ELICITATION AND REDUCTION OF FEAR - BEHAVIORAL AND NEUROENDOCRINE INDEXES AND BRAIN INDUCTION OF THE IMMEDIATE-EARLY GENE C-FOS

The elicitation and reduction of fear were indexed with fear-potentiat ed startle and corticosterone release, and induction of the immediate- early gene c-fos as a marker of neural activity in male Sprague-Dawley rats. Conditioning consisted of pairing one stimulus with footshock, which was withheld when the conditioned stimulus was preceded by a dif ferent modality stimulus, the conditioned inhibitor. On the test day, approximately 60% of the rats were used for c-fos in situ hybridizatio n, and were presented with either the conditioned stimulus alone, the conditioned inhibitor alone, a compound of the two stimuli, or no stim uli, and killed 30 min following the presentation of 10 such stimuli. The remaining rats were tested with the fear-potentiated startle parad igm. Rats displayed reliable fear-potentiated startle and corticostero ne release to the conditioned stimulus, and both measures were reduced when the conditioned stimulus was preceded by the conditioned inhibit or. The ventral bed nucleus of the stria terminalis, septohypothalamic nucleus, some tegmental nuclei, and the locus coeruleus had particula rly high c-fos induction in rats that received the conditioned inhibit or, providing one of the first functional indication that these nuclei might be important in behavioural or endocrine inhibition. Conditioni ng specific c-fos induction in the three groups that received a stimul us on the test day was observed in many hypothalamic areas, the medial geniculate body and the central gray, structures previously involved in fear and anxiety. The cingulate, infralimbic and perirhinal cortex, nucleus accumbens, lateral septum, dorsal endopiriform nucleus, and V entral tegmental area bad higher c-fos induction in rats presented wit h the fearful conditioned stimulus, confirming previous studies. The a mygdala and hippocampus of conditioned rats did not show higher c-fos induction than in rats repeatedly exposed to the context. Many regions displayed c-fos messenger RNA induction in the control condition, sug gesting that processes other than fear and anxiety participate in c-fo s induction. (C) 1997 IBRO. Published by Elsevier Science Ltd.