Effects of Microinjections of a Serotonin Receptor (5-HT2A/C) Agonist and Antagonist into the Amygdala in Rats on Anxiety Behavior and Conditioned Reflex Fear

The role of 5-HT 2A/C receptors in the amygdala in the acquisition, testing, and extinction of conditioned reflex fear and on anxiety behavior was studied. Microinjections of a receptor antagonist (ketanserin, 0.5 μg/0.5 μl) into the basolateral nucleus of the amygdala decreased signs of conditioned reflex fear in the form of freezing and prevented reacquisition, but had no effect on anxiety behavior in rats. Administration of an agonist (DOI, 1 μg/0.5 μl) led to an increase in movement activity, panic-like behavior, and decreased anxiety, with reductions in the signs of fear in the form of freezing. Both the agonist and the antagonist of 5-HT 2A/C receptors were able to accelerate fear extinction in rats with prolonged freezing, which does not extinguish easily. These results provide evidence that 5-HT 2A/C receptors in the amygdala play a significant role in the occurrence and persistence of conditioned reflex fear apparent as freezing.