Transient expression of somatostatin receptors in the rat cerebellum during development

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.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

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.

Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

Comparison of hypocretin/orexin and melanin-concentrating hormone neurons and axonal projections in the embryonic and postnatal rat brain

Hypocretin/orexin (H/O) and melanin-concentrating hormone (MCH) are peptide neuromodulators found in separate populations of neurons located within the lateral and perifornical hypothalamic regions. H/O has been linked to sleep-wakefulness regulation and to the sleep disorder narcolepsy, and both systems have been implicated in energy homeostasis, including the regulation of food intake. In the present study we compared the development of H/O and MCH-expressing neuronal populations with in situ hybridization and immunohistochemistry on adjacent sections in the embryonic and postnatal rat brain. We found that MCH mRNA and protein were present in developing neurons of the hypothalamus by embryonic day 16 (E16), whereas H/O mRNA and protein were not detected until E18. We also identified previously undescribed populations of MCH-immunoreactive cells in the lateral septum, paraventricular hypothalamic nucleus, lateral zona incerta, and ventral lateral geniculate nucleus that may play a specific role in the development of these regions. MCH immunoreactive axonal processes were also evident earlier than H/O stained fibers and at the time H/O immunoreactive processes were first identified in the hypothalamus at E20, extensive MCH axonal fiber systems were already present in many brain regions. Interestingly, however, the density of axonal fibers immunoreactive for H/O in the locus coeruleus reached peak levels at the same developmental age (P21) as MCH immunoreactive axons in the diagonal band of Broca (DBB). The peak of axon density coincided with the developmental stage at which adult patterns of feeding and sleep-waking activity become established. The present results demonstrate developmental differences and similarities between the MCH and H/O systems that may relate to their respective roles in feeding and sleep regulation.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

Stage-specific control of neuronal migration by somatostatin

Developing neurons transiently express somatostatin and its receptors, but little is known about their function at these early stages. As we thought that endogenous somatostatin might control the migratory behaviour of immature neurons, we have examined the effects of somatostatin in cerebellar granule cells of early postnatal mice, because these cells express all five types of somatostatin receptors before the initiation of their migration. Here we show that somatostatin has opposite and stage-specific effects on the migration of cerebellar granule cells. Activation of somatostatin receptors increases the rate of granule cell migration near their birthplace, but decreases the rate near their final destination. Furthermore, somatostatin enhances the size and frequency of spontaneous Ca2+ fluctuations in the early phase of migration, whereas it eliminates spike-like Ca2+ transients in the late phase. Somatostatin-induced changes at both early and late phases are reversed by a blockade of K+ channel activity. These results indicate that somatostatin may provide an essential cue for accelerating the movement of granule cells in the early phase and for terminating the movement in the late phase through altering intracellular Ca2+ concentrations and K+ channel activity.

Aminergic receptors during the development of the human brain: the contribution of in vitro imaging techniques

The development of the human brain is a complex process and, in this regard, the maturation of neurotransmitter systems and their receptors is of special interest. The study of these systems requires methodological approaches with powerful anatomical resolution. In this paper we review the application of visualization procedures to the fine localization, pattern of appearance and functional relevance of monoaminergic receptors in postmortem human brain samples corresponding to different stages of development (fetal, neonatal, infant). Data obtained by using mostly in vitro autoradiography but also in situ hybridization and, very recently, second messenger labeling, are discussed, including the methodological limitations inherent in working with inmature human tissue. From these studies, several conclusions were made. (1) It is possible to visualize, in the human brain with high resolution, the presence of neuroreceptors at early prenatal stages. (2) The anatomical distribution of monoaminergic receptors in the developing human brain is, in general terms, comparable to that found in the adult. (3) During the developmental process, some receptors, which are early and sometimes transiently expressed, play important thophic roles in the regulation of neuronal development: this is the case with the serotonin 5-HT1A receptors, which attain peak levels of hyperexpression over the hippocampus (dentate gyrus, dendritic areas of CA fields) and the raphe nuclei and show a transient expression in the cerebellum, around the 25 week of gestational age. (4) Different patterns of ontogenetic appearance for human receptors have been identified: dopamine D2-like (caudate, putamen, nigra) and 5-HT1A receptors are good examples of prenatal development, while 5-HT1B sites (basal ganglia, neocortex) present a mainly postnatal pattern of appearance. (5) Neurotransmitter receptors at human fetal stages are already functional from the point of view of transducing response.

Involvement of sst2 somatostatin receptor in locomotor, exploratory activity and emotional reactivity in mice

Somatostatin (SRIF) controls many physiological and pathological processes in the central nervous system but the respective roles of the five receptor isotypes (sst1-5) that mediate its effects are yet to be defined. In the present study, we attempted to identify functions of the sst2 receptor using mice with no functional copy of this gene (sst2 KO mice). In contrast with control 129Sv/C57Bl6 mice, sst2 mRNA was no longer detectable in the brain of sst2 KO mice; 125I-labeled Tyr0DTrp8-SRIF14 binding was also greatly reduced in almost all brain structures except for the hippocampal CA1 area, demonstrating that sst2 accounts for most SRIF binding in mouse brain. Invalidation of this subtype generated an increased anxiety-related behaviour in a number of behavioural paradigms, while locomotor and exploratory activity was decreased in stress-inducing situations. No major motor defects could be detected. sst2 KO mice also displayed increased release of pituitary ACTH, a main regulator of the stress response. Thus, somatostatin, via sst2 receptor isotype pathways, appears involved in the modulation of locomotor, exploratory and emotional reactivity in mice.

Visualisation of somatostatin receptor sst(3) in the rat central nervous system

Somatostatin actions are mediated through G-protein coupled receptors named sst(1) to sst(5). We used an affinity-purified polyclonal antibody AS-69, directed against a specific N-terminal peptide sequence of sst(3) to determine the immunohistochemical distribution of the sst(3) receptor in the rat and human brain. The specificity of the antibody was shown by Western blotting experiments using an N-terminal sst(3) fusion protein. Enzymatic deglycosylation experiments were combined to blotting experiments on a sst(3)-transfected cell line and rat brain membrane proteins and with immunocytochemistry on the sst(3)-transfected cell line. These studies showed that the antibody detected the deglycosylated sst(3) receptor protein. Immunohistochemical staining showed that sst(3) immunoreactivity recognised by this N-terminal antiserum was widely distributed throughout the brain with cells and processes labelled in the cerebral cortex, regions of the limbic system (including the hippocampal formation, some amygdaloid regions, some basal ganglia nuclei and regions from the nucleus basalis complex), the habenula, the hypothalamus, the thalamus, different mesencephalic structures (substantia nigra, zona incerta, superior colliculus), the reticular formation, the cerebellum. The distribution of immunoreactivity was in good general agreement with that predicted from the localisation of sst(3) mRNA and radio-ligand binding studies; however, due to the preference of AS-69 towards the deglycosylated receptor, it appears that the sst(3) immunoreactivity detected may correspond largely to the deglycosylated receptor. This study on the immunohistochemical distribution of the sst(3) receptor in the brain may provide a better understanding of the central actions of somatotropin release-inhibiting factor (SRIF).

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 potentiates voltage-dependent K+ and Ca2+ channel expression induced by nerve growth factor in PC12 cells

It has been proposed that neurotransmitters and neuromodulators may function as neurotrophic factors during the development of the nervous system. Somatostatin (SS) was known to increase neurite outgrowth in PC12 cells, rat pheochromocytoma cell line, and cerebellar granule cells as well as Helisoma neuron. To further investigate a neurotrophic role of SS, voltage-dependent K+ and Ca2+ channel expression was studied using whole-cell patch-clamp in PC12 cells and the effect of SS was compared to that of nerve growth factor (NGF). Cyclic AMP (cAMP) level and mitogen-activated protein (MAP) kinase phosphorylation were also studied following the treatment with SS and/or NGF. Whereas NGF (50 ng/ml) increased continually the current density of the voltage-dependent K+ channel throughout 8 days treatment, SS (1 microM) increased the K+ current density on day 2 to the peak. K+ current density was decreased thereafter and was not different on day 6 from that of undifferentiated cells. Although SS did not increase voltage-dependent Ca2+ current density, it potentiated NGF-induced increase of voltage-dependent Ca2+ channel current density as well as the K+ current density. cAMP level was decreased by NGF and/or SS treatment. An increased phosphorylation of MAP kinase induced by NGF was not changed by SS treatment. These results support functionally that SS may function as a neurotrophic factor in developing nervous system.

Chronic exposure to either somatostatin (SS) or octreotide, a long-lasting SS analogue, affects SS expression in the postnatal visual cortex of the rat

The peptide somatostatin (SS) is widely distributed in the mammalian brain where it modulates neuronal activity through interactions with specific membrane-bound receptor subtypes (ssts). Five different ssts were characterized so far (sst1-5) and their selective agonists were developed on the basis of their binding specificity. SS and ssts are transiently expressed in the developing brain, suggesting a functional role of somatostatinergic systems in neuronal maturation. In the present study, we investigated the effects of chronic exposure to either the SS synthetic analogue, SS-14 or octreotide (a long-acting sst2-preferring analogue) on the maturation of SS-immunoreactivity (-ir) in the primary visual cortex of the rat. SS-ir maturation was investigated both by an evaluation of the number of SS-immunoreactive cells and by radioimmunoassay (RIA) to measure the levels of SS in the postnatal visual cortex. In the visual cortex of normal rats, the number of SS-positive cells markedly increased during the second postnatal week and then significantly decreased until the adult value was reached at the third week. Early and repeated intracerebroventricular (i.c.v.) injections of either SS-14 or octreotide prevented the increase in the number of SS-positive cells, with adult values reached at the end of the first postnatal week. Similarly, administration of either SS-14 or octreotide significantly decreased the SS content of the visual cortex, measured at the end of the second postnatal week. These results show that high local concentrations of either SS-14 or octreotide interfere with SS expression in developing cortical neurons in a restricted postnatal period.

Immunocytochemical localization of somatostatin and autoradiographic distribution of somatostatin binding sites in the brain of the African lungfish, Protopterus annectens

The anatomical distribution of somatostatin-immunoreactive structures and the autoradiographic localization of somatostatin binding sites were investigated in the brain of the African lungfish, Protopterus annectens. In general, there was a good correlation between the distribution of somatostatin-immunoreactive elements and the location of somatostatin binding sites in several areas of the brain, particularly in the anterior olfactory nucleus, the rostral part of the dorsal pallium, the medial subpallium, the anterior preoptic area, the tectum, and the tegmentum of the mesencephalon. However, mismatching was found in the mid-caudal dorsal pallium, the reticular formation, and the cerebellum, which contained moderate to high concentrations of binding sites and very low densities of immunoreactive fibers. In contrast, the caudal hypothalamus and the neural lobe of the pituitary exhibited low concentrations of binding sites and a high to moderate density of somatostatin-immunoreactive fibers. The present results provide the first localization of somatostatin in the brain of a dipnoan and the first anatomical distribution of somatostatin binding sites in the brain of a fish. The location of somatostatin-immunoreactive elements in the brain of P. annectens is consistent with that reported in anuran amphibians, suggesting that the general organization of the somatostatin peptidergic systems occurred in a common ancestor of dipnoans and tetrapods. The anatomical distribution of somatostatin-immunoreactive elements and somatostatin binding sites suggests that somatostatin acts as a hypophysiotropic neurohormone as well as a neurotransmitter and/or neuromodulator in the lungfish brain.

Influence of the neuropeptide somatostatin on the development of dendritic morphology: a cysteamine-depletion study in the rat auditory brainstem

We investigated the functional role of somatostatin during early ontogeny of the brain, when the neuropeptide as well as its receptors are heavily expressed in the auditory brainstem. Rat pups received a daily injection of cysteamine which, when applied at low concentrations, most selectively depletes somatostatin. Neurons from the lateral superior olive, an auditory brainstem nucleus which transiently receives a dense somatostatinergic input, were intracellularly labeled at postnatal day 14 or 18. The dendritic morphology of these neurons was then analyzed quantitatively and compared with neurons from controls. Cysteamine treatment induced a reduction of the number of dendritic end points by more than 50%. At postnatal day 14, for example, controls and somatostatin-depleted animals had an average of 58 and 28 end points, respectively. The number of primary dendrites was also significantly reduced by cysteamine. In contrast, the size of the somata, the orientation of the dendritic trees within the lateral superior olive, the dendritic areas, and the cross-sectional size of the lateral superior olive were not altered. These results indicate that somatostatin depletion during early development has profound effects on the maturation of dendritic morphology. The selective influence on the dendritic trees suggests that somatostatin acts as an endogenous trophic peptide and promotes the achievement of dendritic complexity.

Distribution and pharmacological characterization of somatostatin receptor binding sites in the sheep brain

Somatostatin binding sites have been localized and quantified in the sheep brain using 125I-Tyr0-DTrp8-somatostatin, by quantitative high resolution light microscopic autoradiography. Sections were analyzed by densitometry on radioautographic film, and subsequently on slides coated with photoemulsion. Specific somatostatin binding sites were concentrated in the medial habenula, superior colliculus, dorsal motor nucleus of the vagus nerve, inferior olive, spinal trigeminal nucleus, and cerebellum. In competition experiments, octreotide, a sst2/sst3/sst5 selective agonist only partially displaced 125I-Tyr0-DTrp8-somatostatin in the three cerebellar layers while it was fully active as compared to somatostatin 14 and 28 in the deeper layers of the parietal cortex. Moderate to low somatostatin receptor densities were present in the mesencephalic periaqueductal gray, dorsal raphe, thalamic paraventricular nucleus, interpeduncular nucleus, pineal gland, dorsal tegmental, dorsolateral tegmental and parabrachial nuclei, nucleus of the solitary tract. The distribution of somatostatin binding sites generally correlates with the data obtained on slides dipped in photoemulsion which provided better resolution and more precise localization. In most of the labeled areas, 125I-Tyr0-DTrp8-somatostatin receptor binding was distributed between both neuropil and perikarya. Perikarya bearing 125I-Tyr0-DTrp8-somatostatin receptors were observed in areas which did not display detectable binding sites on film such as the preoptic-anterior hypothalamic complex and arcuate nucleus and in the locus coeruleus. In conclusion, the distribution of 125I-Tyr0-DTrp8-somatostatin binding sites in sheep brain is very reminiscent of other mammals being closer to the human than to rodents.

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.

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.

Somatostatin receptors in the developing rat brain

The distribution of [125I]SRIF-28 ([Leu8,D-Trp22,125I-Tyr25]somatostatin-28), [125I]204-090 ([Tyr3]octreotide) and [125I]CGP 23996 (c[Asu-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) labelled recognition sites was studied by autoradiography in rat brain at embryonic day 18 (E 18) and postnatal day 5 (P 5). These results were compared with mRNA expression of somatostatin receptors SSTR1-5 (named sst1-5 now) as studied by in situ hybridization. [125I]SRIF-28, [125I]204-090 and [125I]CGP 23996 binding displayed different although partially overlapping distributions, and showed an increase between E 18 and P 5, which was less marked for [125I]204-090 binding. -125I-204-090 binding and sst2 receptor mRNA were similarly distributed, whereas [125I]CGP 23996 binding did not correlate with any single somatostatin receptor mRNA. The data suggest that most SRIF receptor subtypes in rat brain are present before birth, but evolve differently.

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.

Cysteamine impairs the development of the acoustic startle response in rats: possible role of somatostatin

Somatostatin is transiently expressed in many regions of the developing brain, among others in auditory brainstem nuclei of neonatal rats. To explore the functional significance of somatostatin during the ontogeny of acoustically elicited behavior, the acoustic startle response (ASR) was measured in developing rats after chronic application of cysteamine, which, when applied in low doses, most selectively depletes somatostatin. Cysteamine treatment drastically reduced somatostatin immunoreactivity in the cochlear nuclear complex and the caudal pontine reticular nucleus, i.e. in structures mediating the ASR. It did not affect the ASR amplitude of postnatal day (P) 13 animals, yet it resulted in a significant reduction of the ASR amplitude at P18. Our results therefore suggest that somatostatin can influence the maturation of sensorimotor information processing.

Embryonic and postnatal mRNA distribution of five somatostatin receptor subtypes in the rat brain

The messenger RNA (mRNA) expression of somatostatin (SRIF) receptors SSTR-1, SSTR-2, SSTR-3, SSTR-4 and SSTR-5 (called sst1-5, now) was studied in rat brain between embryonic day 17 (E17) and post-natal day 5 (P5) by in situ hybridization histochemistry and compared to that of adult rats. sst1 receptor mRNA expression was very low and restricted at E17, spread out at E18, to reach very high levels comparable to that of adult at P5 (e.g. in temia tecta, posteromedial cortical amygdaloid nucleus, subiculum). At E17/E18, sst2 receptor mRNA expression was low and limited (telencephalon); significant levels were present at P5 in allocortex, hippocampus, locus coeruleus, similarly to adult brain. sst3 receptor mRNA was high at E17 in most brain regions, and almost as ubiquitous as in adult brain at P5. sst4 receptor mRNA was apparently absent at E17, with low levels in the hippocampus, amygdala and habenula at E18; a wider distribution, especially in the hippocampus and cerebral cortex was observed at P5, similar to that of adult. sst5 receptor mRNA was not detected at E17 and negligible at E18; low levels were found in the cortex, hippocampus and cerebellum at P5. However, in adult brain, only the cerebellum and hind-brain showed some sst5 receptor mRNA transcripts. The presence and distribution of SRIF receptor mRNAs differs substantially in embryo and adult brain. Some mRNAs are present throughout development, while others proceed only postnatally to the adult form. There are striking differences within and between the different SRIF receptor mRNAs, suggesting a role in neurogenesis for some SRIF receptors (e.g. sst2). However, mRNA and protein levels do not necessarily correlate.

Somatostatin enhances neurofilament expression and neurite outgrowth in cultured rat cerebellar granule cells

Somatostatin and its receptors are transiently expressed at a high level in the cerebellum around birth, before declining to adult levels by 2-3 weeks postnatally. We therefore investigated the neurotrophic effects of somatostatin (SS) on rat cerebellar granule cells in culture by measuring the percentage of cells with processes, the content of mRNA and protein for neurofilament (NF) and mRNA for glutaminase, and the number of viable cells (MTS assay). SS increased the percentage of cells with processes at 8 h after plating. After 1 day in vitro (DIV), SS caused a 2-fold increase in NF mRNA, and a 23% increase in NF protein. The mRNA increase was maximal at DIV1 whereas by DIV7 the NF protein content of control cells reached that of SS-treated cells. SS had no effect on glutaminase mRNA or on the number of viable neurons from either postnatal day 5 or 8 animals. These results demonstrated that SS has a neurotrophic effect on neurite production, including initiation of neurite outgrowth, but no effect on neuronal survival, cell proliferation, or phenotype differentiation (glutaminase expression), and support the possibility that SS plays a role in the differentiation of immature cerebellar granule cells during central nervous system 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.

Co-expression of somatostatin SSTR-3 and SSTR-4 receptor messenger RNAs in the rat brain

In situ hybridization histochemistry was used to study the distribution and possible co-expression of the messenger RNA of the somatostatin receptor subtypes SSTR-4 and SSTR-3 in rat brain. Our results demonstrate that SSTR-3 messenger RNA is widely expressed within the rat brain, while expression of SSTR-4 messenger RNA is restricted to the telencephalon, diencephalon and granular layer of the cerebellum. It is also shown that single neurons can co-express both SSTR-4 and SSTR-3 receptor messenger RNAs. The highest density of SSTR-4 messenger RNA was found in the pyramidal cell layer of the hippocampus, especially in the CA1 and CA2 areas, anterior olfactory nuclei, amygdala, and in layers IV and VI of the cerebral cortex. SSTR-3 messenger RNA displayed a homogeneous distribution in the cerebral cortex and was expressed in the olfactory bulb, pyramidal cells of the hippocampus, granular cell layer of the dentate gyrus, motor and sensory metencephalic nuclei, and the granular and Purkinje cell layers of the cerebellum. Whether SSTR-3 and SSTR-4 messenger RNA can be expressed by the same cell was assessed by the simultaneous use of digoxigenin (SSTR-3)- and isotopic (SSTR-4)-labelled oligoprobes. Co-expression of SSTR-3 and SSTR-4 messenger RNAs was found in neurons of the CA1 and CA2 regions of the hippocampus, in the subiculum and in layer IV of the cerebral cortex. The expression of two receptor subtypes of the same neurotransmitter in a single cell, if confirmed functionally, raises questions about the consequences of the simultaneous activation of these different receptors.

Pharmacological characterization of somatostatin receptors in rat cerebellar nuclei

Rat cerebellar nuclei contain somatotropin release-inhibiting factor (SRIF) receptors that bind [125I][Leu8,D-Trp22,Tyr25]SRIF-28 but do not bind [125I][Tyr0,D-Trp8]SRIF-14. The aim of the present study was to investigate the pharmacological profile of these receptors by means of binding experiments on tissue sections and quantitative autoradiography. Competition experiments indicated the presence of a single class of [125I][Leu8,D-Trp22,Tyr25]SRIF-28 binding sites in the lateral cerebellar nuclei, showing similar affinities for SRIF-14 and SRIF-28, but low affinity for short-chained analogs. The IC50 values for somatostatin analogs to compete with the binding of [125I][Leu8,D-Trp22,Tyr25]SRIF-28 in the lateral cerebellar nuclei ranked as follows: [Leu8,D-Trp22,Tyr25]SRIF-28 approximately SRIF-14 approximately SRIF-28 < CGP 23996 < D-Phe-Phe-Phe-D-Trp-Lys-Thr-Phe-Thr-NH2 (BIM 23052) < SMS 201-995 approximately N-Ahep-(7-10)SRIF-14-Bzl << MK 678 < D-Phe-Phe-Tyr-D-Trp-Lys-Val-Phe-D-Nal-NH2 (BIM 23056) < D-Phe-c[Cys-Tyr-D-Trp-Lys-Abu-Cys]Nal-NH2 (NC 8-12). Optimum binding of [125I][Leu8,D-Trp22,Tyr25]SRIF-28 did not require divalent cations, and was partly inhibited by guanosine 5' triphosphate. It appears from this study that the rat lateral cerebellar nuclei contain a pure population of receptors exhibiting the same binding characteristics as the recently cloned sstr1 somatostatin receptor. These nuclei could thus provide a useful model in which to investigate the characteristics of native sstr1.

Molecular pharmacology of somatostatin receptors

The neuropeptide somatostatin (SRIF) is widely expressed in the brain and in the periphery in two main forms, SRIF-14 and SRIF-28. Similarly, the presence of SRIF receptors throughout the whole body has been reported. SRIF produces a variety of effects including modulation of hormone release (e.g. GH, glucagon, insulin), of neurotransmitter release (e.g. acetylcholine, dopamine, 5-HT), and its own release is modulated by many neurotransmitters. SRIF affects cognitive and behavioural processes, the endocrine system, the gastrointestinal tract and the cardiovascular system and also has tumor growth inhibiting effects. Initially, two classes of SRIF receptors have been proposed on the basis of biochemical and functional studies. However, the recent cloning of five putative SRIF receptor subtypes which belong to the G-protein coupled receptor superfamily suggests that SRIF mediates its various effects via a whole family of receptors. Here we review, in this new context, the molecular pharmacology of the SRIF receptor subtypes present in the brain and in the periphery, and address the question of nomenclature of SRIF receptors.

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.

Ontogeny of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors in the rat cerebellum: a quantitative autoradiographic study

Pituitary adenylate cyclase-activating polypeptide and PACAP receptors are both present in the rat cerebellar cortex, suggesting that PACAP may play an important role in the cerebellum. In the present study, the variation of the concentration of PACAP binding sites in the rat cerebellum was investigated during postnatal development by means of quantitative autoradiography, using [125I]PACAP27 as a radioligand. In the external granule cell layer and the medulla, the density of PACAP binding sites was high at birth, markedly decreased from postnatal day 8 (P8) to P25 and finally vanished at the end of the third postnatal week. In the internal granule cell layer and molecular layer, PACAP binding sites were first detected at P8. In the internal granule cell layer, the density of binding sites slightly decreased during development but remained elevated in adults. Conversely, in the molecular layer, PACAP binding sites rapidly decreased during the second and third postnatal weeks and virtually disappeared after P25. In all four layers of the cerebellar cortex, the autoradiographic labeling was displaced by PACAP27 (IC50 close to 10(-8) M), but was not affected by VIP. No significant changes in IC50 and Hill coefficient were noticed in the various layers throughout development. The present study shows that all four layers of the cerebellar cortex express PACAP binding sites during development. The evolution of the receptor concentration exhibited differential profiles in the various layers but the specificity characteristics of the recognition sites were identical in all four structures.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Developmental patterns of somatostatin-receptors and somatostatin-immunoreactivity during early neurogenesis in the rat

The temporal pattern of distribution of somatostatin receptor was investigated using the somatostatin analogue [125I]Tyr0-DTrp8-somatostatin14 as a ligand and compared with that of somatostatin immunoreactivity during early developmental stages in the spinal cord and the sensory derivatives in rat fetuses. Qualitative and quantitative analysis showed that somatostatin receptors were detected in a transient manner. In the neural tube, they were clearly associated with immature premigratory cells and with the developing white matter. During the time-period examined (from day 10.5 to 16.5), the disappearance of somatostatin receptors followed a ventro to dorsal gradient probably linked to the regression of the ventricular zone. In sensory derivatives, they were expressed in the forming ganglia and their central and peripheral nerves from embryonic day 12.5 to 16.5 inclusive, with a peak around day 14.5 and low levels observed at day 16.5. Competition experiments performed at embryonic day 14.5 demonstrated that somatostatin1-14, somatostatin1-28, and Octreotide displaced specific binding with nanomolar affinities while CGP 23996 was only active at micromalar doses. Such displacements are compatible with the SSTR2 and/or SSTR4 pharmacology. During the time period examined, some transient somatostatin immunoreactive cell bodies and fibers were detected in the neural tube and in the sensory derivatives. These results demonstrate the existence, in neuronal derivatives, of a complex temporal and anatomical pattern of expression of somatostatin receptors, from the SSTR2/SSTR4 subtype(s), and somatostatin immunoreactivity. It appears that the transient expression of somatostatin receptors and/or somatostatin immunoreactivity characterizes critical episodes in the development of a cohort of neurons; a fact that unequivocally reinforces the notion that somatostatin plays a fundamental role during neurogenesis in vertebrates.

Colocalization of binding sites for somatostatin, muscarine and nicotine on cultured neurones of rat neocortex, cerebellum, brain stem and spinal cord: combined autoradiographic and immunohistochemical studies

The cellular localization of binding sites for [125I]1-tyramine somatostatin ([125I]SS) was studied in explant cultures of rat CNS by autoradiography. In cultures from cortex, brain stem and spinal cord many neurones revealed binding sites for the peptide whereas in cerebellar cultures only little binding of [125I]SS was observed. In addition to neurones, astrocytes were also labelled by the peptide. By combined immunohistochemical and autoradiographic techniques, it was demonstrated that the majority of neurones which expressed binding sites for [125I]SS were also immunostained by the monoclonal cholinergic muscarinic or nicotinic receptor antibodies (M 35 and W 6, respectively), providing evidence for a colocalization of cholinergic and somatostatin receptors on the neuronal membrane.

Localization of oxytocin binding sites in the thoracic and upper lumbar spinal cord of the adult and postnatal rat: a histoautoradiographic study

Oxytocin binding sites were detected by autoradiography on films and emulsion-coated sections in the spinal cord of adult and postnatal rats from C8 to L2, using a highly selective 125I-labelled oxytocin antagonist. Oxytocin binding sites were detected on all transverse sections in the dorsal horn, where labelling was scattered over laminae I and II. The autonomic areas, i.e. the intermediolateral cell column, the central grey (lamina X) and the nucleus intercalatus were labelled. Binding in the intermediolateral cell column was most frequently observed on sections from T9 to T11 in adult and T7 to T8 in postnatal rats. In this location, oxytocin binding sites were highly concentrated on cell bodies of putative sympathetic preganglionic neurons; however, not all of these cells were labelled. Diffuse labelling occurred on the dorsal part of the central grey, mainly between T8 and L2. Isolated labelled cells belonging to the nucleus intercalatus were scattered between the central canal and the intermediolateral cell column. In addition, oxytocin binding sites were found on some motoneurons of the lateral group of T12-T13, but only in postnatal rats. The distribution of oxytocin binding sites in the rat spinal cord coincides with that of the oxytocin innervation and strongly suggests a modulatory role of this peptide in sensory and autonomic functions.

Mild sustained effects of neonatal vasopressin and oxytocin treatment on brain growth and behavior of the rat

The lasting effects of a 9-day neonatal exposure to vasopressin and oxytocin were examined in the rat to discover if peptide administration results in organizational effects. When tested in young adulthood, brain growth, not body growth, appeared to be impaired. Basal and challenge tests of urine production, carried out to see the development of the hormonal antidiuretic function of vasopressin, revealed no lasting changes, and therefore did not confirm earlier findings of an induced mild polyurea. Behavioral testing of learning by making use of a one-trail step-through paradigm with a 24-h retention trial--a test that is sensitive to vasopressin--did not show impairments. Open field tests, however, showed enhanced emotionality in the vasopressin-treated females, as well as an initially increased ambulation in the males, and increased grooming in both sexes, the latter also having been reported to be induced by vasopressin administration in the septal areas. Oxytocin treatment did not produce lasting changes. Our conclusion, therefore, is that peripherally circulating vasopressin can affect the organizational development of the rat brain. It remains to be established whether this is an effect obtained through changes in the general peripheral physiology or a reflection of plasticity phenomena at the level of central vasopressin neurotransmission.

Localization and characterization of PACAP receptors in the rat cerebellum during development: evidence for a stimulatory effect of PACAP on immature cerebellar granule cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors are abundant in the brain and particularly in the cerebellum of adult rats. In contrast, the occurrence of PACAP binding sites has not been investigated during ontogenesis. The aim of the present study was to determine the distribution and biochemical characteristics of PACAP binding sites in the rat cerebellum during postnatal development, and to examine the effect of PACAP on immature cerebellar granule cells. Autoradiographic studies revealed that PACAP binding sites are transiently expressed in a germinative matrix of the cerebellar cortex, the external granule cell layer, and in the medulla, from postnatal days 8 to 25. A population of PACAP binding sites persisted in the internal granule cell layer in the mature cerebellum. Emulsion-coated cytoautoradiography, performed on cultured immature granule cells from eight-day-old rat cerebellum, demonstrated that transient PACAP binding sites are expressed by cerebellar immature granule cells. Biochemical characterization of binding revealed the occurrence of two classes of PACAP recognition sites exhibiting, respectively, high (Kd = 0.39 +/- 0.08 nM) and low (Kd = 21.2 +/- 9.4 nM) affinity for PACAP27. The two naturally occurring forms PACAP38 and PACAP27 were equipotent in competing for [125I]PACAP27 binding. In contrast, the [Des-His1]PACAP38 analog was eight times less efficient and vasoactive intestinal polypeptide only induced weak displacement of the binding. Exposure of cultured immature granule cells to PACAP27 resulted in a dose-dependent stimulation of the production of cAMP, indicating that PACAP binding sites represent authentic receptors positively coupled to adenylate cyclase. These results show that PACAP receptors are actively expressed in the cerebellum of rats during postnatal development. The presence of functional PACAP receptors in the external granule cell layer suggests that PACAP may play a role in the control of proliferation and/or differentiation of granule cells.

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.

Facilitation of 2-deoxyglucose uptake in rat cortex and hippocampus slices by somatostatin is independent of cholinergic activity

2-Deoxyglucose (2-DG) uptake is an index of regional glucose utilization which reflects predominantly activity in the axonal terminal of neuronal pathways. The present experiments showed that somatostatin elevated 2-DG uptake in rat cortex and hippocampus slices. Treatment with somatostatin-14 and somatostatin-28 markedly enhanced 2-DG uptake, whereas the amino-terminal fragment of somatostatin-28 did so only slightly. This effect appeared to be mediated by an interaction with somatostatin receptors because cyclo-somatostatin, a somatostatin antagonist, abolished the effect of somatostatin-14. The increase in 2-DG uptake caused by somatostatin-14 was blocked by the calcium channel antagonist, nifedipine, but not by tetrodotoxin, suggesting that the action of somatostatin does not require the initiation of impulse activity, somatostatin enhanced the KCl-induced release of acetylcholine, suggesting that a cholinergic mechanism is involved in the somatostatin-induced cellular responses. We therefore examined whether acetylcholine receptor antagonists block the somatostatin-induced increase in 2-DG uptake. Neither muscarinic nor nicotinic receptor antagonists affected the somatostatin-14-induced response. The present results suggest that somatostatin has a stimulatory effect on energy metabolism and that this effect is independent of acetylcholine receptor mechanism.

Autoradiographic localization of somatostatin mRNA in the adult rat lower brainstem: observation by the double illumination technique

Using in situ hybridization histochemistry and observation with a double (dark and bright) illumination apparatus, the precise localization of preprosomatostatin mRNA was studied in the adult rat lower brainstem and cerebellum. It has previously been hard to localize the somatostatin precursor gene in the adult rat brainstem, because the level of expression is low or undetectable in some brain areas in adulthood, in contrast to the high levels in the neonatal period. The present study in adult rats showed the clear localization of this mRNA in the same areas where it is found in the perinatal period. The results showed that somatostatin neurons in such areas continue the minimal production of the precursor gene even at the adult stage.

Somatostatin receptors are expressed by immature cerebellar granule cells: evidence for a direct inhibitory effect of somatostatin on neuroblast activity

Somatostatin and somatostatin receptors are transiently expressed in the immature rat cerebellar cortex but virtually undetectable in the cerebellum of adults. Although somatostatin binding sites have been visualized during the postnatal period in the external granule cell layer, the type of cell that expresses somatostatin receptors has never been identified; thus, the potential function of somatostatin in the developing cerebellum remains unknown. In the present study, we have taken advantage of the possibility of obtaining a culture preparation that is greatly enriched in immature cerebellar granule cells to investigate the presence of somatostatin receptors and the effect of somatostatin on intracellular messengers on cerebellar neuroblasts in primary culture. Autoradiographic labeling revealed the occurrence of a high density of binding sites for radioiodinated Tyr-[D-Trp8]somatostatin-(1-14) on 1-day-old cultured immature granule cells. Saturation and competition studies showed the existence of a single class of high-affinity binding sites (Kd = 0.133 +/- 0.013 nM, Bmax = 3038 +/- 217 sites per cell). Somatostatin induced a dose-dependent inhibition of forskolin-evoked cAMP formation (ED50 = 10 nM), and this effect was prevented by preincubation of cultured immature granule cells with pertussis toxin. Somatostatin also caused a marked reduction of intracellular calcium concentration. These results show the presence of functionally active somatostatin receptors on immature granule cells. Our data suggest the possible involvement of somatostatin in the regulation of proliferation and/or migration of neuroblasts during the development of the cerebellar cortex.

Molecular cloning of a somatostatin-28 receptor and comparison of its expression pattern with that of a somatostatin-14 receptor in rat brain

The tetradecapeptide somatotropin-release inhibiting factor somatostatin-14 regulates the release of peptide hormones and also functions as neurotransmitter. The octacosapeptide somatostatin-28, the N-terminally extended form of somatostatin-14, shows similar biological activities yet with different potencies. Both peptides most likely function through distinct receptors. Here we report on the molecular and functional characterization of a somatostatin-28 receptor (SSR-28) cloned from a rat brain cDNA library. The nucleotide sequence contains an open reading frame for a protein of 428 amino acid residues with a predicted molecular mass of 47 kDa. Binding assays using radiolabeled somatostatin-14 and membranes from COS cells transfected with the cloned cDNA show that this receptor, SSR-28, has a higher binding affinity for somatostatin-28 (IC50 = 0.24 nM) than for somatostatin-14 (IC50 = 0.89 nM). RNA blot analysis reveals a 4.4-kilobase mRNA in rat cerebellum and at significantly lower abundance in other brain regions. In situ hybridization indicates that SSR-28 mRNA is present in the granular and Purkinje cell layers of the cerebellum and in the large cells of the hypoglossal nucleus of the brain stem. Signals for SSR-28 mRNA do not overlap with those of a previously cloned rat receptor that preferentially binds somatostatin-14 (SSR-14). SSR-14 mRNA is found in the medial cerebellar nucleus, horizontal limb of the diagonal band, various hypothalamic nuclei, and in layers IV and V of the cortex. In the rat cerebellum, SSR-14 and SSR-28 mRNAs are developmentally regulated; the levels of the former are highest around birth and levels of the latter are highest at the adult stage.