Chapter VI Dopamine systems in the primate brain

The relationship between metabolic control and basal ganglia morphometry and function in individuals with early-treated phenylketonuria

Abnormalities of the cortical white matter are the most prominent and widely-reported neurological findings in individuals with early-treated phenylketonuria (ETPKU). Much less is known regarding the effects of ETPKU on gray matter structures in the brain such as the basal ganglia. Previous findings on basal ganglia in ETPKU have been mixed. The current study was designed to further elucidate the effects of ETPKU and elevated phe levels on the morphometry of basal ganglia structures (i.e., putamen, caudate nucleus, nucleus accumbens, and globus pallidus). High resolution magnetic resonance imaging (MRI) data was collected from a sample of 37 adults with ETPKU and a demographically-matched comparison group of 33 individuals without PKU. No overall group differences (ETPKU vs. non-PKU) in basal ganglia volumes were observed. However, within the ETPKU group, poorer metabolic control (as reflected by higher blood phenylalanine levels) was associated with larger putamen volume. Vertex-wise shape analysis revealed that the volume increase was accompanied by shape changes in the middle left putamen. Consistent with this area's role in motor control, a significant correlation between left putamen volume and motor performance was also observed. Additional research is needed to fully understand the cellular level processes underlying this effect as well as to better understand the clinical impact of these morphometric changes and their potential relation to treatment response.

Defining the interconnectivity of the medial prefrontal cortex and ventral midbrain

Dysfunction in dopamine (DA) signaling contributes to neurological disorders ranging from drug addiction and schizophrenia to depression and Parkinson’s Disease. How might impairment of one neurotransmitter come to effect these seemingly disparate diseases? One potential explanation is that unique populations of DA-releasing cells project to separate brain regions that contribute to different sets of behaviors. Though dopaminergic cells themselves are spatially restricted to the midbrain and constitute a relatively small proportion of all neurons, their projections influence many brain regions. DA is particularly critical for the activity and function of medial prefrontal cortical (mPFC) ensembles. The midbrain and mPFC exhibit reciprocal connectivity – the former innervates the mPFC, and in turn, the mPFC projects back to the midbrain. Viral mapping studies have helped elucidate the connectivity within and between these regions, which likely have broad implications for DA-dependent behaviors. In this review, we discuss advancements in our understanding of the connectivity between the mPFC and midbrain DA system, focusing primarily on rodent models.

The VTA dopaminergic system as diagnostic and therapeutical target for Alzheimer's disease

Neuropsychiatric symptoms (NPS) occur in nearly all patients with Alzheimer's Disease (AD). Most frequently they appear since the mild cognitive impairment (MCI) stage preceding clinical AD, and have a prognostic importance. Unfortunately, these symptoms also worsen the daily functioning of patients, increase caregiver stress and accelerate the disease progression from MCI to AD. Apathy and depression are the most common of these NPS, and much attention has been given in recent years to understand the biological mechanisms related to their appearance in AD. Although for many decades these symptoms have been known to be related to abnormalities of the dopaminergic ventral tegmental area (VTA), a direct association between deficits in the VTA and NPS in AD has never been investigated. Fortunately, this scenario is changing since recent studies using preclinical models of AD, and clinical studies in MCI and AD patients demonstrated a number of functional, structural and metabolic alterations affecting the VTA dopaminergic neurons and their mesocorticolimbic targets. These findings appear early, since the MCI stage, and seem to correlate with the appearance of NPS. Here, we provide an overview of the recent evidence directly linking the dopaminergic VTA with NPS in AD and propose a setting in which the precocious identification of dopaminergic deficits can be a helpful biomarker for early diagnosis. In this scenario, treatments of patients with dopaminergic drugs might slow down the disease progression and delay the impairment of daily living activities.

Dopamine Supersensitivity: A Novel Hypothesis of Opioid-Induced Neurobiological Mechanisms Underlying Opioid-Stimulant Co-use and Opioid Relapse

Emergent harms presented by the co-use of opioids and methamphetamine highlight the broader public health challenge of preventing and treating opioid and stimulant co-use. Development of effective therapeutics requires an understanding of the physiological mechanisms that may be driving co-use patterns, specifically the underlying neurobiology of co-use and how they may facilitate (or be leveraged to prevent) continued use patterns. This narrative review summarizes largely preclinical data that demonstrate clinically-meaningful relationships between the dopamine and opioid systems with direct implications for opioid and stimulant co-use. Synthesized conclusions of this body of research include evidence that changes in the dopamine system occur only once physical dependence to opioids develops, that the chronicity of opioid exposure is associated with the severity of changes, and that withdrawal leaves the organism in a state of substantive dopamine deficit that persists long after the somatic or observed signs of opioid withdrawal appear to have resolved. Evidence also suggests that dopamine supersensitivity develops soon after opioid abstinence and results in increased response to dopamine agonists that increases in magnitude as the abstinence period continues and is evident several weeks into protracted withdrawal. Mechanistically, this supersensitivity appears to be mediated by changes in the sensitivity, not quantity, of dopamine D2 receptors. Here we propose a neural circuit mechanism unique to withdrawal from opioid use with implications for increased stimulant sensitivity in previously stimulant-naïve or inexperienced populations. These hypothesized effects collectively delineate a mechanism by which stimulants would be uniquely reinforcing to persons with opioid physical dependence, would contribute to the acute opioid withdrawal syndrome, and could manifest subjectively as craving and/or motivation to use that could prompt opioid relapse during acute and protracted withdrawal. Preclinical research is needed to directly test these hypothesized mechanisms. Human laboratory and clinical trial research is needed to explore these clinical predictions and to advance the goal of developing treatments for opioid-stimulant co-use and/or opioid relapse prevention and withdrawal remediation.

Computational benefits of structural plasticity, illustrated in songbirds

The plasticity of nervous systems allows animals to quickly adapt to a changing environment. In particular, the structural plasticity of brain networks is often critical to the development of the central nervous system and the acquisition of complex behaviors. As an example, structural plasticity is central to the development of song-related brain circuits and may be critical for song acquisition in juvenile songbirds. Here, we review current evidences for structural plasticity and their significance from a computational point of view. We start by reviewing evidence for structural plasticity across species and categorizing them along the spatial axes as well as the along the time course during development. We introduce the vocal learning circuitry in zebra finches, as a useful example of structural plasticity, and use this specific case to explore the possible contributions of structural plasticity to computational models. Finally, we discuss current modeling studies incorporating structural plasticity and unexplored questions which are raised by such models.

Dopamine, vocalization, and astrocytes

Dopamine, the main catecholamine neurotransmitter in the brain, is predominately produced in the basal ganglia and released to various brain regions including the frontal cortex, midbrain and brainstem. Dopamine's effects are widespread and include modulation of a number of voluntary and innate behaviors. Vigilant regulation and modulation of dopamine levels throughout the brain is imperative for proper execution of motor behaviors, in particular speech and other types of vocalizations. While dopamine's role in motor circuitry is widely accepted, its unique function in normal and abnormal speech production is not fully understood. In this perspective, we first review the role of dopaminergic circuits in vocal production. We then discuss and propose the conceivable involvement of astrocytes, the numerous star-shaped glia cells of the brain, in the dopaminergic network modulating normal and abnormal vocal productions.

Cortical thickness mediates the relationship between DRD2 C957T polymorphism and executive function across the adult lifespan

Dopamine (DA) signaling is critical for optimal cognitive performance. Aging is accompanied by a change in the strength of this signaling, with a loss of striatal and extrastriatal D2 binding potential. The reduction in dopamine modulation with age negatively influences various aspects of cognition. DRD2 C957T (rs6277) impacts DA D2 receptor density and availability, with C homozygotes linked to lower striatal DA availability and reduced executive functioning (EF), but also high extrastriatal binding potential. Here, we investigated in 176 participants aged 20-94 years whether: (1) DRD2 C carriers differ from T carriers in cortical thickness or subcortical volume in areas of high concentrations of D2 receptors that receive projections from mesocortical or nigrostriatal dopaminergic pathways; (2) whether the DRD2*COMT relationship has any synergistic effects on cortical thickness; (3) whether the effect of DRD2 on brain structure depends upon age; and (4) whether DRD2-related regional thinning affects executive function performance. We show that DRD2 impacts cortical thickness in the superior parietal lobule, precuneus, and anterior cingulate (marginal after FDR correction), while statistically controlling sex, age, and COMT genotype. Specifically, C homozygotes demonstrated thinner cortices than both heterozygotes and/or T homozygotes in an age-invariant manner. Additionally, DRD2 predicted executive function performance via cortical thickness. The results highlight that genetic influences on dopamine availability impact cognitive performance via the contribution of brain structure in cortical regions influenced by DRD2.

Frontoparietal cortical thickness mediates the effect of COMT Val158Met polymorphism on age-associated executive function

Proper dopamine (DA) signaling is likely necessary for maintaining optimal cognitive performance as we age, particularly in prefrontal-parietal networks and in fronto-striatal networks. Thus, reduced DA availability is a salient risk factor for accelerated cognitive aging. A common polymorphism that affects DA D1 receptor dopamine availability, COMT Val158Met (rs4680), influences enzymatic breakdown of DA, with COMT Val carriers having a 3- to 4-fold reduction in synaptic DA compared to COMT Met carriers. Furthermore, dopamine receptors and postsynaptic availability are drastically reduced with aging, as is executive function performance that ostensibly relies on these pathways. Here, we investigated in 176 individuals aged 20-94 years whether: (1) COMT Val carriers differ from their Met counterparts in thickness of regional cortices receiving D1 receptor pathways: prefrontal, parietal, cingulate cortices; (2) this gene-brain association differs across the adult lifespan; and (3) COMT-related regional thinning evidences cognitive consequences. We found that COMT Val carriers evidenced thinner cortex in prefrontal, parietal, and posterior cingulate cortices than COMT Met carriers and this effect was not age-dependent. Further, we demonstrate that thickness of these regions significantly mediates the effect of COMT genotype on an executive function composite measure. These results suggest that poorer executive function performance is due partly to thinner association cortex in dopaminergic-rich regions, and particularly so in individuals who are genetically predisposed to lower postsynaptic dopamine availability, regardless of age.

Dopamine tunes prefrontal outputs to orchestrate aversive processing

Decades of research suggest that the mesocortical dopamine system exerts powerful control over mPFC physiology and function. Indeed, dopamine signaling in the medial prefrontal cortex (mPFC) is implicated in a vast array of processes, including working memory, stimulus discrimination, stress responses, and emotional and behavioral control. Consequently, even slight perturbations within this delicate system result in profound disruptions of mPFC-mediated processes. Many neuropsychiatric disorders are associated with dysregulation of mesocortical dopamine, including schizophrenia, depression, attention deficit hyperactivity disorder, post-traumatic stress disorder, among others. Here, we review the anatomy and functions of the mesocortical dopamine system. In contrast to the canonical role of striatal dopamine in reward-related functions, recent work has revealed that mesocortical dopamine fine-tunes distinct efferent projection populations in a manner that biases subsequent behavior towards responding to stimuli associated with potentially aversive outcomes. We propose a framework wherein dopamine can serve as a signal for switching mPFC states by orchestrating how information is routed to the rest of the brain.

A PITX3-EGFP Reporter Line Reveals Connectivity of Dopamine and Non-dopamine Neuronal Subtypes in Grafts Generated from Human Embryonic Stem Cells

Development of safe and effective stem cell-based therapies for brain repair requires an in-depth understanding of the in vivo properties of neural grafts generated from human stem cells. Replacing dopamine neurons in Parkinson's disease remains one of the most anticipated applications. Here, we have used a human PITX3-EGFP embryonic stem cell line to characterize the connectivity of stem cell-derived midbrain dopamine neurons in the dopamine-depleted host brain with an unprecedented level of specificity. The results show that the major A9 and A10 subclasses of implanted dopamine neurons innervate multiple, developmentally appropriate host targets but also that the majority of graft-derived connectivity is non-dopaminergic. These findings highlight the promise of stem cell-based procedures for anatomically correct reconstruction of specific neuronal pathways but also emphasize the scope for further refinement in order to limit the inclusion of uncharacterized and potentially unwanted cell types.

Dopamine modulation of learning and memory in the prefrontal cortex: insights from studies in primates, rodents, and birds

In this review, we provide a brief overview over the current knowledge about the role of dopamine transmission in the prefrontal cortex during learning and memory. We discuss work in humans, monkeys, rats, and birds in order to provide a basis for comparison across species that might help identify crucial features and constraints of the dopaminergic system in executive function. Computational models of dopamine function are introduced to provide a framework for such a comparison. We also provide a brief evolutionary perspective showing that the dopaminergic system is highly preserved across mammals. Even birds, following a largely independent evolution of higher cognitive abilities, have evolved a comparable dopaminergic system. Finally, we discuss the unique advantages and challenges of using different animal models for advancing our understanding of dopamine function in the healthy and diseased brain.

Loss of asymmetric spine synapses in prefrontal cortex of motor-asymptomatic, dopamine-depleted, cognitively impaired MPTP-treated monkeys

Parkinson's disease is usually characterized as a movement disorder; however, cognitive abilities that are dependent on the prefrontal cortex decline at an early stage of the disease in most patients. The changes that underlie cognitive deficits in Parkinson's disease are not well understood. We hypothesize that reduced dopamine signalling in the prefrontal cortex in Parkinson's disease is a harbinger of detrimental synaptic changes in pyramidal neurons in the prefrontal cortex, whose function is necessary for normal cognition. Our previous data showed that monkeys exposed to the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), but not exhibiting overt motor deficits (motor-asymptomatic), displayed cognitive deficits in prefrontal cortex-dependent tasks. The present results demonstrate that motor-asymptomatic MPTP-treated monkeys have a reduced dopamine concentration and a substantially lower number (50%) of asymmetric (excitatory) spine synapses in layer II/III, but not layer V, of the dorsolateral prefrontal cortex, compared to controls. In contrast, neither dopamine concentration nor asymmetric synapse number was altered in the entorhinal cortex of MPTP-treated monkeys. Together, these findings suggest that the number of asymmetric spine synapses on dendrites in the prefrontal cortex is dopamine-dependent and that the loss of synapses may be a morphological substrate of the cognitive deficits induced by a reduction in dopamine neurotransmission in this region.

Autologous mesenchymal stem cell-derived dopaminergic neurons function in parkinsonian macaques

A cell-based therapy for the replacement of dopaminergic neurons has been a long-term goal in Parkinson's disease research. Here, we show that autologous engraftment of A9 dopaminergic neuron-like cells induced from mesenchymal stem cells (MSCs) leads to long-term survival of the cells and restoration of motor function in hemiparkinsonian macaques. Differentiated MSCs expressed markers of A9 dopaminergic neurons and released dopamine after depolarization in vitro. The differentiated autologous cells were engrafted in the affected portion of the striatum. Animals that received transplants showed modest and gradual improvements in motor behaviors. Positron emission tomography (PET) using [11C]-CFT, a ligand for the dopamine transporter (DAT), revealed a dramatic increase in DAT expression, with a subsequent exponential decline over a period of 7 months. Kinetic analysis of the PET findings revealed that DAT expression remained above baseline levels for over 7 months. Immunohistochemical evaluations at 9 months consistently demonstrated the existence of cells positive for DAT and other A9 dopaminergic neuron markers in the engrafted striatum. These data suggest that transplantation of differentiated autologous MSCs may represent a safe and effective cell therapy for Parkinson's disease.

Impaired recognition of facial expressions of anger in Parkinson's disease patients acutely withdrawn from dopamine replacement therapy

We have previously reported that acute dopaminergic blockade in healthy volunteers results in a transient disruption of the recognition of facial expressions of anger, whilst leaving intact the recognition of other facial expressions (including fear and disgust) and facial identity processing. Parkinson's disease (PD) is characterised by cell loss in dopaminergic neuronal populations, and hence we predicted that PD would be associated with impaired anger recognition. We reasoned that treatment with dopamine replacement therapy (DRT) could mask any deficit present in PD, and therefore studied facial expression recognition in a group of PD patients transiently withdrawn from DRT. Seventeen PD patients were compared to 21 age- and IQ-matched controls on the Ekman 60 task, which required the forced-choice labelling of 10 exemplars of each of six facial expressions (anger, disgust, fear, sadness, happiness, surprise). In line with our predictions, PD patients showed a selective impairment in the recognition of facial expressions of anger. This deficit was not related to the PD patients' performance on the Benton unfamiliar-face matching task, which was normal, nor was the deficit related to overall disease severity, or to depression symptoms. However, as predicted by simulation theories, impaired anger recognition in PD was related to reduced levels of the anger-linked temperament trait, exploratory excitability. The results extend our previous findings of a role for dopamine in the processing of facial expressions of anger, and demonstrate the power of adopting a phylogenetic, comparative perspective on emotions.

Localization of the GoLoco motif carrier regulator of G-protein signalling 12 and 14 proteins in monkey and rat brain

Regulator of G-protein signalling (RGS)12 and -14 proteins possess the RGS domain, Ras-binding domains and the GoLoco motif. Emerging evidence suggests that these proteins are involved in several cellular functions in addition to stimulation of GTPase activity of G-protein alpha subunits. However, our understanding of the role of the two proteins in brain function remains marginal. Here, we have studied the expression pattern of RGS12 and RGS14 proteins in brain at regional, cellular and subcellular levels. Both proteins were expressed throughout the brain regions, including cortex, hippocampus, striatum, thalamus and substantia nigra. The most intense immunostaining for RGS12 was seen in cortex and that of RGS14 was found in striatum. In cortex, RGS12 and RGS14 proteins were associated with pyramidal and nonpyramidal cell types. Apical dendrites of pyramidal cells were also labelled. RGS12 was found in both nuclear and cytoplasmic compartments. In contrast to RGS12 protein, RGS14 was localized in astrocytes in addition to neurons. Pyramidal cells in the CA1 area showed labelling for both RGS proteins. The presence of RGS12 was predominantly nuclear in the striatum of rat brain; however, the labelling of this protein was non-nuclear in adult monkey brain. To our surprise, in 1-month-old monkey brain the immunostaining pattern of the same protein was changed to nuclear. Non-nuclear staining for RGS12 was also evident in thalamus of adult monkey brain; however, in 1-month-old monkey brain, it was seen into two different populations, one with nuclear and the other with cytoplasmic staining. Both RGS12 and RGS14 were exclusively localized at postsynaptic sites of excitatory synapses. Our results demonstrate a highly dynamic expression pattern of RGS12 and RGS14 proteins in the central nervous system, and support the view that these proteins may participate not only in G-protein receptor signalling pathways but also in other cellular activities.

Neuropharmacology of cognition and memory: A unifying theory of neuromodulator imbalance in psychiatry and amnesia

The case of HM, a man with intractable epilepsy who became amnesic following bilateral medial temporal lobe surgery nearly half a century ago has instigated ongoing research and theoretical speculation on the nature of memory and the role of the hippocampus. Neuropsychological testing showed that although HM had extensive anterograde memory loss he could still acquire motor and cognitive skills implicitly, but could not remember the context of this learning. This has lead to declarative and procedural descriptions of the memory process. Cholinergic and monoaminergic neurotransmitter systems have also been implicated in the memory process and anticholinergic drugs traditionally have been associated with impairment of declarative memory. The cholinergic hypothesis of Alzheimer's disease is a classic example of an application of these neuropharmacological findings. In schizophrenia, preattentive deficits have been amply demonstrated by unconscious priming studies. Memory processes are also impaired in these patients. Dopamine, glutamate and even cholinergic dysfunction has been implicated in the clinical picture of schizophrenia. The present paper will attempt to bring together both the anatomical and pharmacological data from these disparate fields of research under a cohesive theory of cognition and memory. A hypothesis is presented for an inverse relationship between monoaminergic and cholinergic systems in the modulation of implicit (unconscious) and explicit (conscious) cognitive processes. It is postulated that muscarinic cholinergic receptors and monoaminergic systems facilitate unconscious and conscious processes, respectively, and they disfacilitate conscious and unconscious processes, respectively (the purported inverse relationship). In fact, the muscarinic and monoaminergic modulations of a neural network are proposed to be finely balanced such that, if, the activity of one receptor system is modified then this by necessity has effects on the other system. It takes into account receptor subtypes and their effects mediated through excitatory and inhibitory G-protein complexes. For example, m1/D2 and D1/m4 paired receptor subtypes, colocalized on separate neurons would have opposing functional effects. A theory is then presented that the critical underlying pathophysiology of schizophrenia involves a hypofunctional muscarinic cholinergic system, which induces abnormal facilitation of monoaminergic subsystems such as dopamine (e.g., a decrease in m1R function would potentiate D2R function). This extends the idea of an inverted U function for optimal monoaminergic concentrations. Not only would this impair unconscious preattentive processes, but according to the hypothesis, explicit cognition as well including memory deficits and would underlie the mechanism of psychosis. Contrary to current thinking a different view is also presented for the role of the hippocampus in the memory process. It is postulated that long-term explicit memory traces in the neocortex are laid down by phasic coactivation of forebrain projecting monoaminergic systems above some basal firing rate, such as the rostral serotonergic raphe, which projects diffusely to the cortex and according to a modified Hebbian principle. This is the proposed principal function of the hippocampal theta rhythm. The phasic activation of the cholinergic basal forebrain is mediated by projections from a separate cortical structure, possibly the lateral prefrontal cortex. Phasic muscarinic receptor activation is proposed to strengthen implicit memory traces (at a synaptic level) in the neocortex. Thus, the latter are spared by medial temporal surgery explaining the dissociation of explicit from implicit memory.

Factors affecting the clinical outcome after neural transplantation in Parkinson's disease

Intrastriatal grafts of embryonic mesencephalic tissue can survive in the brains of patients with Parkinson's disease, but the degree of symptomatic relief is highly variable and some cases develop troublesome dyskinesias. Here we explored, using clinical assessment and 18F-dopa and 11C-raclopride PET, factors which may influence the functional outcome after transplantation. We observed increased 18F-dopa uptake in the grafted putamen, signifying continued survival of the transplanted dopaminergic neurons, in parallel with a progressive reduction of 18F-dopa uptake in non-grafted regions for the whole patient group. The patients with the best functional outcome after transplantation exhibited no dopaminergic denervation in areas outside the grafted areas either preoperatively or at 1 or 2 years post-operatively. In contrast, patients with no or modest clinical benefit showed reduction of 18F-dopa in ventral striatum prior to or following transplantation, which may have limited graft-induced improvement. We obtained no evidence that dyskinesias were caused by abnormal dopamine (DA) release from the grafts. As has been observed for intrinsic dopaminergic neurons, there was a significant correlation between 18F-dopa uptake and methamphetamine-induced change of 11C-raclopride binding (as a measure of DA release) in the putamen containing the graft. Furthermore, we observed no correlation between 11C-raclopride binding in anterior, posterior or entire putamen under basal conditions or after methamphetamine, and dyskinesia severity scores in the contralateral side of the body. Withdrawal of immunosuppression at 29 months after transplantation caused no reduction of 18F-dopa uptake or worsening of UPDRS motor score, indicating continued survival and function of the graft. However, patients showed increased dyskinesia scores, which might have been caused either by growth of the graft or worsening of a low-grade inflammation around the graft. These findings indicate that poor outcome after transplantation is associated with progressive dopaminergic denervation in areas outside the grafts, a process which may have started already before surgery. Also, that the development of dyskinesias after transplantation is not associated with excessive DA release from the grafts. Finally, our data provide evidence that long-term immunosuppression can be withdrawn without interfering with graft survival or the motor recovery induced by transplantation.

Development of A9/A10 dopamine neurons during the second and third trimesters in the African green monkey

Disruption in the development of dopamine-containing neurons has been postulated to underlie several CNS disorders. However, there have been no quantitative studies on the normal development of primate dopamine neurons. Thus, the fetal maturation of primate midbrain dopamine neurons was examined to establish changes that occur in the A9/A10 groups during the second and third trimesters. Eleven fetal African green monkey midbrains were immunostained for tyrosine hydroxylase (TH-ir) as a marker for dopamine neurons and quantified using stereological techniques (nucleator method). The number and size of defined dopamine neurons and the volume occupied by A9/A10 neurons increased in near linear fashion throughout the term. The estimated number of defined dopamine neurons in each hemisphere rose from approximately 50,000 at embryonic day (E) 70 to 225,000 at birth (E165), similar to the adult population. The size and the area occupied by them at birth were, however, well below the estimated adult levels. Additionally, the younger fetal midbrains had far less diversity in dopamine cell volumes compared with older fetuses and adult brains. Until midway through gestation (E81), clusters of apparently immature midbrain TH-ir cells were observed, but could not be counted. Even though the majority of cells destined to become dopamine neurons are generated in the first trimester, phenotypical maturation of A9 and A10 cell bodies continues steadily throughout gestation and extends well into the postnatal period. These data have relevance to transplantation studies that employ fetal dopaminergic grafts, and to disorders hypothesized to result from damage to developing midbrain dopamine neurons.

Do rats have a prefrontal cortex?

The lack of a single anatomical or functional definition of 'prefrontal cortex' has led to different and, in some respects, controversial views on the existence of a prefrontal cortex in non-primate mammals, in particular in rats. Until the classic paper by Rose and Woolsey [Res. Publ. Assoc. Nerv. Ment. Dis. 27 (1948) 210], the general idea was that a prefrontal cortex is unique to primate species. Rose and Woolsey's 'prefrontal cortex' definition was based upon a single anatomical criterion, i.e. the cortical projection area of the mediodorsal thalamic nucleus. Single criteria, however, do not appear to be sufficient for defining the prefrontal cortex. Therefore, other anatomical and functional characteristics are currently used to identify the prefrontal cortex in different species. Yet, recently the debate about the nature of the prefrontal cortex in non-primate species has been resumed. In the present paper we will compare the structural and functional characteristics of the prefrontal cortex of nonhuman primates and rats. We will argue that rats have a functionally divided prefrontal cortex that includes not only features of the medial and orbital areas in primates, but also some features of the primate dorsolateral prefrontal cortex.

Compulsive use of dopamine replacement therapy in Parkinson's disease: reward systems gone awry?

Dopamine replacement therapy (DRT) is the most effective treatment for Parkinson's disease (PD); it provides substantial benefit for most patients, extends independence, and increases survival. A few patients with PD, however, take increasing quantities of medication far beyond those required to treat their motor disabilities. These patients demand rapid drug escalation and continue to request more DRT despite the emergence of increasingly severe drug-induced motor complications and harmful behavioural consequences. In this article we detail the features of compulsive DRT-seeking and intake in PD, in relation to theories of compulsive drug use.

Monoamine neuron innervation of the normal human brain: an 18F-DOPA PET study

18F-DOPA positron emission tomography (PET) has been used for two decades to study the organization and pathology of the striatal dopamine system in the human brain, particularly in Parkinson's disease. High resolution 3D PET allows a more detailed analysis than previously available and was employed in this study to determine the regional uptake of 18F-DOPA in control brain. Eleven healthy volunteers underwent 18F-DOPA PET with a region of interest (ROI) study performed using individual volumetric MRI's coregistered to the PET ADD image. A Patlak linear graphical analysis was undertaken to obtain influx constant (Ki) values. The highest Ki values were from neostriatal areas, with a rostrocaudal gradient of increasing Ki values from head of caudate nucleus to rostral putamen to caudal putamen. However, Ki values for transaxial slices from dorsal to ventral through the caudate and putamen were uniform. Ventral striatum Ki was 81% with red nucleus and globus pallidus Ki values of approximately 40% of neostriatum. In limbic areas, highest values were obtained from amygdala (35% neostriatal Ki). Neocortical Ki values varied from 22% in temporal pole to 6% in occipital cortex of neostriatal values. Hypothalamic Ki was high (45%) in comparison to thalamus (17%) and retina (17%). 18F-DOPA is taken up by serotonin (raphe, 51%), and noradrenaline (locus coeruleus, 37%) as well as dopamine neurons. These data indicate that 18F-DOPA PET can be used with detailed, anatomically based ROIs as a tool for in vivo analysis of regional changes in monoamine neuron systems throughout the brain in Parkinson's disease and other disorders.

Mechanisms of dopamine activation of fast-spiking interneurons that exert inhibition in rat prefrontal cortex

Prefrontal cortical dopamine (DA) modulates pyramidal cell excitability directly and indirectly by way of its actions on local circuit GABAergic interneurons. DA modulation of interneuronal functions is implicated in the computational properties of prefrontal networks during cognitive processes and in schizophrenia. Morphologically and electrophysiologically distinct classes of putative GABAergic interneurons are found in layers II-V of rat prefrontal cortex. Our whole cell patch-clamp study shows that DA induced a direct, TTX-insensitive, reversible membrane depolarization, and increased the excitability of fast-spiking (FS) interneurons. The DA-induced membrane depolarization was reduced significantly by D1/D5 receptor antagonist SCH 23390, but not by the D2 receptor antagonist (-)sulpiride, D4 receptor antagonists U101958 or L-745870, alpha1-adrenoreceptor antagonist prazosin, or serotoninergic receptor antagonist mianserin. The D1/5 agonists SKF81297 or dihydrexidine, but not D2 agonist quinpirole, also induced a prolonged membrane depolarization. Voltage-clamp analyses of the voltage-dependence of DA-sensitive currents, and the effects of changing [K(+)](O) on reversal potentials of DA responses, revealed that DA suppressed a Cs(+)-sensitive inward rectifier K(+) current and a resting leak K(+) current. D1/D5, but not D2 agonists mimicked the suppressive effects of DA on the leak current, but the DA effects on the inward rectifier K(+) current were not mimicked by either agonist. In a subgroup of FS interneurons, the slowly inactivating membrane outward rectification evoked by depolarizing voltage steps was also attenuated by DA. Collectively, these data showed that DA depolarizes FS interneurons by suppressing a voltage-independent 'leak' K(+) current (via D1/D5 receptor mechanism) and an inwardly rectifying K(+) current (via unknown DA mechanisms). Additional suppression of a slowly inactivating K(+) current led to increase in repetitive firing in response to depolarizing inputs. This D1-induced increase in interneuron excitability enhances GABAergic transmission to PFC pyramidal neurons and could represent a mechanism via which DA suppresses persistent firing of pyramidal neurons in vivo.

Selective reduction by dopamine of excitatory synaptic inputs to pyramidal neurons in primate prefrontal cortex

We have employed in vitro physiological methods to investigate dopaminergic modulation of excitatory synaptic transmission in monkey prefrontal cortex (PFC) circuits. We show that combined activation of D1-like and D2-like dopamine receptors results in the reduction of extracellular stimulation-evoked isolated EPSCs in layer 3 pyramidal neurons. Using paired recordings from synaptically connected pyramidal neurons we have determined the basic properties of unitary synaptic connections between layer 3 pyramids in the primate PFC and, interestingly, we found that dopamine does not reduce synaptic transmission between nearby pairs of synaptically coupled PFC pyramidal neurons. This input specificity may be a critical aspect of the dopaminergic regulation of recurrent excitatory circuits in the PFC.

Functional effect of adeno-associated virus mediated gene transfer of aromatic L-amino acid decarboxylase into the striatum of 6-OHDA-lesioned rats

In animal models of Parkinson's disease, gene transfer of aromatic L-amino acid decarboxylase (AADC) leads to an increase in the capacity of the striatum to decarboxylate exogenous L-DOPA. However, the functional effects of enhanced L-DOPA to dopamine conversion have not been explored. Here, we show that following adeno-associated virus (AAV)-AADC transduction, the transgenic AADC is able to decarboxylate exogenous L-DOPA more efficiently so that a dose of L-DOPA ineffective before gene transfer elicits a motor asymmetry (rotational behavior) following gene transfer. Furthermore, rotation scores showed a strong correlation with AADC activity in the lesioned striatum, thus allowing for behavioral screening of successful gene transfer in the brain. In animals receiving AAV2-AADC, dopamine production was restored to 50% of normal levels 12 weeks after the infusion. Microdialysis experiments demonstrated an in vivo enhanced conversion of L-DOPA to dopamine, but no storage capacity as dopamine was released to the extracellular space in a continuous, nonregulated fashion. In addition to the potential clinical benefit of improving decarboxylation efficiency in Parkinson's disease, our approach may be relevant for the treatment of AADC deficiency, a rare, autosomal recessive disorder causing a severe movement disorder and progressive cognitive impairment.

Structure of longitudinal brain zones that provide the origin for the substantia nigra and ventral tegmental area in human embryos, as revealed by cytoarchitecture and tyrosine hydroxylase, calretinin, calbindin, and GABA immunoreactions

In a previous work, mapping early tyrosine hydroxylase (TH) expressing primordia in human embryos, the tegmental origin of the substantia nigra (SN) and ventral tegmental area (VTA) was located across several neuromeric domains: prosomeres 1-3, midbrain, and isthmus (Puelles and Verney, [1998] J. Comp. Neurol. 394:283-308). The present study examines in detail the architecture of the neural wall along this tegmental continuum in 6-7 week human embryos, to better define the development of the SN and VTA. TH-immunoreactive (TH-IR) structures were mapped relative to longitudinal subdivisions (floor plate, basal plate, alar plate), as well as to radially superposed strata of the neural wall (periventricular, intermediate, and superficial strata). These morphologic entities were delineated at each relevant segmental level by using Nissl-stained sections and immunocytochemical mapping of calbindin, calretinin, and GABA in adjacent sagittal or frontal sections. A numerous and varied neuronal population originates in the floor plate area, and some of its derivatives become related through lateral tangential migration with other neuronal populations born in distinct medial and lateral portions of the basal plate and in a transition zone at the border with the alar plate. Some structural differences characterize each segmental domain within this common schema. The TH-IR neuroblasts arise predominantly within the ventricular zone of the floor plate and, more sparsely, within the adjacent medial part of the basal plate. They first migrate radially from the ventricular zone to the pia and then apparently move laterally and slightly rostralward, crossing the superficial stratum of the basal plate. Several GABA-IR cell populations are present in this region. One of them, which might represent the anlage of the SN pars reticulata, is generated in the lateral part of the basal plate.

Dopamine increases excitability of pyramidal neurons in primate prefrontal cortex

Dopaminergic modulation of neuronal networks in the dorsolateral prefrontal cortex (PFC) is believed to play an important role in information processing during working memory tasks in both humans and nonhuman primates. To understand the basic cellular mechanisms that underlie these actions of dopamine (DA), we have investigated the influence of DA on the cellular properties of layer 3 pyramidal cells in area 46 of the macaque monkey PFC. Intracellular voltage recordings were obtained with sharp and whole cell patch-clamp electrodes in a PFC brain-slice preparation. All of the recorded neurons in layer 3 (n = 86) exhibited regular spiking firing properties consistent with those of pyramidal neurons. We found that DA had no significant effects on resting membrane potential or input resistance of these cells. However DA, at concentrations as low as 0.5 microM, increased the excitability of PFC cells in response to depolarizing current steps injected at the soma. Enhanced excitability was associated with a hyperpolarizing shift in action potential threshold and a decreased first interspike interval. These effects required activation of D1-like but not D2-like receptors since they were inhibited by the D1 receptor antagonist SCH23390 (3 microM) but not significantly altered by the D2 antagonist sulpiride (2.5 microM). These results show, for the first time, that DA modulates the activity of layer 3 pyramidal neurons in area 46 of monkey dorsolateral PFC in vitro. Furthermore the results suggest that, by means of these effects alone, DA modulation would generally enhance the response of PFC pyramidal neurons to excitatory currents that reach the action potential initiation site.

Decreased density of tyrosine hydroxylase-immunoreactive axons in the entorhinal cortex of schizophrenic subjects

BACKGROUND

We recently reported a laminar-specific reduction in the density of tyrosine hydroxylase (TH)-immunoreactive axons in the prefrontal cortex of subjects with schizophrenia. In this report, we extend these investigations to the entorhinal cortex (ERC), another candidate site of dysfunction in this disorder.

METHODS

Using immunocytochemical techniques and blind quantitative analyses, we determined the density of TH-immunoreactive axons in the rostral subdivision of the ERC from seven matched pairs of schizophrenic and control subjects.

RESULTS

The relative density of TH-labeled axons was significantly decreased by over 60% in layers 3 and 6, but not in layer 1, of the ERC in schizophrenic subjects. In contrast, in the prefrontal cortex of the same subjects, labeled axon density was significantly decreased by 62% only in layer 6. Furthermore, the length of TH-labeled axons did not differ between six matched pairs of nonschizophrenic psychiatric and control subjects in any layer of the ERC. Finally, the density of TH-labeled axons in the ERC of cynomolgus monkeys chronically treated with haloperidol was not reduced relative to control animals.

CONCLUSIONS

These findings reveal regional- and laminar-specific alterations in TH-immunoreactive axons that appear to be specific to the pathophysiology of schizophrenia.