Effects of serotonergic terminal lesion in the amygdala on conditioned fear and innate fear in rats

The amygdala and the medial prefrontal cortex (mPFC) are crucial brain structures for anxiety, and it is speculated that the serotonergic neural system in these structures has an important role in regulating anxiety. In our previous study, we indicated that local injections of selective serotonin reuptake inhibitor into the amygdala attenuated anxiety-related behaviors in conditioned fear in rats. In the present study, we investigated the effects of serotonergic terminal lesions in the amygdala and in mPFC induced by local injection of 5,7-dihydroxytryptamine (5,7-DHT), on anxiety-related behaviors in conditioned fear and the elevated plus-maze test in rats. A 5,7-DHT lesion in the amygdala attenuated memory-dependent fear assessed by conditioned fear, but enhanced memory-independent fear assessed by the elevated plus-maze test. These results suggest that the role of the amygdalar serotonergic system in fear is different between memory-dependent and independent fear and, in particular, it is paradoxical that an amygdalar serotonergic lesion exerts a similar effect on memory-dependent fear to SSRI. Moreover, a serotonergic lesion in the amygdala enhanced the retrieval of extinction memory in conditioned fear; however, a serotonergic lesion in mPFC did not bring about any behavioral changes.