Alberto Sanchez

Parkinson's disease as a result of aging

It is generally considered that Parkinsons disease is induced by specific agents that degenerate a clearly defined population of dopaminergic neurons. Data commented in this review suggest that this assumption is not as clear as is often thought and that aging may be critical for Parkinsons disease. Neurons degenerating in Parkinsons disease also degenerate in normal aging, and the different agents involved in the etiology of this illness are also involved in aging. Senescence is a wider phenomenon affecting cells all over the body, whereas Parkinsons disease seems to be restricted to certain brain centers and cell populations. However, reviewed data suggest that Parkinsons disease may be a local expression of aging on cell populations which, by their characteristics (high number of synaptic terminals and mitochondria, unmyelinated axons, etc.), are highly vulnerable to the agents promoting aging. The development of new knowledge about Parkinsons disease could be accelerated if the research on aging and Parkinsons disease were planned together, and the perspective provided by gerontology gains relevance in this field.

Magnesium sulphate injected subcutaneously suppresses autotomy in peripherally deafferented rats

&NA; In rats, recent evidence suggests that injury discharge caused by peripheral nerve section releases excitatory amino acids into the spinal cord which in turn influences decisively the development of autotomy, a self‐mutilation behaviour directed towards the denervated areas. Autotomy has been proposed as a behavioural correlate of the neuropathic pain which occurs in humans after complete nerve lesions. Mg2+ ions have been shown to offer protection from neurological and degenerative disorders in which excitatory amino acids are putatively involved. To ascertain the preventive value of Mg2+ administration on autotomy, male rats underwent unilateral ligation and transection of the sciatic and saphenous nerves 30 min after being injected subcutaneously (s.c.) with 300 or 600 mg/kg MgSO4 or saline. Thereafter, autotomy was monitored for 8 weeks. Serum, lumbosacral (L1–S1) and brain magnesium levels were analyzed 0, 30, 60, 120, 180, 240, 360 min and 24 h after the s.c. injection of 600 mg/kg MgSO4. Serum magnesium levels increased quickly from 1.02 mM (0 time) to 4.52 mM (at 60 min) and dropped afterwards to reach physiological levels at 6 h. Peak increments in L1–S1 and brain Mg2+ levels were smaller (32% and 30%, respectively) although maintained for at least 6 h. Magnesium pretreatment in a significant and dose‐dependent manner (1) largely suppressed autotomy, (2) decreased final autotomy scores, (3) delayed autotomy onset, and (4) decreased the percentage of animals engaged in high autotomy behaviors. The data support a role for excitatory amino acids in determining susceptibility to autotomy and suggest a hopeful way to prevent neuropathic pain in humans after peripheral deafferentation.

Modulation of neuropathic pain in rats by intrathecally injected serotonergic agonists

THE involvement of spinal cord serotonergic influences in the development of autotomy, a proposed behavioural model of denervation pain, was studied in rats subjected to sciatic and saphenous nerve transection 5min after intrathecal injection of 100 or 200 μg of several serotonergic receptor subtype agonists. Injection of 8-OH- DPAT, m-CPP, 2-m-5-HT and a low dose of 5-HT, significantly shifted one or more of the parameters describing autotomy to less intense behaviour. In contrast, the injection of CGS-12066B and DOI intensified autotomy. These results suggest both a modulatory role for spinal cord serotonin in the events occurring shortly after neurectomy and new therapeutic approaches for the prevention of certain pain syndromes, such as phantom limb pain.

Relationship between autotomy behaviour and spinal cord monoaminergic levels in rats

&NA; In the rat, unilateral neurectomy of the sciatic and saphenous nerves causes autotomy, a self‐mutilation behaviour, against the denervated limb that is variable in both its onset and severity. To study some of the possible neurochemical sources of this variability, spinal cord levels of norepinephrine (NE), dopamine (DA), serotonin (5‐HT) and 5‐hydroxyindoleacetic acid (5‐HIAA) were analysed ipsi‐ and contralateral to the lesioned side by high performance liquid chromatography at C5‐T1 and 2L1‐S1. According to the early or late onset and to the slight or intense autotomy behaviour, the animals were assigned to four different groups: early autotomy, early no autotomy, late autotomy, and late no autotomy. Two sham‐operated groups were sacrificed at an early or late stage in the postoperative period. The spinal cord NE content remained unchanged throughout the different experimental situations. The more conspicuous changes observed were:a generalized increase in spinal 5‐HT metabolism in all deafferented groups;a significant and selective increase in lumbosacral 5‐HT and 5‐HIAA levels of the rats that did not self‐lesion for 8 weeks after deafferentation anda significant fall (30–45%) in DA levels at denervated spinal segments of the rats that actively self‐attacked late in the postoperative period. The data suggest that spinal cord serotonergic and dopaminergic influences play an important role in determining the susceptibility to autotomy (and perhaps chronic pain) after peripheral deafferentation.

The degeneration and replacement of dopamine cells in Parkinson's disease: the role of aging

Available data show marked similarities for the degeneration of dopamine cells in Parkinson’s disease (PD) and aging. The etio-pathogenic agents involved are very similar in both cases, and include free radicals, different mitochondrial disturbances, alterations of the mitophagy and the ubiquitin-proteasome system. Proteins involved in PD such as α-synuclein, UCH-L1, PINK1 or DJ-1, are also involved in aging. The anomalous behavior of astrocytes, microglia and stem cells of the subventricular zone (SVZ) also changes similarly in aging brains and PD. Present data suggest that PD could be the expression of aging on a cell population with high vulnerability to aging. The future knowledge of mechanisms involved in aging could be critical for both understanding the etiology of PD and developing etiologic treatments to prevent the onset of this neurodegenerative illness and to control its progression.

The astrocytic response to the dopaminergic denervation of the striatum

Increasing evidence suggests that the dopaminergic degeneration which characterizes Parkinsons disease starts in the striatal dopamine terminals and progresses retrogradely to the body of dopamine cells in the substantia nigra. The role of striatal astrocytes in the striatal initiation of the dopaminergic degeneration is little known. This work was aimed at studying the astrocytic response to the dopaminergic denervation of the striatum. The injection of 6‐hydroxydopamine (25 μg) in the lateral ventricle of adult Sprague–Dawley rats induced a fast (4 h) and selective (unaccompanied by unspecific lesions of striatal tissue or microgliosis) degeneration of the dopaminergic innervation of the striatum which was followed by a selective astrocytosis unaccompanied by microgliosis. This astrocytosis was severe and had a specific profile which included some (e.g. up‐regulation of glial fibrillary acidic protein, GS, S100β, NDRG2, vimentin) but not all (e.g. astrocytic proliferation or differentiation from NG2 cells, astrocytic scars, microgliosis) the characteristics observed after the non‐selective lesion of the striatum. This astrocytosis is similar to those observed in the parkinsonian striatum and, because it is was unaccompanied by changes in other striatal cells (e.g. by microgliosis), it may be suitable to study the role of striatal astrocytes during the dopaminergic denervation which characterizes the first stages of Parkinsons disease.

The degeneration of dopaminergic synapses in Parkinson's disease: A selective animal model.

Available evidence increasingly suggests that the degeneration of dopamine neurons in Parkinsons disease starts in the striatal axons and synaptic terminals. A selective procedure is described here to study the mechanisms involved in the striatal denervation of dopaminergic terminals. This procedure can also be used to analyze mechanisms involved in the dopaminergic re-innervation of the striatum, and the role of astrocytes and microglia in both processes. Adult Sprague-Dawley rats were injected in the lateral ventricles with increasing doses of 6-hydroxydopamine (12-50 μg), which generated a dose-dependent loss of dopaminergic synapses and axons in the striatum, followed by an axonal sprouting (weeks later) and by a progressive recovery of striatal dopaminergic synapses (months later). Both the degeneration and regeneration of the dopaminergic terminals were accompanied by astrogliosis. Because the experimental manipulations did not induce unspecific damage in the striatal tissue, this method could be particularly suitable to study the basic mechanisms involved in the distal degeneration and regeneration of dopaminergic nigrostriatal neurons, and the possible role of astrocytes and microglia in the dynamics of both processes.

Effects of selective neurotoxic lesion of lumbosacral serotonergic and noradrenergic systems on autotomy behaviour in rats

&NA; Male rats underwent unilateral ligation and transection of the sciatic and saphenous nerves 2, 7 or 14 days after being injected intrathecally (at the thoracolumbar junction) with 6‐hydroxydopamine (6‐OHDA), 5,6‐dihydroxytryptamine (5,6‐DHT) or vehicle, and the development of autotomy was monitored. The effects of both neurotoxins on cervicothoracic (C5‐T1) and lumbosacral (L1‐S1) norepinephrine (NE), dopamine (DA) and 5‐hydroxytryptamine (5‐HT) spinal cord levels were analysed by HPLC in separate groups of rats. 6‐OHDA treatment (20 &mgr;g/10 &mgr;1) produced a rapid (from day 2) and significant (90–95%) fall in NE content only at L1‐S1. 5,6‐DHT administration (20 &mgr;g/10 &mgr;l) produced a gradual (68%, 90% and 94%, at 2, 7 and 14 days, respectively) and selective depletion of 5‐HT only at L1‐S1. DA levels remained essentially unchanged after both neurotoxins. No differences in monoamine levels were detected among groups injected with vehicle. The main effects of neurotoxins on autotomy were:a significant delay in the onset of autotomy in the rats injected with 6‐OHDA 2 days before neurectomy;a trend to autotomize earlier and more severely in the rats injected with 5,6‐DHT 7 days before neurectomy andan almost complete suppression of autotomy in the rats injected with 5,6‐DHT 14 days before neurectomy. These results revealed that the expression of autotomy in rats can be modulated by interfering with spinal cord serotonergic activity and suggest new possible avenues for the treatment of certain specific pain diseases, such a phantom limb pain, by using selective agents capable of activating or blocking spinal cord serotonergic receptor subtypes.

Autotomy in rats following peripheral nerve transection is attenuated by preceding formalin injections into the same limb

The influence of an evolving painful inflammatory lesion on the development of autotomy, a behavioural model of denervation pain, was studied in rats suffering sciatic and saphenous nerves transection 30 or 60 min, and 1, 3, 7 or 14 days after being injected with formalin (50 microl, 5%, s.c). Hindpaws pressure and heat nociceptive thresholds and volume of the injected paw were assessed, in non-operated rats, at the above time-points. The main effects on autotomy were: (1) a significant attenuation when formalin injection preceded the neurectomies by 1 day or more, a period characterized by hypalgesia of the injected paw to both mechanical (during the first week) and thermal (spanning up to the third day after formalin) stimuli and inflammation (lasting for 14 days); (2) a significantly earlier onset when formalin was injected 30 min before neurectomies. Possible mechanisms linking nociceptive responsiveness and inflammation to the development of autotomy are discussed.

Striatal astrocytes engulf dopaminergic debris in Parkinson's disease: A study in an animal model

The role of astrocytes in Parkinson’s disease is still not well understood. This work studied the astrocytic response to the dopaminergic denervation. Rats were injected in the lateral ventricles with 6-hydroxydopamine (25μg), inducing a dopaminergic denervation of the striatum not accompanied by non-selective tissue damage. The dopaminergic debris were found within spheroids (free-spheroids) which retained some proteins of dopaminergic neurons (e.g., tyrosine hydroxylase, the dopamine transporter protein, and APP) but not others (e.g., α-synuclein). Free-spheroids showed the initial (LC3-autophagosomes) but not the late (Lamp1/Lamp2-lysosomes) components of autophagy (incomplete autophagy), preparing their autophagosomes for an external phagocytosis (accumulation of phosphatidylserine). Free-spheroids were penetrated by astrocyte processes (fenestrated-spheroids) which made them immunoreactive for GFAP and S100β, and which had some elements needed to continue the debris degradation (Lamp1/Lamp2). Finally, proteins normally found in neurons (TH, DAT and α-synuclein) were observed within astrocytes 2–5 days after the dopaminergic degeneration, suggesting that the intracellular contents of degenerated cells had been transferred to astrocytes. Taken together, present data suggest phagocytosis as a physiological role of striatal astrocytes, a role which could be critical for cleaning striatal debris during the initial stages of Parkinson’s disease.