Heterogeneity of [3H]neurotensin bindings: studies with dynorphin, L-156,903 and levocabastine

Inhibition of neurotensin-induced pancreatic carcinoma growth by a nonpeptide neurotensin receptor antagonist, SR48692

BACKGROUND

Recently, a nonpeptide neurotensin (NT) receptor antagonist, SR48692, was developed that selectively antagonizes the high affinity, biologically active NT binding site. The effect of SR48692 on NT-mediated growth of a human pancreatic carcinoma, MIA PaCa-2, was determined both in vitro and in vivo.

METHODS

(125)I-NT binding and Northern blot analyses were performed for evaluation of the NT receptor in MIA PaCa-2 cells. Intracellular calcium ([Ca2+]i) mobilization and inositol phosphate (IP3) levels were measured. Cell growth studies were performed by counting cell numbers. Athymic nude mice were inoculated with MIA PaCa-2 cells and randomized into four groups to receive either vehicle (NT or SR48692) or NT + SR48692.

RESULTS

MIA PaCa-2 cells possess both a high affinity, SR48692-sensitive and a levocabastine-insensitive NT binding site; Northern blot analysis demonstrated expression of the NT receptor. SR48692 inhibited [Ca2+]i mobilization, IP3 turnover, and MIA PaCa-2 cell growth induced by NT in a dose-dependent fashion. In in vivo experiments, NT significantly increased the size, weight, and DNA and protein content of xenografted MIA PaCa-2 tumors; SR48692 inhibited the effect of NT.

CONCLUSIONS

The novel NT receptor antagonist SR48692 will be a valuable agent to delineate further the cellular mechanisms responsible for peptide-mediated growth of normal and neoplastic gut tissues.

Structure, functional expression, and cerebral localization of the levocabastine-sensitive neurotensin/neuromedin N receptor from mouse brain

This work describes the cloning and expression of the levocabastine-sensitive neurotensin (NT) receptor from mouse brain. The receptor protein comprises 417 amino acids and bears the characteristics of G-protein-coupled receptors. This new NT receptor (NTR) type is 39% homologous to, but pharmacologically distinct from, the only other NTR cloned to date from the rat brain and the human HT29 cell line. When the receptor is expressed in Xenopus laevis oocytes, the H1 antihistaminic drug levocabastine, like NT and neuromedin N, triggers an inward current. The pharmacological properties of this receptor correspond to those of the low-affinity, levocabastine-sensitive NT binding site described initially in membranes prepared from rat and mouse brain. It is expressed maximally in the cerebellum, hippocampus, piriform cortex, and neocortex of adult mouse brain.

Neurotensin receptor expression in the rat forebrain and midbrain: a combined analysis by in situ hybridization and receptor autoradiography

The aim of the present study was to examine the distribution of the levocabastine-insensitive high-affinity neurotensin binding sites in the rat forebrain and midbrain in relation to the distribution of the cloned neurotensin receptor mRNA by using a combination of both high-resolution in vitro receptor autoradiography and in situ hybridization approaches. Groups of cells rich in neurotensin receptor mRNA were observed in the basal forebrain nuclei, the ventral tegmental area, the substantia nigra and in the interfascicular and caudal linear nuclei and the retrorubral field. Cells expressing lower levels of neurotensin receptor mRNA were found in several subdivisions of the cortex; the dentate gyrus; the septofimbrial, suprachiasmatic, medial habenular, and mammillary nuclei; the dorsal part of the lateral septum; the zona incerta; and the dorsomedial and perifornical hypothalamic areas. Most of the brain areas containing neurotensin receptor mRNA demonstrated a selective association of neurotensin binding sites with neuronal cell bodies. In contrast, in several telencephalic and diencephalic structures, the presence of neurotensin binding sites was not correlated with that of neurotensin receptor mRNA, suggesting that neurotensin receptors were mainly located on axon terminals. This study provides a better understanding of the anatomical organization of neurotensin receptor expressing systems in the rat brain and gives further insight into the pre- vs. postsynaptic location of neurotensin receptors in various brain regions. Moreover, it indicates that all neurons expressing the cloned neurotensin receptor harbour neurotensin binding sites on their perikaryal membrane.

Chronic ethanol administration downregulates neurotensin receptors in long- and short-sleep mice

Neurotensin (NT) has been shown to differentially alter many of the physiologic responses to ethanol administration in long-sleep (LS) and short-sleep (SS) mice, which were selectively bred for differences in hypnotic sensitivity to ethanol. These mice have been shown to differ in NT receptor densities in cortical and mesolimbic brain regions and it has been suggested that ethanol actions may be mediated, in part, by neurotensinergic processes. The present study was conducted to further examine this hypothesis by determining the effects of acute and chronic ethanol administration on NT receptor systems in these mice. Scatchard analysis of [3H]NT binding in brain membranes from mice chronically treated with ethanol yielded a one-site model, whereas binding in membranes from control mice were best described by a two-site model. Values for binding capacity (Bmax) were significantly reduced in several brain regions, and binding site density for total, levocabastine-sensitive, and levocabastine-insensitive binding sites were also reduced. The maximum effect was seen after 2 weeks of chronic ethanol consumption. Three weeks after withdrawal from ethanol, Kd and Bmax had returned to control values. Similarly, binding density in all regions for total, levocabastine-sensitive, and levocabastine-insensitive sites had returned to control values within 2 weeks. NT receptor characteristics measured 2 h post-3.0 g/kg ethanol revealed that ethanol caused a rapid downregulation of both subtypes of NT receptors. The finding that both acute and chronic ethanol significantly downregulate the neurotensin receptor systems further supports the hypothesis that ethanol's actions may be mediated in part by neurotensinergic systems.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Regional characterization of brain neurotensin receptor subtypes in LS and SS mice

Neurotensin (NT) receptor subtypes were investigated in nine brain regions from long sleep (LS) and short sleep (SS) mice that were selectively bred for differences in sensitivity to ethanol. Differences in NT receptor densities may mediate, in part, genetically selected differences in ethanol sensitivity between the two lines of mice. The use of [3H] NT at concentrations from 0.02 to 20 nM yielded biphasic binding isotherms as revealed by Scatchard analysis. Membranes from LS ventral midbrain yielded dissociation constants (KD values) of 0.34 and 3.85 nM for the high (NTH) and low (NTL) affinity components, respectively. SS membranes displayed similar KD values, however the maximum number of binding sites (Bmax) for both receptor subtypes were significantly greater in SS than in LS membranes (46.7 vs. 71.5 fmol/mg protein for NTH and 170.2 vs. 208.2 fmol/mg protein for NTL). Using levocabastine, and H1 antagonist with selectivity for NTL, characterization of NTH and NTL binding in nine brain regions was performed. In general, membranes from each brain region of SS mice had higher densities than LS for both receptor subtypes. Significant differences for the total density of receptors and NTL were found in entorhinal cortex, nucleus accumbens, hippocampus, and ventral midbrain. The only region to differ in NTH was the ventral midbrain. Competition experiments using various NT fragments to compete for NTH binding showed the C-terminal amino acids to be essential for binding. The order of potency was NT1-13 = NT8-13 greater than Neuromedin N greater than NT1-8 = NT1-11.