Figure 3. The early- and late-onset responses are suppressed by perfusing a high-Mg ²⁺/low-Ca²⁺ extracellular solution.

A, the top trace shows the voltage response elicited in a UBC by a train stimulus delivered to the MFB. Here and in the next experiments, train frequency was 100 Hz. After changing the extracellular solutions in the bath (control) with the high-Mg²⁺/low-Ca²⁺ extracellular solution (see Methods), both the early- and the late-onset responses disappeared (middle trace). The effect was reversible after wash (bottom trace). The horizontal bar at the bottom, here and in the next figures, indicates the train stimulus length. The scatter plot at the bottom illustrates the spike frequency in the early-onset (filled symbols) and in the late-onset (open symbols) response during the course of the same experiment. B, scatter plot summarizing the average spike frequency in the early-onset (n = 3) and late-onset (n = 4) response following perfusion of the high-Mg²⁺/low-Ca²⁺ extracellular solution. Note that the high-Mg²⁺/low-Ca²⁺ extracellular solution almost completely suppressed the synaptic response.