Membranes prepared after infection of Sf9 cells with recombinant baculovirus containing the rat 5HT(2c) receptor DNA, but not after infection with wild-type virus, expressed high affinity binding sites for 125I- lysergic acid diethylamide and [3H]mesulergine. The receptor site density reached an optimum of 50-70 pmol/mg membrane protein at 60 h postinfection. Extraction of peripheral membrane proteins from the postnuclear membrane fraction with 6 M urea depleted GTPγS-binding 4-fold without decreasing 5HT(2c) receptor binding activity. Urea-extracted Sf9 membranes expressing the 5HT(2c) receptor catalyzed the activation of squid retinal α(q) but not bovine retinal α(t) or bovine α(o)/α(j). Productive interaction of 5HT(2c) receptors with squid α(q) was enhanced by the addition of βγ dimers prepared from either bovine brain or bovine rod outer segment discs. While the addition of serotonin increased 5HT(2c) receptor-catalyzed GTPγS binding to α(q), the unoccupied receptor was also catalytically active. The 5HT(2c) receptor antagonists, mesulergine, mianserin, and ketanserin competitively inhibited 5HT activation of the receptor with predicted rank-order affinities; and mianserin and ketanserin markedly inhibited basal 5HT(2c) receptor activity. Interestingly, this 'inverse agonist' efficacy did not correlate with antagonist affinity for the 5HT(2c) receptor. Baculoviral expression of the 5HT(2c) receptor and urea extraction of postnuclear Sf9 cell membranes have provided a high density of in situ, uncoupled, G-protein- linked receptor useful for reconstitution with purified G-protein subunits. This has allowed for independent manipulation of receptor and G-protein chemical concentrations and has revealed that a G-protein-linked receptor can possess a significant basal catalytic activity and that antagonist compounds can act as inverse agonists of this basal activity at the level of receptor activation of G-proteins.