The G protein coupling behavior of four human 5-hydroxytryptamine receptor subtypes (5-HT1A, 5-HT1B, 5-HT1D, and 5-HT1E) has been studied in membranes from Sf9 cells expressing the individual receptors. The 5-HT1A and 5-HT1B receptors exhibited both high-and low-affinity states for agonist, with the majority of the receptors in a low-affinity state. Addition of purified G protein subunits to membranes expressing either 5-HT1A or 5-HT1B receptors shifted the majority of the receptors to a high-affinity state in the absence, but not in the presence, of guanine nucleotides. The alpha(i1), alpha(i2), alpha(i3), and alpha(0) subunits were able to shift the receptors to a high-affinity state with either beta gamma(brain) or beta gamma(retina) while alpha(t) subunits were inactive regardless of which beta gamma preparation was used. A significantly higher affinity for agonist was observed with both receptors in the presence of alpha(i3) subunits compared with either alpha(i2) or alpha(0) subunits, while a significantly lower concentration of alpha subunits was required for a maximal affinity shift of 5-HT1A receptors compared with 5-HT1B receptors (EC50 values of 6.4 and 12.0 nM, respectively). The 5-HT1D and 5-HT1E receptors exhibited only a single affinity state for agonist. Addition of purified G protein subunits to membranes containing 5-HT1D receptors caused a small increase in affinity for agonist that was only partially reversed by guanine nucleotides while the addition of purified G protein subunits to membranes containing 5-HT1E receptors had no affect on agonist binding. Thus when expressed in an identical membrane environment these four closely related 5-HT1 receptor subtypes exhibit different G protein coupling behaviors.
