Disinhibition in rat superior colliculus mediated by GABA(C) receptors

The stratum griseum superficiale (SGS) of the superior colliculus contains a high concentration of the recently described GABA(C) receptor. In a previ ous study, it was postulated that activation of these receptors on inhibito ry interneurons functions to disinhibit projection cells that relay visual information to the thalamus and brainstem. To test this model, we used in v itro 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 characteriz ation. As expected, bath applications of GABA and muscimol always strongly depressed evoked PSCs at concentrations of >100 and >1 muM, respectively. H owever, at lower agonist concentrations, which most likely activate GABAC b ut not GABAA receptors, effects were not uniform. Evoked responses were sup pressed 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 re sponses (8 of 10 cells) had morphological characteristics of projection cel ls. Finally, all effects of GABA and muscimol at low concentrations were bl ocked by (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid, a specifi c GABAC receptor antagonist, but not by the specific GABAA receptor antagon ist bicuculline. Taken together, these results indicate that in SGS, GABAC receptors are predominantly expressed by GABAergic neurons and that activat ion of these receptors leads to disinhibition of SGS projection cells.