British Journal of Pharmacology (2001) 132, 941 ± 949
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Analysis of S-nitroso-N-acetylpenicillamine eects on dopamine
release in the striatum of freely moving rats: role of endogenous
ascorbic acid and oxidative stress
Pier Andrea Serra, 1Giovanni Esposito, 1M. Rosaria Delogu, 1Rossana Migheli, 1Gaia Rocchitta,
Egidio Miele, 1Maria S. Desole & *,1,2Maddalena Miele
1
1
1
Department of Pharmacology, University of Sassari, viale S.Pietro 43B, 07100 Sassari, Italy and 2The Bethlem and Maudsley
NHS Trust, Bethlem Royal Hospital, Monks Orchard Road, Beckenham, Kent BH3 3BX
1 We showed previously that interaction between NO and iron(II), both released following
decomposition of sodium nitroprusside (SNP), accounted for the late SNP-induced dopamine (DA)
increase in dialysates from the striatum of freely moving rats.
2 In this study, intrastriatal infusion of the NO-donor S-nitroso-N-acetylpenicillamine (SNAP)
(0.2 mM for 180 min) induced a moderate increase in dialysate DA and decreases in ascorbic acid
dialysate concentrations; in contrast, SNAP 1 mM infusion induced a long-lasting decrease in both
DA and ascorbic acid dialysate concentrations. 3-Methoxy-tyramine (3-MT), dihydroxyphenylacetic
acid (DOPAC), homovanillic acid (HVA), and uric acid levels were unaected.
3 Co-infusion of ferrous sulphate [iron(II), 1 mM for 40 min] with SNAP either 1 or 0.2 mM (for
180 min), produced a signi®cant increase in both DA and 3-MT dialysate concentrations, but it did
not aect decreases in dialysate ascorbic acid levels. All other dialysate neurochemicals were
unaected.
4 Co-infusion of ascorbic acid (0.1 mM) with SNAP (1 mM) for 180 min did not modify SNAPinduced decreases in dialysate DA levels. In contrast, co-infusion of uric acid (1 mM) reversed
SNAP-induced decreases in dialysate DA; co-infusion of a superoxide dismutase mimetic delayed
SNAP-induced DA decreases for a short period, while co-infusion of the antioxidant Nacetylcysteine (NAC, 0.1 mM) signi®cantly increased dialysate DA.
5 The results of this study show that SNAP induces concentration-related changes in DA dialysate
levels. At higher concentrations, SNAP induces non-enzymatic DA oxidation, which is inhibited by
uric acid and NAC; ascorbic acid failed to protect dialysate DA from oxidation, probably owing to
its promoting eect on SNAP decomposition; exogenous iron(II) may react with NO generated from
SNAP decomposition, with a consequent increase in dialysate DA and 3-MT, therefore mimicking
SNP eects on striatal DA release.
British Journal of Pharmacology (2001) 132, 941 ± 949
Keywords: NO-donor SNAP; dopamine; in vivo release; ascorbic acid; iron; microdialysis; rat striatum
Abbreviations: ANOVA, analysis of variance; DA, dopamine; DOPAC, dihydroxyphenylacetic acid; HVA, homovanillic acid;
MnTBP, manganese(III) tetrakis (4-benzoic acid) porphyrin; 3-MT, 3-methoxytyramine; NAC, n-acetylcysteine;
NO, nitric oxide; SIN-1, 3-morpholinosydnonimine; SNAP, S-nitroso-N-acetylpenicillamine; SNP, sodium
nitroprusside
Introduction
Nitric oxide (NO) is a widespread and versatile biological
messenger molecule. Roles proposed for NO in CNS
pathophysiology are increasingly multiform and range from
intracellular signaling (Garthwaite & Boulton, 1995; Park et
al., 1998), through neurotransmitter release, neural development, synapse plasticity, killing of cells and invading
pathogens, apoptosis, to tissue damage or protection (Yun
et al., 1996; Murphy, 2000). It is evident that either
damaging, protective, or signalling eects of NO depend on
both the source of NO (neuronal, glial, extracellular,
exogenous) and the timing of NO production (Murphy,
*Author for correspondence at: Department of Pharmacology,
Gynaecology and Obstetrics, Faculty of Medicine, University of
Sassari, Viale S.Pietro 43B, 07100 Sassari, Italy
E-mail: Pharmaco@ssmain.uniss.it
2000). This assumption appears to be also true when dealing
with the biological eects of NO-generating drugs. These
eects vary according to the chemical structure of NOgenerating drugs (Bates et al., 1991; Holm et al., 1998;
Menconi et al., 1998) and the composition of the endogenous
environment in which NO is generated (Millar, 1995; Reiser
et al., 1999; Serra et al., 2000a). Ascorbic acid is a very
important component of the endogenous environment, since
NO readily reacts with ascorbic acid. Neuronal ascorbic acid
concentrations (10 mM) are about 10 times higher than glial
and, respectively, 20 ± 25 times higher than extracellular
concentrations (Miele & Fillenz, 1996; Rice, 2000). Extracellular ascorbic acid levels are also dynamically modulated
by glutamate-mediated activity (Rice, 2000), via glutamateascorbate heteroexchange (Miele et al., 1994). The close
relationship between NO and ascorbic acid has been outlined
942
P.A. Serra et al
in several biological systems (Millar, 1995; Lilley & Gibson,
1997). Ascorbic acid may either generate NO from nitrite
ions (NO27) in the extracellular space (Millar, 1995), protect
NO from destruction by superoxide anions (Dudgeon et al.,
1998; Jackson et al., 1998), or scavenge it (Whiteman &
Halliwell, 1996). In this regard, Karanth et al. (2000) claimed
that ascorbic acid may even act as an inhibitory transmitter
in the hypothalamus by scavenging NO. Neal et al. (1999)
showed that dopamine (DA) released from the retina is
oxidized by NO and that endogenous ascorbic acid protects
DA from oxidation by scavenging NO. Ascorbic acid may
trigger decomposition of the NO-donor sodium nitroprusside
(SNP) both in biological tissue in vitro (Bates et al., 1991;
Reiser et al., 1999) and in the striatal extracellular space in
vivo (Serra et al., 2000a). In addition, ascorbic acid
potentiates decomposition of S-nitroso-N-acetylpenicillamine
(SNAP) in striatal slices in vitro, but not that of 3morpholinosydnonimine (SIN-1) (Reiser et al., 1999).
In a previous study (Serra et al., 2000a), we showed that
intrastriatal infusion of either SNI-1 or SNP produced a NOmediated increase in DA concentration in dialysates from the
striatum of freely moving rats. In addition, we showed that
only SNP decreased dialysate ascorbic acid levels. This
decrease was related to the ascorbic acid triggering of SNP
decomposition (Bates et al., 1991; Reiser et al., 1999) in the
striatal extracellular space. The eects of the NO-donor
SNAP on DA release in terminal ®elds of the dopaminergic
system in vivo have been often con¯icting. West & Galloway
(1996; 1997) showed that SNAP increased DA eux from the
striatum of chloral hydrate-anaesthetized rats. In contrast,
Guevara-Guzman et al. (1994) showed that SNAP decreased
extracellular DA concentration in urethane-anaesthetized rat,
as did NO gas, given directly by dissolution in degassed
perfusion ¯uid. Segieth et al. (2000) showed that SNAP
promoted DA release from the rat hippocampus in vivo at
low concentrations, whilst high concentrations induced longlasting DA decreases. More recently, Trabace & Kendrick
(2000) have shown that short-lasting intrastriatal infusion of
SNAP induced increases in dialysate DA at low concentration, and decreases in dialysate DA at high concentrations;
these DA changes were attributed to SNAP-induced peroxynitrite formation; at high levels, peroxynitrite would reduce
extracellular DA concentration through oxidation, while at
low levels it increases DA levels in a cyclic GMP-dependent
manner. The study of the role of endogenous ascorbic acid
and superoxide anion in SNAP-induced changes in DA
release and metabolism in the striatum of freely moving rats
was therefore deemed of interest.
Methods
Animals
Male Wistar rats (Morini, R. Emilia, Italy), weighing between
280 ± 330 g were used in all experiments. The rats were
maintained under standard animal care conditions (12 : 12 h
light/dark cycle, lights coming on at 0700 h; room temperature 218C), with food and water ad libitum. Prior to the start
of any experiment, the health of the rat was assessed
according to published guidelines (Morton & Griths,
1985). All procedures were speci®cally licensed under the
British Journal of Pharmacology vol 132 (4)
Exogenous NO and striatal oxidative stress
European Community directive 86/609 included in Decreto
No. 116/1992 of the Italian Ministry of Public Health.
Drugs
SNAP, SIN-1, N-acetylcysteine (NAC), ascorbic acid, uric
acid, and ferrous sulphate [FeSO4, iron(II)] were purchased
from Sigma-Aldrich (Milan, Italy); manganese(III) tetrakis
(4-benzoic acid) porphyrin (MnTBP) from Calbiochem
(Darmstadt, Germany).
Drug administration
Iron(II), ascorbic acid and SIN-1 concentrations were chosen
according to Serra et al. (2000a); SNAP according to
Guevara-Guzman et al. (1994) and Trabace & Kendrick
(2000); NAC according to Serra et al. (2000b); MnTBP
according to Petersen et al. (2000)
Microdialysis probe construction
The striatal probe combined two independent microdialysis
probes of concentric design with two separate inlets and a
shared outlet, as previously described (Miele et al., 2000; Serra
et al., 2000b). The probes were constructed using two section
of plastic-coated silica tubing (diameter 0.15 mm; Scienti®c
Glass Engineering, Milton Keynes, U.K.) each placed in the
centre of a semi-permeable polyacrylonitrile dialysis ®bres
(molecular cut-o weight of 12 KD, Filtral 16 Hospal
Industrie, France). Each probe had a ®nal diameter of
0.22 mm. The tips of the dialysis ®bres were sealed and
joined using quick-drying epoxy glue. The two sections of
silica tubing served as inlets; the outlet was made also with a
section of plastic-coated silica tubing, positioned in the centre
of polythene tubing. The semipermeable membrane was
coated with epoxy leaving an active length of 4 mm. The
diameter of the ®nal probe was approximately 0.50 mm. The
striatal probe combining two microdialysis probes of concentric design with two separate inlets and a shared outlet,
allowed separate co-infusion of drugs (Serra et al., 2000b).
Surgery
Stereotaxic surgery was performed under chloral hydrate
(400 mg kg71 i.p.) anaesthesia. The microdialysis probes were
implanted in the right striatum using the following coordinates from the atlas of Paxinos & Watson (1986): A/P
+0.5 mm from bregma, +2.5 mm M/L, and 76.0 mm D/V
from dura. Body temperature during anaesthesia was
maintained at 378C by means of an isothermal-heating pad
(Harvard Apparatus, Kent, U.K.). Following surgery the
animals were placed in large plastic bowls (50655 cm), and
maintained in a temperature- and light-controlled environment, with free access to food and water. Experiments were
carried out 24 h after probe implantation with the animal in
its home bowl. This arrangement allowed the rats free
movement.
Microdialysis procedure
The composition of the Ringer solution used was as follows
(in mM): NaCl 147, KCl 4, CaCl2 1.2, MgCl2 1 (pH 6.0). A
P.A. Serra et al
microinfusion pump (CMA/100, Microdialysis, Sweden)
pumped Ringer solution at a ¯ow rate of 1.0 ml min71 using
two separate syringes connected to the inlets by a length of
polythene tubing; every 20 min, 40 ml dialysate samples were
collected manually in 250 ml micro-centrifuge tubes (Alpha
Laboratories, U.K.) attached to the outlet. Subsequently, a
20 ml aliquot of collected dialysate was injected into the
analytical system. Drugs were added to the Ringer solution
and infused via the striatal probe implanted in the striatum.
Chromatographic analysis
Exogenous NO and striatal oxidative stress
943
induced by the higher SNAP concentration (Figure 1C). 3MT levels were unaected (Figure 1B), as were other
neurochemicals (data not shown).
Effect of intrastriatal co-infusion of ascorbic acid on
SNAP-induced changes in dialysate neurochemical
concentrations
We showed previously (Serra et al., 2000a) that the
peroxynitrite generator SIN-1 increased dialysate DA without changes in dialysate ascorbic acid concentration; in
DA, 3-methoxytyramine (3-MT), dihydroxyphenylacetic acid
(DOPAC), homovanillic acid (HVA), ascorbic acid and uric
acid were quanti®ed by high performance liquid chromatography with electrochemical detection (HPLC-EC) as previously described (Serra et al., 2000a), using an Alltech 426
HPLC pump equipped with a Rheodyne injector, column
15 cm64.6 mm i.d. Alltech Adsorbsphere C18 5U, electrochemical detector Antec CU-04-AZ and Varian Star
Chromatographic Workstation. The mobile phase was citric
acid 0.5 M, Na acetate 1 M, EDTA 12.5 mM, MeOH 10%
and sodium octylsulphate 650 mg l71 (pH=3.0); the ¯ow rate
was 1.3 ml min71. The ®rst sample was collected after 60 min
of stabilization (time 0), then dialysates were collected, at
20 min intervals, for 40 min prior to the start of experiments.
Histology
Following the experiments, rats were killed with an overdose
of chloral hydrate (800 mg kg71 i.p.). The location of each
microdialysis probe was con®rmed by post-mortem histology.
Brains were ®xed in formal saline and 50 mm coronal sections
were made with a cryostat. The slices were stained with cresyl
violet and examined under a microscope.
Statistical analysis
The concentrations in the dialysate were expressed in nM
(DA, 3-MT) or mM (DOPAC, HVA, ascorbic acid, uric acid)
and given as mean+s.e.mean. Drug eects on neurochemicals were statistically evaluated in terms of changes in
absolute dialysate concentrations. Statistical signi®cance was
assessed using analysis of variance (ANOVA) for dierence
between groups and over time. Dierence within or between
groups were determined by paired or unpaired t-tests with
Bonferroni multiple comparison adjustment.
Results
Effect of intrastriatal infusion of t SNAP on DA, 3-MT,
DOPAC, HVA, ascorbic acid and uric acid dialysate
levels
Intrastriatal infusion of SNAP (1 mM for 180 min, n=4)
induced a long-lasting decrease in DA and ascorbic acid
dialysate concentrations (Figure 1A,C), whilst 3-MT (Figure
1B), DOPAC, HVA and uric acid (Figure 2) were unaected.
Intrastriatal infusion of SNAP (0.2 mM for 180 min, n=3)
induced increases in dialysate DA (Figure 1A) and a decrease
in dialysate ascorbic acid (ANOVA P50.02) lower than that
Figure 1 Eect of intrastriatal infusion of SNAP 1 mM (n=4) or
0.2 mM (n=3) on DA (A), 3-MT (B), and ascorbic acid (C) dialysate
concentrations. Dialysates were collected, at 20 min intervals, for
180 min during drug infusion (horizontal black bar) and for 80 min
after discontinuation of drug infusion. Values are given as mean+s.e.
mean. *P50.05 compared with baseline values.
British Journal of Pharmacology vol 132 (4)
P.A. Serra et al
944
addition, we showed that intrastriatal ascorbic acid coinfusion inhibited SIN-1-induced increases in dialysate DA.
The inhibition was related to NO scavenging by ascorbic
Figure 2 Eect of intrastriatal infusion of SNAP 1 mM (n=4) on
DOPAC, HVA and uric acid dialysate concentrations. Dialysates
were collected, at 20 min intervals, for 180 min during drug infusion
(horizontal black bar) and for 80 min after discontinuation of drug
infusion. Values are given as mean+s.e.mean.
Exogenous NO and striatal oxidative stress
acid. In the present study, SNAP induced a concentrationrelated decrease in dialysate ascorbic acid. The study of
ascorbic acid co-infusion on SNAP-induced decreases in
DA dialysate concentrations was therefore deemed of
interest.
Co-infusion of ascorbic acid (0.1 mM for 180 min, n=3)
did not aect SNAP (1 mM)-induced decreases in dialysate
DA (Figure 3A). In addition, ascorbic acid co-infusion
aected neither 3-MT (Figure 3B), uric acid (Figure 3D),
nor DOPAC and HVA dialysate levels (data not shown).
Co-infusion of ascorbic acid (0.1 mM for 180 min, n=3)
inhibited SNAP (0.2 mM)-induced increases in dialysate DA;
during co-infusion, dialysate levels of DA were always in the
range of baseline values. All other neurochemicals were
unaected (data not shown).
Dialysate ascorbic acid concentrations attained during
ascorbic acid/SNAP (1 mM) co-infusion were compared with
those attained with ascorbic acid/SIN-1 (1 mM) co-infusion
(n=3). As shown in Figure 4, dialysate ascorbic acid
concentrations attained during 180 min co-infusion with
SNAP (1 mM) were signi®cantly lower (by about 28 ± 30%)
than those attained during 180 min co-infusion with SIN-1.
Dialysate ascorbic acid concentrations attained during coinfusion with SNAP 0.2 mM did not statistically dier from
those attained during 180 min co-infusion with SIN-1 (data
not shown).
Figure 3 Eect of intrastriatal co-infusion of ascorbic acid 0.1 mM (n=3), uric acid 1 mM (n=3), or MnTBP 0.1 mM (n=4) on
SNAP-induced changes in DA (A), 3-MT (B), ascorbic acid (C), and uric acid (D) dialysate concentrations. Dialysates were
collected, at 20 min intervals, for 180 min during drug infusion (horizontal black bar) and for 80 min after discontinuation of drug
infusion. Values are given as mean+s.e.mean. *P50.05 compared with baseline values. Thin horizontal black bar in (A) indicates
signi®cant decreases for both SNAP/ascorbic acid SNAP/MnTBP groups; in (C), it indicates signi®cant decreases for both SNAP/
uric acid and SNAP/MnTBP groups. +P50.05 compared with SNAP/MnTBP group.
British Journal of Pharmacology vol 132 (4)
P.A. Serra et al
Exogenous NO and striatal oxidative stress
945
(Patel & Day, 1999), in order to assess the role of peroxynitre
in SNAP-induced oxidation of dialysate DA.
Co-infusion of MnTBP (0.1 mM for 180 min, n=3) shortly
delayed SNAP (1 mM)-induced decreases in dialysate DA
(Figure 3A) and attenuated ascorbic acid decreases (Figure
3C). MnTBP co-infusion aected neither 3-MT (Figure 3B),
nor DOPAC and HVA dialysate levels (data not shown); in
contrast, MnTBP signi®cantly increased uric acid levels (up
to 60% of baseline at end of co-infusion) (Figure 3D).
Effect of NAc-cysteine co-infusion on SNAP-induced
changes in DA and ascorbic acid dialysate concentrations
Figure 4 Dialysate ascorbic acid concentrations following intrastriatal co-infusion of ascorbic acid 0.1 mM with SIN-1 1 mM (n=3)
or SNAP 1 mM (n=3). Dialysates were collected, at 20 min intervals,
for 180 min during drug infusion (horizontal black bar) and for
80 min after discontinuation of drug infusion. Values are given as
mean+s.e.mean. +P50.05 compared with ascorbic acid/SIN-1
group.
Effect of uric acid co-infusion on SNAP-induced changes
in DA and ascorbic acid dialysate concentrations
In their very recent paper, Trabace & Kendrick (2000)
suggest that SNAP induces peroxynitrite formation which
results, according to the peroxynitrite level attained,
either in extracellular DA oxidation, or in increases in DA
release from the striatum of freely moving rats. Uric
acid is a natural strong scavenger of peroxynitrite (Hooper
et al., 1998). In addition, one of the scavenging activity of
uric acid is to maintain ascorbic acid in its reduced form in
biological ¯uids (Sevanian et al., 1991). Although in the
present study SNAP infusion failed to modify dialysate
uric acid concentrations (Figure 2), the study of uric
acid co-infusion on SNAP-induced changes in DA and
ascorbic acid dialysate concentrations was deemed of
interest.
In preliminary experiments, it was found that uric acid
dialysate concentrations ranging from 80 to 100 mM could be
attained with the infusion of uric acid 1 mM. Co-infusion of
uric acid 1 mM with SNAP 1 mM for 180 min (n=3) reverted
SNAP-induced decreases in dialysate DA levels (Figure 3A),
whilst 3-MT levels were unaected (Figure 3B); in addition,
uric acid co-infusion attenuated SNAP-induced decreases in
dialysate ascorbic acid (Figure 3C); DOPAC and HVA levels
were unaected (data not shown).
Effect of superoxide dismutase (SOD) mimetic MnTBP
co-infusion on SNAP-induced changes in DA and ascorbic
acid dialysate concentrations
Peroxynitrite is formed by reaction of NO with superoxide
anions. Ascorbic acid, at high physiological concentrations, is
a natural inhibitor of this reaction (Jackson et al., 1998);
however, in this study, co-infusion of ascorbic acid failed to
protect dialysate DA from SNAP-induced oxidation. SOD is
too large a molecule to cross the dialysis membrane used;
therefore, we used the cell-permeant SOD mimetic MnTBP
We showed previously (Serra et al., 2000b) that intrastriatal
infusion of NAC protected DA and L-DOPA, which are both
catechol-containing compounds, from autoxidation. It is well
known that O-methylation of either DA or L-DOPA protects
these compounds from non-enzymatic oxidation (Miller et
al., 1996). The fact that SNAP 1 mM induced decreases in
dialysate DA concentration leaving unaected that of 3-MT,
the DA O-methylated derivative, prompted us to evaluate the
eect of NAC co-infusion on the SNAP-induced dialysate
DA decrease.
Co-infusion of NAC 0.1 mM with SNAP 1 mM for 180 min
(n=4) resulted in a signi®cant increase in dialysate DA levels
(Figure 5A), whilst 3-MT levels were unaected (Figure 5B);
in addition, NAC co-infusion attenuated the decrease in
dialysate ascorbic acid (Figure 5C), whilst DOPAC, HVA
and uric acid levels were unaected (data not shown).
Effect of iron(II) intrastriatal co-infusion on
SNAP-induced changes in dialysate neurochemicals
concentrations
We showed previously (Serra et al., 2000a) that interaction
between NO and iron(II), both released following decomposition of SNP, accounted for the late but quite substantial
SNP-induced dopamine (DA) increase in dialysates from the
striatum of freely moving rats. In addition, we showed
(unpublished observation) that the late SNP-induced increase
in dialysate DA was mimicked by co-infusing the NO-donor
SIN-1 with either iron(II), or potassium ferrocyanide, the
iron-carrier of SNP, therefore con®rming that an exogenous
NO/exogenous iron interaction was responsible for the late
great SNP-induced increase in dialysate DA. The evaluation
of iron(II) co-infusion on SNAP-induced changes in DA
dialysate concentrations was therefore deemed of interest.
Intrastriatal infusion of iron(II) (1 mM for 40 min, n=3)
induced a late moderate increase in dialysate DA concentration, whilst all other neurochemicals (3-MT, DOPAC, HVA,
ascorbic acid, and uric acid) dialysate concentrations were
unaected (data not shown).
When SNAP (1 mM for 180 min) was co-infused with
iron(II) (1 mM for 40 min, n=4), dialysate DA greatly
increased progressively to a peak (316% of baseline) at the
end of 180 min SNAP infusion (Figure 5A). Similarly,
iron(II) co-infusion signi®cantly increased 3-MT dialysate
levels, but in this case the peak (407% of baseline) occurred
early (Figure 5B). Iron(II) co-infusion did not aect SNAPinduced dialysate ascorbic acid decreases (Figure 5C). All
other neurochemicals (DOPAC, HVA and uric acid) were
unaected (data not shown).
British Journal of Pharmacology vol 132 (4)
946
P.A. Serra et al
Exogenous NO and striatal oxidative stress
Figure 5 Eect of intrastriatal co-infusion of NAC (0.1 mM for
180 min, n=4) or iron(II) (1 mM for 40 min, n=4), on SNAP 1 mMinduced changes in DA (A), 3-MT (B), and ascorbic acid (C)
dialysate concentrations Dialysates were collected, at 20 min
intervals, for 180 min during drug infusion (horizontal black bar)
and for 80 min after discontinuation of drug infusion. Values are
given as mean+s.e.mean (n=3). *P50.05 compared with baseline
values.
Figure 6 Eect of intrastriatal co-infusion of iron(II) (1 mM for
40 min, n=3) on SNAP 0.2 mM-induced changes in DA (A), 3-MT
(B), and ascorbic acid (C) dialysate concentrations. SNAP 0.2 mM,
same group as in Figure 1. Dialysates were collected, at 20 min
intervals, for 180 min during drug infusion (horizontal black bar) and
for 80 min after discontinuation of drug infusion. Values are given as
mean+s.e.mean. *P50.05 compared with baseline values; +P50.05
compared with SNAP group.
When SNAP (0.2 mM for 180 min) was co-infused with
iron(II) (1 mM for 40 min, n=3), the early SNAP-induced
increase in dialysate DA concentration was inhibited during
40 min iron(II) co-infusion. Following discontinuation of
iron(II) co-infusion, dialysate DA signi®cantly increased to a
peak (340% of baseline) at the end of 180 min SNAP
infusion (Figure 6A). Similarly, iron(II) co-infusion signi®cantly increased 3-MT dialysate levels, but in this case the
peak (265% of baseline) occurred early (Figure 6B). Dialysate
ascorbic acid concentration signi®cantly decreased during
40 min of iron(II) co-infusion. Following discontinuation of
iron(II) co-infusion, dialysate ascorbic acid showed a trend to
recovery (Figure 6C). All other neurochemicals (DOPAC,
HVA, uric acid) were unaected (data not shown).
British Journal of Pharmacology vol 132 (4)
Discussion
The results of the present study con®rm the results of a
previous study (Serra et al., 2000a) that both ascorbic acid
P.A. Serra et al
and iron(II) play a key role in DA changes induced by NOdonors in dialysates from the striatum of freely moving rats.
Ascorbic acid promotes degradation of both SNAP (Reiser
et al., 1999) and SNP (Bates et al., 1991; Reiser et al., 1999),
but not that of SIN-1 (Reiser et al., 1999). In this study,
SNAP induced a concentration-dependent decrease in
dialysate ascorbic acid levels; we showed previously (Serra
et al., 2000a) that SNP infusion induced time-dependent
decreases in dialysate ascorbic acid levels, whilst SIN-1 did
not aect dialysate ascorbic acid. These ®nding are consistent
with the role of endogenous ascorbic acid as an active
promoter of both SNAP and SNP degradation in vivo.
Following SNAP degradation, and the ensuing NO release,
DA dialysate levels showed concentration-dependent changes:
long-lasting decreases at higher SNAP concentration (1 mM),
and short lasting increases at lower concentration (0.2 m).
These ®ndings are in agreement with those of Trabace &
Kendrick (2000). Decrease in DA dialysate induced by the
higher concentration of SNAP is undoubtedly due to DA
non-enzymatic oxidation (Trabace & Kendrick, 2000), since
dialysate levels of 3-MT, the extracellular O-methylated DA
metabolite which is resistant to oxidation (Miller et al., 1996),
were unaected by SNAP. The question arises as to whether
SNAP-induced oxidation of DA might be mediated by excess
of NO generated from SNAP degradation, by peroxynitrite
formed following endogenous superoxide anion reaction with
NO generated from SNAP degradation, or both. Intrastrial
infusion of either SIN-1, a well-known peroxynitrite generator (Menconi et al., 1998), or SNP, which also induces
peroxynitrite formation in vitro (Keller et al., 1998), resulted
in a long-lasting increase in dialysate DA (Serra et al.,
2000a). These ®ndings seem to exclude a peroxynitrite role in
DA oxidation. However, both the chemical structure of NOgenerating drugs and the composition of the endogenous
environment in which NO is generated must be taken in
consideration. The NO generated in vitro by SNAP is far
greater than that by SIN-1 (Holm et al., 1998). In addition,
NO generation from SNAP degradation, in the present study
in vivo, is likely to be increased by the potentiating eect of
endogenous ascorbic acid on SNAP degradation (Reiser et
al., 1999). NO released in excess by the higher SNAP
concentration might act both as a free radical to promote
extracellular DA oxidation, and as a promoter of peroxynitrite formation by reacting with endogenous superoxide
anion. The SOD mimetic MnTBP delayed SNAP-induced
decreases in dialysate DA for a short period, attenuated
decreases in dialysate ascorbic acid, and increased dialysate
uric acid levels. These convergent ®ndings suggest an initial
formation of peroxynitrite, which might initially participate
to extracellular DA oxidation. Thereafter, NO continuously
generated from SNAP degradation would take the place of
peroxynitrite in the extracellular DA oxidation. This
hypothesis is supported by the fact that: (1) Co-infusion
with uric acid protected DA from SNAP-induced oxidation.
Uric acid is a natural strong scavenger of peroxynitrite, but
not of NO (Hooper et al., 1998). Uric acid is an active
component of the neuronal antioxidant pool (Becker, 1993).
It is capable of inhibiting SNP-and free radical-initiated lipid
peroxidation and DNA damage (Cohen et al., 1984; Keller et
al., 1998); in addition, it forms strong complexes with iron
ions, particularly Fe3+ (Cohen et al., 1984), and inhibits DA
autoxidation (Church & Ward, 1994); (2) Co-infusion of the
Exogenous NO and striatal oxidative stress
947
antioxidant NAC with SNAP completely protected DA from
oxidation with a consequent signi®cant increase in dialysate
DA levels. Wang et al. (1998) showed that NAC is an active
antioxidant also in presence of NO.
Co-infusion of SNAP with iron(II), which is known to
react readily with NO (Stamler et al., 1992; Le Brun et al.,
1997), signi®cantly increased dialysate DA and 3-MT levels.
The latter ®nding further highlights the role of iron in the
NO-donor drug-induced increase in dialysate DA. In a
previous study (Serra et al., 2000a), we showed that NO
released following SIN-1 decomposition increased dialysate
DA. In contrast to SNAP (Holm et al., 1998), SIN-1 also
generates the superoxide anion; thus, as a potential
peroxynitrite generating drug (Menconi et al., 1998), SIN-1
should promote DA oxidation (to 6-hydroxyindole-5-one,
according to Kerry & Rice-Evans, 1999), as SNAP does.
Evidently, this peroxynitrite-induced DA oxidation does not
occur in striatal dialysates following SIN-1 infusion. In a
submitted study, in order to explain the fact that the iron
chelator deferoxamine inhibited the SIN-1-induced increase
in dialysate DA (unpublished observations), we suggested
that the superoxide anion generated by SIN-1 decomposition
would preferentially release iron from storage proteins and
enzymic [4Fe-4S] clusters, rather than react with NO, with a
consequent elevation in free iron levels (Keyer & Imlay, 1996)
The endogenous iron would then react with NO released by
SIN-1 decomposition, to form NO-iron complexes (Stamler
et al., 1992; Le Brun et al., 1997), therefore mimicking the
eect of SNP (which notoriously releases both NO and iron)
on striatal DA release (Serra et al., 2000a).
Endogenous ascorbic, acting as a NO scavenger (Whiteman & Halliwell, 1996), may either protect DA from
oxidation (Neal et al., 1999) or even act as an inhibitory
transmitter in the hypothalamus by scavenging NO (Karanth
et al., 2000). In a previous study (Serra et al., 2000a) we
hypothesized that exogenous ascorbic acid inhibited SIN-1induced increases in dialysate DA by scavenging NO. The
results of this study con®rm this hypothesis, since the DA
releasing eect of the lower SNAP concentration (0.2 mM)
was completely blocked by ascorbic acid co-infusion. It is
likely that exogenous ascorbic acid, besides promoting further
the degradation of SNAP (Reiser et al., 1999), had scavenged
the ensuing NO. Exogenous ascorbic acid exerts antidopaminergic action on striatal function in vivo (Desole et al.,
1987; Rebec & Pierce, 1994; Gulley & Rebec, 1999). The NO
scavenging activity of ascorbic acid might be one of the
mechanisms of this antidopaminergic action.
The results of this study raise the question as to whether
NO-generating drugs might be useful tools for the in vivo
study of the role of endogenous NO in striatal dopaminergic
transmission. The composition of the endogenous environment in which NO is generated may play a key role. When
NO is generated intraneuronally, it has to face an ascorbic
acid concentration of about 10 mM (Rice, 2000), which is
20 ± 25 times higher than that found in vivo in the striatal
extracellular space (Miele & Fillenz, 1996). Therefore, the
neuronal ascorbic acid concentration far exceeds the
extracellular one at which exogenous ascorbic acid acts as a
scavenger of exogenous NO.
In conclusion, NO generated from low SNAP concentrations increases DA dialysate levels; endogenous extracellular
ascorbic acid may promote SNAP decomposition, with a
British Journal of Pharmacology vol 132 (4)
948
P.A. Serra et al
consequent generation of an excess of NO; an excess of NO
may induce both peroxynitrite formation and DA dialysate
non-enzymatic oxidation; these eects are both inhibited by
exogenous uric acid and the antioxidant NAC; exogenous
iron(II) may preferentially react with NO generated from
SNAP decomposition; this interaction results in increases in
Exogenous NO and striatal oxidative stress
dialysate DA, and therefore mimics the SNP eect on striatal
DA release.
The research was supported by University of Sassari (ex 60%
fund).
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(Received October 10, 2000
Revised December 4, 2000
Accepted December 5, 2000)
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