Regulatory peptide receptors: visualization by autoradiography

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Concomitant expression of several peptide receptors in neuroendocrine tumours: molecular basis for in vivo multireceptor tumour targeting

Peptide receptors have been found to represent excellent targets for in vivo cancer diagnosis and therapy. Recent in vitro studies have shown that many cancers can overexpress not only one but several peptide receptors concomitantly. One of the challenges for nuclear medicine in this field in the coming decade will be to take advantage of the co-expression of peptide receptors for multireceptor tumour targeting. In vitro receptor studies can reveal which peptide receptor is overexpressed in which tumour and which receptors are co-expressed in an individual tumour; such knowledge is a prerequisite for successful in vivo development. One group of tumours of particular interest in this respect is the neuroendocrine tumours, which have previously been shown often to express peptide receptors. This review summarises our investigations of the concomitant expression of 13 different peptide receptors, in more than 100 neuroendocrine tumours of the human intestine, pancreas and lung, using in vitro receptor autoradiography with subtype-selective ligands. The incidence and density of the somatostatin receptors sst(1)-sst(5), the VIP receptors VPAC(1) and VPAC(2), the CCK(1) and CCK(2) receptors, the three bombesin receptor subtypes BB(1) (NMB receptor), BB(2) (GRP receptor) and BB(3), and GLP-1 receptors were evaluated. While the presence of VPAC(1) and sst(2) was detected in the majority of these neuroendocrine tumours, the other receptors, more differentially expressed, revealed a characteristic receptor pattern in several tumour types. Ileal carcinoids expressed sst(2) and VPAC(1) receptors in virtually all cases and had CCK(1), CCK(2), sst(1) or sst(5) in approximately half of the cases; they were the only tumours of this series to express NMB receptors. Insulinomas were characterised by a very high incidence of GLP-1, CCK(2) and VPAC(1) receptors, with the GLP-1 receptors expressed in a particularly high density; they expressed sst(2) in two-thirds and sst(1) in approximately half of the cases and lacked CCK(1) and NMB receptors. All gastrinomas had sst(2) and GLP-1 receptors; they expressed GRP receptors in three-quarters of the cases and CCK(1) or VPAC(1) in approximately half of the cases. Most bronchial carcinoids had VPAC(1), while sst(1), sst(2) and CCK(2) were found in two-thirds of the cases and BB(3) in one-third of the cases. These data provide evidence for the vast biological diversity of these neuroendocrine tumours. Moreover, the results represent a basis for starting and/or optimising the in vivo targeting of these tumours by selecting the suitable radiopeptides for tumour diagnosis and/or therapy. Finally, the data strongly encourage concomitant application of several radiopeptides to permit more efficient targeting of these tumours.

Localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick

The distribution and cellular localization of dopamine D1A and D1B receptor mRNAs in the forebrain and midbrain of the domestic chick were examined using in situ hybridization histochemistry with 35[S]-dATP labeled oligonucleotide probes, visualized with film and emulsion autoradiography. Labeling for D1A receptor mRNA was intense in the medial and lateral striatum, and moderately abundant in the pallial regions termed the archistriatum and the neostriatum, in the hypothalamic paraventricular nucleus region, and in the superficial gray layer of optic tectum of the midbrain. D1B receptor mRNA was abundant in the medial and lateral striatum, and in the pallial region termed the hyperstriatum ventrale, and moderately abundant in the intralaminar dorsal and posterior thalamus and in the superficial gray of the optic tectum. At the cellular level, about 75% of neurons in the medial striatum and 59% of neurons in the lateral striatum were labeled for D1A receptor mRNA, whereas about 39% of the neurons in the medial striatum and 21% in the lateral striatum were labeled for D1B receptor mRNA. Large striatal neurons were not labeled for D1A or D1B receptor mRNA. The data suggest that while both D1A and D1B receptors mediate dopaminergic responses in many neurons of the avian striatum, primarily D1A receptors mediate dopaminergic responses in the archistriatum and the neostriatum, while primarily D1B receptors mediate dopaminergic responses in the hyperstriatum ventrale and the thalamus.

Developmental changes in binding sites and reactivity for CGRP and VIP in porcine pulmonary arteries

During postnatal adaptation pulmonary arteries dilate. CGRP and VIP are pulmonary vasodilators. In this report, porcine lungs from newborn to adult were studied. Radiolabeled ligand binding and autoradiography showed CGRP binding sites on the endothelium of pulmonary arteries and veins, which increased postnatally, and VIP binding sites on smooth muscle, which decreased. Isolated conduit arteries relaxed normally (initially endothelium dependent) in response to CGRP from birth. VIP first caused relaxation at 10 days and was endothelium dependent. Age-related changes in receptor binding density were not always reflected in an appropriate alteration in pharmacological response.

Facilitation of GABA release by neurotensin is associated with a reduction of dopamine release in rat nucleus accumbens

The main aim of the present study was to investigate the effects of local perfusion with the tridecapeptide neurotensin on extracellular GABA and dopamine levels in the nucleus accumbens of the halothane-anaesthetized rat, using in vivo microdialysis. In an initial set of characterization studies we examined the Na+ dependence of neurotransmitter release by local perfusion with ouabain, veratridine and tetrodotoxin. Local perfusion with the Na+ ATPase inhibitor ouabain (10 microM) or the Na+ channel agonist veratridine (20 microM) perfused into the nucleus accumbens increased both extracellular GABA and dopamine levels. The Na+ channel antagonist tetrodotoxin (1 microM) consistently decreased (24% of basal) dopamine levels, while even at 10 microM it did not affect GABA. However, tetrodotoxin (10 microM) abolished the veratridine-induced increase in both GABA and dopamine, demonstrating that Na(+)-dependent neuronal activity is involved in this release mechanism. In a second set of experiments a hypothesis for a functional link between neurotensin, dopamine and GABA in the medial nucleus accumbens was tested. Towards this aim, the effects of local perfusion with a high 1 microM concentration of neurotensin into the nucleus accumbens increased both GABA (210% of basal value) and dopamine (145% of basal) release. However, a low (10 nM) concentration of neurotensin again increased GABA release (160% of basal), but decreased that of dopamine (75% of basal value). Furthermore, the local perfusion with the GABAA receptor antagonist bicuculline abolished the neurotensin (10 nM) induced inhibition of dopamine release without affecting the increase in GABA release. These findings suggest that neurotensin modulates both GABA and dopamine neurotransmission in the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

Phylogeny of tachykinin receptor localization in the vertebrate central nervous system: apparent absence of neurokinin-2 and neurokinin-3 binding sites in the human brain

Binding of [125I]Bolton-Hunter labeled tachykinins substance P (BHSP), neurokinin A (BHNKA) and eledoisin (BHELE) to brain sections from several vertebrates was investigated by receptor autoradiography. Densities of BHSP binding sites were low in fish brain, increased in lower vertebrates, were high in birds and rodents, and relatively constant in cat, monkey and human. In contrast, BHELE binding site densities were moderate in fish brain and high in frog, snake, chick, pigeon, mouse and rat brain. Low and very low densities were localized in guinea pig and cat, while no significant BHELE specific binding was found in monkey and human brain. BHSP and BHELE binding sites were distinctly distributed in the vertebrate brains analyzed. Each ligand showed a characteristic regional distribution which was similar from species to species. The affinity profiles of tachykinins for BHSP and BHELE binding sites as analyzed on frog, chick and rat brain sections, corresponded to the NK1 and NK3 receptor types, respectively. No BHNKA binding sites could be detected in any vertebrate brain investigated. In conclusion, marked species variations exist in the density and distribution of tachykinin receptor types in the vertebrate brain. Thus, neurokinin A receptors (NK2 type) seem to be absent in the vertebrate central nervous system and, while substance P receptors (NK1 type) appear to be preserved and increase in density during evolution, the contrary seems to happen for the eledoisin receptors (NK3 type) which are more abundant in lower vertebrates and apparently absent in primate, particularly human brain.

Vasoactive intestinal peptide (VIP) and peptide histidine methionine (PHM) in human eccrine sweat glands: demonstration of innervation, specific binding sites and presence in secretions

Vasoactive intestinal peptide (VIP) and peptide histidine methionine (PHM) immunoreactivities have been detected in alcohol extracts of human axillary skin using sensitive and specific radioimmunoassays. VIP immunoreactivity (7.63 + 2.33 pmol/g, x + SE, n = 9) was more abundant than PHM immunoreactivity (3.86 + 0.56 pmol/g, x + SE, n = 9). Immunocytochemistry of sections of skin revealed a network of VIP/PHM immunoreactive nerve fibres around the perimeter of eccrine but not apocrine sweat glands. In vitro autoradiography of skin sections using 125I-labelled VIP and PHM, demonstrated binding sites on the membranes of eccrine gland secretory cells. The binding of each radiolabelled ligand was eliminated by the presence of a large molar excess of appropriate cold peptide but was unaffected when incubated with related peptide, indicating the presence of specific binding sites for both VIP and PHM. Radioimmunoassay of Sep-pak concentrated human sweat identified the presence of both VIP immunoreactivity (30.6 pmol/l) and PHM immunoreactivity (43.4 pmol/l). Reverse-phase HPLC analysis of axillary skin extracts and sweat, followed by radioimmunoassay of fractions, identified single resolved peaks of VIP and PHM immunoreactivity with identical retention times to synthetic standards. Eccrine sweat glands in human axillary skin have VIP and PHM peptidergic innervation and possess specific binding sites for each peptide which are both secreted to the surface of the skin.

Effects of neurotensin on dopamine release and metabolism in the rat striatum and nucleus accumbens: cross-validation using in vivo voltammetry and microdialysis

The effects of the neuropeptide neurotensin on dopamine release and metabolism in the posteromedial nucleus accumbens and anterior dorsomedial striatum of the anesthetized rat were investigated using in vivo chronoamperometry and intracerebral microdialysis techniques. A dose-dependent augmentation of dopamine efflux as evidenced by increases in the chronoamperometric signal was observed in the nucleus accumbens following intracerebroventricular injections of neurotensin. However, neurotensin failed to alter extracellular concentrations of dopamine in the striatum. The selective effects of neurotensin on mesolimbic dopamine neurons were confirmed using in vivo microdialysis. These results demonstrate that neurotensin can selectively enhance the release and metabolism of dopamine in neurons projecting from the ventral tegmental area to the nucleus accumbens.

Quantitative autoradiography of TRH receptors in discrete brain regions of different mammalian species

The results clearly show marked heterogeneity and ubiquity of the CNS distribution of TRH receptors across several mammalian species including man. The use of high resolution autoradiography coupled with image analysis has permitted the visualization and quantification of TRH receptor density in even very small regions and nuclei of the CNS. This technique will undoubtedly help elucidate the other areas of TRH receptor localization that have thus far escaped detection in mammals and that are yet to be studied in lower vertebrates. Although an attempt has been made to correlate the presence of the peptide, its receptors, and its possible physiological functions, only further detailed physiological/behavioral investigations will ultimately unravel and support the diverse neurotransmitter and trophic roles of TRH in CNS and endocrine function.

Localization of bombesin-like peptides in tumors

The localization of bombesin gene products in neuroendocrine tumors was achieved by a number of techniques used in combination. These included immunocytochemistry, radioimmunoassay, and chromatographic procedures using a variety of region-specific antibodies recognizing separate portions of probombesin. In situ hybridization using cRNA probes was employed to analyze bombesin gene expression at a cellular level. A novel procedure using a divalent form of bombesin and gold-labeled monoclonal antibodies for the localization of bombesin binding sites at the ultrastructural level was employed in this study. Antibodies to neuron-specific enolase and electron microscopy were employed for the determination of neuroendocrine differentiation. Surgical samples of pulmonary (n = 250) and nonpulmonary (n = 28) small cell carcinomas, 49 carcinoids, and 62 atypical lung carcinoids were investigated and compared with 169 control tumors, including lymphomas, adenocarcinomas, squamous cell carcinomas, and non-small-cell undifferentiated tumors. Cell lines cultured from pulmonary small cell carcinoma and smear preparations of pleural effusions from patients with small cell carcinoma of the lung were also investigated. Strong immunostaining for neuron-specific enolase was noted in all neuroendocrine tumors investigated, and no immunoreactivity was noted in control cases. Electron-dense neurosecretory granules were abundant in carcinoid tumors, scattered in small cell carcinoids, and absent in control cases. Immunostaining for bombesin was particularly strong in benign carcinoids, whereas the more malignant neuroendocrine tumors (e.g., small cell carcinomas) stained best with antibodies to the carboxyl-terminal flanking portion of human probombesin (proGRP). These findings were further validated by radioimmunoassay and chromatography of tissue extracts. Specific binding sites for bombesin were demonstrated on the surface of small cell carcinoma cells maintained in culture. In situ hybridization demonstrated mRNA for preprobombesin in all small cell carcinomas investigated, including surgical samples, cytological preparations, and cell lines. Hybridization reactions varied in intensity, with some cells in autoradiograms almost masked by silver grains and others showing much lighter deposits.