There are many examples of a single receptor coupling directly to more than one cellular signal transduction pathway. Although traditional receptor theory allows for activation of multiple cellular effectors by agonists, it predicts that the relative degree of activation of each effector pathway by an agonist (relative efficacy) must be the same. In the current experiments, we demonstrate that agonists at the human serotonin2A (5-HT2A) and 5-HT2C receptors activate differentially two signal transduction pathways independently coupled to the receptors [phospholipase C (PLC)-mediated inositol phosphate (IP) accumulation and phospholipase A2 (PLA2)-mediated arachidonic acid (AA) release]. The relative efficacies of agonists differed depending on which signal transduction pathway was measured. Moreover, relative to 5-HT, some 5-HT2C agonists (e.g., 3-trifluoromethylphenyl-piperazine) preferentially activated the PLC-IP pathway, whereas others (e.g., lysergic acid diethylamide) favored the PLA2-AA pathway. In contrast, when two dependent responses were measured (IP accumulation and calcium mobilization), agonist relative efficacies were not different. These data strongly support the hypothesis termed "agonist-directed trafficking of receptor stimulus" recently proposed by Kenakin [Trends Pharmacol Sci 16:232-238 (1995)]. Concentration-response curves to 5-HT2C agonists were fit well by a three-state model of receptor activation, suggesting that two active receptor states may be sufficient to explain pathway-dependent agonist efficacy. Rational drug design that optimizes preferential effector activity within a group of receptor-selective drugs holds the promise of increased selectivity in clinically useful agents.

Numerous research has pointed out that serotonin2c (5-HT 2C ) receptor, a subtype of 5-HT receptors belonging to the G-protein-coupled receptor superfamily, modulates the activity of mesencephalic dopamine (DA) neurons, the dysfunction of which is involved in devastating diseases such as schizophrenia, Parkinson's disease, and drug addiction. In the present study, using in vivo intracerebral microdialysis and Chinese hamster ovary (CHO) cells expressing 5-HT 2C receptors to identify appropriate 5-HT 2C receptor ligands, we sought to determine whether the property of 5-HT 2C receptors to spontaneously activate intracellular signaling pathways in vitro (constitutive activity) participates in the tonic inhibitory control that they exert on DA release in the rat striatum and nucleus accumbens in vivo. In

Peripheral opioid analgesia is increased substantially after inflammation. We evaluated the hypothesis that an inflammatory mediator, bradykinin (BK), evokes functional competence of the ␦-opioid receptor (DOR) for inhibiting trigeminal ganglia (TG) sensory neurons. We also evaluated whether BK evokes trafficking of the DOR to the plasma membrane. Rat TG cultures were pretreated with BK (

2,5-Dimethoxy-4-substituted phenylisopropylamines and phenethylamines are 5-hydroxytryptamine (serotonin) (5-HT) 2A/2C agonists. The former are partial to full agonists, whereas the latter are partial to weak agonists. However, most data come from studies analyzing phospholipase C (PLC)-mediated responses, although additional effectors [e.g., phospholipase A 2 (PLA 2 )] are associated with these receptors. We compared two homologous series of phenylisopropylamines and phenethylamines measuring both PLA 2 and PLC responses in Chinese hamster ovary-K1 cells expressing human 5-HT 2A or 5-HT 2C receptors. In addition, we assayed both groups of compounds as head shake inducers in rats. At the 5-HT 2C receptor, most compounds were partial agonists for both pathways. Relative efficacy of some phenylisopropylamines was higher for both responses compared with their phenethylamine counterparts, whereas for others, no differences were found. At the 5-HT 2A receptor, most compounds behaved as partial agonists, but unlike findings at 5-HT 2C receptors, all phenylisopropylamines were more efficacious than their phenethylamine counterparts. 2,5-Dimethoxyphenylisopropylamine activated only the PLC pathway at both receptor subtypes, 2,5-dimethoxyphenethylamine was selective for PLC at the 5-HT 2C receptor, and 2,5-dimethoxy-4-nitrophenethylamine was PLA 2 -specific at the 5-HT 2A receptor. For both receptors, the rank order of efficacy of compounds differed depending upon which response was measured. The phenylisopropylamines were strong head shake inducers, whereas their phenethylamine congeners were not, in agreement with in vitro results and the involvement of 5-HT 2A receptors in the head shake response. Our results support the concept of functional selectivity and indicate that subtle changes in ligand structure can result in significant differences in the cellular signaling profile.

1 The serotonin 2C (5-HT 2C ) receptor couples to both phospholipase C (PLC)-inositol phosphate (IP) and phospholipase A 2 (PLA 2 )-arachidonic acid (AA) signalling cascades. Agonists can dierentially activate these eectors (i.e. agonist-directed tracking of receptor stimulus) perhaps due to agonist-speci®c receptor conformations which dierentially couple to/activate transducer molecules (e.g. G proteins). Since editing of RNA transcripts of the human 5-HT 2C receptor leads to substitution of amino acids at positions 156, 158 and 160 of the putative second intracellular loop, a region important for G protein coupling, we examined the capacity of agonists to activate both the PLC-IP and PLA 2 -AA pathways in CHO cells stably expressing two major, fully RNA-edited isoforms (5-HT 2C-VSV , 5-HT 2C-VGV ) of the h5-HT 2C receptor. 2 5-HT increased AA release and IP accumulation in both 5-HT 2C-VSV and 5-HT 2C-VGV expressing cells. As expected, the potency of 5-HT for both RNA-edited isoforms for both responses was 10 fold lower relative to that of the non-edited receptor (5-HT 2C-INI ) when receptors were expressed at similar levels. 3 Consistent with our previous report, the ecacy order of two 5-HT receptor agonists (TFMPP and bufotenin) was reversed for AA release and IP accumulation at the non-edited receptor thus demonstrating agonist tracking of receptor stimulus. However, with the RNA-edited receptor isoforms there was no dierence in the relative ecacies of TFMPP or bufotenin for AA release and IP accumulation suggesting that the capacity for 5-HT 2C agonists to trac receptor stimulus is lost as a result of RNA editing. 4 These results suggest an important role for the second intracellular loop in transmitting agonistspeci®c information to signalling molecules.