Disinhibition in rat superior colliculus mediated by GABAc receptors

The stratum griseum superficiale (SGS) of the superior colliculus contains a high concentration of the recently described GABA(C) receptor. In a previous study, it was postulated that activation of these receptors on inhibitory interneurons functions to disinhibit projection cells that relay visual information to the thalamus and brainstem. To test this model, we used in vitro whole-cell patch-clamp methods to measure effects of GABA and muscimol on EPSCs and IPSCs evoked in rat SGS by electrical optic layer stimulation. The neurons were filled with biocytin for later morphological characterization. As expected, bath applications of GABA and muscimol always strongly depressed evoked PSCs at concentrations of >100 and >1 micrometer, respectively. However, at lower agonist concentrations, which most likely activate GABA(C) but not GABA(A) receptors, effects were not uniform. Evoked responses were suppressed by both agonists in 48% of the neurons, whereas the remaining cells exhibited enhanced responses with increased evoked EPSCs, decreased evoked IPSCs, or both types of change. Most morphologically identified cells with suppressed responses (14 of 17 cells) had morphological characteristics of putative GABAergic interneurons, whereas almost all cells with enhanced responses (8 of 10 cells) had morphological characteristics of projection cells. Finally, all effects of GABA and muscimol at low concentrations were blocked by (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid, a specific GABA(C) receptor antagonist, but not by the specific GABA(A) receptor antagonist bicuculline. Taken together, these results indicate that in SGS, GABA(C) receptors are predominantly expressed by GABAergic neurons and that activation of these receptors leads to disinhibition of SGS projection cells.