GABAC receptor-mediated inhibition is altered, but not eliminated in the superior colliculus of GABAC ρ1 knockout mice

GABAC receptors (GABACRs) are widely expressed in the mammalian subcortical visual system, particularly in the retina and superior colliculus (SC). GABACRs are composed of specific ρ1-3 subunits whose expression varies among visual structures. Thus, ρ1 subunits are most abundant in retina and their loss eliminates GABACR expression and function (McCall et al. J Neurosci 22:4163-4174). In the SC, ρ2 subunit expression may be equal to or stronger than ρ1 subunit expression, however, results across studies vary considerably. To more directly assess the expression of GABACR subunits, we characterized inhibition in the SC of wild type (WT) and GABAC ρ1 Null mice that lack expression of GABAC ρ1 subunits. We used wholecell patch clamp recordings and evaluated GABACR-mediated modulation of electrically-evoked post synaptic currents using either agonists or antagonists in WT mice. In GABAC ρ1 Null SGS cells, IPSCs were shorter in duration and their EPSCs were longer, indicating that a slow GABACR-mediated inhibitory component was reduced in each case. In contrast to retina, GABACR-mediated currents in the SC were altered, but not eliminated in GABAC ρ1 Null mice. In the majority of SC cells in GABAC ρ1 Null mice, GABACR activation could still be induced to alter EPSC peak amplitudes in putative interneurons and in many projection neurons. These results, compared to previously published data, indicate a fundamental difference between retina and SC in the control of GABACR expression and subunit composition. GABAC receptors in SC of ρ1 Null mice 3