I use a bipolar parallel stimulating electrode (connected to a GRASS stimulator and stimulus isolator) to stimulate fibres in a slice and record synaptic responses using whole cell patch clamp. We have off late been generating an additional antidromic spike in every cell that is visible just before the synaptic response. We have since changed our stimulating electrode, tried some grounding configurations to get rid of it, without much success. The antidromic spike disappears on using a bipolar concentric electrode but unfortunately, this does not evoke synaptics of large amplitudes. Has anyone faced this problem or share how the parallel stimulating electrode set up is set up/grounded?