Thus, most TRAPed cells in V1 are excitatory neurons. To determine the time window around a TM injection during which active cells are efficiently TRAPed, we examined V1 in FosTRAP mice that had been stimulated with 1 hr of diffuse bright light at various times relative to the injection (Figure 4A). TRAPing was maximal when light stimulation occurred 23–24 hr after injection. No TRAPing above the level of the dark control occurred when light was

given 6–7 hr before the injection or 35–36 hr after injection (Figures 4B and 4D). Labeling in a control region (S1) was Neratinib price similar across all time points (Figures 4B and 4D). Thus, under these conditions, TRAP appears to be sensitive to neuronal activation that occurs less than 6 hr prior to injection and up to 24–36 hr after injection. A long time window may be desirable in cases where it is beneficial to TRAP cells on the basis of the integration of activity over a long period of time. However, applications that utilize stimuli and experiences of short duration could Olaparib manufacturer benefit from a shorter time window. After injection, TM is metabolized to its principal active form, 4-hydroxytamoxifen (4-OHT; Robinson et al., 1991).

Directly injecting 4-OHT shortened the TRAPing time window to <12 hr (Figure 4D); optimal TRAPing in V1 was observed when light was administered in the hour immediately before injection of 4-OHT, and minimal TRAPing was observed when light was delivered 6–7 hr before or 5–6 hr after the injection. To determine the dependence of TRAP on stimulus duration, we delivered light pulses of varying durations beginning 1 hr before a 4-OHT injection. Relative to mice left in the dark, mice exposed to light pulses of 5, 15, and 60 min in duration had 2.6-, 4.9-, and 8.3-fold more TRAPed cells in V1 (Figures S5A–S5C). Thus, even short (5 min) stimuli are sufficient for TRAPing, although longer duration stimuli increase the total numbers of TRAPed cells. These results are consistent with prior findings

that the induction of Fos protein in V1 is dependent on stimulus duration (Amir and Robinson, 1996). The time course of effector expression after TRAPing determines Tryptophan synthase the earliest time point at which subsequent experimental manipulations are possible. Although this parameter is most likely to be dependent on effector and cell type, we found that it took at least 72 hr following light stimulation and 4-OHT injection for TRAPed V1 cells to express sufficiently high levels of tdTomato to be reliably identified (Figures S5D–S5F). Next, we took advantage of the tonotopic organization of the auditory system to evaluate whether TRAP can provide genetic access to cell populations that are activated by particular features of sensory stimuli. We focused on the cochlear nucleus (CN), all three subdivisions of which receive input from spiral ganglion neurons (SGNs) that carry auditory information from the cochlea.