a noradrenaline precursor, inhibits dopamine release and metabolism in the rat striatum in vivo

The effect of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS) on dopamine (DA) release and metabolism in the striatum was studied in freely moving rats by intracerebral microdialysis techniques. The DA level as well as the levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid were significantly decreased 140 rain after the administration of L-threo-DOPS (50 mg/kg intraperitoneally). The results suggest that L-threo-DOPS inhibits the release and metabolism of DA in the striatum. Key words. In vivo microdialysis; striatum; noradrenaline; L-threo-3,4-dihydroxyphenylserine; dopamine release; 3,4-dihydroxyphenylacetic acid; homovanillic acid; high performance liquid chromatography. It has been reported that a marked decrease of both dopamine-fl-hydroxylase (DBH) and tyrosine hydroxy- lase (TH) activities occur in the brain tissue of Parkinso- nian patients ~. This suggests that lesions not only in dopaminergic but also in noradrenergic neuronal systems in the CNS might be involved in this disease. Nara- bayashi et al. 2 reported that L-threo-3,4-dihydroxy- phenylserine (L-threo-DOPS), that was directly convert- ed to noradrenaline (NA) by aromatic L-amino acid de- carboxylase (AADC) without a DBH intermediate, could ameliorate the freezing symptom. This is often a disabling symptom in Parkinsonian patients with long- standing disease, even when the rigidity and tremor have been well controlled over a long period by L-DOPA. However, little is known about the influence of this drug on dopaminergic neuronal activity in the striatum. In the present study, we investigated the influence of L-threo- DOPS on dopamine (DA) release and metabolism in the rat striatum by measuring the levels of DA, 3,4-dihydroxy- phenylacetic acid (DOPAC) and homovanillic acid (HVA) in this region, using the brain microdialysis method 3, 4. Materials and methods Male Wistar rats weighing 250-300 g were used. Ani- mals were maintained on a 12 h light/dark cycle 0ights on at 07.00 h) with free access to food and water. Rats were anesthetized with pentobarbital (45 mg/kg i.p.) and the dialysis probe was implanted into the striatum (coordi- nates were A: 0.5 mm, L: 3.0 mm, V: 7.0 ram, from the bregma and the dura surface according to the atlas of Paxinos and Watson 5. After the experiment, the position of the dialysis probe was verified by anatomical examina- tion. A dialysis experiment was carried out one day after the implantation of the probe. The dialysis probe was contin- uously perfused at a constant flow rate of 2.5 gl/min with an artificial CSF replacement (NaC1 140mM, KC1 3.35 raM, MgC12 1.15 raM, CaCl2 _1.26 mM, Na2HPO, 1.20 raM, NaHzPO 4 0.3 mM and pH 7.4). About 3 h after the beginning of the perfusion, stable basal levels of DA, DOPAC and HVA in the dialysates were obtained. Then L-threo-DOPS suspended in 0.3 % carboxymethylcellulose sodium (CMC) solution was ad- ministered i.p. at 50 mg/kg and the levels of DA, DOPAC and HVA were measured until 5 h after the injection. DA, DOPAC and HVA were measured by injecting the dialysates injected into the high performance liquid chro- matography coupled to electrochemical detection (HPLC-ECD) system (EICOM Co., Kyoto, Japan). The mobile phase was 0.1 M sodium acetate buffer (pH 4.5) containing 2.5-3.3 mM octanesulfonic acid, 0.02 mM EDTA and 10.0% (v/v) methanol. The column (4.6 x 150 mm, 7 g ODS-C18 resin, EICOM Co.) was used at 25 ~ The graphite working electrode was set at + 600 mV vs a Ag/AgC1 reference electrode (EICOM ECD-100 electrochemical detector) and the flow rate (EICOM EP-10 pump) was 0.90 ml/min. Statistical anal- ysis of the data was evaluated by one-way analysis of variance followed by Student's t-test. The averaged basal levels of DA, DOPAC and HVA were determined from the mean of 3 samples obtained prior to injection, The identity of the DA peak was confirmed by addition of TTX (5 x 10 .7 M) to the perfusion fluid, which caused a disappearance of the DA peak. Basal levels of DA, DOPAC and HVA in the striatum were 49 4- 9 pg/20 min (n = 4), 6674 + 711 pg/20 min (n = 4) and 3169 +856 pg/20 rain (n = 4), respectively. No re- markable changes in DA or its metabolites were seen in the vehicle-injected (control) group, but in the group given L-threo-DOPS (50 mg/kg i.p.) the level of DA as well as the levels of DOPAC and HVA were significantly decreased 140 min after the injection (figs 1 and 2). The decrease in the striatal levels of DA, DOPAC and HVA in response to the administration of L-threo-DOPS was time-dependent until 5 h after the injection. This is the first report demonstrating directly that L- threo-DOPS inhibits both DA release and metabolism in the rat striatum in vivo. A decrease in DA level after the administration of L-threo-DOPS was observed in a clin- ical report on a DBH deficiency 6: the plasma DA level was decreased and NA level was increased after the treat- ment with L-threo-DOPS. In this case, it was speculated