The central aromatic amino acid DOPA decarboxylase inhibitor, NSD-1015, does not inhibit L-DOPA-induced circling in unilateral 6-OHDA-lesioned-rats

Adopting the Rumsfeld approach to understanding the action of levodopa and apomorphine in Parkinson's disease

Dopaminergic therapies dominate the treatment of the motor and non-motor symptoms of Parkinson's disease (PD) but there have been no major advances in therapy in many decades. Two of the oldest drugs used appear more effective than others-levodopa and apomorphine-but the reasons for this are seldom discussed and this may be one cause for a lack of progress. This short review questions current thinking on drug action and looks at whether adopting the philosophy of ex-US Secretary of State Donald Rumsfeld reveals 'unknown' aspects of the actions of levodopa and apomorphine that provide clues for a way forward. It appears that both levodopa and apomorphine have a more complex pharmacology than classical views would suggest. In addition, there are unexpected facets to the mechanisms through which levodopa acts that are either forgotten as 'known unknowns' or ignored as 'unknown unknowns'. The conclusion reached is that we may not know as much as we think about drug action in PD and there is a case for looking beyond the obvious.

l-DOPA promotes striatal dopamine release through D1 receptors and reversal of dopamine transporter

Previous studies have pointed out that l-DOPA can interact with D1 or D2 receptors independent of its conversion to endogenous dopamine. The present study was set to investigate whether l-DOPA modulates dopamine release from striatal nerve terminals, using a preparation of synaptosomes preloaded with [³H]DA. Levodopa (1 µM) doubled the K⁺-induced [³H]DA release whereas the D2/D3 receptor agonist pramipexole (100 nM) inhibited it. The l-DOPA-evoked facilitation was mimicked by the D1 receptor agonist SKF38393 (30-300 nM) and prevented by the D1/D5 antagonist SCH23390 (100 nM) but not the DA transporter inhibitor GBR12783 (300 nM) or the aromatic l-amino acid decarboxylase inhibitor benserazide (1 µM). Higher l-DOPA concentrations (10 and 100 µM) elevated spontaneous [³H]DA efflux. This effect was counteracted by GBR12783 but not SCH23390. Binding of [³H]SCH23390 in synaptosomes (in test tubes) revealed a dense population of D1 receptors (2105 fmol/mg protein). Both SCH23390 and SKF38393 fully inhibited [³H]SCH23390 binding (Ki 0.42 nM and 29 nM, respectively). l-DOPA displaced [³H]SCH23390 binding maximally by 44% at 1 mM. This effect was halved by addition of GBR12935 and benserazide. We conclude that l-DOPA facilitates exocytotic [³H]DA release through SCH23390-sensitive D1 receptors, independent of its conversion to DA. It also promotes non-exocytotic [³H]DA release, possibly via conversion to DA and reversal of DA transporter. These data confirm that l-DOPA can directly interact with dopamine D1 receptors and might extend our knowledge of the neurobiological mechanisms underlying l-DOPA clinical effects.

Cystathionine-β-Synthase: Molecular Regulation and Pharmacological Inhibition

Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used "CBS inhibitors" (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Locomotor response to L-DOPA in reserpine-treated rats following central inhibition of aromatic L-amino acid decarboxylase: further evidence for non-dopaminergic actions of L-DOPA and its metabolites

L-DOPA is the most widely used treatment for Parkinson's disease. The anti-parkinsonian and pro-dyskinetic actions of L-DOPA are widely attributed to its conversion, by the enzyme aromatic L-amino acid decarboxylase (AADC), to dopamine. We investigated the hypothesis that exogenous L-DOPA can induce behavioural effects without being converted to dopamine in the reserpine-treated rat-model of Parkinson's disease. A parkinsonian state was induced with reserpine (3 mg/kg s.c.). Eighteen hours later, the rats were administered L-DOPA plus the peripherally acting AADC inhibitor benserazide (25 mg/kg), with or without the centrally acting AADC inhibitor NSD1015 (100 mg/kg). L-DOPA/benserazide alone reversed reserpine-induced akinesia (4158+/-1125 activity counts/6 h, cf vehicle 1327+/-227). Addition of NSD1015 elicited hyperactive behaviour that was approximately 7-fold higher than L-DOPA/benserazide (35755+/-5226, P<0.001). The hyperactivity induced by L-DOPA and NSD1015 was reduced by the alpha(2C) antagonist rauwolscine (1 mg/kg) and the 5-HT(2C) agonist MK212 (5 mg/kg), but not by the D2 dopamine receptor antagonist remoxipride (3 mg/kg) or the D1 dopamine receptor antagonist SCH23390 (1 mg/kg). These data suggest that L-DOPA, or metabolites produced via routes not involving AADC, might be responsible for the generation of at least some L-DOPA actions in reserpine-treated rats.

L-3,4-Dihydroxyphenylalanine as a neurotransmitter candidate in the central nervous system

Historically, 3,4-dihydroxyphenylalanine (DOPA) has been believed to be an inert amino acid that alleviates the symptoms of Parkinson's disease by its conversion to dopamine via the enzyme aromatic L-amino acid decarboxylase. In contrast to this generally accepted idea, we propose that DOPA itself is a neurotransmitter and/or neuromodulator, in addition to being a precursor of dopamine. Several criteria, such as synthesis, metabolism, active transport, existence, physiological release, competitive antagonism, and physiological or pharmacological responses, must be satisfied before a compound is accepted as a neurotransmitter. Recent evidence suggests that DOPA fulfills these criteria in its involvement mainly in baroreflex neurotransmission in the lower brainstem and in delayed neuronal death by transient ischemia in the striatum and the hippocampal CA1 region of rats.