The firing patterns of hippocampal interneurons are highly dependent on the network state, such as theta oscillations during movement or large-amplitude irregular network activity during sleep (Buzsáki, 2006, Ego-Stengel and Wilson, 2007, O’Keefe and Conway, 1978 and Ranck, 1973). Drug-free behavior-dependent firing patterns of some identified cell types have been determined recently in freely moving rats (ivy cells, PV+ basket cells; Lapray et al., 2012) and in head-fixed mice (O-LM cells, PV+ basket cells; Varga et al., 2012), although for O-LM cells this did not include sleep.

The firing patterns of identified bistratified cells in drug-free animals are unknown. We have recorded the firing of two distinct types of dendrite-targeting neuron in freely moving rats to test the hypothesis that differences in the axonal terminations of SOM-expressing cells are associated with different firing patterns under natural awake behavior and Smad inhibitor sleep. This required the recording and labeling of SOM-expressing interneurons in freely moving rats using

the juxtacellular labeling technique to identify NVP-BKM120 the cells and enabled us to quantitatively dissect the firing dynamics of these cells and compare them to PV+ basket cells (Lapray et al., 2012), which target a different subcellular domain of pyramidal cells. We have recorded the firing patterns of single interneurons using a glass electrode during periods of sleep, movement, and quiet wakefulness. Then, we either moved the electrode into a juxtacellular position or sometimes the cells spontaneously drifted close to the electrode, which made it possible to attempt labeling the cells with neurobiotin for identification of cell types. The

labeled Oxymatrine cells were assessed by immunofluorescence microscopy and tested for the presence of various molecules, including SOM and NPY. Nine identified interneurons (n = 9 rats, one cell each) were immunopositive for SOM, NPY, or both when tested by immunofluorescence microscopy and showed dendritic and axonal arborizations similar to previously described bistratified and O-LM cells (Buhl et al., 1994 and McBain et al., 1994). The recording sites were distributed over an area of 1.7 × 1.4 mm along the rostrocaudal and mediolateral axes (Figure S1A available online). Somata of bistratified cells (n = 4/5 recovered) were located in the vicinity of pyramidal cell somata (Figures 1A and S1A and S1C), had mainly radially oriented dendritic trees (n = 3/5 recovered; see exception Figure S1C), and axon collaterals distributed in strata oriens and radiatum (n = 3/5 recovered). The axonal extent of a well-labeled cell was large (Figure 1A), reaching 2.4 mm mediolaterally and 1.7 mm rostrocaudally, confirming previous results obtained in vivo (Klausberger et al., 2004). Somata (n = 4/4 tested) were immunopositive for NPY (Figures 1B and S1E) and parvalbumin (PV), the latter also expressed in dendrites and axon (Figures 1C and S1D; Table 1).