The distinct temporal origins of olfactory bulb interneuron subtypes.
Segregation of neuron-type-specific synaptic connections in different strata is a characteristic feature shared by the olfactory bulb (OB) and retina. In the mammalian OB, mitral cells form dendrodendritic synapses with granule cells (GCs) in the deep stratum of the external plexiform layer (EPL), whereas tufted cells form dendrodendritic synapses in the superficial stratum. In the search for membrane proteins with strata-specific expression patterns, we found that a leucine-rich repeat membrane protein (5T4 oncofetal trophoblast glycoprotein) was expressed selectively by a subset of superficial GCs. The somata of 5T4-positive GCs were localized in or near the mitral cell layer, and their apical dendrites ramified preferentially in the superficial stratum of the EPL, where tufted cell dendrites ramified. Strata-specific expression of 5T4 was found also in the retina: 5T4 was expressed selectively by rod-bipolar cells and a subset of amacrine cells whose dendrites ramified in a specific sublamina of the inner plexiform layer. During the perinatal and postnatal development of the OB, 5T4 expression paralleled in time the formation of dendrodendritic synapses in the EPL. Odor deprivation during the first postnatal month selectively reduced the thickness of the superficial stratum of the EPL and the number of 5T4-positive GCs. Because 5T4 is known to interact with actin cytoskeleton, these observations suggest that 5T4 is involved in the formation or maintenance of strata-specific dendritic ramification or synaptic connection of subsets of local interneurons.