Study of functional properties of various receptors made from coronal brain slices uses whole cell voltage clamp recordings. A holding potential of -60 mV is established. Bath application of receptor agonists for four different types consistently either elicited inhibitory or excitatory postsynaptic currents. One receptor only elicited a postsynaptic response at a holding potential of 0 mV. Which of the following is most likely to be this receptor?