Learning-dependent synaptic modifications in the cerebellar cortex of the adult rat persist for at least four weeks

Abstract

Several experiments have demonstrated increased synapse number within the cerebellar cortex in association with motor skill learning but not with motor activity alone. The persistence of these synaptic changes in the absence of continued training was examined in the present experiment. Adult female rats were randomly allocated to either an acrobatic condition (AC) or a motor activity condition (MC). The AC animals were trained to traverse a complex series of obstacles, and each AC animal was pair-matched with an MC animal that traversed an obstacle-free runway. These animals were further assigned to one of three training conditions. Animals in the EARLY condition were trained for 10 consecutive days before being killed, animals in the DELAY, condition received the same 10 d of training followed by a 28 d period without training, and animals in the CONTINUOUS condition were trained for the entire 38 d. Unbiased stereological techniques were used to obtain estimates of the number of synapses per Purkinje cell within the cerebellar paramedian lobule. Results showed the AC animals to have significantly more synapses per Purkinje cell than the MC animals in all three training conditions. There were no differences in the number of synapses per Purkinje cell among the EARLY, DELAY, and CONTINUOUS conditions. These data demonstrate that both the motor skills and the increases in synapse number presumed to support them persist in the absence of continued training.

Fig. 1.

An electron micrograph showing a typical synapse (arrow) within the molecular layer of the PML (18,000× magnification).

Fig. 2.

Performance of animals in all three conditions on the acrobatic task (±SEM). The mean time/trial/task significantly decreased in all three conditions as training progressed (see text).

Fig. 3.

Number of synapses per Purkinje cell (±SEM) within the PML. Multiple comparisons (*Student–Newman–Keuls,p < 0.05) showed that the AC animals had significantly more synapses per Purkinje cell than the MC animals in the EARLY (A), DELAY (B), and CONTINUOUS (C) conditions.