OBJECT CODING IN THE DENTATE GYRUS DURING NOVEL OBJECT RECOGNITION

The hippocampus is crucial for learning, memory, and spatial navigation. Activity of hippocampal principal neurons is also modulated by non-spatial features of the environment, including object location and identity. In the dentate gyrus (DG), mossy and granule cells exhibit object-associated changes in their spatial firing maps. However, these responses are heterogeneous and poorly characterized. We examined how object introduction and exchange in a novel object recognition test (NORT) affected the activity of excitatory neurons in the DG. Using in vivo calcium imaging, we monitored neuronal activity in mice exploring a Y-maze, where objects were spatially separated in two maze arms, allowing for detection of object-specific firing. Each session included two trials in the empty maze, exploration of two identical objects, the NORT 1.5 h later in which one object was replaced, and another trial in which the positions of the two objects were switched. We identified a population of object-responsive cells (OCs) that were active only upon object introduction. This population showed higher firing map similarity between trials in which the same object was present in the arm compared to trials with different objects. Recent studies hint at the involvement of dendrite-targeting somatostatin-expressing interneurons (SOM-INs) in novelty-associated responses. Indeed, optogenetic SOM-IN silencing disrupted this object specificity and reduced the separation between object representations. On the behavioral level, opto- and chemogenetic suppression of SOM-INs led to reduced novel object preference. Together, these findings suggest that SOM-IN-mediated inhibition is critically important for object-specific coding in the DG and object discrimination.