Dopamine receptors in human foetal brains: characterization, regulation and ontogeny of [3H]spiperone binding sites in striatum

Transient compartmentalization and accelerated volume growth coincide with the expected development of cortical afferents in the human neostriatum

The neostriatum plays a central role in cortico-subcortical circuitry underlying goal-directed behavior. The adult mammalian neostriatum shows chemical and cytoarchitectonic compartmentalization in line with the connectivity. However, it is poorly understood how and when fetal compartmentalization (AChE-rich islands, nonreactive matrix) switches to adult (AChE-poor striosomes, reactive matrix) and how this relates to the ingrowth of corticostriatal afferents. Here, we analyze neostriatal compartments on postmortem human brains from 9 postconceptional week (PCW) to 18 postnatal months (PM), using Nissl staining, histochemical techniques (AChE, PAS-Alcian), immunohistochemistry, stereology, and comparing data with volume-growth of in vivo and in vitro MRI. We find that compartmentalization (C) follows a two-compartment (2-C) pattern around 10PCW and is transformed into a midgestational labyrinth-like 3-C pattern (patches, AChE-nonreactive perimeters, matrix), peaking between 22 and 28PCW during accelerated volume-growth. Finally, compartmentalization resolves perinatally, by the decrease in transient "AChE-clumping," disappearance of AChE-nonreactive, ECM-rich perimeters, and an increase in matrix reactivity. The initial "mature" pattern appears around 9 PM. Therefore, transient, a 3-C pattern and accelerated neostriatal growth coincide with the expected timing of the nonhomogeneous distribution of corticostriatal afferents. The decrease in growth-related AChE activity and transfiguration of corticostriatal terminals are putative mechanisms underlying fetal compartments reorganization. Our findings serve as normative for studying neurodevelopmental disorders.

Differential effects of age on human striatal adenosine A₁ and A(2A) receptors

The aim of this study was to investigate the effect of age on the distribution of adenosine A₁ receptors (A₁Rs) and adenosine A(2A) receptors (A(2A)Rs) in the striatum of healthy subjects using PET imaging with 8-dicyclopropylmethyl-1-[¹¹C]methyl-3-propylxanthine ([¹¹C]MPDX) and [7-methyl-¹¹C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([¹¹C]TMSX), respectively. We recruited 8 young (22.0 ± 1.7 years) and 10 elderly (65.4 ± 7.6 years) volunteers to undergo [¹¹C]MPDX PET scanning, and 11 young (22.7 ± 2.7 years) and six elderly (60.7 ± 8.5 years) volunteers to undergo [¹¹C]TMSX PET scanning. A dynamic series of decay-corrected PET scans was performed for 60 min following injection of [¹¹C]MPDX or [¹¹C]TMSX. We calculated the binding potential (BP(ND) ) of [¹¹C]MPDX and distribution volume ratio (DVR) of [¹¹C]TMSX in the striatum. The BP(ND) of [¹¹C]MPDX was significantly lower in elderly than in young subjects, both in the putamen and head of the caudate nucleus. The BP(ND) was negatively correlated with age in both the putamen and the head of the caudate nucleus. However, no difference was found between the DVR of [¹¹C]TMSX in the striata of young and elderly subjects, nor was there a correlation between age and the DVR of [¹¹C]TMSX. The effect of age on the distribution of A₁Rs in the human striatum described herein is similar to previous reports of age-related decreases in dopamine D₁ and D₂ receptors. Unlike A₁Rs, however, this study suggests that the distribution of A(2A) Rs does not change with age.

Molecular mechanisms of maternal cannabis and cigarette use on human neurodevelopment

Prenatal development is highly sensitive to maternal drug use due to the vulnerability for disruption of the fetal brain with its ongoing neurodevelopment, resulting in lifelong consequences that can enhance risk for psychiatric disorders. Cannabis and cigarettes are the most commonly used illicit and licit substances, respectively, among pregnant women. Although the behavioral consequences of prenatal cannabis and cigarette exposure have been well-documented in epidemiological and clinical studies, only recently have investigations into the molecular mechanisms associated with the developmental impact of early drug exposure been addressed. This article reviews the literature relevant to long-term gene expression disturbances in the human fetal brain in relation to maternal cannabis and cigarette use. To provide translational insights, we discuss animal models in which protracted molecular consequences of prenatal cannabis and cigarette exposure can be better explored and which enable future evaluation of epigenetic pathways, such as DNA methylation and histone modification, that could potentially maintain abnormal gene regulation and related behavioral disturbances. Altogether, this information may help to address the current gaps of knowledge regarding the impact of early drug exposure that set in motion lifelong molecular disturbances that underlie vulnerability to psychiatric disorders.

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.

Development-related expression of AKAP79 in the striatal compartments of the human brain

The expression of AKAP79 which tethers regulatory proteins within postsynaptic densities has been studied in the two striatal compartments, i.e. patches and matrix, at different stages of the developing human brain by means of immunohistochemistry. The two striatal compartments exhibit various intensities of diffuse immunolabelling and a different number of immunoreactive nerve cells. From the 14th to 20th gestational week a nearly homogeneous distribution of immunoreactive structures in the two compartments of the striatum is seen. Thereafter, a decrease in immunoreactive structures within the matrix is observed (22nd-25th week, intermediate stage). From the 27th week onwards the patch compartment contains distinctly more immunoreactive puncta and nerve cells. Thus, the patches stand out clearly in the immunopreparations. This distribution pattern does not change during proceeding development. AKAP79-immunoreactive nerve cells closely resemble those constituting the class of medium-sized inhibitory projection neurons that receive the dopaminergic input of the striatum. Literature data suggest that AKAP79 may be functionally attributed to dopaminergic inputs. Accordingly, the patterns of AKAP79 expression can at least in part be correlated with the sequential occurrence of dopaminergic innervation. The mature matrix containing a dopaminergic innervation being as dense as in the patches displays distinctly less AKAP79-immunoreactive neurons and puncta than the patches. This discrepancy might indicate that a subpopulation of matrix neurons may, despite dopaminergic input, not express AKAP79.

Changing distribution patterns of synaptophysin-immunoreactive structures in the human dorsal striatum of the fetal brain

Within the striatum two compartments, matrix and patches, can be distinguished by differences in the expression of neuroactive substances, afferent and efferent connections and time of neurogenesis. The present study was done to demonstrate the pattern of synaptophysin (SYN) expression which is indicative of synaptogenesis in the human fetal striatum (15th-32nd weeks of gestation) with special reference to developmental changes. From the 15th to the 22nd gestational weeks an intense diffuse SYN immunolabelling of striatal patches is observed. In the matrix SYN-immunoreactive fiber bundles are seen until the 20th week. Thereafter, the matrix is nearly devoid of SYN-immunoreactive structures. From the 28th week of gestation the matrix contains diffuse SYN immunoreactivity which gradually becomes as intense as that of the patches. The latter, thus, can no longer be delineated in the 30th week. The results show that fibrous SYN immunolabelling most probably indicating intra-axonal transport of synaptic vesicles can only be observed during the first half of gestation. Moreover, it becomes obvious that the patch compartment can selectively be visualized by anti-SYN until the 28th week. This pattern may correspond to the early dopaminergic innervation from the substantia nigra which is known to reach the developing patches. From the 28th week a transition from patchy to diffuse immunolabelling is seen. The increase in matrix labelling may be due to the occurrence of new neuronal contacts. The changeover from patchy to homogeneous SYN immunolabelling takes place distinctly earlier than changes in the distribution of other neuroactive substances described before.

Neurotransmission and the ontogeny of human brain

The early appearance of neurotransmitters in brain tissue refers to their regulative functions on the neuronal circuits. Many neurotransmitters have direct effects on neuronal outgrowth and differentiation during brain development, which precede their role in synaptic information coding. Both the neurotrophic and neurotoxic properties of excitatory amino acids (EAAs) have focused special interest on glutamatergic neurotransmission during brain development. Therefore, this work intends to review and discuss developmental alterations of the EAA neurotransmitter system in the human brain, their relation to human brain maturation and implications for pathological processes during early human brain development.

Ontogeny of the striatal neurons expressing the D2 dopamine receptor in humans: an in situ hybridization and receptor-binding study

D2 dopamine receptor (D2R) gene expression was analyzed by in situ hybridization and D2R ligand autoradiography in the human striatum during ontogeny. D2R mRNA and ([3H]YM-09151-2)-binding sites were detected in the striatum from week 12 of fetal life. At this time, D2R mRNA and binding sites were predominant in the putamen and occurred in a pattern of clusters. D2R-binding sites displayed a similar pattern. The signal in the caudate nucleus was weak from weeks 12 to 16. From week 20 of fetal life, D2R mRNA and D2R-binding sites signals became intense in the ventral striatum. At birth, D2R mRNA became homogeneously distributed while D2R-binding sites kept an heterogeneously distribution. Comparative topological and temporal analysis of the D2R, enkephalin and D1 dopamine receptor (D1R) mRNAs showed a distinct developmental pattern for each mRNA. Before birth, the neurons expressing enkephalin and D1R mRNAs were preferentially distributed in the matrix and in the striosomes, respectively, while the neurons expressing D2R mRNA did not display a preferential localization. At birth, high levels of enkephalin mRNA were restricted to the matrix; D1R mRNA level was homogeneous throughout the striatum. D2R mRNA was heterogeneously distributed in the whole striatum with high signals located both in the striosomes and the matrix. These results demonstrate that functional D2R are expressed as early as week 12 in the striatum with a heterogeneous distribution. Our findings also demonstrate that, in contrast to what was expected from similar studies in rodents, D2R mRNA and enkephalin mRNA do not display identical, overlapping expression patterns in striatal neurons during human ontogeny.

D1 and D2 dopamine receptors in perinatal and adult basal ganglia

There is reason to believe that dopamine is important in developmental programs of the basal ganglia, brain nuclei implicated in motor and cognitive processing. Dopamine exerts effects through dopamine receptors, which are predominantly of the D1 and D2 subtypes in the basal ganglia. Cocaine acts as a stimulant of dopamine receptors and may cause long-term abnormalities in children exposed in utero. Dopamine receptor (primarily D1) stimulation has been linked to gene regulation. Therefore, D1 and D2 receptor densities in perinatal and adult striatum and globus pallidus were examined using quantitative autoradiography. The most striking finding was that pallidal D1 receptor densities were 7-15 times greater in the perinatal cases than in the adult. Pallidal D2 receptor densities were similar at both ages. In both the adult and perinatal striatum, D2 receptor densities were greater in the putamen than in the caudate, and both D1 and D2 receptor densities were modestly enriched in caudate striosomes compared with the matrix. In both caudate and putamen, perinatal D1 receptor levels were within the adult range, whereas D2 receptor levels were only 50% of adult values. The development of D1 and D2 receptors appears to vary across the major subdivisions of the human basal ganglia. The facts that we found such extremely high levels of D1 receptors in the perinatal pallidum, and that D1 receptor activation influences gene regulation, suggest that the globus pallidus could be particularly susceptible to long-term changes with perinatal exposure to cocaine and other D1 receptor agonists or antagonists.

Localization of dopamine receptors and associated mRNA in transplants of human fetal striatal tissue in rodents with experimental Huntington's disease

Huntington's Disease (HD) is characterized by deficits in motor and cognitive functions. This neurodegenerative disease shows an extensive loss of medium-sized spiny projection neurons (GABAergic) within the neostriatum. With the loss of these neurons, there is a concomitant loss of associated receptors, such as those for GABA, glutamate, and dopamine. In the present study, we have addressed the question of whether dopamine receptors are re-established in the lesioned rodent striatum following the transplantation of human striatal cells. Human striatal cell suspension or saline (transplant controls) was injected into the striatum of rats previously lesioned with quinolinic acid (QA). Three nine months following transplantation, the animals were sacrificed and the brains were processed for receptor autoradiography and in situ hybridization of dopamine D1 and D2 receptor subtypes. Our results demonstrate that animals transplanted with human striatal cells show a significant increase in D1 receptors following transplantation when compared to the lesion area in control animals, while D1 receptor mRNA remains unchanged. In contrast to D1 receptor binding, D2 receptor levels are not increased in the lesioned and transplanted area of the striatum when compared to controls; however, D2 receptor mRNA levels are significantly increased. These results demonstrate that at the times the animals were examined, D1 and D2 receptors were differentially regulated. Our results further indicate that human striatal primordium will survive following transplantation and will express D1 receptors and D2 receptor mRNA that are depleted in the QA lesioned rodent striatum. This study compliments and extends previous findings on human striatal cell transplantation in rodent models of HD.

Molecular anatomy of the development of the human substantia nigra

A series of 15 fetal and perinatal human brains (from week 12 of fetal life to day 2 after birth) was studied in order to describe the anatomical and molecular correlates of the substantia nigra ontogeny. In situ hybridization, immunohistochemistry and binding studies were used to detect D2 dopamine receptor (D2R) mRNA, D2R binding sites, dopamine membrane transporter (DAT) mRNA, tyrosine hydroxylase (TH) protein D1 dopamine receptor (D1R) protein and D1R binding sites. Dopaminergic (DA) neurons of the substantia nigra were detected through TH immunoreactivity from week 12. At week 16, the substantia nigra was clearly delineated as a compact group of intermingled neurons and fibers. From week 19, groups of DA neurons were segregated from the pars reticulata. These groups have been divided into the substantia nigra pars compacta, the ventral tegmental area and the retrorubral area. The DA neurons exhibited a gradual increase in size and branching development until birth. From week 12 onward they expressed several other markers of dopamine transmission, i.e., D2R mRNA, D2R binding sites and DAT mRNA. The ventral tegmental area expressed lower levels of mRNA for DAT and D2R than the pars compacta. From week 12, D1R immunoreactivity and D1R binding sites were also present in the substantia nigra pars reticulata. This suggests that projecting striatonigral neurons, known to express the D1R gene, have developed pathways connecting with the substantia nigra by week 12. Our results demonstrate that the developing substantia nigra in human displays early transcriptional and translational activity for the main constituents of dopaminergic transmission from week 12 and receives at this time dopaminoceptive inputs bearing D1 receptors from the striatum.

Evolution of dopamine receptors in the rat after neonatal hypoxia-ischemia: autoradiographic studies

The aim of this work was to follow the evolution of striatal dopamine D1 and D2 receptors after hypoxic-ischemic (H/I) insult in immature rats. SPET imaging of these receptors could be used as an index to assess brain dysfunctions after perinatal H/I without change in cerebral blood flow or neuronal loss. We submitted 1-week-old rat pups to unilateral ligation of the left carotid artery plus 2h exposure to 8% O2. After recovery periods of 1, 2 or 9 weeks, ex vivo and in vitro autoradiographic studies of dopamine receptors were performed on normal appearing brains using specific ligands usable in human SPET imaging. Striatal dopamine D2 receptors tended to decrease bilaterally after one week's recovery. The decrease then reached 40% at 3 weeks of age and at 10 weeks of age the level of receptors had returned to normal values. By contrast, no change in dopamine D1 receptors was seen, whatever the age studied. In conclusion, changes in dopamine D2 receptors could be a valuable index for SPET imaging to evaluate H/I brain damage in the absence of anatomical lesions.