Biochemical Aspects of the Maturation of Serotoninergic Neurons in the Rat Brain

Abstract

At birth, the new-born rat is very immature and the organization of its central nervous system is far from complete. Thus, the cerebellum is hardly visible behind the colliculi, the electrical activity of the cerebral cortex is discrete, myelination has not begun. The poor development of the central nervous system of the neonatal rat can be quantified by comparing its biochemical composition with that of the adult brain. Thus, the water content represents about 90% of the wet weight of the brain at birth whereas it is less than 80% in the adult. By contrast, both lipids and proteins increase significantly in % of wet weight from birth to adulthood (Winick and Noble, 1965; Davis and Himwich, 1973; Agrawal and Davison, 1973). The brain levels of other components, notably amino acids (Lajtha and Toth, 1973; Agrawal and Davison, 1973), monoamines are also strikingly different in the new-born and the adult rat. Since several of these small molecules are considered as putative neurotransmitters in the central nervous system of the adult rat, their presence suggests that they may also have a physiologic role in the immature brain. Indeed, monoamines, namely norepinephrine, dopamine and serotonin (5-HT) are already present inside particular neurons in neonatal rat brain. Histofluorescence techniques show that neuroblasts in the medial zone of the brain-stem, i.e. the raphe system, exhibit typical yellow 5-HT fluorescence as early as the 13th day following conception (Olson and Seiger, 1972).