Involvement of the Sst1 somatostatin receptor subtype in the intrahypothalamic neuronal network regulating growth hormone secretion: an in vitro and in vivo antisense study

The LIM-homeobox transcription factor Isl1 plays crucial roles in the development of multiple arcuate nucleus neurons

Neurons in the hypothalamic arcuate nucleus relay and translate important cues from the periphery into the central nervous system. However, the gene regulatory program directing their development remains poorly understood. Here, we report that the LIM-homeodomain transcription factor Isl1 is expressed in several subpopulations of developing arcuate neurons and plays crucial roles in their fate specification. Mice with conditional deletion of the Isl1 gene in developing hypothalamus display severe deficits in both feeding and linear growth. Consistent with these results, their arcuate nucleus fails to express key fate markers of Isl1-expressing neurons that regulate feeding and growth. These include the orexigenic neuropeptides AgRP and NPY for specifying AgRP-neurons, the anorexigenic neuropeptide αMSH for POMC-neurons, and two growth-stimulatory peptides, growth hormone-releasing hormone (GHRH) for GHRH-neurons and somatostatin (Sst) for Sst-neurons. Finally, we show that Isl1 directly enhances the expression of AgRP by cooperating with the key orexigenic transcription factors glucocorticoid receptor and brain-specific homeobox factor. Our results identify Isl1 as a crucial transcription factor that plays essential roles in the gene regulatory program directing development of multiple arcuate neuronal subpopulations.

Somatostatin triggers rhythmic electrical firing in hypothalamic GHRH neurons

Hypothalamic growth hormone-releasing hormone (GHRH) neurons orchestrate body growth/maturation and have been implicated in feeding responses and ageing. However, the electrical patterns that dictate GHRH neuron functions have remained elusive. Since the inhibitory neuropeptide somatostatin (SST) is considered to be a primary oscillator of the GH axis, we examined its acute effects on GHRH neurons in brain slices from male and female GHRH-GFP mice. At the cellular level, SST irregularly suppressed GHRH neuron electrical activity, leading to slow oscillations at the population level. This resulted from an initial inhibitory action at the GHRH neuron level via K⁺ channel activation, followed by a delayed, sst1/sst2 receptor-dependent unbalancing of glutamatergic and GABAergic synaptic inputs. The oscillation patterns induced by SST were sexually dimorphic, and could be explained by differential actions of SST on both GABAergic and glutamatergic currents. Thus, a tripartite neuronal circuit involving a fast hyperpolarization and a dual regulation of synaptic inputs appeared sufficient in pacing the activity of the GHRH neuronal population. These “feed-forward loops” may represent basic building blocks involved in the regulation of GHRH release and its downstream sexual specific functions.

Single cell transcriptomics of hypothalamic warm sensitive neurons that control core body temperature and fever response Signaling asymmetry and an extension of chemical neuroanatomy

We report on an 'unbiased' molecular characterization of individual, adult neurons, active in a central, anterior hypothalamic neuronal circuit, by establishing cDNA libraries from each individual, electrophysiologically identified warm sensitive neuron (WSN). The cDNA libraries were analyzed by Affymetrix microarray. The presence and frequency of cDNAs were confirmed and enhanced with Illumina sequencing of each single cell cDNA library. cDNAs encoding the GABA biosynthetic enzyme Gad1 and of adrenomedullin, galanin, prodynorphin, somatostatin, and tachykinin were found in the WSNs. The functional cellular and in vivo studies on dozens of the more than 500 neurotransmitters, hormone receptors and ion channels, whose cDNA was identified and sequence confirmed, suggest little or no discrepancy between the transcriptional and functional data in WSNs; whenever agonists were available for a receptor whose cDNA was identified, a functional response was found. Sequencing single neuron libraries permitted identification of rarely expressed receptors like the insulin receptor, adiponectin receptor 2 and of receptor heterodimers; information that is lost when pooling cells leads to dilution of signals and mixing signals. Despite the common electrophysiological phenotype and uniform Gad1 expression, WSN transcriptomes show heterogeneity, suggesting strong epigenetic influence on the transcriptome. Our study suggests that it is well-worth interrogating the cDNA libraries of single neurons by sequencing and chipping.

Subcellular dynamics of somatostatin receptor subtype 1 in the rat arcuate nucleus: receptor localization and synaptic connectivity vary in parallel with the ultradian rhythm of growth hormone secretion

Growth hormone (GH) secretion in male rats exhibits a 3.3 h ultradian rhythm generated by the reciprocal interaction of GH-releasing hormone (GHRH) and somatostatin (SRIF). SRIF receptor subtypes sst(1) and sst(2) are highly expressed in GHRH neurons of the hypothalamic arcuate nucleus (ARC). We previously demonstrated an ultradian oscillation in binding of SRIF analogs to the ARC in relation to GH peaks and troughs. Here we tested the hypothesis that these ultradian changes in SRIF binding are due to differential plasma membrane targeting of sst(1) receptors in ARC neurons using immunocytochemistry and electron microscopy. We found that 87% of sst(1)-positive ARC neurons also synthesized GHRH. Subcellularly, 80% of sst(1) receptors were located intracellularly and 20% at the plasma membrane regardless of GH status. However, whereas 30% of the cell-surface sst(1) receptors were located perisynaptically or subsynaptically following exposure to high GH secretion, this fraction was increased to 42% following a GH trough period (p = 0.05). Furthermore, the relative abundance of symmetric and asymmetric synapses on sst(1)-positive dendrites also varied significantly, depending on the GH cycle, from approximately equal numbers following GH troughs to 70:30 in favor of symmetric, i.e., inhibitory, inputs after GH peaks (p < 0.02). These findings suggest that postsynaptic localization of sst(1) receptors and synaptic connectivity in the ARC undergo pronounced remodeling in parallel with the GH rhythm. Such synaptic plasticity may be an important mechanism by which sst(1) mediates SRIF's cyclical effects on ARC GHRH neurons to generate the ultradian rhythm of GH secretion.

Novel, potent, and radio-iodinatable somatostatin receptor 1 (sst1) selective analogues

The proposed sst(1) pharmacophore (J. Med. Chem. 2005, 48, 523-533) derived from the NMR structures of a family of mono- and dicyclic undecamers was used to design octa-, hepta-, and hexamers with high affinity and selectivity for the somatostatin sst(1) receptor. These compounds were tested for their in vitro binding properties to all five somatostatin (SRIF) receptors using receptor autoradiography; those with high SRIF receptor subtype 1 (sst(1)) affinity and selectivity were shown to be agonists when tested functionally in a luciferase reporter gene assay. Des-AA(1,4-6,10,12,13)-[DTyr(2),DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (25) was radio-iodinated ((125)I-25) and specifically labeled sst(1)-expressing cells and tissues. 3D NMR structures were calculated for des-AA(1,4-6,10,12,13)-[DPhe(2),DTrp(8),IAmp(9)]-SRIF-Thr-NH(2) (16), des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (23), and des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9),Tyr(11)]-SRIF-NH(2) (27) in DMSO. Though the analogues have the sst(1) pharmacophore residues at the previously determined distances from each other, the positioning of the aromatic residues in 16, 23, and 27 is different from that described earlier, suggesting an induced fit mechanism for sst(1) binding of these novel, less constrained sst(1)-selective family members.

Gender-dependent role of endogenous somatostatin in regulating growth hormone-axis function in mice

It has been previously reported that male and female somatostatin (SST) knockout mice (Sst-/-) release more GH, compared with Sst+/+ mice, due to enhanced GH-secretory vesicle release. Endogenous SST may also regulate GH secretion by directly inhibiting GHRH-stimulated GH gene expression and/or by modulating hypothalamic GHRH input. To begin to explore these possibilities and to learn more about the gender-dependent role of SST in modulating GH-axis function, hypothalamic, pituitary, and liver components of the GH-axis were compared in male and female Sst+/+ and Sst-/- mice. Pituitary mRNA levels for GH and receptors for GHRH and ghrelin were increased in female Sst-/- mice, compared with Sst+/+ controls, and these changes were reflected by an increase in circulating GH and IGF-I. Elevated levels of IGF-I in female Sst-/- mice were associated with elevated hepatic mRNA levels for IGF-I, as well as for GH and prolactin receptors. Consistent with the role of GH/IGF-I in negative feedback regulation of hypothalamic function, GHRH mRNA levels were reduced in female Sst-/- mice, whereas cortistatin (CST) mRNA levels were unaltered. In contrast to the widespread impact of SST loss on GH-axis function in females, only circulating GH, hypothalamic CST, and hepatic prolactin receptor expression were up-regulated in Sst-/- male mice, compared with Sst+/+ controls. These results confirm and extend the sexually dimorphic role of SST on GH-axis regulation, and suggest that CST, a neuropeptide that acts through SST receptors to inhibit GH secretion, may serve a compensatory role in maintaining GH-axis function in Sst-/- male mice.

Flow characterization of a microfluidic device to selectively and reliably apply reagents to a cellular network

A three-dimensional microfluidic device has been successfully fabricated and the flow streams characterized for eventual use in studying communication in an in vitro network of nerve cells. The microfluidic system is composed of two layers of channels: a lower layer for the delivery of pharmacological solutions and an upper layer of channels used to direct the flow of the pharmacological solution streams and perfuse the cells with media and nutrients. Flow profiles have been characterized with computational fluid dynamics simulations, confocal fluorescence microscopy, and carbon-fiber amperometry, which have been used to map changes in flow profiles at different bulk flow rates. Ultimately, the microfluidic system and incorporated cell network will show how networked neurons adapt, compensate, and recover after being exposed to different chemical compounds.

Model-projected mechanistic bases for sex differences in growth hormone regulation in humans

Models of physiological systems facilitate rational experimental design, inference, and prediction. A recent construct of regulated growth hormone (GH) secretion interlinks the actions of GH-releasing hormone (GHRH), somatostatin (SRIF), and GH secretagogues (GHS) with GH feedback in the rat (Farhy LS, Veldhuis JD. Am J Physiol Regul Integr Comp Physiol 288: R1649–R1663, 2005). In contrast, no comparable formalism exists to explicate GH dynamics in any other species. The present analyses explore whether a unifying model structure can represent species- and sex-defined distinctions in the human and rodent. The consensus principle that GHRH and GHS synergize in vivo but not in vitro was explicable by assuming that GHS 1) evokes GHRH release from the brain, 2) opposes inhibition by SRIF both in the hypothalamus and on the pituitary gland, and 3) stimulates pituitary GH release directly and additively with GHRH. The gender-selective principle that GH pulses are larger and more irregular in women than men was conferrable by way of 4) higher GHRH potency and 5) greater GHS efficacy. The overall construct predicts GHRH/GHS synergy in the human only in the presence of SRIF when the brain-pituitary nexus is intact, larger and more irregular GH pulses in women, and observed gender differences in feedback by GH and the single and paired actions of GHRH, GHS, and SRIF. The proposed model platform should enhance the framing and interpretation of novel clinical hypotheses and create a basis for interspecies generalization of GH-axis regulation.

Growth hormone-releasing hormone (GHRH) neurons in the arcuate nucleus (Arc) of the hypothalamus are decreased in transgenic rats whose expression of ghrelin receptor is attenuated: Evidence that ghrelin receptor is involved in the up-regulation of GHRH expression in the arc

GH secretagogue (GHS)/ghrelin stimulates GH secretion by binding mainly to its receptor (GHS-R) on GHRH neurons in the arcuate nucleus (Arc) of the hypothalamus. GHRH, somatostatin, and neuropeptide Y (NPY) in the hypothalamus are involved in the regulatory mechanism of GH secretion. We previously created transgenic (Tg) rats whose GHS-R expression is reduced in the Arc, showing lower body weight and shorter nose-tail length. GH secretion is decreased in female Tg rats. To clarify how GHS-R affects GHRH expression in the Arc, we compared the numbers of GHS-R-positive, GHRH, and NPY neurons between Tg and wild-type rats. Immunohistochemical analysis showed that the numbers of GHS-R-positive neurons, GHRH neurons, and GHS-R-positive GHRH neurons were reduced in Tg rats, whereas the numbers of NPY neurons and GHS-R-positive NPY neurons did not differ between the two groups. The numbers of Fos-positive neurons and Fos-positive GHRH neurons in response to KP-102 were decreased in Tg rats. Competitive RT-PCR analysis of GHRH mRNA expression in the cultured hypothalamic neurons showed that KP-102 increased NPY mRNA expression level and that NPY decreased GHRH mRNA expression level. KP-102 increased GHRH mRNA expression level in the presence of anti-NPY IgG. GH increased somatostatin mRNA expression. Furthermore, GH and somatostatin decreased GHRH mRNA expression, whereas KP-102 showed no significant effect on somatostatin mRNA expression. These results suggest that GHS-R is involved in the up-regulation of GHRH and NPY expression and that NPY, somatostatin, and GH suppress GHRH expression. It is also suggested that the reduction of GHRH neurons of Tg rats is induced by a decrease in GHS-R expression.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

Immunohistochemical Distribution and Subcellular Localization of the Somatostatin Receptor Subtype 1 (sst1) in the Rat Hypothalamus

The aim of the present study was to examine the cellular and sub-cellular distribution of the somatostatin (SRIF) receptor subtype sst1 in the rat hypothalamus. Receptors were immunolabeled using an antibody directed against an antigenic sequence in the N-terminus of the receptor. Immunopositive neuronal cell bodies and dendrites were observed throughout the mediobasal hypothalamus, including the medial preoptic area, paraventricular, periventricular, and arcuate nuclei. Immunoreactive axons and axon terminals were also observed in the median eminence, suggesting that sst1 is also located pre-synaptically. Electron microscopic examination of the arcuate nucleus revealed a predominant association of immunoreactive sst1 with perikarya and dendrites. Most immunoreactive receptors were intracellular and localized to tubulovesicular compartments and organelles such as the Golgi apparatus, but 14% were associated with the plasma membrane. Of the latter, 47% were apposed to abbuting afferent axon terminals and 20% localized immediately adjacent to an active synaptic zone. These results demonstrate a widespread distribution of sst1 receptors in rat hypothalamus. They also show that somatodendritic sst1 receptors in the arcuate nucleus are ideally poised to mediate SRIF's modulation of afferent synaptic inputs, including central SRIF effects on growth hormone-releasing hormone neurons documented in this area.

Somatostatin receptor 1 selective analogues: 2. N(alpha)-Methylated scan

Des-AA(1,2,5)-[d-Trp(8)/d-Nal(8),IAmp(9)]SRIF (AA = amino acid, Nal = 3-(2-naphthyl)-alanine, IAmp = 4-(N-isopropyl)-aminomethylphenylalanine, SRIF = somatostatin), with or without a tyrosine or monoiodotyrosine, were scanned with the introduction of a backbone N-methyl group and tested for binding affinity at the five human somatostatin receptors (sst(1)(-)(5)). N(alpha)-Methylation resulted in loss of sst affinity (2- to >5-fold) when introduced at residues Lys(4) (6), Phe(6) (7), Phe(7) (8), Thr(10) (11), and Phe(11) (12) of the parent compound Des-AA(1,2,5)-[d-Nal(8),IAmp(9)]SRIF (4). N(alpha)-Methylation was tolerated at residues Cys(3) (5), d-Nal(8) (9), Thr(12) (13), and Cys(14) (15) with retention of binding sst affinity and selectivity and resulted in an increase in sst binding affinity at positions IAmp(9) (10) and Ser(13) (14). In these series, the d-Trp(8) substitution versus d-Nal(8) is clearly superior. C-Terminally lysine-extended analogues (21-25) retained sst(1) selectivity and binding affinity when compared to their d-Nal(8)- (4) or d-Trp(8)- (3) containing parent. Des-AA(1,2,5)-[d-Trp(8), (N(alpha)Me)IAmp(9)]SRIF (17), Des-AA(1,2,5)-[d-Trp(8),IAmp(9),(N(alpha)Me)Ser(13)]SRIF (19), Des-AA(1,2,5)-[d-Trp(8),IAmp(9),(N(alpha)Me)Cys(14)]SRIF (20), Des-AA(1,2,5)-[d-Trp(8),(N(alpha)Me)IAmp(9),Tyr(11)]SRIF (34), and Des-AA(1,2,5)-[d-Agl(8)(N(beta)Me,2-naphthoyl),IAmp(9),Tyr(11)]SRIF (42) (Agl = aminoglycine) are sst(1) agonists in their ability to inhibit forskolin-induced cAMP production.

Expression analysis of hypothalamic and pituitary components of the growth hormone axis in fasted and streptozotocin-treated neuropeptide Y (NPY)-intact (NPY+/+) and NPY-knockout (NPY-/-) mice

In the fasted and the streptozotocin (STZ)-induced diabetic male rat, hypothalamic growth hormone (GH)-releasing hormone (GHRH) mRNA levels, and pulsatile GH release are decreased. These changes are believed to be due to a rise in hypothalamic neuropeptide Y (NPY) that inhibits GHRH expression. To directly test if NPY is required for metabolic regulation of hypothalamic neuropeptides important in GH secretion, NPY, GHRH and somatostatin (SRIH) mRNA levels were determined in fasted (48 h) and STZ-treated wild-type (NPY(+/+)) and NPY-knockout (NPY(-/-)) mice by ribonuclease protection assay. In addition, pituitary receptor mRNA levels for GHRH (GHRH-R), ghrelin (GHS-R) and SRIH (sst2) were assessed by RT-PCR. Under fed conditions the GH axis of NPY(+/+) and NPY(-/-) did not differ. In the NPY(+/+) mouse, fasting resulted in a 23% weight loss and >250% increase in NPY mRNA accompanied by a significant reduction in both GHRH and SRIH mRNA. These changes were associated with increases in pituitary expression of GHRH-R and GHS-R and a concomitant suppression of sst2. In the NPY(-/-) mouse, fasting also resulted in a 23% weight loss and comparable changes in GHRH-R and sst2, but failed to alter GHRH, SRIH and GHS-R mRNA levels. Fasting resulted in an overall increase in circulating GH, which reached significance in the fasted NPY(-/-) mouse. Induction of diabetes in NPY(+/+) mice, using a single, high-dose, STZ injection (150 mg/kg), resulted in modest weight loss (5%), and a 158% increase NPY expression which was associated with reciprocal changes in pituitary GHS-R and sst2 expression, similar to that observed in the fasted state, but no change in hypothalamic GHRH or SRIF expression was observed. Induction of diabetes in NPY(+/+) and NPY(-/-) mice, using a multiple, low-dose, STZ paradigm (5 consecutive daily injections of 40 mg/kg), did not alter body weight, hypothalamic neuropeptide expression or pituitary receptor expression, with the exception that sst2 mRNA levels were suppressed and GH levels did rise in the NPY(-/-) mouse. These observations demonstrate that NPY is not required for basal regulation of the GH axis, but is required for fasting-induced suppression of GHRH and SRIH expression, as well as fasting-induced augmentation of pituitary GHS-R mRNA. In contrast to the rat, fasting clearly did not suppress circulating GH levels in mice, but resulted in an overall rise in mean GH levels, similar to that observed in other mammalian species. The fact that many of the fasting-induced changes in the GH axis were observed in the high-dose STZ-treated mice, but were not observed in the multiple, low-dose paradigm, suggests STZ-mediated modulation of GH axis function is dependent on the severity of the catabolic state and not hyperglycemia.

NPY neurons express somatostatin receptor subtype 1 in the arcuate nucleus

The regulation of growth hormone (GH) secretion involves hypothalamic somatostatin and its specific receptors (sst1-sst5). sst1 is highly expressed in the arcuate nucleus (AN), and several data suggest that sst1 receptors are preferentially involved in the somatotropic hypothalamic network. Neuropeptide Y (NPY)-containing neurons function as direct transducers for GH feedback. Interestingly, there is an overlap in the distribution of NPY and sst1 containing cells in the AN. To determine whether these NPY cells are target for somatostatin we used a double label in situ hybridization histochemistry. Image analysis revealed that approximately 7% of NPY-hybridizing neurons coexpressed sst1 mRNA. These results further support the evidence for the direct interactions between the somatotropic axis and the neuroendocrine regulatory loops of energy homeostasis.

Putative GH pulse renewal: periventricular somatostatinergic control of an arcuate-nuclear somatostatin and GH-releasing hormone oscillator

Growth hormone (GH) pulsatility requires periventricular-nuclear somatostatin(SRIF(PeV)), arcuate-nuclear (ArC) GH-releasing hormone (GHRH), and systemic GH autofeedback. However, no current formalism interlinks these regulatory loci in a manner that generates self-renewable GH dynamics. The latter must include in the adult rat 1) infrequent volleys of high-amplitude GH peaks in the male, 2) frequent discrete low-amplitude GH pulses in the female, 3) disruption of the male pattern by severing SRIF(PeV) outflow to ArC, 4) stimulation of GHRH and GH secretion by central nervous system delivery of SRIF, 5) inhibition of GH release by central exposure to GHRH, and 6) a reboundlike burst of GHRH secretion induced by stopping peripheral infusion of SRIF. The present study validates by computer-assisted simulations a simplified ensemble formulation that predicts each of the foregoing six outcomes, wherein 1) blood-borne GH stimulates SRIF(PeV) secretion after a long time latency, 2) SRIF(PeV) inhibits both pituitary GH and ArC GHRH release, 3) ArC GHRH and SRIF(ArC) oscillate reciprocally with brief time delay, and 4) SRIF(PeV) represses and disinhibits the putative GHRH-SRIF(ArC) oscillator. According to the present analytic construction, time-delayed feedforward and feedback signaling among SRIF(PeV), ArC GHRH, and SRIF(ArC) could endow the complex physiological patterns of GH secretion in the male and female.

Joint pituitary-hypothalamic and intrahypothalamic autofeedback construct of pulsatile growth hormone secretion

Growth hormone (GH) secretion is vividly pulsatile in all mammalian species studied. In a simplified model, self-renewable GH pulsatility can be reproduced by assuming individual, reversible, time-delayed, and threshold-sensitive hypothalamic outflow of GH-releasing hormone (GHRH) and GH release-inhibiting hormone (somatostatin; SRIF). However, this basic concept fails to explicate an array of new experimental observations. Accordingly, here we formulate and implement a novel fourfold ensemble construct, wherein 1) systemic GH pulses stimulate long-latency, concentration-dependent secretion of periventricular-nuclear SRIF, thereby initially quenching and then releasing multiphasic GH volleys (recurrent every 3-3.5 h); 2) SRIF delivered to the anterior pituitary gland competitively antagonizes exocytotic release, but not synthesis, of GH during intervolley intervals; 3) arcuate-nucleus GHRH pulses drive the synthesis and accumulation of GH in saturable somatotrope stores; and 4) a purely intrahypothalamic mechanism sustains high-frequency GH pulses (intervals of 30-60 min) within a volley, assuming short-latency reciprocal coupling between GHRH and SRIF neurons (stimulatory direction) and SRIF and GHRH neurons (inhibitory direction). This two-oscillator formulation explicates (but does not prove) 1) the GHRH-sensitizing action of prior SRIF exposure; 2) a three-site (intrahypothalamic, hypothalamo-pituitary, and somatotrope GH store dependent) mechanism driving rebound-like GH secretion after SRIF withdrawal in the male; 3) an obligatory role for pituitary GH stores in representing rebound GH release in the female; 4) greater irregularity of SRIF than GH release profiles; and 5) a basis for the paradoxical GH-inhibiting action of centrally delivered GHRH.

Leptin-target neurones of the rat hypothalamus express somatostatin receptors

Hypothalamic leptinoceptive neurones can be visualized by histochemical demonstration of leptin-induced nuclear translocation of the signalling molecule STAT3. We investigated the relationship of the leptinoceptive neurones to the somatostatin signalling system. With double-labelling immunohistochemistry, we studied the colocalization of leptin-activated transcription factor, STAT3, with somatostatin receptor subtypes, sst1, sst2A, sst2B, sst3 and sst4, or the neuropeptide itself, in the rat hypothalamus. Immunoreactivity for all the entities was widely distributed throughout the entire hypothalamus. Despite the wide distribution, only few cases of colocalization of somatostatin with leptin-activated STAT3 were detected in the paraventricular, arcuate and dorsomedial nuclei. A moderate to high degree of colocalization of nuclear STAT3 and all investigated subtypes of somatostatin receptors was found in the lateral and dorsal hypothalamic areas and in the dorsomedial hypothalamic nucleus. Immunoreactivity for sst1, sst2B and sst4 was present in STAT3-containing nuclei of the paraventricular, periventricular, arcuate and ventromedial hypothalamic neurones, as well as in the retrochiasmatic and posterior hypothalamic areas. Despite the wide distribution of sst2A in the rat hypothalamus, few events of colocalization with leptin-activated STAT3 were observed in the dorsomedial nucleus and in the lateral and dorsal hypothalamic areas only. Many leptin-responsive neurones of the dorsal, lateral, periarcuate, perifornical and posterior hypothalamic areas, as well as in the ventromedial and dorsomedial hypothalamic nuclei, displayed sst3 immunoreactivity at their neuronal cilia. These results provide strong anatomical evidence for the direct interaction of leptin and the somatostatin systems in neuroendocrine control loops such as the energy homeostasis, growth or stress response.

Characterisation of [125I]-Tyr0DTrp8-somatostatin binding in sst1- to sst4- and SRIF-gene-invalidated mouse brain

Five somatostatin receptors (sst) have been cloned and mRNAs for the first four (sst1-4) are expressed in many brain regions. In the present work, we compared the distribution of the non-selective ligand [125I]-Tyr0-DTrp8-SRIF14 by autoradiography in 24 brain regions and pituitary in wild type, sst1- to sst4- or SRIF-gene invalidated (KO) mice. [125I]-Tyr0-DTrp8-SRIF14 binding was not significantly modified in sst1 KO mouse brain with the noticeable exception of the substantia nigra and only moderately decreased in pituitary. For sst2 KO mice, a general decrease (>75%) was observed in most regions, with the noticeable exception of the olfactory bulb and CA1 field of the hippocampus. SST3 KO brain displayed a decrease in binding in the external plexiform layer of the olfactory bulb only (-54%). For sst4 KO mice, [125I]-Tyr0-DTrp8-SRIF14 binding levels in the external plexiform (-35%) and glomerular (-39%) layers of the olfactory bulb as well as the hippocampus CA1 field (-68%) were significantly decreased. In SRIF KO mice, a significant increase in binding levels was observed in olfactory bulb, anterior olfactory nucleus, frontal cortex upper layers, lateral septum, CA1 field, zona incerta and lateral hypothalamus, substantia nigra, periaqueductal grey and parabrachial nucleus. Competition with selective ligands (CH275, octreotide or L-779,976, L-796,778, L-803,087, and octreotide or L-817,778, for sst1-5 receptors, respectively) was in accordance with these findings. Moreover, octreotide was still able to compete on residual [125I]-Tyr0-DTrp8-SRIF14 binding sites in sst2 KO pituitary. It is concluded that most [125I]-Tyr0-DTrp8-SRIF14 binding sites in mouse brain and pituitary belong to the sst2 subtype but for the olfactory bulb (sst3 and sst4 receptors), the CA1 of the hippocampus (sst4 receptors) and the pituitary (sst5 and sst1 receptors) in which other subtypes are also expressed. The overall increase in [125I]-Tyr0-DTrp8-SRIF14 binding in SRIF KO mice indicates that SRIF receptors, mostly from the sst2 subtype, are regulated by the endogenous ligand(s).

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.

AMPA-sst2 somatostatin receptor interaction in rat hypothalamus requires activation of NMDA and/or metabotropic glutamate receptors and depends on intracellular calcium

Modulation of glutamatergic transmission by neuropeptides is an essential aspect of neuronal network activity. Activation of the hypothalamic somatostatin sst2 receptor subtype by octreotide decreases AMPA glutamate responses, indicating a central link between a neurohormonal and neuromodulatory peptide and the main hypothalamic fast excitatory neurotransmitter. In mediobasal hypothalamic slices, sst2 activation inhibits the AMPA component of glutamatergic synaptic responses but is ineffective when AMPA currents are pharmacologically isolated. In mediobasal hypothalamic cultures, the decrease of AMPA currents induced by octreotide requires a concomitant activation of sst2 receptors with either NMDA and/or metabotropic glutamate receptors. This modulation depends on changes in intracellular calcium concentration induced by calcium flux through NMDA receptors or calcium release from intracellular stores following metabotropic glutamate receptor activation. These results highlight an unusual regulatory mechanism in which the simultaneous activation of at least three different types of receptor is necessary to allow somatostatin-induced modulation of fast synaptic glutamatergic transmission in the hypothalamus.

SOM230: a new somatostatin peptidomimetic with potent inhibitory effects on the growth hormone/insulin-like growth factor-I axis in rats, primates, and dogs

The goal of this project was to find a somatostatin (SRIF) analog with superior therapeutic potential. Receptor binding studies of new SRIF analogs were used to reveal SRIF substructures that interact with individual human SRIF receptor subtypes (sst1-sst5). Incorporation of these substructures into a stable cyclohexapeptide template led to SOM230, which binds with nanomolar affinity to sst1, sst2, sst3, and sst5. In rats, the inhibitory effect of SOM230 on GH was similar to SMS 201-995 (octreotide) at 1 h, but was 4-fold more potent at 6 h post injection, indicating increased metabolic stability. Treatment of rats with SOM230, at 1 and 10 micro g/kg.h, decreased IGF-I plasma levels, on d 2, by 68% and 90% (P < 0.01); whereas, under SMS 201-995 treatment, plasma IGF-I levels decreased by 28% and 49%, respectively. After a 2-wk infusion of rats, the suppression of IGF-I levels by SOM230 was still pronounced, whereas the response to SMS 201-995 was largely lost. This enhanced effect of SOM230 on IGF-I plasma levels was confirmed in an 8-wk study where both analogs were infused at 50 micro g/kg/h in rats. In rhesus monkey, SOM230 and SMS 201-995 treatment resulted in GH inhibition, with half-maximal inhibitory dose values of 0.5 and 0.4 micro g/kg, respectively, but plasma IGF-I levels were only lowered by SOM230 (-53%). In cynomolgus monkeys, a 2-wk infusion of SOM230, but to a much lesser extent of SMS 201-995, lowered plasma GH levels significantly (from 16.3 to 1.8 ng/ml, P = 0.007). Both in cynomolgus monkeys and beagle dogs, infusion of SOM230, but not SMS 201-995, lowered IGF-I levels significantly. In conclusion, SOM230 has a unique structure, binds almost universally to human ssts, and inhibits potently the GH/IGF-I axis cross-species. SOM230 is a candidate drug for clinical use.

Somatostatins and their receptors in fish

Somatostatin (SRIF) is a multigene family of peptides. SRIF-14 is conserved with identical primary structure in species across the vertebrates. The presence of multiple SRIF genes has been demonstrated in a number of fish species. Notably, three distinct SRIF genes have been identified in goldfish. One of these genes, which encodes [Pro(2)]SRIF-14, has also been identified in sturgeon and African lungfish, and is closely associated with the amphibian [Pro(2),Met(13)]SRIF-14 gene and mammalian cortistatin gene. The main neuroendocrine role of SRIF-14 peptide that has been determined in fish is the inhibition of pituitary growth hormone secretion. The functions of SRIF-14 variant or larger forms of SRIF peptide and the regulation of SRIF gene expression remain to be explored. Type one and two SRIF receptors have been identified from goldfish and type three SRIF receptor from an electric fish. Fish SRIF receptors display considerable homology to mammalian counterparts in terms of primary structure and negative coupling to adenylate cyclase. The identification of the multiple gene family of SRIF peptides and multiple types of SRIF receptors in fish opens a new avenue for the study of physiological roles of SRIF, and the molecular and cellular mechanisms of SRIF actions in fish.

Agonist-induced phosphorylation of somatostatin receptor subtype 1 (sst1). Relationship to desensitization and internalization

The sst1 somatostatin (SRIF) receptor subtype is widely expressed in the endocrine, gastrointestinal, and neuronal systems as well as in hormone-sensitive tumors, yet little is known about its regulation. Here we investigated the desensitization, internalization, and phosphorylation of sst1 expressed in CHO-K1 cells. Treatment of cells with 100 nm SRIF for 30 min reduced maximal SRIF inhibition of adenylyl cyclase from 40 to 10%. This desensitization was rapid (t(12) < 2 min) and dependent on agonist concentration (EC(50) = 2 nm). However, internalization of receptor-bound ligand occurred slowly (t(12) > 180 min). Incubation of cells with SRIF also caused a rapid (t(12) < 2 min) increase in sst1 receptor phosphorylation in a dose-dependent manner (EC(50) = 1.3 nm), as determined in a mobility shift phosphorylation assay. Receptor phosphorylation was not affected by pertussis toxin, indicating a requirement for receptor occupancy rather than signaling. The protein kinase C activator, phorbol 12-myristate 13-acetate also stimulated sst1 receptor phosphorylation whereas forskolin did not. Both agonist- and phorbol 12-myristate 13-acetate-stimulated receptor phosphorylation occurred mainly on serine. These studies are the first to demonstrate phosphorylation of the sst1 receptor and suggest that phosphorylation mediated uncoupling, rather than sequestration, leads to its desensitization.