We have identified a stromal cell-derived inducing activity (SDIA) that promotes neural differentiation of mouse ES cells. SDIA accumulates on the surface of PA6 stromal cells and induces efficient neuronal differentiation of cocultured ES cells in serum-free conditions without use of either retinoic acid or embryoid bodies. BMP4, which acts as an antineuralizing morphogen in Xenopus, suppresses SDIA-induced neuralization and promotes epidermal differentiation. A high proportion of tyrosine hydroxylase-positive neurons producing dopamine are obtained from SDIA-treated ES cells. When transplanted, SDIA-induced dopaminergic neurons integrate into the mouse striatum and remain positive for tyrosine hydroxylase expression. Neural induction by SDIA provides a new powerful tool for both basic neuroscience research and therapeutic applications.

Treatment of Parkinson's disease with L-dopa therapy leads to long-term complications, including loss of drug efficacy and the onset of dyskinesia. Adenosine A2A receptors in striatum are selectively localized to GABAergic output neurons of the striato-pallidal pathway and may avoid such problems. The novel adenosine A2A receptor antagonist KW-6002 has been examined for antiparkinsonian activity in MPTP-treated primates. Oral administration of KW-6002 reversed motor disability in MPTP-treated common marmosets in a dose-dependent manner. However, KW-6002 only modestly increased overall locomotor activity and did not cause abnormal movement, such as stereotypy. The ability of KW-6002 to reverse motor disability was maintained on repeated daily administration for 21 days, and no tolerance was observed. KW-6002 induced little or no dyskinesia in MPTP-treated primates previously primed to exhibit dyskinesia by prior exposure to L-dopa. These results suggest that selective adenosine A2A receptor antagonists represent a new class of antiparkinsonian agents that improve disability without producing hyperactivity and without inducing dyskinesia.

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Adenosine A 2A receptors are abundant in the caudate-putamen and involved in the motor control in several species. In MPTP-treated monkeys, A 2A receptor-blockade with an antagonist alleviates parkinsonian symptoms without provoking dyskinesia, suggesting this receptor may offer a new target for the antisymptomatic therapy of Parkinson's disease. In the present study, a signi®cant neuroprotective effect of A 2A receptor antagonists is shown in experimental models of Parkinson's disease. Oral administration of A 2A receptor antagonists protected against the loss of nigral dopaminergic neuronal cells induced by 6-hydroxydopamine in rats. A 2A antagonists also prevented the functional loss of dopaminergic nerve terminals in the striatum and the ensuing gliosis caused by MPTP in mice. The neuroprotective property of A 2A receptor antagonists may be exerted by altering the packaging of these neurotoxins into vesicles, thus reducing their effective intracellular concentration. We therefore conclude that the adenosine A 2A receptor may provide a novel target for the long-term medication of Parkinson's disease, because blockade of this receptor exerts both acutely antisymptomatic and chronically neuroprotective activities.

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