Postnatal changes in development of serotonin-, neuropeptide Y-, Leu-enkephalin- and substance P- terminals in the...

Postnatal developmental changes were investigated in afferent terminals immunoreactive to serotonin (5-HT), neuropeptide Y (NPY), Leu-Enkephalin (ENK) and substance P (SP) within the locus coeruleus from postnatal rat from day 1 (1 D) to 9 week (9 W) adult using immunohistochemical techniques. Quantitative study using a light microscopic image analyzing system revealed that the number of immunoreactive terminals increased after birth to reach a peak at 5 W, then decreased by 26% of this value and stabilized at 7 W; terminals immunoreactive to 5-HT or NPY increased gradually after birth, while those immunoreactive to ENK or SP increased suddenly at 3 W. Electron microscopic analysis revealed similar changes in the total terminal number during development. Synaptic terminals, on the other hand, increased sharply from 1 D to 3 W, then gradually until 5 W, and remained stable thereafter. These results suggest that surplus afferent terminals are eliminated prior to the establishment of afferent innervation and that 5 W postnatal is the critical time for maturation of the afferent system. Electron microscopy also demonstrated morphological characteristics of terminals immunoreactive to each peptide or 5-HT and developmental changes in their characteristics.