Somatostatin: electron microscope immunohistochemical localization in secretory neurons of rat hypothalamus

Somatostatin and somatostatin receptors: implications for neoplastic growth and cancer biology

Somatostatin agonists (SM-As) are capable of achieving durable symptomatic relief and significant clinical responses in certain tumours. Herein, we review the diverse direct and indirect mechanisms of antineoplastic activity elicited by SM-As as well as the hurdles that complicate their use as monotherapies in a broader range of malignancies. Emphasis is placed on recent clinical attempts to neutralise the IGF-mediated survival factor effects in the bone metastasis microenvironment in advanced prostate cancer. The first clinical trials of this 'anti-survival factor manipulation' strategy utilised the ability of SM-As to suppress the growth hormone-dependent liver-derived IGF-I bioavailability in combination with other drugs, such as dexamethasone, zolendronate and oestrogens, acting systemically and at the bone metastasis microenvironment. These regimens restored androgen ablation responsiveness in stage D3 prostate cancer patients and successfully produced objective clinical responses while only mild toxicities were observed. Furthermore, we focus on the preclinical experimental data of a targeted SM-A coupled to the super-potent doxorubicin derivative AN-201. The resulting conjugate (AN-238) has shown increased antitumour potency with a favourable toxicity profile. The potential use of novel SM-As as anticancer drugs is discussed in relation to data suggesting other direct and indirect treatment approaches pertaining to the somatostatin system.

111Indium Pentetreotide Imaging in the Evaluation of Head and Neck Tumors

OBJECTIVES/HYPOTHESIS

Peptide receptor imaging with Indium pentetreotide is useful in the diagnosis of diffuse neuroendocrine system tumors (DNEST) of the head and neck. Uses of Indium pentetreotide scintigraphy include tumor and metastases detection, familial tumor screening, and surveillance for recurrence. Using target to background ratios (TBR) could generate a comparative scale for these tumors.

STUDY DESIGN

A retrospective study evaluated the size, TBR, conventional imaging, and outcomes of patients imaged with Indium pentetreotide scintigraphy for suspected head and neck DNEST.

METHODS

Patients with head and neck tumors imaged by Indium pentetreotide scintigraphy during a nine-year period were reviewed. Data analyzed were age, sex, scintigraphy, pathology, and conventional radiology. Tumor data included dimension, multiplicity, metastases, and tumor and brain counts.

RESULTS

Fifty-three patients underwent 58 scans. The sensitivity and specificity were 93% and 92%. Several different DNEST were successfully evaluated, including familial paragangliomas and multiple paragangliomas. TBRs were variable depending on type of DNEST.

CONCLUSIONS

Indium pentetreotide scintigraphy is accurate in determining the presence of paragangliomas, carcinoid tumors, esthesioneuroblastomas, small cell neuroendocrine tumors, andmetastases. It is an excellent surveillance tool. Screening patients for familial paragangliomas can be accomplished. No reliable comparative scale to distinguish amongst the various DNEST could be developed using TBR.

Somatostatin and somatostatin receptor physiology

Since the discovery of somatostatin (SST) over three decades ago, its ubiquitous distribution and manifold functions are still being documented. SST is synthesized in the hypothalamus and transported to the anterior pituitary gland where it tonicaly inhibits GH and TSH secretion as well as being responsible for GH pulsatile release. Several internal feedback loops, sleep, exercise, and chemical agents control and influence SST release. SST also impacts the function of a wide variety of cells and organ systems throughout the body. Knowledge of the structures of the SSTs has resulted in recognition of the essential four core conserved residues responsible for their actions. The SSTs act through six separate SST cell surface receptors (SSTRs), members of the family of G protein-coupled receptors. Receptor ligand binding (SST/SSTR) results in cellular activities specific for each receptor, or receptor combinations, and their tissue/cell localization. Understanding the structure/function relationship of the SSTs and their receptors, including the internalization of SST/SSTR complexes, has facilitated the development of a variety of novel pharmacologic agents for the diagnosis and treatment of neuroendocrine tumors and unfolding new applications.

Androgen receptor immunoreactivity in somatostatin neurons of the periventricular nucleus but not in the bed nucleus of the stria terminalis in male rats

Somatostatin (SS) is a neuropeptide responsible for the inhibition of growth hormone (GH) release from the anterior pituitary. The existence of sexual dimorphism in the pattern of GH secretion in several mammalian species strongly suggests a role of sex steroids in the regulation of GH release. It has been recently demonstrated that SS mRNA levels are decreased by gonadectomy in male rats while administration of testosterone reversed the postgonadectomy decrease in hypothalamic SS mRNA levels. The mechanism(s) by which androgens regulate somatostatin neurons are unknown at this time. Both direct and indirect pathways have been suggested. In this study, double-labeling immunocytochemistry was used to address the question of whether SS neurons have androgen receptors in adult male rats. About 70-75% of SS-immunoreactive (-ir) neurons in the periventricular hypothalamic nucleus possess androgen receptors and this co-localization persists throughout the rostrocaudal extent of this nucleus. In the bed nucleus of the stria terminalis, no SS-ir neurons were shown to have androgen receptors. These results suggest that androgens may directly exert their effect upon SS neurons in the periventricular nucleus.

Afferent and efferent connections of the habenula in the larval sea lamprey (Petromyzon marinus L.): an experimental study

The habenula is an integrative center between the striatum and the limbic and motor systems. With the aim of achieving further understanding of the evolution of this structure in vertebrates, we carried out an experimental study of the afferent and efferent connections of the habenula of larval sea lamprey. Experimental procedures included in vivo and in vitro transport after injections of horseradish peroxidase (HRP) into the habenula, telencephalon, pineal organ, dorsal thalamus, and posterior tubercle as well as carbocyanine dye tracing (DiI). The combined results of these experiments showed that the pattern of habenular connections is very simple. Most afferents appear to originate from the lobus subhippocampalis and neighboring area, whereas the only efferents found coursed in the fasciculus retroflexus to the neuropil of the nucleus interpeduncularis. This neuropil comprises a commissural region in the rostral mesencephalon, two long bilateral areas extending in the basal mesencephalon and medulla oblongata to the trigeminal level, and, finally, a caudal commissural zone. The conspicuous habenular commissure contains interhemispheric fibers that appear to form occasional contacts within the habenulae. The lamprey habenula also receives a few immunocytochemically identified fibers (somatostatinergic, catecholaminergic, and serotoninergic fibers) from other sources.

Neuroanatomical connections between corticotropin-releasing factor (CRF) and somatostatin (SRIF) nerve endings and thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of rat hypothalamus

In order to investigate the possible involvement of corticotropin-releasing factor (CRF) and somatostatin (SRIF) on thyrotropin-releasing hormone (TRH) neuronal cell activity in the rat hypothalamic paraventricular nucleus, we have proceeded to the simultaneous localization of CRF or SRIF and TRH. For this purpose, we used a dual immunostaining procedure that employed antibodies to CRF and SRIF and peroxidase-labeled goat anti-rabbit IgG as a first sequence, and antibodies to a cryptic fragment (Phe178-Glu199) of pro-TRH (to label TRH neurons) and alkaline phosphatase-labeled goat anti-rabbit IgG as the second sequence. A rich innervation of the paraventricular nucleus by immunoreactive CRF and SRIF fibers was observed. A large number of CRF and SRIF nerve endings were seen intimate anatomic proximity and often appeared to surround TRH-containing cell bodies. These results strongly suggest that TRH neurons might be regulated by both CRF and SRIF. These interactions might be the neuroanatomical basis for the already observed inhibitory effects of CRF and SRIF on TRH release.

Ultrastructural localization of neuropeptide FF, a new neuropeptide in the brain and pituitary of rats

The octapeptide FLFQPQRF-NH2 or neuropeptide FF ('F8Famide'; FMRFamide-like peptide'; 'morphine-modulating peptide') has been isolated from the bovine brain. In this study, the ultrastructural localization of neuropeptide FF-like immunoreactivity was examined with pre-embedding immuno-electron microscopy in the nucleus of the solitary tract and in the posterior lobe of the pituitary gland of an adult rat. Neuropeptide FF-like immunoreactivity was detected only in neuronal structures of the medial and commissural nuclei of the solitary tract and in the neurohypophysis. In the medulla, the peroxidase-antiperoxidase reaction product was localized in large (100 nm) dense-cored vesicles and in the cytoplasm of the neuronal perikarya, dendrites and axon terminals. In the labeled terminals, small (50 nm) clear vesicles rimmed with the peroxidase-antiperoxidase reaction product were seen. Synaptic contacts of labeled perikarya and dendrites with unlabeled axon terminals were observed. Labeled axon terminals formed contacts with unlabeled dendrites and perikarya. In the posterior lobe of the pituitary gland, neuropeptide FF-like immunoreactivity was localized in nerve terminals frequently associated with blood vessels. The results suggest that neuropeptide FF-like peptides are localized exclusively in neuronal structures and that they are synthesized in cell somata and released from axon terminals. In the brain, neuropeptide FF-like peptides may act as neuromodulators involved in the regulation of autonomic functions. The localization of neuropeptide FF-like immunoreactivity in the neurohypophysis suggests endocrine regulatory functions of these peptides.

Ultrastructural localization of somatostatin-like immunoreactivity in the rat dentate gyrus

Neurons containing somatostatin (SOM) are enriched in the dentate gyrus. We sought to establish the ultrastructural localization of this peptide in the dentate gyrus of the rat brain with a double-bridged peroxidase-antiperoxidase (PAP) method localizing antisera directed against somatostatin (SOM)-28 and SOM-28. Initial light microscopic observations confirmed that the majority of perikarya and thick varicose processes with intense SOM-like immunoreactivity (SOM-LI) were observed in the hilus. Fine varicose processes with SOM-LI were found throughout all layers of the dentate gyrus but were most intense in the outer third of the molecular layer (ML), where an occasional perikaryon with SOM-LI was seen. By electron microscopy, SOM-LI was found in neuronal perikarya, dendrites, axons, and axon terminals. Two types of SOM-containing perikarya were observed. The first type was small (6-10 microns), round or avoid, and had a labeled cytoplasma with abundant Golgi complexes and a dense accumulation of PAP-reaction product. The second type of perikarya was larger (11-16 microns) and had a more abundant cytoplasm than the first type, but the Golgi complexes did not appear labeled. Most (96% of 374) of the synapses on the SOM-labeled perikarya and dendrites were from terminals without SOM-LI which formed nearly equal proportions of asymmetric and symmetric junctions. The remainder of the presynaptic terminals contained SOM-LI and made primarily symmetric synapses. Synaptic junctions from both unlabeled and labeled terminals were primarily on the shafts of the small (0.5-1.5 microns) SOM-immunoreactive dendrites. The terminals with SOM-LI (0.25-1.3 microns) contained many small, clear vesicles and from zero to four large dense-core vesicles. Terminals with SOM-LI were associated 1) with one unlabeled perikaryon or dendrite (49% of 215 in the hilus; 76% of 326 in the ML); 2) with two unlabeled perikarya or dendrites simultaneously (5% hilus; 4% ML); and 3) with one SOM-containing perikaryon or dendrite (6% hilus; 3% ML). In all three types of associations, synaptic contacts on perikarya were few while the majority were with small (distal) dendrites. Moreover, most of the terminals with SOM-LI formed symmetric junctions or lacked membrane specializations but were without any apparent glial intervention in the plane of section analyzed. The remaining SOM-labeled terminals (40% hilus; 17% ML) were without any apparent synaptic relations. However, a few of these terminals were in direct apposition to other terminals, some of which were also SOM-immunoreactive.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide Y-like immunoreactivity in neurons of the solitary tract nuclei: vesicular localization and synaptic input from GABAergic terminals

The ultrastructural localization of neuropeptide Y-like immunoreactivity (NPY-LI) was examined in the medial nuclei of the solitary tracts (mNTS) of adult rat brain. Peroxidase-antiperoxidase (PAP) reaction product was localized extensively to the central lumen of large (100-150 nm), dense-core vesicles. The labeled vesicles were seen in axon terminals of untreated, control animals and in perikarya and dendrites of rats receiving intraventricular injections of colchicine 24 h prior to sacrifice. The labeled terminals were of two types. The first type contained numerous small, clear vesicles that were rimmed with peroxidase product and 1-6 large, dense-core vesicles that were labeled throughout their central lumen. The second type contained a more homogeneous population of labeled large, dense-core vesicles. Axon terminals showing NPY-LI formed either asymmetric synapses with unlabeled dendrites or were without recognized junctions. Within labeled terminals, as well as within perikarya and dendrites, the majority of the dense-core vesicles were located near non-synaptic portions of the plasmalemma that were heavily ensheathed with glial processes. Only a few unlabeled terminals penetrated the glial investments to form synaptic contacts on soma or dendrites containing NPY-LI. These synaptic contacts were of both symmetric and asymmetric types. Combined immunoperoxidase labeling for glutamic acid decarboxylase and immunogold labeling for NPY further established that at least some of the terminals forming symmetric junctions on the NPY-immunoreactive dendrites were GABAergic. These results provide ultrastructural evidence that in the mNTS, NPY-LI is localized principally to large dense-vesicles within neurons whose output is partially regulated by GABA. The preferential distribution of the labeled vesicles along non-synaptic, glial-invested portions of the plasmalemma suggests that neuronal NPY may modulate the activity of nearby astrocytes. Additionally, the localization of NPY-LI in terminals containing a mixed population of synaptic vesicles and forming asymmetric axodendritic junctions suggests that NPY and/or coexisting transmitter may also exert certain known hypotensive effects by excitation of local intrinsic or projection neurons in this brain region.

GABAergic innervation of somatostatin-containing neurosecretory cells of the anterior periventricular hypothalamic area: a light and electron microscopy double immunolabelling study

Double immunolabelling on semithin sections revealed glutamate decarboxylase immunopositive dots surrounding somatostatin-containing cell sections in the rat periventricular hypothalamic area. Up to 12 appositions were observed per cell section with an average number of 2-3 and a unimodal distribution. At the electron microscopical level pre-embedding staining of glutamate decarboxylase showed that most immunoreactive elements consisted of immunolabelled axonal endings. Most of these glutamate decarboxylase immunopositive boutons were found within the neuropil where they frequently made synapses on unidentified dendrites. Some of them were apposed to somatostatin-containing cell bodies that were identified according to the presence of immunolabelled granules using combined immunogold post-embedding staining. In many instances glutamate decarboxylase immunoreactive endings were also found to be involved in synaptic contact with somatostatin-labelled perikarya, or neuronal processes. These contacts provide the morphological basis for a direct GABAergic control of the somatostatin-containing cells regulating the secretion of growth hormone.

Somatostatin in multiple sclerosis

The detection of somatostatin, a 14 aminoacid peptide, in human brain and cerebrospinal fluid (CSF) initiated examinations by radioimmunoassay and immunocytochemical technique to elucidate its origin, localization, function, and possible significance in central nervous system disorders. The present survey deals with these aspects with special reference to multiple sclerosis (MS) and to correlation between disease activity and somatostatin content and variations in CSF.

Neuropeptide Y, enkephalin and noradrenaline coexist in sympathetic neurons innervating the bovine spleen. Biochemical and immunohistochemical evidence

The subcellular distribution of noradrenaline (NA), neuropeptide Y (NPY), Met- and Leu-enkephalin (ENK), substance P (SP), somatostatin (SOM), and vasoactive intestinal polypeptide (VIP) was investigated in homogenates of bovine splenic nerve. The distribution of noradrenergic peptide-containing nerves in the bovine celiac ganglion, splenic nerve and terminal areas in spleen was studied by indirect immunofluorescence histochemistry using antisera to tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), NPY, enkephalin peptides, SP, SOM, VIP, and peptide HI (PHI). After density gradient centrifugation, high levels of NPY- and ENK-like immunoreactivity (LI) were found in high-density gradient fractions, coinciding with the main NA peak. SP, SOM and VIP were found in fractions with a lower density, VIP being also enriched in a heavy fraction; the latter three peptides were present in low concentrations. Immunohistochemistry revealed that staining for NPY-LI and ENK-LI partly overlapped that for TH and DBH in celiac ganglia, splenic nerve axons and terminal areas of spleen. Almost all principal ganglion cells were TH- and DBH-immunoreactive. Many were also NPY-immunoreactive, whereas a smaller number were ENK-positive. In the celiac ganglion patches of dense SP-positive networks and some VIP/PHI- and ENK-immunoreactive fibers were seen around cell bodies. The results indicate that NPY and ENK are stored with NA in large dense-cored vesicles in unmyelinated axons of bovine splenic nerve. SP, SOM and VIP appear in different organelles in axon populations separate from sympathetic noradrenergic nerves.

Electron microscopic immunocytochemical study of somatostatin neurons in the periventricular nucleus of the rat hypothalamus with special reference to their relationships with homologous neuronal processes

The neurons containing somatostatin in the rat periventricular nucleus were studied by using a modified electron microscopic immunocytochemical technique that improves both the penetration of immunoreagents into unembedded immunostained tissues and the preservation of ultrastructural morphology. Inside perikarya and dendrites, immunostaining was not only associated with neurosecretory granules but also with ribosomes and saccules of the cis face of the Golgi apparatus. In the axonal profiles found in this region the labeling was observed both on neurosecretory granule cores and on the limiting membrane of small synaptic-like vesicles. Throughout the periventricular nucleus, both non-synaptic and synaptic relationships were shown between labeled neurons. Non-synaptic relationships mainly consisted of direct apposition of the membranes of neighboring neurons by dendrosomatic, somasomatic or dendrodendritic contacts. These labeled perikarya and dendrites were also synaptically contacted by labeled axonal endings containing numerous aggregated synaptic-like vesicles. The physiological significance of the synaptic and non-synaptic relationships between somatostatinergic neurons is discussed in terms of possible synchronization between homologous neurons of the somatostatin neuroendocrine system and control of these neurons by a central ultra-short loop feedback mechanism.

Ultrastructural morphometric analysis of somatostatin-like immunoreactive neurones in the rat central nervous system after labelling with colloidal gold

Nerve terminals of the rat median eminence, arcuate nucleus and spinal cord were examined in the electron microscope after post-embedding, colloidal gold labelling of immunoreactivity to somatostatin. Strong immunostaining was thus obtained together with adequate morphological preservation. Reactive boutons showed clusters of gold particles essentially confined to dense-cored vesicles. In the median eminence, the positive varicosities made up more than half of all terminals and averaged 735 nm in diameter. Those in the arcuate nucleus and spinal cord were much less numerous and generally smaller (575 nm). The labelled vesicles had mean external diameters of 109, 95 and 79 nm in the median eminence, arcuate nucleus and spinal cord, respectively. Calculations of the likely amounts of somatostatin within the vesicles of the median eminence and arcuate nucleus yielded values of 0.7 and 1.4 mM, corresponding to 190 and 230 molecules of the peptide, respectively. These data support a neurotransmitter or modulator role for somatostatin in the central nervous system.

Immunocytochemical localization of somatostatin-28 in the rat hypothalamus

Somatostatin-28 (SS28) is considered as a precursor of somatostatin-14 (SS14) and also of the remaining NH2 terminus of SS28, the dodecapeptide SS28 which has been recently characterized. In order to study the cellular and subcellular localization of SS28 in the rat hypothalamus, we have conducted a light and electron microscopic immunocytochemical study, using both the peroxidase-antiperoxidase and the immunogold technique. Several antisera which selectively recognize one or more of these 3 somatostatin-related peptides were used. In serial paraffin sections, it was observed that SS28 was contained in the same neuronal cell bodies which also contain SS28. These neurons were exclusively located in the periventricular nucleus. All the antisera used also produced a strong staining in the external zone of the median eminence. At the electron microscopic level, the 3 peptides were exclusively localized in all the dense core vesicles in cell bodies and axons in the periventricular nucleus and terminals in the median eminence. These results strongly suggest that SS28 and SS14 originate from the precursor SS28 and that processing of the precursor occurs in the cell body. They also support the hypothesis that the 3 peptides are released simultaneously under appropriate stimulation.

Immunohistochemical distribution of somatostatin-like immunoreactivity in the central nervous system of the adult rat

The localization and distribution of somatostatin (growth hormone release-inhibiting hormone; somatotropin release-inhibiting factor) have been studied with the indirect immunofluorescence technique of Coons and collaborators and the immunoperoxidase method of Sternberger and coworkers using specific and well-characterized antibodies to somatostatin, providing semiquantitative, detailed maps of somatostatin-immunoreactive cell profiles and fibers. Our results demonstrate a widespread occurrence of somatostatin-positive nerve cell bodies and fibers throughout the central nervous system of adult, normal or colchicine-treated, albino rats. The somatostatin cell bodies varied in size from below 10 micron up to 40 micron in diameter and could have only a few or multiple processes. Dense populations of cell somata were present in many major areas including neocortex, piriform cortex, hippocampus, amygdaloid complex, nucleus caudatus, nucleus accumbens, anterior periventricular hypothalamic area, ventromedial hypothalamic nucleus, nucleus arcuatus, medial to and within the lateral lemniscus, pontine reticular nuclei, nucleus cochlearis dorsalis and immediately dorsal to the nucleus tractus solitarii. Extensive networks of nerve fibers of varying densities were also found in most areas and nuclei of the central nervous system. Both varicose fibers as well as dot- or "dust-like" structures were seen. Areas with dense or very dense networks included nucleus accumbens, nucleus caudatus, nucleus amygdaloideus centralis, most parts of the hypothalamus, nucleus parabrachialis, nucleus tractus solitarii, nucleus ambiguus, nucleus tractus spinalis nervi trigemini and the dorsal horn of the spinal cord. One exception is the cerebellum which only contained few somatostatin-positive cell bodies and nerve fibers. It should be noted that somatostatin-positive cell bodies and fibers did not always conform to the boundaries of the classical neuroanatomical nuclei, but could often be found in areas between these well-established nuclei or occupying, in varying concentrations, only parts of such nuclei. It was difficult to identify with certainty somatostatin-immunoreactive axons in the animals studied. Some pathways could, however, be demonstrated, but further experimental studies are necessary to elucidate the exact projections of the somatostatin-immunoreactive neurons in the rat central nervous system.

Radioautographic localization of somatostatin-14 and somatostatin-28 binding sites in the rat brain

Somatostatin-14 (S14) and its precursor, somatostatin-28 (S28), are widely distributed throughout the rat brain, suggesting that they could act as neurotransmitter or neuromodulator in the central nervous system. The present study was undertaken to study the localization of S14 and S28 receptors in the rat brain determined by "in vitro" radioautography. The study performed on slide mounted frozen brain section with iodinated S14 and S28 analogs revealed an identical distribution of binding sites for the two forms of somatostatin. A good correlation could be observed between receptor distribution and immunohistologically localized neuropeptides except for striatum and hypothalamus. However, receptors were not detectable in the hypothalamus and were found in low concentration in the caudate-putamen nucleus, two regions containing high amounts of S28 and S14, suggesting a high occupancy of receptors in these areas by endogenous peptides or an inverse correlation between receptor and peptide concentrations.

Synaptic contacts of somatostatin-immunoreactive amacrine cells in goldfish retina

Retinal amacrine cells containing somatostatinlike immunoreactivity (SLI) were labeled by the peroxidase-antiperoxidase technique and their connections were analyzed by light and electron microscopy. The labeled processes were found in two distinct plexuses--one near the most proximal border of the inner plexiform layer and the other near the most distal border. They received most (89%) of their input from amacrine cells and the remainder from bipolar cells. A majority (56%) of their output synapses go to processes of amacrine cells, a substantial proportion (38%) go to ganglion cell dendrites, and the remainder go to bipolar cell axon terminals. The relative frequencies of each of the types of contacts were nearly identical in the distal and proximal plexuses. The cells containing SLI are different in their morphology and synaptic connections from any goldfish amacrine cells containing conventional neurotransmitters, but one type of amacrine cell containing SLI resembles certain other peptidergic amacrine cells in the goldfish retina.

Ontogenetic appearance of somatostatin-containing nerve terminals in the median eminence of rats

The development of immunoreactive (ir) somatostatin-containing nerve terminals in the rat median eminence (ME) has been examined electron-microscopically. Nerve fibers containing ir particles scattered throughout the axoplasm are first seen in the external layer of the ME on day 18.5 of gestation, and, on day 21.5 appear to terminate on the basement membrane of the perivascular space of the portal vessels. After birth, the fiber terminals contain several membrane-limited granules, which are labeled with ir PAP particles. Ultrathin, Epon-embedded sections of ME, treated by the protein A gold-labeling method for somatostatin, demonstrate positively labeled granules in the nerve fibers in the postnatal ME, but in the prenatal tissue, no specific gold-labeling is found. These findings show that, in the external layer of the ME, somatostatin storing occurs in the granules in the axonal terminals after birth.

Growth hormone neuroregulation and the clinical relevance of somatostatin

The hypothalamus controls GH secretion from the anterior pituitary using two peptides; somatostatin inhibits GH, but physiologically the most important appears to GHRF, the structure of which has recently been discovered by two groups. This exciting development has not only given us further insight into the control of GH secretion, but also posed interesting questions as to the cause of the abnormal GH responses to various stimuli seen in patients with acromegaly. The other hypothalamic peptide controlling GH secretion, somatostatin has been the subject of intensive research in the last ten years. It is widely distributed and has important physiological actions including those involved in GH secretion and its action as a hypothalamic hormone. The secretion and synthesis of these two hypothalamic hormones is in turn modulated by a number of neurotransmitters, the most important of which appears to be dopamine. Knowledge gained in these studies has enabled the development of useful tools in the diagnosis of growth hormone deficiency as well as the only effective medical treatment for acromegaly. Much remains to be learnt of the physiology of growth hormone releasing factor and as a result further patients will benefit in the future.

Electronmicroscopic study of somatostatin-containing neurons in rat arcuate nucleus with special reference to neuronal regulation

After an intraventricular administration of colchicine, the arcuate nucleus of rat hypothalamus was examined light and electron microscopically by pre-embedding immunohistochemistry for somatostatin. The arcuate nucleus exhibited numerous immunoreactive cell bodies and dense networks of immunoreactive fibers. The fibers appeared to surround immunonegative cell bodies. The immunoreactive cell bodies were multipolar in shape and projected immunoreactive processes to some extent. The immunoreactive cell bodies and fibers received synaptic contacts by immunonegative fiber terminals containing a large number of synaptic clear vesicles. Similarly, immunoreactive somatostatin fibers appeared to terminate upon other immunonegative cell bodies and fibers. The immunoreactive presynaptic terminals contain several labeled granules and numerous synaptic vesicles. In close proximity to these immunolabeled terminals, non-labeled presynaptic terminals were also observed upon the immunonegative cell bodies and fibers. This suggests that in the arcuate nucleus neurons regulated by somatostatin neurons are also under the control of other types of neurons.

Fine structural studies on a type of somatostatin-immunoreactive neuron and its synaptic connections in the rat neostriatum: a correlated light and electron microscopic study

Somatostatin-immunoreactive neurons in the rat neostriatum were studied by correlated light and electron microscopy using the peroxidase-antiperoxidase immunocytochemical technique. Immunoreactivity was localized in neuronal perikarya and processes. The perikarya were of spindle or fusiform shape (average length 16.9 microns) and were found in all parts of the neostriatum. From each neuron there arose two to four straight immunoreactive dendritelike processes, which could frequently be traced as far as about 130 microns from their perikaryon. Immunoreactive varicose axonlike processes were occasionally found, some of which were proximal axons of identified immunoreactive cells. Nine of the light microscopically identified neurons showing somatostatin-immunoreactivity were studied in the electron microscope; two of them had proximal axons with varicosities. Each neuron had an oval or elongated nucleus, which was always indented. These morphological features correspond well to those of certain "medium-size aspiny" neurons classified by Golgi studies. Although the immunoreactive endproduct was diffusely located throughout the neuron, it was characteristically located in the saccules and large granules (diameter 133 nm) of the Golgi apparatus, and large immunoreactive vesicles of similar size to those in the Golgi apparatus frequently occurred in all parts of axon. Very little synaptic input was found on the perikarya and dendrites of somatostatin-immunoreactive neurons. The perikarya and proximal dendrites received both symmetrical and asymmetrical synaptic input, while the distal dendrites usually received boutons that formed asymmetrical contacts. The somatostatin-immunoreactive boutons contained pleomorphic electron-lucent vesicles (diameter 39.3 nm) and a few large immunoreactive granular vesicles; these boutons always formed symmetrical synapses. Their postsynaptic targets were dendritic shafts, spines, and unclassified dendritic profiles. On the other hand, the varicosities of identified proximal axons of somatostatin-positive neurons did not form typical synapses, since they lacked clusters of small vesicles, but some of them were in direct apposition (via membrane specializations) to unlabelled perikarya or dendrites. It is concluded that somatostatin is a useful marker for a particular type of neuron in the neostriatum. The presence of somatostatin immunoreactivity in synaptic boutons is consistent with the view that somatostatin could be a neurotransmitter in the neostriatum.

Some cellular characteristics of somatostatin neurons and terminals in the periventricular nucleus of the rat hypothalamus and median eminence. Electron microscopic immunohistochemistry

Somatostatin neuronal perikarya and their processes, presumably dendrites, in the periventricular nucleus of the rat hypothalamus and terminals in the median eminence were observed by electron microscopic immunohistochemistry. Neuronal perikarya and processes contained immunoreactive dense granules (100-120 nm in diameter) and other cellular components such as polysomes, rER membranes occasionally showed high electron density. Few axo-somatic terminals were found on the somatostatin neurons, but we could detect a number of preterminal axons on immunoreactive processes, presumably dendrites. Therefore, we considered that somatostatin neurons receive mainly neuronal input through axo-dendritic synapses rather than through axo-somatic ones. In the somatostatin terminals in the external layer of the median eminence immunoreactivity was completely restricted on the granules.

Electron immunocytochemistry in vasoactive intestinal peptide (VIP) in the rat brain

In order to clearly identify the cellular elements as well as the organelles containing vasoactive intestinal peptide (VIP), an immunocytochemical localization of this peptide was performed in the rat brain. Ultrastructural localization studies involving use of both pre- and post-embedding staining techniques clearly demonstrated that VIP immunoreactivity was associated with dendrites and axonal processes. In dendrites, the labeling was rather diffuse throughout the cytoplasm whereas in terminals it was observed in large dense core vesicles and not within the small clear vesicles. About 50% of positive endings were observed forming typical synaptic contact. These results support the hypothesis that VIP can act as a neurotransmitter.

Localization of somatostatin-like immunoreactivity in the Golgi apparatus of central and peripheral neurons

With the indirect immunofluorescence technique of Coons and collaborators somatostatin-like immunoreactivity (SLI) was observed in certain neurons of the central and peripheral nervous system of the rat. In the cell bodies a strong SLI was observed with a distribution resembling that of the Golgi apparatus. In addition a weak SLI was diffusely distributed in the cytoplasm. After photography the sections processed for immunocytochemistry were stained with the thiamine pyrophosphatase technique of Novikoff and Goldfischer. The latter technique is assumed to be a specific marker for the Golgi complex. It was found that the strong SLI and the thiamine pyrophosphatase activity had an identical distribution. Thus, one pool of somatostatin appears to be localized to the Golgi apparatus.

Immunohistochemical localization of somatostatin in the human hypothalamus

In order to identify clearly the nervous structures containing somatostatin in the human hypothalamus, an immunohistochemical localization of this neurohormone was performed at light-microscopic level. Using a antiserum specific to somatostatin and the unlabeled antibody peroxidase-antiperoxidase technique, we have found somatostatin in neurons with cell bodies in an area in the anterior hypothalamus corresponding to the infundibular nucleus. Somatostatin-containing fibers were also detected in the neurovascular zone of the pituitary stalk, suggesting that somatostatin is released in that region to reach the capillaries in the pituitary portal plexus. A large bundle of somatostatin fibers extending from the anterior part of the paraventricular nucleus up to the posterior portion of the mammillary bodies has also been detected. The role of these fibers still remains to be clarified.