Differences in membrane structure between excitatory and inhibitory components of the reciprocal synapse in the olfactory bulb

The reciprocal dendro‐dendritic synapse between granule and mitral or tufted dendrites in the external plexiform layer of the olfactory bulb consists of an excitatory mitral‐to‐granule synaptic contact and an adjacent inhibitory granule‐to‐mitral synaptic contact. The pre‐ and postsynaptic membranes of both synaptic contacts were identified in replicas of freeze‐fractured external plexiform layer in rabbits, mice, and chinchillas. At the excitatory synaptic contact there is a prominent specialization in the postsynaptic memberane, represented by an aggregate of homogeneous particles associated with the external half of the membrane. In contrast, the postsynaptic membrane at the inhibitory granule‐to‐mitral synaptic contact lacks evident internal specializations, and the distribution of particles on both fracture faces resembles that at non‐synaptic regions. Less marked differences in particle distribution characterized the cytoplasmic half of the presynaptic membranes. These differences probably reflect diversity in the nature or distribution of membrane proteins at excitatory and inhibitory synapses. Protuberances on the external half of the presynaptic membrane, possibly sites of vesicle interaction with the plasma membrane, surrounded but were not coextensive with both types of synaptic contact. A few gap junctions connected proximal dendrites of mitral or tufted cells with granule cell dendrites.