Neurotransmitter receptors known to mediate their cellular actions through guanine nucleotide regulatory binding proteins (G proteins) comprise a large and diverse gene superfamily and modulate the activity of a variety of important physiological functions. Ethanol has been shown to specifically affect G protein-mediated signal transduction and this finding suggests that changes in G protein-linked systems may be involved in the manifestations of chronic ethanol ingestion. In order to better understand the molecular mechanisms of signal transduction mediated by this family of receptors we have utilized a number of molecular biological approaches including gene cloning, permanent receptor gene expression in cultured cells, and site-directed mutagenesis to study the relationship between receptor structure and function and the mechanisms of receptor regulation. CHO cell lines permanently transfected with a single muscarinic acetylcholine receptor subtype are proving to be of great utility in the characterization of muscarinic agonists with M1 subtype selectivity. These agents are considered as candidate drugs for the treatment of Alzheimer's disease. The ability to modify the amino acid sequence of a receptor using site- directed mutagenesis has allowed for the identification of conserved cysteine residues in muscarinic acetylcholine receptors that are involved in ligand binding and receptor activation by agonists. Also, we have demonstrated that beta-adrenergic and muscarinic acetylcholine receptors expressed in the same cell line are down-regulated in a coordinated fashion by agonists. Since both of these receptor subtypes can be found on peripheral tissues and in the central nervous system, the phenomenon of cross-regulation becomes an important area of study. Lastly, clones encoding a fat cell specific beta-adrenergic receptor (beta3A-subtype) were isolated from a rat brown adipose tissue cDNA library. Expression of this receptor, which is most likely involved in mediated lipolysis in adipose tissue is reduced 60-70% in obese (fa/fa) Zucker rats as compared with lean controls, suggesting that decreased levels of this receptor may play a role in obesity.
