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

Brain Research 987 (2003) 39–48 www.elsevier.com / locate / brainres

Research report

Selective tolerance to the hypothermic and anticataleptic effects of a neurotensin analog that crosses the blood–brain barrier Mona Boules a , *, Beth McMahon a , Rui Wang a , Lewis Warrington a , Jennifer Stewart a , Sally Yerbury a , Abdul Fauq b , Daniel McCormick c , Elliott Richelson a a

Neuropsychopharmacology Laboratory, Mayo Foundation for Medical Education and Research, and Mayo Clinic, 4500 San Pablo Road, Jacksonville, FL 32224, USA b Mayo Chemistry Core Facility, Mayo Clinic, Jacksonville, FL, USA c Mayo Protein Core Facility, Mayo Clinic, Rochester, MN, USA Accepted 30 June 2003

Abstract 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-amphetamineand 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.  2003 Elsevier B.V. All rights reserved. Theme: Neural basis of behavior Topic: Neuropeptides and behavior Keywords: NT69L; Apomorphine; Haloperidol; Cocaine; D-Amphetamine

1. Introduction Neurotensin is an endogenous tridecapeptide neurotransmitter that was discovered by Carraway and Leeman in bovine hypothalami [5]. It is found in the central nervous system as well as in the gastrointestinal tract. Neurotensin exerts potent central nervous system effects including *Corresponding author. Mayo Clinic, 4500 San Pablo Road, Jacksonville, FL 32224, USA. Tel.: 11-904-953-2439; fax: 11-904-9537117. E-mail address: [email protected] (M. Boules). 0006-8993 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0006-8993(03)03227-X

hypothermia [1], antinociception [19], modulation of dopamine neurotransmission [9,16,20,21], and stimulation of anterior pituitary hormone secretion [18,22,25]. Peripherally, neurotensin acts as a paracrine and endocrine peptide of both the digestive and cardiovascular systems. All known effects of NT(1–13) are mediated by the smaller fragment, NT(8–13). For neurotensin to exert its central nervous system effects, it needs to be delivered directly into the brain. This is due in part to its rapid degradation by peptidases upon systemic administration [6]. Although peptides generally do not cross the blood–brain barrier [23], many groups

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have worked on developing a neurotensin agonist that could be delivered systemically and cross this highly selective barrier. Our laboratory has been working on developing neurotensin peptide analogs that are resistant to peptidase degradation and can cross the blood–brain barrier. Because neurotensin has neuroleptic-like effects in animal models, our aim has been to develop a potential, novel antipsychotic for human use. Such a novel neuroleptic would lack the dopamine receptor blocking effects of currently available antipsychotics and therefore, would not likely produce extrapyramidal side effects, including tardive dyskinesia. Previously, we reported on a NT(8–13) analog, called NT69L, that crosses the blood–brain barrier [31]. NT69L causes hypothermia, antinociception, and has neurolepticlike properties, similar to those of atypical neuroleptics. Thus, for this latter property, NT69L blocks the climbing behavior in rats induced by the dopamine agonist apomorphine at a high dose (600 mg / kg) [7]. It also reverses haloperidol-induced catalepsy [7]. Due to its potential use as an atypical neuroleptic, we were interested to learn if repeated injections of NT69L would result in the development of tolerance to its various effects. Here we report on the selective development of tolerance to some, but not to all of the effects of NT69L.

2. Materials and methods

2.4. Catalepsy test The test for catalepsy is well established [7]. The rats were given 1, 3 or 5 daily injections of NT69L (1 mg / kg) or saline. After the last injection of NT69L, the rats were injected 30 min later with 1 mg / kg haloperidol and then tested for catalepsy. The test was performed by simply placing the animal’s forepaws on a suspended metal bar (10 mm in diameter, 11 cm above the counter). The time that elapsed until the animal’s forepaws touched the counter was recorded. The cut-off time, when the animal was removed from the bar, was 3 min.

2.5. Apomorphine-induced climbing Animals were pretreated with either saline or NT69L (1 mg / kg) for 1, 3 or 5 days. Following these pretreatments, animals were administered apomorphine (600 g / kg, s.c.) 30 min later. This compound was dissolved in oxygen-free boiled 0.9% NaCl solution containing 0.1% (w / v) ascorbic acid and 0.1% (w / v) metabisulfite to prevent oxidation. The volume of injection was 1 ml / kg under the loose skin of the back of the animal’s neck. Immediately following these injections, the rats were placed in empty plastic cages for observation. Approximately 5 min later, behavioral monitoring was initiated and it lasted for 1 h. Climbing episodes were quantitated by observing the number of times the rat moved up and down in a vertical position during a 1-min observation period [15].

2.1. Drugs 2.6. NT69L was synthesized as previously described [7]. Haloperidol (H-1512), apomorphine (4393-), D-amphetamine (A-5880) and cocaine (C-5776) were purchased from Sigma (St. Louis, MO, USA).

2.2. Animals Male Sprague–Dawley rats weighing 150–250 g were used for all experiments. Rats were housed in a temperature controlled room with free access to food and water. The animals were kept on a 12 h light–dark cycle. All tests were performed during the light cycle. The rats in each group were used only for one treatment. All procedures were approved by the Mayo Foundation Institutional Animal Use and Care Committee. The treated groups were injected intraperitoneally (i.p.) with the indicated compounds, while the control groups were injected with an equal volume of saline (0.9% NaCl).

2.3. Body temperature measurement Body temperature was measured by means of a thermistor probe inserted approximately 2 cm into the rectum of the rat. Three daily injections of NT69L (1 mg / kg) were given and body temperature was recorded for 4 h daily.

D-Amphetamine-induced

hyperactivity

Animals were pretreated with either saline or NT69L (1 mg / kg) for 1, 3 or 5 days. On the last day, each rat was then placed individually in a plexiglass Opto-Varimex Minor motility chamber (Columbus Instruments, Columbus, OH, USA) for 1 h for acclimation. Following the acclimation period, a 30-min baseline activity was recorded. The rats were then injected with NT69L or saline, placed back in the activity chamber for 30 min, after which time they were injected with D-amphetamine (5 mg / kg) i.p. and activity was recorded for 2 h.

2.7. Cocaine-induced hyperactivity For acute testing, the rats were pretreated with either saline or NT69L (1 mg / kg) followed by injection of cocaine (40 mg / kg) 30 min later. Animals were then tested for activity as described above for the experiments with D-amphetamine. For repeated daily injections and testing, the animals were pretreated with either saline or NT69L (1 mg / kg) for 3 or 5 days followed by injection of cocaine (40 mg / kg) 30 min later on the last day. The rats were then tested for activity as described for amphetamine. Again, there was 1 h for acclimation, a 30-min baseline followed by injection of NT69L or saline. The rats were

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then placed in the activity chamber for 30 min, after which time each animal was injected with cocaine (40 mg / kg) i.p. Over the next 3 h activity was measured. A separate group of rats was injected daily with cocaine and tested for activity as mentioned above.

2.8. Blood glucose and pituitary hormone levels To study the effect of repeated injections of NT69L on blood hormone levels, five control and five treated Sprague–Dawley rats were injected with either saline or with 1 mg / kg NT69L i.p. for 5 days. Rats were sacrificed by decapitation 1 h after the last injection. Blood was collected in cold centrifuge tubes with heparin and kept on ice, centrifuged at 1250 g for 10 min, and plasma was collected and stored at 220 8C until assayed. Glucose was assayed using an enzyme assay at the clinical laboratory in the Mayo Clinic (Jacksonville, FL, USA). Corticosterone and T 4 were assayed using a radioimmunoassay at the clinical laboratory in the Mayo Clinic (Rochester, MN, USA) and the Mayo Clinic (Jacksonville, FL, USA), respectively.

2.9. Statistical analysis Statistical analysis was carried out by one-way analysis of variance (ANOVA) followed by Tukey’s test for multiple comparisons, using SIGMA STAT software. The insets are graphs of the data analyzed by Kendall’s rank correlation

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to assess the evidence for an association between the number of daily doses of NT69L and the response measures (Figs. 1–5). For the temperature experiment, the measure used was the change in temperature at 1 h from baseline, for the catalepsy test it was the total amount of time on the bar over the course of the experiment, and for the activity experiments it was the total amount of activity over the course of the experiments (the sum of the measurements taken). Data in Fig. 6 were analyzed using Wilcoxon rank sum test. P,0.05 was considered significant. S-PLUS statistical software (Insightful, Seattle, WA, USA) was used for the analyses of the insets. The time profile graphs were generated with the use of GRAPHPAD software (San Diego, CA, USA).

3. Results

3.1. Hypothermia After the first daily injection of NT69L (1 mg / kg), there was a marked reduction in body temperature (Fig. 1) as we have reported previously, with an ED 50 of 390 mg / kg [7]. However, body temperature did not deviate from normal after the third or fifth daily injection of this peptide (P50.008). Challenging the rats with five times the dose resulted in a drop of body temperature similar to that found after the first day’s injection (results not shown) indicating development of tolerance.

Fig. 1. Effect of daily injections of NT69L on body temperature. Sprague–Dawley rats were given daily injections of NT69L (1 mg / kg) i.p. for 1, 3 or 5 days. After each injection body temperature was recorded for 4 h by means of a thermistor probe inserted into the rectum of the rat. *, Significantly different (P50.002) from rats injected with saline and rats injected with NT69L for 3 or 5 days. The inset shows the body temperature lowering effects of NT69L for each rat injected for 1, 3 or 5 days. This graph indicates strong evidence of tolerance with repeated daily injections of NT69L (P50.008, Kendall’s rank correlation).

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3.2. Haloperidol-induced catalepsy NT69L blocks haloperidol-induced catalepsy with an ED 50 of 260 mg / kg [7]. Repeated injections of NT69L either for 3 or 5 days resulted in strong evidence of diminished ability of NT69L to block this catalepsy (P5 0.018) (Fig. 2).

3.3. Apomorphine-induced climbing NT69L blocks apomorphine-induced climbing without affecting licking and sniffing, with an ED 50 of 16 mg / kg [7]. After three or five daily injections of NT69L (1 mg / kg) it was equally effective at blocking apomorphine (600 mg / kg) induced climbing (Fig. 3).

3.4.

D-Amphetamine-induced

activity

Administration of NT69L (1 mg / kg) followed by Damphetamine (5 mg / kg) 30 min later blocked the Damphetamine-induced hyperactivity [4]. There was a weak indication that some tolerance might develop to amphetamine-induced hyperactivity only in the first 30 min postinjection, after which there is no indication of tolerance and there was no significant difference between the

negative control (rats injected only with vehicle) and one, three or five daily injections of NT69L (1 mg / kg) (Fig. 4).

3.5. Cocaine-induced activity Single injection of NT69L blocked the cocaine-induced hyperactivity [4]. Repeated (daily for 3 or 5 days) administration (Fig. 5) of NT69L (1 mg / kg) resulted in reversing cocaine (40 mg / kg) induced hyperactivity as did one dose of NT69L. The effects of NT69L (1 mg / kg) partially blocked cocaine-induced hyperactivity after three and five daily injections only in the first 30 min postinjection, after which there was no indication of tolerance and there was no significant difference between the negative control (rats injected only with vehicle) and one, three or five daily injections of NT69L (1 mg / kg) (Fig. 5).

3.6. Plasma glucose and pituitary hormone levels Acute injection of NT69L (1 mg / kg) caused hyperglycemia, increased cortisol and decreased T 4 blood levels 1 h postinjection [2]. After five daily injections of NT69L (1 mg / kg), there was no significance difference from the saline control (Fig. 6A–D), indicating the development of tolerance to the effect of NT69L on glucose and on change in pituitary hormone levels.

Fig. 2. Effect of daily injections of NT69L on the reversal of haloperidol-induced catalepsy. Sprague–Dawley rats were given one, three or five daily injections of NT69L (1 mg / kg) i.p. followed by haloperidol (1 mg / kg) i.p. 30 min later on days 1, 3 or 5. Catalepsy was measured by placing the animal’s forepaws on a suspended metal bar (10 mm in diameter, 11 cm above the counter) and the time that elapsed until the animal’s forepaws touched the counter was recorded. The cut-off time was 3 min. *, Significantly different from rats injected with alcohol / saline and rats injected with NT69L / haloperidol (day1) (P50.001). The inset shows strong evidence of tolerance with repeated daily injections of NT69L (P50.018, Kendall’s rank correlation).

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Fig. 3. Effect of daily injections of NT69L on blocking apomorphine-induced climbing. Sprague–Dawley rats were given one, three or five daily injections of NT69L (1 mg / kg) or saline i.p. followed 30 min later by 600 mg / kg s.c. of apomorphine on day 1, 3 or 5. Apomorphine was dissolved in O 2 -free saline containing 0.05% ascorbic acid to prevent oxidation. Climbing episodes were quantitated by observing the number of times the rat moved up and down in a vertical position during a 1-min observation period. *, Significantly different (P50.001) from rats injected with NT69L for 1, 3 or 5 days. The inset shows very weak evidence of tolerance with repeated daily injections of NT69L (P50.099, Kendall’s rank correlation).

4. Discussion Chronic stimulation of neurotransmitter receptors may lead to adaptive changes that result in a decreased responsiveness to the neurotransmitter. If the agent that is stimulating the receptor is a drug, then the phenomenon that occurs is called tolerance. More generally, tolerance is defined as a decreased responsiveness to a drug after repeated exposure, requiring higher dosages of the drug to cause the same level of effect. At the molecular level, mechanisms explaining tolerance to a drug are desensitization of receptors followed by downregulation, a process whereby the receptor protein on the surface of the cell is taken inside the cell. Thus, tolerance might be due to a decreased coupling of the receptor with second messenger systems or a reduction in the number of ligand recognition sites. However, it could also result from modulating events along the neuronal effector pathways distant from the receptor system [10]. We have shown that prolonged exposure to agonist results in both desensitization and downregulation of neurotensin receptor 1 in cultured cells expressing these receptors [8,13,38]. In the present study, repeated administration of the NT(8–13) analog NT69L resulted in the rapid development of tolerance (after a single dose) to its hypothermic effects, its ability to block the catalepsy caused by

injection of the dopamine receptor antagonist haloperidol, and its effect on blood glucose levels and pituitary hormones. Others have reported the development of tolerance to the hypothermic effect of neurotensin directly injected into brain [10]. On the other hand, blocking effects of NT69L against direct (apomorphine) or indirect ( D-amphetamine and cocaine) dopamine receptor agonists did not show tolerance after several daily injections. Thus, after either a single injection or several daily injections of NT69L, it still blocked apomorphine-induced climbing and partially blocked the hyperactivity caused by injection of D-amphetamine and cocaine only in the first 30 min postinjection. However, with repeated injections there was no evidence of tolerance to the effect of NT69L on amphetamine and cocaine-induced activity after the initial 30 min. Similar results were reported by our group in 6-hydroxydopamine lesioned rats, where repeated injections of NT69L did not diminish its effect in blocking the apomorphine and amphetamine-induced turning behaviors [3]. Thus, it appears that tolerance develops to some, but not to all of the pharmacological effects of NT69L. The development of tolerance to some, but not to all effects of neurotensin has been previously reported by Rinkel et al. [24]. These authors observed development of tolerance to neurotensin on locomotor activity in the open field test following i.c.v.

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Fig. 4. Effect of daily injections of NT69L on blocking D-amphetamine-induced activity. Sprague–Dawley rats were given one, three or five daily injections of NT69L (1 mg / kg) followed 30 min later by amphetamine (5 mg / kg) i.p. to rats that were acclimated to the activity chambers and activity recorded as described in the text. *, Significantly different (P50.001) from saline. **, Significantly different from saline, days 1, 3 and 5 (P50.001). ***, Significantly different from saline, days 1 and 3 (P50.05). The inset shows weak evidence of tolerance with repeated daily injections of NT69L (P50.082, Kendall’s rank correlation).

administration of a high dose (10 mg) of the peptide and to antinociception (hot plate test) after administration of 0.3 mg. However, no tolerance was seen to electric foot shock, after administration of 3 mg of neurotensin. Others [10] reported that rats developed tolerance to neurotensin for body temperature lowering but not its analgesic effect. Neurotensin mediates its effects through its receptors [33]. Roles for the three molecularly-cloned neurotensin receptors in the brain are far from being elucidated. However, it is likely that one neurotensin receptor subtype mediates longterm metabolic effects and another neurotensin receptor subtype is associated with rapid synaptic actions [26]. Using striatal slices, our laboratory found that NT69L increased in a dose-dependent manner potassiumand electrically-evoked [ 3 H]DA release. Preincubation of the slices with NT69L resulted in marked reduction in its effects on potassium-evoked [ 3 H]DA release, with no effects on electrically-evoked [ 3 H]DA release. In contrast, in slices from animals pretreated with NT69L for 5 days NT69L maintained a robust increase in potassium-evoked dopamine release but its effect on electrically-evoked release was reduced by 80% [Wang et al. 2003, submitted for publication]. These results as well as the differential development of tolerance to NT69L strongly suggest that different neurotensin receptors are involved in the facilitation of its pharmacological effects and that modulation of

neurotensin receptor sensitivity, when NT69L is injected intraperitoneally may involve interactions with both neural networks and cellular proteins. This differential development of tolerance to NT69L may also be attributed to differential sensitivity of receptors for neurotensin or dopamine in extrapyramidal versus mesolimbic areas. The site of locomotor hyperactivity induced by dopaminergic drugs such as apomorphine and D-amphetamine is thought to be the mesolimbic system. The inhibition of the locomotor hyperactivity induced by these dopaminergic drugs has been frequently used as an animal model to study mesolimbic dopamine function. The fact that the rats developed tolerance to NT69L for reversal of haloperidol-induced catalepsy, which involves the extrapyramidal region, but not to climbing behavior induced by apomorphine or hyperactivity induced by D-amphetamine or cocaine, suggests that neurotensin receptors on the dopamine neurons projecting to the striatum are more able to be modulated by neurotensin receptor agonists than those on dopamine neurons projecting to the mesolimbic regions. Parallel to our findings, Scatton [27] found a differential regional development of tolerance to increased dopamine turnover upon repeated neuroleptic administration. Tolerance developed in the striatum earlier than that in the mesolimbic system. The increase in dopamine metabolism

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Fig. 5. Effect of daily injections of NT69L on blocking cocaine-induced hyperactivity. Sprague–Dawley rats were given one, three or five daily injections of NT69L (1 mg / kg). followed 30 min later by cocaine (40 mg / kg) i.p. to rats that were acclimated to the activity chambers and activity recorded as described in the text. *, Significantly different (P50.05) from saline. **, Significantly different from saline, days 1, 3 and 5 (P50.05).

in the nigrostriatal system caused by a single injection of a neuroleptic is reduced following repeated treatment [29]. This apparent tolerance does not seem to occur in the mesocortical DA system, after repeated administration of relatively low doses of neuroleptics [28]. Thus, there seems to be a differential regional development of tolerance to increased dopamine metabolism induced by repeated treatment with a neuroleptic. The striatum seems to be most susceptible to the development of tolerance, since the threshold dose of neuroleptic inducing tolerance appears to be lower in the striatum than that in the mesolimbic and mesocortical systems. Longer treatment (40 days vs. 11 days) and larger dose (5–10 times) are needed to induce tolerance in the mesolimbic regions compared to that in the striatum. In contrast, no tolerance develops in the frontal cortex [27]. The difference in development of tolerance to neuroleptics could be due to the development of supersensitivity or to changes in dopaminergic receptor function [30]. Also, the neuronal control of various dopamine pathways are likely to be of a different nature possibly leading to differences in intensity and / or time courses in the response to repeated blockade of dopamine receptors [27,32]. Thus, the difference in response to repeated injections of NT69L in this study might be attributed either to the action

of NT69L on different neurotensin receptor subtypes involved in striatal and hypothalamic versus mesolimbic systems or to the regional differences in sensitivity of possible dopamine receptor subtypes to NT69L. Neurotensin has been reported to reduce the affinity of dopamine D 2 , but not dopamine D 1 receptors in membranal preparations from the nucleus accumbens, tuberculum olfactorium and neostriatum [11,34–37]. There is also in vivo evidence that neurotensin regulates central dopamine transmission by reducing pre- and postsynaptic dopamine D 2 , and enhancing D 1 receptor sensitivity possibly through an antagonistic neurotensin receptor / D 2 interaction [12]. Also, neurotensin has been reported to counteract the D 2 agonist-induced inhibition of g-aminobutyric acid release in the rat nucleus accumbens, via an antagonistic postsynaptic neurotensin / D 2 receptor interaction in freely moving rats [17]. The interaction of the dopaminergic and the neurotensinergic systems with other neurotransmitter systems, such as serotonergic, g-aminobutyric acid, adrenergic, muscarinic and histaminergic may also play a role in the regional development of tolerance. Recently, Hertel et al. [14] reported induction of tolerance to NT69L against D-amphetamine (0.5 mg / kg) after 6 days of twice daily injections of this peptide. However, there are important differences between their study and

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Fig. 6. Effect of five daily injections of NT69L on plasma glucose (A), cortisol (B), TSH (C), and T 4 (D) levels in Sprague–Dawley rats. Values are mean6S.E.M. Each group consisted of 5–10 animals. Rats were injected with either saline (controls) or NT69L (1 mg / kg) i.p. Rats were sacrificed by decapitation at 1 h after injection. In each case, data for NT69L day 1 are from [2]. The blood was collected in 30-ml centrifuge tubes with heparin and centrifuged at 1250 g for 10 min. The plasma was collected and frozen at 220 8C until assayed. Glucose was assayed using an enzyme assay, while corticosterone, TSH, and T 4 were assayed by radioimmunoassays. Wilcoxon rank sum test, shows significant difference between day1 versus day 5 and saline (P,0.01).

ours. First, while we used 1 mg / kg i.p., these researchers used NT69L at three very low dosages: (converted from mmol / kg to mg / kg) 0.041, 0.083 and 0.157 mg / kg s.c. Second, in naive rats, only the middle dosage of NT69L (0.083 mg / kg) had an effect on blocking the hyperactivity caused by D-amphetamine. Third, they then used the highest dosage of NT69L (0.157 mg / kg), which had no effect acutely on D-amphetamine-induced hyperactivity, and gave this amount twice daily for 6 days to induce tolerance. To reconcile their data in the D-amphetamine experiments with our data, one can hypothesize that Hertel et al. [14] using a much lower dosage of NT69L, were studying its effects at a high-affinity neurotensin receptor, while we, using a much higher (10–20 fold) dosage of NT69L, have been studying its effects at a lower-affinity

neurotensin receptor. Additionally, the prediction from their results that a dosage of NT69L above 0.083 mg / kg would not block the effects of D-amphetamine, are not likely due to their use of a low dosage of D-amphetamine. This is because we have shown that at a D-amphetamine dosage (0.75 mg / kg) close to theirs (0.5 mg / kg), NT69L at 1 mg / kg i.p. blocks the hyperactivity caused by this psychostimulant [4]. In conclusion, the present findings indicate that tolerance selectively developed to some (hypothermic effect, hyperglycemia, increased cortisol and decreased T 4 , and the blockade of haloperidol-induced catalepsy), but not to other (apomorphine-induced climbing, and D-amphetamine- and cocaine-induced hyperactivity) of the pharmacological effects of a NT(8–13) analog. These findings

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strongly support the selective role of neurotensin in the modulation of dopamine neurotransmission and further suggest that neurotensin analogs might be useful in modulating certain behavioral effects of psychostimulants.

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