Retrograde tracing techniques influence reported death rates of adult rat nigrostriatal neurons

Investigation of the therapeutic potential of N-acetyl cysteine and the tools used to define nigrostriatal degeneration in vivo

The glutathione precursor N-acetyl-L-cysteine (NAC) is currently being tested on Parkinson's patients for its neuroprotective properties. Our studies have shown that NAC can elicit protection in glutathione-independent manners in vitro. Thus, the goal of the present study was to establish an animal model of NAC-mediated protection in which to dissect the underlying mechanism. Mice were infused intrastriatally with the oxidative neurotoxicant 6-hydroxydopamine (6-OHDA; 4 μg) and administered NAC intraperitoneally (100mg/kg). NAC-treated animals exhibited higher levels of the dopaminergic terminal marker tyrosine hydroxylase (TH) in the striatum 10d after 6-OHDA. As TH expression is subject to stress-induced modulation, we infused the tracer FluoroGold into the striatum to retrogradely label nigrostriatal projection neurons. As expected, nigral FluoroGold staining and cell counts of FluoroGold(+) profiles were both more sensitive measures of nigrostriatal degeneration than measurements relying on TH alone. However, NAC failed to protect dopaminergic neurons 3 weeks following 6-OHDA, an effect verified by four measures: striatal TH levels, nigral TH levels, nigral TH(+) cell counts, and nigral FluoroGold levels. Some degree of mild toxicity of FluoroGold and NAC was evident, suggesting that caution must be exercised when relying on FluoroGold as a neuron-counting tool and when designing experiments with long-term delivery of NAC--such as clinical trials on patients with chronic disorders. Finally, the strengths and limitations of the tools used to define nigrostriatal degeneration are discussed.

The presence of perforated synapses in the striatum after dopamine depletion, is this a sign of maladaptive brain plasticity?

Synaptic plasticity is the process by which long-lasting changes take place at synaptic connections. The phenomenon itself is complex and can involve many levels of organization. Some authors separate forms into adaptations that have positive or negative consequences for the individual. It has been hypothesized that an increase in the number of synapses may represent a structural basis for the enduring expression of synaptic plasticity during some events that involve memory and learning; also, it has been suggested that perforated synapses increase in number after some diseases and experimental situations. The aim of this study was to analyze whether dopamine depletion induces changes in the synaptology of the corpus striatum of rats after the unilateral injection of 6-OHDA. The findings suggest that after the lesion, both contralateral and ipsilateral striata exhibit an increased length of the synaptic ending in ipsilateral (since third day) and contralateral striatum (since Day 20), loss of axospinous synapses in ipsilateral striatum and a significant increment in the number of perforated synapses, suggesting brain plasticity that might be deleterious for the spines, because this type of synaptic contacts are presumably excitatory, and in the absence of the modulatory effects of dopamine, the neuron could die through excitotoxic mechanisms. Thus, we can conclude that the presence of perforated synapses after striatal dopamine depletion might be a form of maladaptive synaptic plasticity.

The combination of oral L-DOPA/rimonabant for effective dyskinesia treatment and cytological preservation in a rat model of Parkinson's disease and L-DOPA-induced dyskinesia

Parkinson's disease is the second most prevalent neurodegenerative disease in the world. Its treatment is limited so far to the management of parkinsonian symptoms with L-DOPA (LD). The long-term use of LD is limited by the development of L-DOPA-induced dyskinesias and dystonia. However, recent studies have suggested that pharmacological targeting of the endocannabinoid system may potentially provide a valuable therapeutic tool to suppress these motor alterations. In the present study, we have explored the behavioral (L-DOPA-induced dyskinesias severity) and cytological (substantia nigra compacta neurons and striatum neuropil preservation) effects of the oral coadministration of LD and rimonabant, a selective antagonist of CB1 receptors, in the 6-hydroxydopamine rat model of Parkinson's disease. Oral coadministration of LD (30 mg/kg) and rimonabant (1 mg/kg) significantly decreased abnormal involuntary movements and dystonia, possibly through the conservation of some functional tyrosine hydroxylase-immunoreactive dopaminergic cells, which in turn translates into a well-preserved neuropil of a less denervated striatum. Our results provide anatomical evidence that long-term coadministration of LD with cannabinoid antagonist-based therapy may not only alleviate specific motor symptoms but also delay/arrest the degeneration of striatal and substantia nigra compacta cells.

Musculotopic organization of the motor neurons supplying the mouse hindlimb muscles: a quantitative study using Fluoro-Gold retrograde tracing

We have mapped the motor neurons (MNs) supplying the major hindlimb muscles of transgenic (C57/BL6J-ChAT-EGFP) and wild-type (C57/BL6J) mice. The fluorescent retrograde tracer Fluoro-Gold was injected into 19 hindlimb muscles. Consecutive transverse spinal cord sections were harvested, the MNs counted, and the MN columns reconstructed in 3D. Three longitudinal MN columns were identified. The dorsolateral column extends from L4 to L6 and consists of MNs innervating the crural muscles and the foot. The ventrolateral column extends from L1 to L6 and accommodates MNs supplying the iliopsoas, gluteal, and quadriceps femoris muscles. The middle part of the ventral horn hosts the central MN column, which extends between L2 and L6 and consists of MNs for the thigh adductor, hamstring, and quadratus femoris muscles. Within these longitudinal columns, the arrangement of the different MN groups reflects their somatotopic organization. MNs innervating muscles developing from the dorsal (e.g., quadriceps) and ventral muscle mass (e.g., hamstring) are situated in the lateral and medial part of the ventral gray, respectively. MN pools belonging to proximal muscles (e.g., quadratus femoris and iliopsoas) are situated ventral to those supplying more distal ones (e.g., plantar muscles). Finally, MNs innervating flexors (e.g., posterior crural muscles) are more medial than those belonging to extensors of the same joint (e.g., anterior crural muscles). These data extend and modify the MN maps in the recently published atlas of the mouse spinal cord and may help when assessing neuronal loss associated with MN diseases.

c-Fos expression in neurons projecting from the preoptic and lateral hypothalamic areas to the ventrolateral periaqueductal gray in relation to sleep states

The ventrolateral division of the periaqueductal gray (vlPAG) and the adjacent deep mesencephalic reticular nucleus have been implicated in the control of sleep. The preoptic hypothalamus, which contains populations of sleep-active neurons, is an important source of afferents to the vlPAG. The perifornical lateral hypothalamus (LH) contains populations of wake-active neurons and also projects strongly to the vlPAG. We examined nonREM and REM sleep-dependent expression of c-Fos protein in preoptic-vlPAG and LH-vlPAG projection neurons identified by retrograde labeling with Fluorogold (FG). Separate groups of rats (n=5) were subjected to 3 h total sleep deprivation (TSD) followed by 1 h recovery sleep (RS), or to 3 h of selective REM sleep deprivation (RSD) followed by RS. A third group of rats (n=5) was subjected to TSD without opportunity for RS (awake group). In the median preoptic nucleus (MnPN), the percentage of FG+ neurons that were also Fos+ was higher in TSD-RS animals compared to both RSD-RS rats and awake rats. There were significant correlations between time spent in deep nonREM sleep during the 1 h prior to sacrifice across groups and the percentage of double-labeled cells in MnPN and ventrolateral preoptic area (VLPO). There were no significant correlations between percentage of double-labeled neurons and time spent in REM sleep for any of the preoptic nuclei examined. In the LH, percentage of double-labeled neurons was highest in awake rats, intermediate in TSD-RS rats and lowest in the RSD-RS group. These results suggest that neurons projecting from MnPN and VLPO to the vlPAG are activated during nonREM sleep and support the hypothesis that preoptic neurons provide inhibitory input to vlPAG during sleep. Suppression of excitatory input to the vlPAG from the LH during sleep may have a permissive effect on REM sleep generation.

Time course changes of the striatum neuropil after unilateral dopamine depletion and the usefulness of the contralateral striatum as a control structure

INTRODUCTION

After unilateral dopamine depletion, some ipsilateral alterations occur and the contralateral structure has been utilized as control.

OBJECTIVE

Our aim is to analyse the evolution of the ultrastructural alterations of the ipsilateral and contralateral striata of the 6-hydroxydopamine lesioned rats to demonstrate that the contralateral striatum should not be used as control structure.

METHODS

After the surgery and the rotation behavior evaluation, animals were killed from 3 to 120 days after lesioning, and their striata were compared with those of aged rats.

RESULTS

The ultrastructural analysis shows increased diameter of the synaptic ending in ipsilateral (since the third day) and contralateral striata (since day 30) and an increase in perforated synaptic contacts.

CONCLUSION

Our data suggest that the contralateral striatum should not be taken as control structure at least after 20-30 days after lesioning, as the alterations found here may result in wrong interpretations when comparing with the ipsilateral-lesioned one.

Readministration of adenoviral gene delivery to dopamine neurons

An approach currently being explored as treatment for Parkinson's disease is gene therapy. An important question concerns the duration of transgene expression in dopamine neurons and the issues of vector persistence, neuronal damage and the feasibility of readministering vector to the same neuronal population. We show, using an adenoviral vector expressing the LacZ reporter gene, that transgene expression declined over time but with minimal loss of dopamine neurons or vector DNA. Readministration of vector resulted in low levels of transgene delivery to the neurons. Moreover, the neurons to which vector had already been delivered were unable to transport the retrograde tracer fluorogold. Our findings indicate that transgene expression declined in dopamine neurons despite the persistence of virus, and the capacity to readminister vector to these neurons was limited.

Protein tyrosine phosphatase inhibition reduces degeneration of dopaminergic substantia nigra neurons and projections in 6-OHDA treated adult rats

The survival of injured adult dopaminergic substantia nigra pars compacta neurons can be promoted by various neurotrophic factors. Most neurotrophic factor receptors are activated by intracellular tyrosine phosphorylation upon ligand binding and are subsequently inactivated or dephosphorylated by protein tyrosine phosphatases. This raised the possibility that tyrosine phosphatase inhibition might improve neuronal survival. Here, we infused the stable water-soluble tyrosine phosphatase-specific inhibitor, peroxovanadium [potassium bisperoxo(1,10-phenanthroline)oxovanadate (V) (bpV(phen))], for 14 days close to the substantia nigra starting immediately after a unilateral moderate injury by injection of the neurotoxin 6-hydroxydopamine (6-OHDA) into the midbrain of adult Sprague-Dawley rats. The dopaminergic nigrostriatal neurons were identified by retrograde tracing with fluorogold 7 days prior to the injury. With infusion of 3 or 10 microm peroxovanadium, 75% of these neurons survived compared to 45% in vehicle-infused rats. Degeneration of the dopaminergic projections to the neostriatum was also reduced by 10 microm peroxovanadium. Twenty minutes after an acute injection of peroxovanadium into the substantia nigra, increased tyrosine phosphorylation in Western blots of nigral extracts was seen in the same protein bands as after injections of brain-derived neurotrophic factor (BDNF) or NT-4. This suggests that peroxovanadium enhances endogenous neurotrophic signalling resulting in improved neuronal survival. The neuroprotective effects of this small molecule protein tyrosine phosphatase inhibitor represent a proof-of-principle for a novel treatment strategy in a model for Parkinson's disease.

Fluorogold labeling of descending brain neurons in larval lamprey does not cause cell death

In our previous double-labeling studies, the fluorescent anatomical tracers Fluorogold (FG) and Texas red dextran amine (TRDA) were used to demonstrate that descending brain neurons, approximately 80% of which are reticulospinal (RS) neurons, in spinal cord-transected larval lamprey regenerate their axons. However, the numbers of FG-labeled descending brain neurons decreased significantly with increasing recovery times, from 2 to 16 weeks. For some FG-labeled mammalian neurons, FG appears to degrade and/or be lost over time, while in other neurons this tracer can kill neurons. In the present study, these possibilities were examined in larval lamprey for FG-labeled descending brain neurons. As in our previous studies, FG was applied to the spinal cord at 40% body length (BL, relative distance from the head) to retrogradely labeled descending brain neurons, and after recovery times of 2, 8, or 16 weeks, HRP, a non-toxic retrograde tracer, was applied to the spinal cord at 20% BL to determine if the numbers of HRP-labeled neurons were reduced. At these three recovery times, the numbers of HRP-labeled descending brain neurons were not significantly different than the numbers of HRP-labeled neurons in control animals that were not labeled with FG. Furthermore, the size and morphology of cell bodies and dendritic trees were not noticeably different in descending brain neurons with and without FG. Thus, in larval lamprey, FG does not appear to kill these neurons, but some FG probably is degraded and/or lost from neurons with increasing recovery times.

Blueberry- and spirulina-enriched diets enhance striatal dopamine recovery and induce a rapid, transient microglia activation after injury of the rat nigrostriatal dopamine system

Neuroinflammation plays a critical role in loss of dopamine neurons during brain injury and in neurodegenerative diseases. Diets enriched in foods with antioxidant and anti-inflammatory actions may modulate this neuroinflammation. The model of 6-hydroxydopamine (6-OHDA) injected into the dorsal striatum of normal rats, causes a progressive loss of dopamine neurons in the ventral mesencephalon. In this study, we have investigated the inflammatory response following 6-OHDA injected into the striatum of adult rats treated with diet enriched in blueberry or spirulina. One week after the dopamine lesion, a similar size of dopamine degeneration was found in the striatum and in the globus pallidus in all lesioned animals. At 1 week, a significant increase in OX-6- (MHC class II) positive microglia was found in animals fed with blueberry- and spirulina-enriched diets in both the striatum and the globus pallidus. These OX-6-positive cells were located within the area of tyrosine hydroxylase (TH) -negativity. At 1 month after the lesion, the number of OX-6-positive cells was reduced in diet-treated animals while a significant increase beyond that observed at 1 week was now present in lesioned control animals. Dopamine recovery as revealed by TH-immunohistochemistry was significantly enhanced at 4 weeks postlesion in the striatum while in the globus pallidus the density of TH-positive nerve fibers was not different from control-fed lesioned animals. In conclusion, enhanced striatal dopamine recovery appeared in animals treated with diet enriched in antioxidants and anti-inflammatory phytochemicals and coincided with an early, transient increase in OX-6-positive microglia.

No evidence for new dopaminergic neurons in the adult mammalian substantia nigra

A recent report by Zhao et al. [Zhao, M., Momma, S., Delfani, K., Carlen, M., Cassidy, R. M., Johansson, C. B., Brismar, H., Shupliakov, O., Frisen, J. & Janson, A. M. (2003) Proc. Natl. Acad. Sci. USA 100, 7925-7930] suggests that dopaminergic neurons, the cell type lost in Parkinson's disease, are continuously generated in the adult substantia nigra pars compacta. Using similar methodological procedures to label dividing cells, we found no evidence of new dopaminergic neurons in the substantia nigra, either in normal or 6-hydroxydopamine-lesioned hemi-Parkinsonian rodents, or even after growth factor treatment. Furthermore, we found no evidence of neural stem cells emanating from the cerebroventricular system and migrating to the substantia nigra. We conclude that it is unlikely that dopaminergic neurons are generated in the adult mammalian substantia nigra.

Vulnerabilities of ventral mesencephalic neurons projecting to the nucleus accumbens following infusions of 6-hydroxydopamine into the medial forebrain bundle in the rat

The terminal arbors of dopaminergic projections in the nucleus accumbens (Acb) core degenerate more rapidly, completely and permanently in a variety of neurotoxic circumstances than do those in the medial shell. It is unknown if this always reflects purely losses of the distal parts of axons from the core (as proposed in methamphetamine intoxication), or whether, in some circumstances, the disproportionate loss of core axons may also stem from an intrinsic vulnerability to degeneration of core-projecting neuronal perikarya. Experiments described here addressed this issue in the following manner. Three days after Fluoro-Gold (FG), a retrogradely transported tracer, had been iontophoresed selectively into the core or medial shell of male Sprague-Dawley rats, each received an infusion of saline vehicle containing or lacking 6-hydroxydopamine (6-OHDA) in the ipsilateral medial forebrain bundle (MFB). Twenty-one days later the brains were processed to exhibit ventral mesencephalic neurons containing FG. Application of an unbiased sampling method revealed substantially greater losses of FG labeled neurons relative to controls in rats that had received 6-OHDA lesions and deposition of FG in the Acb core as compared to the medial shell. Of the few core-projecting neurons that remained in the ventral mesencephalon after these lesions, 54% did not co-localize tyrosine hydroxylase immunoreactivity (TH-ir) and, thus, were not expected to degenerate. The capacity to selectively remove core-projecting dopaminergic neurons may be useful in the determination of molecular correlates of vulnerability and resistance to neurotoxicity and to possibly test the role of the core in reinforcement paradigms.