Ultrastructure of immature synaptic inputs in the lateral superior olive of the rodent brainstem

Neurons of the lateral superior olive (LSO), which compute intensity differences between the two ears, receive two primary inputs, an ipsilaterally arising excitatory input and a contralaterally arising inhibitory input, that are precisely matched for stimulus frequency. Circuit refinement to establish this precise match takes place within the first few postnatal weeks through elimination of single-fiber inputs and concomitant strengthening of the remaining inputs. However, little is known about the ultrastructure of these young synapses and about how changes in physical features of these synapses could contribute to refinement. To characterize pre-hearing postnatal development of somatic synapses in the LSO, we performed transmission electron microscopy and examined synapses in the rodent LSO from birth to hearing onset at postnatal day 13. Synaptic vesicles and mitochondria in putative synaptic boutons were surprisingly scarce at birth. During the second week, bouton enlargement was accompanied by an increase in the number of vesicles and mitochondria. The size of mitochondria also increased, pointing to changes in functional and metabolic needs of synapses. Our results reveal extensive remodeling at individual presynaptic terminals that could strengthen single-fiber inputs and contribute to the development of robust synaptic transmission.