Somato-synaptic variation of GABA A receptors in cultured murine cerebellar granule cells: investigation of the role of the α6 subunit

Abstract

Electrophysiological investigation of cultured cerebellar murine granule cells revealed differences between the GABA A receptors at inhibitory synapses and those on the cell body. Specifically, mIPSCs decayed more rapidly than cell body receptors deactivated, the mean single channel conductance at the synapse (32 pS) was greater than that at cell body (21 pS) and only cell body receptors were sensitive to Zn 2+ (150 μM), which depressed response amplitude by 82±5% and almost doubled the rate of channel deactivation. The GABA A receptor α6 subunit is selectively expressed in cerebellar granule cells. Although concentrated at synapses, it is also found on extrasynaptic membranes. Using a mouse line (Δ α6lacZ) lacking this subunit, we investigated its role in the somato-synaptic differences in GABA A receptor function. All differences between cell body and synaptic GABA A receptors observed in wild-type (WT) granule cells persisted in Δ α6lacZ cells, thus demonstrating that they are not specifically due to the cellular distribution of the α6 subunit. However, mIPSCs from WT and Δ α6lacZ cells differed in both their kinetics (faster decay in WT cells) and underlying single channel conductance (32 pS WT, 25 pS Δ α6lacZ). This provides good evidence for a functional contribution of the α6 subunit to postsynaptic GABA A receptors in these cells. Despite this, deactivation kinetics of mIPSCs in WT and Δ α6lacZ granule cells exhibited similar benzodiazepene (BDZ) sensitivity. This suggests that the enhanced BDZ-induced ataxia seen in Δ α6lacZ mice may reflect physiological activity at extrasynaptic receptors which, unlike those at synapses, display differential BDZ-sensitivity in WT and Δ α6lacZ granule cells (Jones, A.M., Korpi, E.R., McKernan, R.M., Nusser, Z., Pelz, R., Makela, R., Mellor, J.R., Pollard, S., Bahn, S., Stephenson, F.A., Randall, A.D., Sieghart, W., Somogyi, P., Smith, A.J.H., Wisden, W., 1997. Ligand-gated ion channel partnerships: GABA A receptor α 6 subunit inactivation inhibits δ subunit expression. Journal of Neuroscience 17, 1350–1362).