Rita Raisman-Vozari

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer’s disease and Parkinson’s disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimers disease and Parkinsons disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimers disease and Parkinsons disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Levodopa in the treatment of Parkinson's disease: Current controversies

Levodopa is the most effective symptomatic agent in the treatment of Parkinsons disease (PD) and the “gold standard” against which new agents must be compared. However, there remain two areas of controversy: (1) whether levodopa is toxic, and (2) whether levodopa directly causes motor complications. Levodopa is toxic to cultured dopamine neurons, and this may be a problem in PD where there is evidence of oxidative stress in the nigra. However, there is little firm evidence to suggest that levodopa is toxic in vivo or in PD. Clinical trials have not clarified this situation. Levodopa is also associated with motor complications. Increasing evidence suggests that they are related, at least in part, to the short half‐life of the drug (and its potential to induce pulsatile stimulation of dopamine receptors) rather than to specific properties of the molecule. Treatment strategies that provide more continuous stimulation of dopamine receptors provide reduced motor complications in MPTP monkeys and PD patients. These studies raise the possibility that more continuous and physiological delivery of levodopa might reduce the risk of motor complications. Clinical trials to test this hypothesis are underway. We review current evidence relating to these areas of controversy.

Divalent metal transporter 1 (DMT1) contributes to neurodegeneration in animal models of Parkinson's disease

Dopaminergic cell death in the substantia nigra (SN) is central to Parkinsons disease (PD), but the neurodegenerative mechanisms have not been completely elucidated. Iron accumulation in dopaminergic and glial cells in the SN of PD patients may contribute to the generation of oxidative stress, protein aggregation, and neuronal death. The mechanisms involved in iron accumulation also remain unclear. Here, we describe an increase in the expression of an isoform of the divalent metal transporter 1 (DMT1/Nramp2/Slc11a2) in the SN of PD patients. Using the PD animal model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication in mice, we showed that DMT1 expression increases in the ventral mesencephalon of intoxicated animals, concomitant with iron accumulation, oxidative stress, and dopaminergic cell loss. In addition, we report that a mutation in DMT1 that impairs iron transport protects rodents against parkinsonism-inducing neurotoxins MPTP and 6-hydroxydopamine. This study supports a critical role for DMT1 in iron-mediated neurodegeneration in PD.

Evolution of changes in neuronal activity in the subthalamic nucleus of rats with unilateral lesion of the substantia nigra assessed by metabolic and electrophysiological measurements

Cellular expression of cytochrome oxidase subunit I (COI) mRNA has recently been used as a metabolic marker for neuronal activity to study the functional changes in the subthalamic nucleus (STN) in parkinsonism. The previous experimental studies have been performed when the pathological state was stabilized at a maximal level. In order to determine the evolution of changes in neuronal activity in the STN after nigrostriatal denervation, we analysed by in situ hybridization the cellular expression of COI mRNA in the subthalamic neurons at different times, from 6 h to 14 days, after unilateral intranigral microinjection of 6‐hydroxydopamine (6‐OHDA) in rats. In parallel, the time‐dependent changes of the unit neuronal activity of subthalamic neurons have been recorded. Levels of COI mRNA increased by 41% in subthalamic neurons from 24 h after 6‐OHDA intoxication, to 14 days (+26%). Similarly, electrical activity started to increase slightly 24 h after lesion (+20%) and remained significantly higher at 14 days after the lesion (+189%). Changes in neuronal mean discharge rate were associated with changes in the pattern of spiking activity, from a regular firing pattern to an irregular one with a high bursting activity. These results show that: (i) the hyperactivity of the STN represents a very early phenomenon in the physiopathology of parkinsonian syndromes; and (ii) that changes in COI mRNA expression slightly precede changes in electrical neuronal activity.

An immunohistochemical study of the distribution of brain-derived neurotrophic factor in the adult human brain, with particular reference to Alzheimer's disease

Brain-derived neurotrophic factor is a member of the family of neuronal differentiation and survival-promoting molecules called neurotrophins. Neuronal populations known to show responsiveness to the action of brain-derived neurotrophic factor include the cholinergic forebrain, mesencephalic dopaminergic, cortical, hippocampal and striatal neurons. This fact has aroused considerable interest in the possible contribution of an abnormal brain-derived neurotrophic factor function to the aetiology and physiopathology of different neurodegenerative disorders, such as Alzheimers disease. This report describes the cellular and regional distribution of brain-derived neurotrophic factor in post mortem control human brain and in limited regions of the brain in patients with Alzheimers disease, as was revealed by immunohistochemistry. Brain-derived neurotrophic factor is widely expressed in the control human brain, both by neurons and glia. In neurons, brain-derived neurotrophic factor was localized in the cell body, dendrites and axons. Among the structures showing the most intense immunohistochemical labeling were the hippocampus, claustrum, amygdala, bed nucleus of the stria terminalis, septum and the nucleus of the solitary tract. In the striatum, immunoreactivity was more intense in striosomes than in the matrix. Many labeled neurons were found in the substantia nigra pars compacta. The large putatively cholinergic neurons in the basal forebrain showed no immunoreactivity. The general pattern of labeling was similar in individuals with Alzheimers disease. Brain-derived neurotrophic factor-immunoreactive material was found in senile plaques, and some immunoreactive cortical pyramidal neurons showed neurofibrillary tangles, suggesting that brain-derived neurotrophic factor may be involved in the process of neuronal degeneration and/or compensatory mechanisms which occur in this illness.

Differential expression of tyrosine hydroxylase and membrane dopamine transporter genes in subpopulations of dopaminergic neurons of the rat mesencephalon

Dopaminergic (DA) cells of the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) display differences in their topography, biochemistry and susceptibility to pathological processes. Neuronal dopamine concentration is regulated in large part by tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis, and by the dopamine reuptake system. In the present study, TH protein, TH mRNA and dopamine membrane transporter (DAT) mRNA were quantified at cellular level in 4 arbitrary subregions of the rat ventral mesencephalon (lateral, middle, medial SNC and VTA), using in situ hybridization and immunoautoradiography. The distribution of labelling for TH protein and TH mRNA was almost superimposable and close to that of DAT mRNA in mesencephalic neurons. Lower values of cellular expression in TH protein, TH mRNA and DAT mRNA were observed in the lateral part of the SNC compared to the other subregions. TH and DAT expression were correlated in SNC but not in VTA. Indeed DA cells in this region expressed low levels of DAT mRNA in comparison to the middle and medial SNC. These results suggest a heterogeneity of DA metabolism among populations of mesencephalic cells. The relative lower expression of the DAT gene in VTA neurons suggests a less efficient dopamine reuptake capacity, which may partly account for the relative sparing of the mesolimbic system reported in Parkinsons disease and MPTP-treated animals.

Dopaminergic sprouting in the rat striatum after partial lesion of the substantia nigra

The capacity of the dopaminergic nerve system to reinnervate the denervated adult striatum was analyzed in a model of partial 6-hydroxydopamine-induced unilateral lesion of rat substantia nigra pars compacta. Sprouting of dopaminergic fibers entering the ventrolateral part of the striatum from a narrow zone of the external capsule was detected on the lesioned side 4 and 7 months, but not 10 days, after lesioning. Ultrastructural examination of the zone of sprouting revealed hypertrophic dopaminergic fibers and growth-cone-like structures, confirming the existence of an ongoing process of spontaneous regrowth of dopaminergic fibers. The identification of the factors involved in the regrowth of dopaminergic fibers may help to orientate molecular research into new treatments for Parkinsons disease.

Long-term induction of tyrosine hydroxylase expression: compensatory response to partial degeneration of the dopaminergic nigrostriatal system in the rat brain.

Abstract: The present study was undertaken to examine the adaptive changes occurring 1 and 6 months after moderate or severe unilateral 6‐hydroxydopamine‐induced lesions confined to the lateral part of the rat substantia nigra pars compacta (SNC). The expression of tyrosine hydroxylase (TH) enzyme was analyzed in the remaining dopaminergic nigral cell bodies and in the corresponding striatal nerve endings. In the cell bodies of the lesioned SNC, TH mRNA content was increased (+20 to +30%) 6 months after the lesion without changes in cellular TH protein amounts. The depletion of TH protein in the nerve terminal area was less severe than the percentage of cell loss observed in the SNC at 1‐ and 6‐month postlesion intervals. Moreover, the decrease in TH protein in the ipsilateral striatum was less pronounced 6 months after lesion than 1 month after. That no corresponding change in TH protein content was observed in the cell bodies at a time when TH increased in nerve terminals suggests that the newly synthesized protein is probably rapidly transported to the striatal fibers. These results suggest the existence of a sequence of changes in TH expression between cell bodies and fibers, occurring spontaneously after partial denervation of the nigrostriatal pathway.

The alpha2-adrenoceptor antagonist dexefaroxan enhances hippocampal neurogenesis by increasing the survival and differentiation of new granule cells.

The generation of new neurons in the hippocampus is a dynamic process regulated by environmental, endocrine, and pharmacological factors. Since enhancement of hippocampal neurogenesis has been associated with learning and memory, and the locus coeruleus–noradrenergic system has been shown to modulate these cognitive functions, we hypothesized that activation of noradrenergic neurotransmission might enhance neurogenesis in the adult hippocampus. To test this hypothesis in vivo, we induced the release of noradrenaline in the hippocampus by blocking presynaptic inhibitory autoreceptors with the selective alpha2-adrenoceptor antagonist dexefaroxan. Confocal microscopy showed that noradrenergic afferents make contact with proliferating and differentiating cells, suggesting a direct noradrenergic influence on neurogenesis. Chronic systemic treatment of rats with dexefaroxan did not affect cell proliferation per se in the dentate gyrus (as monitored by bromodeoxyuridine-labeling), but promoted the long-term survival of newborn neurons by reducing apoptosis. Dexefaroxan treatment also enhanced the number and complexity of the dendritic arborizations of polysialated neural cell adhesion molecule-positive neurons. The trophic effects of dexefaroxan on newborn cells might involve an increase in brain-derived neurotrophic factor, which was upregulated in afferent noradrenergic fiber projection areas and in neurons in the granule cell layer. By promoting the survival of new endogenously formed neurons, dexefaroxan treatment represents a potential therapeutic strategy for maintaining adult neurogenesis in neurodegenerative conditions, such as Alzheimers disease, that affect the hippocampus.

An immunocytochemical study on the distribution of two G-protein-gated inward rectifier potassium channels (GIRK2 and GIRK4) in the adult rat brain.

G-protein-gated inward rectifier potassium channels mediate the synaptic actions of numerous neurotransmitters in the mammalian brain, and were recently shown to be candidates for genetic mutations leading to neuronal cell death. This report describes the localization of G-protein-gated inward rectifier potassium channel-2 and G-protein-gated inward rectifier potassium channel-4 proteins in the rat brain, as assessed by immunocytochemistry. G-protein-gated inward rectifier potassium channel-2 immunoreactivity was widely distributed throughout the brain, with the strongest staining seen in the hippocampus, septum, granule cell layer of the cerebellum, amygdala and substantia nigra pars compacta. In contrast, G-protein-gated inward rectifier potassium channel-4 immunoreactivity was restricted to some neuronal populations, such as Purkinje cells and neurons of the globus pallidus and the ventral pallidum. The presence of G-protein-gated inward rectifier potassium channel-2 immunoreactivity in substantia nigra pars compacta dopaminergic neurons was confirmed by showing its co-localization with tyrosine hydroxylase by double immunocytochemistry, and also by selectively lesioning dopaminergic neurons with the neurotoxin 6-hydroxydopamine. At the cellular level both proteins were localized in neuronal cell bodies and dendrites, but clear differences were seen in the degree of dendritic staining among neuronal groups. For some neuronal groups the staining of distal dendrites (notably dendritic spines) was strong, while for others the cell body and proximal dendrites were preferentially labelled. In addition, some of the results suggest that G-protein-gated inward rectifier potassium channel-2 protein could be localized in distal axonal terminal fields. A knowledge of the distribution of G-protein-gated inward rectifier potassium channel proteins in the brain could help to elucidate their physiological roles and to evaluate their potential involvement in neurodegenerative processes in animal models and human diseases.