Field postsynaptic potentials (fPSP) are evoked by laser scanning photostimulation with glutamate uncaging in slice.

<p><b>A</b>, One to two glass pipettes were placed in barrel cortex slice to record local field potentials. Responses where evoked with glutamate uncaging at sites positioned in a 16 × 16 grid with a 75 μm spacing. <b>B</b>, Examples of evoked local field potentials. Direct and synaptic responses could be distinguished based on their latency. The numbers refer to the stimulation sites shown in A. <b>C</b>, The black traces are the same as in B. Synaptic events were blocked by 100 nM TTX (in green). Blue dots indicate the stimulation onset and the light blue shade the time window for the rising of direct responses. <b>D</b>, Example of a map reporting with colors the peak amplitude of direct responses evoked 0 to 5 ms after the stimulus onset. Top panels are in control conditions, the bottom panels are in the presence of TTX. Left panels show the negative potentials, the right the positive potentials. The white circles indicate the position of the recording, the rectangles the position of barrels in the slice. All panels are from the same recording. <b>E</b>, Example of a map in control condition (top) and in the presence of TTX (bottom) showing the mean amplitude of local field potentials evoked 5 to 50 ms post stimulus onset. In blue, the isolines of regions with negative potentials; in red, the isolines of regions with positive potentials. The green and purple show regions where responses were larger than 15 μV. Same recording as in D. <b>F</b>, Group-average isoline map of recordings performed in layer 2/3. The isolines show the regions in the map where the mean amplitude of potentials evoked within the 5–50 ms time window were negative (blue) and positive (red). <b>G</b>, Example of a color map reporting the <i>peak</i> amplitude of negative potentials (fPSP map) evoked in control condition (top) and in the presence of TTX (bottom). Same recording as in D and E. <b>H</b>, Group-average map for recording performed in layer 3 prior (left) and after application of LY379268, an agonist of group 2 metabotropic glutamate receptors (right). <b>I</b>, The effect of LY379268 on synaptic events evoked by stimulations in layer 4 or on the direct responses evoked by stimulations in the vicinity of the recording pipette.