The potent neurotoxin kainate activates ion channel-forming receptors. However, it can also activate a G protein-coupled signaling pathway to inhibit transmitter release in central neurons. It remains unclear whether the same receptor complex is involved in both signaling activities. Here we show that in a population of dorsal root ganglion cells, exposure to kainate elicits a G protein-dependent increase in intracellular Ca2+. Furthermore, in these cells a brief exposure to kainate inhibited the K+-induced Ca2+ increase, a process that was sensitive to the G protein inhibitor Pertussis toxin and inhibitors of protein kinase C. This metabotropic action did not require ion channel activity and was not observed in neurons prepared from mice deficient for the ion channel-forming subunit GluR5. These results indicate that GluR5, an ion channel-forming subunit, signals through a second messenger cascade, inhibiting voltage-dependent Ca2+ channels. Thus, such a system represents a noncanonical signaling route of ion channel-forming receptors.

Cysteine string protein-α (CSP-α) is a synaptic vesicle protein that prevents activity-dependent neurodegeneration by poorly understood mechanisms. We have studied the synaptic vesicle cycle at the motor nerve terminals of CSP-α knock-out mice expressing the synaptopHluorin transgene. Mutant nerve terminals fail to sustain prolonged release and the number of vesicles available to be released decreases. Strikingly, the SNARE protein SNAP-25 is dramatically reduced. In addition, endocytosis during the stimulus fails to maintain the size of the recycling synaptic vesicle pool during prolonged stimulation. Upon depolarization, the styryl dye FM 2-10 becomes trapped and poorly releasable. Consistently with the functional results, electron microscopy analysis revealed characteristic features of impaired synaptic vesicle recycling. The unexpected defect in vesicle recycling in CSP-α knock-out mice provides insights into understanding molecular mechanisms of degeneration in motor nerve terminals.