THE EFFECTS OF (-)-BETA-CYCLAZOCINE AND (-BETA-CYCLAZOCINE ON NMDA-EVOKED RESPONSES AND NMDA-MEDIATED CELL-DAMAGE IN CULTURED RAT HIPPOCAMPAL-NEURONS())

Microspectrofluorimetric measurements of excitatory amino acid-evoked rises in intracellular free calcium concentration ([Ca2+](i)), electro physiological measurements of currents through single NMDA receptor-op erated ion channels and estimates of cellular viability following NMDA challenge were employed to examine the interactions of(-)- and (+)-be ta-cyclazocine with the NMDA receptor-channel complex in cultured rat hippocampal neurons. Rises in [Ca2+](i) evoked by NMDA, but not those evoked by kainate, AMPA or 50 mM K+, were reduced by (-)-beta-cyclazoc ine in a concentration- and use-dependent manner with an estimated IC5 0 value of 272 nM. In outside-out patches, (-)-beta-cyclazocine did no t change the magnitudes of unitary NMDA-evoked currents but diminished both the frequency of channel openings and their mean open time. The IC50 for (-)-beta-cyclazocine against NMDA channel open state probabil ity was estimated at 84 nM. The actions of(-)-beta-cyclazocine were co nsistent with a voltage-dependent open channel block of the NMDA chann el with a blocking rate constant of 7.03 . 10(7) M(-1). s(-1) at -40 m V. Neurons exposed to a high concentration of NMDA in vitro were prote cted from death by 1 and 10 mu M (-)-beta-cyclazocine. In all of the a bove assays, (+)-beta-cyclazocine was considerably less potent an NMDA antagonist and neuroprotective agent than (-)-beta-cyclazocine; the I C50 for (+)-beta-cyclazocine against channel open state probability wa s estimated at 14 mu M. The results demonstrate that (-)-beta-cyclazoc ine is a potent and selective inhibitor of NMDA-evoked responses in cu ltured rat hippocampal neurons and an effective neuroprotective agent in vitro.