Effects of ipsilateral cerebellum ablation on acquisition and retention of classically conditioned eyeblink responses in rats.

The ipsilateral cerebellum to the trained eye has been reported to be essential for acquisition and retention of the conditioned response (CR) in rabbit eyeblink conditioning. Although pharmacological studies have suggested its important roles in other species too, to what degree does eyeblink conditioning in rats depend on the ipsilateral cerebellum is not clear. In this work, we ablated the ipsilateral cerebellum in rats before or after conditioning to examine its roles in acquisition and retention of the CR. In the first experiment, rats received ablation of the ipsilateral cerebellum and recovered for more than 3 weeks. They then underwent eyeblink conditioning for 7 days with a tone and a periorbital electrical shock. Consistent with other previous reports, hemicerebellectomized rats showed significant impairment compared to sham-lesioned rats. However, the hemicerebellectomized rats acquired CRs to some degree, and the acquired CR showed adaptive timing. In the second experiment, rats received the hemicerebellectomy after acquiring CR by 7 days of conditioning in a delay paradigm. After more than 3 weeks of recovery, they were again conditioned in a delay paradigm. Rats with ipsilateral cerebellar lesions showed severe impairment in retention of the pre-acquired CR; however, they reacquired CR to some degree during the subsequent reconditioning sessions. These results suggest that the ipsilateral cerebellum plays an important role in rat eyeblink conditioning as well but that other brain regions can partially compensate for its removal.