Abstract
Ascending 5-HT projections from the
median raphe nucleus (MRN), probably to the
hippocampus, are implicated in the acquisition
of contextual fear (background stimuli), as
assessed by freezing behavior. Foreground cues
like light, used as a conditioned stimulus (CS) in
classical fear conditioning, also cause freezing
through thalamic transmission to the amygdala.
As the MRN projects to the hippocampus and
amygdala, the role of this raphe nucleus in fear
conditioning to explicit cues remains to be
explained. Here we analyzed the behavior of
rats with MRN electrolytic lesions in a
contextual conditioning situation and in a fear-potentiated
startle procedure. The animals
received MRN electrolytic lesions either before
or on the day after two consecutive training
sessions in which they were submitted to 10
conditioning trials, each in an experimental
chamber (same context) where they. received
foot-shocks (0.6 mA, 1 sec) paired to a 4-sec
light CS. Seven to ten days later, the animals
were submitted to testing sessions for assessing
conditioned fear when they were placed for five
shocks, and the duration of contextual freezing
was recorded. The animals were then submitted
to a fear-potentiated startle in response to a 4-sec
light-CS, followed by white noise (100 dB, 50 ms). Control rats (sham) tested in the same
context showed more freezing than did rats
with pre- or post-training MRN lesions. Startle
was clearly potentiated in the presence of light CS in the sham-lesioned animals. Whereas pretraining
lesions reduced both freezing and fear-potentiated
startle, the post-training lesions
reduced only freezing to context, without
changing the fear-potentiated startle. In a
second experiment, neurotoxic lesions of the
MRN with local injections of N-methyl-D-aspartate
or the activation of