Multimode neural population coding of diverse innate fear response by mitral and tufted cells.

Neurons that encode odor information are fundamental to innate fear processes, yet how mitral/tufted (M/T) cells encode innate fear remains unknown. Here, we identify three different response patterns of M/T cells in the dorsal olfactory bulb (dOB) during active avoidance elicited by non-dehydrogenated 2,4,5-trimethylthiazole (nTMT) through in vivo calcium imaging and multielectrode recordings in mice, including enhanced responses, suppressed responses, and no response. Remarkably, suppressed response M/T cells encode active avoidance, whereas suppressed and enhanced response M/T cells jointly encode passive freezing. Our results show that cyclin-dependent kinase 5 (Cdk5) deficiency reduces the mean firing rate (MFR) of M/T cells and alters the intrinsic encoding patterns of M/T cells in nTMT-induced abnormal avoidance and freezing behaviors, and these changes can be rescued by activating dOB M/T cells. These results reveal multimodal neural population coding characteristics of innate fear in the dOB.