Further characterization of neurotensin receptor desensitization and down-regulation in clone N1E-115 neuroblastoma cells

The role of NTS2 in the development of tolerance to NT69L in mouse models for hypothermia and thermal analgesia

NT69L is a neurotensin (NT)(8-13) analog that binds the two major NT receptors, NTS1 and NTS2, and elicits similar behavioral effects as endogenous NT. Tolerance develops rapidly to some, but not to all of NT69L's effects, and to date, little is known about the mechanisms responsible for this tolerance. The development of tolerance appears to be more prevalent in behavioral effects mediated by NTS1 than by those mediated by NTS2, including hypothermia and thermal analgesia. However, we hypothesize that both NTS1 and NTS2 have important roles in mediating the effects of NT69L. Here, we investigate the role of NTS2 on NT69L-mediated hypothermia and thermal analgesia with the use of NTS2 knock-out mice. We show that tolerance develops to NT69L-mediated hypothermia and thermal analgesia following sub-chronic treatment in wild-type (WT) mice, and that NTS2 is necessary for the development of that tolerance. Additionally, we suggest potential means by which NTS2 influences these NT69L-mediated behaviors.

Time course of the hypothermic response to continuously administered neurotensin

Intracerebroventricular administration of the tridecapeptide neurotensin is known to elicit hypothermia in rodents for few hours. In the present study, we investigated a continuous intracerebroventricular infusion regimen for prolongation of the hypothermic effect. Male Wistar-Han rats (n=13) received neurotensin 10-50 microg/h for 48 h, while their body temperature was monitored continuously. This protocol led to a dose-dependent decrease of body temperature down to 35-36 degrees C. The nadir of hypothermia lasted for approximately 4h and normothermia was re-established after 12-24h. Furthermore, abundance of neurotensin in the hypothalamus was determined after 6 and 30 h by western blotting. High levels were still found at 30 h, while the rats had already become normothermic at that time. In summary, continuous infusion of neurotensin led to prolongation of the known hypothermic response, however resulted in development of tolerance.

Gene expression profiles highlight adaptive brain mechanisms in corticotropin releasing factor overexpressing mice

Corticotropin-releasing factor (CRF) plays an important role in mediating central and peripheral responses to stress. Alterations in CRF system activity have been linked to a number of psychiatric disorders, including anxiety and depression. Aim of this study was to elucidate homeostatic mechanisms induced by lifelong elevated CRF levels in the brain. We therefore profiled gene expression in several brain areas of transgenic mice overexpressing CRF (CRF-OE), a model for chronic stress. Several genes showed altered expression levels in CRF-OE mice when compared to their wild type littermates and were confirmed by quantitative PCR. Differences in gene expression profiles revealed the presence of previously unrecognized homeostatic mechanisms in CRF-OE animals. These included changes in glucocorticoid signaling, as exemplified by changes in 11beta-hydroxysteroid dehydrogenase type 1, FK506 binding protein 5 and serum/glucocorticoid kinase. Alterations in expression of genes involved in myelination (myelin, myelin-associated glycoprotein), cell proliferation and extracellular matrix formation (Edg2, Fgfr2, decorin, brevican) suggest changes in the dynamics of neurogenesis in CRF-OE. Pronounced changes in neurotensin (NT) receptors 1 and 2 mRNA were identified. Overall downregulation of NT receptors in CRF-OE animal was substantiated by receptor binding studies. Pronounced neurotensin receptor downregulation was observed for NT type 1 receptors in limbic brain areas, suggesting that NT could be implicated in some of the effects attributed to CRF overexpression. These data show that lifelong exposure to excessive CRF leads to adaptive changes in the brain which could play a role in some of the behavioral and physiological alterations seen in these animals.

Selective tolerance to the hypothermic and anticataleptic effects of a neurotensin analog that crosses the blood-brain barrier

NT69L, a neurotensin analog that crosses the blood-brain barrier, reduces body temperature, reverses apomorphine-induced climbing, haloperidol-induced catalepsy, and D-amphetamine- and cocaine-induced locomotor activity in rats. In this study we tested the development of tolerance to these effects of NT69L in rats. The blockade of apomorphine-induced climbing behavior and D-amphetamine- and cocaine-induced hyperactivity seen after a single acute injection did not show significant change with repeated daily injections of NT69L. Thus, for example, NT69L after five daily injections at a fixed dosage was as effective at reversing cocaine-induced hyperactivity as after the first injection. On the other hand, repeated daily injections of NT69L resulted in a diminished hypothermic response and a diminished anticataleptic effect against haloperidol. The effect of NT69L on blood glucose, cortisol, and thyroxine (T(4)) were all back to control levels after five daily injections. Thus, tolerance developed to NT69L after the first injection, when it was tested for causing hypothermia, blockade of haloperidol-induced catalepsy, and change in blood glucose, cortisol and T(4) levels. Since tolerance did not develop to the effects of drugs acting as direct (apomorphine) or indirect (D-amphetamine and cocaine) agonists at dopamine receptors over the course of 5 days, these findings suggest a selective role of neurotensin in the modulation of dopamine neurotransmission. Furthermore, due to the lack of development of tolerance, NT69L or similar analogs might be useful in modulating certain behavioral effects of psychostimulants or have potential use as an antipsychotic drug in humans.

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

Mesolimbic expression of neurotensin and neurotensin receptor during stress-induced gastric mucosal injury

Neurotensin is a neurotransmitter present in the brain and gastrointestinal tract. Intracerebroventricular injection of neurotensin protects rats from gastric mucosal injury caused by cold water restraint (CWR). Direct injection of neurotensin into the nucleus accumbens (NACB), part of the mesolimbic dopamine system, reduces gastric mucosal injury, suggesting that neurotensin confers protection on the mucosa through interaction with the mesolimbic system. The hypothesis is that the concentration of neurotensin in the mesolimbic system decreases during CWR, affecting the expression of neurotensin and the neurotensin receptor. After 1 h of CWR, neurotensin concentration significantly decreased 41% in the NACB and returned toward control concentrations after 2 h of CWR. The concentration of neurotensin mRNA significantly decreased 46% after 1 h CWR and returned toward control after 2 h. In contrast, neurotensin binding sites in the NACB increased from 159 to 228 fmol/mg protein after 1 h of CWR and increased significantly to 280 fmol/mg protein after 2 h CWR, whereas the level of neurotensin receptor mRNA significantly decreased 51 and 50% at 1 and 2 h, respectively. These studies show that neurotensin concentration within the mesolimbic system is transiently reduced by CWR stress and that the number of neurotensin binding sites increases, presumably in response to the decrease in neurotensin.

Desensitization of neurotensin-induced phosphoinositide hydrolysis in transfected CHO cells

The regulation of neurotensin-induced phosphoinositide turnover was studied in transfected CHO cells expressing the rat neurotensin receptor. Stimulation of these cells with neurotensin resulted in an important, but transient, increase in inositol phosphate cell content. Preincubation of the cells with neurotensin dramatically decreased their response to further stimulation. This diminution, which was time-dependent and not related to the availability of phospholipase C substrate, is though to reflect a progressive homologous desensitization of the recombinant neurotensin receptor.

Sodium nitroprusside-induced apoptotic cellular death via production of hydrogen peroxide in murine neuroblastoma N1E-115 cells

Sodium nitroprusside is widely used in pharmacological studies as a potent vasodilator or a nitric oxide donor. The mechanisms of cellular death induced by sodium nitroprusside were investigated in murine neuroblastoma N1E-115 cells. Sodium nitroprusside reduced the cellular viability, and the DNA extracted from treated cells showed a ladder-like intranucleosomal fragmentation pattern, which is an indication of apoptosis. The DNA fragmentations were also visualized by in situ nick translation. The cellular death was attenuated by cycloheximide, indicating that ongoing protein synthesis was essential for the initiation of the degenerative response. However, other nitric oxide donors did not decrease the cellular viability. The nitric oxide scavenger, hemoglobin, had no effect on sodium nitroprusside-induced cellular death. Furthermore, sodium cyanide, which is formed by the metabolism of sodium nitroprusside, did not cause cellular death. On the other hand, hydrogen peroxide, another product of sodium nitroprusside metabolism, reduced the cellular viability and induced DNA fragmentation. In addition, the cell damage induced by sodium nitroprusside was enhanced by a medium without fetal bovine serum. In conclusion, we proposed that hydrogen peroxide is the important toxic species for induction of apoptosis in N1E-115 cells exposed to sodium nitroprusside.

Changes in mouse brain neurotensin receptor density following chronic infusion of neurotensin

Many centrally acting drugs affect neurotensin (NT) systems by increasing levels of the peptide in specific brain regions. If these changes represent increases in extracellular NT levels, then changes in NT receptors would be expected. The focus of this study was to examine the effects of continuous exposure of NT receptors to agonist. Continuous infusion of NT (0.6 or 6 nmol/h) into the lateral ventricle via an osmotic minipump for 3 days caused a significant increase (over saline infusion) in total and low-affinity NT receptor density in the cerebellum of LS mice. High-affinity NT receptor density was increased in the frontal cortex. Seven days of NT infusion (6 nmol/h) caused no changes in NT receptor density.

Rational design of novel neurotensin mimetics: discovery of a pharmacologically unprecedented agent exhibiting concentration-dependent dual effects as antagonist and full agonist

We report the rational design of novel neurotensin mimetics through use of the Multiple Template Approach. This approach is based on our notion that a flexible peptide can be replaced by a partially flexible molecule, identified through testing a comparatively small number of molecules possessing a different intrinsic availability of conformations of the native peptide. The Multiple Template Approach has culminated in the discovery of a pharmacologically unprecedented agent, which behaves as a neurotensin antagonist at low concentration and as a full neurotensin agonist at high concentration.

Receptor mediated internalization of neurotensin in transfected Chinese hamster ovary cells

After association with intact Chinese hamster ovary (CHO) cells expressing the rat neurotensin receptor, tritiated neurotensin was rapidly internalized. Internalization was maximal after 30 min and accounted for about 90% of the total associated ligand. Neurotensin internalization was not observed at 0-4 degrees and was inhibited by an excess of unlabelled neurotensin or by the neurotensin non peptide antagonist, SR 48692. Moreover, the incubation of intact cells for 30 min with 10 nM neurotensin resulted in a significant decrease in the number of the cell surface neurotensin receptors. These results indicate that the endocytosis of membrane bound neurotensin in transfected CHO cells resulted from the internalization of the ligand-receptor complex inside the cell, through an agonist-induced process.