Evidence that long-lasting potentiation in limbic circuits mediating defensive behaviour in the right hemisphere underlies pharmacological stressor (FG-7142) induced lasting increases in anxiety-like behaviour: role of benzodiazepine receptors.

Abstract

The hypothesis that benzodiazepine receptors mediate initiation of lasting behavioural changes induced by FG-7142 was supported in this study. Behavioural changes normally induced by FG-7142 were blocked by prior administration of the competitive benzodiazepine receptor blocker, Flumazenil. When cats were subsequently given FG-7142 alone, the drug produced lasting behavioural changes in species characteristic defensive responses to rodent and cat vocal threat. FG-7142 also induced long-lasting potentiation (LLP) of evoked potentials in a number of efferent pathways from the amygdala in both hemispheres. Flumazenil given prior to FG-7142 blocked LLP in all but one of the amygdala efferent pathways, suggesting benzodiazepine receptor dependence of initiation of LLP. Three physiological changes were most closely correlated with behavioural changes. LLP in the right amygdalo-ventromedial hypothalamic (VMH) and amygdalo-periacqueductal gray (PAG) pathways coincided closely with behavioural changes, as did a reduced threshold for the right amygdalo-VMH evoked potential. Administration of Flumazenil after FG-7142 returned defensive behaviour to pre FG-7142 baseline levels in a drug-dependent manner. At the same time LLP only in the right amygdalo-PAG pathway was reduced by Flumazenil. LLP in other pathways and amygdalo-VMH threshold were unaltered by Flumazenil. Moreover, covariance analyses indicated that increased defensiveness depended solely on LLP in the right amygdalo-PAG. These findings support the view that maintenance of lasting increases in defensive behaviour depend upon LLP of excitatory neural transmission between amygdala and lateral column of the PAG in the right hemisphere. Moreover, FG-7142 may be a useful model of the effects of traumatic stressors on limbic system function in anxiety, especially in view of the recent data in humans implicating right hemispheric function in persisting negative affective states in post-traumatic stress disorder.