Effects of dopamine agonists on dopamine secretion from PC12 cells: lack of functional autoreceptor activity

In an effort to determine if PC12 cells have functional dopamine autoreceptors we found that carbachol-stimulated release of dopamine from undifferentiated PC12 cells was inhibited by the dopamine autoreceptor agonists apomorphine and U-68553B. Studies were conducted to determine the mechanism of this effect. The inhibition of dopamine release by apomorphine or U-68553B did not appear to result from effects on dopamine metabolism. When cells were exposed to the dopamine agonists for 2 minutes no changes in dopa, DOPAC or dopamine were observed. Over this same time interval, apomorphine and U68553B at 10 microM inhibited carbachol-stimulated dopamine release by 45.6% and 57.4% respectively. However, these drugs failed to inhibit the potassium dependent release of dopamine from cells suggesting no direct involvement with ion fluxes. Furthermore, haloperidol did not block the inhibitory effects of U-68553B on PC12 cells. This would appear to preclude activation of a dopamine autoreceptor as a possible mechanism. Kinetic analysis revealed that U-68553B is most likely a non-competitive inhibitor (Ki = 2 microM) of the nicotinic acetylcholine receptor. These data do not provide evidence for functional dopamine autoreceptors on undifferentiated PC12 cells but rather indicate that dopamine agonists may alter dopamine release by alternative mechanisms.