excitotoxic injury. Our studies of the native KA receptors highlight their diversity and the need for more selective drugs