Abstract
We investigated the effects of R(−)-apomorphine and S(+)-apomorphine on dopamine receptors modulating electrically evoked [ 3H]dopamine and [ 3H]acetylcholine release from slices of cat caudate nucleus. R(−)-Apomorphine inhibited the release of both [ 3H]dopamine and [ 3H]acetylcholine with an IC 50 of 20 nM, while S(+)-apomorphine was without inhibitory action on the electrically evoked release of either neurotransmitter at concentrations up to 1 μM. At a concentration of 1 μM, however, S(+)-apomorphine antagonized the inhibition by R(−)-apomorphine, producing a parallel five-fold shift to the right in the concentration-response curve to R(−)-apomorphine. These results indicate that S(+)-apomorphine is devoid of intrinsic activity to stimulate presynaptic dopamine receptors modulating the electrically evoked release of dopamine and acetylcholine. In addition, S(+)-apomorphine has an approximately ten-fold lower affinity for presynaptic dopamine receptors compared to R(−)-apomorphine.
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