Cerebellar Purkinje cells receive GABA-mediated feed-forward inhibitory input from molecular layer interneurons (MLINs; stellate and basket cells) that is thought critical for the precise timing of parallel-fibre evoked action potentials (1). Purkinje cells express mainly α1, β2 and γ2 subunits of the GABA_A receptor and the γ2 subunit is likely responsible for clustering of GABA_A receptors at synapses (2-3). As part of a strategy to use transgenic mice for the investigation of CNS function through reversible modulation of specific brain circuits (4), we selectively inactivated the γ2 subunit gene in Purkinje cells, using mice with Cre recombinase driven from the L7 promoter (L7Cre) crossed with a line harbouring a floxed γ2 subunit allele (fγ2). In situ hybridization on brain sections with an exon 4-specific probe confirmed the selective absence of γ2 subunit mRNA from L7Cre x fγ2/fγ2 Purkinje cells. We made recordings from Purkinje cells in acute cerebellar slices from L7Cre x fγ2/fγ2 mice and their WT littermates. Mice were anaesthetised prior to decapitation. Responses to exogenous GABA were observed in patches taken from Purkinje cells of both groups of mice. Consistent with the loss of a γ2 subunit, currents recorded from L7Cre x fγ2/fγ2 mice showed a lower single-channel conductance and incomplete desensitization. Purkinje cell miniature IPSCs (mIPSCs) were observed in all WT mice, but in no L7Cre x fγ2/fγ2 mice. In accord with the Purkinje cell-selective expression of Cre, MLINs from both groups displayed mIPSCs. Purkinje cells fire simple spikes spontaneously and the activity of interneurons influences spike timing (5). Of the cells recorded, 61 of 68 fired action potentials spontaneously. The mean firing rate of Purkinje cells from the two groups was not significantly different, either at room- or physiological temperature. However, the coefficient of variation (CV; SD/mean) of the inter-spike interval (ISI) differed. At 25°C the CV was 0.20 ± 0.03 for 26 cells from WT and 0.10±0.01 for 9 cells from L7Cre x fγ2/fγ2 mice (mean±sem, P<0.05, Mann-Whitney U test). At 35°C the CVs were 0.14±0.01 and 0.06±0.01, respectively (n=9 and 7, P<0.05). Consistent with a role for MLINs in shaping Purkinje cell firing patterns (5), blockade of GABA_A receptors with SR95531 decreased the CV of the ISI in WT Purkinje cells from 0.20±0.04 to 0.13±0.02 at 25°C and from 0.13±0.02 to 0.08±0.01 at 35°C (both P<0.05, paired t test; n=8 and 4, respectively). SR95531 failed to alter the CV of the ISI in cells from L7Cre x fγ2/fγ2 mice. Our results support the idea that γ2 subunits are needed for targeting of GABA_A receptors to synaptic sites in Purkinje cells. We are currently investigating the behavioural outcome of this loss of normal inhibitory input.