Beta-adrenergic receptors (bAR) activate the stimulatory heterotrimeric G protein (Gs) while D2-like dopamine receptors (D2DR) activate the inhibitory heterotrimeric G protein (Gi). Activation of Gs and Gi cause the stimulation and inhibition of adenylyl cyclase (AC), respectively. Beta2-adrenergic (b2AR) and D4 dopamine (D4DR) receptors are prototypical bAR and D2DR that co-immunoprecipitate when they are exogenously co-expressed in HEK 293 cells suggesting the existence of a signaling complex containing both receptors. Bioluminescence resonance energy transfer (BRET) was used to demonstrate that these receptors as well as G proteins and AC were closely associated as a signaling complex in HEK 293 cells, and to show that activation of Gi results in a conformational change of the heterotrimeric Gi that does not involve the dissociation of the G-alpha-i-subunit from the Gbeta-gamma-subunit complex. To determine if a signaling complex containing both bAR and D2DR exists in vivo, brain tissue was dissolved in a solution of 3-3-cholamidopropyl)dimethylammonio-2-hydroxy-1-propanesulfonate (CHAPSO), and the soluble bAR was precipitated with alprenolol-Sepharose. The precipitate was able to bind the D2DR ligand 125Iiodosulpride, and its binding was blocked by the D2DR antagonist spiperone, but not by the bAR antagonist propranolol, indicating that the precipitate also contained D2DR. If propranolol was present during incubation of the CHAPSO extract with alprenolol-Sepharose, no specific 125Iiodosulpride binding could be detected in the precipitate indicating that the D2DR was associated directly with the bAR or was part of a complex that included the bAR, and not non-specifically bound by the alprenolol-Sepharose. The molecular basis for the therapeutic action of mood stabilizing drugs including carbamazepine, lithium, and valproate is being investigated. These drugs are used to treat bipolar disorder, a serious disease that affects 1% of the population. The most popular hypothesis is that these mood stabilizing drugs reduce the level of cyclic AMP produced in response to neurotransmitter-mediated activation of AC by an as yet ill defined mechanism. HEK 293 cells co-expressing b2AR and D4DR were treated with therapeutic concentrations of these drugs. Both carbamazepine and lithium reduced the isoproterenol-mediated stimulation of AC and attenuated the ability of dopamine to inhibit AC. As with patient treatment the effects of the drugs were not immediate, but required prolonged exposure. The data suggest that the drug effects are the result of a reduction in cell surface b2AR and D4DR (Beta-adrenergic receptors (bAR) activate the stimulatory heterotrimeric G protein (Gs) while D2-like dopamine receptors (D2DR) activate the inhibitory heterotrimeric G protein (Gi). Activation of Gs and Gi cause the stimulation and inhibition of adenylyl cyclase (AC), respectively. Beta2-adrenergic (b2AR) and D4 dopamine (D4DR) receptors are prototypical bAR and D2DR that co-immunoprecipitate when they are exogenously co-expressed in HEK 293 cells suggesting the existence of a signaling complex containing both receptors. Bioluminescence resonance energy transfer (BRET) was used to demonstrate that these receptors as well as G proteins and AC were closely associated as a signaling complex in HEK 293 cells, and to show that activation of Gi results in a conformational change of the heterotrimeric Gi that does not involve the dissociation of the G-alpha-i-subunit from the Gbeta-gamma-subunit complex. To determine if a signaling complex containing both bAR and D2DR exists in vivo, brain tissue was dissolved in a solution of 3-3-cholamidopropyl)dimethylammonio-2-hydroxy-1-propanesulfonate (CHAPSO), and the soluble bAR was precipitated with alprenolol-Sepharose. The precipitate was able to bind the D2DR ligand 125Iiodosulpride, and its binding was blocked by the D2DR antagonist spiperone, but not by the bAR antagonist propranolol, indicating that the precipitate also contained D2DR. If propranolol was present during incubation of the CHAPSO extract with alprenolol-Sepharose, no specific 125Iiodosulpride binding could be detected in the precipitate indicating that the D2DR was associated directly with the bAR or was part of a complex that included the bAR, and not non-specifically bound by the alprenolol-Sepharose. The molecular basis for the therapeutic action of mood stabilizing drugs including carbamazepine, lithium, and valproate is being investigated. These drugs are used to treat bipolar disorder, a serious disease that affects 1% of the population. The most popular hypothesis is that these mood stabilizing drugs reduce the level of cyclic AMP produced in response to neurotransmitter-mediated activation of AC by an as yet ill defined mechanism. HEK 293 cells co-expressing b2AR and D4DR were treated with therapeutic concentrations of these drugs. Both carbamazepine and lithium reduced the isoproterenol-mediated stimulation of AC and attenuated the ability of dopamine to inhibit AC. As with patient treatment the effects of the drugs were not immediate, but required prolonged exposure. The data suggest that the drug effects are the result of a reduction in cell surface b2AR and D4DR (Rebois, R. V., Maki, K, Meeks, J. A., Fishman, P. H., Hebert, T. E. and Northup, J. K. D2-like dopamine and beta-adrenergic receptors form a signaling complex for bimodal regulation of adenylyl cyclase J. Neurochem. submitted).