Somatostatin receptors: distribution in rat central nervous system and human frontal cortex

Effects of somatostatin, a somatostatin agonist, and an antagonist, on a putative migraine trigger pathway

OBJECTIVE

To determine whether somatostatin (SST) could be a cortico-brainstem neurotransmitter involved in producing the headache of migraine.

BACKGROUND

There is evidence to support the idea that a cortico-brainstem-trigeminal nucleus neuraxis might be responsible for producing migraine headache; we have suggested that SST may be one of the neurotransmitters involved.

METHODS

Rats were anesthetised and prepared for recording neurons in either the periaqueductal gray matter (PAG) or nucleus raphe magnus (NRM), as well as the trigeminal nucleus caudalis (TNC). The dura mater and facial skin were stimulated electrically or mechanically. SST, the SST agonist L054264 and the SST antagonist CYN54806 were injected intravenously, by microinjection, or by iontophoresis into the PAG or NRM. Cortical neuronal activity was provoked by cortical spreading depression (CSD) or light flash (LF) and was monitored by recording cortical blood flow (CBF).

RESULTS

Intravenous injection of SST: (a) selectively decreased the responses of TNC neurons to stimulation of the dura, but not skin, for up to 5 h; (b) decreased the ongoing discharge rate of TNC neurons while simultaneously increasing the discharge rate of neurons in either brainstem nucleus and; (c) prevented, or reversed, the effect of CSD and LF on brainstem and trigeminal neuron discharge rates. CSD and LF decreased the discharge rate of neurons in both brainstem nuclei and increased the discharge rate of TNC neurons. These effects were reversed by L054264 and mimicked by CYN54806. Injections of L054264 into the PAG or NRM reduced the response of TNC neurons to dural stimulation and skin stimulation differentially, depending on the nucleus injected. Injections of CYN54806 into either brainstem nucleus potentiated the responses of TNC neurons to dural and skin stimulation, but without a marked differential effect.

CONCLUSIONS

These results imply that SST could be a neurotransmitter in a pathway responsible for migraine pain.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Long-lasting actions of somatostatin on pyramidal cell excitability in the mouse cingulate cortex

Many neurological diseases are related to disturbances of somatostatin- (SOM-) expressing interneurons in the cingulate cortex. Therefore, their role within the circuitry of the cingulate cortex needs to be investigated. We describe here the physiological time course of SOM effects onto pyramidal cell excitability and action potential discharge pattern. Furthermore, we show that the GRK2 inhibitor Gallein had no effect on the reduced SOM-induced response following repetitive SOM applications.

Does somatostatin have a role to play in migraine headache?

Migraine is a condition without apparent pathology. Its cardinal symptom is the prolonged excruciating headache. Theories about this pain have posited pathologies which run the gamut from neural to vascular to neurovascular, but no observations have detected a plausible pathology. We believe that no pathology can be found for migraine headache because none exists. Migraine is not driven by pathology - it is driven by neural events produced by triggers - or simply by neural noise- noise that has crossed a critical threshold. If these ideas are true, how does the pain arise? We hypothesise that migraine headache is a consequence of withdrawal of descending pain control, produced by "noise" in the cerebral cortex. Nevertheless, there has to be a neural circuit to transform cortical noise to withdrawal of pain control. In our hypothesis, this neural circuit extends from the cortex, synapses in two brainstem nuclei (the periaqueductal gray matter and the raphe magnus nucleus) and ultimately reaches the first synapse of the trigeminal sensory system. The second stage of this circuit uses serotonin (5HT) as a neurotransmitter, but the neuronal projection from the cortex to the brainstem seems to involve relatively uncommon neurotransmitters. We believe that one of these is somatostatin (SST). Temporal changes in levels of circulating SST mirror the temporal changes in the incidence of migraine, particularly in women. The SST₂ receptor agonist octreotide has been used with some success in migraine and cluster headache. A cortical to PAG/NRM neural projection certainly exists and we briefly review the anatomical and neurophysiological evidence for it and provide preliminary evidence that SST may the critical neurotransmitter in this pathway. We therefore suggest that the withdrawal of descending tone in SST-containing neurons, might create a false pain signal and hence the headache of migraine.

Intra-ventral tegmental area microinjections of urotensin II modulate the effects of cocaine

Although the peptide urotensin II (UII) has well studied direct actions on the cardiovascular system, the UII receptor (UIIR) is expressed by neurons of the hindbrain. Specifically, the UIIR is expressed by the cholinergic neurons of the laterodorsal tegmentum (LDTg) and the pedunculopontine tegmentum (PPTg). These neurons send axons to the ventral tegmental area (VTA), for which the PPTg and LDTg are the sole source of acetylcholine. Therefore, it was hypothesized that UIIR activation within the VTA would modulate reward-related behaviors, such as cocaine-induced drug seeking. Intra-VTA microinjections of UII at high concentrations (1 nmole) established conditioned place preference (CPP), but also blocked cocaine-mediated CPP (10 mg/kg). When rats received systemic sub-effectual doses of cocaine (7.5 mg/kg) with intra-VTA injections of 1 or 10 pmole of UII CPP was formed. Furthermore, the second endogenous ligand for the UIIR, urotensin II-related peptide, had the same effect at the 10 pmole dose. The effects of low doses of UII were blocked by pretreatment with the UIIR antagonist SB657510. Furthermore, it was found that intra-VTA UII (10 pmole) further increased cocaine-mediated (7.5 mg/kg) rises in electrically evoked dopamine in the nucleus accumbens. Our study has found that activation of VTA-resident UIIR produces observable behavioral changes in rats, and that UIIR is able to modulate the effects of cocaine. In addition, it was found that UIIR activation within the VTA can potentiate cocaine-mediated neurochemical effects. Therefore, the coincident activation of the UII-system and cocaine administration may increase the liability for drug taking behavior.

Somatostatin and its 2A receptor in dorsal root ganglia and dorsal horn of mouse and human: expression, trafficking and possible role in pain

Background

Somatostatin (SST) and some of its receptor subtypes have been implicated in pain signaling at the spinal level. In this study we have investigated the role of SST and its sst2A receptor (sst2A) in dorsal root ganglia (DRGs) and spinal cord.

Results

SST and sst2A protein and sst2 transcript were found in both mouse and human DRGs, sst2A-immunoreactive (IR) cell bodies and processes in lamina II in mouse and human spinal dorsal horn, and sst2A-IR nerve terminals in mouse skin. The receptor protein was associated with the cell membrane. Following peripheral nerve injury sst2A-like immunoreactivity (LI) was decreased, and SST-LI increased in DRGs. sst2A-LI accumulated on the proximal and, more strongly, on the distal side of a sciatic nerve ligation. Fluorescence-labeled SST administered to a hind paw was internalized and retrogradely transported, indicating that a SST-sst2A complex may represent a retrograde signal. Internalization of sst2A was seen in DRG neurons after systemic treatment with the sst2 agonist octreotide (Oct), and in dorsal horn and DRG neurons after intrathecal administration. Some DRG neurons co-expressed sst2A and the neuropeptide Y Y1 receptor on the cell membrane, and systemic Oct caused co-internalization, hypothetically a sign of receptor heterodimerization. Oct treatment attenuated the reduction of pain threshold in a neuropathic pain model, in parallel suppressing the activation of p38 MAPK in the DRGs

Conclusions

The findings highlight a significant and complex role of the SST system in pain signaling. The fact that the sst2A system is found also in human DRGs and spinal cord, suggests that sst2A may represent a potential pharmacologic target for treatment of neuropathic pain.

Activation of somatostatin 2 receptors in the brain and the periphery induces opposite changes in circulating ghrelin levels: functional implications

Somatostatin is an important modulator of neurotransmission in the central nervous system and acts as a potent inhibitor of hormone and exocrine secretion and regulator of cell proliferation in the periphery. These pleiotropic actions occur through interaction with five G protein-coupled somatostatin receptor subtypes (sst(1) (-) (5)) that are widely expressed in the brain and peripheral organs. The characterization of somatostatin's effects can be investigated by pharmacological or genetic approaches using newly developed selective sst agonists and antagonists and mice lacking specific sst subtypes. Recent evidence points toward a divergent action of somatostatin in the brain and in the periphery to regulate circulating levels of ghrelin, an orexigenic hormone produced by the endocrine X/A-like cells in the rat gastric mucosa. Somatostatin interacts with the sst(2) in the brain to induce an increase in basal ghrelin plasma levels and counteracts the visceral stress-related decrease in circulating ghrelin. By contrast, stimulation of peripheral somatostatin-sst(2) signaling results in the inhibition of basal ghrelin release and mediates the postoperative decrease in circulating ghrelin. The peripheral sst(2)-mediated reduction of plasma ghrelin is likely to involve a paracrine action of D cell-derived somatostatin acting on sst(2) bearing X/A-like ghrelin cells in the gastric mucosa. The other member of the somatostatin family, named cortistatin, in addition to binding to sst(1) (-) (5) also directly interacts with the ghrelin receptor and therefore may simultaneously modulate ghrelin release and actions at target sites bearing ghrelin receptors representing a link between the ghrelin and somatostatin systems.

Expression of somatostatin and somatostatin receptor subtypes in Apolipoprotein D (ApoD) knockout mouse brain: An immunohistochemical analysis

Apolipoprotein D (ApoD) is widely distributed in central and peripheral nervous system. ApoD expression has been shown to increase in several neurodegenerative and neuropsychiatric disorders, as well as during regeneration in the nervous system. Like ApoD, in the central nervous system somatostatin (SST) is widely present and functions as neurotransmitter and neuromodulator. The biological effects of SST are mediated via binding to five high-affinity G-protein coupled receptors termed SSTR1-5. Mice lacking ApoD exhibit reduced SST labeling in cortex and hippocampus and increased expression in striatum and amygdala without any noticeable changes in substantia nigra. Changes in SSTRs expressions have been described in several neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. In the present study, using SSTR1-5 receptor-specific antibodies, we mapped their distribution in wild type (wt) and ApoD knockout (ApoD(-/-)) mouse brain. SSTR1-5 expression was observed both as membrane and cytoplasmic protein and display regions and receptor specific differences between wt and ApoD(-/-) mice brains. In cortex and hippocampus, SSTR subtypes like immunoreactivity are decreased in ApoD(-/-) mice brain. Unlike cortex and hippocampus, in the striatum of ApoD(-/-) mice, projection neurons showed increased SSTR immunoreactivity, as compared to wt. Higher SSTR subtypes immunoreactivity is seen in substantia nigra pars compacta (SNpc) whereas lower in substantia nigra pars reticulata (SNpr) of ApoD(-/-) mice brains as compared to wt. Whereas, amygdala displayed SSTR subtypes changes in different nuclei of ApoD(-/-) mice in comparison to wt mice brain. Taken together, our results describe receptor and region specific changes in SST and SSTR subtypes expression in ApoD(-/-) mice brain, which may be linked to specific neurological disorders.

Colocalization of somatostatin receptor subtypes (SSTR1-5) with somatostatin, NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase in the rat hypothalamus

The hypothalamus is a major site of somatostatin (SST) production and action. SST is synthesized in several hypothalamic nuclei and involved in a variety of functions. Using SST receptor (SSTR)-specific antibodies, we localized SSTR subtypes in the rat hypothalamus. In addition, we also demonstrated SSTRs colocalization with SST, NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase (TH). SSTR1 is strongly localized in neurons in all major hypothalamic nuclei as well as in nerve fibers in the zona externa of the median eminence and the ependyma of the third ventricle. SSTR2 is also well expressed in most regions but with a relatively lower abundance in comparison to SSTR1. In contrast, SSTR3 is localized primarily in the paraventricular nucleus, dorsomedial hypothalamic nucleus, arcuate nucleus, and median eminence. SSTR4-like immunoreactivity is mainly confined to the arcuate nucleus, ventromedial hypothalamic nucleus, median eminence, and ependymal cells of third ventricle, with the rare SSTR4-positive neuron in the paraventricular nucleus. SSTR5 is the least expressed subtype occurring only in few cells in the inner layer of the median eminence. Overall, SSTR1 is the predominant subtype, followed by SSTR2, 4, 3, and 5. Combined immunofluorescence, immunocytochemistry, and histochemistry were used to demonstrate SSTRs colocalization with SST, TH, and NADPH-d. SSTRs colocalization with SST, TH, and NADPH-d displays in a region and receptor specificity. Colocalization of SST and NADPH-d with SSTRs in hypothalamic regions was similar, suggesting that SST and NADPH-d producing cells are same. In contrast, TH was selectively coexpressed with SSTRs in the hypothalamus in a receptor-specific manner. Taken together, these data suggest that SSTRs may interact with NADPH-d and TH to exert a physiological role in concert within the hypothalamus.

Expression of sstr1 and sstr2 in rat hypothalamus: correlation with receptor binding and distribution of growth hormone regulatory peptides

With the aim of elucidating the role of individual somatostatin receptors in the central control of growth hormone secretion, we have examined the distribution of sstr1 and sstr2 mRNAs in the hypothalamus of the adult rat by in situ hybridization using 35S-labelled antisense riboprobes. Both receptors were expressed strongly in the preoptic area, suprachiasmatic nucleus and arcuate nucleus. High sstr1, but low sstr2, expression was evident in the paraventricular and periventricular nuclei as well as in the ventral premammillary nucleus. Conversely, moderate to high sstr2, but low sstr1, mRNA levels were detected in the anterior hypothalamic nucleus, ventromedial and dorsomedial nuclei and medial tuberal nucleus. Within the arcuate nucleus, the distribution of cells expressing sstr1 and sstr2 was comparable to that of neurons which bind somatostatin-14 selectively, one third of which have been documented to contain growth hormone-releasing hormone. Within the periventricular nucleus, the distribution of cells expressing sstr1 and, to a lesser extent, sstr2 was reminiscent of that of both [125I]somatostatin-labelled and somatostatin-immunoreactive cells. Taken together, these results imply a role for both sstr1 and sstr2 receptors in the central regulation of growth hormone-releasing hormone and somatostatin secretion, and hence of growth hormone release, by somatostatin.

Transient expression of somatostatin receptors in the brain during development

The study of somatostatin receptors by means of autoradiography in tissue sections revealed high densities of binding sites in the immature central nervous system. In rat cerebral cortex, the receptors are present in the intermediate zone and in association with cells migrating through the cortical plate. Somatostatin receptors in the intermediate zone of fetuses and in the cortical plate of postnatal rats exhibit high and low affinities respectively for the somatostatin analogue MK 678. In the rat cerebellum, the external granule cell layer, a germinal matrix containing interneuron precursors, contains a high density of receptors. These receptors exhibit high affinity for MK 678 throughout the period of cell multiplication. In granule cell cultures from eight-day-old rats, MK 678, octreotide and somatostatin are able to inhibit cAMP formation induced by forskolin or pituitary adenylyl cyclase-activating polypeptide. Somatostatin reduces the intracellular Ca2+ concentration in cultured granule cells; this response desensitizes rapidly. These results suggest that the somatostatin receptors in the external granule cell layer are type 2 receptors (sstr2). A low density of receptors with low affinity for MK 678 was also detected in the external granule cell layer and in the granule cell layer of neonatal rats. In adult rats the cerebellum is devoid of somatostatin receptors. These observations indicate that somatostatin probably exerts morphogenetic activities through different receptor types in several structures of the central nervous system.

Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats

Previous studies have suggested that somatostatin inhibits pancreatic secretion at a central vagal site, and the dorsal vagal complex (DVC) is involved in central feedback inhibition of the exocrine pancreas. The aim of this study was to investigate the effect of exogenous somatostatin in the DVC on pancreatic secretion and the somatostatin receptor subtype(s) responsible for the effect. The effects of somatostatin microinjected into the DVC on pancreatic secretion stimulated by cholecystokinin octapeptide (CCK-8) or 2-deoxy-d-glucose (2-DG) were examined in anesthetized rats. To investigate the somatostatin inhibitory action site, a somatostatin receptor antagonist [SRA; cyclo(7-aminoheptanoyl-Phe-d-Trp-Lys-Thr)] was microinjected into the DVC before intravenous infusion of somatostatin and CCK-8/2-DG. The effects of injection of a somatostatin receptor-2 agonist (seglitide) and combined injection of somatostatin and a somatostatin receptor-2 antagonist (CYN 154806) in the DVC on the pancreatic secretion were also investigated. Somatostatin injected into the DVC significantly inhibited pancreatic secretion evoked by CCK-8 or 2-DG in a dose-dependent manner. SRA injected into the DVC completely reversed the inhibitory effect of intravenous administration of somatostatin. Seglitide injected into the DVC also inhibited CCK-8/2-DG-induced pancreatic protein secretion. However, combined injection of somatostatin and CYN 154806 did not affect the CCK-8/2-DG-induced pancreatic secretion. Somatostatin in the DVC inhibits pancreatic secretion via somatostatin receptor-2, and the DVC is the action site of somatostatin for its inhibitory effect.

Anti-hyperalgesic effect of octreotide in patients with irritable bowel syndrome

BACKGROUND

Octreotide has been found to be beneficial in the treatment of chronic pain, although the mechanisms underlying its therapeutic effect are incompletely understood.

AIMS

To assess the effect of octreotide on perceptual responses to rectal distension in irritable bowel syndrome patients and healthy controls at baseline and following the experimental induction of rectal hyperalgesia.

METHODS

In study 1, rectal perception thresholds for discomfort were determined in seven irritable bowel syndrome patients and eight healthy controls on three separate days using a computer-controlled barostat. Subjects received saline, low-dose and high-dose octreotide in a random double-blind fashion. In study 2, perceptual responses to rectal distension were obtained in nine irritable bowel syndrome patients and seven controls before and after repetitive high-pressure mechanical sigmoid stimulation.

RESULTS

Octreotide increased the discomfort thresholds in irritable bowel syndrome patients, but not in controls, without changing rectal compliance. Repetitive sigmoid stimulation resulted in decreased rectal discomfort thresholds in the patient group only. In irritable bowel syndrome patients, octreotide prevented the sensitizing effect of repetitive sigmoid stimulation on rectal discomfort thresholds.

CONCLUSIONS

Octreotide effectively increased discomfort thresholds in irritable bowel syndrome patients, but not in controls, at baseline and during experimentally induced rectal hyperalgesia. These findings suggest that octreotide exerts primarily an anti-hyperalgesic rather than analgesic effect on visceral perception.

The somatostatin sst2A receptor in the rat trigeminal ganglion

Immunohistochemistry for the somatostatin sst2A receptor was performed on the rat trigeminal ganglion to know its function in the trigeminal nervous system. The immunoreactivity was detected in 9.4% of primary sensory neurons in the ganglion. These neurons were small to medium-sized (range=106.5-1123.2 microm(2); mean+/-S.D.=506.3+/-213.2 microm(2)) and predominantly located in the rostromedial part of the ophthalmo-maxillary division. They were also immunoreactive for calcitonin gene-related peptide and the vanilloid receptor subtype 1. In addition, 13.7% of trigeminal neurons which were retrogradely traced with fluorogold from the nasal mucosa exhibited sst2A receptor-immmunoreactivity. Trigeminal neurons which innervated the facial skin and tooth pulp were devoid of the immunoreactivity. In the brainstem trigeminal sensory nuclear complex, both the neuronal cell body and the neuropil exhibited sst2A receptor-immunoreactivity in the superficial medullary dorsal horn.The present study indicates that sst2A receptor-immunoreactive trigeminal nociceptors innervate the nasal mucosa. They may project to the superficial laminae of the medullary dorsal horn.

Tonic control of peripheral cutaneous nociceptors by somatostatin receptors

The peptide somatostatin [somatotropin release-inhibiting factor (SRIF)] is widely distributed in the body and exerts a variety of hormonal and neural actions. Several lines of evidence indicate that SRIF is important in nociceptive processing: (1) it is localized in a subset of small-diameter dorsal root ganglion cells; (2) activation of SRIF receptors results in inhibition of both nociceptive behaviors in animals and acute and chronic pain in humans; (3) SRIF inhibits dorsal horn neuronal activity; and (4) SRIF reduces responses of joint mechanoreceptors to noxious rotation of the knee joint. The goal of the present study is to show that cutaneous nociceptors are under the tonic inhibitory control of SRIF. This is accomplished using behavioral and electrophysiological paradigms. In a dose-dependent manner, intraplantar injection of the SRIF receptor antagonist cyclo-somatostatin (c-SOM) results in nociceptive behaviors in normal animals and enhancement of nociceptive behaviors in formalin-injected animals, and these actions can be blocked when c-SOM is coapplied with three different SRIF agonists. Furthermore, intraplantar injection of SRIF antiserum also results in nociceptive behaviors. Electrophysiological recordings using an in vitro glabrous skin-nerve preparation show increased nociceptor activity in response to c-SOM, and this increase is blocked by the same three SRIF agonists. Parallel behavioral and electrophysiological studies using the opioid antagonist naloxone demonstrate that endogenous opioids do not maintain a tonic inhibitory control over peripheral nociceptors, nor does opioid receptor antagonism influence peripheral SRIF effects on nociceptors. These findings demonstrate that SRIF receptors maintain a tonic inhibitory control over peripheral nociceptors, and this may contribute to mechanisms that control the excitability of these terminals.

Somatostatin inhibits GABAergic transmission in the sensory thalamus via presynaptic receptors

The action of somatostatin on GABA-mediated transmission was investigated in cat and rat thalamocortical neurons of the dorsal lateral geniculate nucleus and ventrobasal thalamus in vitro. In the cat thalamus, somatostatin (10 microM) had no effect on the passive membrane properties of thalamocortical neurons and on the postsynaptic response elicited in these cells by bath or iontophoretic application of (+/-)baclofen (5-10 microM) or GABA, respectively. However, somatostatin (1-10 microM) decreased by a similar amount (45-55%) the amplitude of electrically evoked GABA(A) and GABA(B) inhibitory postsynaptic potentials in 71 and 50% of neurons in the lateral geniculate and ventrobasal nucleus, respectively. In addition, the neuropeptide abolished spontaneous bursts of GABA(A) inhibitory postsynaptic potentials in 85% of kitten lateral geniculate neurons, and decreased (40%) the amplitude of single spontaneous GABA(A) inhibitory postsynaptic potentials in 87% of neurons in the cat lateral geniculate nucleus. Similar results were obtained in the rat thalamus. Somatostatin (10 microM) had no effect on the passive membrane properties of thalamocortical neurons in this species, or on the outward current elicited by puff-application of (+/-)baclofen (5-10 microM). However, in 57 and 22% of neurons in the rat lateral geniculate and ventrobasal nuclei, respectively, somatostatin (1 microM) reduced the frequency, but not the amplitude, of miniature GABA(A) inhibitory postsynaptic currents by 31 and 37%, respectively. In addition, the neuropeptide (1 microM) decreased the amplitude of evoked GABA(A) inhibitory postsynaptic currents in 20 and 55% of rat ventrobasal neurons recorded in normal conditions and during enhanced excitability, respectively: this effect was stronger on bursts of inhibitory postsynaptic currents(100% decrease) than on single inhibitory postsynaptic currents (41% decrease). These results demonstrate that in the sensory thalamus somatostatin inhibits GABA(A)- and GABA(B)-mediated transmission via a presynaptic mechanism, and its action is more prominent on bursts of GABAergic synaptic currents/potentials.

Advances in understanding neuronal somatostatin receptors

It has long been considered that somatostatin acts as a neuromodulator in the mammalian central nervous system but its precise physiological roles remain elusive. Early studies to identify somatostatin-binding sites revealed a widespread heterogeneous pattern, especially in the CNS. More recently, a family of somatostatin receptors have been identified, of which five genes (sst(1-5)) have been cloned. In this review, we discuss current data describing the localisation of the five receptor types. Recent progress in understanding their function has been made using high-affinity, selective receptor ligands and transgenic animal technology. Finally, the therapeutic potential for somatostatin receptor-selective compounds as analgesics is considered.

Distribution, targeting, and internalization of the sst4 somatostatin receptor in rat brain

Somatostatin mediates its diverse physiological effects through a family of five G-protein-coupled receptors (sst(1)-sst(5)); however, knowledge about the distribution of individual somatostatin receptor proteins in mammalian brain is incomplete. In the present study, we have examined the regional and subcellular distribution of the somatostatin receptor sst(4) in the rat CNS by raising anti-peptide antisera to the C-terminal tail of sst(4). The specificity of affinity-purified antibodies was demonstrated using immunofluorescent staining of HEK 293 cells stably transfected with an epitope-tagged sst(4) receptor. In Western blotting, the antiserum reacted specifically with a broad band in rat brain, which migrated at approximately 70 kDa before and approximately 50 kDa after enzymatic deglycosylation. sst(4)-Like immunoreactivity was most prominent in many forebrain regions, including the cerebral cortex, hippocampus, striatum, amygdala, and hypothalamus. Analysis at the electron microscopic level revealed that sst(4)-expressing neurons target this receptor preferentially to their somatodendritic domain. Like the sst(2A) receptor, sst(4)-immunoreactive dendrites were often closely apposed by somatostatin-14-containing fibers and terminals. However, unlike the sst(2A) receptor, sst(4) was not internalized in response to intracerebroventricular administration of somatostatin-14. After percussion trauma of the cortex, neuronal sst(4) receptors progressively declined at the sites of damage. This decline coincided with an induction of sst(4) expression in cells with a glial-like morphology. Together, this study provides the first description of the distribution of immunoreactive sst(4) receptor proteins in rat brain. We show that sst(4) is strictly somatodendritic and most likely functions in a postsynaptic manner. In addition, the sst(4) receptor may have a previously unappreciated function during the neuronal degeneration-regeneration process.

Immunohistochemical distribution of the somatostatin receptor subtype 5 in the adult rat brain: predominant expression in the basal forebrain

Somatostatin exerts its actions by means of a family of G protein-coupled receptors, five of which have so far been cloned. Whereas mRNAs for receptor subtypes sst(1)-sst(4) have been unequivocally localized in the brain, the data concerning the fifth subtype, sst(5), are contradictory. Moreover, whereas sst(1) and sst(2A) receptor proteins have been localized by immunohistochemistry, the distribution of sst(3)-sst(5) receptor proteins and/or subtype-specific binding remains to be determined in the central nervous system. In the present study, we investigated the distribution of immunoreactive sst(5) in adult rat brain and pituitary and demonstrated the presence of this receptor protein in the central nervous system by using an affinity-purified antibody generated against the C-terminus of the receptor. The specificity of the antibody for sst(5) was established by immunoblotting experiments on membranes prepared from cells transfected with cDNA encoding different somatotropin release inhibiting (SRIF) receptor subtypes as well as from anterior pituitary. In both systems, the antibody specifically recognized a band at approximately 50 kDa molecular mass, corresponding well to the reported size of the cloned receptor (48 kDa). Immunofluorescence in COS-7 cells transfected with individual SRIF receptor subtypes as well as in sections of rat pituitary demonstrated the antibody's applicability to the immunohistochemical detection of sst(5) receptors. In rat brain sections, sst(5) immunoreactivity was predominantly associated with neuronal perikarya and primary dendrites. Immunolabeling was most prominent in the olfactory tubercle, islands of Calleja, diagonal band of Broca, substantia innominata, and magnocellular preoptic nucleus of the basal forebrain as well as in the reticular nucleus of the thalamus. Other, less intensely labeled areas included the cerebral cortex, hippocampus, amygdala, preoptic area as well as the lateroanterior nucleus of the hypothalamus. The present findings provide the first characterization of the anatomic distribution of sst(5) receptors in the rat brain. They demonstrate a prominent expression of these receptors in the basal forebrain, suggesting that they may be involved in the mediation of somatostatin effects on the sleep-wake cycle through their association with cortically projecting subcortical systems.

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

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

Neurogenesis, cell death and regeneration in the adult gymnotiform brain

Gymnotiform fish, like all teleosts examined thus far, are distinguished by their enormous potential for the production of new neurons in the adult brain. In Apteronotus leptorhynchus, on average 10(5) cells, corresponding to approximately 0.2 % of the total population of cells in the adult brain, are in S-phase within any period of 2 h. At least a portion of these newly generated cells survive for the rest of the fish's life. This long-term survival, together with the persistent generation of new cells, leads to a continuous growth of the brain during adulthood. Zones of high proliferative activity are typically located at or near the surface of the ventricular, paraventricular and cisternal systems. In the central posterior/ prepacemaker nucleus, for example, new cells are generated, at very high rates, in areas near the wall of the third ventricle. At least some of these cells differentiate into neurons, express immunoreactivity against the neuropeptide somatostatin and migrate into more lateral areas of this complex. Approximately 75 % of all new brain cells are generated in the cerebellum. In the corpus cerebelli and the valvula cerebelli, they are produced in the molecular layers, whereas in the eminentia granularis the newborn cells stem from proliferation zones in the pars medialis. Within the first few days of their life, these cells migrate towards specific target areas, namely the associated granule cell layers. At least some of them develop into granule neurons. The high proliferative activity is counterbalanced by apoptosis, a mechanism that resembles the processes known from embryonic development of the vertebrate brain. Apoptosis also appears to be used as an efficient mechanism for the removal of cells damaged through injury in the brain of adult Apteronotus leptorhynchus. Since apoptosis is not accompanied by the side effects known from necrosis, this ‘clean’ type of cell death may, together with the enormous proliferative activity in the brain, explain, at least partially, the tremendous capability of teleost fish to replace damaged neurons with newly generated ones. One factor that appears to play a major role in the generation of new cells and in their further development is the neuropeptide somatostatin. In the caudal cerebellum of the gymnotiform brain, somatostatin-binding sites are expressed, at extremely high densities, at sites corresponding to the areas of origin, migration and differentiation of the newborn cells. This pattern of expression resembles the expression pattern in the rat cerebellum, where somatostatin immunoreactivity and somatostatin-binding sites are transiently expressed at the time when the granule cells of the cerebellum are generated. Moreover, after mechanical lesions of the corpus cerebelli, the expression of somatostatin-like immunoreactivity is tremendously increased in several cell types (presumably astrocytes, microglia and granule cell neurons) near the path of the lesion; the time course of this expression coincides with the temporal pattern underlying the recruitment of new cells incorporated at the site of the lesion.

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.

Selective targeting of somatostatin receptor 3 to neuronal cilia

Recently, five members of the somatostatin receptor family have been cloned. However, little is known about their cellular and subcellular localization in the central nervous system. Using specific anti-peptide antisera, we observed somatostatin receptor 3-like immunoreactivity in many brain regions, including the cerebral cortex, hippocampus, hypothalamus, amygdala and cerebellum. In all of these regions (except for the cerebellar cortex), somatostatin receptor 3-like immunoreactivity was selectively targeted to 4-8-microm-long rod-shaped profiles which did not co-localize with axonal or dendritic markers. One immunoreactive profile was always associated with one neuronal cell body. This staining pattern was resistant to colchicine treatment and showed a closely overlapping distribution with somatostatin receptor 3 messenger RNA, suggesting that the receptor protein is not transported over long distances. Electron microscopic analysis revealed that somatostatin receptor 3-like immunoreactivity is localized to the plasma membrane of neuronal cilia which extended into an intercellular pocket and showed a 9+0 filament pattern in their basal body and proximal segments. Thus, somatostatin receptor 3 demonstrates a unique example of a G-protein-coupled receptor not localized to "classical" pre- or postsynaptic sites, but selectively targeted to neuronal cilia. The presence of the somatostatin receptor 3 receptor on neuronal cilia suggests that these presumably non-motile cilia may not merely represent developmental remnants, but rather function as chemical sensors of the immediate milieu.

Distribution of somatostatin receptor subtypes in rat lumbar spinal cord examined with gold-labelled somatostatin and anti-receptor antibodies

Using gold-labelled somatostatin, somatostatin binding sites were predominantly found in laminae I-III, X and on motorneurones of the rat lumbar spinal cord. A comparison with immunohistochemical staining using antisera against somatostatin receptor sequences revealed that the marked binding in laminae I-III coincided with the presence of somatostatin receptor-like immunoreactivity for the receptor subtypes 1, 2 and 3. Binding sites on motorneurones were only paralleled by an immunoreaction for subtype 3. In lamina X, however, the lack of a positive immunoreaction indicates that in this part other subtypes may be present.

Immunocytochemical localization of somatostatin receptor sst2A in the rat spinal cord and dorsal root ganglia

Intrathecal administration of octreotide, a stable somatostatin analogue, provides pain relief in patients, and locally applied somatostatin inhibits firing of nociceptive dorsal horn neurons. In the present study, we have raised polyclonal antibodies that specifically detect the somatostatin receptor sst2A and used these antisera for immunocytochemical localization of the receptor protein in the rat spinal cord and dorsal root ganglia. In the superficial layers of the dorsal horn, sst2A-like immunoreactivity (Li) formed a dense network consisting of neuronal perikarya and dendrites which were often closely apposed by, but not co-contained within, somatostatin-14-immunoreactive nerve fibres and terminals. sst2A-Li was resistant to dorsal rhizotomy and did not colocalize with either substance P or calcitonin gene-related peptide suggesting that sst2A-Li was not located to primary afferents, but rather confined to second-order spinal neurons. The position of sst2A-Li perikarya and dendrites in the dorsal horn appeared to be similar to those containing mu-opioid receptor-Li; however, double labelling experiments revealed no instances of coexistence of these two receptors. sst2A-Li was also observed in the dorsal root ganglia predominantly targeted to the somatic plasmalemma of medium size neurons distinct from those expressing somatostatin-14 or delta-opioid receptors. Thus, the present results not only provide a morphological substrate for spinal octreotide analgesia but also show that somatostatin and opioids are poised to modulate nociceptive transmission by distinct anatomical systems.

Differential distribution of alternatively spliced somatostatin receptor 2 isoforms (sst2A and sst2B) in rat spinal cord

We have recently shown that the cytoplasmic tail of the somatostatin receptor sst2 undergoes alternative splicing giving rise to two isoforms, SSt2A and sst2B. In the present study, we have raised polyclonal antibodies that specifically detect either sst2A or sst2B and used these antisera for immunocytochemical localization of the receptor proteins in the rat spinal cord. sst2A-immunoreactivity formed a dense network consisting of neuronal perikarya and dendrites in the superficial layers of the dorsal horn. In contrast, prominent sst2B-immunoreactivity was found on neuronal perikarya and proximal dendrites throughout the gray matter of the spinal cord. Taken together, we show that alternative carboxy-terminal splicing is involved in cell-specific expression of somatostatin receptor sst2 isoforms in rat spinal cord, and that sst2A and sst2B mediate effects of somatostatin at different cellular sites.

Central somatostatin diminished inhibitory action of central CGRP on pancreatic basal secretion in conscious rats

We examined whether central somatostatin prevents an inhibitory effect of central calcitonin-gene related peptide (CGRP) on pancreatic secretion in conscious male Wistar rats (330-330 g). Rats were prepared with separate cannulas for draining bile and pancreatic juice and with a duodenal cannula and an extrajugular vein cannula. In addition, another cannula was stereotactically implanted into the left lateral cerebral ventricle. Rats were placed in restraint cages and experiments were conducted 4 days after the operation without anesthesia. An injection of CGRP (0.1, 1.0 nmol/10 microl) into the left lateral cerebral ventricle (i.c.v.) inhibited pancreatic secretion dose-dependently. To confirm the inhibitory effect of CGRP (i.c.v.) was mediated via sympathetic nerves, phentolamine was injected intravenously (i.v.) bolus (0.5 mg kg(-1)) 0.5-h before CGRP (i.c.v.), followed by continuous infusion of 0.2 mg kg(-1) h(-1). Phentolamine (i.v.) reversed the inhibition produced by CGRP (i.c.v.). An injection of 4 nmol/10 microl somatostatin (i.c.v.) 5 min prior to CGRP injection diminished the inhibitory effect of CGRP (i.c.v.). It is concluded that centrally administered somatostatin diminished the inhibitory action of CGRP (i.c.v.) on pancreatic secretion, probably via inhibiting autonomic (sympathetic) nerve excitation at the central site.

The localization of somatostatin receptor 1 (sst1) immunoreactivity in the rat brain using an N-terminal specific antibody

The biological actions of somatostatin are mediated via a family of G protein-coupled receptors named sst1 to sst5. We used an affinity-purified polyclonal antibody AS-65, directed against a specific N-terminal peptide sequence of sst1 to determine the immunohistochemical distribution of N-terminal sst1 immunoreactivity in the rat brain. The specificity of the antibody was shown by western blotting experiments using an N-terminal sst1 fusion protein. Enzymatic deglycosylation experiments were combined with blotting experiments on a sst1-transfected cell line and rat brain membrane proteins and with immunocytochemistry on an sst1-transfected cell line. These studies showed that the antibody detected the deglycosylated sst1 receptor protein. Immunohistochemical staining showed that sst1 immunoreactivity (presumably the deglycosylated receptor) recognised by this N-terminal antiserum was widely distributed throughout the brain with cells and processes labelled in the cerebral cortex, regions of the limbic system (including the hippocampal formation and some basal ganglia nuclei), the epithalamus, the thalamus, different subthalamic structures (subthalamic nucleus, zona incerta), the colliculi, the hypothalamus, the reticular formation, the cerebellum and regions of the trigeminal nerve complex. The distribution of immunoreactivity was in good general agreement with that predicted from the localization of sst1 messenger RNA and radioligand binding studies. This study on the immunohistochemical distribution of the sst1 receptor in the brain will provide a better understanding of the central actions of somatostatin at its receptor types.

Visualisation of non-glycosylated somatostatin receptor two (ngsst2) immunoreactivity in the rat central nervous system

The biological actions of the neuropeptides somatostatin-14 and -28 are receptor-mediated. To date, five G protein-coupled receptors sst1 to sst5 have been characterised pharmacologically and their genes have been cloned. In this study, we used an affinity-purified polyclonal antibody (AS-68) raised against a specific N-terminal peptide sequence of sst2 to localise N-terminal sst2-immunoreactive regions in the rat brain and the cervical spinal cord. The specificity of the antiserum was demonstrated by Western and slot blotting experiments using a N-terminal sst2 fusion protein. Further blotting experiments with a sst2(A)-transfected cell line and rat CNS membrane proteins showed that the antibody detected the non-glycosylated and/or non-sialated receptor. A strong signal using an sst2(A)-transfected CHO-K1 cell line was obtained only if the cells had been treated with N-Glycosidase F prior to the immunochemical detection. Two variants of sst2 (sst2(A) and sst2(B)) have been identified by cloning procedures and gene expression studies in the rodents. They differ in their carboxy-termini: AS-68 would, however, be able to recognise the non-glycosylated form of both these variants. We present here the central nervous system distribution of non-glycosylated sst2-immunoreactivity in the rat using this N-terminal antibody. The sst2 non-glycosylated N-terminal like immunoreactivity was distributed throughout the brain with cells and processes labelled in the cerebral cortex and the basal ganglia (neostriatum, substantia nigra), in the limbic system (hippocampal formation, amygdala), in the diencephalon (epithalamus, thalamus, hypothalamus), the superior colliculus, the periaqueductal grey matter and some of the reticular formation nuclei. The distribution of the non-glycosylated sst2-like immunoreactivity detected here was consistent with that predicted from the localisation of sst2 mRNA and SRIF-ligand binding studies.

A quantitative study of neurons which express neurokinin-1 or somatostatin sst2a receptor in rat spinal dorsal horn

The neurokinin-1 and somatostatin sst2a receptors have both been identified on spinal cord neurons. In this study we have estimated the proportions of neurons in different parts of the spinal cord which express these receptors, by using a monoclonal antibody against a neuronal nuclear protein named NeuN and combining the optical disector method with confocal microscopy. The NeuN antibody was initially tested on over 3200 neurons identified with antisera against a variety of compounds, including neuropeptides, enzymes and receptors, and also on astrocytes and oligodendrocytes. All of the neurons, but none of the glial cells that were examined possessed NeuN-immunoreactivity, which suggests that NeuN is a reliable marker for all spinal cord neurons. We found that approximately 45% of neurons in lamina I, 23-29% of those in laminae IV-VI and 18% in lamina X possessed the neurokinin-1 receptor, while the receptor was present on a smaller proportion of neurons in laminae II and III (6% and 11%, respectively). Thirteen percent of lamina I neurons and 15% of those in lamina II expressed the sst2a receptor. To provide further information about the types of neuron which possess the sst2a receptor, we searched for possible co-existence with the neurokinin-1 receptor as well as with GABA and glycine. sst2a and neurokinin-1 receptors were not co-localized on neurons in laminae I and II. All of the sst2a-immunoreactive neurons examined were also GABA-immunoreactive, and 83.5% were glycine-immunoreactive, indicating that the receptor is located on inhibitory neurons in the superficial dorsal horn. These results demonstrate the proportions of neurons in each region of the spinal cord which can be directly activated by substance P or somatostatin acting through these receptors. Levels of receptors can change in pathological states, and this method could be used to determine whether or not these changes involve alterations in the number of neurons which express receptors. In addition, the method can be used to estimate the sizes of neurochemically-defined populations of spinal cord neurons.

Outward current produced by somatostatin (SRIF) in rat anterior cingulate pyramidal cells in vitro

1. A high density of receptors for somatostatin (SRIF) exists in the anterior cingulate cortex but their function is unknown. Whole-cell patch clamp recordings were made from visualized deep layer pyramidal cells of the rat anterior cingulate cortex contained in isolated brain slices to investigate the putative effects of SRIF and to identify the receptor subtype(s) involved. 2. SRIF (1-1000 nM) produced a concentration-dependent outward current which was associated with an increased membrane conductance, was sensitive to Ba2+ (300 microM - 1 mM), and was absent in the presence of a maximal concentration of the GABA(B) receptor agonist, baclofen (100 microM). These observations suggest the outward current was carried by K+ ions. 3. SRIF analogues also elicited outward currents with a rank potency order of (EC50, nM): octreotide (1.8)>BIM-23027 (3.7)>SRIF (20)=L-362,855 (20). BIM-23056 was without agonist or antagonist activity. Responses to L-362,855 were unlike those to the other agonists since they were sustained for the duration of the application. 4. The sst2 receptor antagonist, L-Tyr8Cyanamid 154806 (1 microM), had no effect alone but partially reversed responses to submaximal concentrations of SRIF (100 nM, 44+/-6% reversal) and L-362,855 (100 nM, 70+/-6% reversal) and fully reversed the response to BIM-23027 (10 nM). In contrast, L-Tyr8Cyanamid 154806 did not antagonize the response to baclofen (10 microM). 5. We conclude that SRIF activates a K+ conductance in anterior cingulate pyramidal neurones via an action predominantly at sst2 receptors.

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

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

Morphine and somatostatin analogue reduce c-fos expression in trigeminal subnucleus caudalis produced by corneal stimulation in the rat

The influence of morphine and somatostatin on nociceptor-evoked activation of central trigeminal neurons and cardiovascular reflex responses was assessed in barbiturate-anaesthetized rats. Morphine or the somatostatin analogue, octreotide, was given intracerebroventricularly 20 min prior to application of mustard oil to the corneal surface. The expression of the immediate early gene, c-fos, was used to estimate neuronal activation within the spinal trigeminal nucleus. Morphine reduced the number of Fos-positive neurons produced at the transition region between trigeminal subnucleus caudalis and the upper cervical spinal cord, whereas c-fos expression at the subnucleus interpolaris/caudalis transition was not affected significantly. Morphine also reduced the arterial pressure and heart rate responses to corneal stimulation in proportion to the dose of morphine and required a threshold dose similar to that which reduced c-fos expression. Naloxone prevented the morphine-induced inhibition of c-fos expression and cardiovascular reflex responses to corneal stimulation. Somatostatin analogue reduced the number of Fos-positive neurons at the subnucleus caudalis/cervical cord transition, but not at the subnucleus interpolaris/caudalis transition, an effect that was not prevented by naloxone. Somatostatin analogue did not blunt the cardiovascular responses evoked by corneal stimulation. A subthreshold dose of morphine plus a threshold dose of somatostatin analogue caused a greater inhibition of Fos-positive neurons at the subnucleus caudalis/cervical cord transition, but not in reflex-evoked autonomic responses, than the same dose of either drug alone. Intracerebroventricular administration of morphine and somatostatin analogue inhibit corneal activation of neurons within the superficial laminae at the subnucleus caudalis/cervical cord transition through opioid and non-opioid-dependent neural pathways, respectively. By contrast, the low sensitivity of corneal-responsive neurons at the subnucleus interpolaris/caudalis transition to analgesics suggests that these neurons are not simply a rostral extension of the medullary dorsal horn. Correlation analyses suggest that morphine-induced inhibition of cardiovascular responses to corneal stimulation depend on the activity of neurons at the subnucleus caudalis/cervical cord transition and not on those at the subnucleus interpolaris/caudalis transition region.

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.

Immunohistochemical localization of the somatostatin SST2(A) receptor in the rat brain and spinal cord

The neuropeptide somatostatin is widely distributed in the CNS and is believed to play a role as a neurotransmitter or a neuromodulator. Somatostatin mediates its actions by the binding of the peptide to high affinity membrane receptors. The genes for five somatostatin receptor types have been cloned recently and Northern blotting and in situ hybridization studies have shown that the transcripts of all five types are expressed in the CNS. Here we report the cellular distribution of somatostatin sst2(a) receptor protein in the adult rat CNS, using a polyclonal anti-peptide antibody directed against a portion of the C-terminal domain of the receptor. The specificity of the affinity-purified antibody was demonstrated by Western blotting and immunolabelling of cells transfected with a hemagglutinin epitope-tagged version of the sst2(a) receptor. Immunohistochemistry showed a distinct distribution of the receptor protein in the rat brain. Cells and processes were labelled in a number of areas, including the basolateral amygdala, the locus coeruleus, the endopiriform nucleus, the deep layers of the cerebral cortex, the subiculum, the claustrum, the habenula, the interpenduncular nucleus, the hippocampus and the central grey. In the spinal cord, the substantia gelatinosa showed strongly-labelled cell bodies and their processes. This study provides an improved understanding of the distribution of the sst2(a) receptor in rat brain.

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.

Somatostatin receptors in the central nervous system

Somatostatin was first identified chemically in 1973, since when much has been established about its synthesis, storage and release. It has important physiological actions, including a tonic inhibitory effect on growth hormone release from the pituitary. It has other central actions which are not well understood but recent cloning studies have identified at least five different types of cell membrane receptor for somatostatin. The identification of their genes has allowed studies on the distribution of the receptor transcripts in the central nervous system where they show distinct patterns of distribution, although there is evidence to indicate that more than one receptor type can co-exist in a single neuronal cell. Receptor selective radioligands and antibodies are being developed to further probe the exact location of the receptor proteins. This will lead to a better understanding of the functional role of these receptors in the brain and the prospect of determining the role, if any, of somatostatin in CNS disorders and the identification of potentially useful medicines.

Somatostatin receptors in the rhesus monkey brain: localization and pharmacological characterization

To characterize the nature and distribution of somatostatin (SRIF) receptors, radioligand binding studies and in vitro receptor autoradiography were performed in Rhesus monkey brain using either [125I]LTT-SRIF-28 ([Leu8, D-Trp22, 125I-Tyr25]SRIF-28) alone or in the presence of 3 nM seglitide (to block sst2 sites), [125I]Tyr3-octreotide or [125I]CGP 23996 (c[Asu-Lys-Asn-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) in buffer containing either 120 mM Na+ or 5 mM Mg2+. [125I]Tyr3 -octreotide labelled an apparently homogeneous population of sites in cerebral and cerebellar cortex (Bmax = 27.3 +/- 2.8 fmol/mg protein and 52.6 +/- 8.6 fmol/mg protein, PKd = 9.46 +/- 0.03 and] 9.93 +/- 0.03, respectively). The pharmacological profile of these sites correlated highly significantly with that of human recombinant sst2 receptors (r = 0.996), but not or much less with that of human recombinant sst3 and sst5 receptors (r = 0.12 and 0.45, respectively). [125I]CGP 23996 (in Na(+)-buffer) also labelled an apparently homogeneous population of sites in Rhesus monkey cerebral cortex membranes (Bmax = 3.1 +/- 0.3 fmol/mg protein, pKd = 10.57 +/- 0.08), the pharmacological profile of which was highly significantly correlated with the profiles of human recombinant sst1 and sst4 receptors (r = 0.98 and 0.96, respectively). Using receptor autoradiography, high levels of [125I]LTT-SRIF-28 and [125I]Tyr3 -octreotide recognition sites were found in basal ganglia, molecular and granular layers of the cerebellum and layers III, V and VI of entorhinal cortex. In these regions, the addition of 3 nM seglitide produced a marked decrease of [125I]LTT-SRIF-28 binding. Low levels of [125I]LTT-SRIF-28 binding were observed in subiculum, pituitary and choroid plexus. By contrast, [125I]CGP 23996 labelling in the presence of Mg2+ as well as Na+ ions was highest in pituitary and choroid plexus. However, [125I]CGP 23996 binding was diversely affected by these ionic conditions in several regions of hippocampus and cerebral cortex. Displacement of [125I]CGP 23996 (in Mg(2+)-buffer) with seglitide in the molecular layer of the cerebellum, deep layers of the entorhinal cortex, layers I, II and V of the insular cortex and frontal pole yielded complex competition curves suggesting the presence of two populations of SRIF receptors. By contrast, [125I]CGP 23996 binding (in Mg(2+)-buffer) in the choroid plexus, hilus of the dentate gyrus and stratum oriens and radiatum of the CA3 field of hippocampus was not affected by seglitide up to 10 microM, suggesting only sst1 and/or sst4 sites which have a negligible affinity for seglitide to be present in these structures. Taken together, these results suggest that [125I]CGP 23996 (in the presence of Na+) labels exclusively SRIF-2 receptors (sst1 and/or sst4), whereas in the presence of Mg2+ ions, [125I]CGP 23996 labels both SRIF-2 and SRIF-1 receptors (sst2, sst3 and sst5). The present study also demonstrates the presence and differential distribution of sst2 and sst1/sst4 receptors in the Rhesus monkey brain.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

A quantitative autoradiographical study on the distribution of somatostatin sst2 receptors in the rat central nervous system using [125I]-BIM-23027

The kinetic properties, steady state binding characteristics and autoradiographic distribution of the somatostatin (SRIF) sst2 receptor-selective ligand, [125I]-BIM-23027, have been investigated in the rat central nervous system. Analysis of kinetic, saturation and competition binding data in rat hippocampal membranes was consistent with [125I]-BIM-23207 binding to a single population of non-interacting binding sites. Competition studies, using different SRIF ligands suggested that [125I]-BIM-23027 was binding to sites similar to that of the recombinant sst2 receptor. The rank order of affinity for displacing specific binding was BIM-23027 = SRIF > L-362855 > > BIM-23056. There was a widespread distribution of [125I]-BIM-23027 binding sites in the rat central nervous system. The highest density of binding was observed in the dentate gyrus, medial habenular, amygdala, claustrum and lateral septum as well as in the piriform, cingulate and parietal cortex. The cervical and lumbar spinal cord also displayed moderate levels of binding localized to the substantia gelatinosa. The cellular localization of [125I]-BIM-23027 binding was found to be associated with dendritic terminal fields. In contrast, the cellular signal for sst2 receptor mRNA was restricted to cell somata. The widespread distribution of [125I]-BIM-23027 binding sites within the brain suggests that receptors similar to the recombinant sst2 receptor may mediate a variety of different physiological effects within the central nervous system.

Cellular localisation and co-expression of somatostatin receptor messenger RNAs in the human brain

Genes for five high affinity somatostatin receptors, named sst1-5, have been cloned recently. In this study we describe the tissue distribution and cellular localisation of mRNA encoding sst1, sst3 and sst4 receptors in the human cerebellum, frontal cortex (Brodmann's area 11) and hippocampus. RT-PCR and in situ hybridisation studies indicated a distinct, but partially overlapping pattern of expression of the receptor mRNAs. In situ hybridisation studies using co-expression techniques with probes for sst1, sst3 and sst4 receptor mRNA on paraffin sections revealed the presence of neurones expressing more than one somatostatin receptor mRNA type in both the hippocampus and pyramidal cells of layer V of the frontal cortex.