, 2002, Dehmel et al., 2002 and Kulesza et al., 2003). This IPSP-induced offset firing is mediated by glycine receptors (Kadner and Berrebi, 2008) and three mechanisms have been postulated to explain the offset firing: (1) Coincident excitation and inhibition suppress firing during the sound, but the excitation outlasts the inhibition (discussed in Kulesza et al., 2003). We demonstrate that offset firing is an intrinsic activity of SPN neurons

with the ionic mechanism requiring three crucial elements: sound-evoked IPSPs, a large electrochemical chloride gradient, and the combination of a hyperpolarization-activated cation current, IH, with a T-type calcium current, ITCa. Modeling has suggested that IH could contribute to stimulus duration encoding (Hooper et al., 2002); our results provide experimental evidence for this but also demonstrate the crucial importance of the IPSP high throughput screening Screening Library cost and enhanced chloride gradients, so that the inhibition can provide sufficient hyperpolarization to activate IH in response to physiological sensory

input. We have confirmed this interpretation using sound-evoked SPN single-unit recordings in vivo, characterized the conductances using voltage clamp in vitro, and demonstrated that these conductances are sufficient to explain the results by computational modeling. The result is a physiologically elegant solution to computation of sound termination: a cell-specific conversion of inhibition into excitatory AP firing, which enhances timing accuracy and provides crucial information for downstream duration encoding. The majority of SPN neurons showed AP firing as an offset response following sound stimulation in vivo (64%, extracellular single unit, n = 15, Figures 1E, 1F, and 1I) or following hyperpolarization in vitro (89%, whole-cell patch clamp, n = 70, Figures 1C, 1H, and 1I). A minority of neurons showed no offset firing upon 17-DMAG (Alvespimycin) HCl hyperpolarization

(Figure 1I), but instead showed sustained or onset firing in vivo, as seen in other species (Behrend et al., 2002 and Dehmel et al., 2002). The offset firing characteristically exhibited an intrinsic rhythm observed as multiple distinct peaks in the poststimulus time histogram (PSTH; Figure 1F, inset). The average number of APs in the offset response was 3.5 ± 0.4 (n = 65) with an interspike interval of 1.85 ms ± 0.19 ms (n = 26 cells) between the first two APs and increasing variability for the subsequent APs, as shown in the inset in Figure 1C. A subpopulation of cells (25%; Figure 1I, dotted line) fired only one offset AP; these neurons lacked the very negative ECl (see below) and possessed an IA type potassium current, suggesting that their role may differ. The differences were minor and we did not exclude these cells from the data set to avoid sample bias.