Spontaneous synaptic activity in an organotypic culture of the mouse retina

Many strains of mutant mice die at birth, when the retina is still very immature. The retinas of such mice can be studied in organotypic cultures. After a preceding anatomic study of the synaptic development, the electrical activity of the synaptic circuits within such cultures was studied in wild-type and gephyrin-deficient mice.

Organotypic cultures of newborn mouse retinas were grown for 14 days in vitro. Spontaneous postsynaptic currents (sPSCs) of amacrine cells were measured by using the whole-cell configuration of the patch-clamp technique. GABAergic and glycinergic currents that were isolated with specific antagonists, and retinas from wild-type (geph(+/+)) and gephyrin-deficient (geph(-/-)) mice were compared.

Rapidly decaying sPSCs that were blocked by kynurenic acid were mediated by ionotropic glutamate receptors, whereas sPSCs with significantly higher peak amplitudes and slow-decay kinetics were identified as spontaneous inhibitory postsynaptic currents (sIPSCs) mediated by gamma-aminobutyric acid type A receptors (GABA(A)Rs) and glycine receptors (GlyRs). In gephyrin-deficient (geph(-/-)) cultures, we found no sIPSCs mediated by GlyRs. sIPSCs mediated by GABA(A)Rs expressed in amacrine cells of geph(-/-) retinas decayed significantly faster than GABAergic sIPSCs recorded in amacrine cells of geph(+/+) retinas.

The different decay kinetics of GABA(A)Rs expressed in amacrine cells of geph(+/+) and of geph(-/-) retinas suggests that these cells express at least two types of GABA(A)R subtypes. In amacrine cells of geph(-/-) mice, a specific GABA(A)R subtype that may contain the alpha2 subunit, is impaired by the absence of gephyrin, whereas other GABA(A)Rs appear to function normally.