Differences in the density of GABA-A receptor alpha-2 subunits and gephyrin in brain structures of rats selected for low and high anxiety in basal and fear-stimulated conditions, in a model of contextual fear conditioning.

In this paper we used a model of individual differences in fear responses in rats selected according to their low and high freezing response in the contextual fear test and termed these animals "low- and high anxiety" rats, respectively. We studied differences in the density of GABA-A receptor alpha-2 subunits and gephyrin in the brain structures of low (LR) and high (HR) anxiety rats subjected to extinction trials and re-learning of a conditioned fear response. We found different patterns of spontaneous (western blotting) and fear-stimulated expression of alpha-2 subunits and gephyrin (western blotting and immunocytochemistry) in brain structures of HR and LR animals. Increased basal concentrations of alpha-2 subunits in the amygdala were observed in HR rats (compared to unconditioned control group). The density of alpha-2 subunits in the amygdala negatively correlated with freezing response duration in the aversive context on re-learning in the same group of animals. This finding supports data on the role of GABA-A receptor alpha-2 subunits in the amygdala in modulation of anxiety-like behaviour. Western blotting revealed that exposure of HR animals to fear-conditioned context upon re-test of the conditioned fear test elevated the concentration of alpha-2 subunits in the amygdala and prefrontal cortex. In addition, immunocytochemistry showed that conditioned fear increased the number of cells co-expressing alpha-2 subunits and gephyrin in the basolateral amygdala and dentate gyrus of the hippocampus in the HR group. Together, these findings suggest that animals that are more vulnerable to stress differ in the intracellular mechanisms that control GABA-A receptor trafficking in limbic structures (hippocampus and amygdala), which are involved in the control of emotional behaviour. These data indicate a possible mechanism for the variable effects of benzodiazepines among patients with anxiety disorders. The obtained data may also help to better explain the neurobiological mechanisms that operate in clinical situations where anxious patients subjected to exposure therapy are exposed to an aversive, contextual and conditioning stimulus.