D1 and D2 dopamine receptors in perinatal and adult basal ganglia

Indirect evidence for altered dopaminergic neurotransmission in very premature-born adults

While animal models indicate altered brain dopaminergic neurotransmission after premature birth, corresponding evidence in humans is scarce due to missing molecular imaging studies. To overcome this limitation, we studied dopaminergic neurotransmission changes in human prematurity indirectly by evaluating the spatial co-localization of regional alterations in blood oxygenation fluctuations with the distribution of adult dopaminergic neurotransmission. The study cohort comprised 99 very premature-born (<32 weeks of gestation and/or birth weight below 1500 g) and 107 full-term born young adults, being assessed by resting-state functional MRI (rs-fMRI) and IQ testing. Normative molecular imaging dopamine neurotransmission maps were derived from independent healthy control groups. We computed the co-localization of local (rs-fMRI) activity alterations in premature-born adults with respect to term-born individuals to different measures of dopaminergic neurotransmission. We performed selectivity analyses regarding other neuromodulatory systems and MRI measures. In addition, we tested if the strength of the co-localization is related to perinatal measures and IQ. We found selectively altered co-localization of rs-fMRI activity in the premature-born cohort with dopamine-2/3-receptor availability in premature-born adults. Alterations were specific for the dopaminergic system but not for the used MRI measure. The strength of the co-localization was negatively correlated with IQ. In line with animal studies, our findings support the notion of altered dopaminergic neurotransmission in prematurity which is associated with cognitive performance.

Methamphetamine, Neurotransmitters and Neurodevelopment

Methamphetamine (MA), as massively abused psychoactive stimulant, has been associated with many neurological diseases. It has various potent and neurotoxic properties. There are many mechanisms of action that contribute to its neurotoxic and degenerative effects, including excessive neurotransmitter (NEU) release, blockage of NEU uptake transporters, degeneration of NEU receptors, process of oxidative stress etc. MA intoxication is caused by blood-brain barrier disruption resulted from MA-induced oxidation stress. In our laboratory we constantly work on animal research of MA. Our current interest is to investigate processes of MA-induced alteration in neurotransmission, especially during development of laboratory rat. This review will describe current understanding in role of NEUs, which are affected by MA-induced neurotoxicity caused by altering the action of NEUs in the central nervous system (CNS). It also briefly brings information about NEUs development in critical periods of development.

The development of monoaminergic neurotransmitter systems in childhood and adolescence

Brain maturation extends throughout adolescence well into early adulthood. Knowledge on developmental changes is crucial for age-appropriate pharmacotherapy. This article reviews data on maturational processes with a focus on the noradrenergic, dopaminergic, and serotonergic neurotransmitter systems.The literature was searched with a focus on studies in humans. However, since data in humans are limited animal studies were also included. All reviewed neurotransmitter systems show age-related development processes that differentiate child and adolescent brain function from those of adult brains. Unfortunately, the state of knowledge surrounding development-related changes remains sufficiently sparse, There is a high need for more studies on pediatric psychopharmacology and its biological underpinnings. Safety and efficacy of psychopharmacological medicines cannot be readily extrapolated from adults.

Early human brain development: Starring the subplate

This review summarizes early human brain development on the basis of neuroanatomical data and functional connectomics. It indicates that the most significant changes in the brain occur during the second half of gestation and the first three months post-term, in particular in the cortical subplate and cerebellum. As the transient subplate pairs a high rate of intricate developmental changes and interactions with clear functional activity, two phases of development are distinguished: a) the transient cortical subplate phase, ending at 3 months post-term when the permanent circuitries in the primary motor, somatosensory and visual cortices have replaced the subplate; and subsequently, b) the phase in which the permanent circuitries dominate. In the association areas the subplate dissolves in the remainder of the first postnatal year. During both phases developmental changes are paralleled by continuous reconfigurations in network activity. The reviewed literature also suggests that disruption of subplate development may play a pivotal role in developmental disorders, such as cerebral palsy, autism spectrum disorders, attention deficit hyperactivity disorder and schizophrenia.

Vulnerability of the mesencephalic dopaminergic neurons of the human neonate to prolonged perinatal hypoxia: an immunohistochemical study of tyrosine hydroxylase expression in autopsy material

Experimental studies indicate that hypoxia to the fetus, a common occurrence in many birth complications in humans, results in long-term disturbances of the central dopaminergic (DA) systems that persist in adulthood. Because dysregulation of DA systems is involved in the pathophysiology of many neurological and psychiatric disorders, we investigated the effects of perinatal hypoxia on the mesencephalic DA neurons of the human neonate using immunohistochemistry. We studied the expression of tyrosine hydroxylase (TH), the first and rate-limiting enzyme in catecholamine synthesis, in substantia nigra, and ventral tegmental area of 18 neonates in relation to the age and severity/duration of hypoxic injury estimated by neuropathological criteria. In severe/abrupt perinatal hypoxia, intense TH staining was observed in substantia nigra, ventral tegmental area, and, surprisingly, in the nonpreganglionic Edinger-Westphal nucleus. In severe/prolonged hypoxia, there was a striking reduction or even absence of TH immunoreactivity in all the mesencephalic nuclei. These observations suggest that at early states of perinatal hypoxia, there is a massive increase in dopamine synthesis and release that is followed by feedback blockage of dopamine synthesis through inhibition of TH by the end product dopamine. Early dysregulation of DA neurotransmission could predispose infant survivors of severe perinatal hypoxia to dopamine-related neurological and/or cognitive deficits later in life.

Monoamine oxidases in development

Monoamine oxidases (MAOs) are flavoproteins of the outer mitochondrial membrane that catalyze the oxidative deamination of biogenic and xenobiotic amines. In mammals there are two isoforms (MAO-A and MAO-B) that can be distinguished on the basis of their substrate specificity and their sensitivity towards specific inhibitors. Both isoforms are expressed in most tissues, but their expression in the central nervous system and their ability to metabolize monoaminergic neurotransmitters have focused MAO research on the functionality of the mature brain. MAO activities have been related to neurodegenerative diseases as well as to neurological and psychiatric disorders. More recently evidence has been accumulating indicating that MAO isoforms are expressed not only in adult mammals, but also before birth, and that defective MAO expression induces developmental abnormalities in particular of the brain. This review is aimed at summarizing and critically evaluating the new findings on the developmental functions of MAO isoforms during embryogenesis.

Regional expression of dopamine D1 and D2 receptor proteins in the cerebral cortex of asphyxic newborn infants

Dopamine D(1) and D(2) receptor protein expression was examined by Western blotting in newborn infants dying from cerebral asphyxia between 31 and 42 weeks' gestation, and matched controls. Frontal, occipital, temporal, and motor cortex tissue samples were obtained at autopsy (median postmortem interval 35 hours) and frozen for storage at -80 degrees C. A total of 2 immunoreactive bands were detected with each primary antibody in infant brain, whereas a single band was present in adult human and rat tissue. Immunoreactivity varied between cortical areas for both receptors, but their regional patterns differed significantly. D(1) protein levels were higher in motor and temporal cortex than in frontal or occipital cortex. D(2) protein showed graded expression frontal > motor > occipital > temporal cortex. Asphyxia cases showed lower expression of the upper D(2) immunoreactive band, but no difference in regional pattern. Lower D(2) receptor expression may attenuate stress responses and underlie increased vulnerability to hypoxia at birth.

Postnatal alterations in dopaminergic markers in the human prefrontal cortex

Dopamine in the prefrontal cortex plays a critical role in normal cognition throughout the lifespan and has been implicated in the pathophysiology of neuropsychiatric disorders such as schizophrenia and attention deficit disorder. Little is known, however, about the postnatal development of the dopaminergic system in the human prefrontal cortex. In this study, we examined pre- and post-synaptic markers of the dopaminergic system in postmortem tissue specimens from 37 individuals ranging in age from 2 months to 86 years. We measured the levels of tyrosine hydroxylase, the rate limiting enzyme in dopamine biosynthesis, using Western immunoblotting. We also examined the gene expression of the three most abundant dopamine receptors (DARs) in the human prefrontal cortex: DAR1, DAR2 and DAR4, by in situ hybridization. We found that tyrosine hydroxylase concentrations and DAR2 mRNA levels were highest in the cortex of neonates. In contrast, the gene expression of DAR1 was highest in adolescents and young adults. No significant changes across age groups were detected in mRNA levels of DAR4. Both DAR1 and DAR2 mRNA were significantly lower in the aged cortex. Taken together, our data suggest dynamic changes in markers of the dopamine system in the human frontal cortex during postnatal development at both pre-and post-synaptic sites. The peak in DAR1 mRNA levels around adolescence/early adulthood may be of particular relevance to neuropsychiatric disorders such as schizophrenia in which symptoms manifest during the same developmental period.

Ontogeny of the human central nervous system: what is happening when?

The present paper reviews current data on the structural development of the human nervous system. Focus is on the timing of ontogenetic events in the telencephalon. Neuronal proliferation and migration especially occur during the first half of gestation; the second half of gestation is the period of the existence of the functionally important transient structure 'subplate' and the major period of glial cell proliferation and programmed cell death. Axon and dendrite sprouting and synapse formation bloom during the last trimester of gestation and the first postnatal year. Major part of telencephalic myelination occurs during the first year after birth. Many developmental processes, such as myelination, synapse formation and synapse elimination continue throughout childhood and adolescence. Evidence is emerging that the peak of synapse elimination occurs between puberty and the onset of adulthood. Neurotransmitter systems are present from early foetal life onwards and their pre- and perinatal development is characterized by periods of transient overexpression. The latter is for instance true for the acetylcholinergic, catecholaminergic and glutamate systems. Thus, the development of the human brain is characterized by a protracted, neatly orchestrated chain of specific ontogenetic events. The continuous changes of the nervous system have consequences for vulnerability to adverse conditions, for diagnostics and for physiotherapeutical intervention.

Development of neurotransmitter systems during critical periods

Neurotransmitters are released from neurons and mediate neuronal communication. Neuromodulators can also be released from other cells and influence the neuronal signaling. Both neurotransmitters and neuromodulators play an important role in the shaping and the wiring of the nervous system possibly during critical windows of the development. Monoamines are expressed in the very early embryo, at which stage the notochord already contains high noradrenaline levels. Purines and neuropeptides are probably also expressed at an early stage, in a similar way as they occur during early phylogenesis. The levels of most neurotransmitters and neuromodulators increase concomitantly with synapse formation. Some of them surge during the perinatal period (such as glutamate, catecholamines, and some neuropeptides) and then level off. The interesting question is to what extent the expression of neuroactive agents is related to the functional state of the fetus and the newborn. Monoamines are expressed in the very early embryo, at which stage the notochord already contains high noradrenaline levels. They may have an important role for neurotransmission in the fetus. In the adult mammal, the fast switching excitatory amino acids dominate. However, they also seem to be important for the wiring of the brain and the plasticity before birth. NMDA receptors that are supposed to mediate these effects dominate and are then substituted by AMPA receptors. The main inhibitory amino acids gamma-aminobutyric acid (GABA) and glycine are excitatory in the developing brain by depolarizing developing neurons that have high Cl- concentrations. This seems to be of major importance for the wiring of neuronal circuits. Prenatal or neonatal stress, for example, hypoxia, can affect the programming of neurotransmitter and receptor expression, which can lead to long-term behavioral effects.

Ageing and the nigrostriatal dopaminergic system

BACKGROUND

Brain dopamine has been the focus of numerous studies owing to its crucial role in motor function and in neurological and psychiatric disease processes. Whilst early work relied on postmortem data, functional imaging has allowed a more sophisticated approach to the quantification of receptor density, affinity and functional capacity. This review aims to summarise changes in the nigrostriatal dopaminergic system which accompany normal ageing.

METHODS

A literature search focussed on postmortem and neuroimaging studies of normal ageing within the nigrostriatal dopaminergic tract. The functional significance of age-related effects was also considered.

RESULTS

There are significant reductions in pre- and post-synaptic markers of brain dopamine activity during normal ageing: However the rate of decline (linear or exponential), the effects of gender and heterogeneity and the mechanisms by which these changes occur remain undetermined. Limited data suggest there is a significant association between postsynaptic receptor density and specific aspects of motor and cognitive function.

CONCLUSION

The identification of strategies to improve dopaminergic transmission may delay the onset of motor and cognitive deficits associated with normal ageing. In order to develop effective preventative strategies, the causative mechanisms underlying age-related changes and the interaction between synaptic structure and function need to be more clearly elucidated.

Neurotransmitters and neuromodulators during early human development

BACKGROUND

Neurotransmitters such as monoamines appear in the embryo before the neurones are differentiated. They may have other functions than neurotransmission during embryogenesis such as differentiation and neuronal growth. For example, serotonin may act as a morphogen. A number of neuropeptides are expressed during ontogenesis, but their function has been difficult to establish. Maybe some of them remain as evolutionary residues. Fast-switching neurotransmitters like the excitatory amino acids and the more ionotropic receptors dominate in the human brain, but appear probably later during evolution as well as during ontogeny.

METHODS

The distribution of catecholamines during development has been analysed with a fluorescence method, while most of the other neurotransmitters have been mapped with immunohistochemical methods. The classical method to determine the physiological role of a neurotransmitter or modulator is to study the physiological effect of its antagonist, blocking the endogenous activity. By transgenic technique, the genes encoding for enzymes involved in the synthesis of neurotransmitters can be knocked-out.

MAJOR FINDINGS

Pharmacological blocking of endogenous activity has, for example, demonstrated that adenosine suppresses fetal respiration. Knocking out the dopamine beta-hydroxylase gene results in fetal death, suggesting that noradrenaline is essential for survival. Some neurotransmitters change their effect during embryogenesis, e.g. GABA which is excitatory in the embryo, but inhibitory after birth due to a switch from a high to low chloride content in the nerve cells. It is possible that this is of importance for the wiring of neuronal network in early life. NMDA receptors dominate in the foetus, while kainate and AMPA receptors appear later. At birth, there is a surge of neurotransmitters such as catecholamines, which may be of importance for the neonatal adaptation.

CONCLUSIONS

Neurotransmitters and modulators are not only important for the neural trafficking in the embryo, but also for the development of the neuronal circuits. Prenatal or neonatal stress (hypoxia), as well as various drugs, may disturb the wiring and cause long-term behavioural effects (fetal and neonatal programming).

Developmental and age-related changes of dopamine transporter, and dopamine D1 and D2 receptors in human basal ganglia

The developmental and age-related changes of the dopamine transporter (DAT), and the dopamine D1 and D2 receptor (D1R and D2R) subtypes were investigated in basal ganglia (BG) of human brain. DAT immunostaining was mainly observed in the neuropil, neurons, and glia of the striatum. The DAT-positive neuropil was detectable at 32 GW, a peak being reached at 9-10 years of age, with a decrease to 50-63 years of age. The developmental pattern of DAT immunoreactivity in neuron was similar to that of the neuropil. DAT-positive glia were observed in the BG at 32 GW, which increased slightly at 38-40 GW, and then did not obviously change until 6-8 months after birth. D2R-positive neurons were clearly observed at 19 GW, a peak being reached at 32 GW and 1-3 months of age in the globus pallidus and striatum, respectively, with a decrease after 9-10 years of age. D1R was expressed as early as D2R, but decreased after 6-8 months. Our results suggest that D1R and D2R expression is an intrinsic property of striatal neurons and is independent of dopaminergic innervation. D1R may play a more important role in neuronal maturation of the BG than D2R. D2R may be closely correlated with late neuronal development. The higher expression of DAT during adolescence may be related to function of the BG which learns complex behavioral patterns. The significance of the age-related decreases in DAT, D1R and D2R in the BG remains to be further investigated.

Tyrosine hydroxylase-immunoreactive elements in the human globus pallidus and subthalamic nucleus

In contrast to the well-established dopaminergic innervation of the neostriatum, the existence of dopaminergic innervation of the subthalamic nucleus and globus pallidus is controversial. In the present study, tyrosine hydroxylase (TH)-immunoreactive elements were observed by light microscopy after antigen retrieval in the subthalamic nucleus and in the internal and external segments of the globus pallidus in postmortem human brain. Small islands of apparent neostriatal tissue with abundant arborization of fine, TH-immunoreactive axons in the vicinity of calbindin-positive small neurons resembling neostriatal medium spiny neurons were present in the external segment of the globus pallidus. Large numbers of medium-large, TH-immunoreactive axons were observed passing above and through the subthalamic nucleus and through both pallidal segments; these are presumed to be axons of passage on their way to the neostriatum. In addition, fine, TH-immunoreactive axons with meandering courses, occasional branches, and irregular outlines, morphologically suggestive of terminal axon arborizations with varicosities, were seen in both pallidal segments, including the ventral pallidum, and the subthalamic nucleus, consistent with a catecholaminergic (probably dopaminergic) innervation of these nuclei. This finding suggests that, in Parkinson's disease and in animal models of this disorder, loss of dopaminergic innervation might contribute to abnormal neuronal activation in these three nuclei.

Levodopa induces a cytoplasmic localization of D1 dopamine receptors in striatal neurons in Parkinson's disease

Parkinson's disease is characterized by a massive loss of nigral dopamine neurons that results in a reduction of dopamine concentrations in the striatum. The most commonly used treatment for this disease is levodopa therapy to restore striatal dopamine. This treatment is mediated by dopamine receptors, but the effect of treatment and the disease on receptor distribution is unknown. In this study, the distribution of D1 dopamine receptors was analyzed at the cellular and subcellular level in the striatum of 5 patients with Parkinson's disease (all treated with levodopa) and 4 control subjects. In the control brains, D1 dopamine receptors were mostly detected on the plasma membrane of medium-sized spiny neurons. The quantitative analysis performed at the ultrastructural level in patients with Parkinson's disease revealed an increase in immunostaining in the cytoplasm of medium-sized neurons. This effect was likely the result of the treatment rather than the dopaminergic denervation, as such changes were not observed in the striatum of rats with a unilateral 6-hydroxydopamine nigrostriatal lesion, but were present in normal or lesioned rats treated with a D1 dopamine agonist. Altered localization of D1 dopamine receptors may participate in the occurrence of side effects of levodopa therapy such as dyskinesia and fluctuations in motor performances.

Simultaneous measurement of [18F]FDOPA metabolism and cerebral blood flow in newborn piglets

Available information on the dopamine (DA) metabolism of the immature brain is rare. In order to establish a useful animal model we have performed PET experiments in anesthetized neonatal pigs using 6-[18F]-fluoro-L-DOPA (FDOPA) as tracer. In this study, we have simultaneously determined the cerebral blood flow and the rate constant of FDOPA conversion by the aromatic amino acid decarboxylase, the ultimate enzyme in the synthesis of dopamine. The estimated values of FDOPA decarboxylation in the basal ganglia were similar to values calculated in adult animals and humans. However, in contrast to those studies a significant decarboxylation was also found in the frontal cortex and the cerebellum. HPLC analysis of brain samples also revealed extensive and rapid metabolism of FDOPA in the five investigated brain regions. At 8 min after tracer injection about 80% of FDOPA was already converted to FDA and its metabolites. Surprisingly, a rather high fraction (16-21%) of [18F]-fluoro-3-methoxytyramine was found which may indicate a low storage capacity of vesicular DA at this perinatal stage. It is suggested that the findings are related to the ontogenetic development of the dopaminergic system. The knowledge of the regulation of the DA metabolism in the immature brain may have implications for the understanding of neurodevelopmental effects of perinatal oxygen deprivation.