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Axonal and somatic filtering of antidromically evoked cortical excitation by simulated deep brain stimulation in rat brain

Antidromic cortical excitation has been implicated as a contributing mechanism for high‐frequency deep brain stimulation (DBS). Here, we examined the reliability of antidromic responses of type 2 corticofugal fibres in rat over a stimulation frequency range compatible to the DBS used in humans. We activated antidromically individual layer V neurones by stimulating their two subcortical axonal branches. We found that antidromic cortical excitation is not as reliable as generally assumed. Whereas the fast conducting branches of a type 2 axon in the highly myelinated brainstem region follow high‐frequency stimulation, the slower conducting fibres in the poorly myelinated thalamic region function as low‐pass filters. These fibres fail to transmit consecutive antidromic spikes at the beginning of high‐frequency stimulation, but are able to maintain a steady low‐frequency (6–12 Hz) spike output during the stimulation. In addition, antidromic responses evoked from both branches are rarely present in cortical neurones with a more hyperpolarized membrane potential. Our data indicate that axon‐mediated antidromic excitation in the cortex is strongly influenced by the myelo‐architecture of the stimulation site and the excitability of individual cortical neurones.