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DJ Wiedemann, PA Garris, JA Near and RM Wightman
Department of Chemistry, University of North Carolina, Chapel Hill.
In vivo voltammetry was used to assess the change in stimulated striatal dopamine overflow in response to various treatments with the dopamine receptor antagonist haloperidol. Dopamine overflow was induced with stimulating electrodes implanted in the medial forebrain bundle of anesthetized rats while dopamine concentrations were monitored with Nafion-coated, carbon-fiber microelectrodes implanted in the striatum. An acute challenge of haloperidol (0.5 mg kg-1, i.p.) given to naive animals caused stimulated overflow to increase at all stimulation frequencies (10-60 Hz), with the greatest change, 5-fold, occurring at 30 Hz. These results have been compared to those obtained in a different group of rats given daily injections of haloperidol (0.5 mg kg-1, s.c.) for 30 consecutive days. On the 30th day, dopamine striatal tissue levels and uptake kinetics were not altered by this treatment, but 3,4-dihydroxyphenylacetic acid tissue levels were elevated almost 2- fold. A challenge dose of haloperidol (0.5 mg kg-1, i.p.) administered to the animals treated with chronic haloperidol did not elicit a change in stimulated dopamine overflow. In two other groups, rats were withdrawn from 30-day haloperidol treatment for 3 days or 14 days before experimentation. Stimulated dopamine overflow concentrations in both groups were not significantly different from naive animals. When the withdrawn animals were given a haloperidol challenge (0.5 mg kg-1, i.p.), 15- and 12-fold increases in overflow for 3-day and 14-day withdrawal groups, respectively, were observed at a stimulation frequency of 30 Hz. Thus, chronic treatment with haloperidol induces long-lasting effects on the capacity of dopamine receptors to modulate dopamine release.
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