Effects of differential interference with postnatal cerebellar neurogenesis on motor performance, activity level, and maze learning of rats: a developmental study.

The region of the cerebellum was X-irradiated in infant rats with selected exposure schedules designed to produce animals in which the cerebellar cortex was (a) essentially normal except for agenesis of late forming granule cells with axons situated in the uppermost molecular layer (12--15X), (b) lacking in stellate cells, with a severe reduction in granule cells with axons in the upper molecular layer (8--15X), (c) morphologically disorganized but had only intermediate cell agenesis (4--5X), or (d) disorganized and devoid of practically all postnatally forming interneurons (4--15X). In the first two experiments young adults had to traverse rotating rods that differed in texture and types of obstacles. The 8--15X animals showed no deficits on any of the rods tested. The third study dealt with spontaneous motor performance in the open field at three ages. The 4--5X and 4--15X animals were hypoactive as infants and young adults; this was attributed to their motor deficits. The 8--15X and 12--15X animals were hyperactive in the open field as young adults. The fourth experiment examined intra- and/or intersession habituation. No group differences were found in habituation patterns. In the fifth experiment, using activity wheels, the 4--15X group was hypoactive, and the 8--15X and 12--15X groups were hyperactive as young adults. In the sixth experiment young adults were tested for learning performance in a multiple-unit water maze. The 4--15X group was deficient on single alternation; the 4--5X and 12--15X groups on double alternation. The seventh experiment shed some light on the single alternation deficit of the 4--15X group; only these animals failed to alternate spontaneously in a nonaversive situation. In conclusion, these behavioral results, combined with those of recent morphological investigations, suggest that the cerebellar cortex is hierarchically organized: The basal domain of Purkinje cells and the lower molecular layer are implicated in the coordination of movements; the apical domain of Purkinje cells and the upper molecular layer, in the coordination of actions.