Multiple states of the 5-hydroxytryptamine1 receptor as indicated by the effects of GTP on [3H]5-hydroxytryptamine binding in rat frontal cortex.

In several monoamine receptor systems, agonist but not antagonist binding is found to be associated with both a high- and a low-affinity state of the receptor. Guanine nucleotides, like GTP, can eliminate the high-affinity receptor state such that only the low-affinity state of the receptor is present. Since [3H]5-hydroxytryptamine ([3H]5-HT) is an agonist, its binding may also be associated with multiple states of a receptor. To evaluate this, the binding of [3H]5-HT to rat frontal cortical membranes was measured in the absence and presence of GTP. Three different types of binding experiments were performed: (a) saturation experiments, (b) dissociation experiments, and (c) competition experiments with 5-hydroxytryptamine (5-HT) agonists. When all three types of binding experiments were carried out in the absence of GTP, results from either graphical representations or computer analysis of the data indicated that a two-component model of binding described the data better than a single-component model. By contrast, in the presence of GTP, a one-component model adequately described the data obtained from either saturation or dissociation experiments. Competition of [3H]5-HT (15 nM) binding by three 5-HT agonists (5-methoxytryptamine, 5-HT, and d-lysergic acid diethylamide) was adequately described by a single-component system in the presence of GTP as well, even though all three agonists produced biphasic inhibition curves in the absence of the guanine nucleotide. These experimental results are consistent with the idea that the binding of [3H]5-HT in the absence of GTP is associated with multiple receptor states. Since the presence of multiple states can confound the interpretation of inhibition curves of [3H]5-HT binding caused by agonists, it is important to eliminate the high-affinity state of the receptor by including GTP in the binding assay.