In investigations of the molecular determinants for the recognition of drugs by a serotonin (5-HT) receptor in the brain, the commonality in the reactivity patterns of 5-HT congeners has been identified. On this basis a mechanistic hypothesis is proposed to explain the rank order of the affinity of drugs that bind to the receptor shared by 5-HT and LSD (the 5-HT/LSD receptor). This hypothesis describes the relation between affinity and the intramolecular rearrangement needed to make a 5-HT congener recognizable at the 5-HT/LSD receptor. The rearrangement of the congener aligns the electrostatic orientation vector of its indole portion in the direction defined by the vector in 5-HT. To further probe this mechanistic hypothesis the molecular determinants for the action of a new series of tryptamine derivatives was studied in which a methylenedioxy substituent is placed at the 5.6 or the 4,5 positions. The reactivity characteristics of these molecules are presented. The-oretical predictions for their activity on the 5-HT/LSD receptor are discussed. These predictions are based on the comparison of the electrostatic potentials of these molecules with the requirements for 5-HT-like recognition by the 5-HT/LSD receptor, and on simulations of molecular interactions with a molecular probe (imidazolium cztion) which represents a matching receptor site. The electrostatic nature of the complexes that these molecules form with imidazolium cation is revealed by the decomposition of the stabilization energy. It is shown that the affinity of these new compounds for 5-HT/LSD receptors can be predicted and explained on the basis of the working hypothesis obtained previously. Preliminary experimental data on the binding of 5,6- and 4.5-methylenedioxy derivatives of typtamine to the 5-HT/LSD receptors in brain confirm these findings.
GPR3, GPR6, and GPR12 are three orphan receptors that belong to the Class A family of G-protein-coupled receptors (GPCRs). These GPCRs share over 60% of sequence similarity among them. Because of their close phylogenetic relationship, GPR3, GPR6, and GPR12 share a high percentage of homology with other lipid receptors such as the lysophospholipid and the cannabinoid receptors. On the basis of sequence similarities at key structural motifs, these orphan receptors have been related to the cannabinoid family. However, further experimental data are required to confirm this association. GPR3, GPR6, and GPR12 are predominantly expressed in mammalian brain. Their high constitutive activation of adenylyl cyclase triggers increases in cAMP levels similar in amplitude to fully activated GPCRs. This feature defines their physiological role under certain pathological conditions. In this review, we aim to summarize the knowledge attained so far on the understanding of these receptors. Expression patterns, pharmacology, physiopathological relevance, and molecules targeting GPR3, GPR6, and GPR12 will be analyzed herein. Interestingly, certain cannabinoid ligands have been reported to modulate these orphan receptors. The current debate about sphingolipids as putative endogenous ligands will also be addressed. A special focus will be on their potential role in the brain, particularly under neurological conditions such as Parkinson or Alzheimer's disease. Reported physiological roles outside the central nervous system will also be covered. This critical overview may contribute to a further comprehension of the physiopathological role of these orphan GPCRs, hopefully attracting more research towards a future therapeutic exploitation of these promising targets.
Recent isothiocyanate covalent labeling studies have suggested that a classical cannabinoid, (-)-7'-isothiocyanato-11-hydroxy-1',1'dimethylheptyl-hexahydrocannabinol (AM841), enters the cannabinoid CB2 receptor via the lipid bilayer (Pei, Y., Mercier, R. W., Anday, J. K., Thakur, G. A., Zvonok, A. M., Hurst, D., Reggio, P. H., Janero, D. R., and Makriyannis, A. (2008) Chem. Biol. 15, 1207-1219). However, the sequence of steps involved in such a lipid pathway entry has not yet been elucidated. Here, we test the hypothesis that the endogenous cannabinoid sn-2-arachidonoylglycerol (2-AG) attains access to the CB2 receptor via the lipid bilayer. To this end, we have employed microsecond time scale all-atom molecular dynamics (MD) simulations of the interaction of 2-AG with CB2 via a palmitoyl-oleoyl-phosphatidylcholine lipid bilayer. Results suggest the following: 1) 2-AG first partitions out of bulk lipid at the transmembrane alpha-helix (TMH) 6/7 interface; 2) 2-AG then enters the CB2 receptor binding pocket by passing between TMH6 and TMH7; 3) the entrance of the 2-AG headgroup into the CB2 binding pocket is sufficient to trigger breaking of the intracellular TMH3/6 ionic lock and the movement of the TMH6 intracellular end away from TMH3; and 4) subsequent to protonation at D3.49/D6.30, further 2-AG entry into the ligand binding pocket results in both a W6.48 toggle switch change and a large influx of water. To our knowledge, this is the first demonstration via unbiased molecular dynamics that a ligand can access the binding pocket of a class A G protein-coupled receptor via the lipid bilayer and the first demonstration via molecular dynamics of G protein-coupled receptor activation triggered by a ligand binding event.
