CCK-X receptors in the endothermic mako shark (Isurus oxyrinchus)

By mapping the distribution of cholecystokinin (CCK) receptor types onto an established phylogenetic hypothesis of vertebrate relationships, we tested two hypothesis about the evolution of CCK receptors: (1) A single CCK receptor type, CCK-X, is the ancestral receptor, while CCK-A and CCK-B receptors represent derived receptor types; (2) the evolution of two separate CCK receptors is functionally related to the evolution of endothermy. Specifically, we localized and characterized 125I-CCK-binding sites in the gut and brain of mako shark (Isurus oxyrinchus), a warm-blooded chondrichthyean fish. Competitive inhibition studies of 125I-CCK binding showed that the CCK receptor in the mako shark brain, gallbladder, pyloric stomach, and intestine binds sulfated CCK-8 and sulfated gastrin-17 (gastrin-17-II) with much higher affinity (K(i) ranging from 0.05 to 2.02 nM) than unsulfated gastrin-17 (gastrin-17-I, K(i) ranging from 4.63 to 62.17 nM). These results indicate that the mako shark expresses a single CCK-X receptor in all tissues. Additional competitive inhibition studies showed that the mako CCK-X receptor has very low affinities for the following nonpeptide agonist and antagonists: A71623, L364,718, A57696, A65186.72, Cam-1481, and SR 27897B (specific for some mammalian CCK-A receptors) and L365,260 and CI-988 (specific for some mammalian CCK-B receptors), confirming the pharmacological differences between the CCK-X receptor and the CCK-A and -B receptors. Based on the mapped phylogenetic distribution of CCK receptor types, we conclude that CCK-X is the ancestral receptor type and that two receptor types, e.g. CCK-A and CCK-B, are not part of the suite of characters necessary for evolution of endothermy in fishes.

Cholecystokinin mediation of colonic absorption via peptide YY: foregut-hindgut axis

Peptide YY (PYY), a 36-amino-acid polypeptide, is found in abundance in the colon, a region where its physiologic roles are unknown. Previous studies have revealed a substantial increase in plasma PYY after cholecystokinin (CCK) administration. PYY is released from the hindgut in response to a meal and inhibits CCK release. In this study we evaluated the effects of CCK and PYY on intestinal absorption of water and electrolytes. Colonic, ileal, or jejunal Thiry-Vella fistulas (TVFs) were created in 12 dogs, and intestinal continuity was reestablished. The TVFs were perfused with an isotonic buffer solution containing [14C] PEG as a volume marker. Electrolyte and water transport were measured every 15 minutes, and plasma PYY and CCK levels were measured by radioimmunoassay. Group 1 dogs received an intravenous bolus of MK329, a specific CCK receptor antagonist, at 20 nmol/kg after a standard mixed meal; group 2 colonic TVF dogs received a meal and an intravenous bolus of PYY polyclonal antibody at 1 mg/kg. Postprandially, all three regions of the bowel became significantly proabsorptive for water, sodium, and chloride. In the colon postprandial absorption was abolished by MK329 starting 60 minutes after a meal, whereas specific CCK receptor blockade blunted ileal absorption. CCK receptor blockade did not affect postprandial absorption in the jejunum. Postprandial PYY levels did not rise in MK329-treated animals. PYY antibody reduced colonic absorption during the postprandial phase. Reduction of meal-induced colonic absorption and PYY release by MK329 in awake dogs suggests an important foregut-hindgut hormonal feedback loop. Foregut-derived CCK stimulates hindgut PYY release, which in turn stimulates colonic absorption while inhibiting further CCK release.

Effect of endogenous hypergastrinemia on carcinogenesis in the rat esophagus

We surgically prepared a hypergastrinemia model in rats and studied the effects of hypergastrinemia on chemically induced carcinogenesis in the esophagus. Operations were performed on 5-week-old male Donryu rats as follows: (1) truncal vagotomy plus pyloroplasty (group V), (2) segmental gastrectomy plus pyloroplasty (group G), (3) antrectomy (group A), and (4) no operation (group C) as a control. From the age of 6 weeks, the animals were given 0.003% N-methyl- N-amylnitrosamine (MAN) solution as drinking water for 8 weeks. After 20 weeks of MAN administration, the animals were bled and killed. The average serum gastrin levels in groups V and G were significantly higher than those groups C or A. There were significant differences between C and V in the incidence of carcinoma, and between V and A in the incidence of carcinoma including severe dysplasia. The incidence of histologically identified lesions per animal was determined, and significant differences were observed between C and both V and G in the incidence of carcinoma including severe dysplasia. Furthermore, we also detected gastrin receptors in the esophageal lesions produced by the oral administration of MAN to rats. The results of the present study suggest that endogenous hypergastrinemia has a positive influence on chemically induced carcinogenesis in the rat esophagus.

Numerous cell targets for gastrin in the guinea pig stomach revealed by gastrin/CCKB receptor localization

The localization of the gastrin/CCKB receptor (GR) has recently become a subject of debate, especially since the publication of evidence for its presence in an unsuspected location, namely in the lamina propria, the submucosal layer of the stomach lining. Knowledge of the receptor localization is important because of the critical role of gastrin secretion and its trophic effects on the gastric epithelium. The present study, which utilizes immunohistochemistry and electron microscopy as primary tools, provides unequivocal data concerning the localization of GR in the guinea pig stomach. GR is expressed in parietal cells, on chief cells, and in previously unreported endocrine cells of the stomach. It is not found in the lamina propria. The predominant localization of the receptor in the endocrine cells is on the membranes of cytoplasmic electron-dense secretory granules. The positioning of these cells in the gastric glands suggests that they may be involved in the uptake of gastrin from the circulation. The distribution of GR implies that it may be involved in the regulation of various processes and may mediate various effects of gastrin in the stomach.

Structural and functional changes of rat exocrine pancreas exposed to nicotine

Nicotine, an addictive agent in cigarette smoking, has been implicated in the etiology of pancreatitis and pancreatic carcinoma. Very little experimental data are, however, available regarding the effects of nicotine on the structural and functional changes in the exocrine pancreas. Two groups of rats divided into control and nicotine exposed were used. The animals in the nicotine group were maintained for 28 d with ad libitum water containing 0.77 mM of nicotine. At term, the animals were sacrificed, and pancreas was examined for pathological and functional changes. Nicotine induced cytoplasmic vacuolation and cellular edema in the exocrine pancreas. Plasma levels of glucose and insulin and CCK-8-stimulated amylase release in isolated acini were significantly decreased by nicotine, whereas the total cellular amylase content was significantly increased. Analysis of competitive ligand binding data on membranes from isolated acini showed that the Bmax and Kd values for CCK receptors were not significantly changed by nicotine, p > 0.05. These data indicate that a postreceptor mechanism is involved in the inhibition in stimulus-secretion coupling of enzyme secretion by nicotine. The increase in total cellular amylase content and decreased enzyme secretion by nicotine may be implicated in the induction of pancreatic pathology.

Effect of endogenous hypergastrinemia on gastrin receptor expressing human colon carcinoma transplanted to athymic rats

BACKGROUND & AIMS

The effect of endogenous hypergastrinemia on growth of human colon carcinoma is not known. Our aim was to study the growth of human colon carcinoma in an animal model with endogenous hypergastrinemia.

METHODS

Human colon carcinoma was transplanted to the colon of 40 athymic rats. Of these, 25 underwent gastric fundectomy to accomplish endogenous hypergastrinemia, and 15 were sham operated to serve as controls. The duration of the study was 8 weeks. During the last week, 12 fundectomized animals received a gastrin (cholecystokinin B) receptor antagonist. Metaphase arrest index, local invasion, and distant spread of the tumor were investigated. Expression of gastrin and cholecystokinin B receptor messenger RNA was examined by reverse-transcription polymerase chain reaction.

RESULTS

Tumor spread by direct extension outside the colon was observed in all animals, and liver metastases were observed in 10 of the 25 fundectomized animals. Sham-operated animals showed none of these features. The metaphase arrest index of the tumor did not differ between fundectomized animals given the cholecystokinin B receptor antagonist and sham-operated animals, whereas it was significantly increased in fundectomized animals not given the antagonist. The tumor expressed both gastrin and cholecystokinin B receptor messenger RNA.

CONCLUSIONS

The results indicate that endogenous hypergastrinemia may promote proliferation and spread of human colon carcinoma expressing cholecystokinin B receptor.

Localization and quantification of cholecystokinin receptors in rat brain with storage phosphor autoradiography

The present study evaluated a new imaging technique that demonstrated the application of storage phosphor autoradiography in the localization and quantification of cholecystokinin receptors in rat brains and compared the results with film autoradiography. Cryostat sections were incubated with [125I]Bolton-Hunter-labeled sulfated cholecystokinin octapeptide followed by exposure to a storage phosphor-imaging screen and suitable autoradiography film. To obtain satisfactory images, it took 6 days with film autoradiography vs. 15 hours with the storage phosphor technique. Both film and storage phosphor autoradiograms showed the same cholecystokinin receptor distribution in brain sections; however, the film imaged more details. To reach the lowest possible response ratio between low and high receptor density regions in rat brains, storage phosphor autoradiography was about 240-fold faster than film. In addition, the new technique presented a considerably larger exposure time range for maintaining that ratio. The binding per area showed a linear relationship with the thickness of sections between 5 and 14 microns. In the linear response range, the quantitative results of both methods are comparable. In conclusion, storage phosphor autoradiography is a faster technique for localizing and quantifying peptide receptors in tissue sections but slightly compromised in resolution when compared with film autoradiography.

A study of the mechanism of butachlor-associated gastric neoplasms in Sprague-Dawley rats

Long term administration of butachlor to Sprague-Dawley rats in a previous bioassay, resulted in the induction of gastric neoplasms which occurred only in the highest dose group (3000 ppm in the diet), primarily in females and specifically in the fundic region. The tumors were a composite of highly undifferentiated enterochromaffin-like (ECL) cells and mucus producing cells with morphologic characteristics unlike those previously described in the rat stomach. Mucosal atrophy of marked intensity was a consistent feature of the gastric mucosa in animals from the highest dose group. An additional long term study was conducted in female Sprague-Dawley rats at dietary levels of 0, 100, 1000 and 3000 ppm to explore the mechanism(s) involved in the formation of these neoplasms. Cell proliferation was evaluated in both fundic and pyloric regions of the stomachs of rats at multiple time periods from 14 days to 26 months. Mucosal thickness was determined in the fundic region at the same time intervals as were used for cell proliferation studies. Gastric pH and gastric acid production were measured after approximately 21 months of exposure. Serum gastrin levels were analyzed at 14, 60, and 120 days and at 6, 18 and 20 months. Cholecystokinin (CCK)/gastrin receptor binding studies were conducted on samples of four tumors and pooled fundic mucosa from five animals in the control group. Cell proliferation was increased in both the neck and base regions of the fundic mucosa at nearly all time points measured from 14 days to 26 months. The magnitude of the changes in the base region were substantially greater than those in the neck region. Fundic mucosal thickness was decreased beginning at the 30-day time point and continued at all intervals, being less than one half that of controls at 20 and 26 months. Gastric pH in rats from the highest dose was elevated to nearly twice control levels at 21 months. Gastric acid secretion was dramatically decreased in animals from the 3000 ppm group and was moderately decreased in the 1000 ppm group at 21 months. Hypergastrinemia was observed at the 3000 ppm level only, beginning at 120 days with progression to extremely high levels by 18 months. CCK/gastrin receptor binding was demonstrated in all tumors studied, at levels comparable to or higher than that of the pooled control sample. All changes involved only the fundic region, the site of tumor formation. Tumors occurred only in animals from the 3000 ppm level, the only level at which hypergastrinemia occurred.(ABSTRACT TRUNCATED AT 400 WORDS)

Gastrin-CCK-B type receptors on human T lymphoblastoid Jurkat cells

The presence of specific receptors for gastrointestinal hormones on T cells and their involvement in the immune response are still matters of debate. We reported the effects of gastrin-cholecystokinin (CCK)-related peptides on J.RT3-T3.5 Jurkat cells. A single class of high-affinity binding sites (dissociation constant approximately 0.1 nM) for gastrin and CCK-8 was evident on these cells. These peptides dose-dependently induced a transient increase in intracellular Ca2+ concentration ([Ca2+]i), which was independent of extracellular Ca2-. L-365,260 was 150- to 300-fold more potent than L-364,718 to inhibit radiolabeled ligand binding or peptide-stimulated [Ca2+]i increase, confirming the gastrin-CCK-B nature of the receptor. Gastrin caused a rise in inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] level within 5 s of stimulation. Finally, gastrin increased interleukin (IL)-2 secretion in J.RT3-T3.5 cells. We conclude that 1) J.RT3-T3.5 cells possess "gastrin-CCK-B type" receptors coupled to phospholipase C activation, Ins(1,4,5)P3 formation, and Ca2+ release from intracellular Ca2+ pools, and 2) these receptors could be involved in the regulation of IL-2 production.

Cholecystokinin-B/gastrin receptor: a novel molecular probe for human small cell lung cancer

The brain-gut hormones, gastrin and cholecystokinin, have a trophic effect on the gastrointestinal mucosa in vivo and promote the growth of several neoplastic cell lines. In this study, cholecystokinin-B/gastrin receptor has been demonstrated to provide a novel molecular marker for the diagnosis of small cell lung cancer by using biopsy specimens. Physiological expression of the receptor mRNA is detectable in particular areas of the human brain, stomach, and pancreas but not in the lung. The receptor mRNA was detected selectively in all small cell lung cancer (10 cases) with a RT-PCR assay. By contrast, it was detectable in only 1 of 13 squamous cell carcinomas or 21 adenocarcinomas of the lung. Thus, the cholecystokinin-B/gastrin receptor could be an attractive therapeutic target for small cell lung cancer.

[125I]SNF 8702: a selective radioligand for CCKB receptors

The CCK-8 analogue, SNF 8702, was radioiodinated. [125I]SNF 8702 showed high-affinity specific binding for both guinea pig and mouse brain tissues, whereas no specific binding was seen for guinea pig pancreatic tissue. The properties of the site labeled by [125I]SNF 8702 were characterized by binding inhibition studies for a series of CCKA and CCKB receptor ligands. The binding selectivity profile corresponded to that for the CCKB receptor. The labeled compound is stable for more than 6 weeks during storage at -20 degrees C.

Gastrin receptor genes are expressed in gastric parietal and enterochromaffin-like cells of Mastomys natalensis

Although gastric enterochromaffin-like (ECL) carcinoid tumors are known to develop in patients with long-standing hypergastrinemia, the expression of the gastrin receptor gene in ECL cells has not yet been demonstrated. Therefore, this study was designed to examine gastrin receptor gene expression in ECL cells. Mastomys gastric mucosal cells isolated by enzyme dispersion were separated into 10 fractions (F1-10) by centrifugal elutriation. Each fraction was examined histologically to determine whether they contained ECL and/or parietal cells and Northern blot analysis was used to confirm the presence of histidine decarboxylase and H+, K(+)-ATPase gene expression. ECL cells were found only in fractions 1 and 2, whereas parietal cells were detected in fractions 6-10. Gastrin receptor gene expression was demonstrated in both parietal cell-rich and ECL cell-rich fractions. In addition, the gastrin receptor cDNA sequences obtained from the two of the fractions (F1 and 8) were identical. These results suggest that gastrin receptor genes are expressed in ECL cells as well as in parietal cells and that these receptors are identical.

Postmortem stability of monoamines, their metabolites, and receptor binding in rat brain regions

The effects of postmortem delay, time of storage, and freezing, thawing, and refreezing tissue samples were studied in postmortem rat brain using conditions that reflect the handling of postmortem human brain before neurochemical analysis. The levels of monoamines and metabolites in the striatum and cingulate and occipital cortex were measured using alumina extraction and HPLC methods. Binding of raclopride to dopamine D2, SCH-23390 to dopamine D1, ketanserin to serotonin 5-HT2, 8-hydroxy-2-(di-n-propylamino)tetralin to serotonin 5-HT1A, and cholecystokinin (CCK)-8 to CCK-B sites was measured in tissue homogenates from the striatum or fronto-parietal cortex. An 18-h postmortem delay before dissection and storage resulted in region-specific changes in monoamine and metabolite levels. Binding to striatal D1 and frontoparietal cortex CCK-B sites was reduced over the course of a 27-h postmortem delay. Binding to D2 and 5-HT sites was relatively stable. Storage of tissue for up to 8 months also resulted in region-specific changes in monoamine and metabolite levels. No changes in receptor binding were seen after long-term storage. Freezing, thawing, and refreezing tissue samples resulted in increased levels of striatal 3,4-dihydroxyphenylacetic acid and decreased binding to striatal D2 sites. These results demonstrate time-, temperature-, and storage-dependent regional differences in the stability of monoamines and their metabolites and in binding to various receptor sites. These differences in stability and binding should be accounted for to interpret accurately the effects of neurological disorders on neurotransmitter dynamics in postmortem human brain tissue.

Novel expression of gastrin (CCK-B) receptors in pancreatic carcinomas and dysplastic pancreas from transgenic mice

Transgenic mice bearing the elastase I promoter--SV40 T-antigen fusion gene (ELSV) develop pancreatic acinar cell carcinomas by 3 to 6 months of age. The purpose of the study was to determine if pancreatic carcinomas and dysplastic pancreas from the Tg (Ela-1, SV40E + Ela-1, neo) Bri19 strain of ELSV transgenic mice express gastrin (CCK-B) receptors. To accomplish this, we utilized iodine 125 (125I)-gastrin binding studies, reverse transcription-polymerase chain reaction (RT-PCR), and Southern blot analysis to examine pancreatic carcinomas from 26-week-old male ELSV transgenic mice, dysplastic pancreas from 8-week-old male ELSV transgenic mice, and normal pancreas from 30-week-old nontransgenic male mice (SJL/J) and from 8-week-old nontransgenic male mice (B6SJLF1/J). No saturable gastrin binding to normal nontransgenic mouse pancreas was found. In contrast, saturable gastrin binding was detected at pH 6.5, 22 degrees C, in 9 of 13 pancreatic carcinomas and all 5 dysplastic pancreata. Competitive inhibition 125I-gastrin binding assays showed gastrin bound to a single class of high-affinity receptors (receptor binding affinity [Kd] 0.11 +/- 0.02 nM, binding capacities ranging from 1 to 60 fmol/mg protein for pancreatic carcinomas; Kd: 0.15 +/- 0.04 nM, binding capacities ranging from 1 to 9 fmol/mg protein for dysplastic pancreas). RT-PCR and Southern blot analysis confirmed 125I-gastrin binding studies by demonstrating gastrin (CCK-B) receptor mRNA expression in pancreatic carcinomas and dysplastic pancreas but an absence of mRNA expression in normal nontransgenic mouse pancreas. In conclusion, pancreatic carcinomas and dysplastic pancreas in ELSV transgenic mice novelly express gastrin (CCK-B) receptors. This expression may provide a growth advantage to acinar cells as part of the multistage process of carcinogenesis.

Characterization of cholecystokinin receptors on the human sphincter of Oddi

BACKGROUND

The present in vitro study investigated the interaction between cholecystokinin (CCK) and receptors on human sphincter of Oddi tissue obtained from donated human livers that were being transplanted.

METHODS

Radiolabeled ligands with cholecystokinin receptor specificity, autoradiography, and crystal scintillation counting were used to directly characterize cholecystokinin receptors on tissue sections.

RESULTS

The binding of 125I-BH-CCK-8 to the tissue was saturable, specific, and dependent on time, pH, and temperature. Saturable binding of 125I-BH-CCK-8 was localized on the smooth muscle layer, and binding was inhibited only by cholecystokinin-related peptides. Computer analysis of 125I-BH-CCK-8 binding indicated the presence of two classes of binding sites, one with a high affinity and the other with a low affinity for CCK-8. CCK-8 caused relaxation (half-maximal concentration, 6 nmol/L) and carbachol caused contraction (half-maximal concentration, 10 nmol/L) of circular, cross-sectional strips of the tissue. Longitudinal strips were less responsive. The relative 125I-BH-CCK-8 binding inhibition potency of CCK-8 agreed closely with its relative ability to cause sphincter relaxation. Tetrodotoxin (1 mumol/L) and atropine (1 mumol/L) caused a rightward shift of the dose-response curve for CCK-8-stimulated sphincter relaxation.

CONCLUSIONS

The present results indicate that cholecystokinin receptors on the human sphincter of Oddi are sulfate dependent and mediate sphincter relaxation.

Localization of cholecystokinin binding sites in the adult and developing Brazilian opossum brain

Cholecystokinin (CCK) is now recognized as one of the most abundant peptides in the mammalian central nervous system. We have previously used immunohistochemistry to localize CCK in the adult and developing Brazilian opossum brain. However, little is known about the distribution of CCK binding sites in the developing mammalian brain. Therefore, to further our knowledge of the sites of action for CCK during development, we initiated a series of studies to localize CCK binding sites in the adult and developing Brazilian opossum. This species was chosen because pups are born in a fetus-like state. Receptor autoradiography was performed on coronally sectioned brains of 1 to 60 day postnatal (PN) animals and adults with 125I-Bolton Hunter-CCK-8 as the radioligand. Binding is evident in the 1PN opossum brainstem and is observed in the developing forebrain by 5PN. Region-specific binding increases during development, and binding in the 35PN brain resembles the adult pattern. Binding is evident prior to the detection of CCK-like immunoreactivity in many areas. The facial motor nucleus is identifiable and exhibits high levels of binding in Brazilian opossum pups of 10 to 35 days of age. However, binding is undetectable in the facial motor nucleus of 45 and 60PN pups. In general, the binding patterns for CCK in the adult opossum resemble those of other mammals and likely mediate similar physiological functions. However, some cholecystokininergic pathways appear to be unique to neonatal mammals.

Overexpression of messenger RNA for cholecystokinin-A receptor and novel expression of messenger RNA for gastrin (cholecystokinin-B) receptor in azaserine-induced rat pancreatic carcinoma

Using receptor binding assays, we have previously demonstrated the overexpression of the high-affinity cholecystokinin (CCK) receptor and the novel expression of the gastrin (CCK-B) receptor in the azaserine-induced rat pancreatic carcinoma DSL-6. Since cDNA of both the CCK-A receptor (classical pancreatic CCK receptor) coding region and the CCK-B receptor coding region have recently been cloned and sequenced, we investigated the expression of messenger RNA of these receptors in DSL-6 pancreatic carcinoma. Our results showed that the 32P-labelled cDNA probe of the CCK-A receptor coding region hybridized with an approximately 2.7 kb mRNA from both DSL-6 pancreatic carcinoma and normal rat pancreas. However, the relative expression of the CCK-A receptor mRNA in DSL-6 pancreatic carcinoma was approximately 8-fold of that in normal rat pancreas. The 32P-labelled cDNA probe of the CCK-B receptor coding region hybridized with an approximately 2.7 kb mRNA from DSL-6 pancreatic carcinoma; no hybridizing mRNA could be identified from normal rat pancreas. In summary, the CCK-A receptor mRNA is overexpressed approximately 8-fold and the gastrin (CCK-B) receptor mRNA is novelly expressed in DSL-6 pancreatic carcinoma as compared to normal rat pancreas. These results further confirm our previous findings based on receptor binding assays. The gene overexpression of the CCK-A receptor and the novel gene expression of the gastrin (CCK-B) receptor may be generated by alterations in gene regulation during carcinogenesis, and may play an important role in promoting tumor growth.

Social isolation of rats increases the density of cholecystokinin receptors in the frontal cortex and abolishes the anti-exploratory effect of caerulein

The role of cholecystokinin (CCK) receptors in the development of anxiety caused by social isolation of rats was studied using the elevated plus-maze and receptor binding techniques. The isolation of male Wistar rats significantly reduced their exploratory activity in the elevated plus-maze compared with that of rats kept in groups of four. Caerulein (0.1-5 micrograms/kg s.c.), an agonist at CCK receptors, only at the highest dose (5 micrograms/kg) significantly decreased the exploratory behaviour of rats housed in groups, but not in the isolated rats. By contrast, small doses of caerulein (0.1-0.5 microgram/kg) even tended to increase the behavioural activity of isolated rats in the plus-maze test. In parallel to the behavioural changes, isolation of the rats increased the number of [3H]pCCK-8 binding sites in the frontal cortex, but not in the other forebrain structures (the mesolimbic area, striatum and hippocampus). Isolation did not affect the density of benzodiazepine receptors in the frontal cortex. In conclusion, the isolation of rats for 7 days produced anxiogenic-like effect on the behaviour of rats and increased the number of CCK receptors in the frontal cortex without affecting benzodiazepine receptors.

Visualization of neuropeptide-binding sites on individual telencephalic neurons of the rat

The identification of high-affinity binding sites for neuropeptides on individual target cells is a prerequisite when studying the sites of action and the manner in which peptides act as neuromediators. In situ and in vitro, this can be achieved using newly synthesized, biologically active conjugates of somatostatin or cholecystokinin (sulphated octapeptide) with colloidal gold. Labelled neurons show a peptide-specific, non-overlapping distribution in rat telencephalic structures; i.e, whereas the somatostatin-gold conjugate labels binding sites on neurons and glial cells, cholecystokinin-binding sites are restricted to neurons. Binding of either gold-labelled ligand can be competitively suppressed by excess amounts of the native peptide or its analogues. Neuronal somatostatin-binding sites are visualized on neurons in lamina III and, in particular, in lamina V/VI of the primary somatosensory cortex and in the magnocellular nucleus of the telencephalic cholinergic system. Cholecystokinin-binding sites are localized in the main olfactory bulb, on neurons in the cortical "hindlimb" and "forelimb" region, in the hippocampus, and in the cingulate and visual cortex.