Postnatal development of brainstem gustatory pathways: Development of protein P-38 immunoreactivity and myelination of chorda tympani axons

Previous studies conducted in this lab have shown that morphological changes occur in postsynaptic elements of brainstem gustatory relays during rat's postnatal life. Dendritic length and mitochondrial respiratory enzyme activity increase in the rostral nucleus of the solitary tract (NST) and in the caudal parabrachial nucleus (PBN). These increases are well-related to certain aspects of neurophysiological development in the NST and PBN. Other factors are known to contribute to the development of neurophysiological responses, such as synapse formation and myelination of axons. The present study examined developmental increases in protein P-38 immunoreactivity. Protein P-38 ("Synaptophysin") is a specific integral membrane protein found in small, clear synaptic vesicles. In addition, the time-course for myelination of central chorda tympani (CT) axons was investigated. Results demonstrate that P-38 immunoreactivity increases in the NST and PBN in a temporally-sequential manner during postnatal development. Reliable increases in P-38 immunoreactivity are observed between postnatal days 1 and 10 (P1-P10) in the rostral NST, whereas density of reaction products in the PBN increases from P11-P31. The time-course for myelination of chorda tympani (CT) axons within the medulla does not differ from the time-course for myelination of CT axons in the periphery. These studies confirm that both pre- and postsynaptic constituents of brainstem gustatory relays develop in a temporally-sequential manner. Relationships between these neurological changes and the ontogeny of adult-like taste-guided responses are discussed.