(A) In most layer 5 pyramidal neurons, focal application of 5-HT (100 μM, green), but not drug-free saline (blue), generates a long-lasting inhibition of action potential firing (top). A plot of instantaneous spike frequency for each action potential is shown below. (B) In a minority of layer 5 pyramidal neurons, focal 5-HT application increases action potential generation. (C) Some layer 5 pyramidal neurons display biphasic responses to 5-HT. In all examples shown, DC somatic current injection was used to depolarize neurons and induce action potential generation. Dashed lines show the level of zero Hz. Scale bars in A apply to data in B and C. Unpublished data courtesy of Daniel Avesar and Allan T. Gulledge.