Mitral and tufted cells in the olfactory bulb act as an input convergence hub and transmit information to higher olfactory areas. Since first characterized, they have been classed as distinct projection neurons based on size and location i.e. laminarly arranged mitral cells and diffusely spread tufted cells across both the mitral layer (ML) and external plexiform layer (EPL). New in vivo work has shown that these neurons encode complementary olfactory information, akin to parallel channels in other sensory systems. Yet, many ex vivo studies collapse them into a single class, mitral/tufted (M/T), when describing their physiological properties and impact on circuit. Using immunohistochemistry and whole-cell patch clamp electrophysiology in acute slices from mice, we attempted to align in vivo and ex vivo data, to find a simple classifier of projection neurons in the ML and EPL based on intrinsic firing properties.

We aimed to produce an unbiased classifier of putative mitral (pMC) and putative tufted cells (pTC). Light microscopy and immunohistochemistry in acute and fixed slices confirmed that projection neurons in the ML have disparate soma sizes. As M/T cells have been divided based on soma size we used a diameter-based k-means analysis which returned a divisor that mirrors literature. Applied to electrophysiological data we found that pMCs and pTCs are distinct in several intrinsic parameters like action potential threshold (pMC(n=24) 2.8±0.34 pA/pF; pTC(n=12) 5.5±0.98 pA/pF; unpaired t-test p=0.0025) and relative afterhyperpolarization (pMC(n=20) 17.6±0.94 pA; pTC(n=9) 12.6±1.1 pA; unpaired t-test p=0.007) and homogenous in others. Together these results illustrate the heterogeneity of M/T cells and suggest that they cannot be simply classified based on their intrinsic firing properties alone.

Funding Grants: Royal Society Research RGS\R1\191481 & Isaac Newton Trust/Wellcome Trust ISSF/ University of Cambridge Joint Research (EG); RANNIS 217945-051 (PHP,EG).