Neuromedin N: high affinity interaction with brain neurotensin receptors and rapid inactivation by brain synaptic peptidases

Potential roles of neurotensin on cognition in conditions of obese-insulin resistance

Neurotensin is an endogenous tridecapeptide that can be found in both central and peripheral nervous systems. Under normal physiological conditions, neurotensin is involved in the regulation of pain, body temperature, physical activity, appetite as well as learning and memory. In addition, it plays an important role in fat metabolism. Previous studies have demonstrated that alterations of neurotensin levels were associated with several neuropathological conditions such as Alzheimer's disease, mood disorders, and obesity associated eating disorders. Obesity has been shown to be associated with low-grade systemic inflammation, brain inflammation, and cognitive decline. Several pieces of evidence suggest that neurotensin might play a role in cognitive decline following obesity. However, the underlying mechanisms of neurotensin on cognition under obese-insulin resistant condition are still unclear. In this review, the current available evidence from in vitro, in vivo and clinical studies regarding the role of neurotensin in the physiological condition and obesity in association with cognition are comprehensively summarized and discussed. The studies which report controversial findings regarding these issues are also presented and discussed.

Neuromedin: An insight into its types, receptors and therapeutic opportunities

Neuropeptides are small protein used by neurons in signal communications. Neuromedin U was the first neuropeptide discovered from the porcine spinal and showed its potent constricting activities on uterus hence was entitled with neuromedin U. Following neuromedin U another of its isoform was discovered neuromedin S which was observed in suprachiasmatic nucleus hence was entitled neuromedin S. Neuromedin K and neuromedin L are of kanassin class which belong to tachykinin family. Bombesin family consists of neuromedin B and neuromedin C. All these different neuromedins have various physiological roles like constrictive effects on the smooth muscles, control of blood pressure, pain sensations, hunger, bone metastasis and release and regulation of hormones. Over the years various newer physiological roles have been observed thus opening ways for various novel therapeutic treatments. This review aims to provide an overview of important different types of neuromedin, their receptors, signal transduction mechanism and implications for various diseases.

Synthesis and binding characteristics of [(3)H]neuromedin N, a NTS2 receptor ligand

Neurotensin (NT) and its analog neuromedin N (NN) are formed by the processing of a common precursor in mammalian brain tissue and intestines. The biological effects mediated by NT and NN (e.g. analgesia, hypothermia) result from the interaction with G protein-coupled receptors. The goal of this study consisted of the synthesis and radiolabeling of NN, as well as the determination of the binding characteristics of [(3)H]NN and G protein activation by the cold ligand. In homologous displacement studies a weak affinity was determined for NN, with IC50 values of 454nM in rat brain and 425nM in rat spinal cord membranes. In saturation binding experiments the Kd value proved to be 264.8±30.18nM, while the Bmax value corresponded to 3.8±0.2pmol/mg protein in rat brain membranes. The specific binding of [(3)H]NN was saturable, interacting with a single set of homogenous binding sites. In sodium sensitivity experiments, a very weak inhibitory effect of Na(+) ions was observed on the binding of [(3)H]NN, resulting in an IC50 of 150.6mM. In [(35)S]GTPγS binding experiments the Emax value was 112.3±1.4% in rat brain and 112.9±2.4% in rat spinal cord membranes and EC50 values of 0.7nM and 0.79nM were determined, respectively. NN showed moderate agonist activities in stimulating G proteins. The stimulatory effect of NN could be maximally inhibited via use of the NTS2 receptor antagonist levocabastine, but not by the opioid receptor specific antagonist naloxone, nor by the NTS1 antagonist SR48692. These observations allow us to conclude that [(3)H]NN labels NTS2 receptors in rat brain membranes.

Cardiovascular biomarkers and sex: the case for women

Measurement of biomarkers is a critical component of cardiovascular care. Women and men differ in their cardiac physiology and manifestations of cardiovascular disease. Although most cardiovascular biomarkers are used by clinicians without taking sex into account, sex-specific differences in biomarkers clearly exist. Baseline concentrations of many biomarkers (including cardiac troponin, natriuretic peptides, galectin-3, and soluble ST2) differ in men versus women, but these sex-specific differences do not generally translate into a need for differential sex-based cut-off points. Furthermore, most biomarkers are similarly diagnostic and prognostic, regardless of sex. Two potential exceptions are cardiac troponins measured by high-sensitivity assay, and proneurotensin. Troponin levels are lower in women than in men and, with the use of high-sensitivity assays, sex-specific cut-off points might improve the diagnosis of myocardial infarction. Proneurotensin is a novel biomarker that was found to be predictive of incident cardiovascular disease in women, but not men, and was also predictive of incident breast cancer. If confirmed, proneurotensin might be a unique biomarker of disease risk in women. With any biomarker, an understanding of sex-specific differences might improve its use and might also lead to an enhanced understanding of the physiological differences between the hearts of men and women.

Intrathecal neurotensin is hypotensive, sympathoinhibitory and enhances the baroreflex in anaesthetized rat

BACKGROUND AND PURPOSE

The neuromodulatory effects of the gut-neuropeptide neurotensin on sympathetic vasomotor tone, central respiratory drive and adaptive reflexes in the spinal cord, are not known.

EXPERIMENTAL APPROACH

Neurotensin (0.5 µM-3 mM) was administered into the intrathecal (i.t.) space at the sixth thoracic spinal cord segment in urethane-anaesthetized, paralysed, vagotomized male Sprague-Dawley rats. Pulsatile arterial pressure, splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, ECG and end-tidal CO(2) were recorded.

KEY RESULTS

Neurotensin caused a dose-related hypotension, sympathoinhibition and bradycardia. The maximum effects were observed at 3000 µM, where the decreases in mean arterial pressure (MAP), heart rate (HR) and sSNA reached -25 mmHg, -26 beats min(-1) and -26% from baseline, respectively. The sympathetic baroreflex was enhanced. Changes in central respiratory drive were characterized by a fall in the amplitude of the phrenic nerve activity. Finally, administration of SR 142948A (5 mM), a potent, selective antagonist at neurotensin receptors, caused a potent hypotension (-35 mmHg), bradycardia (-54 beats min(-1) ) and sympathoinhibition (-44%). A reduction in the amplitude and frequency of the phrenic nerve activity was also observed.

CONCLUSIONS AND IMPLICATIONS

The data demonstrate that neurotensin plays an important role in the regulation of spinal cardiovascular function, affecting both tone and adaptive reflexes.

Proneurotensin/neuromedin N secreted from small cell lung carcinoma cell lines as a potential tumor marker

Proteins secreted from specific cancer cells have a high potential for use as tumor markers. We identified secreted proteins produced by 15 different carcinoma cell lines grown in serum-free medium using MS/MS. Proneurotensin/neuromedin N (proNT/NMN) was found in conditioned medium from four of seven small cell lung carcinoma cell lines but not from eight nonsmall cell lung carcinoma cell lines. These results indicate proNT/NMN has potential as a specific tumor marker of small cell lung carcinoma.

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.

Inactivation of neurotensin and neuromedin N by Zn metallopeptidases

The two related peptides neurotensin (NT) and neuromedin N (NN) are efficiently inactivated by peptidases in vitro. Whereas NT is primarily degraded by a combination of three Zn metallo-endopeptidases, namely endopeptidases 24.11, 24.15 and 24.16, in all systems examined, NN is essentially inactivated by the Zn metallo-exopeptidase aminopeptidase M. In this paper we review the work that has led to the identification of the NT- and NN-degrading enzymes and to the purification and cloning of EP 24.16, a previously unidentified peptidase. We provide a brief description of the three NT-inactivating endopeptidases and of their specific and mixed inhibitors, some of them developed in the course of studying NT degradation. Finally, we review in vivo data obtained with these inhibitors that strongly support a physiological role for EP 24.11, 24.15 and 24.16 in the termination of NT-generated signals and for aminopeptidase in terminating NN action. Knowledge of the NT and NN inactivation mechanisms offers the perspective to develop metabolically stable analogs of these peptides with potential therapeutic value.

Effect of gastric bypass and gastric banding on proneurotensin levels in morbidly obese patients

CONTEXT

Neurotensin is produced mainly in the N cells of the ileum and has a role in appetite regulation; levels are decreased in obese subjects and increase after bariatric surgery. Mature neurotensin is very unstable, with a short half-life.

OBJECTIVE

The objective of this study was to compare baseline and postoperative levels of the more stable neurotensin precursor, proneurotensin/neuromedin (pro-NT/NMN), in patients after gastric banding, gastric bypass, and nonoperated controls, respectively, during long-term follow-up.

DESIGN AND SETTING

This was a prospective observational study in a university hospital.

PARTICIPANTS AND MAIN OUTCOME MEASURES

Overnight fasting plasma pro-NT/NMN concentrations were measured with a new sandwich immunoassay in morbidly obese subjects at baseline and 6, 12, and 24 months after gastric banding (n = 8), Roux-en-Y gastric bypass (n = 5), and in nonoperated controls (n = 7).

RESULTS

After gastric bypass and banding, body weight decreased by (mean +/- sd) 29.5 +/- 5.5 and 22.8 +/- 5.9 kg, respectively. The decrease after 3 and 6 months was more pronounced after gastric bypass compared with gastric banding (P < 0.05). Plasma pro-NT/NMN levels in patients after gastric bypass increased from 246.3 +/- 174.3 pmol/liter on admission to 748.3 +/- 429.6 pmol/liter after 24 months (P < 0.01). In contrast, in patients with gastric banding, pro-NT/NMN concentrations remained stable (207.3 +/- 60.5 pmol/liter at admission, 226.6 +/- 116.8 pmol/liter after 24 months). Neither body weight nor plasma pro-NT/NMN levels changed in nonoperated controls.

CONCLUSION

Plasma pro-NT/NMN levels show a more pronounced increase after gastric bypass compared with gastric banding, suggesting that specific bariatric surgical procedures result in distinct alterations of gastrointestinal hormone metabolism. The more stable precursor pro-NT/NMN provides a new tool to quantify neurotensin levels in clinical practice.

Proneurotensin 1-117, a stable neurotensin precursor fragment identified in human circulation

Proneurotensin/neuromedin N (pro NT/NMN) is the common precursor of two biologically active peptides, neurotensin (NT) and neuromedin N (NMN). We have established antibodies against peptide sequences of the NT/NMN precursor and developed a sandwich immunoassay for the detection of pro NT/NMN immunoreactivity in human circulation. Endogenous pro NT/NMN immunoreactivity was enriched by affinity chromatography using antibodies against two different pro NT/NMN epitopes, and further purified by reversed phase HPLC. Mass spectrometry analysis revealed pro NT/NMN 1-117 as major pro NT/NMN immunoreactivity in human circulation. Pro NT/NMN 1-117 is detectable in serum from healthy individuals (n = 124; median 338.9 pmol/L). As known for NT, the release of pro NT/NMN 1-117 from the intestine into the circulation is stimulated by ingestion of an ordinary meal. Investigation of the pro NT/NMN 1-117 in vitro stability in human serum and plasma revealed that this molecule is stable for at least 48 h at room temperature. Since pro NT/NMN 1-117 is theoretically produced during precursor processing in stoichiometric amounts relative to NT and NMN, it could be a surrogate marker for the release of these bioactive peptides.

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Mediation by neurotensin-receptors of effects of neurotensin on self-stimulation of the medial prefrontal cortex

1 Intracortical microinjections of neurotensin (NT) selectively decreased intracranial self-stimulation (ICSS) of the medial prefrontal cortex in the rat. 2 To elucidate whether this effect is mediated by NT receptors or by the formation of NT-dopamine complexes, we investigated the effects on ICSS of intracortical microinjections of neurotensin (1-11), an NT fragment that forms extracellular complexes with dopamine but does not bind to NT receptors. 3 We also studied the effects of the peripheral administration of SR 48692, a selective antagonist of NT receptors, on the inhibition of ICSS produced by the intracortical administration of NT. 4 Unilateral microinjections of neurotensin (1-11) at doses of 10, 20 and 40 nmol into the medial prefrontal cortex did not change the basal ICSS rate of this area. 5 The intraperitoneal administration of SR 48692 at doses of 0.08 and 0.16 mg kg-1 30 min before microinjection of 10 nmol of NT into the medial prefrontal cortex, antagonized the inhibition of ICSS produced by the neuropeptide. 6 These results demonstrate that the inhibitory effect of NT on ICSS is mediated by NT receptors.

Thalamostriatal projection neurons in birds utilize LANT6 and neurotensin: a light and electron microscopic double-labeling study

Based on its location, connectivity and neurotransmitter content, the dorsal thalamic zone in birds appears to be homologous to the intralaminar, midline, and mediodorsal nuclear complex in the thalamus of mammals. We investigated the neuroactive substances used by thalamostriatal projection neurons of the dorsal thalamic zone in the pigeon. Single-labeling experiments showed that many neurons in the dorsal thalamic zone are immunoreactive for neurotensin and the neurotensin-related hexapeptide, (Lys8,Asn9)NT(8-13) (LANT6). Double-labeling experiments, using the retrograde fluorescent tracer, FluoroGold, combined with fluorescence immunocytochemistry for either LANT6 or neurotensin, showed that neurotensin- and LANT6-containing neurons in the dorsal thalamic zone project to the striatum of the basal ganglia. Immunofluorescence double-labeling experiments showed that neurotensin and LANT6 are often (possibly always) co-expressed in neurons in the dorsal thalamic zone. Electron microscopic immunohistochemical double-labeling showed that LANT6 terminals in the striatum make asymmetric contacts with heads of spines labeled for substance P and heads of spines not labeled for substance P, suggesting that these terminals synapse with both substance P-containing and non-substance P-containing medium spiny striatal projection neurons. These findings indicate that LANT6 and neurotensin may be utilized as neurotransmitters in thalamostriatal projections in birds and raise the possibility that this may also be the case in other amniotes.

Proliferogenic effect of neurotensin (NT) and neuromedin-N (NMN) on the rat adrenal cortex: evidence that angiotensin-II mediates the effect of NMN, but not of NT

NT and NMN, two biologically active peptides acting via the same specific receptor, are both able to stimulate in vivo the proliferative activity of rat adrenocortical cells. The present study aimed to investigate whether the mechanism underlying this effect of NT and NMN may involve an enhanced production of angiotensin-II (ANG-II), a potent adrenocortical proliferogenic factor. Metaphases per adrenal section were counted 120 min after vincristine injection. A bolus administration of ANG-II resulted in a marked increase in the number of metaphase-arrested cells 12, 24 and 48 h after the beginning of the experiment; the concomitant administration of saralasin (SAR), a competitive antagonist of ANG-II, completely blocked the proliferogenic effect of ANG-II. NT-evoked rise in the number of metaphases occurred 48 h after administration and was not influenced by the simultaneous SAR injection. On the contrary, NMN injection induced a burst of adrenocortical cell proliferation within 12 h, and this effect was prevented by SAR. These data suggest that ANG-II mediates the proliferogenic effect of NMN, but not that of NT.

Neurotensin and neuromedin N undergo distinct catabolic processes in murine astrocytes and primary cultured neurons

We examined the occurrence of various endopeptidases and exopeptidases and their subcellular partition within soluble and membrane-associated compartments of 15-day-old astrocytes and 4-day-old primary cultured neurons. Peptidases were monitored with chromogenic or fluorimetric substrates and identified by means of specific inhibitors. We assessed the contribution of these peptidases in the catabolism of two related neuropeptides, neurotensin and neuromedin N. Metabolites were separated by HPLC and the identity of the proteolytic activities involved in their formation was established using specific inhibitors. Neuromedin N and neurotensin undergo both quantitative and qualitative differential proteolysis. Initial maximal rates of neuromedin N degradation were higher than those of neurotensin in both cell types. Furthermore, the two peptides were inactivated much more rapidly by the soluble than by the membrane-associated fractions prepared from both cell cultures. Neuromedin N was rapidly broken down by an aminopeptidase M/leucine aminopeptidase attack, leading to the functionally silent Des-Lys1-neuromedin N metabolite. In the astrocytic membrane-associated fraction, neuromedin N underwent an additional minor endoproteolytic cleavage at the Pro3-Tyr4 bond elicited by endopeptidase 24.11, as suggested by the protective effect of its blocking agent phosphoramidon. Unlike neuromedin N, neurotensin totally resisted hydrolysis by aminopeptidases. Primary inactivating cleavages detected in both cell types appeared mainly located at the Arg8-Arg9 and Pro10-Tyr11 bonds, leading to the formations of neurotensin-(1-8) and neurotensin-(1-10) as the major biologically inactive neurotensin catabolites. Endopeptidase 24.15 appeared mainly responsible for neurotensin-(1-8) formation by the soluble fraction of neurons and astrocytes. In contrast, endopeptidase 24.16 was involved in neurotensin-(1-10) formation by both soluble and membrane-associated fractions of the two cell types. An additional cleavage leading to neurotensin-(1-11) formation and ascribed to endopeptidase 24.11 was detected mainly in the membrane-associated fraction from astrocytes. Finally, the secondary processing of neurotensin degradation products indicated that: (a) neurotensin-(1-11) was converted into neurotensin-(1-8) in the membrane fraction prepared from astrocytes; (b) neurotensin-(1-10) was transformed into neurotensin-(1-8) by an unidentified peptidase belonging to the class of metalloenzymes. The significance of distinct quantitative and qualitative catabolic fates of neuromedin N and neurotensin in cultured astrocytes and neurons is discussed.

Compared binding properties of 125I-labeled analogues of neurotensin and neuromedin N in rat and mouse brain

Neurotensin and neuromedin N are two structurally related peptides that are synthesized by a common precursor. The purpose of the present work was to characterize neuromedin N receptors in rat and mouse brain and to compare these receptors with those of neurotensin. A radiolabeled analogue of neuromedin N has been prepared by acylation of the N-terminal amino group of the peptide with the 125I-labeled Bolton-Hunter reagent. This 125I-labeled derivative of neuromedin N bound to newborn mouse brain homogenate with high affinity (KD = 0.5 nM). Cross-competition experiments between radiolabeled and unlabeled neurotensin and neuromedin N indicated that each peptide was able to displace completely and specifically the other peptide from its interaction with its receptor. Independently of the radioligand used, the affinity of neurotensin was always better than that of neuromedin N. Quantitative radioautographic studies demonstrated that the ratio of labeling intensities obtained with 125I-labeled analogues of neurotensin and neuromedin N remained constant in all the brain areas. Our results do not support the existence of a specific neuromedin N receptor in rat and mouse brain and can be explained by the presence of a common receptor for both peptides.

Co-occurrence of gamma-aminobutyric acid, parvalbumin and the neurotensin-related neuropeptide LANT6 in pallidal, nigral and striatal neurons in pigeons and monkeys

Immunohistochemical double-labeling techniques were used to examine the co-localization of the neurotransmitter gamma-aminobutyric acid (GABA), the calcium-binding protein parvalbumin and the neurotensin-related hexapeptide LANT6 in neurons of the striatum and its target areas in pigeons and monkeys. The studies revealed the existence of a population of striatal interneurons apparently containing all three of these substances in both monkeys and pigeons. The results also revealed that GABA and LANT6 were co-localized in numerous pallidal and nigral reticulata neurons that also contained parvalbumin in both species. Examination of diverse other cell groups in avian forebrain and midbrain revealed that parvalbumin and LANT6 were typically co-localized to GABAergic neurons. In light of the presence of pallidal, reticulata and striatal neurons containing these three substances in two widely divergent amniote groups such as pigeons and monkeys, it seems likely that: (1) comparable neuronal populations are present in other avian and mammalian species; and (2) these neuronal populations play a fundamental role in basal ganglia functions that requires these three substances.

Neuromedin N is a potent modulator of dopamine D2 receptor agonist binding in rat neostriatal membranes

In the concentration range of 1-10 nM, neuromedin N produced a significant concentration-related increase in the Kd values of [3H]L-(-)-N-propylnorapomorphine binding sites in rat neostriatal membranes with a peak action at 10 nM (36% increase versus the control group mean value). The Bmax values were not affected by neuromedin N. Neurotensin at 10 nM induced an increase in the Kd values, which was not affected by a threshold concentration of neuromedin N (0.1 nM). In view of the higher potency of neuromedin N versus neurotensin to modulate neostriatal D2 receptors in contrast to the higher potency of neurotensin versus neuromedin N to bind to the cloned neurotensin receptors, it seems possible that the neuromedin N activated neostriatal neurotensin receptors controlling the D2 receptors represent a distinct subtype of neurotensin receptors.

Differential catabolic fate of neuromedin N and neurotensin in the canine intestinal mucosa

We have established the peptidase content of a P2 fraction (enriched in synaptosomes) and plasma membranes prepared from canine intestinal mucosa. Fourteen exo- and endopeptidases were assayed with fluorimetric or chromogenic substrates and identified by means of specific peptidase inhibitors. Post-proline dipeptidyl aminopeptidase IV, aminopeptidase M, and carboxypeptidase A were the most abundant exopeptidases, while aminopeptidases A and B, dipeptidyl aminopeptidase, pyroglutamyl peptide hydrolase I, and carboxypeptidase B displayed little, if any, activity. Endopeptidase 24.11 was the only endopeptidase that was detected in high amount. By contrast, proline endopeptidase exhibited a low activity, while angiotensin-converting enzyme, endopeptidase 24.15, endopeptidase 24.16, and cathepsin B and D-like activities were not detected. The catabolic rates of the two related neuropeptides, neurotensin (NT) and neuromedin N (NN), established that NN was inactivated 16 to 24 times faster than NT by plasma membrane and P2 fractions, respectively. Furthermore, the two peptides underwent qualitatively distinct mechanisms of degradation. A phosphoramidon-sensitive formation of NT(1-10) was detected as the major NT catabolite, indicating that NT was susceptible to an endoproteolytic cleavage elicited by endopeptidase 24.11. By contrast, NN was inactivated by the action of an exopeptidase at its N-terminus, leading to the formation of [des-Lys1]NN. The occurrence of this NN metabolite was prevented by bestatin and actinonin, but not by the aminopeptidase B inhibitor, arphamenine B, indicating that the release of the N-terminal residue of NN was likely due to aminopeptidase M.(ABSTRACT TRUNCATED AT 250 WORDS)

Intramembrane interactions between neurotensin receptors and dopamine D2 receptors as a major mechanism for the neuroleptic-like action of neurotensin

Evidence has been presented that behavioral actions of NT, inducing its neuroleptic-like action, can be explained on the basis of NT-D2 intramembrane receptor-receptor interactions in the basal ganglia, unrelated to the coexistence phenomenon, leading to reduced affinity and transduction of the D2 agonist binding site. By reducing selectively D2 receptor transduction at the pre- and postsynaptic level, the NT receptor appears capable of switching the DA synapses towards a D1 receptor-mediated transduction, illustrating how receptor-receptor interactions can increase the functional plasticity of central synapses (FIG. 12).

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.

Effects of neuromedin-N on the pituitary-adrenocortical axis of dexamethasone-suppressed rats

Neuromedin-N (NMN) (6 micrograms/100 g body weight for 2 d) partially reversed the dexamethasone (Dx)-induced inhibition of ACTH release and the consequent adrenal atrophy and decrease in glucocorticoid (corticosterone) plasma concentration in rats. Dx administration did not alter the level of circulating mineralocorticoid (aldosterone), but NMN (2 or 6 micrograms/100 g body weight for 2 d) significantly increased it. These findings suggest that the mechanism underlying the glucocorticoid (but not the mineralocorticoid) secretagogue action of NMN involves the stimulation of hypophyseal ACTH release. The hypothesis is advanced that the potent mineralocorticoid secretagogue effect of NMN may be mediated either by a direct action on zona glomerulosa cells or by the enhanced release of other regulatory peptides exerting aldosterone stimulating effect.

Phospholipase C activation by neurotensin and neuromedin N in Chinese hamster ovary cells expressing the rat neurotensin receptor

The rat neurotensin receptor cDNA sequence was transfected in Chinese hamster ovary cells and cellular clones which stably express the corresponding protein were isolated and characterized. The Scatchard analysis of the specific binding of [3H]neurotensin indicated a Kd value of 0.45 +/- 0.08 nM and a Bmax value of 3.27 +/- 0.29 pmol/mg of protein. Displacement experiments using peptidic analogs of neurotensin and levocabastine confirmed that the transfected receptor exhibits the binding properties of the neurotensin receptor characterized in the rat brain. Neurotensin stimulated the phosphoinositides hydrolysis in a time- and concentration-dependent manner and this effect was mimicked by neurotensin(8-13) and by neuromedin N. The stimulation of phosphoinositides hydrolysis was not inhibited by pertussis toxin. These results indicate that the transfected cells actively express the rat neurotensin receptor which is functionally coupled to phospholipase C through a pertussis toxin-insensitive GTP-binding protein, and that neuromedin N is able to induce the phosphoinositides turnover by interaction with the neurotensin receptor.

Effects of thiorphan, bestatin and a novel metallopeptidase inhibitor JMV 390-1 on the recovery of neurotensin and neuromedin N released from mouse hypothalamus

The effects of the endopeptidase 24.11 ('enkephalinase') inhibitor thiorphan, the aminopeptidase inhibitor bestatin and a novel metallopeptidase inhibitor JMV 390-1 on the K(+)-evoked release of immunoreactive neurotensin and neuromedin N (iNT and iNN) from mouse hypothalamic slices were examined. (JMV 390-1 inhibits several metallopeptidases including endopeptidases 24.11, 24.15 and 24.16, and aminopeptidase N equipotently with Ki values around 50 nM.) Thiorphan increased the recovery of released iNT nearly 2-fold and had no effect on iNN. Bestatin produced a 4-fold increase in iNN recovery and was inactive on iNT. Finally, iNT and iNN recoveries were increased up to 4- and 5-fold, respectively, by JMV 390-1. These results show that in the mouse hypothalamus endopeptidase 24.11 participates with other metalloendopeptidases to the degradation of endogenously released NT while endogenously released NN is principally degraded by aminopeptidase(s).

A comparison of the effects of mesencephalic injections of neurotensin(1-13) and neuromedin N on brain electrical self-stimulation

Neuromedin N (NM-N), a hexapeptide that shares a four amino acid C-terminal homology with the tridecapeptide, neurotensin (NT), has been suggested as a potential neurotransmitter or neuromodulator that could interact with the NT-sensitive receptors. In this experiment, we compared the effects of an equimolar concentration of NM-N and NT(1-13) injected in the ventral tegmental area (VTA) on brain electrical self-stimulation (SS), a behavior previously shown to be potentiated by VTA injections of NT(1-13). Rats implanted with a stimulating electrode in the mesencephalic central gray and a guide cannula in the VTA were trained to lever press to obtain rewarding electrical stimulations. Functions relating the rate of lever pressing to the stimulation frequency were determined, on separate daily tests, before and after the injection of 3 nmol of NM-N, NT(1-13), or an equal volume of saline vehicle. At this concentration, both NM-N and NT(1-13) produced a significant facilitation of SS when compared to saline vehicle, an effect that was not seen when the peptides were injected outside the VTA. The facilitation of SS by NM-N, however, was much weaker and of a shorter duration than the one produced by NT(1-13). The shorter time course and the weaker behavioral effect of NM-N compared to NT(1-13) are consistent with its lower potency at the NT receptor and its faster rate of enzymatic degradation in the VTA, and suggest that NM-N potentiated the reward-relevant neural signal by acting on mesencephalic NT receptors.

Neurotensin terminals form synapses primarily with neurons lacking detectable tyrosine hydroxylase immunoreactivity in the rat substantia nigra and ventral tegmental area

A light and electron microscopic double antigen localization technique was employed to examine the fine structural relationship between neurotensin-containing axon terminals and dopaminergic neurons in the substantia nigra and ventral tegmental area of the rat. At the light microscopic level, neurotensin-immunoreactive terminals were densely distributed throughout the substantia nigra pars compacta and ventral tegmental area in close proximity to tyrosine hydroxylase-immunoreactive somata and dendrites. On electron microscopic examination, direct synaptic connections were identified between neurotensin-immunoreactive axon terminals and tyrosine hydroxylase-immunopositive perikarya and dendrites. However, only 8.2% and 8.8% of the neurotensin-immunoreactive axonal profiles detected in the substantia nigra and ventral tegmental area, respectively, were found in direct apposition with tyrosine hydroxylase-immunostained elements. In turn, only 9.3% and 10.0% of tyrosine hydroxylase immunoreactive dendrites sampled from the substantia nigra and ventral tegmental area, respectively, were seen in contact with neurotensin immunopositive axon terminals. However, neurotensin-immunoreactive and tyrosine hydroxylase-immunolabelled elements were frequently identified in close anatomical proximity (less than 5 microns) to one another. These results are interpreted in light of the selective association of neurotensin receptors with dopaminergic neurons in the substantia nigra and ventral tegmental area to suggest a predominantly parasynaptic mechanism of action for neurotensin in the ventral midbrain.

Different relative abundance of neurotensin and neuromedin N in bovine ocular tissues

Neurotensin (NT) and neuromedin N (NN) are regulatory peptides encoded by the same gene and located in tandem within a common precursor. Using specific radioimmunoassays for both peptides, their relative abundance in extracts of bovine ocular tissues has been examined. Within the retina, the molar concentration of NN was significantly higher (P less than 0.001) than that of NT. In contrast, within both choroid/sclera and iris/ciliary bodies, the molar concentration of NT was significantly higher (P less than 0.001) than that of NN. These data demonstrate that the theoretical molar ratio of 1:1, which would result from complete processing of both peptides from the common precursor, does not occur in any of the ocular tissues examined. Reverse phase HPLC of extracts of each ocular tissue confirmed the differential abundance of NT and NN. These data would suggest that the common NT/NN precursor is differentially-processed within bovine ocular tissues, a finding which may be of physiological significance.

Facilitation of brain stimulation reward by mesencephalic injections of neurotensin-(1-13)

The effects on brain stimulation reward of neurotensin-(1-13) microinjected at different concentrations (2.5, 5, 10 and 20 micrograms/0.5 microliters) into the ventral mesencephalic region containing mesocorticolimbic dopamine neurons were tested in 12 male rats. Neurotensin lowered the stimulation frequency required to sustain threshold levels of responding for brain stimulation reward, suggesting that this neuropeptide is involved in modulating the activity of dopamine neurons that mediate behaviors motivated by positive reinforces. The magnitude of the facilitatory effect of neurotensin on brain stimulation reward was dependent on the concentration injected and to a significant extent also on whether the peptide was administered in an ascending or a descending order of concentration. The different effects of neurotensin depending on the order of administration may suggest long-lasting effects on the responsiveness of neurotensin receptors in this region after injection of high concentrations of the peptide. Subsequent injection of morphine (2.5-5 micrograms/0.5 microliter) into the same site produced a weaker facilitation of brain stimulation reward than expected, suggesting that local damage after multiple central injections or prior injections of neurotensin itself reduced the responsiveness of dopamine neurons to opiates. Taken together, the results are consistent with data indicating that activation of neurotensin receptors in the ventral mesencephalon stimulates dopamine cell firing and axonal dopamine release in limbic terminal fields and suggest that endogenous neurotensin is involved in the control of behavior motivated by positive reinforcement.

Neurotensin modulates K(+)-stimulated dopamine release from the caudate-putamen but not the nucleus accumbens of mice with differential sensitivity to ethanol

Slices of caudate-putamen (CP) and nucleus accumbens (NA) prepared from Long-Sleep (LS) and Short-Sleep (SS) mice were used to determine the effects of neurotensin (NT) and ethanol on K(+)-stimulated 3H-dopamine (3H-DA) release and to test the hypothesis that ethanol acts, in part, via NT receptor-mediated processes. Slices prepared from either LS or SS CP or NA did not differ in submaximal (25 mM) K(+)-stimulated 3H-DA release but 60 mM K+ induced significantly greater 3H-DA release from LS CP slices compared with SS CP slices. NT had no effect on unstimulated 3H-DA overflow but enhanced 25 mM K(+)-stimulated 3H-DA release from slices of the CP of both lines of mice. Augmentation of DA release by NT from caudate slices was concentration dependent and tetrodotoxin (TTX) insensitive, implicating a role of presynaptic neurotensin receptors located on nigrostriatal DA neurones. In contrast, NT did not enhance K(+)-stimulated 3H-DA release from NA slices from either line of mice. The absence of an NT effect in NA slices was not due to a rapid desensitization of NT receptors but the data were consistent with the absence of presynaptic NT receptors on dopaminergic terminals in the NA. Between-line differences were observed in the effect of ethanol on NT enhancement of 25 mM K(+)-stimulated 3H-DA release from CP slices. Ethanol (100 mM) applied concomitantly with NT blocked the NT enhancement of 3H-DA release from CP slices of LS but not SS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced peptide bond pseudopeptide analogues of neurotensin: binding and biological activities, and in vitro metabolic stability

A series of pseudopeptide analogues of neurotensin was produced by systematically replacing the five peptide bonds in neurotensin-(8-13) with CH2NH (psi, reduced) bonds. All these analogues were synthesized with a free amino terminus (H derivatives) and with a N-terminal tert-butyloxycarbonyl group (Boc derivatives). The compounds were screened in vitro for agonist or antagonist activity and for metabolic stability by testing (1) their ability to inhibit the binding of radiolabelled neurotensin to homogenates of newborn mouse brain; (2) their ability to contract isolated guinea-pig ileum preparations; and (3) their degradation in the presence of rat brain homogenates. All the analogues bound to the mouse brain neurotensin receptor and all exhibited agonist activity in the guinea-pig ileum assay. Only the H- and Boc-[psi 8,9] derivatives were at least as potent as their parent compounds neurotensin-(8-13) and Boc-neurotensin-(8-13) in the binding and biological assays. All the other pseudopeptide analogues with reduced bonds at position 9-10, 10-11, 11-12 and 12-13 showed a marked reduction in potency ranging from 2 to 4 orders of magnitude. All the derivatives that were protected at their N terminus either by the presence of a Boc group or by the presence of a reduced bond at position 8-9 and 9-10 were slowly degraded by rat brain homogenates. The other derivatives were, in contrast, quite rapidly degraded. There was a good correlation between binding and biological potencies for those analogues that were resistant to degradation. Interestingly, the degradation-resistant H-[psi 8,9] compound exhibited higher binding and biological potency then neurotensin. It is therefore expected that this analogue will produce highly potent and long-lasting neurotensin-like effects in vivo, and preliminary experiments indicate that this is indeed the case.

Effects of neurotensin on midbrain dopamine neurons: are they mediated by formation of a neurotensin-dopamine complex?

The effects of neurotensin on midbrain dopamine neuron activity were studied in brain slices using single-unit recording techniques. At low concentrations (0.2-10 nM), neurotensin attenuated dopamine-induced inhibition without a significant effect on the basal firing rate. At higher concentrations (greater than 10 nM), however, it consistently caused an increase in cell activity. At even higher concentrations (greater than 100 nM), a sudden cessation of cell activity preceded by an increase in firing rate was observed. Whether this effect of neurotensin was due to depolarization inactivation or to a toxic effect of the peptide at high concentrations remains to be determined. To determine whether the effects of neurotensin were mediated by formation of a neurotensin-dopamine complex, several neurotensin analogues were studied. Neurotensin (8-13), which binds to both neurotensin receptors and dopamine, mimicked the effects of native neurotensin. Neuromedin N, which competes with neurotensin for the same receptor but does not bind to dopamine, also mimicked the effects. However, neurotensin (1-11), which forms a complex with dopamine but is inactive in competing for neurotensin receptors, was ineffective. In addition, the excitatory effect of neurotensin was not attenuated in the presence of dopamine receptor blockade by sulpiride. These results suggest that formation of a neurotensin-dopamine complex may not account for the action of neurotensin on dopamine cells. When combined with the fact that there is a high density of neurotensin receptors on dopamine cells, our results support the suggestion that the observed effects of neurotensin on dopamine neurons are most likely mediated by an activation of neurotensin receptors.

Neurotensin and neurotensin analogues modify the effects of chronic neuroleptic administration in the rat

The effects of intracerebroventricular neurotensin and the neurotensin analogues neuromedin N and [D-Trp 11]neurotensin on the behavioural responses to chronic neuroleptic administration were investigated in the rat. Chronic (18 weeks) administration of a low dose (12.5 mg/kg, i.m., every 3 weeks) of fluphenazine decanoate alone failed to elicit the vacuous chewing mouth movements (VCMs) which have previously been reported following higher doses of this drug, but VCMs were seen in neuroleptic-treated animals following the additional administration of neurotensin. A higher dose of fluphenazine (25 mg/kg, i.m., every 3 weeks) greatly increased the VCM response, and this potentiation was suppressed to control levels by [D-Trp11]neurotensin, but unaffected by neuromedin N. These findings suggest that alterations in neurotensin may contribute to the deleterious extrapyramidal effects of long-term neuroleptic administration, and that [D-Trp11]neurotensin may attenuate these effects by blockade of neurotensin receptors within the central nervous system.

Neuromedin-N inhibits migrating myoelectric complex and induces irregular spiking in the small intestine of rats; comparison with neurotensin

The effects of neuromedin-N on migrating myoelectric complexes in the small intestine of rats were studied. As neuromedin-N and neurotensin are structurally related peptides a comparison with neurotensin was made. Myoelectric activity was recorded by means of three bipolar electrodes implanted into the wall of the small intestine at 5, 15 and 25 cm distal to the pylorus. The peptides were administered as intravenous infusions to fasted conscious rats. Neuromedin-N at doses of 100-800 pmol kg-1 min-1 caused a dose-dependent disruption of the migrating myoelectric complexes and induced irregular spiking activity (n = 7, P less than 0.05). Neurotensin induced a similar response, but at doses of 1.0-8.0 pmol kg-1 min-1 (n = 5, P less than 0.05). Thus, on a molar basis, neuromedin-N appeared to be about 100-times less potent than neurotensin. Hexamethonium (20 mg kg-1 i.v.) inhibited the migrating motor complexes and induced quiescence, but did not block the effect of neuromedin-N at a dose of 800 pmol kg-1 min-1. Atropine (1 mg kg-1 i.v.) and mepyramine (2 mg kg-1 i.v.) did not affect the migrating motor complexes, nor did they block the effect of neuromedin-N. Simultaneous infusion of neuromedin-N and neurotensin in a 1:1 molar ratio at doses of 2 pmol kg-1 min-1 showed inhibition of the response to neurotensin in eight out of ten experiments. In conclusion, neuromedin-N changes the myoelectric activity in the small intestine from a fasting to a fed pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin and neuromedin N are differently metabolized in ventral tegmental area and nucleus accumbens

Whole homogenates and membrane-bound and cytosoluble fractions prepared from rat ventral tegmental area (VTA) and nucleus accumbens were examined for their content of peptidasic activities and for their ability to metabolize neurotensin and its natural related hexapeptide neuromedin N. No qualitative differences were observed between these two brain regions concerning the presence and the subcellular distribution of a series of activities able to hydrolyze various specific fluorimetric enzymatic substrates. However, aminopeptidase B, endopeptidase 24-15, and endopeptidase 24-11 were significantly lower in the VTA than in the nucleus accumbens membrane preparations, while proline endopeptidase was detected in significantly higher amount only in the cytosolic fraction prepared from nucleus accumbens. Both neurotensin and neuromedin N were metabolized more rapidly in the nucleus accumbens than in the VTA. Furthermore, the degradation rate of neuromedin N was considerably faster than that of neurotensin whatever the cerebral area examined. Studies carried out with highly specific peptidase inhibitors revealed that endopeptidase 24-15 mainly contributed to the catabolism of neurotensin in homogenates and membrane-bound preparations of nucleus accumbens and VTA, while aminopeptidase B appeared predominantly responsible for the rapid disappearance of neuromedin N in both cerebral tissues. The possibility that the different metabolic processes of the two peptide congeners could explain their distinct pharmacological profiles observed after their microinjection in the nucleus accumbens and in the VTA is discussed.

Marked variations of the relative distributions of neurotensin and neuromedin N in micropunched rat brain areas suggest differential processing of their common precursor

Neuromedin N (NN) is a hexapeptide that shares a four amino acid identity with the C-terminus of neurotensin (NT) and exhibits NT-like effects in the central nervous system. Both peptides were recently shown to be encoded in the same precursor molecule. By means of specific and sensitive radioimmunoassays, we compared the distribution of immunoreactive NT and NN (iNT and iNN) in micropunched rat brain structures. The data revealed marked regional variations in the ratio of iNT over iNN. For instance, the ratio value was 4.5 in the posterior hypothalamus and 0.8 in the mammillary bodies. Reverse phase HPLC analysis of extracts of several brain regions showed that iNT and iNN coeluted with synthetic NT and NN, respectively. The results suggest that differential processing of the common neurotensin/neuromedin N precursor occurs in various regions of the rat brain.

Neurotensin and neuromedin N elevate the cytosolic calcium concentration via transiently appearing neurotensin binding sites in cultured rat cortex cells

Through assessment of the changes in the intracellular free-calcium concentration ([Ca2+]i), which was measured using the calcium sensitive dye, fura-2, the character of the neurotensin (NT) binding sites which appeared transiently during the early ontogenetic stage in the rat cerebral cortex was analyzed in primary cultures of cerebral cortex cells from neonatal rats. NT (1-1000 nM) elevated [Ca2+]i of the cells even when extracellular calcium was chelated with 1 mM ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). These findings suggest that the transiently appearing NT-binding sites in the cortex are receptors for NT and that some of the NT-induced increase in [Ca2+]i is due to mobilization from the intracellular calcium store. Further application of NT after 10 min washing caused an increase in [Ca2+]i again. This is in contrast to the findings for cortical slices from adult rats and mRNA-injected oocytes; desensitization due to NT was of long duration and further application of NT failed to activate the neurons which had responded the first time to NT. These facts suggest that the character of the NT-binding sites in the cerebral cortex differs between neonatal and adult rats. In addition, we showed that neuromedin N had a similar property to NT as to mobilization of [Ca2+]i and acted only on NT-responsive cells, suggesting the interaction between NT and neuromedin N at the postsynaptic level via the same receptor.

Enhancing effect of ubenimex (bestatin) on proliferation and differentiation of hematopoietic progenitor cells, and the suppressive effect on proliferation of leukemic cell lines via peptidase regulation

Ubenimex (Bestatin) significantly enhanced the G- and GM-CSF-induced colony formation of human bone marrow cells at concentrations of 0.001, 0.01, 0.1 and 1.0 microgram/ml (21-61% enhancement), but not at 10 micrograms/ml. Ubenimex did not influence the EPO-induced erythroid colony and burst formation between 0.0001-100 micrograms/ml. Against human and mouse leukemic cell lines, the growth-inhibitory activities of ubenimex were dose-dependently observed. Aminopeptidase activities on U937 and TF-1 cells were almost inhibited with 10 and 100 micrograms/ml of ubenimex, respectively. Cross-linking studies of 125I-GM-CSF binding to TF-1 cells demonstrated that the 150-kDa band of 2 major bands was enhanced after incubation with 0.01 microgram/ml ubenimex but decreased after that with 100 micrograms/ml, and that the 95-kDa band was not changed at any concentration of ubenimex. Change in density of the 150-kDa band on ubenimex-treated TF-1 cells was correlated with that in expression of CD10 (neutral endopeptidase) on them, whereas that in expression of CD13 (aminopeptidase N) was not changed at any concentration. These results suggest that one possible mechanism of ubenimex action in hematopoietic progenitor cells is the up-regulation of the high affinity receptor for GM-CSF and that in leukemic cell lines is suppression of amino acid incorporation via peptidase regulation.

Neurotensin high affinity binding sites and endopeptidase 24.11 are present respectively in the meningothelial and in the fibroblastic components of human meningiomas

The presence of neurotensin receptors and endopeptidase 24.11 (E-24.11) in 16 human meningioma specimens, obtained at surgery, was assessed by measuring the binding of 125I-[tyrosyl3]neurotensin(1-13) (125I-NT) and the inhibitor 3H-N(2RS)-3-hydroxyaminocarbonyl-2-benzyl-1-oxopropyl)glycine (3H-HACBO-Gly), for the receptor and enzyme, respectively. E-24.11 activity was also measured. Autoradiography, on the 16 meningiomas, showed that specific 125I-NT labeling (nonspecific labeling was assessed in the presence of excess NT) was exclusively located in the meningothelial regions. In contrast, specific 3H-HACBO-Gly labeling (nonspecific labeling was assessed in the presence of an excess of the E-24.11 inhibitor thiorphan) was exclusively found in fibroblastic regions. No specific labeling of either ligand was found on collagen or blood vessels. In vitro binding assays were performed on membranes of 10 of the 16 meningiomas. In the 4 meningiomas rich in meningothelial cells, 125I-NT specifically bound to one population of sites with Bmax ranging from 57 to 405 fmol/mg protein and Kd around 0.3 nM. These sites share common properties with the brain NT receptor, since the carboxy terminal acetyl NT(8-13) fragment bound to the same sites but with a higher affinity. The carboxy terminal analogue of NT, neuromedin N, also bound to the same sites with a 10-fold lower affinity and the sites were bradykinin and levocabastine insensitive. In the 4 meningiomas rich in fibroblastic cells, 3H-HACBO-Gly specifically bound to one population of sites with Bmax ranging from 251 to 739 fmol/mg protein and Kd around 2.8 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Different ontogenetic profiles of cells expressing prepro-neurotensin/neuromedin N mRNA in the rat posterior cingulate cortex and the hippocampal formation

The ontogeny of the expression of prepro-neurotensin/neuromedin N messenger RNA (prepro-NT/NN mRNA) in the rat posterior cingulate cortex (retrosplenial cortex) and the hippocampal formation was investigated using in situ hybridization histochemistry. In the primordium of the posterior cingulate cortex and the hippocampal formation, prepro-NT/NN mRNA was first expressed on embryonic day 17, and was found in the subiculum, layers II-III in areas 29a and 29b, and layer VI in the posterior cingulate cortex at birth. Expression was also observed in the CA1 field. In the adult rat, the expression of prepro-NT/NN mRNA was reduced in the posterior cingulate cortex, and only a few positive cells were seen here. However, the CA1 field and the subiculum still contained numerous positive cells.

Neurotensin binding to dopamine

Rotating disk electrode voltammetry at glassy carbon electrodes and ultraviolet/visible spectroscopy were used to demonstrate that dopamine binds to neurotensin with a dissociation constant of 7.5 x 10(-8). By measuring the binding constants of various neurotensin analogs, it was found that the -Arg8-Arg9-portion of the neurotensin sequence is critical for binding dopamine. Neurotensin also formed a complex with 4-ethylcatechol, 4-methylcatechol, 3-methoxytyramine, and norepinephrine. Although changes in the side chain did not alter the binding constant, methoxylation of the catechol moiety significantly increased the dissociation constant. These data along with additional studies of dopamine interactions with arginine derivatives suggest that the guanidino groups of arginine and the catechol hydroxyls of dopamine are responsible for mediating the observed binding. It is hypothesized that the capacity of neurotensin to bind directly to dopamine may be partly responsible for its previously observed antagonism of dopamine-induced locomotor activity.

Differential processing of the neurotensin/neuromedin N precursor in the mouse

Posttranslational processing of the neurotensin/neuromedin N (NT/NN) precursor has been investigated in mouse brain and small intestine by means of region-specific radioimmunoassays coupled to chromatographic fractionations. In brain, total NT/NN immunoreactivity measured with a common C-terminal antiserum was 15.72 pmol/g. NT measured with an N-terminal antiserum was 9.74 pmol/g and NN measured with an N-terminal antiserum was 5.98 pmol/g. In small intestine, combined NT/NN immunoreactivity was 108.55 pmol/g, consisting of 66.37 pmol/g NT but only 0.96 pmol/g NN. Gel permeation chromatography and reverse phase HPLC revealed that the large discrepancy in the NT and NN values obtained in small intestinal extracts was due to the presence of a high molecular weight, hydrophobic peptide, which was reactive only with the common C-terminally directed antiserum. Pepsinization of this generated an immunoreactive peptide with similar chromatographic characteristics to NN. In mouse intestine, NN is only partially cleaved from the common NT/NN precursor, resulting in the presence of an N-terminally extended molecular species. This novel molecular species of neuromedin N may be the physiological mediator of certain peripheral biological effects hitherto attributed to neurotensin or neuromedin N.

Calcium-dependent release of neuromedin N and neurotensin from mouse hypothalamus

Neuromedin N (NN), a hexapeptide, was isolated from porcine spinal cord. Its C-terminal tetrapeptide sequence is identical to that of neurotensin (NT) and it exhibits NT-like effects when injected in the central nervous system. Both peptides were recently shown to be encoded in the same precursor molecule. We have just developed a sensitive and specific radioimmunoassay (RIA) for NN and showed that the peptide central nervous system distribution paralleled that of NT, the highest concentrations being found in the hypothalamus. Using this assay and a specific RIA for NT, we show here that NN and NT were simultaneously released from slices of mouse hypothalamus by K(+)-induced depolarization in a Ca(++)-dependent manner. The ratio of released NN over NT was 0.3 and was identical to the ratio of endogenous NN over NT. For both NN and NT, the releasable peptide pool represented 2% of the endogenous peptide pool. HPLC characterization of the releasable and endogenous immunoreactive material reacting with the NN and NT antisera showed that it coeluted with synthetic NN and NT, respectively. The present data further support the hypothesis that NN acts as a neuromodulator in the central nervous system.

Comparison of neuromedin-N and neurotensin on net fluid flux across rat small intestine

Neuromedin-N, a hexapeptide recently isolated and purified from porcine spinal cord, has close sequence homology with the C-terminal region of the tridecapeptide neurotensin. Both peptides have a remarkably similar peripheral distribution. Little is known of the biological activity of neuromedin-N. Neurotensin and peptide histidine methionine are known to stimulate net fluid secretion into rat small intestine. We have therefore tested the effect of neuromedin-N and the hexapeptide neurotensin-(8-13), the smallest fully active analogue of neurotensin in this system, compared with that of neurotensin and peptide histidine methionine. All four peptides reduced net absorption in low doses and caused net secretion in larger doses. However, whereas peptide histidine methionine was active in all areas of the small intestine, neurotensin, neurotensin-(8-13) and neuromedin-N were inactive in the duodenum. In the post-duodenal areas neurotensin was approximately 7 times more active than peptide histidine methionine, 21 times more potent than neuromedin-N and 33 times more potent than neurotensin-(8-13).

Neurotensin binding sites in porcine jejunum: biochemical characterization and intramural localization

Neurotensin is present in high concentrations in the mammalian gut, especially in enteroendocrine cells of the mucosa. Exogenous neurotensin has been shown to alter ion transport by the mucosa and contractile activity of intestinal smooth muscle. The purpose of this study was to determine the distribution of neurotensin binding sites within the intestinal wall. Initially, biochemical characteristics of [125I]neurotensin binding sites were determined within two preparations of the distal porcine jejunum: (1) the mucosa and submucosa, and (2) the circular and longitudinal muscle with their intramural plexuses. Ligand binding data for the preparation including the mucosa and submucosa indicated that [125I]neurotensin bound specifically to two sites having apparent equilibrium dissociation constants of approximately 0.046 and 0.37 nM. A binding site with a dissociation constant of approximately 0.38 nM was confirmed for the preparation of muscle and associated intramural plexuses. Xenopsin and neurotensin were equipotent to neurotensin in competing for these binding sites; neuromedin N was approximately 40 times less potent in the preparation of mucosa and submucosa. Receptor autoradiography was used to determine the distribution of [125I]neurotensin binding sites within the wall of the jejunum. Autoradiograms of [125I]neurotensin bound to cross sections of the proximal and distal jejunum showed that the highest densities of silver grains were associated with the internal submucosal ganglia, external submucosal plexus and myenteric ganglia. A moderate density of silver grains was associated with the circular muscle. The localization of neurotensin binding sites to submucosal ganglia is consistent with observations that neurotensin effects on active anion secretion by the mucosa are blocked by tetrodotoxin. Immunohistochemical localization of neurotensin in the porcine jejunum demonstrated a limited population of neurotensin immunoreactive cells within the mucosal epithelium. It is possible that neurotensin released from these cells in the mucosa as well as neurotensin-related peptides released from enteric neurons may be the endogenous ligands for the binding sites visualized in this study.

The characterization and regional distribution of neuromedin N-like immunoreactivity in rat brain using a highly sensitive and specific radioimmunoassay. Comparison with the distribution of neurotensin

Neuromedin N is a hexapeptide that shares a 4 amino acid homology with the C-terminus of neurotensin and exhibits neurotensin-like effects in the central nervous system. Both peptides were recently shown to be encoded in the same precursor molecule. In this study, a radioimmunoassay for neuromedin N was developed using monoiodo [125I-Tyr4]neuromedin N as the tracer and a rabbit antiserum raised against synthetic [Cys6]neuromedin N coupled to ovalbumin through its Cys residue. The antiserum showed strong structural requirement for the N-terminal sequence of neuromedin N and did not cross-react with neurotensin and other related peptides. The limit of detection of the radioimmunoassay was 0.5 fmol/tube and the IC50 was 5 fmol/tube. Neuromedin N-like immunoreactivity was present in 0.1 N HCl extracts of rat brain at a concentration of 9.3 +/- 1.3 pmol/g of tissue and behaved like synthetic neuromedin N on HPLC. Its concentration was significantly lower than that of neurotensin assayed in the same extracts (15.1 +/- 1.4 pmol/g), and this was not the consequence of lower extraction yield or lower post-mortem stability of neuromedin N as compared to neurotensin. The regional rat brain distribution of neuromedin N-like immunoreactivity paralleled that of neurotensin-like immunoreactivity, being highest in the hypothalamus and lowest in the cerebellum. These data support the proposal of a neuromodulator role for neuromedin N. The highly specific and sensitive radioimmunoassay described here will make it possible to investigate in more detail the regional brain distribution of neuromedin N and to study its release from brain tissues.