GABA_B receptor activation contributes to the inhibitory effect of serotonin 5-HT₃ receptors on cortical circuit responses. A, B, Maximum amplitude maps recorded in slices from a P29 animal after electrical stimulation at the layer 6/white matter boundary. Application of the GABA_B receptor antagonist 2-OH-saclophen (300 μm) slightly increased (B) or did not effect circuit excitation (A). The inhibitory action of the 5-HT₃ receptor agonist M-109 (100 mm) was either partially (A) or in some cases almost completely (B) antagonized by GABA_B receptor blockade. The bottom rowsshow difference plots to demonstrate the size of effects more clearly. The last panels in the top rows inA and B show the time course of drug effects on the averaged maximum amplitudes (first 20 msec after stimulus onset averaged over all 100 positions per trial) and their reversibility.