Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions.

GABAA receptors form heteromeric GABA-gated chloride channels assembled from a large family of subunit genes. In cerebellum, distinct GABAA receptor subtypes, differing in subunit composition, are segregated between cell types and synaptic circuits. The cerebellum therefore represents a useful system to investigate the significance of GABAA receptor heterogeneity. For instance, studies of mice carrying targeted deletion of major GABAA receptor subunit genes revealed the role of alpha subunit variants for receptor assembly, synaptic targeting, and functional properties. In addition, these studies unraveled mandatory association between certain subunits and demonstrated distinct pharmacology of receptors mediating phasic and tonic inhibition. Although some of these mutants have a profound loss of GABAA receptors, they exhibit only minor impairment of motor function, suggesting activation of compensatory mechanisms to preserve inhibitory networks in the cerebellum. These adaptations include an altered balance between phasic and tonic inhibition, activation of voltage-independent K+ conductances, and upregulation of GABAA receptors in interneurons that are not affected directly by the mutation. Deletion of the alpha1 subunit gene leads to complete loss of GABAA receptors in Purkinje cells. A striking alteration occurs in these mice, whereby presynaptic GABAergic terminals are preserved in the molecular layer but make heterologous synapses with spines, characterized by a glutamatergic-like postsynaptic density. During development of alpha1(0/0) mice, GABAergic synapses are initially formed but are replaced upon spine maturation. These findings suggest that functional GABAA receptors are required for long-term maintenance of GABAergic synapses in Purkinje cells.