Behavioral and neurochemical effects of neurotensin microinjection into the ventral tegmental area of the rat

The ventral tegmental area of the rat brain has been shown to possess high densities of neurotensin- and dopamine-containing neuronal perikarya. We recently demonstrated that microinjection of neurotensin into the ventral tegmental area produces behavioral hyperactivity similar to amphetamine-induced increase in exploratory behaviors, but lacking stereotypies. In this study, we report that the threshold dose for neurotensin-induced hyperactivity is 0.10-0.25 micrograms neurotensin/side. Either intracerebroventricular injection of haloperidol (5.0 micrograms/lateral ventricle) or destruction of the mesolimbic dopamine system by 6-hydroxydopamine abolishes the behavioral hyperactivity produced by intraventral tegmental injection of neurotensin (2.5 micrograms/side). Using high pressure liquid chromatography with electrochemical detection, we show that neurotensin injection into the ventral tegmental area increases the concentration of dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the nucleus accumbens and olfactory tubercles, but not in the striatum. This effect is especially profound in the nucleus accumbens where the threshold dose is less than 0.025 micrograms/side. The ratio of 3,4-dihydroxyphenylacetic acid to dopamine increased in the nucleus accumbens and olfactory tubercles in a dose-dependent fashion (0.025 microgram-2.50 micrograms/side). Neurotensin-induced behavioral hyperactivity correlates positively with neurotensin-induced changes in the ratio of 3,4-dihydroxyphenylacetic acid to dopamine. This study indicates that neurotensin acts in the ventral tegmental area to activate the mesolimbic dopamine system. Further, this activation produces behavioral hyperactivity characterized by an increase in exploratory behaviors. The fact that both immunoreactive neurotensin and neurotensin receptors are found in high concentration in the ventral tegmental area supports the possible physiological significance of this peptide-catecholamine interaction.

Effect of neurotensin on gastric function in man

Neurotensin is a peptide recently discovered in the human ileum and it is released into plasma after ingestion of food. Neurotensin was infused intravenously into 12 healthy volunteers at a mean dose of 2.4 pmol/kg/min, the mean rise in plasma levels being 89 +/- 8 pmol/l. An inhibition of both gastric acid and pepsin output, and also a delay in gastric emptying of oral glucose, were observed. Neurotensin may therefore have a physiological role in modulating gastric function in man.

Neurotensin stimulates exocytotic histamine secretion from rat mast cells and elevates plasma histamine levels

1. Neurotensin stimulated histamine release and granule extrusion when applied to isolated rat peritoneal mast cells. 2. This secretory response was prevented by the removal of calcium or energy and was not accompanied by the release of lactic dehydrogenase. 3. The secretory response produced by neurotensin was prevented by prior treatment of mast cells with cromoglycate. 4. The intravenous injection of neurotensin into anaesthetized rats produced a rapid and significant increase in the level of blood histamine that was dependent upon the dose of neurotensin. 5. Treatment of rats with compound 48/80, 24 hr before neurotensin, abolished the elevation in blood histamine caused by neurotensin. The intravenous injection of cromoglycate 1-2 min before neurotensin greatly reduced the response to neurotensin. 6. The intradermal injection of neurotensin (0.03-30 p-mole) increased capillary permeability in rats pre-treated intravenously with Evans Blue. This response was abolished by the antihistamine, diphenhydramine. Increasing the dose of neurotensin to 300 p-mole partially overcame this inhibition by diphenhydramine. 7. Our results demonstrate that neurotensin can elicit an exocytotic secretory response from isolated rat peritoneal mast cells and elevate histamine levels in blood. It is suggested that some of neurotensin's physiological effects may be due to stimulation of mast cell secretion.

The nonpeptide neurotensin antagonist, SR 48692, used as a tool to reveal putative neurotensin receptor subtypes

The nonpeptide neurotensin (NT) antagonist, SR 48692, was recently shown to inhibit NT binding to the cloned rat and human NT receptor and to antagonize NT effects in a variety of in vitro and in vivo assays. Here, we show that, in contrast to its antagonistic action on NT-induced hypomotility in the rat, SR 48692 failed to antagonize NT-induced hypothermia and analgesia in the mouse and rat. We suggest that these effects might be mediated through a subtype of SR 48692-insensitive NT receptor.

Functional interactions between melanin-concentrating hormone, neuropeptide Y, and anorectic neuropeptides in the rat hypothalamus

A growing body of evidence indicates that a number of peptides expressed in the mammalian hypothalamus are involved in the regulation of food intake and energy balance. Among these, melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are potent appetite stimulants, whereas alpha-melanocyte-stimulating hormone (alpha-MSH), neurotensin, and glucagon-like peptide (GLP)-1(7-36) amide have appetite-suppressing properties. However, the functional interactions between pathways involving these neuropeptides remain incompletely understood. In the current study, we describe the functional interactions between orexigenic (appetite-stimulating: MCH and NPY) and anorectic (appetite-suppressing: alpha-MSH, neurotensin, and GLP-1) peptides after intracerebroventricular (i.c.v.) administration in the rat. The i.c.v. administration of GLP-1 completely prevents the orexigenic effects of both MCH and NPY. However, i.c.v. administration of alpha-MSH prevents only the orexigenic effect of MCH, as we have previously shown, but does not prevent the effect of NPY on food intake. Similarly, i.c.v. administration of neurotensin prevents only the orexigenic effect of MCH, but does not prevent the appetite-stimulating effect of NPY. Thus, our study suggests that the functional interactions between these neuropeptides are specific, although the underlying mechanisms are as yet unexplored.

The effect of neurotensin on food consumption in the rat

The effect of neurotensin on feeding behavior were studied in rats. Intracerebroventricular administration of neurotensin (3.3-30 micrograms) produced a dose-related decrease in food intake in 24 h food deprived rats. Acute intracerebroventricular injection of neurotensin (30 micrograms) shortly after the ingestion of a novel flavor did not produce a flavor aversion during testing 48 h later, suggesting that reduction of food intake by low doses of centrally administered neurotensin is not related to a conditioned taste aversion. Intracerebroventricularly administered thyrotropin-releasing hormone (2.2 micrograms) also inhibited food intake and appeared to attenuate slightly the inhibition of food intake induced by 10 micrograms neurotensin.

Peripheral and central administration of xenin and neurotensin suppress food intake in rodents

Xenin is a 25-amino acid peptide highly homologous to neurotensin. Xenin and neurotensin are reported to have similar biological effects. Both reduce food intake when administered centrally to fasted rats. We aimed to clarify and compare the effects of these peptides on food intake and behavior. We confirm that intracerebroventricular (ICV) administration of xenin or neurotensin reduces food intake in fasted rats, and demonstrate that both reduce food intake in satiated rats during the dark phase. Xenin reduced food intake more potently than neurotensin following ICV administration. ICV injection of either peptide in the dark phase increased resting behavior. Xenin and neurotensin stimulated the release of corticotrophin-releasing hormone (CRH) from ex vivo hypothalamic explants, and administration of alpha-helical CRH attenuated their effects on food intake. Intraperitoneal (IP) administration of xenin or neurotensin acutely reduced food intake in fasted mice and ad libitum fed mice in the dark phase. However, chronic continuous or twice daily peripheral administration of xenin or neurotensin to mice had no significant effect on daily food intake or body weight. These studies confirm that ICV xenin or neurotensin can acutely reduce food intake and demonstrate that peripheral administration of xenin and neurotensin also reduces food intake. This may be partly mediated by changes in hypothalamic CRH release. The lack of chronic effects on body weight observed in our experiments suggests that xenin and neurotensin are unlikely to be useful as obesity therapies.

Neurotensin self-injection in the ventral tegmental area

Earlier work with the conditioned place-preference paradigm suggested that neurotensin (NT) acts as a behavioral reinforcer when microinjected into the ventral tegmental area (VTA) of the midbrain. We report here that animals will perform an operant task to obtain microinfusions of NT into the VTA. Rats reliably pressed a lever to obtain NT infusions while neglecting an identical but inactive lever. Substitution of saline for NT initiated response extinction; following the reintroduction of NT, reliable responding resumed. These results extend earlier work suggesting that NT in the VTA can be a positive reinforcer.

Neurotensin: effects of hypothalamic and intravenous injections on eating and drinking in rats

To investigate a role for the brain-gut peptide neurotensin (NT) in ingestive behavior, changes in food and water intake of food-deprived rats were examined following injection of NT into the paraventricular hypothalamic nucleus (PVN) or the mesenteric vein. Unilateral PVN NT (2.5, 5.0, 10.0 micrograms/0.3 microliter) produced substantial dose-dependent reductions in total food intake 0.5, 1, and 4 hr postinjection. In contrast, PVN NT had no effect on water intake and produced no change in grooming, rearing, sleeping, resting or locomotor activity. Bilateral PVN NT at a high dose (10.0 micrograms/side) suppressed consumption of solid or liquid diet in food-deprived rats, but did not affect water intake in water-deprived rats. This specificity is consistent with a role for CNS NT in feeding behavior. Intravenous NT (1-1000 pmole/kg/min for 30 min) did not specifically suppress food intake; however, low doses did increase water intake in food-deprived rats. These findings do not support a role for plasma NT in feeding, but do suggest that it may play a role in drinking behavior.

Neurotensin facilitates dopamine release in vitro from rat striatal slices

Neurotensin, (0.1-10 microM) stimulated the release of [3H]dopamine from rat striatal slices in a calcium-dependent manner, and potentiated the K+-evoked release of [3H]dopamine and endogenous dopamine. This effect was dose-dependent, (1 nM-1 microM) with an EC50 of approximately 10 nM, and was mediated by means of a receptor of similar structure-activity profile to those described in other tissues.

Highly potent neurotensin analog that causes hypothermia and antinociception

The tridecapeptide neurotensin has long been proposed as an endogenous neuroleptic. However, for neurotensin [or neurotensin(8-13) [NT(8-13)], the active fragment] to cause its effects, it must be administered centrally. Here, we report on an analog of NT(8-13), (N-methyl-Arg),Lys,Pro,L-neo-Trp,tert-Leu,Leu (NT69L), which contains a novel amino acid, L-neo5 degrees C (rectal), with a significant effect persisting for over 7 h. NT69L also caused a rapid (within 15 min) and persistent (for over 5 h) antinociceptive effect, as determined by the hot plate test. NT69L was overall the most potent and longest lasting neurotensin analog that has been reported. These studies provide the background for further testing of a stable, potent and long lasting neurotensin analog as a potential neuroleptic.

Neurotensin and cholecystokinin microinjected into the ventral tegmental area modulate microdialysate concentrations of dopamine and metabolites in the posterior nucleus accumbens

Neurotensin and cholecystokinin, neuropeptides which coexist with dopamine in many ventral tegmental neurons, were microinjected into the ventral tegmental area during in vivo microdialysis in the posterior nucleus accumbens. Neurotensin significantly elevated concentrations of dopamine and its metabolites at doses of 10 pmol, 1 nmol, and 10 nmol, while cholecystokinin significantly elevated dopamine metabolite concentrations only at a dose of 10 nmol. These data suggest that neurotensin potently mediates the release of dopamine from the mesolimbic pathway via direct actions on the cell body.

Effect of centrally administered neurotensin on multiple feeding paradigms

Recent studies have suggested that the tridecapeptide, neurotensin, may be an endogenous satiety factor. The present study was undertaken to examine the effects of neurotensin on multiple paradigms known to stimulate feeding. Following a 30 hour starvation period, neurotensin suppressed feeding at the 20 microgram and 10 microgram dose, but not at the 1 microgram dose when compared to saline controls. Norepinephrine (20 micrograms ICV) induced feeding was suppressed at the 20 microgram neurotensin dose but not at the 10 microgram or 1 microgram dose. In contrast, neurotensin did not suppress muscimol induced feeding at any of the doses. Insulin induced feeding (10 units SC) also was not suppressed by neurotensin. Neurotensin suppressed dynorphin induced feeding at the 20 microgram and 10 microgram but not at the 1 microgram dose. Neurotensin suppressed spontaneous feeding (p less than 0.01) in vagotomized rats (2.5 +/- 0.3 g/2 hr) when compared with saline controls (4.2 +/- 0.5 g/2 hr) suggesting that an intact vagus is not necessary for neurotensin's anorectic effect. We conclude that neurotensin may play a role in short-term appetite regulation by a complex interaction with monoamines and neuropeptides, particularly norepinephrine and the kappa opiate agonist, dynorphin.

Neuroanatomical site specific modulation of spontaneous motor activity by neurotensin

Immunohistofluorescent neurotensin (NT) is found in the ventral tegmental area (VTA), and bilateral injection of NT into the VTA produces an increase in exploratory behaviors. The VTA also contains dopaminergic cell bodies with axonal projections to the nucleus accumbens. In this study it was shown that bilateral microinjection of NT (4.0 micrograms/side) into the nucleus accumbens blocked the behavioral hyperactivity produced by intra-VTA injection of NT (2.5 micrograms/side).

Hypothermia elicited by the intracerebral microinjection of neurotensin

Changes in rectal temperature were measured after the intracerebral microinjection of neurotensin (2.5 micrograms/0.5 microliters) at 135 sites in the rat. At 63 of the 135 microinjection sites, the tridecapeptide produced a rapid onset of hypothermia ranging in magnitude from 0.8 to 2.3 degrees C below the baseline rectal temperature. The drop in rectal temperature persisted for 2-4 hours following the microinjection. The greatest concentrations of neurotensin-sensitive sites were found in the medial preoptic region of the hypothalamus and in the periaqueductal gray area, both of which contain relatively large amounts of endogenous neurotensin. Other active sites were found in the ventral thalamus, the dorsomedial hypothalamus, and in the diagonal band of Broca. At no injection site did neurotensin evoke an increase in rectal temperature. These data support the proposition that neurotensin may act endogenously to mediate heat-loss mechanisms in the rat. The data provide further evidence indicating a potent neuromodulatory role for neurotensin.

Centrally administered neurotensin: activity in the Julou-Courvoisier muscle relaxation test in mice

Intracisternal (IC) neurotensin (NT) produces muscle relaxation in the Julou-Courvoisier traction test, a screening procedure utilized for assessing neuroleptic drug activity. A dose-response relationship was not observed. IC administration of thyrotropin-releasing hormone (TRH) totally abolished the effects of NT. Two other peptides, substance P and bradykinin, were inactive in the traction test. These data provide further evidence for CNS effects on NT and indicate that this peptide exerts neuroleptic-like activity in a screening test for antipsychotic agents.

Peptides as central regulators of feeding

During the past decade there has been an increased awareness of the role peptides play as neuromodulators. In this article we review the available data on peptides as central regulators of food ingestion. We stress the possible problems of non-specific effects. We stress that whereas many peptides decrease feeding after central injection, only two families of peptides have been shown to increase feeding after central injection. These are the opioid family and the pancreatic polypeptide-neuropeptide Y family. The putative role of corticotropin releasing factor as the mediator of norepinephrine and serotonin effects on feeding is discussed.

Xenin-25 amplifies GIP-mediated insulin secretion in humans with normal and impaired glucose tolerance but not type 2 diabetes

Glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-stimulated insulin secretion (GSIS). This response is blunted in type 2 diabetes (T2DM). Xenin-25 is a 25-amino acid neurotensin-related peptide that amplifies GIP-mediated GSIS in hyperglycemic mice. This study determines if xenin-25 amplifies GIP-mediated GSIS in humans with normal glucose tolerance (NGT), impaired glucose tolerance (IGT), or T2DM. Each fasting subject received graded glucose infusions to progressively raise plasma glucose concentrations, along with vehicle alone, GIP, xenin-25, or GIP plus xenin-25. Plasma glucose, insulin, C-peptide, and glucagon levels and insulin secretion rates (ISRs) were determined. GIP amplified GSIS in all groups. Initially, this response was rapid, profound, transient, and essentially glucose independent. Thereafter, ISRs increased as a function of plasma glucose. Although magnitudes of insulin secretory responses to GIP were similar in all groups, ISRs were not restored to normal in subjects with IGT and T2DM. Xenin-25 alone had no effect on ISRs or plasma glucagon levels, but the combination of GIP plus xenin-25 transiently increased ISR and plasma glucagon levels in subjects with NGT and IGT but not T2DM. Since xenin-25 signaling to islets is mediated by a cholinergic relay, impaired islet responses in T2DM may reflect defective neuronal, rather than GIP, signaling.

Effects of a novel neurotensin peptide analog given extracranially on CNS behaviors mediated by apomorphine and haloperidol

Neurotensin (NT) is a neuropeptide neurotransmitter in the central nervous system. It has been implicated in the therapeutic and in the adverse effects of neuroleptics. Activity of NT in brain can only be shown by direct injection of the peptide into that organ. However, we have developed a novel analog of NT(8-13), NT69L, which is active upon intraperitoneal (i.p.) injection. Like atypical neuroleptics, NT69L blocked the climbing behavior in rats, but not the licking and sniffing behaviors of a high dose (600 microgram/kg) of the non-selective dopamine agonist apomorphine. Its blockade of climbing was very potent with an ED(50) (effective dose at 50% of maximum) of 16 microgram/kg. Both apomorphine and NT69L caused a long-lasting hypothermia, which was greater with the peptide but not synergistic in combination with apomorphine. The ED(50) of NT69L for hypothermia was 390 microgram/kg. NT69L (up to 5 mg/kg i.p.) did not produce catalepsy. However, when given before haloperidol, NT69L, but not clozapine, completely prevented catalepsy. When given after haloperidol, NT69L, but not clozapine, reversed haloperidol's cataleptic effects with an ED(50) of 260 microg/kg. There was no significant difference between the ED(50)s for hypothermia and anticataleptic effects of NT69L. However, the ED(50) for blocking the effects of apomorphine was significantly lower than the other two. These data suggest that NT69L may have neuroleptic properties in humans and may be useful in the treatment of extrapyramidal side effects caused by typical neuroleptics such as haloperidol.

The hypotensive effect of centrally administered neurotensin in rats

We have evaluated the cardiovascular effects of intracerebroventricular (i.c.v.) injections of neurotensin (NT) in pentobarbital-anesthetized rats. In most animals, the i.c.v. injection of NT (5.4, 10.8 and 16.2 nmol/rat) induced a dose-dependent fall of the arterial blood pressure. This effect was usually rapid in onset (30-60 sec) and of short duration (approximately 1-4 min). It was not preceded nor accompanied by any significant alteration of the heart rate. In about 25% of the animals, the vasodepressor effect of i.c.v. injections of NT was long lasting (30-45 min). Conscious rats were much less sensitive than anesthetized animals. The hypotensive effects of intravenously (i.v.) administered NT was fully maintained in animals made tolerant to the hypotensive effect of centrally administered NT. Similarly, the animals made unresponsive to i.v. injections of NT either by repeated i.v. injections of NT (e.g. tachyphylaxis) or by a chronic treatment with compound 48/80, still responded normally to centrally administered NT. The results suggest the existence of at least two anatomically distinct sites of action through which NT can induce hypotension in rats. One appears to be located in the periphery and the other, in the central nervous system.

Neurotensin injected into the nucleus accumbens blocks the psychostimulant effects of cocaine but does not attenuate cocaine self-administration in the rat

The neuropeptide neurotensin (NT) has been shown to modulate mesolimbic dopaminergic activity. Neurotensin injected into the VTA produces motor stimulation and release of dopamine in the nucleus accumbens. In contrast, when neurotensin is administered into the nucleus accumbens, it produces neuroleptic-like effects such as attenuation of the locomotor activity elicited by psychostimulants. In the present study, the hypothesis that neurotensin injected into the nucleus accumbens might modulate the psychostimulant and reinforcing actions of cocaine was tested. In experiment one, rats were trained to self-administer cocaine intravenously on an FR5 schedule of reinforcement. Following the establishment of baseline responding, rats were implanted with bilateral cannulae in the nucleus accumbens. One week later, rats were injected into the nucleus accumbens with various doses of neurotensin (4.2, 8.4 and 16.7 micrograms, total doses bilaterally) immediately prior to the self-administration session. No significant effects were found with any of the doses of neurotensin tested on the self-administration of cocaine. However, in experiment 2, neurotensin at doses of 4.2 and 16.7 micrograms injected into the nucleus accumbens significantly reduced the locomotor activation induced by an acute injection of cocaine (15 mg/kg i.p.) and a dose of 16.7 micrograms attenuated the locomotor activation induced by amphetamine (0.75 mg/kg i.p.). Thus, neurotensin in the nucleus accumbens appears to specifically modulate the acute locomotor activating properties of cocaine but not cocaine self-administration. Different mechanisms by which NT interacts with dopamine in the nucleus accumbens may provide a means of selectively altering psychostimulant motor actions without affecting psychostimulant reinforcement.