Possible CS and US pathways for rabbit classical eyelid conditioning: electrophysiological evidence for projections from the pontine nuclei and inferior olive to cerebellar cortex and nuclei.

Projections from the lateral region of the pontine nuclei and the dorsal accessory inferior olive to both cerebellar cortex and cerebellar dentate/interpositus nuclei were electrophysiologically examined using single-pulse stimulation and single-unit and population recording. Stimulation of the pontine nuclear region activated population potentials and single units recorded in both cerebellar cortex and deep nuclei. Pontine-evoked activity in cerebellar cortex (Larsell's lobule HVI and adjacent areas) was rather well-defined and strong while pontine-evoked activity in the deep cerebellar nuclei seemed relatively more diffuse and weaker. Short onset latencies for both single units and population potentials were found suggesting direct projections. Similar to previous studies, inferior olive stimulation evoked short-latency responses in cerebellar cortex and nuclei thus suggesting direct projections. More pontine- and olivary-evoked activity was seen in cortex than in the nuclei with slightly more olivary-evoked potentials per recording electrode penetration observed than pontine-evoked activity. Our findings suggest that cortical and nuclear regions of the cerebellum receive converging projections from the pontine nuclei and inferior olive, projections that may carry information about stimuli used during classical conditioning. These findings are discussed in terms of cerebellar circuits that may be involved in classical eyelid conditioning.