Naturally-occurring mutations in the gene encoding class A (or P/Q-type) calcium channels are associated with multiple abnormalities, ranging from migraine headache to motor ataxias to absence epileptic seizures. These heterogeneous neurological phenotypes underscore the central importance of P/Q-type calcium channels-the dominant exocytotic channels in central nervous system. P/Q is not, however, the only type of calcium channel controlling synaptic transmission in the CNS. N-type (or class B) calcium channels usually co-exist with P/Q and, together, they jointly govern the release of many, if not all, transmitters. Whether P/Q and N channels play unique functional roles at the synapse is unclear. Experiments with one P/Q channel mutant mouse, tottering, suggest, however, that the two channels are not functionally redundant and that tottering offers an opportunity to explore their different roles in exocytosis. Homozygous tottering animals display a dramatic neurological phenotype, characterized by ataxia and frequent absence seizures. Our preliminary experiments on tottering demonstrate that a primary consequence of the P/Q channel mutation is a shift in the ratio of P/Q:N channels in some (but not all) nerve terminals. For example, release of the excitatory transmitter glutamate and glutamatergic synaptic transmission at the parallel fiber-Purkinje cell synapse in cerebellum are controlled largely by N-type calcium channels in the mutant, rather than P/Q-type as they are in wild-type animals. As a consequence of these changes in the presynaptic calcium channel complement, excitatory transmission is reduced and G protein-dependent inhibition is enhanced at mutant synapses. In contrast, GABA release from inhibitory nerve terminals appears to be unaffected in tottering animals. On the basis of these observations, we hypothesize that the selective effect of the tottering allele on excitatory transmission leads to an overall decreased excitation of Purkinje cells. Three interacting factors contribute: 1) glutamate release from excitatory inputs is impaired due to the decreased involvement of P/Q channels; 2) the relative increase in N channel-mediated release further enhances susceptibility of these inputs to presynaptic inhibition (because N channels are more effectively modulated by G proteins than are P/Q channels); and 3) unimpaired inhibitory, GABAergic inputs are relatively more efficacious in the face of reduced excitation. As Purkinje cells control cerebellar output via GABAergic inhibitory transmission onto output neurons in deep cerebellar nuclei, we predict that a reduction in Purkinje cell activity will enhance net cerebellar output. Ultimately, such changes would excite thalamus and motor cortex, providing a plausible mechanism for the ataxia and seizures observed in these animals. Experiments proposed here will stringently test this hypothesis through in-depth cellular and synaptic exploration of calcium channels and calcium-dependent exocytosis in tottering cerebellum. Results will provide essential information for understanding the consequences of the mutation on cerebellar circuit behavior and may, in the long term, offer suggestions for new therapeutic interventions into ataxia and other motor disorders.
| Erickson, M A; Haburcak, M; Smukler, L et al. (2007) Altered functional expression of Purkinje cell calcium channels precedes motor dysfunction in tottering mice. Neuroscience 150:547-55 |
| Zhou, Yu Dong; Turner, Timothy J; Dunlap, Kathleen (2003) Enhanced G protein-dependent modulation of excitatory synaptic transmission in the cerebellum of the Ca2+ channel-mutant mouse, tottering. J Physiol 547:497-507 |
