Extent of multiple innervation of cerebellar Purkinje cells by climbing fibers in adult X-irradiated rats. Comparison of different schedules of irradiation during the first postnatal week

Abstract

The multiple innervation of cerebellar Purkinje cells (PCs) by climbing fibers (CFs) that is transient in normal developing rats can be experimentally maintained in cerebella which have been degranulated by repetitive postnatal X-irradiation restricted to the first postnatal week 22. Since the involution of redundant CFs occurs essentially between postnatal days 5 and 10, and given that postirradiation effects last 2–3 days, the question arose to know whether it is possible to further delimit a ‘critical period’ of irradiation within the first week. An estimate of the extent of multiple innervation of PCs by CFs was made in adult rats that had been irradiated according to 5 different schedules: in two groups, rats received X-rays applied repetitively during the first postnatal week (PN0–7 groups); in the 3 other groups, X-rays were delivered either during the first part of the week (early group PN1–3) or during the last part of the week (late groups PN4–7). In addition, two daily doses were tested (150 and 200 r). The CF pathway was electrically stimulated in anesthetized rats at the level of the inferior olive or in the cerebellar white matter. Intracellular recordings of spontaneous and evoked CF responses in PCs allowed to estimate the number of afferent CFs and to calculate the mean value ( m) per PC for each group. The majority of recorded cells was located in lobules VII and VIII and similar results were obtained in these two lobules. In lobule VII, irradiation restricted to the first part of the week (early group) did not impede most of the regression ( m = 1.27 ± 0.06). In contrast, high doses (200 r) applied during the second part of the week (late group) maintained a high index of multiple ( m = 3.05 ± 0.05) innervation, very close to the level obtained in the 0–7 group receiving similar doses on corresponding days ( m = 3.32 ± 0.07). Homologous groups receiving the lower daily dose (150 r) maintained a significantly lower index of multiple innervation.