Ontogeny of somatostatin mRNA-containing perikarya in the rat central nervous system.

The distribution of neuronal perikarya containing somatostatin mRNA in the developing rat brain was investigated with in situ hybridization histochemistry. This study describes the expression of somatostatin mRNA during selected perinatal stages and demonstrates regional changes in somatostatin mRNA expression at the single cell level. The mRNA expression closely parallels previously reported developmental localization of the peptide (Inagaki et al., 1982; Shiosaka et al., 1982). As early as embryonic day 13 (E13), somatostatin mRNA was observed in discrete spinal cord and brainstem regions. At birth, densely hybridized somata could be seen primarily in ventral and caudal brain areas with small scattered neurons in the hippocampus and dorsal neocortex. After birth, somatostatin mRNA increased in forebrain regions, such as the hippocampus, dorsal neocortex, and caudate. By postnatal day 14 (P14), the distribution in the telencephalic and diencephalic regions approached that of the adult brain. Several brain regions manifested large changes in the density of somatostatin mRNA hybridization during development. For example, the cerebellar vermis and brainstem contained somatostatin mRNA perikarya during early postnatal development but decreased in these regions in the adult. During perinatal development, increases in somatostatin mRNA content were the results of increases in both the number of neurons containing somatostatin mRNA as well as in the amount of this mRNA expressed in each cell. As the brain differentiates, the apparent numbers of somatostatin mRNA containing neurons in certain brain regions are reduced. These data provide evidence for transient somatostatinergic neurons during early development in discrete areas of the occipital cortex, pyriform cortex, cerebellum, and brainstem and suggest that this peptide may play a role in the development of these regions.