Enhanced transmission of glutamate current flowing from the dendrite to the soma in rat neocortical layer 5 neurons.

The presence of tetrodotoxin (TTX)-sensitive slowly-inactivating Na+ channels in the dendrites of neocortical layer 5 neurons was tested by focal iontophoresis of glutamate on the dendrite while voltage clamping the soma and proximal dendrite. The glutamate-transmitted current was measured with the voltage clamp circuit. When the soma was depolarized the transmitted current increased indicating voltage-dependent properties in the dendrite. Over 50% of this increased voltage-dependence was blocked by TTX indicating a large portion of the enhanced dendro-somatic current was caused non-inactivating Na+ channel inward rectification. The glutamate-transmitted current measured with a voltage clamp of the soma at firing level was equal to the effective glutamate measured during repetitive firing.