The use of a destabilized form of CreERT2, the

inclusion of endogenous sequences in the Fos and Arc 3′UTRs that contribute to mRNA destabilization, the development of new CreER ligands that are more rapidly absorbed and metabolized than 4-OHT, and the development of drug-dependent recombinases with reduced leakiness and improved inducibility may result in an improved signal-to-noise ratio. Nonetheless, complex experiences, such as exploration of a novel environment, can increase TRAPing above homecage levels ( Figure 6), suggesting that the current version of TRAP has sufficient signal-to-noise ratio FK228 without sensory deprivation. Despite the limitations, we have shown that TRAP provides valuable genetic access to active populations of neurons with feature selectivity in multiple systems. Thus, TRAP can be used in combination with various Cre-dependent effectors to trace connectivity, record activity, and manipulate functions of these select neuronal populations. Although previous

methods, such as TetTag, also enabled genetic manipulation of functionally defined neuronal populations, TRAP’s superior temporal SAHA HDAC nmr resolution and its ability to provide permanent genetic access make it a major advancement that has the potential to enable previously impossible experiments. Methods for mouse production and histology can be found in the Supplemental Experimental Procedures. All mouse procedures were approved by the Stanford University Administrative Panel on Laboratory Animal Care and were in accordance with all applicable regulatory standards. ArcCreER (JAX stock #021881) and FosCreER (JAX stock #021882) mice can be requested from the Jackson Laboratory. In pilot experiments,

we tested a range of TM doses (30–150 mg/kg) and found that TM-induced recombination was highly nonlinear. Low TM doses (30 mg/kg TM) induced minimal recombination, particularly in the less-sensitive FosTRAP mice (data not shown). Given that 150 mg/kg TM induced robust recombination and was Rolziracetam well tolerated by the mice, this dose was used for further studies. Similarly, 15 mg/kg 4-OHT induced minimal recombination in FosTRAP mice, whereas 150 mg/kg 4-OHT was not well tolerated. For additional studies, 50 mg/kg was used. In V1 (see Figure 4), treatment with 150 mg/kg TM and 50 mg/kg 4-OHT produced similar total numbers of TRAPed cells both in the dark condition (4-OHT, 622 ± 110 cells/mm3; TM, 777 ± 191 cells/mm3; t[7] = 0.65, p = 0.53) and when administered at the time point for optimal TRAPing (0 hr 4-OHT, 5184 ± 605 cells/mm3; 24 hr TM, 5736 ± 731 cells/mm3; t[9] = 0.57, p = 0.59). We also found that 4-OHT produced more consistent results than TM. In 5%–20% of mice treated with TM, induction appeared to fail altogether, and so few cells were labeled that the mice were excluded from analysis. Similar failures were never observed in mice treated with 4-OHT.