Behavior of cat abducens motoneurons following the injection of toxic ricin into the lateral rectus muscle.

The aim of this investigation was to study the behavior of identified abducens motoneurons in the chronic cat following a single injection of toxic ricin into the lateral rectus muscle. Lateral rectus electromyographic potentials induced by VIth nerve stimulation disappeared, and abducens antidromic field potentials decreased by 90% 3 days following ricin injection. Several abnormalities and a significant decrease in eye position and velocity sensitivities were observed in motoneuron activity up to 8-10 days following ricin injection. Contrary to a previous report for axotomized abducens motoneurons, no functional sign of recovery was observed. Histological analysis showed a survival of 10-15% of the abducens motoneuron population 10 days following ricin injection. From this time on, recorded motoneurons behaved like controls, but showed a specific retraction signal suggesting an exclusive projection onto the retractor bulbi muscle. Although intermingled in the nucleus with motoneurons, no recorded abducens internuclear interneuron was affected by the ricin during one month following the injection.