Adenosine A(2A) (A(2A)R) and dopamine D(2) (D(2)R) receptors mediate the antagonism between adenosinergic and dopaminergic transmission in striatopallidal GABAergic neurons and are pharmacological targets for the treatment of Parkinson's disease. Here, a family of heterobivalent ligands containing a D(2)R agonist and an A(2A)R antagonist linked through a spacer of variable size was designed and synthesized to study A(2A)R-D(2)R heteromers. Bivalent ligands with shorter linkers bound to D(2)R or A(2A)R with higher affinity than the corresponding monovalent controls in membranes from brain striatum and from cells coexpressing both receptors. In contrast, no differences in affinity of bivalent versus monovalent ligands were detected in experiments using membranes from cells expressing only one receptor. These findings indicate the existence of A(2A)R-D(2)R heteromers and of a simultaneous interaction of heterobivalent ligands with both receptors. The cooperative effect derived from the simultaneous interaction suggests the occurrence of A(2A)R-D(2)R heteromers in cotransfected cells and in brain striatum. The dopamine/adenosine bivalent action could constitute a novel concept in Parkinson's disease pharmacotherapy.

The incorporation of chemical modifications into the structure of bioactive compounds is often difficult because the biological properties of the new molecules must be retained with respect to the native ligand. Ergopeptides, with their high affinities at D(1) and D(2) dopamine receptors, are particularly complex examples. Here, we report the systematic derivatization of two ergopeptides with different peptide-based spacers and their evaluation by radioligand binding assays. Selected spacer-containing ergopeptides with minimal biological alteration and a proper anchoring point were further derivatized with a biotin reporter. Detailed characterization studies identified 13 as a biotin ergopeptide maintaining high affinity and agonist behavior at dopamine receptors, being a useful tool for the study of heteromers involving D(1)R, D(2)R, or D(3)R.

Abstract.Ligands acting at multiple dopamine receptors hold potential as therapeutic agents for a number of neurodegenerative disorders. Specifically, compounds able to bind at D 1 R and D 2 R with high affinity could restore the effects of dopamine depletion and enhance motor activation on degenerated nigrostriatal dopaminergic systems. We have directed our research towards the synthesis and characterisation of heterocycle-peptide hybrids based on the indolo[2,3-a]quinolizidine core. This privileged structure is a water-soluble and synthetically accessible scaffold with affinity for diverse GPCRs. Herein we have prepared a solid-phase combinatorial library of 80 indoloquinolizidine-peptides to identify compounds with enhanced binding affinity at D 2 R, a receptor that is crucial to re-establish activity on dopamine-depleted degenerated GABAergic neurons. We applied computational tools and high-throughput screening assays to identify 9a{1,3,3} as a ligand for dopamine receptors with nanomolar affinity and agonist activity at D 2 R. Our results validate the application of indoloquinolizidine-peptide combinatorial libraries to fine-tune the pharmacological profiles of multiple ligands at D 1 and D 2 dopamine receptors.