Agonist-independent activation of Gz by the 5-hydroxytryptamine1A receptor co-expressed in Spodoptera frugiperda cells -: Distinguishing inverse agonists from neutral antagonists

The human 5-hydroxytryptamine(1A) receptor, when expressed in Spodoptera frugiperda (Sf9) cells, facilitates the binding of [S-35]GTP gamma S to a co-expressed GTP-binding regulatory protein, G(z), consistent with constitutive activity. The antagonists 4-(2'-methoxyphenyl)-1-[2'-(n-2 "-pyridinyl)-p-iodobenzamido]ethyl-piperazine (p-MPPI) and the related fluorobenzamido analogue p-MPPF had little (p-MPPI) or no (p-MPPF) effect on this activity, In contrast, a third antagonist, the neuroleptic spiperone, produced an almost complete suppression. Thus, using G protein activation as an index of receptor activity, p-MPPF was classified as a neutral antagonist, p-MPPI as a partial inverse agonist, and spiperone as essentially a full inverse agonist, As predicted, spiperone displayed properties consistent with a special form of noncompetitive antagonism when used to displace the agonist [I-125]R-(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)amino]tetralin. Our data profile Sf9 cells as a unique vehicle for the characterization of inverse agonists, as these cells support a systematic pairing of mammalian receptors and G proteins, quantitative assays of G protein activation, and unambiguously labeled populations of coupled and uncoupled receptors.