5 Hz; Figure S5J). Together, these data demonstrate that a circuit intrinsic to the OT can generate and maintain persistent gamma oscillations. If the generator of these oscillations is indeed located in the OT, then the pharmacological manipulations that

altered the structure of the oscillations in the intact slice (Figure 3) should alter them in the same way when applied specifically to the OT. First, we tested the effects of the NMDA-R blocker APV on induced gamma oscillations in the isolated OT. In transected slices, bath application of APV substantially reduced the duration (11.1% of control, p < 0.001, n = 8; Figures 7, S6A, S6B, and S6C) and power Veliparib (49.3% of control, p < 0.001, Friedman test, n = 8) of activity in the i/dOT. Moreover,

increasing the strength of afferent stimulation by 3–4× in the presence of APV did not increase the duration of the oscillations (Figure S6D), suggesting that the effect of APV was not merely to reduce the general excitability of the OT circuitry. In sum, these data suggested that NMDA-R mediated glutamatergic transmission in the i/dOT was essential for the persistence of the oscillations. Next, we tested the effects of focal application of the GABA-R blocker PTX to the OT in intact slices. Recall that PTX, when bath-applied to intact slices, eliminated gamma periodicity (Figure 3A). We puffed PTX focally onto either the OT or the lpc with a micropipette while recording activity in the sOT in intact midbrain slices. Both the OT and the Ipc are innervated by GABAergic circuits, as indicated by the presence of parvalbumin immunoreactivity in both structures Cytoskeletal Signaling inhibitor Isotretinoin (Figure 8A). When applied to the OT, puffs of PTX transiently changed gamma oscillations into episodes of high-frequency spiking, mimicking the results of bath application

(Figures 8B, 8C, and S7A; duration: 33% of control, p > 0.5; power: 31% of control, p < 0.001, Friedman test, n = 7). In contrast, puffs of PTX applied to the Ipc in the same slice did not alter the periodic structure of gamma oscillations in the sOT (Figures 8D, 8E, and S7B; duration: 88% of control, p > 0.3; power: 74% of control, p > 0.5, n = 7). These results demonstrate conclusively that inhibition in the OT regulates the gamma periodicity of the midbrain oscillator. This study demonstrates that gamma oscillations can be induced in an in vitro slice preparation of the avian midbrain network and that these oscillations strongly resemble those induced by salient sensory stimuli in vivo. The synaptic mechanisms that regulate the frequency, power, and duration of the oscillations are similar to those that regulate gamma oscillations in mammalian forebrain structures. The source of the midbrain oscillations is the i/dOT. Rhythmic output from the i/dOT entrains periodic burst firing in the cholinergic nucleus Ipc, and the Ipc broadcasts the oscillations to the sOT.