The characterization of the dopaminergic profile of EMD 23,448, and indolyl-3-butylamine: selective actions on presynaptic and supersensitive postsynaptic DA receptor populations.

Abstract

The effects of the dopamine (DA) agonist EMD 23,448 on central normosensitive and supersensitive DA receptors were investigated. EMD 23,448 only slightly inhibits rat striatal DOPA synthesis in vivo and does not inhibit the enhanced striatal DOPA synthesis elicited by acute administration of haloperidol. Also unlike other DA agonists it does not increase striatal acetylcholine levels. However, it inhibits striatal DOPA synthesis in rats with DA receptors rendered supersensitive by chronic treatment with haloperidol. EMD 23,448 also effectively inhibits the enhanced striatal DOPA synthesis elicited by administration of GBL. Furthermore, EMD 23,448 selectively reduces, in a dose-dependant way, DA utilization in nerve terminals of the central caudate and in dotted terminals of the ventral striatum but DA utilization in the substantia nigra is unaffected. The most marked reduction of DA utilization was induced in the anteromedial frontal cortex. These results indicate that EMD 23,448 selectively stimulates presynaptic DA receptors and supersensitive postsynaptic DA receptors. Behavioral experiments in animals with normosensitive and supersensitive DA receptors also indicate that EMD 23,448 effectively stimulates presynaptic and supersensitive postsynaptic DA receptors. Receptor binding studies have shown that EMD 23,448 has a high affinity for the D2 DA receptors, but it ineffectively promotes the coupling of the DA receptors with the guanine nucleotide regulatory protein. However, at supersensitive striatal DA receptors the coupling is shown to be enhanced by EMD 23,448. The selectivity of EMD 23,448 for presynaptic DA receptors might, at least in part, be related to the presence of DA receptor reserves which are sensitive to EMD 23,448. With regard to the selectivity of EMD 23,448 for supersensitive postsynaptic DA receptors an increase in the efficiency of the coupling mechanism upon activation by EMD 23,448 is probably involved.