Transmitter systems involved in neural plasticity underlying increased anxiety and defense--implications for understanding anxiety following traumatic stress

Abstract

Lasting changes in anxiety-like behavior (ALB) may be produced in several ways. These include partial limbic kindling, injection of the beta-carboline FG-7142, and brief, non-injurious, exposure of rodents to cats (predator stress). Both seizures and FG-7142 induce long-term potentiation (LTP) in efferent pathways of the amygdala known to participate in feline defensive behavior. By comparing the behavioral and physiological effects of partial kindling and injection of FG-7142, NMDA-dependent LTP in the right amygdalo-periacqueductal gray (PAG) pathway emerges as being critical to maintained increases in feline ALB. A similar dependence on NMDA-mediated processes is described for lasting increases in rodent ALB following predator stress. The lasting aftereffects of predator stress on a variety of measures parallel many of the symptoms of post-traumatic stress disorder (PTSD). Support is provided for the idea that behavioral changes following FG-7142 and predator stress may model anxiety associated with PTSD. Moreover, it is suggested that both models share mechanisms in common involving the PAG. These mechanisms likely involve initiation of LTP by NMDA receptors, and prolongation of LTP by CCKB receptors. To the extent that response to the stressors reviewed here mimics the symptoms of PTSD, the data implicate NMDA-mediated processes in the creation of what van der Kolk has called permanent emotional memories in PTSD. Their representation may be in the form of NMDA-dependent LTP of transmission within the amygdala and between the amygdala and its efferents. CCK may play a pivotal role in prolonging limbic LTP and anxiety following traumatic stress. Since block of CCKB receptors before and after the stressor prevents lasting increases in ALB, pharmacological intervention to block CCK receptors shortly after a traumatic stressor might be efficacious in mitigating the permanence of these emotional memories.