S(-)-NORNICOTINE INCREASES DOPAMINE RELEASE IN A CALCIUM-DEPENDENT MANNER FROM SUPERFUSED RAT STRIATAL SLICES

The present study demonstrates that S(-)-nornicotine evoked a concentration-dependent increase in dopamine (DA) release from superfused rat striatal slices. The increase in DA release was indicated by an S(-)-nornicotine-induced overflow of endogenous 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatal superfusate and by an S(-)-nornicotine-induced increase in tritium overflow from striatal slices preloaded with [H-3]DA. Low concentrations (0.01-1.0 muM) of S(-)-nornicotine, which did not evoke endogenous DOPAC overflow, also were unable to modulate electrically evoked DOPAC overflow. The increase in DOPAC overflow induced by S(-)-nornicotine was compared with that produced by S(-)-nicotine. Comparing equimolar concentrations (0.1-100 muM) of S(-)-nornicotine and S(-)-nicotine, superfusion with S(-)nornicotine resulted in a significantly greater DOPAC overflow. In contrast to the effect of S(-)-nicotine, S(-)-nornicotine evoked a sustained increase in DOPAC overflow for the entire period of S(-)-nornicotine exposure. Furthermore, DOPAC overflow evoked by S(-)-nornicotine in control Krebs buffer was inhibited by superfusion with a low-calcium buffer, Moreover, in the low-calcium buffer, DOPAC overflow induced by 30 and 100 muM S(-)-nornicotine was not different from that with no S(-)-nornicotine. The results indicate that S(-)-nornicotine, a constituent of tobacco products and a known metabolite of S(-)-nicotine, increases DA release in a calcium-dependent manner in superfused rat striatal slices. It is interesting that unlike S(-)-nicotine, there does not appear to be desensitization to this effect of S(-)-nornicotine.