Hormonal and cognitive factors associated with the exploratory behavior of rats submitted to repeated sessions of the elevated plus-maze

Naive rats submitted to the elevated plus-maze (EPM) display a characteristic increase in open arm exploration and reduced risk assessment behaviors (RABs) after the administration of anxiolytic drugs. Upon re-exposure to the maze, however, the traditional measures of the EPM become resistant to these drugs. This intriguing phenomenon was initially observed for the benzodiazepine chlordiazepoxide and referred as one-trial tolerance (OTT). In this review, we summarized hormonal, cognitive and neuroanatomical data obtained from rats submitted to the test/retest protocol in the EPM. The re-exposure to the EPM is characterized by more prominent RABs and a distinct Fos protein distribution in the brain, particularly in limbic structures involved with the cognitive aspects of fear, such as the ventral regions of the medial prefrontal cortex (mPFC) and amygdala. Interestingly, naive rats treated with midazolam had a significant decrease in the number of Fos-positive neurons in the anterior cingulate cortex, area 1 (Cg1), anterior and dorsal premammillary nuclei of hypothalamus. On the other hand, midazolam caused a significant decrease in the number of Fos-positive neurons in the mPFC, amygdala, dorsomedial nucleus of hypothalamus and raphe nuclei in maze-experienced rats. Cg1 was the only structure targeted by the benzodiazepine in both sessions. Systemically administered midazolam before test or retest sessions reduced the RABs and plasma corticosterone levels in rats submitted to both sessions. Similar behavioral results were obtained with intra-Cg1 infusions of midazolam. The results reviewed here support the view of the crucial role of the RABs in the development of the OTT and point to this mPFC area as an important locus for the anxiolytic-like action of benzodiazepines in rodents.