Neurons of origin of the neurotensinergic plexus enmeshing the ventral tegmental area in rat: retrograde labeling and in situ hybridization combined

The Role of Intra-Amygdaloid Neurotensin and Dopamine Interaction in Spatial Learning and Memory

Neurotransmitter and neuromodulator neurotensin (NT) has been proved to facilitate spatial and passive avoidance learning after microinjected into the rat central nucleus of amygdala (CeA). These previous studies of our laboratory also revealed that neurotensin-1 receptor (NTS1) is involved in the mentioned actions of NT. Extensive literature confirms the interaction between neurotensinergic and dopaminergic systems, and our research group also suppose that the mesolimbic dopaminergic system (MLDS) is involved in the spatial learning and memory-facilitating effect of NT in the CeA. In the present work, NT and dopamine (DA) interaction has been examined in the Morris water maze and passive avoidance tests. Rats received 100 ng NT, 5 µg dopamine D2 receptor antagonist sulpiride in itself, sulpiride as a pretreatment before NT or vehicle solution into the CeA. NT microinjection significantly decreased target-finding latency in the Morris water maze test and significantly increased entrance latency in the passive avoidance test, as was expected based on our previous findings. The DA D2 receptor antagonist pretreatment was able to inhibit both effects of NT. The results confirm the facilitatory effect of NT on spatial learning and memory and let us conclude that these actions can be exerted via the DA D2 receptors.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Neurotensin in reward processes

Neurotensin (NTS) is a neuropeptide neurotransmitter expressed in the central and peripheral nervous systems. Many studies over the years have revealed a number of roles for this neuropeptide in body temperature regulation, feeding, analgesia, ethanol sensitivity, psychosis, substance use, and pain. This review provides a general survey of the role of neurotensin with a focus on modalities that we believe to be particularly relevant to the study of reward. We focus on NTS signaling in the ventral tegmental area, nucleus accumbens, lateral hypothalamus, bed nucleus of the stria terminalis, and central amygdala. Studies on the role of NTS outside of the ventral tegmental area are still in their relative infancy, yet they reveal a complex role for neurotensinergic signaling in reward-related behaviors that merits further study. This article is part of the special issue on 'Neuropeptides'.

The Lateral Preoptic Area: A Novel Regulator of Reward Seeking and Neuronal Activity in the Ventral Tegmental Area

The lateral preoptic area (LPO) is a hypothalamic region whose function has been largely unexplored. Its direct and indirect projections to the ventral tegmental area (VTA) suggest that the LPO could modulate the activity of the VTA and the reward-related behaviors that the VTA underlies. We examined the role of the LPO on reward taking and seeking using operant self-administration of cocaine or sucrose. Rats were trained to self-administer cocaine or sucrose and then subjected to extinction, whereby responding was no longer reinforced. We tested if stimulating the LPO pharmacologically with bicuculline or chemogenetically with Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) modifies self-administration and/or seeking. In another set of experiments, we tested if manipulating the LPO influences cocaine self-administration during and after punishment. To examine the functional connectivity between the LPO and VTA, we used in vivo electrophysiology recordings in anesthetized rats. We tested if stimulating the LPO modifies the activity of GABA and dopamine neurons of the VTA. We found that stimulating the LPO reinstated cocaine and sucrose seeking behavior but had no effect on reward intake. Furthermore, both stimulating and inhibiting the LPO prevented the sustained reduction in cocaine intake seen after punishment. Finally, stimulating the LPO inhibited the activity of VTA GABA neurons while enhancing that of VTA dopamine neurons. These findings indicate that the LPO has the capacity to drive reward seeking, modulate sustained reductions in self-administration following punishment, and regulate the activity of VTA neurons. Taken together, these findings implicate the LPO as a previously overlooked member of the reward circuit.

Examination of pain threshold and neuropeptides in patients with acute suicide risk

INTRODUCTION

One of the main challenges in suicide prevention is the limited understanding of the biological mechanisms underlying suicide. Recent findings suggest impairments in pain processing in acutely suicidal patients. However, little is known about the biological factors that may drive these discrete physiological abnormalities. In this study, we examined plasma peptides involved in analgesic and inflammatory responses and physical pain threshold in acutely suicidal patients.

METHODS

Thirty-seven depressed patients of both sexes hospitalized for severe suicidal ideation or a recent suicide attempt were characterized clinically including history of suicidal ideation and behavior. Psychological and physical pain, and pressure pain threshold was also measured. Plasma levels of β-endorphin, neurotensin, agouti-related protein (AgRP), C-reactive protein (CRP), adrenocorticotropic hormone (ACTH), and brain-derived neurotrophic factor (BDNF) were run in Milliplex multiplex assays.

RESULTS

The number of lifetime suicide attempts was positively correlated with β-endorphin (r = 0.702; p = 0.007), and neurotensin (r = 0.728, p = 0.007) plasma levels. Higher pain threshold was measured in the suicide attempt group as compared to the suicidal ideation group. Pain threshold was strongly and negatively associated with CRP plasma levels (r = -0.548; p < 0.001). In patients reporting chronic pain, lower AgRP levels and lower pain threshold were observed (t = 4.472; p = 0.001).

CONCLUSION

Our results suggest that abnormalities in the opioid and neurotensin systems may underlie the increase in pain threshold found in suicide attempters, and possibly risk for suicidal behavior. Targeting pain circuits and systems may provide therapeutic mechanisms for suicide prevention.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

Determination of neurotensin projections to the ventral tegmental area in mice

Pharmacologic treatment with the neuropeptide neurotensin (Nts) modifies motivated behaviors such as feeding, locomotor activity, and reproduction. Dopamine (DA) neurons of the ventral tegmental area (VTA) control these behaviors, and Nts directly modulates the activity of DA neurons via Nts receptor-1. While Nts sources to the VTA have been described in starlings and rats, the endogenous sources of Nts to the VTA of mice remain incompletely understood, impeding determination of which Nts circuits orchestrate specific behaviors in this model. To overcome this obstacle we injected the retrograde tracer Fluoro-Gold into the VTA of mice that express GFP in Nts neurons. Identification of GFP-Nts cells that accumulate Fluoro-Gold revealed the Nts afferents to the VTA in mice. Similar to rats, most Nts afferents to the VTA of mice arise from the medial and lateral preoptic areas (POA) and the lateral hypothalamic area (LHA), brain regions that are critical for coordination of feeding and reproduction. Additionally, the VTA receives dense input from Nts neurons in the nucleus accumbens shell (NAsh) of mice, and minor Nts projections from the amygdala and periaqueductal gray area. Collectively, our data reveal multiple populations of Nts neurons that provide direct afferents to the VTA and which may regulate specific aspects of motivated behavior. This work lays the foundation for understanding endogenous Nts actions in the VTA, and how circuit-specific Nts modulation may be useful to correct motivational and affective deficits in neuropsychiatric disease.

Distribution of CGRP and CGRP receptor components in the rat brain

BACKGROUND

Calcitonin gene-related peptide and its receptor, consisting of receptor activity-modifying protein 1 and calcitonin receptor-like receptor, are of considerable interest because of the role they play in migraine and recently developed migraine therapies.

METHODS

To better understand the function of this neuropeptide, we used immunohistochemistry to determine a detailed distribution of calcitonin gene-related peptide, receptor activity-modifying protein 1 and calcitonin receptor-like receptor in the rat brain in a region of 0.5-1.5 mm lateral to the midline. We found calcitonin gene-related peptide immunoreactivity in most of the neurons of the cerebral cortex, hippocampus, cerebellum, thalamic nuclei, hypothalamic nuclei and brainstem nuclei. In contrast, receptor activity-modifying protein 1 and calcitonin receptor-like receptor immunoreactivity were found almost exclusively in the neuronal processes in the investigated regions.

CONCLUSION

Overall, the degree of expression of calcitonin gene-related peptide and calcitonin gene-related peptide receptor components in the central nervous system is astonishingly complex and suggestive of many different brain functions, including a possible role in migraine. However, currently, the presence of calcitonin gene-related peptide and the nature of its receptors throughout the brain is an enigma yet to be solved.

Sources of input to the rostromedial tegmental nucleus, ventral tegmental area, and lateral habenula compared: A study in rat

Profound inhibitory control exerted on midbrain dopaminergic neurons by the lateral habenula (LHb), which has mainly excitatory outputs, is mediated by the GABAergic rostromedial tegmental nucleus (RMTg), which strongly innervates dopaminergic neurons in the ventral midbrain. Early reports indicated that the afferent connections of the RMTg, excepting its very strong LHb inputs, do not differ appreciably from those of the ventral tegmental area (VTA). Presumably, however, the RMTg contributes more to behavioral synthesis than to simply invert the valence of the excitatory signal coming from the LHb. Therefore, the present study was done to directly compare the inputs to the RMTg and VTA and, in deference to its substantial involvement with this circuitry, the LHb was also included in the comparison. Data indicated that, while the afferents of the RMTg, VTA, and LHb do originate within the same large pool of central nervous system (CNS) structures, each is also related to structures that project more strongly to it than to the others. The VTA gets robust input from ventral striatopallidum and extended amygdala, whereas RMTg biased inputs arise in structures with a more direct impact on motor function, such as deep layers of the contralateral superior colliculus, deep cerebellar and several brainstem nuclei, and, via a relay in the LHb, the entopeduncular nucleus. Input from the ventral pallidal-lateral preoptic-lateral hypothalamus continuum is strong in the RMTg and VTA and dominant in the LHb. Axon collateralization was also investigated, providing additional insights into the organization of the circuitry of this important triad of structures.

Maternally responsive neurons in the bed nucleus of the stria terminalis and medial preoptic area: Putative circuits for regulating anxiety and reward

Postpartum neuropsychiatric disorders are a major source of morbidity and mortality and affect at least 10% of childbearing women. Affective dysregulation within this context has been identified in association with changes in reproductive steroids. Steroids promote maternal actions and modulate affect, but can also destabilize mood in some but not all women. Potential brain regions that mediate these effects include the medial preoptic area (mPOA) and ventral bed nucleus of the stria terminalis (vBNST). Herein, we review the regulation of neural activity in the mPOA/vBNST by environmental and hormonal concomitants in puerperal females. Such activity may influence maternal anxiety and motivation and have significant implications for postpartum affective disorders. Future directions for research are also explored, including physiological circuit-level approaches to gain insight into the functional connectivity of hormone-responsive maternal circuits that modulate affect.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Modulation of locomotor activation by the rostromedial tegmental nucleus

The rostromedial tegmental nucleus (RMTg) is a strong inhibitor of dopamine neurons in the ventral tegmental area (VTA) reported to influence neurobiological and behavioral responses to reward omission, aversive and fear-eliciting stimuli, and certain drugs of abuse. Insofar as previous studies implicate ventral mesencephalic dopamine neurons as an essential component of locomotor activation, we hypothesized that the RMTg also should modulate locomotion activation. We observed that bilateral infusions into the RMTg of the gamma-aminobutyric acid A (GABAA) agonist, muscimol, indeed activate locomotion. Alternatively, bilateral RMTg infusions of the GABAA receptor antagonist, bicuculline, suppress robust activations of locomotion elicited in two distinct ways: (1) by disinhibitory stimulation of neurons in the lateral preoptic area and (2) by return of rats to an environment previously paired with amphetamine administration. The possibility that suppressive locomotor effects of RMTg bicuculline infusions were due to unintended spread of drug to the nearby VTA was falsified by a control experiment showing that bilateral infusions of bicuculline into the VTA produce activation rather than suppression of locomotion. These results objectively implicate the RMTg in the regulation of locomotor activation. The effect is important because much evidence reported in the literature suggests that locomotor activation can be an involuntary behavioral expression of expectation and/or want without which the willingness to execute adaptive behaviors is impaired.

An update on the connections of the ventral mesencephalic dopaminergic complex

This review covers the intrinsic organization and afferent and efferent connections of the midbrain dopaminergic complex, comprising the substantia nigra, ventral tegmental area and retrorubral field, which house, respectively, the A9, A10 and A8 groups of nigrostriatal, mesolimbic and mesocortical dopaminergic neurons. In addition, A10dc (dorsal, caudal) and A10rv (rostroventral) extensions into, respectively, the ventrolateral periaqueductal gray and supramammillary nucleus are discussed. Associated intrinsic and extrinsic connections of the midbrain dopaminergic complex that utilize gamma-aminobutyric acid (GABA), glutamate and neuropeptides and various co-expressed combinations of these compounds are considered in conjunction with the dopamine-containing systems. A framework is provided for understanding the organization of massive afferent systems descending and ascending to the midbrain dopaminergic complex from the telencephalon and brainstem, respectively. Within the context of this framework, the basal ganglia direct and indirect output pathways are treated in some detail. Findings from rodent brain are briefly compared with those from primates, including humans. Recent literature is emphasized, including traditional experimental neuroanatomical and modern gene transfer and optogenetic studies. An attempt was made to provide sufficient background and cite a representative sampling of earlier primary papers and reviews so that people new to the field may find this to be a relatively comprehensive treatment of the subject.

Protracted maturation of forebrain afferent connections of the ventral tegmental area in the rat

The mesocorticolimbic dopamine system has long attracted the interest of researchers concerned with the unique gamut of behavioral and mental health vulnerabilities associated with adolescence. Accordingly, the development of the mesocorticolimbic system has been studied extensively, but almost exclusively with regard to dopaminergic output, particularly in the nucleus accumbens and medial prefrontal cortex. To the contrary, the ontogeny of inputs to the ventral tegmental area (VTA), the source of mesocorticolimbic dopamine, has been neglected. This is not a trivial oversight, as the activity of VTA neurons, which reflects their capacity to transmit information about salient events, is sensitively modulated by inputs. Here, we assessed the development of VTA afferent connections using the β subunit of cholera toxin (Ctβ) as a retrograde axonal tracer in adolescent (postnatal day 39) and early adult (8-9-week-old) rats. After intra-VTA injections of Ctβ, adolescent and early adult animals exhibited qualitatively similar distributions of retrogradely labeled neurons in the sense that VTA-projecting neurons were present at all of the same rostrocaudal levels in all of the same structures in both age groups. However, quantitation of retrogradely labeled neurons revealed that adolescent brains, compared with early adult brains, had significantly fewer VTA-projecting neurons preferentially within an interconnected network of cortical and striatopallidal forebrain structures. These findings provide a novel perspective on the development of the mesocorticolimbic dopamine system and may have important implications for age-dependent specificity in the function of this system, particularly with regard to adolescent impulsivity and mental health vulnerabilities.

Hypothalamic neurotensin projections promote reward by enhancing glutamate transmission in the VTA

The lateral hypothalamus (LH) sends a dense glutamatergic and peptidergic projection to dopamine neurons in the ventral tegmental area (VTA), a cell group known to promote reinforcement and aspects of reward. The role of the LH to VTA projection in reward-seeking behavior can be informed by using optogenetic techniques to dissociate the actions of LH neurons from those of other descending forebrain inputs to the VTA. In the present study, we identify the effect of neurotensin (NT), one of the most abundant peptides in the LH to VTA projection, on excitatory synaptic transmission in the VTA and reward-seeking behavior. Mice displayed robust intracranial self-stimulation of LH to VTA fibers, an operant behavior mediated by NT 1 receptors (Nts1) and NMDA receptors. Whole-cell patch-clamp recordings of VTA dopamine neurons demonstrated that NT (10 nm) potentiated NMDA-mediated EPSCs via Nts1. Results suggest that NT release from the LH into the VTA activates Nts1, thereby potentiating NMDA-mediated EPSCs and promoting reward. The striking behavioral and electrophysiological effects of NT and glutamate highlight the LH to VTA pathway as an important component of reward.

Comparison of the locomotor-activating effects of bicuculline infusions into the preoptic area and ventral pallidum

Ambulatory locomotion in the rodent is robustly activated by unilateral infusions into the basal forebrain of type A gamma-aminobutyric acid receptor antagonists, such as bicuculline and picrotoxin. The present study was carried out to better localize the neuroanatomical substrate(s) underlying this effect. To accomplish this, differences in total locomotion accumulated during a 20-min test period following bicuculline versus saline infusions in male Sprague-Dawley rats were calculated, rank ordered and mapped on a diagram of basal forebrain transposed from immunoprocessed sections. The most robust locomotor activation was elicited by bicuculline infusions clustered in rostral parts of the preoptic area. Unilateral infusions of bicuculline into the ventral pallidum produced an unanticipatedly diminutive activation of locomotion, which led us to evaluate bilateral ventral pallidal infusions, and these also produced only a small activation of locomotion, and, interestingly, a non-significant trend toward suppression of rearing. Subjects with bicuculline infused bilaterally into the ventral pallidum also exhibited persistent bouts of abnormal movements. Bicuculline infused unilaterally into other forebrain structures, including the bed nucleus of stria terminalis, caudate-putamen, globus pallidus, sublenticular extended amygdala and sublenticular substantia innominata, did not produce significant locomotor activation. Our data identify the rostral preoptic area as the main substrate for the locomotor-activating effects of basal forebrain bicuculline infusions. In contrast, slight activation of locomotion and no effect on rearing accompanied unilateral and bilateral ventral pallidal infusions. Implications of these findings for forebrain processing of reward are discussed.

Effects of neurotensin gene knockout in mice on the behavioral effects of cocaine

RATIONALE

The neuropeptide neurotensin (NT), which has been implicated in the modulation of dopamine signaling, is expressed in a subset of dopamine neurons and antagonism of the NT receptor has been reported to reduce psychostimulant-induced behavior. Gene knockout (KO) of the neurotensin/neuromedin N precursor provides an approach to delineating possible roles of endogenous NT in psychostimulant-induced responses.

OBJECTIVES

Involvement of NT in cocaine responses was examined by comparing acute and conditioned locomotor responses, conditioned place preference, and sensitization in wild-type (WT), heterozygous, and homozygous NT KO mice.

RESULTS

NT KO mice did not differ from their WT or heterozygous littermates in either baseline or acute cocaine-stimulated locomotor activity. The locomotor stimulant effects of cocaine were slightly prolonged in these mice under some, but not all, experimental conditions. The rewarding effects of cocaine as assessed in the conditioned place preference and conditioned locomotion paradigms were also similar between genotypes at all cocaine doses tested.

CONCLUSIONS

These results suggest that endogenous NT is not involved in cocaine-mediated behaviors in most circumstances, but under some conditions, a slight prolongation of the effects of cocaine was observed in the absence of endogenous NT.

Inputs to the midbrain dopaminergic complex in the rat, with emphasis on extended amygdala-recipient sectors

The midbrain dopaminergic neuronal groups A8, A9, A10, and A10dc occupy, respectively, the retrorubral field (RRF), substantia nigra compacta (SNc), ventral tegmental area (VTA), and ventrolateral periaqueductal gray (PAGvl). Collectively, these structures give rise to a mixed dopaminergic and nondopaminergic projection system that essentially permits adaptive behavior. However, knowledge is incomplete regarding how the afferents of these structures are organized. Although the VTA is known to receive numerous afferents from cortex, basal forebrain, and brainstem and the SNc is widely perceived as receiving inputs mainly from the striatum, the afferents of the RRF and PAGvl have yet to be assessed comprehensively. This study was performed to provide an account of those connections and to seek a better understanding of how afferents might contribute to the functional interrelatedness of the VTA, SNc, RRF, and PAGvl. Ventral midbrain structures received injections of retrograde tracer, and the resulting retrogradely labeled structures were targeted with injections of anterogradely transported Phaseolus vulgaris leucoagglutinin. Whereas all injections of retrograde tracer into the VTA, SNc, RRF, or PAGvl produced labeling in many structures extending from the cortex to caudal brainstem, pronounced labeling of structures making up the central division of the extended amygdala occurred following injections that involved the RRF and PAGvl. The anterograde tracing supported this finding, and the combination of retrograde and anterograde labeling data also confirmed reports from other groups indicating that the SNc receives robust input from many of the same structures that innervate the VTA, RRF, and PAGvl.

The role of neurotensin in physiologic and pathologic processes

PURPOSE OF REVIEW

Neurotensin is a 13-amino acid peptide found in the central nervous system central nervous system and the gastrointestinal tract. Since its initial discovery in 1973, neurotensin has been shown to play a role in a wide range of physiologic and pathologic processes throughout the body. Ongoing research efforts continue to clarify the role of neurotensin in various central nervous system and gastrointestinal processes, as well as how disruption of these normal mechanisms may lead to diseases ranging from schizophrenia to colorectal cancer. The goal of this review is to provide an overview of the most recent advances in the field of neurotensin research, in the context of what has been previously published.

RECENT FINDINGS

Because of the seemingly unrelated functions of neurotensin in the central nervous system and the periphery, the scope of the articles reviewed is rather broad. Contributions continue to be made to our understanding of the downstream effects of neurotensin signaling and the complex feedback loops between neurotensin and other signaling molecules. By selective targeting or blockade of specific neurotensin receptors, investigators have identified potential drugs for use in the treatment of schizophrenia, alcoholism, chronic pain, or cancer. Neurotensin-based pharmacologic agents are being used successfully in animal models for a number of these conditions.

SUMMARY

The review highlights the wide array of biological processes in which neurotensin has a role, and summarizes the most recent advances in various fields of neurotensin research. The knowledge gained through this research has led to the development of first-in-class drugs for the treatment of various medical conditions, and it is clear that in the coming years some of these agents will be ready to move from the bench to the bedside in clinical trials.

Periaqueductal gray afferents synapse onto dopamine and GABA neurons in the rat ventral tegmental area

The midbrain central gray (periaqueductal gray; PAG) mediates defensive behaviors and is implicated in the rewarding effects of opiate drugs. Projections from the PAG to the ventral tegmental area (VTA) suggest that this region might also regulate behaviors involving motivation and cognition. However, studies have not yet examined the morphological features of PAG axons in the VTA or whether they synapse onto dopamine (DA) or GABA neurons. In this study, we injected anterograde tracers into the rat PAG and used immunoperoxidase to visualize the projections to the VTA. Immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA was then used to identify the phenotype of innervated cells. Electron microscopic examination of the VTA revealed axons labeled anterogradely from the PAG, including myelinated and unmyelinated fibers and axon varicosities, some of which formed identifiable synapses. Approximately 55% of these synaptic contacts were of the symmetric (presumably inhibitory) type; the rest were asymmetric (presumably excitatory). These findings are consistent with the presence of both GABA and glutamate projection neurons in the PAG. Some PAG axons contained dense-cored vesicles indicating the presence of neuropeptides in addition to classical neurotransmitters. PAG projections synapsed onto both DA and GABA cells with no obvious selectivity, providing the first anatomical evidence for these direct connections. The results suggest a diverse nature of PAG physiological actions on midbrain neurons. Moreover, as both the VTA and PAG are implicated in the reinforcing actions of opiates, our findings provide a potential substrate for some of the rewarding effects of these drugs.

The regulatory role of neurotensin on the hypothalamic-anterior pituitary axons: emphasis on the control of thyroid-related functions

Neurotensin (NT) is a 13 amino acid neurohormone and/or neuromodulator, located in the synaptic vesicles and released from the neuronal terminals in a calcium-dependent manner. This peptide is present among mammalian and nonmammalian species, mainly in the central nervous system and the gastrointestinal tract. Due to its neuroendocrine activity, NT has been related to the pathophysiology of a series of disorders, such as schizophrenia, drug-abuse, Parkinson's disease, cancer, stroke, eating disorders and other neurodegenerative conditions. Moreover, NT participates in the physiology of pain-induction, central blood pressure control and inflammation. NT also plays an important interactive role in all components of the hypothalamic-anterior pituitary circuit, which is mediated by an endocrine, paracrine or/and autocrine manner, towards most of the anatomical regions that define this circuit. A considerable amount of data implicates NT in thyroid-related regulation through this circuit, the exact mechanisms of which should be further investigated for the potential development of more targeted approaches towards the treatment of thyroid-related endocrine diseases. The aim of this study was to provide an up-to-date review of the literature concerning the regulatory role of NT on the hypothalamic-anterior pituitary axons, with an emphasis on the control of thyroid-related functions.

Cortico-Basal Ganglia reward network: microcircuitry

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

Functional implications of glutamatergic projections to the ventral tegmental area

Glutamatergic afferents of the ventral tegmental area (VTA) play an important role in the functioning of the VTA and are involved in the pathophysiology of drug addiction. It has recently been demonstrated that the VTA is densely innervated by glutamatergic axons and that glutamatergic neurons projecting to the VTA are situated in almost all structures that project there. While the projection from the prefrontal cortex is essentially entirely glutamatergic, subcortical glutamatergic neurons innervating the VTA intermingle with non-glutamatergic, most likely GABAergic and/or peptidergic VTA-projecting neurons. The first part of this review focuses on the origins and putative functional implications of various glutamatergic projections to the VTA. In the second part we consider how different neuropeptides via different mechanisms modulate glutamatergic actions in the VTA. We conclude by developing a model of how the glutamatergic afferents might together contribute to the functions of the VTA.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and gamma-aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

Comparative immunohistochemical study of the dopaminergic systems in two inbred mouse strains (C57BL/6J and DBA/2J)

This study investigated possible neurochemical differences in the brain of two inbred mouse strains, C57BL/6J (C57) and DBA/2J (DBA) that in behavioral, memorization and learning tasks under normal and experimental conditions perform differently or often in an opposite manner. The immunohistochemical study, designed to investigate the dopaminergic system, identified many differences within the midbrain A10 area and less marked differences in areas A9 and A8. The number of dopamine transporter (DAT), vesicular monoamine transporter of type 2 (VMT) and tyrosine hydroxylase (TH) immunoreactive cell bodies was significantly higher in the midbrain of DBA mice than in C57 mice (on average +21.5%, P<0.001 in A10: +9.4% in A9, P<0.05: and +5.9% in A8, P<0.1). The distribution patterns of nerve fibres immunoreactive for same antisera also differed significantly in the two strains, especially at prelimbic, infralimbic and anterior cingulate cortical levels. In C57 mice these fibres were scanty whereas in DBA mice they were well represented. In the nucleus accumbens, also the territorial distribution of DAT immunoreactive nerve fibres differed in the two strains. In the midbrain, the galanin immunoreactive axons were more densely distributed in DBA than in C57 mice whereas neurotensin immunoreactive axons were more densely distributed in C57 than in DBA. These distinct immunohistochemical patterns could help to explain why performance differs in the two mouse strains.

Muscimol inactivation of the septo-preoptic complex affects medial forebrain bundle self-stimulation only when directed at the complex's ventrolateral components

Elements of the septo-preoptic basal forebrain complex, particularly the lateral and medial septum, the diagonal band of Broca, and the magnocellular preoptic area, have been linked to medial forebrain bundle (MFB) self-stimulation. This study examines the roles of these areas in MFB self-stimulation by temporarily inactivating them with 25 and 50ng doses of the GABA(A) receptor agonist muscimol. Changes in performance capacity and stimulation reward effectiveness were evaluated with the rate-frequency curve shift paradigm. When infused into the lateral and medial septum and the vertical limb of the diagonal band of Broca, both doses of muscimol were as ineffective as saline in altering either the rats' maximum rate of response for stimulation or the frequency required to maintain half-maximal response rate (required frequency). However, when infused into the horizontal limb of the diagonal band of Broca or the magnocellular preoptic area, muscimol substantially decreased maximal response rate and modestly increased required frequency. Changes in maximum rate were dose-dependent, but changes in required frequency were not. Muscimol infusions contralateral to the stimulated hemisphere were as effective as ipsilateral infusions; bilateral infusions tended to so suppress responding that resulting rate-frequency curves were often invalid. These results suggest a role in MFB self-stimulation for only the ventrolateral components of the septo-preoptic complex, and support past observations of considerable bilaterality in the neural systems that support self-stimulation.

Bidirectional regulation of dopamine D2 and neurotensin NTS1 receptors in dopamine neurons

Several lines of evidence suggest a close association between dopamine (DA) and neurotensin (NT) systems in the CNS. Indeed, in the rodent brain, abundant NT-containing fibres are found in DA-rich areas such as the ventral tegmental area and substantia nigra. Moreover, it has been shown in vivo that NT, acting through its high-affinity receptor (NTS1), reduces the physiological and behavioural effects of DA D2 receptor (D2R) activation, a critical autoreceptor feedback system regulating DA neurotransmission. However, the mechanism of this interaction is still elusive. The aim of our study was thus to reproduce in vitro the interaction between D2R and NTS1, and then to characterize the mechanisms implicated. We used a primary culture model of DA neurons prepared from transgenic mice expressing green fluorescent protein under the control of the tyrosine hydroxylase promoter. In these cultures, DA neurons endogenously express both D2R and NTS1. Using electrophysiological recordings, we show that activation of D2R directly inhibits the firing rate of DA neurons. In addition, we find that NT, acting through a NTS1-like receptor, is able to reduce D2R autoreceptor function independently of its ability to enhance DA neuron firing, and that this interaction occurs through a protein kinase C- and Ca(2+)-dependent mechanism. Furthermore, prior activation of D2R reduces the ability of NTS1 to induce intracellular Ca(2+) mobilization. Our findings provide evidence for bidirectional interaction between D2R and NTS1 in DA neurons, a regulatory mechanism that could play a key role in the control of the activity of these neurons.

Brain neurotensin, psychostimulants, and stress--emphasis on neuroanatomical substrates

Neurotensin (NT) is a peptide that is widely distributed throughout the brain. NT is involved in locomotion, reward, stress and pain modulation, and in the pathophysiology of drug addiction and depression. In its first part this review brings together relevant literature about the neuroanatomy of NT and its receptors. The second part focuses on functional-anatomical interactions between NT, the mesotelencephalic dopamine system and structures targeted by dopaminergic projections. Finally, recent data about the actions of NT in processes underlying behavioral sensitization to psychostimulant drugs and the involvement of NT in the regulation of the hypothalamo-pituitary-adrenal gland axis are considered.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Neurotensin afferents of the ventral tegmental area in the rat: [1] re-examination of their origins and [2] responses to acute psychostimulant and antipsychotic drug administration

The ventral tegmental area (VTA) is involved in reward-related behaviours and the actions of psychostimulant drugs. It is influenced by afferents expressing a variety of neurotransmitters and neuromodulators; the innervation containing neurotensin is among the densest of these. Intra-VTA neurotensin activates dopaminergic neurons and plays an important role in the development of behavioural sensitization to psychostimulant drugs and possibly in schizophrenia. Using gold-coupled wheatgerm agglutinin as retrograde tracer in combination with nonisotopic in situ hybridization for neurotensin mRNA or neurotensin antibodies after colchicine treatment, the present study was undertaken to demonstrate the neurotensinergic neurons projecting to the VTA and determine whether (and in which subpopulations) neurotensin expression is regulated in VTA-projecting neurons after administrations of the psychostimulant drug methamphetamine or the antipsychotic haloperidol. This study reveals the lateral preoptico-rostral lateral hypothalamic continuum and the medial preoptic area as main sources for the neurotensin afferents of the VTA. Fewer neurotensinergic, VTA-projecting neurons are situated in the dorsal raphe, pedunculopontine and laterodorsal tegmental nuclei, lateral hypothalamic area, ventral endopiriform area, lateral septum, accumbens shell, parabrachial nucleus and different parts of the extended amygdala. The number of neurotensinergic VTA-projecting neurons increased significantly only after methamphetamine administration and exclusively in the accumbens shell. It is concluded that the widespread neurotensinergic VTA-projecting neurons, situated in areas involved in different reward-related behaviours, are well suited to convey distinct reward information to the VTA. The up-regulation of neurotensin expression selectively in VTA-projecting neurons in the accumbens shell following methamphetamine administration may be an important factor in the development of behavioural sensitization.

Neurotensin antagonist acutely and robustly attenuates locomotion that accompanies stimulation of a neurotensin-containing pathway from rostrobasal forebrain to the ventral tegmental area

Neurotensin exerts complex effects on the mesolimbic dopamine system that alter motivation and contribute to neuroadaptations associated with psychostimulant drug administration. Activation of abundant neurotensin receptors in the ventral tegmental area (VTA) enhances dopamine neuron activity and associated release of dopamine in the nucleus accumbens (Acb) and cortex. In view of recent anatomical studies demonstrating that 70% of all neurotensin-containing neurons projecting to the VTA occupy the lateral preoptic area-rostral lateral hypothalamus (LPH) and lateral part of the medial preoptic area (MPOA), the present study examined functionality in the LPH-MPOA neurotensinergic pathway in the rat. Disinhibition (resulting ultimately in stimulation-like effects) of LPH-MPOA neurons with microinjected bicuculline (50 or 100 ng in 0.25 microL) produced locomotor activation that was considerably attenuated by systemic administration of the neurotensin antagonist SR 142948 A (0.03 and 0.1 mg/kg). In contrast, locomotion elicited in this manner was completely blocked by SR 142948 A infused directly into the VTA (5.0 and 15.0 ng in 0.25 microL). Baseline locomotion was unaffected by systemic or intra-VTA administration of SR 142948 A and LPH-MPOA-elicited locomotion was unaffected by infusion of SR 142948 A into the substantia nigra pars compacta and sites rostral and dorsal to the VTA. Locomotion was not elicited by infusions of bicuculline into the lateral hypothalamus at sites caudal to the LPH-MPOA, where neurotensin neurons projecting to the VTA are fewer. The results demonstrate the capacity of a neurotensin-containing pathway from LPH-MPOA to VTA to modulate locomotion. This pathway may be important in linking hippocampal and mesolimbic mechanisms in normal behaviour and drug addiction.

On the retention of neurotensin in the ventral tegmental area (VTA) despite destruction of the main neurotensinergic afferents of the VTA--implications for the organization of forebrain projections to the VTA

Neurotensin (NT) modulates ventral tegmental area (VTA) signaling in a manner relevant to psychostimulant drug actions, thus inviting evaluation of psychostimulant effects in conditions of reduced or absent VTA NT. However, in a preliminary study, NT immunoreactivity (-ir) in the VTA was unaffected following destruction of the main concentration of forebrain neurotensinergic VTA afferents in the lateral preoptic-rostral lateral hypothalamic continuum (LPH) and adjacent lateral part of the medial preoptic area (MPOA). This study attempted to determine what measures are necessary to obtain a significant reduction of VTA NT-ir. Large unilateral ibotenic acid lesions were made in several structures containing NTergic, VTA-projecting neurons, including the LPH-MPOA, nucleus accumbens, VTA itself and dorsal raphe. None of these was associated with substantial ipsilateral loss of NT-ir in the VTA, lateral hypothalamus or lateral habenula. Combinations of lesions, such as LPH-MPOA plus VTA and LPH-MPOA plus dorsal raphe, also failed to substantially reduce NT-ir in these structures. Transections of the medial forebrain bundle (mfb) likewise failed to produce a substantial loss of VTA NT-ir measured with immunohistochemistry and radioimmunoassay. Transections of the mfb were carried out in combination with infusions of retrograde and anterograde axonal tract-tracers, revealing that the routes taken by some forebrain NT-ir VTA afferents circumvent mfb transections. All of these results together are consistent with the hypothesis that the connectional organization of forebrain and brainstem, potentially in combination with limited adaptive synaptogenesis, renders the VTA relatively insensitive to moderate losses of neurotensinergic and, perhaps, other peptidergic afferents.

Differential cellular distribution of tonicity-induced expression of transcription factor TonEBP in the rat brain following prolonged systemic hypertonicity

In a previous work performed on cerebral cortex and hippocampus we reported that tonicity-responsive enhancer binding protein (TonEBP), originally identified as a transactivator of osmoprotective genes involved in osmoadaptation of renal cells, was induced in neurons only, but to varying levels, following acute systemic hypertonicity. Whether or not this cellular specificity reflected a unique ability of neurons or a differential time course among brain cells for tonicity-induction of TonEBP was investigated throughout the brain in this study by subjecting the animals to prolonged systemic hypertonicity. In normal rats, TonEBP immunolabeling and TonEBP-mRNA in situ hybridization labeling showed a widespread, uneven and parallel distribution. TonEBP was expressed primarily in the cell nuclei of neurons, where it was heterogeneously distributed in a nucleoplasmic and a granular pool. In rats subjected to prolonged systemic hypertonicity, TonEBP labeling increased in the cell nuclei of neurons only. The tonicity-induced expression of TonEBP for a given cell group of neurons was rather uniform but varied greatly among neuronal cell groups and was positively correlated with the average size of the cell nuclei, as determined by quantitative analysis of digitized images. The detailed distribution of tonicity-induced expression of TonEBP is reported throughout the brain. In normal rats, a very minor proportion of non-neuronal cells, identified as a subset of astrocytes and possibly oligodendrocytes, showed faint nuclear immunolabeling, which however did not increase in hypertonic animals. Ependymocytes, capillary endothelial cells, and microglial cells showed no TonEBP labeling, even in hypertonic animals. Altogether our data indicate that neurons, albeit possibly to a varying extent, are the only brain cells able to use TonEBP-mediated processes for adaptation to a systemic hyperosmotic unbalance.

A cocaine-and-amphetamine-regulated-transcript peptide projection from the lateral hypothalamus to the ventral tegmental area

Cocaine-and-amphetamine-regulated-transcript peptides play a role in the modulation of feeding and psychomotor stimulant-like behaviors. The ventral tegmental area and the lateral hypothalamus are likely structures where cocaine-and-amphetamine-regulated-transcript peptides mediate both of these functions. Although lateral hypothalamus inputs to the ventral tegmental area have long been known, the chemical nature of this pathway remains poorly understood. To address this issue, we tested the possibility that cocaine-and-amphetamine-regulated-transcript peptide-containing neurons in the lateral hypothalamus project to the ventral tegmental area using the retrograde transport of cholera toxin subunit B combined with cocaine-and-amphetamine-regulated-transcript peptide immunostaining. The largest density of retrogradely-labeled neurons in the hypothalamus after cholera toxin subunit B injection in the ventral tegmental area was found, ipsi- and contralaterally, in the lateral hypothalamus/perifornical area, although substantial numbers of retrogradely-labeled cells were also found in the medial preoptic area, lateral preoptic area, paraventricular nucleus, dorsomedial hypothalamus and ventromedial hypothalamus. More than 80% of the retrogradely-labeled cocaine-and-amphetamine-regulated-transcript peptide-immunoreactive neurons in the hypothalamus were found in the lateral hypothalamus/perifornical area both ipsilateral and contralateral to the injection sites. Although retrogradely-labeled neurons were seen in the amygdala, locus coeruleus, and raphe nucleus, none of them displayed cocaine-and-amphetamine-regulated-transcript peptide immunoreactivity. Therefore, the hypothalamic projection to the ventral tegmental area provides a substrate whereby cocaine-and-amphetamine-regulated-transcript peptides could mediate the rewarding aspects of feeding and psychomotor stimulant-like behaviors. These findings, combined with the fact that the lateral hypothalamus receives strong inputs from the shell of the nucleus accumbens and ventral pallidum, suggest that these structures are part of integrative functional loops that control reward and appetitive behaviors.

Endogenous neurotensin in the ventral tegmental area contributes to amphetamine behavioral sensitization

Studies showing psychostimulant-like effects of exogenous neurotensin (NT) infused into the ventral tegmental area (VTA) prompted us to examine the role in the VTA of the endogenous NT in behavioral sensitization to amphetamine. Rats were sensitized to amphetamine by means of a subcutaneous amphetamine (1 mg/kg) injection, and the same dose was injected 7 days later to evaluate the expression of sensitization. The highly selective NT-receptor antagonist SR 142948A was injected into the VTA prior to the first and/or second amphetamine administration. SR 142948A (5 pmol/side) given before the first amphetamine exposure prevented the induction of behavioral sensitization, but did not alter the acute response to amphetamine. SR 142948A given with the second amphetamine administration did not affect the expression of behavioral sensitization. In contrast to administration into the VTA, intraperitoneal administration of SR 142948A (0.03, 0.1, or 0.3 mg/kg) had no detectable effect on the induction of amphetamine sensitization. These results suggest that activation of VTA NT receptors by endogenous NT may contribute to the neuroadaptations underlying behavioral sensitization to amphetamine.

Limbic and motor circuitry underlying footshock-induced reinstatement of cocaine-seeking behavior

The role of limbic, cortical, and striatal circuitry in a footshock reinstatement model of relapse to cocaine seeking was evaluated. Transient inhibition of the central extended amygdala [CEA; including the central nucleus of the amygdala (CN), ventral bed nucleus of the stria terminalis (BNSTv), and nucleus accumbens shell (NAshell)], ventral tegmental area (VTA), and motor circuitry [including the dorsal prefrontal cortex (PFCd), nucleus accumbens core (NAcore), and ventral pallidum (VP)] blocked the ability of footshock stress to reinstate lever pressing previously associated with cocaine delivery. However, inhibition of the basolateral amygdala, mediodorsal nucleus of the thalamus, or the ventral prefrontal cortex had no effect on drug-seeking behavior. These data suggest that footshock stress activates limbic circuitry of the CEA that, via the VTA, activates motor output circuitry responsible for producing lever press responding. Consistent with this notion, the D1/D2 dopamine receptor antagonist fluphenazine blocked footshock-induced reinstatement when infused into the PFCd. Further, inhibition of the NAshell blocked a footshock-induced increase in dopamine within the PFC and concomitantly blocked reinstatement responding. Also supporting the idea of a CEA-VTA-motor circuit in stress-induced reinstatement of cocaine seeking, inactivation of the PFCd was shown to block stress-induced glutamate release within the NAcore while concurrently inhibiting reinstatement responding. Taken together, these data suggest that footshock activates limbic circuitry in the CEA, which in turn activates a VTA dopamine projection to the PFCd. The rise in dopamine within the PFCd initiates reinstatement via a glutamatergic projection to the NAcore.

Specific involvement of neurotensin type 1 receptor in the neurotensin-mediated in vivo dopamine efflux using knock-out mice

Abstract Neurotensin is a tridecapeptide neurotransmitter known to be involved in psychiatric disorders, various physiological processes and several different neurobiological mechanisms, including modulation of accumbal dopamine release. Two neurotensin extracellular binding sites, namely NT1- and NT2-receptor (NT1R and NT2R), have been cloned from the rat brain. These receptors are distinguishable by their different in vitro pharmacological properties but the available pharmacological tools have weak in vivo potency and specificity. The use of genetically engineered knock-out mice has provided a powerful alternative to the classical pharmacological approach to investigate their respective roles. In this study, using in vivo differential pulse amperometry, we show that, in wild-type mice, neurotensin application into the ventral tegmental area dose-dependently evokes dopamine efflux in the nucleus accumbens. This neurotensin-mediated efflux is dramatically decreased in mice lacking NT1R while it is unaffected in NT2R-deleted mice. This finding indicates that a large part of the dopamine efflux evoked by neurotensin in the nucleus accumbens of wild-type mice is mediated via NT1R present in the ventral tegmental area.

3,4-Dihydroxyphenylacetaldehyde is the toxic dopamine metabolite in vivo: implications for Parkinson's disease pathogenesis

In Parkinson's disease (PD), there is a highly selective loss of dopamine (DA) neurons in the substantia nigra (SN) greater than in the ventral tegmental area (VTA). The simplest explanation for selective DA neuron loss in PD is that DA is toxic and, because only DA neurons contain significant amounts of DA, this highly localized synthesis of DOPAL accounts for selective vulnerability of DA neurons. However, the large concentrations of DA required to produce in vivo toxicity cast doubt on its role in PD pathogenesis. Alpha-synuclein (alpha-syn) is the major component of the Lewy body, the pathological marker of PD, and is genetically linked to the disease. Recent studies indicate that alpha-syn neurotoxicity is mediated by a free radical generating metabolite of DA. Here we test the hypothesis that 3,4-dihydroxyphenylacetaldehyde (DOPAL), the monamine oxidase metabolite of DA, mediates DA toxicity in vivo. We injected DOPAL, DA and its oxidative, reduced and methylated metabolites into rat SN and VTA. Five days post-surgery, the injection sites were evaluated in Nissl preparations and with tyrosine hydroxylase (for DA neurons), neuronal nuclear antigen (for neurons) and glial fibrillary acidic protein (for astrocytes) immunoreactivities. Lesion size in SN vs. VTA was compared using morphometry. DOPAL at concentrations as low as 100 ng was toxic to DA SN neurons>DA VTA neurons>glia. Neither DA nor its other metabolites showed evidence of neurotoxicity at fivefold higher doses. However, 20 microg of DA produced lesions in the SN and VTA. We conclude that DOPAL is the toxic DA metabolite in vivo. Implications for a unified hypothesis for PD pathogenesis are discussed.

Effects of neonatal amygdala lesions on [125I] neurotensin binding in specific brain areas of adult rat

Neurotensin has been implicated in the pathophysiology of schizophrenia. The neonatal amygdala lesion in rat has been proposed to be a neurodevelopmental model for some aspects of schizophrenia. [125I] Neurotensin binding was assessed in adult rats using in vitro autoradiography following a lesion of the basolateral amygdala at postnatal day 7 (Pd 7) or postnatal day 21 (Pd 21). The Pd 7 and Pd 21 lesions differentially affected neurotensin receptor densities in the hippocampal complex and (less pronounced) in the dopaminergic cell regions, implying a neurodevelopmental cause. These results may be of relevance for the involvement of neurotensin in the pathogenesis of schizophrenia.

Anatomical substrates of orexin-dopamine interactions: lateral hypothalamic projections to the ventral tegmental area

Dopaminergic projections to the forebrain arising from the mesencephalic ventral tegmentum modulate information processing in cortical and limbic sites. The lateral hypothalamus is crucial for the coordination of behavioral responses to interoceptive cues. The presence of a hypothalamic input to the ventral tegmental area has been known for some time, but the organization of this pathway has received little attention. Among the neuropeptides found in the hypothalamus are the orexins, which are selectively expressed in the lateral hypothalamus and adjacent perifornical area and are critically involved in homeostatic regulatory processes, including arousal and feeding. We examined the anatomical relationships between orexin and dopamine neurons in rats, with particular attention to characterizing the lateral hypothalamic projection to midbrain dopamine neurons. Iontophoretic deposits of the retrograde tracer FluoroGold into the ventral tegmental area revealed a large number of retrogradely-labeled cells that formed a band extending from the medial perifornical area arching dorsally over the fornix and then ventrolaterally into the lateral hypothalamus; approximately 20% of these cells expressed orexin A-like immunoreactivity. Moreover, axons that were anterogradely labeled from the lateral hypothalamus were seen throughout the ventral tegmental area, and were often in close proximity to the dendrites and somata of dopamine neurons. Dopamine and orexin fibers were found to codistribute in the medial prefrontal cortex; orexin fibers were present in lower density in the medial shell of the nucleus accumbens, and the central and posterior basolateral nuclei of the amygdala. We conclude that the lateral hypothalamic/perifornical projection represents an anatomical substrate by which interoceptive-related signals may influence forebrain dopamine function.

Cocaine- and amphetamine-regulated transcript peptide projections in the ventral midbrain: colocalization with gamma-aminobutyric acid, melanin-concentrating hormone, dynorphin, and synaptic interactions with dopamine neurons

To date, cocaine- and amphetamine-regulated transcript (CART) peptides have been found to influence feeding, locomotor activity, and conditioned place preference. A common brain structure that could mediate these effects is the ventral tegmental area (VTA). For a better understanding of the anatomical substrates that might underlie CART peptides' role in these behaviors, we performed a series of experiments to elucidate the source, synaptic connectivity, and neurochemical content of CART peptide-immunoreactive (CARTir) terminals in the rat VTA. Double-labeling immunofluorescence revealed that approximately 15% of CARTir terminals in the VTA contain the hypothalamic neuropeptide, melanin-concentrating hormone (MCH). Furthermore, CART peptides were also found to colocalize with GABA and, to a small extent, with dynorphin in nerve terminals in both the VTA and the substantia nigra (SN). In the VTA, CARTir terminals form both symmetric and asymmetric synapses onto dopaminergic and nondopaminergic distal dendrites, suggesting that various sources contribute to this innervation. About 30% of CARTir terminals in the VTA and only 15% in the SN appose or form synaptic contact with DA neurons, which support our previous data showing that GABAergic basal ganglia output neurons in the substantia nigra pars reticulata (SNr) receive strong CARTir input from the accumbens core. Results of these studies suggest that the most significant behavioral states influenced by CART peptides, feeding and locomotion, may be mediated by direct and/or indirect modulation of VTA dopaminergic neuronal activity.