NEUROBIOLOGIC STUDIES OF NEURONS AND GLIA IN CELL CULTURE

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Profound inactivation of voltage-dependent calcium channels can be demonstrated with 2 Hz stimulation using voltage clamp techniques under conditions where Na and K channels are blocked in cultured spinal cord (SC) and dorsal root ganglion (DRG) neurons. Such inactivation is much more pronounced in DRG as compared to SC neurons. The inactivation is dependent both on steady membrane voltage and on the amount of calcium which enters the neuron. DRG, thus, appears to regulate intracellular calcium less well than SC neurons in response to activity related calcium ingress. DRG neurons also maintain transmitter output very poorly relative to SC neurons. This deficit in function is accentuated in high external calcium. Synaptic boutons of DRG neurons have a relative paucity of mitochondria correlating with this deficit in transmitter output with repetitive activation. We propose that the inactive boutons described in earlier reports may be related to ineffective calcium sequestering mechanisms. These mechanisms may represent one source of long term modulation of synaptic efficacy. Nitrendipine, a dihydropyridine binds with high and low affinity to cultured neurons, but in contrast to its blocking action for Ca++ channels in muscle, nitrendipine, event at micromolar concentrations, has little or no blocking action on neuronal Ca++ channels. Infection of DRG neurons with various strains of herpes virus has different highly specific effects on neuronal excitable mechanisms. Fusion, with electrical coupling, may be induced and blockade of sodium but not calcium channels and of inward but not outward rectification result from viral infection. Acyclovir, which prevents viral replication, blocked the effects of the virus on membrane properties. Chick and rat sensory neurons have voltage-sensitive calcium channels, and in a subpopulation of these cells, a large prolonged post-spike depolarization occurs. Voltage clamp experiments indicate that this after potential is due to a calcium-sensitive chloride conductance whose properties are described.