CABA(A) receptors mediate synaptic and tonic inhibition in many neurons of the central nervous system. These receptors can be constructed from a range of different subunits deriving from seven identified families. Among these subunits, alpha(5) has been shown to mediate GABAergic tonic inhibitory currents in neurons from supraspinal nuclei. Likewise, immunohistochemical and in situ hybridization studies have shown the presence of the alpha(5) subunit in spinal cord neurons, though almost nothing is known about its function. In the present report, using slices of the adult turtle spinal cord as a model system we have recorded a tonic inhibitory current in ventral horn interneurons (VHIs) and determined the functional contribution of the alpha(5) subunit-containing GABA(A) receptors to this current. Patch clamp studies show that the GABAergic tonic inhibitory current in VHIs is not affected by the application of antagonists of the alpha(4/6) subunit-containing GABA(A) receptors, but is sensitive to L-655708, an antagonist of the GABA(A) receptors containing alpha(5) subunits. Last, by using RT-PCR and immunohistochemistry we confirmed the expression of the alpha(5) subunit in the turtle spinal cord. Together, these results suggest that GABA(A) receptors containing the alpha(5) subunit mediate the tonic inhibitory currents observed in VHIs. (C) 2011 Elsevier Inc. All rights reserved.