In vitro studies of dopamine D2 and D3 receptor pharmacology. Full, partial and inverse agonism

Abstract

ABSTRACT. Many receptors mediating the effects of transmitter substances act by coupling to G-proteins. Drugs activating such receptors promote the coupling between the receptor protein and the G-protein; such compounds are referred to as agonists. Drugs that bind to the receptor without promoting the coupling between the receptor and the G-protein are defined as antagonists; such compounds are usually not assumed to influence the receptor activity per se, but are able to block the effect of an agonist. Compounds exerting a limited activation of the receptor are referred to as partial agonists. To what extent a partial agonist will activate the receptor is dependent on the receptor responsiveness; traditionally, the receptor density has been regarded as the most important factor in this context. Recent data however suggest that the interaction between G-protein-coupled receptors and compounds displaying affinity for these receptors may be more complicated than previously assumed. Due to multiple confounding factors, it may be difficult to analyze the functional results of the interaction between a receptor and a drug in vivo; thus, for detailed investigations of drug-receptor interactions, well-characterized in vitro systems are useful. One major purpose of the experiments presented in this thesis has been to examine the utility of two in vitro models - 1) measurement of prolactin release from transfected GH cells, and 2) measurement of arachidonic acid liberation from transfected CHO cells - for studying the functional effects of various compounds displaying affinity for dopamine D2 and/or D3 receptors. These receptors both belong to the receptor family (the D2 receptor family) that is assumed to be the major target for drugs used for the treatment of schizophrenia and Parkinson's disease, respectively.The major findings are the following. First: In D2-transfected prolactin-producing GH cells, certain antipsychotic agents previously regarded as neutral D2 receptor antagonists (haloperidol, flupenthixol) were shown to exert a D2-mediated effect on prolactin release and cAMP formation opposite to that of dopamine, also in the absence of agonist. This is the first functional study showing so-called inverse agonism vis-à-vis a G-protein-coupled receptor in intact cells; subsequently many reports have confirmed that certain agents may counteract a constitutive coupling between a receptor and a G-protein. Second: Also using D2-transfected GH cells, it was shown that the intrinsic activity displayed by a partial agonist is dependent on effector-cell-related factors other than receptor density; thus, in a situation where the receptor number was held constant, the effect on prolactin release displayed by partial agonists was shown to vary from agonism to antagonism depending on which intracellular transduction systems that were being simultaneously activated (agonist-directed trafficking of receptor stimulus). Third: Using D2-transfected fibroblast tumour cells (CHO cells), it was shown that D2 receptors stimulate the liberation of arachidonic acid also in the absence of Ca2+-mobilizing agents; moreover, using this model, it was confirmed that haloperidol may act as an inverse agonist vis-à-vis the D2 receptor. Fourth: Using D3-transfected CHO cells, D3 receptors were shown to inhibit the liberation of arachidonic acid, i.e. to exert an effect opposite to that of D2 receptors. Also, it was shown that a substance generally regarded as neutral D3 antagonist, PNU-99194A, in this model displays the profile of an agonist.The major conclusion of these studies is that the pharmacological profile of compounds interacting with G-protein-coupled receptors may be more complicated to define than previously believed. To try to characterize novel compounds using one experimental model only (in vitro or in vivo) does not seem advisable.