Piribedil and apomorphine: pre- and postsynaptic effects on dopamine synthesis and neuronal activity.

Piribedil and apomorphine can cause a complete but temporary inhibition in firing of the DA-containing neurons. The cells recover to approximately 30 to 40% of the original firing rate, and with subsequent administration of the drug become more and more resistant to further inhibition. At this point they are also resistant to inhibition by other direct- or indirect-acting DA agonists that are normally able to inhibit the firing of DA cells. This effect may be due to an interaction of the DA agonists with the postsynaptic DA receptors, but apomorphine at least is able to inhibit the firing of DA cells directly when iontophoresed onto the DA cell body (Aghajanian and Bunney, 1973). These drugs can block the increase in DA synthesis observed in the absence of impulse flow, indicating a presynaptic site of acttion. Taken together, these results suggest that in interpreting the effects of drugs interacting with the DA systems several points of interaction should be considered.