Biochemical and Pharmacological Differentiation of Neuroleptic Effect on Dopamine D-1 and D-2 Receptors
In dopamine (DA) receptor binding tests two different receptor populations can be measured. By using 3H-haloperidol or 3H-spiroperidol as ligands D-2 receptors are detected. When the thioxanthene neuroleptics 3H-cis (Z)-flupenthixol or 3H-piflutixol are used as ligands D-1 receptors coupled to adenylate cyclase can be detected. In displacement experiments butyrophenones (haloperidol and spiroperidol) and diphenylbutylpiperidines (pimozide) interact with D-2 receptors only, whereas thioxanthenes (cis [Z]-flupenthixol, cis [Z]-piflutixol and cis [Z]-clopenthixol) have equal affinity for D-1 and D-2 receptors. A similar differentiation is also seen in behavioral studies. The methylphenidate antagonistic effect of butyrophenones and diphenylbutylpiperidines is dramatically attenuated by concomitant treatment with an anticholinergic agent or a GABA agonist. The effect of thioxanthenes is almost unchanged. When mice are treated for 12 days with neuroleptics, supersensitivity to methylphenidate is induced. The supersensitivity is reversed by thioxanthenes, whereas butyrophenones and diphenylbutylpiperidines have no effect at all. These differenes do not appear to be due to a change in the affinity for DA receptors because the ability of cis (Z)-flupenthixol, haloperidol, DA or apomorphine to displace 3H-piflutixol or 3H-spiroperidol is the same in supersensitive and control mice. The results clearly demonstrate that neuroleptics acting on both D-1 and D-2 receptors have a behavioral profile different from that of neuroleptics acting exclusively on D-2 receptors.
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