Research reportImmediate early gene and neuropeptide expression following exposure to the predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT)

- Rats freeze more to TMT than both BA and H2O, with freezing to BA greater than to H2O.
- TMT exposed rats displayed freezing bouts longer than 20 s, but BA and H2O did not.
- TMT exposure increased c-fos and egr-1 in the BNST, PVN, and CeA, but not mPFC.
- TMT exposure increased crh in the CeA and PVN, but not the BNST or mPFC.
- Enk increased in the PVN for both TMT and BA exposed rats, but not H2O.

The immediate early gene c-fos and a number of neuropeptides have been widely used to help delineate the neural circuitry of innate fear to predator odors. The present study used in situ hybridization techniques to examine the expression of the immediate early gene transcription factors c-fos and egr-1, and the neuropeptides corticotropin-releasing hormone (crh) and enkephalin (enk) following exposure to the predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT). Rats were exposed to water (H2O), TMT, or the irritating odor butyric acid (BA) and freezing was used to measure fear behavior. Changes in gene expression were analyzed in the medial prefrontal cortex (mPFC), the bed nucleus of the stria terminalis (BNST), paraventricular nucleus of the hypothalamus (PVN), and central nucleus of the amygdala (CeA). Animals froze more to TMT than BA and H2O, and more to BA than H2O. Compared to H2O and BA, c-fos and egr-1 were elevated within the BNST, PVN, and CeA in rats exposed to TMT, but not the mPFC. Crh was also elevated in rats exposed to TMT within the CeA and PVN, but not the BNST or mPFC. Enk was elevated within the PVN in TMT and BA exposed rats compared to H2O exposure. These data indicate that exposure to the predator odor TMT induces similar expression patterns for c-fos and egr-1, but different patterns for crh and enk, with partial overlap of the immediate-early genes and neuropeptides within specific brain regions.