Electrophysiological changes in the fasciculus gracilis of the cat following chronic clioquinol administration

Clioquinol was administered to cats for more than 200 days, in order to investigate the neural mechanisms underlying the sensory disturbances of subacute myelo-opticoneuropathy (SMON). Electrophysiological examination, carried out under urethane-chloralose anesthesia, revealed that there were 3 major abnormalities in the surface potentials of the nucleus gracilis evoked by sural nerve stimulation, i.e., a reduction in the peak-to-peak amplitude, prolongation of the N wave, and a reduction in P wave amplitude. The reduction in P-wave amplitude suggested suppression of presynaptic inhibition. This was confirmed by excitability tests of the presynaptic terminals of sural nerve fibers within the nucleus gracilis. Recordings of orthodromic volleys in the fasciculus gracilis, elicited by sural nerve stimulation, showed an increase in temporal dispersion. Increased temporal dispersion was also evident from recordings of antidromic volleys in the sural nerve. Peripheral axons of primary sensory neurons in the sural nerve and their terminals within the spinal cord showed no significant functional abnormalities in the chronic clioquinol cat. It is suggested that primary axons in the fasciculus gracilis near the nucleus gracilis are affected by chronic clioquinol administration.