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Amperometric glucose biosensors based on the immobilization of glucose oxidase (GOx) on Pt electrodes with electropolymerized o-phenylenediamine (PPD) were implanted in the right striatum of freely-moving rats. Carbon paste electrodes for the simultaneous monitoring of ascorbic acid (AA) and/or tissue O 2 were implanted in the left striatum. A detailed in vivo characterization of the Pt/PPD/GOx signal was carried out using various pharmacological manipulations. Confirmation that the biosensor responded to changing glucose levels in brain extracellular fluid (ECF) was obtained by intraperitoneal (i.p.) injection of insulin that caused a decrease in the Pt/PPD/GOx current, and local administion of glucose (1 mM) via an adjacent microdialysis probe that resulted in an increase in the biosensor current. An insulin induced increase in tissue O 2 in the brain was also observed. Interference studies involved administering AA and subanaesthetic doses of ketamine i.p. Both resulted in increased extracellular AA levels with ketamine also causing an increase in O 2 . No significant change in the Pt/PPD/GOx current was observed in either case indicating that changes in O 2 and AA, the principal endogenous interferents, have minimal effect on the response of these first generation biosensors. Stability tests over a successive 5-day period revealed no significant change in sensitivity. These in vivo results suggest reliable glucose monitoring in brain ECF.

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Swiss mice were given 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 25 mg/kg/day, for 5 consecutive days and killed at different days after MPTP discontinuance. Decreases in striatal tyrosine hydroxylase activity and levels of dopamine and its metabolites were observed 1 day after MPTP discontinuance. Ascorbic acid and glutamate levels had increased, dehydroascorbic acid and GSH decreased, whereas catabolites of highenergy phosphates (inosine, hypoxanthine, xanthine, and uric acid) were unchanged. In addition, gliosis was observed in both striatum and substantia nigra compacta (SNc). Sections of SNc showed some terminal deoxynucleotidyl transferase-mediated 2-deoxyuridine 5-triphosphate nick end labeling (TUNEL)-positive cells. Neurochemical parameters of dopaminergic activity showed a trend toward recovery 3 days after MPTP discontinuance. At this time point, TUNEL-positive cells were detected in SNc; some of them showed nuclei with neuronal morphology. A late (days 6 -11) increase in striatal dopamine oxidative metabolism, ascorbic acid oxidative status, and catabolites of high-energy phosphates were observed concomitant with nigral neuron and nigrostriatal glial cell apoptotic death, as revealed by TUNEL, acridine orange, and Hoechst staining, and transmission electron microscopy. These data suggest that MPTP-induced activation/apoptotic death of glial cells plays a key role in the sequential linkage of neurochemical and cellular events leading to dopaminergic nigral neuron apoptotic death. Parkinson's disease (PD)1 is characterized by a selective loss of neurons in the substantia nigra compacta (SNc) and significant diminution of neostriatal content of dopamine and its major acidic metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Consequently, the functioning of the nigrostriatal dopaminergic system is impaired. Although cellular and molecular pathways leading to neuronal death in PD are still unknown, major biochemical processes such as oxidative stress and impaired energy metabolism are involved (for review, see Ref.

1 We have previously shown that manganese enhances L-dihydroxyphenylanine (L-DOPA) toxicity to PC12 cells in vitro. The supposed mechanism of manganese enhancing eect [an increase in L-DOPA and dopamine (DA) auto-oxidation] was studied using microdialysis in the striatum of freely moving rats. 2 Systemic L-DOPA [25 mg kg 71 intraperitoneally (i.p.) twice in a 12 h interval] signi®cantly increased baseline dialysate concentrations of L-DOPA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and uric acid, compared to controls. Conversely, DA and ascorbic acid concentrations were signi®cantly decreased. 3 A L-DOPA oxidation product, presumptively identi®ed as L-DOPA semiquinone, was detected in the dialysate. The L-DOPA semiquinone was detected also following intrastriatal infusion of L-DOPA. 4 In rats given L-DOPA i.p., intrastriatal infusion of N-acetylcysteine (NAC) signi®cantly increased DA and L-DOPA dialysate concentrations and lowered those of L-DOPA semiquinone; in addition, NAC decreased DOPAC+HVA and uric acid dialysate concentrations. 5 In rats given L-DOPA either systemically or intrastriatally, intrastriatal infusion of manganese decreased L-DOPA dialysate concentrations and greatly increased those of L-DOPA semiquinone. These changes were inhibited by NAC infusion. 6 These ®ndings demonstrate that auto-oxidation of exogenous L-DOPA occurs in vivo in the rat striatum. The consequent reactive oxygen species generation may account for the decrease in dialysate DA and ascorbic acid concentrations and increase in enzymatic oxidation of xanthine and DA. L-DOPA auto-oxidation is inhibited by NAC and enhanced by manganese. These results may be of relevance to the L-DOPA long-term therapy of Parkinson's disease. British Journal of Pharmacology (2000) 130, 937 ± 945

1. Glutamate in the extracellular compartment of the striatum of freely moving rats was monitored at 5 min intervals using microdialysis and an enzyme-based assay. 2. Basal levels of dialysate glutamate were 3-6 + 05 1uM. Local infusion through the dialysis probe of tetrodotoxin (TTX), cadmium chloride or magnesium chloride produced no reduction in basal levels of glutamate; with the latter two there was, instead, an increase. 3. Neuronal activation stimulated by induced grooming was accompanied by an increase in total glutamate efflux of 47 5 + 25-0% above basal level; this increase was not reduced by local infusion of 'ITX. 4. We propose that the ITX-insensitive release of glutamate in response to physiological stimulation is derived from glial cells and is a Ca2+-dependent mechanism triggered by a receptor-mediated release of Ca2+ from internal stores that spreads through the network of astrocytes.Glutamate is the main central excitatory transmitter in the vertebrate nervous system. This function has been extensively investigated using electrophysiological techniques. However glutamate, as well as being a neurotransmitter, has an additional role in energy and nitrogen metabolism. As a result, it is present in relatively high concentration in the cytoplasm of all neurones, as well as glia.

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