Partial agonism at serotonin 5-HT1B and dopamine D2L receptors using a luciferase reporter gene assay

We have used a luciferase reporter gene assay to study the functional responses of two G-protein-coupled receptors in Chinese hamster ovary (CHO) cells. The rank order of potency of drugs for the endogenous 5-HT1B receptor was 5-Hydroxytryptamine (5-HT) > zolmitriptan > dihydroergocristine > (-)lisuride (with no response to bromocriptine). However, only 5-HT and (-)lisuride produced a full functional response, with zolmitriptan and dihydroergocristine achieving 69+/-2% and 50+/-1% of the maximal response. In the same cells stably transfected with the rat dopamine D2L receptor, dopamine and bromocriptine produced a full agonist functional response, whilst (-)lisuride produced a biphasic response curve, indicating activity at both the endogenous 5-HT1B and exogenous dopamine D2L receptors. Using the receptor specific antagonists, pindolol and (+)butaclamol, (-)lisuride was shown to produce 52% of the maximal response at the dopamine D2 receptor relative to dopamine. In comparison to a cAMP accumulation assay, the rank orders of potency and intrinsic activity were the same for all compounds used. These results demonstrate that this reporter gene assay is capable of discriminating both potency and efficacy of drugs and can be used to characterise partial agonists at endogenously and heterologously expressed receptors in CHO cells.