Chapter I Somatostatin receptors

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

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 and somatostatin receptor gene expression in dominant and subordinate males of an African cichlid fish

Somatostatin is a neuropeptide best known for its inhibitory effects on growth hormone secretion and has recently been implicated in the control of social behavior. Several somatostatin receptor subtypes have been identified in vertebrates, but the functional basis for this diversity is still unclear. Here we investigate the expression levels of the somatostatin prepropeptide and two of its receptors, sstR2, and sstR3, in the brains of socially dominant and subordinate Astatotilapia burtoni males using real-time PCR. Dominant males had higher somatostatin prepropeptide and sstR3 expression in hypothalamus compared to subordinate males. Hypothalamic sstR2 expression did not differ. There were no differences in gene expression in the telencephalon. We also observed an interesting difference between dominants and subordinates in the relationship between hypothalamic sstR2 expression and body size. As would be predicted based on the inhibitory effects of somatostatin on somatic growth, sstR2 expression was negatively correlated with body size in dominant males. In contrast sstR2 expression was positively correlated with body size in subordinate males. These results suggest that in A. burtoni social status affects the relationships between somatostatin prepropeptide and receptor gene expression in the hypothalamus and the control of somatic growth.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Somatostatin-like receptors in goldfish: cloning of four new receptors

In this study, four somatostatin-like receptor (Sst) cDNAs were identified from goldfish pituitary, using RT-PCR screening and rapid amplification of cDNA ends (RACE) strategies. These include two type-five like Sst (Sst(5B) and Sst(5C)) and two type-three like Sst receptors (Sst(3A) and Sst(3B)), designated based on their amino acid sequence similarities to the known mammalian and fish Sst(5) and Sst(3). Both Sst(5C) and Sst(3A) mRNAs are widely expressed in all brain regions and pituitary; however, Sst(3B) expression is restricted to forebrain and Sst(5B) expression is mainly detected in pituitary and spinal cord.

Neurochemical characterization of receptor-expressing cell populations by in vivo agonist-induced internalization: insights from the somatostatin sst2A receptor

Characterization of both neurochemical phenotype of G protein-coupled receptor (GPCR)-expressing cells and receptor compartmentalization is a prerequisite for the elucidation of receptor functions in the central nervous system. However, it is often prevented by the diffuse and homogeneous distribution of receptor immunoreactivity. This is particularly true for the somatostatin (SRIF) sst2A receptor, which is largely distributed in the mammalian brain. By using this receptor as a model, we investigated whether receptor internalization, a biochemical property shared by numerous GPCRs, would reveal sst2A-expressing cell populations in the rat dorsolateral septum (LSD), a region in which SRIF might play an important modulatory role. Thirty minutes to 1 hour after intracerebroventricular injection of the sst2A receptor agonist octreotide, numerous sst2A-immunoreactive neurons and processes became apparent due to intracytoplasmic accumulation of intensely stained granules. Double-immunolabeling experiments with synaptophysin and MAP2 provided evidence that internalized sst2A receptors are predominantly localized in the somatodendritic compartment. Revealing sst2A receptor-expressing cell bodies permitted to analyze their neurotransmitter content. Quantitative analysis demonstrated an extensive overlap (approximately 85%) between SRIF- and sst2A-expressing neuronal populations. Additionally, numerous SRIF-immunoreactive axon-like terminals were found in close apposition with sst2A-positive cell bodies and dendrites. Taken together, these data suggest that the sst2A receptor is predominantly expressed in LSD neurons as a postsynaptic autoreceptor, thus providing novel neuroanatomic clues to elucidate SRIF neurotransmission in this region. More generally, in vivo agonist-induced internalization appears as a rapid and powerful tool for the neurochemical characterization of GPCR-expressing cell populations in the mammalian brain.

In vivo internalization of the somatostatin sst2A receptor in rat brain: evidence for translocation of cell-surface receptors into the endosomal recycling pathway

To determine whether cellular compartmentalization of somatostatin receptors can be regulated in vivo, we examined the immunocytochemical distribution of the sst2A receptor (sst2AR) after stereotaxical injections of somatostatin analogs into the rat parietal cortex. Whereas CH-275, a sst1R agonist, failed to induce changes in the diffuse sst2AR immunostaining pattern characteristic of control animals, somatodendritic profiles displaying intracytoplasmic immunoreactive granules became apparent short-term after injection of either somatostatin or the sst2R agonist octreotide. Confocal microscopy revealed that 90% of sst2AR-immunoreactive endosome-like organelles displayed transferrin receptor immunoreactivity. At the electron microscopic level, the percentage of sst2AR immunoparticles dramatically decreased at the plasmalemma of perikarya and dendrites after octreotide injection. Conversely, it significantly increased in endosomes-like organelles. These results demonstrate that sst2ARs undergo, in vivo, rapid and massive internalization into the endocytic recycling compartment in response to acute agonist stimulation and provide important clues toward elucidating somatostatin receptor signaling in the mammalian brain.