The dopamine D3 receptor subtype has been recently targeted as a potential neurochemical modulator of the behavioral actions of psychomotor stimulants, such as cocaine. However, definitive behavioral investigations have been hampered by the lack of highly selective D3 agonists and antagonists. In an attempt to design a novel class of D3 ligands with which to study this receptor system, a series of chemically divergent compounds that possessed various structural features that exist within several classes of reputed D3 agents was screened and compared to the recently reported NGB 2904 (58b). On the basis of these results, a novel series of compounds was designed that included functional moieties that were required for high-affinity and selective binding to D3 receptors. All the compounds in this series included an aryl-substituted piperazine ring, a varying alkyl chain linker (C3-C5), and a terminal aryl amide. The compounds were synthesized and evaluated in vitro for binding in CHO cells transfected with human D2, D3, or D4 receptor cDNAs. D3 binding affinities ranged from K(i) = 1.4 to 1460 nM. The most potent analogue in this series, 51, demonstrated a D3/D2 selectivity of 64 and a D3/D4 selectivity of 1300. Structure-activity relationships for this class of ligands at D3 receptors will provide new leads toward the development of highly selective and potent molecular probes that will prove useful in the elucidation of the role D3 receptors play in the psychomotor stimulant and reinforcing properties of cocaine.

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The dopaminergic receptor profile of a series of trans-1-[(2-phenylcyclopropyl)methyl]-4-arylpiperazines was examined. Aromatic substitution patterns were varied with the goal of identifying a compound having affinities for the D(2) and D(4) receptors in a ratio similar to that observed for the atypical neuroleptic clozapine. The compounds (1S, 2S)-trans-1-[(2-phenylcyclopropyl)methyl]-4-(2, 4-dichlorophenyl)piperazine (5m) and (1S, 2S)-trans-1-[(2-phenylcyclopropyl)methyl]-4-(2, 4-dimethylphenyl)piperazine (5t) were selected for functional antagonists at D(2) and D(4) receptors and had a D(2)/D(4) ratio approximating that of clozapine; they proved inactive in behavioral tests of antipsychotic activity.

The complement system represents an innate immune mechanism of host defense that has three effector arms, the C3a receptor, the C5a receptor (C5aR), and the membrane attack complex.

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