Figure 5.

The biphasic response is associated with a lower level of neuronal activity than the RD response. A1, Examples of two sets of phase shifts obtained after the successive application of glutamate (from 300 nm to 300 μm, 30 s; arrowhead) to the same cell. Classification of the response is determined for a standard application of glutamate (30 μm, 30 s), as indicated by the rectangle. Calibration: 1 min and 10°. A2, The graph reports the amplitude of the initial decrease of the phase shift for cells displaying either a biphasic (n = 5) or RD (n = 4) response. The curve shown is the best fit of the data to the logistic equation described in the Materials and Methods section. Fitting the data to the logistic equation yielded an EC₅₀ of 15 and 45 μm, respectively, for cells with RD and biphasic responses. B1, Traces of current triggered by glutamate (30 μm, 30 s, arrowhead) and recorded from cells displaying a biphasic (left) or a RD (right) response. Note that the RD optical response is associated with a stronger inward current than with the biphasic one. Calibration: 0.5 nA and 30 s. B2, Expanded traces of EPSCs from electrical recording shown in B1, before application of glutamate. Calibration: 100 pA and 50 ms. B3, Cumulative probability plots comparing rise time (bin: 0.2 ms; left) and decay time (bin: 0.5 ms; right) for the cells shown in B1 and B2. Kinetics were significantly longer for RD cells as well for rise time (KS two-sample test, p < 0.001) as for decay time (KS two-sample test, p < 0.001). C1, In the presence of GABA (3 μm), the RD response obtained in control conditions (Cont.; thick line) became a biphasic response (dotted line) after an application of glutamate (30 μm, 30 s, arrowhead) (n = 5). Scale bars correspond to 1 min and 5°. C2, Example of electrophysiological recording from a neuron (displaying tonic activity of action potentials) in culture. Application of GABA (30 μm) hyperpolarized the membrane potential with disappearance of action potentials. Calibration: 1 min and 20 mV. D1, Conversely, in the presence of picrotoxin (30 μm), the biphasic response obtained in control conditions (Cont.; thick line) became an ID response (dotted line) after an application of glutamate (30 μm, 30 s, arrowhead) (n = 6). Calibration: 1 min and 5°. D2, Example of electrophysiological recording from neuron in culture displaying synaptic activity (EPSPs and IPSPs) but no action potential. After application of picrotoxin (30 μm), the electrical activity is characterized by the appearance of an epileptic form of activity (action potentials superimposed on plateau potential). Calibration: 1 min and 20 mV.