Research reportMicrogliosis and down-regulation of adenosine transporter induced by methamphetamine in rats
Introduction
Amphetamines are indirectly acting sympathomimetic compounds that exert powerful psychomotor stimulant effects through activation of brain catecholamine-containing neurons. Although in the past amphetamines have been used clinically in some treatments, dopaminergic neurotoxicity produced by chronic administration of amphetamine in rats has been widely demonstrated [7]. It is well documented that methamphetamine (METH; a methyl derivative of amphetamine) treatment decreases multiple indices of dopamine (DA) terminal integrity, especially in striatum. Striatal changes produced by extended exposure of rats to METH include long-lasting decreases in dopamine (DA) content [10], DA metabolites [16], tyrosine hydroxylase activity [8], and reduced DA uptake [21].
One approach determining that METH-induced reductions in biochemical specific parameters are accompanied by evidence of structural damage is the alteration of two markers of neuronal injury: reactive gliosis (enhanced expression of glial fibrillary acidic protein (GFAP)) and silver degeneration reaction, while loss of neuronal perikarya is not evident [13].
Although the mechanism underlying METH-induced damage is incompletely understood, several important components have been identified. Amphetamines reverse the operational direction of the high-affinity transport sites present in monoaminergic terminals [9], increasing DA concentration in extracellular space [3]. The magnitude of METH-induced striatal overflow of DA has been shown to correspond with the extent of subsequent injury [14]. Oxidative products of DA also play a role in METH-induced toxicity [18].
Several papers involving neurodegeneration studies postulate that an increase in the peripheral-type benzodiazepine receptor (PBR) density can be used as an indirect marker of neuronal damage [1]. In CNS, PBR is restricted to microglia and any injury in CNS concludes with a microgliosis 19, 20.
Furthermore, cells synthesize a group of proteins called heat-shock-proteins (HSP) when exposed to various kinds of stress, such as high or low temperature, heavy metals, toxins and radiation, and this response has been well conserved throughout evolution. In CNS many kinds of injury, such as ischemia, trauma, seizures and Alzheimer's disease induce HSP72 expression in vivo. HSP72, a member of the HSP70 family, which is the most abundant and best conserved subset of eukaryotic stress proteins, is synthesized transiently following stress and is thought to play some functional role as molecular chaperon. Some studies associate the synthesis of HSP72 with a neuroprotective action [17].
There is now considerable evidence that the neuromodulator adenosine (ADO) is one of the brain's natural anticonvulsants. The bursting of nerve cells results in a massive formation and release of ADO, which has a number of actions that may reduce the extent of cell death. These include: hyperpolarization of neurons that would lead to Mg2+ blockade of NMDA receptors, inhibition of excitatory amino acid and acetylcholine release, inhibition of neuronal Ca2+ uptake, blockade of Na+ uptake [11]and compound action potentials, increased astrocytic glycogenolysis and increased cerebral blood flow. These effects of ADO may act in a combined fashion to maintain neuronal homeostasis and help reduce cell death under stressful conditions [6].
The purpose of the present study was to establish new aspects on neurodegenerative process induced by METH. The study has integrated various neuronal parameters, such as:
(a) Degeneration of dopaminergic terminals by quantification of DA reuptake sites.
(b) Microglia proliferation by determination of a specific marker of this cell type (PBR).
(c) 72-kDa heat-shock-protein expression, as marker for cellular stress and
(d) Report of some ADO transport alterations that could increase interstitial ADO concentration.
Section snippets
Animal procedures and drug administration
Male Sprague–Dawley rats, weighing 210–250 g (Harlan Ibérica, Barcelona, Spain), were used. The care and use of these animals were performed according to the protocols approved by a review committee of the University of Barcelona under supervision of the Government of Catalonia.
The animals were housed one per cage. Before treatment, the rats were allowed to acclimatize to an environmental temperature of 30±1°C for 2 h. Rats were injected, s.c., with METH (10 mg/kg) or saline every 2 h,
Results
The scheduled treatment induced a lethality of about 22%. Surviving animals showed bloody secretions from nose and eyes, suggesting an acute hypertensive syndrome. Animals exposed to a low environmental temperature to avoid hyperthermic and lethal syndrome, showed a decrease in dopaminergic toxicity (data not shown).
Discussion
It has been described that chronic administration of METH to rats induces a loss of dopaminergic neurotransmission specifically in striatum [16]. Present study corroborates this finding from the results obtained in []GBR 12935 binding experiments. As described, diphenyl-substituted piperazine derivatives, such as GBR 12935, are the most potent and selective inhibitors of dopamine transporter in corpus striatum. The affinity and specificity of this compound is superior to other ligands used as
Acknowledgements
We are grateful to Mr. R. Rycroft (Linguistic Advice Service of the University of Barcelona) for revising the language of the manuscript. This work was supported by a CICYT Grant(SAF 96-0375).
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