Olfaction is a critical sense for identifying, discriminating, and remembering chemical stimuli in the environment. It is vital in early mammalian life, and in rodents is essential for survival from birth. Projection neurons in the main olfactory bulb (OB) – mitral/tufted cells (M/TCs) – are the only route by which olfactory information from the nose is transmitted to higher processing centres in the brain, transmission which depends on action potential (AP) propagation. Organismal survival therefore depends on the initiation of APs in M/TCs being functional from birth.
However, mouse M/TCs are not fully mature at birth. They are known, for example, to undergo extensive, activity-dependent dendritic pruning in the first postnatal week. Is M/TC AP firing similarly immature and plastic in early life? In fact, how M/TCs acquire the ability to fire mature APs remained entirely unknown. The axon initial segment (AIS) initiates and modulates APF firing, however, aside from one classic study suggesting that the AIS of M/TCs is surprisingly immature at birth, nothing is currently known about the maturation of M/TC excitability.
Here, we combined structural and functional approaches to characterise the development of the axon initial segment (AIS) and excitability in early M/TCs. We observed a progressive lengthening of the AIS revealed by immunostaining of the AIS master organiser molecule Ankyrin-G. Whole-cell patch-clamp electrophysiology recordings showed an overall electrically immature profile of M/TCs, both in passive membrane properties and action potential firing parameters.
Together, these results link structure to function in a hitherto unstudied developmental process that is crucial to the operation of an entire sensory system.