Hippocampal pyramidal cell response to 6-hydroxydopamine lesions of the rat ventral tegmental area

Reduced monoaminergic nuclei MRI signal detectable in pre-symptomatic older adults with future memory decline

Evidence from murine models and human post-mortem studies indicates that monoaminergic nuclei undergo degeneration at the pre-symptomatic stage of Alzheimer’s disease (AD). Analysing 129 datasets from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) and relying on the Clinical Dementia Rating as group-defining instrument, we hypothesised that the MRI signal of monoaminergic nuclei would be a statistically significant predictor of memory decline in participants initially recruited in ADNI as healthy adults. As opposed to a group of cognitively stable participants, participants developing memory decline had reduced signal in the ventral tegmental area at baseline, before any evidence of functional decline emerged. These findings indicate that monoaminergic degeneration predates the onset of memory decline in an AD-centred initiative, with a crucial involvement of very-early changes of a dopaminergic region. This translates into potential informative avenues for pharmacological treatment of pre-symptomatic AD.

Deafferentation-induced abnormal neurofilament phosphorylation in red nucleus neurones

Hippocampal deafferentation has been proposed as a pathogenetic mechanism for neurofibrillary tangle (NFT) formation in human mesolimbocortical dementia. We previously developed a rodent model of hippocampal deafferentation involving bilateral destructive lesions of the ventrotegmental area (VTA), septum of the medial forebrain and entorhinal cortex combined with pharmacological inhibition of serotonin 5-HT2 and dopamine D1 receptors. Unexpectedly, we observed an alteration in phosphorylated neurofilament protein immunoreactivity and argyrophilia in magnocellular neurones of the red nucleus. Here, we determined the neuroanatomical, pharmacological and temporal requirements for this effect on red nucleus neurones. We found that abnormal phosphorylation and argyrophilia were critically dependent on bilateral destruction of the VTA and antagonism of 5-HT2 receptors. Although extensive neurofilament hyperphosphorylation and argyrophilia were observed in red nucleus magnocellular neurones within nine days of treatment, no NFTs were formed and these effects were transitory. Resolution of these cytoskeletal abnormalities was accompanied by increased expression of the calcium binding protein, parvalbumin, suggesting that alterations in intraneuronal calcium levels may modify the deafferentation response.

6-hydroxydopamine-induced lesions of dopaminergic neurons alter the function of postsynaptic cholinergic neurons without changing cytoskeletal proteins

The neuropathological hallmarks of many neurodegenerative diseases are intraneuronal inclusions containing cytoskeletal proteins such as neurofilaments in Lewy bodies in Parkinson's disease and tau in neurofibrillary tangles in Alzheimer's disease. Dysfunction in dopaminergic and cholinergic systems also exist in both Alzheimer's disease and Parkinson's disease. Because the primary pathology in Parkinson's disease is localized to the dopaminergic system, we set out to determine if perturbations in cholinergic systems are a consequence of dopaminergic neuron loss. Therefore, following intracerebral microinjections of 6-hydroxydopamine in rats, the activity of cholinergic neurons was measured by hemicholinium binding in cholinergic terminal fields and perturbations in cytoskeletal proteins were examined in dopaminoceptive neurons using immunocytochemistry. The 6-hydroxydopamine injections robustly reduced the number of monoaminergic cell bodies in the lateral midbrain and dramatically decreased dopamine and its major metabolites in dopaminergic projection sites. This treatment increased hemicholinium binding in the prefrontal cortex (200%) and amygdala (284%); however, despite previous reports to the contrary, there were no increases in immunoreactivity for phosphorylated neurofilaments, microtubule-associated protein (MAP) 2, tau or paired helical filament (PHF) tau. This lack of an increase in cytoskeletal proteins was observed following either injections of moderate doses of the toxin directly into the medial forebrain bundle or after high doses were administered intracerebroventricularly. These results suggest that removal of dopaminergic inputs to the forebrain results in hyperactivity of the cholinergic systems but is not sufficient to induce postsynaptic perturbations in cytoskeletal proteins which occur in neurodegenerative diseases.

The dopaminergic mesencephalic projections to the hippocampal formation in the rat

1. The dopaminergic mesencephalic neurons projecting to the hippocampal formation are distributed in three cell groups: A8 region in the retrorubral field, A9 region in the substantia nigra and A10 region in the ventral tegmental area. 2. Anterograde and retrograde tract-tracing techniques combined with immunohistochemical procedures indicate a topographical organization of mesencephalic dopaminergic projections towards the hippocampal formation. 3. Electrophysiological evidence suggest that dopaminergic mesencephalic neurons could have a regulatory role in suppressing hippocampal excitability. 4. The functional significance of the mesohippocampal dopaminergic system is largely unknown, although it was suggested that this projection could have a role in methamphetamine-produced hypermotility and in modulation of memory processes.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Specific lesions in the extrapyramidal system of the rat brain induced by 3-nitropropionic acid (3-NPA)

The irreversible mitochondrial toxin 3-nitropropionic acid (3-NPA) is a specific inhibitor of succinate dehydrogenase. We performed stereotaxic unilateral injections of 3-NPA into the nigrostriatal dopaminergic pathway in rats in order to examine its specific effects on the dopamine system. The 3-NPA-treated rats displayed unidirectional apomorphineinduced rotations, suggesting that 3-NPA selectively damages dopaminergic neurons when injected into the nigrostriatal pathway. In situ hybridization 7 weeks postinjection indicated a decrease in tyrosine hydroxylase (TH) mRNA to 30% of the noninjected side in the substantia nigra pars compacta (P < 0.05) and decreased to 62% of the noninjected side in the ventral tegmental area (VTA) (nonsignificant) of 3-NPA-lesioned rats. The number of TH mRNA positive cells showed statistically significant decreases in substantia nigra and VTA (P < 0.001) within the lesioned side. In contrast, expression of mRNAs encoding choline acetyltransferase, p75 low-affinity NGF receptor, neurotrophin tyrosine kinase receptors Trk and TrkB, and brain-derived neurotrophic factor showed neuronal sparing in several other regions of the brain. The results suggest that the nigrostriatal dopaminergic system might be selectively vulnerable to 3-NPA and demonstrate that it is possible to employ 3-NPA in a model of partial lesion of the nigrostriatal dopaminergic system resembling early stages of Parkinson's disease.

Denervation induced abnormal phosphorylation in hippocampal neurons

This study demonstrates that combined dopaminergic and cholinergic denervation of the hippocampus results in the appearance of morphologically altered, Tau reactive, apical dendrites of granule cells in the rat dentate gyrus. The denervated granule cells and their apical dendrites also display immunoreactivity to a mitogen-activated protein kinase, ERK-1, and also evidence of abnormal phosphorylation of these dendrites as revealed by SMI-31 immunoreactivity. Dopaminergic denervation alone also causes mitogen activated protein kinase reactivity without the Tau-reactive apical dendrities. These results suggest an analogy to synaptophysin loss and the appearance of dendritic threads described in Alzheimer's disease (AD), as an early stage in the formation of neurofibrillary tangles (NFT). This is the first animal model in which abnormal phosphorylation of Tau has been shown to be produced experimentally in vivo.

Immunoreactive changes resulting from dopaminergic denervation of the dentate gyrus of the rat hippocampal formation

The hypothesis that dopaminergic denervation is a factor in the development of hippocampal neurofibrillary tangles was tested in the rat with bilateral stereotaxic 6-hydroxydopamine (6-OHDA) lesions of the ventral tegmental area (VTA). This led to the postsynaptic appearance of cytoplasmic immunoreactivity to ubiquitin in neurons of the dentate gyrus. An additional postsynaptic morphologic abnormality was seen when the animals were pretreated with the D1 dopaminergic antagonist SCH 23390 and the VTA lesions were combined with septal lesions affecting cholinergic and GABAergic neurons projecting to the dentate gyrus. These changes consisted of a loss of dendritic microtubule associated protein (MAP-2) and tau immunoreactivity and a prominence of remaining dendrites in the molecular layer of the dentate gyrus. Additional investigation will be needed to determine if these transynaptic changes in dentate neurons, resulting from denervation, represent a precursor stage to neurofibrillary tangle development.

Cholinoceptive cells in rat cerebral cortex: somatodendritic immunoreactivity for muscarinic receptor and cytoskeletal proteins

Adult rat telecephalon was surveyed for cells demonstrating immunopositivity for muscarinic receptor (M35 antibody), microtubule-associated proteins, neurofilaments, and brain-spectrin. Neurons immunostained for muscarinic receptor were found in frontal, parietal, temporal, and occipital isocortex where they accounted for approximately 15-16% of all neurons. This labeling involved a large proportion of layer V pyramidal cells, some layer III pyramidal cells and a small proportion of non-pyramidal cells in layers II-VI. In the hippocampus, pyramidal cells, non-pyramidal cells and granular cells were immunoreactive, as were many pyramidal cells in subicular and entorhinal cortices. In every cortical region examined, cells demonstrating muscarinic receptor were morphologically identical to cells stained lightly to moderately for acetylcholinesterase following pretreatment with diisopropylfluorophosphate, and they were found in similar numbers and in a similar laminar distribution. These characteristics further corresponded to those of cells whose somatodendritic compartments were intensely immunostained by antibodies to microtubule-associated proteins (MAP): MAP-1, MAP-2, MAP-5; neurofilament proteins (NF): NF-68kD, NF-160kD, NF-200kD; and brain-spectrin. Double immunostaining using a fluorescence method followed by an avidin-biotin staining procedure revealed that cortical cells which possessed immunoreactivity for muscarinic receptor demonstrated an 80-85% overlap with cells that were immunoreactive for MAP-2 (and tau) or NF-200kD. Following unilateral ibotenic acid lesions of the nucleus basalis, MAP-2 immunostaining was reduced in the ipsilateral isocortex. This significant reduction was most evident in the parietal cortex, exactly where maximal loss of acetylcholinesterase-containing fibers occurred. The same lesion produced no significant difference in immunodensity of muscarinic receptor, MAP-1, MAP-5 NF-68kD, NF-160kD and NF-200kD. Thus, cortical cholinoceptive cells are enriched with cytoskeletal components and cholinergic afferents modulate cortical MAP-2.