Acknowledgment. The financial support of the Na- tional Institutes of Health (Grant No. NS-12389) and the Camille and Henry Dreyfus Foundation (Teacher-Scholar award to LEO) is gratefully acknowledged. NMR and mass spectra were determined on instruments purchased with the assistance of NSF departmental instrumentation grants. We are indebted to Dr. John Daly, NIH, for pro- viding a sample of natural gephyrotoxin. ... Supplementary Material ...