Only ZCZ90 retained its agonism of stretch-induced afferent firing in muscle spindles, and this action is attenuated by PCCG-13. Kainate derivatives. (a) ZCZ49 and (b) ZCZ50 lost the potent agonist action of kainate on stretch-induced afferent firing in muscle spindles, even at 10 μm, while derivative (c) ZCZ90 significantly increased firing at only 1 μm, (d) Agonism of ZCZ90 is blocked by PCCG-13 (10 μm), showing it retains affinity for the PLD-mGluR. ***P = 0.0001 vs. control.