An Overview of In Vitro Assays of 64Cu-, 68Ga-, 125I-, and 99mTc-Labelled Radiopharmaceuticals Using Radiometric Counters in the Era of Radiotheranostics

Radionuclides are unstable isotopes that mainly emit alpha (α), beta (β) or gamma (γ) radiation through radiation decay. Therefore, they are used in the biomedical field to label biomolecules or drugs for diagnostic imaging applications, such as positron emission tomography (PET) and/or single-photon emission computed tomography (SPECT). A growing field of research is the development of new radiopharmaceuticals for use in cancer treatments. Preclinical studies are the gold standard for translational research. Specifically, in vitro radiopharmaceutical studies are based on the use of radiopharmaceuticals directly on cells. To date, radiometric β- and γ-counters are the only tools able to assess a preclinical in vitro assay with the aim of estimating uptake, retention, and release parameters, including time- and dose-dependent cytotoxicity and kinetic parameters. This review has been designed for researchers, such as biologists and biotechnologists, who would like to approach the radiobiology field and conduct in vitro assays for cellular radioactivity evaluations using radiometric counters. To demonstrate the importance of in vitro radiopharmaceutical assays using radiometric counters with a view to radiogenomics, many studies based on ⁶⁴Cu-, ⁶⁸Ga-, ¹²⁵I-, and ^99mTc-labeled radiopharmaceuticals have been revised and summarized in this manuscript.

Nitric Oxide Is Essential for Generating the Minute Rhythm Contraction Pattern in the Small Intestine, Likely via ICC-DMP

Nitrergic nerves have been proposed to play a critical role in the orchestration of peristaltic activities throughout the gastrointestinal tract. In the present study, we investigated the role of nitric oxide, using spatiotemporal mapping, in peristaltic activity of the whole ex vivo mouse intestine. We identified a propulsive motor pattern in the form of propagating myogenic contractions, that are clustered by the enteric nervous system into a minute rhythm that is dependent on nitric oxide. The cluster formation was abolished by TTX, lidocaine and nitric oxide synthesis inhibition, whereas the myogenic contractions, occurring at the ICC-MP initiated slow wave frequency, remained undisturbed. Cluster formation, inhibited by block of nitric oxide synthesis, was fully restored in a highly regular rhythmic fashion by a constant level of nitric oxide generated by sodium nitroprusside; but the action of sodium nitroprusside was inhibited by lidocaine indicating that it was relying on neural activity, but not rhythmic nitrergic nerve activity. Hence, distention-induced activity of cholinergic nerves and/or a co-factor within nitrergic nerves such as ATP is also a requirement for the minute rhythm. Cluster formation was dependent on distention but was not evoked by a distention reflex. Block of gap junction conductance by carbenoxolone, dose dependently inhibited, and eventually abolished clusters and contraction waves, likely associated, not with inhibition of nitrergic innervation, but by abolishing ICC network synchronization. An intriguing feature of the clusters was the presence of bands of rhythmic inhibitions at 4-8 cycles/min; these inhibitory patches occurred in the presence of tetrodotoxin or lidocaine and hence were not dependent on nitrergic nerves. We propose that the minute rhythm is generated by nitric oxide-induced rhythmic depolarization of the musculature via ICC-DMP.

Somatostatin as an Active Substance in the Mammalian Enteric Nervous System

Somatostatin (SOM) is an active substance which most commonly occurs in endocrine cells, as well as in the central and peripheral nervous system. One of the parts of the nervous system where the presence of SOM has been confirmed is the enteric nervous system (ENS), located in the wall of the gastrointestinal (GI) tract. It regulates most of the functions of the stomach and intestine and it is characterized by complex organization and a high degree of independence from the central nervous system. SOM has been described in the ENS of numerous mammal species and its main functions in the GI tract are connected with the inhibition of the intestinal motility and secretory activity. Moreover, SOM participates in sensory and pain stimuli conduction, modulation of the release of other neuronal factors, and regulation of blood flow in the intestinal vessels. This peptide is also involved in the pathological processes in the GI tract and is known as an anti-inflammatory agent. This paper, which focuses primarily on the distribution of SOM in the ENS and extrinsic intestinal innervation in various mammalian species, is a review of studies concerning this issue published from 1973 to the present.

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.

The Potential for Gut Organoid Derived Interstitial Cells of Cajal in Replacement Therapy

Effective digestion requires propagation of food along the entire length of the gastrointestinal tract. This process involves coordinated waves of peristalsis produced by enteric neural cell types, including different categories of interstitial cells of Cajal (ICC). Impaired food transport along the gastrointestinal tract, either too fast or too slow, causes a range of gut motility disorders that affect millions of people worldwide. Notably, loss of ICC has been shown to affect gut motility. Patients that suffer from gut motility disorders regularly experience diarrhoea and/or constipation, insomnia, anxiety, attention lapses, irritability, dizziness, and headaches that greatly affect both physical and mental health. Limited treatment options are available for these patients, due to the scarcity of human gut tissue for research and transplantation. Recent advances in stem cell technology suggest that large amounts of rudimentary, yet functional, human gut tissue can be generated in vitro for research applications. Intriguingly, these stem cell-derived gut organoids appear to contain functional ICC, although their frequency and functional properties are yet to be fully characterised. By reviewing methods of gut organoid generation, together with what is known of the molecular and functional characteristics of ICC, this article highlights short- and long-term goals that need to be overcome in order to develop ICC-based therapies for gut motility disorders.

Expression of β2 adrenoceptors within enteric neurons of the horse ileum

The activity of the gastrointestinal tract is regulated through the activation of adrenergic receptors (ARs). Since data concerning the distribution of ARs in the horse intestine is virtually absent, we investigated the distribution of β2-AR in the horse ileum using double-immunofluorescence. The β2-AR-immunoreactivity (IR) was observed in most (95%) neurons located in submucosal plexus (SMP) and in few (8%) neurons of the myenteric plexus (MP). Tyrosine hydroxylase (TH)-IR fibers were observed close to neurons expressing β2-AR-IR. Since β2-AR is virtually expressed in most neurons located in the horse SMP and in a lower percentage of neurons in the MP, it is reasonable to retain that this adrenergic receptor could regulate the activity of both secretomotor neurons and motor neurons innervating muscle layers and blood vessels. The high density of TH-IR fibers near β2-AR-IR enteric neurons indicates that the excitability of these cells could be directly modulated by the sympathetic system.

Agonist-biased trafficking of somatostatin receptor 2A in enteric neurons

Somatostatin (SST) 14 and SST 28 activate somatostatin 2A receptors (SSTR2A) on enteric neurons to control gut functions. SST analogs are treatments of neuroendocrine and bleeding disorders, cancer, and diarrhea, with gastrointestinal side effects of constipation, abdominal pain, and nausea. How endogenous agonists and drugs differentially regulate neuronal SSTR2A is unexplored. We evaluated SSTR2A trafficking in murine myenteric neurons and neuroendocrine AtT-20 cells by microscopy and determined whether agonist degradation by endosomal endothelin-converting enzyme 1 (ECE-1) controls SSTR2A trafficking and association with β-arrestins, key regulators of receptors. SST-14, SST-28, and peptide analogs (octreotide, lanreotide, and vapreotide) stimulated clathrin- and dynamin-mediated internalization of SSTR2A, which colocalized with ECE-1 in endosomes and the Golgi. After incubation with SST-14, SSTR2A recycled to the plasma membrane, which required active ECE-1 and an intact Golgi. SSTR2A activated by SST-28, octreotide, lanreotide, or vapreotide was retained within the Golgi and did not recycle. Although ECE-1 rapidly degraded SST-14, SST-28 was resistant to degradation, and ECE-1 did not degrade SST analogs. SST-14 and SST-28 induced transient interactions between SSTR2A and β-arrestins that were stabilized by an ECE-1 inhibitor. Octreotide induced sustained SSTR2A/β-arrestin interactions that were not regulated by ECE-1. Thus, when activated by SST-14, SSTR2A internalizes and recycles via the Golgi, which requires ECE-1 degradation of SST-14 and receptor dissociation from β-arrestins. After activation by ECE-1-resistant SST-28 and analogs, SSTR2A remains in endosomes because of sustained β-arrestin interactions. Therapeutic SST analogs are ECE-1-resistant and retain SSTR2A in endosomes, which may explain their long-lasting actions.

Long-term culture and cryopreservation of interstitial cells of Cajal

OBJECTIVE

Interstitial cells of Cajal (ICCs) in the gastrointestinal tract generate and propagate slow waves and mediate neuromuscular neurotransmission. Damage to ICCs has been described in several gastrointestinal motor disorders, and although many studies have examined ICCs in culture, they have been largely limited to freshly dissociated cells or short-term cultures. An efficient and reliable method to establish a source of ICCs is much needed. The aim of this study was to investigate methods for culturing, subculturing, cryopreservation, and recovery of ICCs.

METHODS

ICCs were derived from intestinal segments of domestic rabbits, and immunohistochemistry for c-Kit was used to identify ICCs in culture and after recovery. Recovered ICCs were also examined for motilin receptor expression.

RESULTS

Optimal conditions for ICC culture and cryopreservation were based on cell growth curves and MTT assay. On the basis of these findings, recovered cells were cultured for 7 days and then sorted via flow cytometry based on c-Kit immunoreactivity. The percent of c-Kit positive cells was 64.3%, and the number of ICCs sorted was 6.7 × 10(5). Reverse-transcription polymerase chain reaction and western blotting verified motilin receptor expression in c-Kit-positive ICCs.

CONCLUSIONS

This is the first study to describe the culture, passage, and recovery of ICCs and to show motilin receptor expression. Our results suggest that ICCs play an important role, at least in some species, in initiating the migrating myoelectric complex induced by motilin.

Octreotide ameliorates intestinal dysmotility by interstitial cells of Cajal protection in a rat acute necrotizing pancreatitis model

OBJECTIVES

Intestinal motility is impaired in acute necrotizing pancreatitis (ANP). The aim of present study was to investigate the effects of octreotide on the small intestinal motor function during experimentally induced ANP.

METHODS

L-Ornithine was intraperitoneally injected to induce ANP. Octreotide was administrated subcutaneously every 8 hours. The small intestine migrating myoelectrical complexes and slow waves in vivo were recorded before and after (24, 48, and 72 hours) ANP induction. The morphological alterations of interstitial cells of Cajal (ICCs) in deep muscular plexus were evaluated by immunohistochemistry and Western blots.

RESULTS

Disturbed migrating myoelectrical complex cycle length and decreased dominant frequency of slow waves exacerbated gradually with time. The bolus applications of octreotide per 8 hours attenuated these functional abnormalities. The result of morphological study suggested that octreotide might ameliorate the damage of ICCs at 48 and 72 hours after ANP induction. Decreased expression of c-Kit protein at 72 hours was also attenuated by octreotide.

CONCLUSIONS

The pathogenesis of the ileus in ANP may be related to the sustained deficiencies in ICCs. Octreotide may ameliorate the severity of ileus by minimizing the injury of ICCs.

Comparison of CRF-immunoreactive neurons distribution in mouse and rat brains and selective induction of Fos in rat hypothalamic CRF neurons by abdominal surgery

Mice and rats are widely used in stress-related behavioral studies while little is known about the distribution of the stress hormone, corticotropin-releasing factor (CRF) in the mouse brain. We developed and characterized a novel rat/mouse CRF polyclonal antibody (CURE ab 200101) that was used to detect and compare the brain distributions of CRF immunoreactivity in naïve and colchicine-treated rats and mice. We also assessed whether the visceral stressor of abdominal surgery activated brain CRF neurons using double labeling of Fos/CRF in naïve rats. CRF-ir neurons were visualized in the cortex, bed nucleus of the stria terminalis, central amygdala, hypothalamic paraventricular nucleus (PVN), Barrington's nucleus and dorsolateral tegmental area in naïve rats. CRF-immunoreactive (ir) neurons in the mouse brain were detected only after colchicine. The pattern shows fundamental similarity compared to the colchicine-treated rat brain, however, there were differences with a lesser distribution in both areas and density except in the lateral septum and external subnucleus of the lateral parabrachial nucleus which contained more CRF-ir neurons in mice, and CRF-ir neurons in the dorsal motor nucleus of the vagus were found only in mice. Abdominal surgery in naïve rats induced Fos-ir in 30% of total CRF-ir neurons in the PVN compared with control (anesthesia alone) while Fos was not co-localized with CRF in other brain nuclei. These data indicate that CRF-ir distribution in the brain displays similarity as well as distinct features in mice compared to rats that may underlie some differential stress responses. Abdominal surgery activates CRF-ir neurons selectively in the PVN of rats without colchicine treatment.

Localization and regulation of fluorescently labeled delta opioid receptor, expressed in enteric neurons of mice

BACKGROUND & AIMS

Opioids and opiates inhibit gastrointestinal functions via μ, δ, and κ receptors. Although agonists of the δ opioid receptor (DOR) suppress motility and secretion, little is known about the localization and regulation of DOR in the gastrointestinal tract.

METHODS

We studied mice in which the gene that encodes the enhanced green fluorescent protein (eGFP) was inserted into Oprd1, which encodes DOR, to express an approximately 80-kilodalton product (DOReGFP). We used these mice to localize DOR and to determine how agonists regulate the subcellular distribution of DOR.

RESULTS

DOReGFP was expressed in all regions but was confined to enteric neurons and fibers within the muscularis externa. In the submucosal plexus, DOReGFP was detected in neuropeptide Y-positive secretomotor and vasodilator neurons of the small intestine, but rarely was observed in the large bowel. In the myenteric plexus of the small intestine, DOReGFP was present in similar proportions of excitatory motoneurons and interneurons that expressed choline acetyltransferase and substance P, and in inhibitory motoneurons and interneurons that contained nitric oxide synthase. DOReGFP was present mostly in nitrergic myenteric neurons of colon. DOReGFP and μ opioid receptors often were co-expressed. DOReGFP-expressing neurons were associated with enkephalin-containing varicosities, and enkephalin-induced clathrin- and dynamin-mediated endocytosis and lysosomal trafficking of DOReGFP. DOReGFP replenishment at the plasma membrane was slow, requiring de novo synthesis, rather than recycling.

CONCLUSIONS

DOR localizes specifically to submucosal and myenteric neurons, which might account for the ability of DOR agonists to inhibit gastrointestinal secretion and motility. Sustained down-regulation of DOReGFP at the plasma membrane of activated neurons could induce long-lasting tolerance to DOR agonists.

Abdominal surgery inhibits circulating acyl ghrelin and ghrelin-O-acyltransferase levels in rats: role of the somatostatin receptor subtype 2

Clinical studies are evaluating the efficacy of synthetic ghrelin agonists in postoperative ileus management. However, the control of ghrelin secretion under conditions of postoperative gastric ileus is largely unknown. Peripheral somatostatin inhibits ghrelin secretion in animals and humans. We investigated the time course of ghrelin changes postsurgery in fasted rats and whether somatostatin receptor subtype 2 (sst(2)) signaling is involved. Abdominal surgery (laparotomy and 1-min cecal palpation) induced a rapid and long-lasting decrease in plasma acyl ghrelin levels as shown by the 64, 67, and 59% reduction at 0.5, 2, and 5 h postsurgery, respectively, compared with sham (anesthesia alone for 10 min, P < 0.05). Levels were partly recovered at 7 h and fully restored at 24 h. The percentage of acyl ghrelin reduction was significantly higher than that of desacyl ghrelin at 2 h postsurgery and not at any other time point. This was associated with a 48 and 23% decrease in gastric and plasma ghrelin-O-acyltransferase protein concentrations, respectively (P < 0.001). Ghrelin-positive cells in the oxyntic mucosa expressed sst(2a) receptor and the sst(2) agonist S-346-011 inhibited fasting acyl ghrelin levels by 64 and 77% at 0.5 and 2 h, respectively. The sst(2) antagonist S-406-028 prevented the abdominal surgery-induced decreased circulating acyl ghrelin but not the delayed gastric emptying assessed 0.5 h postinjection. These data show that activation of sst(2) receptor located on gastric X/A-like cells plays a key role in the rapid inhibition of circulating acyl ghrelin induced by abdominal surgery while not being primarily involved in the early phase of postoperative gastric ileus.

The effect of gastric inhibitory polypeptide on intestinal glucose absorption and intestinal motility in mice

Gastric inhibitory polypeptide (GIP) is released from the small intestine upon meal ingestion and increases insulin secretion from pancreatic β cells. Although the GIP receptor is known to be expressed in small intestine, the effects of GIP in small intestine are not fully understood. This study was designed to clarify the effect of GIP on intestinal glucose absorption and intestinal motility. Intestinal glucose absorption in vivo was measured by single-pass perfusion method. Incorporation of [(14)C]-glucose into everted jejunal rings in vitro was used to evaluate the effect of GIP on sodium-glucose co-transporter (SGLT). Motility of small intestine was measured by intestinal transit after oral administration of a non-absorbed marker. Intraperitoneal administration of GIP inhibited glucose absorption in wild-type mice in a concentration-dependent manner, showing maximum decrease at the dosage of 50 nmol/kg body weight. In glucagon-like-peptide-1 (GLP-1) receptor-deficient mice, GIP inhibited glucose absorption as in wild-type mice. In vitro examination of [(14)C]-glucose uptake revealed that 100 nM GIP did not change SGLT-dependent glucose uptake in wild-type mice. After intraperitoneal administration of GIP (50 nmol/kg body weight), small intestinal transit was inhibited to 40% in both wild-type and GLP-1 receptor-deficient mice. Furthermore, a somatostatin receptor antagonist, cyclosomatostatin, reduced the inhibitory effect of GIP on both intestinal transit and glucose absorption in wild-type mice. These results demonstrate that exogenous GIP inhibits intestinal glucose absorption by reducing intestinal motility through a somatostatin-mediated pathway rather than through a GLP-1-mediated pathway.

Activation of neuronal SST₁ and SST₂ receptors decreases neurogenic secretion in the guinea-pig jejunum

BACKGROUND

Vasoactive intestinal peptide (VIP) submucosal neurons, the main regulators of gut secretion, display inhibitory postsynaptic potentials mediated by somatostatin (SOM) acting on SST(1) and SST(2) receptors (SSTR(1), SSTR(2)) in the guinea-pig small intestine. We investigated the implications of this for neurally-evoked mucosal secretion.

METHODS

Mucosal-submucosal preparations from guinea-pig jejunum were mounted in Ussing chambers to measure Cl(-) secretion, measured by short circuit current (I(sc)). All drugs were added serosally. Veratridine (1 μmol L(-1)) was used to stimulate neurons and provide a robust secretory response for pharmacological testing.5-hydroxytrptamine (5-HT, 300 nmol L(-1)) was used to specifically activate non-cholinergic secretomotor neurons, while 1,1-dimethyl-4-phenylpiperazinium (DMPP, 10 μmol L(-1)) was used to stimulate all secretomotor neurons.

KEY RESULTS

Somatostatin (50 nmol L(-1)) induced a tetrodotoxin (TTX, 1 μmol L(-1))-sensitive decrease in secretion. Somatostatin also reduced the veratridine-induced increase in I(sc). The effects of SOM were significantly reduced by blocking SSTR(1) and SSTR(2) individually or together. Blocking SSTR(1) abolished the inhibition produced by SOM. Quantitative PCR demonstrated that SSTR(1) and SSTR(2) were much more highly expressed in the submucosa than the mucosa. Submucosal SSTR(1) expression was several fold higher than SSTR(2). Responses to DMPP (biphasic) and 5-HT (monophasic) were TTX-sensitive. Somatostatin significantly reduced the 5-HT-induced increase in I(sc), and the second, more sustained phase evoked by DMPP.

CONCLUSIONS & INFERENCES

These data suggest that SOM exerts its antisecretory effects by suppressing firing of VIP secretomotor neurons, rather than via a direct action on mucosal enterocytes.

Pattern of Fos expression in the brain induced by selective activation of somatostatin receptor 2 in rats

Central activation of somatostatin (sst) receptors by oligosomatostatin analogs inhibits growth hormone and stress-related rise in catecholamine plasma levels while stimulating grooming, feeding behaviors, gastric transit and acid secretion, which can be mimicked by selective sst(2) receptor agonist. To evaluate the pattern of neuronal activation induced by peptide sst receptor agonists, we assessed Fos-expression in rat brain after intracerebroventricular (i.c.v.) injection of a newly developed selective sst(2) agonist compared to the oligosomatostatin ODT8-SST, a pan-sst(1-5) agonist. Ninety min after injection of vehicle (10 microl) or previously established maximal orexigenic dose of peptides (1 microg=1 nmol/rat), brains were assessed for Fos-immunohistochemistry and doublelabeling. Food and water were removed after injection. The sst(2) agonist and ODT8-SST induced a similar Fos distribution pattern except in the arcuate nucleus where only the sst(2) agonist increased Fos. Compared to ODT8-SST, the sst(2) agonist induced higher Fos-expression by 3.7-times in the basolateral amygdaloid nucleus, 1.2-times in the supraoptic nucleus (SON), 1.6-times in the magnocellular paraventricular hypothalamic nucleus (mPVN), 4.1-times in the external lateral parabrachial nucleus, and 2.6-times in both the inferior olivary nucleus and superficial layer of the caudal spinal trigeminal nucleus. Doublelabeling in the hypothalamus showed that ODT8-SST activates 36% of oxytocin, 63% of vasopressin and 79% of sst(2) immunoreactive neurons in the mPVN and 28%, 55% and 25% in the SON, respectively. Selective activation of sst(2) receptor results in a more robust neuronal activation than the pan-sst(1-5) agonist in various brain regions that may have relevance in sst(2) mediated alterations of behavioral, autonomic and endocrine functions.

5-HT(1A), SST(1), and SST(2) receptors mediate inhibitory postsynaptic potentials in the submucous plexus of the guinea pig ileum

Vasoactive intestinal peptide (VIP) immunoreactive neurons are important secretomotor neurons in the submucous plexus. They are the only submucosal neurons to receive inhibitory inputs and exhibit both noradrenergic and nonadrenergic inhibitory synaptic potentials (IPSPs). The former are mediated by alpha(2)-adrenoceptors, but the receptors mediating the latter have not been identified. We used standard intracellular recording, RT-PCR, and confocal microscopy to test whether 5-HT(1A), SST(1), and/or SST(2) receptors mediate nonadrenergic IPSPs in VIP submucosal neurons in guinea pig ileum in vitro. The specific 5-HT(1A) receptor antagonist WAY 100135 (1 microM) reduced the amplitude of IPSPs, an effect that persisted in the presence of the alpha(2)-adrenoceptor antagonist idazoxan (2 microM), suggesting that 5-HT might mediate a component of the IPSPs. Confocal microscopy revealed that there were many 5-HT-immunoreactive varicosities in close contact with VIP neurons. The specific SSTR(2) antagonist CYN 154806 (100 nM) and a specific SSTR(1) antagonist SRA 880 (3 microM) each reduced the amplitude of nonadrenergic IPSPs and hyperpolarizations evoked by somatostatin. In contrast with the other antagonists, CYN 154806 also reduced the durations of nonadrenergic IPSPs. Effects of WAY 100135 and CYN 154806 were additive. RT-PCR revealed gene transcripts for 5-HT(1A), SST(1), and SST(2) receptors in stripped submucous plexus preparations consistent with the pharmacological data. Although the involvement of other neurotransmitters or receptors cannot be excluded, we conclude that 5-HT(1A), SST(1), and SST(2) receptors mediate nonadrenergic IPSPs in the noncholinergic (VIP) secretomotor neurons. This study thus provides the tools to identify functions of enteric neural pathways that inhibit secretomotor reflexes.

Immunohistochemical expression of somatostatin receptors in digestive endocrine tumours

INTRODUCTION

Somatostatin receptors are expressed in a large number of human tumours. The somatostatin receptors types 1-5 expression in a series including 100 gastro-entero-pancreatic endocrine tumours were analysed.

METHODS

From a prospectively built database of patients with gastro-entero-pancreatic endocrine tumours referred from three institutions, 100 cases with clinical and pathological data were selected. Somatostatin receptors expression by immunohistochemistry with somatostatin receptor types 1-5 antibodies in tissue paraffin sections were studied and correlated with the histological diagnosis according to the WHO classification, location and functional status.

RESULTS

Of the 100 cases, 67 were gastrointestinal tumours, 25 pancreatic and 8 liver metastasis of unknown origin. Thirty-one of them were functioning tumours: 2 insulinomas, 5 gastrinomas, 1 glucagonoma and 23 carcinoids. Somatostatin receptors expression was observed in 94 tumours. The six negative cases were all non-functioning tumours. Somatostatin receptors 2a and 5 were highly expressed (86 and 62%, respectively), and surprisingly found even in poorly differentiated endocrine carcinomas. Somatostatin receptors expression was less frequent in pancreatic than in gastrointestinal tumours. Well-differentiated neoplasms had a higher density of somatostatin receptors. Only SSTR2a showed membrane staining.

CONCLUSIONS

Immunohistochemistry revealed that somatostatin receptors were highly expressed in both primary and metastatic gastro-entero-pancreatic endocrine tumours with heterogeneous staining distribution. It proved to be a reliable technique even in small tumour samples.

Updating old ideas and recent advances regarding the Interstitial Cells of Cajal

Since their discovery by Cajal in 1889, the Interstitial Cells of Cajal (ICC) have generated much controversy in the scientific community. Indeed, the nervous, muscle or fibroblastic nature of the ICC has remained under debate for more than a century, as has their possible physiological function. Cajal and his colleagues considered them to be neurons, while contemporary histologists like Kölliker and Dogiel categorized these cells as fibroblasts. More recently, the role of ICC in the origin of slow-wave peristaltism has been elucidated, and several studies have shown that they participate in neurotransmission (intercalation theory). The fact that ICC assemble in the circular muscular layer and that they originate from cells which emerge from the ventral neural tube (VENT cells), a source of neurons, glia and ICC precursors other than the neural crest, suggests a neural origin for this particular subset of ICC. The discovery that ICC express the Kit protein, a type III tyrosine kinase receptor encoded by the proto-oncogene c-kit, has helped better understand their physiological role and implication in pathological conditions. Gleevec, a novel molecule designed to inhibit the mutant activated version of c-Kit receptors, is the drug of choice to treat the so-called gastrointestinal stromal tumours (GIST), the most common non-epithelial neoplasm of the gastrointestinal tract. Here we review Cajal's original contributions with the aid of unique images taken from Cajal's histological slides (preserved at the Cajal Museum, Cajal Institute, CSIC). In addition, we present a historical review of the concepts associated with this particular cell type, emphasizing current data that has advanced our understanding of the role these intriguing cells fulfil.

The role(s) of somatostatin, structurally related peptides and somatostatin receptors in the gastrointestinal tract: a review

Extensive functional and morphological research has demonstrated the pivotal role of somatostatin (SOM) in the regulation of a wide variety of gastrointestinal activities. In addition to its profound inhibitory effects on gastrointestinal motility and exocrine and endocrine secretion processes along the entire gastrointestinal tract, SOM modulates several organ-specific activities. In contrast to these well-known SOM-dependent effects, knowledge on the SOM receptors (SSTR) involved in these effects is much less conclusive. Experimental data on the identities of the SSTRs, although species- and tissue-dependent, point towards the involvement of multiple receptor subtypes in the vast majority of gastrointestinal SOM-mediated effects. Recent evidence demonstrating the role of SOM in intestinal pathologies has extended the interest of gastrointestinal research in this peptide even further. More specifically, SOM is supposed to suppress intestinal inflammatory responses by interfering with the extensive bidirectional communication between mucosal mast cells and neurons. This way, SOM not only acts as a powerful inhibitor of the inflammatory cascade at the site of inflammation, but exerts a profound antinociceptive effect through the modulation of extrinsic afferent nerve fibres. The combination of these physiological and pathological activities opens up new opportunities to explore the potential of stable SOM analogues in the treatment of GI inflammatory pathologies.

Control of gastric acid secretion in somatostatin receptor 2 deficient mice: shift from endocrine/paracrine to neurocrine pathways

The gastrin-enterochromaffin-like (ECL) cell-parietal cell axis is known to play an important role in the regulation of gastric acid secretion. Somatostatin, acting on somatostatin receptor type 2 (SSTR(2)), interferes with this axis by suppressing the activity of the gastrin cells, ECL cells, and parietal cells. Surprisingly, however, freely fed SSTR(2) knockout mice seem to display normal circulating gastrin concentration and unchanged acid output. In the present study, we compared the control of acid secretion in these mutant mice with that in wild-type mice. In SSTR(2) knockout mice, the number of gastrin cells was unchanged; whereas the numbers of somatostatin cells were reduced in the antrum (-55%) and increased in the oxyntic mucosa (35%). The ECL cells displayed a reduced expression of histidine decarboxylase and vesicle monoamine transport type 2 (determined by immunohistochemistry), and an impaired transformation of the granules to secretory vesicles (determined by electron microscopic analysis), suggesting low activity of the ECL cells. These changes were accompanied by an increased expression of galanin receptor type 1 in the oxyntic mucosa. The parietal cells were found to respond to pentagastrin or to vagal stimulation (evoked by pylorus ligation) with increased acid production. In conclusion, the inhibitory galanin-galanin receptor type 1 pathway is up-regulated in the ECL cells, and the direct stimulatory action of gastrin and vagal excitation is enhanced on the parietal cells in SSTR(2) knockout mice. We suggest that there is a remodeling of the neuroendocrine mechanisms that regulate acid secretion in these mutant mice.

Inhibition of pacemaker currents by nitric oxide via activation of ATP-sensitive K+ channels in cultured interstitial cells of Cajal from the mouse small intestine

We investigated the role of nitric oxide (NO) in pacemaker activity and signal mechanisms in cultured interstitial cells of Cajal (ICC) of the mouse small intestine using whole cell patch-clamp techniques at 30 degrees C. ICC generated pacemaker potential in the current clamp mode and pacemaker currents at a holding potential of -70 mV. (+/-)-S-nitroso-N-acetylpenicillamine (SNAP; a NO donor) produced membrane hyperpolarization and inhibited the amplitude and frequency of the pacemaker currents, and increased resting currents in the outward direction. These effects were blocked by the use of glibenclamide (an ATP-sensitive K+ channel blocker), but not by the use of 5-hydroxydecanoic acid (a mitochondrial ATP-sensitive K+ channel blocker). Pretreatment with ODQ (a guanylate cyclase inhibitor) almost blocked the NO-induced effects. The use of cell-permeable 8-bromo-cyclic GMP also mimicked the action of SNAP. However, the use of KT-5823 (a protein kinase G inhibitor) did not block the NO-induced effects. Spontaneous [Ca2+]i oscillations in ICC were inhibited by the treatment of SNAP, as seen in recordings of intracellular Ca2+ ([Ca2+]i). These results suggest that NO inhibits pacemaker activity by the activation of ATP-sensitive K+ channels via a cyclic GMP dependent mechanism in ICC, and the activation of ATP-sensitive K+ channels mediates the inhibition of spontaneous [Ca2+]i oscillations.

Interstitial cells of cajal are involved in neurotransmission in the gastrointestinal tract

Interstitial cells of Cajal (ICC) are important cells which coordinate gastrointestinal motility. ICC express Kit receptor tyrosine kinase, and Kit immunohistochemistry reveals ICC morphology and distribution in the gastrointestinal musculature. ICC show a highly branched morphology and form unique networks. Myenteric ICC (ICC-MY) are located at the layer of the myenteric plexus and serve as electrical pacemakers. Intramuscular ICC (ICC-IM) and ICC in the deep muscular plexus (ICC-DMP) are distributed within the muscular layers, and are densely innervated by excitatory and inhibitory enteric motor neurons and in close contact with nerve terminals. Recent studies combined with morphological and functional techniques directly revealed that ICC-IM and ICC-DMP are mediators of enteric motor neuro-transmission. These types of ICC express several receptors for neurotransmitters such as acetylcholine and substance P and show responses to excitatory nerve stimulations. ICC also express receptive mechanisms for nitric oxide, which is an inhibitory neurotransmitter in the gastrointestinal tract. They can respond to nitrergic nerve stimulation by cyclic GMP production. Kit mutant mice lack ICC-IM and show attenuated postsynaptic responses after intrinsic nerve stimulation. These findings indicate the importance for ICC in neurotransmission in the gastrointestinal tract.

Involvement of intramuscular interstitial cells of Cajal in neuroeffector transmission in the gastrointestinal tract

Specialized cells known as interstitial cells of Cajal (ICC) are distributed in specific locations within the tunica muscularis of the gastrointestinal (GI) tract. ICC serve as electrical pacemakers, provide pathways for the active propagation of slow waves, are mediators of enteric motor neurotransmission and play a role in afferent neural signalling. Morphological studies have provided evidence that motor neurotransmission in the GI tract does not occur through poorly defined structures between nerves and smooth muscle, but rather via specialized synapses that exist between enteric nerve terminals and intramuscular ICC or ICC-IM. ICC-IM are coupled to smooth muscle cells via gap junctions and post-junctional responses elicited in ICC-IM are conducted to neighbouring smooth muscle cells. Electrophysiological studies from the stomachs and sphincters of wild-type and mutant animals that lack ICC-IM have provided functional evidence for the importance of ICC in cholinergic excitatory and nitrergic inhibitory motor neurotransmission. Intraperitoneal injection of animals with Kit neutralizing antibody or organ culture of gastrointestinal tissues in the presence of neutralizing antibody, which blocks the development and maintenance of ICC, has provided further evidence for the role of ICC in enteric motor transmission. ICC-IM also generate an ongoing discharge of unitary potentials in the gastric fundus and antrum that contributes to the overall excitability of the stomach.

Selective labeling and isolation of functional classes of interstitial cells of Cajal of human and murine small intestine

Specific functions of interstitial cells of Cajal (ICC) have been linked to distinct classes that differ by morphology and distribution. In the small intestine, slow wave-generating ICC are located in the myenteric region (ICC-MY), whereas ICC that mediate neuromuscular neurotransmission occur either throughout the circular muscle layer (intramuscular ICC, ICC-IM) or in association with the deep muscular plexus (ICC-DMP). Selective isolation of ICC to characterize specific properties has been difficult. Recently, neurokinin-1 receptors have been detected in murine ICC-DMP and neurons but not in ICC-MY. Here we identified and isolated ICC-DMP/IM by receptor-mediated internalization of fluorescent substance P and Kit immunofluorescence. Specificity of labeling was verified by confocal microscopy. Mouse and human ICC-DMP/IM were detected in suspension by fluorescent microscopy and harvested for RT-PCR with micropipettes. The isolated cells expressed Kit but not markers for neurons, smooth muscle, or antigen-presenting cells. ICC-DMP expressed neurokinin-1 receptor, M(2) and M(3) muscarinic receptors, P2Y(1) and P2Y(4) purinergic receptors, VIP receptor 2, soluble guanylate cyclase-1 subunits, and protein kinase G. L- or T-type Ca(2+) channels were not detected in these cells. ICC-MY and ICC-DMP were simultaneously detected and enumerated by flow cytometry and sorted to purity by fluorescence-activated cell sorting. In summary, functional classes of ICC have distinct molecular identities that can be used to selectively identify and harvest these cells with, for example, receptor-mediated uptake of substance P and Kit immunofluorescence. ICC-DMP express neurotransmitter receptors and signaling intermediate molecules that are consistent with their role in neuromuscular neurotransmission.

GLP-2 receptor localizes to enteric neurons and endocrine cells expressing vasoactive peptides and mediates increased blood flow

BACKGROUND & AIMS

Glucagon-like peptide-2 (GLP-2) is a nutrient-responsive hormone that exerts diverse actions in the gastrointestinal tract, including enhancing epithelial cell survival and proliferation, mucosal blood flow, and nutrient uptake and suppressing gastric motility and secretion. These actions are mediated by the G-protein-coupled receptor, GLP-2R. Cellular localization of the GLP-2R and the nature of its signaling network in the gut, however, are poorly defined. Thus, our aim was to establish cellular localization of GLP-2R and functional connection to vascular action of GLP-2 in the gut.

METHODS

Intestinal cellular GLP-2R localization was determined with real-time, quantitative reverse-transcription polymerase chain reaction (qRT-PCR) of laser capture microdissected subtissue and fluorescence in situ hybridization and also with double and/or triple immunostaining of human and pig tissue using a validated GLP-2R polyclonal antibody. Superior mesenteric arterial blood flow and intestinal eNOS expression and phosphorylation were measured in TPN-fed pigs acutely (4 h) infused with GLP-2.

RESULTS

We show that the porcine GLP-2R mRNA was expressed in the villus epithelium and myenteric plexus. GLP-2R protein was co-localized by confocal immunohistochemistry with serotonin in enteroendocrine cells and also with endothelial nitric oxide synthase (eNOS)-expressing and vasoactive intestinal polypeptide-positive enteric neurons. In neonatal pigs, GLP-2 infusion dose-dependently stimulated intestinal blood flow and coordinately upregulated the expression of intestinal eNOS mRNA, protein, and phosphorylation (eNOS-Ser1117).

CONCLUSIONS

We conclude that the GLP-2-induced stimulation of blood flow is mediated by vasoactive neurotransmitters that are colocalized with GLP-2R in 2 functionally distinct cell types within the gastrointestinal tract.

Effect of N-methyl-d-aspartate receptor blockade on neuronal plasticity and gastrointestinal transit delay induced by ischemia/reperfusion in rats

Intestinal ischemia impairs gastrointestinal motility. The aims of this study were to investigate the effect of intestinal ischemia on gastrointestinal transit and on the expression of enteric transmitters in the rat, and whether the glutamate N-methyl-d-aspartate receptors influence these effects. Ischemia (1 h), induced by occluding the superior mesenteric artery, was followed by 0 or 24 h of reperfusion. Normal and sham-operated rats served as controls. Serosal blood flow was measured with laser Doppler flow meter. Gastrointestinal transit was measured as time of appearance of a marker in fecal pellets. Immunohistochemistry was used to evaluate the number of neurons immunoreactive for neuronal nitric oxide synthase (NOS) or vasoactive intestinal polypeptide and the density of substance P immunoreactive fibers in the myenteric plexus. The N-methyl-d-aspartate receptors antagonist, (+)-5-methyl-10,11-dihydro-5HT-[a,b] cyclohepten-5,10-imine (MK-801) (1 mg/kg i.v.) or the NOS inhibitor, N-nitro-l-arginine (10 mg/kg i.v.) was administered prior to ischemia. Serosal blood flow was decreased by 70% during ischemia, but it was not altered in sham-operated rats. Gastrointestinal transit was significantly prolonged in ischemic/reperfused rats compared with controls. There was a significant increase in the number of vasoactive intestinal polypeptide and neuronal nitric oxide synthase immunoreactive neurons, and a marked decrease of substance P immunoreactive fibers in ischemia followed by 24 h of reperfusion animals compared with controls. These alterations were not observed in ischemia without reperfusion. A significant delay of gastrointestinal transit and increase of vasoactive intestinal polypeptide neurons were also observed in sham-operated rats. The changes in transmitter expression and gastrointestinal transit in ischemic/reperfused rats were prevented by pre-treatment with the NOS inhibitor, N-nitro-l-arginine or the N-methyl-d-aspartate receptors antagonist, MK-801. This study suggests an involvement of the glutamatergic system and its interaction with nitric oxide in intestinal ischemia/reperfusion. Ischemia/reperfusion might induce local release of glutamate that activates N-methyl-d-aspartate receptors leading to increased production of nitric oxide and adaptive changes in enteric transmitters that might contribute to gastrointestinal dysmotility.

Bombesin receptor subtype-3 is expressed by the enteric nervous system and by interstitial cells of Cajal in the rat gastrointestinal tract

Bombesin receptor subtype-3 (BRS-3), a G-protein-coupled orphan receptor, shares 47% and 55% homology with other known mammalian bombesin receptors. Despite the molecular characterization of BRS-3, its function remains unclear as a consequence of its low affinity for bombesin and the absence of an identified natural ligand. Although the other mammalian bombesin receptors are widely distributed in the gut and central nervous system, expression of BRS-3 in the gastrointestinal tract has not been previously described. We report the expression of BRS-3 mRNA and protein in the tunica muscularis of the rat gastrointestinal tract. The mRNA expression pattern was studied by reverse transcription followed by quantitative polymerase chain reaction. To identify the cellular sites of expression of BRS-3, we performed immunocytochemistry by using a N-terminus-specific affinity-purified antiserum. BRS-3 was found to be widely expressed in the rat gastrointestinal tract at both the mRNA and protein levels. BRS-3-like immunoreactivity (BRS-3-LI) was localized in neurons of the myenteric and submucosal ganglia, being primarily concentrated near the neuronal plasma membrane, and in fibers distributed in the longitudinal and circular muscle layers. In addition, BRS-3-LI was observed in the cell bodies and processes of c-kit+ interstitial cells of Cajal. These data have functional applications for the effects mediated by the activation of BRS-3 on gut motility through distinct neuronal and non-neuronal pathways.

Role of somatostatin receptors on gastric acid secretion in wild-type and somatostatin receptor type 2 knockout mice

Somatostatin, probably acting through somatostatin type 2 receptors (SSTR2), is the main inhibitor of gastric acid secretion. We characterized gastric acid secretion in SSTR2 knockout mice, and used preferential somatostatin receptor agonists to assess the relative role of SSTR1, 2, 3, 4, and 5 on gastric acid secretion. Basal gastric acid secretion and the secretory response to a meal were similar in conscious wild-type and knockout mice. However, under urethane anesthesia, which releases endogenous somatostatin, SSTR2 knockout mice had a basal secretion 11-15-fold higher than wild-type animals (micromol/10 min:1.40+/-0.09 vs. 0.10+/-0.01, p<0.05). Gastrin immunoneutralization or H(2) receptors blockade (cimetidine), but not cholinergic blockade (atropine), reduced the high basal secretion in SSTR2 knockout mice. In SSTR2 knockout mice, gastrin and histamine stimulated acid secretion with similar efficacy, while in wild-type mice histamine was more effective than gastrin. SSTR2 knockout mice showed also a hypersecretory response to pylorus ligation compared with wild-type animals. In wild-type mice, somatostatin-14, SMS 201-995, and the SSTR2-preferential agonist, DC 32-87, inhibited gastrin-stimulated acid secretion with an order of potency SMS 201-995>DC 32-87>somatostatin-14. Preferential agonists for the SSTR1, 3, 4, and 5 were devoid of any effect. None of the compounds tested affected the high basal secretion observed under urethane anesthesia in SSTR2 knockout mice. These results show that gastric antisecretory effects of peripheral somatostatin are mediated solely through SSTR2. In the absence of functional SSTR2 other somatostatin receptors do not compensate for the lack somatostatin-SSTR2-mediated inhibition. Basal acid secretion and the response to a meal are normal in conscious SSTR2 knockout mice, suggesting the presence of somatostatin-independent mechanisms that compensate for the lack of somatostatin-SSTR2-mediated inhibitory responses.

Peripheral PACAP inhibits gastric acid secretion through somatostatin release in mice

1. Studies in rats suggest that PACAP modulates gastric acid secretion through the release of both histamine and somatostatin. 2. We characterized the effects of exogenous PACAP on gastric acid secretion in urethane-anesthetized mice implanted with a gastric cannula and in conscious 2-h pylorus ligated mice, and determined the involvement of somatostatin and somatostatin receptor type 2 (SSTR2) by using somatostatin immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. 3. Urethane-anesthetized wild-type mice had low basal acid secretion (0.10+/-0.01 micromol (10 min)(-1)) compared with SSTR2 knockout mice (0.93+/-0.07 micromol (10 min)(-1)). Somatostatin antibody and PRL-2903 increased basal secretion in wild-type mice but not in SSTR2 knockout animals. 4. In wild-type urethane-anesthetized mice, PACAP-38 (3-270 microg kg(-1) h(-1)) did not affect the low basal acid secretion, but inhibited the acid response to pentagastrin, histamine, and bethanechol. 5. In wild-type urethane-anesthetized mice pretreated with somatostatin antibody or PRL-2903 and in SSTR2 knockout mice, peripheral infusion of PACAP-38 or somatostatin-14 did not inhibit the increased basal gastric acid secretion. 6. In conscious wild-type mice, but not in SSTR2 knockout mice, PACAP-38 inhibited gastric acid secretion induced by 2-h pylorus ligation. The antisecretory effect of PACAP-38 was prevented by immunoneutralization of somatostatin. 7. These results indicate that, in mice, peripheral PACAP inhibits gastric acid secretion through the release of somatostatin and the activation of SSTR2 receptors. There is no evidence for stimulatory effects of PACAP on acid secretion in mice.

Neurotransmitters mediating the intestinal peristaltic reflex in the mouse

The motor, modulatory, and sensory neurotransmitters that mediate the peristaltic reflex in the mouse colon were identified by direct measurement, and their involvement in various pathways was determined by selective receptor antagonists. Mucosal stimulation in the central compartment of a three-compartment flat sheet preparation of mouse colon elicited ascending contraction and descending relaxation in the orad and caudad compartments, respectively. Ascending contraction was accompanied by substance P release, a marker for excitatory neurotransmitter release, into the orad compartment and was partly inhibited by atropine and spantide, and abolished by a combination of the two antagonists. Descending relaxation was accompanied by vasoactive intestinal peptide (VIP) release, a marker for inhibitory neurotransmitter release, into the caudad compartment, and was partly inhibited by VIP10-28 and NG-nitro-L-arginine, and abolished by a combination of the two agents. Somatostatin release increased during descending relaxation: immunoneutralization of somatostatin or blockade of its effect with a selective somatostatin type 2 receptor antagonist inhibited descending relaxation. The delta-opioid receptor antagonist naltrindole augmented descending relaxation and ascending contraction. Calcitonin gene-related peptide (CGRP) release increased in the central compartment and was mediated by concurrent release of 5-hydroxytryptamine (5-HT) because its release was blocked by a 5-HT4 receptor antagonist. Both the latter and the CGRP antagonist CGRP8-37, inhibited ascending contraction and descending relaxation. Thus, the reflex in mouse like that in rat and human intestine is initiated by mucosal release of 5-HT and activation of 5-HT4 receptors on CGRP sensory neurons and is relayed via somatostatin and opioid interneurons to VIP/nitric-oxide synthase inhibitory motor neurons and via cholinergic interneurons to acetylcholine/tachykinin excitatory motor neurons.

A multi-tyrosinated sst1/2 receptor preferring somatostatin agonist inhibits reflex and immune-mediated secretion in the guinea pig colon

Somatostatin and its analogs such as WOC 3B were compared for their ability to alter the release of 5-hydroxytryptamine (5-HT) and prostaglandins and to affect chloride secretory capacity, determined by activity of neural reflexes or by the influence of immune mediators and other secretagogues. In guinea pig colon set up in flux chambers, the multi-tyrosinated sst1/sst2 receptor preferring somatostatin agonist, WOC 3B, inhibited stroking-evoked 5-HT release without affecting basal release. WOC 3B had no effect on stroking-induced or basal prostaglandin E2 release (PGE2). Neither 5-HT nor PGE2 release was dependent on neural input. Tetrodotoxin induced a decrease in basal short circuit current (Isc) indicative of a decrease in chloride secretion. The decrease in basal Isc during neural blockade was highly correlated with the decrease in basal Isc when WOC 3B was used. In piroxicam- and atropine-treated tissues, to eliminate prostaglandins and cholinergic muscarinic input to crypts, WOC 3B further reduced the piroxicam-resistant and not the atropine resistant Isc during brush stroking the mucosa. Somatostatin and WOC 3B reduced the stroking-evoked Isc with similar half maximum concentrations of 1-2 nM. WOC 3B reduced by more than 50% dimaprit-evoked cyclical Isc. The rank order of potencies in inhibiting dimaprit-evoked Isc was: Somatostatin-14=WOC 3B>CH275=DC-32-92>DC-23-48>> >>DC-32-87=DC-32-97. Low nanomolar concentrations of WOC 3B primarily inhibited the neural effects of carbachol and forskolin on Isc without altering their epithelial effects. Equi-molar concentrations (4 nM) of CH275, a somatostatin sst1 receptor agonist, and the somatostatin sst2 receptor agonist, [Tyr(3)]-octreotide, inhibited dimaprit-evoked Isc by 25% and 26%, and their effects were additive. The results suggest that WOC 3B, a somatostatin analogue containing three tyrosine residues, has anti-secretory effects due to activation of somatostatin sst1 and sst2 receptors on enteric neurons.

Somatostatin receptor type 2 mediates bombesin-induced inhibition of gastric acid secretion in mice

Studies in isolated mouse stomach showed that bombesin releases somatostatin. We characterized the effects of exogenous bombesin on gastric acid secretion in mice and determined the involvement of somatostatin and somatostatin receptor type 2 (SSTR2) by using somatostatin immunoneutralization, the SSTR2 antagonist, PRL-2903, and SSTR2 knockout mice. Gastric acid secretion was monitored under basal and pentagastrin-, histamine- or bethanechol-stimulated conditions in urethane-anaesthetized mice. Bombesin (10-40 micro g kg-1 h-1) and somatostatin-14 (20 micro g kg-1 h-1) were infused I.V. 10 and 30 min after PRL-2903 or somatostatin antibody pretreatment, respectively. Urethane-anaesthetized wild-type mice had low basal acid secretion (0.12 +/- 0.01 micro mol (10 min)-1) compared with SSTR2 knockout mice (1.43 +/- 0.10 micro mol (10 min)-1). Somatostatin antibody and PRL-2903 increased basal secretion in wild-type mice but not in SSTR2 knockout animals. In wild-type mice, bombesin inhibited secretagogue-stimulated acid secretion in a dose-dependent manner, and somatostatin-14 inhibited pentagastrin-stimulated secretion. In wild-type mice pretreated with somatostatin antibody or PRL-2903 and in SSTR2 knockout mice, bombesin and somatostatin-14 I.V. infusion did not alter the increased gastric acid secretion. These results indicate that, in mice, bombesin inhibits gastric acid secretion through the release of somatostatin and the activation of SSTR2. These observations strengthen the important role of SSTR2 in mediating somatostatin inhibitory actions on gastric acid secretion.

Enteric motor neurons form synaptic-like junctions with interstitial cells of Cajal in the canine gastric antrum

Morphological studies have shown synaptic-like structures between enteric nerve terminals and interstitial cells of Cajal (ICC) in mouse and guinea pig gastrointestinal tracts. Functional studies of mice lacking certain classes of ICC have also suggested that ICC mediate enteric motor neurotransmission. We have performed morphological experiments to determine the relationship between enteric nerves and ICC in the canine gastric antrum with the hypothesis that conservation of morphological features may indicate similar functional roles for ICC in mice and thicker-walled gastrointestinal organs of larger mammals. Four classes of ICC were identified based on anatomical location within the tunica muscularis. ICC in the myenteric plexus region (IC-MY) formed a network of cells that were interconnected to each other and to smooth muscle cells by gap junctions. Intramuscular interstitial cells (IC-IM) were found in muscle bundles of the circular and longitudinal layers. ICC were located along septa (IC-SEP) that separated the circular muscle into bundles and were also located along the submucosal surface of the circular muscle layer (IC-SM). Immunohistochemistry revealed close physical associations between excitatory and inhibitory nerve fibers and ICC. These contacts were synaptic-like with pre- and postjunctional electron-dense regions. Synaptic-like contacts between enteric neurons and smooth muscle cells were never observed. Innervated ICC formed gap junctions with neighboring smooth muscle cells. These data show that ICC in the canine stomach are innervated by enteric neurons and express similar structural features to innervated ICC in the murine GI tract. This morphology implies similar functional roles for ICC in this species.

Reciprocal paracrine pathways link atrial natriuretic peptide and somatostatin secretion in the antrum of the stomach

Atrial natriuretic peptide (ANP) as well as its receptor, NPR-A, have been identified in gastric antral mucosa, suggesting that ANP may act in a paracrine fashion to regulate gastric secretion. In the present study, we have superfused antral mucosal segments obtained from rat stomach to examine the paracrine pathways linking ANP and somatostatin secretion in this region.ANP (0.1 pM to 0.1 microM) caused a concentration-dependent increase in somatostatin secretion (EC(50), 0.3 nM). The somatostatin response to ANP was unaffected by the axonal blocker tetrodotoxin but abolished by addition of the selective NPR-A antagonist, anantin. Anantin alone inhibited somatostatin secretion by 18+/-3% (P<0.005), implying that endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion. Somatostatin (1 pM to 1 microM) caused a concentration-dependent decrease in ANP secretion (EC(50), 0.7 nM) that was abolished by addition of the somatostatin subtype 2 receptor (sst2) antagonist, PRL2903. Neutralization of ambient somatostatin with somatostatin antibody (final dilution 1:200) increased basal ANP secretion by 70+/-8% (P<001), implying that endogenous somatostatin inhibits ANP secretion. We conclude that antral ANP and somatostatin secretion are linked by paracrine feedback pathways: endogenous ANP, acting via the NPR-A receptor, stimulates somatostatin secretion, and endogenous somatostatin, acting via the sst2 receptor, inhibits ANP secretion.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.

Identification of cells expressing somatostatin receptor 2 in the gastrointestinal tract of Sstr2 knockout/lacZ knockin mice

Somatostatin is found in neurons and endocrine cells in the gastrointestinal tract. The actions of somatostatin are mediated by a family of G-protein-coupled receptors that compose five subtypes (SSTR1-5), each of which is encoded by a separate gene. lacZ "knockin" mice, in which the reporter gene lacZ was engineered into the genomic locus of Sstr2 by gene targeting, were used to examine the expression pattern of Sstr2 and identify potential targets for neurally released and hormonal somatostatin in the gastrointestinal tract. In the body of the stomach, a large proportion of epithelial cells and subpopulations of myenteric neurons expressed Sstr2. Double- or triple-labeling with antisera to H(+)K(+)ATPase (to identify parietal cells) and/or histidine decarboxylase (to identify enterochromaffin-like [ECL] cells) combined with beta-galactosidase staining revealed that both parietal cells and ECL cells expressed Sstr2, and these two cell types accounted for almost all of the Sstr2-expressing epithelial cells. Somatostatin inhibits gastric acid secretion. The presence of SSTR2 on both parietal and ECL cells suggests that somatostatin acting on SSTR2 may reduce acid secretion by both acting directly on parietal cells and by reducing histamine release from ECL cells. In the small and large intestine, subpopulations of neurons in the myenteric and submucosal plexuses expressed Sstr2, and many of the Sstr2-expressing myenteric neurons also showed SSTR2(a) immunostaining. Most of Sstr2-expressing neurons in the myenteric plexus showed nitric oxide synthase (NOS) immunoreactivity. Previous studies have shown that NOS neurons are descending interneurons and anally projecting, inhibitory motor neurons. Thus, somatostatin acting at SSTR2 receptors on NOS neurons might modulate descending relaxation.

Distribution of galanin receptor 1 immunoreactivity in the rat stomach and small intestine

Galanin affects gastrointestinal functions by activating different G protein-coupled receptors. Here, we identified the sites of expression of the galanin receptor 1 (GAL-R1) subtype in the rat stomach and small intestine by using immunohistochemistry with an antibody raised to the third intracellular loop of rat GAL-R1 (GAL-R1(Y225-238)) and confocal microscopy. Antibody specificity was confirmed by (1) the detection of a band at approximately 70 kDa in Western blot of membranes from GAL-R1 transfected cells, (2) the cell surface staining of GAL-R1 transfected cells, which was not detected in control cells, and (3) the abolition of Western signal and tissue immunostaining by preadsorbing the antibody with the peptide used for immunization. GAL-R1 immunoreactivity was localized to the cell surface of enterochromaffin-like cells, and of myenteric and submucous neurons, and to fibers distributed to the plexuses, interconnecting strands, muscle layers, vasculature, and mucosa. A dense network of GAL-R1 immunoreactivity was observed in the deep muscular plexus in very close association with interstitial cells of Cajal visualized by c-kit immunostaining. In the ileum, 81.6% of GAL-R1 myenteric neurons and 70.7% of GAL-R1 submucosal neurons were substance P immunoreactive. Vasoactive intestinal polypeptide immunoreactivity was found in 48.3% of GAL-R1 submucosal neurons, but not in GAL-R1 myenteric neurons. These findings support the hypothesis that GAL-R1 mediates galanin actions on gastrointestinal motility and secretion by modulating the release of other neurotransmitters and contributes to galanin-induced inhibition of gastric acid secretion by means of the suppression of endogenous histamine release.

Expression of 5-HT3 receptors in the rat gastrointestinal tract

BACKGROUND & AIMS

Functional effects mediated via the 5-hydroxytryptamine3 receptor (5-HT3R) can be elicited from both extrinsic and intrinsic neurons innervating the gastrointestinal (GI) tract. Clinically, 5-HT3 antagonists are important in the treatment of emesis and have been used for the treatment of symptoms in functional bowel disease. The aim of the present study was to elucidate the cellular sites of 5-HT3R expression in the rat GI tract using immunohistochemistry.

METHODS

Immunohistochemistry was performed in fixed cryostat sections and whole mounts of stomach and intestine of fasted rats, using an affinity-purified antibody directed to a 19-amino acid sequence of the cytoplasmic loop of the 5-HT3R.

RESULTS

5-HT3R immunoreactivity was localized to numerous neurons of the myenteric and submucosal plexus, concentrated primarily near the neuronal plasma membrane, and to fibers in the circular and longitudinal muscles, submucosa, and mucosa. 5-HT3R immunoreactivity was also expressed by interstitial cells of Cajal and a few endocrine cells. Numerous 5-HT3R-positive myenteric neurons were cholinergic, and few neurons coexpressed VIP or SP immunoreactivity. Fibers immunoreactive for 5-HT3R in the duodenal but not ileal mucosa were markedly reduced by subdiaphragmatic vagotomy or chemical denervation of vagal afferents.

CONCLUSIONS

These findings indicate that 5-HT3Rs are expressed by distinct cells in the GI tract, including functionally distinct classes of neurons, interstitial cells of Cajal, and endocrine cells. The effects of serotonin mediated by 5-HT3Rs involve the activation of neuronal and nonneuronal pathways.

Interstitial cells of Cajal and purinergic signalling

Interstitial cells of Cajal (ICC) in the guinea pig intestine, identified by the tyrosine kinase receptor, c-Kit, have been shown with immunohistochemistry to express nucleotide P2X2 and P2X5 receptors. P2X5 receptors have also been demonstrated on interstitial cells in the mouse ileum. It is speculated that release of ATP from enteric nerves, enteric glial cells or from contracting smooth muscle may provide a feedback mechanism for pacemaker activity in the intestine.

Localisation of mesenchymal tumours by somatostatin receptor imaging

Oncogenic osteomalacia, an acquired hypophosphataemic syndrome associated with mesenchymal tumours, is characterised by hypophosphataemia secondary to inappropriate phosphaturia, reduced concentrations of serum calcitriol, and defective bone mineralisation. Removal of these tumours results in complete reversal of these biochemical defects. However, because these tumours are small, slow-growing, and frequently situated in unusual anatomical sites, conventional imaging techniques often fail to detect them. Since mesenchymal tumours express somatostatin receptors, we postulated that somatostatin analogues would be able to detect these tumours. We did Indium-111 labeled pentetreotide imaging in seven patients with oncogenic osteomalacia. In five patients, we identified a mesenchymal tumour, and clinical improvement occurred after tumour resection. Our findings suggest that 111In-pentetreotide imaging effectively detects occult mesenchymal tumours and facilitates surgical treatment of oncogenic osteomalacia.

The interstitial cells of Cajal and a gastroenteric pacemaker system

In spite of a claim by Kobayashi (1990) that they do not correspond to the cells originally depicted by CAJAL, a particular category of fibroblast-like cells have been identified in the gut by electron microscopy (Faussone-Pellegrini, 1977; Thuneberg, 1980) and by immunohistochemistry for Kit protein (Maeda et al., 1992) under the term of the "interstitial cells of Cajal (ICC)". Generating electrical slow waves, the ICC are intercalated between the intramural neurons and the effector smooth muscular cells, to form a gastroenteric pacemaker system. ICC at the level of the myenteric plexus (IC-MY) are multipolar cells forming a reticular network. The network of IC-MY which is believed to be the origin of electrical slow waves is morphologically independent from but associated with the myenteric plexus. On the other hand, intramuscular ICC (IC-IM) usually have spindle-shaped contours arranged in parallel with the bulk smooth muscle cells. Associated with nerve bundles and blood vessels, the IC-IM possess receptors for neurotransmitters and such circulating hormones as cholecystokinin, suggesting their roles in neuromuscular and hormone-muscular transmissions. In addition, gap junctions connect the IC-MY and IC-IM, thereby realizing the electrically synchronized integrity of ICC as a pacemaker system in the gut. The smooth muscle cells are also coupled with ICC via gap junctions, and the functional unit thus formed enables rhythmically synchronized contractions and relaxations. It has recently been found that a lack of Kit-expressing cells may induce hyper-contractility of the tunica muscularis in vitro, whereas a decrease in Kit expression within the muscle wall causes dysmotility-like symptoms in vivo. The pacemaker system in the gut thus seems to play a critical role in the maintenance of both moderate and normal motility of the digestive tract. A loss of Kit positive cells has been detected in several diseases with an impaired motor activity, including diabetic gastroenteropathy. Pathogenesis of these diseases is thought to be accounted for by impaired slow waves and neuromuscular transmissions; a pacemaker disorder may possibly induce a dysmotility-like symptom called 'gastroenteric arrhythmia'. A knowledge of the structure and function of the ICC and the pacemaker system provides a basis for clarifying the normal mechanism and the pathophysiology of motility in the digestive tract.

Physiology and pathophysiology of the interstitial cell of Cajal: from bench to bedside. III. Interaction of interstitial cells of Cajal with neuromediators: an interim assessment

Interstitial cells of Cajal (ICC) control gastrointestinal motility; some pace slow waves and others act in enteric neurotransmission. This review asks the question, does either class of ICC receive and respond to messages carried by neuromediators from these nerves? Relevant evidence includes the presence of receptors or responses to exogenous neuromediators and responses to endogenous neuromediators. Some pacemaking ICC networks have receptors for or respond to some exogenous neuromediators. None is known to respond to endogenous neuromediators. Intramuscular ICC have receptors for and respond to some neuromediators and are required in mice for responses to the exogenous and endogenous neuromediators nitric oxide and acetylcholine. The mechanisms underlying this requirement remain unclear. ICC pathologies exist, but their origins are unknown.

CCK(B)/gastrin receptor mediates synergistic stimulation of DNA synthesis and cyclin D1, D3, and E expression in Swiss 3T3 cells

In order to develop a model system for identifying signaling pathways and cell cycle events involved in gastrin-mediated mitogenesis, we have used high efficiency retroviral-mediated transfection of cholecystokinin (CCK)(B)/gastrin receptor into Swiss 3T3 cells. The retrovirally-transfected CCK(B)/gastrin receptor binds 125I-CCK-8 with high affinity (Kd = 1.1 nM) and is functionally coupled to intracellular signaling pathways including rapid and transient increase in Ca2+ fluxes, protein kinase C-dependent protein kinase D activation, and MEK-dependent ERK1/2 activation. In the presence of insulin, CCK-8 or gastrin induced a 66.5 +/- 8.8-fold (mean +/- SEM, n = 24 in eight independent experiments) increase in cellular DNA synthesis, reaching a level similar to that achieved by stimulation with a saturating concentration of fresh serum, and much greater than the response to each agonist added alone. CCK-8 also induced a striking increase in the expression of cyclins D1, D3, and E and hyperphosphorylation of Rb acting synergistically with insulin. Similar effects were observed when CCK(B)/gastrin receptor was activated in the presence of EGF or bombesin. Our results demonstrate that activation of CCK(B)/gastrin receptor retrovirally-transfected into Swiss 3T3 induces a potent synergistic effect on DNA synthesis, accumulation of cyclins D1, D3, and E and hyperphosphorylation of Rb in combination with insulin, EGF, or bombesin. Thus, the CCK(B)/gastrin receptor transfected into Swiss 3T3 cells provides a novel model system to elucidate mitogenic signal transduction pathways and cell cycle events activated via this receptor.

Interstitial cells of Cajal--their role in pacing and signal transmission in the digestive system

Interstitial cells of Cajal (ICC) are located in most parts of the digestive system. Although they were discovered over 100 years ago, their function began to be unravelled only recently. Morphological observations have led to a number of hypotheses on the possible physiological roles of ICC: (1) these cells may be the source of slow electrical waves recorded in gastrointestinal (GI) muscles; (2) they participate in the conduction of electrical currents, and (3) mediate neural signals between enteric nerves and muscles. These hypotheses were supported by experiments in which the ICC-containing layer was removed surgically, or when ICC were ablated chemically, and as a consequence the slow waves were absent. Electrophysiological experiments on isolated cells confirmed that ICC can generate rhythmic electrical activity and can also respond to messenger molecules known to be released from enteric nerves. In mice mutants deficient in ICC, or in mice treated with antibody against the protein c-Kit, slow wave activity was impaired. These results support the role of ICC as pacemaker cells. Physiological studies have shown that ICC in certain GI regions are important for signal transmission between nerves and smooth muscle. There is evidence that pathological changes in ICC may be associated with GI motility disorders. The full interpretation of the role of ICC in disease conditions will require much further study on the physiology and pharmacology of these cells.

Monoclonal antibody to rat galanin: production, characterization, and in vivo immunoneutralization activity

A monoclonal antibody (MAb) to galanin was prepared by cell fusion of myeloma Fox-NY and spleen cells from Robertsonian mice immunized with rat galanin. Hybridomas producing high-affinity antibodies were cloned in pristine-primed Balb/c mice. The antibody was purified by affinity chromatography and concentrated to 12 mg IgG/mL by dialysis. Immunoreactivity of the antibody was screened by radioimmunoassay. Ascites fluid contained approximately 10 mg/mL IgG that belong to the subclass of IgG2a as determined by enzyme-linked immunoadsorbent assay (ELISA). The titer of this IgG2a antibody entitled #G65G was 1:10,000 and the ID50 for rat galanin was 1000 fmol/mL as determined by liquid phase radioimmunoassay. Immunohistochemistry showed that this galanin MAb stains densed, beaded processes distributed to the enteric plexuses, where they appear to encircle neuronal cell bodies, to the muscle layer, where they are particularly abundant in the circular muscle layer and in the deep muscular layer, and to the mucosa. In vivo capacity of immunoneutralization by this antibody was tested in male Sprague-Dawley rats fasted for 24 h and anesthetized with urethane. Systemic injection of protein A purified galanin antibody (6 mg/rat) decreased by 70% of the inhibitory effect of intravenous galanin (2 nmol/kg/h i.v.) on gastric acid secretion induced by intracisternal TRH analog. These results show that galanin antibody #G65G is useful for in vivo immunoneutralization of galanin effects and is a valuable tool for immunohistochemical localization of galanin in gastrointestinal tissues.

Interstitial cells of Cajal: primary targets of enteric motor innervation

For many years morphologists have noted the close relationship between interstitial cells of Cajal (ICC) and nerve fibers within the tunica muscularis of gastrointestinal (GI) organs. These observations led to speculations about a role for ICC in mediating neural inputs to the GI tract. Immunohistochemical and functional studies demonstrated the presence of receptors for the neurotransmitters utilized by enteric motor neurons, and changes in second messengers in ICC after field stimulation of intrinsic enteric neurons showed that ICC were functionally innervated in GI muscles. Recent double labeling experiments have shown that both excitatory and inhibitory enteric motor neurons are closely associated with ICC in the deep muscular plexus (IC-DMP) of the small intestine and intramuscular ICC (IC-IM) of the proximal and distal GI tract. Enteric motor neurons form synaptic-like structures with IC-IM and IC-DMP. Far fewer close contacts are found between enteric motor neurons and smooth muscle cells. Experiments on W/W(V) mutants that lack IC-IM in the stomach, lower esophageal sphincter, and pylorus have shown that these ICC are critical components of the neuromuscular junction. Cholinergic excitatory and nitrergic inhibitory neurotransmission are severely decreased in tissues lacking IC-IM, yet there is no loss of cholinergic or nitrergic neurons in W/W(V) mutants. These data suggest that either the post-junctional mechanisms responsible for receiving and transducing neurotransmitter signals are specifically expressed by ICC, or that the large extracellular spaces typically between nerve terminals and smooth muscle cells may not allow effective concentrations of neurotransmitters to reach receptors expressed by smooth muscle cells. These findings indicate an important role for certain classes of ICC in enteric neurotransmission and predict that loss of ICC in human motor disturbances may significantly compromise neural regulation of GI motility.

Cellular biology of somatostatin receptors

Somatostatin, and the recently discovered neuropeptide cortistatin, exert their physiological actions via a family of six G protein-coupled receptors (sst1, sst2A, sst2B, sst3, sst4, sst5). Following the cloning of somatostatin receptors significant advances have been made in our understanding of their molecular, pharmacological and signaling properties although much progress remains to be done to define their physiological role in vivo. In this review, the present knowledge regarding neuroanatomical localization, signal transduction pathways, desensitization and internalization properties of somatostatin receptors is summarized. Evidence that somatostatin receptors can form homo- and heterodimers and can physically interact with members of the SSTRIP/Shank/ProSAP1/CortBP1 family is also discussed.

Interstitial cells of Cajal: primary targets of enteric motor innervation

For many years morphologists have noted the close relationship between interstitial cells of Cajal (ICC) and nerve fibers within the tunica muscularis of gastrointestinal (GI) organs. These observations led to speculations about a role for ICC in mediating neural inputs to the GI tract. Immunohistochemical and functional studies demonstrated the presence of receptors for the neurotransmitters utilized by enteric motor neurons, and changes in second messengers in ICC after field stimulation of intrinsic enteric neurons showed that ICC were functionally innervated in GI muscles. Recent double labeling experiments have shown that both excitatory and inhibitory enteric motor neurons are closely associated with ICC in the deep muscular plexus (IC-DMP) of the small intestine and intramuscular ICC (IC-IM) of the proximal and distal GI tract. Enteric motor neurons form synaptic-like structures with IC-IM and IC-DMP. Far fewer close contacts are found between enteric motor neurons and smooth muscle cells. Experiments on W/W(V) mutants that lack IC-IM in the stomach, lower esophageal sphincter, and pylorus have shown that these ICC are critical components of the neuromuscular junction. Cholinergic excitatory and nitrergic inhibitory neurotransmission are severely decreased in tissues lacking IC-IM, yet there is no loss of cholinergic or nitrergic neurons in W/W(V) mutants. These data suggest that either the post-junctional mechanisms responsible for receiving and transducing neurotransmitter signals are specifically expressed by ICC, or that the large extracellular spaces typically between nerve terminals and smooth muscle cells may not allow effective concentrations of neurotransmitters to reach receptors expressed by smooth muscle cells. These findings indicate an important role for certain classes of ICC in enteric neurotransmission and predict that loss of ICC in human motor disturbances may significantly compromise neural regulation of GI motility.

Postnatal development of somatostatin 2A (sst2A) receptors expression in the rabbit retina

In the retina, somatostatin (SRIF) acts as a neuromodulator by interacting with specific SRIF subtype (sst) receptors. Aim of this investigation was to determine the cellular localization of the sst2A receptor isoform in the postnatal rabbit retina. Receptor immunoreactivity was localized using the antiserum K-230, directed to the C-terminus of the human sst2A receptor. In the postnatal rabbit retina, sst2A receptors were abundantly expressed without significant regional differences. They were localized predominantly to rod bipolar cells, identified with a protein kinase C (PKC) antibody, to amacrine cells, some of which also containing tyrosine hydroxylase (TH), and to presumed rare horizontal cells. Quantitative analysis showed that sst2A-immunoreactive (-IR) bipolar and amacrine cells reached their maximum density and absolute number at the time of eye opening, when the expression pattern of sst2A receptors was similar to that in adult retinas. In the adult retina, 68% of the PKC-IR rod bipolars and 34% of the TH-IR amacrine cells were observed to also express sst2A receptors. The appearance of sst2A receptor immunolabeling prior to eye opening and the developmental profile of sst2A receptor expression are compatible with a role of SRIF in the maturation of retinal circuitries. The partial expression of sst2A receptors in PKC-IR rod bipolar cells and in TH-IR amacrine cells may suggest some type of heterogeneity within these cell populations.