Heterologous Expression of Receptors and Signaling Proteins in Adult Mammalian Sympathetic Neurons by Microinjection

Heterologous expression of receptor proteins provides a means of studying a molecularly defined receptor subtype in isolation from species closely related either by function and/or homology. For example, metabotropic glutamate receptors (mGluRs) comprise a large family (presently eight distinct subtypes, plus splice variants) of G protein-coupled receptors (GPCRs) for which L -glutamate is the endogenous hgand (1 ,2 ). However, identifying a response arising from a single natively expressed mGluR subtype is problematic, because pharmacological agents with the required specificity are not currently available. Moreover, other proteins that interact with L -glutamate, such as ionotropic glutamate receptors (1 or glutamate transporters, are often coexpressed in neurons along with mGluRs, thus confounding the situation further. Finally, even the apparent presence of only a single receptor subtype in a given cell (e.g., as determined by mnnunohistochemistry) is not sufficient to allow unambiguous assignment of a response to a particular receptor subtype, since additional uncharacterized members of a receptor family may exist. Thus, assigning roles to, and characterizing signaling pathways of, specific mGluR subtypes in their native environment is a difficult task.