Ontogeny of the peptidergic system in the rat spinal cord: immunohistochemical analysis

The ontogeny of neuropeptides, such as somatostatin (SRIF), substance P (SP), leucine-enkephalin (LE), and neurotensin (NT) in the spinal cord (including the spinal ganglion) of the rat, was examined by means of the indirect immunofluorescence method. SRIF and SP appear in the early fetal period before the establishment of the spinal synaptic transmission system, and their appearance precedes that of LE and NT, thus suggesting that SRIF and SP might have some important role in the development of the spinal cord. Furthermore, a number of SRIF-positive structures are found during the fetal period in the spinal cord; however, SRIF-positive fibers in the ventral horn, lamina V, VI, and X tend to decrease remarkably in number after birth, while those found in the dorsal horn maintain their immunoreactivity even in the adult rats. These facts suggest that SRIF in the latter area might function as a neurotransmitter, whereas in the former areas, SRIF might have another role in the development of the spinal cord. SP-positive structures also made their appearance during the fetal period. SP-positive fibers continue to increase in number after birth, and they can be seen throughout the entire spinal cord even in the adult rats. It becomes difficult to identify SP-positive neurons as the rats grow. Numerous SP-positive cells are demonstrated, however, by colchicine pretreatment, thus suggesting that this system is functioning actively in the adult rats. LE- and NT-positive structures appear at perinatal stages and they continue to increase in number after birth. These facts suggest that these peptides (SP, LE and NT) might act as neurotransmitters.