Light microscopic radioautographic localization of somatostatin binding sites in the brainstem of the rat

Distribution of Neurotensin and Somatostatin-28 (1-12) in the Minipig Brainstem

Using an indirect immunoperoxidase technique, an in depth study has been carried out for the first time on the distribution of fibres and cell bodies containing neurotensin and somatostatin-28 (1-12) (SOM) in the minipig brainstem. The animals used were not treated with colchicine. The distribution of neurotensin- and SOM-immunoreactive fibres was seen to be quite similar and was moderate in the minipig brainstem: a close anatomical relationship between both neuropeptides was observed. The distribution of cell bodies containing neurotensin or SOM was quite different and restricted. Cell bodies containing neurotensin were found in four brainstem nuclei: nucleus centralis raphae, nucleus dorsalis raphae, in the pars centralis of the nucleus tractus spinalis nervi trigemini and in the nucleus ventralis raphae. Cell bodies containing SOM were found in six nuclei/regions of the brainstem: nucleus ambiguus, nucleus dorsalis motorius nervi vagus, formatio reticularis, nucleus parabrachialis medialis, nucleus reticularis lateralis and nucleus ventralis raphae. According to the observed anatomical distribution of the immunoreactive structures containing neurotensin or SOM, the peptides could be involved in sleep-waking, nociceptive, gustatory, motor, respiratory and autonomic mechanisms.

Activated somatostatin type 2 receptors traffic in vivo in central neurons from dendrites to the trans Golgi before recycling

Understanding the trafficking of G-protein-coupled receptors (GPCRs) is of particular importance, especially when modifications of the neurochemic environment occur as in pathological or therapeutic circumstances. In the central nervous system, although some GPCRs were reported to internalize in vivo, little is known about their trafficking downstream of the endocytic event. To address this issue, distribution and expression pattern of the major somatostatin receptor subtype, the somatostatin type 2 (sst2), was monitored in the hippocampus using immunofluorescence, autoradiographic and immunogold experiments from 10 minutes to 7 days after in vivo injection of the receptor agonist octreotide. We then analyzed whether postendocytic trafficking of the receptor was dependent upon integrity of the microtubule network using colchicine-injected animals. Together, our results suggest that upon agonist stimulation, dendritic receptors are retrogradely transported through a microtubule-dependent mechanism to a trans Golgi domain enriched in the t-SNARE syntaxin 6 and trans Golgi network 38 proteins, before recycling. Because we show that the exit rate from the trans Golgi apparatus back to the plasma membrane (hours) is slower than the entry rate (minutes), the neuronal postendocytic trafficking of sst2 receptor is likely to have functional consequences in several neurological diseases in which an increase in somatostatin release occurs.

Somatostatin receptor type 2 undergoes plastic changes in the human epileptic dentate gyrus

Temporal lobe epilepsy (TLE) is characterized by hippocampal sclerosis together with profound losses and phenotypic changes of different classes of interneurons, including those expressing somatostatin (SRIF). To understand the functional significance of the plasticity of SRIF transmission in TLE, unraveling the status of SRIF receptors is, however, a prerequisite. To address this issue, we characterized expression and distribution of the major SRIF receptor, the sst2 subtype, in hippocampal tissue resected in patients with TLE using complementary neuroanatomic approaches. In patients with hippocampal sclerosis, the number of cells expressing sst2 receptor mRNA as well as sst2 receptor-binding sites and immunoreactivity decreased significantly in the CA1-3, reflecting neuronal loss. By contrast, in the dentate gyrus, sst2 receptor mRNA expression was strongly increased in the granule cell layer, and sst2 receptor-binding sites and immunoreactivity was preserved in the inner but decreased significantly in the outer molecular layer. In this latter region, pronounced changes in SRIF terminal fields were observed. Decreased receptor density in the distal dendrites of granule cells is likely to reflect downregulation of sst2 receptors in response to physiopathologic release of SRIF. Because sst2 receptors have anticonvulsant and antiepileptogenic properties, this phenomenon may contribute to the etiology of TLE seizures.

Somatostatin receptors 2 and 5 are the major somatostatin receptors in insulinomas: an in vivo and in vitro study

Somatostatin (SRIF) receptors (sst) are present on normal pancreatic endocrine beta-cells. However, the use of SRIF analogs in the scintigraphic imaging of insulinomas and in the medical management of these tumors seems to be restricted to a subgroup of patients. The aim of this study was to determine the prevalence of sst expression in vitro and characterize sst subtype binding in insulinomas and its correlation with in vivo sst receptor scintigraphy (SRS). In vitro studies were performed on 27 insulinomas from 25 patients: 22 with benign and three with malignant tumors. Semiquantitative RT-PCR of sst mRNAs was performed for 20 of these insulinomas. Sst2 and sst5 were expressed in 70%, sst1 in 50%, and sst3 and sst4 subtypes only in 15-20% of the tumors. (125)I-Tyr(0)DTrp(8)SRIF(14) binding was assessed by quantitative autoradiography in 18 insulinomas, and competition experiments were performed with SRIF(14) and L797-591, L779-976, L796-778, L803-087, L817-818, selective agonists of the five sst subtypes, and BIM23244, a selective agonist of sst2 and sst5. Significant specific binding was observed in 72% of the insulinomas. Displacement experiments with ligands of higher affinity for each of the sst receptors revealed significant binding with the sst2 and sst5 ligands in 72%, sst3 in 44%, sst1 in 44%, and sst4 in 28% of cases. All insulinomas displaying sst2 binding were also sst5 sensitive. However, the ratio of sst5/sst2 displacement was variable and only equal to that for SRIF(14) in experiments with the sst2/sst5 agonist BIM23244. SRS was performed 10 times in nine patients; it detected 60% of the tumors, including metastases of a malignant insulinoma. All the tumors detected by SRS displayed high levels of (125)I-Tyr(0)DTrp(8)SRIF(14) binding. The mechanisms underlying the loss of expression of sst2/sst5 in a third of insulinomas remains to be determined, but this loss of expression may be involved in beta-cell dysfunction.

Immunohistochemical localization of the somatostatin sst2(b) receptor splice variant in the rat central nervous system

Somatostatin is a neuromodulator in the mammalian CNS. To date, genes for at least five different somatotrophin release inhibiting factor receptors, termed sst1-sst5, have been cloned. The rat sst2 receptor exists in two splice variants, sst(alpha)a) and sst2(b), which differ in their carboxy-termini. When heterologously expressed in Chinese hamster ovary-K1 cells, these splice variants show little difference in their operational characteristics. Recently, the distribution of the sst2(a) receptor was documented, yet at present no data are available about the distribution of the sst2(b) receptor in the CNS. Here, we present the characterization of a novel polyclonal anti-peptide antibody that is selective for the sst2(b) receptor splice variant. The antibody was raised against the unique intracellular carboxy-terminal portion of the receptor protein. Using this affinity-purified antibody in western blotting experiments, the sst2(b) receptor expressed in Chinese hamster ovary-K1 cells was shown to be a glycoprotein with a molecular weight centred at about 85,000. The antibody showed no cross-reactivity to any of the recombinant human sst1-5 receptors, the rat sst2(a) receptor or wild-type Chinese hamster ovary-K1 cells. Employing immunohistochemistry, we investigated the distribution of the sst2(b) receptor in the brain and spinal cord of adult rats. A distinct distribution was found throughout the rostrocaudal axis of the CNS. Somatodendritic as well as axonal staining was observed. Somatodendritic labelling was particularly obvious in the olfactory bulb, cerebral cortex, hippocampal formation, mesencephalic trigeminal nucleus and cerebellum, as well as in cranial and spinal motor areas. The results show that the distribution of the sst2(b) receptor partially overlaps with that of the sst2(b) receptor, although there were differences in a number of brain areas. The location of the sst2(b) receptor implies that it may mediate a modulatory role of somatostatin inhibitory releasing factor on sensory as well as motor functions.

Interrelationships between somatostatin sst2A receptors and somatostatin-containing axons in rat brain: evidence for regulation of cell surface receptors by endogenous somatostatin

Using an antipeptide antibody, we reported previously on the distribution of the somatostatin sst2A receptor subtype in rat brain. Depending on the region, immunolabeled receptors were either confined to neuronal perikarya and dendrites or distributed diffusely in tissue. To investigate the functional significance of these distribution patterns, we examined the regional and cellular relationships between somatostatin axons and sst2A receptors in the rat CNS, using double-labeling immunocytochemistry. Light and confocal microscopy revealed a significant correlation (p < 0.02) between the distribution of somatodendritic sst2A receptor immunoreactivity and that of somatostatin terminal fields, both quantitatively and qualitatively. Furthermore, in regions of somatodendritic labeling, a subpopulation of sst2A-immunoreactive cells was also immunopositive for somatostatin, suggesting that a subset of sst2A receptors consists of autoreceptors. By contrast, in regions displaying diffuse sst2A labeling only moderate to low densities of somatostatin terminals were observed, and no significant relationship was found between terminal density and receptor immunoreactivity. At the electron microscopic level, areas expressing somatodendritic sst2A labeling were found by immunogold cytochemistry to display low proportions of membrane-associated, as compared with intracellular, receptors. Conversely, in regions displaying diffuse sst2A receptor labeling, receptors were predominantly associated with neuronal plasma membranes, a finding consistent with the high density of sst2 binding sites previously visualized in these areas by autoradiography. Double-labeling studies demonstrated that in the former but not in the latter regions, sst2A-immunoreactive somata and dendrites were heavily contacted by somatostatin axon terminals. Taken together, these results suggest that the low incidence of membrane-associated receptors observed in regions of somatodendritic sst2A labeling may be caused by downregulation of cell surface receptors by endogenous somatostatin, possibly through ligand-induced receptor internalization.

Complementarity of radioautographic and immunohistochemical techniques for localizing neuroreceptors at the light and electron microscopy level

To assess relationships between neuropeptide-binding sites and receptor proteins in rat brain, the distribution of radioautographically labeled somatostatin and neurotensin-binding sites was compared to that of immunolabeled sst2A and NTRH receptor subtypes, respectively. By light microscopy, immunoreactive sst2A receptors were either confined to neuronal perikarya and dendrites or diffusely distributed in tissue. By electron microscopy, areas expressing somatodendritic sst2A receptors displayed only low proportions of membrane-associated, as compared to intracellular, receptors. Conversely, regions displaying diffuse sst2A labeling exhibited higher proportions of membrane-associated than intracellular receptors. Furthermore, the former showed only low levels of radioautographically labeled somatostatin-binding sites whereas the latter contained high densities of somatostatin-binding suggesting that membrane-associated receptors are preferentially recognized by the radioligand. In the case of NTRH receptors, there was a close correspondence between the light microscopic distribution of NTRH immunoreactivity and that of labeled neurotensin-binding sites. Within the substantia nigra, the bulk of immuno- and autoradiographically labeled receptors were associated with the cell bodies and dendrites of presumptive DA neurons. By electron microscopy, both markers were detected inside as well as on the surface of labeled neurons. At the level of the plasma membrane, their distribution was highly correlated and characterized by a lack of enrichment at the level of synaptic junctions and by a homogeneous distribution along the remaining neuronal surface, in conformity with the hypothesis of an extra-synaptic action of this neuropeptide. Inside labeled dendrites, there was a proportionally higher content of immunoreactive than radiolabeled receptors. Some of the immunolabeled receptors not recognized by the radioligand were found in endosome-like organelles suggesting that, as in the case of sst2A receptors, they may have undergone endocytosis subsequent to binding to the endogenous peptide.

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.

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.

Localization of the somatostatin receptor SST2A in rat brain using a specific anti-peptide antibody

Biological actions of somatostatin are exerted via a family of receptors, for which five genes recently have been cloned. However, none of these receptor proteins has been visualized yet in the brain. In the present-study, the regional and cellular distribution of the somatostatin sst2A receptor was investigated via immunocytochemistry in the rat central nervous system by using an antibody generated against a unique sequence of the receptor protein. Specificity of the antiserum was demonstrated by immunoblot and immunocytochemistry on rat brain membranes and/or on cells transfected with cDNA encoding the different sst receptor subtypes. In rat brain sections, sst2A receptor immunoreactivity was concentrated either in perikarya and dendrites or in axon terminals distributed throughout the neuropil. Somatodendritic labeling was most prominent in the olfactory tubercle, layers II-III of the cerebral cortex, nucleus accumbens, pyramidal cells of CA1-CA2 subfields of the hippocampus, central and cortical amygdaloid nuclei, and locus coeruleus. Labeled terminals were detected mainly in the endopiriform nucleus, deep layers of the cortex, claustrum, substantia innominata, subiculum, basolateral amygdala, medial habenula, and periaqueductal gray. Electron microscopy confirmed the association of sst2A receptors with perikarya and dendrites in the former regions and with axon terminals in the latter. These results provide the first characterization of the cellular distribution of a somatostatin receptor in mammalian brain. The widespread distribution of the sst2A receptor in cerebral cortex and limbic structures suggests that it is involved in the transduction of both pre- and postsynaptic effects of somatostatin on cognition, learning, and memory.

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.

Operational characteristics of somatostatin receptors mediating inhibitory actions on rat locus coeruleus neurones

1. In order to characterize somatostatin (SRIF) receptor inhibiting spontaneous firing of rat locus coeruleus neurones, and their transduction mechanism(S), extracellular recordings were obtained from a pontine slice preparation of rat brain containing the locus coeruleus (LC). LC neurones were identified by electrophysiological and pharmacological properties; spontaneous firing (characteristically 0.5-5 Hz) was reversibly and concentration-dependently inhibited by exogenously applied noradrenaline. 2. Spontaneous firing of LC neurones was reversibly and concentration-dependently inhibited by SRIF and the N-terminally extended form, somatostatin-28 (SRIF-28), with EC50 values of 15.1 and 19.4 nM, respectively. The synthetic SRIF analogues (octreotide, MK-678, BIM-23027 and L-362,855) also caused concentration-dependent inhibition of LC neurone firing with a rank order of agonist potencies compatible with actions at a receptor resembling the recombinant sst2 receptor. The putative sst3 selective agonist, BIM-23056, was without agonist or antagonist effect. 3. Addition of 100 nM desipramine significantly increased the efficacy of exogenously applied noradrenaline (EC50 values, 2.96 and 0.13 microM, absence and presence of desipramine, respectively) but did not significantly affect SRIF-induced inhibition (EC50 values, 15.6 and 8.0 nM, respectively). Furthermore, application of phenoxybenzamine (3 microM) abolished responses to NA, but did not affect responses to SRIF (EC50 = 14.1 nM). 4. Application of the cyclic AMP analogue, 8-bromoadenosine-cyclic monophosphate (8-Br-cyclic AMP; 500 microM), significantly increased the spontaneous firing rate of all neurones tested (223 +/- 24% over basal rate). Concentration-effect curves for SRIF constructed in the absence and presence of 8-Br-cyclic AMP had similar threshold concentrations, maxima and EC50 values. 5. Incubation of pontine slices in a modified artificial CSF containing 500 ng ml-1 pertussis toxin (PTX) for 18 h prior to extracellular recording affected neither the spontaneous firing of LC neurones, nor the inhibitory responses to muscimol (EC50 2.2 and 1.2 microM, absence and presence of PTX). However, inhibitory responses to SRIF were markedly attenuated. 6. We conclude that the inhibitory actions of SRIF on spontaneous firing of LC neurones are mediated directly by activation of somatodendritic SRIF receptors, and not indirectly by release of noradrenaline. The SRIF receptors involved appear to couple via a pertussis toxin sensitive G-protein, and elicit their response by a mechanism apparently independent of inhibition of cyclic AMP formation. The agonist profile of several selective and novel SRIF analogues suggests the identity of this receptor to be similar to the recombinant sst2 receptor.

Effect of phenylephrine and prazosin on the somatostatinergic system in the rat frontoparietal cortex

Somatostatin (SS) and noradrenaline (NA) are distributed in the rat cerebral cortex, and seizure activity is one of the aspects of behavior affected by both neurotransmitters. Due to the possible interaction between both neurotransmitter systems, we studied whether phenylphrine, an alpha 1-adrenoceptor agonist, and prazosin, an alpha 1-adrenoceptor antagonist, can modulate SS-like immunoreactivity (SS-LI) levels, binding of [125I][Tyr11]SS to its specific receptors, the ability of SS to inhibit adenylate cyclase (AC) activity, and the guanine nucleotide binding regulatory protein G, and G., in the Sprague-Dawley rat frontoparietal cortex. An IP dose of 2 or 4 mg/kg of phenylephrine injected 7 h before decapitation decreased the number of SS receptors and increased the apparent affinity in frontoparietal cortex membranes. An IP dose of 20 or 25 mg/kg of prazosin administered 8 h before decapitation increased the number of SS receptors and decreased their apparent affinity. The administration of prazosin before the phenylephrine injection prevented the phenylephrine-induced changes in SS binding. The addition of phenylephrine and/or prazosin 10(-5) M to the incubation medium changed neither the number nor the affinity of the SS receptors in the frontoparietal cortex membranes. Phenylephrine or prazosin affected neither SS-LI content nor the basal or forskolin (FK)-stimulated AC activities in the frontoparietal cortex. In addition, SS caused an equal inhibition of AC activity in frontoparietal cortex membranes of phenylephrine-and prazosintreated rats compared with the respective control group. Finally, phenylephrine and prazosin did not vary the pertussis toxin (PTX)-catalyzed ADP ribosylation of Gi- and/or Go-proteins. These results suggest that the above-mentioned changes are related to the phenylephrine activation of alpha 1-adrenoceptors or to the blocking of these receptors by prazosin. In addition, these data provide further support for a functional interrelationship between the alpha 1-adrenergic and somatostatinergic systems in the rat frontoparietal cortex.

Oxytocin and vasopressin binding sites in the hypothalamus of the rat: histoautoradiographic detection

Localization of oxytocin and vasopressin binding sites has so far been studied in the rat brain by means of film autoradiographs. The availability of selective iodinated ligands with high specific activity allowed us to develop the histoautoradiographical technique and to reinvestigate at the microscopic scale the distribution of these sites in the hypothalamus. Most oxytocin binding sites were localized in delimited nuclei, e.g., the medial preoptic, the ventromedial, the ventral premammillary, the supramammillary, and the medial mammillary nuclei. In addition, a weak diffuse specific labeling occurred in the medial preoptic and the anterior hypothalamic areas. The vasopressin binding sites (of the V1a type) were detected in delimited nuclei, e.g., the suprachiasmatic, the stigmoid, and the arcuate nuclei, but they were also diffusely distributed in the lateral hypothalamic and the dorsochiasmatic areas. The locations of neurohypophysial peptides binding sites detected by light microscopy are compared with those previously obtained by film autoradiography.

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.

Immunocytochemical and in vitro autoradiographic evidence for a direct somatostatinergic modulation of the enkephalinergic hypothalamoseptal tract of the guinea-pig

The present study was undertaken to determine whether the enkephalinergic hypothalamoseptal tract originating in the magnocellular dorsal nucleus in the guinea-pig brain is under the influence of somatostatin. In the first step, double immunocytochemical labeling of enkephalinergic cells and somatostatinergic fibers was combined at the light and electron microscopic levels in the magnocellular dorsal nucleus. As a second step, an in vitro radioautography was used to determine whether somatostatin receptors are present in the same area. A close relationship between somatostatin nerve endings and enkephalin perikarya was observed at both the light and electron microscopic levels. Contracts were more numerous in the ventral part of the magnocellular dorsal nucleus. Whenever synaptic images were clearly observable, they appeared symmetrical. In the same area, a moderate concentration of G-protein-coupled somatostatin binding sites was also visualized. These results suggest that somatostatin has a regulator role on the enkephalinergic hypothalamoseptal tract, directly at the level of the magnocellular dorsal nucleus.

Histoautoradiographic detection of oxytocin- and vasopressin-binding sites in the telencephalon of the rat

Localization of oxytocin- and vasopressin-binding sites has so far been studied in the rat brain by means of film autoradiographs. The disposal of iodinated ligands with high specificity has allowed us to develop histoautoradiography on emulsion-coated sections and to reinvestigate on a microscopic scale the distribution of these sites in the telencephalon (septum, striatopallidal system, amygdala and hippocampus). This technique showed that oxytocin and vasopressin labelling presented distinct distributions and coincided with delimited zones, corresponding to anatomical subdivisions defined on cytoarchitectural and immunocytochemical bases. Vasopressin sites were seen in the dorsal and intermediate parts of the lateral septum and the juxtacapsular nucleus of the bed nucleus of the stria terminalis. Oxytocin sites were located in the ventral and intermediate parts of the lateral septum, the oval and the principal nuclei of the bed nucleus of the stria terminalis and the septofimbrial nucleus. In the striatopallidal system, vasopressin sites were found in the accumbens nucleus and the fundus striati, whereas oxytocin sites were in the accumbens nucleus, the head, and the posterolateral parts of the caudate-putamen, the striatal cell bridges, and the olfactory tubercle. In the amygdala, vasopressin sites were not found, but oxytocin sites were located in the central, medial, and basomedial nuclei. In the hippocampus, vasopressin sites were located in the dentate gyrus (polymorph and molecular layers), and oxytocin sites, in the subiculum (molecular and pyramidal layers) and in the field CA1 of Ammon's horn (lacunosum moleculare and pyramidal layers). The localization of the binding sites at the microscopic level permitted us to reinvestigate whether or not correlation existed in a same area between innervation, electrophysiological effects, and presence of binding sites.

Modulation by isoproterenol and propranolol of somatostatin receptors in synaptosomes from rat frontoparietal cortex

DL-Propranolol (PRO), a beta-adrenergic blocking agent, and the neuropeptide somatostatin (SS) have central nervous system depressant and anticonvulsive properties. To investigate a possible relationship between these two components, we studied the influence of PRO and DL-isoproterenol (ISO), a beta-adrenergic agonist, on the somatostatinergic system in the rat frontoparietal cortex. The short- (5 h) and long-term (14 days) administration of ISO (5 mg/kg, intraperitoneally (i.p.)), or of PRO (10 mg/kg, i.p.) did not affect somatostatin-like immunoreactivity (SLI) content in the frontoparietal cortex of male Wistar rats. Both short- and long-term ISO administration decreased the number of specific [125I]Tyr11-SS receptors in synaptosomes from frontoparietal cortex (31%, P < 0.05, and 26%, P < 0.02, after short- and long-term administration, respectively) without changing the affinity constant. This decrease in the number of [125I]Tyr11-SS receptors was not due to a direct effect of ISO on these receptors since no decrease in binding was produced by high concentrations of ISO (10(-5) M) when added in vitro. This decrease could be blocked by pretreatment with PRO. Short- and long-term administration of PRO alone produced an increase in the [125I]Tyr11-SS binding in frontoparietal cortex (26%, P < 0.02, and 40%, P < 0.001, after short- or long-term administration, respectively) without changing the affinity constant.

Chronic growth hormone (GH) hypersecretion induces reciprocal and reversible changes in mRNA levels from hypothalamic GH-releasing hormone and somatostatin neurons in the rat

Effects of growth hormone (GH) hypersecretion on somatostatin-(SRIH) and GH-releasing hormone (GHRH) were studied by in situ hybridization and receptor autoradiography in rats bearing a GH-secreting tumor. 6 and 18 wk after tumor induction, animals displayed a sharp increase in body weight and GH plasma levels; pituitary GH content was reduced by 47 and 55%, while that of prolactin and thyrotropin was unchanged. At 18 wk, hypothalamic GHRH and SRIH levels had fallen by 84 and 52%, respectively. In parallel, the density of GHRH mRNA per arcuate neuron was reduced by 52 and 50% at 6 and 18 wk, while SRIH mRNA levels increased by 71 and 83% in the periventricular nucleus (with no alteration in the hilus of the dentate gyrus). The numbers of GHRH- and SRIH-synthetizing neurons in the hypothalamus were not altered in GH-hypersecreting rats. Resection of the tumor restored hypothalamic GHRH and SRIH mRNAs to control levels. GH hypersecretion did not modify 125I-SRIH binding sites on GHRH neurons. Thus, chronic GH hypersecretion affects the expression of the genes encoding for GHRH and SRIH. The effect is long lasting, not desensitizable and reversible.