05, see Experimental Procedures).

Two cells fired independently from the hippocampal θ rhythm (Figure 1A). The four θ-modulated cells fired preferentially between the peak and the descending phase of dCA1 θ (range 187.0–283.7°, where 0° and 360° represent θ troughs; θ phase histograms of single neurons are illustrated in Figure S2). However, statistical analysis showed that these four cells did not form a synchronized population in relation to dCA1 θ (R′ = 1.03, R0.05,4 = 1.09, Moore test). Furthermore, Akt inhibitor the firing of axo-axonic cells did not show statistically significant modulation in phase with dCA1 γ oscillations (p > 0.1, Rayleigh test, n = 6; Figure S3; Table S3). Axo-axonic cells displayed dramatic short-latency excitations in response to noxious stimuli. All axo-axonic cells increased their firing

rates upon hindpaw pinches (+377% of baseline, latency 267 ms, peak 377 ms, n = 6; ranges: 133%–606%, latency 200–400 ms, peak 400–600 ms, respectively; Table 2; individual histograms are shown in Figure S4). This excitation rapidly adapted, and was curtailed at stimulus offset (Figure 5D). Responses to electrical footshocks were similarly pronounced (mean 226% of baseline, latency 50 ms, peak 225 ms, n = 4/4; ranges 133%–606%, 20–100 ms, 20–420 ms, respectively; Figure 1C; Ku-0059436 ic50 Table 2; individual histograms, Figure S5). These neurons exhibited typical axo-dendritic patterns. Their axons formed cartridges. Almost all of large-axon varicosities were in close apposition with ankyrin G-expressing axon initial segments, (n = 6/6 cells), as seen with immunofluorescence (Figure 1D). We analyzed randomly-sampled synapses from two of these cells

using electron microscopy. The vast majority of postsynaptic targets were axon initial segments (95.4%, n = 43 synapses; Figure 1E; Table S1), confirming that these cells were of the axo-axonic type. All axo-axonic cells expressed parvalbumin (PV), sometimes weakly (Figure 1F), but were never calbindin (CB)-positive. Two of 6 neurons densely expressed the GABAAR-α1 subunit TCL on their dendrites (immunohistochemical results are summarized in Table S2). Axo-axonic cells were bitufted. Their dendrites did not branch immediately, were tortuous and sparsely spiny (Figure 1G). Axonal arborizations of all 6 cells were very dense and mostly contained within the dendritic field. Axons were always restricted to the BLA, but could be distributed between lateral and basal nuclei. These results show that the firing of axo-axonic cells of the BLA dramatically increases in response to salient sensory stimuli. However, their spontaneous population activity is not tightly synchronized with hippocampal θ (Figure 5). Next, we studied the firing of parvalbumin-expressing (PV+) basket cells (n = 15). During dCA1 θ oscillations, PV+ basket cells fired at a mean frequency of 11.0 Hz (range 1.8–27.