The transition from externally to internally focused attention is accompanied by the coactivation of a group of brain regions collectively known as the default mode network (DMN). While the DMN was initially considered as a cortical network, recent results have provided evidence that some subcortical structures are also involved in DMN regulation. Here we investigated the subcortical nodes, ventral pallidum (VP) and mediodorsal thalamus (MD), in DMN regulation in the tree shrew, a diurnal mammal that is a close relative of primates. We use wireless multichannel recordings and motion tracking, deep learning-based pose estimation, to perform unsupervised classification of behavioral states in the home cage. We found pronounced gamma oscillation (40-60Hz) in both VP and MD that was coordinated with gamma in the anterior cingulate (AC) cortex specifically during DMN-associated behavioral states. We also found enhancements in the high gamma range (60-150Hz), but only in the subcortical sites. Further, cross-frequency coupling between gamma and delta oscillations were markedly higher during DMN than other behaviors, underscoring the engagement of MD, VP and AC neural circuits during DMN states. Our findings highlight VP regulation in DMN in the tree shrew, consistent with previous rodent studies. We further demonstrate a key role for MD thalamus in DMN regulation, as some previous human fMRI studies suggested. Further, our results extend the homologies in DMN regulation among mammals and underline the importance of both thalamus and basal forebrain to the initiation and maintenance of DMN brain states.