Changes in the Structural Organization of the Nucleus Raphe Pallidus in Conditions of Decreased Endogenous Serotonin During Prenatal Development in Rats

The role of serotonin in the development of the nucleus raphe pallidus (NRP) and the dynamics of changes in serotonin-synthesizing neurons in this nucleus were studied at different points in the postnatal period in Wistar rats in normal conditions and in animals whose intrauterine development proceeded on the background of serotonin deficiency. The NRP was found to include two populations of serotoninergic neurons, differing in terms of their morphological characteristics. At the initial stages of postnatal development (day 5), only large neurons synthesized serotonin, while the synthetic activity of small neurons appeared later, by day 10. In normal animals, increasing age was associated with increases in the size and number of large neurons, which may be evidence for a continuing process of differentiation and/or an increase in the functional loading. The size and number of small neurons showed virtually no change with age. Serotonin deficit during prenatal development led to abnormalities in the structural organization of the NRP. As compared with controls, there were reductions in the sizes and numbers of serotonin-synthesizing neurons of both populations, with no change in size with age. Some neurons developed degenerative changes, reducing the number of cells. The damage seen would alter the serotoninergic innervation of the medullary nuclei responsible for cardiorespiratory regulation, and would thus impair the activity of the cardiovascular and respiratory systems.