Opposing actions of CSW and RasGAP modulate the strength of Torso RTK signaling in the Drosophila terminal pathway

The NMDA receptor antagonistic effects of budipine were assessed using concentration- and patch-clamp techniques on cultured striatal, hippocampal, cortical and superior colliculus neurones. Inward current responses of striatal neurones to NMDA (200 microM) at -70 mV were antagonized by budipine in a concentration-dependent manner (50% inhibitory concentration (IC50) 59.4 +/- 10.7 microM, n = 17) with 24 times lower potency than memantine but similar potency to amantadine. In striatal neurones, budipine blocked outward currents at +70 mV with an IC50 of 827 microM, suggesting that the binding site is less deep in the channel (delta = 0.45) than for memantine. However, more detailed analysis of the fractional block by budipine 300 microM in hippocampal neurones gave a delta-value of 0.90, but revealed that 28% block is mediated at a voltage-independent site. This voltage-insensitive site was accessible in the absence of agonist. Budipine exhibited concentration-dependent open channel blocking kinetics (kappa(on) = 0.71 x 10(4) M(-1) s(-1)) whereas the fast offset rate was concentration-independent (kappa(off) = 0.63 s(-1)). Calculation of the ratio kappa(off)/kappa(on) revealed an apparent Kd value of 88.7 microM. Budipine, memantine and amantadine had similar effects against NMDA-induced currents in cultured hippocampal, cortical and superior colliculus neurones, although amantadine was somewhat more potent in cultured striatal neurones. The relevance of NMDA receptor antagonism to the anti-Parkinsonian effects of budipine remains to be established.