Ontogeny of somatostatin-containing neuron system of the rat cerebellum including its fiber connections: an experimental and immunohistochemical analysis

The somatostatin 2A receptor is enriched in migrating neurons during rat and human brain development and stimulates migration and axonal outgrowth

The neuropeptide somatostatin has been suggested to play an important role during neuronal development in addition to its established modulatory impact on neuroendocrine, motor and cognitive functions in adults. Although six somatostatin G protein-coupled receptors have been discovered, little is known about their distribution and function in the developing mammalian brain. In this study, we have first characterized the developmental expression of the somatostatin receptor sst2A, the subtype found most prominently in the adult rat and human nervous system. In the rat, the sst2A receptor expression appears as early as E12 and is restricted to post-mitotic neuronal populations leaving the ventricular zone. From E12 on, migrating neuronal populations immunopositive for the receptor were observed in numerous developing regions including the cerebral cortex, hippocampus and ganglionic eminences. Intense but transient immunoreactive signals were detected in the deep part of the external granular layer of the cerebellum, the rostral migratory stream and in tyrosine hydroxylase- and serotonin- positive neurons and axons. Activation of the sst2A receptor in vitro in rat cerebellar microexplants and primary hippocampal neurons revealed stimulatory effects on neuronal migration and axonal growth, respectively. In the human cortex, receptor immunoreactivity was located in the preplate at early development stages (8 gestational weeks) and was enriched to the outer part of the germinal zone at later stages. In the cerebellum, the deep part of the external granular layer was strongly immunoreactive at 19 gestational weeks, similar to the finding in rodents. In addition, migrating granule cells in the internal granular layer were also receptor-positive. Together, theses results strongly suggest that the somatostatin sst2A receptor participates in the development and maturation of specific neuronal populations during rat and human brain ontogenesis.

Transient expression of somatostatin receptors in the brain during development

The study of somatostatin receptors by means of autoradiography in tissue sections revealed high densities of binding sites in the immature central nervous system. In rat cerebral cortex, the receptors are present in the intermediate zone and in association with cells migrating through the cortical plate. Somatostatin receptors in the intermediate zone of fetuses and in the cortical plate of postnatal rats exhibit high and low affinities respectively for the somatostatin analogue MK 678. In the rat cerebellum, the external granule cell layer, a germinal matrix containing interneuron precursors, contains a high density of receptors. These receptors exhibit high affinity for MK 678 throughout the period of cell multiplication. In granule cell cultures from eight-day-old rats, MK 678, octreotide and somatostatin are able to inhibit cAMP formation induced by forskolin or pituitary adenylyl cyclase-activating polypeptide. Somatostatin reduces the intracellular Ca2+ concentration in cultured granule cells; this response desensitizes rapidly. These results suggest that the somatostatin receptors in the external granule cell layer are type 2 receptors (sstr2). A low density of receptors with low affinity for MK 678 was also detected in the external granule cell layer and in the granule cell layer of neonatal rats. In adult rats the cerebellum is devoid of somatostatin receptors. These observations indicate that somatostatin probably exerts morphogenetic activities through different receptor types in several structures of the central nervous system.

Age-related change of somatostatin-immunoreactive neurones in the main olfactory bulb of the rat

Somatostatin is found in the olfactory system, including the main olfactory bulb (MOB), and is thought to be one of the neuroactive substances for olfaction. However, somatostatin immunoreactivity in the olfactory system has not been determined during ageing. Hence, we examined the age-related changes of somatostatin-immunoreactive (IR) neurones in the rat MOB over a period of 2 years, at the following various ageing stages: post-natal month 1 (PM 1), PM 3, PM 6, PM 12 and PM 24. In PM 1 group, a few somatostatin-IR neurones were detected in the granule cell layer (GCL), and had slender or oval somata and short processes. At PM 3, somatostatin-IR neurones were observed in the glomerular, external plexiform and GCL. The size of somatostatin-IR somata was larger than that at PM 1. In PM 6 group, the number and size of somatostatin-IR neurones increased, and their processes became longer while running in various directions. At PM 12, somatostatin-IR neurones increased in number, and their processes became markedly longer than those at PM 6. At this stage, somatostatin-IR neurones had multipolar somata, and were the largest in size. In PM 24 group, somatostatin-IR neurones were most numerous. However, the processes of somatostatin-IR neurones were shorter than those at PM 12. This study suggests that the increased number of somatostatin-IR neurones in the MOB of aged rats may play a role to compensate for any decrease of olfactory function.

Calcitonin gene-related peptide receptor expression in the neurons and glia of developing rat cerebellum: an autoradiographic and immunohistochemical analysis

Quantitative autoradiography (using [125I]human alpha-calcitonin gene-related peptide as a ligand) and immunofluorescence (using monoclonal antibodies directed against a purified receptor) followed by confocal analysis were applied to analyse the distribution and cellular localization of the calcitonin gene-related peptide receptor in the rat cerebellum during development. From late embryonic days to the end of the second postnatal week, during the time window of calcitonin gene-related peptide expression in climbing fibers, high levels of calcitonin gene-related peptide binding sites were found in the white matter, where immunolabeling was present in oligodendrocytes. Lower levels were found in the cerebellar cortex, where receptor immunolabeling was found in Bergmann glia in a presumptive cell surface location and, during the second postnatal week, also in the cytoplasm of Purkinje cells. From the end of the second postnatal week to adulthood, when calcitonin gene-related peptide is no longer present in climbing fibers, the number of calcitonin gene-related peptide binding sites increased in the molecular layer, where not only Bergmann glia but also Purkinje cell distal dendritic branchlets were immunolabeled in a presumptive cell surface location. Concomitantly, the number of calcitonin gene-related peptide binding sites sharply decreased in the white matter. The developmental expression of the calcitonin gene-related peptide receptor and the previously described proliferating/differentiating effects of the peptide on glial cells suggest that calcitonin gene-related peptide and its receptor may promote a coordinated development of cerebellar glial cells, an effect driven mainly by the calcitonin gene-related peptide released by climbing fibers. As a result of glia-neuron interactions, an indirect effect on the differentiation of the cerebellar neuronal circuitry is also likely to occur.

Neurogenesis, cell death and regeneration in the adult gymnotiform brain

Gymnotiform fish, like all teleosts examined thus far, are distinguished by their enormous potential for the production of new neurons in the adult brain. In Apteronotus leptorhynchus, on average 10(5) cells, corresponding to approximately 0.2 % of the total population of cells in the adult brain, are in S-phase within any period of 2 h. At least a portion of these newly generated cells survive for the rest of the fish's life. This long-term survival, together with the persistent generation of new cells, leads to a continuous growth of the brain during adulthood. Zones of high proliferative activity are typically located at or near the surface of the ventricular, paraventricular and cisternal systems. In the central posterior/ prepacemaker nucleus, for example, new cells are generated, at very high rates, in areas near the wall of the third ventricle. At least some of these cells differentiate into neurons, express immunoreactivity against the neuropeptide somatostatin and migrate into more lateral areas of this complex. Approximately 75 % of all new brain cells are generated in the cerebellum. In the corpus cerebelli and the valvula cerebelli, they are produced in the molecular layers, whereas in the eminentia granularis the newborn cells stem from proliferation zones in the pars medialis. Within the first few days of their life, these cells migrate towards specific target areas, namely the associated granule cell layers. At least some of them develop into granule neurons. The high proliferative activity is counterbalanced by apoptosis, a mechanism that resembles the processes known from embryonic development of the vertebrate brain. Apoptosis also appears to be used as an efficient mechanism for the removal of cells damaged through injury in the brain of adult Apteronotus leptorhynchus. Since apoptosis is not accompanied by the side effects known from necrosis, this ‘clean’ type of cell death may, together with the enormous proliferative activity in the brain, explain, at least partially, the tremendous capability of teleost fish to replace damaged neurons with newly generated ones. One factor that appears to play a major role in the generation of new cells and in their further development is the neuropeptide somatostatin. In the caudal cerebellum of the gymnotiform brain, somatostatin-binding sites are expressed, at extremely high densities, at sites corresponding to the areas of origin, migration and differentiation of the newborn cells. This pattern of expression resembles the expression pattern in the rat cerebellum, where somatostatin immunoreactivity and somatostatin-binding sites are transiently expressed at the time when the granule cells of the cerebellum are generated. Moreover, after mechanical lesions of the corpus cerebelli, the expression of somatostatin-like immunoreactivity is tremendously increased in several cell types (presumably astrocytes, microglia and granule cell neurons) near the path of the lesion; the time course of this expression coincides with the temporal pattern underlying the recruitment of new cells incorporated at the site of the lesion.

Transient expression of somatostatin sst2 receptors in rat cerebellar nuclei during development

Adult rat cerebellar nuclei contain a single population of [125I][Leu8,D-Trp22,Tyr25]somatostatin-28 binding sites characterized as sst1 receptors. In the present study, we have investigated the evolution of somatostatin receptors in rat cerebellar nuclei during development by means of quantitative autoradiography on tissue sections. The binding of [125I][Leu8,D-Trp22,Tyr25]somatostatin-28, observed in the primordium of the medial cerebellar nuclei at embryonic day 17, reached a maximum at postnatal day 7 or 10 in the different nuclei. Thereafter, the density of binding sites gradually decreased to the adult level. Competition studies were performed using the somatostatin analogues CH-288 and MK-678 as specific sst1 and sst2 ligands, respectively. Partial inhibition of the radioligand binding by CH-288 and MK-678 revealed the presence of a predominant population of sst1 from embryonic day 19-28 day postnatal and a minor population of sst2 receptors. The use of [125I]MK-678 as a radioligand confirmed the presence of a transient population of sst2 receptors, suggesting that somatostatin could act on rat cerebellar nuclei via sst1 and/or sst2 receptors during development.

Somatostatin does not affect multiplication of granule cells in the rat cerebellum

Somatostatin receptor are transiently expressed by immature granule cells of rat cerebellum. The effects of somatostatin and octreotide on cell proliferation were studied in cultured cerebellar explants from 10-day-old rats. Cell multiplication was measured using [3H]thymidine incorporation and flow cytometric analysis of cell cycle parameters. [3H]Thymidine incorporation occurred exclusively in neuroblasts of the external granule cell layer in the presence of insulin (1 microM). The labeling index, the length of the S phase, and the potential doubling time were similar in vivo and in explants. Octreotide (10(-12) to 10(-6)M) had no effect on [3H]thymidine incorporation in cerebellar explants. In addition, somatostatin and octreotide did not modify the proportion of cells in the S, G0-G1, and G2-M phases. The present results demonstrate that somatostatin does not affect cell multiplication in neurons of the external granule cell layer.

Presence of somatostatin sst2 receptors in the developing rat auditory system

A transient expression of somatostatin mRNA as well as of the peptide itself has been described in the developing mammalian auditory brainstem. However, little is known about the presence, and the spatial and temporal pattern of somatostatin (SRIF) receptor subtypes in this system. Therefore, we investigated the distribution of SRIF receptor binding sites labeled with the radioligands [125I]LTT-SRIF-28, [125I]Tyr3-octreotide, and [125I]CGP 23996 (in buffers containing either Mg2+ or Na+ ions) within the developing auditory brainstem of the rat. In addition, we performed in situ hybridization with a 35P-labeled oligoprobe, specific for somatostatin sst2 receptor mRNA. We observed a transient expression of SRIF receptors, labeled with [125I]LTT-SRIF-28, [125I]Try3-octreotide, and [125I]CGP 23996 (only in the presence of Mg2+ ions), in all principal auditory nuclei during neonatal development. In the adult rats, however, only the inferior colliculus displayed significant SRIF receptor binding. A very similar spatiotemporal labeling pattern was found for sst2 receptor mRNA. Our in situ hybridization data, together with those on ligand binding, suggest a predominantly transient expression of sst2 receptors in the auditory system. Since sst2 sites (and possibly sst3 and sst5) as well as SRIF itself appear to be co-expressed during a period when synapse maturation occurs, we suggest that sst2 receptors are involved in this process of the developing auditory system.

Expression of galanin and a galanin receptor in several sensory systems and bone anlage of rat embryos

Using in situ hybridization and immunohistochemistry the expression of, respectively, prepro-galanin (pre-pro-GAL) mRNA and GAL receptor-1 mRNA, as well as GAL-like and GAL message-associated peptide-like immunoreactivities, were studied in rats from embryonic day 14 (E14) to postnatal day 1. GAL expression was observed already at E14 in trigeminal and dorsal root ganglion neurons and at E15 in the sensory epithelia in developing ear, eye, and nose, as well as at E19 during bone formation. Also, GAL receptor-1 mRNA was expressed in the sensory ganglia of embryos but appeared later than the ligand. These findings suggest that GAL and/or GAL message-associated peptide may have a developmental role in several sensory systems and during bone formation.

Ontogeny of somatostatin immunoreactive neurons in the medial cerebral cortex and other cortical areas of the lizard Podarcis hispanica

The ontogeny of somatostatin immunoreactive interneurons in the cerebral cortex of the lizard Podarcis hispanica has been studied in histological series of embryos, perinatal specimens, and adults. Somatostatin immunoreactive interneurons appear in the early stages of lizard cerebral cortex ontogeny, their number increases during embryonary development, reaches a peak in early postnatal life, and decreases in adult lizards. The first somatostatin immunoreactive somata in the lizard forebrain appeared on E36, and they were located in non cortical areas. Then, on E39 and later, somatostatin immunoreactive neurons were seen in the lizard cortex in a rostral-to-caudal spatial gradient, which parallels that of the normal histogenesis of the lizard cerebral cortex. On E39, labelled somata were seen in the medial and dorsal cortex inner plexiform layers; immunoreactive puncta and dendritic processes were detectable in the inner plexiform layer of the medial cortex. On E40, labelled neurons were observed in the inner plexiform layer of the lateral cortex; labelled processes were found in the inner plexiform layers (dorsomedial, dorsal, and lateral cortices) and the outer plexiform layers (medial and dorsomedial cortices). At hatching (P0), some somatostatin immunoreactive neurons populated the external plexiform layer of the dorsomedial cortex. On P28, groups of labelled neurons appeared in the cell layer of dorsal and lateral cortices, reaching the adult-mature pattern of somatostatin immunoreactivity in the lizard cerebral cortex, i.e., labelled somata and dendritic processes populating the inner plexiform layers in addition to an axonic labelled plexus in the outermost part of the outer plexiform layers. Immunoreactive somata and processes occupied all the cortical areas, but they were especially abundant in the dorsomedial cortex. Proliferating Cell Nuclear Antigen (PCNA) immunostaining in the same histological series revealed that the number of PCNA immunoreactive nuclei in the subjacent proliferative neuroepithelium followed an inverse-complementary evolution to somatostatin, suggesting some temporal relationship between somatostatin immunoreactive cells and neurogenesis in the lizard cerebral cortex.

Anatomical distribution of somatostatin receptors in the brainstem of the human fetus

The distribution of somatostatin binding sites was studied in the pons and medulla oblongata of three human fetuses (gestional ages 26, 28 and 30 weeks). The study was carried out by in vitro quantitative autoradiography using either [125I-Tyr0,D-Trp8]somatostatin-14 or [125I-Tyr11]somatostatin-14 as radioligands. Somatostatin binding sites were observed in a number of nuclei subserving sensory, motor or integrative functions within the pons and medulla. In addition, discrete tracts also contained significant amounts of binding sites. Among structures involved in sensory processes, a high density of binding sites (40-60 fmol/ mg wet tissue) was measured in the dorsal cochlear nucleus and in the nucleus tractus spinalis trigemini caudalis. Moderate to high levels of binding sites (30-40 fmol/mg wet tissue) were detected in the other sensory cranial nerve nuclei. A moderate density of sites (15-30 fmol/mg wet tissue) was measured in most motor nuclei, the highest concentrations being observed in the dorsal motor nucleus of the vagus nerve, the facial nucleus, the hypoglossal nucleus and the nucleus ambiguus. The griseum pontis and the nucleus corporis pontobulbaris contained very high (> 60 fmol/mg wet tissue) and high concentrations of somatostatin binding sites, respectively, while the other relay nuclei contained low to moderate levels of binding. In monoaminergic nuclei, very high and moderate to high concentrations of somatostatin binding sites were measured in the nucleus locus coeruleus and in its dorsal subnucleus, respectively. Moderate densities of sites were detected in the ventral subnucleus of the nucleus locus coeruleus and in the different parts of the raphe. In the white matter, low levels of binding were measured in the inferior cerebellar peduncle, the lateral and median lemnisci and the tractus solitarius. Conversely, moderate to high concentrations of somatostatin binding sites were measured in the median and superior cerebellar peduncles. The pyramis contained a very high density of recognition sites. A marked heterogeneity in the density of binding sites was observed within a few structures particularly in the medial accessory olivary of nucleus and the medial longitudinal fasciculus. Selective ligands were used to determine the pharmacological profile of the [Tyr11]somatostatin-14 binding sites in various brainstem regions. In the dorsal cochlear nucleus and the pyramis, all somatostatin binding sites belonged to the SSA subtype. Conversely, in the lateral paragigantocellular nucleus, all somatostatin binding sites belonged to the SSB subtype. The other regions studied contained various proportions of SSA and SSB subtypes. In conclusion, the present study shows that high concentrations of somatostatin receptors are present in many regions of the human fetus brainstem. These data support the concept that somatostatin could be involved in the maturation of brain structures.

The postembryonic development of somatostatin immunoreactivity in the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: a double-labelling study

The neuropeptide somatostatin (SS) is widely distributed in both the central and peripheral nervous system of vertebrates. Its widespread distribution is paralleled by a large variety of diverse functions. While embryonic and perinatal development of SS-like immunoreactivity have been well examined, little is known about the postnatal development of this neuropeptide. Since, in teleosts, neurogenesis persists in many brain regions during adulthood, these vertebrates are well suited to investigate this phenomenon. In the present study, we have, therefore, examined the development of somatostatinergic cells born during adulthood in the central posterior/prepacemaker nucleus (CP/PPn) of Apteronotus leptorhynchus, a weakly electric gymnotiform fish. This was achieved by labelling proliferating cells with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) and by simultaneous immunocytochemical detection of SS-like immunoreactivity. SS-like immunoreactivity is adopted in a period between 2 days and 3.5 days after birth. While the number of BrdU-labelled cells in the CP/PPn decreases 10 days after birth, the percentage of double-labelled cells among the BrdU-labelled cells remains with 1.0-7.6% in the period between 3.5 days and 100 days after birth rather constant. This percentage matches well the fraction of SS-positive cells in the total population of cells present in the CP/PPn.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Somatostatin and leu-enkephalin in the rat auditory brainstem during fetal and postnatal development

A transient expression of the neuropeptide somatostatin has been described in several brain areas during early ontogeny and several opioid peptides, such as leu-enkephalin, have also been found in the brain at this stage in development. It is therefore believed that somatostatin and leu-enkephalin may play a role in neural maturation. The aim of the present study was to describe the spatiotemporal pattern of somatostatin and leu-enkephalin immunoreactivity in the auditory brainstem nuclei of the developing rat and to correlate it with other developmental events. In order to achieve this goal, we applied peroxidase-antiperoxidase immunocytochemistry to rat brains between embryonic day (E) 17 and adulthood. Somatostatin immunoreactivity (SIR) was found in all nuclei of the auditory brainstem, yet it was temporally restricted in most nuclei. SIR appeared prenatally and reached maximum levels around postnatal day (P) 7, when great numbers of immunoreactive neurons were present in the ventral cochlear nucleus (VCN) and in the lateral lemniscus. At that time relatively low numbers of cells were labeled in the dorsal cochlear nucleus, the lateral superior olive (LSO), and the inferior colliculus (IC). During the same period, when somata in the VCN were somatostatin-immunoreactive (SIR), a dense network of labeled fibers was also present in the LSO, the medial superior olive (MSO), and the medial nucleus of the trapezoid body (MNTB). As these nuclei receive direct input from VCN neurons, and as the distribution and morphology of the somatostatinergic fibers in the superior olivary complex (SOC) was like that of axons from VCN neurons, these findings suggest a transient somatostatinergic connection within the auditory system. Aside from the LSO, MSO, and MNTB, labeled fibers were found to a smaller extent in all other auditory brainstem nuclei. After P7, the SIR decreased and only a few immunoreactive elements were found in the adult auditory brainstem nuclei, indicating that somatostatin is transiently expressed in the rat auditory brainstem. Leu-enkephalin immunoreactivity showed a lower number and weaker intensity of labeled structures as compared to SIR, with E18 being the earliest day at which labeled fibers appeared in the SOC. At birth, immunoreactive fibers were also present in the cochlear nuclear complex and in the IC. Leu-enkephalin immunoreactive somata were found only after P12 in the CN and after P16 in the IC. Leu-enkephalin immunoreactivity was not transient, but increased progressively with age until about P21, when the adult levels were reached.(ABSTRACT TRUNCATED AT 400 WORDS)

Somatostatin in the prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: evidence for a nonsynaptic function

Neuropeptides are widely distributed throughout the nervous system and exert a large number of heterogeneous functions. While they are synthesized in the soma, release is thought to take place in axonal terminals of neurons. A good model system to investigate the role of peptides in the nervous system is provided by the central posterior/prepacemaker nucleus (CP/PPn) of pacemaker nucleus (Pn), a medullary cell group controlling the electric organ discharge (EOD). Previous immunocytochemical and in situ-hybridization studies employing topographical criteria indicated that PPn neurons may express the neuropeptide somatostatin (SS). In the present study, we unambiguously identified PPn neurons by in vitro tract tracing. By combining this technique with SS immunocytochemistry, we found that a large portion of retrogradely labelled PPn neurons exhibited SS-like immunoreactivity (72-89%, n = 708 cells in 10 fish examined). Surprisingly, however, neither the proximal PPn axons nor anterogradely labelled terminals innervating the Pn displayed significant amounts of SS-like immunolabelling (n = 10 fish examined in each experiment). These results and the lack of SS binding sites in the Pn [82] suggest that SS expressed by PPn cells is not synaptically released at the target site of their axons, the Pn, but acts via a nonsynaptic mechanism in the CP/PPn proper.

Transient expression of somatostatin immunoreactivity in the olfactory-forebrain region in the chick embryo

The tissue distribution of somatostatin (SST) immunoreactivity was studied in the nasal and forebrain region in the chick embryo. On embryonic day (ED) 3, SST-immunoreactive (ir) cells were first detected in the cells migrating from the olfactory placode. Then, at ED3.5, SST-ir cells and -ir fibers appeared in the olfactory epithelium and olfactory nerve bundles. At ED6-8, one component of the SST-ir fibers was found to separate from the olfactory nerve and it entered the parenchyma of the medial forebrain surface. These SST-ir fibers extended dorsocaudally toward the preseptal area. During this same period, a few SST-ir cells were observed in the medial forebrain adjacent to the SST-ir fibers. SST immunoreactivity in the nasal and forebrain areas was most striking at ED5-8 but a reduction of SST immunoreactivity in the nasal and forebrain areas occurred at ED11 and it virtually disappeared by the day of hatching. These results indicate that the expression of SST in the nasal and forebrain regions is transient in the chick embryo. Since the SST-ir cells did not co-express luteinizing hormone-releasing hormone (LHRH), it, thus, appears that these SST-r cells belong to a different cell population from LHRH neurons that are also found in the olfactory-forebrain axis during embryonic development [23]. However, a close relationship exists between SST-ir cells and -ir neuronal fibers and LHRH neurons. This may play a role in development of LHRH neurons.

Quantitative autoradiographic study of somatostatin receptors in the adult human cerebellum

The evolution of the distribution and density of somatostatin receptors was studied in the human cerebellum during ageing. The brain tissues were collected 3-30 h after death from 20 individuals aged from 28 to 86 years. In vitro autoradiographic experiments were performed on blocks of vermis and of right and left cerebellar hemispheres, using [125I-Tyr0,DTrp8]S14 as a radioligand. In the vermis, the mean concentrations of somatostatin receptors in the molecular layer, the granular layer and the medulla were 140 +/- 9, 150 +/- 22 and 61 +/- 13 fmol/mg proteins, respectively. For each individual, the density of sites in the two lateral lobes was similar. The mean concentrations of somatostatin receptors in the molecular layer, the granular layer and the medulla were 152 +/- 17, 190 +/- 20 and 56 +/- 11 fmol/mg proteins, respectively. The mean level of somatostatin receptors and the type of distribution of the receptors were not correlated to the age of the patients. Different distribution patterns of somatostatin receptors were noted among the patients studied. In the majority of patients (11/20), the density of somatostatin receptors was higher in the granular layer than in the molecular layer. Conversely, in four patients, the density of somatostatin receptors was higher in the molecular layer. The other individuals exhibited similar concentrations of somatostatin receptors in the granular and molecular layers. The present study indicates that the adult human cerebellum contains a high concentration of somatostatin receptors (> 100 fmol/mg proteins) and that the receptor level does not decline during ageing.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin enhances nerve growth factor-induced neurite outgrowth in PC12 cells

The neuropeptide somatostatin has been found to be abundant in numerous developing regions within the central nervous system. In order to understand the role of somatostatin in development, effects of exposure to the neuropeptide were studied in PC12 cells, a well characterized model of neuronal differentiation. Somatostatin increased neurite outgrowth after 2 days in culture and enhanced neurite outgrowth after nerve growth factor (NGF) exposure. This effect was inhibited by somatostatin antibody and pertussis toxin. Somatostatin had no effect on NGF binding or internalization but did cause a decrease in cAMP levels during the time of maximal stimulation of neurite outgrowth. In a protein kinase A-deficient cell line (A126-1B2), somatostatin had no effect on neurite outgrowth. These results indicate that somatostatin may function as a differentiation factor in developing systems through inhibition of cAMP synthesis.

Somatostatin-14 and somatostatin-28 levels are light-driven and vary during development in the chicken retina

The relative levels of somatostatin-14 and somatostatin-28 were determined during both perinatal development and variations in lighting conditions in the chicken retina. During perinatal development of the retina, somatostatin-14 predominated in recently hatched chickens, whereas somatostatin-28 predominated in the retinas of older chickens. In mature chickens, the levels of both somatostatin-14 and somatostatin-28 increased during the light and decreased during the dark. Our results suggest that these two forms of somatostatin are released proportionally and in parallel.

Identification and localization of somatostatin-like immunoreactivity in the cerebellum of gymnotiform fish, Apteronotus leptorhynchus

Previous studies have shown major mismatches between the localization of the neuropeptide somatostatin of its mRNA and the distribution of somatostatin receptors in the cerebellum of gymnotiform fish. By employing highly sensitive immunohistochemical techniques, we re-examined these mismatches in the gymnotiform fish Apteronotus leptorhynchus. We found somatostatin-like immunoreactivity in the corpus cerebelli, valvula cerebelli, eminentia granularis anterior, eminentia granularis posterior, and transitional zone. While labelled somata were predominantly restricted to a region near the boundary between the granule cell layers and molecular layers, immunopositive processes were distributed throughout large areas within the respective molecular layer. These immunopositive cells appear to represent a novel cell type in the cerebellum of gymnotiform fish.

Expression patterns of rat somatostatin receptor genes in pre- and postnatal brain and pituitary

The relative abundances of mRNAs encoding four different somatostatin receptors were examined using PCR techniques during postnatal development of the rat brain and hypophysis. In most tissues, somatostatin receptor 1 and 4 mRNAs are more abundant than those encoding somatostatin receptor 2 and 3. Transcript levels of somatostatin receptor subtype 4 are relatively high in the cortex, hippocampus, and striatum, those of subtype 1 in the cortex and brainstem, and those of subtype 3 in the cerebellum. In situ hybridization revealed the presence of significant amounts of somatostatin receptor 1 mRNA, as early as prenatal day 14, in the trigeminal ganglion and in the neuroepithelial layers surrounding the lateral, third, and fourth ventricles. In the developing cortex a morphological change in the sites of somatostatin receptor 1 gene expression occurs; mRNA is present superficially in the cortex at prenatal stages, appears in all layers shortly after birth, and in adult rats is restricted to the deep cortical layers. In the cerebellum, somatostatin receptor 1 mRNA levels are highest around birth, declining thereafter. In contrast, cerebellar somatostatin receptor 3 transcripts are absent at birth, become detectable around postnatal day 7, and reach a maximal level during maturation.

Developmental expression of somatostatin receptors in the rat retina

The ontogeny of somatostatin receptor binding was studied in developing rat retina using the iodinated derivative of the somatostatin analog, SMS 204-090. Specific binding of the ligand was seen as early embryonic day (E) 15 in the region of the inner neuroblastic layer. At E19 binding was localized to the ganglion cell and developing inner plexiform layers. At postnatal day (P) 2, there was diminished binding on autoradiography in this region. At P11, binding was more intense in the inner plexiform layer, and there was discernible binding in the outer plexiform layer. In the adult retina, the binding was seen clearly in two distinct bands corresponding to the inner plexiform layer and the outer plexiform layer. There was a single saturable binding site with the dissociation constant (Kd) of 0.25 +/- 0.04 nM. Binding sites were fairly constant throughout development except for a significant decline during the first postnatal week (Bmax = 1.8). These results demonstrate the early appearance of somatostatin receptors in the rat retina with high levels present embryologically followed by a brief decline in the early postnatal period with a return to high levels by synapse formation (P11). These receptor data parallel previous reports of the appearance of the somatostatin mRNA and peptide in rat retina.

Developmental expression of the proenkephalin and prosomatostatin genes in cultured cortical and cerebellar astrocytes

Astrocytes were prepared from rats of 4 ages, embryonic day 20, postnatal days 3 and 8, and adult, in order to study the developmental time course of expression of enkephalin and somatostatin (SS). Glial fibrillary acidic protein (GFAP) content was constant in both cortical and cerebellar astrocytes prepared from all ages. SS mRNA and peptide decreased over this developmental time course in cerebellar astrocytes; the time course of changes in SS mRNA paralleled that for rat cerebellum. Proenkephalin (PE) mRNA increased about 3-fold in cerebellar astrocytes from embryonic day 20 to adult but remained constant in cortical astrocytes; in contrast, PE mRNA showed a 10- to 12-fold increase in rat cerebellum and cortex developmentally. For both cerebellar and cortical astrocytes, free met-enkephalin decreased from embryonic day 20 to adult, whereas total met-enkephalin (measured following trypsin-carboxypeptidase B digestion of the extracts) increased. These results suggest (1) that there is a developmental regulation of the expression of both enkephalin and SS peptides in astrocytes, and (2) that the regulation occurs at the level of transcription for SS but at the level of precursor processing for PE. Possible trophic functions for astrocyte-derived peptides early in CNS development are discussed.

Somatostatin receptors in the human cerebellum during development

The ontogeny of somatostatin receptors (SRIF-R) was studied in the human cerebellum from mid-gestation to the 15th month postnatal. The brains were collected 3-26 h after death, from 18 fetuses and infants, and from 4 adults aged from 48 to 82. SRIF-R were characterized by membrane-binding assay and their localization was determined by in vitro autoradiography. Both techniques were conducted with two radio-ligands: [125I-Tyr0, DTrp8]S14 and D-Phe-Cys-125I-Tyr-DTrp-Lys-Thr- ol (125I-SMS 204-090). Membrane-binding studies carried out with each radioligand showed the presence of a single population of saturable, high affinity binding sites. Neither were the Kd values for either ligand (assessed by Scatchard analysis) changed appreciably during development, mean Kd values being 0.36 +/- 0.04 nM and 0.56 +/- 0.11 nM for [125I-Tyr0,DTrp8]S14 and 125I-SMS 204-090, respectively. Although inter-individual fluctuations of the Bmax were observed, the concentration of SRIF-R in the cerebellum of fetuses and infants up to 8 months appeared to be at least 2- to 10-fold higher than in the adult cerebellum. No appreciable differences in the Bmax values were found using either radioligand. The highest density of SRIF-R was observed in the cerebellar cortex of fetuses, in particular in the external granule cell layer (EGC), where stem cells of the granule cells are generated and enter the differentiation process. A high density of SRIF-R also occurred in the internal granule cell layer.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitters as Neurotrophic Factors: a New Set of Functions

At the start of this review, factors were deemed trophic if they stimulated mitosis, permitted neural cell survival, promoted neurite sprouting and growth cone motility, or turned on a specific neuronal phenotype. The in vitro evidence from cell cultures is overwhelming that both neurotransmitters and neuropeptides can have such actions. Furthermore, the same chemical can exert several of these effects, either on the same or on different cell populations. Perhaps the most striking example is that of VIP, which can stimulate not only mitosis, but also survival and neurite sprouting of sympathetic ganglion neuroblasts (Pincus et al., 1990a,b). The in vivo data to support the in vitro experiments are starting to appear. A role for VIP in neurodevelopment is supported by in vivo studies that show behavioral deficits produced in neonatal rats by treatment with a VIP antagonist (Hill et al., 1991). The work of Shatz' laboratory (Chun et al., 1987; Ghosh et al., 1990) suggests that neuropeptide-containing neurons, transiently present, serve as guideposts for thalamocortical axons coming in to innervate specific cortical areas. Along similar lines, Wolff et al. (1979) demonstrated gamma-aminobutyric acid-accumulating glia in embryonic cortex that appeared to form axoglial synapses and suggested the possibility that gamma-aminobutyric acid released from the glia might play a role in synaptogenesis by increasing the number of postsynaptic thickenings. Meshul et al. (1987) have provided evidence that astrocytes can regulate synaptic density in the developing cerebellum. The work of Zagon and McLaughlin (1986a,b, 1987) has shown that naltrexone, an antagonist of the endogenous opioid peptides, affects both cell number and neuronal sprouting. Lauder's laboratory (Lauder et al., 1982) has shown a role for 5-HT in regulation of the proliferation of numerous cell types. These studies illustrate another important point, that neurotransmitters and neuropeptides function in communication not only between neurons, but also between neurons and glial cells, and between glial cells. Given that astrocytes can express virtually all of the neural receptors and can produce at least some of the neurotransmitters and neuropeptides, they must now be considered equal partners in the processes of intercellular communication in the nervous system, including the trophic responses. The actions of neurotransmitters and neuropeptides have to be considered in terms of a broad spectrum of actions that range from the trophic actions described in this review, to the classic transmitter actions, to potential roles in neurotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of peptides in human hypothalamus in relation to sudden infant death syndrome (SIDS)

The brains of mammals are not mature at birth, in particular in humans. Growth and brain development are influenced by the hormonal state in which the hypothalamus plays the major regulatory role. The maturation of the hormonal patterns leads to the physiological establishment of chronological variations as revealed by the circadian variations of both hypothalamic peptides and pituitary hormones (as illustrated for hypothalamic-pituitary-thyroid axis by the determination of thyro-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) circadian rhythms in the rat (Jordan et al., 1989)). It has been established that hypothalamic peptide variations are regulated by hormonal feed-back and amine systems, with the maturation of the latter also being dependent upon the whole functional maturation of the brain. Though these systems have been studied in the rat, very little information is currently available with regard to the human brain. The only biochemical or immunohistochemical information published to date concerns either the fetus or the adult. We have studied four main peptidergic systems (somatostatin-releasing inhibiting factor (SRIF), thyrotropin-releasing hormone (TRH), luteinizing hormone-releasing hormone (LHRH) and delta sleep inducing peptide (DSIP) in post-mortem adults and infants and in sudden infant death syndrome (SIDS) brains either by autoradiography and/or immunochemistry of radioimmunology. From a technical point of view, human brain studies display certain pitfalls not present in animal studies. These may be divided into two subclasses: ante- and post-mortem. Ante-mortem problems concern mainly sex, laterality, nutritional and treatment patterns while post-mortem problems concern post-mortem delay and conditions before autopsy and hypothalamic dissection. This might induce dramatic changes in morphological, immunochemical and autoradiographic evaluations. The matching of pathological subjects with controls is particularly difficult in the case of SIDS because of the rapid changes which take place in physiological regulatory processes during the first year of life. Thus, the treatment of hypothalamic tissue samples both for immunochemistry, radioimmunology and autoradiographic studies required techniques which must be rigorously controlled. For example, SRIF studies were carried out with three different antibodies, which gave similar results. The use of different technical procedures as well as different antibodies is discussed. These types of differences might explain, at least in part, the discrepancy observed until now. As previously described in the fetus (Bugnon et al., 1977b; Bouras et al., 1987), we confirmed that in the infant hypothalamic SRIF immunoreactive cell bodies are present in the paraventricular and suprachiasmatic nuclei and in the periventricular area.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient expression of somatostatin messenger RNA and peptide in the hypoglossal nucleus of the neonatal rat

The postnatal developmental expression of somatostatin mRNA and peptide in the rat hypoglossal nucleus was analyzed using immunocytochemical and in situ hybridization techniques. Both the neuropeptide and its cognate mRNA were found to be transiently present within a subpopulation of hypoglossal motoneurons during the neonatal period. At the day of birth, a large population of perikarya situated in caudal, ventral regions of the hypoglossal nucleus expressed somatostatin. By postnatal day 7, the number of hypoglossal somata which expressed somatostatin had diminished considerably, and by 2 weeks postnatal, only few such cell bodies were found. By 3-4 weeks postnatal, somatostatin peptide- and mRNA-containing hypoglossal motoneurons were rarely observed, and in the adult, they were never detected, despite the use of colchicine. A double-labeling co-localization technique was used to demonstrate that somatostatin, when present perinatally, always coexisted with calcitonin gene-related peptide in hypoglossal motoneurons. The latter peptide, in contrast to somatostatin, was expressed in large numbers of somata throughout the entire hypoglossal nucleus and persisted within the motoneurons throughout development into adulthood. These results demonstrate that somatostatin is transiently expressed in motoneurons of the caudal, ventral tier of the hypoglossal nucleus in the neonatal rat. The developmental disappearance of somatostatin is most likely not due to cell death; hypoglossal somata continue to express calcitonin gene-related peptide, with which somatostatin coexisted perinatally, a high levels throughout development. Thus, it appears that the regulation of somatostatin expression in hypoglossal neurons occurs at the level of gene transcription or mRNA stability/degradation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral dysplasias as expressions of altered maturational processes

This review identifies the fundamental anatomical and physiological processes that provide the substrates of maturation in the developing nervous system. Cerebral malformations may be viewed from the perspective of aberrations in one or more of these processes. The maturational processes are generally sequential but with considerable temporal precision and overlap: 1) neuronogenesis and gliogenesis, including neural induction by the notochord and primary segmentation of the nervous system; 2) programmed cell death of excess neuroblasts and glioblasts; 3) neuroblast migration; 4) formation and growth of neurites; 5) development of membrane polarity and excitability; 6) synaptogenesis; 7) biosynthesis of neurotransmitters and other secretory products; and 8) myelination of axons. The anatomical and synaptic organization of the fetal brain differs greatly from the mature state. An excess of axonal collaterals, dendritic branches, spines and synapses are formed and later selectively deleted Transitory neurons, specialized glial cells, synaptic circuits and transient neurotransmitter systems serve as functional elements for a limited time. The clinical expression of these transitory features of the fetal brain are incompletely understood, particularly in the context of embryonic cerebral plasticity.

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.

Regional distribution of somatostatin binding sites in the human hypothalamus: a quantitative autoradiographic study

Using in vitro quantitative autoradiography and [125I]Tyr0-D-Trp8SRIF 14 as radioligand, we characterized the detailed distribution of somatostatin binding sites in human hypothalamus of both infants and adults. Guanosine triphosphate pretreatment, before incubation, allowed us to detect higher [125I]Tyr0-D-Trp8SRIF 14 binding site densities in hypothalamic structures such as preoptic and anterior hypothalamic areas and ventromedial and dorsomedial nuclei. In contrast, guanosine triphosphate was without effect in the other hypothalamic regions. The regional effects of guanosine triphosphate pretreatment were not different in infant and adult hypothalamus. Scatchard analysis showed that in a guanosine triphosphate-sensitive region (preoptic area) and a guanosine triphosphate-insensitive area (infundibular nucleus), [125I]Tyr0-D-Trp8SRIF 14 bound to a single class of binding sites. Affinities were similar in both regions, not modified by guanosine triphosphate pretreatment and not different in the adult (1.5 +/- 1.2 nM vs 3.2 +/- 2.1 nM for preoptic area and infundibular nucleus, respectively) and infant (0.9 +/- 0.5 nM vs 2.4 +/- 1.7 nM for preoptic area and infundibular nucleus). [125I]Tyr0-D-Trp8SRIF 14 binding sites were widely distributed in the anterior, mediobasal and posterior hypothalamus. Somatostatin 28 was twice as potent as somatostatin 14 to displace [125I]Tyr0-D-Trp8SRIF 14 binding in the preoptic area and infundibular nucleus. However, IC50s were 30 times lower in the preoptic area as compared with the infundibular nucleus. In adult as well as in infant, high densities were found mainly in the diagonal band of Broca, preoptic area and infundibular nucleus. Intermediate densities were localized in the anterior hypothalamic area, ventromedial, dorsomedial and lateral mammillary nuclei. The dorsal hypothalamic area, the paraventricular and medial mammillary nuclei displayed low but measurable densities. The only marked difference in the distribution of [125I]Tyr0-D-Trp8SRIF 14 binding sites in adult vs infant was observed in the medial and tuberal nuclei where the concentrations were seven-fold higher in adult hypothalamus.

Somatostatin mRNA and molecular forms during development of the rat retina

The developmental control of rat retinal somatostatin (SS) expression was determined by Northern blot hybridization. The amount of SS transcript is higher than adult levels at embryonic day 16 and declines during late prenatal development. This pattern parallels the developmental expression of somatostatin immunoreactivity. The expression of somatostatin-immunoreactive peptide products using gel filtration chromatography was determined over the same period. Three molecular weight forms occur during development: a high m.wt. product and two smaller peptides which migrate with somatostatin-28 (SS28) and somatostatin-14 (SS14). SS14 is the peptide that predominates at all developmental time points, and is the only form detectable in the adult retina. The transient prenatal increase of somatostatin message and peptide suggests a role for this peptide in the developing retina.

Functional maturation of somatostatin neurons and somatostatin receptors during development of mouse hypothalamus in vivo and in vitro

Ontogenesis of somatostatin (SRIF) neurons and receptors was studied in fetal hypothalamic cell cultures kept in serum-free medium, and compared to the in vivo developmental pattern. Initial rise in neuronal content of SRIF occurred later in vitro than in vivo. In vitro, K(+)-induced SRIF release was only present after synaptogenesis. SRIF binding sites were measurable as early as 1 day after birth and at an equivalent time in culture, after 6 days in vitro (DIV); their affinity was in the nanomolar range. In cultured cells, binding reached a maximum at two weeks in vitro and decreased sharply thereafter as a consequence of binding site occupancy by the endogenous ligand. Indeed, pretreatment with cysteamine decreased SRIF concentration in the neuronal cultures and twice as many binding sites as in control cultures of 21 DIV were measured. Competition kinetics using unlabelled SMS 201-995 to displace [125I]SRIF revealed two distinct binding sites in the neuronal preparations (IC50 = 11 +/- 3 pM and 4.5 +/- 0.8 nM). In contrast, only the lower affinity site was present on glial cell preparations (1.7 +/- 0.4 nM). SRIF inhibited adenylate cyclase activity in glia and neurons, and the onset of SRIF coupling to the second messenger occurred earlier in vitro than in vivo. Pertussis toxin pretreatment was equally effective in neuronal and glial cell preparations to decrease SRIF binding and to inhibit adenylate cyclase activity.

Somatostatin gene expression in the developing monkey frontal and cerebellar cortices

Somatostatin (SRIF) mRNA was determined in the developing monkey frontal and cerebellar cortices by the dot blot and the northern blot analyses at embryonic day 120 (E120), embryonic day 140 (E140), newborn stage (Nb), postnatal day 60 (P60) and adult stage (Ad.) At E120, at which time the migration of the cortical neurons had already been completed, SRIF mRNA was detectable with 50% of the maximal value at E140 in the cerebral frontal cortex (von Bonin and Bailey's area FD). After E140, the level of mRNA gradually declined to the adult level by P60 with 25% of the maximal value. In the cerebellum, SRIF mRNA was highly expressed at E120. The level decreased to 18% of the maximum at E140. Between the newborn and adult stages, there existed no positive signal of the mRNA. In contrast, both fetal and adult liver tissues contained no amounts of SRIF mRNA. We discussed the physiological meanings of the enhanced SRIF gene expression in the developing monkey cerebral and cerebellar cortices.

Expression of somatostatin receptors is impaired in the cerebellum of developing Brattleboro rats

Somatostatin (SRIF) receptors are expressed in the external granule cell layer of the rat cerebellum during early postnatal life. The aim of the present study was to investigate the distribution and biochemical characteristics of SRIF binding sites in the cerebellum of homozygous (vasopressin deficient) Brattleboro rats, which exhibit a selective impairment of their granule cell layer. This study has been conducted in 13-day-old rats by means of membrane-binding assay and autoradiography using [125I-Tyr0,DTrp8]S14 as a radioligand. In the cerebellum of homozygous Brattleboro rats, Scatchard plot analysis revealed the existence of a single class of SRIF receptors with similar Kd values as in Long-Evans or heterozygous Brattleboro rats (180-200 pM). Conversely, a marked reduction of the concentration of SRIF binding sites was observed in Brattleboro rats as compared to heterozygous or Long-Evans rats. In homozygous Brattleboro rats, autoradiographic studies revealed that the concentration of SRIF receptors was reduced in all lobules of the cerebellum as compared to Long-Evans. In addition, the magnitude of the decrease of receptor concentration was greater than the loss of granule cells observed in the homozygous Brattleboro rat. These results indicate that the expression of SRIF receptors by immature granule cells of the cerebellum is markedly reduced in Brattleboro rats. Whether the impairment of SRIF receptors in diabetes insipidus rats can directly be ascribed to vasopressin deficiency remains to be determined.

Pharmacological characterization of somatostatin receptors in the rat cerebellum during development

Somatostatin (SRIF) receptors (SRIF-Rs) are transiently expressed in a germinative lamina of the rat cerebellum, the external granule cell layer. The appearance of SRIF-Rs coincides with the expression of SRIF-like immunoreactivity in the cerebellum. However, the cellular location of SRIF-Rs does not overlap with the distribution of SRIF-like immunoreactivity, with the latter being restricted to ascending fibers arising from the brainstem, to perikarya within the white matter, and to some Purkinje cells. The characterization of SRIF-Rs in the immature (13-day-old) rat cerebellum was conducted by means of binding experiments in membrane-enriched preparations and autoradiography, using two radioligands, [125I-Tyr0,D-Trp8]SRIF-14 [( 125I-Tyr0,D-Trp8]S14) and 125I-SMS 204-090. The pharmacological profile of cerebellar SRIF-Rs was compared with that of adult cortical SRIF-Rs. Saturation studies performed in 13-day-old rat cerebellum showed that the KD values for [125I-Tyr0,D-Trp8]S14 and 125I-SMS 204-090 binding were 0.35 +/- 0.04 and 0.39 +/- 0.01 nM, respectively. The corresponding Bmax values were 52.7 +/- 4.8 and 49.9 +/- 5.3 fmol/mg of protein, a result indicating that radioligands with high specific radioactivity (2,000 Ci/mmol) bind to a single class of high-affinity sites (SS1). Competition studies showed that different D-Trp-substituted analogs displaced [125I-Tyr0,D-Trp8]S14 binding with Hill coefficients less than 1, a finding indicating the existence of different subtypes of binding sites. When [Tyr0,D-Trp8]S14 was used as a competitor, two sites were resolved by Scatchard analysis in both 13-day-old cerebellum and adult cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of somatostatin in mouse brain. I. Immunocytochemical studies

The postnatal development of the distribution of somatostatin immunoreactive (SOMLI) neurons and fibers in the forebrain of the Balb/C mouse and their relationship to cholinergic afferents have been examined. SOMLI was first discernable in the hypothalamus on postnatal day (PND) 3 and increased gradually to reach adult levels by PND 30. In the limbic system, SOMLI is detectable at birth. In all other structures of the forebrain, SOMLI could be observed by PND 3 but the distribution, density and morphology of the immunoreactive neurons evolved over the following 2-3 weeks. In general, SOMLI cells and fibers increased for 1-3 weeks after their initial appearance and subsequently declined to achieve adult levels. The distribution pattern of SOMLI elements in adult mouse brain was similar to previous reports in rat with a few notable differences in thalamus, olfactory structures and, to a lesser degree, cortex and hippocampus. The temporal pattern of SOMLI expression in extrahypothalamus forebrain regions, during development, suggests a role of this peptide in differentiation and synapse formation. Such an hypothesis receives further support from neonatal lesions of the basal forebrain which resulted in transient cortical cholinergic deafferentation, a delay of cortical differentiation and a transient increase in the number of SOMLI cells in cortex.

Developmental appearance of somatostatin in the rat cerebellum: in situ hybridization and immunohistochemistry

The postnatal appearance of the mRNA for preprosomatostatin, as well as the presence of somatostatin 28 and somatostatin 28, was examined in the rat cerebellum. RNA blotting analysis indicates a dramatic decrease in the level of this mRNA during cerebellar maturation. In situ hybridization reveals that many cells contain this mRNA at early postnatal stages, with progressively fewer cells being labeled after postnatal day 20. Immunohistochemical analysis indicates that there is a changing pattern in the distribution of these somatostatin peptides through cerebellar development. Initially, cells in the granule cell layer are stained, and fiber immunoreactivity is localized primarily in this same layer. During the second postnatal week, this granule cell layer staining decreases, and some Purkinje cells display immunoreactivity, particularly in the flocculus and paraflocculus. This Purkinje cell staining decreases by postnatal day 30, but immunoreactive cells are still present in the granule cell layer. In contrast, in the adult cerebellum, very few of these cells remain immunoreactive.

Distribution of somatostatin mRNA in the rat nervous system as visualized by a novel non-radioactive in situ hybridization histochemistry procedure

The cellular localization of preprosomatostatin mRNA in the rat brain and sensory ganglia has been examined in detail using a newly developed highly sensitive non-radioactive in situ hybridization histochemistry procedure. An alkaline phosphatase labelled anti-sense 30mer oligodeoxynucleotide probe was used for detection of somatostatin mRNA. This probe readily demonstrated somatostatin gene expression throughout the rat CNS with very high contrast and good cellular localization. As a result, we visualized numerous somatostatin mRNA-positive cells in many CNS areas which had previously not been shown to contain a mRNA signal. This method detected a number of somatostatin mRNA-positive cells, in the mitral cell layer of accessory olfactory bulb, the glomerular layer of the main olfactory bulb, the dorsal part of the lateral septum, superficial gray layer of superior colliculus, inferior colliculus, anterior ventral cochlear nucleus, granular layer and Purkinje cell layer of cerebellum, and substantia gelatinosa of medulla and spinal cord, all areas where signal detection using radiolabelled in situ probes has previously been rather difficult. The principle advantages of the present method include the very precise cellular resolution of signal, the rapid reaction time and low background. The sensitivity of the present method seems to be at least equivalent to most immunocytochemical procedures and more sensitive than most isotopic in situ hybridization methods.

Neuropeptide-immunoreactive cells and fibers in the developing primate cerebellum

Using the avidin-biotin-peroxide immunohistochemical method, we studied the distributions of somatostatin (SOM)-, cholecystokinin-8 (CCK-8)- and substance P (SP)-like immunoreactivities in the cerebellum of macaque monkeys at embryonic day 120 (E120), E140, newborn, postnatal day 60 (P60) and adults. During the embryonic stages, there were many SOM-, CCK- and SP-immunoreactive structures in the external granular layer, Purkinje cell layer and white matter, SP-immunoreactive mossy fibers and their terminals were distributed in the granular layer and white matter. During these stages, there were SOM-immunoreactive Purkinje cells, Golgi cells and a few cells in the molecular layer, and CCK-immunoreactive Purkinje cells and cells in the molecular layer. At the newborn stage, all of the immunoreactivities in the external granular layer decreased and the number of immunoreactive fibers increased in the white matter. At P60 stage, SOM- and CCK-immunoreactive fibers were observed around Purkinje cells, which seem to be the fiber terminals of basket cells. Many SOM, CCK and SP fibers were distributed in the white matter. In the adult stage, we observed little neuropeptide-immunoreactivity in the cerebellum. The high concentration of the neuropeptide-immunoreactive fibers and cells in the earlier stages suggests that the neuropeptides may be involved in the development of the primate cerebellar cortex.

In situ hybridization analysis of the somatostatin-containing neuron system in developing cerebellum of rats

We used in situ hybridization histochemistry to examine the postnatal development of the somatostatin (SRIF) synthesizing system in the cerebellum of rats. There are numerous hybridizing neurons from 1 to 9 days after birth. These occur throughout the cerebellum including the developing medulla and cortex except in the external granular cell layer. The lateral cerebellar nucleus also contains SRIF gene-containing cells. The intensity of the signals for SRIF mRNA in the cerebellum decreases with age. There is a drastic decrease in SRIF mRNA in the lateral cerebellar nucleus. SRIF cells cannot be detected in the lateral cerebellar nucleus of adult rats, whereas a small, yet significant number of SRIF cells are scattered in the cerebellar medulla. However, the cerebellum of adult rats still contains a significant number of labeled cells in the granular cell layer, although the intensity for SRIF mRNA decreases from 14 days after birth to adulthood. SRIF gene-expressing cells in the cerebellar cortex are located primarily in the granular cell layer and appear to correspond to Golgi cells judging from their characteristic features. These results are consistent with our previous immunohistochemical study on the decrease of SRIF immunoreactivity in the cerebellum of adult rats. These findings, together with a recent study of transient SRIF receptor-expressing cells in the developing cerebellum suggest that SRIF acts during cerebellar development.

Ontogenesis of enkephalinergic afferent systems in the opossum cerebellum

Enkephalin (ENK) immunoreactive climbing fibers, mossy fibers and a beaded plexus of axons are present in the adult opossum's cerebellar cortex. We have used the indirect antibody peroxidase-antiperoxidase technique to study the ontogeny of enkephalinergic axons in the cerebellum of pouch young opossums from postnatal day (PD) 1 to PD 83. On PD 1, ENK axons are present in the intermediate layer of the cerebellar anlage. At PD 18, after a period of 'waiting', ENK fibers form clusters throughout the cerebellar cortex primarily within the nascent Purkinje cell layer. By PD 40, axon terminals with a climbing fiber phenotype circumscribe Purkinje cells; immature mossy fiber rosettes are present within the internal granule cell layer. A third axon phenotype, beaded ENK fibers can be distinguished on PD 68. Between PD 40 and PD 68, the distributions of ENK climbing and mossy fibers overlap in vermal lobules II-VIII and X, whereas in the hemispheres climbing fibers predominate. However, by PD 83, ENK positive climbing fibers are no longer evident in lateral folia. These results indicate that early arriving ENK axons are present before the differentiation of their cellular targets. Further, a transient appearance of ENK in discrete populations of developing climbing fibers suggests several developmental events: (1) cell death in the inferior olive, (2) collateral regression, or (3) a transient expression of this peptide, that may be characteristic of this chemically defined system of axons.

Acridine orange-RNA fluorescence of maturing neurons in the perinatal rat brain

Cytoplasmic RNA was demonstrated in neurons of the developing rat brain using acridine orange (AO) as a histochemical marker. Fetuses of 18 days and postnatal rats of 1, 7, 14, and 21 days as well as adults several months old were studied. Neuroblasts of the germinal matrix exhibited minimal or no orange-red AO-RNA fluorescence, but immature nerve cells in migration within the cerebral hemispheres of the rat showed a weak but definite orange colour. This finding contrasts with the absence of AO-RNA fluorescence in migrating human neuroblasts. Neurons of the neocortical plate showed uniformly strong fluorescence. In the hippocampus, the most pronounced increase in AO-demonstrated RNA was in pyramidal and granule cells during the first postnatal week. The cerebellum showed a paradoxically stronger fluorescence of granule cells in the 18-day fetus than at birth, and almost no AO-RNA fluorescence of granule cells at 21 days of age or in adults. Motor neurons showed the strongest fluorescence of all neurons. It is likely that the increase in cytoplasmic RNA in neurons corresponds to the onset of neurotransmitter biosynthesis, but transitory fetal neuropeptides may explain stronger fluorescence of some neurons in young individuals. The reliable and simple AO method provides a supplementary means of studying one aspect of neuronal maturation.

Somatostatin expression in the cerebellar cortex during postnatal development. An immunohistochemical study in the rat

The distribution of somatostatin-immunoreactive (SOM-IR) elements in the cerebellar cortex of the rat has been studied at different stages of postnatal development (from birth to day 30) and in adult animals using immunohistochemistry. The results showed that in vermis of new born animals there are three main groups of SOM-IR structures within the cortex which subsequently spread along the Purkinje cell layer. In addition, both in the vermis and in the lateral lobes, numerous more evenly distributed SOM-positive cells and fibers could be seen. SOM-IR Golgi cells, Purkinje cells and climbing fibers could then be recognized during the subsequent developmental stages. In the vermal zone, SOM-IR Purkinje cells formed patches, which seemed to be part of a sagittal columnar or band-like organization. This was most obvious between days 5 and 21 of postnatal development. Subsequently there was a reduction in the number of immunoreactive Purkinje cells but a patchy disposition remained. In addition high numbers of SOM-IR Purkinje and Golgi cells and also climbing fibers were identified in the flocculus and paraflocculus at all stages of development studied, and they were also seen in the adult rats in these regions. In the lateral lobes expression of SOM-like immunoreactivity (LI) decreased and almost completely disappeared in adult animals. The present results demonstrate that a SOM or a SOM-LI peptide can be transiently detected in many Purkinje and Golgi cells in the cerebellar cortex, suggesting a role in events related to developmental processes. However, in some regions and structures SOM-LI can be seen also in adult animals.