An in vivo comparison of the capacity of striatal versus extrastriatal brain regions to form dopamine from exogenously administered L-dopa

Abstract

We used the technique of cerebral microdialysis to monitor the metabolism of exogenously administered L-dopa and compared dopamine and dopamine metabolite formation in the striatum (a site containing abundant dopamine nerve terminals and dopa-decarboxylase (DDC) activity) versus the cerebellum and occipital cortex (sites with limited dopaminergic innervation and DDC activity). The concentrations of dopamine and the major dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) increased in each brain region following L-dopa perfusion; however, dopamine formation was 90% less in the occipital cortex as compared to the striatum and 95% less in the cerebellum. DOPAC formation was 57% less in the occipital cortex and 74% less in the cerebellum. HVA formation was 42% less in the occipital cortex and 70% less in the cerebellum. The levels of the L-dopa metabolite 3-O-methyldopa and the serotonin metabolite 5-hydroxyindoleacetic acid (5HIAA) were identical in the striatum, occipital cortex, and cerebellum both before and after L-dopa administration. We conclude that brain areas with marked reductions in dopamine nerve terminals and DDC activity have a diminished capacity to synthesize dopamine and also lack storage mechanisms to protect the newly synthesized dopamine from degradative metabolism. The relevance of these findings to the use of L-dopa in treating Parkinson's disease is discussed.

Simultaneous measurement of [18F]FDOPA metabolism and cerebral blood flow in newborn piglets

“…The chromatographic analysis of the brain tissue provides additional indirect evidence for the presence of AADC and COMT activity in all _ve regions studied[ Using HPLC six metabolites were reproducibly found in all brain regions studied[ In addition to the data given in Table 1\ preliminary experiments were performed at 3 and 49 min p[i[[ They demonstrate that among these six metabolites the sum of the two monocarboxylic acid FDOPAC and FHVA consistently showed the majority of 07 Interestingly\ none of the 5 detected peaks in brain corresponds to sulfoconjugated metabolites as also found in rat studies with FDOPA\ 04\05 while un~uorinated conjugated DOPAC and HVA has been found in the rat brain[ 19\15 It is known that these sulfoconjugated metabolites of FDOPA are present in plasma[ 03\30 In human plasma but not in rats\ the sulfoconjugated form is the dominant pool of DOPA\ of endogenous catechols and some of their metabolites 45 the mesenteric organs being the main sulfoconjugating source[ 14 Thus the mutual occurrence of sulfoconjugated FDOPA metabolites in plasma "data not shown# does not allow to draw conclusions regarding brain sulfoconjugation in pigs[ Our results of FDOPA metabolism in neonatal pigs di}er from studies performed in adult rats where none of the decarboxylated metabolites was seen in the cerebellum[ 04 In another study using ð 2 HŁ!DOPA a 84) lower AADC activity was found in cortex than in the striatum[ 06 The formation of dopamine\ DOPAC and HVA from exogenous L!DOPA in the occipital cortex and cerebellum of rats was 69) and 79) less than in striatum [ 4 Generally the di}erences in metabolite pattern between the various brain regions are much smaller than expected from studies performed in adult animals[ This fact con_rms our PET results[ Therefore neither of these regions may be used as a reference region during compartment analysis of FDOPA! PET data as it is suggested for adult humans[ 20\22 Obviously a high {{unspeci_c|| AADC activity is present in these regions which may not be related to dopaminergic neurons[ 01 It has been described that the capacity to decarboxylate injected L!DOPA to DA is acquired rapidly by the developing rat CNS and peaks at neonatal day 3[ 00 The biological signi_cance of this enzyme acitivty is unclear[ It may re~ect the synthesis of trace amines which serve as neurotransmitters as well[ Also DOPA itself has been suggested to be a neurotransmitter[ 31 Then the AADC could therefore serve as the enzyme limiting the action of it[ In conclusion\ we have shown in our study that PET measurements with FDOPA may serve as a useful tool to estimate the activity of AADC in the immature porcine brain and to get insight in the regulation of dopamine synthesis during ontogenesis[…”

“…Dopamine "DA# is a major neurotransmitter in the mammalian central nervous system which has been implicated in the etiology of neurological and psychiatric diseases[ 0\08 Disturbances in its metabolism\ which occur during early postnatal development under conditions of hypoxia\ are expected to contribute to the etiology of epilepsy 2 is expected to be the rate!limiting enzyme[ 33\40 Therefore research interest on regulation of dopamine synthesis in the past has mainly focused on this enzyme[ However increasing experimental evidence was obtained that AADC is regulated as well[ 06\46\47 FDOPA!PET has been used in humans to measure the activity of the AADC[ 16\22 Data allowing insights in the regulation of DA synthesis during early postnatal development are di.cult to obtain in humans[ In order to establish a useful animal model we have performed PET experiments with neonatal pigs using FDOPA as tracer[ A prerequisite for the investigation of cerebral dopamine metabolism in the immature brain is to guarantee steady!state conditions in regard to cerebral energy metabolism[ The monitoring of the process is possible by measuring regional cerebral blood~ow because cerebral circulation is tightly regulated and the distribution of blood~ow within the brain is precisely controlled to match perfusion with local metabolism even in the newborn brain [ 26 [4) iso~urane in a gas mixture of 69) nitrous oxide and 29) oxygen via an endotracheal tube[ Polyethylene catheters "inner diameter 0[9 mm# were advanced through the femoral arteries into the abdominal aorta in order to record arterial blood pressure and withdraw reference samples for the coloured microsphere technique[ A further polyethylene catheter "inner diameter 9[4 mm# was inserted into the superior sagittal sinus and advanced to the con~uence sinuum in order to obtain brain venous blood samples[The left ventricle was cannulated retrogradely via the common carotid artery with a polyurethane catheter "PU 2[4 Ch\ Sherwood\ U[K[# Arterial\ left ventricular\ and central venous catheters were connected with pressure transducers "P12Db\ Statham Instruments\ Puerto Rico#[ Body temperature was monitored by a rectal thermoprobe advanced for about 09 cm\ and was maintained throughout the general instrumentation at 2729[2>C using a feedback controlled heating lamp[ Blood gases\ brain tissue pO 1 \ EEG and ECG were also monitored during the studies[ Before and during the PET scan the cerebral blood~ow was measured using the reference sample method with coloured microspheres as described previously[ 44 Brie~y\ vortexed and sonicated microspheres "649\999Ð899\999# were injected in saline within 19 s into the left catheterized heart ventricle followed by~ushing with 0 ml saline[ Withdrawal of arterial reference sample started 04 s before injection and continued for 1 min with a rate of 0[4 ml:min "syringe pump SP109iw\ WPI#[ After euthanasia of the animals\ brain samples were rapidly taken out and digested with 2ml:g 3N KOH "containing 3) Tween 79 in deionized water# for a minimum of 3 h at 59>C[ Vacuum _ltration was performed through 7 mm pore PE!membrane _lter "Costar# followed by rinsing with 0) Tween 79 and 69) ethanol subsequently[ The dye was recovered by adding 049 ml of dimethylformamide[ The dye absorption was measured by a diode!array UV:visible spectrophotometer "Beckman Model 6499# at 299Ð799 nm witha1nmoptical band width[…”