Interactions of beta-adrenergic receptors with a membrane protein other than the stimulatory guanine nucleotide-binding protein

Is depression a disorder of a receptor superfamily? A critical review of the receptor theory of depression and the appraisal of a new heuristic model

The monoamine hypothesis of depression and its direct derivation, the receptor theory, have constituted for several years a frame of reference for researchers working in the field of biological psychiatry. Although most of the data are derived from animal findings and must be considered inconclusive in view of various controversies, some guidelines may be identified: these would suggest that changes in postsynaptic beta-adrenoreceptors, presynaptic alpha 2-adrenoreceptors, as well as in type 2 serotonin receptors and dopaminergic autoreceptors may be involved in the mode of action of antidepressant drugs and, consequently, in the pathophysiology of depression. Nowadays, any attempt to correlate depression with the dysfunction of a single neurotransmitter or receptor is no longer tenable, since it is clear that depression is a heterogeneous disorder which involves abnormalities in the interactive relationships between neurotransmitters and receptors. If, on the one hand, this new model has opened up new fields of research and has led to the investigation of new systems,egthe GABAergic and GABA B receptors, on the other hand, it has been strongly limited by the lack of research tools and reliable peripheral CNS models forin vivostudies. A possible approach to this unresolved dilemma may be provided by molecular biology techniques, which have permitted the identification of the genes and sequencing of the primary structure of several membrane receptors. It is now established that receptors may be grouped into four superfamilies; in depression, there exists compelling evidence of alterations mainly in receptors belonging to the G-protein-coupled family: it is plausible that depression may be related to a disorder of the G-protein-coupled receptor superfamily. Such an hypothesis would represent an attempt to unify the different receptor abnormalities found in depression or following antidepressant treatments, and to shift from the monoamine paradigm to a new heuristic model. In addition, it would accommodate the various dysfunctions likely to be encountered and would open up new theoretical perspectives in the treatment of depression.

Functional expression of adrenergic and opioid receptors in Xenopus oocytes: interaction between alpha 2- and beta 2-adrenergic receptors

We functionally expressed alpha 2-adrenergic, beta 2-adrenergic, and delta-opioid receptors in Xenopus laevis oocytes. We detected receptor function as changes in currents carried by adenosine 3',5'-cyclic monophosphate (cAMP)-regulated chloride channels provided by the cystic fibrosis transmembrane conductance regulator (CFTR) and recorded by two-electrode voltage clamp. Co-application of forskolin and isobutylmethylxanthine (IBMX) or IBMX alone produced currents with a reversal potential indicative of chloride ions only in oocytes previously injected with mRNA encoding CFTR. Isoproterenol produced concentration-dependent responses in oocytes injected with mRNA encoding beta 2-adrenergic receptors and CFTR, and co-administration of propranolol antagonized these responses. Similarly, the alpha 2-adrenergic agonist UK14304 increased IBMX-induced currents only in oocytes injected with mRNA encoding alpha 2-adrenergic receptors and CFTR, and idazoxan antagonized these enhancements. The delta-opioid agonist DADLE produced concentration-related, naloxone-reversible increases in IBMX- and forskolin-induced currents only in oocytes injected with mRNA encoding delta-opioid receptors and CFTR. In oocytes co-injected with alpha 2, beta 2, and CFTR mRNAs, isobolographic analysis revealed an additive interaction between alpha 2- and beta 2-adrenergic receptors. These studies establish the oocyte as a cell system for studying the interactions among cAMP-modulating G protein-coupled receptors and provide another example of alternative coupling of alpha 2-adrenergic and delta-opioid receptors to G proteins, possibly Gs proteins, other than Gi proteins.

Interaction of beta-adrenergic receptors with the inhibitory guanine nucleotide-binding protein of adenylate cyclase in membranes prepared from cyc- S49 lymphoma cells

beta-Adrenergic receptors on membranes prepared from L6 myoblasts, wild-type S49 lymphoma cells, and an adenylate cyclase-deficient variant (cyc-) of S49 lymphoma cells bind the agonist [3H]hydroxybenzylisoproterenol ([3H]HBI) with high affinity. In each case the agonist [3H]HBI is associated with a larger complex than is the antagonist [125I]iodopindolol, and the binding of [3H]HBI can be inhibited by GTP. These observations suggest that there is an agonist-dependent association of the receptor with a guanine nucleotide-binding protein. The goal of the present experiments was to investigate the possibility that an interaction of beta-adrenergic receptors with the inhibitory guanine nucleotide-binding protein of adenylate cyclase was responsible for these observations. Treatment of S49 cells with pertussis toxin decreased the extent of pertussis toxin-catalyzed [32P]ADP-ribosylation of a 41,000-dalton protein, measured in vitro, and decreased the inhibition of adenylate cyclase activity observed in the presence of somatostatin or analogues of GTP. Isoproterenol-stimulated adenylate cyclase activity was potentiated following treatment of wild-type S49 cells and L6 myoblasts with pertussis toxin. Although the ability of receptors on membranes prepared from L6 myoblasts to bind the agonist [3H]HBI was not affected by treatment of cells with pertussis toxin, treatment of cyc- S49 cells with pertussis toxin markedly decreased the ability of receptors to bind [3H]HBI. The observed inhibition of the binding of the agonist [3H]HBI to beta-adrenergic receptors on membranes prepared from cyc- S49 cells after treatment with pertussis toxin could be explained by an interaction between beta-adrenergic receptors and the inhibitory guanine nucleotide-binding protein. Such an interaction may represent a mechanism through which stimulation of the activity of adenylate cyclase by beta-adrenergic receptors can be regulated or through which beta-adrenergic receptors can affect the activity of cyclic AMP-independent cellular processes.