Fig. 1

The bidirectional modulation of fear extinction by thalamic dual firing (tonic and burst) in MD. The mediodorsal thalamic nucleus (MD), a part of the basolateral limbic system, has strong interconnections with the medial prefrontal cortex (mPFC) as well as with the amygdala (Amy), and these regions together form a tight triangular circuitry. Tetrode recordings in freely moving mice revealed that the increased frequency of tonic firing of MD neurons is positively correlated with the development of fear extinction and that frequency of T-type Ca²⁺-dependent burst spikes was increased in the PLCβ4^–/– mice, which showed impaired fear extinction. Moreover, electrical microstimulation with a protocol increasing tonic firing in the MD facilitates fear extinction in both of wild type and PLCβ4^–/– mice. These results suggest that an increase of tonic firing in the MD is necessary for fear extinction. In contrast to the role of tonic firing, electrical microstimulation with a protocol mimicking burst spikes in the MD suppresses fear extinction (Lee et al., 2012)