The anxiolytic action of mGlu2/3 receptor agonist, LY354740, in the fear-potentiated startle model in rats is mechanistically distinct from diazepam

Abstract

The fear-potentiated startle paradigm has been characterized for drugs that act via ionotropic (NMDA and AMPA/kainate receptor) glutamate receptor mechanisms. Previous studies have shown that the potent systemically active mGlu2/3 receptor agonist, LY354740, effectively reduced the expression of fear-potentiated startle responses in rats. The present study examined the effects of LY354740 in a pre- versus post-fear conditioning paradigm and compared the effects to diazepam. Diazepam (0.3, 0.6, and 1.0 mg/kg ip) attenuated both pre- and post-fear conditioning startle responses in a dose-related manner. In contrast, LY354740 (0.03, 0.3, and 3.0 mg/kg ip) did not disrupt preconditioning startle responses at doses that attenuated post-fear conditioning responses. The benzodiazepine antagonist, flumazenil, at a dose (2 mg/kg sc) that did not alter fear-potentiated startle per se, selectively reversed suppression of fear responses to diazepam (0.6 mg/kg ip) while not affecting fear suppression induced by LY354740 (0.3 mg/kg ip). At a dose of 1 mg/kg ip, the mGlu2/3 receptor antagonist, LY341495, did not disrupt fear-enhanced startle per se, but completely reversed the postconditioning anxiolytic effects of LY354740 in this model. This dose of LY341495 had no effect on fear suppression by diazepam. These results demonstrate that fear suppression by diazepam and LY354740 involves different neuronal mechanisms. While diazepam acts via the facilitation of GABAergic transmission, LY354740 induces its actions via the glutamatergic system, specifically mGlu2/3 receptor activation. Furthermore, in contrast to disruption of fear conditioning as well as fear suppression by diazepam, LY354740 had selective effects on fear expression, suggesting anxiolytic actions without the associated memory impairment.