Learning-related changes in cellular activity within mouse dentate gyrus during trace eyeblink conditioning

Because the dentate gyrus serves as the first site for information processing in the hippocampal trisynaptic circuit, it an important structure for the formation of associative memories. Previous findings in rabbit had recorded populations of cells within dentate gyrus that may bridge the temporal gap between stimuli to support memory formation during trace eyeblink conditioning, an associative learning task. However, this previous work was unable to identify the types of cells demonstrating this type of activity. To explore these changes further, we did in vivo single-neuron recording in conjunction with physiological determination of cell types to investigate the functional role of granule cells, mossy cells, and interneurons in dentate gyrus during learning. Tetrode recordings were performed in young-adult mice during training on trace eyeblink conditioning, a hippocampal-dependent temporal associative memory task. Conditioned mice were able to successfully learn the task, with male mice learning at a faster rate than female mice. In the conditioned group, granule cells tended to show an increase in firing rate during conditioned stimulus presentation while mossy cells showed a decrease in firing rate during the trace interval and the unconditioned stimulus. Interestingly, populations of interneurons demonstrated learning-related increases and decreases in activity that began at onset of the conditioned stimulus and persisted through the trace interval. The current study also found a significant increase in theta power during stimuli presentation in conditioned animals, and this change in theta decreased over time. Ultimately, these data suggest unique involvement of granule cells, mossy cells, and interneurons in dentate gyrus in the formation of a trace associative memory. This work expands our knowledge of dentate gyrus function, helping to discern how aging and disease might disrupt this process.