Figure 1.

Specific PIT exposure triggers strong ERK1/2 signaling responses selectively in D1R-MSNs of the NAc-S. A, All mice received Pavlovian and instrumental training. A single PIT test was given to one group of mice (Group PIT, black) whereas it was omitted for the other group (Group No PIT, red). B–D, Pavlovian training (B) produced a gradual increase in magazine entries during presentation of the conditioned stimulus. The lever press rate gradually increased across instrumental training (C). Group PIT exhibited a higher lever press rate when the stimulus predicted the same outcome as the response (Same) than when the stimulus predicted a different outcome (Different) or when there was no stimulus (Baseline) D, Immediately after the test, all mice were perfused and brain samples processed for optimal phosphorylation signal (see Materials and Methods). E, F, Three-staining immunofluorescence (F) was conducted to label eGFP (identifying D2R-MSNs), phospho-ERK1/2 (p-ERK1/2; identifying activated neurons) and DARPP-32 (D-32; identifying all MSNs) in the NAc-S of trained drd2-eGFP mice exposed (Group PIT) or not exposed (Group No PIT) to the PIT test. Almost all p-ERK1/2-activated neurons were identified as MSNs (D-32 immunoreactive, data not shown). For quantification (E), neurons were classified as eGFP or Non-eGFP, depending on the presence or absence of D2R-eGFP. PIT exposure induced large numbers of phospho-ERK1/2-immunoreactive neurons in the NAc-S, which were almost entirely excluded from D2R-eGFP fluorescence. Error bars denote ±1 SEM.