Quantitative light and electron microscopic study of cerebellar granule cells and parallel fiber varicosities in adult tottering (tg/tg), leaner (tgla/tgla) and compound heterozygous (tg/tgla) mice

Abstract

The murine gene, tottering (tg) encodes the []1A subunit of neuronal P/Q-Type Ca²⁺ channels. Two spontaneously occurring mutations of the gene, tottering (tg) and leaner (tg[]), have been characterized. Mice that are homozygous (tg/tg and tg[]/tg[]) and compound heterozygous (tg/tg[]) for these mutations exhibit cerebellar ataxia of differing severity. Morphologic abnormalities observed in the cerebella of these mice are also variable depending on mutant genotype. One of the most pronounced abnormalities is in the ultrastructure of parallel fiber varicosities which have markedly increased cross-sectional areas and synaptic indices. In this study, parallel fiber varicosities of tottering, leaner and coisogenic wild-type mice were evaluated for a variety of morphometric parameters in photomontages prepared from transmission electron micrographs of the molecular layer of the cerebellar vents. Significantly lower densities of parallel fiber varicosities in the anterior and posterior vermis. and parallel fiber varicosity synapses with Purkinje cell dendritic spines in the posterior vermis were found in leaner mice compared with wild-type mice. In addition, significantly greater mean cross-sectional area per parallel fiber varicosity in both the anterior and posterior vermis was found in leaner mice relative to wild-type mice. Significantly greater promotions of parallel fiber varicosities with synaptic indices greater than 1:1 were found in both leaner and tottering mice compared with wild-type mice, and in leaner mice compared with tottering mice for the posterior vermis. In addition, granule cell density in the cerebellar cortex of tottering, heterozygous tgltg[] and wild-type mice was estimated by light-microscopic methods. No significant differences were found between wild-type and mutant mice for this parameter. In conclusion, significant findings from this study indicate the incidence of multisynaptic parallel fiber varicosities in the cerebellar molecular layer of adult tottering and leaner mice is greater than that of wild-type mice, and correlates with the severity of their neorologic phenotype.