Serotoninergic activation of the basolateral amygdala and modulation of tonic immobility in guinea pig

Abstract

Tonic immobility (TI), also known as death feigning or animal hypnosis, is a reversible state of motor inhibition that is not only triggered by postural inversion and/or movement restraining maneuvers but also by repetitive stimulation and pressure on body parts. Evidence has demonstrated that the basolateral nucleus of the amygdala (BLA) is particularly associated with defensive behavior that involves the emotional states of fear and anxiety. In addition, some reports have demonstrated that serotonin (5-HT) released in the amygdala is increased during states of stress and anxiety, principally in the BLA. In the present study, we investigated the effects of serotonergic activation of the BLA on the duration of TI. The results showed that the microinjection of 5-HT (3.0 μg) into the BLA decreased the duration of TI. Similarly, the administration of a 5-HT 1A agonist (0.1 μg of 8-hydroxy-dipropylaminotretalin) or 5-HT 2 agonist (0.1 μg of α-methyl-5-HT) into the BLA reduced the TI duration. The effect of 5-HT 2 agonist was reversed by pretreatment with a dose that had no effect per se (0.01 μg) of ketanserin (5-HT 2 receptor antagonists) into the BLA. Moreover, the activation of 5-HT 1A and 5-HT 2 receptors in the BLA did not alter the spontaneous motor activity in the open field test. The results of the present study indicate that the serotonergic system of the BLA possibly produces a reduction in fear and/or anxiety that reduces the TI duration in guinea pigs, but this is not due to increased spontaneous motor activity induced by serotonergic activation, which might affect TI duration non-specifically.