BACKGROUND AND PURPOSE The cannabinoid CB1 receptor is primarily thought to be functionally coupled to the Gi form of G proteins, through which it negatively regulates cAMP accumulation. Here, we investigated the dual coupling properties of CB1 receptors and characterized the structural determinants that mediate selective coupling to Gs and Gi. EXPERIMENTAL APPROACH A cAMP‐response element reporter gene system was employed to quantitatively analyze cAMP change. CB1/CB2 receptor chimeras and site‐directed mutagenesis combined with functional assays and computer modelling were used to determine the structural determinants mediating selective coupling to Gs and Gi. KEY RESULTS CB1 receptors could couple to both Gs‐mediated cAMP accumulation and Gi‐induced activation of ERK1/2 and Ca2+ mobilization, whereas CB2 receptors selectively coupled to Gi and inhibited cAMP production. Using CB1/CB2 chimeric receptors, the second intracellular loop (ICL2) of the CB1 receptor was identified as primarily responsible for mediating Gs and Gi coupling specificity. Furthermore, mutation of Leu‐222 in ICL2 to either Ala or Pro switched G protein coupling from Gs to Gi, while to Ile or Val led to balanced coupling of the mutant receptor with Gs and Gi. CONCLUSIONS AND IMPLICATIONS The ICL2 of CB1 receptors and in particular Leu‐222, which resides within a highly conserved DRY(X)5PL motif, played a critical role in Gs and Gi protein coupling and specificity. Our studies provide new insight into the mechanisms governing the coupling of CB1 receptors to G proteins and cannabinoid‐induced tolerance.

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TABLE I RELIABLE CONTROLLER DESIGN RESULTSSimilarly, by solving the ARE's in (42) and (43) in Corollaries 3.7, we have an output feedback controller 6 s of the form (4) and (5) Both of these two controllers 6 a and 6 s can provide internal stability and guaranteed disturbance attenuation for the closed-loop system not only when both control channels are operational but also when any of these two control channels experiences an outage.The design results are given in Table I. The two values of the closed-loop disturbance attenuation are computed for each of the two controllers. Namely:o: when there is no outage; c : when there is a controller failure.The "Design " in Table I is the value of used in solving the two corresponding design equations.The actual achievable values of (namely o and c) for the closed-loop system are all less than and quite close to the value of for which the design equations have solutions and the conditions in the Corollaries are satisfied. This indicates that degree of conservativeness in the design method is not very severe.From Table I, it would seem that the actual system performance would be better when some controller failure occurs, contrary to the desirable property of graceful degradation of performance. This is so, however, because a controller failure (modeled as an actuator outage and/or sensor outage) effectively eliminates one column and/or one row of the closed-loop transfer function matrix. This is similar to an observation made in [3]. ACKNOWLEDGMENTThe authors wish to thank the reviewers for many useful suggestions on the initial manuscript of the present work. Syst., vol. 15, no. 6, pp. 37-48, 1995. [11] M. Kinnaert, R. Hanus, and P. Arte, "Fault detection and isolation for unstable linear systems," IEEE Trans. Automat. Contr., vol. 40, pp. 740-742, Apr. 1995. [12] C.-C. Tsui, "A general failure detection, isolation and accommodation system with model uncertainty and measurement noise," IEEE Trans. Automat. Contr., vol. 39, pp. 2318-2321, Nov. 1994. REFERENCES Design of Performance Robustness for Uncertain Linear Systems with State and Control DelaysJ. S. Luo, P. P. J. van den Bosch, S. Weiland, and A. GoldenbergeAbstract-The linear systems considered in this paper are subject to uncertain perturbations of norm-bounded time-varying parameters and multiple time delays in system state and control. The time delays are uncertain, independent of each other, and allowed to be time-varying. The integral quadratic cost criterion is employed to measure system performance. Using solutions of Lyapunov and Riccati equations, a linear state feedback control law is proposed to stabilize the perturbed system and to guarantee an upper bound of system performance, which is applicable to arbitrary time delays.