Abstract
Rats were treated for 21 d with the selective D1 dopamine receptor antagonist SCH23390, the selective D2 dopamine receptor antagonist spiperone, the nonselective dopamine receptor antagonist cis-flupentixol, or a combination of SCH23390 and spiperone. In addition, a group of rats received L-prolyl-L-leucyl-glycinamide (PLG) for 5 d after the 21 d chronic spiperone treatment. Chronic treatment with SCH23390 resulted in a significant increase in D1 dopamine receptor density with no change in the D2 dopamine receptor density. Conversely, spiperone treatment resulted in a significant increase in D2 dopamine receptors and no change in D1 dopamine receptor density. PLG treatment had no effect. SCH23390 plus spiperone treatment resulted in a significant increase in both D1 and D2 dopamine receptor densities. However, although in vitro cis-flupentixol has an equal affinity for D1 and D2 dopamine receptors, only the D2 dopamine receptor density increased after chronic treatment with cis-flupentixol. In vivo treatment with the protein-modifying reagent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which irreversibly inactivates D1 and D2 dopamine receptors, was used to investigate the paradoxical, selective D2 dopamine receptor up-regulation induced by cis-flupentixol treatment. In vivo treatment with cis-flupentixol before EEDQ administration prevented the D1 and D2 dopamine receptor reductions induced by EEDQ. However, cis-flupentixol protected, in a dose-dependent manner, a greater percentage of D2 dopamine receptors than of D1 dopamine receptors from EEDQ-induced modification. These data indicate that, in vivo, cis-flupentixol preferentially interacts with D2 dopamine receptors and could explain why only D2 dopamine receptors were up-regulated following chronic treatment with cis-flupentixol. Rats were tested for their cataleptic response to the administered drug over the course of the chronic drug treatment. Catalepsy scores of rats receiving spiperone decreased over the course of treatment, with a significant reduction in catalepsy occurring by treatment day 5. The profound catalepsy observed in rats receiving SCH23390 did not change over the 21 d of treatment. Rats receiving cis-flupentixol demonstrated tolerance to its cataleptogenic effects, with a significant reduction in catalepsy observed by treatment day 7. During the 3 week treatment, the time between drug injection and a full cataleptic response to cis-flupentixol increased from 20 to 60 min, suggesting a tolerance to the D2, but not D1, dopamine receptor antagonism by cis-flupentixol.(ABSTRACT TRUNCATED AT 400 WORDS)