Bidirectional Alterations in Cerebellar Synaptic Transmission of<i>tottering</i> and <i>rolling</i>Ca²⁺ Channel Mutant Mice

Abstract

Hereditary ataxic mice, <i>tottering</i>(<i>tg</i>) and <i>rolling</i> Nagoya (<i>tg</i>^<i>rol</i>), carry mutations in the P/Q-type Ca²⁺ channel α_1A subunit gene. The positions of the mutations and the neurological phenotypes are known, but the mechanisms of how the mutations cause the symptoms and how the different mutations lead to various onset and severity have remained unsolved. Here we compared fundamental properties of excitatory synaptic transmission in the cerebellum and roles of Ca²⁺ channel subtypes therein among wild-type control, <i>tg</i>, and <i>tg^rol</i>mice. The amplitude of EPSC of the parallel fiber–Purkinje cell (PF-PC) synapses was considerably reduced in ataxic<i>tg^rol</i>. Although the amplitude of the parallel fiber-mediated EPSC was only mildly decreased in young non-ataxic <i>tg</i> mice, it was drastically diminished in adult ataxic <i>tg</i> mice of postnatal day 28–35, showing a good correlation between the impairment of the PF-PC synaptic transmission and manifestation of ataxia. In contrast, the EPSC amplitude of the climbing fiber–Purkinje cell (CF-PC) synapses was preserved in <i>tg</i>, and it was even increased in<i>tg^rol</i>, which was associated with altered properties of the postsynaptic glutamate receptors. The climbing fiber-mediated EPSC was more dependent on other Ca²⁺ channel subtypes in mutant mice, suggesting that such compensatory mechanisms contribute to maintaining the CF-PC synaptic transmission virtually intact. The results indicate that different mutations of the P/Q-type Ca²⁺ channel not only cause the primary effect of different severity but also lead to diverse additional secondary effects, resulting in disruption of well balanced neural networks.