ABSTRACT
The previous chapters have described the receptors for the neurotransmitters, co-transmitters and neuromodulators of the autonomic nervous system and their 476 477 478classifications. With few exceptions, these receptors belong to a G protein-coupled superfamily which display common features. Many of the properties of these receptors have already been mentioned, including their coupling to G proteins, the second messengers involved, and features of their structures important for agonist and antagonist binding. This chapter is intended to assemble this information on the major receptor subtypes already considered. It is beyond the scope of this book to deal with each receptor type individually and all aspects of their structures, coupling and transduction mechanisms to the final tissue response. Instead, an attempt has been made to summarize the main generalizations that appear to apply to the majority of receptors already considered. Most of these receptors have now been cloned and their amino acid sequences determined and, as a consequence, the following common features have emerged.
They are located on the extracellular surface of the cell membrane with seven transmembrane spanning hydrophobic domains (Gm7).
They are coupled to a guanine nucleotide binding protein (G protein).
The pharmacological effects of agonist interactions are mediated through two second messenger systems: cyclic adenosine 3’,5’-monophosphate (cAMP), levels of which may be elevated or supressed, or stimulation of phosphoinositide turnover to generate inositol triphosphate (IP3) and diacylglycerol (DAG).
Some responses may also be produced by direct coupling through the G protein to ion channels without the intermediate second messenger.
