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Effects of intra-VTA injection of neurotensin on local cerebral glucose utilization in freely moving rats☆,☆☆1
Peptides 21 (2000) 1751–1753
Effects of intra-VTA injection of neurotensin on local cerebral glucose utilization in freely moving rats夞,夞夞 Francesco E. Pontieri*, Maurizia Rasura, Alessandra Scontrini, Francesca R. Buttarelli Department of Neuroscience, University ‘La Sapienza’, Rome 00185, Italy
Abstract The [14C]2-deoxyglucose method was applied to measure the effects of the injection of neurotensin (7 g) in the ventral tegmental area on local cerebral glucose utilization in the rat. Injection of neurotensin produced significant increases of glucose utilization in the shell of the nucleus accumbens and in the olfactory tubercle. These results indicate that stimulation of neurotensin receptors in the ventral tegmental area produces functional changes that are confined to the regions receiving mesolimbic projections within the rostral extended amygdaloid complex. These findings extend our understanding on the effects of neurotensin in the limbic system, with particular regard to reward pathways. © 2000 Published by Elsevier Science Inc. Keywords: Neuropeptides; Deoxyglucose; Cerebral metabolism; Nucleus accumbens; Shell; Dopaminergic system; Mesolimbic pathways.
1. Introduction Neurotensin (NT) is a 13 amino acid peptide originally isolated from calf hypothalamus [2]. Like many other neuropeptides, it fulfills a dual function of neurotransmitter or neuromodulator in the nervous system and of local hormone in the periphery [6,15,19,20]. NT is a neuromodulator of dopamine transmission and of anterior pituitary hormone secretion, and exerts potent hypothermic and analgesic effects in the brain. In the periphery, NT is a paracrine and endocrine modulator of the digestive tract and of the cardiovascular system of mammals and acts as a growth factor on a variety of normal or cancer cells. Three NT receptors have been identified; two of them belong to the family of G protein-coupled receptors [7,16], whereas the third one [23] is an entirely new type of neuropetide receptor and is identical to gp95/sortilin, a 100 kDa-protein with a single transmembrane domain. Central dopamine transmission plays a key role in control of motor, cognitive and emotional functions. Dopamine transmission in the mesolimbic system, and the shell of the nucleus accumbens in particular, is enhanced by adminis夞 Supported by grants from MURST 60% and 40% (Neurobiologia delle tossicodipendenze e dei meccanismi di gratificazione naturali). 夞夞 We would like to dedicate this work to Vittorio Erspamer. * Corresponding author. Tel.: ⫹3906-49914708; fax: ⫹3906-4440790. E-mail address: [email protected] (F.E. Pontieri). 0196-9781/00/$ – see front matter © 2000 Published by Elsevier Science Inc. PII: S 0 1 9 6 - 9 7 8 1 ( 0 0 ) 0 0 3 2 6 - 0
tration of drugs of abuse belonging to different pharmacological classes [12,13,18]. This effects is accompanied by increased energy metabolism in the very same area [9,11, 12,14]. There is evidence that NT increases the firing rate of dopamine neurons in the ventral tegmental area (VTA) [21], and the release of dopamine in the nucleus accumbens [8]. Moreover, intra-VTA injection of NT stimulates locomotor activity [4], and produces conditioned place preference [8]. Finally, NT is self-administered in the VTA [8]. In the present study we investigated the effects of intra-VTA injection of NT on local cerebral glucose utilization in order to identify whether the pattern of changes of cerebral energy metabolism produced by the peptide were similar to those reported previously with drugs of abuse acting on the mesolimbic dopamine system.
2. Methods The experiments were performed on male Sprague Dawley rats weighing 280 –300 g. They were housed in single cages under standard temperature and humidity on a 12 h light/dark cycle (light on 08.00 –20.00), with free access to food and water. All procedures were carried out according to the NIH Guide for the Care and Use of laboratory animals. Rats were anesthetized with ketamine (100 mg/kg, intraperitoneal (i.p.)) and placed in a stereotaxic apparatus. The skull was exposed and a small hole drilled to expose the
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dura on the left side. Each rat was implanted with a guide cannula aimed at level of the left ventral tegmental area (coordinates anterior ⫺3.0 mm from bregma, lateral 1.0 mm, ventral ⫺8.5 mm from dura) [10]. Four days after implant of the cannula, rats were anesthetized with halothane (2% in oxygen) and polyethylene catheters were inserted into one femoral artery and vein, then tunneled subcutaneously (s.c.) to exit at the nape of the neck [3]. Five hours after surgery, NT (Sigma, Italy) was injected into the left ventral tegmental area (7 g in 2 l PBS). The [14C]2deoxyglucose procedure was begun ten min after neurotensin injection, and carried out according to the original procedure [17]. Briefly, a pulse of [14C]2-deoxyglucose was injected i.v. (100 g/kg, specific activity 50 –55 mCi/mmol, Amersham International, UK). Timed arterial blood samples were collected, immediately centrifuged and tested for plasma glucose concentrations (Beckman II Glucose Analyzer, USA) and 14C concentrations (Beckman, USA). Approximately 45 min after the administration of the tracer, the animals were killed by the i.v. injection of sodium pentobarbital, the brains were rapidly removed, frozen at ⫺40°C in isopentane, and stored at ⫺70°C until sectioning. Cryostatic coronal sections were thaw-mounted on glass coverslips and autoradiographed on Kodak Min-R X-rays films (Kodak, Italy), along with a set of calibrated [14C]methylmethacrylate standards (Amersham International, UK). The autoradiograms were analyzed by quantitative densitometry using a computerized image processing system (Scion Image for Windows). Local tissue 14C concentrations were determined from the optical densities and a calibration curve obtained from densitometric analysis of the calibrated standards. The rates of glucose utilization were then calculated from the local 14C concentrations and the time courses of the arterial plasma glucose and [14C]2-deoxyglucose concentrations, by means of the operational equation of the method [17]. Local rates of cerebral glucose utilization were calculated in 26 discrete brain areas. Three-way ANOVA for repeated measures on 2 factors (brain side and brain structures) was applied on the results. Huynh-Feldt correction was used for the repeated measures. We performed post-hoc test because the initial aim of this study was to identify cerebral areas that most contributed to the significant treatment effect or to the significant treatment per structure interaction. Thus, independently for each brain area, rates of glucose utilization were compared by uncorrected paired t test for assessing the statistical significance of the difference between values of the injected versus the non-injected side.
Table 1 Effects of the injection of neurotensin in the ventral tegmental area on local cerebral glucose utilization in the rat (mol/100 g/min). Structure
Ipsilateral
Contralateral
Medial Prefrontal Cortex Nucleus Accumbens Core Nucleus Accumbens Shell Olfactory Tubercle Caudate (Dorsolateral) Caudate (Dorsomedial) Caudate (Ventral) Sensory Motor Cortex Anterior Cingulate Cortex Lateral Septum Globus Pallidus Thalamus (Ventromedial) Entopeduncular Nucleus Central Amygdala Basolateral Amygdala Hypothalamus Lateral Subthalamic Nucleus Medial Habenula Medio-Lateral Habenula Lateral Habenula Substantia Nigra Compacta Substantia Nigra Reticulata Hippocampus Superior Colliculus (Ext.) Superior Colliculus (Deep)
95 ⫾ 7 82 ⫾ 7 105 ⫾ 5* 100 ⫾ 4* 121 ⫾ 3 123 ⫾ 6 103 ⫾ 2 105 ⫾ 4 122 ⫾ 5 77 ⫾ 5 62 ⫾ 3 135 ⫾ 8 58 ⫾ 5 55 ⫾ 4 109 ⫾ 7 67 ⫾ 5 99 ⫾ 8 70 ⫾ 4 94 ⫾ 11 116 ⫾ 16 81 ⫾ 6 59 ⫾ 4 79 ⫾ 8 85 ⫾ 5 100 ⫾ 6
95 ⫾ 10 85 ⫾ 8 83 ⫾ 6 89 ⫾ 5 125 ⫾ 2 126 ⫾ 4 108 ⫾ 3 111 ⫾ 6 119 ⫾ 5 77 ⫾ 6 63 ⫾ 3 138 ⫾ 7 62 ⫾ 5 57 ⫾ 3 106 ⫾ 8 69 ⫾ 6 103 ⫾ 4 69 ⫾t8 93 ⫾ 8 102 ⫾ 6 77 ⫾ 8 62 ⫾ 5 78 ⫾ 6 84 ⫾ 6 102 ⫾ 8
Values represent means ⫾ SEM, n ⫽ 4. * P ⬍ 0.05 different from contralateral side, paired t-test.
(P ⬍ 0.001) interactions indicating that the unilateral injection of NT in the VTA changed the rates of glucose utilization in a number of defined areas, asymmetrically in the two hemispheres. The areas which mostly contributed to the effect of NT were the shell of the nucleus accumbens and the olfactory tubercle. Mean values of glucose utilization of these areas were 25% and 12% higher in the injection side than in the contralateral non-injected hemisphere (Table 1, Fig. 1).
3. Results The repeated measures analysis of variance showed significant treatment (P ⫽ 0.025) and brain side (P ⬍ 0.001) main effects, and significant side per treatment (P ⬍ 0.001), structure per treatment (P ⫽ 0.001), and side per structure
Fig. 1. Pseudocolor transformation of coronal section of a rat brain at the level of the nucleus accumbens, showing the increased rates of glucose utilization in the shell of the nucleus accumbens and olfactory tubercle ipsilateral to intra-VTA injection of neurotensin. I ⫽ ipsilateral to intraVTA injection of neurotensin, C ⫽ contralateral to intra-VTA injection of neurotensin.
F.E. Pontieri et al. / Peptides 21 (2000) 1751–1753
4. Discussion The results of the present study show that local injection of NT into the ventral tegmental area produces selective changes in local cerebral glucose utilization in the rat. These findings provide the first report of changes of local cerebral metabolism following injection of NT. Previous reports indicate that injection of NT into the VTA produces behavioral effects consistent with rewarding properties of the peptide, such as conditioned place preference and self-administration [8]. In this study, we demonstrate that the changes of local cerebral glucose utilization following intra-VTA injection of NT are confined to the shell of the accumbens and the olfactory tubercle. Conversely, NT failed to modify energy metabolism in the core of the accumbens and in the medial prefrontal cortex (the site of projection of mesocortical dopamine fibers). The shell of the accumbens and the olfactory tubercle represent the most rostral portions of the extended amygdala, that play a role in emotional and motivational functions [1,5,22]. Within the nucleus accumbens, the selectivity of effects of NT are similar to those seen previously with several drugs of abuse [9,11,12,14], that consistently stimulate dopamine transmission in the shell [12,13,18]. The present results, therefore, show that the cerebral functional changes produced by injection of NT into the VTA are strictly confined to the most limbic portions of the mesolimbic projection areas, and suggest that the effects of NT are most potent on reward pathways. These findings are consistent with previous reports on the effects of intra-VTA injection of NT on conditioned place preference and of self-administration of NT in the VTA [8]. In conclusion, the results of this study provide evidence of functional changes by intra-VTA injection of NT that may be relevant to the rewarding properties of the peptide.
References [1] Alheid G, Heimer L. New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid and corticopetal components of substantia innominata. Neuroscience 1988;27:1–39. [2] Carraway R, Seeman LE. The isolation of a new hypotensive peptide, neurotensin, form bovine hypothalami. J Biol Chem 1973;248:6854 – 61. [3] Crane AM, Porrino LJ. Adaptation of the quantitative 2-[14C]-deoxyglucose method for use in freely moving rats. Brain Res 1989;499: 87–92. [4] Feifel D, Reza TL. Effects of neurotensin administered into the ventral tegmental area on prepulse inhibition of startle. Behav Brain Res 1999;106:189 –93.
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[5] Heimer L, De Olmos J, Alheid GF, Zaborsky L. “Perestroika” in the basal forebrain: opening the border between neurology and psychiatry. Prog Brain Res 1991;87:109 – 65. [6] Kitabgi P, Checler F, Mazella J, Vincent JP. Pharmacology and biochemistry of neurotensin receptors. Rev Clin Basic Pharm 1985; 5:397– 486. [7] Kitabgi P, Kwan CY, Fox JE, Vincent JP. Characterization of neurotensin binding to rat gastric smooth muscle receptor sites. Peptides 1984;5:917–23 [8] McBride WJ, Murphy JM, Ikemoto S. Localization of brain reinforcement mechanisms: intracranial (i.c.) self-administration and i.c. place-conditioning studies. Behav Brain Res 1999;101:129 –52 [9] Orzi F, Passarelli F, La Riccia M, Di Grezia R, Pontieri FE. Intravenous morphine increases glucose utilization in the shell of the rat nucleus accumbens. Eur J Pharmacol 1986;302:49 –51. [10] Paxinos G, Watson C. The rat brain atlas in stereotaxic coordinates. Sydney: Academic Press, 1987. [11] Pontieri FE, Conti G, Zocchi A, Fieschi C, Orzi F. Metabolic mapping of the effects of WIN55212–2 intravenous administration in the rat. Neuropsychopharmacology 1999;21:773– 6. [12] Pontieri FE, Tanda G, Orzi F, Di Chiara G. Effects of nicotine on the nucleus accumbens and similarity to those of addictive drugs. Nature 1996;382:255–7. [13] Pontieri FE, Tanda G, Di Chiara G. Intravenous cocaine, morphine and amphetamine preferentially increase extracellular dopamine in the “shell” as compared to the “core” of the rat nucleus accumbens. Proc Natl Acad Sci USA 1995;92:12304 – 8. [14] Pontieri FE, Colangelo V, La Riccia M, Pozzilli C, Passarelli F, Orzi F. Psychostimulant drugs increase glucose utilization in the shell of the rat nucleus accumbens. NeuroReport 1994;5:2561– 4. [15] Roste`ne WH, Alexander MJ. Neurotensin and neuroendocrine regulation. Front Neuroendocrinol 1992;18:115–73. [16] Schotte A, Leysen JE, Laduron PM. Evidence for a displaceable non-specific [3H]neurotensin binding site in rat brain. NaunynSchmiedeberg’s Arch Pharmacol. 1986;333:400 –5. [17] Sokoloff L, Reivich M, Kennedy C, DesRosiers MH, Patlak CS, Pettigrew KD, Sakurada O, Shinohara M. The [14C]deoxyglucose method for the measurement of local cerebral glucose utilization: theory, procedure and normal values in the conscious and anesthetized albino rat. J Neurochem 1977;28:897–916. [18] Tanda G, Pontieri FE, Di Chiara G. Cannabinoid and heroin activation of mesolimbic dopamine transmission by a common 1 opioid receptor mechanism. Science 1997;276:2048 –50. [19] Vincent JP, Mazella J, Kitabgi P. Neurotensin and neurotensin receptors. Trends Pharmacol Sci 1999;20:302–9. [20] Vincent JP. Neurotensin receptors: binding properties, transduction pathways, and structure. Cell Mol Neurobiol 1995;15:501–11. [21] Werkman TR, Kruse CG, Nievelstein H, Long SK, Wadman WJ. Neurotensin attenuates the quinpirole-induced inhibition of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area. Neuroscience 2000;95:417– 23. [22] Zahm DH, Brog JS. On the significance of subterritories in the “accumbens” part of the rat ventral striatum. Neuroscience 1992;50: 751– 67. [23] Zsu¨rger N, Mazella J, Vincent JP. Solubilization and purification of a high affinity neurotensin receptor from newborn human brain. Brain Res. 1994;639:245–52.
Effects of intra-VTA injection of neurotensin on local cerebral glucose utilization in freely moving rats夞,夞夞 Francesco E. Pontieri*, Maurizia Rasura, Alessandra Scontrini, Francesca R. Buttarelli Department of Neuroscience, University ‘La Sapienza’, Rome 00185, Italy
Abstract The [14C]2-deoxyglucose method was applied to measure the effects of the injection of neurotensin (7 g) in the ventral tegmental area on local cerebral glucose utilization in the rat. Injection of neurotensin produced significant increases of glucose utilization in the shell of the nucleus accumbens and in the olfactory tubercle. These results indicate that stimulation of neurotensin receptors in the ventral tegmental area produces functional changes that are confined to the regions receiving mesolimbic projections within the rostral extended amygdaloid complex. These findings extend our understanding on the effects of neurotensin in the limbic system, with particular regard to reward pathways. © 2000 Published by Elsevier Science Inc. Keywords: Neuropeptides; Deoxyglucose; Cerebral metabolism; Nucleus accumbens; Shell; Dopaminergic system; Mesolimbic pathways.
1. Introduction Neurotensin (NT) is a 13 amino acid peptide originally isolated from calf hypothalamus [2]. Like many other neuropeptides, it fulfills a dual function of neurotransmitter or neuromodulator in the nervous system and of local hormone in the periphery [6,15,19,20]. NT is a neuromodulator of dopamine transmission and of anterior pituitary hormone secretion, and exerts potent hypothermic and analgesic effects in the brain. In the periphery, NT is a paracrine and endocrine modulator of the digestive tract and of the cardiovascular system of mammals and acts as a growth factor on a variety of normal or cancer cells. Three NT receptors have been identified; two of them belong to the family of G protein-coupled receptors [7,16], whereas the third one [23] is an entirely new type of neuropetide receptor and is identical to gp95/sortilin, a 100 kDa-protein with a single transmembrane domain. Central dopamine transmission plays a key role in control of motor, cognitive and emotional functions. Dopamine transmission in the mesolimbic system, and the shell of the nucleus accumbens in particular, is enhanced by adminis夞 Supported by grants from MURST 60% and 40% (Neurobiologia delle tossicodipendenze e dei meccanismi di gratificazione naturali). 夞夞 We would like to dedicate this work to Vittorio Erspamer. * Corresponding author. Tel.: ⫹3906-49914708; fax: ⫹3906-4440790. E-mail address: [email protected] (F.E. Pontieri). 0196-9781/00/$ – see front matter © 2000 Published by Elsevier Science Inc. PII: S 0 1 9 6 - 9 7 8 1 ( 0 0 ) 0 0 3 2 6 - 0
tration of drugs of abuse belonging to different pharmacological classes [12,13,18]. This effects is accompanied by increased energy metabolism in the very same area [9,11, 12,14]. There is evidence that NT increases the firing rate of dopamine neurons in the ventral tegmental area (VTA) [21], and the release of dopamine in the nucleus accumbens [8]. Moreover, intra-VTA injection of NT stimulates locomotor activity [4], and produces conditioned place preference [8]. Finally, NT is self-administered in the VTA [8]. In the present study we investigated the effects of intra-VTA injection of NT on local cerebral glucose utilization in order to identify whether the pattern of changes of cerebral energy metabolism produced by the peptide were similar to those reported previously with drugs of abuse acting on the mesolimbic dopamine system.
2. Methods The experiments were performed on male Sprague Dawley rats weighing 280 –300 g. They were housed in single cages under standard temperature and humidity on a 12 h light/dark cycle (light on 08.00 –20.00), with free access to food and water. All procedures were carried out according to the NIH Guide for the Care and Use of laboratory animals. Rats were anesthetized with ketamine (100 mg/kg, intraperitoneal (i.p.)) and placed in a stereotaxic apparatus. The skull was exposed and a small hole drilled to expose the
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dura on the left side. Each rat was implanted with a guide cannula aimed at level of the left ventral tegmental area (coordinates anterior ⫺3.0 mm from bregma, lateral 1.0 mm, ventral ⫺8.5 mm from dura) [10]. Four days after implant of the cannula, rats were anesthetized with halothane (2% in oxygen) and polyethylene catheters were inserted into one femoral artery and vein, then tunneled subcutaneously (s.c.) to exit at the nape of the neck [3]. Five hours after surgery, NT (Sigma, Italy) was injected into the left ventral tegmental area (7 g in 2 l PBS). The [14C]2deoxyglucose procedure was begun ten min after neurotensin injection, and carried out according to the original procedure [17]. Briefly, a pulse of [14C]2-deoxyglucose was injected i.v. (100 g/kg, specific activity 50 –55 mCi/mmol, Amersham International, UK). Timed arterial blood samples were collected, immediately centrifuged and tested for plasma glucose concentrations (Beckman II Glucose Analyzer, USA) and 14C concentrations (Beckman, USA). Approximately 45 min after the administration of the tracer, the animals were killed by the i.v. injection of sodium pentobarbital, the brains were rapidly removed, frozen at ⫺40°C in isopentane, and stored at ⫺70°C until sectioning. Cryostatic coronal sections were thaw-mounted on glass coverslips and autoradiographed on Kodak Min-R X-rays films (Kodak, Italy), along with a set of calibrated [14C]methylmethacrylate standards (Amersham International, UK). The autoradiograms were analyzed by quantitative densitometry using a computerized image processing system (Scion Image for Windows). Local tissue 14C concentrations were determined from the optical densities and a calibration curve obtained from densitometric analysis of the calibrated standards. The rates of glucose utilization were then calculated from the local 14C concentrations and the time courses of the arterial plasma glucose and [14C]2-deoxyglucose concentrations, by means of the operational equation of the method [17]. Local rates of cerebral glucose utilization were calculated in 26 discrete brain areas. Three-way ANOVA for repeated measures on 2 factors (brain side and brain structures) was applied on the results. Huynh-Feldt correction was used for the repeated measures. We performed post-hoc test because the initial aim of this study was to identify cerebral areas that most contributed to the significant treatment effect or to the significant treatment per structure interaction. Thus, independently for each brain area, rates of glucose utilization were compared by uncorrected paired t test for assessing the statistical significance of the difference between values of the injected versus the non-injected side.
Table 1 Effects of the injection of neurotensin in the ventral tegmental area on local cerebral glucose utilization in the rat (mol/100 g/min). Structure
Ipsilateral
Contralateral
Medial Prefrontal Cortex Nucleus Accumbens Core Nucleus Accumbens Shell Olfactory Tubercle Caudate (Dorsolateral) Caudate (Dorsomedial) Caudate (Ventral) Sensory Motor Cortex Anterior Cingulate Cortex Lateral Septum Globus Pallidus Thalamus (Ventromedial) Entopeduncular Nucleus Central Amygdala Basolateral Amygdala Hypothalamus Lateral Subthalamic Nucleus Medial Habenula Medio-Lateral Habenula Lateral Habenula Substantia Nigra Compacta Substantia Nigra Reticulata Hippocampus Superior Colliculus (Ext.) Superior Colliculus (Deep)
95 ⫾ 7 82 ⫾ 7 105 ⫾ 5* 100 ⫾ 4* 121 ⫾ 3 123 ⫾ 6 103 ⫾ 2 105 ⫾ 4 122 ⫾ 5 77 ⫾ 5 62 ⫾ 3 135 ⫾ 8 58 ⫾ 5 55 ⫾ 4 109 ⫾ 7 67 ⫾ 5 99 ⫾ 8 70 ⫾ 4 94 ⫾ 11 116 ⫾ 16 81 ⫾ 6 59 ⫾ 4 79 ⫾ 8 85 ⫾ 5 100 ⫾ 6
95 ⫾ 10 85 ⫾ 8 83 ⫾ 6 89 ⫾ 5 125 ⫾ 2 126 ⫾ 4 108 ⫾ 3 111 ⫾ 6 119 ⫾ 5 77 ⫾ 6 63 ⫾ 3 138 ⫾ 7 62 ⫾ 5 57 ⫾ 3 106 ⫾ 8 69 ⫾ 6 103 ⫾ 4 69 ⫾t8 93 ⫾ 8 102 ⫾ 6 77 ⫾ 8 62 ⫾ 5 78 ⫾ 6 84 ⫾ 6 102 ⫾ 8
Values represent means ⫾ SEM, n ⫽ 4. * P ⬍ 0.05 different from contralateral side, paired t-test.
(P ⬍ 0.001) interactions indicating that the unilateral injection of NT in the VTA changed the rates of glucose utilization in a number of defined areas, asymmetrically in the two hemispheres. The areas which mostly contributed to the effect of NT were the shell of the nucleus accumbens and the olfactory tubercle. Mean values of glucose utilization of these areas were 25% and 12% higher in the injection side than in the contralateral non-injected hemisphere (Table 1, Fig. 1).
3. Results The repeated measures analysis of variance showed significant treatment (P ⫽ 0.025) and brain side (P ⬍ 0.001) main effects, and significant side per treatment (P ⬍ 0.001), structure per treatment (P ⫽ 0.001), and side per structure
Fig. 1. Pseudocolor transformation of coronal section of a rat brain at the level of the nucleus accumbens, showing the increased rates of glucose utilization in the shell of the nucleus accumbens and olfactory tubercle ipsilateral to intra-VTA injection of neurotensin. I ⫽ ipsilateral to intraVTA injection of neurotensin, C ⫽ contralateral to intra-VTA injection of neurotensin.
F.E. Pontieri et al. / Peptides 21 (2000) 1751–1753
4. Discussion The results of the present study show that local injection of NT into the ventral tegmental area produces selective changes in local cerebral glucose utilization in the rat. These findings provide the first report of changes of local cerebral metabolism following injection of NT. Previous reports indicate that injection of NT into the VTA produces behavioral effects consistent with rewarding properties of the peptide, such as conditioned place preference and self-administration [8]. In this study, we demonstrate that the changes of local cerebral glucose utilization following intra-VTA injection of NT are confined to the shell of the accumbens and the olfactory tubercle. Conversely, NT failed to modify energy metabolism in the core of the accumbens and in the medial prefrontal cortex (the site of projection of mesocortical dopamine fibers). The shell of the accumbens and the olfactory tubercle represent the most rostral portions of the extended amygdala, that play a role in emotional and motivational functions [1,5,22]. Within the nucleus accumbens, the selectivity of effects of NT are similar to those seen previously with several drugs of abuse [9,11,12,14], that consistently stimulate dopamine transmission in the shell [12,13,18]. The present results, therefore, show that the cerebral functional changes produced by injection of NT into the VTA are strictly confined to the most limbic portions of the mesolimbic projection areas, and suggest that the effects of NT are most potent on reward pathways. These findings are consistent with previous reports on the effects of intra-VTA injection of NT on conditioned place preference and of self-administration of NT in the VTA [8]. In conclusion, the results of this study provide evidence of functional changes by intra-VTA injection of NT that may be relevant to the rewarding properties of the peptide.
References [1] Alheid G, Heimer L. New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid and corticopetal components of substantia innominata. Neuroscience 1988;27:1–39. [2] Carraway R, Seeman LE. The isolation of a new hypotensive peptide, neurotensin, form bovine hypothalami. J Biol Chem 1973;248:6854 – 61. [3] Crane AM, Porrino LJ. Adaptation of the quantitative 2-[14C]-deoxyglucose method for use in freely moving rats. Brain Res 1989;499: 87–92. [4] Feifel D, Reza TL. Effects of neurotensin administered into the ventral tegmental area on prepulse inhibition of startle. Behav Brain Res 1999;106:189 –93.
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[5] Heimer L, De Olmos J, Alheid GF, Zaborsky L. “Perestroika” in the basal forebrain: opening the border between neurology and psychiatry. Prog Brain Res 1991;87:109 – 65. [6] Kitabgi P, Checler F, Mazella J, Vincent JP. Pharmacology and biochemistry of neurotensin receptors. Rev Clin Basic Pharm 1985; 5:397– 486. [7] Kitabgi P, Kwan CY, Fox JE, Vincent JP. Characterization of neurotensin binding to rat gastric smooth muscle receptor sites. Peptides 1984;5:917–23 [8] McBride WJ, Murphy JM, Ikemoto S. Localization of brain reinforcement mechanisms: intracranial (i.c.) self-administration and i.c. place-conditioning studies. Behav Brain Res 1999;101:129 –52 [9] Orzi F, Passarelli F, La Riccia M, Di Grezia R, Pontieri FE. Intravenous morphine increases glucose utilization in the shell of the rat nucleus accumbens. Eur J Pharmacol 1986;302:49 –51. [10] Paxinos G, Watson C. The rat brain atlas in stereotaxic coordinates. Sydney: Academic Press, 1987. [11] Pontieri FE, Conti G, Zocchi A, Fieschi C, Orzi F. Metabolic mapping of the effects of WIN55212–2 intravenous administration in the rat. Neuropsychopharmacology 1999;21:773– 6. [12] Pontieri FE, Tanda G, Orzi F, Di Chiara G. Effects of nicotine on the nucleus accumbens and similarity to those of addictive drugs. Nature 1996;382:255–7. [13] Pontieri FE, Tanda G, Di Chiara G. Intravenous cocaine, morphine and amphetamine preferentially increase extracellular dopamine in the “shell” as compared to the “core” of the rat nucleus accumbens. Proc Natl Acad Sci USA 1995;92:12304 – 8. [14] Pontieri FE, Colangelo V, La Riccia M, Pozzilli C, Passarelli F, Orzi F. Psychostimulant drugs increase glucose utilization in the shell of the rat nucleus accumbens. NeuroReport 1994;5:2561– 4. [15] Roste`ne WH, Alexander MJ. Neurotensin and neuroendocrine regulation. Front Neuroendocrinol 1992;18:115–73. [16] Schotte A, Leysen JE, Laduron PM. Evidence for a displaceable non-specific [3H]neurotensin binding site in rat brain. NaunynSchmiedeberg’s Arch Pharmacol. 1986;333:400 –5. [17] Sokoloff L, Reivich M, Kennedy C, DesRosiers MH, Patlak CS, Pettigrew KD, Sakurada O, Shinohara M. The [14C]deoxyglucose method for the measurement of local cerebral glucose utilization: theory, procedure and normal values in the conscious and anesthetized albino rat. J Neurochem 1977;28:897–916. [18] Tanda G, Pontieri FE, Di Chiara G. Cannabinoid and heroin activation of mesolimbic dopamine transmission by a common 1 opioid receptor mechanism. Science 1997;276:2048 –50. [19] Vincent JP, Mazella J, Kitabgi P. Neurotensin and neurotensin receptors. Trends Pharmacol Sci 1999;20:302–9. [20] Vincent JP. Neurotensin receptors: binding properties, transduction pathways, and structure. Cell Mol Neurobiol 1995;15:501–11. [21] Werkman TR, Kruse CG, Nievelstein H, Long SK, Wadman WJ. Neurotensin attenuates the quinpirole-induced inhibition of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area. Neuroscience 2000;95:417– 23. [22] Zahm DH, Brog JS. On the significance of subterritories in the “accumbens” part of the rat ventral striatum. Neuroscience 1992;50: 751– 67. [23] Zsu¨rger N, Mazella J, Vincent JP. Solubilization and purification of a high affinity neurotensin receptor from newborn human brain. Brain Res. 1994;639:245–52.