| Octopamine (OA) is found in high concentrations in insects and has been physiologically compared to the vertebrate fight-or-flight hormones. The existence of OA receptors has been demonstrated by a number of techniques and most OA receptors are coupled to an OA-sensitive adenylate cyclase (OSAC). Studies of this enzyme provide a reliable method for characterizing the pharmacology and biochemistry of the OA receptor. The objectives of the present study were: (1) to identify a high-affinity ligand for the OA receptor, (2) to identify an insect cell-line model system for studying the OA receptor, and (3) to investigate the presence of phosphoinositide hydrolysis-coupled OA receptors.; The imidazoline, 2,3-xylylaminomethyl-2{dollar}spprime{dollar}-imidazoline, XAMI, was shown to have the highest reported affinity for insect OSAC, with a K{dollar}sb{lcub}rm a{rcub}{dollar} 187, 158, 165, and 81 times more effective than OA in cockroach nerve cord, brain and hemocytes and tobacco hornworm nerve cord, respectively. XAMI-mediated cAMP production is dependent upon GTP and enhanced by forskolin, suggesting that XAMI activates adenylate cyclase (AC) via interaction with a receptor-G-protein complex. Additivity studies suggest that XAMI interacts specifically with the OA receptor. These effects of XAMI are reversible, exhibit agonist-induced densensitization and are pharmacologically similar to other OA-mediated effects. Thus, because of its specificity and high affinity, XAMI may be a good general probe for the OA receptor.; Sf9 cells from Spodoptera frugiperda were shown to contain a G-coupled OSAC. The pharmacology of this receptor is similar to that of other Lepidopteran OA receptors but differs from the cockroach receptor, suggesting the existence of species-specific OA-receptor subtypes. Therefore, Sf9 cells can be used as a readily available, homogeneous model system for studying Lepidopteran OA receptors.; Using a radioreceptor assay, the ability of OA agonists to elevate inositol 1,4,5-trisphosphate (IP{dollar}sb3{dollar}) levels in cockroach brain slices was demonstrated. This effect is blocked by phentolamine and is not an indirect consequence of elevated intracellular cyclic AMP levels. This is the first report of OA-mediate inositol phosphate hydrolysis. |
