Successive episodes of synapses production in the developing rat nucleus tractus solitarii

In the rat nucleus tractus solitarii (NTS), synaptogenesis is thought to occur both pre- and postnatally. The present study was performed to precisely define the timetable of synapse formation in the NTS after birth. Changes in synapse morphology and densities were analyzed between postnatal day 3 (P3) and P28 using electron microscopy and ethanol phosphotungstic acid (E-PTA) staining. The proportion of morphologically immature synapses was high at P3 (38%) and P14 (30%) and low (8-14%) at the other ages investigated (P7, P21, and P28). Synaptic density significantly increased between P7 and P14 (60%) and between P21 and P28 (54%), but did not significantly change between P3 and P7 and between P14 and P21. Mean synaptic diameter also increased over the first postnatal month. Significant increases in synaptic size occurred between P3 and P7 (28%) and between P14 and P21 (15%). The present data indicate that, in the NTS, synaptogenesis occurs over a protracted period of time and involves distinct successive episodes of synapse production.