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.

The ontogeny of the somatostatin (SRIF) neuron system in the forebrain and diencephalon the rat was investigated by means of the indirect immunofluorescence method. SRIF-positive cells first appear in the primordium of the hypothalamus surrounding the fasciculus mammillothalamicus of the fetus at gestational day 14 (10-12-mm embryo). At gestational days 16-17 (14-17-mm embryo, SRIF-positive cells appear in the developing piriform cortex and entopeduncular nucleus. The fetus at gestational days 18-19 (17-26-mm embryos) showed a remarkable increase in the number of SRIF-positive cells and numerous groups of such cells are detectable in various forebrain and diencephalic areas such as the hypothalamic periventricular zone, zona incerta, area lateral to the commissura posterior (ACP), area between the optic tract and capsula interna, pars retrolenticularis (AOR), n. caudatus putamen, hippocampus, somatic sensory cortex, and n. accumbens, etc. At gestational day 20, SRIF-positive cells newly appear in the septum, olfactory bulb, diagonal band of Broca, claustrum, lateral preoptic area, and lateral habenular nucleus. The majority of SRIF-positive structures found in the forebrain and diencephalon increase in number during the perinatal stage (between gestational day 21 and postnatal day 2) and more or less maintain their immunoreactivity even in adult rats. However, SRIF-positive cells located in the AOR, ACP, and lateral septal area, etc., begin to decrease in number during the perinatal stage and no or only a few SRIF-containing cells are identified in these areas of infant and adult rats.

The distribution of somatostatin (SRIF)-immunoreactive structures in the central nervous system of the bull frog (both with and without treatment of colchicine) was studied, using the indirect immunofluorescence technique of Coons and co-workers (Coons, '58). SRIF-containing cells were observed in more than ten areas including the spinal cord. These SRIF-positive cells showed segmental distribution, in that SRIF-positive neurons were identified in various areas at various brain levels. An extensive network of SRIF-positive fibers was found in most parts of the central nervous system. The distribution of a catecholamine (CA)-containing neuron system in the frog brain is also presented in this study. The possible interactions between SRIF and CA neurons systems are briefly discussed.

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The ontogeny of the neurotensin (NT) neuron system in the forebrain and diencephalon of the rat was investigated by means of indirect immunofluorescence. NT-positive structures first appear in the primordium of the olfactory bulb, anterior amygdaloid area, piriform cortex, amygdaloid complex, nucleus tractus diagonalis, lateral preoptic area, and lateral hypothalamus, etc., of the fetus of gestational day 16. From this day, NT-positive structures appear gradually in various areas of the forebrain and diencephalon and reach their maximum content on postnatal day 7. In contrast, NT-positive structures decrease slightly in number as the rat grows. However, even in adult rats, numerous NT-positive structures are still identifiable in various areas of the forebrain and diencephalon.