Figure 2. Circuits underlying novelty responses to social and inanimate objects in rodents.

(A) Schematic representation of a sagittal view of a rodent brain showing midbrain circuits and interconnected areas that respond to novel and familiar social information. Novel social stimuli strongly activate VTA (red) DA systems that project to the NAc (orange), which is a hub integrating and consolidating socially-relevant information from the HPC (purple), and processing social reward via projections from the DR (gray). The LH (green) sends GABA inputs to the VTA that contribute to the social novelty response. The VTA also sends DA projections to the IPN (blue), which, via the habenulo (yellow) -interpeduncular axis, has an important role in resolving familiarity-based information. (B) Novel inanimate object stimuli activate midbrain DA systems in the VTA, but most importantly, in the SN lateral (light red) that projects to the TS (light orange). The VTA receives glutamate and GABA projections from the LH that also process salient information to novel inanimate objects. The habenulo-interpeduncular axis guides responses to familiar inanimate objects. Additional midbrain nuclei including the MnR (light gray) also play a role in response to novel objects. DR, dorsal raphe; HPC, hippocampus; IPN, interpeduncular nucleus; LH, lateral hypothalamus; MHb, medial habenula; MnR, median raphe; NAc, nucleus accumbens; SN, substantia nigra; TS, tail of the striatum; VTA, ventral tegmental area.