Mesocortical Projections Research Articles

Functional diversity of dopamine axons in prefrontal cortex during classical conditioning

Midbrain dopamine neurons impact neural processing in the prefrontal cortex (PFC) through mesocortical projections. However, the signals conveyed by dopamine projections to the PFC remain unclear, particularly at the single-axon level. Here, we investigated dopaminergic axonal activity in the medial PFC (mPFC) during reward and aversive processing. By optimizing microprism-mediated two-photon calcium imaging of dopamine axon terminals, we found diverse activity in dopamine axons responsive to both reward and aversive stimuli. Some axons exhibited a preference for reward, while others favored aversive stimuli, and there was a strong bias for the latter at the population level. Long-term longitudinal imaging revealed that the preference was maintained in reward- and aversive-preferring axons throughout classical conditioning in which rewarding and aversive stimuli were paired with preceding auditory cues. However, as mice learned to discriminate reward or aversive cues, a cue activity preference gradually developed only in aversive-preferring axons. We inferred the trial-by-trial cue discrimination based on machine learning using anticipatory licking or facial expressions, and found that successful discrimination was accompanied by sharper selectivity for the aversive cue in aversive-preferring axons. Our findings indicate that a group of mesocortical dopamine axons encodes aversive-related signals, which are modulated by both classical conditioning across days and trial-by-trial discrimination within a day.

Functional diversity of dopamine axons in prefrontal cortex during classical conditioning.

Midbrain dopamine neurons impact neural processing in the prefrontal cortex (PFC) through mesocortical projections. However, the signals conveyed by dopamine projections to the PFC remain unclear, particularly at the single-axon level. Here, we investigated dopaminergic axonal activity in the medial PFC (mPFC) during reward and aversive processing. By optimizing microprism-mediated two-photon calcium imaging of dopamine axon terminals, we found diverse activity in dopamine axons responsive to both reward and aversive stimuli. Some axons exhibited a preference for reward, while others favored aversive stimuli, and there was a strong bias for the latter at the population level. Long-term longitudinal imaging revealed that the preference was maintained in reward- and aversive-preferring axons throughout classical conditioning in which rewarding and aversive stimuli were paired with preceding auditory cues. However, as mice learned to discriminate reward or aversive cues, a cue activity preference gradually developed only in aversive-preferring axons. We inferred the trial-by-trial cue discrimination based on machine learning using anticipatory licking or facial expressions, and found that successful discrimination was accompanied by sharper selectivity for the aversive cue in aversive-preferring axons. Our findings indicate that a group of mesocortical dopamine axons encodes aversive-related signals, which are modulated by both classical conditioning across days and trial-by-trial discrimination within a day.

Stress-induced alterations of mesocortical and mesolimbic dopaminergic pathways

Our ability to develop the cognitive strategies required to deal with daily-life stress is regulated by region-specific neuronal networks. Experimental evidence suggests that prolonged stress in mice induces depressive-like behaviors via morphological, functional and molecular changes affecting the mesolimbic and mesocortical dopaminergic pathways. Yet, the molecular interactions underlying these changes are still poorly understood, and whether they affect males and females similarly is unknown. Here, we used chronic social defeat stress (CSDS) to induce depressive-like behaviors in male and female mice. Density of the mesolimbic and mesocortical projections was assessed via immuno-histochemistry combined with Sholl analysis along with the staining of activity-dependent markers pERK and c-fos in the ventral tegmental area (VTA), nucleus accumbens (NAc) and medial prefrontal cortex (mPFC). Our results show that social stress decreases the density of TH+ dopaminergic axonal projections in the deep layers of the mPFC in susceptible but not resilient male and female mice. Consistently, our analyses suggest that pERK expression is decreased in the mPFC but increased in the NAc following CSDS in males and females, with no change in c-fos expression in both sexes. Overall, our findings indicate that social defeat stress impacts the mesolimbic and mesocortical pathways by altering the molecular interactions regulating somatic and axonal plasticity in males and females.

Dopamine Axon Targeting in the Nucleus Accumbens in Adolescence Requires Netrin-1

The fine arrangement of neuronal connectivity during development involves the coordinated action of guidance cues and their receptors. In adolescence, the dopamine circuitry is still developing, with mesolimbic dopamine axons undergoing target-recognition events in the nucleus accumbens (NAcc), while mesocortical projections continue to grow toward the prefrontal cortex (PFC) until adulthood. This segregation of mesolimbic versus mesocortical dopamine pathways is mediated by the guidance cue receptor DCC, which signals dopamine axons intended to innervate the NAcc to recognize this region as their final target. Whether DCC-dependent mesolimbic dopamine axon targeting in adolescence requires the action of its ligand, Netrin-1, is unknown. Here we combined shRNA strategies, quantitative analysis of pre- and post-synaptic markers of neuronal connectivity, and pharmacological manipulations to address this question. Similar to DCC levels in the ventral tegmental area, Netrin-1 expression in the NAcc is dynamic across postnatal life, transitioning from high to low expression across adolescence. Silencing Netrin-1 in the NAcc in adolescence results in an increase in the expanse of the dopamine input to the PFC in adulthood, with a corresponding increase in the number of presynaptic dopamine sites. This manipulation also results in altered dendritic spine density and morphology of medium spiny neurons in the NAcc in adulthood and in reduced sensitivity to the behavioral activating effects of the stimulant drug of abuse, amphetamine. These cellular and behavioral effects mirror those induced by Dcc haploinsufficiency within dopamine neurons in adolescence. Dopamine targeting in adolescence requires the complementary interaction between DCC receptors in mesolimbic dopamine axons and Netrin-1 in the NAcc. Factors regulating either DCC or Netrin-1 in adolescence can disrupt mesocorticolimbic dopamine development, rendering vulnerability or protection to phenotypes associated with psychiatric disorders.

Depressive-Like Behaviors Are Regulated by NOX1/NADPH Oxidase by Redox Modification of NMDA Receptor 1.

Involvement of reactive oxygen species (ROS) has been suggested in the development of psychiatric disorders. NOX1 is a nonphagocytic form of NADPH oxidase whose expression in the nervous system is negligible compared with other NOX isoforms. However, NOX1-derived ROS increase inflammatory pain and tolerance to opioid analgesia. To clarify the role of NOX1 in the brain, we examined depressive-like behaviors in mice deficient in Nox1 (Nox1-/Y). Depressive-like behaviors induced by chronic social defeat stress or administration of corticosterone (CORT) were significantly ameliorated in Nox1-/Y Generation of ROS was significantly elevated in the prefrontal cortex (PFC) of mice administrated with CORT, while NOX1 mRNA was upregulated only in the ventral tegmental area (VTA) among brain areas responsible for emotional behaviors. Delivery of miRNA against NOX1 to VTA restored CORT-induced depressive-like behaviors in wild-type (WT) littermates. Administration of CORT to WT, but not to Nox1-/Y, significantly reduced transcript levels of brain-derived neurotrophic factor (bdnf), with a concomitant increase in DNA methylation of the promoter regions in bdnf Delivery of miRNA against NOX1 to VTA restored the level of BDNF mRNA in WT PFC. Redox proteome analyses demonstrated that NMDA receptor 1 (NR1) was among the molecules redox regulated by NOX1. In cultured cortical neurons, hydrogen peroxide significantly suppressed NMDA-induced upregulation of BDNF transcripts in NR1-expressing cells but not in cells harboring mutant NR1 (C744A). Together, these findings suggest a key role of NOX1 in depressive-like behaviors through NR1-mediated epigenetic modification of bdnf in the mesoprefrontal projection.SIGNIFICANCE STATEMENT NADPH oxidase is a source of reactive oxygen species (ROS) that have been implicated in the pathogenesis of various neurological disorders. We presently showed the involvement of a nonphagocytic type of NADPH oxidase, NOX1, in major depressive disorders, including behavioral, biochemical, and anatomical changes in mice. The oxidation of NR1 by NOX1-derived ROS was demonstrated in prefrontal cortex (PFC), which may be causally linked to the downregulation of BDNF, promoting depressive-like behaviors. Given that NOX1 is upregulated only in VTA but not in PFC, mesocortical projections appear to play a crucial role in NOX1-dependent depressive-like behaviors. Our study is the first to present the potential molecular mechanism underlying the development of major depression through the NOX1-induced oxidation of NR1 and epigenetic modification of bdnf.

Simultaneous Detection of c-Fos Activation from Mesolimbic and Mesocortical Dopamine Reward Sites Following Naive Sugar and Fat Ingestion in Rats.

This study uses cellular c-fos activation to assess effects of novel ingestion of fat and sugar on brain dopamine (DA) pathways in rats. Intakes of sugars and fats are mediated by their innate attractions as well as learned preferences. Brain dopamine, especially meso-limbic and meso-cortical projections from the ventral tegmental area (VTA), has been implicated in both of these unlearned and learned responses. The concept of distributed brain networks, wherein several sites and transmitter/peptide systems interact, has been proposed to mediate palatable food intake, but there is limited evidence empirically demonstrating such actions. Thus, sugar intake elicits DA release and increases c-fos-like immunoreactivity (FLI) from individual VTA DA projection zones including the nucleus accumbens (NAC), amygdala (AMY) and medial prefrontal cortex (mPFC) as well as the dorsal striatum. Further, central administration of selective DA receptor antagonists into these sites differentially reduce acquisition and expression of conditioned flavor preferences elicited by sugars or fats. One approach by which to determine whether these sites interacted as a distributed brain network in response to sugar or fat intake would be to simultaneous evaluate whether the VTA and its major mesotelencephalic DA projection zones (prelimbic and infralimbic mPFC, core and shell of the NAc, basolateral and central-cortico-medial AMY) as well as the dorsal striatum would display coordinated and simultaneous FLI activation after oral, unconditioned intake of corn oil (3.5%), glucose (8%), fructose (8%) and saccharin (0.2%) solutions. This approach is a successful first step in identifying the feasibility of using cellular c-fos activation simultaneously across relevant brain sites to study reward-related learning in ingestion of palatable food in rodents.

Dopaminergic Meso-Cortical Projections to M1: Role in Motor Learning and Motor Cortex Plasticity

Although the architecture of a dopaminergic (DA) system within the primary motor cortex (M1) was well characterized anatomically, its functional significance remained obscure for a long time. Recent studies in rats revealed that the integrity of DA fibers in M1 is a prerequisite for successful acquisition of motor skills. This essential contribution of DA for motor learning is plausible as it modulates M1 circuitry at multiple levels thereby promoting plastic changes that are required for information storage: at the network level, DA increases cortical excitability and enhances the stability of motor maps. At the cellular level, DA induces the expression of learning-related genes via the transcription factor c-Fos. At the level of synapses, DA is required for the formation of long-term potentiation, a mechanism that likely is a fingerprint of a motor memory trace within M1. DA fibers innervating M1 originate within the midbrain, precisely the ventral tegmental area (VTA) and the medial portion of substantia nigra (SN). Thus, they could be part of the meso-cortico-limbic pathway – a network that provides information about saliency and motivational value of an external stimulus and is commonly referred as “reward system.” However, the behavioral triggers of the release of dopamine in M1 are not yet identified. As alterations in DA transmission within M1 occur under various pathological conditions such as Parkinson disease or ischemic and traumatic brain injury, a deeper understanding of the interaction of VTA/SN and M1 may reveal a deeper insight into a large spectrum of neurological disorders.

GL.01 Dopamine in health and disease

The original synthesis of dopamine in the laboratories of H.E. Dale in South London, followed some 50 years later by the seminal mapping of the mesencephalic dopamine (DA) pathways into...

The Glutamatergic Component of the Mesocortical Pathway Emanating from Different Subregions of the Ventral Midbrain

The mesocortical pathway projecting from the ventral tegmental area (VTA) to the prefrontal cortex (PFC) plays a critical role in a number of cognitive and emotional processes. While this pathway has been traditionally viewed as dopaminergic, recent data indicate that a considerable proportion of rostromedial VTA neurons possess markers for glutamate transmission. However, the relative density of the glutamatergic projection to the PFC from these rostromedial regions is unknown. In the present study, anterograde tracer injections into 4 ventral midbrain subregions were coupled with immunohistochemical analysis of labeled axons in PFC for markers of dopamine (DA; tyrosine hydroxylase [TH]) and glutamate (vesicular glutamate transporter 2; VGLUT2). We found that while tracer injections into the interfascicular nucleus produced labeled fibers in the PFC that were mainly TH positive, tracer injections into the rostral linear nucleus, rostral VTA, and parabrachial pigmented nucleus produced labeled fibers in PFC that contained mainly VGLUT2-positive rather than TH-positive varicosities. When viewed in the light of the previously documented strong γ-aminobutyric acidergic component, it would seem that the rostromedial mesocortical projection is actually an amino acid pathway that in addition has a DA component.

Dementia in Lewy body syndromes

It is well-known that Lewy body Parkinson disease (PD) gives rise to a motor disorder; that this is associated with degeneration of meso-striatal dopaminergic neurons; and that meso-striatal dopaminergic neurons interact with frontal-subcortical loops. When one turns to the dementia of PD (PDD) or that of dementia with Lewy bodies (DLB)—2 syndromes that are presently only distinguished by an arbitrary cutoff in the timing of motor compared to cognitive dimensions, but are quite possibly manifestations of the same pathologic process—the heart, for many, has often led the mind in assuming that cognitive decline must also relate to dysfunction of (nonmotor) frontal-subcortical loops or dopaminergic degeneration of meso-limbic or meso-cortical projections. Aside from possible prejudice in this assumption, there may be notable methodologic problems in being too focused on a system-specific hypothesis for cognitive decline in a neurodegenerative disease. Focusing exclusively on imaging the dopaminergic system in PDD/DLB, for instance, and finding correlations with behavior could indicate a causal relationship, but if imaging and behavioral change are each a function of disease duration or severity of …

Fate of (D-Ala2)-deltorphin-I-like immunoreactive neurons in 6-hydroxydopamine lesioned rat brain.

The use of a polyclonal antiserum specific to C-terminal tetrapeptide amide of (D-Ala2)deltorphin-I, a naturally occurring amphibian skin opioid peptide, has already demonstrated the presence of immunoreactive neurons in rat midbrain. Double immunostaining identified these neurons as a subpopulation of the mesencephalic dopaminergic neurons that were also tyrosine hydroxylase-immunopositive and calbindin-D28kD- negative, namely, the neurons predominantly affected in Parkinson disease. We followed the fate of these neurons after a monolateral injection of 6-hydroxy-dopamine into rat brain. Almost all the immunopositive neurons and their nigrostriatal, mesolimbic and mesocortical projections on the side ipsilateral to the lesion disappeared. Only a few scattered immunopositive neurons within the substantia nigra, pars compacta, and those of supramammillary nucleus remained unaffected. The consistent overlap of dopamine and this new molecule provides a further key to identifying the mammalian counterpart of these amphibian skin opioid peptides.

The neurokinin-3 (NK3) and the neurokinin-1 (NK1) receptors are differentially targeted to mesocortical and mesolimbic projection neurons and to neuronal nuclei in the rat ventral tegmental area.

Tonic activation of neurokinin-3 (NK(3)) receptors in dopamine neurons of the ventral tegmental area (VTA) has been implicated in the pathophysiology of schizophrenia. This psychiatric disorder is associated with a dysfunctional activity in VTA projection neurons that can affect cognitive function at the level of the medial prefrontal cortex (mPFC) as well as motor and motivational states controlled in part by mesolimbic output to the nucleus accumbens (Acb). To determine the relevant sites for NK(3) receptor activation within this neuronal network, we used confocal and electron microscopy to examine NK(3) receptors (Cy5; immunogold) and retrograde labeling of fluorogold (FG, FITC; immunoperoxidase) in the VTA of rats receiving either Acb or mPFC injections of FG. Comparison was made with neurokinin-1 (NK(1)) receptors, which are also present, but less abundant then NK(3) receptors, in dopaminergic and GABAergic VTA neurons. There were no observable differences between NK(3) and NK(1) receptors in their primary locations in the cytoplasm and on the plasma membrane of VTA somata and dendrites with or without FG. Dendrites labeled with FG retrogradely transported from mPFC, however, contained more NK(3) or less NK(1) immunogold particles (plasmalemmal + cytoplasmic) then those retrogradely labeled following FG injection in the Acb. Moreover, only the NK(3) receptors were detected in neuronal nuclei in the VTA and in the nuclei of human HEK-293T NK(3)-transfected cells. The enrichment of NK(3) receptors in mesocortical projection neurons and nuclear distribution of these receptors may provide insight for understanding the selective antipsychotic effectiveness of NK(3) antagonists.

Positive and Negative Symptoms in Schizophrenia: The NMDA Receptor Hypofunction Hypothesis, Neuregulin/ErbB4 and Synapse Regression

Carlsson has put forward the hypothesis that the positive and negative symptoms of schizophrenia are due to failure of mesolimbic and mesocortical projections consequent on hypofunction of the glutamate N-methyl-d-aspartate (NMDA) receptor. The hypothesis has been recently emphasized in this Journal that the loss of synaptic spines with NMDA receptors, which can be precipitated by stress, can explain the emergence of positive symptoms such as hallucinations and that this synapse regression involves molecules such as neuregulin and its receptor ErbB4 that have been implicated in schizophrenia. In this essay these two hypotheses are brought together in a single scheme in which emphasis is placed on the molecular pathways from neuregulin/ErbB4, to modulation of the NMDA receptors, subsequent changes in the synaptic spine's cytoskeletal apparatus and so regression of the spines. It is suggested that identification of the molecular constituents of this pathway will allow synthesis of suitable substances for removing the hypofunction of NMDA receptors and so the phenotypic consequences that flow from this hypofunction.

Region and Sex Differences in Constituent Dopamine Neurons and Immunoreactivity for Intracellular Estrogen and Androgen Receptors in Mesocortical Projections in Rats

Many cortical and prefrontal functions show sex differences in their development, adult capacity, and dysfunction in disorders like schizophrenia. Correlations between circulating gonadal hormones and certain prefrontal functions have also been identified in humans and experimental animal models. Although multiple mechanisms may be involved, such hormone sensitivities/sex differences could be related to gonadal steroid actions on another regulator of cortical/prefrontal cortical function, the mesocortical dopamine system. Thus, although it is well known that perturbations in prefrontal dopamine signaling induce behavioral deficits, it is also known that several endpoints of these afferents are sensitive to gonadal steroids and/or are sexually dimorphic. This study explored possible substrates for this in two ways: by comparing the distributions of immunoreactivity for intracellular estrogen (alpha and beta) and androgen receptors among retrogradely labeled dopaminergic and nondopaminergic mesocortical neurons projecting to prefrontal, premotor, and primary motor cortices, areas in which male rat dopamine axons are differentially hormone-sensitive; and by comparing anatomical data in males and females. These analyses revealed region-, cell-, and sex-specific specializations in receptor localization that paralleled established patterns of mesocortical hormone sensitivity, including the androgen sensitivity of dopamine axons and dopamine-dependent functions in prefrontal cortex. It was also found that the proportions of dopamine neurons making up mesocortical projections were approximately 30% in males, whereas in females, significantly more constituent cells were dopaminergic. Together, these features may be part of the neurobiology giving mesocortical afferents their hormone sensitivities and/or sex differences in physiology, function, and dysfunction in disease.

The hippocampal-SN/VTA loop in functional imaging: insights into the motivational aspects of novelty

Event Abstract Back to Event The hippocampal-SN/VTA loop in functional imaging: insights into the motivational aspects of novelty E. Duzel1, 2* 1 University College London, United Kingdom 2 Otto-von-Guericke University Magdeburg, Germany Novelty provides a fundamental learning signal that motivates goal-directed exploratory behavior and promotes encoding and plasticity in long-term declarative memory. The goal of this talk is to illuminate how these effects of novelty are linked to functional properties that it shares with reward-processing within dopaminergic circuitry. Functional magnetic resonance imaging data from our lab show a number of commonalities between novelty and reward; (i) both activate the SN/VTA, (ii) in both cases this activation can be elicited already by anticipation, (iii) both share common coding principles (such as adaptive coding) within mesolimbic and mesocortical projection regions of the SN/VTA. However, there are also striking functional differences. For instance, unlike rewards, SN/VTA activation by novelty is correlated with the personality trait score of novelty-seeking but not reward-dependence. A direct assessment of the functional interaction of novelty and reward suggests that novelty may act as a motivational signal for exploration in the search for rewards rather than being rewarding itself. Motivational effects of novelty on exploration are also evident in its contextual effects: familiar stimuli that occur in the temporal context of novel events remain more salient learning signals than familiar stimuli that occur in a context without novel stimuli. I will integrate these observations to suggest that the hippocampus interacts with the SN/VTA in novelty-processing to contextually motivate exploratory behavior and that this exploratory drive is associated with improved declarative memory formation even though novelty itself is not rewarding. Conference: 10th International Conference on Cognitive Neuroscience, Bodrum, Türkiye, 1 Sep - 5 Sep, 2008. Presentation Type: Oral Presentation Topic: Symposium 8: Dopaminergic modulation of memory and cognitive control Citation: Duzel E (2008). The hippocampal-SN/VTA loop in functional imaging: insights into the motivational aspects of novelty. Conference Abstract: 10th International Conference on Cognitive Neuroscience. doi: 10.3389/conf.neuro.09.2009.01.038 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Nov 2008; Published Online: 27 Nov 2008. * Correspondence: E. Duzel, University College London, London, United Kingdom, Emrah.Duezel@dzne.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers E. Duzel Google E. Duzel Google Scholar E. Duzel PubMed E. Duzel Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Synaptic Characteristics Between Cortical Cells in the Rat Prefrontal Cortex and Axon Terminals from the Ventral Tegmental Area that Utilize Different Neurotransmitters

Projections from the ventral tegmental area (VTA) have been demonstrated to terminate in the prefrontal cortex (PFC) and to be dopaminergic and/or γ-aminobutyric acidergic (GABAergic), forming a neural circuit implicated in certain memory and cognitive processes. However, it has not been determined whether γ-aminobutyric acid (GABA) and dopamine (DA) are localized in certain types of axon terminals from the VTA to the PFC. To determine the synaptic characteristics made by postsynaptic prefrontal cortical structures and mesoprefrontal fibers utilizing either GABA or DA, we performed a double-labeling method for electron microscopy, in which we combined peroxidase markers for anterograde tract-tracing with postembedding immunogold labeling for tyrosine hydroxylase, DA, and GABAin rats. The anterograde tract-tracing studies showed that tegmentocortical fibers from the VTA terminated as both symmetric and asymmetric axon terminals with the predominantly symmetric synaptic type in the prelimbic cortex of the rat. Furthermore, a study using the combination of anterograde tract-tracing and postembedding immunocytochemistry indicated that tegmentocortical axon terminals forming symmetric synapses were either GABAergic or dopaminergic, whereas a small fraction of tegmentocortical terminals ending as asymmetric synapses were not immunopositive for DA or GABA. These findings indicate that the mesocortical projections to the PFC exert an inhibitory effect on the spontaneous activity of PFC cells via symmetric synapses that use DA and GABA as neurotransmitters and that these projections also have as yet unknown effects via asymmetric synapses using other neurotransmitters.

Functional Anatomy of Humor: Positive Affect and Chronic Mental Illness

Functional Anatomy of Humor: Positive Affect and Chronic Mental Illness

Dopamine innervation of the monkey mediodorsal thalamus: Location of projection neurons and ultrastructural characteristics of axon terminals

Dopamine (DA) axons and receptors have recently been identified in the primate thalamus, including the mediodorsal thalamic nucleus (MD). In order to determine whether the DA innervation of the primate MD shares the anatomical features of the mesocortical or nigrostriatal DA projections, we performed tract-tracing and immunocytochemistry studies in macaque monkeys ( Macaca fascicularis) to identify the location of the DA neurons that project to MD and immuno-electron microscopy to determine the distribution of the dopamine transporter (DAT) in axons within the MD. Similar to the mesocortical projection, retrogradely-labeled, tyrosine hydroxylase-containing neurons were present in dorsal tier ventral mesencephalic nuclei, such as the ventral tegmental area and the dorsal portion of the substantia nigra pars compacta. In contrast, no dual-labeled neurons were present in the ventral tier nuclei, the primary origin of the nigrostriatal DA pathway. In addition, like the DA projection to the prefrontal cortex, DAT immunoreactivity was predominantly localized to the pre-terminal portion of axons in the MD, and was infrequently found in association with synaptic vesicles, in contrast to nigrostriatal DA axons. These findings indicate that the DA projection to the MD shares anatomical features with the mesocortical DA system, suggesting that the functional properties of DA neurotransmission in the MD might be more similar to those in the cortex than in the striatum.

Neurodevelopment and Schizophrenia

The evidence of neuroleptics' antipsychotic action due to their antagonism of dopamine receptors led to the hyperdopaminergic theory of schizophrenia. But the functional disturbance of the dopamine neurons should explain the increase and the reduction of some symptoms at the same time through the D2 receptors blockade. The prodromical manifestations of schizophrenia include abnormal movements which precede and predict the beginning of the disease. They also suggest a dopamine deficit. An injury in the mesocortical dopamine projection during the neural development could result in a functional impairment of the prefrontal cortex which causes the cognitive deficit symptoms. It would result also in an excessive compensatory subcortical dopamine activity which would cause the psychotic symptoms. The evidence in behalf of this physiopathology is complex. The mechanism of the subcortical compensation of the prefrontal dopamine deficit remains unknown. It is necessary to find out which is the exact mechanism of the injury that causes the mesocortical projection loss in the schizophrenia. We have suggested that the injury would depend on the excitotoxicity induced by the perinatal hypoxia which would provoke the selective death of a dopamine neurons subset which are very sensitive during the critical period of the mesencephalic development.

Ultrastructural localization of Leu5-enkephalin immunoreactivity in mesocortical neurons and their input terminals in rat ventral tegmental area.

Enkephalin (ENK) immunoreactivity is widely distributed in the ventral tegmental area (VTA), where endogenous ENK and dynorphin opioid peptides are known to have opposing actions in reward, stress, cognition, and fear-related behaviors. Many neurons in the VTA give rise to mesocortical projections terminating in the medial prefrontal cortex (mPFC), and these projections have been implicated to varying extents in all these functions. To determine whether there is a synaptic basis for ENK and/or dynorphin modulation of cortically projecting neurons within the VTA, we combined retrograde tract-tracing from the mPFC with dual immunocytochemical-labeling electron microscopy in the rat VTA. The retrograde tracer Fluorogold (FG) was microinjected into mPFC. At optimal survival periods, sections through the VTA were processed for immunolabeling of anti-FG and a Leu(5)-ENK antibody recognizing both ENK and dynorphin peptides. Over 26% of the retrogradely labeled neuronal somatodendritic profiles (n = 177) were contacted by ENK-immunoreactive axonal profiles including small axons and axon terminals. The axon terminals varied in their subcellular distribution of ENK immunoreactivity and also differed in forming either inhibitory-type (symmetric) or excitatory-type (asymmetric) synapses. Many of the axonal profiles also were apposed to FG-labeled somata or dendrites without forming recognizable synapses. Approximately one-third of the mesocortical neuronal perikarya also showed sparsely distributed somatodendritic ENK-immunoreactivity. Our results provide ultrastructural evidence that ENK and possibly dynorphin in the rat VTA have distributions consistent with involvement in diverse physiological actions affecting the output of mesocortical neurons, some of which also contain one or both peptides.