Conditioned c-Fos in mouse NTS during expression of a learned taste aversion depends on contextual cues

c-Fos expression in the nucleus tractus solitarius (NTS) of the rat has been found to follow administration of a variety of pharmacologically diverse unconditioned stimuli (US), and it has been proposed that NTS is a critical structure in transduction of the US during taste aversion learning. Before conditioning, the conditioned stimulus (CS) taste does not induce c-Fos in NTS, but following pairing of the CS and US, subsequent CS presentation induces c-Fos in NTS. Although it has been suggested that the shift in the c-Fos response following conditioning represents a molecular correlate of taste aversion learning, i.e. the formerly neutral CS now predicts the toxicity associated with the US, the data presented here suggest a more cautious interpretation of c-Fos expression in NTS. In mice, post-conditioning c-Fos expression to the CS depends on contextual cues: when conditioning and testing occur in a novel environment, CS saccharin causes an increase in c-Fos expression, and when conditioning and testing occur in the home cage, CS saccharin produces a decrease in c-Fos expression relative to controls. Furthermore, we show that merely placing an animal into a novel environment is sufficient to drive c-Fos expression in NTS. These data suggest that c-Fos expression in NTS can be driven by a number of different stimuli and conditions, and that these responses may depend on context-dependent activation of forebrain structures shown to drive conditioned c-Fos expression in NTS.