Naturally occurring cell death is an important feature of neuronal network development: the absence of adequate postsynaptic target neurons during a critical period may result in the death of presynaptic neurons, the degree of death varying inversely with the size of the target population. Studies of mouse mutants with abnormal cerebellar development provide support for this neuron/target relationship in circuits within the CNS. In the present study we analysed the inferior olivary cell population in two cerebellar mutant mice, nervous (nr/nr) and leaner (Cacna1ala/la). In these mice Purkinje cell degeneration begins near the end of the first postnatal month. In nervous mice the loss starts at postnatal day 20 (P20) and by the end of second month almost 90% of the Purkinje cells in the hemisphere and 50% in the vermis have disappeared. In leaner mice Purkinje cell loss starts after P30 and by P60 almost 50% of these cells are lost. We report here a loss of one third of inferior olivary neurons in the nervous mutation while the entire population appears intact in the leaner mouse. These results allow better definition of the end of the period of target dependency of inferior olive neurons. Their implications for the cell-cell interactions in the developing olivo-cerebellar system are discussed.
