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

Pharmacological analysis of extracellular dopamine and metabolites in the striatum of conscious as/agu rats, mutants with locomotor disorder

The as/agu rat is a spontaneously occurring mutation which exhibits locomotor abnormalities, reduced tyrosine hydroxylase levels in the substantia nigra and lower extracellular levels of dopamine. The animal could represent a model of some human locomotor disorders. High-potassium medium evoked a 460% rise of dopamine levels in control rats but double this in mutants. Amphetamine increased extracellular dopamine by 710% in controls and 1480% in mutants. Clorgyline produced a small increase of dopamine levels in controls but an 1170% increase in mutants. The uptake inhibitor nomifensine increased dopamine levels by 910% in controls but only 270% in mutants. After treatment with benserazide plus L-DOPA, an acute injection of L-DOPA evoked a release of dopamine which was twice as large in the as/agu rats compared with controls. The results show reduced extracellular dopamine in as/agu rats when the locomotor disorder is apparent, but there has been little loss of tyrosine hydroxylase. The responses to drugs are qualitatively different from those obtained using 6-hydroxydopamine.Overall, the effects of compounds affecting aminergic neurons suggest that one possible mechanism for the neuronal abnormality in as/agu rats is a defective regulation of dopamine release from striatal terminals.

Microdialysis study of the effects of the antiparkinsonian drug budipine on L-DOPA-induced release of dopamine and 5-hydroxytryptamine by rat substantia nigra and corpus striatum

The purpose of this study was to determine if systemic treatment with the antiparkinsonian drug budipine was capable of influencing the release of dopamine newly synthesised from L-DOPA in the substantia nigra and corpus striatum of the monoamine-depleted rat. Dual probe microdialysis was therefore employed in freely moving animals pretreated with reserpine (4 mg/kg i.p. 18-20 h earlier) and alpha-methyl-p-tyrosine (200 mg/kg i.p. 45 min earlier). Budipine (10 mg/kg i.p.) alone evoked a small but significant increase in basal dopamine efflux in nigra, though not in striatum, but did not affect the spontaneous outputs of DOPAC, 5-HT, or 5-HIAA in either structure. A threshold amount of L-DOPA (25 mg/kg i.p.) stimulated the release of dopamine, DOPAC, and 5-HT (but not 5-HIAA), both in nigra and striatum. The L-DOPA-induced releases of dopamine and DOPAC were greatly accentuated by pretreatment with budipine (10 mg/kg i.p. 45 min earlier), which delayed rather than potentiated the nigral and striatal effluxes of 5-HT. A higher dose of L-DOPA (100 mg/kg i.p.) did not significantly raise the outputs of dopamine or 5-HT, but greatly magnified that of DOPAC. In these experiments, pretreatment with budipine (10 mg/kg i.p.) facilitated the formation of DOPAC from L-DOPA, without increasing the extracellular concentration of dopamine. We conclude from these findings that budipine, at a therapeutically relevant dose, potentiates the release of dopamine newly synthesised from L-DOPA from either end of the nigrostriatal dopamine axis. This effect of budipine could be related to the drug's recently described ability to increase the activity of the converting enzyme, aromatic L-amino acid decarboxylase, and could explain the clinical efficacy of budipine as an adjunct to L-DOPA therapy of Parkinson's disease in man. The significance of 5-HT release to the antiparkinsonian L-DOPA, and the delay in this release caused by budipine, remain to be established.

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

Available information on the dopamine (DA) metabolism of the immature brain is rare. In order to establish a useful animal model we have performed PET experiments in anesthetized neonatal pigs using 6-[18F]-fluoro-L-DOPA (FDOPA) as tracer. In this study, we have simultaneously determined the cerebral blood flow and the rate constant of FDOPA conversion by the aromatic amino acid decarboxylase, the ultimate enzyme in the synthesis of dopamine. The estimated values of FDOPA decarboxylation in the basal ganglia were similar to values calculated in adult animals and humans. However, in contrast to those studies a significant decarboxylation was also found in the frontal cortex and the cerebellum. HPLC analysis of brain samples also revealed extensive and rapid metabolism of FDOPA in the five investigated brain regions. At 8 min after tracer injection about 80% of FDOPA was already converted to FDA and its metabolites. Surprisingly, a rather high fraction (16-21%) of [18F]-fluoro-3-methoxytyramine was found which may indicate a low storage capacity of vesicular DA at this perinatal stage. It is suggested that the findings are related to the ontogenetic development of the dopaminergic system. The knowledge of the regulation of the DA metabolism in the immature brain may have implications for the understanding of neurodevelopmental effects of perinatal oxygen deprivation.

Vesicular transport regulates monoamine storage and release but is not essential for amphetamine action

To assess the role of exocytotic release in signaling by monoamines, we have disrupted the neuronal vesicular monoamine transporter 2 (VMAT2) gene. VMAT2-/- mice move little, feed poorly, and die within a few days after birth. Monoamine cell groups and their projections are indistinguishable from those of wild-type littermates, but the brains of mutant mice show a drastic reduction in monoamines. Using midbrain cultures from the mutant animals, amphetamine but not depolarization induces dopamine release. In vivo, amphetamine increases movement, promotes feeding, and prolongs the survival of VMAT2-/- animals, indicating that precise, temporally regulated exocytotic release of monoamine is not required for certain complex behaviors. In addition, the brains of VMAT2 heterozygotes contain substantially lower monoamine levels than those of wild-type littermates, and depolarization induces less dopamine release from heterozygous than from wild-type cultures, suggesting that VMAT2 expression regulates monoamine storage and release.